ML033650380
| ML033650380 | |
| Person / Time | |
|---|---|
| Site: | Humboldt Bay |
| Issue date: | 09/05/2002 |
| From: | Cluff L, Page W, Swan F Geomatrix Consultants, Pacific Gas & Electric Co |
| To: | NRC/FSME |
| References | |
| +sisprbs20051109, -RFPFR | |
| Download: ML033650380 (34) | |
Text
DATA REPORT A GEOLOGIC MAPPING IN THE ISFSI SITE AREA HUMBOLDT BAY ISFSI PROJECT PREPARED BY DATE
,_ id Co sultanft to:
Frank H. Swan. Ph.D. Geomatrix Consultants. Inc.
Printed Name Organization
- D:4ATE VERIFIED BY William D. Page. ICEG
. PG&E Geosciences Dept.
Printed Name Organization APPROVED BY DATE Lloyd S. Cluff. RG. CEG PG&E Geosciences Dept.
Printed Name Organization Humboldt Bay ISFSI A-1 of 33 Data Report A, Rev. 0
DATA REPORT A GEOLOGIC SMAPPING IN THE ISFSI SITE AREA HUMBOLDT BAY ISFSI PROJECT TABLE OF CONTENTS Page
1.0 INTRODUCTION
............ A-4 2.0 METHODOLOGY ............ A-4 3.0 RESULTS .............. A-6 3.1 PHYSIOGRAPHIC SETTNG .............................................. A-6 3.2 STRATIGRAPHY .............................................. A-8 3.2.1 Rio Del Formation (Late Pliocene to Early Pleistocene) ..................... A-8 3.2.2 Scotia Bluffs (?) Formation (Early Pleistocene) ................................... A-9 3.2.3 Hookton Formation (Middle to Late Pleistocene) ................................ A-9 3.2.4 Buhne Point Terrace and Relict Paleosol (Late Pleistocene) .............A-10 3.2.5 Surficial Deposits (Holocene) ........................... .................... A-I 1 3.3 STRUCTURE ............................................... A-12 3.3.1 Little Salmon Fault ................................................ A-12 3.3.2 Bay Entrance Fault ............................................... A-12 3.3.3 Buhne Point Fault ............................................... A-13 3.3.4 Discharge Canal Fault .............. ................................. A-14
4.0 REFERENCES
......... A-15 List of Tables Table A-I Soil profile descriptions List of Photos Photo A-1 View looking west from Buhne Point showing the bluff along the north side of the Buhne Point terrace.
Photo A-2 Buhne Point terrace and ISFSI site.
Photo A-3 Oblique aerial photographs Buhne Point terrace taken during the trenching activities by Earth Sciences Associates (circa 1975).
Humboldt Bay ISFSI A-2 of 33 Data Report A, Rev. 0
Photo A-4 Discharge Canal fault in bluff west of the Discharge Canal List of Figures Figure A-I Topographic map of Humboldt Bay showing the location of the Humboldt Bay ISFSI site.
Figure A-2 Geologic map of the ISFSI site area.
Figure A-3 Comparison of the shoreline shown on 1858 and 1959 surveys.
Figure A-4 Generalized stratigraphic section at the ISFSI site.
Figure A-5 Stratigraphic section of the uppermost lower Hookton and the upper Hookton Formation exposed in Woodward-Clyde Consultant's trenches 11-T6a, 11-T6B, and 11-T6c.
Figure A-6 Surface traces of the Little Salmon fault zone south of the ISFSI site.
Figure A-7 Structure contour map of the Buhne Point fault Figure A-8 Structure contour map on top of Unit F.
I:Project\5000s\5 11 7.009\Data reports\Report MDATA REPORT A.doc Humboldt Bay ISFSI A-3 of 33 Data Report A, Rev. 0
DATA REPORT A GEOLOGIC MAPPING IN THE ISFSI SITE AREA HUMBOLDT BAY ISFSI PROJECT
1.0 INTRODUCTION
The Humboldt Bay Power Plant and ISFSI site are on the east flank of Buhne Point, a small headland on the eastern shore of Humboldt Bay (Figure A-1). Geologic mapping was performed to characterize the stratigraphic and structural setting of the ISFISI site area. (Figure A-2). Buhne Point is situated within the Little Salmon fault zone and has been uplifted and tilted gently to the southeast. The site is underlain by a thick sequence of late Tertiary and Quaternary sedimentary rocks, and is capped by a late Pleistocene terrace. Information on the site stratigraphy and structure is used to: (1) to evaluate the foundation properties of the ISFSI pad; (2) to evaluate the stability of cut slopes and existing slopes along the sea cliff to the west of the pad; and (3) to evaluate the potential for surface-fault rupture in the pad foundation.
Results of the geologic mapping are presented in this Data Report.
The geologic map (Figure A-2) synthesizes data from the ISFISI site investigations and previous geological and geotechnical studies for the Humboldt Bay Power Plant by Earth Science Associates (1976-1977) and Woodward-Clyde Consultants (1980). The surface mapping was augmented by trenches in the ISFSI study area (Humboldt Bay ISFSI Data Report D) and subsurface borehole data (Humboldt Bay ISFSI Data Report B). F. H. Swan and J.R.
Wesling compiled the existing geological information. Field verification of the geologic data and additional geologic mapping was performed primarily by J.R. Wesling. F. H. Swan assisted in the field verification. The geologic mapping and preparation of this Data Report were performed under Geomatrix Consultants, Inc. work plan ISFSI-GEO-006, Geologic Investigation for the Humboldt Bay Power Plant Independent Spent Fuel Storage Installation Site, Revision 0, February 15, 2000. W.D. Page (PGandE Geoscience Department) supervised the implementation of Geomatrix's work plan.
2.0 METHODOLOGY Prior to conducting the field investigations, a preliminary geologic map was compiled based on existing data in PG&E files, previous consulting reports (e.g., Earth Sciences and Associates, 1976-1977; and Woodward-Clyde Consultants, 1980), and other available sources (e.g., Carver Humboldt Bay ISFSI A-4 of 33 Data Report A, Rev. 0
and Burke, 1992). The map depicted the distribution of the principal stratigraphic units, the attitude of bedding, and the locations of geologic structures (known and inferred faults, fractures and folds). Surficial deposits, including areas of artificial fill, were also indicated.
