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FINAL TECHNICAL REPORT
COVERING THE PERIOD APRIL 15, 1998 to APRIL 14, 2001
HOLOCENE SLIP RATE OF THE GREEN VALLEY FAULT AT FREEBORN CREEK,
FAIRFIELD, CALIFORNIA
Principal Investigator: Glenn Borchardt
Soil Tectonics, P.O. Box 5335, Berkeley, California 94705-0335
Date Submitted: July 14, 2001
Sponsored by the United States Geological Survey, Department of the Interior under U.S.G.S.
Award No. 98HQGR1039
Program Objective: Program Element II, Components II.3 and II.5
Technical Officer: Dr. John Unger, U.S. Geological Survey
Effective Date of Contract: April 15, 1998
Expiration Date of Contract: April 14, 2001
Amount of Contract: $35,000
Research supported by the U.S. Geological Survey (USGS), Department of the Interior, under
USGS award number 98HQGR1039. 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.
HOLOCENE SLIP RATE OF THE GREEN VALLEY FAULT AT FREEBORN CREEK,
FAIRFIELD, CALIFORNIA
Final Technical Report to the United States Geological Survey
Award No. 98HQGR1039
Glenn Borchardt
1
William V. McCormick
2
John Baldwin
3
1
Soil Tectonics, P.O. Box 5335, Berkeley, CA 94705
2
Kleinfelder, Santa Rosa, CA
3
W.A. Lettis and Associates, Walnut Creek, CA
2001
iii
CONTENTS
ABSTRACT 4
INTRODUCTION 5
TECTONIC SETTING ........................................................................................................5
REGIONAL GEOLOGY .....................................................................................................6
SITE GEOLOGY .................................................................................................................6
METHODS 7
RESULTS AND DISCUSSION 7
TRENCH T-1 .......................................................................................................................7
TRENCH T-2 .......................................................................................................................8
TRENCH T-3 .......................................................................................................................8
TRENCH T-4 .......................................................................................................................8
TRENCH T-5 .......................................................................................................................9
TRENCH T-6 .......................................................................................................................9
CONCLUSIONS 9
ACKNOWLEDGEMENTS 10
REFERENCES 11
TABLES 13
FIGURES 19
PLATES 26
4
HOLOCENE SLIP RATE OF THE GREEN VALLEY FAULT AT FREEBORN CREEK,
FAIRFIELD, CALIFORNIA
98HQGR1039
Glenn Borchardt
William McCormick
John Baldwin
Soil Tectonics, P.O. Box 5335, Berkeley, California 94705-0335
510-654-1619; FAX: 815-327-5331; gborchardt@usa.net
Components II.3: "Determine the nature and rates of crustal deformation," and II.5:
"Identify active faults, define their geometry, and determine the characteristics and dates of past
earthquakes".
ABSTRACT
Due to the difficulty of getting permission to access the Freeborn Creek site, an alternate,
less desirable site was studied at Jameson Creek near the junction of I80 and State Highway 12.
Four traces of the N22W trending Green Valley fault (GVF) had been mapped in this area, with
three of them being on land owned by the City of Fairfield. Our best hope of obtaining a Holo-
cene slip rate for the GVF was along Trace No. 2, which was a southerly extension of a fault at
the contact between the Markley Sandstone and the Sonoma Volcanics discovered by
CALTRANS north of State Highway 12. Geomorphically, the suspect trace was defined by a
N22W trending linear swale with a back-facing scarp on its eastern side. The trace projected to-
ward the western side of an apparent faceted pressure ridge south of I80.
Although there were suitable Holocene-age strata, excavations across all three traces
failed to uncover signs of Recent activity. Minor Riedel shears (N20E to N40E) offsetting pre-
Holocene units in Trenches T-3 and T-6 were found on the eastern side of the site. Analysis of
cataclasis along these offsets is in progress. Thin-section data will be compared to that from
creeping and non-creeping faults to determine if catastrophic events are possible along the
Green Valley fault, which lies along Trace No. 4, offsite less than 200 m to the east.
5
INTRODUCTION
The 1989 Loma Prieta earthquake served notice to the Bay Area that the long seismic
quiescence following the great San Andreas earthquake of 1906 is over. As part of the San An-
dreas fault system, the Green Valley fault (GVF) traverses the central portion of the rapidly de-
veloping Solano County corridor between Vallejo and Fairfield (Fig. 1). Along its length, this
fault intersects several major transportation routes, rail lines, power transmission lines, pipelines,
and levees. It was first recognized as an active fault by Sims and others (1973) and based on its
youthful tectonic geomorphology (Bryant, 1982, 1992), the state has zoned it to prevent devel-
opment across the active trace (CDMG, 1993) (Fig. 2). Its activity has been confirmed by trench-
ing studies for development (Carey and Wigginton, 1990; Nichols and Rice, 1972; Ries, 1995;
Rowley and McRae, 1985). The GVF has possible fault lengths ranging from 28 to 82 km and is
considered by many geologists to be the northern extension of the Concord fault (Borchardt and
Baldwin, 2001). The Concord-Green Valley structure appears capable of M7 events.
