FINAL TECHNICAL REPORT

Paleoearthquake Chronology along the Northern West Napa Fault Zone, Napa County, California

U. S. Geological Survey National Earthquake Hazards Reduction Program Award Number 07HQGR0081

Principal Investigator: Kevin B. Clahan1 [email protected]

Contributors: John R. Wesling2, Cooper Brossy3

1 Fugro William Lettis & Associates, Inc. 7/F., Guardian House 32 Oi Kwan Road Wanchai, Hong Kong (ph) 415-488-7382, [email protected]

2Office of Mine Reclamation California Department of Conservation 801 K Street; MS 09-06 Sacramento, CA 95814 Phone: (916) 324-0744; Fax (916) 445-6066

3 Fugro William Lettis & Associates, Inc. 1777 Botelho Dr., Suite 262 Walnut Creek, CA 94596

Program Elements III and IV Keywords: Quaternary Fault Behavior, Geologic Mapping, Age Dating

May 2011

Research supported by the U.S. Geological Survey (USGS), Department of the Interior, under USGS award number 07HQGR0081. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government.

Award 07HQGR0081

Paleoearthquake Chronology along the Northern West Napa Fault Zone, Napa County, California

Kevin B. Clahan1 1Fugro (Hong Kong) Limited, Fugro William Lettis & Associates, Inc., 7/F., Guardian House 32 Oi Kwan Road Wanchai, 842, Hong Kong (852) 2894 5828 phone, [email protected]

ABSTRACT

This investigation looked in to the paleoseismic history of the Northern West Napa fault zone at two fault normal stream channel locations in Napa County, California. Napa Creek and an ephemeral drainage referred to as Alston Creek, both traverse a well defined lineament of the Northern West Napa fault zone. Natural stream channel bank exposures of these two drainages exhibit evidence for Holocene rupture of the Northern West Napa Fault zone. The Alston Creek exposure, which is located within Alston Park, Napa, California, consists of fine grained alluvial units overlying a clast supported cobble channel deposit. The units are offset across a 2.5 m wide, westerly dipping, shear zone as indicated by clayey gouge, slickensides, offset alluvial units, and aligned clasts. Detailed mapping and radiocarbon dating of three detrital charcoal samples from Alston Creek indicates that there has been at least two surface faulting event in the past 3000 years.

Napa Creek, approximately 2.5 km south of Alston Creek, also traverses the same well defined lineament of the Northern West Napa fault zone. Earlier mapping of the exposure (Wesling and Hanson, 2008) describes late Holocene alluvium against Miocene to Pliocene Sonoma Volcanic deposits across a sub-vertical shear zone approximately 1.5 m wide. This study attempted to characterize the upper 1 to 2 m of the exposed section, however, severe anthropogenic alteration of the ground surface and prohibited a more detailed analysis. Previous radiocarbon dating of the exposed offset alluvial units in the Napa Creek exposure indicate two possible rupture events have occurred in the last 600 years.

Additional work for this study included geologic and geomorphic mapping of an offset terrace sequence along Redwood Creek in the Alston Park area. The mapping reveals a series of six vertically uplifted and abandoned stream terraces. This terrace sequence was apparently uplifted as a result of displacement along the West Napa fault zone which bounds the terraces to the east. The maximum apparent vertical displacement of the alluvial terraces may be on the order of several tens of meters based on this and earlier mapping. Radiometric carbon dating of detrital redwood tree material from the upper stream terrace in the sequence at Alston Park was inconclusive. Future work including mapping and cosmogenic age dating of the abandoned terraces has the potential to yield a long-term vertical and potentially lateral slip rate for the Northern West Napa fault.

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TABLE OF CONTENTS

Abstract...... i 1.0 INTRODUCTION ...... 1 2.0 GEOLOGIC AND SEISMOTECTONIC SETTING ...... 1 3.0 ALSTON PARK ALLUVIAL TERRACES ...... 2 4.0 ALSTON CREEK FAULT EXPOSURE ...... 3 5.0 CONCLUSIONS...... 4 6.0 REFERENCES ...... 5

LIST OF TABLES

Table 1. Radiocarbon Results for the Alston Creek Exposure

LIST OF FIGURES

Figure 1. Quaternary Active Faults in the Northern San Francisco Bay Area Figure 2. Map Showing Recency of Activity and Reaches of the West Napa Fault Figure 3. Oblique Aerial Photograph Alston Park Area, Napa County California Figure 4. Quaternary Geologic Map of the Alston Park Area, Napa County, CA Figure 5. Alston Creek Stream Bank Log Figure 6. Alston Creek Photographic Log Figure 7. Site Photographs, Alston Park

