By Robert J. McLaughlin, Barry C. Moring, Christopher S. Hitchcock, and Zenon C. Valin

Pamphlet to accompany

Scientific Investigations Map 3395

2018

U.S. Department of the Interior

U.S. Geological Survey

Prepared in Cooperation with Pacific Gas and Electric Company

Framework Geologic Map and Structure Sections along the 

Bartlett Springs Fault Zone and Adjacent Area from Round 

Valley to Wilbur Springs, Northern Coast Ranges, California
U.S. Department of the Interior

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

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Suggested citation:

McLaughlin, R.J., Moring, B.C., Hitchcock, C.S., and Valin, Z.C., 2018, Framework geologic map and structure sections along 

the Bartlett Springs Fault Zone and adjacent area from Round Valley to Wilbur Springs, northern Coast Ranges, California: U.S. 

Geological Survey Scientific Investigations Map 3395, 60 p., https://doi.org/10.3133/sim3395.

ISSN 2329-132X (online)
iii

Contents

Introduction.....................................................................................................................................................1

Present and Previous Studies ............................................................................................................1

Acknowledgments................................................................................................................................1

Geologic Setting.............................................................................................................................................2

Lithologic and Structural Features of the Metamorphic Basement Terranes.....................................4

Franciscan Complex.............................................................................................................................4

Central Belt ...................................................................................................................................4

Pomo Terrane (Upper Cretaceous to Upper Jurassic).................................................4

Marin Headlands-Geysers Terrane (Upper Cretaceous–Upper Jurassic) ...............4

Snow Mountain Volcanic Terrane (Lower Cretaceous–Valanginian or younger)...5

Eastern Belt...................................................................................................................................5

Yolla Bolly Terrane (Upper and Lower Cretaceous to Upper Jurassic, Tithonian) ..6

Pickett Peak Terrane (Lower Cretaceous, Aptian–Barremian)...................................7

Mendocino Pass Terrane (Lower Cretaceous, Valanginian).......................................7

Coast Range Ophiolite (Upper and Middle Jurassic) .....................................................................8

Great Valley Complex ....................................................................................................................................8

Elder Creek Terrane of the Great Valley Complex (Lower Cretaceous and Upper to Middle 

Jurassic) ...................................................................................................................................8

Structural Relations.......................................................................................................................................8

Major Faults...........................................................................................................................................8

Coast Range Fault........................................................................................................................8

Bartlett Springs Fault Zone ........................................................................................................9

Northern Bartlett Springs Fault Zone ..............................................................................9

Displacement on the Northern Bartlett Springs Fault.........................................9

Lake Pillsbury Releasing Bend ..............................................................................10

Seismicity of the Northern Bartlett Springs Fault Zone ....................................11

Fault Creep along the Northern Bartlett Springs Fault......................................13

Southern Bartlett Springs Fault Zone............................................................................15

Faults of the Middle Mountain and Sanhedrin Mountain Areas ..............................16

Faults of the Clear Lake Area..........................................................................................16

Faults Northeast of the Bartlett Springs Fault Zone ...................................................17

Major Folds ..........................................................................................................................................18

Wilbur Springs Dextral Hook ...................................................................................................18

Antiforms and Synforms ..................................................................................................19

Bartlett Springs Synform ........................................................................................19

Wilbur Springs Antiform .........................................................................................20

Pacific Ridge Antiform ............................................................................................20

Bartlett Mountain Antiform ....................................................................................20

Middle Mountain Synform......................................................................................20

Description of Map Units...........................................................................................................................20

Unconsolidated Deposits ..................................................................................................................20

Alluvial Deposits..................................................................................................................................20

Terrace Deposits.................................................................................................................................21

Fan Deposits ........................................................................................................................................21

Glacial Deposits ..................................................................................................................................21

Spring Deposits...................................................................................................................................21

Volcanic Rocks....................................................................................................................................22

Fluvial and Lacustrine Fill ..................................................................................................................22
Figures

1. Map showing location of the map area, the Lake Pillsbury study area of Ohlin and

others (2010), the Bartlett Springs Fault Zone, and other major faults east of the

San Andreas Fault in the Coast Ranges of northern California............................... sheet 1

2. Map showing quadrangles in map area and sources of geologic data....................sheet 2

3. Location of map area in northern California plotted on 30-m hillshade base,

showing principal access roads and nearby towns.................................................. sheet 1

4. Generalized geologic map of northern California showing location of map area,

the Lake Pillsbury study area of Ohlin and others (2010), the Bartlett Springs

Fault Zone, the Maacama Fault Zone, and the distribution of principal belts of 

the Franciscan Complex..................................................................................................sheet 1

5. Map showing major structural features in map area....................................................sheet 2

6. Geologic maps showing progressive deformation of the northern California

margin during transition from subduction to right-lateral strike slip........................sheet 2

7. Schematic reconstruction of Bartlett Springs Synform configuration prior to its

dismemberment along the Northern Bartlett Springs Fault Zone.............................sheet 2

8. Map showing Wilbur Springs Dextral Hook in context of regional structure and

long-term transpressional deformation along the Northern Bartlett Springs

Fault Zone...................................................................................................................................sheet 2

9. View southeast at serpentinite diapir in Bartlett Springs Fault Zone north of Coyote

Rocks and Lake Pillsbury..................................................................................................................11

10. Views southeast across Bartlett Springs Fault Zone north of Coyote Rocks, distinguished

by serpentinite diapir within stream-cut exposure along Salmon Creek................................. 12

11. Map of double-differenced locations for epicenters of the main shock and aftershocks of

the August 10, 2016, M 5.1 earthquake on the Northern Bartlett Springs Fault......................14

12. Seismicity cross sections showing hypocentral depth distribution of earthquakes along

sections t1, t2-6, t-3, t-4, and t-5..............................................................................................sheet 2

13. Histogram showing main shock and aftershocks defining the August 10, 2016, earthquake

sequence on the Northern Bartlett Springs Fault Zone...............................................................15

14. Map showing locations and locality numbers of fossils listed in table 1

(https://doi.org/10.3133/sim3395) and plotted on the geologic map (sheet 1)................... 33

Marine Overlap Deposits...................................................................................................................22

Great Valley Complex ........................................................................................................................22

Elder Creek Terrane...................................................................................................................22

Coast Range Ophiolite........................................................................................................................24

Franciscan Complex...........................................................................................................................25

Central Belt .................................................................................................................................25

Central Belt Mélange terrane.........................................................................................25

Eastern Belt.................................................................................................................................26

Yolla Bolly terrane.............................................................................................................26

Pickett Peak.......................................................................................................................27

Medicine Pass...................................................................................................................27

References Cited..........................................................................................................................................27

Table

1. Paleontological data from Tertiary and Mesozoic rocks of the map area from

Round Valley to Wilbur Springs, northern Coast Ranges, California .................................34

iv
1

Introduction

This geologic mapping investigation was undertaken by the 

U.S. Geological Survey (USGS) in cooperation with Pacific Gas and 

Electric Company (PG&E) under a joint USGS-PG&E CRADA 

agreement to determine the extent, complexity, architecture, and 

evolution of the Bartlett Springs Fault Zone between Clear Lake 

and Round Valley, California. The Bartlett Springs Fault Zone is the 

eastern-most known active member of the San Andreas transform 

margin in northern California (fig. 1, sheet 1) and is of particular 

interest for its apparent long-lived history as a Miocene and older 

subduction-margin fault that, more recently, was reactivated as an 

active, creeping member of the San Andreas Fault system. The 

northern part of the Bartlett Springs Fault Zone is apparently still 

influenced by subduction of the Gorda Plate beneath North America, 

but it is also accommodating strike-slip displacement associated with 

Pacific Plate interaction with North America (fig. 1, sheet 1). South 

of the map area, the Bartlett Springs Fault Zone has been shown to 

step into and merge with active faults of the eastern San Francisco 

Bay region; to the north of the map area and Round Valley, the fault 

zone steps into several other fault zones, such as the Lake Mountain 

Fault (Herd, 1978), that connect with offshore thrust faults of the 

Cascadia subduction margin (Carver, 1987; Clarke, 1990; Clarke and 

Carver, 1992; Lienkaemper, 2010; McLaughlin and others, 2000). 

PG&E owns or operates hydro-electric facilities and infrastructure 

in northern California that are directly or indirectly impacted by the 

Bartlett Springs Fault Zone, so the geologic framework of the region 

along and adjacent to the fault zone as depicted in this study are of 

interest to PG&E for planning and upgrading purposes.

Present and Previous Studies

Previous geologic mapping in the area of this investigation 

has occurred sporadically for more than six decades, with focus 

on different aspects of the framework geology at different times, 

and most of the mapping has not been formally published (see 

fig. 2, sheet 2). These studies include investigations in the 1960s 

to 1970s, mostly at 1:62,500 scale, by the California Department 

of Water Resources (CDWR) for construction of tunnels to 

divert and transport water from the Eel River drainage system; 

quadrangle mapping in the 1950s and 1970s by Ph.D. students at 

the University of Texas, Austin, and Stanford University; mapping 

by John Suppe and students at Princeton University in the 1960s 

to 1980s; and some M.S. thesis mapping of a few small areas in 

the 1970s and 1980s by students at California State University, 

Hayward, and at San Jose State University.

A large part of this geologic mapping investigation has involved 

the compilation of previously unpublished mapping and its merging 

with published mapping by the USGS in the Covelo 30ʹ × 60ʹ 

quadrangle to the north (Jayko and others, 1989), with 1:30,000-scale 

(Ohlin and others, 2010) and 1:62,500-scale (Etter, 1979) mapping 

in the Lake Pillsbury region, and with 1:24,000-scale mapping in the 

Clear Lake region to the south (for example, Hearn and others, 1995; 

McLaughlin and others, 1990; McLaughlin, 1978).

In 2012–14, new geologic reconnaissance mapping was 

conducted across the Bartlett Springs Fault Zone corridor in 

Mendocino National Forest and adjacent areas north of Clear 

Lake. For this new field work, we traversed the region on Forest 

Service and county roads (fig. 3, sheet 1), collecting structural, 

stratigraphic, and lithologic data and sampling for petrographic 

work. We used Google Earth and high-altitude air-photo imagery 

to delineate approximate boundaries, distribution, and continuity 

of mélange terrains, intact rock units, and ultramafic rocks 

composing the Mesozoic and Tertiary basement of the map area. 

We also delineated major landslides and linear features associated 

with faulting (fig. 4, sheet 1; sheets 1, 2). For some traverses, a 

computer operated from our field vehicle enabled us to enter field 

data into a digital database in the field. For other traverses, field 

data was not entered until return to the office.

Christopher Hitchcock conducted mapping of the Quaternary 

geology in northern Clear Lake basin, Potter Valley, and associated 

subsidiary basins and drainages in 2012–14. This mapping, 

delineating the stratigraphy of Quaternary fluvial, stream, and 

terrace deposits, was integrated with the mapping of McLaughlin 

and Moring during the compilation process. Major landslide 

deposits mapped by McLaughlin and Moring and separately by 

Hitchcock were combined in the map database. Only large, major 

landslide deposits are shown on this geologic map. The reader 

is herein referred to detailed landslide and slope stability maps 

by the California Geological Survey (http://www.conservation.

ca.gov/cgs/geologic_hazards/landslides) and the USGS (https://

www2.usgs.gov/natural_hazards/) to determine more accurately 

the hazard posed by landsliding in this region. Faults interpreted 

as geomorphically youthful, active parts of the Bartlett Springs 

Fault Zone, shown by red fault lines on the geologic map, are 

incorporated from Lienkaemper (2010).

In this report, geologic ages and designations of Mesozoic 

periods and epochs are taken from the Geologic Time Scale of the 

Geological Society of America (Walker and others, 2012; Geologic 

Time Scale, v. 4.0, Geological Society of America, https://doi.

org/10.1130/2012.CTS004R3C). Cenozoic and Quaternary epochs 

are based on their assignments by the U.S. Geological Survey 

Geologic Names Committee, 2010.

Acknowledgments

The mapping investigation was funded from a Cooperative 

Research and Development Agreement (CRADA) between the 

Earthquake Hazards Program of the U.S. Geological Survey 

(USGS) and Pacific Gas and Electric Company (PG&E) as a 

follow-up to an earlier USGS-PG&E CRADA-funded geologic 

mapping investigation of the Bartlett Springs Fault Zone in the 

Lake Pillsbury area (Ohlin and others, 2010). The authors have 

appreciated the continued interest and encouragement from Scott 

Steinberg (PG&E Geosciences Department) in this investigation. 

Steinberg also provided access to several unpublished reports 

prepared by PG&E geotechnical consultants, as well as 

photographs taken from a helicopter reconnaissance of the region 

by Steinberg and Hitchcock in 2012 that were helpful in our map 

compilation.

Diane Moore (USGS Earthquake Science Center) provided 

the senior author with expert consultation and corroboration on 

mineralogy and petrography of serpentinitic fault rocks along the 

Bartlett Springs Fault Zone at several localities in the map area, as 

well as help with numerous petrographic issues with the Franciscan 

Complex.

Large wildfires that prompted extended road closures in 

Mendocino National Forest during the course of our mapping 
2

study made it critical to establish good communications with the 

U.S. Forest Service. We are grateful to them for their cooperation, 

particularly personnel in the Upper Lake (A. Downhour and D. 

Mackintosh), Willows (R.P. Mikulovsky), and Red Bluff (D.R. 

Elder and J.A. Delafuente) offices, for providing us with updated 

Forest Service road maps, road conditions, gate access, and contact 

information for private landowners within Mendocino National 

Forest. We thank several landowners in the map area for providing 

access to their land in the course of mapping, as well as information 

on the drivability of access roads. Northeast of Willits, we thank the 

Adams family, owners of Emandel Resort for access to their land 

and for providing information on access along other parts of the Eel 

River. In Rice Valley and the McCloud Creek area, the Kompf family 

and Dave Rinker were helpful in allowing access to their land in Rice 

Valley and sharing a long-term knowledge of this area.

This mapping investigation has cooperated with the Geology, 

Minerals, Energy, and Geophysics Science Center (GMEG) Delta 

project (R. Graymer, USGS project lead), a 3–D framework geologic 

mapping investigation that includes the area between Wilbur Springs 

to the north and Lake Berryessa to the south, as well as the subsurface 

of southwestern Sacramento Valley and the San Francisco Bay part of 

the Sacramento-San Joaquin Delta.

The report was peer reviewed for scientific content by P.A. 

Stone and B. Melosh (both at GMEG Science Center, USGS).

Geologic Setting

The area of this geologic mapping investigation is in 

the northern coast ranges of California. Complexly deformed 

Mesozoic and lower Cenozoic rocks of the Franciscan Complex, an 

accumulation of sedimentary, metamorphic, and igneous rocks that 

were assembled in a subduction zone and accreted to the western 

continental margin between the Late Jurassic and Miocene, underlie 

the region. Oceanic rocks of the Coast Range ophiolite that form 

the basement of coeval fore-arc strata of the Great Valley complex 

structurally overlie the Franciscan Complex. Subduction continues at 

present in the modern Cascadia subduction margin northwest of the 

map area but has been largely replaced by dextral strike-slip faulting 

associated with northward propagation of the San Andreas transform 

system in the map area.

A consequence of the very long history of deposition of strata in 

the Franciscan Complex is that these rocks were intensely deformed 

during several episodes of subduction-accretion that formed three 

broad, westward-younging structural belts. These belts, the Eastern, 

Central, and Coastal Belts, are younger, but overlap in age westward, 

and pressure/temperature (P/T) conditions from metamorphic mineral 

assemblages in the rocks in each belt imply that the depth to which 

the rocks were subducted increases eastward. Due in part to oblique 

subduction-accretion, some of the Franciscan rocks, particularly 

oceanic basaltic rocks and their overlying pelagic sedimentary 

covers in the Central Belt, have undergone relatively large northward 

translation along the eastern Pacific Ocean margin since the Jurassic. 

This complex structural framework was followed in the Early to 

middle Cretaceous (Hauterivian–Albian), and even later in the Late 

Cretaceous (Maastrichtian) to early Tertiary (early Eocene), by uplift 

of the subduction complex, extensive detachment faulting in the 

overlying ophiolitic and lower sedimentary section of the fore arc, 

and unroofing of the deeply subducted Eastern and Central Belts of 

the Franciscan Complex. This unroofing of the Coast Ranges and 

exposure of deeply subducted Franciscan rocks to erosion has been 

linked to shallowing of Farallon Plate subduction during the Laramide 

orogeny (Saleeby, 2003; Dickinson and others 1979). In northern and 

central California, this was accompanied at depth by east-directed 

emplacement of Franciscan rocks as tectonic wedges into the western 

Great Valley fore arc and the mafic igneous and metamorphic rocks of 

the Sierra Nevada batholith (Ernst and McLaughlin, 2012; Wentworth 

and others, 1984; Wentworth and Zoback, 1990).

The Franciscan Complex is structurally overlain in the map 

area by the Jurassic Coast Range ophiolite, which is highly disrupted 

as the result of (1) extensional shearing and serpentinization of the 

structurally low ultramafic section of the ophiolite during its upward 

migration from mantle to midcrustal levels in the Jurassic and (2) later 

contraction and extension that corresponded with unroofing of the 

Franciscan Complex in the Late Cretaceous and Tertiary. Although 

still debated, the ophiolitic rocks are widely considered to have 

formed in a fore-arc, supra-subduction setting east of the paleotrench 

in which the Franciscan Complex was assembled (Shervais and 

others, 2005a, b; Ernst and McLaughlin, 2012). Serpentinite matrix 

mélange that forms a large part of the ultramafic section of the Coast 

Range ophiolite and is attributed to low-angle extension (Jayko 

and others, 1987) may result both from the Jurassic and the Late 

Cretaceous–early Tertiary deformation.

Unmetamorphosed sedimentary rocks of the Upper Jurassic 

Great Valley fore arc were initially deposited unconformably on 

extrusive and high-level intrusive rocks of the ophiolitic basement of 

the map area. The Upper Jurassic to Lower Cretaceous (Hauterivian 

and younger) part of this sedimentary section, however, was 

subsequently deformed by folding and attenuated with the underlying 

ophiolite.

This deformation resulted in incorporation of the lower Great 

Valley sedimentary section into a regionally extensive thick zone 

of mélange (here mapped as ophiolitic mélange; earlier mapped as 

Grizzly Creek mélange terrane near Wilbur Springs by McLaughlin 

and Ohlin, 1984) consisting of rocks of the Coast Range ophiolite 

tectonically mixed with Upper Jurassic and Lower Cretaceous 

strata of the Great Valley sedimentary section (McLaughlin and 

others, 1990; also see photos of “Grizzly Creek mélange terrane” 

in McLaughlin and Ohlin, 1984). Significant penetrative shearing 

in the serpentinized ultramafic rocks of the map area may reflect 

upward migration of these rocks to the ocean floor from mantle 

depths in the Jurassic. Continued unroofing of ophiolitic basement is 

reflected in the Late Jurassic (Oxfordian–early Tithonian) deposition 

of coarse megabreccia derived from the Coast Range ophiolite at 

the base of the Great Valley complex sedimentary section. Exposure 

of serpentinite on the seafloor during early Great Valley complex 

evolution further led to deposition of sedimentary serpentinite (unit 

Kssp) in Valanginian–Hauterivian time.

Detachment faulting rooted along the serpentinized base 

of the Coast Range ophiolite (Jayko and others, 1987), which 

involved both the ophiolite and the lower Great Valley complex 

(McLaughlin and others, 1990), produced ophiolitic mélange 

(unit KJom). The timing of extension associated with formation 

of the ophiolitic mélange was Early Cretaceous (Valanginian) or 

younger, based on the youngest fossils present in sedimentary 

slabs and blocks in the mélange (McLaughlin and others, 1990). 

The somewhat younger (Valanginian–Hauterivian) sedimentary 

serpentinites and associated complex folding in the overlying 
3

lower Great Valley section could reflect extensional deformation 

in the stratigraphically lower part of the section. Erosion of 

the unroofed Coast Range ophiolite apparently resulted in 

deposition of ophiolite-derived sediment in the fore-arc basin as 

olistostromal deposits of serpentinitic breccias and sandstones 

(”sedimentary serpentinites” of this report). Early Cretaceous 

(Hauterivian) cold-seep-related fossil faunas are associated with 

some of the sedimentary serpentinites, but fossils this young are 

not known as part of the ophiolitic mélange (McLaughlin and 

others, 1990; Campbell and others, 1993; Campbell and others, 

2002, 2008; Kiel and others, 2008).

Others (for example, Macpherson and Phipps, 1988)

have interpreted both the serpentinized ultramafic rocks and 

ophiolitic mélange units of this report to be olistostromal 

and explain their penetrative fabrics as formed by large-scale 

gravitational collapse. This implies that penetrative deformation 

of the serpentinized ultramafic rocks (unit Josp) and ophiolitic 

mélange (unit KJom) was a process that occurred in the shallow 

lithosphere. Although near-surface sedimentary processes 

are clearly associated with deposition of Lower Cretaceous 

(Hauterivian) sedimentary serpentinites (unit Kssp) in the 

lower Great Valley complex, tectonic shearing and boudinage 

in the serpentinized ultramafic rocks (unit Josp) and ophiolitic 

mélange (unit KJom) lack clear evidence of a sedimentary origin 

and, as pointed out, some of the penetrative deformation in the 

serpentinized ultramafic rocks was likely imposed during upward 

migration of the serpentinite from the mantle to the ocean floor 

in the Jurassic and later. Detachment and attenuation faulting 

in the serpentinized ultramafic rocks and ophiolitic mélange 

units is here viewed as too deep-seated to be easily relatable to 

near-surface processes. The mélange process may be linked to 

progressive shallowing of subduction and unroofing of the Coast 

Ranges that continued into the Late Cretaceous and early Tertiary 

during the Laramide orogeny.

Both the ophiolitic mélange and the sedimentary serpentinite 

in the Lower Cretaceous section of the Great Valley complex 

resulted from deep unroofing of the Coast Range ophiolite and 

(or) buried mafic basement beneath the northern Sacramento 

Valley (Jayko and others, 1987; Wentworth and others, 1984; 

Macpherson and Phipps, 1988). In contrast to sedimentary 

processes involved with deposition of the sedimentary 

serpentinite, the ophiolitic mélange, instead, may have formed as 

a series of low-angle detachments associated with extension at 

depth. Extension that triggered unroofing of ophiolitic basement 

must have followed Valanginian basinal deposition of Great 

Valley strata incorporated into the ophiolitic mélange. In the 

Hauterivian, deposition of unroofed and recycled ophiolitic 

debris occurred in the fore arc, including olistostromal deposits 

and sedimentary serpentinite. Fluids associated with cold-seeprelated fossil faunas in the sedimentary serpentinites may well 

have been channeled along detachment structures.

