ML20065L532
| ML20065L532 | |
| Person / Time | |
|---|---|
| Site: | Summer  |
| Issue date: | 06/30/1982 |
| From: | Peck L, Pitcher D, Secor D SOUTH CAROLINA ELECTRIC & GAS CO. |
| To: | |
| Shared Package | |
| ML20065L526 | List: |
| References | |
| NUDOCS 8210190457 | |
| Download: ML20065L532 (153) | |
Text
{{#Wiki_filter:, [ FOURTH TECHNICAL REPORT [ [ [ [ ! ( Geologic Studies in an Area ; [ of Induced Seismicity at [ l Monticello Reservoir, South Carolina i [ ( : ( ! l [ i [ [ I
!Pd p
13 8 M ' J "dPDR 8% [ June, l982 f ;
E E FOURTH TECHNICAL REPORT E GEOLOGICAL STUDIES IN THE AREA 0F INDUCED SEISMICITY AT MONTICELLO RESERVOIR, SOUTH CAROLINA by Donald T. Secor, Jr., Principal Investigator I and L. S. Peck, D. M. Pitcher, D. H. Simpson and W. A. Smith j June, 1982 I Sponsored by the U. S. Geological Survey Contracts No. 14-08-0001-19124 and 19833 I I I The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied of the U. S. Government. I - I -
i PREFACE This report presents the results of geological and geophysical studies that have been underway in the area of induced seismicity at Monticello Reservoir, South Carolina, since March 1, 1980. The purpose of this study is to provide the geological information necessary for understanding the origin of the induced seism'. city. In particular, the study was directed toward determining: 1) if there are laterally extensive faults in the region that might eventually slip, thus producing a large carthquake; 2) if there are lit! alogical boundaries other than fault contacts in the vicinity of the reservoir contro*. ling the distribution of seismic activity, and; 3) if there are foliations or systematic jointing directions alcng which slip might be initiated by the stress and pore pressure changes related to reservoir impoundment. The data base for making the above evaluations was obtained by:
- 1) preparing 1:24,000 scale geological maps of the Chapin, Little Mountain,
'Jenkinsville and Po= aria quadrangles (Plates 1-4); 2) measuring 125 kilometers of surface magnetic profiles (as an aid in locating Mesozoic diabase dikes, Plates 5-8); and 3) measuring the orientations of approximately 100 joint fractures at each of 50 localities in the study area (Plates 9-12) . The overall results and conclusions of this study are summarized in Chaptar I which is modified from a paper that will be published in the Journal of Geophysical Research [Secor and others, in press]. The results of geological field studies in the Chapin, Little Mountain, Jenkinsv111e and Pomaria quadrangles are presented in Chapters 2-5, respectively. These ' chapters are modified from University of South Carolina H.S. theses by David H. Simpson [1981], Lawrence S. Peck [1981], William A. Smith [1982] and David M. Pitcher [1982], respectively. Geological maps (1:24,000 scale) for I I
~
11 the above four quadrangles are presented in Plates 1-4. The surface magnetic l ( profiles for the quadrangles are presented in Plates 5-8, and the results of joint orientation studies are presented in Plates 9-12. We wish to thank R. E. Dooley, P. D. Fullagar, D. C. Prowell, A. W. Snoke, ( Pradeep Talwani and M. D. Zoback for providing us with research results prior to publication. D. C. Prowell of the U. S. Geological Survey arranged for the drilling of four shallow boreholes which were useful in the evaluation of the Wateree Creek fault zone. The cooperation of R. B. Whorton of the South Carolina Electric and Gas Company, William Smith of Dames and Moore, and N. K. Olson and Paul Nystrom of the South Carolina Geological Survey is greatly appreciated. Regional geological studies in the vicinity of Monticello Reservoir were supported by National Science Foundation grant EAR 8020474. The following University of South Carolina students assisted with geological and geophysical
- field studies: Mel Blackwell, Ken Bramlett, Bruce Crawford, Steve Durgin, J. C. Hare, Jack Horkowitz, Chris Jones, Phil Kimbrell, '
Sarah Logan, Mark Snyder, and John Willis. Many of the illustrations were
. drafted by Tania Lush, and the manuscript typing was done by Joyce Goodwin.
1 ( [ [ [ [ .
l iii TABLE OF CONTENTS Chapter Page PREFACE . . . . . . . . . . . ... .... . . . . . . . . . . 1 LIST OF FIGURES . . . . . . . .. .. .. . . . . . . . . .. . vii LIST OF TABLES . . . . . .... . .. . . . . . . . . . .. . x I GEOLOGY OF THE AREA 0F INDUCED SEISMIC ACTIVITY AT MONTICELLO RESERVOIR, SOUTH CAROLINA: A
SUMMARY
. . . . . . 1 ABSTRACT. . . . .. .... . .. . . . . . . . . .. . 1 INTRODUCTION . ... ... .... . . . . . . . . . . 2 ROCK UNITS. . . ... . .. ... . . . . . . . . . .'. 5 DEFORMATIONAL HISTORY AND MAJOR AND MINOR STRUCTURES. . 14 DISCUSSION. . . . . .... . .. . . . . . . . . .. . 20 II THE WATEREE CREEK FAULT ZONE . ..... . . . . . . . . . . 27 ABSTRACT. . . . . .. ... . .. . . . . . . . . . . . 27 -
INTRODUCTION. . . . . ... . .. . . . . . . . . . .. 28 AREAL GEOLOGY . . .. ... . ... . . . . . . . .. . 29 EVIDENCE FOR THE WATEREE CREEK FAULT ZONE . . . . .. . 33 Regional Relations . . ... . . . . . . . . . . . 33 The Spring Hill Roadcut. ... . . . . . . . . . . 35 Determination of the Net Slip Direction. . . . . . 38 Results of Drilling Investigations . . . . . . . . 40 Interaction of a Diabase Dike with the Wateree Creek Fault Zone . . . . . . . . . . . . . 43
~
SUMMARY
. . . . . . . ... . ... . . . . . . . . . . 46 I
I I I
iv Chapter Page-III GEOLOGY OF THE LITTLE MOUNTAIN AND CHAPIN QUADRANGLES, SOUTH CAROLINA: A STUDY OF THE STRATIG2APHIC RELATIONSHIP-48 ( BETWEEN-THE CAROLINA SLATE BELT AND THE CHARLOTTE BELT . .. ABSTRACT ... . . . .. . . ............. 48 INTRODUCTION .. . . ... . ..... ........ 49 REGIONAL GEOLOGY . . ... ... ........... 50 The Charlotte Belt .. ... ........ .. . 50 The Carolina Slate Belt ........ . . . .. 52 Regional Deformation . ... ....... . . . . 55 DESCRIPTION OF THE ROCKS . . ..... ...... . . 56 Charlotte Belt Units . . ... ... .. . . . .. 56 Amphibolite (Ga). . ..... . . .. . . .. 56 Biotite schist and biotite paragneiss . (Gfgn). 58 Undifferentiated granitic orthogneiss (Gggn). 59 Carolina State Belt Stratigraphic Units. . . . .. 60 Impure quartzite, metatuff, and ( white mica schist (6tq) . . . . ... . . .. 60 Metavolcanic tuff breccia unit (Gtb). .. .. 63 Evenly laninated to massive metamudstone-wacke unit (Gmw). ...... . .. . . .. . 64 Amygdaloidal greenstone (Gg). ..... ... 65 Post-Metamorphic Intrusive Rocks . .... ... . 66 Gabbro (Cgb) .. ...... .... . ... 66 Diabase Dikes (JTrd) . . ....... ... . 66 STRUCTURAL GEOLOGY. . .. . ........... . . . 68 Ductile Deformational Chronology . ........ 68 Faulting . . . . .. . . ......... . .. . 72 DISCUSSION AND CONCLUSIONS. .... .......... 76 ni., . .i... .. .. .- .
l Chapter Page IV THE BEDROCK GEOLOGY OF THE JENKINSVILLE 7h' QUADRANGLE, SOUTH CAROLINA . . . . .. . . ... . . . . . . . . . . . . 80 INTRODUCTION. . . .. . .. .. . . . . . . . . . . . . 80 REGIONAL GEOLOGY. . . . . .... . . . . . . . . .. . 81 i ROCK DESCRIPTIONS . . . . . .. . . . . .. . . . . . . 83 l Amphibolite (Ga) . .. . . . . . . . . . ... . . 83 Felsic Gneiss (Gfgn) . . .. . . . . . . . ... . 85 Sillimanite Schist (Gss) . . . . . . . . .. . . . 87 Newberry Plutonic Complex (Sn) . . . . . . . . . . 87 Quartz Diorite (Cd). . . . . . . . . . . . . .. . 91 Winnsboro Plutonic Complex (Cw). . . . . . . . .. 92 Diabase Dikes (Jd) . .... . .. . . . . .. .. 96 REGIONAL RELATIONSHIPS AND DISCUSSION ,. . . . . . . . 100 CONCLUSIONS . . . .. .. .. .. . .. . . . . . .. . 104 V THE BEDROCK GEOLOGY OF THE P0 MARIA 7 ' QUADRANGLE, SOUTH CAROLINA . . . . .. . . . . ... . . . . . . . .. . 106 INTRODUCTION. . . .. . . .. . . . . . . . . . . . . . 106
. REGIONAL GEOLOGY. . . . .. .. . . . .. . . . . . . . 106 ROCK DESCRIPTIONS . . . ... . . . .. . . . . . . . . 108 Amphibolite (Ga) .. . . . . . . .. . . . . .. . 109 Felsic Gneiss (Gfgn) . . .. . .. . . . . .. . . 111 Biotite paragneiss. ... . .. . . . ... . 112 Granitic orthogneiss. . . . .. . . . . .. . 113 E Newberry Granite (Sn). . . . . .. . . . . .. . . 114 Winnsboro Plutonic Complex (Cw). . . . . . . . . . 119 Pomaria Granite (Cp) . . . . . . .. . . . .. . . 124 Other Rock Types . . . ... . .. . . . . ... . 125 I
b.
' vi J
[ Chapter Page STRUCTURAL GEOLOGY. . ... .... .. . . .. . .. . 126 CONCLUSIONS . . . .. ... .... . . . . . . .. . . 129 REFERENCES CITED. . .. ... . .. .... . . . .. . ... . 131 u l 3 c 3- . c t l .. . _ _ _ _ _ _ _ _ _ _ _ . _ _ . . _ _ _ _ . __ - . . _ - -
l vii I LIST OF FIGURES Figure Page 1 Distribution of induced seismic activity in the vicinity of Lake Monticello during the period June, 1978-September. l 1979, inclusive (modified frota Talvani and others [1980]). W Dots indicate the locations of induced earthquakes. The locations of the.U. S. Geological Survey boreholes #1 and E
#2 are indicated. The asterisk indicates the location of E the Virgil C. Su=mer nuclear station. Geological symbols and contacts are from Figure 2. . . . . ... . . . ...... 4 2 A geological map of the Chapin, Little Mountain, Jenkinsville and Pomaria quzdrangles (U. S. Geological Survey 71s' topographic series) . . . . . . . . . . . ...... 6,7 3 Lithologic log for Monticello #1 and #2, compiled from core and cutting samples . . . ...... . ... . . . . . . . . 11 4 Aeromagnetic map of the Chapin, Little Mountain, Jenkinsville and Pomaria quadrangles (modified from g U. S. Geological Survey [1978]). C-atour interval 40 gammas, Dotted lines indicate the positions of geological contacts g
from Figure 2 . . . . . . . . . . . . . . . . . . .... . . . 12 5 Lower hemisphere, equal-area projections of structural data
~
- 1. rom the region south of Monticello Reservoir. (a) 63 L1 elongation lineations from the slate belt portion of the E Chapin and Little Mountain quadrangles. Contours: 1, 5, 3 15 and 25%. (b) 41 LO intersection lineations from the slate belt portion of !fie Chapin and Little Mountain quadrangles. Contours: 1, 5. 15, 25 and 35%. (c) Poles to 713 Si foliation planes from the slate and Charlotte belts in the Chapin and Little Mountain quadrangles.
Contours: 0.5, 2, 5 and 10%. (d) 106 Loxi lineations from the Wateree Creek fault zone in the vicinity of Spring Hill in the Chapin quadrangle. Contours: 1, 5, 12 and 20%.
"A" is axis of drag folding, "B" is the net-slip vector for the Wateree Creek fault zone. . . ....... . . . . . 15 6 Geological cross sections across the Pomaria, Jenkinsville and Chapin quadrangles. See Figure 2 for location of section lines . ............... . . . . . . . . 17 7 Lower hemisphere, equal-area projections of poles to joint l fractures measured at several localities in the Jenkinsville 5 quadrangle. The nu=ber in parentheses is the number of l
measurements represented on each projection. Contours: 1, 3, 6, 9 and 12%. . . . . . . . . . . . . . . . . . . . . . 21,22 I
viii g are Page g 8 Diagram illustrating the projection of the contact between J Gmw and 6tb in the axial zone of the Delmar synclinorium onto the plene of the Wateree Creek fault zone. . . . . .. . . 34 9 Detailed geologic sketch map and cross sections of the Spring Hill roadcut in the central part of the Chapin quadrangle (Fig. 2, and Plate 1). . . .. . . . . . . . ... . 36 10 Location map for the Spring Hill roadcut and vicinity . . . . . 39 3 11 Geologic map showing the configuration of a diabase dike j in the vicinity of the Wateree Creek fault zone in the south-central part of the Chapin quadrangle . . . . . . . .. . 41 { 12 Core logs for drill holes 1, 2, and 3, from the Wateree T Creek fault zone near the Spring Hill roadcut (Fig. 10) .. . . 42 g 13 Map showing major geologic subdivisions of the southern g Appalachians (modified from Will.iams [1978]). Box encloses area shown in Figure 14. . . ... . . . . . . . .. . 51 14 Geologic map of the western Carolina slate belt of South Carolina (map modified from Overstreet and Bell [1965a]; Carpenter [1976]; Brown [1971]; Pirkle [1977]; Pirkle [1978]). Box encloses the Chapin and Little Mountain I quadrangles (see also Fig. 2) ................54
- 15 Lower hemisphere, equal-area projections of 42 L i elongation lineations measured in the Charlotte belt portion of the Little Mountain and Chapin quadrangles.
Contours: 1, 5, 15, and 25%. . . . . . .. . . . . . . . . . . 70 16 Lower hemisphere, equal-area projection of poles to 79 S0 bedding planes measured in the slate belt portion { of the Little Mountain and Chapin quadrangles. Contours: J 1, 2, 5, 10, and 15%. . . . . . . . . . . . . . . . . . . . . 71 17 Lower hemisphere, equal-area projection of poles to 438 Sy foliation planes measured in the slate belt portion of the Little Mountain and Chapin quadrangles. Contours: , 2, 5, 10, and 15% . . . . . . . . . . . . . . . 73 18 Lower hemisphere, equal-area projection of poles to 275 St foliation planes measured in the Charlotte belt portion of the Little Mountain and Chapin quadrangles. Contours: h, 2, 5, 10, and 15% . . . . . . . . . . . . . . . 74 1 1 I l
~ E Figure Page 19 APQ diagram showing the modal variation for the Newberry E pluton (Sn e) and the c.uartz diorite (Cd, +). Diagram 5 after Streckeisen [1967: where AG - alkali feldspar granite, AQS - alkali feldspar quartz syenite, AS - 3 alkali feldspar syenite, GA and GB - Granite A field and B field, QS - quartz syenite. S - syenite, QM - quartz 5
monzonite, M - monzonite, GD - granodiorite, QMD - quartz monzodiorite, MD - monzodiorite, T - tonalite QD - quartz diorit;. and D - diorite Modes were obtained by counting 1000 point s each . . . . . . . . . . . . . . . . . . . . . . . 89 20 APQ diagram showing the modal variation within the Winnsboro pluton (Cw) for seven thin sections and sf x stained slabs from the Jenkinsville quadrangle. Modes were obtained by counting 1000 points each. . . . . . . . . . . 93 21 An olivine-plagioclase-pyroxene-quartz diagram showing the modal variation of seven diabase dikes from the Jenkinsville quadrangle. Modes were obtained by counting 1000 points each. . . . . . . . . ... . . . . . . . . . . . . 99 22 Lower hemisphere, equal-area projection of pc,les to 138 Si foliation planes from~the Jenkinsv111e quadrangle. Contours: 2, 4, 6, 8 and 11%. . . . . . . . . . 102 23 APQ diagram showing the medal variation for the Newberry pluton (+) in the Pomaria quadrangle. Diagram after Streckeisen [ 1967]. . . . . . . . . . . . . . . . . . . . . . 117 24 Aeroradiation map of the Pomaria quadrangle. Contour interval is 50 counts per second. From the U. S. g Geological Survey [1978]. Dotted lines are geological 5 contacts from Plate IV. .... ....... . . . . . . . . .121 25 APQ diagram showing the modal variation for the Winnsboro pluton (o) and the Pomaria pluton (=) in the Pomaria quadrangle. Diagram af ter Streckeisen [1967] . . . . . . . . 123 26 Lower hemisphere, equal-area projection of poles to 97 Si foliation planes measured in the Pomaria quadrangle. Contours: 1, 3, 9 and 15%. ......... . . . . . . . . .128
- I I
a I
l I LIST OF TABLES j Table Page 1 Mineral assemblages of samples from the Charlotte belt portion of the Chapin and Little Mountain quadrangles . . . . . 57 l 2 Mineral assemblages of samples from the slate belt portion of the Chapin and Little Mountain quadrangles . . .. . 61 } 3 Mineral assemblages of samples from the post-tectonic intrusive rocks of the Chapin and Little Mountain quadrangles . . . . . . . . . .. . . . . . . . . . . . . .. . 67 4 Metamorphic mineral assemblages for amphibolite (Ga) south of the Winnsboro pluton (Cw)/amphibolite (Ga) boundary in the Jenkinsville quadrangle . . . . . . . . . . . 84 5 Metamorphic mineral assemblages for amphibolite (Ga) I enclaves and xenoliths within the Winnsboro pluton (Cw) in the Jenkinsville quadrangle. . . . . . . . . . . . . . . . 86 6 Modal mineralogical data for the Winnsboro pluton (Cw), quartz diorite (Cd), and the Newberry pluton (Sn) in I the Jenkinsville quadrangle. Modes were obtained by , counting 1000 points per thin section . . . . . . . . . . . . 90 l l 7 Modal data for six diabase dikes (Jd) in the Jenkinsville g quaurangle. Modes were obtained by counting 1000 points g per thin section. . . . . . . ....... . . . . . . . . . 98 I 8 Metamorphic mineral assemblages for amphibolites (Ga) from the Pomaria quadrangle . . . . . . . . . . . . . .. . . 110 l 9 Metamorphic mineral assemblages for biotite paragneiss and granitic orthogneiss within the felsic gneiss unit (Gfgn) of the Pomaria quadrangle. . . . . . . . . . . . . . . .115 l 10 Modal mineralogical data for the Newberry, Pomaria and I Winnsboro plutons in the Pomaria quadrangle. Modes were obtained by counting thin sections, with the numbers of points indicated in the table . . . . . . . . . . . . . . . . .118 1 B - l 1 I l
xi LIST OF PLATES (in pocket) Plate 1 Geologic map of the Chapin quadrangle, South Carolina (U. S. Geological Survey 7h' topographic series). See Figure 2 for legend. 3 Modified from Simpson [1981] and Peck [1981]. 5 2 Geologic map of the Little Mountain quadrangle, South Carolina 3 (U. S. Geological Survey 7h' topographic series). See Figure 2 for g legend. Modified from Peck [1981]. 3 Geologic map of the Jenkinsville quadrangle, South Carolina (U. S. Geological Survey 7 ' topographic series). See Figure 2 for legend. Modified from Smith [1982]. 4 Geologic map of the Pomaria quadrangle, South Carolina (U. S. Geological Survey 7 ' topographic series). See Figure 2 for legend. Modified from Pitcher [1982]. 5 Surface magnetic profiles across the Chapin quadrangle, based on field measurements made every 25' along the indicated section lines. 6 Surface magnetic profiles across the Little Mountain quadrangle, based on field measurements made every 25' along the indicated I section lines. 7 Surface magnetic profiles across the Jenkinsville quadrangle, based on field measurements made every 25' along the indicated section lines. 8 Surface magnetic profiles across the Pomaria quadrangle, based on field measurements made every 25' along the indicated section lines. 9 Lower hemisphere, equal-area projections of poles to joint fractures measured at localities in the Chapin quadrangle. The number in 3 parentheses is the number of measurements represented on each 5 proj ection. Contours: 1, 3, 6, 9 and 12%. 10 Lower hemisphere, equal-area projections of poles to joint fractures measured at localities in the Little Mountain quadrangle. The number in parentheses is the number of measurements' represented on each projection. Contours.: 1, 3, 6, 9 and 12%. 11 Lower hemisphere, equal-area projections of poles to joint fractures measured at localities in the Jenkinsville quadrangle. The number E in parentheses is the number of measurements represented on each g projection. Contours: 1, 3, 6, 9 and 12%. 12 Lower hemisphere, equal-area projections of poles to joint fractures measured at localities in the Pomaria quadrangle. The number in parentheses is the number of measurements represented on each projection. Contours: 1, 3, 6, 9 and 12%. I
l CHAPTER I GEOLOGY OF THE AREA 0F INDUCED SEISMIC ACTIVITY AT MONTICELLO RESERVOIR, SOUTH CAROLINA: A
SUMMARY
ABSTRACT I This study provides geological background information necessary for an I evaluation of the earthquake hazard in an area of induced seismic activity at Monticello Reservoir, South Carolina. This region contains a thick stratified sequence of Proterozoic Z and Cambrian metasedimentary and metavolcanic rocks. In the early to middle Paleozoic, this sequence was recrystallized and deformed under metamorphic conditions that ranged from greenschist to amphibolite facies and experienced at least two episodes of folding. The region has been intruded by late- to post-kinematic granitoid plutonn of Silurian and Carboniferous ages, and by numerous northwest-trending diabase dikes of Late Triassic and Early Jurassic age. The region south of Monticello Reservoir in the Carolina slate belt experienced two episodes of faulting in the late Paleozoic and/or early to middle Mesozoic. The older group of faults trends approximately east, has only small displace-ments, and is characterized by extensive silicification of the fault zones. The younger group of faults trends approximately north, has experienced dip-slip displacements up to 1700 m, and is characterized by carbonate mineraliza-tion in the fault zones. Both seta of faults are cut by an undeformed diabase dike of Late Triassic or Early Jurassic age. The induced seismic activity I around Monticello P servoir is occurring in a heterogeneous quartz monzonite pluton of Carboniferous age. Although laterally extensive faults have not been found in the vicinity of the reservoir, the pluton contains large enclaves of country rock and is cut by numerous, diversely oriented small faults and B 5
I joints. These local inhomogeneities in the pluton, together with an irregular stress field, are interpreted to control the diffuse seismic t activity around the reservoir. In view of the apparent absence of lengthy faults, it is unlikely that a large magnitude earthquake will occur in 1 response to the stres.s and pore pressure changes related to the impoundment l . of Monticello Reservoir. INTRODUCTION One of the most thoroughly documented cases of induced seismic activity in the eastern United States is located near the Virgil C. Su=mer nuclear station in Fairfield County, South Carolina [Talvani, 1979; Talvani and t ! o thers, 1980]. Two artificial reservoirs (Pr.rr Shoals and Monticello) have I . been impounded in this region (Fig.1). Parr Shoals Reservoir was impounded l in the valley of the Broad River in 1914, and had a volume of .005 km 3, l ! a surface area of 7.5 km2 and a maximum depth of 11 m. In 1976, as a result of modification of Parr Shoals Dam, the volume, surface area and maximum depth were increased to .039 km3 , 18 km2 and 13 m respectively. Monticello Reservoir was impounded in the valley of Frees Creek during late 1977 and early 1978 and has a volume of approximately 0.5 km3, a surface area of 27 km 2 and a maximum l depth of 48 m. Most of the induced seismic activity (Fig. 1) is temporally and spatially associated with Monticello Reservoir. Prior to the impoundment of Monticello Reservoir, this region had a very low level of seismic activity. Filling of the reservoir commenced on December 3,1977, and was completed on February 8,1978. About three weeks af ter filling was initiated, the frequency of local earthquakes abruptly increased by a factor of approximately 100. Thelargestevents(g 2.6-2.9) occurred during the first two years following impoundment. Although the 5 I
3 h general level of seismic activity has gradually decreased with time, it is
.still substantially greater than the pre-impoundment level of seismicity.
Monticello Reservoir has been the focus of an intense research effort aimed at evaluating the seismic hazard and developing-a capability for predicting earthquakes [Talwani and others, 1978, 1980; Talwani, 1979; ( Zoback,1979; Secor,1980; secor and others,1981; Zoback and Hickman, in press]. The reservoir is located in a geologically complex area of the Charlotte belt in the Piedmont province of central South Carolina (Fig. 2, and Plates 1-4). The region is characterized by low relief, deep chemical ( weathering and extensive forest cover. Reliable geological maps can only be made by painstakingly traversing the intricate network of gullies and streams within'the area. Although several geological studies had been completed in i the vicinity of the reservoir [Kesler, 1936, 1972; McCauley,1961; Overstreet and Bell, 1965a; McKenzie and McCauley, 1968; Secor and Wagener, 1968; Wagener, 1970, 1973, 1977a; Costain and others, 1976, 1977; South Carolina Electric and Gas Company,1977, p. 2.5 2.5-324; Glover and others,1977; Bourland and Farrar, 1980], these were mostly of a topical or reconnaissance nature and the structure and distribution of rock units was poorly known at the time of impoundment. The present study was initiated in order to provide , I a geological framework necessary for the interpretation of the seismic activity. In particular, our studies are directed toward determining: 1) if ;
- there are laterally extensive faults in the region that might eventually slip, thus prcducing a large earthquake; 2) if there are lithological boundaries 5 '
other than fault contacts in the vicinity of the reservoir controlling the distribution of seismic activity and; 3') if there are foliations or systematic jointing directions along which slip might be initiated by the stress and
Il I I 81'22.5 81 15' 34* 23.5* N - e cw i E g T.
. L g . ,
. Vnd .
0 ,*,
.A3* , ,n d -Y
/ . .s', ,. ., i o e *4'. . ,'*[.', s'
. . ' '[h'l.s.4
' . * ./w 1
7
, .;*.gk.,.
- d. ',; '. .
l 7.. ' . E"
. . y . v 4 , , 4
- 4. ,
1 AKE MoNilCELIO Cw g PARR s : . 3: . 3 shoals ustavoin
.#',.2 . .* * . J1d
. nd 3
^p-
. 5
. "h . ,
a <i. l _ ~ ~ - Q*, s *, # --
~ n
<. -~,
PAna 5HoAts DAM n d,3
' g) Q92 '
0 W1d 5 Kilom.tersk ' 'J
' I;;Ip. '
6Jte ' fea po 15 ' I I Figure 1. Distribution of induced seismic activity in the vicinity of Lake Monticello during the period June, 1978-September, 1979, inclusive (modified from Talwani and others [1980]). Dots indicate the locations of induced earthquakes. The locations of the U. S. Geological Survey boreholes #1 and
#2 are indicated. The asterisk indicates the location of the Virgil C. Summer nuclear station. Geological symbols g and contacts are from Figure 2. g I
5 pore-pressure changes related to reservoir Lapoundment. In order to make the above evaluations, we prepared detailed geological maps of the Chapin, Little Mountain, Jenkinsville and Pomaria quadrangles (U. S. Geological Survey 7 1/2' topographic series) surrounding the reservoir (Fig. 2, and [ Plates 1-4). These maps constitute the main data base for the following geological synopsis. ROCK UNITS The southern one-third of the study area is underlain by greenschist-facies metavolcanic and metasedimentary rocks of the Carolina slate belt, and the northern two-thirds of the area is underlain by amphibolite-facies metaigneous and metasedimentary rocks and post-kinematic plutonic rocks of the Charlotte belr (Fig. 2, and Plates 1-4) . The stratified rocks in the central and southern part of the study area constitute a sequence several kilometers thick, and the retaplutonic rocks form lenticular masses in this sequence. This lith 9 stratigraphic assemblage is considered by many geologists [e.g. Butler and Ragland,1969b; Seiders and Wright, 1977; Black,1978,1980; Whitney and others,1978] to have accumulated during the evolution of a Proterozoic Z co Cambrian magmatic arc. The older parts of this sequence are units of felsic gneiss (Gfgn) and amphibolite (Ga). Relict compositional
-layering and volcanic textures in these rocks suggest that they nave resulted from the metamorphism of quartzofeldspathic sedimentary and volcanic rocks (Gfgn) interlayered with mafic tuffs and/or flows (Ga). The felsic gneiss and amphibolite units have been injected by numerous partially concordant sheets of granitoid intrusive rocks (Gggn) related to overlying coeval
{' extrusive felsic metatuffs (6tq) . Both the intrusive granitoid orthogneiss t
EXPLANATION LITHOLOGIC UNITS SYMBOLS
.....J.,
J.<mw; . sirat gr.phic or '- i
=< Jd in.ha., Jit.
net - J. tone and ocedWJ4be intTusate contact # ! g , 7 . {g Amygdaleid43 = = = = un.tlictised g greenstone 3 - Fault zone 4 O g Cgb i;.bbroic m t. Ctb Inter-eJi.e. pelatuf f breccia
- . . - . .a u. iri.4
- C 4 '
~
m O ~ i. f,'(i s',"!I',J.,
..-{ g .: letsic metatoff AtIituJe or 70 minnsboro
,';[, . W,,,7; p g ,g,,,( g,,p g ,, aAy n _g # .A' gO
$g and quart:lte E
compositional i rerin, .,
- m "? !s tur4matici roi ia t s.n.is.: 4 e
o O Cggn lem.cratic
. s p.
is!IincJgletti 2 8 8 / St granatotJ anelse in settical (right) 4 32 C - 5 ;..;::** g4 ~. .* .*, $ gg***, . .Newberry (fgn Feinic biotite Bearing anJ lunge of 8 g,g anJ g
# I0
% ** f ***
e, .y..,* plutonic compten gneiss (granstic) lineations - Q _
- l,* :: ,:.*:.
