Geologic Map and Map Database of Western Sonoma, Northernmost Marin, 
and Southernmost Mendocino Counties, California


By M.C. Blake, Jr., R.W. Graymer, and R.E. Stamski




Pamphlet to accompany
MISCELLANEOUS FIELD STUDIES MAP MF-2402
Version 1.0























2002
U.S. Department of the Interior
U.S. Geological Survey

Contents

Geologic explanation and acknowledgements	1
Introduction	1
Stratigraphy	1
	Mesozoic and Tertiary terrane complexes	1
	Description of Terranes	2
	Tertiary Stratigraphy	10
	Quaternary Surficial Deposits	11
	Paleontology	11
	Radiometric Ages	11
Structure	12
	Structural History	12
Description of Map Units	15
Acknowledgements	23

Digital publication and database description	24
Introduction	24
For those who don't use digital geologic map databases	24
MF-2402 Digital Contents	24
	PostScript plotfile package	25
	PDF plotfile package	25
	Digital database package	25
TAR files	26
PostScript plotfiles	27
PDF plotfiles	27
Obtaining the Digital Database and Plotfile Packages	28
	To obtain TAR files of database or plotfile packages from the USGS web pages
	28
Obtaining plots from a commercial vendor	28
Obtaining plots from USGS Map On Demand Services	28
Revisions and version numbers	28
Digital database format	29
Converting ARC export files	29
Digital compilation	29
Base maps	29
Faults and landslides	29
Spatial resolution	29
Database specifics	30
	Lines	30
	Areas	32
	Points	33

References Cited	35


Geologic Explanation and Acknowledgements




Introduction

This report contains a new geologic map at 1:100,000 
scale and an associated set of geologic map databases (Arc-
Info coverages) containing information at a resolution 
associated with 1:62,500 scale.  The map and map 
database, along with this interprative text, are based on the 
integration of previous work with new geologic mapping 
and field checking by the authors (see Sources of Data 
index map on the map sheet or the Arc-Info coverage wso-
so and the textfile wso-so.txt).  The descriptive text 
(below) contains new ideas about the geologic structures 
in the area, including the active Hayward-Rodgers Creek-
Healdsburg Fault system and other active faults in the 
area, as well as the geologic units and their relations, 
including the origin of the Franciscan Complex mlange 
and the Coast Range ophiolite.  
	Together, the map (or map database) and unit 
descriptions in this report describe the composition and 
distribution of geologic materials and the orientation and 
distribution of geologic structures within the study area at 
regional scale.  Such regional geologic information is 
important for analysis of earthquake shaking, landslide 
susceptibility, engineering materials properties, mineral 
resources and hazards, and groundwater resources and 
hazards.  It is also vital for answering questions about the 
geologic history and development of the California Coast 
Ranges.
The information, however, is not sufficiently 
detailed for site-specific evaluations.  Those seeking 
detailed information of that kind should consult the more 
detailed earthquake-hazards maps produced by the 
California Division of Mines and Geology or contact a 
licensed geologist or engineering geologist.
The map area includes about two-thirds of 
Sonoma County and a small part of Marin County west 
of longitude 12245'W and north of latitude 3815'N, plus 
a tiny part of southern Mendocino County along the north 
edge of the map.  This area includes the Pacific coastline 
in the western part, low rolling hills in the southern part, 
rugged mountainous terrain in the northern and central 
parts, and the broad flatlands of Santa Rosa and Alexander 
Valleys in the eastern part.  The northern and central 
mountains are largely uninhabited, but the eastern valleys 
are undergoing rapid development.
The map in this report is modified from and 
supercedes USGS Open-File Report 71-44 (Blake and 
others, 1971).  Two factors led to the decision to remap 
the area.  First, there has been almost 30 years of new 
geologic information accumulated for the map area.  Two 
areas within the map, in particular, have received much 
attention since the earlier work: Cazadero/Ward Creek in 
the central part of the map, where the medium- to high-
grade blueschist rocks and minerals have been studied in 
detail (Liou and Maruyama, 1987; Maruyama and Liou, 
1988, 1989; Erickson, 1995; Wakabayashi, 1992a), and 
the Geysers geothermal field in the northeast corner of the 
map, which was carefully studied in relation to the 
production of geothermal steam for generating electricity 
(McLaughlin, 1978, 1981; McLaughlin and Ohlin, 1984; 
Eberhart-Phillips and Oppenheimer, 1984; Oppenheimer, 
1986; Dalrymple, 1992; Hulen and Nielson, 1993; 
Stanley and others, 1998).  Second, tectonic theories for 
the California Coast Ranges have changed since the 
publication of the earlier map, and the new tectonic 
models have a strong influence on the depiction of 
through-going active faults, volcanic fields, folds, and 
other mapped features.
The digital nature of the geologic map 
information available in this publication is important for 
three reasons.  First, the geologic map data can be 
digitally combined with other map datasets (topography, 
groundwater information, landslide distribution) for rapid 
and complex regional analysis of geologic resources and 
hazards.  Second, digital maps are much more easily 
updated than traditional printed maps.  Third, digital 
publication provides an opportunity for regional planners, 
local, state, and federal agencies, teachers, consultants, and 
others who are interested in geologic data to have the new 
information long before a traditional paper map could be 
published.

Stratigraphy

Mesozoic and Tertiary terrane complexes

The bedrock units in the San Francisco Bay region are 
made up of two components: amalgamated, highly 
deformed tectonostratigraphic terranes that are displaced, at 
least in part, hundreds to thousands of kilometers from 
their position of origin (allocthonous or 
parautochthonous); and generally younger, less deformed 
rocks that overlie the amalgamated terranes and are 
roughly in their original location (except for transport 
along the San Andreas fault system).  In most of the 
region, the older set of rocks is Mesozoic and the younger 
is Tertiary, but in the map area the amalgamated terranes 
include some rocks as young as Miocene.  The young age 
of rocks within these displaced terranes reflects additional 
complexity in the geologic and structural history of the 
map area that is not found in any other part of the region.
	
The amalgamated terranes can be grouped into 
three related rock complexes.  One of these, the Great 
Valley complex, is made up of the Jurassic Coast Range 
ophiolite, which in the map area consists mostly of 
serpentinite, gabbro, diabase, basalt, and keratophyre 
(altered silicic volcanic rocks), and the Jurassic and 
Cretaceous Great Valley sequence, composed of sandstone, 
conglomerate, and shale.  Although the sedimentary rocks 
and ophiolite have been tectonically separated almost 
everywhere in the map area, the Great Valley sequence was 
originally deposited on the ophiolite.  This depositional 
relationship is extrapolated from contacts exposed outside 
the map area: in the Berkeley Hills (Jones and Curtis, 
1991; Graymer and others, 1996) and Solano County in 
the San Francisco Bay region, and elsewhere in California.  
This complex represents the accreted and deformed 
remnants of arc-related Jurassic oceanic crust with a thick 
sequence of overlying turbidites, at least in part related to 
the North American forearc (parautochthonous).
	The second set of amalgamated terranes is the 
Franciscan Complex.  These terranes are composed of 
weakly to strongly metamorphosed graywacke, argillite, 
basalt, chert, limestone, and other rocks.  The rocks of the 
Franciscan Complex in the map area are mostly derived 
from Jurassic to Cretaceous oceanic crust and pelagic 
deposits overlain by Upper Jurassic to lower Tertiary 
turbidites.  Although most Franciscan Complex rocks are 
little metamorphosed, high-pressure, low-temperature 
metamorphic minerals are common in rocks that crop out 
as mlange blocks (Bailey and others, 1964) and in several 
fault-bounded lenses within the map area.  High-grade 
metamorphic blocks in sheared but relatively 
unmetamorphosed argillite matrix (Blake and Jones, 1974) 
reflect the multi-stage history of the Franciscan Complex.  
The eastern and central parts of the complex were 
subducted beneath the Coast Range ophiolite, a process 
that continued until after the deposition of the Franciscan 
Complex sandstone exposed south of the map area that 
contains Campanian (Late Cretaceous) fossils (Blake and 
others, 2000).  The youngest parts of the Franciscan 
Complex, including Tertiary sedimentary rocks, must 
have accreted in late Eocene or younger time, after 
deposition of the Tertiary strata.  Their original 
relationship to the older Fransiscan complex rocks and the 
Great Valley complex is not well understood.  Because 
much of the Franciscan Complex was accreted beneath the 
Great Valley complex, the contact between the two 
Mesozoic complexes is everywhere faulted (Bailey and 
others, 1964), and the Franciscan Complex presumably 
underlies the entire San Francisco Bay area east of the San 
Andreas Fault.  Subsequent deformation, described below, 
has obliterated the original subduction contact almost 
everywhere.
	A third complex crops out in the map area only 
west of the San Andreas Fault Zone at Bodega Head, 
which is composed of granitic rocks of the Salinian 
complex.  This is the northernmost outcrop of the 
Salinian complex, which extends southwards between the 
San Andreas and Sur-Nacimiento Fault Zones about 500 
km to the Big Pine Fault east of Santa Barbara (Ross, 
1972).  The complex in the San Francisco Bay region is 
made up of Late Cretaceous (81.6-91.6 Ma, Curtis and 
others, 1958; 80-117 Ma, Kistler and Champion, 1991) 
granitic rocks intruded into high-grade metamorphic rocks, 
Late Jurassic (161 Ma, James and others, 1993) gabbroic 
rocks, and Late Cretaceous (Campanian to Maastrictian) 
sedimentary rocks (Branner and others, 1909; Hall and 
others, 1959; Clark, 1981; Brabb and others, 2000).  
	Both the Franciscan and the Great Valley 
complexes have been further divided into a number of 
fault-bounded tectonostratigraphic terranes (Blake and 
others, 1982, 1984). When the terranes were first defined, 
the prevailing methodology was to identify separate 
terranes if there was any doubt about stratigraphic linkage 
between structurally separated entities.  As a result of 
further research, many additional data, in particular new 
fossil localities, are known and the distribution and nature 
of the original terranes have been greatly modified in this 
report (see below).  The distribution of terranes in the map 
area is shown in map of terranes on the map sheet as well 
as in the Arc-Info coverage wso-terr.

Description of Terranes
GREAT VALLEY COMPLEX

Healdsburg terrane (gvh)
KJgvc, KJgvs, Jv, Jd, sp

Rocks assigned to the Healdsburg terrane crop out in four 
fault-bounded lenses.  The type area is near Healdsburg, 
where as much as 3,000 m of Lower Cretaceous 
conglomerate depositionally overlie Upper Jurassic shale 
over keratophyre, basalt, gabbro, and peridotite of the 
Coast Range ophiolite.  No depositional contact between 
the basal shale and the underlying ophiolitic rocks has 
been observed in the map area, but west of Dry Creek 
orientation of thin shale and sandstone beds are concordant 
with the approximate trend of the basal contact, 
suggesting that the original depositional contact may be 
preserved there.
	The Healdsburg terrane is distinguished from 
coeval Great Valley sequence rocks by its abundant 
conglomerate containing clasts mostly of light-colored 
(often pink) rhyolite porphyry and rhyolitic welded ash-
flow tuff, plus minor quartzite (quartz arenite) pebbles 
(Blake and others, 1984; Jayko and Blake, 1993).  Biotite 
from two cobbles near Black Point south of the map area 
in Marin County yielded a K-Ar age of about 145 Ma 
(Berkland, 1969). The source for this Late Jurassic 
rhyolite remains unknown but, based on sedimentologic 
and petrologic analyses, appears to be somewhere south of 
the present Sierra Nevada magmatic arc where a source for 
the quartzite (quartz arenite) is available in the Peninsula 
Range of southern California (Jayko and Blake, 1993). 
	Restoration of Miocene and younger offset on the 
Hayward-Petaluma Valley /Rodgers Creek fault system 
proposed by Graymer (1999) juxtaposes the rocks of the 
Healdsburg terrane north of San Francisco Bay with rocks 
of similar composition and age east of San Jose (the 
Berryessa Formation, Wentworth and others, 1998a).  The 
latter rocks were included in the Healdsburg terrane by 
Jayko and Blake (1993) based on similarity of age and 
clast composition.  Further restoration across the 
Calaveras Fault Zone brings the combined 
Healdsburg/Berryessa rocks into proximity with unnamed 
rocks of similar composition but unknown age in the 
Quien Sabe area south of Gilroy (Leith, 1949).  Because 
this offset between Black Point and Gilroy would account 
for all faults of the San Andreas system that cut the 
Healdsburg terrane, the separation between Gilroy and the 
Peninsula Range must have taken place on pre-San 
Andreas faults.

Del Puerto terrane (gvd)
Jk, Jv, sp

The main body of Great Valley complex rocks that have 
been assigned to the Del Puerto terrane (Blake and others, 
1984) lies some 100 km southeast of the map area in the 
eastern Diablo Range.  There, the basal part of the 
stratigraphic sequence in the terrane is composed of 
dismembered ophiolite and a thick accumulation of silicic 
volcanic rocks (keratophyre and quartz keratophyre), 
overlain by silicic tuff and tuffaceous sandstone of the 
Upper Jurassic Lotta Creek Formation and Upper Jurassic 
to Lower Cretaceous turbidites.  These rocks are overlain 
by Upper Cretaceous and Paleocene strata that overlap 
unconformably eastward onto Sierran basement, which 
requires the Del Puerto terrane there to be, at least partly, 
in place (autochthonous).
	In the map area, a single small body of Late 
Jurassic rock in the Camp Meeker quadrangle is herein 
assigned to the Del Puerto terrane.  Although the rock 
body is small, fault bounded, and isolated from other Del 
Puerto terrane bodies, this assignment is made because of 
1) the presence of a sliver of silicic tuff similar to the 
Lotta Creek Formation and 2) the absence of much silicic 
volcanic detritus in the Upper Jurassic and Lower 
Cretaceous strata (which would be suggestive of 
Healdsburg rather than Del Puerto terrane).
	These Del Puerto terrane rocks are stucturally 
interleaved with rocks of the Franciscan Complex and 
Healdsburg terrane and are more than 100 km from the 
main body of Del Puerto terrane.  Restoration of San 
Andreas fault system offsets described for the Healdsburg 
terrane, however, brings the Del Puerto terrane rocks of 
the map area into proximity with the equivalent rocks in 
the Diablo Range.

Elder Creek terrane (gve)
KJgv, Jv, sp

Great Valley complex rocks east of the Maacama Fault at 
Black Mountain (Jimtown and The Geysers quadrangles) 
belong to the Elder Creek terrane, which is marked by the 
absence of much silicic volcanic rock and the presence of 
ophiolite breccia at the base of the sedimentary sequence.  
The Great Valley complex rocks there are composed of 
serpentinite, sheeted diabase, and pillow basalt overlain by 
ophiolite breccia, basaltic sandstone, and shale 
(McLaughlin and Ohlin, 1984).
	The Great Valley complex rocks at Black 
Mountain are interleaved with Franciscan Complex rocks, 
and so have undergone considerable fault offset.  However, 
Black Mountain is in the same general area as the rest of 
the Elder Creek terrane north and east of the map area, the 
easternmost part of which is coextensive with Great 
Valley rocks that overlap Sierran rocks.  Therefore, the 
Black Mountain rocks must be considered largely in place 
with respect to North America (parautochthonous).
	The ophiolitic rocks that form the base of the 
Black Mountain section are chemically distinct from the 
bulk of the Coast Range ophiolite.  Giaramita and others 
(1998) showed that the geochemical signature of the Black 
Mountain rocks is more akin to mid-ocean ridge basalts 
than most Coast Range ophiolite rocks (which have a 
forearc signature, see below), although the high Th/Ta 
ratio still suggested to them that the Black Mountain 
rocks formed in a (rifted) forearc.

Point Arena terrane (gvp)
KJsb, Ka, Ks, TKu, Tg

The rocks west of the San Andreas Fault Zone, from Fort 
Ross northward, are assigned to the Point Arena terrane.  
This terrane is composed of a basal mafic volcanic section 
derived from the upper part of ophiolite crust (Phillips and 
others, 1998) structurally overlain by Upper Cretaceous 
and lower Tertiary turbidites.  These rocks have been 
assigned to the Great Valley complex (Wentworth and 
others, 1998b) based on the presence of sheeted sills in the 
ophiolitic rocks, the absence of blueschist facies 
metamorphism, and the structural relationship between the 
ophiolitic rocks and the overlying strata.  
However, there are major differences between the 
rocks of the Point Arena terrane and other Great Valley 
complex terranes, including the lack of Jurassic and Lower 
Cretaceous strata, the presence of tropical mollusks (Elder 
and others, 1998), and the presence of mafic-clast 
conglomerate.  Furthermore, chemical analysis of the 
ophiolitic rocks in the Point Arena terrane suggest that 
they were formed in a back-arc environment (Phillips and 
others, 1998) rather than the forearc environment 
suggested by the chemistry of most Coast Range 
ophiolite rocks (see below).