The geologic map was prepared following generally accepted techniques (e.g., Compton, 1985) in accordance with the Geomatrix work plan cited above.
The geologic information was compiled on a topographic base map at a scale of i inch equals 50 feet that was provided by PG&E Geosciences Department. During the field investigation, PG&E provided supplemental survey data to provide better topographic control in the vicinity of Buhne Point to the northwest of the plant site.
The information shown on the preliminary geologic map was verified or adjusted based on field mapping of the available exposures, examination of hand-dug pits, examination of two new trenches at the ISFSI site (Geomatrix, 2002b), and the results of subsurface borehole (Geomatrix, 2002a). Exposures and reported field measurements (e.g., bedding attitudes) were located on the topographic maps using several methods. These methods included visual inspection of aerial photographs and local topography, and survey by Brunton compass and tape. Surveyed coordinates' of key features (e.g., locations of selected boreholes, Geomatrix's trenches, selected previous trenches, and measured stratigraphic sections and soil profiles) were provided by PG&E Geosciences Department under Work Plan HBPP-ISFSI-GEO-8 (Geosciences Work Plan, Survey of Geologic Trenches and Test Pits for the HBPP ISFSI Site, Revision 0, July, 12, 2000).
Detailed geologic and pedogenic-soil profile descriptions were made at three locations (Figure A-2). One is in the scarp about 300 feet west of the ISFSI site; another is in the bluff about 450 feet northeast of the ISFSI site; and the third is in trench GMX-T2 at the southwest comer of the ISFSI site. The characteristics and nomenclature used to describe the pedogenic soil profiles are those followed by the U.S. Department of Agriculture Soil Survey Staff (1951),
with modifications to the nomenclature for field descriptions as introduced by the soil survey staff in 1982 (Guthrie and Witty, 1982). A summary of the physical properties and Unless noted otherwise, survey coordinates in the ISFSI study area were measured relative to the Plant Site coordinate grid, where true north is 34° east of Plant north and 0 elevation corresponds to mean lower low water (MLLW). MLLW is 3.7 feet below mean sea level (MSL) and 6.9 feet below mean higher high water (MHHW).
Humboldt Bay ISFSI A-5 of 33 Data Report A, Rev. 0
terminology used to describe the soil profiles is provided by Birkeland (1984). The soil profile descriptions are presented on Table A-1.
Attitudes (strike and dip) of bedding, faults and fractures were compiled from available sources (Earth Sciences and Associates, 1976-1977; and Woodward-Clyde Consultants, 1980). Where possible these measurements were verified in the field during the reconnaissance mapping and additional measurements were added to the field map. Measurements were made using a hand-held "Brunton" style compass. Where it was possible to verify the previously reported measurements, they were generally found to be consistent with the new measurements.
Therefore, no distinction is made between the previously reported and the new measurements on the final map (Figure A-2). The accuracy of the strike measurements is estimated to be about 45%; the accuracy of the dips is <5°.
3.0 RESULTS Results of the geologic mapping are presented on Figure A-2, which also shows the locations of previous and new trenches and borings near the ISFSI site. The geologic mapping progressed from compilation of existing data and examination of existing exposures in the ISFSI study area to the incorporation of data from detailed examination of trenches that cross the ISFSI site.
Lithologic and structural data were compiled from pre-existing documents, field checked, and supplemented by additional mapping. Particular emphasis was given to compiling and verifying geologic data from Woodward-Clyde Consultants (1980) and Earth Sciences Associates (1976-1977). The existing data were integrated with the new data to prepare the geologic map of the Plant Site and ISFSI study area (Figure A-2).
Section 3.1 describes the physiographic setting of the ISFSI site. Section 3.2 describes the site stratigraphy. The pedogenic soils on the terrace surface are used to assess the minimum age of the near-surface deposits. Section 3.3 describes the faulting related to the Little Salmon fault zone.
3.1 PHYSIOGRAPHIC SETTING The ISFSI site and Humboldt Bay Power Plant are located on a low hill, referred to in this report as Buhne Point hill, on the eastern side of Humboldt Bay opposite the entrance of the bay (Figure A-1). The hill, which has a maximum elevation of about 75 feet (23 meters),
extends east of Buhne Point for about 1600 feet (480 meters) and is 150 to 600 feet (50 to 180 Humboldt Bay ISFSI A-6 of 33 Data Report A, Rev. 0
meters) wide (Figure A-2). The hill is capped by an erosional remnant of an uplifted terrace and is an outlier of Humboldt Hill, a northwest-trending ridge that extends southeast of the site.
Humboldt Hill is a large fault-ramp anticline situated along the leading edge of the hanging wall of the Little Salmon fault zone. Buhne Point hill, where the ISFSI site is located, is bordered on the north by a bluff that drops off steeply (graded slope of about 1:1) to the shore of Humboldt Bay. The eastern and southern sides of the hill are bordered by a low tidal marsh.
The western side of the hill is bordered by the village of King Salmon, which is built on fill over tidal marsh and beach deposits that extend more than 1600 feet (0.5 kilometers) into the bay. The westernmost part of the hill forms Buhne Point.
The present hill is only a small remnant of the much larger hill that existed in 1850 when Buhne Point was first described as a navigational aid into the entrance of the bay. The first detailed map of the area, which was made in 1858 (Figure A-3), shows a flat-iron-shaped hill having steep bluffs along its northern and southwestern sides. The flat terrace surface slopes gently away from the bluffs to the southeast. The present shoreline is about 1/4 mile southeast of the 1858 shoreline (Figure A-3). The dramatic coastal retreat and loss of most of Buhne Point hill to wave erosion began when the entrance to the bay was deepened and jetties were placed adjacent to the entrance (Figure A-1) to insure a permanent deep-water access for ships during the late 1800s. The bluff retreat was arrested when riprap was placed along the base of the bluff in the early 1950s to prevent further wave erosion (Photo A-1).
Buhne Point hill was formed by the combination of tectonic uplift associated with the Little Salmon fault zone and erosion. As described above, the present north flank of the hill was formed by modem coastal erosion. The scarp along the southwestern side of the hill is interpreted to be the eroded fault-line scarp associated with down-to-the southwest displacement on the Buhne Point trace of the Little Salmon fault zone. The northeastern margin of the hill that is apparent on the 1858 map (Figure A-3) appears to be related, at least in part, to down-to-the-northeast displacement on a small secondary fault, the Discharge Canal fault. The bluff on the northwest side of Buhne Point hill in 1858 was the eroded sea cliff that faced the ocean across from the natural entrance to the bay. This bluff has since retreated to its present position at the northern side of the plant site.
