ML18219A545

Geologic Investigation of the Gillespie Dam Alternate Siting Area, Arizona Nuclear Power Project, Volume Ii.

ML18219A545
Person / Time
Site:	Palo Verde
Issue date:	08/20/1975
From:	Fugro
To:	Office of Nuclear Reactor Regulation, NUS Corp
References
73-080-EG
Download: ML18219A545 (346)

Text

GEOLOGIC INVESTIGATION OF THE GILLESPIE DAM ALTERNATE SITING AREA ARIZONA NUCLEAR POWER PROJECT VOLUME II Conducted for:

NUS Corporation 14011 Ventura Boulevard Sherman Oaks, California Project No. 73-080-EG August 20, 1975

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CONTENTS

~Pa e VOLUME I 2.5 GEOLOGY 2.5.1 INTRODUCTXON.

2.5.2 ALTERNATE SITING AREA GEOLOGY (5-mile radius) 2.5.2.1 Physiography.

2.5.2.2 Stratigraphy. 10 2.5.2.3 Structure 27 2.5.2.4 Geologic History. 46 2.5.2.5 Engineering Geologic Evaluation of Features Which Could Affect Category I Structures.

2.5.3 GROUNDWATER 55 2.5.4 GEOPHYSXCAL SURVEYS 55 2.

5.5 REFERENCES

CXTED. 56 APPENDIX 2A RADXOMETRIC DATING OF TERTIARY ROCK UNITS IN AND AROUND THE GILLESPIE DAM ALTERNATE SITING AREA APPENDIX 2B DRILLING PROGRAM AT GILLESPIE DAM ALTERNATE SXTXNG AREA APPENDIX 2C DOWNHOLE GEOPHYSICAL INVESTIGATIONS AT GILLESPIE DAM ALTERNATE SITING AREA APPENDIX 2D INVESTXGATXON OF A POTENTIAL SITE ON THE NORTHWEST SIDE OF THE GILLESPIE BASALT FLOW APPENDIX 2E GEOMORPHOLOGICAL INVESTIGATIONS IN THE GILLESPXE DAM ALTERNATE SITING AREA VOLUME IX APPENDIX 2F TRENCHING PROGRAM, GILLESPIE DAM ALTERNATE SXTING AREA APPENDIX 2G SHALLOW REFRACTI ON SEI SMXC SURVEYS g G ILLESPXE DAM ALTERNATE SITXNG AREA fuaao

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VOLUME II (continued)

APPENDIX 2H GROUND MAGNETIC SURVEY OF ENTERRADO FAULT, GILLESPIE DAM ALTERNATE SITING AREA APPENDIX 2I JOINT TREND ANALYSIS IN THE GILLESPIE DAM ALTERNATE SITING AREA APPENDIX 2J HYDROLOGIC DATA FROM THE GILLESPIE DAM ALTERNATE I+

SITING AREA APPENDIX 2K .ENGINEERING TEST DATA, GILLESPIE DAM ALTERNATE SITING AREA

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TABLES APPENDIX 2A Table 2A-1 Radiometric Dates from Rock Units in the Gillespie Dam Alternate Siting Area Table 2A-.,2 Pertinent Radiometric Dates from Rock Units C

Outside the Gillespie Dam Alternate Siting Area APPENDIX 2J Table 2J-1 Water Levels in Drill Holes, Gillespie Dam

. Alternate Siting Area, 1973-1974 APPENDIX 2K Table 2K-1 Rock Density and Specific Gravity of Some Bedrock and Basement Geologic Units Table 2K-2 Shear Modulus from Shallow Refraction Seismic Surveys Table 2K-3 In Situ Moisture Content, Dry Density and Unified Soil Classification:of Samples from Engineering Test Pits Table 2K-4 Summary of Compaction Test Results for Samples from Engineering Test Pits fuaao

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FIGURES 2.5 GEOLOGY Figure 2.5-1 Location of Gillespie Dam Alternate Siting Area Figure 2.5-2 Generalized Stratigraphic Column, Gillespie Dam Alternate Siting Area Figure 2.5-3 Photolineaments in the Gillespie Dam Alternate Siting Area, Figure 2.5-4 Major Photolineaments Crossing Gillespie Dam Alternate Siting Area Figure 2.5-5 Location of Engineering Seismic Lines and Test Pits APPENDIX 2A Figure 2A-1 Location of Radiometric Dating Samples APPENDIX 2B Figure 2B-1 Location of Drill Holes and Drill Hole Cross Sections Figure 2B-2 Condensed Drill Hole Lithologic Log, GDDH-2 Figure 2B-3 Condensed Drill Hole Lithologic Log, GDDH-3 Figure 2B-4 Condensed Drill Hole Lithologic Log, GDDH-4 Figure 2B-5 Condensed Drill Hole Lithologic Log, GDDH-5 Figure 2B-6 Condensed Drill Hole Lithologic Log, GDDH-6 Figure 2B-7 Condensed Drill Hole Lithologic Log, GDDH-7 Figure 2B-8 Condensed Drill Hole Lithologic Log, GDDH-8 Figure 2B-9 Condensed Drill Hole Lithologic Log, GDDH-9 Figure 2B-10 Condensed Drill Hole Lithologic Log, GDDH-10 Figure 2B-ll Condensed Drill Hole Lithologic Log, GDDH-12 Figure 2B-12 Condensed Drill Hole Lithologic Log, GDDH-13 Figure 2B-13 Condensed Drill Hole Lithologic Log, GDDH-14

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FIGURES (continued)

Figure 2B-14 Condensed Drz,ll Hole Lithologic Log, GDDH-15 Figure 2B-15 Condensed Drill Hole Lithologic Log, GDDH-16 Figure 2B-16 Condensed Drj.ll Hole Lithologic Log, GDDH>>17 Figure 2B-17 Condensed Drill Hole Lithologic Log, GDDH-18 Figure 2B-18 Condensed Drill Hole Lithologic Log, GDDH-19 Figure 2B-19 Condensed Drill Hole Lithologic Log, GDDH-20 Figure 2B-20 Condensed Drill Hole Lithologic Log, GDDH-21 Figure 2B-21 Condensed Drill Hole Lithologic Log, GDDH-22 Figure 2B-22 Condensed Drill Hole Lithologic Log, GDDH-23 Figure 2B-23 Condensed Drill Hole Lithologic Log, GDDH-24 Figure 2B-24 Drill Hole Cross-Section, GDDH-6 to GDDH-8 Figure 2B-25 Drill Hole Cross-Section, GDDH-12 to GDDH-13 Figure 2B-26 Drill Hole Cross-Section, GDDH-5 to GDDH-19 Figure 2B-27 Drill Hole Cross-Section, GDDH-19 to GDDH-23 APPENDIX 2C Figure 2C-1 Geophysical Drill Hole Logs, GDDH-2 Figure 2C-2 Geophysical Drill Hole Logs, GDDH-3 Figure 2C-3 Geophysical Drill Hole Logs, GDDH-4 Figure 2C-4 Geophysical Drill Hole Logs, GDDH-5 Figure 2C-5 Geophysical Drill Hole Logs, GDDH-6 Figure 2C-6 Geophysical Drill Hole Logs, GDDH-7 Figure 2C-7 Geophysical Drill Hole Logs, GDDH-8 Figure 2C-8 Geophysical Drill Hole Logs, GDDH-9 Figure 2C-9 Geophysical Drill Hole Logs, GDDH-10 Figure 2C=10 Geophysical Drill Hole Logs, GDDH-12

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FIGURES (continued)

Figure 2C-ll Geophysical Drill Hole Logs, GDDH-13 Figure 2C-12 Geophysical Drill Hole Logs, GDDH-14 Figure 2C-13 Geophysical Drill Hole Logs, GDDH-15 Figure 2C-14 Geophysical Drill Hole Logs, GDDH-16 Figure 2C-15 Geophysical Drill Hole Logs, GDDH-17 Figure 2C-16 Geophysical Drill Hole Logs, GDDH-18 APPENDIX 2D Figure 2D-1 Location of Drill Holes and Geophysical Survey Lines in Proposed Site Area Figure 2D-2 Gravity Map of Proposed Site Area Figure 2D-3 Basal Elevation of Gillespie Basalt in Proposed Site Area Figure 2D-4 Isopach Map of Gillespie Basalt in Proposed Site Area APPENDIX 2E Figure 2E-1 Location of Geomorphic Profiles on Alluvial Fans Figure 2E-2 Geomorphic Profiles, A-A'nd Figure 2E-3 Profiles, B-B" and B-B'eomorphic Profiles, C-C'nd B-Beomorphic Figure 2E-4 of Gila River Terraces and Major D-C'ocation Figure 2E-5 Photolineaments Figure 2E-6. Surveyed Terrace Profile Between Arlington and Gillespie Basalt Flows Figure 2E-7 Flood Plain and Terrace Profiles Between Gillespie and Sentinel Basalt Flows APPENDIX 2F Figure 2F-1 Location of Trenches and Faults Figure 2F-2 Detailed Trench and Fault Maps

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FIGURES (continued)

Figure 2F-3 Trench Log, GDDT-1 Figure 2F-4 Trench Log, GDDT-3 Figure 2F-5 Trench Log, GDDT-4 Figure 2F-6 Trench Log, GDDT-6a Figure 2F-7 Trench Logy, GDDT 6b Figure 2F-8 Trench Log, GDDT-8 Figure 2F-9 Trench Log, GDDT-9a Figure 2F-10 Trench Log, GDDT-9b, NW Wall Figure 2F-ll Trench Log, GDDT-9b, SE Wall Figure 2F-12 Rrench Log, GDBT-1 Figure 2F-13 Trench Log, GDBT-2 Figure 2F-14 Trench Log, GDBT-3 Figure 2F-15 Trench Log, GDBT-4 Figure 2F-16 Trench Log, GDBT-5 Figure 2F-17 Trench Log, GDBT-6 Figure 2F-18 Trench Log, GDBT-7 Figure 2F-19 Trench Log, GDBT-8 Figure 2F-20 Trench Log, GDBT-9 Figure 2F-21 Trench Log, GDBT-10 I Figure 2F-22 Trench Log, GDBT-ll Figure 2F-23 Trench Log, GDBT-12 Figure 2F-24 Trench Log, GDBT-13 Figure 2F-25 Trench Log, GDBT-14 Figure 2F-26 Trench Log, GDBT-16 Figure 2F-27 Trench Log, GDBT-17 Figure 2F-28 Trench Log, GDBT-18

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FIGURES (continued)

Figure 2F-29 Trench Log, GDBT-20 Figure 2F-30 Trench Log, GDBT-21 Figure 2F-31 Trench Log, GDBT-22 Figure 2F-32 Trench Log, GDBT-23 Figure 2F-33 Trench Log, GDBT-24 Figure 2F-34 Trench Log, GDBT-25a Figure 2F-35 Trench Log, GDBT-25b Figure 2F-36 Trench Log, GDBT-26 Figure 2F-37 Trench Log, GDBT-27a Figure 2F-38 Trench Log, GDBT-27b Figure 2F-39 Trench Log, GDBT-27c Figure 2F-40 Trench Log, GDBT-29.-

Figure 2F-41 Trench Log, GDBT-30 Figure 2F-42 Trench Log, GDBT-31 Figure 2F-43 Trench Log, GDBT-32 Figure 2F-44 Trench Log, GDBT-33 Figure 2F-45 Trench Log, GDBT-34 Figure 2F-46 Trench Log, GDBT-35 APPENDIX 2G Figure 2G-1 Seismic Line Locations Figure 2G-2 Seismic Profile, GDSL-1 Figure 2G-3 Seismic Profile, GDSL-2 Figure 2G-4 Seismic Profile, GDSL-3 Figure 2G-5 Seismic Profile, GDSL-4 Figure 2G-6 Seismic Profile, GDSL-5 Figure 2G-7 Seismic Profile, GDSL-6

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FIGURES (continued)

Figure 2G-8 Seismic Profile, GDSL-7 Figure 2G-9 Seismic Profile, GDSL-8 Figure 2G-10 Seismic Profile, GDESL-1 Figure 2G>>,,gl Seismic Profile, GDESL-2 Figure 2G-12 Seismic Profile, GDESL-3 Figure 2G-13 Seismic Profile, GDESL-4 Figure 2G-14 Seismic Profile, GDESL-6 APPENDIX 2H Figure 2H-1 Ground Magnetic Survey Line Locations APPENDIX 2I Figure 2I-1 Location of. Joint Analysis Field Stations Figure 2I-2 Joint Trends at Joint Analysis Field Stations GDJA-1 and GDJA-2 Figure 2I-3 Joint Trends at Joint Analysis Field Stations GDJA-3 and GDJA-4 Figure 2I-4 Joint. Trends at Joint Analysis Field Stations GDJA-5 and GDJA-6 Figure 2I-5 Joint Trends at Joint Analysis Field Stations GDJA-7 and GDJA-8 Figure 2I-6 Joint. Trends at Joint Analysis Field Stations GDJA-9 and GDJA-10 APPENDIX 2K Figure 2K-1 Cumulative Grain Size Curve, GDEP-1 Figure 2K-2 Cumulative Grain Size Curve, GDEP-2 Figure 2K-3 Cumulative Grain Size Curve, GDEP-3 Figure 2K-4 Cumulative Grain Size Curve, GDEP-4 Figure 2K-5 Cumulative Grain Size Curve, GDEP-6

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PLATES 2.5 GEOLOGY Plate 2. 5-1 Geologic Map and Cross-Section APPENDIX 2H-1 2'late Ground Magnetic Intensity Profiles fuaaa

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APPENDIX 2F Trenchin Pro ram Gilles ie Dam Alternate Satin Area

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2F-l APPENDIX 2F Trenchin Pro ram,'illes ie Dam Alternate .Siting Area A. Introduction A trenching program consisting of nine bulldozer trenches and thirty-five backhoe trenches was used to investigate photolineaments and faults. Bulldozer or large track-mounted front-end loaders were used to cut trenches where the rock was too hard or the terrane too steep for backhoes. Many trenches in bedrock were excavated by a large track-mounted cable backhoe. The locations of trenches and the features they were designated to investigate are shown on Figures 2F-l and 2F-2. A log of each trench is given Figures 2F-3 through 2F-46.

B. 'rench logging methods Three methods were used to log trenches after they were excavated and their walls shored and cleaned. A detailed graphical log was obtained by fastening markers to the trench wall at five-foot intervals along leveled string datum lines. Every contact or structure in the trench wall was measured to the nearest tenth of a foot relative to the datum line and markers and then drawn on graph paper; The second method, photologging, was often used on bulldozer trench walls which could not be shored and were too high for detailed .logging. Leveled datum fuIIaII

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lines and five-foot markers {occasionally with vertial lines) were again used for reference. Polaroid photos were taken at each five-foot interval from a constant distance and angle to produce photos with a constant scale at the level line. The photos were mosaiced .and geologic units and structures drawn on the mosaic or a transparent overlay.

When a trench showed no significant features, it was sketchlogged after its length and depth were determined.

C. Results The purpose and results of most of the trenches are given in Section 2.5.1.2.3 and will not be repeated here. Trench GDBT-15 was a small, insignificant test pit which was left unlogged. Trench GDBT-19 was incor-porated into trench GDBT-21. Trench GDBT-28 was never used to designate a trench. Trench GDBT-7 was rendered unnecessary by trenches GDBT-4 and 5.

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APPENDIX 2G Shallow Refraction Seismic Surve Gilles ie Dam Alternate Sitin Area

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2G-1 APPENDIX 2G Shallow Refraction Seismic Survey Gilles ie Dam Alternate Siting Area A. Introduction Thirteen shallow refraction seismic surveys were con-ducted as" part of the alternate siting area investigation.

Eight of the surveys were designed to identify possible subsurface features associated with lineaments or buried faults; the other five were designed to give subsurface soils engineering data. Figure 2G-1 shows the seismic line locations.

B. Method A sledge hammer was used to provide seismic energy and a single'channel seismograph was used to collect the seismic data. Only compression wave velocities were collected for the geologic seismic surveys, but both compression wave and shear wave velocities were collected for engineering seismic surveys.

C., Results Seismic lines GDSL-1 and 2 (Figs. 2G-2, 2G-3) were used to investigate two photolineations trending west. off the southwest flank of the Gillespie basalt flow. Neither seismic line shows any significant anomalies at the intersection with the lineaments.

Seismic line GDSL-3 and GDSL-4 (Figs. 2G-4, 2G-5) were designated to investigate the contact relationships fuaaa

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2G-2 between metamorphic rocks north of Windmill Wash and volcanic rocks to the south. No stratigraphic or structural relationships could be confidently inferred from the results.

Seismic lines GDSL-5 and GDSL-6 (Figs. 2G-6, 2G-7) were located to investigate the eastward projection of the photolineation observed in granitic hills to the west. Neither line revealed any significant- structure near the point where they crossed the lineament pro-jection.

Seismic line GDSL-7 (Fig. 2G-8) was used to investigate a buried structure (the Enterrado fault) between drill holes GDDH-16 and -18. The results tentatively suggested a fault structure about 150 feet northwest of GDDH-18.

Seismic line GDSL-8 (Fig. 2G-9) was located to investi-gate a possible northwest, projection of the Escondido fault, but the results were inconclusive.

The shear modulus of shallow soils along the engineering seismic lines was calculated from the compression and shear wave velocities (Appendix 2K). The shear modulus of soils about two feet in depth are characteristic of low density unconsolidated soils, while those from depths of gour to ten feet apparently represent more dense and cemented soils.

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D. Discussion The results og the eight geologic seismic surveys were generally inconclusive. Probably the two most important reasons for this were: (1) the alluvial fan deposits contain many high velocity caliche horizons, which mask and confuse results from deeper strata, and (2) the sledge hammer usually did not provide enough energy to penetrate the alluvial fan deposits and underlying rock deeply enough to clearly identify geologic structures.

However, most of the photolineations and buried structures were also investigated by other methods which identified the nature of these geologic features.

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Arizona Nuclear Power Project Gillcspie Datn

, Alternate Siting Area SCALE SEISMIC LINE LOCATIONS 1000 0 '000 Figure 2G-l AttPP-7 5-383'<

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APPENDIX 2H Ground Ma netic Survey of Enterrado Fault Gilles ie Dam Alternate Satin Area

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2H-1 APPENDIX 2H Ground Ma netic Survey of Enterrado Fault Gz.llespz.e Dam Alternate Sate.ng Area r

A. Introduction As discussed i.n Appendix 2B, a structural-lithologic

. discontinuity was observed between drill holes GDDH-6 and GDDH-8. Drill hole data from drill holes GDDH-16, 17 and 18, dug between the two older holes, suggested that the feature was a large, steeply dipping fault located between drill holes GDDH-16 and GDDH-18 (Appendix 2B, Fig. 2B-25). Shallow refraction seismic line GDSL-7 (Appendix 2G) was run between drill holes GDDH-6 and GDDH-16. Its results were largely inconclusive, but suggested a lithologic discontinuity about 150 feet northwest of GDDH-18. The inferred fault is named the Enterrado fault because it is buried by undisturbed old (pre-Gillespie basalt) alluvial fan deposits.

B. Procedure After laboratory tests indicated that sufficient con-trast in magnetic susceptibility existed between units on opposite sides of the Enterrado fault, an initial ground magnetic survey of two lines was run by the J. W. Cooksley Company. Both lines showed recogniz-able response at the anticipated fault location, so nine additional lines were surveyed (j'ig. 2H-1).

Readings were usually taken at 25-foot intervals along funaa

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2H-2 the eleven lines, giVing about 24,100 line feet of coverage.

C. Results The total field intensity ground magnetic survey successfully traced the Enterrado fault about 3,500 feet along its strike. The reduced and corrected data were plotted in profile (Plate 2.5-2) to facilitate areal correlation. The indicated fault location is probably accurate within + 50 100 feet.

D. Discussion The Enterrado fault extends at least 1,500 feet north-east of Line 0+00 to Line 15+00 NE and about 2,000 feet southwest to Line 20+00 SW (Plate 2.5-2). Between Lines 20+00 SW and 5+00 NE, the fault strikes about N65 E but between Lines 5+00 NE and 15+00 NE, the strike

', apparently changes to N60 E. The fault apparently dips northwest at an angle of 60 or more. Northeast of Line 15+00 NE the magnetic data are inadequate to determine the occurrence of the fault.

The peak magnetic amplitudes are broader'between Lines 5+00 NE and 5+00 SW than they are further southwest.

This suggests that the alkali basalt section on the southeast side of the fault becomes more dike-like to the southwest,

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2H-3 The magnetic data imply that, another structure inter-sects the Enterrado fault at nearly right angles between Lines 20+00 $ W and 25+00 SW. The magnetic response at station 4+75 NW, Line 25+00 SW, may repre-sent a segment of the Enterrado fault which has experienced as much as 750 feet of apparent right-lateral dis-placement along the intersecting structure.

The location and trend of the intersecting structure is approximately on the projection of the Turban lineament (Fig. 2.5-3) which trends north-northwest through the

,Gila Bend Mountains and is associated with several faults; it is also on a possible northward projection of the Escondido fault (Plate 2.5-1). The 750 feet of apparent right-lateral displacement aligns the displaced segment of the Enterrado fault with a fault which trends N70 0 W and juxtaposes granitic basement against alkali basalt south of Windmill Wash. This fault and the Enterrado fault may be segments of an originally continuous fault which were displaced by a younger intersecting fault (Plate 2.5-1).

About 1,600 feet of vertical displacement, east side down, along the intersecting structure would produce 750 feet, of apparent right-lateral displacement if the Enterrado fault dips 65 tc the northwest, This is about twice the minimum vertical displacement inferred for the Enterrado fault and several times

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2H-4 that inferred for the Escondido fault.

Trench GDBT-34 (Appendix 2F, Fig. 2F-1) was dug across a nearby fault (possibly a northernmost exposure of the Escondido fault) whichis on the projection of both the intersecting structure and the Turban lineament.

Trench GDBT-35 (Appendix 2F, Figs. 2F-l, 2F-47) was dug across the Enterrado fault between drill holes

,GDDH-16 and GDDH-18. Both trenches exposed alluvial.

fan deposits older than the Gillespie basalt (potassium-argon dated at about 3.3 million years) lying undis-turbed across the faults.