/
, ., Amphibolite anJ Asis or eterturacJ p t.ng.
* . f.,,{.
,. ;Q.l.l.,*7,'
besablenJe snriss synelane / l Figure 2. A geological map of the Chapin, Little Mountain, Jenkinsville and j Pomaria quadrangles (U. S. Geological Survey 71' 5 topographic series). l l 1 0% 1 l l l M M M M M M M M M & M M M M M M M M M
1 7 I . Q s...'....,--
.. . . , . .... ... ..........~.......b~p,s-
= . . . . . . . . * . .*....y.
. ,,,s,.,
,e,:. .n. v . :.,, ~ .e~ ,, ~ p,,
,,J.i.d, u
- pn' *, '. .*=. .. .,i ,%. . * ... ;. g . . r. .* .* . .:. * .
- A *:* . . .. .... .;
-.r
.': **...9 s.W.*.%*.': ...*. m.*
u
%. :. .: ' * * * *.q* * .*Je,p <u.n, n :.~.,. ', ', ,, *.:
. : * . . . l* ' * . y n ~~C. .we '.>' :,,'v.,:,.
I .
* ', . . ... .*".* , . . . ..,..*C..fgn
... .. s' t*: ...
.~s *
. 7 1 , . t=
. . . ,;.*. ..e y ,ca. , s, .. ,, .
*s
- =e,, .M m- ~ .c,, v. ^ .2,,,: v,k Q
. .el.e.
- *.:. '.=... *: ;. -;... . ... . . . . v, , ,,,r.'
Sn..,*,.*.. ...ll .
... ?ie +
- f. :c' , c, v,,. s.i.k.
. ,.,-l; :..*.
- 4. .l..
,:........... ....+. ...... *...,..,..,.m.. ~ 4;. . ....... . t a
. , .,. p d . , 'a. . ,.,u, s. ,. .c .; . < %. .
t .. g ... *
. v- * . s . v?
. .; t
]: ;:%;:.. 1 , ,w
.l.' %, . . '. *. . : . * . * : .a. . .- . . p'. N,. c:*;. .H,.\l> . . W,',S . .&o m, ,n
~-o p *&.
e o ~ a'.. l. :. :%. :C.
. .y lgn. ...f .. --
l y ~* c,,
* \p.. , . .
.* .: .*:*l :e.M. .. .V .,.' .h. e 9 .
j q:*: gfan ,s..;. * .../
,4 < ;.':. :.:, M. QN,$.\Citto.,r# \.e ~,p.y a q ,, 0,, L ~d. ;~.r e: , :, .
. .f v a , , ,
. v. . . A,,,,~..
. ~
---,#...,*.*..'.3 **(go W'A's
%% *,s . *.. *N . e., ,l:,-v 1,: ~. . : .
,e .f ~: i s,.,Q)
V ,~, N , -, Ji'~.,d :~ , ,n, . , ,
.l.'..; ggn ,;; LAKE 2. c:
- s. ~
,,~y~,~~.,~p,~,- ,
.',:q,-
E .. g)ve M%
- .y-:.:
- , ~
,y , , ,2 s, f<!,',:y....,.,..,.,,,,-',,.:.,~,.',7,~,
an,,,,
.,,, . ,,,,,y ,::d, . ',,::, .' ~,. , ~q ,s, ,',,-'.
,,~,
MONTICELLO C,;:,..:, .,d11'~, .,::,;,,, ,n,,.,p:, ~,. , sn ,
~:,:. , ~,:,:.C w. ::,~:,:.
~,.,~,s~,~ ,~ , , ,
p:v;. ,;,o ,M,..,,,,,.,~ p ra,s , s .i f:, - ~.' ,:: :.~' , ,2, ~.'. ~;', ::, 2,,,~;, ~.. :.', 2,;2, .
- ), , . , ~
Q:, V ~. b. - . ' ' . : 1, Vf:q , #.* ic,'; Vi '>- ,- M e n 4'Q' U h U *:*c,. ; h
" ' - ' Jid -
J 1' d'. ',%' . ,>, :.*:N,', ,,'", E V. Jf' d :\.%. # , . .. . . ,.. M. , .,l .6 U' ' v, .,C ,i , . .'
.> . q'., i
- V,: > N:
.,,*:,,,;s .
;,y,,,.,,.,,,,,- .s,
,, - . ....;;,j
. p;;,,;,; g
' U' ,,'2'
.Sqv',,.+
.N', ..
9,; , ,; ;,;n , a; ; ; ,, ; ,_ A m,.c, f..,,1..q n)9 n,..,,; .i ,~ ,J. ,, n
, y,, .; ._, ;', d.
n...
.'::: lU,,l.t,#. .D. .:.,'s,:,'., . N,,,. , D. d. ,*, p . :.,.&. . .:.V. , L U, (> M.O, , . : ,.Q,.e. 3' ' , ,. ';&lNN
%..h,,t.s t.e,;; .. .q,,- . ., ... . . 3.2.4':.N,,.,,
.' ,.. .' .:. e, '.s,.,. . s. . ..',q,.... '::vlRGit C. 5UMMER #a - N
- N ::s 's c
' . t. .~ ',..,s.
*, n q N._P. - . '
- 1. ,A1.lO.N ;, ' ' ,<'a' * , , '.
E ~ N,. :.w- .: .. ,. s.- ~, ~e. . .N. . .v, .. .:.,1, . e, . . , . ,"., . y 2. .:.,N U.C. .pa ,p y ,.p $m, L.t.A.R.
.ms :
~~ * <-n~<. u
, , . . .,...,...n.,,...g n , np . , ~ , ,.
Ca ** ~--- n
,,.r. :w., a:.,:.s;.,p.g'. , . - . ~:.,;..- Q,
,.o .~n , o ,&.~,.:,.3;, ~r.y: a:.,:y:.
2:p. p. ,,
. . .s, ..r . .N. . .,.o.p, . . o..p -:.::.... , ; ,~,:... :g+y , r , , ,.. , 74. : . ;.:.w' na m. .
I oo" n. > Q:l, g: .":::::::::::::::.::::r.w.:::::!.(:i.N:~.;*
.4?.:.:.:G:;,':5:i::i:::.;l:;W::@ : .
m a m e. \EE ,lk,I ,HC a..@ 5 k, , ?. 7.'P da <a.A .
- . ,R ... mR i.!..'.,. .:fQ. .[:*. [b.e\n
-'?g> s o; o,h,. ,
,;4.s
- c..,*/ 9 g:....
........?..*.*'.D.A.M w.:. .... ' * * *
; .'. . .. ' Ji f... d '. / ...
'.li.d g .
*W', .. b", I
.g* t- - k_ ~ Q ',gn o * ' '
I . .y
- t. g . .
. :: .:$HOALS ** ::W.
t~ . . .
.M . . . .
'p .
< :.. v. .q
..c.. ~ .":. .... .. ........
.'...:.t . .
. . ./. : . . . .. . . . , . . . . . . . . . . . , %. . , . W $. s 1 d ':='. * * .".
* '. *l:, . } Yd'l. * :.* *'a :.***,9 d?** ,
4 px . .- ,<m.::::n%n'y.f:V."Q:u.'l.*4.%;'&., -*fM). < ...-a 4Gn <f a'IH . . ,+1 ti,..Mt. x.:::::: == Ca%* v.*' .b.w. :. t5.' .??. .n -..,
- 4,.
.: ::t:::,,~,,- s..
Cggn ~& Cggnc.n:m-:.: ::Ca.* . vV. .% 's**~v. 2 '- . . '* . ~ .s' . * .V'*.*s .Y
.~... .
w E wJs ' CCs J
- B*h"<!<*.*'.
. {.;*
th:'C'g'gn
. z ' . . m - ~. s .l. '19 3"'. M'M . ,--~;..-
- r:: *.5.@NC.s - z -[-(#2*Ms.;;;**...
s'-+n* - - - :.'b" : *.k, *f
- N. , 7.'.....
N
, q., M. 'c.n'<O.l. .: . , . V - '. - .W:.::, h. ::: . . . ~n: a'. . ::; e
- 74 is,6:ta:y:; .:.. .h.m;t.::*..c.yf, -m. , 3 c5.a - .- w C.9sa"S.si?..v%.
s.. ::@n
, 9gn .
- rsv' i.,... . p:. .:.C..a:.e..
.. - .e:. - n p. , y
.r.. ... ,wC.a; :-r . .4. D:L.
......s.-
i .. . . ~. .,> ,- . i. n,-,qan :. 2. .+., s.: ": ,,.w
........~m,.n. .
p : :h~ nn ... .. .... . . .. . . . . . . .,
.<....p. . . . g e . .c,.-u ..
d ~,
. . . . a .
........ c.
g.- e. :. m ~ ._Cggn.. ;4;;f.(an. .7 ...-A...f Cggn ob .. nd G9',
' ,. y. 7 , .. . . ~g,..............g W.#.,jj;'6'_a[ : 3f.f "i ,:.>* . . CB
'3'K I
p
.-; .. ,, #,q,.*,Tel*']'~,
-~
- - 4. 2. , .
~ u i D . _. .f.
~ . , ... -- -
p9, . . e .4.p: - ." .' .' .. . . .;n,,.w. - S c9~og, . m.,. ...,.*p'-;;.
~- i
- ,. . m.
. : . .. ro s. .
,,.vN TAINy"a: ?,{. .
-ri :,,,:: .
-a .r LITTLE MOV./ ,: n.y' ;,w. ... p.SPRING -- ----- .wHILL . 3wROADCUTW. ...n$. .o.u 'm;,m
*/ . : . .p f M, ."..; ..
., Cogn .-
gOk
.....r.. .-
', .m. . . .. . r. t.. h. . .. .. ..' .. @: , . ; M.e, D 's I nd W 'u- 7 d Ob 3 .? #
'n N L.? .-'. ' ' 'n' d
^*
.:: .-. .f.i:V, .:... : . s. .. .'
- 2..:Q.....
. if-:,... .,c...
. . 4," *; & 1j S r' -
~
.. d;* + *'.-:* )*: . ..Cmw%
4 re 'i., .s 4 Ob 8 i ." . . . M Cb * . . . ,.. J ,1 d ' ?s
$.ew&:;-W s naA
.:.+*' y 5...re--::, ...2l g 'Cu.
9 ..e'.- (*n*j~p? N -
. C. m .9 y ;. .y..,.:.~......-:.~
- .n,, ?.,. :.&. :.A :}'..',.. .
- s. ..a
~ . - .. ..- .* ..:-
.,/ n d .s.M: W. ,-A.','.. . : h,l& :~? 4: '.. ." . , ..
i .. 4. W.W.i
. ':. f
..f C.p:W 'i ::eT- U ' W:?...FTPE)Cg "
ob
.s/ 5.~.
. ,.",. . . . .:. .: . m i Ctb -4
'O. .T. e.'.,c.3
. gf:.....
y . ::: ~ - - -. . . . . :.: .: . . .f. .Cm- :a: .. w:" ;\ n*.1. s E M~-C - n ; "& v
- so ob G.W..jb'...:n .,> ;. ~ :.n : :# -w'?:d',::C9W -m 4
- .Wa
.u.W(mw '7:
' *i.. * .G y.@s . .:."D,jU t'~ @ d u.a z.. :. ..: : .'1* . :: * ..,- - .. : .:a- ..
,a H,C A'm Y, I . e -- - g " , .,'. T...y g, . Cm.:.:..;
o . 34' 22 5 82* GEOLOGY s
- 80*
l l OF THE CH APIN , LliiLE
- N N MOUNTAIN, PCM ARI A AND CHARbOTTE 9 1 % N BELT 4' '
347 9 s 34 o JENKINSVILLE QUADR ANGLES, / I SOUTH CAROLINA
. .r' A
O f' CAR LWA A @ / SLATE SELT I KILOMETERS
. 34* 75' Y+s ,
80 0 2 4 6 8 81* 3 0' al* 15' 5 Figure 2. I
I (Gggn) and the felsic metatuffs (Gtq) are characteristically sodie-rich, although the granitoid orthogneiss also includes a wider compositional spectrum from tonalite to granite. The thickness of orthogneiss sheets ranges from a few centimeters to a kilometer. Only a few of the thickest and most continuous sheets are shown on Figure 2 and on Plates 1-4. In many places the felsic metatuff unit (6tq) has been silicified and accessory pyrite has been introduced. Locally (as on Little Mountain), the silicification is nearly complete and the rock is quartzitic. These highly altered rocks in the felsic metatuff unit are interpreted to mark a volcanic exhalative horizon that is found widaly in the Carolina slate belt in South Carolina and Georgia [Espenshade and Potter, 1960;.Kesler, 1972; Hartley,1976]. Immediately above the exhalative horizon is a 1-2 km-thick sequence of intermediate to felsic lithic metatuff breccia (6th) . This volcaniclastic deposic is in part probably a product of episodic subaqueous debris flows as evidenced by its coarse-grained and poorly sorted charactec and by the local occurrence of water-rounded pebbles of volcanic rock and quartzite in poorly sorted tuff breccia. The lithic metatuff breccia unit grades into an overlying sequence of metamudstone, metasiltstone and metawacke (Gew) through an interval of a i few hundred meters in which the several lithologies are interbedded. Elsewhere in the Carolina slate belt, well-preserved sedimentary structures in the Gew unit suggest that it is a turbidite sequence deposited below wave base [ Brown, 1971; Kearas and others, 1981]. The Gmv unit contains a sequence of , amygdaloidal greenstone (Gg), 0-100 m thick, near its base, which is inter-preted to have originally been a series of intermediate to mafic flows or flow breccias. Most of the northern part of the study area is underlain by late- to post-kinematic granitoid plutonic rocks. Two distinctive groups of plutonic I I
9 rocks can be recognized: the "Newberry" and the "Winnsboro" plutonic complexes. The "Newberry" plutonic complex (Sn), which is found in the northwestern corner of the study area, is predominantly composed of medium-grained biotite quartz monzonite with some coarse-grained hornblende quartz monzonite. This unit also crops out extensively in regions north and west of the study area [McCauley, 1961a, Wagener, 1977a]. The "Newberry" plutonic complex has an irregular shape, consisting of a group of thick, partially concordant sheet intrusions enclosing numerous large enclaves of felsic gneiss (Efgn). In some places, the "Newberry" quartz monzonite exhibits a faint compositional layering caused by subtle variations in the biotite content. A weak subhorizontal parting is evident in weathered outcrops. This parting may be of tectonic origin, or it may be the result of weathering and exfoliation. The initial 87Sr/86Sr ratio for the Newberry quartz monzonite is 0.7024 l 0.0003, and the Rb-Sr whole-rock age is 415 9 m.y. [Fullagar, 1981]. The northeastern and north-central parts of the study area are underlain by a sparsely porphyritic, medium- to coarse-grained, biotite-hornblende quartz monzonite (Cw), which is here interpreted to be a part of the "Winnsboro" plutonic complex [Wagener, 1970, 19 77a] . The proportions of feldspar phenocrysts, biotite, and hornblende in the quartz monzonite are variable, and thin sheets of granite, aplite and pegmatite also are found. Numerous xenoliths and a few large enclaves of gneiss and amphibolite are l present (Fig. 2, and Plate 3) . The results of drilling and detailed geological mapping at the V. C. Summer nuclear station indicate that, in this area, approximately 30%-70% of the volume of the pluton is occupied by enclaves or large xenoliths of gneiss and amphibolite up to a few hundred meters in length [ South Carolina Electric and Gas Company,1977, Figs. 2.5-15, 2.5-16, I
10 2.5-17]. Two 1.0-1.1 km-deep boreholes were drilled into the area of induced seismicity along the west side of Lake Monticello by the U. S. Geological Survey (Monticello #1 and #2, Ngs. 1, 2). These boreholes were used to measure stresses and to determine the character of the rock in the region of seismic activity. Visual inspection of core and cuttings from Monticello #2 indicates that enclaves and xenoliths of gneiss and amphibolite are concen-trated mostly in the top 830 m of the hole and coastitute approximately 15% of the total volu=e of the pluton (yig. 3) . Cores and cuttings from Monticello #1 indicate that there the pluton is relatively free of inclusions (Fig. 3). Several extensive outcrop areas of amphibolite are found in the eastern part of the Jenkinsville quadrangle north of the main amphibolite belt (Fig. 2, and Plate 3) . These are interpreted to be enclaves surrounded l by quartz monzonite. There almost certainly are additional large enclaves of ! country rock within the "Winnsboro" plutonic complex that were not detected , during our field studies because of the thick mantle of residual soil and the sparse exposures. The irregular character of the magnetic field over the "Winnsboro" (Fig. 4) may be partly a result of the compositional inhomogeneity of the quartz monzonite, but it may also indicate irregularity in the subsurface distribution of enclaves and xenoliths. Leucocratic dikes of aplite and pegmatite are extremely abundant in the amphibolite unit (Ga) where it is in contact with the "Winnsboro" in the southern part of the Jenkinsv111e quadrangle. These dikes locally constitute more than 50% of the total volume 07 of exposed rock. The initial Sr/86Sr ratio of the "Winnsboro" is 0.7047 0.0004 and its Rb-Sr whole-rock age is 295 4 m.y. [Fu11 agar,1971; Fullagar and Butler,1979]. Several Rb-Sr and K-Ar mineral ages in the range 309-286 I m.y. were determined for the quartz monzonite in the foundation rock of the I I
11 s , 1 k COLL AR ELEV.-158m COLL AR ELEV.- 146m F?_2: F 2 2
. v. ; f, 1:11 1, 5~
_ _ - ~_2._ : h 41;s,b};,- ~;n? 2;
'n, ~ ~, n,;:7, ?. ,'s),"1 '
4,,,z,,y ,: ,' , A,y dr:t
' 7,s, '.n: 7 , ;,
- ~yv..,d"';,,;
s,.~
',N, x'tx., ,*
A,,y. i ,- ?, ,. ,
'.n, 5,v.'~ ,
o.il,q' 4. . A, ?.o ,
~.s s
-+=.r=rt
'sy,1.,- v-rmrs -ws ;* - ..
-:~ goos
//;,. ., ,*. 800- ./, f ., ,
'.sA, _,4, s o;. ,~1,,s,~1,r, *
=l .*, *, . .
, p 2g,:. ..
10 0 in, ,'.h,0 s 7~ ' 100- # .* .b,',,.
,,2 B27 '.2,"v,b
', 1, a"n ,1 M,/q,5,,
,,,t. ***a*
- ,-y- t-4 a n,4 :- ,q,q,,;g,,,. ',Q**m:
;~y!,y ; op ,
o estr,t s Jis-M
- N,V ."1l:'elt
..s.,'.,s '.e.
W :e:Vii:. p leks
. , s 0, .
'dlllE't ",. *.J.M. :,7 . ifI ~,If,'
WsN ,~- i c f's~'
.p,-,p 4,-,~p e
,~t,i , ,4 900* sb,1,4',~-4 1
,p..., 900- ,P ,~D ,e t
;u m,, L-,; e ,y ,n*
, b c,5,~;- .;~p,p ,s :v bs 200-,,to u
,3, gp;aq,p,p . ,4
, p ,1,,s,1 200-r. ,p,,p ;,, , 4,
,- r u,g~,i,4 q ,q
, . , , , , gg o :ylet,e
,p ,;,,;
rv,1-4, bi g. .p, k, ,,p- avg ~epv
, ,M,u ,
~
,4y- ,y y, :~y ,,v6 A,,p ',p', ,9 'p,~v, p;u,. ,
L,p ,p a, ,~, , p , d
, ,4,p q~,,4 <
-4~ M,g....,
L,;,p q, qs,p ,;q,, c ;, :q~,k,v,, g P,
,~P;id c~.
,k ~p so
',d- ,t,qu,;,
, - 4,~d
,b /,,p,,
,4 4~,
q"vyt L ;q?q,
,?,,p,- 1000- qup an~g,~
',bq,,p ,p ,;, 1000-4,,,pc,; ;~;
4c,, p,s o qqa, ", s , 1,p q,p
,qid' i
4,b, , b,I"b300- 4'p,~ ,U ,' 300- ,e- ,
- 'b ,pvg - ,
,q~4, q",I,?
q
, q, ,?,o ,
1
,',;s,"b,v,b;
> >,,, v a N =yn,e
~
L,To,t,y, q L
;,d q,,p qu;, 4 b,~b4 ,
f W m 1"P "D c 4, ,~ : 4 E Q: qu, , ,p , 4-4,,b,. Q: N W g L,p;,s,,p;, t ,s '< ' W s s ?s ~ W ;.,q,,,, g Ld ,'~;.
- q,y;a,,',., W 8'k-k, 6' E w 0e s,"J"k
'('i$p I f_ _. - _- WEATHERED f 8E't
,i
-: -:- . QUARTZ l2
~
;y ,{'{s,!'f t' ~2l - P, ,. ~by k,
~ ,
s, :-Z _ - . - MONZONITE d t Lp ,7,t, 4 ',iq , k, s . % ,7,D U,, 4'"',$'- N d' b 't'f-',If{ QUARTZ
':',$'(,t'U:kf5 .'(-"1i<
Q. Q. ?
?4~ / ']"-f,,'
W . i,eP ? W MONZON:TE ANO
.< I'tE.,{k , ;, b ,12, GRANITE Q r't~vS,y;t, Q , , ss.e,p,',,-
s 4~9 s, q ,}, J,}, s,.s-s~ L,},, q ,,, ,, = ' . " * ,i L;q,
;, ,.p i ,I, p .*.;"
t /. . ';
;**.*** AMPHIBOLITE Soo- ,1"r a a~p t 500- c , * = *. ; *%
;.y,y,;. ,
':,J;n,~ ,,N, ;J:
r 1:.r , s
. ,,a.rs,i ,
ykpg,v r,r ,1
,'NII?,
, y q,I, O CORED INTERVAL s,,5,U n-,c ~ynse # s, y-o'" y', "y',
- I,' 55's's f,'s';t se r rJ .*. r.*
FR ACTURED INTERVAL USUALLY WITM r,f,%' ,i M r L'p p,'? ' i ~3, ZEOLITF AND/OR l
- od{,pk c/$$ "$ CARBONATE {
600-m 600- ';q,b{,N,y
, ~ ;- MINERALIZATION
,,)q },.
N ' s,i w WATER ZONE l,f,3I'Il,f, (Jff,,p'5Y
,y
~Y'I'6,~~
t-N NO SAMPLE an~ ,,
-+ , twi- :";"'W"
-, ~ , , ,
,, , .?-
V VotD q,:4,,,y,4,i
-,. ,, n, t op',ta
.v.,y
~ p ,pn, 70 0- . :J , ' '
CORE LOG FOR 700~ '5,"I'v6':
-
- CORE LOG FOR
'!': 6$?' ', MONTICELLO #1 l0':s. ', MONTICELLO #2
',If ,I:-2 _1 . ::.y Figure .. Lithologic log for Monticello #1 and #2, compiled from core and cutting samples.
12
- g. g i
. . I0 mo'~e!sttIV 0 k'f $,.,f 0Q g
.0
,,,,,,,,- .. ,. .:, 'h" , #f "' ,
- }h.
'( ,
~
$ h j
a . . , , l
/ / f} 4 1J} sar
~
- r. f 'c3=s )
j -
^
e$ 1 f+ ... r- LR g
. m g
CON!out INTERVAL. 0 GAMM A$ otouures f us ceoioc c su,.. rne Figure 4. Aeromagnetic map of the Chapin, Little Mountain, Jenkinsv111e I and Pomaria quadrangles (modified from U. S. Geological Survey 3 [1978]). Contour interval 40 gammas. Dotted lines indicate g the positions of geological contacts from Figure 2. I I
l V. C. Summer nuclear station at the south end of Lake Monticello [ South Carolina Electric and Gas Company, 1977, Table 2.5-3; Fullagar and Kish, 1981]. Several small plugs and sheets of gabbroic 1 sck (Cgb, Fig. 2, and Plate 2) intrude the rocks of the Carolina slate belt in the southern part of the Little Mountain quadrangle. These clearly are post-metamorphic because the associated contact metamorphism overprints the regional greenschist- .I facies metamorphism and anneals the S foliation. They are tentatively i interpreted to be of Carboniferous age because mafic plutonic rocks are associated with Carboniferous granitic rocks elsewhere in the Piedmont [Waskom and Butler, 1971]. The youngest rocks in the study area are a series of steeply dipping olivine diabase dikes. These dikes are 0-10 m thick, up to several kilometers long, and trend N.15W.-N.30W. Paleomagnetic studies have been conducted or. 24 different dikes from the South Carolina Piedmont [Dooley and Smith, in press]. Their average pole position (83.9 E., 66.1 N.) is similar to that reported f r other North American diabases of early Mesozoic age [deBoer, 1967; Smith and Noltimier,1979; Sutter and Smith,1979]. Although previous studies have indicated that many of the diabase dikes in the Piedmont have anomalously old K-Ar whole-rock ages because of excess argon contamination [Dooley and Wampler,1977], eleven dikes in the present study have K-Ar p whole-rock ages in the range 207-180 m.y. , in excellent agreement with the I paleomagnetic results (The K-Ar whole-rock ages were determined by R. E. Dooley using facilities at the Georgia Institute of Technology.). An anomalous age j of 288 m.y. was determined for one dike from the Salem Crossroads area north l l of Monticello Reservoir. The paleomagnetic pole positions and most of the l . l l <
14 K-Ar whole-rock ages, therefore, indicate a Late Triassic and/or Early Jurassic age for the diabase dikes in the study area. DEFORMATIONAL HISTORY AND MAJOR AND MINOR STRUCTURES I The rocks in the study area have been affected by two strong episodes of folding (Di , D2 ), and are cut by a variety of later faults and joints. The earliest deformation (D1 ) was an episode of tight to isoclinal passive folding. The associated regional metamorphism (M1 ) ranges from greenschist facies in the Carolina slate belt to amphibolite facies in the j Charlotte belt. The Chapin synclinorium is a major northeast plunging F1 fold crossing the southern part of the study area. The initial compositional layering in the stratified rocks (S ) 0has been strongly transposed by Si slaty cleavage. Si contains a strong elongation lineation (L 1) which parallels the lox 1 intersection lineation. The average orientations of L1 and L0x1 in the study area (Figs. Sa, b) indicate that the Chapin synclinorium plunges 11C to the N.75E. The 415 m.y.-old "Newberry" plutonic complex contains rotated xenoliths which carry the Si foliation and hence the time of , Di must be Silurian or older. Whole-rock K-Ar studies of slates from the l Carolina slate belt in North Carolina, which have never been heated signifi-cantly above the argon retention temperature, indicate that the most probable , time for Mi is Early Ordovician [Kish and others,1979]. The Si foliation has been extensively reoriented by mesoscopic to l macroccopic F2 flexural flow folds (Fig. Sc) . In most places F2 f lds are coaxial with Li and L0x1 The intensity of F2 folding increases progressively from south to north in the study area. The time of D2 is thought to be pre-295 m.y. because 2F -style folds have been observed in rotated xenoliths in the "Winnsboro" plutonic complex. I
15 N N i . n i d (a) (b) l l N N ! of I*~- !..q_ q el - m _.3 I L .. , f';m _,x.' S
*A (c) (d)
Figure 5. Lower hemisphere, equal-area projections of structural data from the region south of Monticello Reservoir. (a) 63 Lt elongation lineations from the slate belt portion of the Chapin and Little Mountain quadrangles. Contours: 1, 5, 15 and 25%. (b) 41 L intersection lineations from the slate belt portion of keChapinandLittleMountain quadrangles. Contours: 1, 5, 15, 25 and 35%. (c) Poles 7 to 713 Si foliation planes from the slate and Charlotte
] belts in the Chapin and Little Mountain quadrangles.
Contours: 0.5, 2, 5 and 10%. (d) 106 L0x1 lineations from y the Wateree Creek fault zone in the vicinity of. Spring Hill 55 in the Chapin quadrangle. Contours: 1, 5, 12 and 20%.
? "A" is the axis of drag folding, "B" is the net-slip vector for the Wateree Creek fault zone.
16 The post-D2 brittle faults in the study area are divided into several groups on the basis of their orientation and the character of the mineraliza-tion that has taken place in the fault zones. I The oldest group of faults, striking approximately east and dipping steeply, is characterized by repeated episodes of brecciation and silicifi-cation. The adjacent vall rocks are cut by numerous irregular extensional veins partly infilled with comb quarts. The displacemert along faults of this group has apparently been small, for generally the same rock unit is I present on both sides of the fault surface. Faults containing silicified breccia are widespread in the Appalachian Piedmont [Conley and Dru=mond, 1965; Odom and Hatcher, 1980]. Those in the eastern Piedmont of South Carolina are thought to be of late Paleozoic or early Mesozoic age because they cut Carboniferous metagranitic rocks near the Fall Line and are them-I selves cut by Upper Triassic and/or Lower Jurassic diabase dikes. The east-trending silicified fault zones are displaced by a later set of steeply dipping faults trending N. to N.15 W. The Wateree Creek fault zone in the central and southern parts of the Chapin quadrangle (Fig. 2, and Plate 1) is the most thoroughly documented example of this north-trending I fault set. The Wateree Creek fault zone displaces stratigraphic units on the flanks of the Chapin synclinorium in a manner suggesting down-to-the-east, predominantly dip-slip displacement (Fig. 2). The apparent offset of the axis of the Chapin synclinorium in cross section also indicates predominantly dip-slip displacement of approximately 1700 m (coyare A-A' , B-B', and C-C' , Figs. 2 and 6). In the vicinity of the Wateree Creek fault zone, the L1 and lox 1 lineations have been dispersed along a portion of a small circle on the l stereonet (Fig. Sd) by fault-related drag. The axis of the drag fold derived I I
, S E ,.