Coast Range ophiolite (cro)

	The ophiolitic rocks that make up the basal part 
of all the Great Valley complex terranes in the map area 
include most of the rock types that make up a typical 
ophiolite suite, including serpentinite, pyroxenite, gabbro, 
diabase, and massive and pillowed basalt.  The ophiolite 
formed between about 164 and 173 Ma (Blake and others, 
1992; Hagstrum, 1997; Hopson and others, 1997), but its 
origin is a matter of debate.  As summarized recently by 
Dickinson and others (1996), three models of ophiolite 
generation have been suggested for the Coast Range 
ophiolite: mid-ocean ridge volcanism at equatorial 
latitudes associated with seafloor spreading, back-arc 
spreading related to west-directed subduction under an 
ocean-island arc west of North America, and fore-arc 
spreading related to east-directed subduction under North 
America.  Geochemical evidence suggests that most Coast 
Range ophiolite rocks formed in a fore-arc setting 
(Shervais, 1992; Shervais and Kimbrough, 1985; 
Dickinson and others, 1996; but see also MacPherson and 
others, 1985; Giaramita and others, 1998).  Paleomagnetic 
inclinations show that they formed in roughly the same 
latitude relative to North America that they now occupy, 
which seems to rule out large northward transport.  In 
addition, there are two belts of ocean-island arc volcanics 
in the Sierra Nevada Foothills that are roughly the same 
age as the Coast Range ophiolite [including the Logtown 
Ridge Formation, Copper Hill volcanics of Clark (1964), 
and Gopher Ridge Volcanics (Lindgren and Turner, 1894; 
Imlay, 1961; Behrman, 1978; Graymer and Jones, 1991)] 
and two similar belts in the Klamath Mountains (the 
Hayfork and Western Klamath terranes, Murchey and 
Blake, 1993).  These ocean-island arc rocks lie between 
the Coast Range ophiolite and preexisting North 
American crust.  An additional coeval belt of arc volcanic 
rocks is preserved as roof pendants within the Sierra 
Nevada (the Goddard terrane of Nockelberg, 1983).  The 
presence of ocean-island arc rocks between Coast Range 
ophiolite and North American rocks suggests that there 
was at least one ocean-island arc between the ophiolite and 
North America when the ophiolite formed, which would 
preclude its formation in the forearc related to east-directed 
subduction beneath North America.  
We thus propose a fourth model that satisfies all 
these geochemical, paleomagnetic, and geologic 
constraints (fig. 1).  In this model, the Coast Range 
ophiolite formed in the fore-arc of an ocean-island arc 
related to east-dipping, ocean-ocean subduction some 
distance west of North America.  The Gopher Ridge 
Volcanics are the remnant of an island arc related to the 
Coast Range ophiolite, whereas the Goddard terrane 
represents the remnant of the continental arc related to 
east-directed, ocean-continent subduction beneath North 
America.  Ophiolite and island-arc rocks were transported 
more or less directly eastward and accreted to the 
continental margin after deposition of Kimmeridgian strata 
that overlie the arc rocks in the Sierra Foothills.  During 
this accretion, arc volcanic rocks and overlying 
sedimentary rocks underwent penetrative deformation and 
greenschist-facies metamorphism, prior to intrusion of 
latest Jurassic (Tithonian) unfoliated plutonic rocks 
(Saleeby and others, 1989; Graymer and Jones, 1994; 
Nockelberg, 1983).  The accretion probably coincides with 
early deformation of the Coast Range ophiolite section 
and deposition of locally overlying ophiolite breccia 
(Robertson, 1989).  The deformed ophiolite and the 
ophiolite breccia are overlain by unmetamorphosed 
Tithonian strata (Knoxville Formation), marking the close 
of the deformational period.  This Late Jurassic (late 
Kimmeridgian to early Tithonian) period of accretion and 
deformation has been called the Nevadan orogeny (Hinds, 
1934).
Serpentinite that is structurally interleaved with 
Franciscan Complex mlange or that contains high-grade 
metamorphic blocks has previously been mapped as part 
of the Franciscan Complex (for example, Coleman, 
1996).  Therefore, it is important to point out that we 
consider all serpentinite in the map area part of, or derived 
from, the Coast Range ophiolite.  We base this 
conclusion primarily on 1) the structural position of the 
Coast Range ophiolite in the upper plate of the 
subduction zone associated with accretion of the 
Franciscan Complex, which would have brought the basal 
(serpentinite generating) part of the Coast Range ophiolite 
into proximity with Franciscan Complex rocks, but 
would not have brought the base of Franciscan Complex 
ophiolite to higher structural levels and 2) on the absence 
of intermediate parts of Franciscan Complex ophiolitic 
crust (static and cumulate gabbro) in the Coast Ranges 
(see Blake and others, 2000, for a more complete 
discussion of the serpentinite).

FRANCISCAN COMPLEX

In the northern Coast Ranges, the Franciscan Complex is 
made up of three structural belts: an Eastern Belt 
composed of coherent medium- to low-grade blueschist 
metamorphic rocks (lawsonite-albite and lawsonite-jadeite-
quartz sub-facies); a Central Belt composed of slabs of 
coherent Jurassic and Cretaceous low-grade metavolcanics 
and metasediments (prehnite-pumpellyite facies) 
interleaved within mlange; and a Coastal Belt composed 
mostly of Late Cretaceous and early Tertiary, low-grade 
metasediments (laumontite facies).  The position of these 
belts probably reflects the order of accretion of Franciscan 
Complex terranes from easternmost to westernmost.  The 
first-accreted Eastern Belt rocks were also farthest 
subducted and most metamorphosed, whereas Coastal Belt 
rocks, which are generally farthest west, are least 
metamorphosed.  The distribution of Franciscan Complex 
rocks in the map area is not nearly as simple as it is 
farther north, but we have retained the belt nomenclature 
as general categories in which to group the Franciscan 
terranes.  Disruption of the simple belt pattern that is 
observed farther north is probably due to post-accretion 
faulting, including, but not limited to, offset on faults of 
the San Andreas Fault system.

Eastern Belt

Cazadero terrane (fcz)
KJfm, KJfmgs

	The easternmost Franciscan Complex rocks in 
the northern Coast Ranges make up the Pickett Peak 
terrane, which is composed of coherent (not mlange), 
glaucophane-schist facies, textural zone 2B and 3 
metagraywacke, metachert, and metabasalt.  K/Ar, Rb/Sr, 
and Ar/Ar whole-rock and single-mineral ages range from 
about 135-110 Ma (Lanphere and others, 1978; McDowell 
and others, 1984; Wakabayashi, 1999a), whereas white-
mica Ar-Ar ages range from about 121-125 Ma 
(Wakabayashi, 1999a).  
	In the map area, medium-grade blueschist makes 
up the newly-named Cazadero terrane.  These rocks form a 
more than 50-km-long, northwest-trending slab of textural 
zone 2B-3A (Jayko and others, 1986) glaucophane- and 
jadeite-bearing metagraywacke (the Skaggs Springs schist 
of Wakabayashi, 1992b) that runs from the Healdsburg 
quadrangle to the Gube Mountain quadrangle.  They also 
form a set of smaller (less than 15 km long) lenses of 
low- to medium-grade (type II and III of Coleman and Lee, 
1963) metabasalt and metasediments (the Ward Creek 
Schist of Coleman and Lee, 1963) interleaved with 
mlange and Great Valley complex rocks in the Cazadero 
and Fort Ross quadrangles.  K/Ar radiometric dates from 
the Ward Creek schist range from 135+7 to 150+7.5 Ma 
(Lee and others, 1964; Coleman and Lanphere, 1971) in 
addition to an Ar/Ar age of 142.60.5 Ma (Wakabayashi 
and Deino, 1989), and the Skaggs Springs schist has 
yielded an Ar/Ar age of about 141 Ma (Wakabayashi, 
1999a).  These rocks were previously correlated with the 
Pickett Peak terrane of the northern Coast Ranges (Blake 
and others, 1984), but they differ from those rocks in that 
they are largely metagraywacke rather than metapelite, are 
higher grade (higher P/T), and are older than most Pickett 
Peak terrane rocks.

Yolla Bolly terrane (fyb)
KJfm, KJfmgs

Although not extensive in the map area, one of the most 
widespread and distinctive Franciscan Complex units in 
the San Francisco Bay region is the Yolla Bolly terrane.  
This terrane consists of metagraywacke, metachert, and 
metabasalt, all containing abundant blueschist-facies 
minerals such as lawsonite, jadeitic pyroxene, and 
metamorphic aragonite. In addition, the metagraywacke is 
characterized by a weak to pronounced foliation (textural 
zone 2 of Blake and others, 1967).
	No fossils are known from the Yolla Bolly rocks 
of the study area, but similar metachert from the Diablo 
Range and northern Coast Ranges (Sliter and others, 
1993; Isozaki and Blake, 1994) yielded ages that range 
from Early(?) to Late Jurassic, and the overlying 
metagraywacke in the Diablo Range is latest Jurassic 
(Tithonian; Crawford, 1976).  The blueschist 
metamorphism shows that these rocks have been 
subducted, with the age of the graywacke, derived from arc 
volcanics associated with a subduction zone, presumably 
marking the time when the oceanic rocks approached and 
entered the trench (Wentworth and others, 1998a).  The 
radiometric age of metamorphic minerals in Yolla Bolly 
terrane rocks is 92-100 Ma (Maruyama and others, 1992; 
Mattinson and Echevarria, 1980), marking the peak of 
subduction zone metamorphism.  Thus, Yolla Bolly 
terrane rocks were resident in the subduction zone for at 
least 50 million years after the Late Jurassic time of their 
approach and entry into the trench.
	The outcrops of Yolla Bolly terrane rocks in the 
Two Rock quadrangle comprise the northern tip of a 
north-northwest-trending slab of jadeite-bearing 
metagraywacke with a pronounced foliation (textural zone 
2B of Jayko and others, 1986).  These rocks have been 
correlated with the type Yolla Bolly terrane of northern 
California (Blake and others, 1984) based on similarities 
in lithology, sandstone composition, and metamorphic 
state.  In addition, a complex set of northwest-trending 
slabs of type I and II basalt and metabasalt with minor 
metagraywacke (textural zone 2A) in the Duncans Mills, 
Cazadero, and Fort Ross quadrangles are tentatively 
assigned to the Yolla Bolly terrane, even though they were 
previously thought to be a lower-grade equivalent of the 
Ward Creek schist (Coleman and Lee, 1963). 

Central Belt

Novato Quarry terrane (fnq)
Kfss

The Novato Quarry terrane forms a relatively 
narrow and discontinuous, northwest-trending belt between 
the Hayward Fault, southeast of the map area, and the San 
Andreas Fault.  Though mostly southeast of the map area, 
Novato Quarry terrane rocks form a small thrust wedge 
interleaved with mlange in the Bodega Head quadrangle.
This terrane consists largely of thin-bedded 
turbidites with local channel deposits of massive 
sandstone (see Blake and others, 1984, for discussion of 
depositional environments as well as photographs of 
typical outcrops). Although the strata are in many places 
folded and locally disrupted (broken formation), they are 
nearly everywhere well bedded. 
The Novato Quarry terrane sandstone contains 
metamorphic prehnite and pumpellyite and is arkosic with 
abundant K-feldspar, indicating derivation from a granitic 
or rhyolitic source area.  Several specimens of Inoceramus 
schmidti of Late Cretaceous (Campanian) age have been 
found in this terrane (Bailey and others, 1964).  Novato 
Quarry terrane sandstone composition is similar to the 
arkosic part of the Coastal belt, but Coastal belt strata are 
generally more poorly bedded and more disrupted and 
Novato Quarry terrane rocks are older.
The age of the Novato Quarry terrane rocks 
constrains Central Belt Franciscan Complex sandstone 
deposition to have continued at least into Campanian 
time, with subsequent accretion at the subducting plate 
margin.

Devils Den Canyon terrane (fdd)
Kfss

Three slabs of coherent graywacke northeast of Alexander 
Valley (Jimtown, Geyserville, Asti, and The Geysers 
quadrangles) are herein assigned to the Devils Den Canyon 
terrane.  These rocks include those designated Devils Den 
Canyon terrane by McLaughlin and Ohlin (1984), rocks 
outside their study area, and rocks they correlated with the 
Geysers terrane (which is considered herein part of the 
Marin Headlands terrane).  These rocks were also 
previously considered part of the Rio Nido terrane (Jayko 
and Blake, 1984) and both Rio Nido and Central terranes 
(Blake and others, 1984).
	The graywacke of the Devils Den Canyon terrane 
is feldspathic-lithic, locally displays a very slight 
incipient foliation (textural zone 2A of Jayko and others, 
1986), and contains abundant, large (more than 1 mm) 
grains of detrital white mica.  Microscopic pumpellyite is 
abundant, but lawsonite and prehnite are not.  McLaughlin 
and Ohlin (1984) noted that some Devils Den Canyon 
terrane graywacke is rich in chert detritus, but point 
counts by Jayko and Blake (1984) showed no such 
distinction relative to other Central Belt terranes.  No 
fossils are known from the Devils Den Canyon terrane, so 
its age relative to other Franciscan Complex terranes is 
not constrained.
The Devils Den Canyon terrane rocks are grouped 
together because of their geographic proximity and their 
lithologic similarity.  They are distinguished from other, 
similar graywacke of the Lake Sonoma terrane and the 
Coastal Belt terranes because of their structural position 
and incipient foliation.  They contain less lithic detritus 
than the graywacke of the Marin Headlands terrane and 
Central "terrane".  We have discarded the Rio Nido terrane 
designation, and other rocks previously assigned to that 
terrane are now considered part of the newly named Lake 
Sonoma terrane and the Coastal Belt terranes (see 
Undivided Central and Coastal Belt below for discussion).

Lake Sonoma terrane (fls)
Kfss, gs

	The positively identified extent of the newly 
named Lake Sonoma terrane is one body of coherent 
graywacke and minor chert and greenstone southwest of 
Lake Sonoma (Warm Springs Dam, Tombs Creek, and 
Bigfoot Mountain quadrangles).  The graywacke is 
moderately lithic and unfoliated (textural zone 1 of Jayko 
and others, 1986).  Small to large lenses of intercalated 
chert and greenstone are common in parts of the terrane 
and are also unfoliated. 
Lake Sonoma terrane graywacke has previously 
been considered part of the Coastal Belt (Bailey and others, 
1964) and the Rio Nido terrane (Blake and others, 1984; 
Jayko and Blake, 1984).  The lithology of the graywacke 
in this terrane is indistinguishable from that of the lithic 
facies of the Coastal Belt (Jayko and Blake, 1984).  
However, these rocks have yielded a fossil of Late 
Cretaceous (Turonian) age (Bailey and others, 1964).  That 
age is too old to permit the Lake Sonoma terrane rocks to 
be part of the Coastal Belt, because graywacke from the 
latter is underlain by younger pelagic deposits of Late 
Cretaceous (Campanian-Maastrichtian) to middle Eocene 
age (Sliter, 1984 and written commun., 1990).  The Lake 
Sonoma terrane graywacke is also similar to that of the 
Devils Den Canyon terrane, but is provisionally 
distinguished herein based on lower structural position, 
lack of foliation, and presence of greenstone and chert.  As 
noted above, we have discarded the Rio Nido terrane 
designation (see Undivided Central and Coastal Belt below 
for discussion).

Marin Headlands terrane (fmh)
Kfgwy, KJfch, KJfgcs, KJfgc, Jfgs

Marin Headlands terrane is the most widely dispersed 
Central Belt terrane in the San Francisco Bay area, 
cropping out at Mount Diablo in Contra Costa County, 
in San Mateo County, in San Francisco, in several areas 
of Marin County, and as two large bodies in the map area: 
a complexly faulted slab more than 25 km long in the 
northeast corner of the map area (Asti and The Geysers 
quadrangles) and a narrow, tightly folded and faulted body 
west of Healdsburg (Guerneville, Geyserville, and Warm 
Springs Dam quadrangles).
	Rocks of this terrane have been previously 
mapped as Geysers terrane (McLaughlin and Ohlin, 1984), 
Central terrane (Blake and others, 1984), and Central and 
Nicasio Reservoir terranes (Jayko and Blake, 1984).  
However, Murchey and Jones (1984) showed that chert 
bodies from the rocks in the map area contain radiolarians 
that match almost all the radiolarian assemblages known 
from the Marin Headlands type area.  Furthermore, like 
the type area, the rocks assigned to this terrane are 
distinguished by coherent stacked sequences of pillowed 
greenstone, ribbon chert, and lithic graywacke.  
	The age and tectonic history of the Marin 
Headlands terrane is also well constrained.  The chert 
layers range in age from late Early Jurassic (Toarcian to 
Pleinsbachian) to late Early or early Late Cretaceous (late 
Albian to Cenomanian), although the oldest cherts are 
missing in the map area (the basal chert here is early 
Middle Jurassic, Bajocian; Murchey and Jones, 1984).  
Graywacke that overlies chert south of the mapped area 
has yielded ammonites of Albian to Cenomanian age 
(Bailey and others, 1964).  Geochemical and 
paleomagnetic analysis of the pillow basalt and chert 
(Karl, 1984; Gromme, 1984; Shervais, 1989; Murray and 
others, 1990; Hagstrum and Murchey, 1993) has revealed 
a history of Early Jurassic oceanic crust being 
progressively overlain by pelagic deposits (chert), while 
being together brought closer to North America by plate 
motion until close approach to the subduction margin in 
Albian or Cenomanian time, when arc-related detrital 
sediments overwhelmed pelagic deposition.  Soon after, 
the upper oceanic crust and overlying sediments were 
accreted to the continental margin at about 10-15 north 
latitude and have been transported north to their current 
position (about 37-39 north latitude) by transform offset 
along the continental margin.
	The history of the Marin Headlands terrane 
provides an attractive model for the tectonic history of all 
Franciscan Complex terranes:  oceanic rocks (including 
MORB and seamounts), overlain by pelagic and detrital 
sediments, brought into proximity with North America by 
convergent plate motion, more or less subducted, accreted, 
and moved northward to their current position on the 
continent by transform offset along the continental 
margin.