Another major pulse of uplift and unroofing of Franciscan 

Complex basement in the region occurred between the latest 

Cretaceous (Maastrichtian) and late Paleocene, as indicated 

by local remnants of Paleocene to Eocene fore-arc-basin strata 

preserved near Covelo and Rice Valley and in the Lower Lake 

area that are unconformable on underlying Great Valley strata. 

In Rice Valley, a local conglomerate unit at the base of upper 

Paleocene strata that is unconformable on the Upper Cretaceous 

section contains clasts of lawsonitic metagraywacke derived from 

the Franciscan Complex. This relation suggests (for example, 

Berkland, 1973) that the Franciscan source of the lawsonitic 

metasandstone was exhumed from subducted depths greater than 

25 km between the latest Cretaceous (~66 Ma) and late Paleocene 

(~56 Ma).

In the late Miocene (~8–5 Ma), following this major 

tectonism over the subduction margin, faulting in the northern 

Coast Ranges between Covelo and Clear Lake began to link 

southward to the northwardly propagating transform system. 

Faults of the transform system to the south extended into the 

accretionary margin inboard of the present San Andreas Fault 

and Mendocino Triple Junction. The Bartlett Springs Fault Zone 

appears to be a fault zone present in the Miocene and older 

subduction margin that provided a pre-existing zone of crustal 

weakness that accommodated the transition from dominant 

subduction convergence to dominant strike slip, with northward 

propagation of the main San Andreas transform and its more 

inboard segments (the Hayward-Calaveras-Rodgers Creek-West 

Napa-Maacama and the Concord-Green Valley-Bartlett Springs 

Fault Zones).

Starting about 4 Ma, a 70-km-wide band of the northern 

California margin began its northwest-directed transition from 

a margin dominated by head-on to oblique subduction-driven 

convergence to one dominated by transpression and transtension 

associated with steep faults of the northwardly propagating San 

Andreas Fault system. The modern Bartlett Springs Fault Zone 

appears to have developed largely during this period of transition 

from subduction to strike-slip tectonics. Favorably oriented faults 

of the Bartlett Springs Fault Zone that initially were active as part 

of the subduction margin in the Miocene were re-occupied by 

steeper dipping, discontinuous, stepping strike-slip faults of the 

transform margin.

The geologic map and accompanying structure sections 

delineate the course of the active Bartlett Springs Fault Zone 

over a distance of ~110 km, from the vicinity of Clear Lake 

northwestward to Covelo. The fault zone is divided into 

northern and southern segments, separated by a prominent 

right step, across which the two fault zone segments overlap. 

The northern Bartlett Springs Fault Zone appears to follow a 

complex northwest-oriented path of crustal weaknesses that 

were established in the subduction margin realm, prior to 

initiation of strike-slip faulting. The southern Bartlett Springs 

Fault Zone (fig. 5, sheet 2), which includes the southwest side 

of a 2.5-km-wide right step southeast of State Highway 20 near 

Grizzly Spring, departs from the prominent zone of pre-transform 

deformation and follows a less maturely developed path that cuts 

across the structural grain of the older pre-transform margin. 

The southeasternmost mapped extent of the northern Bartlett 

Springs Fault Zone segment east of the right step, separating it 

from the southern segment, overlaps the southern segment for 

~5 km. The overlapping part of the northern segment, however, 

does not exhibit geomorphically youthful fault features (see later 

description of northern and southern Bartlett Springs Fault Zone). 

Farther southeastward, this overlapping segment of the northern 

Bartlett Spring Fault Zone may die out as a bedding planeparallel fault within strata of the Mesozoic Great Valley complex, 

though its full extent is unknown and the focus of more detailed 

ongoing investigations by the USGS.
4

Lithologic and Structural Features of the 

Metamorphic Basement Terranes

Franciscan Complex

The Franciscan Complex is considered to represent 

the “type” subduction-zone rock assemblage of the eastern 

Pacific basin rim (for example, Ernst and McLaughlin, 2012). 

Subdivisions of the Franciscan Complex are based on their field 

and aerial map expression, integration of numerous informal 

local unit names applied to various parts of the Franciscan at 

different times, and regional geologic mapping and geophysical 

investigations (Jayko and others, 1989; McLaughlin and others, 

2000; Blake and others, 1992, 2000; Lanphere and others, 2007, 

2011). The Correlation of Map Units (sheet 2) summarizes the 

tectonostratigraphic framework of these rocks as used in this 

report. Regionally, the Franciscan Complex is divided into three 

thick, eastwardly inclined and folded, sheet-like belts, containing 

rocks that generally become younger but overlap in age 

westward. These belts, referred to from northeast to southwest as 

the Eastern, Central, and Coastal Belts (Irwin, I960; Bailey and 

others, 1964), include rocks that were metamorphosed at high 

P/T conditions (for example, blueschist facies, deeply subducted 

to tens of kilometers) in the Eastern Belt, at intermediate P/T 

conditions (for example, low greenschist facies burial conditions) 

in the Central Belt, and at much lower P/T conditions in the 

Coastal Belt (for example, zeolite to prehnite-pumpellyite facies), 

where maximum structural burial was only to a few kilometers 

(Ernst and McLaughlin, 2012). Rocks assigned to the Eastern and 

Central Belts of the Franciscan Complex are present in the map 

area, but the Coastal Belt is at the surface only to the west of the 

map. If conceptual views of crustal structure are valid, however, 

it may underlie rocks of the Central Belt at some depth in the 

western part of the map area.

Several subsidiary map units are recognized in the Eastern 

and Central Belts, which we consider to be fault-bounded and 

which we refer to as tectonostratigraphic terranes of the Eastern 

or Central Belts. We also recognize subunits of mélange in both 

the Eastern and Central Belts that may be of different age and 

which are mapped as different units. Mélanges in both belts, 

however, contain similar high-grade blueschist blocks (for 

example, glaucophane schist, eclogite, and amphibolite) that 

may be derived from the same or similar protoliths. The matrix 

material of these mélange terranes in the Eastern and Central 

Belts differ in metamorphic grade, consistent with metamorphism 

of the belts in which they occur.

Central Belt

The Central Belt is the westernmost belt of the Franciscan 

Complex exposed in the map area. West of Ukiah valley and 

U.S. Highway 101, it is underlain structurally by the Coastal 

Belt beneath a regionally warped low-angle fault referred to 

as the Coastal Belt Thrust. In the map area, the Central Belt 

comprises at least 30 to 40 percent of the surface exposures 

of Franciscan Complex rocks, but it is inferred to structurally 

underlie the entire area west of Sacramento Valley. The Central 

Belt is predominantly a large-scale mélange that, along with 

meter- to tens-of-meter-scale sheared blocks of blueschist, 

eclogite, amphibolite, metasandstone, metachert, metaigneous, 

and ultramafic rocks, also encloses numerous kilometer- to tensof-kilometers-scale slabs of broken metasandstone and argillite 

± pelagic chert or limestone ± basaltic volcanic rocks ± other 

mafic to ultramafic parts of the Mesozoic ocean floor. Many slabs 

enclosed by mélange are composed solely of metasandstone and 

argillite. These various metasedimentary slabs have metamorphic 

signatures that vary from strongly to moderately foliated 

blueschist grade to unfoliated low- to medium-greenschist 

grade (for example, pumpellyite and prehnite-pumpellyitebearing) and minor zeolite-grade (laumontite-bearing) rocks. The 

surrounding mélange matrix is no higher than low-greenschist 

grade (pumpellyite-bearing) to perhaps incipient blueschist grade 

(traces of lawsonite) locally.

In the map area, we divide the Central Belt into (1) an 

undivided mélange matrix unit (Central Belt Mélange terrane) 

and (2) numerous large slab-like bodies of rock enclosed by 

the undivided mélange matrix. These slabs are mapped either 

as generic lithotectonic terrane units or, locally, as named 

terranes (Pomo terrane, Marin Headlands-Geysers terrane, 

and Snow Mountain volcanic terrane), where a distinctive 

tectonostratigraphy is evident.

Pomo Terrane (Upper Cretaceous to Upper Jurassic)

This unit is recognized only east of Potter Valley, in 

the eastern part of the Central Belt, along the contact with 

serpentinite and structurally overlying Cretaceous strata of 

the Great Valley complex of Middle Mountain. The rocks of 

the Pomo terrane appear to represent slabs of a dismembered 

seamount (Berkland, 1972, 1978). The lower part of this section 

consists of alkalic basaltic pillow flows, flow breccias, and 

intrusive diabase, overlain locally by, or intercalated locally with, 

radiolarian chert. Radiolarians in the chert are Jurassic, equivalent 

in age to a part of the nearby Geysers chert section. However, the 

basaltic rocks are overlain unconformably by a thin carbonatecemented bioclastic basalt-clast breccia, sandstone, and shale 

containing a latest Cretaceous (Maastrichtian) mollusk fauna 

that, in turn, is faulted against serpentinite and overlying Upper 

Cretaceous Great Valley complex strata of the Middle Mountain 

block or enclosed by undivided mélange of the Central Belt.

Marin Headlands-Geysers Terrane (Upper Cretaceous–

Upper Jurassic)

The Marin Headlands-Geysers terrane is named for the 

two areas south of the area of this map where these rocks have 

been studied in great detail (McLaughlin and Pessagno, 1978; 

Pessagno, 1977; Murchey, 1984; Murchey and Jones, 1984; 

Hagstrum and Murchey, 1993). In the Marin Headlands area 

and at The Geysers, east of Healdsburg, the terrane consists 

of a sequence of oceanic basaltic flows and breccia overlain 

by a section ~67 m (The Geysers) to 80 m (Marin Headlands) 

thick of pelagic radiolarian chert that, in turn, is depositionally 

overlain by metasandstone turbidites derived from the Cretaceous 

continental margin. The radiolaria in the chert sections of the 

Marin Headlands-Geysers terrane have been studied in great 

detail and shown to represent a condensed pelagic section 

deposited on basaltic ocean floor at equatorial latitudes, far 
5

from the continental margin, between the Middle Jurassic 

(Pliensbachian, ~190 Ma) and the early Late Cretaceous (early 

Cenomanian or late Albian, ~100 Ma). The terrane is widely 

considered to represent the remnants of an extensive large 

oceanic igneous province such as an oceanic plateau, similar 

to the large plateaus preserved on the west side of the Pacific 

Ocean basin. The overlying metasandstone, of low-greenschist 

(pumpellyite) grade in these areas, preserves the initial encounter 

of the oceanic rocks with the North American Plate subduction 

margin. The metasandstone turbidites were deposited on the 

oceanic basalt and chert no earlier than Cenomanian time in the 

Marin Headlands, based on an ammonite (Murchey and Jones, 

1984) and recent dating of detrital zircons in the metasandstone 

(maximum depositional age of ~100 Ma; McPeak and others, 

2015). At The Geysers, the chert section is virtually identical in 

age to the Marin Headlands based on the radiolarian zonation 

(Murchey and Jones, 1984; Hagstrum and Murchey, 1993). The 

overlying metasandstone at The Geysers is petrographically 

similar to the Marin Headlands based on point-count data 

(McLaughlin and Ohlin, 1984), but an analysis of detrital 

zircons in the metasandstone showed that the youngest zircon 

population is 114 Ma (data of G. Gehrels, cited in Dumitru and 

others, 2015), considerably older than the youngest radiolaria 

in the underlying Geysers area chert section. This is explained 

by recognizing that the detrital zircon data from The Geysers 

provides only a maximum depositional age for the metasandstone 

overlying the chert and only coincidentally will date actual 

deposition of sediment transport systems. However, the similarity 

of framework grain compositions in metasandstones from the 

Marin Headlands and The Geysers (McLaughlin and Ohlin, 

1984) is taken to indicate that the clastic rocks in both areas 

were derived from a similar set of source regions but may have 

interacted at somewhat different times with the oceanic rocks 

due to the large original aerial extent of the chert-basalt terrane 

(suggested by distribution of similar rocks from Baja California 

to southern Oregon) or possibly the oceanic rocks had an 

orientation that was oblique to the margin during accretion.

We have identified several semi-intact, large slabs of broken, 

complexly intercalated basalt, radiolarian chert, and metasandstone 

that extend into the map area from The Geysers region to the south 

and are surrounded by other rocks of the Central Belt. We have 

assigned these rocks to the Marin Headlands-Geysers terrane, 

where we have radiolarian ages (table 1; sheet 1) from the cherts, 

and the basalt-chert-metasandstone relations seem consistent 

with their correlation to the Marin Headlands-Geysers terrane. 

Many other large to small blocks of basalt, undated chert, and 

metasandstone that are enclosed by mélange and adjacent to or 

near the rocks we assign to the Marin Headlands-Geysers terrane 

may also be pieces of this terrane. However, they are instead 

designated only as blocks of chert, basalt, or metasandstone where 

there is no data to confidently correlate the rocks with the Marin 

Headlands-Geysers terrane.

Snow Mountain Volcanic Terrane (Lower Cretaceous–

Valanginian or younger)

The Snow Mountain volcanic terrane is an unusual 

sequence of metamorphosed rhyolitic to basaltic volcanic and 

volcaniclastic rocks interpreted to have formed in a seamount 

setting (Macpherson, 1983). These rocks were earlier mapped 

and assumed to be a structural outlier of a part the Stonyford 

section of the Coast Range ophiolite (Brown, 1964). Subsequent 

mapping and petrologic and geochemical study of these rocks by 

Macpherson (1983), however, showed that the Snow Mountain 

rocks have undergone low-blueschist-grade metamorphism and 

thus were subducted with rocks of the Franciscan Complex. 

Map relations and modeling of the geometry of a thin sheet of 

ultramafic rocks beneath the Snow Mountain rocks (Griscom, in

Brown and others, 1981), suggest that they structurally overlie 

or are enclosed by mélange of the Central Belt that bounds these 

rocks to the north, west, and south. Rocks of the Yolla Bolly 

and Pickett Peak terranes of the Eastern Belt also structurally 

overlie the Central Belt regionally. However, metavolcanic 

rocks like those described from the Snow Mountain volcanic 

terrane have not been recognized in the Yolla Bolly terrane, and 

metavolcanic rocks of the Pickett Peak terrane are of higher 

blueschist grade than the Snow Mountain volcanic terrane. We 

therefore assign the Snow Mountain volcanic terrane to the 

Central Belt, although we acknowledge that the Snow Mountain 

rocks could actually represent a seamount accreted with the Yolla 

Bolly terrane rather than part of the Central Belt. Early work (for 

example, Brown, 1964) also assumed a Jurassic age for the Snow 

Mountain volcanic terrane based on an erroneous correlation of 

metacherts associated with the Snow Mountain volcanic terrane 

with the radiolarian fauna of cherts in the Coast Range ophiolite 

at Stonyford. Efforts to separate radiolarians from metachert in 

the Snow Mountain volcanic section have been unsuccessful (B. 

Murchey, oral commun., 2015). Consequently, at the time of this 

writing, reliable age data for the Snow Mountain volcanic terrane 

do not exist.

Eastern Belt

The Eastern Belt represents the structurally highest belt of 

the Franciscan Complex and was the earliest assembled of the 

three belts. It apparently was subducted to deeper levels, with 

accompanying higher blueschist-grade metamorphism than in 

the Central and Coastal Belts (Ernst and McLaughlin, 2012; 

Dumitru and others, 2010, 2015; Blake and others, 1988). In 

northern California, development of the higher blueschist-grade 

metamorphic mineral assemblages was commonly accompanied 

by textural reconstitution of the metasedimentary rocks to 

textural zones 2–3 (Blake and others, 1967) and conversion of 

the metasedimentary rocks to slate, phyllite, and schist, with 

increasing textural grade. In general, the increase in textural 

reconstitution and in P/T relations based on metamorphic 

mineral assemblages has been shown to increase structurally 

upward in parts of the Eastern Belt that are thrust beneath the 

Klamath Mountains (Blake and others, 1967). This structurally 

upward increase in metamorphic grade (“inverted-metamorphic 

zonation”) is interrupted in places by low-angle faults that may 

repeat or truncate the gradual upward progression from lowto high-metamorphic mineral assemblage and textural grade 

(Blake and others, 1967; Worrall, 1981). In parts of the Diablo 

Range south of San Francisco Bay, this relation is not so clear; 

here, high P/T neoblastic blueschist minerals, including jadeitic 

pyroxene, are present in metasandstones that are not highly 

reconstituted (Raymond, 2014; Ernst, 1971; Wentworth and 

others, 1999). This suggests that kinematic relations between 
6

metamorphic mineral facies and development of metamorphic 

fabric do not correspond everywhere to equivalent burial depths.

In the map area, we divide the Eastern Belt into the three 

tectonostratigraphic terranes described below.

Yolla Bolly Terrane (Upper and Lower Cretaceous to Upper 

Jurassic, Tithonian)

The Yolla Bolly terrane is named for rocks north of the 

map area in the Yolla Bolly Mountains Wilderness area of the 

northern Coast Ranges (Blake and Jayko, 1983; Jayko, 1984; 

Blake and others, 1985; Worrall, 1981). These rocks represent 

the structurally lower part of the Eastern Belt. Previous workers 

have used various local names (such as the Taliaferro Complex 

of Suppe, 1973; the Pacific Ridge Complex of Suppe and Foland, 

1978; and the East Clear Lake and Little Indian Valley terranes of 

McLaughlin and Ohlin, 1984) to describe the character of these 

rocks in different areas. We include all of these locally named 

rock units in the Yolla Bolly terrane of this report. Inclusion of 

rocks in the Little Indian Valley terrane (McLaughlin and Ohlin, 

1984) in the Yolla Bolly terrane, however, raises some unresolved 

issues with the depositional age assignment of the Yolla Bolly 

terrane.

The Yolla Bolly terrane is divided into two major units 

in the map area: a structurally low mélange (unit fym) and a 

higher unit (unit fys) of more or less coherent rocks (broken 

formation). The mélange unit comprises slaty, sheared, and 

foliated metasandstone and argillite containing blocks and slabs 

of metagreenstone, metachert, and metaconglomerate, rare blocks 

of high-grade blueschist and amphibolite locally containing 

garnet, and rare lenses of metaserpentinite found along shear 

planes associated with the mélange fabric. The mélange is 

interpreted as a continuous, thick, fault-bounded, folded layer 

(see structure sections, sheet 2) that overlies structurally lower 

mélange of the Central Belt. Given this relation, some blocks 

in this mélange of largely Eastern Belt sedimentary rocks 

may contain some high-grade blueschist blocks incorporated 

(possibly plucked from) the underlying mélange of the Central 

Belt during emplacement of the Yolla Bolly terrane (and eastdirected wedging of the Central Belt beneath the Eastern Belt). In 

particular, high-grade blueschist and amphibolite blocks and slabs 

in the area of Goat Mountain along the crest of the Pacific Ridge 

antiform (see map, sheet 1) are here viewed as a part of the Yolla 

Bolly terrane mélange unit. This interpretation differs from the 

suggestion of others (Suppe and Foland, 1978; Ernst and others, 

1970; Coleman and Lanphere, 1971) that the Goat Mountain 

rocks represent structurally overlying thrust remnants of a highergrade-blueschist metamorphic terrane, perhaps part of a schuppen 

complex (Suppe and Foland, 1978).

The structurally higher part of the Yolla Bolly terrane 

consists primarily of metasandstone and argillite complexly 

intercalated locally with a unit of thin metabasalt flows and 

breccias overlain by tuffaceous metachert. The sequence of 

metabasalt-metachert-metasandstone is further characterized 

by development of a slaty cleavage that is in many places axial 

planar to isoclinal folding of the rocks. These rocks, together 

with the mélange, are further characterized as having a planar 

metamorphic fabric and have undergone textural reconstitution 

varying from high textural zone 1 to high textural zone 2 (Blake 

and others, 1967) during subduction. Textural reconstitution 

was accompanied by development of neoblastic pumpellyite 

in textural zone 1 to low textural zone 2 metaclastic rocks and 

pumpellyite + lawsonite ± jadeite ± sodic amphibole in rocks 

of low to high textural zone 2 (Ernst and McLaughlin, 2012; 

McLaughlin and Ohlin, 1984; Ohlin and others, 2010; Blake and 

others, 1967; McLaughlin and others, 1990). The metamorphic 

fabric appears to have developed prior to development of the 

mélange, and for this reason we believe the mélange is related to 

post-depositional tectonic processes such as uplift and unroofing 

of the Franciscan Complex and (or) to tectonic wedging.

Locally, some unusual alkalic and mafic intrusive rocks are 

present in the map area and elsewhere in the Yolla Bolly terrane 

that have been studied petrologically and dated by 40Ar/39Ar and 

U-Pb methods (Layman, 1977; Echeverria, 1980; Mattinson 

and Echeverria, 1980; Mertz and others, 2001). In the Monkey 

Rock area, these intrusive rocks intrude slaty metasandstone and 

argillite and metachert and display chilled margins (Layman, 

1977; Ohlin and others, 2010). North of the map area, similar 

field relations are also present in a slab of Yolla Bolly terrane 

rocks enclosed in Central Belt mélange at Island Mountain 

(McLaughlin and others, 2000, table 1; Koski and others, 1993; 

Stinson, 1957). An intrusion at Leech Lake Mountain north of the 

map area yielded an ~119 Ma 40Ar/39Ar age (Mertz and others, 

2001); whereas, a U-Pb age of 95 Ma on zircon was determined 

for gabbroic intrusive rocks near Ortigalita Peak in the Diablo 

Range south of the map area (Mattinson and Echeverria, 1980). 

A single blueschist metamorphic event that followed folding of 

wall-rock greywacke and post-folding intrusion of the gabbro 

was dated at 92 Ma (Turonian) in the Ortigalita Peak area. 

Although the timing of these intrusions into widely separated 

rocks correlated with the Yolla Bolly terrane extends over 24 

m.y., these rocks have similar alkalic geochemistry (Ohlin and 

others, 2010).

Fossils and detrital zircons from metasedimentary rocks, 

including metachert and metasandstone of the Yolla Bolly 

terrane, imply a long depositional history (from ~155 to 94 Ma) 

beginning in the Late Jurassic (Kimmeridgian–Tithonian), based 

on radiolarians in the metacherts and several localities of Buchia

in the Little Indian Valley area northwest of Wilbur Springs. 