NW \ A'
^ so C- Ca E9 99" e0 .. ..... .. .. .:.:... .. ..w;::rg; ,: . . .,. .,., ' ;p;m. ::.M::. ......
- .:w,: . 7c. . . :.:l .. '. .". :::::.
. . ~..
g-'- :::::.:!!.?;::.*4?:c:.
- ':li: :. :. .' :.:. . . s: : & .~ . . . .
..:C.,,. ,..... ;
..v.::
,~,~':..:* :::. :::..:o::::::ii.i.5., .
- s. ' : i :J:!i.::.'c.
- t
': : i::
i;ii:i:::.
- 1:'::!:i :::=::::.:..:.,::.:=::' '"
w .2
=
M 1 JL g 4
"" ** C <* C C,q C,b Cg (mw C' 0 '
0 ~ W ~ :' . . '.S9, M Y '- . ; .\; : .
% *.:i .':...' . : V !!..::....':
. :,:: .:o.,: . .... .. . . :. . .l:. . ':. . .' -
...........r
.,- C,s
. . . . .. . . . . . -i-
.. .. . . . . . . '....C.
' # 'I'*. . . . #. . . ,,
- i! . \'
; C,, s m m ::: .i'..ii .: :: . .:-:
.... l'. - -2 KILOME T E RS
]
~ ~m ! O 2 4 6 8 10 i NO VE RitC AL E X AGGER ATION i
e ) I o 4 I Figure 6. Geological cross sections across the Pomaria, Jenkinsville and Chapin
- quadrangles. See Figure 2 for location of section lines. -
i ' I 4 4 d J l 9s 1 M i r i 1 i t
18 I from the data in Figure 5d (4 S.3 W.) is subhorizontal, indicating predomi-nantly dip-slip displacement in the fault zone. A portion of the Wateree Creek fault zone is exposed in a large roadcut near Spring Hill in the east-central part of the Chapin quadrangle (Fig. 2) . The average dip of fault surfaces in this outcrop is 78 to the west, indicating that at this level of exposure the faults in the Wateree Creek zone are high-angle reverse. Thoroughly brecciated rock is exposed in several places along the fault zone. Samples recovered from several core holes drilled into the fault zone below the level of weathering indicate that veins and open spaces in the breccia have been infilled predominantly with I carbonate mincrals and accessory quartz. The apparent brittle character of the breccia and the mineralogy of the associated vein fillings suggests that the faulting occurred at low temperatures and pressures within a few kilometers of the earth's surface. Detailed field investigations, including backhoe I trenching, in the southern part of the Chapin quadrangle indicate that a diabase dike, petrologically similar to the Upper Triassic and Lower Jurassic diabase dikes, intrudes across the Wateree Creek fault zone and is apparently not displaced along the fault zone (Fig. 2, and Plate 1) . Therefore, the faulting is thought to have occurred prior to the Early Jurassic. The north-trending Su=mers Branch fault zone in the Little Mountain I quadrangle (Fig. 2, and Plates I, 2) offsets stratigraphic units on the flanks of the Chapin synclinorium in a manner suggesting down-to-the-west, dip-slip displacement. The Summers Branch fault zone is interpreted to be similar to the Wateree Creek fault zone, although actual outcrops of rock from within the fault zone have not been found. During excavation for the foundations of the V. C. Summer nuclear I I
s, i l9 station '(located in the "Winnsboro" complex), a distinctive group of fractures - (. ' containing druses and vein fillings of laumontite were found. Although there is considerable variability to the attitudes of the mineralized fractures, (.-
.two' steeply dipping sets that strike N.60 E. and N.28 W. can be recognized
( .[ South Carolina Electric and Gas Company,1977, p. 2.5-38 to 2.5-40, and Fig. 2.5-25]. Fractures of the northeast-trending set have undergone oblique-slip displacements up to_2 m. Fractures of the northwest-trending set displace those of the northeast set and have undergone oblique-slip displacements up to several centimeters. Undeformed laumontite crystals fill openings along h both sets. A K-Ar age of 45 5 m.y. has been determined on laumontite crystals from the N.60 E. trending fracture set. This age is regarded as a minimum estimate of the time of fault movement [ South Carolina Electric and < -Gas Company, 1977, p. 2.5-47]. [ Fractures and alteration zones containing zeolite and carbonate minerals f were found at several levels in Monticello #2, and in some cases were asso-ciated with a substantial _ influx of groundwater (Fig. 3). Zeolite minerali-
-zation has also been observed nearby along northwest-trending fractures in the spillway to the Parr Shoals dam, and in a borehole in the "Winnsboro" plutonic complex east of the study area [Costain and others,19773 These local occurrences of zeolite mineralization in fractures are thought to be a manifestation of a Mesozoic deformational event, perhaps associated with the emplacement of diabase dikes, that has affected a large area in the central ~
Piedmont of North and South Carolina [Privett, 1973a, 1973b, 1974a, 1974b, b 1977; Brown and Gilbert, 1977; Butler, 1977; Wagener, 1977b]. ( 'In most outcrops in the study area, the rocks are cut by one or more joint sets in which the individual fractures have little or no lateral
20 displacement. The joints are usually 10-200 cm in length, although in most places a few longer joints are present which extend beyond the limits of exposure. Short joints of ten are arranged in echelon patterns located near theterminath.onofanearbylongjoint. Approximately 100 joint measurements have been made at each of 50 localities in the study area. In general, the orientations of the sets and the joint frequency are variable from one locality to another (Fig. 7, and Plates 9-12), although in most places one or more sets are oriented at a high angle to Si and Q xy or Lt . The controls on orientation and frequency are not well understood, although the orientations of other fabric elements and the extent of lithological heterogeneity probably are important. In general, joint frequency is lowest in the "Winnsboro" plutonic complex, and highest in the low-grade metamorphic rocks of the Carolina slata belt. However, even in otherwise homogeneous structural and lithological domains, joint orientation and frequency are variable. DISCUSSION In order to make judgments about the possible earthquake hazard at I Monticello Reservoir, it is necessary to evaluate the above geological infor-mation in light of the ambient stress field in the region of seismic activity. i i Information about the stresses is available from the following sources:
- 1) composite-fault-plane solutions derived from seismic data [Talwani and others,1980; South Carolina Electric and Gas Company,1980], 2) stress measurements made in the U. S. Geol'ogical Survey boreholes, Monticello #1 and
#2, by the hydraulic-fracturing technique [Zoback,1979; Zoback and Hick =an, in press], and 3) stress measuremnts made in the foundation excavation of I
the V. C. Summer nuclear station by the overcoring technique [ South Carolina I I
'N N l .-
g 0 2t o I $ $h . S hg o < :., i m> pj -1 JJ-2 N42E58SE S j O N60E60SE Si N (126) N (103) ,
= l l
o o ; l O PJ-2 JJ-3 i N40E60SE Si N64E575E Si l N (11 9) N (103)
! 0 a ,
(' o O 1 l O l
,. s g 'g PJ -3 9o #
0 o JJ-4 N29E65SE Sj NO S N N (103) j (170) l 0 3x 0 e !
~
g O (* - 0 JJ -1 Ja-s l
# N75E65NW l NO S Si (98) i (103) l Figure 7. Lower hemisphere, equal-area projections of poles to joint fractures measured at several localities in the Jenkinsville quadrangle. The number in parentheses is the number of measurements represented on l each projection. Contours: 1, 3, 6, 9 and 12%. l l
l L --- - -.
N N N o . g o e pq O g h l
& M, h "
1266 4. I 1250 1263 l N64E26SE Sj (100) NO Sj (99) NO Si (100) N N N b O
, g h@eoo
~
- oa i e , ~
9@ O I 1259 1265 1268 N32E535E Sj (10 0) N17E36NW Sj (100) N17W60NE Sj (100) 34*22'30" , JENKINSVILLE N 7.5' QUADRANGLE
*-1250 I
I f 70 s
;;_irJJ-4 32ee g
1265 1P_F.* l 1260 p; 3 ; N82E41NW S1 (10 0) PJ - 1p9 b, 34 15,' PJ-V *-J J - 3 JJ-2* i o e 5 ' e I Figure 7.
~
23 E Electric and Gas Company, 1977, p. 2.5-226]. [ The seismic studies at Monticello Reservoir indicate that the epicenters of in ~ 2ced earthquakes are occurring in three groups (Fig.1) located in areas underlain by rocks of the "Winnsboro" plutonic complex. One large ( - group is located at the southwest corner of Lake Monticello, near both the V. C. Summer nuclear station and the site of Monticello #2. Another large group is located in the west-central part of the lake around the site of Monticello #1. A third smaller group is located at the north end of the lake. Almost all of the foci are located at depths of less than 2 km, and composite-( fault-plane solutions indicate that the mechanism of faulting is predocinmntly thrusting with the P axes approximately horizontal and having directions ranging from N.34 E. through due east to S.76 E. [Talwani and others,1980]. Analysis of the source characteristics of some of the earthquakes s_uggests that the seismicity is occurring along pre-existing fractures [Duc, 1980; ( Talwani, 1981]. The apparent scatter in the locations of the earthquake foci suggest that the foci are not located along a single major fault but instead . are located along numerous small fractures pervading the rock. Variability in the orientation of nodal planes for various sub-groups of earthquakes [Talwani and others,1980] suggests variability in the arientiation of ( pre-existing fractures and also perhaps temporal and/or spatial variability in the orientation of the principal stresses. [ The above observations are further reinforced by stress mea urements and fracture orientation studies made in the U. S. Geological Survey boreholes Monticello #1 and #2 which are located in the two clusters of most intense ( seismic activity [Zoback, 1979; Zoback and Hickman, .: press]. Hydraulic fracturing stress measurements made in these holes indicate that within a ( r
24 few hundred meters of the earth's surface the rock is in a state of incipient failure by thrust faulting. At greater depths the rock is generally not critically stressed, although the magnitudes of the horizoncal principal stresses vary e.zratically with depth [Zoback and Hickman, in press]. Tracture studies made with a borehole televiewer [Zoback and Hickman, in press] indicate that the rock at Monticello #2 is much more intensely fractured than the rock at Monticello #1. Although a weak preferred orientation is evident in some restricted intervals of the boreholes, in general the fracture orientations are variable and erratic. Co=parison of fracture orientations with the orien-tations of nodal planes derived from composite fault plane solutions indicates that there are fractures having an orientation favorable for reactivation in the regions of seismicity [Talvani,1981; Zoback and Hickman, in press]. Several determinations of the magnitudes and directions of the horizontal principal stresses were made by the strain-relief, overcoring technique in the foundation excavation for the V. C. Summer nuclear station [ South Carolina Electric and Gas Company, 1977, p. 2.5-226]. The magnitude of the greatest horizontal principal cor:pressive stress ranged from +31 to +97 bars, and its direction ranged from N.35 W. through due north to N.15 E. These directions are approximately perpendicular to the direction of greatest horizontal compressive principal stress inferred from composite-fault-plane solutions [Talvani and others,1980; South Carotina Electric and Gas Company,1980, Appendix IV, Tables 1 and 3]. ! One of the primary objectives of this study was to determine if there are laterally extensive faults or other major lithological boundaries in the vicinity of Monticello Reservoir having orientations favorable for reactiva-tion in connection with the induced seismic cetivity. Because the directions I I
e ' 25 C
.of the principal stresses are variable, it is difficult to predict the planar orientations that would be most susceptible to reactivation. However, the average direction for the greatest horizon:al compressive principal stress derived from composite-fault-plane solutions is N.71oE. The most favorable
~ orientations for thrust faults resulting from a greatest compressive principal
{ stress having this orientation would be N.190W.300SW. or N.190W.300NE. In the Chapin and Little Mountain quadrangles two sets of major faults having orientations E.-W.650S. (silicified faults) and N.15cW.780SW. (Wateree Creek and Summers Branch faults) are present. These are not orientations favorable
~
for reactivation, and furthermore, no field evidence has been found indicating { that long faults .are present in the vicinity of the reservoir. The northern-
'most outcrop on the Wateree Creek fault zone (Fig. 2, and Plate 1) is located ,
in the Chapin quadrangle approximately 8 km south of Monticello Reservoir. Several Jurassic and/or Triassic diabase dikes striking N.15oW. to N.30cW. were found in the region surrounding Monticello Reservoir (Figs. 1,
-2, and Plates 1-4), and it is possible that similar dikes occur in the
[ subsurface in the regions of seismic activity. However, the diabase dikes are steeply dipping to vertical and hence are oriented at a high angle to the planes preferred for the development of thrust faults. Thus, it is unlikely that the intrusive contacts between diabase dikes and the enclosing granitic { rocks would function as guides for the development of new long fractures. -1 ( The only other major lithological boundaries near the region of seismic activity are the contacts between the "Winnsboro" plutonic complex and (- surrounding rocks. These strike almost perpendicular to the preferred-faulting directions and are interpreted to be irregular intrusive contacts having numerous apophyses extending outward into surrounding rocks. Because h ( . E.. . .
26 of their orientation, irregularity, and intrusive character, these contacts probably retain substantial cohesion and would be unlikely to function as guides for new long fractures. I Another objective of this study was to determine if there are foliation directions along which slip might be initiated by the stress and pore-pressure changes related to reservoir impound =ent. Almost all of the seismic activity is located'in the "Winnsboro" plutonic complex which underlies Monticello Reservoir. The "Winnsboro" is post-kinematic and is generally unfoliated, although in some places an emplacement related irregular gneissic layering is present. In addition, the "Winnsboro" complex contains numerous foliated xenoliths and enclaves of felsic gneiss and amphibolite having a maximum Because the larger enclaves are sparsely I length on the order of 1-2 km. distributed, and because the foliation in them is folded and diversely oriented, it is unlikely that the enclaves could functio'n as guides for the development . of new long fractures. The above geological and geophysical studies indicate that the "Winnsboro" plutonic complex contains numerous diversely oriented small fractures and lithological inhomogeneities having a maximum length on the order of 1-2 km. These local inhomogeneities, together with an irregular stress field, are interpreted to control the diffuse seismic activity that is occurring around i Monticello Reservoir. In view of the apparent absence of long faul5.s in l l the vicinity of the reservoir, it is unlikely that a large magnitude earthquake l will occur in response to the stress and pore pressure changes [Zoback and Hickman, in press] related to reservoir impoundment. I
27 CHAPTER II THE WATEREE CREEK FAULT ZONE ABSTRACT I The Wateree Creek fault zone is a region of north trending high angle reverse faulting located in the Chapin quadrangle in the Piedmont province of central South Carolina. The location and character of this fault are I established from the following evidence: 1) offset of stratigraphic contacts and pre-existing structural features, 2) abrupt termination of a prominent aeromagnetic anomaly, 3) occurrence of fault breccia along the zone, 4) drag } of pre-existing rock fabric elements adjacent to the zone, and 5) observation of three of the major component faults in the zone in the Spring Hill roadcut. Structural analysis of the Wateree Creek fault zone indicates that the % 1700 m displacement is predominantly dip-slip and down to the east. The youngest geologic feature which is clearly offset by the Waterce Creek fault zone is an east trending silicified breccia zone of unknown age. Preliminary studies indicated that a major N.20 W. trending diabase dike contains a dextral en echelon offset in the region of the Wateree Creek fault zone, however, a series of detailed magnetometer profiles combined with backhoe trenching have shown that the diabase locally intrudes across the fault zone. Therefore most, if not all, of the displacement along the Wateree Creek fault zone occurred prior to the emplacement of the diabase dike. Neither the silicified breccia zone nor the diabase dike have yet been accurately dated. However we suggest an early to middle Mesozoic age for the Wateree Creek fault zone based on a presumed age of late Paleozoic to early Mesozoic for the silicified breccia zone and a presumed Late Triassic or Early Jurassic age for the diabase. I I -
28 INTRODUCTION A major zone of brittle faulting of probable Mesozoic age occurs in the Chapin quadrangle, northwest of Columbia, South Carolina, and is especially well exposed in a portion of the Carolina slate belt. This structure is referred to as the Wateree Creek fault zone and outcrops as 'several parallel fault surfaces with varying displacement across a zone 250 m bide. The major movement on the fault was probably Mesozoic and predates the intrusion of a l Late Triassic or Early Jurassic diabase dike. The fault offsets Paleozoic-rock units and structures, the slate belt / Charlotte belt boundary, and an earlier east-west trending fault containing silicified breccia. 4 This study is part of a project initiated by workers at the University of South Carolina in 1978 to study the geology and geophysics of the area l surrounding the Virgil C. Sum =er nuclear station near Monticello, South Carolina (Fig. 1) . The station is eight kilometers north of the northernmost confirmed outcrop of the Wateree Creek fault zone. Beyond purely academic c. goals, there is a need to gather as much data as possible to aid in an assessment of the earthquake risk potential for such man-made structures in this area. Moreover, considerable low-level induced seismic activity is associated with an adjacent pump-storage reservoir (Monticello Re.servoir) [Talvani, 1979; Secor,1980], and the Wateree Creek fault zone may be one of the important geologic boundary constraints influencing this activity. This study was geared toward understanding the faulting using data gathered from six avenues of research. The project goals were to study the evidence for faulting and determine if a fault actually existed, to analyze the nature and displacement of the faulting, and to bracket the age, if possible, using available constraints. The first of the six avenues of I
a a 29 i> research was an analysis of the regional geologic data gathered by detailed -- field mapping. This provided the prima facie evidence for faulting , including the offsets of geologic contacts and the existence of deformed l structures plus certain geologic features (such as diabase dikes) which 4 could be studied in greater detail to consider their relationships to the fault. Some of this information could then be used to approximate the net-slip on the fault. The next step was a detailed study of a roadcut exposure I of the fault zone, referred to as the Spring Hill roadcut. This roadcut contains all the elements available for establishing the maximum age for the I faulting. A structural study of the rotation of a pre-existing L g ; I intersection lineation by drag associated with the Wateree Creek fault zone was carried out in order to establish a net-slip direction independant of that i- - found in the study of the regional relationships. Four holes were drilled : within the fault zone to observe the extent of faulting and drag folding, to determine the dip direction of the fault surfaces within the zone, and to
.l-determine if zeolite minerals suitabic for age dating occurred in the breccia zone. Detailed magnetometer profiles were used to determine outcrop patterns I of a Jurassic (?) diabase dike in a region where it crosses the fault zone to , determine the age relations. Finally, backhoe trenching was undertaken to E determine if magnetic peaks actually represented diabase and to permit study 1-of fresh diabase in key locations in or near the fault zone.
AREAL GEOLOGY ^;
; Much of the information concerning the age and net slip of the Wateree .
7 Creek fault zone is derived from the apparent displacement and deformation of .
.s_
g
r 30 I to present this evidence, some discussion of the areal geology is necessary. The reader is referred to the following sources for additional documentation of the regional geological information su=marized below: Overstreet and Bell [1965]; Secor and Wagener [1968]; Secor and Snoke [1978]; Secor [1980]; Snoke and others [1980]. The boundary between the Carolina slate belt and the Charlotte belt trends N.75 E. across the northern part of the Chapin quadrangle (Fig. 2) . In this region, the oldest part of the stratigraphic section outcrops in the In South Carolina the Charlotte belt may be distinguished I Charlotte belt. from the Carolina slate belt by its higher grade of regional metamorphism (epidote amphibolite and amphibolite, as opposed to greenschist) and by the presence of stratiform sheet intrusions of granitic orthogneiss up to 1 km in thickness (Gggn, Fig. 2). The stratigraphic sequence is a thick (N10 km) section of metasedimentary and metavolcanic rocks that are thought to have been deposited in a late Precambrian and Cambrian volcanic arc [ Butler and Ragland, 1969b; Hills and Butler,1969; Fullagar,1971; Glover and Sinha, 1973; St. Jean,1973; Butler and Fu11 agar, 1975; Black and Fu11 agar, 1976; Seiders and Wright, 1977; Black,1978; Carpenter and others, 1978; Whitney and others, 1978]. The oldest stratigraphic units are sequences of amphibolite (Ea) and biotite schist and paragneiss (Gfgn) which occur in the Charlotte belt in the northern part of the Chapin quadrangle. To the south, in the Carolina slate belt, the f following successively younger stratigraphic units outcrop in order from north felsic metavolcanic rock and quartzite (Gtq); intermediate to t to south: g felsic metavolcanic tuff breccia (6tb); and metamudstone and metawacke (Gmw). 5 The Gmw unit contains lenticular layers of amygdalcidal andesitic greenstone (Gg) at or near its base. The distribution of the above stratigraphic units I I
31 I on the geologic map (Plate 1, and Fig. 2) is primarily related to the cumulative effects of two strong deformational episodes (D1 , D2) which affected the area during Faleozoic time. f The first deformation (D )1was an episode of regional metamorphism and
- passive folding that resulted in the formation of a major F1 synclinorium (the Chapin synclinorium), the axis of which passes through the central part The Dg deformation i
of the Chapin quadrangle trending N.75 E. (Fig. 2). produced a prominent metamorphic foliation (S t ) which is the dominant fabric element in both the slate and Charlotte belts and which approximately parallels
- the axial surface of the Chapin synclinorium. In many places the S1 ' foliation l
l contains a prominent elongation lineation (L1 ) and/or an intersection lineation between compositional layering and St (L0x1}
- L1 and L0x1 are parallel to each other (Fig. Sa, 5b), and in the region of the Chapin syncli-norium, they plunge gently to the northeast parallel to the axis of the synclinorium. The timing of the D; deformation is known only within broad jllW l
limits. It must postdate the youngest stratigraphic unit which carries the Di fabric (s520 m.y. [ Hills and Butler,1969], and it must predate the emplacement of a 415-385 m.y. se. ries of granitic plutons in the Charlotte belt which do not carry the Di f abric [Fu11 agar,1971; Fullagar and others, 1971; Butler and Fullagar, 1978; Fullagar, 1981]. In middle to late Paleozoic time the rocks in the central and northern parts of the Chapin quadrangle ware refolded by a series of mesoscopic and macroscopic flexural folds (D 2 , F2 ) (Fig. Sc) . The style and axial surface inclination of these later folds is variable, and more than one distinct sub-episode may be present [Bourland and Farrar, 1980]. In most places the axes of F2 folds are parallel to L0x1 and Ly so that F2 folding did not I l
T 32 result in the dispersion or reorientation of lox 1 "" 1* Two distinct sets of steeply dipping to vertical faults formed in the area in latest Paleozoic and/or Mesozoic time. The oldest set trends east to northeast and the fault zones were extensively silicified during movement. These fault zones are characterized by silicified breccia and by the presence of numerous irregular gash veins partly infilled with comb quartz. The direction and magnitude of the displacement associated with the east to northeast trending faults is unknown, but the displacement is generally too small to produce measurable offsets of major stratigraphic units at the 1:24,000 scale. The youngest set of faults is oriented approximately north-south and is characterized by intense calcite veining and by dip-slip displacements up to 1700 meters. The Wateree Creek feult zone belongs to this youngest group, and in the central part of the Chapin quadrangle the . Wateree Creek fault zone clearly offsets stratigraphic units, the axis of the Chapin synclinorium, and a prominent east trending silicified breccia zone (Plate 1, and Fig. 2) . A diabase dike of probable Late Triassic or Early Jurassic age contains a dextral en echelon offset in the region of the Wateree l Creek fault zone in the south-central part of the Chapin quadrangle. However our investigations in this region indicate that this offset is the result of l a primary irregularity in the shape of the dike and that the dike was emplaced after most and perhe.ps all of the displacement had occurred on the Wateree Creek fault zone. The geological history of the Wateree Creek fault zone is of considerable importance because of its proximity to Lake Monticello and the Virgil C. Summer nuclear station. In the following section we present the results of field investigations concerning the geclogical character, age, and displacement of the Wateree Creek fault zone. I
l l 33 EVIDENCE FOR THE WATEREE CREEK FAULT ZONE Regional Relations Several aspects of the regional geology / geophysics indicate that a north trending fault occurs along Wateree Creek. In the south central part of the Chapin quadrangle, a very prominent east-trending magnetic anomaly, I coinciding with the axis of the Chapin synclinorium, is abruptly terminated at Wateree Creek (Fig. 4) . In this same region the stratigraphic contacts I between the metamudstone and metawacke unit (Gmw) and the intermediate to felsic metavolcanic tuff breccia sequence (6tb), on the flanks of the Chapin 1 . synclinorium, are clearly offset in a manner suggesting predominantly dip-l slip displacement with the eastern block down relative to the western block (Plate 1, and Fig. 2) . Projection of the traces of the stratigraphic contact l between Emw and 6tb in the limbs of the Chapin synclinorium onto the, plane I of the Wateree Creek fault zone (Fig. 8) also indicates predominantly dip-slip displacement with a net slip of approximately 1700 m. In the central j part of the Chapin quadrangle a prominent cast-trending silicified breccia zone is similarly offset (Plate 1, Fig. 2). Throughout most of the Chapin l quadrangle the mesoscopic rock fabric contains a prominent subhorizontal lineation (L0x1 and/or L g) . At several places along Wateree Creek this l lineation has an anomalous steeply plunging orientation that is interpreted l to be a censequence of drag in and adjacent to the Wateree Creek fault zone. Finally, at several locations along the zone, non-silicified fault breccia occurs, and in the vicinity of Spring Hill steeply dipping to vertical north trending faults occur in outcrop. The above observations suggest that the . l Wateree Creek fault zone formed late in the tectonic history of the region j under conditions of relatively low temperature and confining pressure such I L L. _. o
I I Wg ,EB
- / , f
> ! \ ' ' .
V/ I NetSlip Vector l Mudstone/ Volcanic rock contact
- -- -- Eroded portion of contact WB onWestern fault Block wB j
EB on Eastern fault Block j
/
/ EB /
/
! /
\ /
t ~ /- /,, g cts !\ /7 i P. SlipVector i i I g,, {mw
- l' B y -fi 1:
d, i' Erb 13 N 'l Etb T6ff Breccia l ll <mw Modstone and Wockes
'.'yt Gib l
Figure 8. Diagram illustrating the projection of the contact between Gmv and 6tb in the axial zone of the Delmar synclinorium onto the plane of the Wateree Creek fault zone. I
~
m l t 35 l that widespread brittle deformation occurred. These conclusions are further l substantiated by observations made in a large roadcut located about two l kilometers northwest of Spring Hill in the central part of the Chapin quadrangle (Fig. 2, and Plate 1) . l The Spring Hill Roadcut I The Spring Hill roadcut is the best exposure for viewing the character-istics of the rocks in and adjacent to the Wateree Creek fault zone, and l l observations made here have played a central role in the formulation of our I B conclusions about the zone. The saprolite exposed in the Spring Hill roadcut l is thoroughly weathered and very friable. The quality of this exposure has I degraded substantially during the past 18 months, and in order to permanently l document the evidence for t'.e Wateree Creek fault zone, a detailed geological l sketch map and cross sections have been prepared for this important locality l l (Fig. 9). The cross sections of the northwest and southeast faces of the l roadcut were made in the field from an uncontrolled photo-mosaic. The I geological relationships shown on the road surface are reconstructed from observations made in the faces of the roadcut. The roadway is paved and the l actual geologic relationships in the roadway have not been observed. The southwestern end of the Spring Hill roadcut is interpreted to be west of the Wateree Creek fault zone. A gently dipping intrusive contact l (A, Fig. 9) between massive diorite and spotted hornfels occurs at the west end of the cut. The precursor to the spotted hornfels is interpreted to have been interlayered mudstone and metavolcanic rock from near the top of the 6tb unit (Fig. 2) in the north limb of the Chapin synclinorium. Relict L lapilli and relict L and L g lineations (% 12 N.75 E.) locally can be Ox1 { recognized in the spotted hornfels. The contact metamorphism is interpreted E r .
. DOCUMENT .