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1.0 INTRODUCTION

The West Napa fault has long been recognized as a potentially important, second-order element of the San Andreas system of faults (Figure 1). The fault is the northernmost element of a series of relatively short north-to-northwest-trending en echelon faults that also includes the Franklin and Southhampton faults. The southern portion of the West Napa fault (near the City of Vallejo) is recognized to be active (in the Holocene) and is designated an Earthquake Hazard Zone by the State of California. The northern extent of the fault is less well defined but exhibits recent geomorphological features as far north as St. Helena (Clahan et al., 2005). A growing body of geologic, geophysical, and seismologic data in the northern and eastern Bay Area indicates that the fault is a principal element in the tectonic regime north of San Francisco Bay (Langenheim et al., 2010; Wagner et al., in press; Wesling and Hansen, 2008).

The location and occurrence of the Mt. Veeder earthquake, which caused millions of dollars in damage and resulted in Napa County being declared a federal disaster area, is believed to have occurred on the West Napa fault (Langenheim et al., 2006). Recent geophysical and geological work by the USGS and CGS suggest that the westward-dipping West Napa fault is a major structural boundary within the Northern San Francisco Bay area (Langenheim et al., 2010).

This study is an effort to develop a better understanding of the paleoseismic characteristics of the northern part of the Northern West Napa fault through detailed mapping of offset stream bank exposures and uplifted marine terraces. Napa Creek and an ephemeral drainage referred to as Alston Creek, both traverse a well defined lineament of the Northern West Napa fault zone. Natural stream channel bank exposures of these two drainages exhibit evidence for Holocene rupture of the Northern West Napa Fault zone. This study attempted to map the upper 1 to 2 m of the previously unmapped section of the Napa Creek exposure, however, severe anthropogenic alteration of the ground surface and prohibited a more detailed analysis. The mapping results of the Alston Creek exposure and the Alston Park terrace Quaternary terraces are presented in this report.

2.0 GEOLOGIC AND SEISMOTECTONIC SETTING

The West Napa fault is the northernmost element of a series of relatively short north-to- northwest-trending en echelon faults that also includes the Franklin and Southhampton faults (Figure 1). These faults lie between two major traces of the San Andreas fault system: the Hayward-Rodgers Creek-Maacama fault zone on the west and the Calaveras and Concord-Green Valley fault zones on the east. The West Napa fault was first mapped by Weaver (1949) and has since been recognized as a northwest-trending zone of anastomosing fault traces that are mapped for a distance of 30 to 35 km along the western margin of the Napa Valley (Fox and others, 1973; Helley and Herd, 1977; Wagner and Bortugno, 1982; Fox, 1983). This roughly corresponds to the northern part of the Carquinez to Napa River and Napa River to Yountville sections of the fault as shown on Figure 2. Presently, only the southern portion of the fault (i.e., the northern part of the Carquinez to Napa River section) is considered to exhibit sufficiently strong evidence of Holocene activity and is well enough defined to be designated as an Alquist- Priolo Earthquake Fault Zone by the State of California.

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Fault kinematic studies of the northern Bay Area indicate the West Napa fault zone may transfer slip from the northern Calaveras fault on the southeast to the Maacama fault zone on the northwest (Sowers, et al., 2010). The transfer of slip from the northern Calaveras fault apparently is accommodated through a series of faults in the northern East Bay Hills referred to as the Contra Costa Shear Zone (CCSZ; Brossy et al., 2010). A sequence of uplifted marine terraces along the Carquinez Strait appears to be displaced across the northern Southhampton fault, which projects to the southern end of the West Napa fault (Kelson et al., 2005). Detailed mapping by Brossy et al. (2010) north of the Carquinez Strait reveals geomorphic features and air-photo lineaments along the northwestward projection of the Southampton fault that strongly suggests that it extends northward through the city of Vallejo, and likely merges or transfers slip across a stepover to the West Napa fault near the latitude of the Town of Vallejo (Brossy et al., 2010)