Our recent study of the associated Concord fault, on trend to the south, yielded a horizon-
tal slip rate of 3.2 mm/yr (Borchardt and others, 2001; Borchardt and others, 1999), a value simi-
lar to the ongoing aseismic slip of 3-3.5 mm/yr measured between 1980 and 1999 at nearby sites
in Concord (Galehouse, 1999). Although the vertical slip rate at Galindo Creek was 0.45 mm/yr,
there was no clear evidence of catastrophic events occurring at that site. Creep on the GVF is
similar to that on the Concord fault, being episodic and averaging between 4 and 5 mm/yr for
14.7 years (Galehouse, 1999). There have been no historic earthquakes on the GVF; the largest
on the Concord fault was a M5.4 in 1955. So far, their similar seismic and aseismic behaviors
point toward the two faults acting as a single structure.
The present study originally was to determine the Holocene slip rate of the only trace in
the middle of the GVF at Freeborn Creek. Unfortunately, permission to excavate could not be
obtained from the landowner, A.D. Seeno Construction Company. During the negotiations, most
of the site was destroyed by the construction of infrastructure for new homes. A replacement
wetland was excavated within the northern setback zone in which we planned to excavate. The
alternate site described in this report was not nearly as desirable, for several reasons. First, the
site is at the northern end of the fault, not in the middle, where it would be the best representa-
tive. Second, the subparallel Cordelia fault, which lies only 1.7 km to the east, probably takes up
part of the slip on the fault system. Third, the official Earthquake Fault Zone map of the State of
California shows four subparallel, discontinuous traces within the fault zone at Jameson Creek.
Fortunately, the Jameson Creek site was owned by the City of Fairfield. Permission to excavate
was readily obtained.
TECTONIC SETTING
Detailed geodetic studies employing both trilateration and satellite-based techniques indi-
cate that 31 to 38 mm/yr of northwest dextral shear is occurring between the Pacific and North
American plates across a 100-km-wide zone in the northern Coast Ranges west of the Great Val-
ley (Williams and others, 1994). At the latitude of Jameson Creek, most of the plate motion in
the San Francisco Bay Area is accommodated by three major right-lateral strike-slip fault sys-
tems that include the San Andreas fault, the Hayward-Rodgers Creek-Healdsburg fault system,
and the Concord-Green Valley-Cordelia fault system (Fig. 1). The San Andreas fault has a slip
6
rate of about 24±2 mm/yr (Niemi and Hall, 1992; Prentice, 1989) and the Hayward-Rodgers
Creek fault system has a slip rate of about 8±2 mm/yr (Lienkaemper and Borchardt, 1996;
Schwartz and others, 1992). The remaining slip predicted by the plate motion model (up to 6
mm/yr) apparently is distributed across the Green Valley, Cordelia, and other faults.
Early slip rate estimates for the Concord-Green Valley fault system ranged from 8±2
mm/yr (Kelson and others, 1992) to 5±3 mm/yr (WGCEP, 1999). However, our recent paleose-
ismic study at Galindo Creek (Borchardt, and others, 1999) showed that that part of the Concord
fault had a geologic slip rate of only 3.2±0.4 mm/yr during the last 6,000 years (Borchardt, and
others, 2001). Aseismic slip in the area also has been about 3 mm/yr during the last two decades,
possibly indicating that catastrophic ground surface rupture may not occur on this part of the
fault. No clearly indisputable evidence for discrete event horizons was found at Galindo Creek.
The hazard produced by the Green Valley fault may be lower than previously thought. A geolog-
ic slip rate on the Green Valley fault would help us test that hypothesis.
REGIONAL GEOLOGY
The Green Valley fault lies along the eastern side of the coastal mountains north of the
Sacramento River. It roughly forms the boundary between Tertiary bedrock of the hills and late
Pleistocene and Holocene alluvium of Suisun Bay. In places, the fault subdivides the Tertiary
bedrock, leaving slivers of it on the east side of the fault. One example of this exists just north
of Jameson Creek where the Markley Sandstone on the west appears to be in fault contact with
Sonoma Volcanics on the east (Cole and Pratt, 1991).
SITE GEOLOGY
The Markley Sandstone and Sonoma volcanic outcrops like those to the north are absent
on the south side of the creek. The area may have been planated or eroded as a result of the ten-
dency of Jameson Creek to produce dextral meander bends, perhaps in response to right-lateral
movement along the GVF. A pressure ridge south of the site apparently consists of Tehama For-
mation sandwiched between two traces of the GVF (Fig. 2).