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APPENDIX 2I Joint Trend Analysis in the Gillespie Dam Alternate Siting Area

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QPPHNDQX 2I Jo'j,"nt Tren'dA'n'a'1:si's'n'he

'i'lies 'ie Dean 5:1'te'r'n'ate Sitj.n 'rea A. Introduction Joint trend analysis can yield evidence of the nature of stresses to which rock units have been subjected.

I Because the nature of these stresses and the geologic structures they produce vary during geologic time, comparison of local joint systems with regional joint

'trends of known age can help establish the geologic interval during which the joints were formed. Because other geologic structures often parallel important.

joint sets, recognition of important joint trends can establish likely trends of other geologic structres.

B. Method The stations at. which joint trends were studied (Fig.

2I-1) were selected on the basis of good exposure of important rock types, lack of tilting, and accessibility.

Joint trends were studied in the following rock units:

(1) metasedimentary unit, (2) gneissic granitic rock, (3) granitic rock, (4) older alkali basalt, and (5) Gillespie basalt. The trends of about 100 randomly selected joints were recorded at each station and transferred to rose diagrams (figs. 2I-2 through 2I-6).

C. Results The metasedimentary rocks which compose the oldest fuuao

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geologic unit in the alternate siting area have a domin-ant joint trend of about N40 W (Fig. 2I-2) in addition to another joint set (not shown) which strikes roughly parallel to the variable trend of the metamorphic foliation.

Gneissic granitic basement rock has a dominant joint set trending about N55 W, plus a variety of subordinate joint sets with trends ranging from west to west-northwest and east to east-northeast (Fig. 2I-2).

Granitic basement rock show a dominant joint set trending N20-35 W with a secondary set trending north-northeast and other minor sets, including one trending east-west (Figs. 2I-3, 2I-4).

The older alkali basalt unit has two strong joint sets, one trending Nl5-35 W and the other trending N70 E to S85oE, and various minor sets with northeast trends (Fig. 2I-5).

The Gillespie basalt has subdominant joint sets trending about N30 W and northeast with a minor set trending east-west (Fig. 2I-7).

D.: Discussion All rock units older than the Gillespie basalt (potassium-argon dated at about 3,3 million years) show a dominant northwest joint trend with secondary northeast and

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2r-3 east-west joint trends. Rehrj.g and Heidrick (1972) indicate that joints in Lagamide granitic stocks in the Basin and Range province of Ar'izona have dominant trends of NNW + 20o and ENE + 20 and a minor trend of east-west. Thus, the dominant joint patterns in Precambrian (?) granitic basement rock in the siting area probably were developed during Laramide tectonism.

Joints developed in the older alkali basalt bedrock may have been controlled by the older Laramide structures.

The dominant northwest trends in Precambrian gneissic granite and metasedimentary rocks are more westerly than those in unfoliated granitic rocks and older alkali basalt and may have originated during Precambrian time. The joint pattern in the Gillespie basalt is dominated by columnar jointing resulting from shrinkage during cooling.

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LO lo go 20 8 70 60 50 40 30 20 IO 0 10 20 30 40 50 60 70 80 Percent of joints B. Granitic basement (GDJA-4)

Lo ca t i on shown on Arizona Nuclear Power Project Figure 2 I-I Gillespie Dam Alternate Siting Area JOINT TRENDS AT JOINT ANALYSIS FIELD STATIONS GDJA-3 AND 'GDJA-4 Figure 2I-3 ANPP-7 S-3842

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Location shown on Arizona Nuclear Power Project Figure 2 I-i Gillespie Dam Alternate Siting. Area JOINT TRENDS AT JOINT ANALYSIS FIELD STATIONS GDJA-5 AND GDJA-6 Figure 2I-4 ANPP-75-3844

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~eo 70 60 50 40 30 20 lo 0 lo 20 30 40 50 60 70 80 Percent of joints B. Gillespie basalt (GDJA-IO)

Location shown on Arizona Nuclear Power Project Figur e 2I-I i4'..

Gillespie Dam Alternate Siting Area JOINT TRENDS AT JOINT ANALYSIS FIELD STATIONS GDJA-9 AND GDJA-10 Figure 2I-6 AHPP 78-8848

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APPENDIX 2J H drolo ic Data from the Gilles ie Dam Altern'at'e Sz.tan Area

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APPENDXX 2J H drolo ic Data from the Gilles ie Dam lternate Sitin Area A. Introduction Hydrologic data were gathered on a fairly regular basis from holes drilled prior to September, 1973.

Except for data gathered during Decembers of 1973 and 1974, collection of further data and data analysis was deferred by common agreement with ANPP, so work could be concentrated on the Palo Verde Nuclear Generating Station.

B. Method and Results Water levels were collected from siting area drill holes with a double wire electric well sounder. The results are shown in Table 2J-1.

C. Discussion Water levels measured in the alternate siting area are highly variable within and between holes. Many water level readings were made within a few weeks or months after the holes were drilled and before the levels stabilized. The water level in many earlier

.holes (GDDH-2 through GDDH-9) is relatively constant

-for the last two readings.

For the last two readings all water levels occur within bedrock except in drill holes'DDH 2~ GDDH-22, and

-GDDH-23. The variation between water levels in

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2J-2 adjacent holes may suggest lithologic or structural discontinuities which result in lack of integration of the groundwater'ystem.

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TABLE 2J-1 Water Levels in Drill Holes, Gillespie Dam Alternate Siting Area, 1973-1974 Drill Holes Collar Drill Hole Elevation 6/23/73 8/5/73 8/25-30/73 9/6/73 9/14/73 12/6/73 12/6/74 GDDH-2 948 875.4 864 878.6 877.3 875.3 870 869.4 GDDH-3 957 910.5 905.4 905.5 903 901 GDDH-4 994 848. 9 829.3 793.5 850.5 818.7 787 778.4 GDDH-5 955 877.6 866.0 875.9 877 872.9 GDDH-6 987 934.6 906.1 880 Caved IkDDH-7 1021 980 920 (Dry) Dry Dry Dry Dry b

CDDH-8 1014 887 885 887 885 883 GDDH-9 1074 951. 8 923.3 922.3 923.5 920 921.9 GDDH-10 1069 992 989.6 GDDH-12 964 850 844.7 GDDH-13 1049 907.1 GDDH-14 888 Dry Dry GDDH-15 942 718 710.6 GDDH-16 1006 974 876.7 GDDH-17 1001 970 959.1 GDDH-18 989 879 Destroyed

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TABLE 2 J-1 (Continued)

Water Levels in Drill Holes, Gillespie Dam Alternate Siting Area, 1973-1974 Drill Hole Collar Drili Hole Elevation .6/23/73 8/5/73 8/25-30/73 9/6/73 9/14/73 12/6/73 12/6/74 954 784 (Mud)

GDDH-19'DDH-20 971 773 GDDH-21 943 733 (Mud)

GDDH-22 983 843.9 GDDH-23" 965 921.9 QGDDH-24 947 827 (Dry) a

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APPENDIX 2K Engineerin Test Data Gilles ie Dam Alternate Sitin Area

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2K-1 APPENDIX 2K En ineerin Test Da'ta Gillese Dam A'lternate'it'ing Area A. Introduction The poorly sorted alluvial fan deposits which form most of the soils in the alternate siting area include abundant gravel and numerous boulders. These large clasts prevented collection of undisturbed drive and Pitcher samples from drill holes so a detailed laboratory investigation was not. possible. A few preliminary engineering data are presented herein; should the Gillespie Dam area warrant. further investigation in the future, more complete data can be generated.

B. Results The specific gravity and density of some of the bedrock and basement geologic units were determined from NC drill core samples (Table 2K-1). The data are generally typical of the sampled rock types. The density variation in the mixed pebble breccia unit is the result of compositional variation; in GDDH-19 the unit is composed primarily of volcanic detritus while in GDDH-4 it is composed primarily of metasedimentary debris.

The shear modulus of shallow soils was calculated from shear and compression wave veloce,ties determined by five shallow refraction seismic surveys (Table 2K-2).

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Engineering tests on samples collected from backhoe pits along the same five seismic lines include in situ moisture content, dry density (Table 2K-3, compaction (Table 2K-4) and sieve analysis (Figs. 2K-l through 2K-5). The results of these tests were utilized to determine the Unified Soil Classification of soils from each test pit (Table 2K-3, Figs. 2K-l through 2K-6).

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2K-3 TABLE 2K-1 Rock Density and Specific Gravity of Some Bedrock and Basement Geologic Units Drill Dry Specific Hole No. Depth Stratigraphic Unit Density (pcf) Gravity GDDH-4 90- 92 Mixed pebble breccia 127.3 2 '1 (poorly lithified)

GDDH-4 289-291 Mixed pebble breccia 129.3 (well lithified)

GDDH-6 222-223 Old alkali basalt 171.7 2.88 GDDH-6 317-318 Tuffaceous sandstone 122.3 (fine-grained)

GDDH-6 334-336 Tuf faceous sandstone 150. 6 2. 72 (gravelly)

GDDH-6 374-376 Granitic basement 164.2 2.75 GDDH-18 273-275 Granitic basement 165.3 2.71 GDDH-19 72- 74 Gillespie basalt 174.3 2.99 GDDH-19 134-136 Mixed pebble breccia 138.3 (poorly lithified)

GDDH-19 178-179 Mixed pebble breccia 135.6 2.73 (well lithified)

GDDH-21 12- 13 Gillespie basalt 167. 3 3. 00 Note: All core specimens were oven dried 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> prior to testing.

Drill hole locations shown on Fig. 2.5-5.

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2K-4 TABLE 2K- 2 Shear Modulus from Shallow Refraction Seismic Surveys Seismic Line Depth Shear Modulus (psi)

GDESL-1 21 4,320 GDESL-1 10' 23,330 GDESL-2 I 9,820 GDESL-2 87,460 GDESL-3 100 7,500 GDESL-3 70,660 GDESL-4 9,440 GDESL-4 I 8,650 GDESL-6 2 I 12,030 GDESL-6 7 I 49,080 Note: For location of seismic lines refer to Fig. 2.5-5.

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2K-5 TABLE 2K-3 In Situ Moisture Content, Dry Density and Unified Soil Classification of Samples from Engineering Test Pits Unified Soil Classification of Sample Dry Density Moisture Combined Samples Location Depth (pcf ) Content from each Test Pit GDEP-1 2 I 2.5 SP-SC GDEP-1 115.1 4.0 100 GDEP-2 123.3 2.4 SM-SC GDEP-2 121.9 2.0 GDEP-3 2 I 91.9 5.0 SC GDEP-3 10' 105.5 7.3 GDEP-4 I 97.3 6.3 SC GDEP-4 4 I 76.4 11. 0 GDEP-6 2' 110.3 3.1 SC GDEP-6 I 137.7 3.1 Note: Engineering test pit locations shown on Fig. 2.5-5.

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2K-6 TABLE 2K-4 Summary of Compaction Test Results for Samples from Engineering Test Pits Optimum Optimum Sample Moisture Content Dry Density Location Depth I g) (pcf )

GDEP-1 0-10 128. 0 GDEP-2

'-10 7.5 136.0 GDEP-3 GDEP-4

'-4'-7'.0

'-10 11.

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124.0 115.0 GDEP-6 7.0 136.0 The above tests were conducted in accordance with ASTM Standard D-1557 Method D.

Engineering test pit locations shown on Fig. 2.5-5.

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.a.a I 0 oe ot ! ""! 0 I C I . ~ I' CtO " '1 '04 0 4 C 4 4 I C. I O 0 H .001 .005 .01 .050.1 0.5 1.0 50 10 SO r- GRAIN-SIZE IN MILLI METERS D2 A GRAVEL UQ O SILT OR CLAY SAND DI FINE MEDIUM COARSE FINE COARSE I H hJ R CD SYMBOL PIT SAMPLE SAMPLE L I QUID PLASTIC PLASTICITY SOIL M I H cn-~ NUMBER NUMBER INTERVAL LIMIT LIMIT INDEX TYPE td .1&2 2-10 SM-SC O 23?I 'll I CD Cil I O Ctt CD Cil CD I I I lj ~ ~ ~ l' HYDROMETER U.S. STANDARD SIEVE NUM8ER STANDARD SIEVE OPENING ~ I I~ ~~ 200 100 60 40 20 10 4 /8 /4 I I/2 3 100 W4t 4 4 '4 0 454 Wt PS J 5 tlti 55 OIC1 I ~ < ~ si '" I+0 I tie W 1 + '.S-S I i CJ ii 5 i 5 cc iWW 90 ~4 I:JCCC :5 a~ ;a 4t 'tJW<t '+. . hi t S ~ C 4 4 e Itli 0" CSSj wa<<apTi>> I ht I 5 i Fo-iiaij :;jj< '7 f W4 4 4 ~ 80 4 I ISCJC Pi.i.W i 5 I 4~ 5 a wa+ tt owwe e* '00 .ao 44 j 4 w" <I w'eir at 445 je:4 iehiI 4 i'd 55 4 iaiii ~ 5 4 ~ i<i 5.5 SJP i"I".' I al 4 t t e I\ I 44 4 je W iC ~ i.,c-.:+I+'. 5 Ol +0 +I' i 70 1"-:ita 4 .s' C. I Sii oe 04 <P4 4 04 Wt tthj'i4 4 W< 4 et Jjii C 4 4 4 <Si twj[wo ttt I 1, . Iih.i 5 j I 1 .e . '. '. 60 't PP.th 0 .i.iiiii t 4 ~ 00 4$ tiW I toe< I0 0 c. 4 ~ poe ~ I 5 SC iii 4 II 5 lP"::;:",.: I oeoii. <<5 i Sa po +wi'1 4% ii t OW OW to 4 5- +4 tt Sl 04 ~ 4 tP it ~ 50 t4'4 W 50 4 W ~c 4 a 4'4<4 JPi t OOW i:5 . i I "5 i I I'X'< i I t'4' tt h " .I hPJ 4 ttFO 4 4< I 4, I I jjcosc I tw 4 4 '.'TT e<o i~ii CSC 55 ~ 45<We itj Cii i4. C44. 4 + e~ i. + 1 I 40 0 . 4041 i4 4t ~ a OJ 4 I +J 4 55 I B-- -'-:.::: w st .55 <4 ,at tit 14 ,a ' T': ~ e~ I ,I 1,1 t I. jI ,I 4 "-"Si A E5 aiot 4 L1 t.' 1 4444< i<<<i<<Pa e: ~ ~ 4 at4 + WP, I 30 ~ 5<h" '4' t4t I I OW " +0 0 4 54 i 0" I 01 0 0 I i:," il, I I~ iiiei ~ .Jj C Ii 5 5 054 wetpia I <> 4 4< 5 ~ I Ia .I. ',' J."5 'I'0 C Si i4a '<'t JP 4 20 oaip Caw 4 Ta +  :" ~ 'I " j 4" i "I t'0 C I I .Pj ~ 4t 4 ~ ~ 1 ~ ih ieae '5 haoi ~ I< I.hc 1 tat 'I ' a i <+ rtl 4 ea 4 n .,':5<h<JJ i i<o 4 '5 Jh 4 h 4 "t tI 5. 0 4 ~.P'! C C 10 I' II httl -', ~ .'- I I-ittt'i 44t I4 lh I Ja i. J. J lc -2Pw cco .a I ~ os'J eii;iiiPi ~ 5". I 5 ' o 4 <k 4C I i'5 I i.l..iJ" i.,C <t 4 4JP 41 4 I-c iPWj "WPaaf g C 5 C 0 .001 005 .01 .05 0.1 0.5 'I.O 50 IO 50 H GRAIN-SIZE IN MILLIMETERS I td A N SAND GRAVEL U Q 0 p SILT OR CLAY FINE MEDIUM COARSE FINE COARSE h) I ICJJ H R ~~ n~ o PIT SAMPLE SAMPLE LIQUID PLASTIC PLASTICITY SOIL I D1 ~ SYMBOL NUMBER NUMBER INTERVAL LIMIT LIMIT INDEX TYPE lA H cn Ld 152 2-10 SC 'hS n I 555 A G i aa o EP 1 o I ~ ~ gi lil 5i i t ~ i' ~ } ~ l 5 )i m m m m M w m m m m M m HYDROMETER U.S. STANDARD SIEVE NUMBER STANDARD SIEVE OPENING ~ I I~ 'i4 I 200 100 40 40 20 10 4 ~B 100 aa t>I af <<0!I'ec:, 'C t it I I e- <<j + ~ " >11 "I ~0 1

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~ << 04 W t Z I 04.j 4 ii  ; icij<<" La.. 'I . 4 4 4 04 ej 0 ot I O ~ ~04 00 ff I ii 04 0 I f. C IX te jjj 30 4> 4 ' ait I ~ 4 i4 00 W+~ fe. ac I ~ W~ W 0 ~ Wi C 4 0 + Ci I 4 at Wt 0 waae 20 . I ott 0 aia I ~ oeoa 0 af ~" t I~ jii. I 0'. ~i-4 it W 4 0 3 0 II I 4ic i I olI ia ~ aw 4 4 4 ~C 4 + C C ICC 10 I I ooo 4tta n 4 ae C. W tt >> Ci tj4. 004 I ~ '. t tft 0 0 'I >4 >WOW -C . C ~ ~ itt >'CC%C W 4 I 0 Ã 0 + 0 '05 V .001 005 .01 0.1 0.5 1.0 50 10 50 GRAIN-SIZE IN MILLIMETE RS N SAND GRAVEL Q 0 S)LT OR CLAY FINE COARSE FINE MEDIUM COARSE t7 Q ~ n~ I' o o PIT SAMPLE SAMPLE LIQUID PLASTIC PLASTICITY SOIL wR SYMBOL NUMBER NUMBER INTERVAL LIMIT LIMIT INDEX TYPE . 8 2-4 SC O titan 2 o n 'tf C 0 o o I I I N % % W R % W H R % % N M % H W % W HYDROMETER U.S. STANDARD SIEVE NUM8ER STANDARD SIEVE OPENING ~ I cf ~~ 'l4 I 200 100 60 40 20 10 4 ~8 100 tf  : " ,:,Cii 1 Of ~ ' i\ttf 4 tei a .f'fT taI444 1 a tat ':+'f I t<< jii C.'", ~ ", 4 Iic. i4 4 4 Ot ,I I ';i:;j 4 4 4 ~ 4 i ~ I  :, I ' I I I c.ic a I i4.C 5"I . I Ff " IC'" e 90 . ~ I ' c' I!if!'Ili it 4, 4 Ci 4 aoI )) ~ 54 . I Sc 4 5 ~4 'ft't I tt e 44 I a -4 ' ~  ! ~ t <<II i.i 4! I54 'Ii ,.;tt!. 5 'I -I ' ~ 4C t tf 80 41 ca ))C -Cf I, t t<1<<<< .3 ,  ;: ia ";1:A + ela f'if +af . I )C Ci !i<<'<< I ~ Ct t tt4 I, ~4 4i4 I cac . I '4-43 4 at ~ 4 ~ I; 11 'r I'I ~ t 5 L'i! f 70 4<<, 44 Ktf 4 ~i'.34 Cjc 44 44 Cc C 5 5.1 x 0 i oaiI 44 - 11 . 4 44 4t 5 tt44 'iaaa 115).I fc\i 114 itt A-tafD ~ 4 I !1 4 4 t44t a t C 4 aaa 60 tt C 4'i 11 4\ ta 4~ tttt I C 4 I-I ~4 ., ~ cft I N 'I C 4 I i~, fjf 44 444 eo 4 I CC SO Ot ) "4 I at ~ 4" C <<WC 4 ttit i C "I aai I!I 'ata Z CC aa f 454 ,4- ' 111 51, C 4 .4 t ) I1 I 4 t Catt Cfe 144 Cf H ffcc 1,1" if4<< t ~ I 4e oat it 40 ota ~ ':r ~ t it teat ftltt ~ 5 ~ 4 441 I I CC 4t at J<<O f'-I:<<I I

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r. jaj SAND GRAVEL 0 SILT OR CLAY Q~ FINE MEDIUM COARSE FINE COARSE 18 M

~~ o~ SAMPLE SAMPLE LIQUID PLASTIC PL A ST IC I T Y SOIL CJf + e SYMBOL PIT NUMBER NUMBER INTERVAL LIMIT LIMIT INDEX TYPE 1&2 2 -10 SC 't3 Va hi 'I) 'll O I CD C I A CD A CD I I' l GEOLOGIC. INVESTIGATION OF THE GILLESPIE DAM ALTERNATE SITING AREA ARIZONA NUCLEAR POWER PROJECT VOLUME I Conducted for: NUS Corporation 14011 Ventura Boulevard Sherman Oaks, California Project No. 73-080-EG August 20, 1975 WEIRD I I I II gl ~ ~ I II CONTENTS Pacae VOLUME I 2.5 GEOLOGY 2.

5.1 INTRODUCTION

2.5.2 ALTERNATE SITING AREA GEOLOGY (5-mile radius)

2. 5. 2. 1 -Physiography.