Central "terrane" (mlange, fm)
fsr, KJfs

All of the previously described Franciscan Complex 
terranes in the map area are tectonically enclosed in an 
argillite matrix mlange that has been called the Central 
terrane (Blake and others, 1982, 1984).  Most of the 
matrix consists of sheared mudstone (argillite) and lithic 
sandstone, within which are mixed numerous blocks and 
slabs of coherent graywacke, greenstone, chert, 
metamorphic rocks, limestone, serpentinite, and other 
rocks.  Although treated as a single terrane, the mlange is 
actually the result of the tectonic and (or) sedimentary 
mixing of rocks derived from several terranes:  the rocks 
that would form the sheared matrix from one or more 
terranes, the chert, greenstone and metamorphic rocks 
from other Franciscan Complex terranes, and the 
serpentinite and scarce gabbro from the Coast Range 
ophiolite.  In particular, most of the chert blocks that crop 
out in the mlange in Marin County, south of the mapped 
area, can be assigned with confidence to the Marin 
Headlands terrane based on similarity of radiolarian faunas 
(Murchey and Jones, 1984).
	In a few places in the map area, such as the sea 
cliffs south of Goat Rock in the Arched Rock quadrangle, 
the southwestern part of Pine Flat Road in the Jimtown 
quadrangle, and along King Ridge Road in the Cazadero 
quadrangle, coherent bodies of lithic graywacke and 
conglomerate crop out.  These rocks are identical in 
composition to the graywacke that forms part of the 
mlange matrix (Jayko and Blake, 1984).  The graywackes 
have yielded several Late Jurassic fossils (Bailey and 
others, 1964).  Rocks of similar age and composition are 
well exposed at the abandoned quarry at Greenbrae in 
Marin County south of the study area (Blake and others, 
2000).  There, it is possible to see preserved slabs of 
interbedded graywacke, mudstone, chert, and tuffaceous 
greenstone that have yielded both megafossils and 
microfossils (radiolaria and dinoflagellates) of Late 
Jurassic and Early Cretaceous age (Blake and Jones, 1974; 
Murchey and Jones, 1984).  In Greenbrae, the strata are 
progressively more disrupted by normal faulting outward 
toward the edge of the coherent body.
Despite their similar ages, the radiolarian fauna 
found in the Marin Headlands chert blocks in the mlange 
is different from that found in chert interbedded in the 
matrix source rock.  This difference in chert faunas has led 
to the concept that the mlange matrix is derived from a 
deep-water, continental-margin deposit into which the 
accreting terranes were introduced by tectonic or 
sedimentary processes.  The idea that the matrix material 
is derived from sediments related to the continent is 
supported by the observation that chert pebbles from 
conglomerate associated with matrix graywacke have 
yielded Late Triassic to Early Jurassic radiolaria (Seiders 
and Blome, 1984), older than those known from any 
Franciscan chert, but equivalent to many chert bodies in 
the Sierra Foothills (for example, see Graymer and Jones, 
1994).  Deformation during accretion resulted in the 
interleaving of the rocks that would become mlange 
matrix and the accreted terranes.  Deformation during 
subsequent uplift has led to both the almost complete 
disruption of the original sedimentary character of the 
matrix and the inmixing of exotic blocks derived from the 
accreted terranes, such as the chert blocks from Marin 
Headlands terrane (Blake and Wentworth, 1999).  Only in 
a few locations, like the seacliffs south of the mouth of 
the Russian River, is the sedimentary layering of the 
mlange matrix source rock preserved.
During the period of deformation and inmixing of 
accreted terrane rocks, graywacke from accreted terranes 
was probably also sheared into the mlange matrix at the 
boundary of coherent graywacke slabs.  For example, the 
boundary between Marin Headlands terrane graywacke and 
adjacent mlange is gradational in the sea-cliff exposures 
at Rodeo Cove south of the mapped area (see Blake and 
others, 2000).  Such inmixing of other Franciscan 
Complex graywacke into the mlange matrix is limited, 
however, as indicated by a matrix composition that is 
generally more lithic than graywacke found in coherent 
slabs of other terranes (Jayko and Blake, 1984).
The mechanism by which the mlange blocks 
(knockers) were originally incorporated into the matrix 
rock is still an issue of some debate.  The two contrasting 
models can be briefly described as follows:

1.	The sedimentary model suggests that blocks 
(olistoliths) were transported into the depositional 
environment of the matrix material by gravity driven 
debris slides.  The trench associated with the 
subduction zone provides an area of suitably steep 
slope for this theory and the converging plates bring 
the displaced terranes into proximity of the 
continental margin mlange matrix.  The resulting 
olistostrome then underwent the deformation described 
above, disrupting the original depositional character 
of the matrix/block relationships.  
2.	The tectonic model suggests that primary 
incorporation of blocks into mlange was by tectonic 
processes.  During and after accretion, lenses of rock 
derived from incoming exotic terranes were interleaved 
with continental margin deposits by faulting.  
Subsequent deformation during uplift further broke up 
the lenses of exotic rocks, forming the mlange 
blocks observed today.

We prefer the tectonic model for the Franciscan 
Complex mlange for the following reasons:

1. No original depositional relationship between block 
and matrix has been observed, although areas of 
relatively undisrupted matrix graywacke are known.  
Several workers have documented contacts reported as 
depositional within the mlange, but those contacts 
are either between different lithologies within a single 
mlange block or coherent parts of the mlange 
matrix strata (such as the graywacke on chert contact 
reported by Erickson, 1995).  Presumed contacts are 
the result of progressively increasing shearing at the 
transition from a coherent block to mlange (such as 
that reported by Gucwa (1975) and found at Rodeo 
Cove and Greenbrae).
2. Radiolarians in the blocks are of similar age to or 
younger than those in the matrix.  If the blocks were 
deposited as olistoliths, they would have had to be 
lithified prior to redeposition.  This implies that they 
should be appreciably older than the matrix, like 
those cherts found in the pebbles within matrix 
conglomerates.  Likewise, mlange blocks of 
limestone are known to contain Late Cretaceous 
(Cenomanian and Turonian) fossils (Bailey and 
others, 1964; Sliter, 1984), much too young to be 
redeposited as olistoliths in Late Jurassic-Early 
Cretaceous time.
3. High-grade metamorphic blocks are incorporated into 
low-grade mlange matrix.  The tectonic model 
provides the mechanism (fault offset) to transport 
material from the deeper part of the subduction zone 
back into the upper part of the accretionary prism, 
intermixing high-grade material with lower grade 
rock.  The sedimentary model requires that blueschist 
metamorphism was complete before formation of 
olistoliths, suggesting a tectonic history of deep 
subduction, uplift to the surface, erosion and 
deposition, shallow subduction and accretion, and a 
second period of uplift to the surface.  The complex 
series of events required by the sedimentary model 
would have had to take place quickly, because, 
although the age of the high-grade blocks is about 
160 Ma, metamorphic minerals in veins in the blocks 
are as young as 135 Ma (Kelley, 1982; McDowell 
and others, 1984; Wakabayashi and Deino, 1989; 
Nelson, 1991; Wakabayashi, 1992a, 1999a) which 
overlaps with, and in some cases is younger than, the 
Late Jurassic (Tithonian, 141-146 Ma, van Eysinga, 
1978) age of the matrix sediments.

Explanations for the origin of the mlange 
blocks that are more complex can be proposed in order to 
retain the sedimentary model.  For example, some authors 
have proposed distinguishing several distinct mlange 
terranes within the Central Belt.  Perhaps the younger 
chert and limestone blocks are limited to mlange derived 
from a younger olistostrome.  No young fossils have been 
found in lithic graywacke associated with the mlange, 
however, and no coherent graywacke of appropriate age 
(younger than Turonian limestone blocks) and 
composition to be part of a younger olistostrome is 
known in the Franciscan Complex (Marin Headlands 
terrane graywacke is similar in composition, but is too 
old).  Furthermore, young mlange blocks are not 
concentrated in specific zones in the map area, making the 
idea of multiple mlanges less likely.  The model of a 
single mlange derived from a young olistostrome with 
Late Jurassic graywacke included as detritus explains the 
presence of old fossils and the generally similar 
distribution of mlange blocks throughout the area but 
does not explain the lack of known young lithic 
graywacke or the compositional similarity between the 
Late Jurassic lithic graywacke and mlange matrix.  
The presence of serpentinite blocks in the 
mlange also suggests that blocks of the Coast Range 
ophiolite may have been incorporated into the mlange 
during uplift and disruption, although the correlation of 
the serpentinite blocks with the Coast Range ophiolite is 
unproven (see Blake and others, 2000).  Macpherson and 
others (1990) argued that many of the volcanic blocks in 
the mlange may also come from the Coast Range 
ophiolite.

Undivided Central and Coastal Belt (fcl)
TKfs

As noted above, graywacke of the Lake Sonoma terrane is 
lithologically indistinguishable from that of the Coastal 
Belt lithic facies, but the age of the Lake Sonoma terrane 
graywacke is Late Cretaceous (Turonian) whereas the 
Coastal Belt graywacke is Late Cretaceous (Maastrichtian) 
and Tertiary.  Blake and others (1984) noted the age 
distinction, and divided the similar graywacke in the map 
area into Coastal terrane (younger) and Rio Nido terrane 
(older) based on differing K-feldspar content.  However, 
reexamination of the K-feldspar distribution (Bailey and 
others, 1964; Blake, unpub. data) reveals no distinction 
between rocks previously mapped as Coastal terrane and 
Rio Nido terrane.  In addition, laumontite, which indicates 
the very low grade metamorphism associated with the 
Coastal Belt, has been found by the authors in rocks 
previously mapped as Rio Nido terrane.  Therefore, at this 
time, among rocks previously mapped as Rio Nido or 
Coastal terrane, it is possible to categorize only those 
graywacke bodies that are stratigraphically well controlled 
by fossils to specific terranes or belts with any confidence.  
	As a result, we have designated all those bodies 
of graywacke similar to Lake Sonoma terrane and Coastal 
Belt lithic facies graywackes that lack known fossils as 
undivided Central and Coastal Belt.  Because the Rio Nido 
area is underlain by rocks thus designated as undivided, we 
have abandoned the name Rio Nido terrane for those rocks 
known to be older and renamed them Lake Sonoma terrane 
after the large reservoir in the area of the critical Turonian 
fossil locality.  More detailed studies of laumontite 
distribution or more fossil data would probably make 
separation of the presently undivided rocks possible, but 
that is beyond the scope of this work.

Coastal Belt (fco)
TKfss

The Coastal Belt has been subdivided into terranes in areas 
north of the map area, but although Coastal Belt rocks in 
the map area were previously designated part of the 
Coastal terrane (Blake and others, 1984), we found 
insufficient evidence to extend those terrane distinctions 
into the map area.  The Coastal Belt rocks in the map area 
are very low grade (laumontite facies) and consist of 
moderately lithic to quartz-feldspathic wacke, underlain 
locally by minor greenstone, chert, and limestone.  Bailey 
and others (1964) reported a Late Cretaceous (Turonian) 
age from one of the limestones, but that locality 
(Annapolis quadrangle) is now thought by us to be a 
block in an infaulted lens of Central Belt mlange.  
Pelagic limestone interbedded with chert and basalt at the 
base of the section on Wheatfield Fork of the Gualala 
River (Tombs Creek quadrangle) has yielded fossils of 
middle Eocene age (W. Sliter, written commun., 1990).  
In the same structural block north of the map 
area, the sandstone contains fossils of Late Cretaceous to 
late Eocene (possibly Oligocene) age (Evitt and Pierce, 
1975; Bachman, 1978).  There, sandstone overlies basalt 
and pelagic deposits of Late Cretaceous (Campanian to 
Maastrichtian) age (Bachman, 1978; Sliter, 1984) along a 
sheared contact.  Farther north, where the Coastal Belt is 
divided into three terranes, McLaughlin and others (1980) 
reported Coastal Belt rocks as young as middle Miocene 
overlying ophiolitic rocks of Late Cretaceous (Coniacian 
to Campanian) age from one terrane (King Range terrane).
	The Coastal Belt sandstones within the map area 
are mostly massive, but where fine-grained interbeds are 
present, the bedding is almost everywhere moderately to 
completely disrupted by extensional deformation, creating 
lenses (boudins) of competent sandstone in a sheared 
mudstone and siltstone matrix.
	The Coastal Belt rocks are probably derived from 
Late Cretaceous and early Tertiary seafloor and overlying 
Cretaceous and Tertiary pelagic and detrital sediments that 
were accreted in late Eocene to Miocene time.  Systematic 
variation in sandstone composition suggest that Coastal 
Belt detritus was derived from a number of different 
sources, including two different batholithic sources as well 
as an arc-volcanic source and Central Belt Franciscan 
Complex sources (Kramer, 1976; Underwood and 
Bachman, 1986; McLaughlin and others, 1994).  Detailed 
geochemical studies reveal that the source of the coeval 
Tyee Formation in Oregon was the Idaho Batholith (Heller 
and others, 1985), so Underwood and Bachman (1986) 
proposed that Coastal Belt detritus was derived from the 
Idaho Batholith, Sierra Nevada batholith, and eastern 
Oregon/Washington volcanic arc rocks.  
Paleontologic (Sliter, 1984) and paleomagnetic 
(Harbert and others, 1984) data suggest that deposition of 
the Late Cretaceous limestone took place at about 20 N 
latitude and that these Coastal Belt rocks were transported 
to their accretion to North America by relative motion of 
the Farallon plate.  Accretion, extensional deformation, 
and erosional unroofing of the Coastal Belt was completed 
before the deposition of the largely undeformed, 
unconformably overlying Pliocene Ohlson Ranch strata.

Tertiary Stratigraphy

Franciscan Complex detritus in the Paleocene strata 
overlying Great Valley complex rocks in Rice Valley 
(Berkland, 1973) and the eastern Diablo Range (Bartow, 
1985), as well as the presence of unmetamorphosed early 
Eocene quartzofeldspathic strata overlying Franciscan 
Complex metamorphic rocks in San Mateo County 
(Pampeyan, 1993), indicate that much of the tectonic 
activity that brought the two Mesozoic complexes 
together in the San Francisco Bay area was complete by 
early Tertiary time.  In the map area, however, early 
Tertiary overlapping strata are missing, and the oldest 
overlapping strata are the late Miocene Burdell Mountain 
volcanics (Tb) of Blake and others (2000).  
	Early Tertiary strata in the map area are instead 
parts of the Coastal Belt (Franciscan Complex) and the 
Point Arena terrane (Great Valley complex).  The 
Paleogene strata do not overlap bounding faults, and are 
unrelated to the juxtaposed rocks in that their age and 
lithology are incompatible with being part of the same 
depositional system.  For example, Campanian graywacke 
of the Novato Quarry terrane was probably deposited in an 
arc-trench environment, whereas coeval pelagic limestones 
in the Coastal Belt were deposited far offshore. The 
Coastal Belt terranes didn't experience arc-trench 
deposition until post-Maastrichtian time, and there are no 
coeval Novato Quarry terrane rocks (the Novato Quarry 
terrane had probably been accreted by that time).  The 
incompatible stratigraphies show that the Mesozoic and 
Tertiary terranes have experienced a history distinct from 
adjoining rocks and, therefore, were formed apart from 
them and have been juxtaposed with the other Mesozoic 
terranes in the area by post-Eocene accretion and transform 
offset.  The Salinian complex rocks at Bodega Head lack 
any Tertiary strata whatsoever, but stratigraphic relations 
at Point Reyes south of the map area show that the 
Salinian complex terranes have also undergone similar 
post-Eocene offset.
	The lack of pre-late Miocene strata overlapping 
the Central and Eastern Belt terranes of the Franciscan 
Complex and Healdsburg and Del Puerto terranes of the 
Great Valley complex in the map area is puzzling.  The 
age of the graywacke in the Franciscan Complex suggests 
that Central Belt terrane accretion was complete by Late 
Cretaceous (Campanian or Maastrichtian) time, because 
graywacke deposition is thought to reflect approach to the 
subduction zone.  Furthermore, Tertiary stratigraphic 
relations south of the map area indicate that Franciscan 
and Great Valley complex rocks were exposed at the 
surface as early as Paleocene time (for example, see 
Graymer and others, 1994; Wentworth and others, 1998a; 
Brabb and others, 2000).  The Mesozoic rocks in the map 
area must either have become unroofed later than similar 
rocks farther south, have been uniformly uplifted until late 
Miocene time so that earlier Tertiary deposition never 
took place, or have undergone pervasive uplift and total 
erosion of previously overlying pre-late Miocene strata in 
Miocene time.  After restoration of post-late Miocene 
right-lateral fault offset related to the San Andreas Fault 
System (see below), Mesozoic rocks in the map area are 
adjacent to middle Miocene and older strata in the Berkeley 
Hills.  Those strata do not contain recognizable Franciscan 
Complex detritus, however, suggesting that the map area 
was either not undergoing extensive erosion that would 
produce a large amount of detritus or that at that time 
sediment was transported westward and not recorded in the 
onshore stratigraphic record.
	Whatever the reason for the lack of pre-late 
Miocene strata in the map area, by late Miocene time the 
Mesozoic rocks were at the surface.  In the southeastern 
part of the map area, Mesozoic rocks are overlain by ~12 
Ma Burdell Mountain volcanics of Blake and others 
(2000).  Rocks in the northern part of the map area were 
being eroded and the sediment transported eastward to form 
alluvial deposits that now make up the Orinda Formation 
in the Berkeley Hills (Wakabayashi, 1999b).  Most of the 
map area continued to be free of sedimentary accumulation 
until the present, except for four areas.  
The southeastern part of the area was covered by 
a latest Miocene to Pliocene marine embayment to 
produce the Wilson Grove Formation.  Coeval estuarine 
deposits (sand and gravel of Cotati) and alluvial deposits 
(Petaluma Formation) adjoining the Wilson Grove 
Formation on the east suggest a transitional marine-
nonmarine depositional environment.  The Petaluma 
Formation at the surface is presently separated from the 
marine and estuarine rocks by the Petaluma Valley Fault, 
and restoration of the southern margin of these formations 
suggests about 35 km of right offset since deposition.  
These marine and nonmarine rocks extend under the 
Quaternary deposits of Santa Rosa Valley, as revealed by 
well logs (D. Zigler, Chevron Petroleum, retired, written 
commun.).  Basalt flow rock is interlayered with the basal 
part of the sequence in the southeasternmost part of the 
Wilson Grove Formation and the sand and gravel of Cotati 
(Two Rock quadrangle), suggesting that onset of 
deposition was accompanied by volcanism.  The basalts 
have yielded radiometric ages of about 6-8 Ma (Fox and 
others, 1985b), so they are coeval with and probably 
associated with the development of the Sonoma Volcanic 
field farther east.  It remains undetermined whether there 
exists a causal relationship between the development of 
the Sonoma Volcanic field and the initiation of 
deposition.
The eastern part of the map area (east of the 
Rodgers Creek-Healdsburg Fault) was overlain in latest 
Miocene to Pliocene time by Sonoma Volcanics and 
interleaved volcaniclastic strata of the Glen Ellen 
Formation.  Volcanic centers and detrital sources were all 
east of the map area, as shown by clast imbrication, clast 
source, and other flow-direction indicators within both the 
Sonoma volcanics and Glen Ellen Formation 
(McLaughlin and others, 1999).
The northeastern part of the map area developed a 
small basin probably related to a right-step asperity in the 
right-lateral Maacama Fault Zone.  The timing of the 
development of this basin is poorly constrained, because 
the infilling sedimentary deposits (unit Tls) lack diagnostic 
fossils.
The northwestern part of the map area east of the 
San Andreas Fault Zone was overlapped by shallow 
marine deposits in Pliocene time in a narrow embayment 
to form the Ohlson Ranch Formation.
The overall environment of the map area in late 
Tertiary time, then, was probably one of low-lying coastal 
hills and shallow marine embayments flanked on the east 
by an active volcanic center with related west-facing 
alluvial fans.  Most of the uplift that formed the steep 
topography seen today postdates Pliocene time, except 
perhaps in the north-central part of the map area where 
Tertiary strata are not present.
 	The Tertiary stratigraphic relationships in the 
mapped area also reveal significant late Tertiary and 
Quaternary fault offset.  The Glen Ellen and the younger 
part of the Petaluma-Cotati-Wilson Grove deposystems 
were roughly coeval, but were distinct in their detritus and 
depositional environment.  The boundary between the two 
deposystems is the Rodgers Creek-Healdsburg Fault Zone.  
The juxtaposition of these fault-separated rock bodies with 
significantly different stratigraphies suggests that they 
originally formed in separate depositional basins or widely 
separated parts of a large basin and have since been 
juxtaposed by large offsets on the separating faults.