Deposition continued through the Early and middle Cretaceous 

(Albian) and into earliest Late Cretaceous (Cenomanian) based 

on radiolarians in the metacherts, on megafossils (an Inoceramus, 

see Ohlin and others, 2010), on detrital zircon data in the map 

area and to the north and south (Dumitru and others, 2010, 2015), 

and on the ages of mafic intrusive rocks described above. Several 

localities in the Yolla Bolly terrane that have previously yielded 

Buchias of Late Jurassic to Early Cretaceous age (Tithonian–

Valanginian), have been shown to be enclosed by metasandstone 

that yielded much younger (111–104 Ma) detrital zircons 

(Dumitru and others, 2015), prompting the proposal that all of 

the Jurassic and Early Cretaceous fossils from the Yolla Bolly 

terrane clastic section are redeposited and not accurate indicators 

of maximum depositional age. The Buchias from the Little Indian 

Valley area, however, were found to be in strata that yield detrital 

zircons consistent with a Tithonian–Valanginian (~152–134 Ma) 

maximum depositional age (T. Dumitru, written commun., 2015). 

The proportion of age-equivalent older sedimentary rocks that are 

present elsewhere in the Yolla Bolly terrane is poorly constrained 

at present. A better understanding of structural complexity 

(particularly fold history) will be necessary to place emerging 
7

detrital zircon age data into proper context with paleontologic 

ages in the map area and more regionally.

Pickett Peak Terrane (Lower Cretaceous, Aptian–Barremian)

The Pickett Peak terrane is the structurally highest unit of the 

Franciscan Complex in the map area; the rocks were subdivided into 

the metamorphic rocks of Black Butte and Bald Mountain and the 

metamorphic rocks of Lake Pillsbury by Ohlin and others (2010). 

Here we collectively describe all of these rocks under the Pickett 

Peak terrane. These rocks structurally overlie mélange of the Central 

Belt, as well as metasedimentary rocks of the Mendocino Pass terrane 

(described below) along low-angle faults inferred to be thrusts. The 

low-angle fault separating metamorphic rocks of the Pickett Peak 

terrane from the Mendocino Pass terrane might also have a later 

extensional history associated with uplift and unroofing of the Coast 

Ranges (for example, see Jayko and others, 1987).

Metamorphic rocks of the Pickett Peak terrane are present 

in the map area as an isolated slab at Bald Mountain, but to the 

northeast, metamorphosed metasedimentary and metavolcanic 

rocks also underlie Black Butte. Southeast of Black Butte, the 

unit is contiguous with rocks previously mapped as the South 

Fork Mountain Schist (Ghent, 1965; Blake and others, 1967; 

Lehman, 1974; Bishop, 1977; Brown and Ghent, 1983). These 

rocks are also considered to be a part of the Pickett Peak terrane 

of the Eastern Belt (Irwin and others, 1974; Blake and others, 

1982), which is in fault contact with rocks of the Coast Range 

ophiolite to the east. Pickett Peak terrane rocks of Black Butte 

and Bald Mountain are dominantly metasandstone and phyllite 

and, from regional map relations and drill-hole data (CDWR, 

1966), have a structural thickness of as much as 3 km. A 500- 

to 1,000-m-thick lens of pillowed, upright metavolcanic rocks 

occurs in the high Coast Ranges at Black Butte in an abandoned 

cirque formed during Pleistocene glaciation (Bishop, 1977;

Irwin, 1960). These rocks are reconstituted to textural zone 2 of 

Blake and others (1967), but reconstitution increases to textural 

zone 3 northward, with increasing topographic elevation. The 

textural zone 2 metasandstone contains only small amounts of 

pumpellyite and lawsonite. At Bald Mountain, the metasandstone 

is largely medium to coarse grained with lenses of stretched-pebble 

conglomerate that structurally overlies finer grained, laminated 

metasandstone and argillite containing blocks and lenses of foliated 

mafic volcanic rocks and gabbro. Finer grained, lower Pickett 

Peak terrane rocks are well exposed in the headwaters of the Eel 

River and the Cushman Lake-Hells Half Acre area (see map, 

sheet 1). The coarser-grained metasandstone at Bald Mountain 

is exposed along the Hull Mountain Road and north of Devils 

Rock Garden. A large mass of fine-grained to pegmatitic, foliated 

metadiabase, possibly intrusive into the Bald Mountain rocks, is 

present just north of Bald Mountain peak at Coyote Rock in the 

Plaskett Ridge 7.5ʹ quadrangle (fig. 3, sheet 1). The thickness 

of the structural outlier of Pickett Peak rocks at Bald Mountain 

is approximately 700–1,000 m, significantly less than at Black 

Butte. Crosscutting foliations and associated fold trends indicate 

the Pickett Peak terrane to have undergone multiple periods of 

deformation, although only one high P/T metamorphic event is 

evident petrographically (Ohlin and others, 2010). Petrographic 

evidence suggests that the fold trends and planar fabrics in Pickett 

Peak rocks at Bald Mountain are younger than the high P/T 

metamorphic event in these rocks (Ohlin and others, 2010).

Numerous Early Cretaceous (~Aptian–Valanginian), whole rock 

K-Ar dates, generally interpreted as maximum metamorphic ages 

because of incomplete recrystallization of grains in these rocks to new 

metamorphic minerals, have been reported for metasandstone from 

the Black Butte and Bald Mountain rocks. Suppe and Armstrong 

(1972) dated three samples of metagraywacke in the vicinity of Black 

Butte that yielded ages of 127, 125, and 123 Ma. Lehman (1974)

reported ten whole-rock dates from south of Black Butte ranging 

from 129 to 119 Ma and three dates from the Bald Mountain area of 

138, 133, and 127 Ma. Both Lehman (1974) and Lanphere and others 

(1978) indicate that their ages cluster around 120 Ma, suggesting a 

regional metamorphic event in the Early Cretaceous (~Aptian). This 

agrees with recent U/Pb detrital zircon ages from metachert and with 

40Ar/39Ar metamorphic ages—both from the South Fork Mountain 

Schist of the Pickett Peak terrane (Dumitru and others, 2009). 

This metamorphic age is considerably older than the Cenomanian 

depositional age (about 99–94 Ma) of lawsonitic metasandstones 

in the Yolla Bolly terrane, implying that the Yolla Bolly terrane 

was subjected to later high P/T metamorphism, as well as younger 

metamorphic textural reconstitution than seen in the Pickett Peak 

terrane. The later blueschist metamorphism in the Yolla Bolly 

terrane may have overprinted earlier blueschist metamorphism in the 

Pickett Peak rocks, which is consistent with field and petrographic 

observations (Ohlin and others, 2010).

Mendocino Pass Terrane (Lower Cretaceous, Valanginian)

This unit is present only in the northeastern part of the map 

area, in two fault-bounded wedges between mélange of the Central 

Belt and structurally overlying schistose rocks of the Pickett Peak 

terrane. Ohlin and others (2010) tentatively assigned these rocks to 

the Pickett Peak terrane as possibly correlative with the Valentine 

Spring Formation of Worrall (1981) and Blake and others (1992), 

but here, we describe the rocks separately as the Mendocino 

Pass terrane of the Eastern Belt. Regionally, the Mendocino Pass 

terrane occurs as a structural wedge composed of slightly sheared, 

moderately foliated metasandstone and interbedded argillite, with 

rare structurally interleaved lenses of metachert and greenstone. 

The unit is distinguished by its uniform, coherent nature, in contrast 

with the underlying mélange of the Central Belt, and by a moderate 

foliation, differing from the pronounced foliation and recrystallized 

metamorphic character of structurally overlying schist in the Pickett 

Peak terrane. Buchias that occur in a small, structurally isolated slab 

of the metasedimentary rocks of Mendocino Pass terrane along the 

road between Hells Half Acre and Bean Rock, about 0.4 mi (0.6 

km) south of Calamese Rock, are Early Cretaceous (Valanginian) 

in age (see fossil localities, table 1; also see Ohlin and others, 2010, 

and locality 29 of Blake and Jones, 1974). The fossils occur with 

clasts of volcanic breccia, as much as 12 cm in diameter, dispersed 

in sheared metasandstone. Given the recent demonstration that some 

Buchia localities in Eastern Belt Franciscan rocks are redeposited 

(Dumitru and others, 2010, 2015), the fossils in this breccia unit 

may be problematic for dating deposition, which could actually be 

considerably younger than the fossils. Correlation of this isolated 

structural slab to the Mendocino Pass terrane is based on the uniform 

and characteristic metamorphic grade and composition of the 

metasandstone, its structural position beneath the metasandstone 

and metavolcanic rocks of the Pickett Peak terrane, and its position 

above mélange of the Central Belt. The thrust-fault contact between 

the Mendocino Pass terrane and overlying Pickett Peak terrane 
8

rocks is marked by shearing and (or) serpentinite. Relations found in 

similar rocks to the northeast (Bishop, 1977; Blake and others, 1967), 

however, suggest that the Mendocino Pass terrane rocks may have 

once increased in metamorphic grade gradually structurally upward 

toward the base of the Pickett Peak terrane, in the sense of Blake and 

others (1967). In the map area, significant parts of this intervening 

metamorphosed section are apparently missing and possibly were 

structurally removed by later low-angle faulting that post-dated 

metamorphism and accompanied unroofing and exhumation of the 

Franciscan Complex. If so, then metamorphism of the Mendocino 

Pass and Pickett Peak terranes must have post-dated the Early 

Cretaceous (Valanginian, 140–136 Ma) or younger depositional age 

of the metasedimentary rocks of Mendocino Pass and predated the 

later structural erosion occurring during unroofing and exhumation of 

the Franciscan Complex.

Coast Range Ophiolite (Upper and Middle 

Jurassic)

The Coast Range ophiolite consists of layered and unlayered 

ultramafic and gabbroic rocks, mafic sills and dikes, pillowed basaltic 

flows and flow breccias, pelagic chert, and local mafic aquagene 

tuff that are broadly interpreted as the oceanic basement of the 

Great Valley complex. These ophiolitic rocks locally underlie the 

Elder Creek terrane of the Great Valley complex depositionally. We 

interpret the Coast Range ophiolite here to have formed in a mantle 

wedge within a supra-subduction setting, rather than at a midocean-ridge spreading center (for example, Ernst and McLaughlin, 

2012). Much of the original section of the Coast Range ophiolite 

is missing in the map area and the ophiolite is highly modified by 

Cretaceous and younger tectonism, uplift, and erosion that gave rise 

to sedimentary serpentinite (unit Kssp), ophiolitic mélange (unit 

KJom), and megabreccia (unit Jgb) at the base of the Great Valley 

(Elder Creek terrane) sedimentary section. Some serpentinized 

ultramafic rocks may have been remobilized as diapirs and locally 

extruded onto the seafloor in the Cretaceous and even later along 

younger faults during Tertiary and Quaternary deformation events. 

One such diapir is present along the Bartlett Springs Fault Zone at 

Coyote Rocks just north of Lake Pillsbury, where the serpentinite 

has been mobilized and intrudes Pleistocene terrace and fan deposits 

along the Bartlett Springs Fault Zone (Ohlin and others, 2010; Moore 

and others, 2015).

Great Valley Complex

Elder Creek Terrane of the Great Valley Complex 

(Lower Cretaceous and Upper to Middle Jurassic)

As defined on the geologic map of the Red Bluff 30ʹ × 60ʹ 

quadrangle (Blake and others, 2000), the Elder Creek terrane consists 

of the Middle to Upper Jurassic oceanic basement of the Great 

Valley complex (the Coast Range ophiolite plus serpentinite matrix 

mélange), overlain unconformably by Upper Jurassic to Lower 

Cretaceous strata of the Great Valley complex, locally exhibiting a 

basal, coarse megabreccia derived from the Coast Range ophiolite at 

the base. These rocks are collectively folded, sheared, and complexly 

faulted. Overlapping younger Cretaceous strata of the Great Valley 

complex are less severely deformed. In the map area, Upper Jurassic 

to Lower Cretaceous sedimentary serpentinite locally makes up a 

significant component of the Elder Creek terrane.

To the south of the map area, between Lake Berryessa and San 

Francisco Bay, Elder Creek terrane rocks appear to be divisible into 

three fault-bounded structural domains (Graymer and others, 2002a): 

(1) a domain of disharmonically folded and broken, thin- to mediumbedded mudstone, sandstone, and conglomeratic turbidites, underlain 

by (2) a discontinuous domain of sheared and broken serpentinized 

peridotite with sheared screens of microgabbro or amphibolitic dike 

rocks overlain in places by cumulate gabbro ± microgabbro ± basaltic 

intrusives ± pillowed basalt flows and flow breccia, in turn, underlain 

by or grading laterally into (3) a domain of ophiolitic mélange, 

including blocks of basaltic rocks, gabbro, intact slabs of less-sheared 

serpentinite and gabbro, and also slabs of intact but complexly folded 

or sheared Elder Creek terrane sedimentary rocks, all of which are 

enclosed in a penetratively sheared and gouged serpentinitic or 

argillitic matrix. In the map area, we have not separately mapped a 

domain of disharmonic folding but recognize its presence. It is not 

clear on our map if there is a distinct separation between the folded 

and mélanged domains everywhere that can be mapped regionally 

at the same structural levels or whether there is structural repetition 

vertically or lateral transitioning between domains 1–3 defined above. 

On our map, disharmonically folded, broken ophiolitic and mélanged 

ophiolitic domains are incorporated into a single unit of ophiolitic 

mélange.

Structural Relations

Major Faults

Coast Range Fault

Throughout the California Coast Ranges, the fundamental fault 

boundary between subducted Mesozoic rocks of the Franciscan 

Complex and overlying exhumed basement of the Great Valley 

complex (the Coast Range ophiolite) is referred to as the Coast Range 

Fault. The Coast Range Fault was originally characterized (Bailey 

and others, 1970) as the hanging-wall-thrust boundary between the 

subducted rocks of the Franciscan Complex and Mesozoic strata 

deposited in the overlying Great Valley fore arc, with the Coast 

Range ophiolite representing the Jurassic oceanic basement on which 

the Great Valley complex was deposited. Later work, however, has 

shown that this fault boundary has a more complex history that 

involved major low-angle normal faulting associated with unroofing 

of the subduction complex (Jayko and others, 1987), followed by 

east-vergent, tectonic wedging initiated during the Late Cretaceous 

and early Tertiary Laramide orogeny (Ernst and McLaughlin, 2012), 

followed by an even later period of transpression. Characterization of 

the Coast Range Fault as a Mesozoic thrust, therefore, is a misleading 

oversimplification. Here and on other recent maps and reports by 

others (Platt, 1986; Jayko and others, 1987; Blake and others, 2000), 

this boundary is referred to as the Coast Range Fault, recognizing that 

the fault has a complex history involving subduction overprinted by 

major later extensional and compressional deformation.

Much disruption of the Coast Range Fault was imposed in the 

Tertiary, beginning before and continuing during the development of 

the San Andreas transform system. The origin(s) and evolution of the 
9

later disruption of the Coast Range Fault is very poorly understood, 

but it has played a key role in the development and evolution of the 

Northern Bartlett Springs Fault Zone, which has evolved from the 

Coast Range Fault. The later development of the Bartlett Springs 

Fault system substantially obliterated initial kinematic relations along 

major parts of the Coast Range Fault southwest of Wilbur Springs. 

For this reason, we label the Coast Range Fault along the west side of 

the Coast Range ophiolite only north of the Wilbur Springs antiform, 

although we acknowledge that segments of the Coast Range Fault 

are likely incorporated into, but overprinted by, Tertiary transpression 

along the Bartlett Springs Fault Zone.

Bartlett Springs Fault Zone

The Bartlett Springs Fault Zone, a major active and creeping 

fault, is the easternmost major fault of the San Andreas transform 

system in this part of northern California. Though it was recognized 

as seismically active in the 1980s (Depolo and Ohlin, 1984; Bolt and 

Oakeshott, 1982; Eberhart-Phillips, 1988), its role as a significant, 

active, creeping member of the San Andreas Fault system linking the 

active Cascadia subduction margin north of Cape Mendocino with 

the inboard part of the transform margin has been recognized only in 

recent years (Lienkaemper, 2010; Murray and others, 2014; Lozos 

and others, 2015). Details of how the Bartlett Springs Fault Zone 

evolved from an active member of the former subduction margin 

south of the Mendocino Triple Junction to its present role as a rightlateral strike-slip fault is poorly understood, partly because the faultzone history has evolved over a long time span beginning as early as 

the Miocene, and it appears to have reactivated parts of Paleogene 

and older fault boundaries associated with subduction.

Detailed descriptions of numerous segments of the Bartlett 

Springs Fault Zone have been presented by Ohlin and others 

(2010) in the Lake Pillsbury region. For the more regional map of 

this report, however, we divide the Bartlett Springs Fault Zone into 

the Northern and Southern Bartlett Springs Fault Zone segments 

(fig. 5, sheet 2) that differ in their long-term history and expression 

as crustal features. We contrast the character of these segments of 

the fault zone and discuss how the two segments may have evolved 

structurally. The Southern Bartlett Springs Fault Zone is also 

referred to as the Berryessa Fault, which is described mainly on the 

basis of its late Quaternary expression and history (Lienkaemper, 

2010). The Southern Bartlett Springs Fault Zone also includes 

several previously named local faults, including the Wilson Valley 

and Hunting Creek Faults (Lawton, 1956).

The Bartlett Springs Fault Zone continues southeastward 

and is now linked to faults of the eastern San Francisco Bay 

dextral fault system, having a Miocene and younger cumulative 

displacement of 170 km (McLaughlin and others, 1996; Graymer 

and others, 2002b). The fault zone apparently represents the 

northeastern boundary of the Humboldt Plate of Herd (1978), 

although Herd did not recognize the significance of strike-slip 

faulting along the Bartlett Springs Fault Zone and, instead, 

assigned the northeast boundary of his Humboldt Plate to the 

Maacama Fault south of Round Valley. Of further significance 

to the long-term evolution of the Bartlett Springs Fault is that it 

appears to follow and to have reactivated segments of the broad, 

highly sheared east fault boundary of the Central Belt of the 

Franciscan Complex, particularly in the eastern Coast Ranges north 

of Round Valley (fig. 4, sheet 1).

Northern Bartlett Springs Fault Zone

The Northern Bartlett Springs Fault Zone extends across 

nearly the entire length of this map area, from its northern boundary 

in Round Valley to Cache Creek, south of Highway 20 and 

southwest of Wilbur Springs. North of Highway 20, the Northern 

Bartlett Springs Fault Zone consists of numerous straight to 

somewhat curvilinear northwest-trending, right- and left-stepping 

fault segments and splays that we interpret to follow the dextrally 

sheared and translated axial area of the Bartlett Springs Synform 

(figs. 5, 6, 7, sheet 2). The fault zone is also partly aligned along the 

southwest limb of a major antiform (the Wilbur Springs Antiform) 

northeast of the fault and is subparallel to a relict part of the Bartlett 

Springs Synform and the en echelon Pacific Ridge Antiform farther 

to the northeast (figs. 5, 6, 7, sheet 2). The original synformal and 

antiformal geometry of bedrock in the southern part of the map area 

becomes increasingly obscured northwestward between Wilbur 

Springs and Lake Pillsbury due to dextral dismemberment of the 

Bartlett Springs Synform along the Northern Bartlett Springs Fault 

Zone (McLaughlin and Moring, 2013).

The Northern Bartlett Springs Fault Zone cuts upper 

Pleistocene–Holocene deposits in Lake Pillsbury basin, which 

appears to have formed as a right-stepped or releasing bend structure 

along the creeping section of the fault. Just north of the map area in 

Round Valley near Covelo, the fault zone again takes a right step 

that has resulted in the Round Valley pull-apart structure, filled 

with Holocene and Pleistocene sediment (Jayko and others, 1989). 

Southeast of Lake Pillsbury, the Northern Bartlett Springs Fault Zone 

structurally entrains or isolates thin accumulations of Pleistocene 

or younger alluvial deposits, stream terraces, alluvial fans, and 

abandoned meanders as indicated by geomorphic features in Rice 

and Twin Valleys, Soap Creek, Bartlett Creek, Wolf Creek, and 

North Fork Cache Creek. Northeast of State Highway 20 and High 

Valley, the fault zone is aligned along and truncates the east side of 

fluvial and lacustrine sediments of the Pliocene–Pleistocene Cache 

Formation, which forms a prominent structural basin southwest of 

the Northern Bartlett Springs Fault (McLaughlin and others, 1990; 

Hearn and others, 1995; Rymer, 1981). Southeast of Highway 20 

and Grizzly Spring, youthful fault morphology steps ~2.5 km to 

the southwest across North Fork Cache Creek to the Southern 

Bartlett Springs Fault, which is aligned mostly with various bedrock 

contacts between ultramafic rocks of the Coast Range ophiolite and 

sedimentary units of the lower Great Valley complex. The Cache 

Formation is also involved in the faulting in the right-stepped area 

between the Northern and Southern Bartlett Springs Faults (Gaines, 

2008). Southeast of the step to the Southern Bartlett Springs Fault, 

the Northern Bartlett Springs Fault continues southeastward into the 

Lower Cretaceous section of the Great Valley complex. No data are 

presently available to evaluate the possibility that some Quaternary 

faulting may be partitioned farther to the southeast along the Northern 

Bartlett Springs Fault. Previous mapping of Quaternary fault features 

by Lienkaemper (2010) does not recognize young faulting on the 

Northern Bartlett Springs Fault southeast of the step-over to the 

Southern Bartlett Springs zone.

Displacement on the Northern Bartlett Springs Fault

Although clearly definitive field criteria are lacking, geologic 

relations along the Northern Bartlett Springs Fault Zone suggest 

large, long-term dextral displacement estimated at ≥37–50 km at 
10

~3–4 mm/yr since the Miocene (McLaughlin and others, 2010; 

McLaughlin and Moring, 2013). Estimate of the approximate amount 

and timing of displacement on the Northern Bartlett Springs Fault 

Zone is based on a fragmentary knowledge of Late Cretaceous to 

Neogene paleogeography and on the structural evolution of the 

northern California margin. This area has transitioned from a largely 

convergent, east-dipping, thrust-fault-dominated subduction margin 

in the latest Cretaceous to early Cenozoic to one of west-stepped 

subduction and east-vergent crustal wedging in the Paleogene. This 

was accompanied by major Paleogene uplift, unroofing, and erosion 

of previously subducted rocks of the Franciscan Complex and the 

western Great Valley fore arc. The distribution and configuration 

of Coast Range ophiolite, Great Valley complex, and Franciscan 

Central Belt rocks west of Sacramento Valley, at Wilbur Springs 

and in the Geysers-Clear Lake region, suggest that convergence and 

crustal wedging was oblique to the margin at this time, resulting in 

dextral displacement and folding along the Great Valley fore-arc and 

subduction margin (Franciscan Complex) interface (McLaughlin and 

others, 1988). Between the Eocene and middle Miocene, this was 

followed by development of a west-stepped fore-arc basin deformed 

by Miocene and younger northwest-southeast-oriented transpressional 

folds (figs. 6, 7, sheet 2). Regionally, the transpressional folding 

appears to have developed ahead, or north, of the northwardly 

propagating Mendocino Triple Junction and San Andreas Fault 

system (fig. 6). The Northern Bartlett Springs Fault traverses and 

right-laterally dismembers the Bartlett Springs Synform that is 

subparallel to, and probably formed with, folding that developed 

between the Eocene and middle Miocene. The reconstructed former 

Bartlett Springs Synform forms the southwest flank of a southeastplunging antiformal warp (the Wilbur Springs Antiform) along the 

current southwest margin of the Great Valley. The dismembered, 

former Bartlett Springs Synform and southwest flank of the Wilbur 

Springs Antiform, together with several subparallel warps in the 

core of the antiform (fig. 6; fold axes 1–3 in fig. 7), collectively 

define a composite structure that is here referred to as the “Wilbur 

Springs Dextral Hook” (figs. 6, 7, 8, sheet 2). These structures will be 

discussed further in the section on folding.