2.5.2.2 Stratigraphy. 10 2.5.2.3 Structure 27

2. 5. 2. 4 Geologic History. 46 2.5.2.5 Engineering Geologic Evaluation of Features Which Could Affect Category I Structures. 51 2.5.3 GROUNDWATER 55

2. 5. 4 GEOPHYSICAL SURVEYS 55

2. 5. 5 REFERENCES CITED. 56 APPENDIX 2A RADIOMETRIC DATING OF TERTIARY ROCK UNXTS IN AND AROUND THE GILLESPXE DAM ALTERNATE SITING AREA APPENDIX 2B DRILLING PROGRAM AT GILLESPXE DAM ALTERNATE SITING AREA APPENDIX 2C DOWNHOLE GEOPHYSICAL XNVESTIGATIONS AT GXLLESPIE DAM ALTERNATE SITXNG AREA APPENDIX 2D INVESTIGATION OF A POTENTIAL SITE ON THE NORTHWEST SIDE OF THE GILLESPXE BASALT FLOW APPENDIX 2E GEOMORPHOLOGICAL INVESTXGATIONS IN THE GILLESPIE DAM ALTERNATE SITING AREA VOLUME II APPENDIX 2F TRENCHING PROGRAM SITING AREA i G ILLESP IE DAM ALTERNATE APPENDIX 2G SHALLOW REFRACTION SEISMIC SURVEYS, GILLESPIE DAM ALTERNATE SITING AREA

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VOLUME XI (continued)

APPEND XX 2H I GROUND MAGNET C SURVEY OF ENTERRADO FAULT g GXLLESPIE DAM ALTERNATE SITXNG AREA APPENDIX 2X JOINT TREND ANALYSIS XN THE GXLLESPXE DAM ALTERNATE SITING AREA APPENDIX 2J HYDROLOGIC DATA FROM THE GILLESPXE DAM ALTERNATE SXTING AREA APPENDIX 2K ENGINEERING TEST DATA, GILLESPXE DAM ALTERNATE SITING AREA

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TABLES APPENDIX 2A Table 2A-1 Radiometric Dates from Rock Units in the Gillespie Dam Alternate Siting Area Table 2A-2 Pertinent Radiometric Dates from Rock Units Outside the Gillespie Dam Alternate Siting Area APPENDIX 2J Table 2J-1 Water Levels in Drill Holes, Gillespie Dam Alternate Siting Area, 1973-1974 APPENDIX 2K Table 2K-1 Rock Density and Specific Gravity of Some Bedrock and Basement Geologic Units Table 2K-2 Shear Modulus from Shallow Refraction Seismic Surveys Table 2K-3 In Situ Moisture Content, Dry Density'and Unified Soil Classification.;of Samples from Engineering Test Pits Table 2K-4 Summary of Compaction Test Results for Samples from Engineering Test Pits

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FIGURES 2.5 GEOLOGY Figure 2.5-1 Location of Gillespie Dam Alternate Siting Area Figure 2.5-2 Generalized Stratigraphic Column, Gillespie Dam Alternate Siting Area Figure 2,.5-3 Photolineaments in the Gillespie Dam Alternate Siting Area.

Figure 2.5-4 Major Photolineaments Crossing Gillespie Dam Alternate Siting Area Figure 2.5-5 Location of Engineering Seismic Lines and Test Pits APPENDIX 2A Figure 2A-1 Location of Radiometric Dating Samples APPENDIX 2B Figure 2B-1 Location of Drill Holes and Drill Hole Cross Sections Figure 2B-2 Condensed Drill Hole Lithologic Log, GDDH-2 Figure 2B-3 Condensed Drill Hole Lithologic-Log, GDDH-3 Figure 2B-4 Condensed Drill Hole Lithologic Log, GDDH-4 Figure 2B-5 Condensed Drill Hole Lithologic Log, GDDH-5 Figure 2B-6 Condensed Drill Hole Lithologic Log, GDDH-6 Figure 2B-7 Condensed Drill Hole Lithologic Log, GDDH-7 Figure 2B-8 Condensed Drill Hole Lithologic Log, GDDH-8 Figure 2B-9 Condensed Drill Hole Lithologic Log, GDDH-9 Figure 2B-10 Condensed Drill Hole Lithologic Log, GDDH-10 Figure 2B-ll Condensed Drill Hole Lithologic Log, GDDH-12 Figure 2B-12 Condensed Drill Hole Lithologic Log, GDDH-13 Figure 28-13 Condensed Drill Hole Lithologic Log, GDDH-14 flERRO

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FIGURES (continued)

Figure 28-14 Condensed Drill Hole Lithologic Log, GDDH-15 Figure 2B-15 Condensed Dxill Hole Lithologic Log, GDDH-16 Figure 2B-16 Condensed Dxj.ll Hole Lithologic Log, GDDH-17 Figure 28-17 Condensed Drill Hole Lithologic Log, GDDH-18 Figure 2B-18 Condensed Drill Hole Lithologic Log, GDDH-19 Figure 2B-19 Condensed Drill Hole Lithologic Log, GDDH-20 Figure 2B-20 Condensed Drill Hole Lithologic Log, GDDH-21 Figure 2B-21 Condensed Drill Hole Lithologic Log, GDDH-22 Figure 2B-22 Condensed Drill Hole Lithologic Log, GDDH-23 Figure 2B-23 Condensed Drill Hole Lithologic Log, GDDH-24 Figure 2B-24 Drill Hole Cross-Section, GDDH-6 to GDDH-8 Figure 2B-25 Drill Hole Cross-Section, GDDH-12 to GDDH-13 Figure 2B-26 Drill Hole Cross-'Section, GDDH-5 to GDDH-19 Figure 2B-27 Drill Hole Cross-Section, GDDH-19 to GDDH-23 APPENDIX 2C Figure 2C-1 Geophysical Drill Hole Logs, GDDH-2 Figure 2C-2 Geophysical Drill Hole Logs, GDDH-3 Figure 2C-3 Geophysical Drill Hole Logs, GDDH-4 Figure 2C-4 Geophysical Drill Hole Logs, GDDH-5 Figure 2C-5 Geophysical Drill Hole Logs, GDDH-6 Figure 2C-6 Geophysical Drill Hole Logs, GDDH-7 Figure 2C-7 Geophysical Dxi.ll Hole Logs, GDDH<<8 Figure 2C-8 Geophysical Drill Hole Logs, GDDH-9 Figure 2C-9 Geophysical Dxi:ll Hole Logs< GDDH-10 Figure 2C-10 Geophysical Drill Hole Logs, GDDH.-12

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FIGURES (continued)

Figure 2C-ll Geophysical Drill Hole Logs, GDDH-13 Figure 2C-12 Geophysical Drill Hole Logs, GDDH-14 Figure 2C-13 Geophysical Drill Hole Logs, GDDH-15 Figure 2C-14 Geophysical Drill Hole Logs, GDDH-16 Figure 2C-15 Geophysical Drill Hole Logs, GDDH-17 Figure 2C-16 Geophysical Drill Hole Logs, GDDH-18 APPENDIX 2D Figure 2D-1 Location of Drill Holes and Geophysical Survey Lines in Proposed Site Area Figure 2D-2 Gravity Map of Proposed Site Area

-Figure 2D-3 Basal Elevation of Gillespie Basalt in Proposed Site Area Figure 2D-4, Isopach Map of Gillespie Basalt 'in Proposed Site Area APPENDIX 2E Figure 2E-1 Location of Geomorphic Profiles on Alluvial Fans Figure 2E-2 Geomorphic Profiles, A-A'nd Figure 2E-3 Profiles, B-B" arid B-B'eomorphic Profiles, C-C'nd B-Beomorphic Figure 2E-4 of Gila River Terraces and Major D-C'ocation Figure 2E=5 Photolineaments Figure 2E-6 Surveyed Terrace Profile Between Arlington and Gillespie Basalt Flows Figure 2E-7 Flood Plain and Terrace Profiles Between Gillespie and Sentinel Basalt Flows APPENDIX 2F Figure 2F-1 Location of Trenches. and Faults Figure 2F-2 Detailed Trench and Fault Maps fuaaa

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FIGURES (continued)

Figure 2F-3 Trench Log, GDDT-1 Figure 2F-4 Trench Log, GDDT-3 Figure 2F-5 Trench Log, GDDT-4 Figure 2F-6 Trench Log, GDDT-6a Figure 2F-7 Trench Log, GDDT-6b Figure 2F-8 Trench Log, GDDT-8 Figure 2F-9 Trench Log, GDDT-9a Figure 2F-10 Trench Log, GDDT-9b, NW Wall Figure 2F-ll Trench Log, GDDT-9b, SE Wall Figure 2F-12 Rrench Log, GDBT-1 Figure 2F-13 Trench Log, GDBT-2 Figure 2F-14 Trench Log, GDBT-3 Figure 2F-15 Trench Log, GDBT-4 Figure 2F-16 Trench Log, GDBT-5 Figure 2F-17 Trench Log, GDBT-6 Figure 2F-18 Trench Log, GDBT-7 Figure 2F-19 Trench Log, GDBT-8 Figure 2F-.20 Trench Log, GDBT-9 Figure 2F-21 Trench Log, GDBT-10 I Figure 2F-22 Trench Log, GDBT-ll Figure 2F-23 Trench Log, GDBT-12 Figure 2F-24 Trench Log, GDBT-13 Figure 2F-25 Trench Log, GDBT-14 Figure 2F-26 Trench Log, GDBT-16 Figure 2F-27 Trench Log, GDBT-17 Figure 2F-28 Trench Log, GDBT-18

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FIGURES (continued)

Figure 2F-29 Trench Log, GDBT-20 Figure 2F-30 Trench Log, GDBT-21 Figure 2F-31 Trench Log, 'GDBT-22 Figure 2F-32 Trench Log, GDBT-23 Figure 2F-33 Trench Log, GDBT-24 Figure 2F-34 Trench Log, GDBT-25a Figure 2F-35 Trench Log, GDBT-25b Figure 2F-36 Trench Log, GDBT-26 Figure 2F-37 Trench Log, GDBT-27a Figure 2F-38 Trench Log, GDBT-27b Figure 2F-39 Trench Log, GDBT-27c Figure 2F-40 Trench Log, GDBT-29; Figure 2F-41 Trench Log, GDBT-30 Figure 2F-42 Trench Log, GDBT-31 Figure 2F-43 Trench Log, GDBT-32 Figure 2F-44 Trench Log, GDBT-33 Figure 2F-45 Trench Log, GDBT-34 Figure 2F-46 Trench Log, GDBT-35 APPENDIX 2G Figure 2G-1 Seismic Line Locations Figure 2G-2 Seismic Profile, GDSL-1 Figure 2G-3 Seismic Profile, GDSL-2 Figure 2G-4 Seismic Profile, GDSL-3 Figure 2G-5 Seismic Profile, GDSL-4 Figure 2G-6. Seismic Profile, GDSL-5 Figure 2G-7 Seismic Profile, GDSL-6

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Figure 2G-8 Seismic Profile, GDSL-7 Figure 2G-9 Seismic Profile, GDSL-8 7

Figure 2G-10 Seismic Profile, GDESL-l, Figure 2G-ll Seismic Profile, GDESL-2 Figure 2G-12 Seismic Profile, GDESL-3 Figure 2G-13 Seismic Profile, GDESL-4 Figure 2G-14 Seismic Profile, GDESL-6 APPENDIX 2H Figure 2H-1 Ground Magnetic Survey Line Locations APPENDIX 2I Figure 2I-1 Location of Joint Analysis Field Stations Figure 2I-2 Joint Trends at Joint Analysis Field Stations GDJA-1 and GDJA-2 Figure 2I-3 Joint Trends at Joint Analysis Field Stations GDJA-3 and GDJA-4 Figure 2I-4 Joint Trends at Joint Analysis Field Stations GDJA-5 and GDJA-6 Figure 2I-5 Joint Trends at Joint Analysis Field Stations GDJA-7 and GDJA-8 Figure 2I-6 Joint Trends at Joint Analysis Field Stations GDJA-9 and GDJA-10 APPENDIX 2K Figure 2K-1 Cumulative Grain Size Curve, GDEP-1 Figure 2K-2 Cumulative Grain Size Curve, GDEP-2 Figure 2K-3 Cumulative Grain Size Curve, GDEP-3 Figure 2K-4 Cumulative Grain Size Curve, GDEP-4 Figure 2K-5 Cumulative Grain Size Curve, GDEP-6

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PLATES 2.5 GEOLOGY Plate 2.5-l Geologic Map and Cross-Section APPENDIX 2H Plate 2H-. l Ground Magnetic Intensity Profiles

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2.5 GEOLOGY 2.

5.1 INTRODUCTION

This report presents the geologic and foundation engineering information developed during investigation of the Gillespie Dam alternate siting area, located about 44 miles west-south-west of Phoenix, Arizona, and 11 miles south of the Palo Verde Nuclear Generating Station (Fig. 2.5-1).

During preliminary studies to identify an alternate site to the Palo Verde Nuclear Generating Station, the Gillespie Dam area was found to have favorable characteristics for a nuclear power plant site. A program of geologic and engineering investi-gations was therefore initiated to determine both the geology and foundation characteristics of the area and the feasibility of constructing a nuclear power plant in the area.

The program was conducted by Fugro, Inc. under .the supervision of Jack J. Schoustra and Jay L. Smith with John D. Scott as Project Manager. Darryl Miller and Dennis Duffy are Project Geologist and Project Engineer, respectively.

The investigation included:

o Research of pertinent published and unpublished geologic, seismologic, and hydrologic literature of Arizona.

o Consultation with numerous geologists from universities and public agencies who have expertise in particular subjects.

o Examination of existing and specially prepared aerial photo-graphs and other remote sensing imagery.

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o Reconnaissance and detailed geologic mapping of the site area (5-mile radius) and site vicinity (25-mile radius) at scales of l.inch, 2,000 feet, 1,000 feet and 500 feet to the inch.

o Thirteen shallow refraction seismic surveys totaling 29,600 linear feet.

o Detailed gravity and magnetic geophysical surveys of selected areas.

o Excavation of nine bulldozer trenches and 35 backhoe trenches totaling about 1,375 feet and 3,940 linear feet, respectively. Detailed logs were made of all excavations.

o Boring of 22 drill holes to depths ranging from 63 to 780 feet with samples and detailed logs recording geologic and engineering information.

o Downhole geophysical logging of 18 drill holes by high-resolution methods.

o Potassium-argon dating of 19 volcanic and pyroclastic rock samples.

o Petrographic analysis of thin-section samples of various lithologic units.

o Detailed geomorphic study of alluvial fans and river terraces.

o Engineering testing of foundation materials for static and dynamic properties. Types of tests included:

o Grain size and compaction of soils excavated from test pits.

o Moisture-density of drill hole soil samples.

o Density and specific gravity of rock units.

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o Preliminary hydrologic data were collected from drill holes.

Assistance for particular areas of this investigation was pro-vided by the following consultants to Fugro:

James W. Crosby III Borehole Washington State University Geophysics Laurance Lattman Photogeology University of Cincinnati Michael Sheridan Petrography Arizona State University Roy J. Shlemon Geomorphology Private Consultant John Sumner Geophysics University of Arizona The Gillespie Dam alternate siting area is in the Basin and Range physiographic and structural province of southwestern Arizona. The physiography within 25 miles of the Gillespie Dam area is characterized by sharp irregular mountain ranges of low to moderate rel'ief which are separated by broad, nearly flat, alluviated basins. The mountains comprise granitic and metamorphic rocks of Precambrian or Laramide age to volcanic and sedimentary rocks of middle Tertiary age. Alluvium and volcanic rocks in the broad basins range from late Miocene to Holocene age. The physiographic, geologic, and tectonic setting of the site vicinity (25-mile radius) is described in detail in section 2.5 of the Palo Verde Nuclear Generating Station PSAR, and will not be repeated in this report.

The major geologic units in the alternate siting area (5-mile radius) include:

o A complex of granitic and metamorphic rocks of Precambrian or Laramide age which form the basement rocks.

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o A bedrock sequence of middle to late Tertiary sedimentary and volcanic rocks unconformably overlying the basement rocks.

o Surficial deposits of Pliocene to Holocene age, consisting primarily of several generations of'lluvial fan and river terrace deposits, and small basalt flows near Gillespie Dam and Gila Bend.

The bedrock sequence unconformably overlies the basement com-plex along an erosional surface of moderate relief, and is essentially flat-lying except for local slight to moderate tilting. A number of faults displace bedrock and basement lithologic units in the Gillespie Dam alternate siting area.

None of these faults displaces overlying surficial deposits.

Old alluvial fan and river terrace deposits overlain by the Gillespie basalt (radiometrically dated at an average age of about 3.3 million years) may be traced away from the basalt flow and are undisturbed where they cross faults or lineaments, indicating no faulting has occurred within the area for more than 3 million years.

Investigations of the Gillespie Dam alternate siting area indicate that:

o It is situated in a region of diffuse seismicity with low magnitude earthquakes.

o There are no capable faults within five miles of the siting area.

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o Geologic conditions in the siting area are favorable for plant construction.

o Foundation conditions in the siting area are generally favorable for plant construction. However, in certain areas minor soil recompaction may be necessary, and in other areas Category I foundations would encounter bedrock.

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2. 5. 2 ALTERNATE SITING AREA GEOLOGY (5-mile radius)

A. Description of Land forms The Gillespie Dam alternate siting area is on the north flanks of the Gila Bend Mountains at the west edge of the Phoenix Basin. Its borders include Arlington Valley to the north, the western extremity of the Buckeye Hills and the Gila River to the east, I

and parts of the Gila Bend Mountains to the south and west (Fig. 2.5-1). A small basalt flow, referred to as the Gillespie basalt flow, is just west of Gillespie Dam.

The mountain ranges included in the alternate siting area have diverse trends and topography. The Buckeye Hills (Fig. 2.5-1) comprise a low, narrow, east-west trending range with steep irregular topography developed on granitic rocks and local Tertiary lava flows.

The Gila Bend Mountains (Fig. 2.5-1) include steep, irregular granitic mountains of moderate height along the west side of the Gila River and steep-walled, smooth-crested mountains carved from Tertiary lava flows. The range culminates in Woolsey Peak (elevation 3,170 feet) in the southwest part of the siting area.

In the vicinity of Webb Mountain on the west side of the alternate siting area, the range includes low, irregular mountains developed on metamorphic rock.

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Local pediments and widespread alluvial fans of several generations form the flanks of the ranges, abutting the mountains with a sharp break-in-slope and descending gently toward the Gila River or its longer tributary washes. Older fans and pediments are pro-gressivly more dissected than similar younger features and some are graded to river terraces along the Gila BR River.

The Gila River flows west along the north side of the Buckeye Hills, but near the northeast edge of the siting area it turns suddently southward (Fig. 2.5-1).

It leaves the Phoenix Basin at Gillespie Dam, flowing

-south through a narrow gorge cut in tilted volcanic bedrock at the west end of the Buckeye Hills. The river floodplain is locally bordered by remnants of three river terrace deposits, standing 20, 40 and 80 feet above it.

The Gillespie basalt flow is roughly circular in plan view and about two to three miles in diameter. Its upper surface slopes gently away from a centrally located vent which is about 1,200 feet in elevation.

The base of the flow overlies deposits of the two

'fall upper river terraces, old alluvial fan deposits, and bedrock of the Buckeye Hills and Gila Bend Mountains.

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B. Origin of Landforms Although the trends of the Buckeye Hills and eastern Gila Bend Mountains depart from the more characteristic northwest topographic trends of the Basin and Range

, Province, the outline of the ranges in the siting area almost certainly developed during the Basin and Range orogeny. This orogeny culminated during middle Tertiary time and commonly produced the tilted fault-block ranges and basins which dominate the present topography. Xncreased uplift and relief resulted in rapid erosion of the ranges and aggradation of the basins.

Pediments and alluvial fans developed along the mountain fronts, acting as surfaces of transportation across which eroded detritus moved toward the basins.

Extensive fine-grained deposits accumulated in the closed Phoenix Basin, interfingering with alluvial fan deposits near the ranges. These older pediments, alluvial fans and basin deposits have since been largely removed by erosion or buried by younger collu-vial, alluvial and fluvial deposits.

With the integration of the Gila River drainage system through the area in late Tertiary time, aggradation I

and degradation occurred in response to changes in level of the river. As indicated by development of the river terraces, stream incision and degradation

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of alluvial surfaces have been the dominant response to broad regional warping and/or climatic change.

The Gila River once flowed out of the Phoenix Basin in a broad curve which may have passed as much as 10,000 feet feet west of the tilted volcanic bedrock knob at the west end of Gillespie Dam. However, the Gillespie basalt and the Arlington basalt, a few miles upstream, flowed across the river floodplain when it was at the "40-foot" terrace leve, constricting the river's course and sharpening the bend. The Gillespie basalt dammed the river and forced it to cut a new outlet through the old volcanic bedrock, thus forming the narrow gorge at Gillespie Dam. The damming probably did not last more than a few hundreds to thousands

'f years, for there is little evidence of a lake. Scattered river gravel found locally on both the Arlington and Gillespie flows (Plate 2.5-1) are, however, interpreted as being of overflow or flood origin.

There is no evidence that the sharp bends in the Gila P

River are directly .related to late Tertiary tectonism. The siting area shows no evidence of ground subsidence, collapse or uplift since before eruption of the Gillespie basalt flows. Potassium-argon dates'from the Gillespie basalt flow range from 1.3 to, 4.2 million years, averaging about 3.3 million years (Appendix 2A). Similar dates from the Arlington and Gila Bend basalt flows average 2.2 'million years and 4.5 million years, respectively (Appendix 2A).

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10 For this study rock units in the siting area have been cate-gorized as:

o Basement complex includes Precambrian metamoprhic arid granitic rocks; o Bedrock includes Tertiary volcanic, volcanoclastic and sedimentary rocks; o Surficial deposits includes late Tertiary and Quaternary II alluvial fan and fluvial deposits and the Gillespie and Gila Bend basalt flows.

The distribution of rock units is shown on the geologic map and geologic cross section (plate 2.5-1). Stratigraphic relations are indicated by the stratigraphic column (Fig.

2.5-2). The following sections describe the rock units of the siting area, progressing from oldest to youngest.

A. Basement Complex

'he basement complex rock units are the oldest in the siting area and form the regional geologic basement upon which bedrock and surficial deposits accumulated.

. In the basence of radiometric dates, these units are assigned a Precambrian age on the basis of regional geologic relationships and the predominant opinion among geologists who previously worked in the region (Ross, 1929; Heindl and Armstrong, 1963; Wilson, et al, 1969). The basement complex has been subdivided flERRD

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into three major units.

1. Metasedimentary rocks A suite of folded, well foliated metamoprhic rocks is exposed in the western part of the siting area from. Windmill Wash to the east side of Webb Mountain (Plate 2.5-1). The lithology consists primarily of meta-sedimentary slate, phyllite, and schist with inter-bedded quartzite.and" metavolcanic amphibolite.