The dominant feature of the present topography is the Buhne Point terrace (Qpht on Figure A-2). This planar geomorphic surface has a gentle (2 to 4 degrees) southeast tilt (Photo A-2).
The small flat area that is inset below the Buhne Point terrace at the western end of the Humboldt Bay ISFSI A-7 of 33 Data Report A, Rev. 0
hill appears to be man-made because it is not evident on the 1858 survey map and because the strata at the present ground surface are not weathered indicating that the soils have been removed. Other parts of Buhne Point terrace also have been modified in several places during construction activities for the power plant. For example, low-angle oblique aerial photographs in PG&E archive files (Photos A-3a and A-3b) show grading activities from south of the old security fence to the edge of the bluff on the north side of the terrace. Parts of the Buhne Point terrace surface (Qpht on Figure A-2) in the vicinity of the ISFSI site may have been lowered by up to two to three meters (5 to 10 feet) with the largest amount of lowering occurred along the edge of the bluff and decreasing toward the security fence. In several places the disturbed areas are underlain by up to 4 feet of fill. The proposed ISFSI site is located near the old security fence in the area of disturbed ground.
3.2 STRATIGRAPHY The proposed ISFSI site is underlain by a thick sequence of late Pliocene and Quaternary deposits. Three lithostratigraphic formations separated by unconformities were encountered at the site as shown on Figure A-4. From oldest to youngest, these are the Rio Dell Formation, the Scotia Bluffs Formation and the Hookton Formation, which is divided into lower and upper members. The following descriptions of the Rio Dell, Scotia Bluffs and lower Hookton formations are based on Woodward-Clyde Consultants' 1980 report. In addition to the data from the Woodward-Clyde study, the description of the upper member of the Hookton Formation includes information obtained from the ISFSI site geotechnical borings and trenches, and from surface outcrops in the Buhne Point area.
At Buhne Point, the southeastward-dipping upper Hookton sediments are truncated by the Buhne Point terrace. Remnants of a relict paleosol, which is described in Section 3.2.4 and on Table A-1, are preserved locally where the terrace surface has not been disturbed. In the flats surrounding Buhne Point hill, the Hookton deposits are unconformably overlain by Holocene colluvial, landslide, alluvial, and estuarine deposits. Extensive areas of the site have been graded and the natural soils/surface weathering profile have been removed or buried by man-made fill in most places.
3.2.1 Rio Del Formation (Late Pliocene to Early Pleistocene)
The Rio Dell Formation is a homogeneous marine mudstone that is encountered in boreholes 1700 feet (520 meters) beneath the site. The formation is about 2000 feet (600 meters) thick.
Humboldt Bay ISFSI A-8 of 33 Data Report A, Rev. 0
Regionally the Rio Dell Formation is time transgressive; marine fossils indicate the age ranges from late Pliocene to Pleistocene. Near the site, the uppermost Rio Dell is estimated to be 1.1 +/-
0.2 million years old (Woodward-Clyde Consultants, 1980), making its age early Pleistocene.
3.2.2 Scotia Bluffs (?) Formation (Early Pleistocene)
At the site, the Rio Dell Formation is unconformably overlain by more than 1100 feet (340 meters) of shallow-water sandy marine sediments that are probably correlative with the Scotia Bluffs Formation of Ogle (1953). The deposits consist mostly of silty sand and sandy silt interbedded with clay-rich beds. The clay beds provide excellent marker horizons that can be easily identified on the borehole lithologic logs and geophysical logs, particularly the natural gamma-ray logs. In the site area, Woodward-Clyde Consultants (1980) subdivided the formation into eight units, which are labeled 0 though V (from youngest to oldest). The precise age of the Scotia Bluffs (?) Formation has not been determined, but it was probably deposited between about 1.1 million years ago (the estimated age of the upper Rio Del Formation) and about 780 thousand years ago (i.e., older than the Brunhes/Matuyama magnetic reversal 2 ). Therefore, the Scotia Bluffs (?) is early Pleistocene in age.
3.2.3 Hookton Formation (Middle to Late Pleistocene)
The Hookton Formation consists of middle to late Pleistocene interbedded shallow marine, estuarine, and fluvial deposits that unconformably overlie Scotia Bluffs (?) and older formations. In the vicinity of Buhne Point, the Hookton Formation is divided into a lower unit and an upper unit (Figures A-4 and A-5). The lower Hookton Formation deposits consist of alternating sand, silty sand, gravelly sand, silty clay, and clay that are about 870 to 900 feet (265 to 275 meters) thick. The thickness of the lower Hookton beds vary across the Buhne Point and Little Salmon faults, indicating that folding and faulting occurred during deposition of the lower Hookton Formation. Laterally persistent clay beds that are typically overlain by gravelly sands provide useful marker horizons. A distinctive clay bed, unit F, near the top of the lower Hookton Formation is a particularly useful marker horizon that has been identified in borings across the site and in the western end of Trench 1 -T6c at the northwest end of Buhne Point. The age of the uppermost part of the lower Hookton Formation is about 160,000 +/-
2 Woodward-Clyde (1980) used an age of about 700,000 years for the polarity transition between the Matuyama and Brunhes polarity epochs. Based on recent dating using advanced potassium-argon techniques, the date of this transition is now placed at 780,000 years (Baksi and others, 1992). Previously, this boundary was thought to be at 760,000 years (Izett and others, 1988), and before that it was placed at 730,000 years (Mankenen and Dalrymple, 1979).
Humboldt Bay ISFSI A-9 of 33 Data Report A, Rev. 0
40,000 years, based on amino racemization dates on fossil shell material collected from a Caltrans road cut near the northern end of Humboldt Hill about 3000 feet (900 meters) south of the site (Woodward-Clyde Consultants, 1980). The age of the unit F clay is estimated to be 310,000 + 70,000 years based on stratigraphic interpolation between the amino acid racemization age of the clay above unit F (top of the lower Hookton Formation) and the age of the base of the Hookton Formation (Woodward-Clyde Consultants, 1980). The upper part of the lower Hookton Formation consists of very dense, poorly to well graded sand and silty sand with occasional gravel overlying the unit F clay bed.