Approximate paleogeographic reconstruction of the former 

Mesozoic to Eocene Great Valley and later Neogene fore-arc 

basins, coupled with interpreting the sequence of subsequent 

transpressional folding and dextral dismemberment of the 

Bartlett Springs Synform, provides a crude basis for estimating 

long-term displacement along the Northern Bartlett Springs 

Fault (figs. 6, 7; see also later discussions of the Bartlett Springs 

Synform and Wilbur Springs Dextral Hook). In restoring the 

pre-Northern Bartlett Springs Fault configuration of the Bartlett 

Springs Synform, we relate similarities in the stratigraphies of 

several remnants of Great Valley complex, dismembered along the 

Northern Bartlett Springs Fault, that formerly were associated with 

the westernmost edge of the Great Valley fore-arc basin. Important 

relations in these stratigraphic remnants include correlative 

fossil assemblages of Lower Cretaceous (Hauterivian) cold-seep 

deposits, the presence of Paleocene and Eocene strata deposited 

unconformably on Cretaceous and older Great Valley complex 

rocks, and Miocene marine strata deposited in the former Neogene 

fore-arc basin (Nilsen and Clarke, 1989). We consider the slip 

restoration along the Bartlett Springs Fault in figure 7 to provide 

the minimum estimate of fault displacement (~38–45 km) needed 

to restore the original geometry of the former Bartlett Springs 

Synform, although larger displacement >50 km is also possible. 

Although not present in Rice Valley or farther south, Miocene 

estuarine strata in Round Valley, associated with the uplifted and 

eroded southeastward extension of the Eel River fore-arc basin 

(Nilsen and Clarke, 1989), are interpreted to provide a maximum 

post-folding (~middle Miocene, 15–12 Ma) age for initiation 

of Northern Bartlett Springs faulting and a minimum long-term 

displacement rate of 2.5–3.9 mm/yr. A permissive larger minimum 

displacement of 50 km since 15–12 Ma yields a slip rate of 

3.3–4.2 mm/yr. This admittedly crude estimate in the range 

of long-term slip (~2.5–4.1 mm/yr or ~3.35±0.85 mm/yr) is 

low in comparison to the rate of Holocene slip of 6±2 mm/yr 

(Lienkaemper, 2010; Lienkaemper and others, 2014) but similar to 

repeated Global Positioning System (GPS) measurements across 

the fault that yielded a rate of 3.1±0.2 mm/yr along the creeping 

segment of the Northern Bartlett Springs Fault at Lake Pillsbury 

(Murray and others, 2014).

Lake Pillsbury Releasing Bend

The releasing bend or pull-apart that forms Lake Pillsbury 

basin has been considered to be a possible barrier to through-going 

fault rupture (DePolo and Ohlin, 1984; Geomatrix Consultants, 

1986; Lienkaemper, 2010; Sibson, 1985). The structural geometry 

of the Gravelly Valley (Lake Pillsbury) basin is well constrained 

from geologic mapping by Ohlin and others (2010). Gravity 

and aeromagnetic data (Langenheim and others, 2007, 2011) 

further indicate that the basin is about 2.5 km wide and 5 km long 

and underlain by a roughly 400-m-thick section of sediments 

(Langenheim and others, 2007). Magnetic data shows the fault 

trace below Lake Pillsbury as continuous, with a 30° right bend 

(Langenheim and others, 2007). Faulted, basinward-tilted, and 

uplifted Pleistocene gravels are exposed in road cuts at Logan Spring, 

along the Northern Bartlett Springs Fault southwest of Scott Dam.

Several prominent flights of old terraces that border the east 

and southwest sides of Gravelly Valley (Ohlin and others, 2010) are 

recognized as “strath terraces” (planar terraces eroded into bedrock). 

These strath terraces are interpreted to record a period during which 

ongoing subsidence of the Gravelly Valley alluvial basin was 

punctuated by temporally stable times, allowing the Eel River to 

erode bedrock along the east and west sides of the basin. Based on the 

degree of weathering observed along these strath surfaces the terraces 

are estimated to be upper and middle Pleistocene (tens to hundreds of 

thousands of years old; Ohlin and others, 2010).

Lidar-derived elevation data collected along the Northern 

Bartlett Springs Fault Zone (Metro Engineering, 2011) were used to 

evaluate Quaternary deposits in the Lake Pillsbury basin as part of 

this mapping effort. Some previously mapped Quaternary deposits 

(Ohlin and others, 2010) in the vicinity of Scott Dam were remapped 

by Hitchcock in this study using lidar data and field checked during 

field reconnaissance work in December 2014.

Tectonic geomorphology along the Northern Bartlett Springs 

Fault Zone suggests a dominantly right-lateral sense of displacement, 

including a mix of east- and west-facing scarps and dextral stream 

deflections. Evidence of secondary displacement on normal faults 

is present locally at Gravelly Valley (Lake Pillsbury basin), which is 

bounded by right-stepping normal faults. Secondary normal faults 

are also present at a right step in the fault at Round Valley and at 

Twin Valleys near Bartlett Springs, where fault-bounded sediments 

are ponded on the down-thrown side of the fault trace. Evidence for 

reverse faulting is indicated just north of Lake Pillsbury at the location 
of a fault-creep-monitoring array, where left-stepping cracks in the 

asphalt road crossing the main fault zone and creep array clearly 

indicate active transpressional surface deformation.

A 2- to 3-m-wide shear zone, composed largely of diapiric 

serpentinite, forms a natural exposure of the western strand of the 

Northern Bartlett Springs Fault (fig. 9) and offsets the uppermost 

Pleistocene terrace gravels of Salmon Creek, north of Lake 

Pillsbury, and the creep array (Moore and others, 2015; Ohlin and 

others, 2010; Lienkaemper, 2010). The prominent, 183-m-long, 

6.1-m-high stream-bank exposure of the fault is located about 

250 m north of Coyote Rocks (figs. 9, 10). The exposure is 

evident in terrestrial lidar scanning (TLS) imagery and at least 

three lithologic units are exposed in the stream bank (fig. 10). 

The oldest unit (fig. 10C, unit 1) is serpentinitic bedrock located 

within the several-meter-wide fault zone. Moore and others 

(2011, 2015) interpreted this unit as tectonically entrained from 

considerable depth and to have risen to the ground surface along 

an active strand of the Northern Bartlett Springs Fault Zone (the 

Coyote Rocks strand). The next youngest unit (fig. 10C, unit 2) 

is a moderately well stratified sequence of sand and gravel with 

lenses of sand and imbricated gravels recording deposition in a 

bar-and-swale setting similar to the morphology of the adjacent 

streambed. Stratification of unit 2 defines an apparent dip of 

about 30° S. Soil profiles are absent within unit 2, indicating 

rapid deposition. Unit 2 is overlain by unit 3 (gravel and sand 

similar lithologically to unit 2) along a prominent, wavy, angular 

unconformity that truncates stratification. A soil profile is 

developed in unit 3, but it does not obscure the sandy texture and 

common gravel strings in the unit. Units 2 and 3 are present on 

both sides of the fault zone (figs. 9, 10).

Near-surface fault orientation was documented at Lake 

Pillsbury by Geomatrix Consultants (1986) in a trench near 

the above-described exposures across the Coyote Rocks Fault 

strand. The trenching exposed a fault plane dipping 65° NE. with 

slickensides plunging at 15° SE. Assuming that slip was dextral, 

this suggests east-side-down, oblique-normal displacement. This 

relation, together with the active transpression exhibited at the 

creep array site a short distance to the southeast and the overall 

releasing bend geometry of Lake Pillsbury basin, points to a high 

degree of along-strike fault-zone complexity.

Seismicity of the Northern Bartlett Springs Fault Zone

Microseismic data predating 2012 indicate that the Northern 

Bartlett Springs Fault Zone dips steeply east (at about 70–85°) 

between Lake Pillsbury and Bartlett Springs (figs. 11, 12; also see 

fig. 5, sheet 2), and some focal mechanisms from the microseismicity 

suggest normal (transtensional) strike slip, consistent with eastside-down displacement. An east-side-down sense of displacement, 

however, is inconsistent with the apparent long-term, west-down 

sense of displacement suggested by higher elevations and erosional 

relief on the east side of the Northern Bartlett Springs Fault Zone, as 

well as mapped structural relations across the fault zone southeast of 

Lake Pillsbury. The modern topographic relief might be interpreted 

as merely reflecting a previous compressional tectonic setting now 

overprinted by normal faulting. As outlined below, however, it is 

likely that focal mechanisms that indicate steeply dipping fault planes 

for many of the small earthquakes (generally ≤M 3.0) along the 

Northern Bartlett Springs Fault have large uncertainties in the dip and 

dip direction, inherent in fault plane solutions. We, therefore, view 

11

Figure 9. View southeast at serpentinite diapir in Bartlett Springs Fault Zone north of Coyote 

Rocks and Lake Pillsbury. Diapir and faulting cuts tilted and flat-lying Pleistocene deposits. See 

discussion of diapir.
12

Figure 10. Views southeast across Bartlett Springs Fault Zone north of Coyote Rocks, distinguished by serpentinite diapir 

within stream-cut exposure along Salmon Creek. Fault cuts Quaternary fluvial gravels, an overlying paleosol, and younger 

flat-lying stream terrace deposits. A. Terrestrial lidar point cloud image, looking southeast across Salmon Creek, from 

a survey across the Bartlett Springs Fault Zone. B. A color-relief map from merged terrestrial and aerial lidar datasets, 

showing location of serpentinite diapir exposure along Salmon Creek and expression of surface fault scarp in the Bartlett 

Springs Fault Zone. Heavy white line indicates approximate northwest (A) and southeast (B) ends of stream-cut view in 

figure 10C. C. Orthorectified photomosaic of stream-cut exposure of three Quaternary units cut by serpentinite diapir and 

Bartlett Springs Fault. See text for discussion of stratigraphy (numbered units).

N

BARTLETT SPRINGS FAULT

Serpentinite diapir

BARTLETT SPRINGS FAULT

Serpentinite diapir

A

B

A

B
the dip directions based only on the focal mechanisms as suspect, 

especially when compared to mapped structural relations across the 

Northern Bartlett Springs Fault.

Earthquakes along the Northern Bartlett Springs Fault 

commonly range in magnitude from <M 1.0 to >M 4.0, with 

hypocenters that reach ≥12 km (fig. 11; figs. 1, 4, sheet 1)—

significantly deeper than earthquakes on active faults to the west, 

such as the Maacama Fault. Focal mechanisms of Northern Bartlett 

Springs Fault earthquakes (mostly ≤M 3.0) favor dominant dextral 

strike slip, but as indicated above, include focal mechanisms having 

normal slip components. Although earthquakes large enough to 

have multiple aftershocks have not previously been recorded, a M 

5.1 event followed by numerous aftershocks (figs. 11,12, sheet 2) 

within the area of most frequent previous microseismicity occurred 

on August 10, 2016. The focus of this earthquake was 14.3 km 

(moderately deep for a strike-slip event). Based on the orientations 

of faults mapped at the surface, the first motion data for this 

earthquake favor a right-lateral fault plane oriented parallel to the 

surface trace of the Northern Bartlett Springs Fault, dipping steeply 

to the southwest (80°, with a dip uncertainty of 45°) and up on the 

northeast side (https://earthquake.usgs.gov/earthquakes/eventpage/

nc72672610#focal-mechanism). We plot the location of the doubledifferenced M 5.1 event together with its aftershocks and associated 

double-differenced previous microseismicity in figure 11 (Waldhauser 

and Schaff, 2008; see http://ddrt.ldeo.columbia.edu). Doubledifferenced relocation of the main shock places the hypocenter at a 

depth of 11.3 km (fig. 11; fig. 12, sheet 2).

Notably, the hypocenter of the 8/10/16 main shock and 

aftershocks (see fig. 13) and earlier Northern Bartlett Springs Fault 

microseismicity are mainly east of the surface traces of the Northern 

Bartlett Springs Fault Zone. In seismicity cross sections (fig. 12, 

sheet 2), these earthquakes are seen to define a distinct ~70–85° 

northeast-dipping zone of seismicity south of Lake Pillsbury that 

extends to within a few kilometers of the surface beneath the 

Northern Bartlett Springs Fault. We interpret this to indicate that the 

fault zone is predominantly a steeply northeast-dipping right-lateral 

reverse structure that is up to the northeast, in spite of the steep 

 0 10 20 30 40 50 60 70 80 90

Distance (ft) along stream bed from A A B

 0

10

20

Distance (ft) above stream bed

30

N. 2° W. S. 2° E.

1

2

2

3 3

C SERPENTINITE DIAPIR IN 

BARTLETT SPRINGS FAULT

Figure 10.—Continued

southwest dip implied by the 8/10/16 earthquake focal mechanism. 

The 45° uncertainty in dip from the 8/10/16 event first-motion data, 

with a focal mechanism dip direction opposite to that indicated by 

aftershock and other microseismicity distribution, suggests that the 

focal mechanism dip direction is incorrect. A moment tensor data 

plot further suggests a steep northeast dip for the favored northweststriking fault plane (https://earthquake.usgs.gov/earthquakes/

eventpage/nc72672610#moment-tensor). We therefore consider 

the fault geometry defined by depth location of the main shock 

and associated microseismicity as a more reliable indicator of the 

geometry of the Northern Bartlett Springs Fault Zone than the dip 

suggested by first-motion data. The fault-zone geometry implied 

by the depth distribution of the double-differenced earthquakes 

(figs. 11, 12), moreover, is consistent with the structural framework 

indicated by our geologic mapping along the fault. The distribution of 

microseismicity associated with the northeast and southwest sides of 

the Northern Bartlett Springs Fault north of Lake Pillsbury, however, 

indicates that the fault-zone geometry is more complex in that area. 

The local releasing-bend geometry of the Northern Bartlett Springs 

Fault and associated fault splays, such as the Logan Springs Fault, 

that offset Pleistocene deposits at Lake Pillsbury (fig. 5, sheet 2; Ohlin 

and others, 2010), as well as local restraining bends along the length 

of the Northern Bartlett Springs Fault Zone, further point to faultzone complexity. These surface features indicate local extensional 

and compressional patches that possibly are, in part, related to vertical 

and lateral irregularities in fault orientation and fault rock textural and 

compositional variations and fluid distribution at depth. The observed 

variations in slip character along the length of the Northern Bartlett 

Springs Fault Zone might be characteristic of dipping fault zones that 

have experienced long-lived structural histories in which the forces 

driving fault slip have changed over time. In the case of the Northern 

Bartlett Springs Fault, this change was likely associated with the 

transition from dominant subduction to right-lateral strike slip.

Fault Creep along the Northern Bartlett Springs Fault

Recent geodetic work (Murray and others, 2014) has 

documented fault creep (fig. 5, sheet 2; fig. 7) along a segment of the 

13
Figure 11. Map of double-differenced locations for epicenters of the main shock (large triangle) and aftershocks (smaller triangles) of 

the August 10, 2016, M 5.1 earthquake on the Northern Bartlett Springs Fault. In addition, we plot the double-differenced distribution of 

post-1984 microseismicity (small dots); epicenters are color indexed according to their hypocentral depth in kilometers. Earthquake 

locations are from the Real-Time Double-Difference Earthquake Locations for Northern California (Waldhauser and Engebreth, 2016). Fold 

axes and surface traces of the Northern Bartlett Springs Fault are from map (sheet 1) and figure 5 (sheet 2). Seismicity cross-section lines 

show earthquake hypocenters explained in figure 12 (sheet 2): cross section t1 corresponds to a part of structure cross section B–B’–B”; 

cross section t3, to a part of structure cross section C–C’; and cross section t5 to a part of structure cross section E–E’. 

39°30'0"

39°22'30"

39°15'0"

123°0'0" 122°52'30" 122°45'0"

Bartlett Mountain Antiform

Middle Mountian Synform

unnamed 

synform

Pacific Ridge 

 Antiform

0

0 2 4 6 MILES

2 4 6 8 10 KILOMETERS 

Lake 

Pillsbury

8/10/16 M 5.1

Mainshock NORTHERN BARTLETT SPRINGS FAULT ZONE

Serpentinite 

diapir 

intruding 

Quaternary 

fan deposits

Fault Creep 

array, dextral 

cracks in 

pavement

0–2 

2–4

4–6

6–8

8–10

10–15

15–20

Hypocenter 

Depth Ranges (km)

EXPLANATION

t1

t2-6

t3

t4

t5

14
Northern Bartlett Springs Fault at the north side of Lake Pillsbury

basin just north of the lake and south of the serpentinite diapir

described above (figs. 9, 11, 12). Here, the creeping fault segment

has produced a 1.5-m-wide zone of left-stepped (transpressional) en

echelon cracks where the fault zone crosses the asphalt road around

the lake (Moore and others, 2015; Ohlin and others, 2010). Repeated

measurements of the alignment array across the zone of creep indicate

a creep rate of 3.1±0.2 mm/yr (Murray and others, 2014), about half

of the estimated Holocene slip rate of 6±2mm/yr (Moore and others,

2015; Lienkaemper, 2010; Lienkaemper and others, 2014) for the

northern plus the southern segments of the Bartlett Springs Fault.

Southern Bartlett Springs Fault Zone

Southeast of State Highway 20 near Grizzly Spring, a 2.5-km

right step separates the northern and southern Bartlett Springs

Faults (see southeastern corner of fig. 4 [sheet 1]and fig. 5 [sheet

2]). Here, youthful fault morphology steps to the southwest across 

North Fork Cache Creek to the Southern Bartlett Springs Fault 

(Lienkaemper and others, 2014) from where the Cache Formation

is bounded on the northeast by the Northern Bartlett Springs Fault.

Faulting along the Northern Bartlett Springs Fault continues to the

map boundary along the northeast side of the step as en echelon

bedrock faults, but evidence of youthful geomorphic expression of

the fault zone dies off southeast of the step to the Southern Bartlett

Springs Fault. The Northern Bartlett Springs bedrock faulting

nevertheless continues southeastward beyond the map area along

the southwest side of the Bartlett Springs Synform and the Wilson

and Kennedy Faults (see fig. 5, sheet 2), subparallel or oblique to

bedding and folding in the Jurassic and Lower Cretaceous Great

Valley complex and its contacts with the Coast Range ophiolite. 

This Northern Bartlett Springs bedrock faulting is superposed 

on, or associated with, long-term transpression and major dextral 

drag that dismembered the Bartlett Springs Synform in the Wilbur 

Springs area (fig. 5, sheet 2).

Because the Southern Bartlett Springs Fault Zone is only present 

in the southernmost part of the map area, an extensive description 

of the fault is not provided here, but the reader can access detailed 

descriptions of late Quaternary features and data from paleoseismic 

investigations of the Southern Bartlett Springs Fault Zone in Lienkaemper (2010) and references therein.

Quaternary faulting in the stepover area between the Northern 

and Southern Bartlett Springs Faults has recently been documented 

(Lienkaemper and others, 2014). Cache Creek flows through the 

central part of a right-stepped pull-apart depression cut by several 

intermediately spaced faults between the Hunting Creek Fault 

segment of the Southern Bartlett Springs Fault and the Wilson Valley 

segment of the Northern Bartlett Springs Fault (map, sheet 1; fig. 

5, sheet 2). This pull-apart depression is interpreted to exhibit ~6 

km of incremental Quaternary dextral offset of Cache Creek, where 

it flows through the depression in the stepover (Lienkaemper and 

others, 2014). A 2- to 3-km-long oblique-dextral fault exhibiting steep 

dipping scarps as much as 2.5 m high (the Kuikui Fault) within this 

stepover was trenched in 2011 (Lienkaemper and others, 2014), and 

one short strand of this fault was found to have vertically displaced 

the youngest soil layer 7 cm, which is a minimum offset for the 

broader fault zone of the stepover area (Lienkaemper and others, 

2014). Dextral fault creep is documented both on the Hunting Creek 

Fault segment of the Southern Bartlett Springs Fault Zone and near 

Highway 20 on the Northern Bartlett Springs Fault Zone north of the 

15

Figure 13. Histogram 

showing main shock 

and aftershocks defining 

the August 10, 2016, 

earthquake sequence 

on the Northern Bartlett 

Springs Fault Zone. 

Aftershocks continued in 

the area of the August 10 

main shock at a low level 

until September 16, after 

which microseismicity 

lapsed for about 10 

days before resuming 

in the area. For our 

purposes, the distinct 

lapse in microseismicity 

following September 16 

approximately defines 

the end of the August 10 

earthquake sequence.

August 10, 2016, Earthquake

and aftershock sequence

45

40

35

30

25

20

15

10

5

0

10-Aug

17-Aug

24-Aug

31-Aug

7-Sept

14-Sept

21-Sept

28-Sept

5-Oct

12-Oct

19-Oct

September 16,

aftershock 

sequence ended

Date (2016)

Earthquake frequency (number of earthquakes)
stepover, based on the modeling of repeated measurements across 

GPS site arrays (Murray and others, 2014; Lienkaemper, 2010).

The Southern Bartlett Springs Fault Zone appears to continue 

southward to the map boundary subparallel to the Northern Bartlett 

Springs Fault Zone, but in contrast to the Northern Bartlett Springs 

Fault Zone, the southern fault zone crosses many Mesozoic bedrock 

units at a steep angle with little, if any, apparent offset in most 

places. The faulting within the map area and beyond to the southeast 

is expressed primarily geomorphically, showing little evidence 

of significant strike-slip or dip-slip displacement of bedrock unit 

contacts (compared to probable displacements of as much as tens 

of kilometers along the Northern Bartlett Springs Fault), especially 

where the southern fault zone crosses contacts between the Coast 

Range ophiolite and stratified units of the Great Valley complex. 