Foliation generally parallels bedding planes, fold axes, and the contact with a metadiorite. The unit probably represents a eugeosynclinal sequence that has been metamorphosed to greenschist facies.

2. Metadiorite A low-grade, poorly foliated, metamorphosed quartz diorite composes most of Webb Mountain (Plate 2.5-1). The diorite probably intruded the metasediments during or after the main pulse of metamorphism (Fig. 2.5-2).

3. A granitic complex is widely exposed in the southeastern part of the siting area (Plate 2.5-1).

The rocks consist of granite and quartz monzonite, frequently with a gneissic texture. They are often closely jointed and crudely foliated and are locally sheared or intruded by numerous mafic, felsic and pegmatite dikes. The rocks often contain small to large xenoliths of the older metamorphic rocks which they probably intruded or engulfed (Fig. 2.5-2).

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12 B. Bedrock The nine bedrock units unconformably overlie rocks of the basement complex along an ancient erosional surface of moderate to high relief. Bedrock stratigraphy is.

complicated by: (1) lateral facies changes and lensing stratigraphy in several units, and (2) by numerous unconformities. Most of the bedrock sequence has been radiometrically dated during this study yielding ages ranging from late Oligocene to middle Miocene.

l. Arkosic conglomerate - This unit is well exposed from Turban and Spring Mountains north to the Escondido Fault (Plate 2.5-1). The conglomerate is red-brown, well indurated, poorly sorted and composed primarily of medium to coarse arkosic and tuffaceous sand with abundant granitic pebbles and boulders up to four feet in diameter (Fig.

2.5-2). The unit's thickness varies from zero near basin margins to a maximum of about 50 feet.

Portions of the unit are composed of well sorted coarse sand with occasional large granitic boulders, indicating deposition of alluvium by high velocity runoff of short duration such as presently occurs in desert washes. Metamorphic clasts, occasional ash balls and yellowish inter-beds of reworked tuff also occur in the conglomer-ate. The ash and tuff indicate nearby volcanic activity. Deposition probably occurred in a

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13 granitic valley with fairly restricted drainage.

2. Lahar This unit is exposed around the lower slopes of Woolsey Peak and extends about 1-1/2 miles east of the peak within a basin of granitic basement rocks (Plate 2.5-1). Several lahars are represented in Che unit; they range in color from pink brown to light purple gray. The deposits are generally well indurated, poorly sorted and com-.,

posed of more or less abundant angular to sub-rounded volcanic gravel to boulders in a tuffaceous fine-grained or sandy matrix. The thickness varies due to localized deposition and basin floor irregularities, reaching a maximum of about 250 feet. East of Spring Mountain, the unit is locally overlain by an alkali basalt flow about 20 feet thick. The unit pinches out eastward against granitic basement highs which probably prevented the lahars from entering the valley where the arkosic conglomerate unit accumulated.

A contact between the lahar and conglomerate units is not visible so their stratigraphic relation-ship is unclear. However, their similar strati-graphic positions and the pyroclastic materials in the conglomerate suggest that the lahar unit may be nearly contemporaneous with or slightly younger than the conglomerate (Fig. 2.5-2).

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3. Tuffaceous sandstone This unit is widely exposed east, west, and north of Spring Mountain (Plate 2.5-1), where it disconformably overlies the arkosic conglomerate, lahars, and the thin alkali basalt flow. The sandstone is commonly red-brown, well indurated, poorly to well stratified, and

'I moderately sorted. It is composed primarily of tuffaceous silty fine to coarse sand with disseminated granitic, volcanic, metamorphic and tuff fragments. Occasional gravelly to bouldery lenses and.red-brown or yellow-green reworked tuff beds, both as much as several feet thick, are interbedded with the sandstone. The reworked tuffs are common near the basal contact, but become less frequent higher in the unit. Near granitic highlands, the sands become arkosic and contain abundant, granitic boulders. The beds composing this unit are laterally discontinuous and extensively interfingered with each other.

In most cases, the bedding is nearly horizontal except along some faults or near the irregular bedrock contact where differential compaction has produced significant dips. The unit's maximum thickness is about 300 feet. A distinctive ledge-forming, biotite-rich quartz-latite welded tuf f about five to ten feet thick occurs near the top of the unit and can be recognized over several fuuaa

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15 miles. This tuff has been potassium-argon dated at 28.8 + 0.5 million years (Appendix 2A). Small outcrops of the tuffaceous sandstone unit also occur on the southwest side of the Gilj.espie basalt flow, where complex Tertiary faulting and alkali basalt intrusion have complicated the stratigraphic relations. The stratigraphy of the unit'uggests that it was deposited in a lake or floodplain in a single large basin formed by burial of the divide separating the basins containing the lahar and arkosic conglomerate units.

4. Crossbedded sandstone This unit is locally exposed around the lower flanks and to the east of Turban Mountain (Plate 2.5-1). The sandstone overlies and is complexly interfingered with the upper part of the tuffaceous sandstone unit (Fig.

2.5-2). This sandstone is medium gray-brown, moderately to well indurated, well sorted, and forms large-scale crossbeds. It is composed of speckled black-and-white, medium-grained, tuffa-ceous sand containing arkosic lenses which vary from zero to 200 feet thick. This unit, inter-fingers with overlying alkali basalt flows and always has a reddish baked zone about five feet thick where a volcanic flow overlies it. The sandstone probably originated as dune sands along the lee side of the basin in which the tuffaceous

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16 sandstone unit accumulated.

5. Older alkali basalt This unit is widely exposed in the southern part of the siting area (Plate 2.5-1). Because of its great lateral extent, it is found overlying granitic basement, the cross-bedded sandstone, and tuffaceous sandstone units (Fig. 2.5-2). This unit consists of numerous light to dark blue-gray flows, derived from many local vents. The rock readily fractures along

,widespread closely-spaced flow structures and the flows are separated by flow breccia and occasional red-brown scoriaceous zones about five to ten feet thick.. Many reddish pheonocrysts are visible in hand specimen and, on petrographic examination, these appear to be altered olivine associated with titanaugite phenocrysts in a groundmass of trachytic plagioclase laths, intergranular augite and opaque minerals. Many samples have late-stage biotite flakes. The unit thickness varies from 250 to 800 feet. Near its base the unit is potassium-argon dated at 26.7 + 0.4 million years (Appendix 2A). A thin flow of nearly identical basalt is sandwiched between the lahar and tuf-faceous sandstone units west of Spring Mountain.

'6. Tholeiitic basalt This unit is widely exposed in the southern part of the siting area (Plate flSRD

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17 2.5-1). It commonly overlies the alkali basalt along the 50-foot gradational contact (Fig. 2.5-2);

locally it unconformably caps granitic basement rock. The unit is red-brown to black, very dense, blocky, and aphanitic. Petrographically it is composed of stubby plagioclase laths with minor augite, opaque minerals, and occasional olivine.

The unit comprises several flows, at least one of which was probably vented at Woolsey Peak. The composite thickness is 200-400 feet. The flows have been tilted gently south to southwest forming high, smooth, accordant mountain crests. Two samples of the basalt yielded potassium-argon dates of 23.5 + 1.2 and 20.7 + 0.5 million years.

(Appendix 2A).

Younger alkali basalt This unit. crops out on both sides of the Gila River near Gillespie Dam and forms several small knobs and ridges at or near the northwest margin of the Gillespie basalt flow (Plate 2.5-1). It consists of several flows which are similar to the older alkali basalt, except that they are darker gray and usually contain fewer altered olivine phenocrysts, but have abundant augite phenocrysts. The unit appears to be several hundred feet thick at Gillespie Dam where it overlies granitic basement and is tilted about. 10o to 30 to the west. The stratigraphic funaa

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18 relation of the unit to other bedrock units has not been observed (Fig. 2.5-2), but is inferred from several potassium-argon dates which range from 19.1 + 0.9 to 19.6 + 0.4 million years (Appendix A).

Mixed pebble breccia This unit occurs only in the subsurface and extends more than seven square miles from beneath the west side of the Gillespie basalt flow westward across Windmill Wash to the metamorphic basement rocks in the western part of the siting area (Appendix 2B, Figs. 2.B-l, 24).

The breccia is medium brown to dark gray, well indurated, and poorly to moderately sorted. It is composed of medium to coarse quartz and lithic sand. Clasts of tholeiitic basalt, alkali basalt or metamorphic rocks are common while granitic and tuff fragments occasionally occur. The clasts are predominantly small to medium-sized pebbles with occasional cobbles and boulders; granitic and metamorphic clasts increase approach-ing highlands of those compositions. Calcareous fracture and vug fillings are common; thin mudstone layers occasionally occur. The basal contact of the breccia has never been encountered although the maximum depth drilled exceeds 780

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19 The unit is younger than the tholeiitic basalt (of which it contains fragments), but its age and stratigraphic relations to the younger alkali basalt are undetermined (Fig. 2.5-2). The breccia

-unit bears many similarities to lithified fanglom-erates found elsewhere in the vicinity. Such fanglomerates occur in the Palo Verde site area where they include a basalt flow and are radio-metrically dated at 16.7 million years (Appendix 2A). Basalts capping similar fanglomerates south of both the Belmont Mountains and Vulture Mountains have been radiometrically dated at about 14.6, and 14.2 million years, respectively (Appendix 2A).

9. Fanglomerate This unit is exposed only on the southeast. side of the Gillespie basalt flow near Beehive Wash. It is light gray brown, poorly to moderately indurated, poorly sorted and poorly stratified. It contains abundant boulders of locally derived granitic and Tertiary volcanic rocks. The unit lies undisturbed across faulted tuffaceous sandstone and granitic basement rocks.

Because it is unfaulted, the fanglomerate is inferred to be younger than the mixed pebble breccia which is faulted (Fig. 2.5-2).

C. Surficial Deposits Surficial deposits in the siting area consist mostly Paaa

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20 of alluvial fan and river terrace deposits and the Gillespie and Gila Bend basalts. Where observed in natural exposures or in trenches, these deposits lie undisturbed across faults which displace bedrock units.

Differentiation of alluvial deposits by age is often difficult and depends on use of several criteria simultaneously (Appendix 2E). Five generations of alluvial fan deposits and three of river terrace deposits were recognized near the Gillespie basalt flow where they were most intensively studied. The two oldest fan and terrace deposits predate the Gillespie basalt flow (potassium-argon dated at about 3.3 million years).

l. Oldest (pre-Gillespie basalt) alluvial fan deposits These deposits occur around Woolsey Peak and north of Turban and Spring Mountain in the siting area (Plate 2.5-1). Near Spring Mountain they overlie a north to northeast-sloping pediment surface beveled across bedrock units and are generally 30-50 feet thick. The unit is usually light gray-brown, non-indurated and poorly sorted. It is composed of mixtures; of silt, sand

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gravel and boulders. The clasts include tholeiitic and alkali basalts, welded tuff and tuffaceous sandstone. Because an extremely hard gray-white petrocalcic horizon occurs through the upper ten

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to 15 feet, of the deposit, its upper surface is generally preserved as smooth-crested ridges, although washes crossing it are entrenched as much as 150 feet into underlying bedrock. The jan surface is composed almost entirely of aaliche rubble and large, dense tholeiitic basalt boulders which are weathering out of the petrocalcic horizon and are black with desert varnish.

Surrounding Woolsey Peak the proximal ends of the fans grade into steep talus cones, but near Spring Mountain erosion in the form of pediment moating and scarp retreat has isolated the fan-head from its source area. The distal ends of the fans are truncated by washes and/or buried by deposits of the next younger alluvial fan unit (Fig. 2.5-2).

2. Old (pre-Gillespie basalt) alluvial fan deposits These deposits occur southwest of, and under, the Gillespie basalt flow (Plate 2.5-1). Their composition is often similar to the oldest alluvial fan deposits, but, being more widespread, they locally include abundant granitic and metamorphic clasts near source highlands of those compositions.

The maximum thickness is about 150 feet. Compared to the oldest alluvial fan deposits, their surface is topographically lower, incised up to 50 feet

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22 by washes, broadly preserved with a "mottled" appearance on air photos, less calichified (caliche rubble is locally present). It has moderate desert I

varnish and clasts of less resistant lithologies, although tholeiitic basalt boulders are numerous.

Southwest of the Gillespie basalt flow, the slope and drainage direction of the fan surface is north-west to east, nearly transverse to that of the oldest alluvial fan.

Detailed geomorphic profiles (Appendix 2E) indicate that the surface of the old alluvial fan deposits passes immediately under the western edge of the Gillespie basalt flow. This is confirmed by stream bank exposures along the flow edge, detailed logs of trenches cut through rhe flow (Appendix 2F, Fig.

2F-5), and drill hole logs of wells drilled through the basalt (Appendix 2B, Figs. 2B-1, -26, -27). All show basalt overlying old alluvial fan deposits, often with a baked zone in the fan deposits just below the contact. The age of these fan deposits is thus known to be older than. the Gillespie basalt flow (Fig. 2.5-2), so they can be used for areal correlation and dating of faults.

Drill holes GDDH-19 through 24 on the northwest side of the Gillespie basalt flow indicate that the basalt overlies gently sloping, old alluvial fllBRD

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23 fan deposits which were graded to fine-grained basin fill deposits and possibly to Gila River deposits about 80 feet above the present river channel (Appendix 2B, Fig. 2B-27; Appendix 2D, Fig. 2D-3). The old alluvial fan deposits are thus probably roughly correlative with deposits of the "80-foot" river terrace discussed below.

Possibly correlative alluvial fan deposits occur between the east side of the Gila Bend Mountains and the Gila River (Plate 2.5-1). These deposits also interfinger with Gila River deposits as high as 80 feet above the present river channel.

However, they are strongly calichified and deeply incised, possibly because 'they have not been pro-tected by a basalt cap.

3. Old river deposits These deposits occur as remnants of paired river terraces (Plate 2.5-1) which border the modern flood plain and stand 40 and 80 feet above the modern channel (Appendix 2E).

The deposits consist of silty fine sand to cobbles.

Many of the clasts are well rounded and consist of distinctive rocks such as quartzite, rhyolite, and meta-diabase whose provenance is many miles upstream.

The deposits 80 feet above the present river channel represent the upper level of a major fllSRD

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24 fluvial aggradation. These high terrace deposits are poorly preserved and strongly calichified.

Old pediment-alluvial fan systems along the east side of the Gila Bend Mountains were graded to and

'interfingered with these deposits as they accumulated. In drill hole GDDH-23 (Appendix 2B, Figs. 2B-l, -27) fine-grained basin-fill deposits overlie and/or interfinger with possible Gila River deposits at the 80-foot level (Fig.

2.5-2). This suggests that at least the upper part of the basin-fill sequence in the western end of the Phoenix Basin (including the Palo Verde site area) accumulated after the Gila River entered the area.

Better preserved, less calichified river terrace deposits occur about 40 feet above the present river channel. These deposits veneer a strath terrace surface beveled on older alluvium or rock after the river cut down from the 80-foot level.

The Gillespie basalt was apparently extruded over the 40 and 80-foot terrace deposits shortly after the river cut down to the 4U-foot level, for there is little evidence of pre-basalt incision of pediment-alluvial fan systems which were graded to the higher 80-foot river level. However, following basalt extrusion, fairly extensive pediments, alluvial fans, and tributary stream

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25 terraces were graded to the 40-foot terrace level.

5. Gillespie and Gila Bend basalts The Gillespie basalt forms a small basalt flow along the west side of the Gila River in the vicinity of Gillespie Dam (Fig. 2.5-1; Plate 2.5-1). It overlies portions of old alluvial fan deposits, old river teirace deposits, the younger alkali basalt unit and granitic basement rock. The flow basalt is dark gray to black and very dense with few vesicules except for. occasional scoriaceous zones; it is slightly fractured and locally has columnar jointing.

Petrographically the unit is an olivine basalt, with olivine phenocrysts in a groundmass of matted plagioclase with intergranular calcic augite and opaque minerals. The basalt is 15-30 feet thick at its periphery and its surface is intensely calichified. The basalt is actually a composite of several flows and three or four flow fronts have been identified on the upper surface.

Pillow lavas exposed in the northeast side of the basalt suggest that it flowed into the Gila River as it covered the old "40-foot" flood plain.

Ten radiometric dates have been obtained from the basalt (Appendix 2A). They range from 4.2 + 0.4 million years for a sample near the bottom edge of the basalt to 1.3 + 0.4 million years for a sample

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26 taken near the vent. Five of the dates fall between 3.3, 3.5 million years and the average of all dates is 3.3 million years.

The Gila Bend basalt forms an irregular basalt flow which is included in the southwestern corner of the siting area (Fig. 2.5-1; Plate 2.5-1). It is compositionally similar to the Gillespie basalt, but it is structurally more complex.

Potassium-argon dates of about 2.5, 4.5 and 6.5 mill'ion years from the Gila Bend basalt flow (Appendix 2A) tend to confirm that it is a composite of several generations of basalt flows.

6. Post-Gillespie basalt alluvial deposits - Slightly before, during, and after extrusion of the Gillespie basalt, extensive pediments, alluvial fans and tributary stream terraces were regraded to the 40-foot river terrace level (Plate 2.5-1). Sub-sequent dissection has reduced these alluvial fan deposits to linear ridges with moderately incised surfaces. The fan deposits are moderately to heavily calichified with moderate desert varnish on their surface (Fig. 2.5-2).

A period of incision (probably in Quaternary time) followed by aggradation and renewed incision resulted in the formation of a terrace about 20 feet above the present river channel. The silty

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27 terrace deposits locally contain potshards of the Hohokam Xndians. The potshards represent a mix of Sacaton and Santa Cruz cultures. They are confidently dated by archeologists as 1,100 B.P., which is the maximum age of the 20-foot terrace (Fig. 2.5-2).

Young alluvial fans were graded to the 20-foot terrace. The surface of the deposits is slightly incised and dissected by washes graded to the modern river (Fig. 2.5-2). Modern washes are forming small alluvial fans at the border of the present river flood plain (Fig. 2.5-2).

2.5.2.3 Structure Structural elements recognized in the siting area include stratification, foliation, folds, joints, faults, and igneous intrusive and flow structures. Structures at the surface were recognized and studied by detailed geologic mapping and trench logging. Subsurface structures were inferred from drill hole lithologic and geophysical logs and from gravity, magnetic, and seismic survey data. Structures in areas which are inaccessible or have poor exposures were inferred from areal geologic relationships. Lineaments identified from air photos or other remote sensing imagery were closely studied in the field.

A. Basement Complex Structures The metasedimentary unit south and east of Webb Mountain (Plate 2.5-1) was originally composed of well-stratified deposits. This stratification has fuIIaa

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28 been obscured by folding and folidation. The unit appears to be bent into isoclinal folds whose axial surfaces nearly parallel the foliation. The folidation dips southwest. at angles ranging from 45 to nearly PI y'ex/ical. The foliation trend is arcuate in plan view, with strikes ranging from N45 E in the southern end of the unit to N10 W near Webb Mountain. The contact with the metadiorite unit at Webb Mountain (Plate 2.5-1) parallels the foliation and is- apparently intrusive. The metadiorite unit and the granitic complex are often foliated to a gneissic texture.

The granitic rocks locally contain large foliated xenoliths, probably representing engulfed blocks of intruded metamorphic rocks.

Mafic, felsic and pegmatite dikes and sills are common in the granitic complex. Most of the felsite and pegmatite bodies roughly parallel the foliation in gneissic granite. Some mafic intrusions intersect the gneissic foliation and appear to be related to mid-Tertiary volcanism. Occasional shear zones in the granitic complex can be trac'ed for only short distances due to lack of exposures.

Jointing is well-developed in the basement complex (Appendix 2X). The dominant joint trends are N35 -45 0 W in the metamorphic rocks, N25 0 -35 o W in slightly fol'iated granitic rocks. Other subordinate trends

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29 also occur, especially in gneissic granite.

B. Bedrock Structures Except in the tuffaceous sandstone unit, stratification is poorly developed in bedrock sedimentary and volcanic units. The arkosic conglomerate, lahar, crossbedded sandstone, and mixed pebble breccia are composite units SR formed by accumulation of discontinuous local deposits.

The strata of the alkali basalt and tholeiitic basalt units consist of discontinuous flows of various thickness and extent. Beds of the tuffaceous sandstone unit range from a few inches to a few feet in thickness and are generally subhorizontal and laterally traceable for a few thousand feet. Where they overlie an irregular contact, these beds locally dip as much as 15 Highly variable attitudes characterize well-developed close-spaced flow structures in the old alkali basalt.

The flow structures and numerous local dikes and volcanic necks indicate that the old alkali basalt was extruded from many local sources. Diabasic dikes on the north face of Woolsey Peak indicate that it was a major vent area during extrusion of the tholeiitic

'fall basalt.

The southern and eastern sections of the Gila Bend Mountains have probably been tipped south and southwest, respectively, resulting in the sloping surfaces of the D

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30 thoeliitic basalt flows and in the tilting and folding of the Sil Murk Formation on the southern flank of the range (Heindl and Armstrong, 1963).

Joints in the volcanic rock units are well developed along trends of N20 -30 W to NSO E (Appendix 2X).

Sedimentary rock units have similar but less developed joint sets.

C. Structure of Surficial Deposits Most of the alluvial fan and river deposits in the siting area are poorly to moderately stratified; locally they may be unstratified or well stratified.

The strata are composed of discontinuous lenses, especially transverse to the drainage direction. For this reason lithologic and geophysical 'logs of the

'urficial deposits seldom show any correlation between drill holes (Appendices 2B, 2C). Alluvial fan strata may dip nearly 20 near the fan head, but the dip progressively decreases down fan, approaching horizontal o -0 near the Gila River. The average dip is about 2 -4 downstream. No faults and few joints occur in the alluvial deposits.

The Gillespie basalt overlies some old alluvial fan deposits. Drill hole logs and gravity survey data (Appendix 2D, Fig. 2D-3) indicate that on its north-west, side, the flow base is smoother than its upper surface and follows the gentle riverward slope of

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31 the alluvial fan on which it rests. The basalt is crudely stratified by superposition of numerous local lava flows and by flow structures defined by stretched and aligned vesicles. Partial ring dikes and volcanic necks occur around the central vent from which the flows were extruded. Columnar jointing is character-istically widespread and well developed. Pillow-lava structures occur on the northeast side of the basalt, indicating that the basalt flowed into the Gila River.