More recent geologic and geophysical studies conducted prior to and following the 2000 M 5.2 Mt Veeder (Yountville) earthquake bring into question the total length of the West Napa fault and the recency and level of activity on the northern part of the fault. As described below, evidence of late Holocene faulting has been identified along the northern part of the fault that has been considered to be inactive in the Holocene. Preliminary studies along this part of the fault have identified displaced Holocene alluvium and geomorphic features indicative of Holocene fault activity. Recent mapping and geophysical studies by the U.S. Geological Survey (USGS; Fox, 1983; Langenheim et al., 2006, 2010), California Geological Survey (CGS; Wagner et al., in press; Clahan et al., 2004, 2005), and others (Wesling and Hanson (2008) suggest that traces of the West Napa fault may extend northward to the latitude of the town of St. Helena or possibly Calistoga in the northern Napa Valley, well past the previously mapped northern termination in the Yountville Hills (Helley and Herd, 1977) . This recent work suggests that the predominant range bounding fault zone extends up the Dry Creek Valley and continues along the western edge of the Napa Valley to the north. Secondary faulting off this main trace of the West Napa fault zone potentially underlies the Napa Valley and extends into the Yountville Hills. The much greater length and youthful appearance of the fault suggests that the maximum magnitude of the fault may be somewhat larger than previously thought.

Evidence of Holocene activity has been previously recognized along the northern West Napa fault, north of the Napa River Bridge, but has not been systematically and fully documented. The southern part of the fault, southeast of the Napa River, has long been recognized as exhibiting evidence of Holocene faulting (Bryant, 1982; Jennings, 1994; Wesling and others, 2001). The occurrence of Holocene faulting along the middle to northern part of the fault has important implications for evaluating the earthquake recurrence, fault segmentation, maximum magnitude assessments, and surface fault rupture hazard.

3.0 ALSTON PARK ALLUVIAL TERRACES

Alston Park is located along the western margin of the Napa Valley in northern Napa, California. The park is comprised of a series of stepped alluvial terraces and fan deposits located between Redwood Creek on the south and west sides of the park and Dry Creek Road along the east (Figures 3 and 4). Private development borders the park to the north. The terraces are associated with the incision and deposition of Redwood Creek as it flows east out of the Coast Ranges and

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into Napa Valley. Redwood Creek makes an abrupt right lateral bend near its confluence with the Napa Valley. This bend may be attributed to incision into less indurated Eocene or early Miocene age Domengine sandstone bedrock which lies unconformably on top of Mesozoic Great Valley Sequence Sandstone (Clahan et al., 2004). Bedrock faulting has not been mapped at the location of the right lateral bend in Redwood Creek, however, tectonic uplift along the West Napa fault to the east may have contributed to the orientation of Redwood Creek by sequentially uplifting alluvial terraces that have obstructed the course of the drainage.

The alluvial terraces vary in age and elevation with the higher and older terraces located to the north and then diminishing in age and elevation to the south (Figure 4). A total of six discernable terraces were mapped with the Alston Park area. The level of dissection, elevation, and consolidation of deposits lessens from north to south. The elevation of the oldest alluvial terrace in the sequence (Qt6), located along the northern margin of Alston Park is approximately 280 feet above mean sea level. The youngest alluvial terrace is located to the south where Redwood Creek enters the Napa Valley floor at approximately 130 feet above mean sea level.

All of the terraces are truncated along their eastern margin by a strong geomorphic lineament related to the Holocene active trace of the West Napa fault as described below. Immediately north of the Alston Park area the fault appears to bifurcate into the Yountville reach and the Dry Creek – St. Helena reach (Figure 2) as first described in Wesling and Hanson (2010).

4.0 ALSTON CREEK FAULT EXPOSURE

The well defined geomorphic lineament of the West Napa fault that cuts the eastern margin of the alluvial terraces near Dry Creek Road in Alston Park (Figures 3 and 4) is transected by a relatively small, ephemeral drainage referred to as Alston Creek. Alston Creek drains the dissected older surfaces of alluvial terraces Qt5 and Qt6. The relatively young Alston Creek has incised into Quaternary alluvial terraces on the west side of the fault and deposited an alluvial fan of material across and to the east of the fault trace. The older Quaternary alluvial terraces on the west are apparently offset up to about 25 m above the fan surface to the east.

A detailed log of the Alston Creek bank exposure of the fault was created by carefully cleaning and mapping the exposure over the course of several days. We used hand tools to clean the exposure, and map in all course grained material, stratigraphic units and structural features using tape and compass techniques tied to a local reference grid. Figure 5 is a log of the southern creek bank exposure of Alston Creek as it crosses the mapped trace of the WNF.