As discussed above in part, Lienkaemper (2010) and Lienkaemper 

and others (2014) suggested that Cache Creek, parts of the PliocenePleistocene Cache Formation, and Clear Lake Volcanics could 

be right-laterally offset along the Southern Bartlett Springs Fault 

Zone as much as 5–6 km and that, farther south, volcanic rocks 

correlated with the Mesozoic Coast Range ophiolite and modern 

stream drainages are right-laterally offset 1–2 km by western 

traces of the Southern Bartlett Springs Fault Zone (Lienkaemper, 

2010; Lienkaemper and others, 2014). The suggested bedrock 

offsets, however, are unsupported by any data confirming age or 

compositional equivalence for the Quaternary and Mesozoic volcanic 

rocks correlated across the fault zone. No sedimentologic data is 

available to establish continuity of paleoflow direction and other 

sedimentologic affinities in the Pleistocene and Pliocene sedimentary 

units cut by the Southern Bartlett Springs Fault Zone, with the 

exception of the thesis work of Gaines (2008). Major long-term 

displacement associated with the Northern Bartlett Springs Fault 

must be partitioned southward to other faults, perhaps to the presently 

inactive Northern Bartlett Springs Fault southeast of the stepover to 

the Southern Bartlett Springs Fault. Alternatively, some slip could 

be accommodated by folding above blind transpressional reverse 

faults. Additional investigations will be needed to establish specific 

structures involved with slip partitioning and to test and determine the 

timing of suggested long-term displacements.

Faults of the Middle Mountain and Sanhedrin Mountain Areas

The Middle Mountain area (fig. 4, sheet 1) is a narrow 

northwest-trending ~45 km long synformal fault block made up 

of a structurally thinned section of lower Eocene–Paleocene strata 

overlying Lower and Upper Cretaceous strata of the Great Valley 

complex, underlain locally by structurally thinned igneous rocks of 

the Coast Range ophiolite. The block is flanked on the southwest by 

rock units of the Central Belt of the Franciscan Complex (including 

the Pomo terrane) and to the northeast by both Central Belt and Yolla 

Bolly terrane rocks of the Franciscan Eastern Belt (map, sheet 1). The 

Yolla Bolly terrane rocks to the northeast form a transpressionally 

warped structural slab, the Sanhedrin Mountain slab, that is more than 

85 km long and overlies the Central Belt northeast of Clear Lake. The 

Sanhedrin Mountain slab is antiformally warped northeast of Clear 

Lake (Bartlett Mountain Antiform, cross sections and figs. 5, 7, sheet 

2), as suggested by the distribution of metachert and metavolcanic 

rocks in the slab, and it is largely devoid of seismicity (see map, sheet 

1; seismicity distribution on fig. 5, sheet 2).

The Middle Mountain synformal block, in contrast, exhibits 

a significant linear zone of microseismicity largely between 2 

and 8 km deep, with epicenters in part aligned with the Bucknell 

Creek Fault that bounds the east side of the Middle Mountain 

block (map, sheet 1; fig. 5, sheet 2). Northwest of this alignment 

with the Bucknell Creek Fault, however, the linear trend of 

microearthquake epicenters veers southwest, away from the 

Bucknell Creek Fault and the eastern boundary of the Middle 

Mountain block, to extend beneath the central Middle Mountain 

area, where no faults are mapped at the surface (fig. 5, sheet 2). 

Hayes and others (2006) interpreted the seismicity beneath the 

northern Middle Mountain block to be associated with active 

dike intrusion in the lower crust. Thomas and others (2013), 

however, pointed out that this interpretation is inconsistent with 

the orientation of the seismicity relative to maximum regional 

principal stress and argued that dike intrusion in the lower 

crust should produce compressive stress in the upper crust that 

inhibits shallow seismicity. Alternatively, Thomas and others 

(2013) proposed that the seismicity is more likely related to 

incipient fault development, as also suggested earlier by McLaren 

and others (2007). The fault zone inferred from distribution 

of double-differenced hypocenters of the microearthquakes 

dips vertically to steeply to the northeast at 70–80°, and the 

earthquakes have focal mechanisms consistent with dextral 

normal and reverse slip (Thomas and others, 2013). Departure of 

the earthquakes from the mapped northern extent of the Bucknell 

Creek Fault presumably is associated with a northwest splay from 

the Bucknell Creek Fault at depth that has not ruptured to the 

surface; or if it has, existing surface geologic mapping does not 

document the fault splay.

Faults of the Clear Lake Area

The southwestern part of the map area includes Clear 

Lake, the Quaternary Clear Lake Volcanic Field, and extensive 

coeval and post-volcanic Pliocene, Pleistocene, and Holocene 

fluvial and lacustrine sediments and underlying bedrock units of 

the Franciscan Complex (map, sheet 1; fig. 5, sheet 2). Several 

significant northwest- and west-northwest-trending fault zones, 

as well as one east-northeast- and one north-northeast-trending 

fault zone, extend through this area and cut the Quaternary 

volcanics and clastic sedimentary rocks. The most significant faults 

include the Scotts Valley, Big Valley, Wight Way, Clover Valley, 

Collayomi, Konocti Bay, Borax Lake, Sulphur Bank, and Cross 

Springs Faults (fig. 5, sheet 2). Microseismicity is associated with 

some of these faults, such as the Collayomi and Konocti Bay Faults 

(Eberhart-Phillips, 1988; Bufe and others, 1981), and some of 

this seismicity includes long-period events (near Mount Konocti) 

linked to magma activity at depth (Pitt and others, 2002).

The Cross Springs Fault is an east-side-down normal fault 

that splays southwest from the Northern Bartlett Springs Fault 

and bounds the northwest side of the main depositional basin of 

the Cache Formation (Rymer, 1981; Hearn and others, 1995). 

The Wight Way Fault also is a normal fault that cuts Pleistocene 

lacustrine and fluvial sediments southwest of Lakeport and is 

considered to be one of the structures controlling subsurface 

distribution of aquifers southwest of Clear Lake (Rymer, 1981; 

Hearn and others, 1988, 1995). The curvilinear trend of the Big 

Valley Fault, which subparallels the southwest margin of Clear 

Lake and the western side of Big Valley, suggests that it may be 

associated with subsidence (possibly volcanogenic) along the 

west margin of the Clear Lake Volcanic Field (map, sheet 1).

16
The Konocti Bay Fault is mapped mainly within the Clear Lake 

Volcanics (Hearn and others, 1988, 1995), is intimately associated 

with the distribution of eruptive units, and displaces volcanic units 

within the volcanic field (Hearn and others, 1995). The orientation of 

the fault zone also aligns with faults mapped beneath Clear Lake that 

vent CO2 into the lake (Sims and Rymer, 1976) and that align with the 

Clover Valley and Bucknell Creek Fault Zones farther to the north, 

which separate the Franciscan Eastern Belt on the northeast from the 

Middle Mountain block and Franciscan Central Belt to the southwest. 

The Borax Lake and Sulphur Bank Fault Zones to the northeast of 

the Konocti Bay faulting control the configuration of the elongated 

northeast and southwest arms of Clear Lake, and these faults also 

expose faulted, mineralized, and hydrothermally altered metachert, 

metasandstone, and metavolcanics of the Franciscan Eastern Belt. 

This relation strongly suggests that the Konocti Bay Fault Zone is the 

likely southeastern extension of the Clover Valley-Bucknell Creek 

Fault Zones that, in places, cut through the suture between Franciscan 

Central and Eastern Belt basements (as illustrated in structure sections 

C–Cʹ and D–D ʹ, sheet 2). The Borax Lake Fault Zone, furthermore, 

exhibits a prominent youthful pull-apart basin (Borax Lake) that we 

interpret as indicating that the fault zone has been active since latest 

Pleistocene time.

The Collayomi Fault that is near the southwest boundary 

of the map area is well known from previous mapping of the 

framework geology of the Geysers-Clear Lake geothermal region 

by McLaughlin (1978) and also from mapping of the Clear Lake 

Volcanics by Hearn and others (1995). The Collayomi Fault Zone 

acts as the northeast structural boundary of The Geysers Geothermal 

Field, where steam is produced from a fracture-controlled reservoir 

largely in slabs of greywacke in the Franciscan Central Belt. The 

Collayomi Fault Zone extends along the northeast side of a major 

slab of Coast Range ophiolite and separates the ophiolite and rocks 

mainly of the Franciscan Central Belt on the southwest from a thick 

section of attenuated and folded Coast Range ophiolite and Upper 

Jurassic to Upper Cretaceous strata of the Great Valley complex to the 

northeast (McLaughlin, 1978; McLaughlin and Ohlin, 1984). These 

rocks are covered by Quaternary volcanics in Clear Lake basin to 

the northeast. It is not understood in detail how the thick section of 

covered Coast Range ophiolite and Great Valley complex relates to 

the Middle Mountain area northwest of Clear Lake, except that the 

synformal Middle Mountain block clearly aligns with these rocks 

and is broadly equivalent in age. Where exposed east-southeast of 

the Collayomi Fault Zone outside of the map area, the Great Valley 

complex rocks have been interpreted as attenuated along imbricate 

low-angle normal faults (Jayko and others, 1987). This is contrary 

to earlier interpretations explaining younger-over-older low-angle 

faulting in the Great Valley rocks by out-of-sequence thrusting (Swe 

and Dickinson, 1970; Suppe, 1979).

We suggest that the younger-over-older faulting may best be 

explained by attenuation associated with incremental unroofing 

of the Coast Ranges (1) partly in the Late Jurassic, producing 

ophiolitic fault-scarp breccias in the basal Great Valley complex; 

(2) again in the Early Cretaceous (Hauterivian) when detrital 

serpentinites were deposited as part of the lower Great Valley 

complex in the Wilbur Springs-Knoxville areas (McLaughlin and 

others, 2016); and (3) also in the latest Cretaceous to early Tertiary, 

when the Franciscan Complex was unroofed during westward 

stepout and shallowing of slab subduction (Ernst and McLaughlin, 

2012). Unroofing of the Franciscan may have been driven by 

contraction and uplift in the roof-thrust zone of east-vergent 

tectonic wedges of the structurally low Franciscan Coastal and 

Central Belts that were thrust beneath the structurally higher 

Eastern Belt. Tectonic wedge-driven contraction induced warping 

and uplift in the roof of the wedge complex, followed by exposure, 

erosion, and sedimentary recycling of the structurally higher rocks 

of the Eastern and Central Belts, as well as attenuation faulting 

in the western Great Valley (Jachens and others, 1995; Godfrey, 

1997). Ages of the strata involved in younger-over-older faulting 

in the Clear Lake area suggest that much of it occurred during 

this latter (Late Cretaceous–early Tertiary) period of low-angle 

extension.

Faults Northeast of the Bartlett Springs Fault Zone

In the southeastern part of the map area northeast of the 

Northern Bartlett Springs Fault Zone, several complex north- to 

northwest-striking zones of bedrock faulting are mapped, some 

of which exhibit youthful physiographic expression, but they are 

not well studied (map, sheet 1; fig. 5, sheet 2). These fault zones 

mainly flank the northeast and southwest sides of the Wilbur 

Springs Antiform, cutting rocks of the Coast Range ophiolite 

and overlying Great Valley complex and local Quaternary basin 

fills of Bear Valley and smaller depressions. One of the faults 

(Pacific Ridge Fault Zone) transects the Yolla Bolly terrane of the 

Franciscan Eastern Belt along the southwest flank of the Pacific 

Ridge Antiform (see structure section D–D ʹ ) and is subparallel in 

trend to the Northern Bartlett Springs Fault Zone.

The Bad Ridge Fault Zone is mapped as a wide zone of 

diffuse short fault segments entirely within the broad sheet 

of ultramafic rocks of the Coast Range ophiolite northeast of 

Little Indian Valley (map, sheet 1; fig. 5, sheet 2; McLaughlin 

and others, 1990). The faults of this zone locally exhibit scarps 

that bound or flank small basin-like depressions with thin 

accumulations of locally derived Quaternary sediment. Mineral 

springs also vent along some of these faults (Slowey and Rytuba, 

2008). The crustal geometry of this diffuse fault zone is largely 

unknown, except that the overall character of the faulting is 

extensional. No data are available to evaluate whether these 

faults are associated with active faulting beyond suggestions 

from geomorphology. The relation of the Bad Ridge faulting to 

the other faults northeast of the Bartlett Springs Fault Zone is 

unknown.

The Bear Valley-Resort Fault Zone is mapped as a straight 

N. 15–20° W. fault that is subparallel to and disrupts bedding 

in the enclosing Jurassic and Lower Cretaceous section of the 

Great Valley complex (map, sheet 1; fig. 5, sheet 2; McLaughlin 

and others, 1990). To the northwest, the fault is mapped along 

the southwest side of the Quaternary fill of Bear Valley, in part 

from alignment of fluid seeps and subtle inflections in tributaries 

to Bear Creek (J. Rytuba, personal commun., 2015; Slowey and 

Rytuba, 2008). Entrenchment of the sediments in Bear Creek 

along the linear northeast side of Bear Valley possibly is associated 

with a buried right-stepped fault along the east side of the valley, 

which, if present, would suggest that Bear Valley is a dextral 

pull-apart basin. Further investigations are necessary to determine 

if displacement along the Bear Valley-Resort Fault Zone is right 

lateral and, if so, what the timing constraints are for the faulting. 

To the southeast of Bear Valley, the Bear Valley-Resort Fault 

Zone extends to the Wilbur Hot Springs area to merge with westnorthwest-trending faults of the Little Indian Valley Fault Zone.

17
The Little Indian Valley Fault Zone aligns with the northeast 

side of the Bartlett Springs Synform, whose southwest side is 

highly disrupted and sheared along the Northern Bartlett Springs 

Fault Zone (map, sheet 1; fig. 5, sheet 2). Both the Bear ValleyResort and Little Indian Valley Fault Zones are oriented along the 

northeast and southwest limbs, respectively, of the Wilbur Springs 

Antiform and together represent integral parts of the Wilbur 

Springs Dextral Hook (discussed below). Displacement along the 

Little Indian Valley Fault Zone contributed to dismemberment of 

the Bartlett Springs Synform along the Northern Bartlett Springs 

Fault Zone. The Little Indian Valley Fault is mapped as offsetting 

rocks of the Coast Range ophiolite and lower Great Valley 

complex, including detrital serpentinite that is folded around the 

axis of the Wilbur Springs Antiform. The merged Bear Valley and 

Little Indian Valley Fault Zones, in turn, are the locus of past and 

present hydrothermal venting and mineralization over the Wilbur 

Springs Antiform, but these diffuse fault zones do not extend 

southeast of the main body of detrital serpentinite in the core of 

the Wilbur Springs Antiform. Instead, a zone of structurally higher, 

diffuse, east-west-oriented faults traverses across the axis and 

nose of the antiform (map, sheet 1; fig. 5, sheet 2). We view the 

distribution of these faults around the nose of the Wilbur Springs 

antiformal structure as associated with flexural slip and dextral 

drag of Great Valley complex strata and ophiolitic rocks into the 

Northern Bartlett Springs Fault Zone. As such, all of these faults 

may have experienced displacement associated with initiation 

and continued right-lateral displacement along the Northern 

Bartlett Springs Fault Zone. The significance of these structures 

as potentially active in the modern context of seismic hazard is 

presently unclear but should be considered in the modeling of 

crustal structure over this area.

Major Folds

Wilbur Springs Dextral Hook

The regional geometry of the contact between the Franciscan 

Complex and structurally overlying Coast Range ophiolite and 

Great Valley complex is dominated in the map area by a structure 

here referred to as the Wilbur Springs Dextral Hook (figs. 7, 

8, sheet 2). This major structure comprises a paired southeastplunging antiform (Wilbur Springs antiform) flanked to the 

southwest by a synform (Bartlett Springs Synform) that has been 

highly attenuated and dismembered by dextral strike-slip faulting 

along the Northern Bartlett Springs Fault and the southwest side 

of the Great Valley fore-arc basin (figs. 6, 7, 8, sheet 2). The 

dextral hook in this area is defined by prominent right-lateral 

shearing of the former Bartlett Springs Synform along the 

Northern Bartlett Springs Fault, which truncates the southwest 

limb of the Wilbur Springs Antiform. Combined transpressional 

folding and extensive right-lateral shearing that accompanied or 

post-dated folding define the dextral hook.

Figure 7, a reconstruction of the northeastern Coast Ranges, 

is focused over the Northern Bartlett Springs Fault Zone between 

Wilbur Springs and Round Valley and emphasizes structural 

features associated with the Wilbur Springs Dextral Hook. The 

Wilbur Springs Antiform, Bartlett Springs Synform (figs, 6, 7, 

8) and Northern Bartlett Springs Fault Zone represent integral 

parts of the structural hook. The antiformal character of the 

northeast side of the Wilbur Springs Dextral Hook is exhibited 

by the Wilbur Springs Antiform, but instead of being coupled 

on the southwest with a simple synformal fold, the southwest 

limb of the antiform is truncated and right-laterally attenuated 

along the Northern Bartlett Springs Fault Zone, parallel to 

what is interpreted as the axial trend of a dismembered former 

synform (the Bartlett Springs Synform; previously discussed 

in-part in the Northern Bartlett Springs Fault Zone section). The 

extensive shearing and dismemberment of the synform extends 

from Wilbur Springs to Round Valley (~55–60 km), forming 

the “hook” component of the Wilbur Springs Dextral Hook. 

The structure provides relatively clear evidence of right-lateral 

displacement along the Northern Bartlett Springs Fault Zone 

and the regional contact between the Franciscan Complex and 

western side of the Mesozoic Great Valley fore arc. Because 

the Northern Bartlett Springs Fault Zone evolved from faulting 

rooted in the pre-transform subduction margin (see earlier section 

on Northern Bartlett Springs Fault Zone), the timing of initiation 

and evolution of the dextral hook is critical to an understanding 

of how the northern San Andreas system evolved from the 

subduction margin. The long history of the western side of the 

Great Valley fore arc as a part of the subduction margin, followed 

by its reactivation as a folded transpressional boundary and even 

later by right-lateral strike-slip associated with the Northern 

Bartlett Springs Fault Zone, however, forms a complex pattern of 

superposed structures that obscure evidence of the earliest strikeslip faulting and the timing of initiation of the related Wilbur 

Springs Dextral Hook.

Distinctive fossil cold-seep-related Lower Cretaceous 

(Hauterivian) and older Great Valley complex strata in Rice 

Valley correlate with similar strata in the Wilbur Springs area ~50 

km to the southeast (Campbell and others, 1993, 2002; Kiel and 

others, 2008; McLaughlin and others, 2010). These strata, along 

with overlying Paleocene–Eocene deposits in the Round Valley, 

Rice Valley, Middle Mountain, and Lower Lake areas, indicate 

that, prior to the Miocene, the outer Great Valley fore arc and its 

ophiolitic basement must have extended across much of the area 

that now exposes the Eastern and Central Belts of the Franciscan 

Complex (fig. 6A, sheet 2).

Units of the Coast Range ophiolite, ophiolitic mélange and 

Great Valley complex overlain by Paleogene strata in Round Valley, 

are now separated ~56 kms along the Northern Bartlett Springs Fault 

from a downwarped synformal sliver in Rice Valley having similar 

stratigraphy. Estuarine coal-bearing middle Miocene strata present 

in Round Valley (Clark, 1940), however, are not present in Rice 

Valley, perhaps indicating that initial dextral displacement of these 

rocks occurred between the Eocene and middle Miocene, although 

in our earlier discussion of long-term fault displacement we favored 

the Miocene for initiation of strike slip. We suggest that right-lateral 

separation may have begun in the Miocene but somewhat earlier than 

the middle Miocene estuarine deposition of strata in Round Valley.

Our reconstruction of the pre-Northern Bartlett Springs Fault 

configuration of the Bartlett Springs Synform (fig. 7, sheet 2) places 

Round Valley adjacent to and west of Rice Valley between the Eocene 

and middle to late Miocene along approximately the same outer part 

of the Great Valley fore arc (fig. 6A, B, sheet 2). A synclinally folded 

Paleocene conglomerate unit in the Rice Valley section contains 

clasts derived from blueschist-facies Franciscan metasandstone 

18
(Berkland, 1973), indicating that Paleocene deposition post-dated or 

accompanied major latest Cretaceous to Paleocene unroofing of rocks 

buried >20 km before being folded, deformed, and isolated as part of 

the Bartlett Springs Synform and the Wilbur Springs Dextral Hook. 

This crude timing suggests that the Wilbur Springs Dextral Hook 

formed no earlier than the Late Paleocene.

The Bartlett Springs Fault Zone is also linked southward with 

the Hayward-Calaveras Fault System of the eastern San Francisco 

Bay area, which has accommodated ~170 km of ~12 Ma and younger 

cumulative dextral displacement partitioned to the Calaveras Fault 

from the San Andreas Fault southeast of Hollister (McLaughlin and 

others, 1996). If the Bartlett Springs Fault Zone connected to the 

northern Hayward-Calaveras Fault system 12 Ma, then it is possible 

that the Northern Bartlett Springs Fault formed in the late middle 

Miocene. Folding associated with the Wilbur Springs Dextral Hook, 

thus, would have occurred earlier.

Recent studies in the southern Great Valley and Sierra Nevada 

batholith have described as much as 90° of vertical-axis clockwise 

rotation accompanying extensional collapse of the Tehachapi 

Mountains and southern Sierra Nevada batholith that occurred in the 

Late Cretaceous (fig. 8, sheet 2). Rotation and extensional collapse of 

the batholithic rocks was interpreted as driven by shallow subduction 

of an over-thickened Farallon Plate during the Laramide orogeny 

(Saleeby, 2003; Chapman and others, 2010, 2012). Shallowed 

subduction of the Farallon Plate in the Late Cretaceous to Paleocene 

is proposed to have produced east-directed tectonic wedging, uplift, 

and unroofing of the Franciscan Complex in the northern Coast 

Ranges (Ernst and McLaughlin, 2012). Major Late Cretaceous 

rotation of the batholithic rocks in the southern Sierra Nevada 

batholith was taken up in the adjacent southern Great Valley complex 

and its ophiolitic basement, as well as in the underthrust Franciscan 

Complex of the accretionary margin. Accommodation of the Late 

Cretaceous (approximately 95–85 Ma, Chapman and others, 2012) 

clockwise rotation might conceivably have extended into northern 

California and been accommodated as dextral transpression along 

the contact between the northern Great Valley complex and the 

Franciscan Complex. However, as noted earlier, folding and downwarping of the Bartlett Springs Synform (“hook” component of the 

Wilbur Springs Dextral Hook) into underlying ophiolitic basement 

of the Great Valley and unroofed Franciscan Complex rocks was no 

earlier than late Paleocene, much later than the timing of southern 

Sierra Nevada batholith rotation. The proposed timing of rotation, 

therefore, is too early to relate to transpressional deformation along 

the Great Valley-Franciscan interface in northern California. The 

timing of rotation in southern California also corresponds to the 

timing of deposition of some pelagic Campanian–Cenomanian 

rocks of the Coastal and Central Belts of the Franciscan Complex 

on oceanic basement, far from the continental margin, that did 

not accrete to northern California and become unroofed until the 

Paleogene (<60 Ma). It must be concluded that Cretaceous clockwise 

rotation in the southern Great Valley is not related to initiation of 

transpressional deformation associated with the Wilbur Springs 

Dextral Hook. This timing, instead, is more closely related to the 

timing of tectonic wedging and unroofing of the Franciscan Complex 

in northern California, which appears to have predated initiation of 

transpressional deformation and the Wilbur Springs Dextral Hook 

(fig. 8, sheet 2).