Four discontinuous traces of the GVF have been mapped in a broad zone within the
Jameson Creek area (Figs. 2 and 3). Trace No. 1, farthest to the west, is 0.64 km long, extending
from the west side of the pressure ridge to Jameson Creek at N22W. Trace No. 2 bisects the
Jameson Creek study site, crosses the creek at N22W, and parallels the sidehill where (Cole and
Pratt, 1991) found it at the contact between the Markley Sandstone and the Sonoma Volcanics
(Fig. 4). This trace continues N22W for about one kilometer, where it heads north to join Trace
No. 3 on a continued N22W trend. Trace No. 3 just enters the site from the north. Trace No. 4
lies along the eastern side of the pressure ridge south of I80. It continues northerly for about one
kilometer, crosses Jameson Creek east of the study site, and terminates at the base of an outcrop
of Sonoma Volcanics (Fig. 2). At this point, Trace No. 4 apparently forms a left step with Trace
No. 3 within the Sonoma Volcanics.
We thought that Trace No. 2 was the best prospect for obtaining a slip rate at the site
(Fig. 4). This trace was known to be at the major contact between the Markley Sandstone and the
Sonoma Volcanics where there was some undocumented evidence for Holocene offset north of
State Highway 12 (Cole and Pratt, 1991). The geomorphology of the pressure ridge south of I80
7
indicated that its western side was faceted, probably by a simple N22W continuation of a fault
known to have aseismic slip less than a kilometer to the south (Galehouse, 1999). On aerial
photos and on the large-scale topographic map prepared by the City of Fairfield (Fig. 3), it
appeared that Trace No. 2 proceeded across the site through a N22W-trending swale. Before the
site was graded, the eastern side of the swale was bounded by an elevated area that could have
been the head of an offset fan or a continuation of the pressure ridge south of I80 (Figs. 2, 3, and
4).
METHODS
During November 2000, we excavated six trenches at the site, crossing Traces 1, 2, and
3 (Fig. 3 and Plates 1 through 6). Six small charcoal samples were collected, air dried, and ex-
amined in detail for quality. Only unabraded samples with distinct plant structure were used for
14
C analysis via accelerator mass spectroscopy. Four soil profiles were measured and sampled
(Table 1 and Table 2), and two charcoal samples were dated (Table 3).
Trenches were excavated with a rubber-tired, 36” backhoe at depths between 2 and 3.5-
m. The phreatic surface was encountered at an elevation of 25.3 m (at a depth of 2.5 m) in
trench T-1. One wall of each of the trenches was cleaned and logged at 1:40 scale by using
standard methods (Borchardt, 1993; Lienkaemper and Borchardt, 1996). Clasts larger than 3
cm were measured individually. The 90’ datum for the site was the top of the storm drain cover
just north of trench T-1, which was labeled “FI #3, Grate 90.00, Inv 85.25” (Fig. 3). An elec-
tronic water level was used to tie trench elevations to this point, which was taken as 27.4 m
above sea level. Trench positions and elevations for the remaining trenches were measured
from the nearest grate.
RESULTS AND DISCUSSION
TRENCH T-1
Trench T-1 was across Trace No. 2, where we expected to encounter evidence for Holo-
cene offset within the N22W-trending swale (Plate 1). Despite caving problems induced by high
water, we were able to examine and log the Holocene channel fill that had formed within the
swale at station 28. The thalweg of the 5.5-m wide channel fill was at 24.5 m above sea level.
The channel that formed it had cut through an olive gray silty clay 17CBb5 horizon that extended
throughout most of the 70-m long trench. An earlier channel fill had formed at a similar eleva-
tion at station 45, with its eastern margin possibly being isochronous. Young, 2-m wide, <1 m
deep channel fills formed at stations 23, 7, and 11 after the main channel was abandoned. The
thalwegs of these channels were at 26.10, 26.81, and 27.38 m, respectively, implying that there
might have been a steady increase in base level with time.
Analysis of the soil stratigraphy revealed evidence for five paleosols, with the two
youngest paleosols being older than the main channel fill at station 28. Where grading has been
minimal (in the center of the swale), the depth to the 5Bkb2 horizon was about 80 cm. Precipita-
tion since 1950 averaged 570 mm/yr (22.5 in/yr) at Fairfield. As a rule of thumb, Bk horizons do
not form in soils that have an annual precipitation greater than 500 mm/yr (Birkeland, 1999).
This rule may have to be modified slightly for the Fairfield climate. The 3CB horizon in Soil
8
Profile No. 5 in trench T-4 had a charcoal age of 4.58 ka. Thus the 570-mm/yr rainfall was not
effective enough to prevent the overlying Bk horizon from forming in the clay alluvium. Bk ho-
rizon formation ceased at Union City when annual rainfall increased to 470 mm/yr after 7 ka
(Borchardt and Lienkaemper, 1999). Since pedogenic carbonate forms here at present, its ab-
sence is not a useful indicator of late Holocene soil development in clayey parent materials at
Jameson Creek. None of the soil or sedimentary units in trench T-1 was found to have shears or
offsets indicative of active faulting.
TRENCH T-2
This trench essentially is an eastward extension of trench T-1. Sequence units developed
in trench T-1 have been correlated to this trench. The major unconformity exists where young
flood deposits (A and 2ABb1 horizons) have buried a calcareous paleosol (15ABb5, 15Bkb5,
and 17CBb5 horizons) (Plate 2). None of these units were sheared or offset.