Faulted fine grained alluvial units overlying and partially underlying a clast supported cobble rich deposit are exposed in the stream cut (Figures 5, 6, and 7). The fault zone is approximately 2 m wide although additional faulting may occur further to the west as the exposure degraded upstream. The overall orientation of the mapped fault exposures at Alston Creek is about N10º to 18ºW, with varying dips but generally to the southwest or near vertical. The pronounced cobble rich deposit appears to be warped and offset vertically within and west of the fault zone. A 2 m wide zone of aligned clasts bounded by minor clay gouge is present within the cobble rich deposit. The top and bottom of the cobble rich unit are offset along with the overlying silty clay unit and bioturbated surficial silt unit in some places. An older fine grained deposit underlies the

3 cobble rich deposit within the fault zone. The contact of the cobble rich deposit and the underlying fine grained deposit appears to have more relative offset than the top of the cobble rich deposit suggesting more than one rupture event of this lower unit.

Detrital charcoal in the faulted silty clay at the base of the cobble rich deposit has been radiocarbon dated and calibrated as being approximately 7020 BP. The fine grained matrix of the cobble rich deposit was dated and calibrated as being 2960 BP (Table 1).

5.0 CONCLUSIONS

The Northern West Napa Fault at the latitude of Alston Park in Napa, California shows compelling evidence for Holocene activity. The alluvial units exposed in Alston Creek are offset and warped across a 2 m wide fault zone. The fault zone is generally westerly dipping with a significant vertical component of west side up which corresponds to deeper structural data as described by Langenheim et al. (2010).

The fault zone mapped in the stream bank exposure of Alston Creek is located at the base of the slope of an east facing fault scarp that may show evidence for up to approximately 25 m of relative vertical displacement. The fault scarp forms the eastern margin of numerous vertically uplifted alluvial terraces associated with Redwood Creek to the west. These terraces range in age and elevation. Dating of these surfaces has the potential to reveal a long term vertical slip rate.

Other investigations in the area as described by Wesling and Hanson (2008) support the conclusion of Holocene activity in the area just south of Alston Park. Additional work remains to establish Holocene activity to the north of Alston Park. Strong geomorphic evidence exists within the Dry Creek Valley for Holocene activity and an orientation towards St. Helena to the north. Additional investigations along this reach of the fault may reveal important information about the activity and tectonic characteristics of the Northern West Napa fault.

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6.0 REFERENCES

Brossy, C., Kelson, K, and Ticci, M., 2010, Digital Compilation of Data for the Contra Costa Shear Zone for the Northern California Quaternary Fault Map Database: Collaborative Research with William Lettis & Associates, Inc. and the U.S. Geological Survey: Final Technical Report submitted to the U.S. Geological Survey NEHRP, Award no. 07HQGR0063.

Bryant, W.A., 1982, West Napa fault zone; Soda Creek fault (East Napa fault): California Division of Mines and Geology Fault Evaluation Report FER-129, 9 pp.

Clahan, K.B., Wagner, D.L., Saucedo, G.J., Randolph-Loar, C.E., and Sowers, J.M., 2004, Geologic map of the Napa 7.5′ Quadrangle, Napa County, California: A digital database: California Geological Survey, scale 1:24,000.

Clahan, K.B., Wagner, D.L., Bezore, S.P., Sowers, J.M., and Witter, R.C., 2005, Geologic map of the Rutherford 7.5′ Quadrangle, Sonoma and Napa counties, California: A digital database: California Geological Survey, scale 1:24,000.

Fox, K.F., Jr , Sims, J.D., Bartow, J.A., and Helley, E.J., 1973, Preliminary geologic map of eastern Sonoma County and western Napa County, California: U.S. Geological Survey Miscellaneous field Studies Map MF-483, scale 1:62,500.

Fox, K.F., Jr., 1983, Tectonic setting of Late Miocene, Pliocene, and Pleistocene rocks in part of the Coast Ranges North of San Francisco, California: U.S. Geological Survey Professional Paper 1239, 33 p.

Helley, E.J. and Herd, D.G., 1977, Map showing Quaternary displacement, northeastern San Francisco Bay area region: U.S. Geological Survey Miscellaneous Filed Stidies Map MF- 881, scale 1:125,000.

Jennings, C.W., 1994, Fault activity map of California and adjacent areas with locations and ages of recent volcanic eruptions: Department of Conservation, California Geological Survey, Geologic Data Map No. 6, scale 1:750,000.