D. Faults A'number of faults have been recognized in bedrock units.- Most of them have normal or reversect dip-slip displacement and steep to vertical dips. The most common fault trends are N-NW and ENE-E. The faulting style is characteristic of mid-Tertiary Basin

.and Range tectonism. The fault trends generally parallel Laramide structures and reflect the control exercised by older Laramide structures during the

'Basin and Range Orogeny. Limited exposures make it difficult to find or trace smaller faults in the area of Miocene volcanic rocks; however, the stratigraphy is known well enough to allow identification of faults with, large vertical displacement. Detailed investi-gation of mappable faults indicates that they are older than late Tertiary in age.

The age, trend, displacement, and extent of known or

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32 suspected faults in bedrock were investigated by 75 backhoe trenches and nine bulldozer trenches. A 22-hole drilling program, combined with seismic and magnetic surveys located a large buried bedrock fault. The results of each fault investigation are described in detail below.

The longest traceable fault in the siting area is called the Escondido fault (Appendix 2F, Fig. 2F-1) because of its total lack of topographic expression.

The fault was traced by detailed mapping for about two 'miles. The traceable segment is terminated to the south by colluvial cover and lack of fractur-ing in bedrock exposures. To the north, the fault is terminated by burial beneath alluvial fan deposits. The fault is best exposed on both sides of Escondido Wash, just north of Turban Mountain.

Here the tuffaceous sandstone and overlying older alkali basalt units are juxtaposed along a four-foot wide breccia zone containing dip-slip slickensides.

The fault zone is nearly vertical but its trace is curvi-linear, the strike varying from N70 0 W to N35 0 W.

Wherever the fault is visible, its displacement is dip-slip with the alkali basalt stratigraphically down about 200 feet on the north side of the fault and juxtaposed against tuffaceous sandstone.

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33 Trench GDDT-8 (Appendix F, Figs. F-l, F-8) was cut near a ridge crest north of Spring Mountain. It exposed undisturbed, loose, stratified oldest (pre-Gillespie basalt) alluvial fan deposits which lay under a (9-foot thick petrocalcic horizon and above. bedrock displaced by the Escondido fault. These oldest alluvial fan deposits predate the old (pre-Gillespie basalt) alluvial fan deposits (Section 2.5.1.2.2).

The minimum age of Escondido fault, therefore, is older than the Gillespie basalt (potassium-argon dated at about 3.3 million years) by several tens or hundreds of thousands of years.

A fault juxtaposing the older alkali basalt and tuffa-ceous sandstone units occurs on the north side of Beehive Wash, north of Spring Mountain, and may represent the last northern exposure of the Escondido fault. Trenches GDBT--33 and 34 (Appendix 2F, Fig. 2F-1) were excavated 0

across the fault. The fault trends N3 E and is nearly vertical; the type of displacement is dip-slip, east side down, but the amount is not, known. Trench GDBT-34 exposed moderately stratified old (pre-Gillespie basalt,)

alluvial fan deposits continuously overlying the fault demonstrating that the minimum age of the fault is considerably older than the Gillespie basalt,. The fault is on the projected north-northwest trend of the Turban lineament, a large photolineament which is associated fuuaa

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34 with faults near Turban Mountain (Fig. 2.5-4).

The existence of another large structure, the Enterrado fault, was first suggested when two closely spaced drill holes, GDDH-6 and 8 (Appendix 2B, Fig. 2B-l),

penetrated entirely different lithologies. Drill holes GDDH-16, 17 and 18 (Appendix 2B, Figs. 2B-1, 2B-24) were located between GDDH-6 and 8 to more closely define the nature and location of the fault.

The fault occurs between drill hole GDDH-16, which penetrated to 300 feet in mixed pebble breccia, and GDDH-18, which penetrated 280 feet through alkali basalt, tuffaceous sandstone, and granitic basement rock.

Drill hole GDDH-8, 850 feet northwest of GDDH-16, penetrated 780 feet of mixed pebble breccia. This evidence suggests that the structure is a normal fault with the north side displaced down about 800:feet.

Trench GDBT-35 (Appendix 2F, Fig. 2F-1) exposed undisturbed strata of old (pre-Gillespie basalt) alluvial fans overlying the Enterrado fault between drill holes GDDH-16 and 18. The fault is, therefore, older than the Gillespie basalt (potassium-argon dated at about 3. 3 million years.

Geophysical surveys were used to better define the location and trend of the Enterrado fault. Seismic line GDSL-7 (Appendix 2G) was run between drill holes GDDH-6 and -16. Its results were largely inconclusive but suggested a lithologic discontinuity about 150 faao

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35 feet northwest of GDDH-18. A ground magnetic survey traced the Enterrado fault along a general trend of 0

N65 E for 1,500 feet. northeast and 2,000 feet southwest of the drill holes (Appendix 2H).

The magnetic survey also suggested that the Enterrado fault was possibly displaced by a buried, nearly per-pendicular structure 2,000 2,500 feet southwest of the drill holes (Appendix 2H). This possible inter-secting structure is nearly on trend with both the Turban lineament (Fig. 2.5-3) and a possible north-ward projection of the Escondido fault (Appendix 2F, Fig. 2F-l).

The Enterrado fault has possibly been displaced 750'eet northward on the west side of the intersecting structure. This would locate the northward displaced segment on the N70 E trend of a nearly vertical fault which juxtaposes alkali basalt against, granitic basalt rock south of Windmill Wash (Appendix 2F, Fig. 2F-1).

The fault, where exposed, has at least 50 feet of displacement, north side down, and the trend appears to be N70 E, following the granite-basalt contact. for about two miles. The fault is locally buried by apparently undisturbed old (pre-Gillespie basalt) alluvial fan deposits. This latter fault and the Enterrado fault may be segments of an originally continuous fault that was displaced by the intersecting structure.

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36 Another fault discovered during detailed mapping is exposed about 1,500 feet south of the Escondido fault near trench GDDT-8. Trenches GGDT-9a and 9b, (Appendix 2F, Figs. 2F-l, 2F-9, 2F-10, 2F-ll) exposed one large- fault with an attitude of N60 W, 87 0 SW, and two smaller step faults about 50 feet to the south.

I Approximately 40 feet of dip-slip displacement, north side down,- has occurred along the larger fault and about 15 feet along the smaller faults. Alkali basalt is brecciated within four feet of the main fault and is juxtaposed against tuffaceous sandstone and welded tuff. As at GDDT-8, the faulted bedrock is overlain by oldest (pre-Gillespie basalt) alluvial fan deposits which are capped by a very hard petrocalcic horizon 15-30 feet thick. Strata are difficult to identify in this intensely calichified fan deposit, but there is no visible evidence of faulting, jointing or fracturing. Thus, the fault appears to predate oldest (pre-Gillespie basalt) alluvial fan deposits so it, like the Escondido fault, is considerably older than the Gillespie basalt.

The fan surface near trenches GDDT-9a and 9b slope about 'nine degrees northward. This is several times the average alluvial fan slope in the siting area and is steeper than adjacent correlative fan surfaces (except for the isolated fan segment immediately

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37 to the west). Uplift and rapid erosion are suggested by isolation of the fan head from Spring Mountain, its old source area, and by stream meanders entrenched as much as 200 feet below the fan surface.

The steep slope probably resulted f. om normal, near-surface depositional processes at the fan head.

Correlative fan deposits around Woolsey Peak grade directly into talus and colluvial deposits with surface slopes approaching 20 degrees. Some of these fan deposits have been isolated from their source by erosion which has left them standing above younger fan surfaces. The bouldery composition of the fan deposits at trenches GDDT-9a and 9b is similar to that occurring near a transition from alluvial fan to mass wasting deposits. The fan surface profile shows no sudden break-in-slope but steepens gradually and projects about halfway up Spring Mountain, the old source area. The isolation of the fan head from its source can be best explained by pediment moating, a fairly common arid-region erosional process. The erosion and possible uplift which has entrenched the streams may be regional, rather than local, in nature.

East of Escondido fault, a small fault is exposed in a 25-foot high bank of Escondido Wash, south of where

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38 Beehive Wash cuts through a granitic ridge partially buried by the southern part of the Gillespie basalt flow (Appendix 2F, Fig. 2F-1). The north-trending fault displaces the cross-bedded sandstone unit abopt'2-18 inches. The fault has vertical dip-slip displacement and many discontinuous horizontal and vertical splays with intraformational terminations and contains no gouge. The sandstone, like all the adjacent bedrock, has been pedimented and is covered by 10-15 feet of weakly caliche-cemented, stratified alluvium. Trenches GDBT-25a and -25b (Appendix 2F, Fig. 2F-1) show undisturbed pediment alluvium over-lying faulted sandstone. The pediment alluvium was graded to a major terrace along Beehive Wash which in turn was probably graded to the "40-foot" river terrace (Appendix 2E). The old river level represented by this terrace was apparently the local base level both before and after the eruption of the Gillespie basalt. The age of the pediment alluvium and the possible fault in the sandstone relative to the Gillespie basalt is therefore uncertain; but by analogy with other faults in bedrock, this fault is probably older than the Gillespie basalt and may be diagenetic in origin.

About 20 feet of faulted tuffaceous arkosic sand-.

stone and conglomerate are exposed in the north bank flIIAD

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39 of Beehive Wash near the southeast corner of the Gillespie basalt flow (Appendix 2F, Fig. 2F-l).

The attitude of this bedrock unit is N10 E, 15 0 NW.

The bedrock has been displaced about ten feet along three nearly vertical faults which have vertical slickensides, but no gouge zones, and strikes of 0 0 N60 , 70 0 and 75 W. The tilted, faulted bedrock is J

overlain by apparently undisturbed subhorizontal alluvial deposits which are in turn overlain by the Gillespie basalt flow. Bulldozer trench GDDT-1 (Appendix 2F) was dug at the point where the projections of all three faults converge. The trench exposed about 15 feet of subhorizontally stratified alluvial deposits which were overlain along a local baked zone by Gillespie basalt. The fan deposits were not faulted, jointed or fractured nor was the basalt-fan contact displaced. The age of the faults, therefore, predates the deposition of an alluvial fan which is older than the Gillespie basalt (potassium-argon dated at about 3.3 million years).

About 2,000 feet southwest of GDBT-l, on the south side of a low ridge south of Beehive Wash, two faults were found exposed in the tuffaceous sandstone unit during detailed mapping. Twenty trenches (Appendix 2F) were dug to trace these short (less than 1,000 feet) and discontinuous faults that complexly offset

40 or terminated each other.. The dominant fault trends are N70 -80 W, and N20 W to N10 E. Fault displacements range from a few inches to a few tens of feet. The fault zones commonly have dip-slip or oblique slickensides and are only 1/4 inch wide, occasionally increasing to about one inch in width.

In the absence of visible stratigraphic offset, the fault planes can often be confused with joints.,

In GDBT-29 (Appendix 2F, Figs. 2F-2, 2F-40) an indurated bouldery fanglomerate lay undisturbed over one of the faults. This fanglomerate is apparently correlative with a similar unit which occurs under the Gillespie basalt at, about the same elevation in trench GDDT-3 (Appendix 2F, Figs. 2F-2, 2F-4).

Detailed mapping identified a fault in the bank of a northerly tributary of Beehive Wash, about 2,400 feet, west of trench GDBT-1 (Appendix 2F, Fig.

2F-1). Four backhoe trenches (Appendix 2F, Fig.

2F-2) were cut, across the fault exposing several additional faults which displaced the tuffaceous sandstone unit and juxtaposed it against. granitic basement rock. A few tens of feet of dip-slip displacement has occurred along each of these nearly vertical faults. Fault planes are usually less than one inch thick and have nearly vertical slickensides. The originally discovered fault has

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41 a slightly sinuous trend of N10 E toward a bluff capped by 20 feet of Gillespie basalt. The basalt overlies an indurated bouldery fanglomerate which in turn overlies the tuffaceous sandstone and gran-itic basement rock. Trench GDDT-3 (Appendix 2F, Figs. 2F-2, 2F-4) was cut where it would cross projections of several of the faults. The trench, exposed about ten feet of the indurated fanglomer-ate which was,not faulted, jointed or fractured.

The naturally exposed cap of Gillespie basalt is also undisturbed. The faults are older than the indurated fangloemrate which is older than Gillespie basalt.

West from Escondido fault a poorly exposed fault was discovered on a hillside north of Spring Mountain. At the base of the hill the fault dis-places tuffaceous and arkosic conglomeratic redbeds, and near the top it juxtaposes granitic basement and alkali basalt. Trenches GDBT-27a, b and c (Appendix 2F, Fig. 2F-1) were excavated across the fault projection, but only the latter two exposed the fault. The fault trends N76 W, and displacement along Xt is on the order of several tens of feet, north side down. The fault plane is defined by a two inch wide gouge zone with no slickensides. In trench GDBT-27c the faulted bedrock is overlain by deposits of a stream terrace which is graded through

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42 Beehive Wash to the "40-foot" river terrace (Appendix 2E). The fault was not encountered in trench GDBT-27a, but. more than ten feet of old or oldest alluvial fan deposits lie undisturbed across the fault projection. Deposits of both fans pre-date the Gillespie basalt flow, so the fault is considerably older than the Gillespie basalt.

A fault offsetting a well-indurated boulder fan-glomerate is exposed on the south bank of Windmill Wash, north of Woolsey Peak (Appendix 2F, Fig.

2F-1). The fanglomerate contains many large meta-morphic clasts from the metamorphic highlands to the north, but it is otherwise similar to and possibly correlative with the arkosic conglomerate unit or the Sil Murk Formation. The fanglomerate is tilted to an attitude of N10 W, 24 0 W and is unconformably overlain by subhorizontally stratified old (pre-Gillespie basalt) alluvial fan deposits (Section 2.5.1.2.2).

The alluvial fan deposits overlying the fault pro-jection were .exposed by trenches GDDT-6a and 6b (Appendix 2F, Fig. 2F-1). They were not. faulted, fractured, jointed, or otherwise disturbed. The fault is, therefore, older than the old (pre-Gillespie basalt) alluvial fan deposits.

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43 E, Photolineaments Photolineaments are defined by a series of linear geo-logic or topographic features. They are often controlled 1

by geologic structures such as folds or faults and so require investigation to determine the nature and age of any geologic controls. Photolineaments in the alternate siting area have been located by means of black-and-white and color vertical air photos of various scales, high altitude photography, and side-looking airborne radar and satellite imagery. In many cases there was no visible evidence of faulting asso-icated with these photolineaments when they were examined in the field.

A small east-trending photolineament is defined by aligned stream drainages in the granitic highlands south of Windmill Wash (Fig. 2.5-3). Field mapping revealed no offset of quartz dikes which crossed it.

Seismic lines GDSL-5 and 6 (Appendix 2G) showed no significant stratigraphic discontinuities where they crossed the projection of the photolineament.

A photolineament in the metasedimentary unit was de-fined by stream drainages aligned with an apparent 200-foot lateraldisplacement of a dark amphibolite bed.

Backhoe trenches GDBT-30, 31 and 32 (Appendix 2F, Figs.

2F-l, 2F-41, 2F-42, 2F-43) were excavated in phyllite along a probable projection of the fault. The trenches

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44 exposed severa,l small, sinuous, altered or mineralized breccia zones which roughly parallel the bedding and are apparently the result of interbed slippage during folding. The trend of these breccia zones is nearly at right angles to the photolineament. Therefore, they are not the cause of the photolineament.

Two small parallel photolineaments trending west off of the southwest flank of the Gillespie basalt flow were observed in air photos. They were investigated by seismic lines GDSL-1 and 2 (Appendix 2G). Although the results were largely inconclusive, neither seismic line showed any -anomalies where they cross the photo-lineaments. Trenches GDDT-4 and 5 (Appendix 2F, Fig.

2F-1) were dug across the southern photolineament.

Trench GDDT-4 exposed about five to ten feet of basalt overlying a local baked zone at the top of a 10-12 foot section of strongly calichified, stratified, old (pre-Gillespie basalt) alluvial fan deposits.

The fan deposits were not faulted, fractured, or jointed, nor was the basalt-fan contact displaced.

This evidence suggests that the photolineation is not structurally controlled, but is probably a purely fortuitous alignment of natural surficial features.

Trench GDDT-5 (Appendix 2F, Fig. 2F-1) was dug in old (pre-Gillespi;e basalt} alluvial fan deposits west of GDDT-4 and exposed poorly calichified fan deposits.

The fan deposits were not visibly stratified, so

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45 the trench was not useful in investigation of the photo-lineament. On the basis of the results obtained from trench GDDT-4, another proposed trench (GDDT-7) on the northern photolineament was deemed necessary.

The Gila River photolineament passes near the south-eastern side of the siting area (Fig. 2.5-4; Appendix 2E, Fig. 2E-5). Xt trends about N70 E along the Gila and- Salt Rivers and is associated with faulted mid-Tertiary rocks both within and outside the siting area.

There is no evidence from natural exposures that the Gila Bend basalt flow (potassium-argon dated at 2.5-6.5 million years) is displaced where this photolineament crosses it just south of the siting area.

The Gila Bend photolineament (Fig. 2.5-4) passes through the east side of the siting area, roughly paralleling the north-northwest trend of the Gila River Valley. Xt may be associated with tilted, younger alkali basalts (potassium-argon dated at about 19 million years) .at Gillespie Dam.

Profiles developed from detailed leveling surveys and 20- and 40-foot. contour maps along the surface o+ the "40-foot" terrace which borders the Gila River (Appendix 2E, Figs. 2E-5, 2E-6, 2E-7) suggest that the terrace I

is not displaced where intersected by'hese two major lineaments. The terrace deposits are approximately equiva-lent in age to the Gillespie basalt flow (potassium-argon fllBRa

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46 dated at about 3.3 million years) which locally overlies them.

2.5. 2.4 Ge'o'lo ic Hi's'to'r The geologic history of the Gillespie Dam alternate siting area is simple on a broad scale but complex in detail. There is no geologic record for enormous segments of its past, but the geologic evidence is varied and abundant for about the last 30 million years. The alternate siting area's geologic history is conveniently divided into four parts: pre-Laramide, Laramide Revolution, Basin and Range Orogeny, and post- Basin and Range.

A. Pre-Laramide Regional warping, faulting, and subsidence produced the Sonoran geosyncline in Arizona during Early I

Precambrian time. Sedimentation and intermittent volcanism produced several tens of thousands of feet of deposits during protracted subsidence of the geo-synclinal trough. At the end of the Early Precambrian time, the Matzatzal Revolution (radiometrically dated at about l,200 - l,500 million years) destroyed most of the geosyncline. The geosynclinal deposits, represented in the alternate siting area by the meta-sedimentary unit, were deformed by thrust-faulting,,

steep reverse faulting, and by major northeast-trending folds. They were subjected to regional metamorphism,'uring and after which they were intruded or engulfed by gabbroic or large granitic plutons.

These are probably represented in the alternate siting U IIR KI f

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47 area by the metadiorite unit and the granitic complex.

There f.s no recognizable geologic record for roughly the next billion years; either no rocks weie formed in ghe area, or, more probably, they have been removed by uplift and erosion. Elsewhere in Arizona during this interval, lat.e Precambrian sediments were deformed and intruded by diabase during the Grand Canyon Disturbance which closed the Precambrian Era. Paleozoic seas then covered parts of Arizona and intermittent deposition resulted in Paleozoic sections locally thicker than 7,800 feet. The'esozoic Nevadan Revolution had little effect on most of Arizona, but volcanism increased, punctuating sedimentation in southern and western Arizona.

B. Laramide Revolution The Laramide Revolution culminated in late Mesozoic and early Cenozoic time. Within the Basin and Range Province in southern and western Arizona, Laramide tectonism included intense folding and faulting, and widespread volcanic and plutonic igneous activity.

Laramide structures commonly trend northwest in Arizona and were often reactivated or otherwise influenced structural development during the following Basin and Range Orogeny. Although some of the granitic rocks may be Larhmide in age, no Laramide rocks are known to occur in the siting area. Many of the fuIIaII

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48 northwest-trending structures such as faults and joints may have developed during this interval.

C. Basin and Range Orogeny The inception of Basin and Range 'orogenic activity tends to blend with the completion of Laramide activity in early Tertiary time. The orogeny culminated in mid-Miocene to mid-Pliocene time with large scale tile-block faulting of the basement complex and Tertiary volcanic-sedimentary bedrock sequences. The major topographic elements in the siting area probably developed during this time. Hundreds of thousands of feet of vertical fault displacement occurred; much of it along northwest-trending Laramide or Precambrian structures. Probably most of the bedrock faults in the alternate siting area were formed at this time.

From early Tertiary through middle Tertiary time, sedimentary and volcanic rocks accumulated in restricted valleys in eroded highlands. In the alternate siting area these highlands were composed of Precambrian basement rocks which shed detritus into the basins forming the arkosic conglomerate. Over these deposits lahars, lacustrine (?) tuffaceous sandstone, eolian (?)

crossbedded sandstone and tuffs were laid down. A welded tuff near the top of this section yielded a potassium-argon date of 28.8 million years (Appendix

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49 Beginning in mid-Tertiary time, the sedimentary sequence and basement,'rocks were buried by several hundred to a few thousand feet of lava flows with local interbeds of alluvial, pyroclastic, or lahar deposits. These volcanic rocks probably originated from many local dikes or vent areas. An older alkali basalt, tholeiitic basalt, and younger alkali basalt have been recognized in the siting area. This volcanic sequence yields radiometric dates ranging from 26.7 million years near the base to 19.1 million years near the top (Appendix 2A). Bouldery breccias and fanglom-erates (such as the mixed .pebble breccia in the siting area) were derived from the volcanic rocks in middle to late Tertiary time. In the site vicinity, basalt flows dated at. 16.7 and 14.2 14.6 million years (Appendix 2A) are associated with these deposits. The breccias and fanglomerates locally lie undisturbed across faults in the volcanic-sedimentary sequence; elsewhere (as in the alternate siting area) they are

,offset as much as several hundred feet of faults.