Quaternary Surficial Deposits

Quaternary surficial deposits in the map area are mostly 
undivided.  The exceptions are beach sand and marine 
terraces along the Pacific coast, river terraces along the 
Russian River and other major drainages, and older 
alluvial fan deposits in Santa Rosa Valley (Healdsburg and 
Sebastapol quadrangles).  The older alluvial fan deposits 
were previously mapped as Glen Ellen Formation (Blake 
and others, 1971; Knudsen and others, 2000), but we have 
reclassified them based on three factors: 1) they are much 
less deformed than Glen Ellen Formation, 2) they are not 
as lithified, and 3) they lack the interbedded tuff that is 
widespread in much of the Glen Ellen Formation.  For a 
more detailed map of the Quaternary deposits, especially 
the late Pleistocene and Holocene deposits, see the recent 
regional Quaternary map of Knudsen and others (2000).

Paleontology

Many different kinds of fossils have proved invaluable in 
understanding the geology of the map area: Buchia in the 
Franciscan and lower Great Valley complex rocks, 
Inoceramus in the Franciscan Complex, ammonites and 
mollusks in the Point Arena terrane, radiolarians in the 
Franciscan Complex chert and Great Valley complex tuff, 
foraminifers in the Great Valley complex shales and 
Franciscan Complex limestones, palynomorphs in the 
Coast Belt, and mollusks, marine and nonmarine 
vertebrates, and diatoms in the Tertiary rocks.
	A partial list of references to paleontological 
reports in the map area and surrounding areas includes 
White (1885), Stirton (1939), Weaver (1944), Gealey  
(1951), Travis (1952), Matsumoto (1960), Bailey and 
others (1964), Wentworth (1967), McNitt (1968), 
Christensen (1973), Blake and Jones (1974), Bachman 
(1978), McLaughlin (1978), McLaughlin and others 
(1980), Murchey and Jones (1984), Elder and others 
(1998), McFadden (1998), and Naidu (1999).  Preparation 
of a digital database of fossil localities and associated 
paleontologic information is being prepared by workers at 
the U.S. Geological Survey and University of California, 
Berkeley.

Radiometric Ages

Several different types of rock bodies in the study area 
have yielded radiometric ages.  The volcanic rocks at and 
near the base of the Wilson Grove Formation and the sand 
and gravel of Cotati (Tb) have been studied: the most 
recent report of ages is Fox and others (1985b).  Several 
silicic tuffs that crop out in the map area have been 
studied by Sarna-Wojcicki (1976) and Sarna-Wojcicki and 
others (1979).  The granitic rocks at Bodega Head have 
been dated by Kistler and Champion (1991).  Dates from 
the metamorphic rocks in the Eastern Belt as well as 
metamorphic blocks in the Central Belt mlange have 
been published by several workers (including 
Wakabayashi, 1992a; Coleman and Lanphere; 1971, Lee 
and others, 1964).  An overview of radiometric ages in the 
map area is provided by Kelley (1982), and an overview of 
Franciscan Complex ages is found in Wakabayashi 
(1999a).

Structure

The mapped structures fall into two general categories.  
The older structures are faults that trend generally about N 
60 W and are associated with terrane boundaries.  Most of 
these faults dip steeply to the east.  The sense of offset on 
these faults is difficult to determine because the 
stratigraphies on either side are at least in part unrelated.  
Parallel faults within coherent terrane slabs are mostly 
reverse faults that duplicate the internal stratigraphy.  
The older structures are truncated by a second set 
of structures that trend about N30W.  These folds and 
faults are associated with the faults of the San Andreas 
Fault system, including the San Andreas, Tolay, Rodgers 
Creek-Healdsburg, and Maacama fault zones.  The sense of 
offset on these faults is dominantly right lateral, but also 
includes a significant fault-normal compressional 
component.  The compression is shown by folding and 
uplift of young strata (QTge, QTor, Qt) adjacent to the fault 
zones, especially in the eastern part of the map area.  
An exception to the overall sense of 
transpression in the younger system is present at right 
steps in the fault zones, such as at Little Sulphur Creek 
(Maacama Fault Zone) and Santa Rosa Valley 
(Tolay/Petaluma River Fault Zone).  In those areas, small 
fault basins have been formed due to localized 
transtension.  In the Santa Rosa Valley area, these basins 
have been overlapped and covered by Quaternary alluvial 
fan deposits, but are known from gravitational anomalies 
(Chapman and Bishop, 1988).  This overlap of basins 
suggests that right-stepping, right-lateral offset on this 
fault zone must have died out before Pleistocene time.  
Note also that near the younger fault zones, the 
older (terrane bounding) structures are deflected to be 
parallel to the younger structural trend.  Because of this 
reorientation, older structures may have been reactivated 
by younger deformation.
	Also present in the map area are many structures 
that are not depicted in this report because of their small 
size and complexity.  These structures include folded 
foliation within coherent Eastern Belt terrane slabs; tight 
folds, shear zones, and extensional fabric in coherent 
Central and Coastal Belt terrane slabs; disruption and shear 
fabric in mlange matrix; orientation of bedding or 
foliation in mlange blocks; and small folds in Great 
Valley complex strata.  
Structural History

The structures in the map area result from a 
complicated structural history that includes late Mesozoic 
to early Cenozoic subduction and accretion, at least two 
periods of subsequent uplift and detachment faulting, 
followed by oblique strike-slip and reverse faulting that 
continues today.
	The earliest structural relationships in the map 
area are those that juxtapose the multiple terranes of the 
Franciscan Complex and the Great Valley complex.  
Franciscan Complex terranes were transported by oceanic 
plate motion and accreted to North America at a 
subduction zone margin.  The Great Valley complex 
formed on the hanging wall of the subduction zone.  
Because the structurally highest Franciscan Complex 
rocks are blueschists, Franciscan Complex accretion must 
have been accomplished by subduction and underplating of 
incoming terranes to the continental margin, rather than 
obduction and overthrusting.  Therefore the subduction 
zone hanging wall rocks (Great Valley complex) were 
originally structurally highest throughout the Coast 
Ranges.
The metamorphic age of the Cazadero terrane is 
approximately 130-160 Ma, about the same as the 
depositional age of the Yolla Bolly and mlange matrix 
graywackes and the oldest part of the Great Valley 
complex sediments, and is older than other Franciscan 
Complex graywackes in the Central and Coastal Belts.  
Therefore, the Cazadero terrane entered the subduction zone 
before other Franciscan Complex terranes, prior to 160 
Ma.  
Structural relationships in the northern Coast 
Ranges (Wentworth and others, 1984; Blake and others, 
1967) suggest that the Cazadero terrane was once the 
structurally highest and innermost Franciscan Complex 
terrane in the map area, although the metamorphic grade 
of Cazadero terrane rocks shows that it was subducted 
deeper than any other Franciscan Complex terrane, as 
much as 20 km (P=9.5-12 kb, T=360-380C; Maruyama 
and Liou, 1988; Moore and Blake, 1989; Oh, 1990; Oh 
and others, 1991).  Its current structural position, 
interleaved with terranes of the Central Belt and the 
Healdsburg terrane of the Great Valley complex, along 
with the lack of Cazadero terrane rocks along the eastern 
margin of the Coast Ranges, suggest that it was 
originally accreted south of the Great Valley, and has since 
been transported north, along with the Healdsburg terrane.
	The model for the history of Franciscan Complex 
terranes suggested by the Marin Headlands terrane implies 
that the age of the graywacke sediments in any terrane 
marks the time of approach of that terrane to the 
subduction zone and the associated volcanic arc detrital 
source.  The graywacke age of the Yolla Bolly terrane 
suggests that it approached the continent in Late Jurassic 
(Tithonian, 141-146 Ma) time, coeval with deposition of 
mlange matrix and early Great Valley complex 
sediments, the latter and possibly the former in the 
hanging wall of the subduction zone.  The Yolla Bolly 
terrane's subduction resulted in metamorphic ages of about 
90-110 Ma and metamorphism less than that of the 
Cazadero terrane, but greater than that of any Central Belt 
terrane.
The order of accretion of the structurally lower 
Central Belt terranes is more problematical because of lack 
of age control in the Devils Den Canyon terrane and other 
terranes outside the map area.  However, known 
graywacke ages suggest that Marin Headlands terrane 
approached the subduction zone first, in late Early to early 
Late Cretaceous time (Albian to Cenomanian, 106-97 
Ma), followed by the Lake Sonoma terrane in middle Late 
Cretaceous time (Turonian, 94-88 Ma) and the Novato 
Quarry terrane in late Late Cretaceous time (Campanian, 
76-68 Ma).  All of these terranes lack the blueschist facies 
metamorphism characteristic of the Eastern Belt terranes, 
so they were never subducted as deeply as the higher-grade 
rocks.
The structural stacking of the oldest, most 
subducted terranes over the youngest and least subducted 
suggests that compressional forces at the convergent 
subduction margin thrust deeply subducted rocks 
progressively upward to the top of the accretionary prism.  
Tagami and Dumitru (1996) used zircon fission-track data 
to show that Yolla Bolly terrane rocks cooled from 300C 
to 100C between 100-70 Ma while Central Belt terranes 
were being accreted.  Wakabayashi and Unruh (1995) 
proposed that Central Belt terrane subduction at that time 
was accompanied by thrusting of Eastern Belt terranes to 
higher structural levels together with detachment and 
attenuation of the overlying Great Valley complex rocks 
(first described by Jayko and others, 1987).
	Presumably, the rocks that would become the 
matrix for the mlange were formed between the 
subduction zone and North America, overlying oceanic 
crust related to the Coast Range ophiolite in the hanging 
wall.  One of the many unique features of the mlange is 
that it is interleaved with elongate slabs of coherent Great 
Valley and Franciscan complex rocks.  These relationships 
extend north of the Mendocino Triple Junction into 
southwest Oregon and, thus, predate the San Andreas fault 
system.  The interleaving of slabs of mlange matrix 
sediments with the accreted terranes of the Central Belt 
probably occurred, at least in part, as the terranes entered 
the oblique subduction zone, dragging slabs of the 
hanging wall down.  Blueschist facies metamorphism of a 
small percentage of mlange matrix (Cloos, 1983) 
supports the idea that matrix was dragged into the 
subduction zone, though only a small percentage reached 
blueschist depths.  Serpentinite and other ophiolitic rocks 
that crop out as blocks in the mlange may have been 
dragged into the subduction zone at this time as well.  
Transpressive deformation in the accretionary prism above 
the trench probably further intercalated the mlange with 
ophiolite and accreted terranes at this time.
	As noted above, the period of accretion and 
crustal thickening was accompanied by detachment and 
attenuation.  The previously stacked terranes were 
significantly thinned, and previously buried ophiolite and 
Franciscan Complex rocks were brought to the surface.  
This thinning resulted in the almost complete attenuation 
of the Coast Range ophiolite in the map area, leaving 
only the dismembered fragments of the ophiolite present.  
Attenuation also took place between the Franciscan 
Complex terranes, as evidenced by the lateral discontinuity 
of some terranes in the region (for example, the Novato 
Quarry terrane is found structurally below the Healdsburg 
terrane in the map area, separated only by mlange, but in 
Marin County, south of the map area, coherent slabs of 
several terranes are present between the Novato Quarry and 
Healdsburg terranes).  
The timing of detachment and attenuation varied 
throughout the Coast Ranges.  Cowan and Page (1975) 
showed that blueschist facies Franciscan Complex rocks 
were unroofed by late Late Cretaceous time (Campanian, 
76-68 Ma) in the southern Coast Ranges, whereas Krueger 
and Jones (1989) and Harms and others (1992) showed that 
the first period of regional attenuation probably initiated 
60-70 Ma in the Diablo Range and the northern Coast 
Ranges.  They suggested that extension was complete by 
late Oligocene time based on the age of strata that 
overlapped extensional faults (Page, 1970), but in some 
parts of the San Francisco Bay area, erosional and 
possibly structural unroofing persisted into the middle 
Miocene, as suggested by the unconformable contact of 
middle Miocene strata on Franciscan Complex rocks in 
the Diablo Range (Osuch, 1970; Graymer and others, 
1996) and on Great Valley complex strata in Marin (Blake 
and others, 2000).  Before attenuation was completed, 
regional uplift of buried layers to the surface had been 
accomplished by the early Eocene throughout most of the 
San Francisco Bay region, as indicated by the presence of 
ophiolite and Franciscan Complex detritus in sedimentary 
strata of that period both south and east of the mapped area 
(for example, the Domengene Sandstone in the Cordelia 
area contains detritus derived from the Coast Range 
ophiolite; Graymer and others, 1999) and by Eocene 
sediments that lie unmetamorphosed on Franciscan 
Complex basement (for example, the Whiskey Hill 
Formation in San Mateo County; Pampeyan, 1993).  
Tagami and Dumitru (1996) used apatite fission-track data 
to show that Yolla Bolly rocks were lifted through the 
90C level about 40 Ma.  The overall picture, then, is one 
of uplift and attenuation that started as early as 100 Ma (as 
suggested by zircon fission-track data), had unroofed 
Franciscan Complex rocks regionally by Eocene time (37-
57 Ma), and may have continued locally as late as middle 
Miocene time (20 Ma).
The attenuation of this period probably 
obliterated most of the subduction-related thrust faults in 
the mapped rocks.  For example, the original subduction 
related thrust fault between the Franciscan Complex and 
Coast Range ophiolite was reactivated as a detachment 
fault throughout most of its extent (Krueger and Jones, 
1989), and many of the other rock units in the map area 
are also bounded by normal faults.  However, the timing 
of offset on most of the faults is poorly constrained, so 
some early faults may remain unrecognized.  The tectonic 
model of incorporation of blocks into the Franciscan 
Complex mlange suggests that much of the tectonic 
mixing associated with mlange was accomplished during 
attenuation, and disruption of coherent parts of the 
mlange matrix in the map area by normal faulting 
supports this idea (Blake and others, 2000).
The entire period of extensional faulting in the 
accretionary prism was accompanied by continued east-
directed oblique subduction.  By Eocene time, the Coastal 
Belt terranes were in position to begin receiving arc-
derived detritus near the subduction zone.  Coastal Belt 
rocks were accreted from late Eocene to middle Miocene 
time (40-20 Ma), perhaps associated with the end of 
attenuation and detachment in the accretionary prism.
The accretion of the Coastal Belt rocks was also 
coeval with the initiation of tectonic wedging that 
followed attenuation in the Central Belt.  First described 
by Wentworth and others (1984), tectonic wedging 
describes east-dipping reverse faulting in the upper crust 
that soles into a basal decollement in the mid-crust that 
rooted into the subduction zone.  The result is wedge-
shaped blocks of upper crust being driven eastward.  This 
process probably produced much of the repetition of 
structural levels seen in the map area, as originally gently 
east-dipping bodies of Franciscan and Great Valley 
complex rocks have been compressed, steepened, and 
stacked onto originally higher structural levels.  The best 
example of this repetition is the stacking of Franciscan 
Complex rocks structurally over Great Valley complex 
rocks that originally formed the highest structural level in 
the Coast Ranges, first described in Napa County east of 
the map area (Weaver, 1949).  In the map area, Franciscan 
Complex rocks wedged over Great Valley complex rocks 
have previously been documented in the Geysers area 
(Jimtown and The Geysers quadrangles; McLaughlin, 
1978) and the Camp Meeker quadrangle (Christensen, 
1973).  Our work suggests that all of the slabs of Great 
Valley complex rocks in the map area have a similar 
structural position. 
	By late Miocene time, the regional tectonic stress 
again changed to transpression associated with the 
development of the San Andreas fault system, although 
upper crustal tectonic wedging continued.  Jones and 
others (1994) described a significant component of 
compression normal to the San Andreas fault system and 
suggested that the pervasive tight folding and imbricate 
faulting of the Miocene and Pliocene strata in the San 
Francisco Bay area is due to this compression (see also 
Graymer, 2000).  In the map area, this type of 
compressive deformation is present in the Glen Ellen 
Formation and Sonoma Volcanics at the eastern edge of 
the map and is probably responsible for uplift of the 
shallow marine Ohlson Ranch Formation to its present 
position more than 300 m above sea level.  It is 
important to note that compressive deformation is not 
uniformly distributed throughout the map area, as shown 
by the relatively undeformed and only modestly uplifted 
(less than 150 m) the shallow marine Wilson Grove 
Formation.
	In addition to compressive deformation, there is 
strong evidence of large amounts of right-lateral offset in 
late Miocene and later time.  The San Andreas Fault Zone 
in central California has undergone about 290-315 km of 
total right-lateral strike slip (Wentworth and others, 
1998b; Dickinson, 1997).  In the San Francisco Bay 
region, this amount is distributed onto several splays of 
the San Andreas Fault system.  About 115-140 km of the 
offset passed west of San Francisco Bay and feeds into the 
San Andreas Fault Zone in the map area, and about 175 
km of offset passed east of San Francisco Bay (Fox and 
others, 1985b; McLaughlin and others, 1996; Jachens and 
others, 1998).  Of that eastern offset, 75 km feeds into 
faults east of the map area and 100 km feeds into the 
Tolay/Petaluma River Fault Zone (35 km) and the 
Rodgers Creek/Healdsburg/Maacama Fault Zone (65 km) 
via the Hayward Fault Zone (modified from Graymer, 
1999; Blake and others, 2000; see the index map of faults 
on the map sheet or the Arc/Info coverage wso-flt for fault 
names).  Additional offset feeds into the San Andreas 
Fault Zone in the map area from the San Gregorio Fault 
Zone that joins the San Andreas Fault Zone in southern 
Marin County.  The San Gregorio Fault Zone has 
undergone about 155-175 km of right offset (Clark and 
others, 1984; Jachens and others, 1998), so the San 
Andreas Fault Zone in Marin County south of the map 
area has undergone about 270-315 km of right-lateral slip.  
As much as 105 km of that offset may splay from the San 
Andreas Fault Zone and pass west of the Point Arena 
terrane on a submarine fault zone (Wentworth and others, 
1998b), although Jachens and others (1998) used 
aeromagnetic evidence to argue that the proposed offshore 
location of that fault conflicts with the distribution of 
magnetic rocks at depth.  The problem may be resolved by 
correlation of the granitic rocks at Bodega Head with 
similar rocks at Montara Mountain in San Mateo County 
(Kistler and Champion, 1991).  The Bodega Head/Montara 
Mountain correlation requires a major fault between Point 
Reyes and Bodega Head with 105 km of right offset.  
Because this fault would splay from the San Andreas Fault 
Zone farther south than the splay proposed by Wentworth 
and others (1998b), it can pass west of the large unfaulted 
magnetic body described by Jachens and others (1998).  In 
the map area, then, the San Andreas Fault Zone may have 
only accumulated 165-210 km of right slip.
	As a result of the large amounts of right-lateral 
slip along the continental margin related to the San 
Andreas Fault system (including the San Gregorio Fault 
Zone) since late Miocene time (less than 22 Ma), Salinian 
complex granitic rocks have been brought north as much 
as 490 km, although granitic rocks at Bodega Head, as 
well as Great Valley complex Point Arena terrane rocks, 
may have only traveled 340-385 km.  Salinian rocks are 
thought to be offset from correlative rocks in the Mojave 
Desert in southern California based on similar age and 
chemistry (especially Sr isotope ratios which are sensitive 
to crustal contamination and are therefore good indicators 
of intrusive environment; Kistler and others, 1973).  
The restoration of San Andreas Fault system 
offset is not enough, however, to account for the presence 
of tropical mollusks in the Late Cretaceous (late 
Campanian-early Maastrichtian, about 70-67 Ma) rocks in 
the Point Arena terrane.  If those mollusks were restricted 
to tropical latitudes, the Point Arena terrane Cretaceous 
rocks must have been transported hundreds of kilometers 
from the south prior to the development of the San 
Andreas Fault system (Wentworth and others, 1998b; 
Elder and others, 1998).  A similar Late Cretaceous to 
early Tertiary, south to north transport history is recorded 
in the Coastal Belt of the Franciscan Complex (Sliter, 
1984; Harbert and others, 1984).
	The Tolay/Petaluma River Fault Zone has 
probably been inactive at least since Pleistocene time as 
indicated by overlap of Pleistocene alluvial deposits in 
Santa Rosa Valley.  Principal active faulting in the map 
area is thought to be focused on the San Andreas, 
Healdsburg, and Maacama fault zones (Hart and Bryant, 
1999).  The San Andreas Fault in the map area 
experienced 4.5 to 5 m of right-lateral surface fault rupture 
during the 1906 earthquake (Galloway, 1966; Lawson, 
1908).  The Healdsburg/Rodgers Creek Fault is thought to 
be the northern extension of the Hayward Fault, which 
generated a large earthquake in 1868, although no fault 
rupture occurred in the map area during that event.  The 
more modest 1969 Santa Rosa earthquakes (M 5.6 and M 
5.7) were generated by the Rodgers Creek Fault (Budding 
and others, 1991).  For more about active faults in the 
region, see Working Group on California Earthquake 
Probabilities (1999).