Upper Hookton Formation deposits consist primarily of silt and clay alternating with thinner sand and gravel lenses. Distinctive marker horizons were not identified in the upper Hookton Formation that could be correlated across the Little Salmon and Bay Entrance faults, but the deposits are significantly thicker on the down-thrown sides of these faults. As exposed in trenches and in the sea-cliff along the north side of the Buhne Point terrace, the deposits underlying the terrace commonly contain distinctive layers having sharp contacts. The textures of the strata vary somewhat laterally, but individual layers commonly can be traced for several feet to tens of feet. The clayey bay mud deposits tend to be more laterally persistent than the interbedded sandy and silty layers. The two layers of bay mud (clay and silt beds) in the upper Hookton Formation are separated by a 30- to 33-foot-thick sandy and silty deposit whose texture varies laterally from silty sand to low to high plasticity silt. These lateral variations are interpreted to be facies changes. Deposits overlying the uppermost clay bed are predominantly sandy and silty clay, well to poorly graded sand, and silty and clayey sand. These deposits, as well as the upper part of the highest bay mud clay and silt bed, were exposed in Woodward-Clyde Consultants (1980) trenchelI-T6a and Geomatrix (2002b) trenches GMX-TI and GMX-T2. A layer of clayey man-made fill overlies the upper Hookton Formation across most of the ISFSI site. The fill ranges from 0 to 10.4 feet (0 to 3.2 m) thick but is typically 2 to 3 feet thick 3.2.4 Buhne Point Terrace and Relict Paleosol (Late Pleistocene)
The Buhne Point terrace (Qpt on Figure A-2) dips gently (2 to 4 degrees) to the southeast. A strongly developed pedogenic soil has formed in the near surface deposits beneath the terrace surface. This relict paleosol crops out in exposures on the steep slopes to the northeast and southwest of the ISFSI site and in the southwest end of trench GMX-T2. Based on these exposures, the relict paleosol appears to be concordant with the tilted terrace surface. It has a well-developed argillic horizon, reddish brown (i.e., 7.5YR hue) colors, clay films, and strong Humboldt Bay ISFSI A-10 of 33 Data Report A, Rev. 0
structure (Table A-1). The presence of a relatively thick, strongly developed argillic horizon (Bt horizon) and the reddish color indicate that the soil on the Buhne Point terrace is correlative with Class II (83,000- and 103,000-year-old) soils developed on marine terraces in the Humboldt Bay area (Carver and Burke, 1992). In particular, the degree of soil development on the terrace surface at Buhne Point is similar to the soil at the South Port Landing quarry on Table Bluff where a thermoluminescence age of 103 ka was obtained for the strata beneath the terrace (Berger and others, 1991).
The Buhne Point terrace is interpreted to have formed during a high-stand of sea level during the late Pleistocene, most likely during the marine oxygen-isotope stage 5c or 5a. The ages of the oxygen isotope stage 5 marine terraces along the California coast are well documented (e.g., Hanson and others 1992); stage 5e marine terraces are dated at 120,000 to 125,000 years and the stage 5c terraces are approximatelylO5,000 years old and stage 5a formed approximately 80,000 years ago. The soils data described above suggest the terrace has been emergent at least since stage 5a and possibly longer. This is consistent with previous estimates of the age of the Buhne Point terrace by Woodward-Clyde Consultants (1980) who interpreted the age as post upper Hookton Formation sediments, which were deposited after deposition of a clay bed containing shell material having a 160,000 + 40,000 year old amino acid racemization age, and prior to 37,000 years ago as determined from radiocarbon dating of wood samples from Trench I1 -T6a (i.e. older than the effective range of radiocarbon dating in 1980). The strongly developed soil on the Buhne Point terrace supports an age of greater than 80,000 years old.
3.2.5 Surficial Deposits (Holocene)
Holocene surficial deposits in the ISFSI site area include alluvial/estuarine marsh sediments, colluviumn on the slopes, and shallow landslides (Figure A-2). The alluvial/estuarine deposits underlie the flat area southwest of the Buhne Point terrace in the King Salmon Avenue area and east of the Discharge Canal. Colluvium derived from the fault line scarp along the southwest side of Buhne Point terrace probably interfingers with the alluvial/estuarine sediments.
Small landslides occur along the bluffs that border the Buhne Point terrace (Figure A-2). They are most abundant on the sea cliff adjacent to Humboldt Bay on the north side of the terrace.
Most of the landslides are very shallow (< 6 feet thick) (< 2 meters thick), "veneer,"
translational landslides. However, the two northwesternmost landslides along the sea cliff Humboldt Bay ISFSI A-l I of 33 Data Report A, Rev. 0
appear to be somewhat deeper (15 to 20 feet) (or 5 to 7 meters) and have rotational movement.
The landsliding post-dates the grading of the sea cliff and placement of the riprap along the shoreline, which were completed during the late 1950's. No large landslides were observed along the bluff.
3.3 STRUCTURE Four traces of the Little Salmon fault zone are mapped in the vicinity of the Humboldt Bay ISFSI site Figure A-6). These include two primary fault traces, the Little Salmon and Bay entrance faults; and two subsidiary faults in the hanging wall of the Bay Entrance fault, the Buhne Point and Discharge Canal faults. The Little Salmon fault corresponds to the middle trace of the Little Salmon fault zone to. the southeast, and the Bay Entrance fault corresponds to the eastern trace of the Little Salmon fault zone to the southeast. The Little Salmon, Bay Entrance and Buhne Point faults all dip to the northeast and displace the late Pleistocene Hookton Formation down to the southwest (Figures A-6). The Discharge Canal fault dips steeply to the southwest and has down-to-the-northeast displacement.
3.3.1 Little Salmon Fault The location of the Little Salmon fault near the site is based on borings and seismic lines conducted by Woodward-Clyde Consultants (1980)(Figure A-1). The fault strikes about N450 W and dips about 250 NE. The fault projects to the surface about 2.2 kilometers (1.4 miles) southwest of the ISFSI site (outside of the area shown on Figure A-2).