D. Post-Basin and Range Interval Basin and Range tectonism locally continued, at a diminished rate through the end of Pliocene time.

With the tectonic development of the ranges and intermontane basins into essentially their present form, alluvial fans and pediments grew at the range bases. The major faults delineating the ranges and

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50, basins have been largely concealed by these post-orogenic deposits in the alternate siting area. Large amounts of debris from the eroding mountains were transported across I

these pedhrlent-fan systems into the basins, and considerable thicknesses of basin fill accumulated. In the alternate siting area, alluvial deposits probably do not exceed a few hundred feet in depth, but in the Phoenix Basin to the east, 2,000 7,000 feet of fine-grained basin fillhas accumulated.

Through-flowing drainage developed in late Pliocene time as the basins were filled. The Gila River was inte-grated through the alternate siting area, flowing west through the Phoenix Basin then south around the west end of the Buckeye Hills. The highest river level was about 80 feet above the present channel and was the local base level for a generation of old alluvial fans. The Gila River subsequently incised to 40 feet above the present channel. This new level again acted as a local base level and extensive pediment-alluvial fan systems were graded to it.

Shortly after the river reached the "40-foot" level, the Gillespie and Arlington basalts (radiometrically dated at about 3.3 million and 2.2 million years, respectivelyt. were extruded, flowing across the ancient flood plain. The flows constricted the river forcing it into a sharp bend where it flows out of the Phoenix

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51 Basin. The Gillespie basalt flow dammed the river for a few hundred or thousand years and forced it to cut a new channel to the 'east of the 'original channel in the younger alkali basalt unit at the west end of the Buckeye Hills. The river remained at, the 40-foot level for some time after eruption of the basalt flow, and post-basalt terraces such as those along Beehive Wash were graded,to it.

A second period of incision occurred in post-basalt (probably Quaternary) time. A period of minor aggrada-tion followed by slight incision formed a low terrace about 20 feet above the modern river flood 'plain. This terrace contains potshards of Hohokam Indian culture which are archeologically dated at about 1,100 B.P.

The general degradational trend during Pleistocene time has resulted in slow stripping away of old alluvial fan surfaces and incision of. tributary streams.

2.5.2.5 En ineerin Geolo ic Evaluation of Features Which Could Affect Cate or I Structures A. There is no reported or observed physical evidence indicating response or failure of the bedrock or alluvial deposits during prior earthquakes. The unbroken surface profiles of alluvial fan and river terrace deposits which are overlain by the Gillespie basalt flow indicate that the area has been tectonically stable and unfaulted since extrusion of the basalt (radiometrically dated at about 3.3 million years). Historic seismicity indicates

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52 the site has only been subjected to very mild ground shaking during the last 100 years. Ground shaking due to historic seismicity would not have been adequate to adversely influence earth materials at the site.

B. Any Category I'oundations in Area A (Fig. 2.5-5) would be in undeformed alluvial fan deposits with a minimum thickness of about 60 feet. With the exception of collapse potential in shallow soils discussed in Section E below, these sediments are firm, consolidated, and continuous; they show no evidence of shears, joints, folds, faults or other tectonic features.

C. No zones of structural weakness, or crushed or dis-turbed materials have been identified in the alluvial fan sediments underlying Area A or the Gillespie basalt (Fig. 2.5-5). Zones of alteration and irregular h

weathering profiles also are not present in these sediments.

D. Any Category I foundations in Area B (Fig. 2.5-5) will probably encounter rock above a depth of 60 feet; however, there is no evidence of unrelieved residual stress in the rock or sediments.

E. The soils found in Area A (Fig. 2.5-5) are locally poorly sorted and of low density. These local low dense'.ty soils indicate potential earth collapse upon addition of moisture while .under load. A few borings fuIIaa

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53 penetrating these soi,lp suggest that no such low density soils occur below a depth of l0 feet, so no problems axe anticipated in excavating and recompacting these soils.

Except for these shallow, low density soils, no sedi-ments in the alternate siting area appear to be unstable because of mineralogy, lack of consolidation, water content, or potential liquefaction during seismic events.

Although granular soils do overlie bedrock, the water table is below the soil-bedrock interface (Appendix 2J).

F. With the exception of the collapse potential of shallow low density soils discussed above, areas susceptible to differential consolidation, cratering, and fissuring are not anticipated in Areas A or B (Fig. 2.5-5). Such natural features as tectonic depressions, cavernous or karst, terranes, salt bodies, and other massive solu-ble deposits were not observed in the siting area. Mining claims in the vicinity of Webb Mountain have been worked on a small scale, but no large-scale or underground mining activities are anticipated in the siting area.

G. The irregularly graded (poorly sorted) alluvial fan deposits which form most of the soils in the siting area include abundant gravel and numerous boulders.

These large clasts make i,t difficult to collect undis-

-turbed drive and Pitcher samples from drill holes.

Therefore, a detailed laboratory investigation was not

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54 attempted for this study, HoWeyer, specific gravities and rock densities (Appendix 2K) were obtained from NX core samples of bedrock units which underlie the soils and from the gzllespie basalt.

The shear and compressional wave velocities obtained from five shallow refraction seismic surveys (Fig.

2.5-5) were used to compute the preliminary shear modulus of shallow soil along the seismic lines (Appendix 2K).

Soil samples were obtained from backhoe pits dug on the same five seismic survey lines (Fig. 2.5-5).

Laboratory tests performed on the samples include sieve analysis, in situ moisture content, dry density and compaction (Appendix 2K).

H. Ground subsidence oi earth fissuring related to ground-water withdrawal have not been reported or observed in the alternate siting area, nor are they likely to pose a problem in the future. Existing groundwater levels usually occur within bedrock units, not in the unlithi-fied soils. The bedrock units are not susceptible to consolidation or compression, should lowering of the groundwater table occur.

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55 2.5.3 GROUNDWATER A detailed hydrologic invests,gati;on has not, been performed.

I Existing groundwater conditions are discussed in Appendix 2J.

Coolant for plant operations is expected to be developed from sewage effluent from the City of Phoenix. For this reason, significant demands upon the groundwater regime are not anti-cipated. Injection of fluids into the subsurface materials is not anticipated.

2. 5. 4 GEOPHYSICAL SURVEYS The following geophysical surveys were conducted in the alter-nate siting area:

A. Eight shallow refraction seismic surveys to measure compressional wave velocities in subsurface materials and five shallow refraction seismic surveys to measure both shear and compressional wave velocities.

B. Local total field intensity ground magnetic survey.

to investigate subsurface structures.

C. Down hole geophysical logs for correlation purposes.

The locations and results of these surveys are given in Appendices 2C, 2G, and 2H.

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56 2.

5.5 REFERENCES

CITED

l. Heindl, L. A., 1959, and A~strong, C. A., 1963, Geology and groundwater conditions in the Gila Bend Indian Reservation, Maricopa, County, Arizona: U. S. Geol.

Survey Water $ upply Papex 1647-A, v. 64, p. 443-458.

2. Rehrig< W. A., and Heidrg,ch, T. L., 1972, Regional fracturing in Laramide stocks of Arizona and its relationship to poxphyry copper mineralization:

Ecen. Geol., v. 67, no. 2, p. 198-213.

3. Ross, C. P., 1923, The lower Gila region, Arizona: A geographic, geologic and hydrologic reconnaissance with a guide to desert watering places: U. S. Geol.

Survey Watex Supply Paper 498, 237 p.

=4. Wilson, E. D., Moore, R. T., and Cooper, J. R., 1969, Geologic Map of Axizona, Arizona Bur. Mines and U. S.

Geol. Survey, cooperative map.

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APPENDIX 2A Radiometric Datin of Tertiar Rock Units, In "and Around the

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2A-l APPENDIX 2A Radiometric Datin of Tertia'r Ro'ck Units In and Around the Gillespie'am Alternate Siting Area A. Introduction Twenty samples of late Tertiary rocks were obtained for radiometric dating. Such dates facilitate inter-pretation of geologic history and age of faulting in the siting area. Dates from outside the siting area which relate to siting area geology are also discussed.

Basement rocks of Precambrian or Laramide age were not dated because they are much older than the Late Tertiary ages which are of concern to nuclear power plant siting.

All data were obtained by whole-rock potassium-argon dating of suitable samples. Dating analysis was done by the Laboratory of Isotope Geochemistry, Department of Geosciences, University of Arizona, Tucson, Arizona, and by Geochron Laboratories, Cambridge, Massachusetts.

B. Results The locations and radiometric dates of samples from the siting area are presented in Figure 2A-l and Table 2A-l, respectively. Pertinent radiometric dates from geologic units outside the Gillespie Dam area are given in Table 2A-2.

The Tertiary bedrock sequence in the alternate siting area is well-dated from the welded tuff in the upper fuaao

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2A-2 part of the tuffaceous sandstone unit through the younger alkali basalt unit. The upper part of the tuffaceous sandstone unit is about;28.8 million years old, while the base of the older alkali basalt which I

overlies it is about 26.7 million years in age. A s'ample of tholeiitic basalt gave a date of 23.5 million years. Dates from flows and dikes of the younger alkali basalt range from 19.1 to 21.6 million years.

C. Discussion The date obtained from sample GDAS-19 (Table 2A-1) is apparently anomalous because it suggests that the top of the tholeiitic basalt is older than the under-lying tholeiitic basalt and older alkali basalt units.'lso, a duplicate sample (GDAS-18) yielded a date of about 20.7 million years, which is consistent with the observed stratigraphy.

Dates from the Palo Verde Hills volcanic sequence (Table 2A-2) indicate that it is probably correlative with and younger than the younger alkali basalt unit in the Gillespie Dam area.

The stratigraphic relations and age of the mixed pebble breccia unit. are undetermined in the Gillespie Dam area, but posse;bly cox'relative indurated fanglomerates in the Palo Verde site area, contain a basalt interbed radio-metrically dated at 16.7 + 0.3 million years (Table 2A-2).

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2A-3 Local exposures of sirgi,lar, tilted Tertiary fanglomer-ates are capped by basalts south of the Belmont and Vulture Mountains. These basalts are radiometrically dated at about 14,2 and 14.6 million years, respectively (Table 2A-2). By analogy, the mixed pebble breccia unit may be about 15 million years old.

The ranges of dates obtained from the Arlington, Gillespie and Gila Bend basalt flows overlap considerably.

Geomorphic evidence suggests that the Gila Bend flow could easily be composed of several flows which differ considerably in age (Table 2A-2). However, the Arlington and Gillespie flows each appear to have been formed during a short volcanic episode. Dates from the Arlington and Gillespie basalts tend to cluster around the respective average dates of 2.2 and 3.3 million years.

Although the radiometric dates suggest that the Gillespie basalt is older than the Arlington basalt, several lines of geomorphic evidence suggest that it is somewhat younger. This discrepancy may be partially explained by. the fact that all but one sampl'e of the Arlington basalt (Table 2A-2) were dated by the Laboratory of Isotope Geochemistry, while all but two samples from the Gillespie basalt were dated by Geochrqn Laboratories (Table 2+-1), The dates returned by Geochron were consistently older than those returned by the other laboratory, even on duplicate samples. Xt has been fuaaII

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noted in the past (Dr. Paul Damon, Laboratory of Isotope Geochemistry, personal communication} that Geochron's dates from Plio-Pleistocene flows have been older by about one million years than those of the Laboratory of Isotope Geochemistry.

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2A-5 TABLE 2A-1 Radiometric Dates from Rock Units in the Gillespie Dam Alternate Siting Area Sample Number Dating Age in Gillespie Dam Fu ro No. Lab No.* ~

m. Rock Unit, GDAS-1 R-2634 3.4 + 0.4 Gillespie basalt GDAS-2 R-2633 4.2 + 0.4 Gillespie basalt GDAS-3 R-2632 3.4 + 0.4 Gillespie basalt GDAS-4 R-2635 3.8 + 0.4 Gillespie basalt GDAS-5 R-2636 3.5 + 0.4 Gillespie basalt GDAS-6 R-2637 3.3 + 0.4 Gillespie basalt GDAS-7 R-2638 19.3 + 1.0 Younger alkali basalt GDAS-8 R-2639 19.6 + 1.0 Younger alkali basalt GDAS-9 R-2640 19.1 + 0.9 Younger alkali basalt GDAS-10 R-2641 3.0 + 0.4 Gillespie basalt GDAS-11 73-4 2.62 + 0.45 Gillespie basalt GDAS-12 73-25 28.8 + 0.5 Tuffaceous sandstone GDAS-13 73-7 1.31 + 0.43 Gillespie basalt GDAS-14,. 73.26 26.7 + 0.4 Older alkali basalt GDAS-15 73-27 19.6 + 0.4 Younger alkali basalt GDAS-16 73.8 19.9 + 0.5 Younger alkali basalt GDAS-17 R-? 23.5 + 1.2 Tholeiitic basalt GDAS-18 74-159 20.7 + 0.5 Tholeiitic basalt GDAS-19 R-2802 28.5 + 1.5 Tholeittic basalt GDAS-20 R-2803 21.6 + 1.1 Younger alkali basalt

• Samples with dating lab numbers beginning with "R-" were analyzed by Geochron Laboratories; other samples were dated by the Laboratory of Isotope Geochemistry.

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2A-6 TABLE 2A-2 Pertinent Radiometric Dates from Rock Units Outside the Gillespie Dam Alternate Siting Area Sam le Number Pugro Dating Age in No. Lab No. m>> Gillespie Dam Rock Unit P-1 73-21 19.1 + 0.4 Palo Verde Hills Volcanic Sequence P-2 73-31 19.9 + 0.4 Palo Verde Hills Volcanic Sequence P-3 73-29 19.8 + 0.4 Palo Verde Hills Volcanic Sequence 73-30 19.1 + 0.7 Palo Verde Hills Volcanic Sequence 73-33 20.3 + 0.7 Palo Verde Hills Volcanic Sequence P-6 73-32 17.7 + 0.6 Palo Verde Hills Volcanic Sequence P-7 73-22 19.4 + 0.4 Palo Verde Hills Volcanic Sequence P-8 73-94 16.7 + 0.3 Basalt interbed in fanglomerate 73-10 14.6 + 0.4 Basalt capping fanglomerate near Belmont Mountains IB-1 73-11 14.2 + 0.2 Basalt capping fanglomerate near Vulture Mountains AR-1

• R-2661 5.6 + 0.7 Arlington basalt AR-1 73-149 1.96 + 0.12 Arlington basalt Ak-1 73-149 1.79 + 0.24 Arlington basalt AR-2 73-151 2.22 + 0.20 Arlington basalt AR-3 73-150 1.24 + 0.24 Arlington basalt AR-4 73-9 2.25 + 0.36 Arlington basalt AR-5 73-3 3.19 + 0.19 Ax'lington basalt fU>>RI>>

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2A-7 TABLE 2A-2 (cont. )

Sam le Number Fugro Dating Age in No. Lab No.* mo ~ Gilles ie Dam Rock Unit.

AR-6 73-152 2.10 + 0.23 Arlington basalt GB-1 R-2585 2.5 + 0.9 Gila Bend basalt GB-2 R-2584 4.5 + 0.9 Gila Bend basalt GB-3 R-2583 6.5 + 0.1 Gila Bend basalt

• Samples with dating lab numbers beginning with "R-" were analyzed by Geochron Laboratories; other samples were dated.

by the Laboratory of Xsotope Geochemistry.

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APPENDIX 2B Drillin Pro ram at Gillespie Dam Alternate Sitz.n Area

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2B-1 APPEND1X 28 DrillinA'lt'er:na't'e Pro r'am 'a'O'i'1'les ie S:i.'t'i:n' Ar'ea Dam A. I'ntroductzon Subsurface geology zn the Gillespie Dam siting area was investigated by a 22-hole drilling program. A drill rig 7

Fugro geologist accompanied each to super-vise the drilling operation and log samples. The locations of the drill holes are shown on Figure 2B-l.

Results of downhole geophysical studies are given in Appendix 2C.

B. Sampling and logging methods Three types of sampling were part of the drilling pro-gram. Whenever the rock was coherent enough, continuous NX core samples were taken. Pitcher samples, either continuous or separated by five to ten feet rotary drilled intervals, were obtained whenever possible from unconsolidated deposits. Where neither of the above techniques was feasible, rotary drilling was employed, and, in some cases, bagged wash samples were taken.

A drill hole log typically gives the depth, type, number, percent recovery, and a detailed lithologic description of each sample. Xt often includes drilling rates, drillers~ comments, and other pertinent remarks.

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2B-2 C. Results The results of the drills.ng program are presented in the form of condensed lithologic logs (Figs 2B-2 through 2B-23). Major geologic units are indicated by upper-case designations and include granitic basement, tuff-aceous sandstone, alkali basalt, mixed pebble breccia, unconsolidated fan (or alluvial) deposits, and Gillespie basalt. Each major unit or its important subunits are described in detail.

D. Discussion A 10-hole drilling program was originally designed to explore the subsurface geology near proposed plant sites in the valley of Windmill Wash, west, of the Gillespie basalt flow. Drill hole 1 was used to determine foundation characteristics at the meteoro-logical tower site (Fig. 2B-1) and was not drilled by Fugro. Drill holes GDDH-2, 3, and 4 (Fig. 2B-1) were located along an east-west line to provide a representa-tive geologic cross section across the valley (Fig.

2B-24). Drill holes GDDH-5, 6, 7, 8, 9, 10, and ll (Fig 2B-1) were located in the southern part of the valley near highlands of granitic basement or Tertiary bedrock.

A number of faults were recognized during detailed geologic mapping of Tertiary volcanic-sedimentary bedrock. None of the faults which could be traced to

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2B-3 beneath old alluvial jan deposits were observed to displace those deposits, eon though the deposits pre-date the Gillespie basalt flow (potassium-argon dated at about 3. 3 mzllion years} .

I The proposed plant sites were relocated northward, away from the faulted bedrock. As a result, proposed drill hole GDDH-ll was not drilled and several new holes were located further north. Drill holes GDDH-12 and 13 (Fig. 2B-1) were placed on east and west projections, respectively, of the line through GDDH-2, 3 and 4 (Fig. 2B-24). Drill hole GDDH-12 penetrated the western edge of the Gillespie basalt flow to examine the relation of the basalt to underlying units particularly alluvial fan deposits (Fig. 2B-26). Drill hole GDDH-13 was part of an attempt. to examine a pro-posed buried bedrock fault bounding the metamorphic highlands west of Windmill Wash. However, the bedrock stratigraphy was not defined well enough to identify any displacement between GDDH-13 and GDDH-4. Drill holes GDDH-14 and 15 (Fig. 2B-1) were drilled further north, with the latter on the northwest side of the Gillespie basalt flow.

Drill holes GDDH-6 (387 feet deep} and GDDH-8 (780 feet deep} penetrated entirely different bedrock sequences (Fags. 2B>>1} suggesting that a large bedrock structure occurred between them. Drill holes GDDH-16, 17 and 18

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2B-4 (p igs. 2B-l, 2B-25) Were drilled to more closely def ine the nature and location of the structure. Drill hole F

and geophysical data now indicate that the structure is a buried fault which lies between GDDH-16 and 18.

Drill holes GDDH-19 to .24 (Fig. 2B-1) were part of a proposed plant site investigation on the northwest side of the Gillespie basalt flow. They are discussed in more detail in Appendix 2D.

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L I THOLOGIC OATA ELEV -948'NCONSOLIDATED 0 ~ ~ FAN DEPOSIT O

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~ SILTY SAND AND 5ANDY 5IL TS 'ilyhf brown; loose or very efifftongu/or, poorly oar/ed, volcanic, fo dence 8 00 metamorphic',and guar/z c/acts; occaeional bouldere.

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E O SANDY CJPAVELSired-brown/ angular /o eubrounded guar/7, volconic, and metamorphic claoto.

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CLAYEY CRAWL/red-brown/ angu!or volcanic c/auto.

MIXED PEBBLE BRECCIA I20 LITIIIC WACKE WITH BRECCIA INTEPBEDS/medium to dark gray, cemented; thinly bedded; angu/ar; poor ly l60 eor/ad, vole onic clast'.

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light red; moderately cemented; IOO thinly bedded; 25-N'%ny'ulor, poorly oortad, granitic, volcanic, metamorphic claefa. VOLCANIC PEBBLE 8RECCIAI II'ght fun; poorly cemenfed; crumblee in Ha0; volcanic breccia clout'ery weathered.