~
PAGE . PULLED. . L ANO.34- l NO.OF PAGES -( nusw D PAGE LLEGS1 D WD COW,nED M. PDR CF 014R i D del 1ER COW REQUE51ED ON -- 3 3 - hPA31100 LARGE10PLM 4t=>c0~,aDm.,OR @ 014R -_ - hkVIDON APimdRE CARDe 36\ DMO bD~O} 1
3 37 to have been overprinted on deformed and regionally metamorphosed strata during emplacement of a small stock of massive diorite in post Di time. The orientations of Di fabric elements in the southwestern end of the roadcut is similar to the orientations observed elsewhere in the Chapin synclinorium and {- hence the southwestern end of the cut is interpreted to be outside the region affected by drag associated with the Wateree Creek fault zone. The rocks in the northeastern two-thirds of the Spring Hill roadcut are broken by two distinct groups of faults. The oldest group occur in a 5-10 meter thick zone, oriented NN.70 W.65 SW., and characterized by intense silicification and brecciation (B, C, Fig. 9) . Numerous irregularly ~ oriented I extensional veins, partly infilled with comb quartz, occur adjacent to the i silicified fault zone on the southwest side in the diorite. The eilicified fault breccia at locality B (Fig. 9) is intsrpreted to be the easternmost end j or silicified fault zone that extends at least 5 km to the west of a ma' (Plate 1, and Fig. 2). The youngest group of faults in the Spring Hill roadcut (D, E, F) oriented NN.30E.780NW., are part of the Wateree Creek fault zone. These north trending faults cleaily displace all of the bedrock map units as well as the west trending silicified fault zone, but they do not displace a thin irregular layer of surficial colluvium of unknown age found in the southeast face of the roadcut (G). The roadcut is divided into four blocks l by the faults of the Wateree Creek fault zone (see insert, Fig. 9). Block 2 displays many of the same rock types and structures as block 1, however, a thin felsic dike in the diorite (H, I) and the silicified fault zone (B, C) display left separations of 5 and 30 meters, respectively, along fault D. ( Blocks 3 and 4 contain metamudstone breccia and metamudstone respectively, and it is not possible to correlate specific geological features across faults [ L . .
i i 38 I I i E and F. The orientations of the Li and lox 1 lineations in block 4 (%58 N.77 E) clearly differ from the regional average (14 N.78 E.) and their anomalous orientation is interpreted to be the result of rotation and/or iI , drag within the Wateree Creek fault zone. The fault zone therefore continues - beyond the northeastern end of the Spring Hill roadcut and may contain addi-tional blocks and fault surfaces. Determination of the Net-Slip Direction In the Spring Hill roadcut and at several other locations along the Wateree Creek fault zone the L g and Q xt lineations are anomalously oriented as a result of drag folding and rotation in the fault zone. When a pre-existing foliation (in this case S i )is refolded, linear elements in the E foliation plane may be dispersed along a small circle on a stereonet, the center of which coincides with the axis of secondary folding. It is assumed here that the net-slip direction is, perpendicular to the axis of secondary (drag) folding in the plane of the fault. Measurements of Li and lox 1 were taken from within a circular area of radius one mile surrounding a prominent creek exposure of the Wateree Creek fault zone just to the east of the Spring Hill roadcut (Fig. 10). This data set, containing 106 lineation measurements from both within and outside the area affected by fault drag, is plotted in Figure 5d. The points fall on a portion of a small circle having an axis oriented 40S.30W., and assuming an average orientation of N.30E.780NW. for the Wateree Creek fault zone, the indicated direction of net slip is 770N.640W. The rotation of L1 and L0x1 lineations by drag in and adjacent to the Waterce Creek fault zone therefore indicates that the fault displacement is predominantly dip slip, with a minor component of lef t lateral slip, a result in agreement with the displacement inferred from the apparent offset of the axis of the Chapin synclinorium (Fig. 8). I
; -Q76. a b,y r ,
1
\} fM !!
1 ! L "E
\l hho -
i at a&' n
+o
;# 3r; L-; aw xev e
h ,.)
- Drill sites h
% , s,ua , r t L g ; ;
+ aooacui j l 2 y Wateree Creek Fault .
j
'x ,
zone ................. L g E siiicitiea roui s-
}}) (.. :
j F hx
- 48
, ,, ('V [)
r urs/ m <nm L W ris _ 1e. t_m_ _, 1 ~ s,ms em _e__emio.
F 40 Results of Drilling Investigations All of the natural or roadcut exposures of the Wateree Creek fault zone have undergone some degree of chemical weathering, and the thoroughly brecciated rock along and adjacent to fault surfaces is particularly susceptible to alteration. In order to obtain fresh samples of cataclastic
- s. g rocks for petrographic study, and in hopes of finding datable secondary 3 minerals (such as zeolites) associated with the faulting, it was decided to drill four shallow core holes into the fault zone. Three of these holes were located along the projected trace of the fault zone near Wateree Creek, aboilt 0.3 km south of the Spring Hill roadcut (Fig. 10). The fourth hole was located adjacent to an outcrop of partly weathered fault breccia about 3.5 km south of the Spring Hill roadcut (Fig.11) .
Mudstone, brecciated to varying degrees, was encountered in all of the drill holes at location 1, 2, and 3 near the Spring Hill roadcut (Fig. 10). . Most of the rock encountered in holes 1 and 2 was relatively intact mudstone, although thin brecciated zones were encountered at several levels. The rock in hole 3 was much more intensely brecciated. The plunge of the Li and Igxi lineations varies between 250 and 800 in holes 1-3. No systematic variation in the plunge of the linea,tions with depth could be detected in the cores j (Fig. 12). From X-ray diffraction studies of whole-rock powder and from examination of thin sections, the mineral assemblage in both intact and brecciated rock was determined to be: quartz-m2scovite-chlorite-calcite - dolomite. Several episodes of veining and brect:iation can be inferred from I mesoscopic inspection of the core. The vein fillings are predominantly carbonate with a trace of quartz. No zeolite minerals were recognized. In thin section, the carbonate appears to have recrystallized following the last episode of brecciation. Aside from some twinning of the carbonate, no I I
I , I gg I > ' n l l F
,'~~y 5
]
3
.I J,b.' *. _-
i g u
%b M M.]y# _
aWc Vo j( + bO
- G r
"na ) '/
t e 1 Ao anA c c . i 0 5e5 O A A K I
,1 .4 ' )
3 3:,& 2 (/,
/
1 3 u N5 N 1O4 i
- j '
A K Y
- r n
)
- e n 0 5
- S F E '
o oi T Y - siG one o - o t u R Ao C \ u t t l
-i t n S i
s t vu t tr R .t S
- f. M _) 3,g(\ o
- \
] \
f _ hho - ) . E o c * .
- eg
,* V c. g \ .
c i S P 5 g\ 5 g _.- evc ni l c\n O f.
. eg. h' O 3
f g h t cm o u 4 O gm W _ ria ,. . 4 I ys
, -)
anp ,
, 0 7 .
g li -
\ s ; .
J69\ pP ts 1 .
, , )
pyh V M ) w *i, . v a o . .u ,' row ,% n :(j
- ),.:d\.;M::n@MQt C.
t fi t / . , * . .
- j/ <! l s j, s n
l otg t fh hWe ea et h c g s,
>l y
- * .j, i ,
t
. k dw o r
( r
;o,M . l J
Uy 3%
;. h i.
x C0 3
.l c,],*
- g.u.-'v. 'U'CC f
Ceo t c
.%,,;4,. c N ,
f, y 0, 3 ~' * ' hrn ,
~ -
aef , p ei
, ( o .,
- k. b i
nCu q ea uet rr g s t f. 3 3
~-
*.~-
~
7 v .
\
~ h' .
,- - - w aki - . r
., pi n /.s e
d o , a ,- , 'l t o ", rfn aa nuo 0 g >,c U . ,
', 3 5
3
%y e
H D E f w f [ Q
%w, 1
g lf lt
.e z a
,. ~ "~. . s o
w h,
.il l- ,.
8
~
~ '
V k od
^ c.n i
ni ea s
-(
- k' P. . 'f o b ~
bgI . .M,.' ia ~ nse .
,(* .
~' ~ 1 , ]
,.* s o
g t 1
'l 't 1 ;
- [ i.
hd ei k e ', A 4 1
42 I I Plunge of Lon and Rock Type l 10
- 2
- 'd
( g
- e.
. . _ 7,<
)k - - oc g
, 2I- o 3 43 < 4T E
25 b*
*c* 7 p
5 u bf-,r -
~
l 47 k 3I *C
'I O
21 M,s 5
,s
*1 m(5 n
l
; G 7C 82 . aQ t ss .-
E ( 4, o :/ 21
/ ::
i
. o = , a f.
i 9
,-35
'2d v3 ... -
9 5 3 m = 0.C -
.j" -
" 'O+ "
l m
- ..., 67[ 7.#
7 - 3 *
- J0 ?/ c ,
@5
\ '
. *1 . .c j 4? 65 , go .
j 50 . .t< l 45 - i $hJ m
; [v 3
r E 50 5 5 2Nj s r a
- sI
- D so g
- - - E S2' - - v n Core 3 2 1 3 Number Terrane ill Mudstone Brecciated Mudstone l Figure 12. Core logs for drill holes 1, 2.and 3, from the Wateree Creek fault zone near the Spring Hill roadcut (Fig. 10).
I
43 f . evidence for post recrystallization cataclasis or ductile deformation was observed. At several locations in the cores, carbonate veins are clearly offset by small normal faults. following the S0 compositional layering. This I
. displacement sense seems compatible with the slip that would be expected to
( result' from down to the east movement along the Wateree Creek fault zone.
' Hole 4 was drilled into brecciated metavolcanic rock of intermediate composition along the west side of the Wateree Creek fault zone in the south limb of. the Chapin synclinorium (Fig.11) . The mineral assemblage here is quartz-albite-muscovite-chlorite-epidote-calcite-dolomite. All of the rock recovered from this hole down to a total depth of 64' is thoroughly 1
brecciated and characterized by intense calcite veining. As in the first three holes, there seems to have been multiple ~ episodes of veining and brecciation. In thin section, the carbonate seems to have recrystallized following the last episode of cataclasis and strain. However, the brecciated rock in hole 4 is relatively weak and friable. Even at the bottom of the hole, at a depth of 62' and more than 40' below the base of the weathered zone, the core recovery rate was only about 50% of the amount drilled.
'The above studies of the petrological character of the cataclastic rocks in the Wateree Creek fault zone suggest that the faulting occurred under
. conditions of relatively low temperature and confining pressure such that carbonates were the only mineral components that were sufficiently mobile to contribute appreciably to vein fillings.
Interaction of' a Diabase Dike with the Wateree Creek Fault Zone Some considerable antiquity for the Wateree Creek fault zone is suggested
- by the lack of a fault scarp and the fact that the faults do not displace a layer of surficial colluvium in the Spring Hill roadcut. Regional geological l .ir. .- .
_ - m
P
. 44
- studies have indicated that a major diabase dike experiences a dextral offset in the vicinity of the Wateree Creek fault zone in the south-central part of the Chapin quadrangle [Secor,1980], (also see Plate 1 and Fig. 2) . In hope of more precisely determining the time of faulting, we have undertaken detailed geological and geophysical studies in the region of apparent offset of the diabase dike. The diabase dike is very susceptible to chemical weathering, and natural outcrops are known from only a few locations (Fig. 11). However, previous studies [Secor,1980] had indicated that most diabase dikes in this region of the Carolina slate belt are characterized by strong positive magnetic anomalies of magnitude 100-1500 gammas. Therefore, in order to precisely map the con-figuration of the diabase in the vicinity of the Wateree Creek fault zone, a series of closely spaced detailed surface magnetometer profiles were measured. The locations of these profiles and the configuration of the diabase dikes inferred from the measurements are shown in Figure 11. In order to be absolutely certain that the magnetic anomalies were related to diabase dikes, several trenches were excavated at critical locations (Fig.11) . The results at trench #1 were indeterminate because surficial colluvium extended to the deepest levels of the excavation. Only mudstone saprolite was recovered from trench #2. The amplitude of the magnetic anomaly at this locality is only 150 gammas, and the dike may be very thin or may not extend completely to the surface. Hornfels, presumably resulting from contact metamorphism of the diabase was encountered in trench #3. Diabase saprolite and residual boulders of diabase were encountered in trenches #4 and #5. Trench site #5 is located at a particularly important place where the diabase dike from the east block appears to penetrate across the fault zone and into the west block. Three separate holes;were actually excavated at
45 trench site #5. The first was located ten meters west of Waterce Creek on - strike with a creek outcrop of diabase saprolite interpreted as being in the - east block. Diabase saprolite was encountered in the hole, and fresh residual i boulders of diabase were recovered from a depth of six meters. The diabase j _ saprolite contained thin quartzo-feldspathic veins similar to north trending vertically oriented veins observed in the adjacent creek outcrop of diabase saprolite. Some slickensided surfaces were observed in the diabase saprolite removed from the excavation, however these are interpreted as weathering phenomena or as excavation related deformation structures, because no deforma-tion structures were observed in the fresh diabase recovered from the trench. Two additional holes were excavated along the projected strike of the diabase _. dike about 100 meters northwest of Wateree Creek. Diabase saprolite and __ I residual boulders of hornfels were recovered from these holes. The excavations Z at trench site #5 clearly indicate that the diabase dike from the east block penetrates well into the Wateree Creek fault zone, and the continuity of the i associated magnetic anomaly on to the northwest indicates that the dike continues for more than 1 km into the west block. Although the diabase dike contains a dextral en echelon offset near the ' Wateree Creek fault zone, the detailed geological and geophysical studies described above demonstrate that this offset actually occurs west of the g fault zone, and that the dike was emplaced af ter most or all of the displace-ment had occurred along the fault. The geometrical irregularity of the diabase dike in the region immediately west of the Wateree Creek fault zone - , is here interpreted to be the result of a discontinuity in the stress field I caused by the fault, and not a direct result of the fault displacement. The
, evidence therefore indicates that the Wateree Creek fault zone was already in -
l
46 existence when the diabase dike was emplaced. At present there is no con-clusive evidence indicating that slip occurred along the fault after the , emplacement of the diabase. If such slip did occur, the resulting horizontal separation of the diabase must be less than a few tens of meters. SEDfARY The location and character of the Wateree Creek fault zone is established from the following evidence: 1) offset of stratigraphic contacts and pre-existing structural features, 2) abrupt termination of a prominent aeromagnetic anomaly, 3) the occurrence of fault breccia at several ' locations along the zone, 4) drag and rotation of pre-existing fabric elements within the zone, 5) observation of three of the component faults in the zone in the Spring Hill roadcut. Examination of cataclastic rocks from four shallow holes drilled into the Wateree Creek fault zone suggest that the brittle faulting occurred at relatively low temperature and confining pressure such that carbonate minerals were the only components sufficiently mobile to contribute appreciably to gash vein fillings. Study of the rotation of pre-existing rock fabric elements, as well as the offset of the axis of the Chapin syncli-norium indicates that the 1700 m displacement is predominantly reverse dip-slip (down to the east), with a minor component of lef t-lateral slip. The youngest geological feature that is clearly offset by the Wateree Creek fault zone is an east trending silicified breccia zone. Similar appearing silicified breccia zones have been described from many places in the E Piedmont [Conley and Drummond, 1965; Overstreet and Bell, 1965a; Secor and Wagener,1968; Snipes and others,1979; Odom and Hatcher,1980], but their absolute age is uncertain. East trending silicified breccia zones clearly 8 5
..m_----- __
postdate Carboniferous deformation and regional metamorphism in the Kiokee belt of South Carolina [Secor and Snoke, 1978]. Silicified breccia zones I are associated with some of the faults bordering Wsozoic grabens in the Piedmont, however some of the border faults are thought to be reactivated Paleozoic faults [ Butler and Dunn,1968; Glover and others, 1980], and it is uncertain whether the silicified breccias are related to late Paleozoic or Mesozoic movement. Snipes and others [1979] have described a silicified I breccia zone in northwestern South Carolina which clearly offsets a Mesozoic diabase dike. Based on the above considerations, the time of earliest movement on the Wateree Creek fault zone is certainly post Carboniferous and probably post early Mesozoic. The oldest geological feature that clearly cuts the Wateree Creek fault zone is a northwest trending diabase dike. This dike is similar to many others that have been observed in this region and throughout the Piedmont province. These diabase dikes are difficult to date radiometrically because they contain excess argon [Dooley and Wampler, 1977]. They are commonly considered to be Upper Triassic and/or Lower Jurassic based on their paleomagnetic pole position [deBoer, 1967; Smith and Noltimier, 1979; Sutter and Smith,1979; Dooley and Smith, in press]. The geological evidence therefore indicates that the main period of movement along the Wateree Creek fault zone was post-Carboniferous and
. pre-Jurassic. Although we cannot completely rule out the possibility that a limited amount of more recent movement has occurred, there is, at present, no evidence indicating such movement.
lI il lI
. I CHAPTER III GEOLOGY OF THE LITTLE MOUNTAIN AND CHAPIN QUADRANGLES, SOUTH CAROLINA: A STUDY OF THE STRATIGRAPHIC RELATIONSHIP BETWEEN THE CAROLINA SLATE BELT AND THE CHARLOTTE BELT ABSTRACT s
Located about 50 kilometers northwest of Columbia, South Carolina, the Little Mountain and Chapin quadrangles straddle the boundary between the Charlotte belt and the Carolina slate belt. Field mapping in this area has I revealed that the slate belt stratigraphically overlies the Charlotte belt. Throughout the entire study area the Charlotte and slate belts are separated by a large tabular intrusion, the Little Mountain pluton, which cuts the study area approximately in half striking about N.70 E. South of the intrusion, within the slate belt portion of the study area, outcrop distribu-tion is controlled by a major F1 synclinorium, the Chapin 'synclinorium, which . extends across the entire southern portion of the study area trending N.75 E. and plunging to the northeast at about 11 . This structure is offset by two north-south faults, the Wateree Creek and the Su=mers Branch faults, which also offset the Charlotte belt- slate belt boundary, the Little Mountain pluton, and units within the Charlotte belt portion of the study area. Offsets of stratigraphic contacts on both limbs of the Chapin syncli-norium indicate that both the Wateree Creek and the Summers Branch faults are dip-slip faults. The Little Mountain pluton and several Charlotte belt units north of the intrusion are offset in the same direction as the slate belt units on the northern limb of the Chapin synclinorium. For that reason the Little Mountain pluton is considered to be concordant with bedding (S0 )- Additionally, this indicates that the pluton and the offset Charlotte belt I I
49 I . I units north of it stratigraphically underlie the slate belt units which 1 comprise the Chapin synclinorium. I INTRODUCTION The Little Mountain and Chapin 7 1/2 minute quadrangles, located about fif ty kilometers northwest of Columbia, South Carolina, straddle the boundary of the Carolina slate belt and the Charlotte belt. Because of its location relative to this major boundary, the area provides an excellent opportunity to study the relationships between these belts. Hopefully this study, as part of an ongoing systematic effort to map this part of the Appalachian Piedmont in detail, can contribute to a better understanding of the Piedmont province of the southern Appalachians. Additionally, these quadrangles are adjacent to Monticello Reservoir, South Carolina, which is one of the best documented examples of reservoir induced seismicity in the United States [Duc and others, 1978; Talwani and others,1978; Talwani, 1979; Talwani and others, 1980]. It is hoped that this study and additional field mapping north of the study area will provide a firm geologic base for interpretation of the large quantity of geophysical data which has been amassed in the induced seismicity study. The purpose of this study is to delineate and describe the lithologic units within the study area, determine structural and stratigraphic relation-ships between and within these units and consider the nature of the Charlotte I belt - slate belt boundary. The Little Mountain quadrangle was mapped by Larry Peck and the Chapin quadrangle was mapped by David Simpson. Field mapping in this area consisted mainly of traversing stream beds. Other outcrops were found along dirt roads, railroad cuts, roadcuts, and along the shores of Lake Murray, but generally exposure .in the area is poor. In
y 50 addition to field mapping, several core holes drilled in the Chapin quadrangle provided information on a major north-south fault zone, the Wateree Creek fault zone. Also, information was obtained from rock staining and standard petrographic studies. REGIONAL GEOLOGY , The southern Appalachian Piedmont has been subdivided into several northeast trendia; belts on the basis of unique lithologic, structural, and/or metamorphic characteristics [ King,1955] (Figure 13) . The Little Mountain- Chapin area straddles the boundary between two of these subdivisions, the Charlotte belt and the Carolina slate belt.
- The Charlotte Belt The Charlotte belt is a belt of medium to high grade metamorphic rocks extending from North Carolina to Georgia. The lithologies of this belt are 5 predominantly intrusive, but amphibolites, hornblende gneisses, biotite gneisses and schists also occur and are considered to have been derived from sedimentary, pyroclastic and volcanic protoliths. These country rocks have been affected by several generations of plutonism. The oldest intru-sions range in age between 705 and 520 m.y. [ Hills and Butler, 1969; Fullagar,1971] and generally carry a metamorphic overprint. Several intrusions of this generation have been interpreted as epizonal intrusions genetically related to the metavolcanic rocks of the Charlotte and slate belts [Fullagar,1971; Whitney and others,1978; Weisenfluh and Snoke, 1978]. The second generation of intrusions, radiometrically dated at 415 1 '
to 385 m.y., are considered to be late kinematic [Fullagar, 1971], and I I
- C U LJ W W M f
~.
..' NORTH T E NNE SSE E ,
^
, , s esj U
#~ ""
- p @
/, __
.g-
;;gs r
i c;/ .'
~(:-
'sN e .
- ^W i[ E
, r, o iiii ma.- p ....../
. :+-
M l.g / 4^ _. j J ,jf,;;^ ,97 / 'L;gn :n AL ABAM A. T f3 3 b - 94 ;. cp [
?b
[ l
<> ;.;.g'- {f i
gO
# dE
*g: ::"h@ "ssib
;.:.:.::!!5I :;:yj:..
c
"')'""
Y ' l e :'# g, -
@p% SOUTH G7 g 4 C A R OLIN A
/ f:!+T
; [ tfg 8 $>, '*- BEL AlH
\ / 44 l~y -
' di' ( BELT
,i k c Oh j
\ s
\ p* et.O \
g P@b p M b GEORGIA 0 MWyRS M U Figure 13. Map showing major geologic subdivisions of the so ithern Appalachians (modified from Williams [1978]). Box encloses area shown in Figure-14.
52 the third generation of plutons, emplace'd between 325 and 265 m.y., are f post kinematic. They do not carry a metamorphic fabric and resulted in contact metamorphism of the rocks which they intrude [Fullagar and Butler, 1979]. These plutons are probably related to a major Hercynian deformational event which resulted in penetrative amphibolite facies metamorphism in the Kiokee and Raleigh belts and small scale F2 folding [Secor nd Snoke, 1978] in the slate belt. In addition to these three periods of plutonism the Charlotte belt has been intruded by many thin mafic dikes which at some locations compose 50-75% of the rock occurring as " mafic dike swarms" [0verstreet and Bell,1965]. A wide age range can be attributed to .these through various cross-cutting relationships with rocks of the different major intrusive episodes, texttiral and metamorphic characteristics. The final period of igneous activity in both the Charlotte and slate belts was the emplacement of Upper Triassic and/or Lower Jurassic diabase dikes [Dooley and Smith, in press]. The Carolina Slate Belt To the southeast of the Charlotte belt is the Carolina slate belt, a northeast trending zone of low to medium grade metamorphic rocks extending about 650 kilometers from central Virginia through the Carolinas into eastern Georgia. It is flanked on the northwest by medium to high grade metamorphic rocks of the Charlotte belt while to the southeast it disappears under Cretaceous and Tertiary age Coastal Plain sediments except where it is bounded by medium grade metamorphic rocks of the Kickee belt in Georgia and southern South Carolina and the Raleigh belt of North Carolina and Virginia (Fig. 13). The slate belt is a thick sequence of metavolcanic and metasedimentary rocks which have been locally intruded by igneous I I
53 I rocks of varying age and composition. Two major lithologic sequences are widely distributed throughout the slate belt. The older sequence, called the Uwharrie Formation [Conley and Bain, 1965; Stronquist and Sundelius, 1969] in North Carolina, the Persimmon Fork and Wildhorse Branch Formations [Secor and Wagener, 1968] and terranes II and IV [Secor and Snoke, 1978] in South Carolina, and the Lincolnton metadacite and felsic pyroclastic sequence [ Carpenter,1976; Whitney and others, 1978] in Georgia, consists mainly of intermediate to felsic metavolcanic rocks. The younger sequence, on the other hand, consists mostly of metaclastic rocks with some thin layers of metatuffs and meta-basalts. It is called the Albemarle Group [Conley and Bain, 1965; Stromquist and Sundelius, 1969] in North Carolina, the Richtex Formation [Secor and Wagener, 1968] and terranes I and III [Secor and Snoke, 1978] in South Carolina and the upper sedimentary sequence [ Carpenter, 1976; Whitney and o thers , 1978] in Georgia. The distribution of these units within western South Carolina, including the study area, is shown in Figure 14. These sequences have not been unequivocally correlated by complete detailed mapping, but the equivalence is widely accepted even though the structural and stratigraphic relationships within the units are controversial [see Secor and Wagener, 1968; Daniels, 1974; and Costello and others,1981]. The Carolina slate belt is generally interpreted to have been deposited in a magmatic are environment. Whether the arc formed on oceanic crust [ Whitney and others, 1978; Black,1978] or on continental crust [ Butler and Ragland,1969; Hatcher,1972; Glover and others, 1978] is still a matter of contention, however. Radiometric dating of slate belt metavol-canic rocks has indicated that the are was active during the late ) l Precambrian and early Cambrian [Fullagar,1971; Butler and Fullagar, ' 1 B s,
l GENERAllZED GEOLOGIC MAP g,s, , OF [, o
-,oo u ,
tn .L.'A
,< uc
,s
-g s-,,
s
'J E,~, ' ,ff
-! -,s
, mu
[ THE CENTRAL SOUTH CAROLINA PIEDMONT '''''A' mu - _. o , . ; .. .-
~
.. . .:.'h utT s' e d W; ^..::::..
g' . _ g!I[
, ., , : . o.. i
.NW
-i."P.'..
g mu I *
'k "
.. , , ' -)I -oooc
.f..
k .h. P.... g ,
..e mi . . no-g / ;"'.,/,'..*;*,'y,Y
,, ,, .:.e.:.*
.m,va *
-u... "
. ::' * M':' '
*f /,*.* .
O
. g+Q d:7l%,gk p htt 9 g COLUMBIA
* .. .g *. - '.'i
- f5"
. . . .: :- : '.' ,,h. :,/,': y
- .. , s.
, / 7.'.* ';f .,'*',
4 .
- ' / J' e BATESBURG
... ~.:
M.f'c.:Q,./;:'** .a E .:
.9; .y "
1 ,/
- g::,d..L{U:::'.': #'
N , ;.
.::r:- *
- a,* * :: : w-i AROLINA ' ** . ' *
**,.t'.'~~
T o e*+ casseo ! SL ATE BELT Qi' . %
,.?* # CO ,gs, c, , g,,,
p.MgC, h DErotMEo ca ANtif mo *I
# lI "I l META 5EDIMENTAlf ROCKS
, I (GaEEH5CH15T PACIE5)
KIOKEE BELT
/- I h l MEIAVOLCANIC ROCKS j l 1
" " . (GaEEN5CH151 FACsES) mv [
BELAIR BELT GUS A e. l mu l "r[s a !"
/
Figure 14. Geologic map of the western Carolina slate belt of South Carolina (map modified from Overstreet and Bell [1965a]; Carpenter [1976]; Brown [1971]; Pirkle [1977]; Pirkle [1978 ]) . Box encloses the Chapin and Little Mountain quadrangles (see also Fig. 2). y M LM M M M M M M M M O 8 M M M M S U M
w 55 t i 1975; Black,1978]. Fossil evidence supports a Cambrian age based on trilobites found in the slate belt of North Carolina [St. Jean, 1973] and South Carolina [Samson and others, 1982]. Regional Deformation
- Both the Charlotte and slate belts have undergone several episodes of deformation. Bourland and Farrar [1980] reported eviden, e for six distinct deformational events accompanying two major thermal events in the Charlotte and slate belts around the Winnsboro pluton, South Carolina.
The oldest deformational event in the slate belt, the Virgilina Deformation, was a late Precambrian or early Cambrian period of folding and faulting. This event affected only the oldest, northernmost, portion of the slate belt in Virginia and North Carolina [ Glover and Sinha, 1973; Black and Fullagar, 1976], the southern portion being too young to have been affected by it. A later event was the first and the strongest deformational event (D 1) to affect the Charlotte and slate belts in South Carolina. It accompanied a period of regional metamorphism being characterized in the slate belt by greenschist facies metamorphism, tight isoclinal folding and formation of penetrative slaty cleavage and in the Charlotte belt by amphibolite facies metamorphism, isoclinal folding and axial plane foliation [Secor and Snoke, 1978; Bourland and Farrar, 1980]. The timing of this event has been bracketed by radiometric dating of the prekinematic plutons and post kinematic plutons which crosscut deformational fabric, establishing a time frame of 520 to 300 m.y. [Fullagar, 1971]. The exact timing of the D i . event, however, is quite controversial. An Acadian age has been indi-cated by U-Pb zircon dates in the slate belt of North Carolina [Seiders 1 and Wright, 1977; criggs and others, 1978]. A Taconic age has been l l f _ _ -
B 56 suggested by Rb-Sr dating of metamorphic minerals in metavolcanic rocks [ Black and Fullagar,1976] and K-Ar dating of phy111tes [Kish and others, 1979] in the slate belt of North Carolina. Recent radiometric dating of rocks from the undeformed Newberry Granite north of this study area which contains xenoliths carrying the Si foliation indicated a cooling date of 415 9 m.y. [Fullagar, 1981] supporting a Taconic age for Di. DESCRIPTION OF THE ROCKS Nine mappable lithologic units have been differentiated within the Little Mountain and Chapin quadrangles as shown on the geologic map of this area (Fig. 2). Seven of these units are of sedimentary or volcanic origin; the remaining two being of intrusive igneous origin. Metamorphosed rocks within the Charlotte belt and within the slate belt will be discussed separate.ly, and post-tectonic intrusive rocks will be considered as a third group. Charlotte Belt Units Three Charlotte belt units have been differentiated within the study area. 'Ihey include a unit of amphibolite of volcanic and/or sedimentary origin (Ea) and a unit of metasedimentary biotite schist and biotite paragneiss (Esp) . Also, granitic orthogneisses (Gggn) intrude much of the Charlotte belt portion of the study area. Mineral assemblages of semples from these units are shown in Table 1. Amphibolite (Ga). Amphibolite is found extensivr',y throughout the Charlotte belt portion of the study area, occurring as large lenticular sheets which are locally migmatized by granitic gneisses. Charlotte belt amphibolites have been interpreted as metamorphosed basaltic flows I I
57 I I $ 3 m o 0 $
$ N O a 5 i 5 0 5 . e 1 3
# 5 a 2 4 % . 8 E e # E t 3 d S T I E ? % a % & i & &
Sample Rock No. Unit I 265 X X T T Ga 353 X X X T X T - Ga 288 X X X X X X X Gfgn 369 X X X X X X Efgn
?68 X X X X X Gggn 298 X X X X X X Eggn 313 X X T X X T Eggn 601 X X T X T T Gggn I X indicates presence of a mineral T indicates presence in trace amounts I
I Table 1. Mineral assemblages of sanples from the Charlotte belt portion of the Chapin and Little Mountain quadrangles. I I I I _ -- _
58 [McCauley,1961b; Overstreet and Bell,1965a] with less mafic amphibolites
'being interpreted as metasedimenta'ry rocks and metatuffs [0$ rstreet and Bell, 1965a]. Found within the amphibolite are tabular intrusions of I
orthogneisses, thin layers of biotite schist and layers of biotite paragneiss. Locally the amphibolite grades into the schist or paragneiss indicating a sedimentary or pyroclastic origin. The metalava flows within this unit may be genetically related to the amygdaloidal greenstones found in the slate belt. The large volume of suspected metalava flows present in the Charlotte belt compared to the relatively small amount in the slate belt (Fig. 2) argues against this correlation. More likely, they occupy different levels in the stratigraphic colu=n and the reduced volume of metabasalt in the slate belt represents a vaning of mafic mag =c generation through time. Amphibolites within the study area are fine to medium grained rocks with a typical mineral assemblage of blue-green hornblende', plagioclase, epidote, quartz, and minor amounts of biotite and opaque oxides. The hornblende crystals are elongate and anhedral. Plagioclasa occurs as anhedral crystals. Biotite schist and biotite paragneiss (Efgn) . Like the amphibolite,
~
the unit composed of biotite schist and biotite paragneiss outcrops extensively within the Charlotte belt portion of the study area. Within this unit are thin layers of amphibolite as well as large tabular intrusions of granitic orthogneiss. The biotite schist is most often found in asso-ciation with these intrusions. Poor exposure, complex deformational history, questionable protolith interpretation, and rapid facies variations all combine to make structural and stratigraphic interpretations within the Charlotte belt difficult at best. Due to the fact that they are interfingered and interlayered this unit and the amphibolite are considered I
s9 E !
