U.S. DEPARTMENT OF THE INTERIOR	U.S. GEOLOGICAL SURVEY
TO ACCOMPANY MAP OF97-98

QUATERNARY GEOLOGY OF CONTRA COSTA COUNTY, AND SURROUNDING PARTS OF 
ALAMEDA, MARIN, SONOMA, SOLANO, SACRAMENTO, AND SAN JOAQUIN COUNTIES, 
CALIFORNIA:  A DIGITAL DATABASE

By E.J. Helley and R.W. Graymer


DISCUSSION

INTRODUCTION

	Contra Costa County is located at the northern end of the Diablo Range of Central California.  
It is bounded on the north by Carquinez Strait, through which flows 27 percent of California's surface 
water runoff.  San Francisco Bay forms the western boundary, the San Joaquin Valley borders it on 
the east and the Livermore Valley forms the southern boundary.  Contra Costa is one of the nine Bay 
Area counties with streams that are tributaries to San Francisco Bay.  Most of the county is 
mountainous with steep rugged topography.  Mount Diablo, in the center of the county, is one of the 
highest peaks in the Bay Area, reaching an elevation of 1173 meters (3,849 ft).  Contra Costa County 
is covered by twenty-five 7.5' topographic Quadrangles shown on the index map (ccq_quad or Sheet 
2).  However, two of the quadrangles (Hayward and Petaluma Point) contain no Quaternary 
deposits in Contra Costa County, and so are not discussed herein.
	The Quaternary deposits in Contra Costa County comprise two distinct depositional 
environments.  One, forming a transgressive sequence of alluvial fan and fan-delta deposits, is 
mapped in the western four-fifths of the county.  The second, forming a combination of eolian dune 
and river delta deposits, is mapped in the San Joaquin Valley in the eastern part of the county.


MAPPING METHODS

	Geological units were mapped on 1:24,000 scale U.S. Geological Survey topographic maps 
using 1939 black and white aerial photographs showing the county before much development had 
taken place.  The mapping was supplemented with 1:12,000- and 1:24,000-scale color aerial 
photography flown in 1965 and 1974 respectively.  The mapping was also aided by observations made 
on turn of the century topographic maps at 1:62,500-scale with 25 foot (7.6 m) contour intervals.  These 
maps were most useful in recognizing natural stream channels which have since been modified.  
The position of water boundaries, the stream channel to water transition, the length and position of 
mapped stream channels (see below), the distribution of artificial levees, and the apparent 
distribution of artificial fill were plotted to conform with the most recent 1:24,000 scale topographic 
maps.
	Mapping units were delineated by:  1) landform morphology, 2) relative topographic position, 
3) relative preservation of surface morphology, 4) tonal contrasts on aerial photographs, 5) relative 
soil profile development (compiled from the U.S. Soil Conservation Service, 1917), and 6) other 
features such as differences in vegetation density and type.  Landform morphology refers to the 
shape of a particular landscape element.  Examples are the distinctive conical shape of alluvial fans 
and the shape of levees that usually border active stream channels, sloping away from them to blend 
into flat basin deposits.  Other criteria listed in the description of geological units are also used to 
distinguish one element from another, but they are particularly useful for delineating units within a 
specific landscape element.  A surface on an alluvial fan, for example, might be differentiated from 
another because of its higher topographic position, greater drainage density, and stronger soil 
profile development.  Geologic units defined this way are called allostratigraphic units (American 
Association of Petroleum Geologists Bulletin, 1983).
	The ca 1850 A.D. shoreline of Nichols and Wright (1971) is inferred to be the contact between 
estuarine mud (Qhbm) and continental deposits.  At the 1850 shoreline, the thickness of the estuarine 
mud (Qhbm) is zero.  The boundary between the estuarine mud and the time equivalent peaty muck 
(Qhpm) in the northeast section of the county is arbitrarily placed at the confluence of the 
Sacramento and San Joaquin Rivers.
	The geological units shown on this map provide information on texture, environment of 
deposition, and age.  The contacts between units are, in most cases, not sharp but diffuse and may 
span a few meters.  Very little information is available regarding the thickness of the geologic 
units.
	No bedrock units have been mapped; bedrock outcrop is simply labeled "br".  Other geologic 
features, such as folds and faults, have not been mapped.  A few large landslides are shown, but this 
map does not show most of the landslides in the area.  See Nilsen and others (1979) for a landslide 
inventory, and Graymer, Jones, and Brabb (1994) for a geologic map emphasizing bedrock units and 
geologic structures.