3.3.2 Bay Entrance Fault The Bay Entrance fault is the closest of the main traces of the Little Salmon fault zone to the ISFSI site. As inferred from borings, the fault strikes N50 W and dips approximately 60E (Woodward-Clyde Consultants, 1980). The fault projects to the surface about 0.5 kilometers (1600 feet) west of the proposed ISFSI site (Figure A-6) and is outside of the area shown on Figure A-2. The fault appears to have a right-slip component that is about 50 percent of the dip-slip separation, based on analysis of boring and geophysical data (Woodward-Clyde Consultants, 1980).
The base of the Hookton Formation is displaced about 440 meters (1440 feet) (dip-slip), and the upper Hookton Formation is displaced about 885 feet (270 m) (Woodward-Clyde Consultants, 1980, Figure C- 10 and Table 2). Progressive separation of the older beds in the Humboldt Bay ISFSI A-12 of 33 Data Report A, Rev. 0
Hookton Formation indicate the fault was active during the deposition of the Hookton Formation. The long-term, dip-slip displacement rate on the Bay Entrance fault southwest of the ISFSI site is 1 to 2 millimeters per year.
To the south of the plant site, the Bay Entrance fault corresponds to the East trace of the Little Salmon fault zone (Figure A-6). In a quarry exposure directly south of College of the Redwoods, this trace displaces lower Wildcat sedimentary rocks (Pullen Formation) over late Pleistocene and Holocene sediments (Carver and Burke, 1987). To the south, at Salmon Creek, this trace deforms a late Holocene alluvial terrace. Based on the offset terraces, Carver and Burke (1987) estimate the late Holocene slip rate to be 2 to 3 millimeters per year.
3.3.3 Buhne Point Fault The location of the Buhne Point fault is based on the analysis of the site borings (Figure A-7).
The fault strikes about N45-70'W. The fault dips about 350 NE down to elevation -900 feet, where the dip flattens and dips less than 20° (Figure 4-21). Below elevation -1000 feet, the fault dip steepens to about 45° and probably continues to steepen until it merges with the Bay Entrance fault. The fault plane is about 140 to 160 meters (460 to 520 feet) below the ISFSI site.
The projected surface trace of the Buhne Point fault is parallel to and southwest of the southwest margin of the Buhne Point terrace, about 200 meters (600 feet) southwest of the ISFSI site (Figure A-7). The 15- to 50-foot- (5 to 15 meter-) high scarp along the southwest side of the Buhne Point terrace is interpreted to be an eroded fault-line scarp associated with the Buhne Point fault. Erosion and grading during plant construction have modified the scarp, but it reflects the general trend of the surface trace.
The Buhne Point fault has progressively greater vertical separation of older horizons. It displaces the Scotia Bluffs (?) Formation 233 feet (71 m) (vertical separation on unit Q); the base of the Hookton Formation 160 feet (49 m), and the unit L clay in the lower part of the Hookton 69 feet (21 m), (Woodward-Clyde Consultants, 1980, figure C-8). Structure contours on the top of unit F in the vicinity of the ISFSI site (Figure A-8) indicate the vertical displacement on the top of unit F in the upper part of the lower Hookton Formation ranges from 20 to 33 feet (6 to 10 m).
Humboldt Bay ISFSI A-13 of 33 Data Report A, Rev. 0
The upper Hookton underlying the terrace at Buhne Point is tilted 2 to 4 degrees to the southeast, indicating continued deformation and faulting on the Bay Entrance and Buhne Point faults during the late Pleistocene, (i.e., during the past 80,000 years). Based on the displacement of unit F (160,000 +/- 40,000 years), the long-term-average slip rate on the Buhne Point fault (dip slip) is about 0.1 millimeter per year. This slip rate is an order of magnitude lower than the slip rate on the Little Salmon and Bay Entrance traces of the fault zone.
Woodward-Clyde Consultants (1980) excavated Trenches 11 -T6b and 11 -T6c across the scarp that borders the Buhne Point terrace (Figure A-2). Both trenches exposed zones of fractures and small displacement faults in the upper part of the lower Hookton Formation. The fractures and small faults are similar to those observed in the hanging wall of other reverse faults that were investigated during regional fault investigations (Woodward-Clyde Consultants, 1980; Carver, 1987b). For example, Woodward-Clyde Consultants (1980) mapped similar features in the hanging wall of the McKinleyville fault about 15 miles north of the plant site. The fractures and small displacement faults are inferred to represent deformation in the hanging wall along the leading edge of a reverse fault, suggesting a fault lies within a few tens of feet of the present topographic scarp. Based on the structure contours on the top of the unit F clay (Figure A-8), a small splay branches from the main trace of the Buhne Point fault to the northwest towards Buhne Point. The vertical displacement on the splay fault is about 10 feet (3 m).
3.3.4 Discharge Canal Fault A small fault, informally referred to as the Discharge Canal fault, displaces the upper Hookton Formation. The fault is partially exposed in a hand-dug pit in the bluff about 250 feet (75 meters) west of the Discharge Canal for the power plant (Figures A-2 and Photo A-4). In this exposure, a sand layer is clearly displaced down-on-the-northeast by numerous closely spaced, steeply dipping to near-vertical (70'S to 900) faults that generally strike N500 W. The fault also was exposed in trenches BP-2 and BP-3 located east of the ISFSI site directly west of the discharge canal (Figure A-2). Logs of these trenches (Earth Sciences Associates, 1977) show a sand layer in upper Hookton Formation that is deformed into a steep "monocline"(down-on-the northeast) that trends N700 W. The vertical separation across the feature is greater than or about equal tolo feet (>3m) (i.e., the limit of the exposure in trench BP-2). The surface trace defined by these exposures corresponds to a 10-foot (3-m) down-to-the-northeast step in the top of unit F (Figure A-8). Based on the location of the offset in unit F relative to the surface trace, the fault dips 70 to 80 degrees to the southwest. The Discharge Canal fault is interpreted to be a Humboldt Bay ISFSI A-14 of 33 Data Report A, Rev. 0
backthrust on the hanging wall of the Buhne Point fault. The "monocline" is either folding above the tip of a blind reverse fault or hanging wall deformation above a backthrust that "daylights" (or is covered by young bay sediments) to the northeast. Another small fault crops out in the sea cliff about 150 feet (45 meters) east of the mapped trace of the Discharge Canal fault (Figure A-2) where a 0.3- to 0.6-foot thick sand layer in the upper Hookton Formation is abruptly truncated by a zone of faint, closely space shears. The fault strikes N320 W and dips 770 to the southwest. Assuming reverse slip, the displacement is greater than about 5 feet (1.5 m) (i.e., greater than the height of the exposure).