\
l luu to be stratigraphically equivalent. l The biotite paragneiss and biotite schist unit consists of fine to medium grained inequigranular foliated rocks. They are predominantly quartz, plagioclase, and biotite. Additionally, they may contain blue-green hornblende, potassium feldspar, muscovite, and opaque oxides in varying percentages (Table 1) . There is wide mineralogical variation within this unit. Undifferentiated granitic orthogneisses (Gggn). Two major types of granitic orthogneiss are found in the Charlotte belt portion of the study area. Rock staining has revealed ~ one type as potassium poor while the other is potassium rich. The first type occurs almost exclusively within the largest intrusion in the study arer., the Little Mountain pluton. This pluton is a large tabular intrusion cutting the study area approximately in half, trending about N.70 E. along the northern boundary of the slate belt. The length of its outcrop area is unknown, but it crosses the study area for 24 km with an average width of about 1.5 km. Due to its low potassium content this rock may be genetically related to the potassium poor metavolcanic rocks of the slate belt. If so, it is almost as old as the slate belt rocks, having intruded its own volcanic ejecta. In the Little Mountain pluton are several small masses of granitic orthogneiss which are potassium rich. Within this large pluton the orthogneisses are not differentiated due to extremely poor exposure in that part of the study area. This second type of orthogneiss occurs most of ten, however, deeper in the Charlotte belt as smaller concordant tabular intrusions. The ages of these rocks are unknown. Additionally, since no outcrop revealing a cross-cutting relationship between the two gneisses has been I I
60 found, their relative ages are also uncertain. If they are the same age they could represent different phases of the same parent magma or could have re1ulted from two different magmas. Except for the differences in feldspar composition, the rocks of this unit are mineralogically similar (Table 1) . A typical mineral assemblage consists of quartz (55%), feldspar (33%), white mica (10%), ~ opaque oxides (2%), and epidote (trace) . Textura11y, they reveal a complex deformational history, but have considerable variation between samples. The rocks vary widely in the degree of foliation visible and in grain size. Rocks in the north are generally fine to medium grained and equigranular. but are sometimes porphyritic. The rocks in the Little Mountain pluton are medium grained and almost always porphyritic. I Carolina Slate Belt Stratigraphic Units Four stratigraphic units have been mapped in the slate belt portion of the study area. These units include: a unit composed of impure quartzite, metatuff and white mica schists (6tq); a metavolcanic debris flow sequence (Gtb); an evenly laminated to massive mudstone and wacke unit (Gmw); and amygdaloidal greenstone (Gg) . The first two units probably correlate with the Persimmon Fork Formation of Secor and Wagener [1968] and the other two probably equate to their Richtex Formation. Mineral assemblages of samples from these units are shown in Table 2. Impure quartzite, metatuff, and white mica schist (6to) . The stratigraphically lowest slate belt unit within the study area is a unit composed of impure quartzites, metatuffs, and white mica schists. This unit extends as a h to lh km wide band across the entire study area adjacent to the southernmost Charlotte belt unit, the Little Mountain pluton (Fig. 2, I I I
l 61 I a E o a N o m n 3 o 3 8 5 E " 8 2 3 " o 8 ~ a 1
!s u t t 8 0 0 3 ? e 0 8
- e I Sample c
a cr ao. a s a
.o so 2:: a o
- o u Rock No. Unit 147 X X X X X X Gmw 229 X X T X X X Gmw 281 X X X X T X Gg 400 X X X X X X Eg 104 X X X X X 6tb
'160 X X X X stb 167 X X X X X X Gb 169 X X X T X 6tb I 194 X X X X X X X Gtb i
213 X X X X X X 6tb 319 X T T X X X Etb 4' X X X X X X X Etg 128 X I X X X X T T stq 306 X X X T T X T Gtq 347 X X X X X X 6tg 354 X X X X X T T 6tg I X indicates presence of a mineral T indicates presence in trace amounts Table 2. Mineral assemblages of samples from the slate belt portion of the I- Chapin and Little Mountain quadrangles. I E
r 62 and Plates 1, 2). The most prominent feature in this northernmost slate belt unit is the Little Mountain- Cannon Hills Monadnock, a feldspathic quartzite ridge about 715 km in length, rising up to 60 meters above the surrounding area. Several layers of kyanite-quartzite occur within the monadnock, some of which are over 300 meters long and 30 to 60 meters wide. The protolith was probably layers of tuff which were enriched in silica by hydrothermal fluids [Espenshade and Potter,1960] possibly resulting from emplacement of the Little Mountain pluton. The resulting altered cherty clay-rich tuff was then probably transformed into lenses of kyanite-quartzite during D1 greenschist facies metamorphism as described by'Hartley [1976] at Graves Mountain, Georgia. In the field it is of ten difficult to differentiate between the quartzite and locally silica enriched metatuff. White mica schist occurs as layers interbedded with layers o quartzite in the monadnock. Rocks of this unit dip consistently to the southeast at about 45 degrees. The metatuffs of this unit are texturally similar to those of the overlying tuff breccia unit but dif fer mineralogically because they contain biotite instead of, or in addition to, muscovite and chlorite found in the tuff breccia unit. In some layers within the 6tq unit the micas are highly concentrated, forming layers of white mica schist and chlorite schist. Quartzites within this unit occur as medium grained equigranular mosaics generally consisting of quartz (65%), kyanite (5%), white mica (20%), opaque oxides (mostly pyrite) (5%), potassium feldspar (5%), and chlorite (trace). Locally, kyanite occurs in greater concentrations forming lenses of kyanite quartzite. The contact between this unit and the overlying tuff breccia (6th) I I
e 63 P L j unit is a gradational one. Indeed, this unit may be part of the tuff breccia r sequence representing a transition between the low grade metamorphic rocks ' < of the slate belt and the higher grade rocks of the Charlotte belt [McCauley, ( 1961a], or it may have been altered as a result of emplacement of the Little P L Mountain pluton. Metavolcanic tuff breccia unit (6tb). Overlying the impure quartzite / metatuff, and white mica schist unit is a sequence of metavolcanic tuff I breccias' . It extends as a 2.5 km wide band across the entire study area k - south of the impure quartzite, metatuff, and white mica schist unit, down F L the western edge of the Little Mountain quadrangle and across most of the southern boundary of the study area (see Figure 2, and Plates 1 and 2) . This unit forms the limbs of a plunging major F1 synclinorium, hereafter referred to as the Chapin synclinorium. L-The northern limb of this structure is equivalent to terrane IV and the southern limb is equivalent to terrane II of Secor and Snoke [1978]. An unusual layer of metafelsic tuff containing rounded crystals of blue quartz, outcrops along the shore of Lake Murray at the center of the southern f-h boundary of the Little Mountain quadrangle. This outcrop correlates with the same distinctive lithology in the same relative position within the sequence on the southern edge of terrane II (Fig. 14), confirming terrane II as an anticlinorium sharing a' common limb with the Chapin synclinorium to ' its north, f Although this unit is extremely heterogeneous with rapid lithologic L change both vertically and laterally, a mineral assemblage typical of this unit consists of plagioclase (50%), quartz (20%), white mica (16%), y chlorite (2%), epidote (8%), and opaque oxides (4%). The matrix of rocks of this unit consists of fine grained subangular to subrounded quartz,
, _ . _ _ _ . _ _ . = _ . . _ _ _ _ _ _ _ _ _ _ - . . _ . , _ . , , . . _ , _ - . .
w 64 plagioclase and opaque oxides and platy micas. Alignment of mica grains within the matrix delineates Si foliation which is also the orientation of quartz stringers and stringers of opaque oxides found in some samples. Relict lapilli commonly found in rocks of this unit are revealed by microscopic study to be fine grained, pancake shaped quartz-feldspathic masses differing from the matrix by their lack of micas or other minerals. They range in size from microscopic to over 10 cm in diameter, but are normally less than 15 mm in diameter. Large euhedral to anhedral quartz I and feldspar grains are also found. The upper contact of this unit is gradational with metatuff breccia interbedded with metamudstone through an interval of about a hundred meters. Evenly laminated to massive metamudstone-wacke unit (Gmw). Graded layers of metamudstone or vacke ranging in thickness from 1s to 3 meters comprise most of this unit. These are interbedded with thin bedded evenly laminated metamudstone and an occasional thin bed of meta lapilli tuff clearly showing the orientation of bedding (S0 ). This unit is the youngest stratigraphic unit in the study area. It forms the core of the northeast plunging Chapin synclinorium and has an outcrop area which extends from I beyond the eastern boundary of the study area across both quadrangles, pinching out just before reaching the western limit of the Little Mountain quadrangle. Convergence of stratigraphic contacts combined with structural data from So measurements confirms this as a major F1 synclinorium. This , t ! structure controls outcrop patterns throughout the entire slate belt portion of the area. The width of outcropping of this unit ranges from 6 km in the southeastern part of the Little Mountain quadrangle to about 1.2 km where a block between the Wateree Creek and Summers Branch faults has been uplifted and eroded in the western half of the Chapin quadrangle. I I
c 65 Rocks of 'this unit typically consist of a very fine grained equigranular subrounded quartz and plagioclase matrix making up about fifty percent of the
- . rock and larger fine grained crystals of subrounded to subangular quartz and plagioclase. The matrix contains white mica and chlorite growths among the
( quartz and feldspar grains. Alignment of these minerals forms the Si folia-tion within this unit. Additionally, the matrix is frequently cut by quartz [ stringers oriented along St . The plagioclase crystals composing about 25%
, of the coarse grains are zoned and are larger than the much more abundant large quartz grains. Typical mineral assemblages. for this unit consist of
{ quartz (60%), plagioclase (20%), white mica (10%), epidote (7%), opaque oxides (2%) (mostly pyrite and limonite af ter pyrite),- and chlorite (1%) . Amygdaloidal greenstone (Gg) . Occurring within and often at the base of the metamudstone-wacke unit are layers of amygdaloidal greenstone. This unit'may have' originated as layers of basaltic lava flows or flow breccias
~
[McCauley,1961; Overstreet and Bell,1965a] although the quartz content may indicate a less mafic protolith. The layers tend to increase in abundance and thickness toward the east. The best exposures of amygdaloidal greenstone f within the study area are along Wateree Creek in the Chapin quadrangle. Thin section study of rocks of this unit reveals a matrix consisting mainly of very fine grained chlorite, epidote,' quartz, and albite crystals. Additionally, the matrix contains larger crystals of broken subhedral albite, and anhedral epidote, fibrous white mica and trace amounts of opaque oxides. The amygdule . fillings consist mostly of quartz and epidote and range up to 1 5 mm in diameter. [ c
- i. _ -- - - - --
7 66 l Post-Metamorphic Intrusive Rocks Two units of post-metamorphic intrusive rocks have been mapped in this area. They are: gabbroic rocks (Cgb) and Upper Triassic and/or Lower Jurassic diabase dikes (JTrd). The gabbros are considered to be related to the 325 to 265 m.y. period of plutonism due to their lack of metamorphic fabric and the presence of hornfels aureoles around them. Mineral assemblages of samples from these units are shown in Table 3. Gabbro (Cgb) . All of the gabbro intrusions in the study area are found within the metamudstone-wacke unit (Gmw) in the Little Mountain quadrangle. ' Two small gabbro plugs intrude the southwestern corner of this area. Evidence for two other gabbroic intrusions exists in the southeastern part of the Little Mountain quadrangle. Although they have not been seen in outcrop, gabbro boulders, sometimes over one meter in diameter, occur extensively as float rock in association with hornfels also occurring as float. The float can be traced for over three kilometers trending about N.40 E. until it terminates at the Summers Branch fault. Another area of gabbroic float rock occurs in the same vicinity but can not be traced. It is therefore interpreted as a small plug. The gabbro plugs at the western edge, of the metamudstone unit have fine grained ophitic textures whereas the gabbro in the eastern part of the Little Mountain quadrangle is medium grained. Subhedral plagioclase crystals are surrounded by anhedral crystal of pyroxene with opaque oxides and some broken subhedral olivine crystals. Diabase dikes (JTrd). All other rock units and geologic structures within the study area are cut by diabase dikes of Upper Triassic and/or Lower Jurassic age [Dooley and Smith, in press]. These dikes trend about N.20 W. with a vertical or near vertical dip. .They range up to ten meters I I
~
l 67 I l I g 8 3 A > 0 h 2 3 8 & 4 & i l l I E H 2 H l a o 5 e4 U 3 e4 o 5h o l 55 ss=91e No. Rock Unit 404 X X X X X Cgb 414 X X X X JTrd 423 X X X JTrd 1 435 X X X X JTrd I X indicates presence of a mineral T indicaces presence in trace amounts 1 I e
' . ,,g.
_ g b _
-. 6- g - ,
- e e '4, 4 f +
*"[
+. -
*g+ '
, *- , , . < , _g
71 I N ll I I . O , l
' ~
)G D i L .
1
/ 1 l
\
.l . E f I Figure 16. Lower hemisphere, equal-area projection of poles to 79 SO I bedding planes measured in the slate belt portion of the Little Mountain and Chapin quadrangles. Contours: 1, 2, 5, 10 and 15%. I I
72 Additionally, th;y describe two other events which predate emplacement of the undeformed Winnsboro pluton, establishing a older age limit of 301 4 m.y. [Fullagar,1971] for the D2 event. Secor and Snoke [1?78] described D2 in the slate and Kiokee belts as Hercynian in age. Snoke and others [1980] bracketed this event with a younger age limit in the slate belt established by emplacement of the prekinematic Clouds Creek pluton dated at 313 2 m.y. and a younger limit set by the synkinematic Edgefield Granite dated at 254 11 m.y. in the Kiokee belt. Enough overlap exists between these two
~
periods of time so that the "D 2" deformational styles seen all the way across the slate belt and into the Charlotte belt could be at least in part synchronous; this is unlikely, however, since D2 within the study area had to end i l before emplacement of the 301 m.y.-old Winnsboro pluton. The D2 event is characterized in both belts within the Little Mountain-
. Chapin area by folding of S0 andt S . These folds vary in style, orientation, and size. Poles to Si foliations from the Charlotte belt and from the clate belt are plotted in Figures Sc, 17 and 18. No penetrative cleavage phase of D2 is recognized within the study area, although F2 folds are abundant.
Within this area the axis of F2 folds generally parallel Lgand L 0x1 lineations, but they do vary locally probably due to sub-epi:,odea of D2 synchronous to events described by Bourland and Farrar as mentioned above. Faulting The study area has been affected by two periods of brittle faulting. The first of these resulted in a group of generally east-west trending l l silicific i faults which are late Paleozoic and/or Mesozoic in age (see Chapter II). The.y are characterized by quartz brecciation and open cavities in which euhedral quartz has crystallized. Although these faults I I
l 73 l I l l ll \ N \ j l / ; , 1 1 L V i y l I .
/(.
( i I e;a , ; i g s A? cN :
\
)
I
~
Figure 17. Lower hemisphere, equal-area projection of poles to 438 Si j foliation planes measured in the slate belt portion of the f Little Mountain and Chapin quadrangles. Contours: 1, 5 2, 5, 10 and 15%. I I
76 I I N I I g , i I
,{- a
/f.-
w -Q; e
/
o . i
'/*
'7
/ ,
i h g Figure 18. Lower hemisphere, equal-area projection of poles to 275 Si I foliation planes measured in the Charlotte belt portion g of the Little Mountain and Chapin quadrangles. Contours: 3 1, 5 2, 5, 10 and 15%. I I
iB 75 can of ten be traced for several kilometers, there seems to be little appreciable movement along them as no displacement of contacts or structures has been observed. The seccnd period of brittle faulting is characterized by major high angle reverse faults trending generally north-south. These faults postdate the east-west faulting but predate emplacement of Jurassic (?) diabase dikes (see Chapter II). Two of these faults have been recognized within the study I area: the Wateree Creek fault located in the center of the Chapin quad-rangle, and the Summers Branch fault trending along the eastern boundary of - the Little Mountain quadrangle. Of the two, the Wateree Creek fault is
) better exposed and has been the subject of a detailed study (Chapter II).
The Summers Branch fault, on the other hand, is poorly exposed, its existence 1 l being supported primarily by the offset of stratigraphic contacts. Offsets l
- are most dramatic in the metamudstone-wacke unit (Gmw) forming the core of the Chapin synclinorium where the width of this unit changes from over 5 kilometers west of the fault to about 1.2 km east of the fault (see Figure 2 and Plates 1 and 2). The axial surface of the Chapin synclinorium dips to the northwest, accounting for a much larger apparent offset of geological contacts on the southern limb of the synclinorium than on its northern limb.
The Chapin synclinorium is broken by the Wateree Creek and. Summers Branch faults in such a way that the block between the two faults is the upthrown block for both faults. Judging from the outcrop width of the Gew unit in I the core of the Chapin synclinorium, the block east of the Wateree Creek fault is upthrown relative to the block west of the Summers Branch fault. Additional evidence for the Summers Branch fault comes from the truncation of a Carboniferous gabbroic int.rusion in the southwest corner of the Little Mountain quadrangle (Fig. 2 and Plates 1 and 2). I
76 DISCUSSION AND CONCLUSIONS The nature of the boundary between the Carolina slate belt and the Charlotte belt and the relationship between these two belts are matters of i fundamental importance in Piedmont geology and have been of long standing I controversy. In south-central North Carolina the belts are bounded by the Gold Hill and Silver Hill faults [Stromquist and Sunde11us,1969]. In Georgia, a portion of the boundary has been interpreted as an unconformity [Scheffler, 1976; Whitney and Thurmond, 1979]. Elsewhere, similar f stratigraphic sequences have been recognized on each side of the boundary I leading to the interpretation of the boundary as a metamorphic gradient from greenschist facies rocks in the slate belt to amphibolite facies rocks in , the Charlotte belt [0verstreet and Bell,1965a; Secor and Wagener,1968; l Griffin,1972; Stromquist and Sundelius,1979; Bourland and Farrar,1980]. l ~ t 'Ihese differing interpretations of the nature of the boundary have led to l ! the videly accepted view that a combination of relationships are involved g i g [Chowns,1976]. Work which has been done in or near the study area include McCauley [1961], Secor and Wagener [1968], and Bourland and Farrar [1980]. Secor and Wagener [1968] traced stratigraphic units across the boundary demon-strating that it is a metamorphic gradient. Bourland and Farrar [1980], working immediately northeast of the study area, arrived at the same I conclusion based upon structural interpretations. McCauley [1961], on the other hand, suggested that the Carolina slate belt and Charlotte belts in Newberry County were derived from different pre-metamorphic protoliths because of geochemical differences between the two belts. I I
77 The' Carolina slate belt is generally considered to have been deposited (- 'in an island are environment. . Whether the arc developed, at least in part, { ..on sialic continental crust [ Butler and Ragland,1969; Hatcher,1972; , 1
- Glover and others,1978] or oceanic crust [ Whitney and others, 1978; Black, l l
b 1978] is still a matter of contention. Studies in modern and ancient island arcs have revealed a consistent sequence of magma genesis beginning with g potassium-poor magmas generated at shallow depths along a Benioff zone with increases in potassium content of magmas generated at greater depths along the Benioff zone [Dickinson and Hatherton,1967; Hietanen, 1975]. This ( increase is reflected in both plutonic and volcanic rocks with an increase in potassium content in volcanic rocks stratigraphically higher in the {. -volcanic pile [ Jakes and White, 1972]. When' it was first discovered that the Little Mountain pluton extended across the entire study area, the area's value for consideration of the h origin of the Charlotte belt- slate belt boundary was doubted, but then
'some fundamental questions arose. Why is this pluton tabularY ,Why is it oriented along regional strike? Also, why is this intrusion located along the Charlotte belt- slate belt boundary?
One explanation for the shape and orientation of the Little Mountain pluton is that it is pre- or synkinematic, its shape and orientation being' a consequence of regional ductile deformation. This would also account for j l the shcpe of the smaller orthogneiss bodies within the Charlotte belt ) portion of the study area. Another explanation for the shape of the Little Mountain pluton is that it was emplaced along a major stratigraphic or ( structural feature such as an unconformity or a fault. This in considered unlikely,'however, _ since no evidence has been found to indicate the existence of such features along the boundary within the study area. c i
F 78 Moreover, this explanation does not account for the similar shape and orientation of the smaller orthogneiss bodies north of the Little Mountain pluton. A more likely explanation than either of the above is the mechanism considered for emplacement of the Main Donegal Granite, Ireland [ Pitcher and Berger, 1972; Pitcher, 1979] in which lateral spreading of a granitic diapir along dominant country rock fabric, in this case bedding,' occurs as upward motion is arrested. Either this mechanism for emplacement of a single magma body or emplacement of the pluton as a lacolith from a much larger magma body would have the same result, forming a concordant tabular pluton. Emplacement as sills would also account for the tabular concordant nature of the smaller intrusions to the north. Emplacement along bedding is supported by field data. Where the contact between granite and country rock can be seen it dips to the southeast. Offsets of the pluton by the Summers Branch and Wateree Creek faults also indicate a ' dip to the southeast whereas S Omeasurements indicate the same orientation for metasedimentary rocks found just south of the pluton. Similar fault offsets indicate that Charlotte belt rocks also dip toward i the southeast. This orientation has important implications for establishing a stratigraphic section of the study area. Although the reason for its emplacement in what would become an important boundary is uncertain, the Little Mountain pluton's orientation indicates that Charlotte belt rocks
- structurally underlie the slate belt.
Due to its lower structural position and generally more mafic composi-tion, the Charlotte belt is considered to represent the lower part of a magmatic arc. The amphibolite unit (Ga) in the Charlotte belt may (at least in part) represent a mafic basement on which the are was constructed,
79 although it is also possible that the amphibolite (Ga) and felsic gneiss (Efgn) units represent arc-related volcanic-sedimentary rocks. The eruptive felsic tuffs and metasedimentary rocks of the Carolina slate belt are interpreted to represent the upper part of the same arc. The leucocratic granitoid gneisses (Gggn, Little Mountain Pluton) in the Charlotte belt may represent hypabyssal intrusive rocks equivalent to eruptive felsic tuffs in the Carolina slate belt. The hydrothermally altered and silicified felsic volcanic rocks (6tg unit) along the border between the slate and Charlotte belts may represent a volcanic exhalative horizon genetically and temporally associated with the emplacement of the adjacent hypabyssal intrusive rocks of the Little Mountain Pluton. In the Little Mountain- Chapin area, the boundary between the Carolina slate belt and Charlotte belt is therefore characterized by the following:
- 1) an intrusive boundary between hypabyssal felsic plutonic rocks and hydrothermally altered and silicified felsic tuff, and
- 2) a metamorphic gradient between the amphibolite facies rocks of the Charlotte belt and the greenschist facies rocks of the Carolina slate I belt.
The former is a stratigraphic feature of Proterozoi: Z or Cambrian age, whereas the latter is a metamorphic feature of probable Ordovician (Taconian) age. Because these features are of different ages, there is no reason to expect that they are genetically related to each other. Although the intrusive boundary coincides with the metamorphic gradient in the Little Mountain- Chapin area, elsewhere these features may not coincide, and the metamorphic gradient characterizing the boundary between the Carolina slate and Charlotte belts may occur at a different stratigraphic level. Clearly additional detailed geologic studies are needed along the border between the slate and Charlotte belts in adjacent regions to test the above interpretations.
F-80 CHAPTER IV THE BEDROCK GEOLOGY OF THE JENKINSVILLE 7 ' QUADRANGLE, SOUTH CAROLINA INTRODUCTION Geological studies were undertaken in the Jenkinsv111e' quadrangle to I provide a geological framework needed for the evaluation of possible earth-quake hazards in an area of induced seismic activity at Lake Monticello, South Carolina, which might pose a threat to the safe operation of the s 8.rgil C. Summer nuclear station. Monticello Reservoir, associated with the l, , Virgil C. Summer station (Fig.1), is one of the most thoroughly documented cases o_f induced seismicity within the United States [Duc and others, 1978; Talwani and others, 1978, 1980; Talwani,1979; Taiwani and Rastogi,1979]. A marked increase in earthquake activity began shortly after the impoundment and infilling of the reservoir. Many earthquake events, with magnitudes l l ranging from <1.0 to 3.0, apparently occur at depths less than 1 kilometer. These earthquakes are interpreted as resulting from thrust type fault motion on naturally occurring fracture planes. . Changes in pore pressure resulting from impoundment are thought to be the triggering mechanism for the earthquakes [Talwani and Rastogi,1979; Zoback,1979; Talwani and others, 1980]. A major zone of brittle faulting is located alonF Wateree Creek several kilometers south of the Monticello Reservoir [Secor and others, 1981b; Simpson,- 1981; see also Chapter 11]. A hypothetical extension of the fault zone projects 1 to 2 kilometers west of the Virgil C. Summer nuclear station, on Monticello Reservoir, through the region of the most intense seismic activity. Because the estimation of the largest probable induced earthquake is critically dependent on whether the Wateree Creek fault zone I L I
81 is present in the area of. seismic activity and on whether it has an orientation that would render it susceptible to reactivation, it is important to understand the structure of the rocks around the reservoir as thoroughly as possible. For this reason the ' writer unde;took the preparation of a (. detailed geologic map of the Jenkinsville 7's' quadrangle which covers the region aroun'd the reservoir.(Fig. 1, and.? late III). A general familiarity with the regional geology of the Appalachian Piedmont is assumed in this report. Some particularly relevant aspects of the regional geology are [- . . reviewed in the following section. REGIONAL GEOLOGY The Piedmont of the southern Appalachians can be subdivided into several diotinctive northeast trending lithological belts (Fig.13) of probable ( Paleozoic age [Crickmay,1952; King,1955; Overstreet and Bell,1965a, 1965b; Hatcher, 1972]. The study area is located approximately 5 kilometers ( north of the boundary between the Charlotte and Carolina slate belts (Fig. 2) . The Carolina slate belt and the Charlotte belt consist of thick sequences of~metasedimentary and metavolcanic rocks which are interpreted as having been deposited in an island are environment during Proterozoic Z and early Cambrian time [ Butler and Ragland,1969; Seiders,1978; Black,1978,1980; L Whitney and others,-.19783. ' The Carolina slate belt and the Charlotte belt were subjected to greenschist and amphibolita facies regional metamorphism, respectively, during the main metamorphic event (M1), which was probably Ordovician as is suggested by K-Ar studies of slates from the Carolina slate belt in North Carolina [Kish and others,1979]. Several generations of plutonism and deformation have affected the l country rocks of this region. The oldest intr.usions range in age between ie
~} t r*
4 m ; i iiir .
82 l 545 to 495 m.y. [Fu11 agar,1971,1981] and usually e.xhibit a metamorphic
- overprint produced by the Mi event during the Ordovician. The second generation of intrusions range in age from 415 to 390 m.y. and are considered to have post dated the Mi event. Several of the second generation plutons, i however, were affected by an M2 event which appears to be quite variable in s
intensity throughout the region [Fullagae,1971; Fu11 agar and others,1971; Butler and Fu11 agar,1978; Kish and others,1979]. The third generation of intrusions, which range in age between 320 and 280 m.y. [Fu11 agar,1971; Fu11 agar and Butler,1979; Fu11 agar,1981], do not exhibit any metamorphic > fabric and are therefore considered post metamorphic.
- The youngest rocks to intrude the Charlotte and slate belts are the Late Triassic and/or Early Jurassic diabase dikes. These dikes, which are -
usually olivine tholeiites, are typically oriented N.10* W. to N. 30'W.