TRENCH T-3
Partly due to the removal of soil horizons by grading, this trench (Plate 3) appears to have
no units in common with trench T-2 to the west. For example, the prominent MnOx layer at the
27-m elevation in the eastern end of T-2 was not found at the 27-m elevation in T-3. Unfortu-
nately, the area between the two trenches had been excavated previously for the installation of a
storm drain (Fig. 3). We were not able to examine this area directly. However, if fault trace No. 3
crossed this area, evidence for it probably would have been seen in trench T-5 (Plate 5).
Minor Riedel shearing appeared in the lower 2/3 of the trench at station 4. Shears dipped
steeply to the east and appeared to have a N40E trend. Charcoal sample 00B507, having a
14
C
age of 17.6 ka (Table 3), was obtained from the offset B22kt horizon. The shearing was studied
in detail on the south wall of the trench (Fig. 5) to obtain evidence for or against cataclasis, frac-
turing of silt and sand grains that would indicate that the shears formed during a catastrophic
event (Fig. 6).
TRENCH T-4
This 25-m long trench was excavated across Trace No. 1 (Fig. 2, Plate 4). The log shows
18 sedimentary units, with the oldest unit consisting of olive yellowish brown silty clay much
like that at the base of Trenches 1 and 2. The youngest part of the section consisted of a 6-m
wide channel fill having its thalweg just below the 28-m elevation (Plate 4). Charcoal samples
00B501 through 00B504 were obtained from a clay fine sand unit between the 28.5 and 29.5-m
elevations. As mentioned, sample 00B504, from the base of the 3CB unit had a
14
C date of 4.58
ka, showing that the overlying Bk horizon developed after the early Holocene dry period (7-10
ka) suggested for the Bay Area by Borchardt and Lienkaemper (1999).
None of the units in T-4 was sheared or offset. Fault Trace No. 1 either does not extend
this far south, or it must be considered non-existent. The area between trenches T-4 and T-1 re-
cently was excavated by David Gius of Wallace-Kuhl & Associates, Inc. We observed no shears
or offset soils or sediments in that area.
9
TRENCH T-5
This trench was excavated for two purposes: 1) examine the possible extension of the
Riedel shear seen in T-3 and 2) intercept Trace No. 3 where it may have gone undetected be-
tween Trenches 2 and 3. Unfortunately, the upper soil horizons had been removed by grading
(Plate 5). Fortunately, the lower sedimentary units were sufficient for evaluating age and tectonic
history. These Bkt, CBt, and 2Crk horizons were flat lying, brown (7.5YR4/2m) silty clays to
gravelly sands at first estimated to be greater than 10 ka and later shown to be greater than 17.6
ka via correlation from trench T-3 (Table 3, Plate 3). A few rare pebbles of vesicular basalt indi-
cated probable derivation from the Sonoma Volcanics. The few siltstone clasts were yellowish
brown (7.5YR5/8m) throughout. Siltstone clasts in the younger materials found in trenches T-1
through T-4 had yellowish brown centers, but they were white on the outside. Some of the calcite
nodules were up to 6 cm in diameter.
There were no shears or offsets in this trench. Either Trace No. 3 has not reached this far
south, or it is nonexistent.
TRENCH T-6
This trench was excavated to intercept the Riedel shears found in T-3. The sedimentary
units here were similar to those in T-5, in that they were strong brown to yellow brown silty
sands to gravel and also had carbonate accumulations to 6 cm (Plate 6). However, instead of be-
ing flat lying, these tended to dip in either direction from the N20E, 70W-dipping main trace at
station 3. This trace was encrusted with calcite that was 6 cm wide near the graded surface at the
28-m elevation, but only millimeters wide at the 26-m elevation at the base of the trench—a
possible indicator of minor anticlinal folding. Minor shears between stations 5 and 7 were vertic-
al, also being encrusted with calcite. Two shears east of the main trace had strikes from N24E to
N30E and dips from 51 to 72W (Plate 6).
The units in this trench are considered to be older than 17.6 ka. Shearing extends to the
graded surface, not allowing for an estimation of its involvement with the soil. Most of the shears
have produced no discernable offset, although movement along the main trace seems to be re-
lated to the development of opposing dips of units to the west and east. This main trace may have
been responsible for the difficulty of correlating units between trenches T-2 and T-3 (Fig. 3).
However, due to the age of the units and the trend of the faulting, we do not consider this trace to
be suitable for determining the Holocene activity of the GVF.
CONCLUSIONS
Despite what appeared at first to be good geomorphic and geologic evidence, trenching at
Jameson Creek did not uncover Holocene offset along Trace No. 2. Extension of the initial
trench 30 m to the west and 70 m to the east also showed no evidence for offset. Trenching 30 m
to the either side of Trace No. 2, likewise, revealed no strike-slip offset along the N22W trend of
the GVF. Trenches T-3 and T-6, however, did uncover Riedel shearing with trends between
N20E and N40E in pre-Holocene sediments. The offsets were suitable for the analysis of catacla-
sis, which eventually may show whether or not the tectonic movement occurred during cata-
strophic events, but they were considered unsuitable for determining the slip rate of the GVF.