4.0 REFERENCES
Baksi, A. K., Hsu, V., McWilliams, M. O., and Farrar, E., 1992, Ar-40/Ar-39 dating of the Bruhnes-Matuyama geomagnetic field reversal: Science, v. 256, p. 356-357.
Berger, G. W., Burke, R. M., Carver, G. A., and Easterbrook, D. J., 1991, Test of thermnoluminesence dating with coastal sediments form northern California: Chemical Geology (Isotope Section), v. 87, p. 21-37.
Birkeland, P.W., 1984, Soils and Geomorphology, Oxford University Press, New York, 372 p.
Burke, R. M., and Carver, G. A., eds., 1992, A look at the southern end of the Cascadia subduction zone and the Mendocino triple junction: Guidebook for the Pacific Cell Friends of the Pleistocene Field Trip to Coastal Northern California, June 5-7, 256 p.
Carver, G. A., 1987, Late Cenozoic tectonics of the Eel River basin region, coastal northern California, in Schymiczek, H., and Sushsland, R., eds., Tectonics, Sedimentation and Evolution of the Eel River and Associated Coastal Basins of Northern California: San Joaquin Geological Society Miscellaneous Paper 37,.p. 61-72.
Carver, G. A., 1987, Geologic criteria for recognizing individual paleoseismic events in compressional tectonic environments, in Crone, A. J., ed., Directions in Paleoseismology: U. S. Geological Survey Open File Report 87-673, p. 115-128.
Carver, G. A., 1992, Late Cenozoic tectonics of coastal northern California, in Carver, G. A.,
and Aalto, K. R., eds., Field Guide to the Late Cenozoic Subduction Tectonics and Sedimentation of Northern Coastal California, GB-71. Pacific Section, American Association of Petroleum Geologists, p. 1-11.
Carver, G. A., and Burke, R. M., 1987, Late Pleistocene and Holocene paleoseismicity of the Little Salmon and Mad River thrust systems, northwestern Califomia-implications to the seismic potential of the Cascadia subduction zone (abs.): Geological Society of America, Abstracts with Program, v. 19, p. 614.
Humboldt Bay ISFSI A-15 of 33 Data Report A, Rev. 0
Carver, G. A., and Burke, R. M., 1988, Trenching investigation of northwestern California:
Final Report for U. S. Geological Survey National Earthquake Hazards Reduction Program, 5 3 p.
Carver, G. A., and Burke, R. M., 1992, Late Cenozoic deformation on the Cascadia subduction zone in the region of the Mendocino triple junction, in Burke, R.M., and Carver, G.A.,
eds., A Look at the Southern End of the Cascadia Subduction Zone and Mendocino Triple Junction: Pacific Cell, Friends of the Pleistocene Guidebook for the Field Trip to Northern Coastal California, p. 31-63.
Carver, G. A., Burke, R. M., and Kelsey, H. M., 1986, Quaternary deformation in the region of the Mendocino triple junction: U. S. Geological Survey National Earthquake Hazards Reduction Program, Unpublished Final Report, 48 p Compton, R.R., 1962, Manual of Field Geology: John Wiley and Sons, Inc., New York, 378 p.
Earth Science Associates (ESA), 1976-1977, Humboldt Bay Power Plant site geology investigations: Report Prepared for Pacific Gas and Electric Company, 100 p., 35 figs.,
19 plates, plus appendixes A through G.
Geomatrix Consultants, Inc. (Geomatrix), 2002a, Data Report B - Boring logs, Humboldt Bay Power Plant ISFSI site: Data report prepared for Pacific Gas and Electric Company Geosciences Department, submitted by Geomatrix Consultants, Inc., Revision 0, 64 p.
Geomatrix Consultants, Inc. (Geomatrix), 2002b, Data Report D - Trenches in the ISFSI site area, Humboldt Bay Power Plant ISFSI site: Data report prepared for Pacific Gas and Electric Company Geosciences Department, submitted by Geomatrix Consultants, Inc.,
Revision 0, 32 p.
Geomatrix Consultants, Inc. (Geomatrix), 2002c, Certification of geologic data obtained by Woodward-Clyde Consultants and Earth Sciences Associates that are used in the analysis of the Humboldt Bay ISFSI site, Humboldt Bay ISFSI Project, Seismic Hazards Analysis: Report prepared for Pacific Gas and Electric Company Geosciences Department, submitted by Geomatrix Consultants, Inc., Revision 0, 20 p.
Guthrie, R.L., and Witty, J.E., 1982, New designations for soil profile horizons and layers and the new Soil Survey Manual: Soil Science Society of America Journal, v. 46, p. 443-444.
Hanson, K. L., Wesling, J. R., Lettis, W. R., Kelson, K. I., and Mezger, L., 1994, Correlation, ages, and uplift rates of Quaternary marine terraces-south-central coastal California:
Geological Society of America Special Paper 292, p. 45-71.
Humboldt Bay ISFSI A-16 of 33 Data Report A, Rev. 0
Izett, G. A., Obradovich, J. D., and Mehnert, H. H., 1988, The Bishop ash bed (Middle Pleistocene), and some older (Pliocene and Pleistocene), chemically and mineralogically similar ash beds in California, Nevada and Utah: U. S. Geological Survey Bulletin 1675, 37 p.
LACO Associates, 1999, Final Report of Seismicity Study Phase 3 at College of the Redwoods, Eureka Campus: Prepared for College of the Redwoods, Eureka, California, by LACO Associates Consulting Engineers, Eureka, California, 24 p. plus figures.
Mankenen, E. A., and Dalrymple, G. B., 1979, Revised geomagnetic time-scale for the interval 0-5 m.y. B.P.: Journal of Geophysical Research, v. 84, p. 615-626.
Munsell Color Co., 1988, Soil color charts: Baltimore, Maryland.
Ogle, B. A., 1953, Geology of the Eel River Valley area, Humboldt County, California:
Division of Mines, Department of Natural Resources, State of California, Bulletin 164, 128 p.
Pacific Gas and Electric Company Geosciences Department, 2000, Geomatrix Consultants work plan - Geologic Investigation for the Humboldt Bay Power Plant Independent Spent Fuel Storage Installation Site: Work Plan ISFSI-GEO-006, Revision 0, March 23, 2000, 6 p. plus 1 attachment (1 p.) and 1 fig.