Description of Map Units

SURFICIAL DEPOSITS

af	Artificial fill (Holocene (Historic))Man-made deposit of various materials.  
Some are compacted 
and quite firm, especially in earthen dams, but other fills made before 1965 are 
nearly everywhere not 
compacted and consist simply of dumped materials.  Only two bodies of this unit 
are shown on the map, 
one in the Warm Springs Dam and one in the Arched Rock quadrangle.  The many other 
bodies of this 
unit are too small to be shown at map scale
Qal	Alluvial fan and fluvial deposits (Quaternary)Alluvial fan deposits are 
brown or tan, medium 
dense to dense, gravelly sand or sandy gravel that generally grade 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.  This unit also includes floodplain deposits: 
medium to dark gray, dense, 
sandy to silty clay.  Lenses of coarser material (silt, sand, and pebbles) may be 
locally present.  In 
addition, this unit includes natural levee deposits: loose, moderately sorted 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.  This unit also includes stream channel deposits: poorly 
sorted to well-sorted sand, 
silt, silty sand, or sandy gravel with minor cobbles.  Cobbles are more common in 
the mountain valleys.  
At the mouth of the Russian and Gualala Rivers and the two esteros in the Valley 
Ford quadrangle, this 
unit also includes bay mud (Qm)  
Qm 	Bay mud (Quaternary)Water saturated estuarine mud, predominantly gray, 
green, and blue clay and silty 
clay underlying marshlands and tidal mud flats.  The mud also contains a few 
lenses of well-sorted, fine 
sand and silt, a few shelly layers (oysters), and peat.  This unit is mapped 
separately only in the Bodega 
Head quadrangle
Qs	Beach and dune sand (Quaternary)Fine-grained, very well sorted, well-
drained, eolian deposits.  They 
occur mainly in large 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 bay 
mud (Qm).  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 20-15 
ka, and probably 
ceased to accumulate after sea level reached its present elevation (about 6 ka)
Qls	Landslide deposits (Quaternary)Poorly sorted clay, silt, sand, gravel, 
boulders, and rock masses.  
Only a few large landslides are shown.  For a more complete map of landslide 
deposits, see Nilsen and 
others (1979)
Qt	Alluvial and marine terrace deposits (Pleistocene)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.  Alluvial 
terrace deposits lie on flat 
surfaces cut into bedrock a few meters that are up to several tens of meters above 
Qal deposits. Marine 
terrace deposits lie on one or more flights of flat surfaces that are cut into 
bedrock and lie up to 275 m 
above present sea level.  These terrace deposits are the remnants of an older 
alluvial system that have been 
lifted above present depositional levels by tectonic uplift associated with 
regional transpression
Qpoaf	Older alluvial fan deposits (Pleistocene)Roughly horizontal beds of 
buff siltstone and claystone, 
buff to gray, fine- to coarse-grained lithic sandstone, pebbly sandstone, pebbly 
mudstone, and pebble to 
cobble conglomerate.  Clasts include silicic to intermediate volcanics, obsidian, 
varicolored chert, 
graywacke and metagraywacke, quartzite, metachert, hornfels, quartz, charcoal, and 
petrified wood.  
Previously mapped (Blake and others, 1971; Fox, 1983) as Glen Ellen Formation 
based on similar clast 
composition (especially obsidian), we differentiate Qpoaf from QTge based on the 
lack of deformation and 
volcanics interbedded in Qpoaf and because Qpoaf is softer and less resistant than 
QTge.   Qpoaf deposits 
probably formed as a large alluvial fan system at the west edge of hills and 
ridges underlain by Glen Ellen 
Formation and Sonoma Volcanics during uplift and transpression related to the 
Rodgers Creek/Healdsburg 
Fault Zone.  Much Qpoaf detritus is probably reworked from the Glen Ellen 
Formation.  This unit is at 
least 65 m thick.  It is incised and overlain by Qal.  This unit may be coeval in 
part with Qt  and related in 
part to the same older alluvial system

ROCKS EAST OF AND WITHIN THE SAN ANDREAS FAULT ZONE

	Clear Lake VolcanicsIn the northeast corner of the map area (The Geysers 
quadrangle) are a few small 
outliers of the large Clear Lake volcanic complex that crops out mostly in Lake 
County to the northeast.  
This very young volcanic center (Pliocene to Holocene) is the northernmost 
manifestation of northward 
younging volcanism thought to be related to the initiation of the San Andreas 
Fault system (Fox and 
others, 1985b).  The near-surface magma and remnant heat from plutonic rocks 
related to the volcanic 
center is probably driving the hydrothermal activity at The Geysers. For a more 
complete description of 
the Clear Lake Volcanics, see Hearn and others (1995). In the map area, the Clear 
Lake volcanics include:
Qr		Rhyolite and rhyodacite (Pleistocene)Porphyritic biotite rhyolite and 
rhyodacite flows.  Contains 
parts of the rhyolite of Alder Creek and rhyodacite of Cobb Mountain of Hearn and 
others (1995).  
Rhyolite has yielded radiometric ages of 1.11+0.02 to 1.15+0.02 Ma, and rhyodacite 
has yielded ages of 
1.05+0.02 to 1.06+0.02 Ma (Hearn and others, 1995)
Qob		Olivine basalt (Pleistocene)Olivine-rich basalt flows, dikes, and 
diatreme breccia.  Equivalent to 
the olivine basalt of Caldwell Pines of Hearn and others (1995).  The basalt has 
yielded a radiometric age 
of 1.66+0.12 Ma (Hearn and others, 1995)

QTge	Glen Ellen Formation (Pleistocene(?) and Pliocene)Brown- to buff-weathering, 
interbedded siltstone, 
fine- to coarse-grained sandstone, pebbly and cobbly sandstone, conglomerate, and 
tuff.  Sandstone is tuffaceous 
or feldspathic arenite.  Coarse clasts include mafic to silicic volcanics, 
obsidian, pumice, varicolored chert, and 
graywacke.  The eastern part of the Glen Ellen Formation (mapped as QTget) 
contains more interbeds of tuff.  
Radiometric ages from interbedded tuff show that the Glen Ellen Formation is 3.1 
Ma or younger (McLaughlin 
and others, 1999).  The basal part of the Glen Ellen Formation is coeval with and 
interfingers with the upper 
part of the Sonoma Volcanics

Tor	Ohlson Ranch Formation (Pliocene)Mostly horizontal, thick beds of buff 
weathering, white to light-gray, 
well-consolidated, soft, quartz-lithic arenite.  Also includes buff-weathering 
gray siltstone, pebbly to cobbly 
sandstone and conglomerate, red lithic sandstone, and white tuff.  A tuff bed near 
the base of the unit in the 
Plantation quadrangle has been correlated (Sarna-Wojcicki, 1976) with the Nomlaki 
Tuff, which has yielded a 
radiometric age of 3.3 + 0.4 Ma (Everndon and others, 1964).  Locally concentrated 
Pliocene marine molluscan 
fossils have also been identified from this unit (Peck, 1960).  This formation 
lies in angular unconformity over 
Franciscan Complex rocks and in at least one location (Soda Springs Road in the 
Annapolis quadrangle) includes 
slabs as much as 10 m long of the underlying rock.  For a more complete 
description of the formation, see 
Higgins (1960).  The unit has been locally subdivided into:
Tors		Sandstone
Torc		Conglomerate

Tls	Sedimentary rocks of Little Sulphur Creek (Pliocene and (or) 
Miocene)Confined to a narrow band 
in the northeast part of the map area (Asti, The Geysers, and Jimtown 
quadrangles), this unit is composed of 
massive to distinctly bedded boulder to pebble conglomerate and breccia, lithic 
sandstone, siltstone, and 
mudstone.  Coarse clasts in places exceed 3 m in length and include varicolored 
chert, blueschist, graywacke, 
gabbro, serpentinite, diabase, and basalt, probably derived from surrounding 
Mesozoic rocks.  One clast of 
middle Miocene fossiliferous arkose has also been found (McLaughlin and Nilsen, 
1982), but no similar rock is 
known from the surrounding area.  Freshwater ostracod and gastropod fossils have 
been reported from this unit, 
as have brackish or marine foraminifers, but none of these fossils provide good 
age control (Gealey, 1951; 
McNitt, 1968; McLaughlin and Nilsen, 1982).  A Miocene or younger whale vertebra 
has also been reported 
(McNitt, 1968), but given the partly nonmarine nature of the unit, it is possible 
that this fossil was reworked 
from an older unit, perhaps the same as the fossiliferous arkose clast.  
McLaughlin and Nilsen (1982) interpreted 
this unit to have formed in a fault basin associated with a right step in the 
Maacama Fault Zone and suggested 
the unit was 3 Ma or younger based on the timing of initiation of San Andreas 
Fault system activity at this 
latitude.  However, it is possible that the fault basin was related to a pre-San 
Andreas structure.  Therefore, the 
age of the unit can only be constrained with confidence to post-date the middle 
Miocene age of the included 
arkose clast

	Sonoma Volcanics (Pliocene and Miocene)In the eastern part of the map area 
are a few outliers of the 
large, young Sonoma Volcanic complex, the bulk of which lies farther east.  The 
complex includes rhyolite, 
dacite, andesite, and basalt tuff; glass; flow rock; pyroclasitic breccia; and 
intrusives that were probably derived 
from several eruptive centers; along with interbedded volcaniclastic sedimentary 
rocks.  Radiometric ages from 
the Sonoma Volcanics range from 2.9+0.2 Ma to 6.95+0.2 Ma (Fox , 1983; Fox and 
others, 1985a).  The 
Sonoma Volcanics, together with the Clear Lake Volcanics, Donnell Ranch volcanics 
of Youngman (1989), and 
Burdell Mountain volcanics of Blake and others (2000), are thought to have formed 
as part of the northward 
younging series of volcanic centers related to initiation of the San Andreas Fault 
system (Fox and others, 
1985b).  For more about the Sonoma Volcanics, see Fox (1983) and Sarna-Wojcicki 
(1976).  In the map area, 
the Sonoma Volcanics are divided into:
Tsa		AndesitePlagioclase and plagioclase-hornblende porphyry andesite flow 
rock.  Also includes minor 
plagioclase-quartz porphyry dacite flow rock
Tsb		BasaltPlagioclase porphyry basalt flow rock.  Also includes minor 
vesicular andesite flow rock and 
volcanoclastic conglomerate

Twg	Wilson Grove Formation (late Pliocene to late Miocene)Mostly massive or 
thick-bedded, buff-
weathering, light-gray, fine-grained quartz-lithic arenite.  Also locally includes 
beds of mollusk- and gastropod-
shell hash, pebble to boulder conglomerate, and tuff.  Fossils from the Wilson 
Grove Formation range in age 
from late Miocene to late Pliocene (C. Powell, oral commun.; Travis, 1952; Naidu, 
1999).  The lower part of 
the unit overlies and is interbedded with late Miocene and early Pliocene basalt 
flow rock (Tb).  At least part of 
the Wilson Grove Formation is probably the marine equivalent of the estuarine sand 
and gravel of Cotati and the 
alluvial Petaluma Formation.  A distinctive tuff marker is distinguished locally:
Twgt		Tuff (late Miocene)White, water-lain tuff and pumice breccia.  
Informally named Roblar tuff by Sarna-
Wojcicki (1992), this tuff has yielded K/Ar ages ranging from 5.68+0.68 to 
6.26+0.1 (Bartow and others, 1973; 
Sarna-Wojcicki, 1976; Fox and others, 1985a)

Tc	Sand and gravel of Cotati (Pliocene and late Miocene)Thick-bedded to 
massive, white quartz arenite, 
buff-weathering siltstone, and pebble conglomerate.  Coarse clasts include 
varicolored chert; gray or brown 
laminated chert; red, gray, and white quartz- and plagioclase-porphyry volcanics; 
black basalt; blueschist; 
graywacke; greenstone; quartz; and black hornfels.  Locally present within the 
sandstone beds are estuarine 
mollusk, crustacean, and gastropod fossils.  The laminated chert clasts may be 
derived from middle Miocene 
Monterey or Claremont Formation cherts (Fox, 1983), although the nearest outcrop 
of these units is several 
tens of kilometers to the southeast.  However, offset on the Hayward-Rodgers 
Creek/Healdsburg and the 
Hayward-Tolay/Petaluma River Faults have probably moved the sand and gravel of 
Cotati and Wilson Grove 
Formation about 60 km from the southeast since deposition of the sand and gravel 
of Cotati.  The sand and 
gravel of Cotati overlies and is interbedded with late Miocene and early Pliocene 
basalt flow rock (Tb), overlies 
the  rhyolite intrusive breccia of the Donnell Ranch volcanics of Youngman (1989; 
Tdb), and interfingers on the 
west with the marine Wilson Grove Formation.  The age of the sand and gravel of 
Cotati is not as well 
constrained as that of the Wilson Grove Formation, but must postdate the 
underlying Donnell Ranch volcanics 
(Tdb, about 9.3 Ma) and include the age of interlayered basalt (Tb, 4.260.27 Ma).  
Because it interfingers with 
the Wilson Grove Formation, we consider the sand and gravel of Cotati to be the 
estuarine equivalent of at least 
part of the Wilson Grove Formation.  The sand and gravel of Cotati is probably 
also the estuarine equivalent of 
at least part of the alluvial Petaluma Formation.  The sand and gravel of Cotati 
is in places indistinguishable 
from the Petaluma Formation, although some beds may be distinguished by having 
estuarine fossils or by 
having small, very spherical pebbles in conglomerate.  Because the sand and gravel 
of Cotati is so similar in 
places to the Petaluma Formation, we have mapped the boundary between the two 
units at the Petaluma Valley 
Fault, which separates verifiable sand and gravel of Cotati outcrops from the main 
extent of Petaluma 
Formation.  However, the two formations probably interfingered during deposition, 
and therefore some of what 
is mapped herein as sand and gravel of Cotati might actually be Petaluma Formation