10
Trench T-4, across Trace No. 1, also did not have evidence for tectonic offset. The lineaments
that were considered possible faults by previous investigators seem to be the result of cut and fill
features formed during the Holocene.
The main, and only trace of the GVF at this latitude appears to lie along Trace No. 4,
which is east of the study site. Trace No. 4 bounds the east side of the pressure ridge south of
I80, continues north to cross Jameson Creek, I80, and State Highway 12. Upon reaching the So-
noma Volcanics it takes a left step, possibly contributing to the uplift of an elongate hill between
two north-trending portions of the fault. The traces on either side of this hill may be suitable as
trench sites for determining the dates of individual events, but probably would be poor sites for
determining the slip rate of the GVF.
ACKNOWLEDGEMENTS
We are extremely grateful to the City of Fairfield, especially Charles Beck, Gene Cor-
tright, and Peter Wright for their permission to access the property and for their helpful attitude
and professionalism. Many thanks to the field crew, who included Patrick Drumm and Brooks
Ramsdell from Geolith, Inc., Jeff Richmond and Byron Anderson from Kleinfelder, Inc., and
Dave Sullivan from Lumos Associates. We appreciate the inclusion of our fault-oriented sam-
ples in the cataclasis study by Susan Cashman of Humboldt State University. Thanks to David
Gius of Wallace-Kuhl & Associates, Inc. for his invitation to view the unfaulted soils and se-
diments excavated between our trenches T-4 and T-1.
We are also grateful to the U.S. Geological Survey (USGS) for supporting this research
under USGS award number 98HQGR1039. The views and conclusions contained in this docu-
ment are those of the authors and should not be interpreted as necessarily representing the offi-
cial policies, either expressed or implied, of the U.S. Government.
11
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Resources Conservation Service, 326 p.
Soil Survey Staff, 1999, Soil taxonomy: A basic system of soil classification for making and in-
terpreting soil surveys, USDA-SCS Agriculture Handbook 436 U.S. Government Printing Of-
fice Washington DC, 869 p.
WGCEP (Working Group on California Earthquake Probabilities), 1999, Earthquake Probabili-
ties in the San Francisco Bay Region: 2000 to 2030 - A Summary of Findings
(http://geopubs.wr.usgs.gov/open-file/of99-517/), Open File Report 99-517, 36 p.
Williams, S. D. P., Svarc, J. L., Lisowski, M., and Prescott, W. H., 1994, GPS measured rates of
deformation in the northern San Francisco Bay region, California, 1990-1993: Geophysical
Research Letters, v. 21, p. 1511-1514.
13
TABLES
Table 1. Description of profiles representative of soil development in Trench T-4 at the Jameson
Creek study site, Fairfield, California. Abbreviations and definitions are available (Soil Sur-
vey Staff, 1993, 1998, 1999).
Described by Glenn Borchardt on November 19, 2000 in the north wall of Trench T-4 at an ele-
vation of ~108’ (via GPS). The site is on an alluvial plain across the N22W trend of Fault Trace
No. 1 of the Green Valley fault. Mediterranean climate. Vegetation is grassland. Slope <1
o
. As-
pect N. Moderately well drained, with the soil moist below 1 m. The soil is slightly acid in the
surface, gradually becoming moderately alkaline at 110 cm. Current water table >2.5 m.
____________________________________________________________________________
Horizon Depth, cm Description
____________________________________________________________________________
Soil Profile No. 1 (station 27.3 m)
(latitude 38
o
12.385’ and longitude 122
o
09.346’)
A 0-54 Very dark brown (10YR2/2m, 5/1d) clay; coarse to medium strong
angular blocky structure; very sticky and plastic when wet, firm when moist, and extremely hard
when dry; common fine to medium roots; few fine continuous random tubular pores; gradual
wavy boundary; pH 6.3; conductivity 110 uS; Sample No. 00B511.
14B1k 54-110 Very dark gray (10YR3/1m, 4/1d) clay with few fine prominent
white (10YR8/1) mottles due to calcite filaments and nodules to 1 cm; coarse to medium strong
angular blocky structure; very sticky and plastic when wet, firm when moist, and extremely hard
when dry; few fine to medium roots; few fine continuous random tubular pores; violent efferves-
cence; clear wavy boundary; pH 7.6; conductivity 180 uS; Sample No. 00B512.
14B2k 110-172 Dark grayish brown (10YR4/2m, 6/2d) clay with many fine to me-
dium prominent white (10YR8/1) mottles due to calcite filaments and nodules to 2 cm and few
fine prominent yellow (10YR7/8md) mottles due to iron oxides; coarse to medium strong suban-
gular angular blocky structure; very sticky and plastic when wet, firm when moist, and extremely
hard when dry; few fine roots; few fine continuous random tubular pores; extremely violent ef-
fervescence; gradual wavy boundary; pH 8.1; conductivity 330 uS; Sample No. 00B513.