~ '
and are steeply dipping. The nature of the Charlotte and Carolina slate belt boundary is a fundamentally important relationship in Piedmont geology and has been a long standing controversy. Thurmond and Whitney [1979] interpret a portion of the , boundary in Georgia as an unconformity. Stromquist and Sundelius [1969] interpret the boundary as a fault. Elsewhere the boundary is regarded as a metamorphic gradient [Secor and Wagener,1968; Overstreet and Bell,1965a; Bourland and Farrar,1980]. Peck [1981], south of the study area, considered the boundary as a metamorphic gradient representing a transition between a
~
mafig asement complex (Charlotte belt) on which the slate belt arc was constructed and the lower part of that arc. A combination of the various ; suggested relationships may be involved along the boundary. e
7 83 ROCK DESCRIPTIONS Several rock units have been differentiated within the study area (Plate III). These include amphibolite (Ga), felsic gneiss (Gfgn), sillimanite schist (Gss), quartz monzonite (Sn), quartz diorite (Cd), quartz monzonite (Cw), and diabase dikes (Jd) . I Amphibolite (Ga) The amphibolite unit is found primarily within the southern portion of the study area (Fig. 2, and Plate III). 'Ihe amphibolite in adjacent areas has been interpreted as metamorphic basaltic-andesitic flows by McCauley [1961] and overstreet and Bell [1965a], with less mafic amphibolites interpreted as metasedimentary rocks and metatuffs [0verstreet and Bell, 1965a]. Bourland and Farrar [1980] interpret the amphibolites located to the immediate east of Jenkinsville to be of intrusive origin rather than of extrusive origin, based on its relationship with the surrounding country rock and its gabbroic texture. Amphibolites within the study area are intarpreted as resulting from the metamorphism of mafic tuffs and/or flows because they are interlayered with metasedimentary rocks and because relict volcanic textures have locally been observed. Numerous sheets of granitoid orthogneiss, ranging in size from several centimeters to several meters, intrude the amphibolite within the study area. These are too thin and numerous to be mapped separately from the amphibolite. Peck [1981] was able to delineate several thick sheets of orthogneiss within the amphibolite in the Little Mountain- Chapin area (Fig. 2, and Plate II) . Within the study area the amphibolites are generally fine to medium grained consisting of varying amounts of hornblende, plagioclase, sphene, and minor epidote, quartz, and opaque oxides (Table 4) . The hornblendes I l
84 I l l w $ < u
& d w 3 w I b
- 2 "
s 3 8 E w n o e w w a s 3 SAMPLE s m - s g NME N s E e }~ $ a
=
e S b c U 5 E 8 l= $ 4 6 0 X X - X X 0 0 0 0 - I 1 1 15 0 X X - X 0 - - - - - 25 0 X X 0 X X - - X 'O O l l 28A X X X - X 0 - 0 X 0 0 l 94 0 X X - X 0 - - - 0 - ' 98A 0 X X - X X - 0 0 0 - 148 X X X - X X - - X - - 158 X X X X X 0 - 0 X 0 - 163B 0 X X - X X - - 0 0 - 178 0 X X X X 0 0 0 0 0 - 189 0 X X 0 X 0 - 0 0 - - X = mineral present in amounts >1.0% 0 = mineral present in amounts <1.0%
- = mineral not present Table 4. Metamorphic mineral assemblages for amphibolites (Ga) south of the Winnsboro pluton (Cw)/amphibolite (Ga) boundary in the l,
Jenkinsv111e quadrangle. l I I
+
85 ( are generally short and stubby euhedral to subhedral crystals, which have a preferred orientation, and comprise approximately 55 to 70% of the rock volume. Hornblende is pleochroic with x = yellow or yellow green; y = green or olive; k. * = dark blue green or dark olive green. The composition of ( the plagioclase, based on the Michel-Levy method, varies from An 26 to An58 . (oligoclase to labradorite) with an average composition of An34 (andesine). Plagioclases, which are usually short, subhedral crystals oriented subparallel to the foliation, comprise approximately 22 to 35% of the total volume of the rock. Sphene, which comprises 2 to 4% of the rock, occurs as isolated blebe commonly exhibiting its characteristic wedge shape crystal form. Minor minerals consist of epidote (3 to 7%), anha.dral quartz (<1 to 2%), opaque oxides (<1 to 2%), potassium feldspar (<1%), biotite (<1%> , and small amounts of apatite and zircon. l The small amphibolite units mapped within the quartz monzonite (Cw) are interpreted as enclaves of the amphibolite. The mineralogy of these enclaves { generally agrees with that of the main body of amphibolite with a few minor differences (Table 5). The enclaves commonly exhibit a reduction of plagioclase and hornblende grain size as well as minor growth of pyroxenes in a few sites, a slight increase in the calcium content of plagioclase, and a general decrease in the amount of epidote. Felsic Gneiss (Gfgn) The felsic gneiss unit, which occurs in the extreme southeastern part ( of the study area (Fig. 2, and Plate III), consists of a fine to medium grained biotite-muscovite-quartzofeldspathic schist and gneiss with minor amounts of potassium feldspar, hornblende and opaque oxides. The dominant p leucocratic minerals are quartz and muscovite, whereaa ,e s , the e primary mafic mineral. [ -
86 I I e n < a I o sa m S w 2 m 5 = m e x s = 5 s m LE umm U 5 E! E 3 U 8 h
=
h h e y e h, h h5 g d E-3 8 5 t B e m a m b c s = x ~ o g X X X 0 0 - - 0 - - X - 41 - 61 X X X - X 0 - - 0 - - - - 61A X X X - X 0 - - 0 - - - - 70 X X X - X 0 - 0 0 - - - - 102 X X X - X X - 0 0 0 - - - 108 X X X X - X 0 - X - - - - 246 - X X 0 - - - 0 X 0 0 X - 341 0 X X - X 0 - - 0 - - - - 342 0 X X - X - - - 0 - - 0 - 496 0 X X - X - - - - - - X = mineral present in amounts >1.0% 0 = mineral present in amounts <1.0%
- = mineral not present I
Table 5. Metamorphic mineral assemblages for amphibolite (Ga) enclaves E and xenoliths within the Winnsboro pluton (Cw) in the B Jenkinsville quad angle. I I. 9
87 l l Field relationships between the felsic gneiss unit (Gfgn) and the amphibolite unit (Ga) suggest that they may be stratigraphically equivalent to one another. This interpretation is based on the observations that the l contacts between the units are very transitional and gradational and the L occurrences of minor amphibole rich layers, which vary in. thickness from 0.5 to several meters, are intercalated within the felsic gneiss unit. Similar observations have been made by Peck [1981] in the Little Mountain and Chapin quadrangles to the south. l Sillimanite Schist (Gss) l The sillimanite schist occur in the central southeastern part of the study area (Plate III) . The schists, which characteristically outcrop as small boulders and form several small resistant hilltops within the study l area, are interpreted as enclaves within the Winasboro pluton. Sillimanite I l occurrences have also been noted adjacent to this area by Wagener [1966, 1970], Secor and Wagener [1968], Farrar [1977], and Bourland and Farrar [1980]. The sillimanite schists are composed of approximately 32% accicular sillimanite, 40% potassium feldspar, 20% quartz, 8% muscovite and minor l amounts of sphene and plagioclase. ! l Newberry Plutonic Complex (Sn) The Silurian quartz monzonite, located in the northwestern corner of the l study area (Fig. 2, and Plate III), is part of the extensive Newberry complex (Sn) extending west through Newberry County [Kesler, 1936; McCauley, 1961; l Overstreet and Bell,1965a, 1965b; Wagener, 1977a]. Outcrops of the pluton l within the study area usually occur as a yellowish fine grained intensely weathered rock which can readily be distinguished from the reddish saprolite I of the Winnsboro pluton (Cw) to the south and east. Unweathered exposures I l l _
88 l of this rock, found approximately 3 kilometers northwest of the study area within the Blair quarry and along nearby roads, cutcrop as a massive fine grained light to medium grey colored rock. The Newberry plutonic complex also outcrops extensively in the adjacent Pomaria quadrangle (Fig. 2, and Plate IV), and a thorough petrographic description of it is presented in Chapter V. For purposes of the present study, modal analyses of two samples were used to determine the mineralogy of the pluton. These analyses plot in the quartz monzonite compositional field (Fig. 19). The mineralogy
~
consists primarily of plagioclase, potassium feldspar, quartz, and biotite with accessory muscovite and opaque oxides (Table 6) . The plagioclase, which has an An composition that ranges from An14 t An28 ( ligoclase) is usually subhedral, to anhedral. Associated with the plagioclase are minor occurrences of myrmekite with vermicular quartz. Subhedral to anhedral potassium feldspars and anhedral quartz, which exhibit undulatory extinction, are the remaining primary minerals that occur in the rock. The amount of biotite is usually three times more abundant than muscovite. Trace amounts of secondary epidote and chlorite also occur in the rock. The Newberry pluton occasionally contains a distinct foliation manifested by an alignment of biotite. This weak foliation may be attributable to either flow or metamorphic phenomena. Fu11 agar and Kish [1981] and Fu11 agar [1981] report a Rb-Sr whole-rock age determination for the Newberry pluton of 415 9 m.y. with an initial Sr87/Sr86 ratio of 0.7024 0.0003. This extremely lov Sr 87/Sr86 initial ratio for the Newberry pluton suggests that the whole rock age is an emplacement age rather than an age of metamorphism. I
89 5 I lI . O I I ~
- s 60 j.
- I GA GB GD AG T E
,S - - k 20 AQ QS , QMD OD QM % !' 5
/ AS / 5 / M \ MD \D\ P A 65 35 10 90 1
Figure 19. APQ diagram showing the modal variation for the Newberry pluton (Sn, e) and the quartz diorite (Cd, +). Diagram af ter Streckeisen [1967] where AG - alkali feldspar ^ ll granite, AQS - alkali feldspar quartz syenite, AS - alkali B feldspar syenite, GA and GB - Granite A field and B field, QS - quartz syenite, S - syenite, QM - quartz monzonite, M - monzonite, GD - granodiorite, QMD - quartz monzodiorite, l I
- MD - monzodiorite, T - tonalite, QD - quartz diorite and D - diorite. Modes were obtained by counting 1000 points each.
- I
'I l lI
I 90 8
$ w d $ g n w
- w w
" d 8' w y
N a 5 U w 8, w U H SAMPLE G $ g 8 @ $ h @ NUMBER $ , @ S a y p g 5 m [ w h
=
m m x o- w = x v o
~ I 16 42 29 12 6 10 - -
tr 1 29 61 10 11 10 tr 3 tr - - 1 tr - 33 42 27 16 11 tr 1 - - - 1 tr -
'E 36 40 33 17 1 4 4 tr - -
tr tr - I 48 37 33 19 8 tr 1 tr tr er - tr - 52 41 32 16 6 tr 4 tr tr tr tr er - 325 40 31 17 2 8 - - tr - - tr - l E Cd l 249 56 1 7 18 15 - tr 2 - tr er - I 279 55 1 6 9 14 - - 1 - - tr 15 422 48 1 5 18 7 - - 4 - - tr 19 l Sn BQ 42 40 13 3 - 1 - 1 - NEWB 39 30 18 2 - 1 - - Table 6. Modal mineralogical data for the Winnsboro pluton (Cw), quartz diorite (Cd), and the Newberry pluton (Sn) in the Jenkinsv111e quadrangle. Modes were obtained by counting 1000 points per thin section. I
~
l
$h_x 91 Quartz Diorito (Cd) Several'small bodies of hornblende quartz diorites ( pyroxene) are
~
found within the' southern half of the Winnsboro pluton (Cw, Plate III) . The rocks are generally dark colored fine to medium grained with a ramifying network of net veins composed generally of quartz and feldspar. In one location the quartz diorite has been forcefully intruded by an apophysis of the Winnsboro pluton. This is interpreted to indicate that the quartz diorite may range in age from pre- to syn- plutonic emplacement. Associations of quartz diorite with granitoid plutons have also been observed in other l localities in South Carolina [0verstreet and Bell,1965a; Secor and Snoke, 1978]. Modal analyses were calculated on several samples to determine the mineralogy of the rock. The mineralogy consists primarily of plagioclase, quartz, biotite, .hornblende, pyroxene, and minor accessory minerals (Table 6) . The rocks generally have an equigranular igneous texture. The plagioclase, which has a composition that ranges from An 38 t An48 (andesine), forms medium grained anhedral to subhedral grains, whicli occasionally are zoned. The pyroxene (clinopyroxene) is usually identified by its cleavage and the l fact that it is colorless in thin sections. The biotite has a yellow (x) j 1 to brown (z) pleochroism. Hornblende occurs as subhedral to anhedral crystals and exhibits pleochroism with x = yellow or olive, y = green or olive, and z = darker olive or green. The opaque oxides occur as small blebs generally associated with the biotite and hornblende. Quartz generally has undulatory extinction and an interstitial growth form. Minor accessory minerals include epidote, hematite, and chlorite. The modal analyses plot well into the quartz diorite compositional field (Fig. 19).
r l l Winnsboro Plutonic Complex (Cw) The Carboniferous pluton is apparently an extension of the Winnsboro plutonic complex in the east [Wagener,1970, M77a] and extends through the - middle and north parts of the study area (Fig. 2, and Plates III and IV) . Detailed and reconnaissance mapping of the Winnsboro pluton complex has been conducted by Kesler [1936]; Wagener [1966,1968,1970,1973, and 1977a]; Secor and Wagener [1968]; Farrar and Becker [1977]; and Bourland and Farrar [1980]. Detailed petrographic analyses were done by Wagener [1973] and Farrar and Becker [1976]. Further mapping and petrography of the pluton within the study area were done by South Carolina Electric and Gas Company [1977] in conjunction with the building of Monticello Reservoir and the Virgil C. Summer nuclear station. The quartz monzonite usually consists of medium to coarse grained megacrystic potassium feldspar with variable amounts of biotite, hornblende, and quartz. The Winnsboro pluton also occurs as a homogeneous fine to medium grained rock. Modal analyses of several thin sections and stained rock slabs were studied to determine the petrography and possible variations within the pluton. Modal analysis was done by counting 1000 points per thin section and slab. The results of the modal analy' sis are represented in Figure 20. The rocks plot mostly below the 20% quartz line which separates the monzogranites (>20% quartz) from quartz monzonites (<20% quartz). The mineralogy of the quartz monzonite consists primarily of plagioclase, potassium feldspar, quartz, biotite, hornblende, and minor amounts of accessory minerals such as muscovite, sphene, epidote, hematite, chlorite, and opaque oxides (Table 6). The plagioclase crystals, which have a composition that varies from An27 to An32 ( lig clase), generally are I
L 93 E Q E F , I { 60 I i G'A GD f GB h AG T l l 4
) 4 20 OMD OD AO O5 .
i OM 5
/ AS / 5 / M \ MD \ D ,\ p A 35 10 90 65
{ Figure 20. APQ diagram showing the modal variation within the Winnsboro g pluton (Cw) for seven thin sections and six stained slabs fr = '6* 3*"ki"*vi
- 9"adra"8 *- " de* "are obtai"ad 67 5 counting 1000 points each.
I t I - L F t P
94 subhedral to anhedral. Some of the plagioclase crystals exhibit normal oscillatory zoning with cores of a more calcic composition grading outward to a more sodic hmposition. Plagioclase crystals also exhibit sausseritiza-tion forming epidote and sericite. Microcline, commonly perthitic, is generally a subhedral megacrystic mineral with an average length of 0.3 to 1.1 cm long. Several crystals have been observed to be greater than 2 cm long, but this is very rare. The quartz is com=only rutilated and exhibits undulatory extinction. Biotite is pleochroic yellow to dark brown or olive brown. Hornblende, generally subhedral, is pleochroic with x = yellow orange or yellow brown, y = olive, and z = dark green or green. Minor amounts of accessory minerals such as muscovite are also present. Several Rb-Sr and K-Ar mineral and mineral-whole rock ages obtained , from the quartz monzonite in the foundation rock of the Virgil C. Summer nuclear station have been determined. These dates range from 309 to 286 m.y. and probably reflect a time of cooling rather than an age of pluton emplace-ment [ South Carolina Electric and Gas Company,1977; Fu11 agar and Kish,1981]. A Rb-Sr whole-roch age of 295 4 m.y. and an asicial Sr67/Sr 86 ratio of 0.7047 0.0004 have been determined for the Winnsboro pluton east of the present study area [Fu11 agar and Butler, ~1979; Fu11 agar,1981]. The U. S. Geological Survey drilled two boreholes, Monticello #1 and #2, into areas of induced seismicity west of Monticello Reservoir (Fig. 2) in order to determine _i_n n situ pore pressures and stresses [Zoback, 1979; Zoback and Hickman, in press]. The Monticello #1 core (1.1 km deep) is relatively free of inclusions whereas 15 to 20% of the total volume of the Monticello #2 core (1.2 km deep) consisted of enclaves and xenoliths of gneiss and amphibolite (Fig. 3) which are mostly concentrated in the top 330 meters [Secor and others, in press].
95 h Throughout the main body of the pluton, numerous xenoliths and er.claves t occur which are in various stages of assimilation. The most persistent f xenoliths are the mafic-rich country rock (amphibolite) . This is probably due to the fact that the felsic gneiss country rock contains a relatively i high proportion of low melting temperature constituents so that xenoliths of these rocks would be more likely to disintegrate. The xenoliths vary in size l from 1 cm to several meters in length. Many of the xenoliths carry a foliation (SI ) which is usually parallel to the elongation of the xenolith. This suggests that the shape of the inclusions could be controlled by the So or St l foliation. Since the quartz monzonite commonly outcrops as loose boulders, measurements were not taken on the orientation of many of the xenoliths I found within these boulders. A few measurements obtained from the more reliable outcrops indicated that the direction of elongation of the xenoliths was of ten parallel or subparallel to the contact of the pluton. Schlieren l features or protoclastic foliations were seen in several places and are very common where abundant xenoliths occur. These are generally biotite-rich and ! l occasionally contain hornblende. There are probably more large enclaves within the Winnsboro pluton which were not detected during mapping owing to l lack of exposure. Subsurface distribution of large enclaves within the , l Winnsboro pluton as well as compositional variations within the quartz monzonite could account for the irregular character of the magnetic field i I over the pluton (Fig. 4). I The boundary between the Winnsboro plutonic complex (Cw) and the Newberry l plutonic complex (Sn) is approximate. It is based primarily on the changes j in color between the Winnsboro (red) and Newberry (yellow) saprolite. In the streams this contact is also manifested by the last occurrence of the Carboniferous pluton boulders. Examination of.the rocks on either side of
I 96 the contact reveals no exposures which show xenoliths of one within the other. The small enclave of the Newberry plutonic complex (Sn) within the Winnsboro plutonic complex (Fig. 2, and Plate III) was mapped primarily on the observations of saprolite color changes and grain size variations. The southern boundary between the Winnsboro plut.onic complex (Cw) and the amphibolite (Ga) is based primarily on the first occurrences of quartz monzonite boulders and saprolite. These boulders, which consist of the medium to coarse grained megacrystic quartz monzonite, usually contain abundant xenoliths of the amphibolite. The xenoliths, which vary in size and shape, are occasionally agmatized and highly disrupted. The agmatites were formed as a result of either the injection of magma into the relatively rigid country rock or the engulfing of country rock by the magma. Physical conditions of pluton emplacement are very difficult to determine based on field relaticnships and the lack of geochemical data. Sinha and Marz [1976] suggest a depth of emplacement, for the Winnsboro complex to the east, around 12 to 14 km at 700 to 800 C. Since the contact metamorphic effects in the amphibolite are very difffcult to differentiate from the regional metamorphic effects, a contact aureole has not been delineated. t Farrar and Becker [1976] and Farrar [1977] als; report similar observations for the Winnsboro complex in the east. Diabase Dikes (Jd) Numerous diabase dikes are intruded into the study area (Fig. 2, and Plate III). The trends of these range from N.30oW. to N.15oW. with an average of approximately N.200W. The average dips of the dikes are vertical , to near vertical. Thicknesses vary from 0.1 meters to 10 meters with an average of 1 meter. Diabase is best exposed in streams where they occur as I~ l
97 boulder trains or as in-situ blocks. Columnar cooling joints are normal + and parallel to the trend of the dike walls, which increases the susceptibility - of the dike to weathering. With progressive weathering, the boulders are reduced in size to rounded to subangular boulders surrounded by clays. Modal analyses of seven diabase dikes were used to determine the mineralogy of the rock. The diabase, generally a fine to medium grained rock with an ophitic to subophitic texture, consists primarily of plagioclase, olivine, and clinopyroxene with minor arounts of opaque oxides, serpentine, and chlorite (Table 7). The plagioclase, which occurs as randomly oriented euhedrcl stellate crystals, has an An composition averaging An59 (labradorite). The clinopyroxene occurs as colorless to pale green or brown anhedral crystals which were identified as an augite. The augite is commonly altered to a chlorite and sometimes to opaque oxides. Olivine occurs as colorless i euhedral to subhedral crystals which commonly have serpentine-filled frac-tures. The major accessory mineral is euhedral to subhedral titanomagnetite. A ternary plot of the total volume percent of plagioclase, pyroxene, I and olivine is shown in Figure 21. Wiegand and Ragland [1979] characterize the diabase dikes of eastern North America, based on calculating the norms, into three main groups: the olivine normative tholeiite, the low TiO2 i quartz normative tholeiite, and the high TiO2 quartz normative tholeiite. A plot of modal analysis can show a correspondence to either the olivine or quarta tholeiites, although it cannot distinguish between the three groups. These seven dikes fall well into the olivine tholeiite composition. A paleomagnetic study of dikes in the study area and adjacent areas of the central Piedmont has recently been completed by Dooley and Smith [1981]. Briefly, Dooley and Smith obtained an average pole position which agrees wi.th other well established Late Triassic- Early Jurassic pole positions I I
98 I I e
$ w w w
5 m I 8 E E w d d g g g g, w H SAMPLE h NUMBER 3 g g g h g h
- o. e n. o u m =
Jd 5 67 18 12 2 tr tr er 22 62 17 18 2 tr er - 71 64 17 15 3 , 1 tr tr 88 50 20 27 2 1 tr - 115 57 13 27 1 1 tr - 199 61 24 13 tr tr tr tr I
. I Table 7. Modal data for six diabase dikes (Jd) in the Jenkinsville quadrangle. Modes were obtained by counting 1000 points j l per thin section. E l
- I I
1 I r I I
99
- I
- I ' l 1
I OLIVINE
- 5 I . 40 30 .-
l , 20 l
- I 10
)
I l ll PYROX. PLAG. ; j i g 10 . JOUARTZ !I i i An olivine-plagioclase-pyroxene-quartz diagram showing .:l jg Figure 21. the modal variation of seven diabase dikes from the Jenkinsville quadrangle. Modes were obtained by counting ja 1000 points each. !I il !I
- I
e 100 from North America [see deBoer, 1967; Watts, 1975; Smith and Noltimier, 1979; Sutter and Snith,1979; Bell and others,1980]. Conventional single analycis whole-rock K-Ar apparent age determinations for several of the dikes have ages which range from 207 to 180 m.y. [ Robert E. Dooley, 1981, personal communication], which agree with the paleomagnetic results. REGIONAL RELATIONSHIPS AND DISCUSSION
. Rocks within the Carolina slat'e belt and the Charlotte belt have been subjected to at least two deformational episodes. The timing of the oldest and strongest deformational event (D 1 ) is not well constrained although it apparently predates the emplacement of the 415 to 390 m.y. series of plutons within the Charlotte belt [Fullagar, 1971, 1981]. This is based on the '
observation that xenoliths which carry the Si fabric are found within the 415 m.y. Newberry pluton, suggesting that Di occurred prior to 415 m.y. ago. Kish and others [1979] suggest that the age of metamorphism was probably Ordovician based on whole-rock K-Ar studies of slates from the Carolina slate belt in North Carolina. The Di event was accompanied by greenschist facies metamorphism in the Carolina salte belt and amphibolite facies metamorphism within the Charlotte belt. The Di event also produced the major penetrative metamorphic foliation (SI ) as well as the Chapin synclinorium, a major large-scale fold (F ),1 to the south [ Peck,1981; Secor and others, in press 3. The amphibolites and felsic gneiss units within the study area are located on the northern limb of the Chapin synelinorium. The axis of the synclinorium trends N.75 E. The Si foliation usually contains a strong elongation, L , i which generally plunges to the northeast, parallel to the axis of the synclinorium. I I
8 101
'A second deformational event-(D 2 ) also affected the area during the middle to late Paleozoic time. Bourland and Farrar [1980] describe the D2
( event as forming a variable series of mesoscopic and macroscopic F2 folds ( which are generally coaxial with the Li elongation lineations [ Peck,1981]. Poles to St from the study area and the Chapin-Little Mountain area are shown in Figures Sa,17,18, and 22 and indicate that S i has been refolded during the D2 event. Peck [1981] reports abundant occurrences of F2 folds in the Little Mountain- Chapin area. A few F 2folds have been recognized within amphibolite xenoliths within the Winnsboro pluten in the study area, indicating that the time of F2 folding at least in part predat'd the time of n emplacement of the Winnsboro pluton (295 m.y.). The intrusion of the Winns'oro b pluton (Cw) within the study area apparently only caused local deformation limited to the immediate vicinity of
~
the pluton. Rb-Sr whole-rock age determinations of 295 4 m.y. [Fullagar and' Butler,1979; Fu11 agar,1981] for the Winnsboro pluton in the east and 309 to 286 m.y. for the pluton in the study area [ South Carolina Electric and Cas Company,1977; Fullagar and Kish,1981] provide an upper (younger) age bracket for the deformational events (D ,I D )2Preceding the pluton emplacement. Two periods of brittle faulting have affected the area south of the study area [ Peck, 1981; Simpson, 1981]. The oldest set of faults are typically east-west trending and usually contain brecciated and silicified country rocks. The amount of displacement is unknown although the faults can be traced for
.several kilometers. The age of these'silicified faults is thought to be of.
late Paleozoic or early' Hesozoic age because they cut Carboniferous rocks in the Kiokee belt of South Carolina and are themselves cut by early Jurassic dikes [Secor and Snoke,1978; Secor and others, in press].
r~ 102 I
- 4 I N
I i i i aa . 2:.s - I
;;s ;;:ii ' ii...y 7 5
.. .ti: 5 a = c
*[ r L,.
% 9thh u ' e I- -7 I
."-. ...iitss T "id a......= 7?iii !!!!!. g g;=5p!O !"%t" n f
- ejsi3 siliun y 3 s
Aa3 g ; I e 4, i 4 ;t I l I 1 Figure 22. Lower hemisphere, equal-area projection of poles to 138 Si foliation planes from the Jenkinsville quadrangle. Contours: 2, 4, 6, 8 and 117.. I I l;
7 103 I Younger north-south trending high angle reverse faults offset the older east-west trending silicified faults. The north-south trending Wateree Creek fault zone, south of the study area (Fig. 2, and Plate I), has been the subject of a detailed study by Simpson [1981]. The fault is characterized as a high angle reverse fault which 1) offsets the stratigraphy, the axis of the Chapin synclinorium, and the east-west trending silicified faults, and 2) has a dip slip displacement of approximately 1700 meters. The interaction of a Jurassic diabase dike with the Wateree Creek fault zone was also studied [Simpson, 1981] to determine a possible constraint on the timing of the last fault movement. Backhoe trenching and detailed magnetometer traverses across the dike and fault zone suggest that the faulting occurred prior to the diabase dike emplacement (early Jurassic) . The Summers Branch fault zone to the west (Fig. 2, and Plates I and II) appears to be similar to the Wateree Creek fault zone b'ased primarily on the offset of stratigraphic contacts [ Peck, 1981] . I. Within the study area, numerous outcrops that exhibit fracturing and zeolite mineralization have been observed. These fractures are commonly variable in length, displacement, and orientation. Two sets of steeply dipping zeolite filled fractures were discovered during the excavation of the Virgil C. Summer nuclear plant. These fracture sets, which were oriented N.28cE. and N.600W., have undergone oblique slip displacement ranging from several centimeters to a few meters. Numerous fractures with zeolite mineralization, have also been observed in the quarry, east of Monticello Reservoir (Fig. 1) and at the Parr Shoals spillway. Monticello #2 had several zones which exhibited fracturing along with zeolite and carbonate mineralization (Fig. 3). A K-Ar age of 45 5 m.y. has been determined from zeolites along fractures at the nuclear excavation site and is interpreted I E
104 as a minimum age of movement along the fractures [ South Carolina Electric and Gas Company, 1977; Wagener, 1977b]. These occurrences of ::eolite filled fractures are interpreted as a Mesozoic deformational event [Privett, 1973, 1974, 1977; Brown and Gil' cert, 1977; Butler, 1977; Ragland, 1977; Wagener, 1977b]. Zoback and Hickman [in press] conducted a study on possible mechanisms for induced seismicity caused by i=poundment of Monticello Reservoir. Examinations of in situ properties in the Monticello #1 and #2 wells revealed that the earthquakes are resulting from high horizontal stresses which cause thrust type fault motion on naturally existing fracture planes and that the changes in fluid pressure act as a triggering mechanism. Talwani and Rastogi [1979] report a mechanism of pore pressure diffusion which reduces the effective strength in highly stressed rocks and triggers failure. CONCLUSIONS This study was undertaken because there was concern that the Wateree Creek fault zone and the induced seismic activity at Monticello Reservoir might pose a threat to the safe operation of the Virgil C. Summer nuclear s tation. The induced saismicity around the Monticello Reservoir appears to j be occurring within the Winnsboro plutonic complex, based on observations of geophysical and geological investigations. The Winnsboro pluton includes variable amounts and distributions of xenoliths and enclaves of country rock. Based on borehole examinations, it is suspected that the irregular character
- of the magnetic field over the pluton indicates that several subsurface inhomogeneities exist around the area of high seismic activity. Stress measurements within the Monticello #2 borehole indicate that only the tcp few hundred meters of rock are critically stressed [Zoback and Hickman, in I
7 105 I press]. The greatest principal stress is horizontal and thrust type fault motion is thought to be occurring on the naturally existing fracture planes. The numerous and diversely oriented fractures and enclaves within the Winnsboro pluton probably control the seismic activity around Monticello Reservoir. It is unlikely that a steeply dipping to vertical fault, such as
- l
'5 the Wateree Creek fault, would be reactivated by the horizontal orientation of the greatest principal stress as observed in Monticello #2.