Tp	Petaluma Formation (Pliocene and late Miocene)Only the northwesternmost part 
of the outcrop area of 
this formation is within the map area (Two Rock quadrangle), although oil-well 
data show that it extends under 
Santa Rosa Valley (D. Zigler, written commun.).  Gray weathering, brown and green 
pebble- and cobble- 
conglomerate, gritstone, lithic and quartz-lithic arenite, and mudstone.  Coarse 
clasts include varicolored chert, 
quartz- and plagioclase-porphyry rhyolite and andesite, vesicular andesite, 
laminated rhyolite, white tuff, basalt, 
quartz, graywacke, greenstone, and laminated chert.  Locally present within the 
sandstone and conglomerate are 
land-mammal fossils, and lacustrine and estuarine ostracods (Liniecki-Laporte and 
Anderson, 1988) have been 
found in the mudstone.  Mammalian fossils originally described as late Pliocene 
(Stirton, 1939) are now known 
to be late Miocene (late Hemphilian, McFadden, 1998).  However, early Pliocene 
mammalian fossils have been 
found elsewhere within the unit (early Blancan; Bartow and others, 1973; Davies, 
1986).  East of the map area, 
the Petaluma Formation also includes a tuff bed correlated with Twgt (Sarna-
Wojckici, 1976) and a layer of 
basalt flow rock dated at 8.520.18 Ma (Fox and others, 1985a).  The Petaluma 
Formation overlies and 
interfingers with the Donnell Ranch volcanics of Youngman (1989) at its base (Fox, 
1983; Youngman, 1989) 
and is coeval with (and may interfinger with) the lower part of the Sonoma 
Volcanics at its top.  It is probably 
the nonmarine equivalent of the estuarine sand and gravel of Cotati and marine 
Wilson Grove Formation.  As 
described above, the sand and gravel of Cotati and the Petaluma Formation are in 
part indistinguishable, and 
strata that include estuarine ostracods that were previously mapped as Petaluma 
Formation would be considered 
by us to be interfingered sand and gravel of Cotati.  The Petaluma Formation lies 
mostly east of the Petaluma 
River Fault and has been offset about 35 km from the Wilson Grove and sand and 
gravel of Cotati west of the 
fault.  It has also been offset about 25 km by movement on the Rodgers Creek-
Healdsburg Fault from its 
original depositional position

Tb	Undivided basalt (Pliocene and Miocene)Black basalt, plagioclase porphyry 
basalt and andesite, and 
vesicular basalt and andesite flow rock.  In the map area, outcrops of Tb have 
yielded radiometric ages ranging 
from 4.260.27 Ma to 7.830.29 Ma (Fox and others, 1985a).  However, Tb also 
includes older basalt which is 
intruded by Tdb.  We correlate this older basalt with either an older part of the 
Donnell Ranch volcanics of 
Youngman (1989) or the about 12 Ma Burdell Mountain volcanics of Blake and others 
(2000; the nearest 
verified outcrop of Burdell Mountain volcanics is at Meecham Hill, Cotati 
quadrangle, just southeast of the map 
area)

Tdb	Donnell Ranch volcanics of Youngman, 1989 (Miocene)Plagioclase-porphyry 
rhyolite pumice-tuff 
vent breccia.  This unit has yielded a K/Ar age of 9.360.38 Ma (A. Sarna-
Wojcicki, written commun., 2000) 
and is correlated with the Donnell Ranch volcanics of Youngman (1989) and the 
volcanics of Berkeley Hills, 
southeast of the map area, based on similar age and composition.  It has been 
offset from these correlated 
volcanics by movement on the Hayward-Petaluma River and Hayward-Rodgers Creek-
Healdsburg Faults

Franciscan Complex 

Coastal Belt

TKfss	Sandstone (late Eocene to Late Cretaceous, Maastrichtian)Mostly 
massive, brown-weathering, dark-
green or gray, feldspathic and feldspathic-lithic wacke.  Sandstone in places 
contains detrital biotite and (or) large 
slate chips.  This unit also locally includes disrupted, thin-bedded sandstone and 
blue-gray weathering, greenish-
gray or dark-gray shale or slate.  Structural disruption ranges from shearing 
evident in the fine-grained beds to 
dismemberment into lenses of sandstone (boudins) in a sheared shale matrix.  The 
zeolite mineral laumontite is 
visible in veins and shear zones throughout the unit.  This unit has yielded 
Paleocene and Eocene palynomorphs 
north of the map area (Bachman, 1978) and overlies middle Eocene to Late 
Cretaceous basalt and pelagic 
limestone (Bachman, 1978; Sliter, 1984).  The feldspathic-lithic part of the unit 
(not mapped separately) is 
lithologically indistinguishable from TKfs and parts of Kfss

TKfgs	Greenstone (middle Eocene to Late Cretaceous)Orange-red weathering 
greenstone derived from pillow 
and massive basalt and basalt breccia.  Original rock texture (pillows, bedding) 
well preserved.  Includes middle 
Eocene to Late Cretaceous (Campanian to Maastrichtian) pelagic limestone locally.  
Depositionally underlies 
TKfss

Coastal or Central Belt

TKfs	Sandstone (late Eocene to Late Cretaceous, Turonian)Mostly massive, brown- 
and orange-weathering, 
green to gray feldpathic-lithic wacke.  Contains detrital biotite and muscovite in 
places.  Locally includes 
disrupted thin beds of sandstone and dark-gray shale and slate.  Laumontite is 
visible in veins and shear zones in 
places.  This unit lacks known fossils and other stratigraphic control and is 
lithologically indistinguishable from 
TKfss and parts of Kfss

Central Belt

Kfss	Sandstone (Late Cretaceous, Turonian to Campanian)Massive to distinctly 
bedded, brown-weathering, 
green, gray, or white, feldspathic and feldspathic-lithic wacke.  In the Bodega 
Head quadrangle, this unit consists 
of distinctly bedded white and gray, K-feldspar- and biotite-bearing feldspathic 
wacke and dark-gray shale.  
Similar rocks south of the map area contain Late Cretaceous (Campanian) fossils 
(Bailey and others, 1964).  In 
the Lake Sonoma area (Warm Springs, Tombs Creek, and Bigfoot Mountain 
quadrangles), the unit consists of 
massive to distinctly bedded, buff-weathering, gray, K-feldspar- and biotite-
bearing feldspathic-lithic wacke, 
pebbly wacke, and conglomerate, and dark-gray concretionary shale.  Coarse clasts 
include graywacke, shale, 
limestone (concretions, some fossiliferous), greenstone, and chert.  The unit in 
this area overlies chert and 
greenstone and has yielded a Late Cretaceous (Turonian) Inoceramus labiatus 
(Bailey and others, 1964).  Other 
than this fossil and the presence of chert, this unit is indistinguishable from 
TKfs and the feldspathic-lithic part 
of TKfss.  In the northeasternmost part of the area (Jimtown, Geyserville, Asti, 
The Geysers, and Highland 
Springs quadrangles), this unit is composed of distinctly bedded, brown-
weathering, greenish-gray, muscovite- 
and K-feldspar-bearing feldspathic-lithic wacke and dark-gray, mica-bearing 
siltstone and slate.  The sandstone in 
this area has locally an incipient foliation (textural zone 2A of Jayko and 
others, 1986), bears muscovite instead 
of biotite, and has not yielded age-diagnostic fossils but is otherwise similar to 
the wacke near Lake Sonoma as 
well as TKfs and the feldspathic-lithic part of TKfss.  These rocks are mapped as 
Kfss because of their structural 
position east of Lake Sonoma

Kfg	Greenstone (Late(?) Cretaceous)Massive and pillowed greenstone, lacking 
evidence of metamorphism in 
hand sample, and minor chert.  This unit crops out in small lenses at the base of 
Late Cretaceous (Turonian) 
sandstone in the Lake Sonoma area

Kfgwy	Sandstone (Late and Early Cretaceous, Cenomanian and (or) late 
Albian)Thick-bedded, buff-
weathering, gray lithic wacke and dark-gray shale.  The unit locally includes 
lenses of pebble and cobble 
conglomerate and pebbly sandstone and mudstone.  Coarse clasts include basalt, 
diabase, gabbro, andesite, 
varicolored chert, blueschist, and serpentinite.  This unit overlies Cenomanian or 
late Albian chert (KJfc), and 
similar sandstones in Marin and San Francisco Counties (south of the map area) 
have yielded both Cenomanian 
and Albian fossils.  This unit is distinguished from other Cretaceous Franciscan 
Complex sandstones by the 
large amount of lithic detritus and small amount of K-feldspar (0-6%).  It is 
similar in composition to coherent 
parts of KJfs, but differs by age and preserved depositional contact with chert

KJfgcs	Greenstone, chert, and sandstone (Cretaceous and Jurassic)Disrupted and 
interleaved graywacke 
(Kfgwy), chert (KJfc), and greenstone (Jfgs).  Individual rock bodies too small to 
show at map scale

KJfc	Chert (Cretaceous and Jurassic)Thin-bedded red chert with buff-weathering, 
dark-gray shale and slate 
partings.  Radiolarians from this unit range from Late or Early Cretaceous 
(Cenomanian or late Albian) to 
Middle Jurassic (Bajocian; Murchey and Jones, 1984)

KJfgc	Greenstone and chert (Cretaceous and Jurassic)Disrupted and interleaved 
chert (KJfc) and greenstone 
(Jfgs).  Individual rock bodies too small and complicated to show at map scale.  
Minor amounts of graywacke 
(Kfgwy) also present locally

KJfs	Graywacke and mlange (Cretaceous and Jurassic)Massive to distinctly bedded, 
brown-, orange-, and 
white-weathering, green to gray, lithic wacke and dark-gray or black siltstone, 
shale, and slate, grading into 
mlange consisting of sheared argillite and graywacke matrix enclosing blocks and 
lenses of sedimentary, 
metamorphic, and volcanic rocks (see fsr below for a more complete description of 
mlange).  Because contacts 
between coherent graywacke and mlange are gradational (derived from different 
amounts of shearing) and because 
of the size and amount of cover in the map area, it was not possible in this study 
to differentiate everywhere 
between coherent graywacke and mlange.  Where observed, coherent graywacke masses 
within the unit are 
indicated by a green hachure, while large observed blocks that are too small to 
map at 1:100,000 scale are 
indicated by diamonds (high-grade) and triangles (low-grade), and structural 
bodies known to be comprised 
primarily of mlange are mapped separately (fsr).  Coherent graywacke bodies also 
locally include conglomerate, 
pebbly sandstone, and rare thin beds of red and white chert.  Coarse clasts 
include black chert, quartzite, hornfels, 
granite, rhyolite, lithic wacke, blueschist, greenstone, green chert, marble, 
amphibolite, and quartz-mica schist.  
In many places, the graywacke contains conspicuous large chips of black and green 
shale.  Coherent sedimentary 
rocks range from completely unfoliated to moderately foliated (Textural Zones 1-2A 
of Jayko and others, 1986).  
Late Jurassic (Tithonian) fossils have been found in both the coherent graywacke 
and the mlange matrix (Bailey 
and others, 1964; D.L. Jones, written commun.).  Interbedded chert from the Asti 
quadrangle has been sampled, 
but not yet processed for microfossils.  Interbedded chert in similar coherent 
graywacke in Marin County has 
yielded Late Jurassic to Early Cretaceous microfossils (Murchey and Jones, 1984).  
Chert pebbles from 
conglomerate have yielded Late Triassic to Early Jurassic microfossils (Seiders 
and Blome, 1984).  Graywacke in 
this unit is distinguished from most other Franciscan Complex graywacke by its 
high lithic content.  It is 
similar in composition to Kfgwy but is significantly older, is in places more 
foliated, and lacks the 
depositionally underlying sequence of chert (KJfc) and greenstone (Jfgs)

fsr		MlangeSheared argillite, graywacke, and minor green tuff matrix 
enclosing blocks and lenses of graywacke, 
chert, metachert, greenstone, serpentinite, silica-carbonate rock, blueschist 
(metasediment and metabasalt), 
eclogite, amphibolite, limestone, and quartz-mica schist.  Enclosed blocks and 
lenses range in size from pebbles 
to several hundred meters.  The matrix graywacke has yielded Late Jurassic 
(Tithonian) fossils.  It is identical in 
age and composition to coherent graywacke in unit KJfs, and is probably derived 
from tectonic dismemberment 
of that unit.  High-grade blocks in the map area have yielded metamorphic ages of 
about 138-150 Ma (K/Ar; 
Kelley, 1982; Lee and others, 1964), similar to the age of Cazadero terrane 
blueschists.  Chert blocks from the 
map area have been sampled but not yet analyzed for microfossils.  Chert blocks in 
Marin County south of the 
map area are almost all similar to coherent chert of the Marin Headlands terrane 
(Murchey and Jones, 1984).  
Mlange blocks are probably derived from tectonic detachment of pieces of 
surrounding coherent Franciscan and 
Great Valley complex terranes.  Blocks about 5-50 m. in size are shown where 
recognized by diamonds (high-
grade metamorphic rocks) and triangles (low-grade and unmetamorphosed rocks), 
whereas the largest mlange 
blocks are mapped as separate units:
gs			Greenstone blockIncludes massive and pillowed greenstone and 
basalt.  Also locally includes greenstone 
and basalt breccia and diabase
ch			Chert blockIncludes massive and thinly bedded red, green, and 
white chert and metachert
gwy			Graywacke block
m			High-grade metamorphic rock blockIncludes blueschist metabasalt 
and metasediment, amphibolite, 
eclogite, and quartz-mica schist
sp			Serpentinite block

Jfgs	Greenstone (Jurassic)Massive and pillowed basalt metamorphosed to 
greenstone.  Amygdaloidal in places.  
Geochemical analysis suggests that these rocks originated as MORB, and 
paleomagnetic analysis suggests a near 
equatorial origin (Hagstrum and Murchey, 1993).  This unit depositionally 
underlies chert (KJfc), and therefore 
predates Middle Jurassic chert deposition.

Eastern Belt

KJfm	Metagraywacke (Cretaceous and Jurassic)Blueschist facies metagraywacke.  In 
the map area, this unit 
mostly consists of brown-weathering, blue, schistose (textural zone 2B-3A of Jayko 
and others, 1986), white-
mica- and glaucophane-bearing metagraywacke.  These rocks crop out both as a large 
coherent belt in the central 
part of the map area (the Skaggs Springs schist of Wakabayashi [1992b]), a much 
smaller coherent body in the 
Cazadero and Fort Ross quadrangles, and small slabs and lenses within mlange 
(fsr) in the Healdsburg, 
Jimtown, and Geyserville quadrangles.  The Skaggs Springs schist has yielded an 
Ar/Ar metamorphic age of 
about 141 Ma (Wakabayashi, 1999a).  This unit also includes the northernmost tip 
of a belt of brown-
weathering, gray, foliated (textural zone 2A of Jayko and others, 1986), jadeite-
bearing metagraywacke in the 
Two Rock quadrangle.  Similar jadeite-bearing metagraywacke elsewhere in the Coast 
Ranges has yielded 
radiometric metamorphic ages of 90-110 Ma (Wakabayashi, 1999a; Mattinson and 
Echevarria, 1980) and has 
yielded Late Jurassic (Tithonian) fossils

KJfmg	Metabasalt (Cretaceous and Jurassic)Blueschist facies metabasalt.  In 
the map area, this unit consists of 
medium-grade (Type III), blue and green, glaucophane-epidote-garnet-sphene-
bearing, foliated metabasalt (the 
Ward Creek schist of Coleman and Lee, 1963) and lower grade (Type II), green, 
glaucophane-muscovite-
lawsonite-sphene-bearing, nonfoliated metabasalt.  Type III metabasalt is 
interlayered with pink, green, brown, 
or blue metachert, dark-brown or bluish-black metashale and metaironstone, and 
white, aragonite-bearing 
metacarbonate.  The original textures (pillows, vesicles, amygdules) are generally 
preserved in Type II 
metabasalt, but are mostly obliterated in Type III.  The contact between Type II 
and III metabasalts is 
gradational.  In some structural blocks, Type II metabasalt grades into Type I 
greenstone, which lacks 
blueschist-facies metamorphic minerals, but Type III metabasalt does not crop out 
in those areas.  Structural 
blocks containing Type II and III metabasalt are indicated on the map by a black 
hachured pattern, whereas those 
blocks containing only Type I and II metabasalt are unhachured.  Geochemical 
analysis of the higher grade part 
of the unit suggests it was derived from a seamount (Coleman and Lee, 1963) that 
was subducted to a maximum 
depth of about 20 km (P=9.5-12 kb, T=360-380C; Maruyama and Liou, 1988; Moore and 
Blake, 1989; Oh, 
1990; Oh and others, 1991).  Type III metashale has yielded K/Ar metamorphic ages 
of 130-135 Ma (Lee and 
others, 1964), and Ward Creek metabasalt (presumably Type III) has yielded an 
Ar/Ar metamorphic age of 
142.60.5 Ma (Wakabayashi and Deino, 1989).  Other reported metamorphic ages for 
higher grade (type not 
specified) rocks of this unit are 1357 Ma to 1507.5 Ma (K/Ar; Coleman and 
Lanphere, 1971), 130-140 Ma 
(Ar/Ar, K/Ar; Wakabayashi, 1999a), and 133.16.7 to 153.67.7 Ma (K/Ar, Rb/Sr; 
Kelley, 1982)

Great Valley complex

KJgv	Sandstone, shale, and conglomerate (Early Cretaceous and Late 
Jurassic)Distinctly bedded, 
brown-weathering, dark-gray to white, biotite- and muscovite-lithic wacke and 
siltstone, dark-gray siltstone and 
shale, and pebble to boulder conglomerate.  Coarse clasts include quartz- and 
plagioclase-porphyry volcanics, 
granitic rocks, gray rhyolite tuff, and chert.  Sparse Late Jurassic (Tithonian) 
and Early Cretaceous (Valanginian) 
molluscan fossils have been found throughout the unit west of the Maacama Fault 
(Bailey and others, 1964; 
Travis, 1952; Christensen, 1973; Gealey, 1951), whereas east of the Maacama Fault 
the strata have yielded 
Early Cretaceous (Hauterivian-Barremian) fossils (McLaughlin and Ohlin, 1984; D.L 
Jones, written commun.).  
Largely subdivided into:
KJgvs		Sandstone, siltstone, and shale
KJgvc		Conglomerate
Jk	Knoxville Formation (Late Jurassic)Distinctly-bedded black shale and thin 
beds of biotite-lithic wacke 
that contain only Late Jurassic fossils.  Mapped only in the Camp Meeker 
quadrangle