DESCRIPTION OF MAP UNITS

	The Quaternary deposits of Contra Costa County are described from youngest to oldest, 
beginning from the Bay and ascending the alluvial fans to bedrock.

af	Artificial Fill (Historic)--Man made deposit of various materials and ages.  Some are  
compacted and quite firm, but fills made before 1965 are nearly everywhere not compacted 
and consist simply of dumped materials.
alf	Artificial Levee Fill (Historic)--Man made deposit of various materials and ages, 
forming artificial levees as much as 20 feet (6.5 meters) high.  Some are compacted and 
quite firm, but fills made before 1965 are almost everywhere not compacted and consist 
simply of dumped materials.  The distribution of levee fill conforms to levees shown on the 
most recent U.S. Geological Survey 7.5 minute quadrangle maps.
GP	Gravel Pits (Historic)--Excavations in stream channels and Holocene alluvium for the 
purpose of extracting sand and gravel for aggregate in construction industries.
Qhasc	Artificial Stream Channels (Historic)--Modified stream channels, usually where streams 
have been straightened and realigned, but also including those channels in the San 
Joaquin Valley and delta that are confined within artificial dikes and levees.
Qhbm 	Bay Mud (Holocene)--Water saturated estuarine mud, predominantly gray, green and 
blue clay and silty clay underlying marshlands and tidal mud flats of San Francisco Bay 
and Carquinez Strait.  The upper surface is covered with cordgrass (Spartina sp.) and 
pickleweed (Salicornia sp.).  The mud also contains a few lenses of well-sorted, fine sand 
and silt, a few shelly layers (oysters), and peat.  The mud interfingers with and grades 
into fine-grained deposits at the distal edge of Holocene fans, and was deposited during the 
post-Wisconsin rise in sea-level, about 12 ka to present (Imbrie and others, 1984).  
Estimated thickness:  0-40 m.  In places it rests unconformably on bedrock.
Qhpm 	Peat and Peaty mud (Holocene)--Water saturated peat and mud deposited in tidal wetlands 
in response to the post-Wisconsin rise in sea level.  These deposits are the time equivalents 
of the bay mud (Qhbm), and they are found only in the northeast section of the county.  The 
unit consists in large part of the decomposed remains of roots and rhizome, particularly 
Scirpus acutus  (tule), Pharagmites australis  (common reed), Distichlis spicata (salt 
grass) and Typha slatifolia (cattail).
Qhsc 	Stream Channel Deposits (Holocene)--Poorly to well-sorted sand, silt, silty sand, or sandy 
gravel with minor cobbles.  Cobbles are more common in the mountainous valleys.  Many 
stream channels are presently lined with concrete or rip rap.  Engineering works such as 
diversion dams, drop structures, energy dissipaters and percolation ponds also modify the 
original channel.  Many stream channels have been straightened, and these are labeled 
Qhasc.  This straightening is especially prevalent in the lower reaches of streams 
entering the estuary.  The mapped distribution of stream channel deposits is controlled by 
the depiction of major creeks on the most recent U.S. Geological Survey 7.5 minute 
quadrangles.  Only those deposits related to major creeks are mapped.  In some places 
these deposits are under shallow water for some or all of the year, as a result of reservoir 
release and annual variation in rainfall.
Qhl 	Natural Levee Deposits (Holocene)--Loose, moderately to well-sorted sandy or clayey silt 
grading to sandy or silty clay.  These deposits are porous and permeable and provide 
conduits for transport of ground water.  Levee deposits border stream channels, usually 
both banks, and slope away to flatter floodplains and basins.  Levee deposits are best 
developed along San Pablo and Wildcat Creeks on the Richmond Bay Plain, along the 
valley of Walnut Creek and along Marsh Creek in the San Joaquin Valley.  Abandoned 
levee systems have also been mapped.  
Qhfp	Floodplain deposits (Holocene)--Medium to dark gray, dense, sandy to silty clay.  Lenses 
of coarser material (silt, sand, and pebbles) may be locally present.  Flood plain deposits 
usually occur between levee deposits (Qhl) and basin deposits (Qhb), and are prevalent in 
the Walnut Creek-Concord Valley.
Qhb	Basin Deposits (Holocene)--Very fine silty clay to clay deposits occupying flat-floored 
basins at the distal edge of alluvial fans adjacent to the bay mud (Qhbm).  Also occupying 
flat areas in the Brentwood dune field where the basin deposits bury older eroded sand 
dunes (Qds).
Qhaf1	Younger Alluvial Fan Deposits (Holocene) --  Brown, poorly-sorted, dense, sandy or 
gravelly clay.  Small fans at mountain fronts have a probable debris flow origin.  Larger 
Qhaf1 fans away from mountain fronts may represent the modern loci of deposition for 
Qhaf.
Qhaf	Alluvial Fan and Fluvial Deposits (Holocene)--Alluvial fan deposits are brown or tan, 
medium dense to dense, gravely sand or sandy gravel that generally grades upward, to 
sandy or silty clay.  Near the distal fan edges, the fluvial deposits are typically brown, 
never reddish, medium dense sand that fines upward to sandy or silty clay.  The best 
developed Holocene alluvial fans in Contra Costa County are on the Richmond Bay Plain 
and the fans of Sand and Deer Creeks in the Brentwood Area.  All other alluvial fans and 
fluvial deposits are confined to narrow valley floors.  Several Holocene fans along the 
south shore of the Carquinez Strait have bulbous surface morphology, are short, overlap 
older Pleistocene surfaces, and may be debris flows.
Qds	Dune Sand (Pleistocene and Holocene)--Fine-grained, very well-sorted, well-drained, 