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-gQ:0 'Q<.g 200 MIXED PEBBLE BRECCIA; light grey to'red-brown; c'emenfed; thin to thickly bedded; 30-70 % angular, poorly eorfed, volcanic claefa with +5% metumorphica .:~n'.o'.O-'. ond gronitice,'ome highly weofhered claate. 300 9:::,j'.:oD. d: .:.00"oo 0 .,o.". GRAVELLY LITHIC WACKE WITH BRFCCIA INTER8ED5; 400 red-brown; cemenfed; thinly bedded; poorly oorfad, volccrnic and metamorphic lithic ound. MIXED PEBBLE BRECCIA WITH LITHIC WACK; rad-brown g cemented; thick to thinly bedded; I5-70% angular to aubangular, poorly d'or fed, volconic, metamorphic und 500 lgranitic breccia clcrefe; Iifhic cond matrix, occaaionolly claote drop below I5%. GRAVELLY LITHIC WACKE; comenfed; thinly beddact. red-brown to gray; MIXED PEBBLE BRECCIA WITH LITHIC WACKF INTFRBED5. 600 QQ GRAVELLY LITHIC WACKE; fhinly bedded. red-brown,'emenfed; 0,:-'MIXED PEBBLE 8RECCIA WITH LITHIC WA C gE INTFRB EDS. ~TD-680 700 80 Arizona Nuclear Power Project SAMPLE DEPTH Gillespie Dam LOCATION (FEET) Alternate Siting Area Rotary and Pitcher Sampling CONDENSED DRILL HOLE LITHOLOGIC, LOG, GDDK-3 Diamond Coro Figure 2B-3 AHPP-1 5-3788 I I I I I I I I I I L I THO LOGIC DATA ELEV 9'94'NCONSOLIDATED FAN DEPOSIT O ,0 ~ ~ 'o. 'o O 0'. '7 B C 40 'o O .0 light red- brown -medium brown; ~ E TRAVELS, SANDS, SILTY; r ~ ongulorl poorly sorted, fina to coarse, vole'onic and O 0 metamorphic sands, grovels, and slits'. Q 'c 80 .O. o O ' MIXED PEBBLE BRECCIA

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I 20 .:5jj'.:.'p,'~ojj.. ('.- fl'$>:.:.OO MIXED PEBBLE BRECCIA WITH LITHIC WACKE INTERBED5'I brown; cemented; thinly bedded; IS-SO% angular, poorly sorted, voleonic and metamorphic c!ostsl cccasionolly breccia clasts drop below IS%, Q. 'goo' 200 TUFFACEOUS SILT&ONE;greenish- yellow tan; moderotely '+QSQ:Q t~o: cemented; thinly bedded; S-IOP'ark mafic(P) grains. -os";.oQ:.:0: MIXED PEBBLE BRECCIA - soma os above. OQ5)'+e 5 ~ .>':g: 280 "': 6'"n-'. TD "294 S20 Arizona Nuclear Power Project SAMPLE LDEPTH '"Gillespie Darn LOCATION, (FEET) Alternate Siting Area CONDENSED DRILL HOLE g Diomond Core LITHOLOGIC LOGE GDDH-4 Figure 2B-4 AllPP-1 0-3790 I I I I I i I I I I I L I THOLOG1C DATA N N

• SILTY SAND AND $ 4NDY SILTY; red-brovvn; loose to very stiff; subangular, poorly sorted, volcanic'nd metamorphic

~ ~ o ~ ~ c/asts. ~ ~ ~ ~GRAVELLYSAND'5 AND SANDYGRAVEL5'Ilight'rown; medium dense,'ngular, poorly sorted, volcanic li thic sands +0 c A><c moderate colichlfication. and'ravels,'ceo/ a )h 'V <hOt Vy)1 t t >~s h a< c 7 h C ALKALI BASALT ALKALIBA5'ALT I=LOW5'ND BRECCIASI red fo medium gray-green,' ho'locrystalline, porphyritic (ferromagnesi on phenocrysts); mocterately to intensely fractured; flow 80 h C.~)C) breccias common> cinder P matrix. h TUI=IACEOU5 LITHIC WACKE; light-medium brown,'ell indurated; thinly bedded; subangular, moderately sorted, volcanfc lithic frauen ts. ALKALI BA5ALT BRECCIA ~TUFFACEOVS'ITHIC WACKEI light-medium brown,'ell I20 indvrateo", thinly bedded; subangular i modarotely sorted, volcanic lithic tragments. l60 ALKALI BA5ALT BRECCIA h c c >c > h V r'3> Ic' 1 C' 200 TD-802 240 280 Arizona Nuclear Power Project SAMPLE DEPTH Gillespie Dam LOCATION ( F EE T) Alternate Siting Area Rotory and Pitcher Sampling CONDENSED DRILL HOLE LITHOLOGIC LOG, GDDH-5, Diomond Core Figure 2B-5 AkPP-7 3 3792 I, I -I I I I I LITHOLOGIC DATA ELEV -987'NCONSOLIDATED 0 ~ R~' ~ FAN DEPOSIT ~ ~ ~ 5II.TY 5AND Ah!D 5ANDY 5ILT5 WITHGRAVELIII'ght brown; loose to very stiff; highly calcareous; angular, poorly sorted, occasional c'obblas or boulders. 50 7 p V r) lr 5ANDYGRAVEL5 AND GRAVFLLY5ANDS; red-brown; loose to dense.'ngular, poorly to moderately sorted volcanics, quartz, and'eldspar. L c v A ALKALI BASALT v 1 C IOO c ALKALI BA5ALT I=LOW'O'ND BRECCIAL; purple-gray; L>< r hol ocrystalli no, porphyritic (ferronragnesian I 4Ah v 4 )c phenoci ysts)I locally vesicular; moderotaly to intensely I50 A h C l V fractured; flow breccias common. ALKALI BASALT WITH INCLUDED O'EDIMENT5'I bascrlt as obove; throughly mixed wi>h cr red-browne fine r> v grained, feldspathi c wac/ce. V C L C 4 c 4 v v 200 rVP'67, A v A P V V vv4cA V 7 r v v 1 P r v r ALKALIBA5ALT I=LOW5'ND BRECCIAL-same crs above. C 250 C vc r 7(1 q 1 c 300 r I A TUFFACEOUS SANDSTONE l GRANITIC PEBBLE CONGLOMERATE WITH LITHIC

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~hite to brown; thin to thickly bedded; 20-80Ã WACKE'NTERBEDS; ~ df poorly sorted, oubrounded, granitic clasts. 550 aUARrZ LATiTE PORPHYRY; red; holocrystalline; flow ~ "" ', ~ 'i ~; banding; phenocrysfcr of quartz and feldspar. r I ( wIr 'ie')r 'L I ~% I / ir5 GRANITIC PEBBLE CONGLOMERATE WITH LITHIC WACKE i iiili i /~ w~r/Nr lh INTERBEDS- same as above, )r r lrrr GRANITIC BASEMENT ROCK MONZONITE - QUART2 MONZONITE PORPHYRY; white; porphyl'Ihc'/Ã- hypidiomorphic. Arizona Nuclear Popover Project SAMPLE DEPTH Gillespie Darn TD-887'00 LOCATION (FEET) Alternate Siting Area Rotary and Pitcher Sampling CONDENSED DRILL HOLE LITHOLOGIC LOG, GDDH-6 Diamond Core Figure 2B-6 AHPP-7 5-3794 l I l I Ii I' I I I I I L1THO LOGIC DATA I 0 . a e UNCONSOLIDATED FAN DEPOSIT e 15 ~ ~ SANDS, SILT/, AND GRAYFLSI brown,'oose fo dc'noel ,o, angular, poorly oorfcd'i!ts, fine fo coarse sonCk, gravelo,'ocal moderafe calichificafion,'ocally clayey. d 0 ' ~ ~ '0' I V 7 ALKALI BASALT C h rV ~n > rV l '7 t'7 V 7 V ) ) ~ 60 h q V n ALKALI I3ASALT FLOWS'ND SRFCCIASI lighf purple-7 V c 7I g 7 @ray fe rcd-brown; holocrysfollinc, porphyritic n C ~ 7 I (terromagnesion phenocrysfo)l locolly vesicular; 7 7~Vs I C7t. modorofely fo infeneely fracfured; flow breccia V '7 ( common. 75 C TUFFACEOU5> FELDSPA7HIC, SANDSTONE; lighf red; cemented; fhinly bedded; oubangulor fo rounded, 90  ::::::.i:::;:~!".::::,. moderate!y fo well eorfed, fine send, quortz, fcldhpor, VV' v n <c'LKALIBASALT FLOW h TD- IOO I05 12[) Arizona Nuclear Power Project SAMPLE DEPTH . GilleSI)ie Dln11 LOCATION (FEET) Alternate Siting Area Rotary and Pitcher Sampiing CONDENSED DRILL HOLE LITHOLOGIC LOG, GDDH-7 Diamond Core Figure 2B-7 ANPP-]y-2198 I I I Ij i L I I I I I LITHOLOGIC DATA / 0 d p'...: 'd. '.'o.d .'o.. "d.O ~ ...O 0 'Q o:. c..d ~ 'o. qd4:

4. ~ UNCONSOLIDATED FAN DEPOSIT GRAYELLVSAhlDS; brown; denset subangular, poorly sorted, granitic, volcanic and mes'amorphic c lasts.

L GRAYELS AND SANDY GJi'AYELS; brown; loose to o'ense; ~ subangulor, poorly sorted, vole'onic riravels. MIXED PEBBLE BRECCIA IOO 200 VOLCANIC PEBBLE BRECCiA;medium gray; cemented; thinly bedded; 20-7$ Po of angulor, poorly sorted, volcanic and tuff fragments; occasionol interbedh of red-brown, moderotely sorted, volcanic lithic sand and silt. 300 o'o~oO-'g'Q<o

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o~.'.: 4::4'OP@.:o.p '.~0+ ..:" GRADATIONAL LITHOLOGIC CHANGE 400 500 'io.; 'Q,'ao, ..:.:Q.'o~0" MIXED PEBBLE BRECCIAl light grays cemented, thinly 600 bedded; 20-AY of angular, poorly sorted, volcanic, metamorphic, and gronitic clasls; occasional interbeds of lig'ht brown, poorly to moderately sorted, fine to .".cX~.o.:o c'oarse quartz, feldspar ond lithic sand. 'o.~g:.:: i': 700

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-",O.o,.;~.O:. 0o'; 70 80$ -780'AMPLE Arizona Nuclear Power Project DEPTH Cillespie Darn LOCATION (FEET) Alternate Siting Area mm Rotary ond Pitcher Sampling CONDENSED DRILL HOLE LITHOLOGIC LOG, GDDH-8 gg Diamond Core Figure 2B-8 ANPP-1 5-3198 I l l I I l I I I I l l l l lj I I gi L I THOLOGIC DATA I UNCONSOLIDATED FA N DEPOSIT ~ ~ ~ o, ~ 0. ~ o 25 o ~ 0' ~ 0 ~ ~ D 5AND, 5'lI T, oRAVEL; red brown,'oose fo dense or very sf/'ff; slightly fo highly calcareous; angular fo 50 subrounded, poorly fo well sorfed, volcanic ond grani fic clasfs, cobbles of volconics + I diomefer, guar fz increosi ngly obundonf w/'th de p th. ~ p ~ 0 75 o'h/ w / GRANITIC BAS EMENT ROC K )/N/ W/g~ / Nl -( -/-/-i- i)-/i/i /I/ r~ / /i<<-i 'very weofhercd; red-brown / ~/ I ~ QUARTZ MONZONITf; /~/ t I i/N/'/ white,'oft (crumbly); coarsely crystalline t decompose IOO grani fe inploce, grading fresher downward, locolly Ir~l/ / i -/~/ I/. sfoined red. <~ /i I/ w g /I '/i/i %/K ~(/I ~/ il I 25 g/%% / /g / Nl / /(~/ l / ~//4i yWI l/ i/w w ~ g el~/hi I/~/ i w%//~ ii/ QUARTZ MONZONITf-MONZONITEI white fo I/'ghf fan, I50 I /y i/l /~ ir 1/r /iver ri/- -~/i i i/> i)ihli loco/ red stain; medium fo coarse grained, L/ // i/ la hypidiomorphic gronular, moderafe fo very fracfured'. ~)/I r I/q/ l N g / ili-i I /I / / /i~/ I / i/X/%- /t %L l75 TD -IFZ 20$ Arizona Nuclear Power Project SAMPLE DEPTH Gillespie Dam LOCATlON (FEET) Alternate Siting Area CONDENSED DRILL HOLE Rotary and Pitcher Samplinp LITHOLOGIC LOG, GDDH-9 Diamond Core Figure 2B-9 ANPP-lb-3SOO I I I I I I I' I I I L I THOLOGIC DATA I 0 UNCONSOLIDATED FAN DEPOSIT I 20 ~ ~ ~ 0 ~ SILTY SANDS AND TRAVELS; light fan to gray; highly calcareous; loose,'ubangular, poorly sorted. volcanic ~ ~ and granific fragments; occasional volcanic boulders. 'll v ALKALI BASALT V C 40 v t L C v n V l f') ) t. )C A> V V C L 60 0 n V C'C, ALKALIBA5'ALT FLOWS AND 8RECCIASI purple - gray ~L7 A Io red; ho!o crystalline, porphyritic (ferromarlneoian h v7hn C phenocrysfs); locolly vesicular, very to intensely h h fractured; flow breccia common. V 80 WC< C V C' 3 n V V< ht )V + >C h v IOO MIXED PE88LE CONGLOMERATE; mottled orange - red-C* brown,'emented; thinly bedded; subrounded, poorly C) n sorfed, volcanic and mefamorphic clasfs. VCA A A I 20 A ) AV v L V 7 V v C C A ALKALI BASALT FLOI/Y5'ND &RECCIAS h L 7 C I40 7AL V lr )r v C V V C C L TD -/$ 2 Arizona Nuclear Power Project SAMPLE DEPTH Gillespie Dam LOCATION (FEET) Alternate Siting Area II Rotary and Pitahar Sampling CONDENSED DRILL HOLE LITHOLOGIC LOG, GDDH-10 Diamond Core Figure 2B-10 ANPP-75-3803 I I I I I I I I I , L I THOLOG I C DATA ELEY. -964 0 o tt ~ tt' O 0 CLAYEYAND GRAVELLYSILT WITH 5'AND: brown; verystiH; highly colcoreous; calichefied basalt and alluvjal Fan depo oi ts. GILLESPIE BASALT BA6'ALT; dork gray to black; holocrystolline, ophonitic'; vesicular very to'intense!y FracturedI locally scoriaceous; zeolite(PS vug filling. 60 '0 UNCONSOLIDATED FAN DEPOSIT '0' ~ ~ ~ ~ 0' ~ 0 ~ .,~ ~ re ~ ~ ~ ~ GRAVELS AND GRAVELLY 5AND WITH 5IL7; red-brown to block; loose,'ngular, poorly sorted, volcanic; gran/'tie,and 90 tp r ~ ~ ~ minor metamorphic Ihehic rock Fragments. ~ ' ~ 5ANDY 5ILT WITH GRAVEL; light brown; hard; highly colcareous; volcanics abundont. MIXED PEBBLE BRECCIA '1'.o""" I20 ob ..rO .. O.."q. VOLCANIC PEBBLE BRECC!3 WITH LI7HIC WACKE IN TE'RBFDS; red-brown- gray ',cementedp thinly bedded; l50 IS -2$ % of ongulor, poorly sorted, volconic clasts. .'P..; " TD -/69 I80 2IO 24 Arizona Nuclear Power Project SAMPLE DEPTH t. Gillespie Darn Ig LOCATION . (FEET) Rotortr ond Pitcher Somptintt Diomond Core 1 r Alternate Siting Area CONDENSED DRILL HOLE LITHOLOGIC LOG i GDDH-12 Figure 2B-ll ANPP-7 5-3804 l I l l, L l' I l I l, i i )I I L I THOLOGIC DATA UNCONSOLIDATED FAN DEPOSIT o.'o'. CLAYEYAND SANDY GRAVELI bghf red'-brown,'oose fo e 0

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.. o-.o .:.'o . 0 'O. dense; very colccrreous; angular, poorly sorted,mcfcrmorphi (p'hyllife and shist) clasfs. o'4 METAMORPHIC 8RAVELilight brown - light gray g loose; IOO >cr'o '.."Q..:e; calc'oreous; angular, moderately sorts', Icrrg e chlorite O schist clasfs; very little sand, silt, or clay. .C SILTY, SANDY GRAVEL W/TH CLAY; Iighf brown -lig'hf gray; O, loose'- moderofcly dense; poorly camcnfcd; some O U poorly sorfecf. phyllifa, schisf, and quartz coliche,'crngular, fragments. 200 MIXED PEBBLE BRECCIA IvIETAMORPHIC PBBLE BREN/A; red-brown -lighf gray; moderately fo well cemeri fad; crudely sfrafified; c'alcorcous; angular, poorly sorted metamorphic (schist and phyllife) clasfs; occasionol boulders and volcanic clasfs,'oderofely weafhered fo 300 PEBBLE'RECCIAilighf to medium brown to 93.'IXED cemented; crudely stratified;'colcorcous,'O fo gray,'ell 7$ P~ angulor- subcrngular, poorly sorted metamorphic and volcanic clasfs in subequal am ounfs', Iocolly porous with calcite filled covifies and fractures; some gronitic'lasfs below 207. 400 Strong'ly froctured with calcite filling and slic'kan-sides(P) Large calcife filled frocfure. Tuffcrccous(7/ clcry Icryar, I -2 fh/ ck. Mixed pebble breccia-same crs above, cxc'cpf volccrni 'pp clasts increased fo 7$ fo of gravel. Tholeiific bosalfl 2 fhick; boulder or flow'. TD-$37 600 700 Arizona Nuclear Power Project SAMPLE DEPTH Gillespie Dam LOCATION (FEET) Alternate Siting Area Rotary ond Pitcher Sompling CONDENSED DRILL HOLE LITHOLOGIC LOG, GDDH-13 ~ Diomond Core Figure 2B-12 ANPP 7 5-3806 I l I I l I i g I I II l L I THOLOGIC DATA FLEV.-888 0 ~ c t ~ UNCONSOLIDATED FAN DEPOSIT ~ O ,C SANDY SILT WITH 4JPAYELI brovvnt very sfiff; angular> poorly eorfed, silf crnd fina fo medium sand; cfuarfz, granific e, mefamorphics. cf MIXED PEBBLE BRECCIA o~g:.oj'0'.0: o a:o'. ~ ~'.~'o.0 60 o.':O,." .< i:i: ': .'P:o~ .'0'. ~ ...:>0."o.:Oo u ".'o MIXED PEB8Lf BRECCIA/ red-brovvn to grays loosely', Pg. cemenfedj fhinly beddedr angular, poorly sortedi ~ 90 ~'.D.o.,p '.0'.>>..O.'.O.'. Q'og O'.>> p Og ' .4 O... Oy p grcrnific, volcanic and mofomorphic cfasfs; bcrdly P'.p broken vp. "o':O ~: ig'0.'d~.>>'.g

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I 20 :CO'0:o:, . 8 ~ .P'.'Pp';::C",:>

Oig:0: Q~ 'p~o.'

I50 LITHIC VYACKEI purple - gray; cemented; fhlnly beddedt ongular, poorly sorfedt fine fo cdarse Iifhir sand; volcanic, mofamorphic, granifio claefs. IBO TD-/8/ Arizona Nuclear Power Project SAMPLE DEPTH Gillespie Dam LOCATION (FEET) Alternate Siting Area Rotary and Pitcher Sampling CONDENSED DRILL HOLE LITHOLOGIC'LOGi GDDH-14 Diamond Core Figure 2B-13 ANPP-78-3808 ~ l ~ l j l I L I THOLOGIC DATA Q t 0 ELE'V-942 GRAVELLY SANDY 5'IL7; light tonl sof calichefied basalt rubble tt highly calcareous; ono'lluviol fan deposits. 0 + + + + + + + G I LL E S P I E B A S A LT + + + + + + + O 30 + + + + + + + + + + + BA5'AL7; medium gray,'holocrystalline, 0 + + + aphanitic,'esicular CJ + + + + + + + 60 + + + + + + + + Basalt sc ori a + + + + + + + UNCONSOLIDATED FAN DEPOSIT 90 I GRAVELLY SAND5 AND 6'IL7$ red-brovvn to gray-green; loose to dense or very stiff; angular, poorly sorted, ~ ~ ~ ~ ~ ~ 'o volcanic and metamorphic clasts. MIXED PEBBLE BRECCIA 'jQ:'.o.:09; "jj" ,;O>'.o.~:Dn<<..: I 20 ",Q0Q:.g:ow:~- I50 MIXED PEBBLE 8RFCCIAI brown to gray- white; cemented, thinly hooded; angular, poor!y sorted, volcanic ond metamorphic clasts. I 80 'a"'q4~'"0' " .+0'9:i'.KC.,i. 2IO O' 7D-29/ Arizona Nuclear Power Project SAMPLE DEPTH Gillespie Dam LOCATION ( F EE T) Alternate Siting Area Rotary ond Pitcher Sampling CONDENSED DRILL HOLE LITHOLOGIC LOG, GDDH-15 Di om ond Core Figure 2B-14 ANPP "7 5-38 10 I' I I I I I I I I I I i I I L l THOLOGlC DATA d UNCONSOLIDATED FAN DEPOS IT ~, ~ ~ r SILTY 5AND5 AND GRAVELY; brown; loose to dense'poorly moderately sorted, granitic and volcanic clasts. t 40 ! o op:o;. c'.0 'p O."o. 54VDV GRAVEL WITH COBBLE5/black -brown,'oose/ angular, volcanic and granitic clasts. ",g:,o'.:~Qjj':.','.g~d::,,<:,4o MIXED PEBBLE BRECCIA ,.~gougQ.:- ..:g:,'.e.Q'.e O.te'.O9 .o;c " .o"-+'iO;gQo; 80 "Q h VOLCANIC PEBBLE BRECCIA WITH LITHIC WACKE /NTER8ED5; light gray; cemented; thinly bedded; /5-40% of angulare poorly sorted, volcanic c/as/s; locolly breccia ciao/s < /5/~. I20 i~':~'0'.:: o:. w(,N'f) I60 5.. PjfI / ELD5PATHIC lITHIC WACKE; light brown; cement'ed; thinly bedded; angular, poorly sorted, feldspar and lithic sand; breccia ciao/s 5-/0%. 'olcanic 200 240 'i)$ "B.l6 v i L!THIC WACKE WITH VOLCANIC BRECCIA INTERBE'D5/ red-brovvn; moderately hard /o hardt thinly bedded; angular to eubangular, very fine to medium sand; i e ee 5-/0% volcanic clasts. 280 rP Pi>~ <<ed'. TD -902'AMPLE Arizona Nuclear Power Project DEPTH Gillespie Darn LOCATION ( F E ET) Alternate Siting Area IIII Rctcry cnd pitcher Sampling CONDENSED DRILL HOLE LITHOLOGIC LOG, GDDH-16 Diamond Core Figure 2B-15 ANPP-75 3812 I' I I I I L I THO'LOGIC DATA I o o UNCONSOLIDATED FAN DEPOSIT a ~ ~ 0 o ~ I Oi IO 0 D .o 'O 0 o. SWDY GRAVEL; black'-brown> loose; ancfular, volcanic and granitic clos]s. ~ ~ o' '0' ~ '.0. ~ 20 o . 0 ~ D. ~ o ~ ~ ~ a ~ D ' o '0 L ALKALI BAS ALT gL3,I L ) L v I rt 7 r h V C 1 1 L 7VC Ay y C L147V I ~L. ~-Lg L n sr a. C' 1~ ALKALI BASALT; lj'rihf pink - gray ) holocrysfolline,, porphyri fic (ferromagnesion phenocrysfs); very fo infensely fractured. Ly L 50 VL v L ~ C hh V~h 7 V L L r V C 60 V C, 7 1 C TD-63 70 Arizona Nnciear Power Project SAMPLE DEPTH Gillespie Darn LOCATION ( F EE T) Alternate Siting Area Rotary and Pitcher Sampling CONDENSED DRILL HOLE LITHOLOGIC LOGg GDDH-17 Diamond Cor e Figure 2B-16 ANPP-7 5-3S I4 l'j I, Ii I I gi I gi, l gi i ~ I ll ! L I THOLOG1C DATA ELEV.- '(to00,0 ' o.o 989'NCONSOLIDATED FAN DEPOSIT '0'.< 5ANDY, 5/LTY, 6RAVEL/ ton; loose,'highly calcareous; L eubangular clasts. A ( v ( r ALKALI BASALT v 7 40 7 () C."h.h rn ALKALI BA5'ALT FLOWS ANP 8RECCIA; medium gray; A r holocrystalli ne, porphyritic (ferromagnesian phenocrysts)/intensely fractured; flow brecclas 7v(n'( common. 1 l C. 80 "-':::(':::':9:: 'Q FELpspATHIC HrACKE5 A/l/g BA5ALT COBBLE BRECCIA; red-brown; cemented; thin to thickly bedded; angular, '2 g, I g A V poor'to well sorted sondstones and breccias. rv l.pr lr n l'. r v ra,v p 20 I n vt <~< hv<v( lrvnv ~c ALKALI BA5ALT FLOWS'NP 8RECCIA -same as above. ~~" ~n< A i 7 160 n

poorly sorted volcanic, granitic i and metamorphic clasts; occasional boulders. ~TD-209 2IO Arizona Nuclear Power Project SAMPLE DEPTH Gillespie Darn LOCATION (FEET) Alternate Siting Area Diamond Core CONDENSED DRILL HOLE LITHOLOGIC LOG, GDDH-20 Figure 2B-19 ANPP-15-3820 I I I L I THOLOGIC DATA 0 ELEV. -9M BA5ALT RUBBLE/ light brown and black; loose to hard; c'emented; very calcareous (petroca/ci c zone); '0 subangular to oubrounded, poorly sorted basalt clasts. 0 GILLESPIE BASALT O BA 5AL T; dork gray - b/ockl bord to very hard; ncnco/careousl .C thin flow structures; ophanitic, olivine phenocrysts; locolly vesicu/ar or scorloceous,'alcareous and clayey U O vug and fracture filling. 60 /roctured zones, cloyey silt fracture filling. 90 0 UNCONSOLIDATED FAN DEPOSIT '0 S/L T, SAND, NAVEL; red-brown,'oose to dense, poorly . ~ ' to moderotely cemented/ calcareous; angular to

0. subrounded, poorly sorted, volcanic, and metamorphic I20 clasts.