The boundary between the Winnsboro plutonic complex and the adjacent country rock is an intrusive contact with a narrow zone (300 meters to 1 kilcmeter) of migmatitic rock between the two units. The boundary does not appear to be offset and is regarded as coherent across the study area. Therefore there is no evidence that the Wateree Creek fault zone is present within the study area. Fault breccia, located within Chapin quadrangle approximately eight kilometers south of the Virgil C. Summer nuclear station, is the northernmost control point of the Wateree Creek fault zone. It is concluded that it is unlikely that a large magnitude local earthquake will occur in response to the pore pressure and stress changes 'I related to the impoundment of Monticello Reservoir. This conclusion is based primarily on the absence of any through going faul s and the small volume of critically stressed rock near the surface in the seismically active region. I E .I 8 I E
106 CHAPTER V , THE BEDROCK GEOLOGY OF THE P0 MARIA 7h' QUADRANGLE, - SOUTH CAROLINA INTRODUCTION The Pomaria 7 ' quadrangle is located about sixty-five kilometers northwest of Columbia, South Carolina. It lies entirely within the Charlotte belt, a linear belt of amphibolite-facies metamorphic rocks containing numerous igneous plutons extending southwest-northeast across central Gouth Carolina (Fig. 13). The southern edge of the quadrangle is approximately six kilometers north of the Charlotte belt- Carolina slate belt boundary (Fig. 2). The mapping of this quadrangle was undertaken as part of a project to complete detailed geologic maps of four quadrangles surrounding Monticello Reservoir in western Fairfield County (Fig. 2) . This lake is the site of one of the most thoroughly documented cases of induced seismicity in the eastern United States [Talvani, 1979; Talvani and others,1978; Talwani and others, 1980]. The purpose cf the project is to provide the geologic data necessary to understand the cause of the tremors. Although a number of other geologic studies have been made in the area [McCauley,1961; Wagener,1977; South l Carolina Electric and Gas Company, 1977], this is the first attempt to complete a detailed geologic map of the quadrangle. 1 REGIONAL GEOLOGY The Piedblont of South Carolina is divided into several lithologic belts aW i [ King, 1955; Hatcher, 1972]. The Charlotte belt, within which the study area is located, is a belt of amphibolite facies metasedimentary and felsic to l I 5
107 j 1 l mafic metavolcanic rocks. A number of pre- to post-metamorphic plutons intrude these metamorphic rocks. The Charlotte belt is interpreted to be part of a magmatic are active during Proterozoic Z and Cambrian times J [ Butler and Ragland, 1969; Whitney and others,1978] . Regional magnetic and l gravity highs over the area have been interpreted as indications that the Charlotte belt and much of the Carolina slate belt are underlain by mafic crust [ Hatcher and Zietz, 1980]. I The rocks to the south in the Carolina slate belt are greenschist facies metasedimentary and felsic to intermediate metavolcanic rocks which can be divided into mappable stratigraphic units. Peck [1981] described the slate belt units found to the south of the study area in the Chapin and Little Mountain quadrangles (see Chapter III) . The metasedimentary and metavolcanic I rocks have been locally intruded by igneous rocks of various ages, although these intrusions are less abundant than in the Charlotte b61t. The rocks of the slate belt were deposited during the Proterozoic Z and Cambrian time, as indicated by geochronological studies [ Carpenter and others, 1978] and recently confirmed by the discovery of numerous Middle Cambrian trilobites by University of South Carolina graduate student Sara Samson near the Clouds Creek pluton in the Batesburg quadrangle [Samson and others, 1982]. Meta-morphism in the Carolina slate belt was probably roughly contemporaneous with that in the Charlotte belt. Whole-rock K-Ar studies in the slate belt of North Carolina indicated a probable early Ordovician age for this 1 1 I episode of regional metamorphism [Kish and others,1979]. Three different generations of igneous activity have been defined within the Charlotte and Carolina slate belts [Fullagar, 1971; Fullagar, 1981]. l Members of all three groups occur in the study area. The first predates the main metamorphic event and is represented by the granitic' gneiss unit (6ggn) E
I 108 in the Little Mountain quadrangle and the granitic orthogneiss within the felsic gneiss unit (Gfgn) in the present study area. These carry the Si foliation. A zircon age of 550 m.y. [ James Wright, personal cummunication, 1982] has been determined for the Gggn unit in the Chapin quadrangle southeast of the study area. Previous Rb-Sr dating by Fullagar [1971] had placed the group in the age range of 545-495 m.y. The second generation is the 415 to 385 m.y.-old group of plutons. In the study area it is represented by the 415 m.y.-old Newberry granite (Sn) [Fullagar,1981]. These postdate the main tuetamorphic event and do not contain a penetrative deformation fabric. The final episode of igneous activity is a group of 320 to 280 m.y.-old plutons, which are represented by the Winnsboro pluton (Cw) and possibly the Pomaria granite (Cp). These do not exhibit signs of penetrative deformation and are considered to be post-metamorphic in age. This group probably includes a quartz syenite body discovered near the Pomaria quadrangle-Newberry East quadrangle boundary. All of the above units are cut by a number of northwest trending diabase dikes. These are the youngest rocks present, and are of probable Late Triassic- Early Jurassic age. ROCK DESCRIPTIONS Six rock units have been identified within the Pomaria quadrangle, two metamorphic and four igneous. These are amphibolite (Ga) and felsic gneiss l (Gfgn), both Cambrian in age; Newberry granite (Sn), Silurian in age; the ! Winnsboro pluton (Cw), Carboniferous; Pomaria granite (Cp), possibly Carboniferous; and diabase dikes (Jd), Jurassic or Triassic in age. A description of each follows. l l I1
, 109 Amphibolite (Ga) The amphibolite unit is limited to the southeastern portion of the study area with the exception of a few lenses within the felsic gneiss unit (Gfgn) (Fig. 2, and Plate IV). It includes both amphibolite and hornblende gneisses with occasional biotite paragneiss or granitic orthogneiss layers too thin to be mapped separately. A foliation is generally well defined in the rrak, though in a few locations the amphibolite is massive, with no distinct mineral orientation at either the megat.copic or microscopic level. The well foliated amphibolites interlayered with paragneisses are interpreted to be metamorphosed mafic tuffs. Some of the massive amphibolites may be meta-morphosed mafic flows or mafic intrusive rocks. One sample of massive amphibolite (number 4903 in Table 1) contained considerable amounts of pyroxene, which otherwise is limited to contact aureoles of the granites. Bourland and Farrar [1980] interpreted some of the amphibolites near thd Winnsboro plutonic complex to the east of the study area to be of intrusive origin. Hornblende is the predominant mineral present in the amphibolites, comprising from 50-75% of the rock. The crystals are subhedral to anhedral, prismatic in b ui pleochroic with x = pale green to yellow green, y = olive gr( m e ~ a, and z = dark blue green to dark green. Plagiuclase usually occurs in short, unzoned, subhedral crystals with no apparent orientation with respect to foliation. Using the Michel-Levy method, plagioclase compositions were determined to range from An28 (oligoclasc) to An65 (labrcdorite). ust values fell in the range from An35 to An40 (andesine). Plagioclase generally makes up from 15 to 30% of the rock. Anhedral quartz is present in amounts ranging up to 8% of the rock volume, although it more commonly comprises only 2-4% of the total. Quartz
I 110 I I
. s - g
* $ w w
$ Y 3 8 N y 8 N g C y E SAMPE s g m g m
g g g gj g. m
, g g
"" g e
!= $
m E m S m t w
~
e o 2 o s m B 0601 X X X X X T 0 X 0 X X 0 1804 X X X X X 0 0 X 0 0 X 0 2001 X X X 0 0 0 0 X 0 0 X 0 B 2605 X X X 0 X 0 0 X 0 0 X X 4903 X X X 0 0 0 X X 0 0 T X 5906 X X 'X T X 0 X X 0 X X 0 6704 X X X X 0 0 0 X 0 X X 0 6809 X X X 0 T 0 0 0 0 0 X 0 7003 X X X X T 0 0 X 0 T X 0 7004 X X X 0 X 0 0 X X 0 X 0 8 X = mineral present in amounts greater than 1% T = mineral present in amounts less than 1% 8 0 = mineral not present
\ E Table 8. Metamorphic mineral assemblages for amphibolites (Ga) from the Pomaria quadrangle.
I
~
I
111 I . nearly always exhibits moderate undulose extinction as the result of strain. Epidote is present in nearly all the amphibolites, occurring in small subhedral grains. Often it is congregated in small aggregates. The amphibolite contains up to 7% of this mineral. Garnet is present in only a few of the samples. It occurs as large, highly fractured crystals which show considerable alteration around the rims, with associated growth of plagioclase, quartz, and chlorite. Inclusions are common but appear to lack a preferred orientation. Garnet was not reported as occurring in amphibolites in the Jenkinsv5.11e quadrangle [ Smith, 1982] or the Little Mountain quadrangle [ Peck,1981], although in all other respects these are very similar compositionally to amphibolites in the Pomaria quadrangle. From its occurrence garnet is interpreted to be metastable during the last episode of deformation. Other components of the amphibolite include sphene (0-3%), biotite (0-2%), secondary chlorite (0-3%), opaque oxides (trace-4%), potassium feldspar (0-trace), and trace amounts of muscovite, apatite, zircon, and pyrite. Amphibolite which occurs as xenoliths within the Winnsboro pluton and within approximately 100 meters of the granite-country rock contact show signs of contact metamorphism. Minerals show a large reduction in grain size and quartz has recrystallized into polygonal, relatively unstrained subgrains. Clinopyroxene, diopsidic in composition, appears with a corresponding decrease in the amount of epidote present. I Felsic Gneiss (Gfgn) The felsic gneiss unit is composed of two main rock types - biotite paragneiss and granitic orthogneiss, with minor amounts of amphibolite and hornblende gneiss. These are often interlayered, with biotite paragneiss 1 1
112 predominating in the southern portion of the quadrangle, and granitic orthogneiss becoming mor common northward. This interlayering of thin sheets of each rock type along with the generally poor exposure prevent I mapping these as separate units. The petrography of the paragneiss and orthogneiss will be discussed individually, however. This unit is essentially the equivalent of the Charlotte belt gneiss unit of Wagener [1973], McCauley [1961], and South Carolina Electric and Gas Company [1977]. The only difference is that the amphibolite has been E separated out as a separate unit whenever possible. Biotite paragneiss, The biotite paragneiss always carries a distinct foliation defined by alignment of biotite and muscovite. Strike of the paragneiss is quite consistent over the southern portion of the study area, averaging about N.70 E. Felsic stringers and pods parallel to foliation occur ficequently within the paragneiss. These are granitic in nature, being composed almost entirely of quartz, plagioclase, and some potassium feldspar. It is unknown whether these are related to synmetamorphic differentiation or the intrusion of the granitic orthogneiss. I The biotite paragneiss is highly variable in composition, ranging from a hornblende-biotite gneiss to a muscovite schist. Although biotite was present in all samples, the amount varied from 2-25% of the rock volume. It is pleochroic, from light brown to dark brown or green-brown. Plagioclase is generally the most abundant mineral present. As in the amphibolite, it occurs as short subhedral crystals with no apparent preferred orientation. Michel-Lnvy determinations of composition yielded an average value of An34 (andesine). I I
113 Quartz occurs in subhedral grains wMch exhibit undulose extinction. In general, it is more abundant in the paragneiss than in the amphibolite, comprising as much as 15% of some samples. It is considerably more abundant in the granitic stringers and pods occurring within the paragneiss. Muscovite is found in most but not all samples of the paragneiss. It is exceptionally abundant in a small area in the southwestern corner of the quadrangle, from near Taylors Crossroads eastward for about three kilometers. In portions of this area the rock is a muscovite schist, with that mineral comprising nearly 50% of the rock. Protolith composition must have been more aluminous in this area. Elsewhere, muscovite totals less than 10%. Ocher minerals occasionally present in the paragneiss include hornblende, potassium feldspar, sphene, and epidote. Opaque oxides in minor amounts are = found in all samples. In the large enclave of the felsic gneiss unit in the vestern part of the study area (Plate IV), the biotite paragneiss shows signs of contact metamorphism from the surrounding Newberry granite. A high-r .de assemblage of biotite, quartz, plagioclase, garnet, sillimanite, green spinel, and opaques is present. Garnet crystals are euhedral and contain many large inclusions. The sillimanite and spinel occur within what appear to be outlines of crystals which have been totally altered. This rock is part of a band of slightly higher grade rocks which stretch east-west across the quadrangle. This band will be discussed in more detail at a later point. Granitic orthogneiss. The granitic orthogneiss is found throughout the felsic . gneiss unit but is most abundant in the northern part. It is inter-layered with biotite paragneiss and some amphibolite. It occuro as concordant sheets of granitic material which exhibit sharp, well defined contacts with the other rock types.
114 The orthogneiss is composed primarily of quartz, plagioclase, and potassium feldspar. Common accessory minerals include muscovite, biotite, and opaque oxides, with minor amounts of epidote, garnet, zircon, and I secondary chlorite (Table 9). The foliation in the rock is defined by elongated quartz grains. Large quartz grains present in the original rock have become elongkted during deformation, recrystallizing into less strained subgrains. These elongated quartz aggregates distinguish the orthogneiss from the flow-foliated portions E of the Pomaria granite. Potassium feldspar is usually more abundant than plagioclase. Both occur as subhedral crystals with no apparent orientarion with respect to foliation. Plagioclase is often highly altered and in places only the outline of the former crystal remains. The potassium feldspar is in the form of - microcline. As in the amphibolite, accessory garnets are altered along fractures and do not appear to be completely stable under terminal metamorphic conditions. Newberry Granite (Sn) The Newberry granite is widespread over the northern portion of the study area. It is an extensive plutonic complex, extending from west of Newberry eastward for some 30 kilometers into the Salem Crossroads and Jenkinsville quadrangles. The Newberry has apparently been intruded as a number of sheets, partly concordant to concordant with the foliation in the area. These vary considerably in thickness, upwards from a few meters. This, along with the extensive weathering of the granite, prevents a very accurate representation of the outcrop pattern on the geologic map of the quadrangle. It is certain that even in the areas shown as all Newberry I I
115 h. { w El I g d $ E w SAMPLE-y.
$ . o '. d i S
- l E E U
o z m w m @ m w e !$ o I o 0305 X 'O X. T X 0 T 0 0 X X 1001 X 0 X 0 X X 0 0 X 0 X hl 1101 X 0 X 0 X X 0 0 0 0 X 1802 X 0 X 0 X X 0 0 X X X BIOTITE 4101 X 0 X 0 X 0 0 0 0 0 X PARAGNEISS 6602 X 0 X 0 X X 0 0 0 0 X 3502 X X X X X 0 X X 0 X X h 6604 X X X 0 X 0 X X 0 0 X
-1801 .X 0 -X X X X 0 X 0 X X 2301 X 0 X X X X 0 0 0 0 X GRANITIC 6007 X 0 X X X X 0 0 0 0 X ORTH 0 GNEISS h 6301 X 0 X X 0 X 0 0 0 0 X 6802 X 0 X X 0 0 0 X T 0 X X = mineral present in amounts-greater than 1%
T'= mineral present in amounts less than 1% ( 0 = mineral not present
- [
~
Table 9. . Metamorphic mineral assemblages for biotite paragneiss and granitic orthogneiss within the felsic gneiss unit (Gfgn) of (' the Pomaria quadrangle. 1'
116 granite there are enclaves of metamorphic rock that have not been recognized because of sparse exposure and extensive weathering. I The Newberry granite is a generally quite homogeneous, fine- to medium-grained rock which compositionally lies near the quartz monzonite-monzogranite boundary on the APQ diagram (see Figure 23) . It is composed primarily of quartz, plagioclase, potassium feldspar, ana biotite. Minor r amounts of muscovite and opaques are generally present, with traces of epidote, ( sphene, and zircon. Hornblende was not observed in thin section, although
~
it was present in a hand sample taken from a boulder near the western edge of the Pomaria quadrangle just north of South Carolina highway 34. - Five thin sections were point counted to determine mineral percentages. The results of the point counts are shown in Table 10. In all cases plagioclase was the most abundant mimral, comprising between 34 and 44% of the rock volume. It occurs as subhedral crystals, with larger grains of ten showing extensively sericitized cores. Zoning is also prevalent in the larger cryctals, with calcic cores and more soda-rich rims. Average plagioclase composition fell in the oligoclase range (An24)* Potassium feldspar was always the second most abundant mineral present, with 29 to 40% of total volume. It occurs as subhedral microcline crystals. Quartz totaled between 13 and 24% of the rock. Most samples fell in the range of 19 to 20% quartz. This mineral occurs as small anhedral grains which show at most slight undulose extinction. The only other major mineral present was biotite, with 3 to 8% total. Distinct orientation of biotite crystals I was evident in only one sample, number 0403, which was collected from a thin (less than 1 meter ' thick) sheet of granite within cmphibolite. This preferred orientation probably is the result of flow during emplacement. I B
117 s s O 60 GA GB GD AG ,
.T O 4 A / AS / S / M \ MD \D\
90 65 35 P, 10 Figure 23. APQ diagram showing the modal variation for the Newberry pluton (+) in the Pomaria quadrangle. Diagram after Streckeisen [1967]. E
-..u.--- ,
118 I
$ e '
M I d 2 E @ SAMPLE NUMBER y
$ $ $ M $ S d
E l= U E NUMBER OF
@, h ]g 5 @ $ El O $ $ POINTS NEWBERRY GRANITE 42 33 4 1 T 0 0 0 1000 BL-1 19 1 20 38 34 5 1 2 0 0 T 0 1000 WS-1 20 44 31 4 T 0 0 0 ,0 700 2905 1 2 0 0 0 0 600 B
0403 19 37 34 7 1 24 34 29 8 2 2 1 0 T 0 1000 0901 13 42 40 3 1 1 0 0 0 0 1000 BQ* . P0 MARIA GRANITE 42 28 28 T 1 0 0 0 T 1000 6203A 1 6203B 32 39 27 0 1 1 0 0 0 0 1000 6203 27 24 46 T 0 3 0 0 T 0 700 WINNSBORO GRANITE 14 44 22 12 T 3 1 4 0 0 1000 3303 16 53 5 25 T 1 0 0 0 0 1000 4101 20 44 17 13 2 4 0 0 0 0 1000 4601
*From Smith [1982].
B Table 10. Modal mineralogical data for the Newberry, Pomaria and Winnsboro plutons in the Pomaria quadrangle. Modes were obtained by g counting thin sections, with the numbers of points indicated in 5 the table.
~
I I e -
119 The Newberry pluton as a whole does not show any evidence of deformation. There is an indistinct compositional layering noted at Lone Star Industries'
.g Blair quarry (in the Blair quadrangle), but this is nonuniform over any j distance and certainly does not represent a throughgoing fabric. Fullagar
[1981] reports a Silurian Rb-Sr whole rock age of 415 9 m.y. for this b granite. The initial 87Sr/ Rb 86 ratio of .7024 is extremely low and implies a lower crustal origin [Fu11 agar, 1971]. s The Newberry granite is distinct from the Winnsboro and Pomaria igneous a bodies in several ways. It is generally finer-grained, in most cases has a - higher muscovite content, and contains a few xenoliths. Many of the i " xenoliths" in the Blair quarry are mafic mineral clots composed almost entirely of biotite. Pitcher [1979] suggests clots of this nature are the N result of re-equilibration between liquid and crystal fractions during ascent
- and emplacement. This results in the removal of the mafic constituents into 5
the dark clots which then settle out. , The Newberry, like the Winnsboro, seams to have at least two distinct facies distinguished by differences in grain-sizes. There are areas in which the grain-size of the Newberry is noticeably larger, just as there are = locations where a fine-grained phase of the Winnsboro exists. Speer, Becker, and Farrar [1980] contend that in many cases intrusive contacts exist between _ the different facies of the Winnsboro plutonic complex and this may also be the case with the older Newberry granite. e Winnsboro Plutonic Complex (Cw) r a The Winnsboro pluton is found in the central section of the study area (Fig. 2, and Plate IV). It is apparently the same pluton which extends from the Winnsboro area westward into the Jenkinsville quadrangle as mapped by _ ___m_-___m
120 i Smith [1981]. Extensive work has been done on this complex by a number of l workers, including Wagener [1970, 1973, 1977], Farrar [1977], Farrar and Becker [1977], and South Carolina Electric and Gas Company [1977]. The Winnsboro has been determined to be of Carboniferous age as the result of Rb-Sr and K-Ar dating [ South Carolinc Electric and Gas Company, 1977; Fullagar,1981; Fullagar and Kish,1981]. These dates range from 309 to 286 m.y., thus placing the Winnsboro in the group of 300 million year old postmetamorphic plutons. There is no evidence of deformation within the granite body, although a weak fabric defined by alignment of tabular potassium feldspar crystals has been observed near the contact with surrounding country rocks. In these areas xenoliths are also aligned parallel to subparallel with the contact. These features are probably the result of flow during emplacement. Outcrop of this unit is very sparse due to extensive weathering. Unweathered material has been found in only a few streams in the western portion of the quadrangle where the pluton is highly charged with xenoliths. Elsewhere, only a few residual boulders remain. The streams in the area, however, form a distinctive drainage pattern by which the extent of the pluton can be approximated. Stream valleys are very wide and the sides have a gentle - slope. The pluton is also very well defined on the aeroradiation map The body is not as (Fig. 24), where it shows as a distinct radiation high. well defined on the aeromagnetic map (Fig. 4) . This may be due to the large number of xenoliths and enclaves present. Drilling near the site of the V. C. Summer nuclear station revealed significant amounts of gneiss and
~
amphibolite present at depth [ South Carolina Electric and Gas Company,1977]. The most striking feature of the Winnsboro' pluton is the abundance of xenolitas, especially near the contact with surrounding metamorphic rocks. I I
121 A j gj:Ry] y :4 %l g
~
gg
- U** 0 'N 0 i 2 'b f 4 km ~
N Figure 24. Aeroradiation map of the Pomaria quadrangle. Contour interval is 50 counts per s'econd. From the U. S. Geological Survey [1978]. Dotted lines are geological contacts from Plate IV.
=
122 In contrast with the Newberry granite, the Winnsboro does not exhibit sharp contacts with surrounding rocks, but rather seems to have been injected along fractures and joints. The mapped contact was based upon the last appearance of significant amounts of the medium-grained granitic material within the country rock. As one traverses inward into the pluton, the number of xenoliths decrease, although a number of xeno11ths and enclaves were observed well into the interior portion of the body. Compositionally, the pluton generally falls into the quartz monzonite or monzogranite field on the APQ diagram [Wagener,1973; Smith,1982). Modal analyses taken on three thin sections produced a vide scattering of points on the diagram (see Figure 25). Upon staining of the rock slabs for potassium feldspar, it was shown that distribution of the feldspars is very uneven. Potassium feldspar percentage in the slabs was estimated to be considerably higher than that in the thin sections, and thus tb points in the figure are misleading. Due to the uneven potassium feldspar distribution, a larger nu=ber of point counts are needed to accurately determine the composition of this plutonic complex. , The major minerals present in the Winnsboro are plagioclase, potassium feldspar, quartz, and biotite. Common accessories are muscovite and opaque oxides, with occasional epidote and hornblende. Plagio,clase is the most abundant occurring mineral. Crystals are generally subhedral, highly zoned, and often exhibit considerable alteration - and sericitization within the cores. Average composition was determined to fall within the oligoclase range. Quartz is present as anhedral, relatively unstrained grains. Myrmekitic intergrowth with potassium feldspar is common. I The potassium feldspar occurs in the form of anhedral to subhedral microcline, I with the larger crystals most often showing tha-distinct crystal faces. 1
l-L: ] 123 I L Y-o t I i [: , 60 GA G.B GD AG T c -
/ 20 l
{ O OS OWsD OD ( / ' 5 A
/ AS / 5 / M \ MD \D\ P- l 90 65 35 10
[ Figure 25. APQ diagram showing the modal variation for the Winnsboro pluton (o) and the Pomaria pluton (=) in the Pomaria ( quadrangle. Diagram after Streckeisen [1967]. [ l L - - - - - - - -
p 124 Biotite is quite common in the rock, comprising as much as 25 percent of some samples. .The high percentages shown in Table 10 are thought to be the result of contamination due to the assimilation of country rock during
. emplacement. All three thin sections analyzed come from areas with significant amounts of xenoliths present. In weathered material taken from areas where few or no xenoliths were observed, biotite appeared to be significantly less abundant.
Hornblende is present in one of the thin sections and was noted in several hand samples. This mineral has been identified in a number of the postmetamorphic plutons of the southern Piedmont [Speer, Becker, and Farrar, 1 1980]. Within the Winnsboro pluton its distribution appears to be quite W uneven. In the case of the Bald Rock batholith in northern South Carolina, van Gelder and McSween [1981] suggest the presence of hornblende is due to the assimilation of mafic country rocks. This msy possibly be the case with the Winnsboro as well, as within the study area hornblende was observed only where the granite contained xenoliths. Pomaria Granite (Cp) The Pomaria granite is a small, roughly circular body centered near the a g town of Pomaria. Primary exposure is at an abandoned quarry just north of U. S. highway 176. The cnly other exposure is a few boulders within the town itself. Excellent constraint on the extent of the granite to the south is provided by exposures of biotite paragneiss on U. S. 176 and the road leading to the quarry. The granite is assumed to be postmetamorphic due to the occurrence of undeformed material within it. Most cf the rock, however, carries a weak to strong fabric, presumably related to flow during emplacement. The granite I
125 t clearly intrudes surrounding country rocks as is shown by the presence of I xenoliths. Although interpreted here as Carbonifercus in age, no dating on j ( l this body has yet been performed. It is presumed to be Carboniferous solely on the basis of lack of deformation throughout the body. The Pomaria granite is extremely inhomogenous as the results of point counting show in Table 10 and the APQ diagram in Figurc 25. Large variations l l in both composition and grain-size occur over very short distances. Within any cne sample, however, the granite is relatively equigranular. The average l composition plots well into the granite B field owing to the high quartz content, in one case 42% of the rock. The other major minerals present are plagioclase and potassium feldspar. The results of staining for potassium l l content as well as the point counts indicated large variations in the amounts of potassium feldspar and plagioclase present. Both of these minerals occur in generally subhedral crystals. Myrmekite is common. Ons. unusual feature of the rock is the near absence of biotite. It is quite leucocratic, with no more than 3% dark minerals, mostly opaques. Other minerals present include muscovite and traces of sphene and garnet. The garnet was observed in only one sample which was taken from an area where xenoliths were abundant. It is unclear whether the garnet is the result of contamination or was an original I component of the granite. Other Rock Types l Several diabase dikes cross the study area. These have an orientation from about N.15 W. to N.30 W. and are steeply dipping to vertical. Paleo-magnetic study of a number of dikes in the central South Carolina Piedmont l by R. E. Dooley and W. A. Smith [1982] indicated a Late Triassic and/or Early Jurassic age. Compositionally, the diabase dikes are olivine tholeiites. l l
h 126 g l
- i. ,
g A quartz syenite has recently been discovered just to the west of the boundary between the Pomaria and Newberry East quadrangles in the area south of Keitts Crossroads. It is possible that the body extends slightly into the study area. The quartz syenite is unfoliated and contains numerous xenoliths of varying composition. Minerals present in the rock include potassium feldspar, quartz in myrmekite, plagioclase, orthopyroxene, clinopyroxene, opaque oxides, and secondary hornblende. This composition indicates a very hot, dry magma. Further work needs to be done on this bcdy to determine its extent and relationships with surrounding roc'ks. The only other rock types present in the study area are numerous postmetamorphic aplite and pegmatite dikes which occur in all the different units. , 8 STRUCTURAL GEOLOGY With the complex geology in the Charlotte belt, it is difficult to determine the deformational history of the area. In addition, the lack of exposure limits the amount of structural data available. One foliation (S I) is observed in the metamorphic rocks. It is quite consistent in orientation over the extreme southern portion of the quadrangle but becomes more variable as the granites are approached. This foliation is the result of the earliest recognizable deformational episode (D1 ). Timing of Di must be Silurian or older, as the 415 m.y.-old Newberry pluton contains rotated xenoliths which carry the Si foliation. The D1 deformational event also produced tight to isoclinal F1 folds with axial planes parallel to foliation. These F1 folds have been observed in the biotite paragneiss in many parts of the quadrangle. I In several locations the Si foliation has been disrupted by F2 f Ids. The axial planes of the F2 folds are nearly perpendic: tar to the Si foliation. I I
c 127 h Figure 26'is a plot of poles to Si foliation in the Pomaria quadrangle showing the folding of S t. Rotated xenoliths within the Winnsboro pluton have been observed to contain F2 folds. As the Winnsboro is undeformed, the D2 deformational event forming the folds must be older than the 300 m.y.-old ( g'ranite. s Faulting in the area of study is limited to small ductile offsets, although a number of brittle faults have been discovered to the south in both the Little Mountain and Chapin quadrangles. One of these is the north-south Summers Branch fault mapped by Peck [1981]. If projected several more kilometers to the north, it would extend into the southeastern corner of the Pomaria quadrangle. However, no evidence for the existence of this fault in the study area has been found. A diabase dike of probable Late Triassic or Early Jurassic age has been found to cut across the north-south Wateree Creek fault zone in the Chapin quadrangle [Secor and others,1981]. This indicates h .that most of the movement on the fault occurred prior to about 190 m.y. ago.
~ In the metamorphic rocks to the north of the Winnsboro pluton, the metamorphic grade seems to be slightly higher than in the rocks to the south.