eolian deposits of northeastern Contra Costa County.  They occur mainly in two large 
northwest-southeast trending sheets, as well as many small hills, most displaying 
Barchan morphology.  Dunes display as much as 30 m of erosional relief and are presently 
being buried by basin deposits (Qhb) and peaty mud (Qhpm).  They probably began 
accumulating after the last interglacial high stand of sea-level began to recede about 71 ka, 
continued to form when sea level dropped to its Wisconsin minimum about 18 ka, and 
probably ceased to accumulate after sea level reached its present elevation (about 6 ka).  
Atwater (1982) recognized buried paleosols in the dunes, indicating periods of non-
deposition.
Qms	Merritt Sand (Pleistocene and Holocene) -- Fine-grained, very well sorted, well-drained 
eolian deposits of western Alameda County.  An area of Merritt Sand outcrops in the 
western part of the Oakland East quadrangle.  It is probably time-correlative with Qds, but 
displays different morphology.  The Merritt Sand forms large sheets up to 15 meters high 
with yardang morphology.
Qls	Landslide deposits (Pleistocene and/or Holocene) -- Poorly sorted clay, silt, sand, and gravel.  
Only a few very large landslides have been mapped.  For a more complete map of landslide 
deposits, see Nilsen and others (1979).
Qpaf	Alluvial Fans and Fluvial Deposits (Pleistocene)--Brown dense gravely and clayey sand 
or clayey gravel that fines upward to sandy clay.  These deposits display various sorting 
and are located along most stream channels  in the county.  All Qpaf deposits can be 
related to modern stream courses.  They are distinguished from younger alluvial fans 
and fluvial deposits by higher topographic position, greater degree of dissection, and 
stronger soil profile development.  They are less permeable than Holocene deposits, and 
locally contain fresh water mollusks and extinct late Pleistocene vertebrate fossils.  They 
are overlain by Holocene deposits on lower parts of the alluvial plain, and incised by 
channels that are partly filled with Holocene alluvium on higher parts of the alluvial 
plain.  Maximum thickness is unknown but at least 50 m.
Qpaf1	Alluvial Terrace Deposits (Pleistocene)--Qpaf1 and Qpaf2 are the first and second 
Qpaf2	Pleistocene alluvial terraces, respectively.  Deposits consist of crudely - bedded, clast - 
supported, gravels, cobbles, and boulders with a sandy matrix.  Clasts as much as 35 cm 
intermediate diameter are present.  Coarse sand lenses may be locally present.  
Pleistocene terrace deposits are cut into Qpaf alluvial fan deposits a few meters and lie up 
to several meters above Holocene deposits.  These terrace deposits are so similar in 
lithology and texture as to be almost identical.  They are distinguished for mapping 
purposes based on topographic position (the highest is the oldest) and degree of dissection 
and soil profile development.  The highest terrace displays the thickest argillic B horizon 
as well as the dark A horizon (Mollic epipedon).  The sequence is best observed along both 
banks of Arroyo Valle south of Livermore.  The terraces on left (south-west) bank are more 
continuous than on the right (north-east).  Each terrace represents the cutting and filling of 
previous deposits.
Qmt	Marine Terrace Deposits (Pleistocene)--Three small outcrops of marine terraces are 
located about 5 m above present mean sea level and are characterized by a thin (<0.5 m 
thick) bed of oysters at their base.  The terraces are located on the south shore of San Pablo 
Bay in the extreme northwest Contra Costa County at Lone Tree Point, Wilson Point, and 
an unnamed outcrop in between.  The oyster beds unconformably overlie the Cierbo 
Sandstone of Miocene Age, and are in turn overlain by about 5 m of greenish-gray silty 
mudstone.  The oysters have been dated by the Uranium-Thorium method (Helley and 
others, 1993) and are of last interglacial age, approximately 125 ka.
Qpoaf	Older Alluvial Fan deposits (Pleistocene) -- Brown dense gravely and clayey sand or 
clayey gravel that fines upward to sandy clay.  These deposits display various sorting 
qualities.  All Qpoaf deposits can be related to modern stream courses.  They are 
distinguished from younger alluvial fans and fluvial deposits by higher topographic 
position, greater degree of dissection, and stronger profile development.  They are less 
permeable than younger deposits, and locally contain fresh water mollusks and extinct 
Pleistocene vertebrate fossils.
Qtu	Undifferentiated Continental Gravels (Plio-Pleistocene)--Semi-consolidated to 
unconsolidated poorly sorted gravel, sand, silt and clay distributed in isolated patches 
throughout the country.  These deposits are unrelated to modern drainages, and are most 
abundant in the Walnut Creek-Concord Valley and in patches that appear to represent an 
ancestral drainage emanating from the north face of Mt. Diablo flowing northwesterly 
down the Clayton-Concord valley, finally entering Carquinez Strait just west of the 
Concord Naval Weapons Depot.  Their main distinction is not being related to modern 
drainage or Pleistocene drainage.  Thickness varies but most outcrop areas exceed 50 m.  
No soil profile development is preserved at most localities due to erosion.  These deposits 
probably represent the late Cenozoic uplift of the Coast Range.
QTl	Livermore gravels (Pliocene and Pleistocene) -- Poorly to moderately consolidated, indistinctly 
bedded, cobble conglomerate, gray conglomeratic sandstone, and gray coarse-grained 
sandstone.  Also includes some siltstone and claystone.  Clasts are mostly graywacke, chert, 
and metamorphic rocks probably derived from the Franciscan complex. 