Soil Survey Staff, 1951, Soil survey manual: U.S. Department of Agriculture Handbook No.
18, 503 p.
U.S. Coast and Geodetic Survey, 1858, Preliminary survey map of Humboldt, California.
U.S. Geological Survey, 1959, Cannibal Island Quadrangle California, Humboldt Co., 7.5 Minute Series (Topographic), scale 1;24,000.
U.S, Geological Survey, 1959, Eureka Quadrangle California, Humboldt Co., 7.5 Minute Series (Topographic), scale 1;24,000.
U.S. Geological Survey, 1959, Fields Landing Quadrangle California, Humboldt Co., 7.5 Minute Series (Topographic), scale 1;24,000.
Witter, R.C., Patton, J., Carver, G.A., Kelsey, H.M., Garrison-Laney, C., Koehler, R.D., and Hemphill Haley, E., 2002, Upper-plate earthquakes on the western Little Salmon fault and contemporaneous subsidence of southern Humboldt Bay over the past 3,600 years, northwestern California: U.S. Geological Survey, National Earthquake Hazards Reduction Program Final Technical Report, Award No. O1HQGRO125, 44 p.
Woodward-Clyde Consultants, 1980, Evaluation of the potential for resolving the geologic and seismic issues at the HBPP Unit No. 3: Summary to Pacific Gas and Electric Company, San Francisco, 74 p. plus appendix volume.
Humboldt Bay ISFSI A-17 of 33 Data Report A, Rev. 0
TABLE A-1 SOIL PROFILE DESCRIPTIONS Page 1 of4 HDorizon' (cp) Color2 Texture3 Structure4 Clay Films6 Boundary7 Moist Mottles (moist) Moist Wet PROFILE JW-11 * - 100 METERS WEST OF THE ISFSI SITE A 0-35 10YR 2/2 -- 1 2 fg fr ss, ps N.O. a-c, s BAt 35-52 7.5YR 4/4 -- I-scl 2-3 m-c fi ss-s, p 1-2 n pf & po c,s sbk Bt 52-96 7.5YR 4/6 -- sc] 2-3 c pr fi s, p 2-3 n-mk po & c, w breaking pf to 2-3 m-c sbk 2Bt2 96-130 7.5YR 4/6 -- scl 2-3 c pr fi-vfi s, p 2-3 mk-k pf & c-g, s breaking po to 2-3 m sbk 2Bt3 130-180 7.5YR 5-6/8 c 1-2 d, 7.5YR 7/4 scO 1-2 c pr fi ss-s, p 1-2 n pf c-g, s 34 mk-k po 3Bt 180-230 10-7.5YR c 1-2 d-p, 10YR 7/4 sicl 1-2 c pr fi-vfi s, p 1-2 n pf a-c, w 5/8 2-3 n-mk po 3Cox 230-270+ 10YR 7/4 f 2 d, 10YR 6/8 & sicl M fi ss-s, p vI n-mik po --
7.5YR 6/8
- Soil profile exposed in steep south-southwest facing escarpment below the Buhne Point terrace.
Location shown on Figure A-2.
Humboldt Bay ISFSI A-18 of 33 Data Report A, Rev. 0 1:\Project\5000s\51 17.009\Data reports\Report A\DR- A_RevOCFINAL.doc
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TABLE A-1 SOIL PROFILE DESCRIPTIONS Page 2 of 4 Det Coor Tete
_________ Stutr DepthColor2 ~~~~~~~~~~~~~Consistences Horizon' (cm) _______Texture3 Structure4 Consistence5 Clay Films6 Boundary7 Moist Mottles (moist) Moist Wet PROFILE JW-12
- A130 METERS EAST OF ISFSI SITE A 0-20 10YR 2/1 -- sicl I m gr fr ss-s, p -- c,s A2 20-43 1OYR 3/1 -- sicl I m sbk fi s, p vl npf c, w breaking to 2 m gr B 43-75 7.5YR 4/6 -- sicl-sic 1-2 c-vc pr fi-vfi s-vs, p- I n pf C, s breaking vp to 1 m sbk 2Bt 75-140 7.5YR 5-6/8 m 2 d, 7.5YR 7/3 sic 2 m pr fi s-vs, vp 3 mnk-k pf & po c-g, S breaking to 3 m sbk 2Bt 140-197 7.5YR 6/6 f 2 d, 7.5YR 7/3 sic I cpr vfi s-vs, p- 2-3 k pf & po g,s breaking VP to 2 m abk 2BCtc 197-232 7.5YR 6/6 f-c 2 f, 7.5YR 6/4 c I c pr & vfi-efi vs, Vp 3 k pf & po a-c, s abk 3Cox 232-257 1OYR 5-6/4 -- sI m fi ss, ps -- a, s 4Cox 257-327+ 1OYR 6/4 c-rn 2-3 d-p, 7.5YR 5/8 sicl m s,p -- --
- Soil profile exposed in steep north-northwest facing escarpment below the Buhne Point terrace.
Location shown on Figure A-2.
Humboldt Bay ISFSI A-19 of 33 Data Report A, Rev. 0
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TABLE A-1 SOIL PROFILE DESCRIPTIONS Page 3 of 4
- Soil profile exposed in northwest wall of trench GMX-T2 (station 180 ft); relict paleosol formed on the Buhne Point terrace.
Location shown on Figure A-2.
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TABLE A-1 SOIL PROFILE DESCRIPTIONS Page 4 of 4 Explanation for Soil Descriptions Master horizons: A = a surface horizon characterized by the accumulation of organic matter and typically as a zone of elluviation of clay, sesquioxides, silica, gypsum, carbonate, and/or salts; B = a subsurface horizon characterized as having a redder color, stronger structure development, and/or accumulation of secondary illuvial materials, such as clay, sesquioxide, silica, gypsum, and/or salts; C = a subsurface horizon that may appear similar or dissimilar to the parent material and includes unaltered material and material in various stages of weathering.
Modifiers of master horizons: b = buried soil horizon; j = used in conjunction with other modifiers to denote incipient development of that particular feature or property; ox = oxidized (for C horizon only); p = plowing or other disturbance; t = accumulation of clay; w = color or structural B horizon.