14B3k 172-240 Very dark grayish brown (10YR3/2m, 6/1d) silty clay with few
fine prominent white (10YR8/1) mottles due to calcite filaments and nodules to 1 cm and few
fine prominent yellow (10YR7/8md) mottles due to iron oxides; coarse to medium strong angu-
lar blocky structure; very sticky and plastic when wet, firm when moist, and very hard when dry;
very few fine roots; few fine continuous random tubular pores; violent effervescence; gradual
wavy boundary; pH 7.5; conductivity 600 uS; Sample No. 00B514.
14BCk 240-245+ Very dark grayish brown (10YR3/2m, 6/2d) silty clay with com-
mon fine to medium prominent black (10YR2/0) mottles due to mangans; massive to medium
moderate subangular to angular blocky structure; very sticky and plastic when wet, firm when
14
moist, and extremely hard when dry; few fine continuous random tubular pores; violent efferves-
cence; pH 7.5; conductivity 1100 uS; Sample No. 00B515.
*ESTIMATED AGE: t
o
= 4.6 ka
t
b
= 0 ka
t
d
= 4.6 ky
Soil Profile No. 2 (station 1 m) (latitude 38
o
12.375’ and longitude 122
o
09.346’)
AB 0-30 Very dark brown (10YR2/2m, 4/2d) clay; coarse to medium strong
subangular to angular blocky structure; very sticky and plastic when wet, firm when moist, and
very hard when dry; many fine roots; common fine continuous random tubular pores; gradual
smooth boundary; pH 5.4; conductivity 370 uS; Sample No. 00B521.
B 30-102 Black (10YR2/1m, 4/1d) clay; coarse strong angular blocky struc-
ture; very sticky and very plastic when wet, firm when moist, and very hard when dry; common
fine roots; few fine continuous random tubular pores; abrupt wavy boundary; pH 6.3; conductivi-
ty 420 uS; Sample No. 00B522.
Bk 102-134 Dark gray (10YR4/1m, 6/1d) clay with common medium promi-
nent white (10YR8/1md) mottles due to calcite filaments and few fine faint yellow (10YR7/8md)
mottles due to iron oxides; medium to coarse strong subangular to angular blocky structure;
sticky and plastic when wet, firm when moist, and hard when dry; few fine roots; few fine conti-
nuous random tubular pores; extremely violent effervescence; clear wavy boundary; pH 7.5;
conductivity 300 uS; Sample No. 00B523.
2CB 134-186 Yellowish brown (10YR5/4m, 6/4d) sand to gravelly sand with
few fine faint brownish yellow (10YR6/8md) mottles due to iron oxides; massive structure;
slightly sticky and nonplastic when wet, very friable when moist, and soft when dry; common
fine continuous random tubular pores; abrupt smooth boundary; pH 7.5; conductivity 630 uS;
Sample No. 00B524.
3CB 186-210 Dark grayish brown (2.5Y4/2m, 6/2d) fine sand; massive structure;
nonsticky and nonplastic when wet, very friable when moist, and soft when dry; few fine conti-
nuous random tubular pores; very few thin clay films coating pores; abrupt smooth boundary;
charcoal sample 00B504 was dated at 4.58 ka; pH 7.6; conductivity 670 uS; Sample No.
00B525.
4CB 210-235 Dark grayish brown (2.5Y4/2m, 6/2d) light silty clay loam; me-
dium to coarse moderate angular blocky structure; slightly sticky and slightly plastic when wet,
very friable when moist, and slightly hard when dry; few fine to medium continuous random tu-
bular pores; abrupt smooth boundary; pH 7.6; conductivity 560 uS; Sample No. 00B526.
15
18CB 235-247+ Light brownish gray (2.5Y6/2m, 8/2d) silty clay loam with com-
mon fine distinct brownish yellow (10YR6/8md) mottles due to iron oxides; medium weak angu-
lar blocky structure; sticky and plastic when wet, firm when moist, and very hard when dry; few
fine to medium continuous random tubular pores; pH 7.5; conductivity 1010 uS; Sample No.
00B527.
ESTIMATED AGE: t
o
= 4.6 ka
t
b
= 0 ka
t
d
= 4.6 ky
______________________________________________________________________________
*
Pedochronological estimates based on available information. All ages should be considered
subject to +50% variation unless otherwise indicated (Borchardt, 1992).
14
C dates were used to
substantiate ages and durations in bold face.
t
o
= date when soil formation or aggradation began, ka
t
b
= date when soil or strata was buried, ka
t
d
= duration of soil development or aggradation, ky
16
Table 2. Description of profiles representative of soil development on either side of the Riedel
shear in Trench T-3 at the Jameson Creek study site, Fairfield, California. Abbreviations and de-
finitions are available (Soil Survey Staff, 1993, 1998, 1999).