Significantly later, well-documented major ~15 to 5 Ma 

clockwise rotation of the southwestern Tehachapi Mountains, western 

Transverse Ranges, and Salinian block were associated with early 

to middle Miocene displacements of the San Andreas Fault System 

(Colgan and others, 2012).

We suggest that the Franciscan Complex in northern California 

was wedged into the western side of the Great Valley complex and 

its ophiolitic basement north of the Tehachapi Mountains in the Late 

Cretaceous–Paleogene (~70–56 Ma). Simultaneously, the Franciscan 

wedge may also have been pushed obliquely northward by the 

clockwise rotation of the southern Sierra Nevada, accompanied by 

rapid uplift (Chapman and others, 2012). In northern California, 

however, significant transpressional deformation along the Great 

Valley-Franciscan interface mainly followed latest Cretaceous and 

Paleogene uplift and unroofing of the Franciscan tectonic wedge 

and its roof-thrust system composed of the Coast Range ophiolite, 

ophiolitic mélange, and Great Valley complex. The timing of 

transpressional deformation—resulting in uplift and folding of the 

northern California Neogene fore arc and the initiation of the Wilbur 

Springs Antiform, Bartlett Springs Synform, and the resultant Wilbur 

Springs Dextral Hook (fig. 8, sheet 2)—corresponds approximately 

with San Andreas Fault system-related early Miocene and younger 

rotation and northward translation of the southwestern Tehachapi 

Mountains, western Transverse Ranges, and Salinian block (Colgan 

and others, 2012).

Antiforms and Synforms

All of the fold structures discussed below are associated with 

major slab-like fault blocks in the Franciscan Complex or in the Great 

Valley complex and Coast Range ophiolite. The folds are antiformal 

or synformal warps in these fault blocks. Other folding in the Great 

Valley complex and younger deposits is largely subsidiary to the 

more fundamental warps in the major fault blocks. We therefore 

focus our discussion of folding around the regional distribution of the 

antiformal and synformal structures. Structural relations associated 

with most of these structures have been discussed, in part, in the 

descriptions of the major faults bounding the structural blocks that 

incorporate the antiforms and synforms, so the following descriptions 

are sometimes shortened to avoid repetition.

Bartlett Springs Synform

The original extent of the Bartlett Springs Synform, as 

discussed in previous sections, is largely dismembered along the 

Northern Bartlett Springs Fault Zone as an integral part of the Wilbur 

Springs Dextral Hook structure (figs. 6, 7, 8, sheet 2), but a part of 

the synform remains essentially intact northeast of the fault zone 

southwest of Wilbur Springs (map, sheet 1; fig. 5, sheet 2). A large 

slab of ophiolitic mélange, remnants of lower Great Valley complex 

strata, overlying Paleogene strata and Miocene marine and estuarine 

strata distributed along the Northern Bartlett Springs Fault Zone 

between Wilbur Springs, Rice Valley, and Covelo are interpreted as 

parts of the former Bartlett Springs Synform that were dismembered 

along the Northern Bartlett Springs Fault Zone. Reconstruction of 

these dismembered deposits as a former northwestward extension 

of the Bartlett Springs Synform suggests ~38–47 km of cumulative 

dextral displacement and perhaps ~3mm/yr of long-term slip since 

the Miocene (~15–12 Ma) along the Northern Bartlett Springs Fault 

Zone (figs. 6, 7, 8). The timing of initiation of this dismemberment 

is poorly constrained and, therefore, this estimate of slip rate is 

considered highly speculative.

19
Wilbur Springs Antiform

The Wilbur Springs Antiform is a major southeast-plunging 

antiformal structure adjacent to and northeast of the Bartlett Springs 

Synform that folds the lower Great Valley complex and Coast Range 

ophiolite over underlying Yolla Bolly terrane rocks of the Franciscan 

Complex. The northeast flank and crest of the antiform where the 

Coast Range ophiolite overlies the Franciscan Complex is clearly 

seen in down-plunge views to the southeast from Little Indian Valley. 

The axial trend of the antiform apparently is truncated obliquely 

with dismembered parts of the Bartlett Springs Synform along the 

Northern Bartlett Springs Fault Zone northwest of Little Indian 

Valley (figs. 5, 6, 7, sheet 2). The axial area of the Wilbur Springs 

Antiform northwest and southeast of Little Indian Valley corresponds 

with dominant high, dissected topography northeast of the Northern 

Bartlett Springs Fault Zone. Together with the Pacific Ridge Antiform 

to the northeast of the Wilbur Springs Antiform, these transpressional 

structures appear to exert some degree of control on the modern 

topography. This relation may be significant to our understanding of 

active deformation along the Northern Bartlett Springs Fault Zone 

that is shown from microseismicity to dip steeply northeast, beneath 

the transpressional axes of the Wilbur Springs and Pacific Ridge 

Antiforms in this area (fig. 11; fig. 12, sheet 2).

Pacific Ridge Antiform

The Pacific Ridge Antiform is a narrow, elongate northweststriking, topographic high northeast of the Northern Bartlett Springs 

Fault Zone that is subparallel to, but stepped northeast from, the 

Wilbur Springs Antiform (figs. 5, 6, 7, sheet 2; map, sheet 1). The 

Pacific Ridge Antiform is flanked to the northeast by a complex 

synformal structure between Pacific Ridge and Snow Mountain, but 

the southwest flank of the fold appears to root into the steep-dipping 

Pacific Ridge Fault along the contact between Yolla Bolly terrane 

mélange in the core of the antiform and overlying metasandstone and 

argillite to the southwest (see structure section D–D ʹ, sheet 2). The 

faulting in this southwest limb of the antiform aligns with a narrow 

band of serpentinite along the fault, which extends southeastward 

into the main Coast Range ophiolite along the northeast limb of the 

Wilbur Springs Antiform. Merging of the narrow serpentinite band 

along the Pacific Ridge Fault with ultramafic rocks of the main Coast 

Range ophiolite to the southeast mimics the geometry of the larger 

scale Wilbur Springs Dextral Hook discussed above.

Bartlett Mountain Antiform

The Bartlett Mountain Antiform is inferred within the Sanhedrin 

Mountain block of Yolla Bolly terrane that overlies the Central Belt 

southwest of the Northern Bartlett Springs Fault Zone, northeast of the 

Middle Mountain Synform and Clear Lake. The basis for inferring an 

antiformal structure in the Sanhedrin Mountain block is partly from 

the mapped distribution of the metachert (unit cy) plus metavolcanic 

(unit vy) lithofacies of the Yolla Bolly terrane, particularly where these 

complexly folded rocks are seen to wrap around the southeastern 

side of the Sanhedrin Mountain block on the northeast side of Clear 

Lake. A broad northwest-trending antiformal warp beneath the central 

Sanhedrin Mountain block is further suggested from the structurally 

low distribution of mélange mapped in the central part of the 

Sanhedrin Mountain block (see structure sections, sheet 2). Presence 

of a broad antiformal warp is consistent with expectations of such a 

transpressional structure intervening between synformal structures 

northeast of the Northern Bartlett Springs Fault Zone and the Middle 

Mountain Synform southwest of the Sanhedrin Mountain block. We 

acknowledge, however, that the depicted Bartlett Mountain antiformal 

axis is likely oversimplified, as the structure of the Yolla Bolly terrane 

rocks of the Sanhedrin Mountain block is clearly more complex in 

detail than can be depicted.

Middle Mountain Synform

Mapping of the Middle Mountain structural block in some detail 

in the 1960s and 1970s (CDWR, 1966, 1968, 1969; Berkland, 1969, 

1972, 1978) shows it to be composed of a complexly attenuated 

section of Coast Range ophiolite, Great Valley complex, and 

Paleogene strata that, in turn, are warped into the southeast-plunging 

Middle Mountain Synform. The Middle Mountain synformal block 

does not appear to continue to the thick section of Great Valley 

complex and Coast Range ophiolite that underlies the Clear Lake 

Volcanic Field, as rocks of the Franciscan Central Belt intervene. 

However, similarities in the character of the Mesozoic units in these 

areas and alignment of the Middle Mountain block with the Clear 

Lake Mesozoic–Tertiary section across several major northwesttrending faults (fig. 4, map, sheet 1; fig. 5, sheet 2; McLaughlin and 

others, 1990) suggest that the Middle Mountain block and Clear Lake 

Mesozoic sections may be linked across some of the seismically 

active faults of the two areas and have an as-yet-unconstrained strikeslip history.

20

DESCRIPTION OF MAP UNITS

UNCONSOLIDATED DEPOSITS

af Artificial fill (Holocene)—Clay, silt, sand, rock fragments, organic matter, and man-made debris 

used in road and land fills, in dam construction, and for other anthropogenic purposes

Qls Landslide deposits (Holocene and Pleistocene)—Unconsolidated to weakly consolidated rock 

and soil debris, which has moved downslope by various landslide processes, including 

rotational slumping, surface creep, flowage, or combinations thereof

ALLUVIAL DEPOSITS

Qal Alluvial deposits (Holocene and Pleistocene)—Poorly sorted, unconsolidated sand, silt, gravel, 

and boulders deposited in modern stream and river channels and on adjacent low-lying 

flood plains. Include lake deposits of clay and silt and basin deposits (unit bcl of Hearn 

and others, 1995) along the margins of Clear Lake. Mapped locally, where age relations 

based on superposition with younger and older Quaternary deposits is unknown
21

Qoa Older alluvial deposits (Holocene and Pleistocene)—Weakly sorted to unsorted, weakly consolidated silt, clay, sand, and gravel deposits that may locally display clast imbrication 

and crossbedding, but the original depositional environment is unknown; moderately to 

extremely dissected; preserved along major drainages and valley margins

TERRACE DEPOSITS

Qt Alluvial terrace deposits (Holocene and Pleistocene)—Poorly to moderately well sorted, weakly 

to moderately consolidated sand, silt, gravel, and boulder deposits; deposited along or 

adjacent to major streams. Locally divided into the following units:

Qty Younger alluvial terrace deposits and surfaces (Holocene?)—Stream terraces elevated 

above the active stream channel with minimal dissection; delineated by flat geomorphic 

surfaces and in places includes erosional bedrock platforms (strath terraces) that may not 

be overlain by alluvial deposits

Qto Older alluvial terrace deposits and surfaces (Pleistocene)—Alluvial terrace deposits that 

are elevated above, and more dissected than, young alluvial terrace deposits. Associated 

with flat-lying surfaces eroded into older deposits and bedrock. Commonly composed of 

a thin (<5 m thick) layer of gravel and cobbles

Qtvo Very old alluvial terrace deposits and surfaces (Pleistocene)—Alluvial stream-terrace 

remnants associated with flat but highly dissected surfaces typically preserved well above 

active stream channels. Commonly composed of highly weathered mixture of silt with 

scattered gravel to cobbles

FAN DEPOSITS

Qf Alluvial fan deposits (Holocene and Pleistocene)—Poorly sorted, weakly consolidated sand, 

silt, gravel, and boulder deposits; deposited where streams disgorge onto flat plains at the 

mouths of canyons. Locally divided into the following units:

Qfy Younger alluvial fan deposits and surfaces (Holocene?)—Poorly sorted, weakly consolidated sand, silt, gravel, and boulder deposits of alluvial fan systems at different elevations, 

locally overlying or inset into older deposits

Qfo Older alluvial fan deposits and surfaces (Pleistocene)—Alluvial fan deposits that are elevated, dissected, and incised by younger fans and alluvial terrace deposits

Qfvo Very old alluvial fan deposits and surfaces (Pleistocene)—Topographically high, dissected, 

and uplifted alluvial fan deposits. Silt and clay with poorly cemented gravel lenses and 

moderately consolidated sand layers; mapped as continental deposits by Cardwell (1965) 

in the Potter Valley area

GLACIAL DEPOSITS

Qgd Glacial deposits, present locally (Pleistocene)—Unsorted gravel and boulder deposits and silt; at 

high elevations, in places accompanied by striae on bedrock, small cirques, and moraines. 

Distinctive flat but irregular surfaces viewed on aerial photographs, seen as far south as 

Snow Mountain along high divide east of Lake Pillsbury. Described by Holway (1914) 

and in several other early investigations summarized in Irwin (1960)

SPRING DEPOSITS

Qsn Hydrothermal deposits (Pleistocene and younger)—Local sinter-like spring deposits along faults 

and silica carbonate rock, a product of hydrothermal fluid reaction with serpentinite. Rocks 

are composed of a mix of Ca-Fe-Mg carbonate plus quartz. Hydrothermal spring sinter is 

mapped only along the Logan Fault at Lake Pillsbury, associated with serpentinite. Elsewhere, 

silica carbonate rock alteration of serpentinized ultramafic or other mafic rocks accompanies 

epithermal precious or base-metal mineralization (especially Hg, Au, Ag, Pb, and Zn). The 

hydrothermal fluids associated with the silica carbonate and related hydrothermal spring sinter 

is considered to have been heated by magmas associated with Pleistocene volcanism

Qtrv Travertine (Pleistocene and younger)—Layered CaCO3 associated with mineral springs and 

seeps, commonly forming mineralized terrace deposits along faults or stratigraphic 

contacts, deposited from CaCO3-saturated low-temperature fluids. Some travertine 

deposits may exhibit evidence of a mixed fluid history that included hot fluid circulation, 

evidenced by local occurrences of siliceous sinter
VOLCANIC ROCKS

Qv Clear Lake Volcanic rocks (Pleistocene)—Siliceous to andesitic volcanics of the Clear Lake 

region; includes rhyolites to basaltic andesites that are vitric to porphyritic and erupted 

as flows and flow breccias, with only minor explosive eruptive activity. These rocks have 

been described and mapped in great detail by other workers (Hearn and others, 1988, 

1995; Donnelly-Nolan and others, 1981; Hammersley and DePaolo, D.J., 2006). Age of 

volcanism is ~2.1 Ma to 300 ka

FLUVIAL AND LACUSTRINE FILL

QTc Cache Formation (Pleistocene and upper Pliocene)—Nonmarine fluvial and lacustrine deposits of gravel, sandstone, siltstone, and mudstone in the greater Clear Lake region, 

representing paleo-rivers and streams, lakes, and ponds. Gravels and silts locally contain 

vertebrate and plant fossils. Lacustrine sediments contain plants, freshwater fish, pollen, 

and ostracodes and locally include tephra layers (Rymer, 1981; Sims and others, 1981). 

Gravels of the Cache Formation southeast of Clear Lake are overlain by 1.45 Ma basalt 

and contain pebbles of obsidian derived from eroded 4.5 Ma Pliocene Sonoma Volcanics 

sources near Santa Rosa, suggesting fluvial transport from the south-southwest and lack 

of a well-developed divide between Clear Lake basin and the Napa-Santa Rosa areas in 

early Pleistocene

MARINE OVERLAP DEPOSITS

Tm Sandstone and lignitic shale of the Covelo area (middle Miocene)—Largely estuarine to shallow marine and locally coal bearing; correlated with the marine Temblor Formation in the 

southern Diablo Range of central California (Clark, 1940)

Te Marine sandstone and shale (Eocene)—Sandstone locally massive and spheroidally weathering. 

Deposited in nearshore to shelf or inner bathyal marine setting

Tep Sandstone, shale, and conglomerate (Eocene and upper Paleocene)—Deposited in nearshore 

to shelf marine setting. In most places unit is depositional on older rocks, locally with 

angular discordance, but in places, contacts with older rocks are sheared along low-angle 

normal faults. Locally contains the following units:

Tcg Conglomerate (Eocene and upper Paleocene)—Near base of unit. Varies in clast content. 

At Rice Valley, a local unit of conglomerate contains clasts of metasandstone derived 

from Eastern Belt Franciscan Complex metaclastic rocks that were exhumed from >20 

km depth (based on presence of lawsonite in the clasts) before their incorporation in the 

conglomerate

GREAT VALLEY COMPLEX 

Ku Sandstone, siltstone, and mudstone (Upper Cretaceous, Cenomanian and younger)—Marine 

turbidites, locally channelized and conglomeratic with submarine slumps or mass wasting 

deposits; deposited in shelf to bathyal settings of Great Valley fore-arc basin 

Kuls Limestone (Upper Cretaceous)—Locally bioclastic; forms concretions in mudstone sections or 

lenses several meters in length, mapped in the Middle Mountain area. Correlates with 

similar fossiliferous limestone lenses in the Whispering Pines quadrangle and on the 

Guenoc Ranch southwest of Clear Lake that formed in submarine cold-seep settings 

(K. Campbell, oral commun., 2004; Hepper, 2004; Richmond, 2005) and are associated 

with mudstones having Campanian radiolarian and planktonic foraminifer faunas (E.A. 

Pessagno, written commun., 1976; W.V. Sliter, written commun., 1976)

Kul Sandstone, siltstone, and mudstone with concretionary limestone and conglomerate (Upper 

and Lower Cretaceous, Cenomanian to Hauterivian)—Limestone interbedded with 

sandstone, siltstone, mudstone, and conglomeratic turbidites; deposited in outer shelf to 

bathyal settings; mapped in the Rice Valley area

ELDER CREEK TERRANE

Kl Siltstone, mudstone, minor sandstone, conglomerate, and sedimentary serpentinite (Lower 

Cretaceous, Albian to Hauterivian)—Marine strata deposited in submarine fan, slope, 

and basin plain settings, locally with submarine-seep and diapir-related mass-wasting 

deposits. Mudstone locally contains unmapped carbonate lenses in the Middle Mountain 

22
area, described by Berkland (1969) as containing Early Cretaceous dinoflagellates. 

Included in this unit are exposures of Great Valley-affinity mudstone and siltstone 

containing both Early Cretaceous (Berriasian) invertebrate and middle Cretaceous 

radiolarian (Aptian–Albian) faunas, exposed in a small structural window through the 

Pickett Peak terrane of the Franciscan Complex Eastern Belt in Estell Creek (fossil 

locality nos. 26, 27; table 1). Based on fossils and well-bedded unmetamorphosed 

character (Lehman, 1974), we interpret the Estell Creek unit as part of the Elder Creek 

terrane. See Surpless and others (2006) and McLaughlin and others (2016) for additional 

age constraints from U-Pb dating of detrital zircons in other sandstones of the Elder 

Creek terrane. Elsewhere in the map area, Lower Cretaceous strata of the Elder Creek 

terrane includes the following units:

Klcg Conglomerate (Lower Cretaceous, Albian to Hauterivian)—Mapped in Clear Lake Highlands quadrangle as lenses and channels

Klls Bioclastic limestone (Lower Cretaceous, Hauterivian)—Containing fossil cold-seep-related 

macrofauna, locally intercalated in sedimentary serpentinite

Kssp Sedimentary serpentinite (Lower Cretaceous)—Coarse submarine debris-flow deposits and 

fine-grained serpentinitic sandstone and mudstone turbidites, locally with intercalated 

bioclastic limestone; occurs at several horizons; locally includes rare boulder-size blocks 

of high-grade blueschist metamorphosed from garnet-amphibolite. Hornblende from 

amphibolite blocks in ophiolitic mélange and sedimentary serpentinite in the Wilbur 

Springs area has 40Ar/39Ar plateau ages of 165.6–167.5 Ma, similar to published ages of 

high-grade blocks within the Franciscan Complex and to crystallization ages in the Coast 

Range ophiolite (Shervais and others, 2011)

KJs Argillite, siltstone, and sandstone (Lower Cretaceous, upper Valanginian, to Upper Jurassic, 

Tithonian)—Thin bedded, locally with concretionary carbonate lenses, commonly 

containing Early Cretaceous and Late Jurassic Buchias and other macrofauna. Includes 

the Knoxville Formation of Lawton (1956). Considered equivalent to lithologically 

identical but highly disrupted and sheared rocks incorporated in ophiolitic mélange (unit 

KJom). About 35 km south of the map area, along Pope Creek, a siliceous tuff ~10 

cm thick is interbedded with argillite overlying basal megabreccia of the Great Valley 

complex. Sparse zircons in the tuff, believed to represent remnants from tephra air fall, 

were U-Pb dated at the Stanford-USGS SHRIMP lab at 151.1±2.5 Ma (McLaughlin and 

others, 2016), consistent with ages assigned to Buchias collected nearby in the lower part 

of the Elder Creek terrane

Jgb Megabreccia (Upper Jurassic, Tithonian to Kimmeridgian)—Sedimentary megabreccia, 

composed of gabbroic and basaltic clasts, with minor interbedded turbiditic basaltic 

sandstone. Present only locally at base of Elder Creek terrane sedimentary section. 

Angular blocky to sandy mafic igneous debris is considered to be derived from 

depositionally underlying Coast Range ophiolite. Gabbroic and basaltic breccia clasts are 

derived largely from mafic dikes, basaltic flows, and gabbroic rocks of the underlying 

Coast Range ophiolite. Megabreccia resembles breccias described from submarine fault 

scarps and along active strike-slip faults. Similar Upper Jurassic breccia occurs along the 

southwest side of Sacramento Valley at the base of the Elder Creek terrane in the Elder 

Creek and Paskenta areas (Blake and others, 1992, 1987; Robertson, 1990; Hopson and 

others, 2008).

KJom Ophiolitic mélange (Lower Cretaceous)—Fault bounded; present in structurally lower part of 

Elder Creek terrane; comprised of lower Great Valley strata, tectonically mixed with 

underlying rocks of the Coast Range ophiolite (see, for example, photos of “Grizzly 

Creek mélange terrane” in McLaughlin and Ohlin, 1984). Unit age is based on inferred 

time of tectonic mixing and penetrative shearing; ages of rocks in the unit indicate 

mixing was no earlier than the Valanginian (~134 Ma) and probably coeval with Albian 

to Hauterivian (~113–134 Ma) deposition of youngest Elder Creek terrane strata and 

sedimentary serpentinites (Hauterivian, ~131–134 Ma). Unit forms a thick zone of 

mélange, showing evidence of extensional (for example, boudinage with phacoid-shaped 

intact masses of serpentinized peridotite enclosed in a penetratively sheared serpentinitic 

matrix) and contractional deformation (for example, complex folding locally at outcrop 

scale within blocks and slabs of clastic strata); contraction thus appears to predate 

latest extension. Matrix areas of mélange may be either sheared serpentinite or argillite. 