Kelson, K.I., Unruh, J.R., and Baldwin, J.N., 2005, Late Quaternary deformation in the 12 northeastern East Bay hills, San Francisco Bay region [abs.]: Geological Society of America Abstracts with Programs, v. 37, no. 4, p. 106

Langenheim, V.E., Graymer, R.W., and Jachens, R.C., 2006a, Geophysical setting of the ML 5.2 Yountville, California, earthquake: Implications for seismic hazard in Napa Valley, California: Bulletin of the Seismological Society of America, v. 96, no. 3, pp. 1192-1198.

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Langenheim, V.E., Graymer, R.W., Jachens, R. C., McLaughlin, R.J., Wagner, D.L., and Sweetkind, D.S., 2010, Geophysical framework of the northern San Francisco Bay region, California: Geosphere, v. 6; no. 5; p. 594-620; doi: 10.1130/GES00510.

Sowers, J.M., Kelsey, H.L., and Unruh, J.R., 2010, Mapping, Assessment, and Digital Compilation of the Connectiion Between the Rodgers Creek and Maacama Faults, Sonoma County, for the Northern California Quaternary Fault Map Database: Collaborative Research with William Lettis & Associates, Inc., and the U.S. Geological Survey: Final Technical Report submitted to the U.S. Geological Survey NEHRP, Award no. G09AP00058.

Wagner, D.L. and Bortugno, E.J., 1982, Geologic map of the Santa Rosa Quadrangle, California: California Division of Mines and Geology Regional Geologic Map Series Map 2A, scale 1:250,000.

Wagner, D.L., Saucedo, G.J., Clahan, K.C., Fleck, R., Langenheim, V.E., McLaughlin, R.J., Sarna-Wojcicki, A.M., Allen, J.R., Deino, A.L., 2010, Geology, Geochronology and Paleogeography of the Southern Sonoma Volcanic Field and Adjacent Areas, Northern San Francisco Bay Region, California: Geological Society of America, Geosphere, in press.

Weaver, C.E., 1949, Geology of the Coast Range immediately north of the San Francisco Bay Region, California: Geological Society of America Memoir 35, 242 p.

Wesling, J.R., Hanson, K.L., and Swan, F.H., 2001, Investigation of the West Napa fault, San Francisco Bay Area, California (abs.): Seismological Research Letters, v. 72, no. 2, pp. 226-227.

Wesling, J.R., and Hanson, K.L., 2008, Digital compilation of West Napa fault data for the Northern California Quaternary Fault Map Database: Final Technical Report submitted to the U.S. Geological Survey NEHRP, Award no. 05HQAG0002.

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TABLE 1

RADIOCARBON RESULTS FOR THE ALSTON CREEK EXPOSURE Northern West Napa Fault Zone Napa County, California

Lab Sample Testing Material Measured Δ13C Conventional 2 Sigma Number Name Pretreatment Age Age Calibration Cal BC (organic 6020 to 7020 +/- 7050 +/- 50 243692 AC003 sediment): -23.4 AMS 50 BP BP 5840 (Cal acid washes BP 7970 to 7790) (wood): 105.8 +/- 106.3 +/- 0.3 243691 AC002 -27.3 AMS acid/alkali/acid 0.3 pMC pMC Cal BC 1310 (organic 2960 +/- 2960 +/- 40 to 1040 (Cal 243690 AC001 sediment): -24.7 AMS 40 BP BP BP 3260 to acid washes 2990)

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YUBA CO COLUSA CO

SUTTER CO LAKE CO MENDOCINO CO

YOLO CO SACRAMENTO CO

Maacama

fault NAPA CO

SONOMA CO

Rogers Creek fault Green Valley fault

West SOLANO CO

Napa fault

San Andreas fault SACRAMENTO CO

Concord fault MARIN CO

SAN JOAQUIN CO

Explanation

(Jennings, 1994) CONTRA COSTA CO

Historic Hayward fault Holocene late Quaternary SAN FRANCISCO CO Calaveras fault Quaternary County boundaries SAN JOAQUIN CO

ALAMEDA CO SAN MATEO CO

05 10 15202530354045kilometers

NORTHERN WEST NAPA FAULT Quaternary Active Faults in the Northern San Francisco Bay Area

Figure 1 American Canyon

Carquinez Strait

NORTHERN WEST NAPA FAULT Map Showing Recency of Activity and Reaches West Napa Fault, California (after Wesling and Hanson, 2008) Figure 2 Alston Park

Alston Creek Exposure

Oblique aerial photograph looking to the southeast of Alston Park area, Napa County, CA. Arrows denote West Napa fault scarp. Uplifted alluvial terraces associated with Redwood Creek are seen west of fault scarp (see Figure 4 for detailed map). Location of Alston Creek stream bank exposure is shown where it crosses the scarp (see Figure 5 for detailed log).