Sillimanite is present in some rocks, and the schistosity common in the paragneiss to the south of the granites is no longer present. This schistosity returns to the rocks as one progresses northward into the Blair quadrangle. Overall composition of the metamorphic rocks remains fairly (- L constant. Possibly the additional heat brought in by the granites could be responsible for the higher grade. Certainly both the Winnsboro and Newberry h-plutons contact metamorphosed surrounding country rocks. The sillimanite { has been found only in the immediate vicinity of the granites. Pyroxene is
'also found in the contact aureoles.
h D f --
128 f I. t I I
*f...,gjj e
--.s. ....
f . :' .f ,*. , * * * . . . . . l I
*a.*,
y..s.
....s.. .
[ .t. . * .* .' . * . C *e.?
; .,ry,.- P .*.*.* *
- J.Cc l
. I. '.,'. 3. . . .s . . ... - ,
.;.. . J.;;'.p r..,.,
. - d.f,,,.t.
.a
. . . .. ....e..... .
w::. >....... *
' j ,;' . * . * . ' . * . * . * . * .
,*..N. .. - * ,.;., ' .. ..
** t 4
...: .. .i.g* . . .- . .
* :: . O.y-
. . w.
.~,.... .....
2."I.C. ta..,,..; : ' . . .q . . .s. f- n. ' ; , ' ." *.,.",g<"- .
.=.*
. i' .1 J.
j P,.'i. ., ..... .. .:.
- 9. ... ....
..a.....
* *** . . . . . . . . . *..*.=' .
- p
.r .
***a .
. . MI :..... .
. .b. .'[. . i 1. . . . .. ... ,, ...* ..r. .'
Q23!"'Q.. .
- r. '.':=..:,,.*.,. ;
;-... . e .. .
v s. ~ ......x . . . . .....
.$j.$. . ic' . . ], ..-,. .,.'. ; . .s
*i,,M '
-4.-a. .+ j$, .28..f. Q', ...
**.> . /..
~
..,....:.~ . ,,
> .fy;.s.e. y* *' ' '
' .* 'r". , * . :
- v. .e.',
. .. .. .. ...w..
. .. . - ..g .;;. .
g
...1 .
i
. , a ./ .
31.-Q.y........
. .. . . . .t, ......
a-
- !.!:::::.... . ~. .
N g.:.!.:! ::' . . . .;, c .. I Figure 26. Lower hemisphere, equal-area projection of poles to 97 Si foliation planes measured in the Pomaria quadrangle. Contours: 1, 3, 9 and 15%. I
129 8- Another possibility is that this area forms the core of an antiformal structure. The increased burial depth along with the additional heat of the granites might easily be enough to slightly increase the metamorphic grade. Rocks to the north in the Blair quadrangle closely resemble the paragneiss, orthogneiss, and amphibolite in the Pomaria quadrangle. Strike once again becomes consistent, with dips to the southeast. If this is an antiform, the northern limb of the structure would be overturned. CONCLUSIONS The Pomaria 73' 5 quadrangle is located within the high-grade metamorphic region known as the Charlotte belt. Six mappable rock units, two of them metamorphic and four igneous, occur in the study area. Contacts between units are largely approximate and in the case of the contact between the main amphibolite unit (Ea) and the felsic gneiss unit (Gfgn) it 'may be gradational. Three distinct episodes of igneous activity occur in the study area. Granitic rocks are considerably more abundant than originally believed. The Winnsboro pluton stretches east-west across the quadrangle while a large part of the northern third of the study area is underlain by Newberry granite. The study area has experienced at least two phases of deformation. The earliest and strongest produced the amphibolite facies metamorphism and the Si foliation. The second produced F2 folding but no overprinting clea3 age or foliation. The time of the deformational events is somewhat constrained I by the dated igneous rocks present. - Several problems remain to be solved in the area. One is the determina-tion of whether the amphibolite facies metamorphic rocks of the Charlotte - belt are in any part correlative with the greenschist facies metamorphic rocks of the Carolina slate belt. Although McCauley [1961] concluded from
I 130 geochemical studies that the rocks in the two belts are not correlative, further work of this type needs to be done. Another area of needed study is 4 the dating and mapping of the quartz syenite on tle western border of the quadrangle. Recent field work revealed that this body is of mappable size. Rocks of this type elsewhere in the Piedmont are often associated with mafic plutons and more detailed mapping in the Newberry East quadrangle could determine if this is the case here as well. Age dating also needs to be done on the small Pomaria granite to see if it is a member of the post-metamorphic group of plutons. I B B I I I B B B B B B L__-- _ _ _ _ _ - _ _ _ ._ _ _ _ _ . _ __ _ . - _ _ _ _ _ _ _ _ _ _ _ _ . _ _ _ _ _ _ _ _ _ _ _ _ _ . . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ -
131 l
\
-REFERENCES CITED Bell, Henry, III, K. G. Book, D. L. Daniels, W. E. Huff, Jr. and P. Popenoe, Diabase dikes in the Haile-Brewer area, South Carolina, and their i magnetic properties in Shorter contributions to geophysics, 1979, U. S. Geol. Survey Prof. Paper 1123, C1-C18, 1980. l Black, W. W., Chemical characteristics and Rb/Sr ages of metavolcanics from the Carolina slate belt of North Carolina (abstract), Geol. Soc. Am. i Abstr. Programs, 10, 162-163, 1978. l l
] , Chemical characteristics of metavolcanics in the Carolina slate J belt, in Proceedings, the Caledonides in the USA, edited by D. R. Wones, Department of Geological Sciences, Virginia Polytechnic Institute, mem. 2, Blacksburg, Virginia, 271-278, 1980. -
1 Black, W. W. and P. D. Fullagar, Avalonian ages of metavolcanies and plutons of the Carolina slate belt near Chapel Hill, N. C. (abstract) , Geol. Soc. Am. Abstr. Programs, 8_, 136, 1976. Bourland, W. C. and S. Farrar Tectogenesis of the rocks surrounding the Winnsboro intrusive complex, S. C. Geology, S. C. Geol. Survey, 2 , 1-18, 1980. Briggs, D. F., M. C. Gilbert and L. Glover, III, Petrology and regional significance of the Roxboro metagranite, North Carolina: Geological l Society of America Bulletin, 89_, 511-521, 1978. l Brown, H. S. and N. J. Gilbert, Geologic history of a fault system in the J Charlotte belt, northe n South Carolina (abstract), Geol. Soc. Am. Abstr. Programs, 9_, 122-123, 1977. Brown, T. S., Geology of the Owdoms quadrangle, South Carolina (M.S. thesis), Univ. of South Carolina, Columbia, S. C., 40 pp., 1971. Butler, J. R., Comments on zeolite formation in the Carolina Piedmont I (abstract), Geol. Soc. Am. Abstr. Programs, 9,, 124-125, 1977. Butler, J. R. and P. C. Ragland, Petrology and chemistry of meta-igneous 1 rocks in the Albemarle area, North Carolina slate belt, Am. J. Sci., 267, 700-726, 1969b. Butler, J. R. and P. D. Fullagar, Lilesville and Pageland plutons and associated meta-rhyolites, eastern Carolina slate belt (abstract), Geol Soc. Am. Abstr. Programs, 7,, 475, 1975. 3-
, Petrochemical and geochronological studies of plutonic rocks in the southern Appalachians: III. Leucocratic adamellites of the Charlotte belt near Salisbury, North Carolina, Geol. Soc. America Bull., 8_9_, 460-466, 1978.
J
132 a Carpenter, F_ ' H., General geology of the Carolina slate belt along the Georgia-South Carolina border; in Chowns, T. M., Compiler, Stratigraph , structure and seismicity in slate belt rocks along the Savannah River: Georgia Geological Survey, Guidebook 16, 9-12, 1976. Carpenter, R. H., A. L. Odom and M. E. Hartley, III, Geochronology of the g southern portion of the slate belt (abstract), Geol. Soc. Amer. Abstr. g Programs, 10,, 164, 1978. Chowns, T. M. , Introduction, stratigraphy, structure and seismicity in slate belt rocks along the Savannah River, Georgia Geological Society, s Guidebook 16, 2-8, 1976. Conley, J. F. and G. L. Bain, Geology of the Carolina slate belt west of the Deep River-Wadesboro Triassic Basin, North Carolina, Southeastern Geoloav, 6, 117-138, 1965. - Conley, J. F. and K. M. Drummond, Ultramylonite, zones in the western Carolinas,
~
201-211, 1965. Southeastern Geology, Costain, J. K., 6_, L. Glover, III and A. K. Sinha, Evaluation and targeting of I geotherm1 energy resources in the' southeastern United States, National Technical Information Service, U. S. Dept. of Commerce, Springfield, Virginia, VPI-SU-5103-2, 170 pp., 1976.
. Evaluation and targeting of geothermal energy resources in the southeastern United States, National Technical Information Service, U. S. Dept. of Commerce, VPI-SU-5103-4, 94 pp., 1977.
Costello, O. P., D. T. Secor, Jr. and A. W. Snoke, Structural relationships I as a key to stratigraphic sequence in the Carolina slate belt, Lake Murray, S. C., Southeastern Geology, 1981. Crickmay, G. W., Geology of the crystalline rocks of Georgia, Georgia Dept. Mines, Mining and Geology Bull. 58, 54 p., 1952.
'Daniels, D. L., Geologic interpretation of geophysical maps, central Savannah River area, South Carolina and Georgia: U. S. Geological Survey, Geophysical Investigations Map GP-893, 1:250,000, 3 sheets, 1974.
deBoer, J., Paleomagnetic-tectonic study of Mesozoic dike swarms in the Appalachians, J. Geophys. Res., 72, 2 2237-2250, 1967. Dickinson, W. and T. Hatherton, Andesitic volcanism and seismicity around the Pacific, Science, 157, 801-803, 1967. Dooley,.R. E. and J. M. Wampler, K-Ar relationships in dolerite dikes of Georgia (abstrac.t), Geol. Soc. Am. Abstr. Programs, 9, 134, 1977. I
l l 133 l Dooley, R. E. and W. A. Smith, Paleomagnetic results of early Mesozoic l diabase dikes from the central Piedmont of South Carolina (abstract), Geol. Soc. Amer. Abstr. Programs, H , 15, 1982. l
, Paleomagnetism of early Mesozoic diabase dikes in the South Carolina Piedmont, Tectonophysics, in press.
Duc, A. C., Source properties of induced earthquakes at Monticello Reservoir, South Carolina (M.S. thesis), Univ. of South Carolina, Columbia, S. C., 151 pp., 1980. Duc, C., D. Amick and P. Talwani, Induced seismicity at Monticello Reservoir: ( 50th annual meeting, eastern section of the Seismological Society of America, Weston, Mass., 1978. Espenshade, G. H. and D. B. Potter, Kyanite, sillimanite and andalusite deposits of the southeastern states, U. S. Geol. Survey Prof. Pap. 336', 121 pp., 1960. Farrar, S. , Winnsboro plutonic complex in Costain, J. K. , Glover, L. , III, Sinha, A. K., Evaluation and targeting of geothermal energy resources in the southeastern United States, National Technical Information Service, U. S. Dept. of Commerce, VPI-SU-5103-4, A14-A25, 1977. Farrar, S. S. and S. W. Becker, Winnsboro plutonic complex, in Evaluation and targeting of geothermal resources in the southeastern United States (Progress Report), Blacksburg, Virginia Polytechnic Institute and State University, VPI-SU-5104-1, A44-A58, 1977. Fullagar, P. D. , Age and origin of plutonic intrusions in the Piedmont of the southeastern Appalachians, Geol. Soc. Am. Bull., g , 2845-2862, 1971.
, Summary of Rb-Sr whole-rock ages for South Carolina, S. C. Geol., S. C. Geol. Survey,_2_5, 5 29-32, 1981.
Fullagar, P. D., R. E. Lemmon and P. C. Ragland, Petrochemical and geochronological studies of plutonic rocks in the southern Appalachians: I. The Salisbury pluton, Geol. Soc. America Bull., g , 409-416, 1971. Fullagar, P. D. and J. R. Butler, 325 to 265 m.y.-old granitic plutons in the Piedmont of the southern Appalachians, Am. J. Sci., 279, 161-185, 1979. Fullagar, P. D. and S. A. Kish, Mineral age traverses across the Piedmont of South Carolina and North Carolina, in Geologic investigations of the Kings Mountain belt and adjacent areas in the Carolinas; edited by l J. W. Horton, Jr., J. R. Butler, and D. M. Milton, Carolina Geol. Soc. Field Trip Guidebook, S. C. Geol. Survey, 155-165, 1981. l Glover, L., III, and A. K. Sinha, The Virgilina Deformation, a late Precambrian to early Cambrian (?) orogenic event in the central Piedmont of Virginia and North Carolina, American Journal of Science, 273-A, j 234-251, 1973. l 1 l l .
134 Glover, L., III, A. K, Sinha and J. K. Costain,' Evaluation and targeting of geothemal energy resources ~ in South Carolina, National Technical Information Service, U. S. Department of Commerce, VPI-SU-5104-1, 130 pp., 1977. Glover, L., III, D. G. Mose F. B. Poland, A. R. Bobyarchick aad W. C. Bourland, Grenville basement in the Piedmont of Virginia: Implications for orogenic models (abstract), Geol. Soc. Amer. Abstr. Programs, g, 169, 1978. Griffin, V. S., Progress Report on a ge'ologic study in Abbeville and McCormick I counties, South Carolina, Geologic Notes. S. C. State Development Board, Division of Geology, 16_, 59-76, 1972. Hartley, M. E., III, Graves Mountain, in Chowns. T. M., compiler, Stratigraphy, structure, and seismicity in slate belt rocks along the g Savannah River, Georgia Geol. Soc. Guidebook g, 42-52, 1976. g Hatcher, R. D., Jr. Developmental model for the southern Appalachians, Geol. Soc. Am. Bull., 8_3,,3 2735-2760, 1972. Hatcher, R. D., Jr. and I. Zietz, Tectonic implications of regional aero-magnetic and gravity data from the southern Appalachians, in Wones, D. R., ' ed., The Caledonides in the U.S.A., Virginia Polytechnic Institute, Dept. of Geological Sciences Special Publication 2, 235-244, 1980. Hietanen, A., Generation of potassium-poor magmas in the northern Sierra Nevada and the Svecofennian of Finland, Journal Research, U. S. Geological Survey, 3, 631-645, 1975. Hills, F. A. and J. R. Butler, Rubidium-strontium dates for some rhyolites from the Carolina slate belt of the North Carolina Piedmont (abstract),
- Geol. Soc. Am. Special Paper 121, 445 pp., 1969.
Jakes, P. and A. White, Major and trace element abundances in volcanic rocks of orogenic areas, Geol. Soc. Am. Bull., 8_3,, 29-40, 1972. Kearns, F. L., M. P. Katuna, D. P. Lawrence and R. G. Chalcraft, l Paleoenvironmental interpretation of a portion of the Carolina slate l ' belt, Saluda County, South Carolina (abstract), Geol. Soc. Am. Abstr. Programs, 13_, 11, 1981. Kealer, T. L., Granitic injection processes in the Colu=bia quadrangle, g 4 1936. South Carolina, Jour. Geol., 4_4,, 3
, The Little Mountain syncline in the South Carolina Piedmont, Southeastern Geology, 14,, 195-201, 1972.
King P. B., A geologic section across the southern Appalachiens: an outline of the geology in the segment in Tennessee, North Carolina, and South Carolina, in Guides to Southeastern Geology: Geological Society of America, edited by R. J. Russell, 332-373, 1955. I
f L 135 I L r Kish, S. A., J. R. Butler and P. D. Fullagdr, The timing of metamorphism and L deformation in.the central.and eastern Piedmont of North Carolina (abstract), Geol. Soc. ' Am. Abstr. Programs, 11, 184-185, 1979. Kish, S. A. and P. D. Fullagar, Summary of geochronological data from Late { Paleozoic plutons from high grade metamorphic belts of the eastern Piedmont of North Carolina, South Carolina, and Virginia, in [ Geological investigations of the eastern Piedmont, southern Appalachians: L Carolina Geological Society Field Trip. Guidebook, South Carolina Geological Survey, 61-64, 1978. h McCauley, J. F., Relationships between the Carolina slate belt and the Charlotte belt in Newberry County, South Carolina, Geol. Notes, S. C. Geol. Survey,
~5, 59-79, 1961a.
[. , Rock analyses in the Carolina slate belt and the Charlotte belt of Newberry County, South Carolina, Southeastern Geology, 3_, 1-20, 1961b. McKenzie, J. C., The Geology of Little Mountain, South Carolina (M.S. thesis), Un1v. of South Carolina, Columbia, South Carolina, 1962. McKenzie, J. C. and J. F. McCauley, Geology and kyanite resources of Little Mountain, South Carolina, S. C. Geol. Survey Bull., 3_7, 7 13 pp., 1968. [ Odom, A. L. and R. D. Hatcher, Jr., A char'acterization of faults in the Appalachian foldbelt, U. S. Nuclear Reg. Com., Washington, D. C., NUREG/CR-1621, 314 pp., 1980. Overstreet, W. C. and H. Bell, III, The crystalline rocks of South Carolina, U. S. Geol. Survey Bull., 1183, 126 pp., 1965a. [ , Geologic map of tbe crystalline rocks of South Carolina, U. S. Geol. Survey, Misc. Geol. Inv. Map I-413, scale 1:250,000, 1965b. { Peck, L. S., Geology of the Little Mountain and Chapin quadrangles, South Carolina: A study of the stratigraphic relationship between the Carolina [ slate belt and the Charlotte belt (M.S. thesis), Univ. of South Carolina, Columbia, S. C., 50 pp., 1981. ( Pirkle, W. A., Geology of the Red Hill quadrangle, Edgefield County, South Carolina, Geologic Notes, South Carolina Geological Survey, 21,75-84, 1977.
, Preliminary report on the geology of the Parksville quadrangle, McCormick and Edgefield Counties, South Carolina, Geologic Notes, r South Carolina Geological Survey,_2_2, 2 95-104, 1978.
L Pitcher, D. M., The bedrock geology of the Pomaria 7h' quadrangle, South Carolina (M.S. thesis), Univ of South Carolina, Columbia, S. C., ( 45 pp., 1982. c L [. _ - _ _ _ _ _ _ _ _ _ _ _ _
r-~z 136 Pitcher, W. S..-The nature, ascent and emplacement of granitic magmas, Journal of the Geological Society cf London, 136, 627-662, 1979. Pitcher, W. S. and A. R. Berger, The geology of Donegal: a study of granite emplacement and unroofing, Wiley-Interscience, 419 p., 1972. Privett, D. R., Paragenesis of an unusual hydrothermal zeolite assemblage in a diorite-granite contact zone, Woodleaf, Powan County, North Carolina,
~
Southeastern Geology, 5 1_5_, 105-118, 1973a.
, Laumontization near the northern margin of the Liberty Hill batholith, southern Chester County, South Carolina, Geol. Notes, S. C. Geol. Survey, 17_, 49-54, 1973b.
, Widespread laumontization in the central Piedmont of North Carolina and southern Virginia (abstract), Geol. Soc. Am. Abstr.
Programs, 6_, 389-390, 1974a.
, Laumontization in Kings Mountain belt rocks, Cherokee County, South Carolina, Geol. Notas, S. C. Geol. Survey,_18, 31-35, 1974b.
. Widespread zeolitization in the central Charlotte belt, north-central York County, South Carolina (abstract), Geol. Soc. Am,-
Abstr. Programs, 9, 176, 1977. Ragland, P. C., The use of zeolites to determine the age of last motion on a fault: A case history (abstract), Geol. Soc. Am. Abstr. Programs, 9, l 176, 1977. e St. Jean, J. , A new Cambrian trilobite from the Piedmont of North Carolina, Am. Jour. Sci., 273-A, 196-216, 1973. Samson, S. L., D. T. Secor, Jr., A. W. Snoke, and A. R. Palmer, Geological a implications of recently discovered Middle Cambrian trilobites in the E Carolina slate belt (abstract), Geol. Soc. Am. Abstr. Programs, M, 1982. Scheffler, P. K., The McCormick County, South Carolina, earthquake of 2 August 1974: Geological and geophysical investigations (M.S. thesis), Univ. of South Carolina, Columbia, S. C., 1976. Secor, D. T., Jr., Geological studies in an area of induced ceismicity at Monticello Reservoir, South Carolina, First. Technical Report to the U. S. Geol. Survey, Contract no. 14-08-0001-19124, 34 pp., 1980. Secor, D. T., Jr. and H. D. Wagener, Stratigraphy, structure, and petrology of the Piedmont in central South Carolina, Geol. Notes, S. C. Geol. E Survey, 12_, 67-84, 1968. E I
137 Socor, D. T. , Jr. and A. W. Snoke, Stratigraphy, structure and p1'itonism in
'( the central South Carolina Piedmont; in Snoke, A. W., editor, Ceological investigations of the eastern Piedmont, southern Appalachians:
Carolina Geological Society Field Trip Guidebook for 1978 South Carolina Geological Survey, 65-123, 1978. { Secor, D. T., Jr., L. S. Peck and D. H. Simpson, Geological studies in an area of induced seismicity at Monticello Reservoir. South Carolina, '[ Second Technical Report to the U. S. Geol. Survey, Contract no. 14-08-0001-191? ^ , 32 pp. , 1981. f Secor, D. T., Jr., L. S. Peck D. M. Pitcher, D. C. Prowell, D. H. Simpson, W. A. Smith and A. W. Snoke, Geology of the area of induced seismic activity at Monticello Reservoir, South Carolina, Jour. Coophys. Res., in press. Seiders, V. M., A chemically bimodal calc-alkalic suite of volcanic rocks, Carolina volcanic slate belt, central North Carolina, Southeastern [ Geology, 19, 241-265, 1978. Seiders, V. M. and J. E. Wright, Geology of the Carolina slate belt in the ( Asheboro, North Carolina, area, in Field Guides for Geological Society of America, Southeastern Sectional Meeting, Winston-Salem, North Carolina, edited by E. R. Burt, N. C. Dept. Natural and Economic Resources, 1-34, 1977. { Simpson, D. H., The Wateree Creek Fault Zone (M.S. thesis), Univ. of South I Carolina, Columbia, South Carolina, 35 p., 1981. Sinha. A. K. and B. A. Morn, Winnsboro peraluminous plutonic complex, in Evaluation and targeting of geothermal resources in the southeastern '( United States (Progress. Report), Blacksburg, Virginia Polytechnic Institute and State University, VPI-SU-5104-1, B1-B22, 1976. 1 Smith, T. E. and H. C. Noltimier Paleomagnetism of the Newark trend igneous [ rocks of the north central Appalachians and the opening of the central Atlantic Ocean, Am. Jour. Sci., 279, 778-807, 1979. ( Smith, W. A., The bedrock geology of the Jenkinsville 7h' quadrangle, South Carolina (M.S. thesis), Univ. of South Carolina, Columbia, S. C., 46 pp., 1982. Snipes, D. S., M. W. Davis and P. R. Manoogian, Cross Plains fault in the Piedmont of northwestern South Carolina (abstract), Geol. Soc. Am. Abstr. Programs, _11,, 1 213, 1979. Snoke, A. W., D. T. Secor, Jr. and S. A. Kish, Deformed Hercynian granitic . rocks from the Piedmont of South Carolina, Am. Jour. Sci., 280, 1018-1034, 1980. t ( ._..
138 ! South Carolina Electric and. Gas Company, Virgil C. Summer nuclear station l final safety analysis report, Columbia, S. C., 2, 1977. (
, Virgil C.~ Summer nuclear station unit 1, Supplemental seismological investigation, Columbia, S. C.,
l 1980. Speer, J. A., S. W. Becker and S. Farrar. Field relations and petrology of the postmeccmorphic, coarse-grained granitoids and associated rocks of the southern Appalachian Piedmont in Wones, D. R., ed., The Caledonides in the U.S.A., Virginia Polytechnic Institute, Dept. of the Geological Sciences Special Publication 2, 137-148, 1980. Streckeisen, A. L., Classification and nomenclature of igneous rocks: Neues Jb. Miner. Abh., 107, 104-240, 1967. Stromquist A. A. and H. W. Sundelius, Stratigraphy of the Albemarle Group of l the Carolina slate belt in central North Carolina, United States E Geological Survey Bulletin, 1274-B, B B-22, 1969. Sutter, J. F. and T. E. Smith, Ar/ 'Ar ages of diabase intrusions from the Newark trend basins in Connecticut and Maryland: Initiation of central Atlantic rifting, Am. Jour. Sci., 279, 808-831, 1979. Talwani, P., Induced seismicity and earthquake prediction studies in South Carolina, Eighth Technical Report to the U. S. Geol. Survey, Contract, no. 14-08-0001-16, 36 pp., 1979. .
. Association of induced seismicity with pre-existing fractures at Monticello Reservoir, South Carolina (abstract), Earthquake Notes, 52, 28-29, 1981.
Taiwani, P., D. Stevenson, J. Sauber, B. K. Rastogi, A. Drew, J. Chiang and D. Amick, Seismicity studies at Lake Jocassee, Lake Keovee. and Monticello Reservoir, South Carolina, Seventh Technical Report to the U. S. Geol. Survey, contract no. 14-08-0001-14553, 151 pp., 1978. Talwani, P. and B. K. Rastogi, Mechanism for reservoir induced seismicity, Abstracts of papers presented at 74th Annual Meeting of Seismological Soc. Am., Earthquake Notes, 49, 59, 1979. Talwani, P. , B. K. Rastogi and D. Stevenson, Induced seismicity and earthquake prediction studies in South Carolina, Tenth Technical Report to the U. S. Geological Survey, Contract no. 14-08-0001-17670, 212 pp., 1980. Thurmond, C. and J. Whitney, Structural and stratigraphic development of the Charlotte and northern slate belts within the Chennattit quadrangle, Georgia and South Carolina (abstract), Geol. Soc. Am. Abstr. Programs, M,215,1979. U. S. Geological Survey, Aeromagnetic survey of Northwest South Carolina, Sheets 8 and 13, U. S. Geol. Survey Open File Map 78-847, scale 1:250,000, 1978. I
139 j van Gelder, S. M. and H. Y. McSween, Intrusion of the Bald Rock batholith into Kings Mountain belt and Charlotte belt rocks, South Carolina, in Horton, J. W. , Jr. , Butler, J. R. and Milton, D. J. , eds. , .I Carolina Geological Society Field Trip Guidebook'1981, South Carolina Geological Survey, 147-154, 1981. Wagener, H. D., Coarse-grained sillimanite quartz rock associated with the Winnsboro granites, Fairfield County, South Carolina, Geologic Notes, Div. of Geology, S. C. State Development Board, 10,, 47-49, 1966.
. Notes on metamorphic rocks of the eastern Piedmont of South Carolina, Winnsboro area, Geologic Notes Div. of Geology, S. C. State Development Board, H , 25-29, 1968.
, Geology of the southern two-thirds of the Winnsboro 15-minute quadrangle, South Carolina, S. C. Geol. Survey, MS-17, I 34 pp., 1970.
. Petrology of adamellites, granites and related metam' orphic rocks of the Winnsboro quadrangle, South Carolina, The Citadel Monograph
.I Series, X_, 75 pp., 1973.
, The granitic stone resources of South Carolina, S. C. Geol.
Survey, Mineral Resources Series, 5, 65 pp., 1977a.
, Zeolites as indicators of time of latest movement on Piedmont I faults: the Virgil C. Summer nuclear station (abstract), Geol. Soc. Am.
Abstr. Programs, 9_, 192-193, 1977b. I Waskom, J. D. and J. R. Butler, Geology and gravity of the Lilesville granite batholith, North Carolina, Geol. Soc. Am. Bull., 82_, 2827-2844, 1971. Watts, D. R., A paleomagnetic study of four Mesozoic diabase dike swarms of I the southern Appalachian Mountains (M.S. thesis), Ohio State University, Columbus, Ohio, 115 p., 1975. Weigand, P. W. and P. C. Ragland, Geochemistry of the Mesozoic dolerite dikes from eastern North America: Contr. Mineralogy and Petrology, g , 195-214, 1970. i Weisenfluh, G. A. and A. W. Snoke, An epizonal trondjemite-quartz keratophyre complex near Calhoun Falls, South Carolina, Geologic Notes, S. C. Geol. Survey, ,2_2, 2 48-73, 1978. Whitney, J. A., T. A. Paris, R. H. Carpenter and M. E. Hartley, III, Volcanic . evolution of the southern slate belt of Georgia and South Carolina: A primitive oceanic island arc, Jour. Geol., 8_6_, 173-192, 1978. Williams, H., Tectonic lithofacies map of the Appalcchian orogen, St. Jobs, Newfoundland. Canada Memorial University of Newfoundland, 1:1,000,000, 1978. I
140 Zoback, M. D., In-situ study of the mechanism of reservoir triggered earth-quakes in the southeastern. United States, in Proceedings of research conference on intra-continental earthquakes, Chrid, Yugoslavia, 1979. Zoback, M. D. and S. Hickman, InDsitu study of the physical mechanisms controlling induced seismicity at Monticello Reservoir, South Carolina, Jour. Geophys. Res., in press. I I I I I l I'
~
l I1 l
. .. _ m ;.,7_ . . . . . _ _ _ . _ . _ _ _
. DOCUVIENT .
~
PAGE . f PU _ LED g l ANO.9 - NO. OF PAGES h . D PAGE EEG32 D w ucor<rav m. m cv ot e D E11ER cop (REoVE51En ON - 1 3 ElbGnooiAnstmau g = > c o ,< , u .
=a. Sb .
I k' LWD ON APERidRI CARD ND 3,W\%. o%% 0% m a se m n-YS
-..._._.. _ _ _}}