2 Color: From Munsell soil color chart (Munsell Color Company, 1988); Dry colors were difficult to determine given very wet weather during fieldwork;
-- = not observed; Abundance: f = few, c = common, m = many; Size: I = fine, 2 = medium, 3 = large; Contrast: f = faint, d = distinct, p =
prominent.
3 Texture: sl = sandy loam; Is = loamy sand; s = sand; I = loam; scl = sandy clay loam; sc = sandy clay; cl = clay loam; sil = silt loam; sicl = silty clay loam; sic = silty clay.
4 Structure: Grade: m - massive; sg = single grain; vI = very weak; I = weak; 2 = moderate; 3 = strong. Size: f= fine; m = medium; c = coarse; vc = very coarse. Type: pi = platy; gr = granular; abk = angular blocky; sbk = subangular blocky; cpr = columnar; pr = prismatic.
5 Consistence: Moist consistence: lo loose; vfr = very friable; fr = friable; fi firm; vfi = very firm; efi = extremely firm. Wet Consistence: so
= nonsticky; vss = very slightly sticky; ss = slightly sticky; s = sticky; vs = very sticky; po = nonplastic; vps = very slightly plastic; ps = slightly plastic; p = plastic; vp = very plastic.
6 Clay Films: Frequency: vl = very few; I = few; 2 = common; 3 = many; 4 = continuous. Thickness: n = thin; mk = moderately thick.
Location: br = clay bridges holding mineral grains together; pf = faces of peds; po = lining or filling tubular or interstitial pores; co = colloidal stains on mineral grains; N.O. =none observed; -- = not observed.
' Boundary with lower horizon. Distinctness: va = very abrupt; a = abrupt; c = clear; g = gradual; d = diffuse. Topography: s = smooth; w =
wavy; i = irregular; b = broken.
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AV Photo A-1 View looking west from Buhne Point showing the bluff along the north side of the Buhne Point terrace.
The bluff is protected from erosion by riprap along U,j the shoreline of Humboldt Bay. Photograph JW-2-1 taken on March 9, 2000.
9 0
8 U Humboldt Bay ISFSI Z4:j Data Report A, Rev. 0 A-22 of 33
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S:A51 00s\51 17\51 17.009\task10\2_0401 da\-photoA-02(06).ai Photo A-2 Buhne Point trace and ISFSI site. View to east. Photographs JW-2-5 and JW-2-8 taken on March 10, 2000.
" Humboldt Bay ISFSI fia4 Data Report A, Rev. 0 A-23 of 33
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S:AS1O0s\51 1-A51 17.009\task-10\2O_0401-dra\o.t.A-03(03,04).aI (A) (B)
Meteorlogical Tower I
Photo A-3 Oblique aerial photographs of Buhne Point terrace taken during trenching activities by Earth Sciences Associates (circa 1975). (A) View to the west-southwest. (B) View to the east-southeast.
I Humboldt Bay ISFSI 1S0 Data Report A, Rev. 0 A-24 of 33
OD e~~I
/'C 7; '; X -. 2. i IT~~~~
g - e ;i F ° l 1 2 i ~~~~~~~~~~~3 km In Figure A-1 Topographic map of Humboldt Bay showing the location of the Humboldt Bay ISFSI site. The base map is a composite of parts of the Cannibal Creek, Eureka and Fields Landing U.S.G.S. 7.5' topographic quadrangles.
Humboldt Bay ISFSI 1hi Data Report A, Rev. A A-26 of 33 (
0 0.5 Mile 0 I 1 Kilomer 0 1 Kilometer co, EXPLANATION a, Coastline from U.S.Geological Survey (1959) Fields Landing 0
7.5' Quadrangle, California 9
Figure A-3 Comparison of the shoreline shown on 1858 and 1959 surveys. The 1959 shoreline is superimposed on the 1858 map shows the extensive coastal erosion and the retreat of Buhne Point that occurred prior to placement of riprap along the shoreline of Humboldt Bay.
C>
Humboldt Bay ISFSI I;rd A Data Report A, Rev. 0 A-28 of 33 Gv )
APPROXIMATE AGE (years)
. UNITS
+- 283,000
+-160,000 + 40,000 4 310,000 +/- 70,000
< 600,000 +/- 100,000 ra i~ ' 1,100,000 +/- 200,000 I
Figure A-4 Generalized stratigraphic section at the ISFSI site (after Woodward-Clyde Consultants, 1980, Figure 7).
a Humboldt Bay ISFSI V-3 Data Report A, Rev. 0 A-29 of 33
Depth (meters) lot 21
-_-Disturbed Soil - Unit 5 I bUnit 8
-Unit 4 30 Trench 11-T6a.
Trench 11-T6b Trench 11-T6ci
> ' Unit F Mz Ic i
'- Unconformity 32 t -*-Unit7 0
z 0
F=
--- Unit 3 0r a
Zi-i= -I U-Unexposed Section
,r I >_ -*_Disturbed Soil
. EXPLANATION I :imiI Sand (s)
-- Unit 2 Trench E-.17.tj Silt (sit) 11-T6b 1C H Silty clay/clayey silt (sit-cl/cI-slt)
_--Unit 6 Trench 11-T6c 4 i l-. Gravel (Gr)
- Unit 1 U, 12 --- Unconformity
-Unit 5 Note: Section Isa composite from trenches excavated Inthe Buhne Point terrace.
V
-- Unit F 10 9 14 Figure A-5 Stratigraphic section of the uppermost lower Hookton and upper 6;1 Hookton Formation exposed in Woodward-Clyde Consultants' trenches I:- 11-T6a, 1 -T6b, and 11-T6c (after Woodward-Clyde Consultants, 1980, Figure C-28).
L, Humboldt Bay ISFSI F0j4 Data Report A, Rev. 0 A-30 of 33
(0 -~~~~~~~~ 0 10
>~~~~~~~~~al (proecte urface s- traces)fteLtl amnfutzoesuho h SS ie
> - ~~Humboldt Bay lSFS1 i ME ~~Data Report A, Rev. 0 c,, A-3 1 of 33
S:\51 00s\5117\511 7.009\task_1 0\02-0401dra\ifig-A-07(11 ).ai Figure A-7 Structure contour map of the Buhne Point fault (modifiedfrom Woodward-Clyde Consultants, 1980; Figure C-18).
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