REFERENCES CITED

Atwater, B.F., 1982, Geologic maps of the Sacramento - San Joaquin Delta, California:  U.S. 
Geological Survey Miscellaneous Field Studies Map MF-1401, scale 1:24,000, 21 sheets, 15 p.
Graymer, R.W., Jones, D.L., and Brabb, E.E., 1994, Preliminary geologic map emphasizing 
bedrock formations in Contra Costa County, California:  A digital database:  U.S. 
Geological Survey Open-File Report 94-622.
Helley, E.J., Fitzpatrick, J.A. and Bischoff, J.F., 1993, Uranium series dates on oyster shells from 
elevated marine terraces of San Pablo Bay, California: U.S. Geological Survey Open-File 
Report 93-286.
Imbrie, J., Hays, J.D., Martinson, D.G., McIntyre, A., Mix, A.C., Morley, J.J., Pisias, N.G., Prell, 
W.L., and Shackleton, N.J., 1984, The orbital theory of Pleistocene climate:  Support from a 
revised chronology of the marine d18O record in Berger, A.L., and others, eds., 
Milankovitch and climate, Part 1: D. Reidel Publishing Company, p. 269-305. 
Nichols, D.R. and Wright, N.A., 1971, Preliminary map of historic margins of marshland, San 
Francisco Bay, California:  U.S. Geological Survey Open-File Report, scale 1:125,000.
Nilsen, T.H., Wright, R.H., Vlasic, T.C., and Spangle, W.E., 1979, Relative slope stability and 
lands-use planning in the San Francisco Bay Region, California:  U.S. Geological Survey 
Professional Paper 944, 96 p.
U.S. Soil Conservation Service, 1917, Reconnaissance soil survey of the San Francisco Bay Region, 
California:  Washington, Government Printing Office.