NORTHERN WEST NAPA FAULT Oblique Aerial Photograph Alston Park Area, Napa County, CA (after Wesling and Hansen, 2008)

Figure 3 122°21'30"W 122°21'0"W 122°20'30"W

Area not mapped

122°21'30"W Qhf

Qhf br?

Qls 38°19'30"N Qht2? Qht2? Qoa 122°20'0"W

Qht1 w br w af w w af

br 38°20'0"N Qhf Qt5? Qoa Qht1

Qoa Qht1? Qhf af Qhf

Qep Qht2 Qt5 Qt6

Qoa af af w Qt6 Qhf Qoa Qht2 Qhf Alston Creek fault exposure (Figure 5) Area not mapped af

Qt5 Qht1 Alston Creek Qhf Qht1

Qep 38°19'0"N Qoa

af Qht2 af Area not mapped 122°21'0"W af af Qt4

Qt3b Qep 38°19'30"N

w af Redwood Creek Qhc 122°19'30"W Qt3b Qht1 Qhf

af Qt3a Qt3b af

Qht1 Qht2 38°18'30"N 0 1000 feet

0 400 meters

122°20'30"W 122°20'0"W 122°19'30"W 38°19'0"N

Explanation

Geologic Units Symbols af Artificial Fill Stream Terraces (Youngest to Oldest) Lineament, dashed and queried where uncertain w Water body Qht1 Geologic contact, dashed and Qep Ephemeral pond Qht2 queried where uncertain

Qt3a/ Qls Landslide Qt3b Intermittent stream Limit of detailed mapping Qhc Active Stream Channel Qt4

Qt5 Qhf Holocene floodplain Northern West Napa Fault

Qoa Quaternary older alluvium Qt6 Quaternary Geologic Map of the Alston Park Area, Napa County, CA br Bedrock Figure 4 Vertical Distance (meters) 2 1 0 0+00

? ? Red shadedareadenotes gouge. located, queriedwhere uncertain. located, dashedwhere approximately Fault -Solidwhereaccurately Shear fabric Krotovina hachured where dashed,gradational Contacts approximatelylocated +020 +040 +06+00 5+00 4+00 3+00 2+00 1+00 2 Symbols Root Charcoal sample South Bank N54 3 Explanation Horizontal Distance(meters) 4 ˚ E 1 5a 4 3 1 2 5

well sorted, moderately bioturbated, occassionalrounded pebbles upto 12mm. Unit 2- Yellowish brown (10YR5/4)sandysilt(ML)-med. stiff, finegrained sand, Unit 5a- Yellowish brown clay (10YR5/4)silty (CL)- soft, wet, orange mottling. Unit 5- Yellowish brown clay (10YR 5/6)silty (CL)-med. stiff, moist, withcoarse sand. cobbles withclay matrix, cobbles are rounded to subrounded upto 0.6mdiameter. Unit 4-Grayish brown (10YR5/2)clayey cobbles -dense, volcanic clastsupported to 6cm, moderately bioturbated, abruptlower contact. Unit 3-Brown clay (10YR5/3)silty (CL)-med. stiff, occassionalsubangularclastsup heavily bioturbated, gradational lower boundary. Unit 1- Yellowish brown (10YR5/4) clayey silt(ML)-med. stiff, abundantorganics, ?

? Geologic Units

(2960 +/-40yrsB.P.) AC001 ?

? ?

(7020 +/-50B.P.) AC003 ?

? ? 5

N18 ?

˚ AC002 (Modern) ? W N10 5a

˚ W ? Debris (exposurecovered) Alston CreekStream BankLog,

NORTHERN WEST NAPA FAULT

?

Napa County,CA ?

?

?

?

? ?

7+00 0 1 2 Figure 5 Vertical Distance (meters) FAULTFAULT ZZONEONE

Photo mosaic of Alston Creek stream bank exposure south wall. Individual shearing and offset units and materials are schematically presented within the fault zone.

NORTHERN WEST NAPA FAULT Alston Creek Photographic Log, Napa County, CA

Figure 6 South facing photo of Alston Creek stream bank exposure along fault scarp (note geologist for scale).

Northeast facing photo overlooking Alston Creek stream bank exposure from fault scarp.

NORTHERN WEST NAPA FAULT Site Photographs, Alston Park Napa County, CA

Figure 7