Described by Glenn Borchardt on November 27, 2000 at latitude 38
o
12.392’ and longitude 122
o
09.233’ in the north wall of Trench T-3 at an elevation of ~102’ (via GPS). The site is on an al-
luvial plain across the N22W trend of Fault Trace No. 3 of the Green Valley fault. Mediterranean
climate. Vegetation is grassland. Slope <1
o
. Aspect N. Well drained to moderately well drained,
with the soil moist below 1 m. The soil is neutral in the surface, gradually becoming moderately
alkaline at 225 cm. Current water table 2.8 m. Part of the surface has been removed and replaced
during grading.
____________________________________________________________________________
Horizon Depth, cm Description
____________________________________________________________________________
Soil Profile No. 3 (station 3 m)
AB 12-40 Dark grayish brown (10YR4/2m, 5/1d) clay; medium strong angu-
lar blocky structure; very sticky and very plastic when wet, firm when moist, and very hard when
dry; common fine roots; few fine continuous random tubular pores; clear smooth boundary; pH
7.0; conductivity 630 uS; Sample No. 00B531.
B11kt 40-127 Grayish brown (2.5Y5/2m, 5/2d) silty clay with common fine dis-
tinct white (10YR8/1md) mottles due to nodules to 3 cm and common medium prominent yellow
brown (10YR6/8md) mottles due to iron oxides; medium strong subangular to angular blocky
structure; sticky and plastic when wet, firm when moist, and very hard when dry; few fine roots;
common fine continuous random tubular pores; few medium thick clay films on ped faces and
calcite nodules; extremely effervescent; diffuse smooth boundary; pH 7.4; conductivity 1880 uS;
Sample No. 00B532.
B21kt 127-180 Grayish brown (2.5Y5/2m, 5/2d) silty clay with many fine distinct
white (10YR8/1md) mottles due to nodules to 3 cm and common medium prominent yellow
brown (10YR6/8md) mottles due to iron oxides; medium strong subangular blocky structure;
sticky and plastic when wet, firm when moist, and very hard when dry; many fine continuous
random tubular pores; few medium thick clay films on ped faces and calcite nodules; extremely
effervescent; diffuse smooth boundary; pH 7.5; conductivity 1740 uS; Sample No. 00B533.
B3k 180-225 Light brownish gray (2.5Y6/2m, 7/2d) silty clay with many fine
distinct white (10YR8/1md) mottles due to nodules to 3 cm and common medium prominent yel-
low brown (10YR6/8md) mottles due to iron oxides; medium strong subangular blocky structure;
sticky and plastic when wet, friable when moist, and very hard when dry; common fine conti-
nuous random tubular pores; very few thin clay films on ped faces and calcite nodules; extremely
effervescent; diffuse smooth boundary; pH 7.5; conductivity 1760 uS; Sample No. 00B534.
17
CB 225-300 Light brownish gray (2.5Y6/2m, 7/2d) medium sand with common
medium prominent yellow brown (10YR6/8md) mottles due to iron oxides; massive to medium
strong subangular blocky structure; slightly sticky and slightly plastic when wet, very friable
when moist, and very hard when dry; few fine roots; common fine continuous random tubular
pores; vermiculite in sands; pH 8.0; conductivity 550 uS; Sample No. 00B535.
ESTIMATED AGE: t
o
= 17.6 ka
t
b
= 0 ka
t
d
= 17.6 ky
Soil Profile No. 4 (station 5 m)
B12kt 20-75 Light yellowish brown (2.5Y5/4m, 7/2d) silty clay with common
medium distinct white (10YR8/1md) mottles due to calcite nodules to 3 cm and common fine
prominent brownish yellow (10YR6/8md) mottles due to iron oxides; medium strong subangular
to angular blocky structure; sticky and plastic when wet, firm when moist, and very hard when
dry; common fine continuous random tubular pores; few medium thick clay films coating pores,
ped faces, and calcite nodules; extremely effervescent; gradual smooth boundary; pH 7.6; con-
ductivity 460 uS; Sample No. 00B536.
BCt 75-135 Light yellowish brown (2.5Y5/4m, 6/2d) silty clay loam with many
fine prominent brownish yellow (10YR6/8md) mottles due to iron oxides; medium strong suban-
gular blocky structure; sticky and plastic when wet, firm when moist, and very hard when dry;
common fine continuous random tubular pores; few medium thick clay films coating pores and
ped faces; gradual smooth boundary; pH 7.8; conductivity 380 uS; Sample No. 00B537.
B22kt 135-165 Light yellowish brown (2.5Y5/4m, 6/2d) silty clay with common
medium distinct white (10YR8/1md) mottles due to calcite nodules to 2 cm and common fine
prominent brownish yellow (10YR6/8md) mottles due to iron oxides; medium strong subangular
to angular blocky structure; sticky and plastic when wet, firm when moist, and very hard when
dry; common fine continuous random tubular pores; common medium thick clay films coating
pores, ped faces, and calcite nodules; extremely effervescent; clear wavy boundary; charcoal
sample 00B507 was dated at 17.6 ka; pH 7.9; conductivity 480 uS; Sample No. 00B538.