MIXED PEBBLE BRECCIA O l50

"<.'Q4: . g:.'a.:"

O V MIXED PEBBLE BRECCIA; light to medium bronnt soft to modorotely hard; poorly to we// cemented; poorly stratified, angular to subrounded, poorly sorted, I80 volcanic, metamorphic, and grani ti c gravel; occasionol "oo'.:~":".. ': boulders; many disaggregoted, clayey zones; upper contoct unclear. \ .':0()'oQ p.'co:D. ".Q j+':-:i"g 2IO TD-2!0

24) Arizona Nuclear Power Project

,SAMPLE LOCATION DEPTH ( FEE T) D" Gillespie Darn Alternate Siting Area Diamond Core CONDENSED DRILL HOLE LITHOLOGIC LOG, GDDH-21 Figure 2B-20 ANPP-7 5-3822 I 1 I i g I' ~ Li THOLOGIC DATA ELF V. - 983 r ~BA5ALTRUB/3LEin petrocolcrc'atrix. ta GILLESPIE BASALT O .C C 0 BASALT/ medium to dork gray; bord to very hardg U noncalcareous; thin flow structures; aphanitic, olivine crnd pyroxene phenocrysts; vasicu/err/ numerous fractured and reddish scoriaceous rones; calcite, 80 zeolite, or clay fillings of fracture~ ond vesicles. Soft zona I20 UNCONSOLIDATED FAN DEPOSIT o ' ~CLAYEY SILT; red-brown,'tiff; pooi ly to moderotely cementedl calcareous/ unstratifi ed; some rounded I60 o ~ ~ vole'anic grave/. ~CARAVEL, SANO, SILT WITH CLAY; light to oork brown; 0 .. O . ~ loose to dense or stiff; poorly to moderately cemented; locally calcareous; poorly to mEgdarotely sorted, angu/ar ~ to subrounded. volcanic, metamorphic, and granitic C Q~ clasts; possible occasional river grcrvel and scrnd ~V 200 be/ow /50'. t3. :jj4 jQ'PoO'O.':O:~ ..o,:U:C?P:.:Q '::0:O':0'.ah~:,": .:0".' + MIXED PEBBLE BRECCIA kdIXED PEBBLE BRECCIA; Il'ghI Io dork brown; ooII uncemented cloyey zones alternating with moderately hcrrd cemented calcareous zones; poorly 240 ':0:(),j) ..'j.'j sorted, subangular to subrounded, volcanic, metamorphic and granitic gravel; uncemented zones may represent deeply weathered breccia. TO- 246. S'80 32 Arizona Nuclear Power Project SAMPLE DEPTH Gillespie Dam LOCATION (FEET) Alternate'Siting Area CONDENSED DRILL HOLE Diamond, Core LITHOLOGIC LOG, GDDH-22 Figure 2B-21 AKPP-7 5-3824 I 1 l l l r l ~ l ~ ~ I I L I THOLOGIC DATA 0 ELEV. -965;(opprox) ~8asall rubble in petrocolcic matrix. GILLESPIE BASALT '0 0 oj 0 C 0 CD 60 BA5AL7;dork gray lo blackj hard fo very hard; noncalcareous; aphanific, olivine and pyroxene ~ phenocrysfs; vesicular; locaf/g fractured and reddish scorioceous zones; calcite ond clay fi /ling of fractures and vesicles. 90 I 20 UNCONSOLIDATED ALLUVIAL DEPOSITS 5'IL7 AND CLAY('wif'h sand or grovel inferbeds)/ brown; moderotely sfiffto stiff; calcareous; micaceous; medium to thin bedded; poorly sorfed, subangular fo rounded, volconic, mefamorphic, and granific grave/; 150 possible Gi/a R'iver gravel below /95; possible Q. paleoso/ of /26'- /9$ L ~ ~ 060 O light gray-brown; medium dense fo dense; .'AND; none'olcoreous; moderofely sort ed, subrounded to rounded, cO~L fine to very fine groined, quartz, feldspar, and lithic sand. 0 180 MI X ED PE BB LE BR EC CIA Q. MIXED PEBBLE /3RECCIA; brown fo red-browne soft i uncamenfed clayey zone~ wifh occasional hard cernenfed calcareous zones: poorly sorfed, subangular fo 2IO rounded volcanic metamorphic and grantic gravel; uncemenfed zones may represent deeply weofhered breccia. TD-2/5' Arizona Nuclear Power Project SAMPLE DEPTH Gillespie Darn LOCATION ( F EE T) Alternate Siting Area CONDENSED DRILL HOLE Diomond Core LITHOLOGIC LOGP GDDH-23 Figure 2B-22 AMP P-2 6-36 26 P l I I I I I l I i I 1' L I THOLOG IC DATA I 0 ELEV. -94' BA5'ALT RUBBLEtin light brown sandy petrocalcic matrix. 0 GILLESPIE BASALT 20 o C o BASALTIdark pray,'ery hard; noncalcareoue; moderately fo very fractured wifh coliche fracture I'illing; 40 aphanitic, olivine and pyroxene(P) phenocryotz; sfrafification defined by stretched veelcles Cips 40-$ 0. 60 UNCONSOLIDATED FAN DEPOSIT or .C . ~ o' I ~ 4 . ~ o.o ro CLAYEYSANDY 4RAVELI bro'wn; loosel calcareous,'oorly 4 0 or ~ 4 sorted, cubanyular fo eubroonded volcanic and o ~ .4 ~ o metamorphic gravel. o':c?"..'g:~'"..:: ..o:.".~'. ".:og.,: MIXED PEBBLE BRECCIA 80 ~:jag'.ogo4p o. '0:o ~ .e b.oO: ~p.:g:o'O.o': 100 'r. 'O.4;":O:~jO";. "..:;.:.Qc o.': MIXED PE88LE BRECCIA; red-brown to fo moderafely hard; c'olcoreoos,'oorly sorted, gray-brown,'roft o'0":.4'~

• subangular fo eubrounded volcanic, metamorphic, and

o:~.O~:~'. 'granitic grovel in siIty or clayey matrix,'ecol well

.'"..';C':O Oo .'o:.."' cemented zonee,'ay represent deeply weathered o.'"4:Q j +. '0':4 g breccia.

O. o':c 120 ""O.

TP-/22'40 A'rizona Nuclear Power Project SAMPLE DEPTH Gillespie Dam LOCATION (FEET) Alternate Siting korea Diamond Core CONDENSED DRILL HOLE LITHOLOGIC LOG, GDDH-24 Figure 2B-23 ANPP-7 5-3825 APPENDIX 2C Downhole Geo hysical Investi ations at Gxllespie Dam Alternate Sate.n Area I I V I 2C-1 APPENDIX 2C Downhole Geo h. sical Investi ations at A. Introduction The,.drilling program at Gillespie Dam siting area included downhole geophysical logging of drill holes GDDH-2 through GDDH-18 (Appendix 2B, Fig. 2B-1). These studies were carried out by the Geohydrology Section, Branch of Civil and Environmental Engineering, Washington State University. B. Logging Methods Because the following logs were considered most applicable to stratigraphic analysis in the siting area, they were originally run,in each drill hole: (1) gamma-gamma log, (2) neutron-gamma log, (3) neutron epithermal neutron log, (4) natural gamma log. Caliper logs were obtained when drill holes appeared to stand well without casing. Spontaneous potential and single point resistivity logs were often acquired, temperature logs were occasionally obtained. C. Results Geophysical log suites for drill holes GDDH-2 through GDDH-18 are presented in Figures 2C-1 through 2C-16. On the logs it is possible to distinguish between relatively consolidated and unconsolidated fan deposits, but the contact is generally gradational and cannot fllaRD l I I I ~ )) ~ I s 2C-2, be clearly defined. The contact, which is defined by changes in gamma activity, porosity, and density, is complicated by liqu>d-air interfaces and breakouts in the hole we'll. Ho diagnostic sedimentary units are recognized in the logs. Certain volcanic flows and flow breccias are recog-nizable on the logs, as are granitic rocks of the "'basement complex. However, these materials are sufficiently weathered so that contacts with adjacent sediments appear gradational. On the natural gamma log, basalts show a very low. level of activity, granitic rocks a'high level, and rhyolitic materials an intermediate level. !'iscussion / Th'e stratigraphy of the site area does not lend itself to analysis by downhole geophysical techniques. Most of the logs illustrate rather heterogeneous conditions and indicate virtually no similarities between adjacent holes. This is to be expected where the drilled materials are largely fanglomerates and there is little or no lateral continuity of individual strata. I i g I I ~ l ~ gi I APPENDIX 2D Investi ation of a Potential Site on the Northwest Side of the Gxlles ze Basalt Flow I I y h I 1 I I, I l 2D-1 APPENDIX 2D Xnvesti ation of a Potential Site on the Northwest Side of the'Gil'les ie Basalt 'Flow A. I'ntroduction A potential plant site on the northwest side of the Gillespie basalt flow was proposed by Bechtel Power Corporation. The geologic characteristics of the potential site were investigated by a site drilling program in conjunction with geophysical surveys (Fig. 2D-1). Lithologic logs of drill holes GDDH-19 through GDDH-24 are presented in Appendix 2B. No downhole geophysical logs were obtained. Gravity and ground magnetic survey data were gathered by Mr. Ken Koenen and analyzed by Dr. John S. Sumner. B. Results The geophysical data were obtained early in the investigation of the potential site, but. were generally not reliable. The gravity data (Fig. 2D-2) correctly indicated that the base of the basalt flow.".was flatter than the upper surface. However, the basalt thickness estimated from the gravity data usually varied greatly from that actually penetrated during drilling. The magnetic data were not useful in interpreting the structure of the basalt flow because of the basalt's variable magnetic character,'he location, surface elevation, and basalt thickness at each drill hole were used Co infer the configuration I i li I i I I! t I ('l I I 2D-2 of the basalt flow base g'ig. 2D-3) . This was in turn used with the topographic map to produce an isopach map-of the basalt in the proposed site area (Fig. 2D-4). All drill holes except GDDH-23 penetrated alluvial fan deposits underlying the basalt. In drillhole GDDH-23, fine-grained basin fill deposits, similar to those encountered at the Palo Verde Nuclear Generator Site, occurred below the basalt (Appendix 2B, Fig. 2B-27). These basin filldeposits contained inter-fingering coarse alluvial fan deposits and overlie a paleosol which in turn overlies coarse alluvial fan and Gila River (?) deposits. These coarse deposits occur as high as 830 feet, the elevation at which the "80-foot" river terrace should occur. C. Discussion The drill hole data show that the basalt overlies a generally smooth alluvial fan surface which dips less than 1 northeast. The slope apparently decreases, becoming nearly level to the northeast near drill hole GDDH-23 (Fig. 2D-3). This alluvial fan was probably graded to basin fill deposits, such as those found in GDDH-23, as they accumulated. The stratigraphic relations j.n drill hoj.e GDDH-23 suggest that deposition of the uppermost part of the fine-grained basin fill occurred during and after the I integration of the Gila River drainage through the area. guava I' I l I' I I' I I I I I 6 GDDH-23 GDML"5 GDDH -2I GDDH-GDML- I l5 GDDH-I9 GDDH-22 GDML-4 GDDH-20 <<x '.m GDML-5 GDDH-24 EXPLANAT-ION 0 Q G GDDH-l9 Drillhole location N O ~GDML- G Geophysical survey line XH Ue ~ n~ Gravity contour, interval: 6 n O.l Milligals e U -. e SCALE I 00 ~ Q O IOOO 0 IOOO p O (feet) O I I I I'i I I I' APPENDIX 2E Geomorphological Investi ations in the Gillespie Dam Alternate Satin Area g l, ~ pi ~ I ) i 2E-l APPEND/X 2E Geomor holo ica,l Xnvesti ations in the Gilles ie Dam Alternate Sitin Area A. introduction Laterally extensive, well-defined strata whose age can be confidently determined are an important asset to a nuclear power plant site. Such strata are used to establish areal correlation and minimum ages of faulting. While the datable Arlington basalt overlies extensive, well-defined beds of fine-grained basin fill in the Palo Verde site area, the datable Gillespie basalt overlies discontinuous coarse-grained alluvial fan and fluvial deposits which have complex cut-and-fill and interfingering relationships. Geomorphological studies, therefore, can make valuable contributions by differentiating alluvial deposits and establishing their age relative to the Gillespie basalt. B. Methods Several techniques were used to determine characteristics by which the various alluvial deposits could be differentiated. Because characteristics of each deposit are variable, several characteristics must be considered simultaneously to adequately differentiate the deposits. The deposit characteristics which were studied include: l ~ r l f g r g t l' ~ l I 2E-2

l. Shape, location and drainage direction (geomorphic profiles)<

2. Relation of deposit to source area,

3. Depth of stream incision below original surface,

4. Preservation of original surface, amount of dissection,

5. Type of drainage pattern,

6. Surface color and texture,

7. Surface soil characteristics (desert varnish, caliche),

8. Deposit composition on surface and at depth (pebble counts),

9. Sedimentary characteristics (size range, sorting, roundness, etc.),

10. Stratigraphic relations.

C. Results Geomorphological analysis allowed differentiation of five generations of alluvial fan deposits and four of fluvial deposits. The oldest generations of fan and terrace deposits predate the Gillespie basalt (potassium-argon dated at about 3.3 million years). The deposit characteristics are summarized in Section 2.5.1.2.2. The location and shape of geomorphic profiles is shown in Figures 2E-1 through 2E-7. D. Discussion An important contribution of geomorphological studies l I l 1 t I' I t r 2E-3 was confirmation that deposits of a fairly extensive alluvial fan extend beneath the Gillespie basalt. Pebble counts demonstrated essentially identical composition of alluvial fan deposits exposed in trenches GDDT-4 (cut through the Gillespie basalt) and GDDT-5 (Appendix 2F, Fig. 2F-1). Geomorphic profiles (Figs. 2E-1 through 2E-4) constructed along low, ridge-like remnants of the original surface, projected directly beneath the edge of the basalt flow even where the deposits buried by the flow were not exposed. The surface and deposits of this fan can be used as a dated s'tratigraphic marker to establish areal correlations and the minimum ages of faults which project beneath it. Three dominant Gila River terrace levels have been recognized 0 W N and described between Arlington and Gila Bend.

• The oldest and. highest level is" extremely well'issected, poorly preserved, and commonly is buried by younger alluvium. In the vicinity of Gillespie Dam, these h'igh fluvial gravels occur at or above approximately 80 feet above the present stream level. South of Powers Butte, the gravels occur up to 80 to 100 feet above the present level, and between Gillespie Dam and Enterprise Ranch they are fo>>nd at approximately 80 to 90 feet above the present level. Because many of the terrace remnants in this area occur at about the 80 foot altitude, this level has been designated the "80-foot" terrace. Naturally, this altitude is subject fllllRD

I I 1 f I I I 2E-4 to variation depending upon: o degree of dissection, o amount of stripping and subsequent burial, G gradient of present and former river levels (which is locally subject to change), o 'mount of transverse gradient on the original terrace surface (because of the natural concave cross section of a stream valley, terrace deposits of the same level become higher above the stream at greater distances from the stream), o relative accuracy of the 20- and 40-foot contour interval topographic maps. A second major terrace exists between Arlington and Gila Bend which is lower, younger, less dissected, and better preserved than the "80-foot" terrace. It locally exhibits variations in altitude for the same reasons as outlined above for the "80-foot" terrace. Between the Arlington and Gillespie basalt flows and south toward Enterprise Ranch (about S miles) the younger terrace has been surveyed at approximately 40 feet above the present river level and has therefore been designated the "40-foot" terrace. Both the "40-foot" and "80-foot" terrace deposits consist of well-sorted fluvial sand and gravel with cobbles as much as six inches or more 'in diameter. No discernible differences in lithology, size, sorting, or roundness have been observed between the two terrace deposits. Except for differences in degree of calichification, the two PJBRD I I I I I I I I I 2E-5 terraces are distinguished on the basis of morphology. Bordering the modern floodplain between Arlington and Gila Bend is the youngest terrace, occurring at approxi-mately 20 feet above the present river level. This "20-foot" terrace is distinctly different from the "40-foot" and "80-foot" terraces, consisting almost entirely of red-brown silt. It contains potsherds,'ndicating that it is of Holocene age. The old river terrace deposits bordering. the Gila River and standing about 40 to 80 feet above the present river channel are locally buried by the Gillespie basalt (Fig. 2E-5). The poorly preserved 80-foot terrace deposits h 1 represent the upper limit of a major aggradation by the Gila River. Used together, geomorphic profiles (Figs. 2E-l, 2E-2, 2E-3), drill hole cross sections (Appendix 2B, Figs 2B-26, 2B-27), and basalt flow base contours (Appendix 2D, Fig. 2D-3), indicate that the old (pre-Gillespie basalt) alluvial fan passes beneath'he basalt flow and was probably graded to the 80-foot river terrace level. These fan deposits are only slightly incised where protected by the Gillespie basalt flow, indicating that the basalt was probably. extruded so soon after the river cut down to the 40-foot 'terrace level that tributary drainages had not had time to become re-graded to the new 40-foot level. fuuaa I i I I I I I I I I I I 2E-6 The upper part of Beehive Wash (Plate 2.5-1) probably originally drained t~ the Gila River along the northwest side of the granitic ridge buried by the southern part of the Gillespie basalt flow. Extrusion of the basalt flow diverted the wash drainage sout:h through a saddle in the granitic ridge and into the lower part of the present Beehive Wash drainage. Following drainage, diversion, lateral erosion developed a relatively wide wash valley which was apparently graded to a level about 40 feet above the present river channel. This suggests that the river continued to flow near the 40-.foot terrace level for some time after extrusion of the Gillespie basalt. Scattered Gila River gravel locally occurs on the eastern edge of the Gillespie flow (Plate 2.5-1) and the southern edge of the Arlington flow. These scattered pebbles and cobbles, averaging 1/2- to 1-inch in diameter with a maximum diameter of 3 inches, are interpreted as being of , flood or overflow origin. The extrusion of the Arlington and. Gillespie basalts constricted and/or dammed the Gila River and very probably caused flooding or overflow on the riverward margins of the flows. Detailed leveling surveys were run along the moderately well-preserved surface of the 40-foot river terrace. from the Arlington basalt flow to the Gillespie basalt flow (Fig. 2E-6). Spot elevations (from level survey and 20- and 40-foot contour maps) were determined along the terrace from the Gillespie basalt flow to the Sentinel flBRD I I I I' I I I I 2E-7 basalt flow (Fig. 2E-7). The slope of the terrace profile remains constant, paralleling the modern floodplain through " the distance covered by both studies. The pro'file shows no breaks or warping where it t crosses the Gila River and Gila Bend lineaments (Figs. 2E-5, 2E-6, 2E-7) indicating ~ that within the resolution of the data no major movement has occurred along the lineaments since the formation of / the 40-foot terrace. fuaaa l r i I! I I I I Arizona Nuclear Power Project Gillespie Dam Alternate Siting Area SCALE LOCATION OF GEOMORPHIC PROFILES 1000 0 1000 ON ALLUVIALFANS (feet) Figure 2E-l I I I I' i ~ r I l'