Jsv	Keratophyre and quartz keratophyre tuff (Late Jurassic)This unit is only 
mapped in one small area in 
Camp Meeker quadrangle.  Highly altered, distinctly bedded, red, green, and white 
intermediate and silicic tuff.  
Feldspars are almost all replaced by albite.  Contains Late Jurassic (Tithonian) 
radiolarians and bright green 
celadonite crystals.  Probably correlative with tuff of the Lotta Creek Formation 
of the western San Joaquin 
Valley hundreds of kilometers southeast of the map area.  This unit also includes 
minor amounts of augite-
porphyry breccia.  These rocks are probably the altered remnants of tuff and 
volcanic detritus derived from a 
volcanic arc and deposited on the Coast Range ophiolite.  The presence of Lotta 
Creek Formation tuff here is 
indicative of the large amounts of fault offset experienced by the California 
Coast Ranges


	Coast Range ophiolite (Late and Middle Jurassic)Consists of:
Jv		Mafic and intermediate volcanic rocksMostly massive basalt, also 
includes amygduloidal and 
plagioclase-porphyry basalt, pillow basalt, diabase, keratophyre, and amygduloidal 
and plagioclase-hornblende-
porphyry andesite.  Coast Range ophiolite volcanic rocks are distinguished from 
Franciscan Complex volcanic 
rocks by lack of metamorphism and alteration, by association with intrusive rocks, 
and by lack of associated 
ribbon chert.
Jd		Mafic and intermediate intrusive rocksDiabase, gabbro, and diorite.  
Also includes minor intrusive 
keratophyre.  The Franciscan Complex terranes contain very few outcrops of 
intrusive rocks, especially plutonic 
rocks like gabbro and diorite, so presence of this unit is indicative of Coast 
Range ophiolite affinity
Ju		Ultramafic rocksPeridotite and pyroxenite.  More or less serpentinized, 
but rocks of this unit generally 
maintain relic plutonic fabric (bastite).
sp		SerpentiniteMainly sheared serpentinite, but also includes massive 
serpentinized harzburgite.  In places, 
pervasively altered to:
sc			Silica carbonate rock
spm		Serpentinite matrix mlangeSheared meta-serpentinite containing large 
blocks and lenses (as much as 10 
m. or more in diameter) of high-grade metamorphic rocks, graywacke, and 
greenstone.  Only large lenses are 
mapped.  In the eastern part of the Warm Springs Dam quadrangle, serpentinite, 
greenstone, and blueschist are 
tightly interleaved at outcrop scale

ROCKS WEST OF AND WITHIN THE SAN ANDREAS FAULT ZONE


Tsm	Sandstone and mudstone of the Fort Ross area (early Miocene)Gray-white, K-
feldspar-bearing 
arkose overlain by black, somewhat fissile clayey siltstone.  The sandstone is 
interbedded with black mudstone.  
The siltstone contains thin beds and laminae of fine-grained sandstone and has 
yielded fish scales and early 
Miocene (Saucesian) foraminifers (Wentworth, 1967)

Great Valley(?) complex

Tg	German Rancho Formation of Wentworth and others (1998b) (Eocene and 
Paleocene)Distinctly 
bedded, fine- to medium-grained, K-feldspar-bearing feldspathic arenite, mudstone, 
and conglomerate.  Coarse 
clasts consist mostly of granitic rocks, amphibolite, schist, gneiss, quartzite, 
and porphyry volcanics.  Eocene 
and Paleocene fossils have been collected from many places in the unit (for a 
recent discussion, see Wentworth 
and others, 1998b; Elder and others, 1998; McDougall, 1998), although the 
Paleocene section occurs only south 
of Black Point.  North of Black Point, the base of the unit is marked by red and 
green mudstone yielding early 
Eocene microfossils (Wentworth and others, 1998b; McDougall, 1998) and the 
Paleocene section is missing.  
This absence is attributed by Wentworth and others (1998b) to attenuation faulting

TKu	Undifferentiated German Rancho and Gualala Formations (Eocene to Late 
Cretaceous)Because 
of poor exposure, fault offset, lack of fossil age control, and similarity of 
lithologies, it has not been possible 
to differentiate between these two formations in a fault bounded sliver just west 
of the San Andreas Fault Zone 
(Stewarts Point, Annapolis, and Plantation quadrangles).  Outcrops of this unit 
are primarily deeply weathered 
arkose

	Gualala Formation of Wentworth and others (1998b) (Late Cretaceous and 
Paleocene?)In the 
map area, everywhere divided into:
Ka		Anchor Bay memberGreenish-gray, fine- to medium-grained, plagioclase-
bearing feldspathic arenite and 
mafic-clast pebble to boulder conglomerate.  Laumontite veins are found throughout 
the sandstone.  Coarse 
clasts include mostly basalt, diabase, gabbro, diorite, quartz diorite, and 
andesite, although pyroxenite is locally 
present in boulder conglomerate.  Molluscan fossils of Late Cretaceous age have 
been found throughout the 
member (Elder and others, 1998), but foraminifers of Late Cretaceous to late 
Paleocene age have been found in 
the same section, interleaved both with each other and with Late Cretaceous 
molluscan fossils (Wentworth and 
others, 1998b; McDougall, 1998).  This apparent conflict is still unresolved
Ks		Stewarts Point memberGray, fine- to medium-grained, K-feldspar and 
plagioclase-bearing feldspathic 
arenite and conglomerate.  Coarse clasts include porphyry volcanics and granitic 
rocks.  This unit has yielded 
Late Cretaceous (late Campanian or younger) molluscan fossils and foraminifers 
(Wentworth and others, 1998b; 
McDougall, 1998; Elder and others, 1998)

KJsb	Spilite near Black Point of Wentworth and others (1998b) (Cretaceous? or 
Jurassic?)Massive 
to pillowed basalt flows and sheeted diabase sills, metamorphosed to low-grade 
greenschist facies (albite-chlorite-
clinozoisite; Phillips and others, 1998).  Wentworth and others (1998b) correlate 
this unit with the Coast Range 
ophiolite, but the geochemical studies of Phillips and others (1998) suggest a 
back-arc magmatic affinity, which 
differs from the Coast Range ophiolite (fore-arc affinity).  Jachens and others 
(1998) correlate the positive 
magnetic anomaly related to this unit with that related to the Logan gabbro in San 
Mateo County to the south 
(offset by the San Andreas/San Gregorio Fault Zone), but there is no unequivocal 
match between any strata in 
the area underlain by Logan gabbro and strata underlain by this unit.  This unit 
was originally assigned to the 
Cretaceous (Wentworth, 1967), but no direct evidence is available and both 
correlated mafic bodies are Jurassic 
(James and others, 1993; Blake and others, 1992; Hagstrum, 1997; Hopson and 
others, 1997)
 
Salinian complex

Kgr	Quartz diorite of Bodega Head (Cretaceous)White or gray, hornblende-biotite 
quartz diorite.  Includes 
intermediate and felsic, fine-grained to pegmatitic dikes and pods.  Plagioclase- 
and K-feldspar-porphyry quartz 
diorite present locally.  This unit has yielded a Rb/Sr age of 80-92 Ma (Kistler 
and Champion, 1991)
Acknowledgments

We would like to acknowledge our many colleagues at the U.S. Geological Survey, 
University of California, and Stanford 
University for their many contributions over the years.
	We are grateful to the following U.S. Geological Survey paleontologists who 
have examined our fossils and 
provided ages necessary to establish the stratigraphic sequence and structure: 
David Bukry (Cretaceous and Tertiary 
nannoplankton), Kristin McDougall (Tertiary foraminifers), William Sliter 
(Cretaceous and Eocene foraminifers), John Barron 
(Tertiary diatoms), Charles Powell, II (Tertiary mollusks), and Bonita Murchey 
(Mesozoic radiolaria).
	We are also very grateful to managers and staff of Chevron, EXXON, UNOCAL, 
ARCO, and Shell Petroleum 
Companies who have provided reports, maps, picked slides, and residues for about 
25,000 microfossil localities in the San 
Francisco Bay Region.
	Finally, we are indebted to C.M. Wentworth, W.P. Irwin, and E.E. Brabb of the 
USGS and D.L. Jones of the 
University of California for their thorough reviews of the manuscript for this 
report, as well as C.L. Powell, II, for his 
review of stratigraphic nomenclature, Z.C. Valin for his review of the digital 
database, and J.L. Zigler for her thorough map 
and manuscript edit.

Digital Publication and Database Description





Introduction

This publication includes, in addition to cartographic and 
text products, geospatial (GIS) databases and other digital 
files.  These files are published on the Internet through the 
U.S. Geological Survey (USGS) web sites.  The database 
files are particularly useful because they can be combined 
with any type of other geospatial data for purposes of 
display and analysis.  The other files include digital files 
that support the databases, and digital plot files that can be 
used to display and print the cartographic and text products 
included in this publication.
Following is the digital publication and database 
description.  It contains information about the content and 
format of the digital geospatial databases used to create 
this digital geologic map publication.  This 
information is not necessary to use or 
understand the geologic information in the 
map and preceding geologic description.  The 
digital publication and database description contains 
information primarily useful for those who intend to use 
the geospatial databases.  However, it also contains 
information about how to get digital plot files of the map 
and geologic pamphlet via the Internet, as well as 
information about how the map sheet and pamphlet were 
created, and information about getting copies of the map 
sheet and text from the USGS.
In addition, in 1999, the USGS adopted policies 
regarding revision of publications, introducing the concept 
of version numbers similar to those used in the computer 
industry.  See the section called "Revisions and version 
numbers" for information about the version system and 
about how to access a revision list explaining changes 
from version 1.0, if any have been made.
The digital map database, compiled from 
previously published and unpublished data and new 
mapping by the authors, represents the general 
distribution and orientation of bedrock and surficial 
deposits in the mapped area.  Together with the 
accompanying text (available as digital files wsomf.txt, 
wsomf.pdf, or wsomf.ps), it provides current information 
on the geologic structure and stratigraphy of the map area.  
The database delineates map units that are identified by 
general age and lithology following the stratigraphic 
nomenclature of the USGS.  The scale of the source maps 
limits the spatial resolution (scale) of the database to 
1:62,500 or smaller.  The content and character of the 
database, as well as two methods of obtaining the 
database, are described below. 

For those who don't use digital geologic map 
databases

For those interested in the geology of the mapped area 
who do not use an ARC/INFO compatible Geographic 
Information System (GIS), we have provided two sets of 
plotfiles containing images of much of the information in 
the database.  Each set contains an image of a 1:100,000 
scale geologic map sheet and explanation and an 
explanatory pamphlet.  There is a set of images in 
PostScript format and another in Adobe Acrobat PDF 
format (see the sections "PostScript plot files" and "PDF 
plot files" below).
	Those interested who have computer capability 
can access the plot file packages in either of the two ways 
described below (see the section "Obtaining the digital 
database and plotfile packages").  However, it should be 
noted the plot file packages do require gzip and TAR 
utilities to access the plot files.  Therefore additional 
software, available free on the Internet, may be required to 
use the plot files (see section "Tar files").
	Those without computer capability can obtain 
plots of the map files through USGS plot-on-demand 
service for digital geologic maps (see section "Obtaining 
plots from USGS Map On Demand Services") or from a 
commercial vendor (see section "Obtaining plots from a 
commercial vendor").



MF-2402 Digital Contents

This  report includes three digital packages.  The first is the PostScript 
Plotfile Package, which consists of PostScript plot 
files of a map sheet containing a geologic map and explanation and a pamphlet 
containing a geologic description.  The second 
is the PDF Plotfile Package, which contains the same plotfiles as the first 
package but in Portable Document Format (PDF).  
The third is the Digital Database Package, which contains the geologic map 
database itself and the supporting data, including 
base maps, map explanation, geologic description, and references.


Postscript plotfile package

This package contains the images described here in PostScript format (see below 
for more information on PostScript plot 
files):

wsomap.ps	A PostScript plotfile containing an image of the geologic map and base 
maps at a scale of 
1:100,000 along with a map explanation including terrane map, index maps, and 
correlation chart

wsomf.ps	A PostScript plotfile that contains an image of the pamphlet containing 
detailed unit descriptions 
and geological information, a description of the digital files associated with the 
publication, plus 
references cited

PDF plotfile package

This package contains the images described here in PDF format (see below for more 
information on PDF plot files):

wsomap.pdf	A PDF file containing an image of the geologic map and base maps at a 
scale of 1:100,000, along 
with a map explanation including terrane map, index maps, and correlation chart

wsomf.pdf	A PDF file that contains an image of the pamphlet containing detailed 
unit descriptions and 
geological information, a description of the digital files associated with the 
publication, plus 
references cited

Digital database package

The database package includes geologic map database files for the map area.  The 
digital maps, or coverages, along with their 
associated INFO directory have been converted to uncompressed ARC/INFO export 
files.  ARC export files promote ease of 
data handling, and are usable by some Geographic Information Systems in addition 
to ARC/INFO (see below for a discussion 
of working with export files).  The ARC export files and the associated ARC/INFO 
coverages and directories, as well as the 
additional digital material included in the database, are described below:

ARC/INFO		Resultant	Description of Coverage
export file		Coverage	
--------------		-----------		--------------------------------
wso-geol.e00	wso-geol/	Polygon and line coverage showing faults, depositional 
contacts, and rock units 
in the map area

wso-strc.e00	wso-strc/	Point and annotation coverage showing strike and dip 
information

wso-blks.e00	wso-blks/	Point and annotation coverage showing location of high- 
and low-grade blocks in 
Franciscan rock units

wso-grst.e00	wso-grst/	Polygon coverage showing areas of medium- to high-grade 
blueschist facies 
metamorphism within unit KJfmg

wso-ss.e00	wso-ss/	Polygon coverage showing areas of coherent greywacke in unit 
KJfs (graywacke 
and mlange)

The database package also includes the following ARC coverages and files:

ARC Coverages, which have been converted to uncompressed ARC/INFO export files

ARC/INFO		Resultant	Description of Coverage
export file		Coverage	
--------------		-----------		--------------------------------

wso-quad.e00	wso-quad/	Polygon, line, and annotation coverage showing index map 
of quadrangles in the 
map area

wso-corr.e00	wso-corr/	Polygon, line, and annotation coverage of the 
correlation table for the units in 
this map database.  This database is not geospatial

wso-so.e00	wso-so/	Polygon, line, and annotation coverage showing sources of 
data index map for 
this map database

wso-terr.e00	wso-terr/	Polygon and line coverage of the index map of 
tectonostratigraphic terranes in 
the map area (terranes are described above)

wso-xsl.e00	wso-xsl/	Line and annotation coverage of the cross-section line 
A-A'

wso-xsa.e00	wso-xsa/	Polygon, line, and annotation coverage of SW- to NE-
trending cross section.  
Note:  This coverage is NOT georeferenced


ASCII text files, including explanatory text, ARC/INFO key files, PostScript plot 
files, and an ARC Macro Language file 
for conversion of ARC export files into ARC coverages

wsomf.ps	A PostScript plotfile that contains an image of the pamphlet containing 
detailed unit descriptions 
and geological information, a description of the digital files associated with the 
publication, plus 
references cited

wsomf.pdf	A PDF version of wsomf.ps

wsomf.txt	A text-only file containing an unformatted version of wsomf.ps without 
figures

wsofig1.tif	A TIFF file of figure 1 from wsomf.ps

wsoso.txt	ASCII text-only file containing sources of data related to coverage wso-
so

	import.aml	ASCII text file in ARC Macro Language to convert ARC export files 
			to ARC coverages in ARC/INFO

	mf2402d.met	A parsable text-only file of publication level FGDC metadata 
for this report

	mf2402e.rev	A text-only file describing revisions, if any, to this 
publication

The following supporting directory is not included in the database package but is 
produced in the process of reconverting the 
export files into ARC coverages:

	info/	INFO directory containing files supporting the databases


Tar files

The three data packages described above are stored in TAR (UNIX tape archive) 
files.  A TAR utility is required to extract the 
database from the TAR file.  This utility is included in most UNIX systems and can 
be obtained free of charge over the 
Internet from Internet Literacy's Common Internet File Formats Webpage 
(http://www.matisse.net/files/formats.html).  All 
TAR files have been compressed and may be uncompressed with gzip, which is 
available free of charge over the Internet via 
links from the USGS Public Domain Software page 
(http://edcwww.cr.usgs.gov/doc/edchome/ndcdb/public.html).  In 
addition, several common proprietary freeware programs such as Stuffit Expander 
(http://www.aladdinsys.com/expander/index.html) and shareware programs such as 
WinZip (http://www.winzip.com) can 
handle both TAR file extraction and gzip uncompression.  When the TAR file is 
uncompressed and the data is extracted from 
the TAR file, a directory is produced that contains the data in the package as 
described above.  The specifics of the TAR files 
are listed below:

Name of	Size of	Directory	Data package
compressed	compressed	produced when	contained
TAR file	TAR file	extracted from
	(uncompressed)	TAR file
------------	---------------------	----------------------	---------------
-------------

mf2402a.tgz	5.9 MB (27.4 MB)	wsops	PostScript Plotfile Package

mf2402b.tgz	6.4 MB (6.5 MB)	wsopdf	PDF Plotfile Package

mf2402c.tgz	3.4 MB (33.7 MB)	wsogeo	Digital Database Package



PostScript plot files

For those interested in the geology of the map area who 
don't use an ARC/INFO compatible GIS system, and for 
those who want a copy of the USGS format cartographic 
product, we have included a separate data package with two 
PostScript plot files.  One contains a color plot of the 
geologic map at 1:100,000 scale, along with a terrane 
map, index maps, correlation chart, and map explanation 
(wsomap.ps).  A second PostScript file (wsomf.ps) 
contains the geologic and digital publication description 
and discussion (this pamphlet).
	The PostScript images of the geologic maps and 
map explanation are 50 inches high by 34.5 inches wide, 
so it requires a large plotter to produce paper copies at the 
intended scale.  In addition, some plotters, such as those 
with continual paper feed from a roll, are oriented with the 
long axis in the horizontal direction, so the PostScript 
image will have to be rotated 90 degrees to fit entirely 
onto the page.  Some plotters and plotter drivers, as well 
as many graphics software packages, can perform this 
rotation.  The geologic description is on 8.5- by 11-inch 
pages.
	The PostScript plotfiles for maps were produced 
by the 'postscript' command with compression set to zero 
in ARC/INFO version 7.1.1.  The PostScript plotfiles for 
pamphlets were produced in Microsoft Word 6.0 using the 
Destination PostScript File option from the Print 
command.