Individually mapped blocks and slabs in mélange include the following units:

23
omv Mafic volcanic rocks, including pillow basalt, diabase, and noncumulate gabbroic 

intrusive rocks—Locally abundant in Morgan Valley area; locally includes minor blocks 

of mafic breccia that may correlate with megabreccia (unit Jgb)

oms Undivided marine clastic rocks, commonly argillitic (Lower Cretaceous and 

Upper Jurassic)—Sheared and isoclinally folded at outcrop scale; locally contains 

concretionary carbonate lenses containing Late Jurassic and Early Cretaceous Buchias

and rarely conglomerate

omd Diabase, mapped locally—Sheared and coarse to fine grained with ophitic texture; may 

display chill zones; probably represents dike screens, but an in-situ origin is not evident

omg Gabbro, mapped locally—Sheared and medium- to fine-grained, largely noncumulate rocks 

with granular textures, with clinopyroxene and secondary amphibole, probably derived 

from deep-seated intrusives; evidence of an in-place origin is lacking

omc Radiolarian chert, mapped locally (Upper Jurassic, Kimmeridgian)—Locally tuffaceous; 

contains Late Kimmeridgian radiolarian fauna in Little Indian Valley area (McLaughlin 

and others, 1990)

omb Blueschist, mapped locally

omun Blocks of unknown lithology—Mapped aerially but not examined in the field

COAST RANGE OPHIOLITE

Joc Pelagic chert (Upper to Middle Jurassic, Tithonian to Bajocian)—Locally tuffaceous; 

depositional on basalt flows and may correlate with radiolarian chert in blocks of mafic 

volcanic rocks in ophiolitic mélange unit. Radiolarians from tuffaceous chert in the 

Stonyford section of the Coast Range ophiolite (Shervais and others, 2005a) are Middle 

to Late Jurassic (Kimmeridgian to Bajocian); in the nearby Geysers-Clear Lake region, 

correlative pelagic tuffaceous radiolarian chert in the Coast Range ophiolite is Late 

Jurassic (Kimmeridgian to Tithonian) (McLaughlin and others, 1988)

Jov Basaltic pillowed flows and flow breccias (Middle Jurassic)—Includes shallow-level diabase 

feeder dikes and sills that intrude pillow flows and breccias locally. Probably equivalent 

to some extrusive basaltic rocks in ophiolitic mélange unit

Jod Diabase dikes and sills (Middle Jurassic)—Mafic dikes and sills, with ophitic and microgranular 

textures. Structurally underlie or locally intrude the lower part of basaltic flows and flow 

breccias and, in part, the same age or younger; may correlate with some blocks of diabase 

in ophiolitic mélange unit

Jog Gabbro and ultramafic rocks (Middle Jurassic)—Gabbro, coarsely crystalline to medium grained 

locally with layered (cumulate) texture. Structurally underlies intrusive and extrusive parts of 

the Coast Range ophiolite; in places, includes plagiogranite and intrusive hornblende gabbro 

dikes or sills. Locally, unit grades downward into layered (cumulate) ultramafic rocks; pods 

and lenses of chromitite locally occur in cumulate layering transitional from dunite to gabbro. 

U-Pb ages on zircon (Hopson and others, 1981; McLaughlin and others, 1988), 40Ar/39Ar 

ages on glass (Shervais and others, 2005b), and K-Ar ages from hornblende (Lanphere and 

others, 1971) in cumulate and noncumulate gabbroic rocks of the Coast Range ophiolite in the 

northern Sacramento Valley (Paskenta and Elder Creek), and its structural outliers in the nearby 

Clear Lake area (Harbin Springs) range from 155 Ma (K-Ar, hornblende) to 169–163 Ma (UPb, zircon)

Josp Serpentinized ultramafic rocks (Middle Jurassic)—Layered ultramafic cumulate rocks that 

are transitional upward, or that alternate laterally, with tectonized harzburgite, dunite, 

peridotite, and rare screens of cumulate gabbro. The serpentinized ultramafic rocks are 

generally penetratively sheared and commonly form folded sheet-like zones of serpentinitematrix mélange. A penetrative shear fabric and widespread boudinage have extensively 

overprinted any primary layering in these serpentinized rocks. In least sheared parts of 

the serpentinized ultramafic unit, primary compositional layering in dunite and peridotite 

may be locally preserved. Rare screens of noncumulate microgabbro or diabase, which 

weather into pinnacle-like topographically resistant exposures above an enclosing softer 

serpentinite matrix, may have been intrusive into the ultramafic rocks prior to their shearing 

into the enclosing mélange (for example, Macpherson and Phipps, 1988). The serpentinized 

ultramafic rocks mapped here as parts of the Coast Range ophiolite are incorporated in 

places into ophiolitic mélange (unit KJom) as intact kilometer-scale or smaller mélange 

slabs and sheared mélange matrix. (See Correlation of Map Units, sheet 2)

24
FRANCISCAN COMPLEX

CENTRAL BELT

Central Belt Mélange terrane (lower Tertiary and Upper Cretaceous)

fcm Undivided mélange (lower Tertiary and Upper Cretaceous)—Unit contains both large 

broken slabs of intact to semi-intact rock up to several kilometers in length and much 

smaller mappable blocks (“knockers”) from 1 m to hundreds of meters in length, all 

enclosed by a mélange matrix of crushed, penetratively sheared and boudinaged argillite 

and sandstone. The large, intact slab-like bodies consist largely of sandstone and basalt, 

or a composite of interlayered basalt, chert, and sandstone. Areas underlain by extensive 

mélange matrix commonly exhibit numerous earth flows. The blocks are identified where 

known, but many blocks visible aerially were not accessed on the ground and are labeled 

only as blocks of unknown composition. Undivided mélange includes blocks too small to 

show at map scale, but locally, the lithologies of small blocks are delineated by symbols 

where accessed. Some chert in large slabs of interlayered basalt, chert, and sandstone 

contains radiolarians that correlate in age with parts of the Marin Headlands-Geysers terrane 

(McLaughlin, 1978; McLaughlin and Pessagno, 1978; Murchey and Jones, 1984). Other 

smaller mélange blocks of sandstone, basalt, and chert may also correlate with the Marin 

Headlands-Geysers terrane and probably represent dismembered fragments of that terrane. 

Interlayered basalt, chert, sandstone, and rare conglomerate in the large coherent slabs, some 

of which may have radiolarian or other fossil control, are assigned to the Marin HeadlandsGeysers terrane. Smaller, individual mélange blocks of basalt, chert, and sandstone without 

any biostratigraphic age control are delineated only as generic blocks of their respective 

lithologies (ss, c, or v). Some of these mélange blocks are of higher metamorphic grade than 

the Marin Headlands-Geysers terrane and possibly are derived from metachert, metabasalt, 

and metasandstone in the Eastern Belt. We have included two additional tectonostratigraphic 

terranes in the Central Belt (the Pomo terrane and Snow Mountain volcanic terrane) that occur 

only in the map area and are distinguished by their distinctive tectonostratigraphy and igneous 

characteristics. Individually mapped undivided mélange slabs and blocks on the geologic map 

include the following units:

ss Sandstone and argillite (in part, Upper Cretaceous, Cenomanian or younger; middle 

Cretaceous, Aptian or younger; and possibly, Upper Jurassic, Tithonian or younger)

c Radiolarian chert (Upper Cretaceous to Middle Jurassic)

v Basaltic volcanic rocks (Jurassic)

cgl Conglomerate (Lower Cretaceous and (or) Upper Jurassic or younger)—Rare, occurs as block in mélange northeast of Willis Ridge in northwestern part of map

db Diabase (Cretaceous or Jurassic?)

ms Metasedimentary rocks (Lower Cretaceous? or younger)

b Blueschist blocks (Jurassic)

un Blocks of unknown lithology—Viewed aerially but not accessed on ground

Pomo terrane (Upper Cretaceous to Upper Jurassic)

spo Basaltic breccia, sandstone, and shale (Upper Cretaceous, Maastrichtean)

vpo Alkalic pillow basalt, pillow-breccias, and minor diabase and chert (Jurassic)

Marin Headlands-Geysers terrane (Upper Cretaceous to Lower Jurassic)

smg Sandstone and argillite (Upper Cretaceous, Cenomanian? or younger)

cgmg Conglomerate (Upper Cretaceous, Cenomanian? or younger)—Large, thick, coarse, 

cobbly to bouldery lenses interbedded with unit smg southwest of Camelback Ridge and 

serpentinite sheet (unit Josp) in southwest corner of the map

cmg Radiolarian chert (Upper Cretaceous, lower Cenomanian to Lower Jurassic, Pliensbachian)

vmg Basaltic volcanic rocks (Lower Jurassic)

Snow Mountain volcanic terrane (Lower Cretaceous, Valanginian or younger?)

vsm Basaltic to rhyolitic volcanics and intrusive rocks

dsm Diabasic intrusive rocks, largely sill-like—Locally mapped as intruding basaltic to 

rhyolitic rocks

psm Porphyritic basaltic rocks—Possibly includes intrusive sills and non-pillowed flows

25
ssm Metasandstone and argillite—Locally intercalated with volcanic rocks; lawsonitebearing, with minor chert

EASTERN BELT

Yolla Bolly terrane (Upper and Lower Cretaceous to Upper Jurassic, Tithonian)

fym Mélange of the Yolla Bolly terrane (Upper Cretaceous)—Includes the following blocks:

by Blueschist

cgy Conglomerate

cy Metachert (Early Cretaceous, Albian or Aptian, to Middle Jurassic, Aelenian)—Also 

structurally interlayered with unit vy in unit fys; inferred correlation of cy blocks to cy

interlayered in fys is speculative

gy Gabbro

lsy Limey mudstone—Mapped in one area of Mendocino National Forest

vy Metavolcanic rocks (Middle Jurassic, Aelenian or older)—Also structurally 

interlayered with unit cy in unit fys; inferred correlation of vy blocks to vy interlayered 

in fys is speculative

mspy Metaserpentinite blocks or lenses

uny Unidentified blocks

fys Metasandstone and argillite, locally conglomeratic (Upper and Lower Cretaceous, 

Cenomanian to Aptian, to Late Jurassic, Tithonian)—Rhythmically interbedded, thick- to 

thin-bedded, turbiditic, slaty metasandstone and argillite, reconstituted to high textural zone 1 to 

textural zone 2 (Blake and others, 1967), commonly displaying graded bedding. Metasandstone 

locally with interbedded, thin lenses of dark chert-pebble-rich metaconglomerate. Rocks 

generally are highly folded isoclinally, with a prominent axial planar cleavage at high to low 

angle to bedding. Petrographically, metasedimentary rocks that are reconstituted to textural 

zone 2 commonly contain neoblastic lawsonite and phengitic white mica growing parallel to 

platy partings and in places deformed along the parting. Very rarely, jadeitic pyroxene and (or) 

sodic amphibole is present in some lithic metasandstone with significant mafic igneous detritus. 

Structurally low, fine-grained metasedimentary rocks, reconstituted only to textural zone 1 in 

the Little Indian Valley area, do not contain lawsonite or other identifiable blueschist minerals; 

nevertheless, they still display a prominent slaty parting and have yielded numerous Lower 

Cretaceous and Late Jurassic (Tithonian–Valanginian) fossils (largely Buchia sp.)

cy Metachert (Lower Cretaceous, Albian to Aptian, to Middle Jurassic, Aalenian)—Red to green, 

tuffaceous radiolarian metachert with a prominent axial planar slaty parting, locally folded 

isoclinally. Overlies a thin unit of metabasaltic flows and breccias that is, in turn, faulted at 

the base. Analysis of radiolarians from the metachert have yielded somewhat different age 

relations. West of Covelo, along Hwy 162, a section of metachert studied by Isozaki and Blake 

(1994) was found to contain a Middle Jurassic (Aalenian) to Late Jurassic (Tithonian) fauna. 

Elsewhere, in the Sanhedrin Mountain area (Ohlin and others, 2010), along Hwy 20 north of 

Clear Lake, near Clear Lake Highlands, and near Borax Lake to the south (sheet 1), radiolarians 

range from Late Jurassic to Middle Cretaceous (Aptian or Albian) in Yolla Bolly terrane 

metacherts (this report and Ohlin and others, 2010). Although this unit is assigned the same unit 

symbol as metachert blocks in Mélange of the Yolla Bolly terrane (fym), direct correlation with 

cy blocks in unit fym is largely speculative

vy Metavolcanic rocks (Middle Jurassic, Aalenian or older)—As described in Ohlin and others 

(2010), mafic metavolcanic rocks of the Yolla Bolly terrane collectively mapped as greenstone 

include tuff, basaltic pillow flows, breccias, and diabase. In the Sanhedrin Mountain block, 

sparse lenses of metavolcanic rocks are locally interleaved structurally with thick lenses 

of metachert and metasandstone and argillite. We interpret these contacts of interleaved 

metasandstone, metachert, and metavolcanics rocks as probable faults. In the structurally lower 

part of the Sanhedrin Mountain block, adjacent to the Bartlett Springs Fault Zone, sheared 

lenses of tuffaceous greenstone are intersheared with lenses of conglomerate and feldspathic 

metasandstone. In the Hull Mountain block east of the Bartlett Springs Fault Zone, porphyritic 

pillow basalt and tuff are also locally present and well exposed in lower Mendenhall Creek, 

interlayered with metasandstone and folded at outcrop scale. Along Forest Service Road 

M1 north of Hull Mountain and south of Monkey Rock, a zone of metatuff is interlayered in 

metasandstone. The fine-grained metatuff locally contains blue sodic amphibole, light-green 

hornblende (edenite?), pumpellyite, and stilpnomelane, compatible with the metamorphic 

26
grade of the enclosing metasandstone and argillite. Although this unit is assigned the same map 

symbol as blocks of metavolcanic rocks that occur in mélange of the Yolla Bolly terrane (unit 

f ym), the implied correlation of the metavolcanic blocks to the metavolcanic rocks interleaved 

with unit cy in unit f ys is highly speculative

miy Mafic intrusive rocks (Early Cretaceous)—Ohlin and others (2010) described gabbroic dikes 

and sills that intrude Cenomanian(?) and older metasandstone and argillite in the Hull 

Mountain block, near Monkey Rock, and at a locality along the Hull Mountain road 1 km to 

the southeast. The intrusive rocks were earlier shown to have well-preserved chilled contacts 

(Layman, 1977; Echeverria, 1980) and they trend northwest discontinuously for 1 km. The 

largest intrusive body is 20 m thick (Layman, 1977). The intrusives exhibit chilled margins and 

the adjacent metasandstone shows evidence of baking along contacts with metasedimentary 

rocks. Metamorphic minerals present in the metagabbro include lawsonite, pumpellyite, albite, 

sodic amphibole, carbonate minerals, stilpnomelane, chlorite, quartz, and white mica (Layman, 

1977), indicating that intrusion predated blueschist-grade metamorphism

Pickett Peak terrane (Lower Cretaceous, Aptian to Barremian)

fpp Undivided metasedimentary and metavolcanic rocks and metachert

cpp Metachert

vpp Basaltic metavolcanic rocks

Mendocino Pass terrane (Lower Cretaceous, Valanginian)

fmp Foliated metasandstone, argillite, and minor undivided metachert and greenstone

cmp Metachert and greenstone blocks

27

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32
33

Figure 14. Map showing locations and locality numbers of fossils listed in table 1 (https://doi.org/10.3133/sim3395) and plotted on the 

geologic map (sheet 1). The geology is shown in grayscale to provide a context for locating the fossil occurrences on the geologic map. 

Where fossil localities are numerous and closely spaced, locality numbers may be grouped. Some specific fossil localities or groups of 

localities cited in the text discussion are also labeled on the map.

19

1-18

20-21 22-23

24

28

29

30

25

26-27

33

35

36,40 38-39

41,45-46,48,57

60,62,69

43-44

61,64

63,65-67,70

31

47,52-54

49-51,56,59

80

68,71 73

81

32 34

37 42

92 95-97

100,102

99

101

113

104,107-112

114,116,118,120,123

87

88

90 93 91

94 98

76-78

84 85

86

72

74

75 79

82

83

133

140 162,163

166 167

168-172,174-190

134-143,145-161

173

164-165

129-132

115,117,119,

121,125-127

89

103 105,106

Round Valley

Wilbur 

Springs

Lower 

Lake

Borax

Lake

Little 

Indian 

Valley

Stonyford

Snow 

Mountain

Lake 

Pillsbury

Pomo

terrane

Estell

Creek

Rice

Valley

Clear

Lake
Table 1. Paleontological data from Tertiary and Mesozoic rocks of the map area from Round Valley to Wilbur Springs, northern Coast Ranges, California. Figure 14 shows the fossil 

localities on a page-size figure in grayscale, to help the user find fossil localities on the map (sheet 1). Some localities of Mesozoic fossils that plot on the geologic map within or surrounded by Quaternary deposits are in isolated areas of bedrock underlying alluvium that are too limited to show at the scale of the geologic map (for example, small bedrock windows in stream bottoms, or bedrock interspersed in flights of grouped fluvial terraces of varying age). Some localities might also be associated with Quaternary erosional surfaces on bedrock that do not include actual deposits and instead are only geomorphic surfaces. We include the coordinates for some fossil localities in the Franciscan Complex and Great Valley Complex that are outside of the area of this map (designated as map no. 999) but are in adjacent or nearby areas of the northern Coast Ranges. Refer to the cited sources of the fossil data for more information on the geology of these areas.[Map_No, location number shown on the geologic map (sheet 1) and fig. 14, as well as included in the database. ID, field or laboratory identification number from sample collector. Lat and Long, latitude (N.) and longitude (W.). Fossil and age, identified fossils and ages. Source, source of fossil data. Lith, lithology of enclosing rock. Unit, lithologic or geologic formational source of fossil, where known]

Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

1 143 39.7420 -123.2809 Ammonite: Pachydiscus; Ammonoid: Nostoceras; 

Mollusks: Inoceramus; Upper Cretaceous

Clark, 1940 Sandstone, shale, 

and conglomerate

2 144 39.7400 -123.2924 Ammonite: Pachydiscus; Ammonoid: Nostoceras; 

Mollusks: Inoceramus; Upper Cretaceous

Clark, 1940 Sandstone, shale, 

and conglomerate

3 161A 39.7332 -123.2775 Ammonite: Pachydiscus; Ammonoid: Nostoceras; 

Mollusks: Inoceramus; Upper Cretaceous

Clark, 1940 Sandstone, shale, 

and conglomerate

4 161 39.7330 -123.2763 Ammonite: Pachydiscus; Ammonoid: Nostoceras; 

Mollusks: Inoceramus; Upper Cretaceous

Clark, 1940 Sandstone, shale, 

and conglomerate

5 ~(149,148,153) 39.7293 -123.2546 Ammonite: Pachydiscus; Ammonoid: Nostoceras; 

Mollusks: Inoceramus; Upper Cretaceous

Clark, 1940 Sandstone, shale, 

and conglomerate

6 ~(149,150) 39.7283 -123.2597 Ammonite: Pachydiscus; Ammonoid: Nostoceras; 

Mollusks: Inoceramus; Upper Cretaceous

Clark, 1940 Sandstone, shale, 

and conglomerate

7 146 39.7281 -123.2572 Macrofossils: Brachysphingus liratus Gabb; Turritella infragranulata Gabb; Tellina undulifera

Gabb; Eocene

Clark, 1940 Sandstone and shale

8 ~105 39.7268 -123.2619 Macrofossils: Brachysphingus liratus Gabb; Turritella infragranulata Gabb; Tellina undulifera 

Gabb; Eocene

Clark, 1940 Sandstone and shale

9 156 39.7266 -123.2530 Macrofossils: Brachysphingus liratus Gabb; Turritella infragranulata Gabb; Tellina undulifera 

Gabb; Eocene

Clark, 1940 Sandstone and shale

10 89 39.7265 -123.2602 Macrofossils: Brachysphingus liratus Gabb; Turritella infragranulata Gabb; Tellina undulifera

Gabb; Eocene

Clark, 1940 Sandstone and shale

11 85,85A 39.7257 -123.2629 Ammonite: Pachydiscus; Ammonoid: Nostoceras; 

Mollusks: Inoceramus; Upper Cretaceous

Clark, 1940 Sandstone, shale, 

and conglomerate

12 158 39.7234 -123.2451 Ammonite: Pachydiscus; Ammonoid: Nostoceras; 

Mollusks: Inoceramus; Upper Cretaceous

Clark, 1940 Sandstone, shale, 

and conglomerate

13 92,92-1 39.7230 -123.2468 Macrofossils: Brachysphingus liratus Gabb; Turritella infragranulata Gabb; Tellina undulifera 

Gabb; Eocene

Clark, 1940 Sandstone and shale

34
Table 1.—Continued

Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

14 49 39.7221 -123.2595 Ammonite: Pachydiscus; Ammonoid: Nostoceras; 

Mollusks: Inoceramus; Upper Cretaceous

Clark, 1940 Sandstone, shale, 

and conglomerate

15 168 39.7146 -123.2384 Macrofossils: Desmostylus tooth; Ostrea appressa; 

Ostrea; Natrica sp.; Corbicula (?); Pecten; Middle (to Early?) Miocene

Clark, 1940 Shallow marine 

to estuarine conglomerate, sandstone, shale, and coal

16 169-c 39.7126 -123.2837 Macrofossils: Desmostylus tooth; Ostrea appressa; 

Ostrea; Natrica sp.; Corbicula (?); Pecten; Middle (to Early?) Miocene

Clark, 1940 Shallow marine 

to estuarine conglomerate, sandstone, shale, and coal

17 50[~50-1,50-3] 39.7028 -123.2426 Macrofossils: Brachysphingus liratus Gabb; Turritella infragranulata Gabb; Tellina undulifera

Gabb; Eocene

Clark, 1940 Sandstone and shale

18 56 39.6975 -123.2691 Macrofossils: Desmostylus tooth; Ostrea appressa; 

Ostrea; Natrica sp.; Corbicula (?); Pecten; Middle (to Early?) Miocene

Clark, 1940 Shallow marine 

to estuarine conglomerate, sandstone, shale, and coal

19 164 39.6833 -123.2626 Macrofossils: Desmostylus tooth; Ostrea appressa; 

Ostrea; Natrica sp.; Corbicula (?); Pecten; Middle (to Early?) Miocene

Clark, 1940 Shallow marine 

to estuarine conglomerate, sandstone, shale, and coal

20 120 39.6818 -123.2344 Macrofossils: Brachysphingus liratus Gabb; Turritella infragranulata Gabb; Tellina undulifera

Gabb; Eocene

Clark, 1940 Sandstone and shale

21 123 39.6811 -123.2317 Macrofossils: Brachysphingus liratus Gabb; Turritella infragranulata Gabb; Tellina undulifera

Gabb; Eocene

Clark, 1940 Sandstone and shale

22 M6600 39.6628 -122.9853 Pelecypods: Buchia pacifica (Jeletzky); Early 

Cretaceous (middle Valanginian)

Collected by J.C. Maxwell, 

Univ. of Texas, Austin, 1976; identified by D.L. Jones, 1977

Sheared argillite Franciscan Complex, Central 

Belt, “Poison Rock” mélange at Hayden Rock

35
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

23 BBH-15 39.6623 -122.9871 Radiolaria: Late Jurassic (Tithonian) Collected by H.N. Ohlin; identified by C.D. Blome Radiolarian chert block in mélange Franciscan Com- plex, Central Belt 

(Poison Rock mélange); near Hayden Rock

24 1 39.6042 -122.9343 Mollusks: Buchia, Lower Cretaceous, Valanginian D.L. Jones, in Ohlin and others, 

2010

Metasandstone Franciscan Eastern 

Belt, Yolla Bolly terrane

25 M6090 39.5942 -122.8536 Pelecypod: Buchia; Late Jurassic (Tithonian) Collected by Calif. Dept. of 

Water Resources geologists, as listed in Blake and Jones, 1974 

Mélange Franciscan Complex, Central Belt 

mélange

26 M6601 39.5919 -122.8356 Pelecypods: Buchia cf. B. uncitoides? (Pavlow), 

bivalve fragments; Early Cretaceous (Berriasian?), Radiolaria: sparse, appears to correlate with Early Cretaceous (upper Aptian or lower Albian or younger) section of Dry Creek

Macrofossils collected by J.C. 