CB 165-300 Light brown gray (2.5Y6/2m, 7/2d) medium sand with many fine
to medium distinct brownish yellow (10YR6/8md) mottles due to iron oxides; medium weak
subangular blocky structure; nonsticky and nonplastic when wet, very friable when moist, and
very hard when dry; few fine continuous random tubular pores; pH 7.8; conductivity 680 uS;
Sample No. 00B539.
ESTIMATED AGE: t
o
= 17.6 ka
t
b
= 0 ka
t
d
= 17.6 ky
______________________________________________________________________________
18
Table 3. Charcoal samples dated along the Green Valley fault at Jameson Creek.
T- Field
No.
Lab.
No.
Station Elev. Wall Conv.
Age
Std.
Dev.
Cal.
BC
Cal.
Age
Std.
Dev.
m m yr
B.P.
yr yr ka ky
4 00B504 Beta-
155515
4.2
28.5 N 4110 50 4580 4.58 0.06
3 00B507 Beta-
155516
6.9
27.0 N 14700 110 17600 17.6 0.1
*All
14
C analyses performed by Beta Analytic, Inc., 4985 S.W. 74 Court, Miami, FL
33155 (www.radiocarbon.com).
19
FIGURES
Fig. 1. Principal active faults of the eastern San Francisco Bay Area showing the location of
the Jameson Creek study site along the Green Valley fault (modified from Borchardt
and Baldwin, 2001).
Fig. 2. Alquist-Priolo Earthquake Fault Zone in the vicinity of the Jameson Creek site. Four
traces (numbered) of the Green Valley fault zone were mapped in the area. Aseismic
slip of 3-3.5 mm/yr has been measured south of the site at Point C.
Fig. 3. Site map of the Jameson Creek study site showing the locations of trenches exca-
vated in 2000 (from base maps prepared by the City of Fairfield).
Fig. 4. Aerial photo showing the location of Trace No. 2, once considered the best prospect
for determining the slip rate at the Jameson Creek study site. The scar of Cole and
Pratt (1991) Trench No. 5 across the contact between the Markley Sandstone and the
Sonoma Volcanics is clearly visible north of State Highway 12.
Fig. 5. Detailed log of the south wall of Trench T-3 showing the locations of oriented sam-
ples obtained for analysis of cataclasis.
Fig. 6. Scanning electron micrograph of a thin section along one of the Riedel shears (loca-
tion B-1) in Trench T-3, showing the boundary of a phyllosilicate-rich gouge in the
fault zone. Studies of grain sizes and preferred grain orientations in the gouge and
the adjacent sand are in progress (Susan Cashman, personal communication, 2001).
Grain shattering and orientation will be compared to other micrographs taken along
creeping and non-creeping faults.
20
Fig. 1. Principal active faults of the eastern San Francisco Bay Area showing the location of
the Jameson Creek study site along the Green Valley fault (modified from Borchardt
and Baldwin, 2001).
21
Fig. 2. Alquist-Priolo Earthquake Fault Zone in the vicinity of the Jameson Creek site. Four
traces (numbered) of the Green Valley fault zone were mapped in the area. Aseismic
slip of 3-3.5 mm/yr has been measured south of the site at Point C.
22
Fig. 3. Site map of the Jameson Creek study site showing the locations of trenches excavated in 2000 (from base maps prepared by
the City of Fairfield) (see enlargement below).
23
Fig. 4. Aerial photo showing the location of Trace No. 2, once considered the best prospect for determining the slip rate at the Ja-
meson Creek study site. The scar of Cole and Pratt (1991) Trench No. 5 across the contact between the Markley Sandstone
and the Sonoma Volcanics is clearly visible north of State Highway 12.
24
Fig. 5. Detailed log of the south wall of Trench T-3 showing the locations of oriented samples obtained for analysis of cataclasis.
25
Fig. 6. Scanning electron micrograph of a thin section along one of the Riedel shears (location B-1) in Trench T-3, showing the
boundary of a phyllosilicate-rich gouge in the fault zone. Studies of grain sizes and preferred grain orientations in the
gouge and the adjacent sand are in progress (Susan Cashman, personal communication, 2001). Grain shattering and orien-
tation will be compared to other micrographs taken along creeping and non-creeping faults.
26
PLATES
Plate 1. Log of the north wall of Trench T-1 excavated across suspect fault trace No. 2 at
Jameson Creek.
Plate 2. Log of the north wall of Trench T-2 excavated between suspect fault traces No. 2
and No. 3 at Jameson Creek.
Plate 3. Log of the north wall of Trench T-3 excavated across a Riedel shear east of suspect
fault trace No. 3 at Jameson Creek.
Plate 4. Log of the north wall of Trench T- 4 excavated across suspect fault trace No. 1 at
Jameson Creek.
Plate 5. Log of the north wall of Trench T-5 excavated across suspect fault trace No. 3 at
Jameson Creek.
Plate 6. Log of the north wall of Trench T-6 excavated across the projection of the Riedel
shear in Trench T-3 at Jameson Creek.
27
28
Fig. 3 enlargement. Site map of the Jameson Creek study site (base from the City of Fairfield).
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