PDF plot files

We have also included a second digital package containing 
PDF versions of the PostScript map sheet and pamphlet 
described above.  Adobe Acrobat PDF (Portable Document 
Format) files are similar to PostScript plot files in that 
they contain all the information needed to produce a paper 
copy of a map or pamphlet and they are platform 
independent.  Their principal advantage is that they require 
less memory to store and are therefore quicker to download 
from the Internet.  In addition, PDF files allow for 
printing of portions of a map image on a printer smaller 
than that required to print the entire map without the 
purchase of expensive additional software.  All PDF files 
in this report have been created from PostScript plot files 
using Adobe Acrobat Distiller.  In test plots we have 
found that paper maps created with PDF files contain 
almost all the detail of maps created with PostScript plot 
files.  We would, however, recommend that those users 
with the capability to print the large PostScript plot files 
use them in preference to the PDF files.
	To use PDF files, the user must get and install a 
copy of Adobe Acrobat Reader.  This software is available 
free from the Adobe website (http://www.adobe.com).  
Please follow the instructions given at the website to 
download and install this software.  Once installed, the 
Acrobat Reader software contains an on-line manual and 
tutorial.
	There are two ways to use Acrobat Reader in 
conjunction with the Internet.  One is to use the PDF 
reader plug-in with your Internet browser.  This allows for 
interactive viewing of PDF file images within your 
browser.  This is an easy way to quickly look at PDF 
files without downloading them to your hard disk.  The 
second way is to download the PDF file to your local hard 
disk and then view the file with Acrobat Reader.  We 
strongly recommend that large map images be 
handled by downloading to your hard disk, 
because viewing them within an Internet browser tends to 
be very slow.
	To print a smaller portion of a PDF map image 
using Acrobat Reader, it is necessary to cut out the 
portion desired using Acrobat Reader and the standard cut 
and paste tools for your platform and then to paste the 
portion of the image into a file generated by another 
software program that can handle images.  Most word 
processors (such as Microsoft Word) will suffice.  The 
new file can then be printed.  Image conversion in the cut 
and paste process, as well as changes in the scale of the 
map image, may result in loss of image quality.  
However, test plots have proven adequate.  Superior 
quality can be obtained by using image processing 
software that can open PDF files (like Adobe Photoshop 
Elements) to crop and print a portion of the map.

Obtaining the Digital Database and Plotfile 
Packages

The digital data can be obtained from

	http://pubs.usgs.gov/mf/2002/mf2402

Besides providing easy access to the entire digital database, 
the Western Region Web page also affords easy access to 
the PostScript and PDF plot files for those who do not 
use digital databases.

Obtaining plots from a commercial vendor

Those interested in the geologic map, but who use neither 
a computer nor the Internet, can still obtain the 
information.  Many vendors can download the plotfiles via 
the Internet.  Important information regarding file formats 
is included in the sections  "Tar files," "PostScript plot 
files," and "PDF plot files" above, so be certain to provide 
a copy of this document to your vendor.


Obtaining plots from USGS Map On Demand 
Services

The USGS provides a plot-on-demand service for map 
files, such as those described in this report, through Map 
On Demand Services.  In order to obtain plots, contact 
Map On Demand Services:

U.S. Geological Survey 
Information Services
Box 25286
Federal Center
Denver, CO 80225-0046

(303) 202-4200
1-888-ASK-USGS

FAX:  (303) 202-4695

e-mail:  infoservices@usgs.gov

Be sure to include with your request the MF report 
number.

Revisions and version numbers

From time to time, new information and mapping, or 
other improvements, will be integrated into this 
publication.  Rather than releasing an entirely new 
publication, the USGS has adopted a policy of using 
version numbers similar to those used in the computer 
industry.  The original version will be labeled Version 
1.0.  Subsequent small revisions will be denoted by the 
increase of the numeral after the decimal, while large 
changes will be denoted by increasing the numeral before 
the decimal.  Information about the changes, if any, that 
have been made since the release of Version 1.0 will be 
listed in the publication revision file.  This file will be 
available at the publication web site (see above) and also 
will be included in the digital database package.  A 
simplified version of the revision list will be included in 
the publication metadata.  

Digital database format
	
The databases in this report were compiled in ARC/INFO, 
a commercial Geographic Information System 
(Environmental Systems Research Institute [ESRI], 
Redlands, California), with version 3.0 of the menu 
interface ALACARTE (Fitzgibbon and Wentworth, 1991; 
Fitzgibbon, 1991; Wentworth and Fitzgibbon, 1991).  
The files are in either GRID (ARC/INFO raster data) 
format or COVERAGE (ARC/INFO vector data) format.  
Coverages are stored in uncompressed ARC export format 
(ARC/INFO version 7).  ARC/INFO export files (files 
with the .e00 extension) can be converted into ARC/INFO 
coverages in ARC/INFO (see below) and can be read by 
some other Geographic Information Systems, such as 
MapInfo via ArcLink and ESRI's ArcView (version 1.0 
for Windows 3.1 to 3.11 is available for free from ESRI's 
web site:  http://www.esri.com). The digital compilation 
was done in version 7.1.2 of ARC/INFO with version 3.0 
of the menu interface ALACARTE (Fitzgibbon and 
Wentworth, 1991; Fitzgibbon, 1991; Wentworth and 
Fitzgibbon, 1991).

Converting ARC export files

ARC export files are converted to ARC coverages using 
the ARC command IMPORT with the option COVER.  
To ease conversion and maintain naming conventions, we 
have included an ASCII text file in ARC Macro Language 
that will convert all of the export files in the database into 
coverages and create the associated INFO directory.  From 
the ARC command line type:

	Arc:  &run import.aml

ARC export files can also be read by some other 
Geographic Information Systems.  Please consult your 
GIS documentation to see if you can use ARC export files 
and the procedure to import them.

Digital compilation

The geologic map information was digitized from stable 
originals of the geologic maps at 1:62,500 scale.  The 
author manuscripts (pen on mylar) were scanned using an 
Altek monochrome scanner with a resolution of 800 dots 
per inch.  The scanned images were vectorized and 
transformed from scanner coordinates to projection 
coordinates with digital tics placed by hand at quadrangle 
corners.  The scanned lines were edited interactively by 
hand using ALACARTE, color boundaries were tagged as 
appropriate, and scanning artifacts visible at 1:24,000 
were removed. 

Base maps

Base map layers were derived from published digital maps (Aitken, 1997) obtained 
from the USGS Publications Website (http://pubs.usgs.gov/of/1997/of97-500/ ).  
Please see the website for more detailed information about the original databases.  
Because the base map digital files are already available at the website mentioned 
above, they are not included in the digital database package.

Faults and landslides

This map is intended to be of general use to engineers and 
land-use planners.  However, its small scale does not 
provide sufficient detail for site development purposes.  In 
addition, this map does not take the place of fault-rupture 
hazard zones designated by the California State Geologist 
(Hart and Bryant, 1999).  Similarly, because only some of 
the landslides in the mapped area are shown, the database 
cannot be used to completely identify or delineate 
landslides in the region.  For a more complete depiction of 
landslide distribution, see Nilsen and others (1979), Ellen 
and others (1997), and Wentworth and others (1997).

Spatial resolution

Uses of this digital geologic map should not violate the 
spatial resolution of the data.  Although the digital form 
of the data removes the constraint imposed by the scale of 
a paper map, the detail and accuracy inherent in map scale 
are also present in the digital data.  The fact that this 
database was edited at a scale of 1:62,500 means that 
higher resolution information is not present in the dataset.  
Plotting at scales larger than 1:62,500 will not yield 
greater real detail, although it may reveal fine-scale 
irregularities below the intended resolution of the database.  
Similarly, where this database is used in combination 
with other data of higher resolution, the resolution of the 
combined output will be limited by the lower resolution 
of these data.




Database specifics

What follows is a brief and simple description of the main databases included in 
this report and the data in them.  For a 
comprehensive look at the database structure and content, please see the FGDC 
Metadata file, mf2402d.met, included in the 
database package and available separately at the publication web page.
The map databases consist of ARC coverages and supporting INFO files, which are 
stored in a Universal Transverse 
Mercator (UTM) projection (table 1).  Digital tics define a 2.5 minute grid of 
latitude and longitude in the geologic coverages 
corresponding with quadrangle corners and internal tics.  Note that coverage wso-
xsa/ is not georeferenced.

Table 1.  Map Projection File
The maps are stored in UTM projection.  The following is an annotated projection 
file of the type used in Arc/Info.

PROJECTION UTM	
UNITS METERS	
ZONE 10
SPHEROID CLARKE1866
DATUM NAD27
PARAMETERS
END

The content of the geologic database can be described in terms of the lines, 
points, and areas that compose the map.  Each 
line, point, or area in a map layer or index map database (coverage) is associated 
with a database entry stored in a feature 
attribute table.  Each database entry contains both a number of items generated by 
Arc/Info to describe the geometry of the 
line, point, or area and one or more items defined by the authors to describe the 
geologic information associated with that 
entry.  Each item is defined as to the amount and type of information that can be 
recorded.  Descriptions of the database items 
use the terms explained in table 2.

Table 2.  Field Definition Terms

ITEM NAME	name of the database field (item)
WIDTH	maximum number of digits or characters stored
OUTPUT	output width
TYPE	B-binary integer, F-binary floating point number, I-ASCII integer, C-ASCII 
character string
N. DEC.	number of decimal places maintained for floating point numbers

Because some of the database structure is similar for all coverages, some 
descriptions apply to all coverages in the 
publication.  In that case, the notation <coverage> has been used to indicate the 
description is valid for any included coverage.  
The precise description for a particular coverage can be made by substituting the 
name of the coverage for <coverage>.  For 
example, <coverage>-ID means that the description is the same for every coverage.  
The specific notation for a single 
coverage can be derived by replacing <coverage> with the coverage name (such as 
WSO-GEOL-ID for the coverage wso-geol).

Lines 

The lines (arcs) are recorded as strings of vectors and are described in the arc 
attribute table (the format of the arc attribute table 
is shown in table 3).  They define the boundaries of the map units, the boundaries 
of open bodies of water, and the map 
boundaries.  These distinctions, including the geologic identities of the unit 
boundaries, are recorded in the LTYPE field 
according to the line types listed in table 4.


Table 3.  Content of the Arc Attribute Tables

ITEM NAME	WIDTH	OUTPUT	TYPE	N. DEC	

FNODE#	4	5	B		starting node of arc (from node) 
TNODE#	4	5	B		ending node of arc (to node)
LPOLY#	4	5	B		polygon to the left of the arc
RPOLY#	4	5	B		polygon to the right of the arc
LENGTH	4	12	F	3	length of arc in meters
<coverage>#	4	5	B		unique internal control number
<coverage>-ID	4	5	B		unique identification number
LTYPE	35	35	C		line type (see table 4)
FAULTNAME	35	35	C		name of fault, if any, only in WSO-GEOL


Table 4.  Line Types Recorded in the LTYPE Field

wso-geol and wso-terr
--------------------------------
wso-so and wso-quad
--------------------------------
contact, approx. located           		        
contact, certain		
contact, concealed                   		
contact, concealed, queried	
contact, inferred                    	
contact, inferred, queried           	
fault, active                        	
fault, approx. located               	
fault, certain                        	
fault, concealed                     	
fault, concealed, queried             	
fault, inferred                       	
fault, inferred, queried              
map boundary,                        
reverse fault, approx. located
reverse fault, certain
reverse fault, concealed
scratch boundary                   
thrust fault, approx. located        
water boundary,
leader
map boundary
quad boundary
water boundary
source, boundary
		
	
Table 4a.  Fault Names Recorded in the FAULTNAME Field

wso-geol ONLY
--------------------------------
Bloomfield Fault
Healdsburg Fault Zone
Maacama Fault Zone
Petaluma Valley Fault
San Andreas Fault Zone
Tolay Fault

Note, not every line type listed is present in every coverage.  For example, wso-
terr only has some of the fault types listed.

The geologic linetypes are ALACARTE line types that correlate with the geologic 
line symbols in the ALACARTE line set 
GEOL.LIN according to the ALACARTE lines lookup table (GEOL.LUT).  For more 
information on ALACARTE and its 
linesets, see Wentworth and Fitzgibbon (1991).

Areas 

Map units (polygons) are described in the polygon attribute table (the format of 
the polygon attribute table is shown in table 
5).  In the geologic coverages (wso-geol) and the correlation coverage (wso-corr), 
the identities of the map units from 
compilation sources are recorded in the PTYPE field by map label (table 6).  Map 
units are described more fully in the 
accompanying text file.  In other coverages, various areal information is recorded 
in the PTYPE field (data source region 
number, assemblage number, terrane label, quadrangle name).  Note that ARC/INFO 
coverages cannot contain both point and 
polygon information, so only coverages with polygon information will have a 
polygon attribute table, and these coverages 
will not have a point attribute table.

Table 5.  Content of the Polygon Attribute Tables

ITEM NAME	WIDTH	OUTPUT	TYPE	N. DEC	
	
AREA	4	12	F	3	area of polygon in square meters
PERIMETER	4	12	F	3	length of perimeter in meters
<coverage>#	4	5	B		unique internal control number
<coverage>-ID	4	5	B		unique identification number
PTYPE	35	35	C		unit label


Table 6.  Unit labels (see also Description of Map Units)

Jd                                  					
Jfgs                               
Jk                                  
Jsv
Ju                                  
Jv                                  
KJfc                              
KJfgc                               
KJfgcs
KJfm                                
KJfmg
KJfs                                
KJgv                                
KJgvc                               
KJgvs                               
KJsb
Ka                                  
Kfg
Kfgwy                               
Kfss                                

Kgr                                 
Ks
QTge                                
QTget                               
Qal                                 
Qls                                 
Qm                                  
Qob                                 
Qpoaf                               
Qr                                  
Qs                                  
Qt                                  
TKfgs
TKfs
TKfss
TKu
Tb
Tc
Tdb
Tg                                  
Tls                                 

Tor
Torc
Tors
Tp                                  
Tp?
Tsa
Tsb                                 
Tsm                                 
Twg                                 
Twgt
af
ch                                   
fsr                                 
fsr?
gs                                  
gwy                                 
m                                   
sc                                  
sp                                  
spm                                  
water                               



Note, not every unit label listed is present in every coverage.  For example, 
queried units are not present in the Correlation of 
Map Units coverage (wso-corr).


Points 

Data gathered at a single locality (points) are described in the point attribute 
table (the format of the point attribute table is 
shown in table 7).  The identities of the points from compilation sources are 
recorded in the PTTYPE field by map label 
(tables 8, 9).  Note that ARC/INFO coverages cannot contain both point and polygon 
information, so only coverages with 
point information will have a point attribute table, and these coverages will not 
have a polygon attribute table.

Table 7.  Content of the Point Attribute Tables

ITEM NAME	WIDTH	OUTPUT	TYPE	N. DEC	
	
AREA	4	12	F	3	area of polygon in square meters
PERIMETER	4	12	F	3	length of perimeter in meters
<coverage>#	4	5	B		unique internal control number
<coverage>-ID	4	5	B		unique identification number
PTTYPE	35	35	C		unit label
DIP	3	3	I		dip of bedding or foliation (structure 
coverage only)
STRIKE	3	3	I		strike of bedding or foliation (structure 
coverage only)

Table 8.  Point Types Recorded in the PTTYPE Field for structure coverage (wso-
strc)

wso-strc	
-----------------------------
approx bedding                  
bedding                    
bedding w/tops                        
crumpled bedding                   
flat bedding                      
foliation                             
foliation and bedding                 
ot bedding                       
ot bedding estimated from afar        
vert bedding                
vert foliation and bedding     
vert joint


The geologic point types in the structure coverage are ALACARTE point types that 
correlate with the geologic point 
symbols in the ALACARTE point set ALCGEOL.MRK according to the ALACARTE point 
lookup table.  For more 
information on ALACARTE and its pointsets, see Wentworth and Fitzgibbon (1991).  
The point types in the block coverage 
(wso-blks) indicate specific rock types as listed in table 8a.



Table 8a.  Block Types Recorded in the PTTYPE field and their corresponding 
metamorphic grade as shown by symbol on 
the map (wsomap.ps or wsomap.pdf)

wso-blks
----------------------------------------
high/low grade
-------------------
wso-blks
----------------------------------------
high/low grade
-------------------
bio qtz lith sandstone
blsch metabasalt
blueschist
blueschist?
chert
chert breccia
eclgte + blscht + bio schst
eclogite
foliated green lithic wacke
gar omph eclog w/blsch ovrprnt
garnet blueschist
green chert w/rads
green ribbon chert w/rads
greenstone
greenstone and metachert
greenstone and metatuff
greenstone breccia
greenstone?
greywacke
high grade block
high grade block?
low grade block                     
metachert                            

Low
High
High
High
Low
Low
High
High
High
High
High
Low
Low
Low
Low
Low
Low
Low
Low
High 
High
Low
High

metagreenstone
metagreenstone and gneiss           
metagreywacke
mica schist
mica-lithic wacke
phyllite
pillow greenstone
plag porph andesite
pyrite blueschist
red chert
red chert breccia w/rads
red chert w/rads
red limestone
red ribbon chert
red ribbon chert w/rads
ribbon chert
ribbon chert and chert breccia
schist
schistose greenstone
serpentinite
sheared greywacke
sheared arg w/sp + gs
yellow chert-tuff breccia
High
High
High
High
Low
High
Low
Low
High
Low
Low
Low
Low
Low
Low
Low
Low
High
High
Low
High
High
Low




			
			
				
	



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