Maxwell, 1976; identified by D.L. Jones, 1977. Radiolaria reported in D.H. Lehman, 1974 (table 7); identified by E.A. Pessagno from same locality

Macrofossils and radiolaria, of mixed 

age, present in thin carbonate beds and concretions in thin bedded shaley turbidites in structural window through Pickett Peak terrane 

Probable Great 

Valley sequence, imbricated between underlying rocks of Franciscan Central Belt mélange (and possibly, Eastern Belt Yolla Bolly terrane) and overthrust rocks of Franciscan Eastern Belt, Pickett Peak terrane

27 M6603 39.5919 -122.8356 Same loc. as M6601—Pelecypod: Buchia uncitoides (Pavlow); Early Cretaceous (Berriasian), but 

see M6601

Collected by J.C. Maxwell, 

1976; identified by D.L. Jones, 1977

Thin carbonate beds 

and concretions in well-bedded shaley turbidites of Estell Creek

Probable Great Valley sequence

28 2 39.5764 -122.9721 Radiolaria: Parvicingula(?) procera, Pessagno; 

Parvicingula sp., ?Podubursa sp., Pseudodictyomitra sp.; Late Jurassic, Tithonian

C.D. Blome in Ohlin and others, 

2010

Chert in mélange Franciscan Complex, Central Belt

29 3 39.5491 -122.9193 Mollusks: Inoceramus; Lower Cretaceous, Cenomanian (?) Blake and Jones, 1974; cited in Ohlin and others, 2010 Metasandstone Franciscan Eastern Belt, Yolla Bolly 

terrane

30 4 39.5308 -123.0112 Radiolaria: Archeodictyomitra sp., aff. A.simplex 

Pessagno; pseudodictyomitra sp.; Thanarla sp.; Lower Cretaceous, probably Aptian-Albian

C.D. Blome in Ohlin and others, 

2010

Metasandstone Franciscan Eastern 

Belt, Yolla Bolly terrane

36

Table 1.—Continued
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

31 MR0171 39.4158 -122.7153 Radiolaria: cones, very large squashed spheres 

with stubby spines; no reliable age assignment

Collected by R.D. Brown; described in Brown and others, 

1981. Radiolaria examined by E.A. Pessagno

Radiolarian chert at 

base of volcanicrich 300- to 500-ft-thick sedimentary unit

Snow Mountain/St. 

John Mountain volcanic complex of Central Belt of Franciscan Complex

32 ChertB 39.4122 -122.5859 Radiolaria: Zones 3-8 to 7-10 Tethyan UAZ of 

Baumgartner and others 1995; Middle and Upper Jurassic (lower Bajocian to upper Kimmeridgean)

Collected and identified by B.L. 

Murchey as reported in Shervais and others, 2005 

Radiolarian chert 

sequence in Stonyford volcanic complex

Stonyford volcanic 

complex of Coast Range ophiolite

33 5 39.4092 -122.9520 Mollusks: Buchia piochii, Late Jurassic, Tithonian R.W. Imlay in W.P. Irwin, 1960; 

cited by Ohlin and others, 2010

Shale and sandstone 

slab

Ophiolitic mélange 

34 ChertC 39.4047 -122.5963 Radiolaria: Zones 8-10, Tethyan UAZ of 

Baumgartner and others 1995; Middle to Upper Jurassic (middle Callovian-upper Kimmeridgean) 

Collected and identified by B.L. 

Murchey as reported in Shervais and others, 2005 

Radiolarian chert 

sequence in Stonyford volcanic complex

Stonyford volcanic 

complex of Coast Range ophiolite

35 6 39.3933 -122.9361 Mollusks: Buchia sp., Probably Late Jurassic, 

Tithonian based on proximity to B. piochii loc.

Etter, 1979; cited in Ohlin and 

others, 2010

Shale and sandstone 

slab

Ophiolitic mélange 

36 33 39.3791 -123.0838 Mollusks: Pelecypods; Dinoflagellates; U. Cretaceous, Campanian J.O. Berkland, Calif. Dept. of Water Resources Shale and thick bed- ded sandstone37 ChertA 39.3766 -122.6198 Radiolaria: Zones 3-6 to 6-6, Tethyan UAZ of 

Baumgartner and others 1995; Middle Jurassic (lower Bajocian-upper Bathonian)

Collected and identified by B.L. 

Murchey as reported in Shervais and others, 2005 

Radiolarian chert 

sequence in Stonyford volcanic complex

Stonyford volcanic 

complex of Coast Range ophiolite

38 18 39.3750 -123.0607 Microfossils: Lower Cretaceous J.O. Berkland Sandstone and shale, 

thin bedded

39 29 39.3736 -123.0667 Microfossils: Lower Cretaceous Shell Oil Co. Gray sandstone and 

shale

40 30 39.3726 -123.0797 Arenaceous foraminifera, nondiagnostic Shell Oil Co. Shale and thick bedded sandstone41 25 39.3720 -123.1082 Mollusks: oysters, pectens, U. Cretaceous J.O. Berkland Volcanic sandstone, 

conglomerate

42 ChertD 39.3713 -122.6032 Radiolaria: Zones 9-10, Tethyan UAZ of 

Baumgartner and others 1995; Upper Jurassic (middle Oxfordian to upper Kimmeridgean)

Originally collected and 

described by E.A. Pessagno, 1977; revised by B.L. Murchey in Shervais and others, 2005

Radiolarian chert 

sequence in Stonyford volcanic complex

Stonyford volcanic 

complex of Coast Range ophiolite

43 19 39.3696 -123.0479 Dinoflagellates: Ascodinium, Lower Cretaceous, 

Albian 

J.O. Berkland Massive sandstone 

and shale

37

Table 1.—Continued
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

44 20 39.3665 -123.0516 Dinoflagellates: Ascodinium, Lower Cretaceous, 

Albian 

J.O. Berkland Massive sandstone 

and shale

45 22 39.3664 -123.1062 Ammonite: Guadryceras, Microfossils, , U. Cretaceous, Maestrichtean J.O. Berkland Volcanic sandstone, conglomerate46 28 39.3599 -123.1027 Mollusks: Meekia, U. Cretaceous J.O. Berkland, Calif. Dept. of 

Water Resources

Volcanic sandstone, 

conglomerate

47 M4350 39.3588 -122.8697 Pelecypods: Nuculana sp., Tellina?, undet. Fragments; Age indeterminate Collected by J.O. Berkland, 1970; idenified by W.O. Addicot, 1970 Sandstone in bed of McLeod Creek, Rice Valley Unnamed formation of presumed early Tertiary age48 23 39.3585 -123.1036 Mollusks: oysters, pectens, U. Cretaceous J.O. Berkland Volcanic sandstone, 

conglomerate

49 M6765 39.3558 -122.8256 Pelecypods: Entolium (?) sp; Jurassic or Cretaceous Collected by J. Suppe, 1977; identified by D.L. Jones, 1977 No lithologic data provided No information50 M6766 39.3553 -122.8297 Pelecypods: Buchia sp., striate form, same as 

Buchia sp. in M6564; Late Jurassic or Early Cretaceous

Collected by J. Suppe, 1977; 

identified by D.L. Jones, 1977

No lithologic data 

provided

No information

51 M6767 39.3550 -122.8311 Pelecypods: Buchia cf. B. fischeriana (D’Orbigny); 

Late Jurassic (Tithonian)

Collected by J. Suppe, 1977; 

identified by D.L. Jones, 1977

No lithologic data 

provided

No information

52 M4304 39.3547 -122.8688 Gastropods: Brachysphingus mammilatus (Clark 

and Woodford), Calyptraea sp., Molopophorus?, Neverita cf. N. globosa (Gabb), Turbonilla sp., Turrid-Crassispira-like, Turritella sp., undet. internal molds-several taxa; Pelecypods: Acila decisa (Conrad), Arcid? fragment, Cardiidfragment, Diplodonta cf. D. cretacea (Gabb), Mytilus sp., Nuculana gabbi (Gabb)?, Ostrea sp.-fragment, Pitar?, Solen cf. S. stantoni(Weaver), Spisula sp., Spisula meganosensis (Clark and Woodford), Tellina longa (Gabb); late Paleocene, “Meganos stage”

Collected by J.O. Berkland, 

1970; identified by W.O. Addicot, 1970 

Tertiary marine 

strata, 4-ft-thick red-weathering silty limestone

Unnamed formation 

of presumed early Tertiary age

53 M4351 39.3544 -122.8669 Gastropods: Scaphander costatus (Gabb); Crustaceans: broken appendages and fragment of crab 

carapace

Collected by J.O. Berkland, 

1970; identified by W.O. Addicot, 1970 

Tertiary marine 

sandstone stratigraphically above (E. of) M4303, M4304

Unnamed formation 

of presumed early Tertiary age

54 M4352 39.3544 -122.8675 Gastropods: Scaphander?, Trochid-Bathybembixlike, Undet. Internal molds-2 taxa; Pelecypods: 

undet. Molds; Age indet.

Collected by J.O. Berkland, 

1970; identified by W.O. Addicot, 1970 

Tertiary marine 

sandstone stratigraphically above (E. of) M4303, M4304

Unnamed formation 

of presumed early Tertiary age

38

Table 1.—Continued
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

55 M4303 39.3542 -122.8693 Gastropods: Brachysphingus mammilatus (Clark 

and Woodford), Calyptraea cf. C. diegoana, Cryptoconus injucundus (Hanna)?, Ficopsis sp., Neverita cf. N. globosa (Gabb), Scaphander costatus (Gabb), Turbonilla sp., Turritella maganosensis (Clark and Woodford), T. cf. T. merriami brevitabulata (Merriam and Turner), Whitneyella meganosensis (Turner); Pelecypods: Acila sp.-fragment, Diplodonta cretacea (Gabb), Nuculana gabbi (Gabb), Pitar sp., Schedocardia hartleyensis (Clark and Woodford), Solen cf. S. stantoni (Weaver), Sphenia meganosensis (Clark and Woodford)?, Spisula sp., Tellina longa (Gabb), Verericardia sp.-fragment; late Paleocene, “Meganos Stage” of Clark and Vokes, 1936.

Collected by J.O. Berkland, 

1970; identified by W.O. Addicot, 1970 

Tertiary marine 

strata, 4-ft-thick red-weathering silty limestone

Unnamed formation 

of presumed early Tertiary age

56 M6769,M6769A 39.3528 -122.8197 Pelecypods: Buchia cf. B. fischeriana (D’Orbigny), 

Buchia piochii (Gabb); Late Jurassic (Tithonian)

Collected by J. Suppe, 1977; 

identified by D.L. Jones, 1977

No lithologic data 

provided

No information

57 24 39.3526 -123.1009 Mollusks: oysters, pectens, U. Cretaceous J.O. Berkland Volcanic sandstone, 

conglomerate

58 M6771 39.3525 -122.8594 Cephalopod: Phylloceras sp., Pelecypod: Buchia 

sp. Juv. cf. B. piochii (Gabb); Late Jurassic (Tithonian)

Collected by J. Suppe, 1977; 

identified by D.L. Jones, 1977

No lithologic data 

provided

No information

59 M6768 39.3511 -122.8267 Pelecypods: Buchia pacifica (Jeletzky); Early 

Cretaceous (early to middle Valanginian)

Collected by J. Suppe, 1977; 

identified by D.L. Jones, 1977

No lithologic data 

provided

No information

60 7,27 39.3471 -123.0587 Mollusks: Inoceramus schmidti, Canadoceras, 

Baculites, U. Cretaceous, Campanian

J.O. Berkland, Calif. Dept. of 

Water Resources

Limestone lens

61 26 39.3456 -123.0374 Microfossils Lower Cretaceous Shell Oil Co. Shale in Bucknell 

Creek bed

62 27 39.3455 -123.0586 Mollusks: Inoceramus, U. Cretaceous J.O. Berkland, Calif. Dept. of 

Water Resources

Limestone lens

63 M6770 39.3436 -122.8503 Cephalopod: Ammonite-indet. heteromorph, 

Belemnite-Acroteuthis sp.; Pelecypods: Entolium operculiformis, clams; Early Cretaceous (Hauterivian or Barremian)

Collected by J. Suppe, 1977; 

identified by D.L. Jones, 1977

No lithologic data 

provided

No information

64 8 39.3434 -123.0365 Microfossils, Lower Cretaceous J.O. Berkland? Shell Oil Co. Shale

39

Table 1.—Continued
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

65 kiel-rice.vly 39.3406 -122.8538 Brachiopods: Peregrinella whitneyi; Bivalves: 

Pecten complexicosta, “Modiola” major; Gastropods: Paskentana paskentaensis, Retiskenea? Tuberculata; Early Cretaceous (probably late Berriasian-early Hauterivian)

Collected by J.O. Berkland, 

1973; Campbell and others, 2008; identified by Campbell and others, 2008; Kiel and others, 2008

Small, isolated 

carbonate lenses in sandstone and shale, along margin of Rice Valley basin; interpreted as fossil coldseep deposits

Lower Great Valley 

sequence

66 M6395 39.3406 -122.8531 Brachiopods: Peregrinella whitneyi (Gabb); Early 

Cretaceous (Berriasian-Hauterivian)

Collected by J. Suppe, 1975; 

identified by D.L. Jones and J.W. Miller, 1975

No lithologic data 

provided

See also Kiel and 

others, 2008

67 M6396 39.3397 -122.8542 Brachiopods: Peregrinella whitneyi (Gabb); Early 

Cretaceous (Berriasian-Hauterivian)

Collected by J. Suppe, 1975; 

identified by D.L. Jones and J.W. Miller, 1975

No lithologic data 

provided

See also Kiel and 

others, 2008

68 M6763 39.3394 -122.7964 Pelecypods: Buchia cf. B. fischeriana (D’Orbigny); 

Cephalopod: Belemnite- Cylindroteuthis sp.; Late Jurassic (Tithonian)

Collected by J. Suppe, 1977; 

identified by D.L. Jones, 1977

No lithologic data 

provided

No information

69 9 39.3387 -123.0667 Pollen, U. Cretaceous, Maastrichtian? J.O. Berkland Thick-bedded sandstone and shale70 M6394 39.3356 -122.8483 Pelecypods: Buchia uncitoides (Pavlow); Early 

Cretaceous (Berriasian)

Collected by J. Suppe, 1975; 

identified by D.L. Jones and J.W. Miller, 1975

No lithologic data 

provided

71 M6764 39.3350 -122.7969 Pelecypods: Buchia piochii (Gabb); Late Jurassic 

(Tithonian)

Collected by J. Suppe, 1977; 

identified by D.L. Jones, 1977

No lithologic data 

provided

No information

72 10 39.3279 -123.0503 Mollusks: Turitella, Paleocene J.O. Berkland, Calif. Dept. of 

Water Resources

Sandstone

73 M6987 39.3181 -122.8561 Pelecypods: Buchia sp.; Late Jurassic or Early 

Cretaceous

Collected by S. Etter, Univ. 

Texas, Austin, 1978; identified by D.L. Jones

Conglomeratic sandstone in ophiolitic 

mélange

Lower Great Valley 

sequence; Etterʻs blue slides formation (Etterʻs unit Jbs); ophiolitic mélange of this report

74 11 39.3167 -123.0718 Microfossils Late Cretaceous, post Cenomanian J.O. Berkland Thick-bedded Sandstone and shale75 247 39.2861 -123.1625 Radiolaria: in overturned partial stratigraphic 

sequence: late Middle or early Late Jurassic (~MH-4B) to Early Cretaceous (probably Albian, ~MH-6 or MH-7B) 

Collected by J. Stanford (Stanford, 1991); Identified by Y. 

Isozaki, Yamaguchi Univ., Japan

28 m of radiolarian 

chert in mélange terrane of basalt, chert, greywacke

Franciscan Complex, Central Belt

40

Table 1.—Continued
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

76 12 39.2841 -123.0414 Mollusks: Buchia elderensis, Late Jurassic J.O. Berkland, Calif. Dept. of 

Water Resources

Mélange

77 13 39.2835 -123.0448 Mollusks: Inoceramus sp., U. Cretaceous J.O. Berkland Sandstone and shale78 14 39.2786 -123.0404 Mollusks: Meekia sella, U. Cretaceous J.O. Berkland Sandstone, volcanic 

conglomerate

79 225 39.2737 -123.1422 Radiolaria: Latest Jurassic to early Early Cretaceous (~MH-4 to MH-5B) Collected by J. Stanford (Stan- ford, 1991); identified by Y. 

Isozaki, Yamaguchi Univ., Japan

38 m of radiolarian 

chert in mélange terrane of basalt, chert, greywacke

Franciscan Complex, Central Belt

80 M6564 39.2731 -122.7297 Pelecypods: Buchia pacifica (Jeletzky); striated 

Buchia (new?) sp., B. elderensis (Anderson); this is a mixed collection- Early Cretaceous (Valanginian) and Late Jurassic (Tithonian); depositional age is thus Early Cretaceous (Valanginian) or younger

Collected by J. Suppe, 1975; 

identified by D.L. Jones and J.W. Miller, 1977

B. pacifica is 

from matrix of metasandstone; other Buchias are from a silicified concretion

Franciscan Complex, Eastern 

Belt, mélange of Yolla Bolly terrane, Goat Mountain area

81 M6397 39.2564 -122.7975 Pelecypods: Buchia pacifica Jeletzky; Early Cretaceous (Valanginian) Collected by J. Suppe, 1975; identified by D.L. Jones and 

J.W. Miller, 1975

No lithologic data 

provided

82 675 39.2548 -123.1590 Radiolaria: Latest Jurassic to early Early Cretaceous (~MH-4 to MH-5B) Collected by J. Stanford (Stan- ford, 1991); Identified by Y. 

Isozaki, Yamaguchi Univ., Japan

4.5 m of radiolarian 

chert in mélange terrane of basalt, chert, greywacke

Franciscan Complex, Central Belt

83 676 39.2400 -123.1573 Radiolaria: in partial stratigraphic sequence: Early 

Jurassic (Pliensbachian-Bajocian,~MH-1 to MH-2B) to Middle Jurassic (Bathonian, ~ MH-3 or MH-4B)

Collected by J. Stanford (Stanford, 1991); Identified by Y. 

Isozaki, Yamaguchi Univ., Japan

36.4 m of radiolarian chert overlying 

basalt in mélange terrane of basalt, chert, greywacke

Franciscan Complex, Central Belt

84 15 39.2361 -122.9533 Spores: Lower Cretaceous J.O. Berkland Sandstone and shale, 

thin bedded

85 16 39.2354 -122.9654 Mollusks: Inoceramus, L. Cretaceous J.O. Berkland Shale86 17 39.2141 -122.9446 Dinoflagellates: Lituosphaeridium, Lower Cretaceous J.O. Berkland, Calif. Dept. of Water Resources Sandstone and shale, thin bedded87 31 39.2078 -122.9288 Microfossils Middle Eocene, probably Domengine 

or Capay

Shell Oil Co. Sandstone and shale, 

thin bedded 

88 32 39.2046 -122.9232 Mollusks: Buchia piochii, Upper Jurassic J.O. Berkland, Calif. Dept. of 

Water Resources

Mélange

89 M6407 39.1508 -122.5064 Pelecypods: Buchia piochii (Gabb)?; Late Jurassic 

(Tithonian?), based on B. piochii

Collected by J. Suppe, 1975; 

identified by D.L. Jones and J.W. Miller, 1975

No lithologic data 

provided

No information

41

Table 1.—Continued
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

90 OG81-57 39.1427 -122.9832 Radiolaria: Late Jurassic (Tithonian) Collected by H.N. Ohlin; identified By C.D. Blome Radiolarian chert in basaltic volcanic 

rocks

Franciscan Complex, Central Belt 

(probably Marin Headlands-Geysers terrane)

91 MG81-171 39.1424 -122.8110 Radiolaria: Late Jurassic (Tithonian) to Early 

Cretaceous (early Valanginian)

Collected by R.J. McLaughlin; 

identified By C.D. Blome

Radiolarian chert in 

mélange

92 M6398 39.1417 -122.6394 Pelecypods: Buchia cf. B. piochii (Gabb) sp. Juvenile; Late Jurassic (Tithonian) Collected by J. Suppe, 1975; identified by D.L. Jones and 

J.W. Miller, 1975

No lithologic data 

provided

93 HS309(MR1076) 39.1413 -122.9146 Radiolaria: Late Jurassic (late Kimmeridgean/ early Tithonian-Emiluvia Hopsoni Zone 2 of 

Pessagno, 1977)

Collected by H.N. Ohlin; identified By C.D. Blome Radiolarian chert in mélange Franciscan Com- plex, Central Belt

94 OG81-47 39.1368 -122.9827 Radiolaria: Late Jurassic (Tithonian) Collected by H.N. Ohlin; identified By C.D. Blome Radiolarian chert in basaltic volcanic 

rocks

Franciscan Complex, Central Belt 

(probably Marin Headlands-Geysers terrane)

95 M6399 39.1353 -122.6217 Pelecypods: Buchia cf. B. piochii (Gabb), Inoceramus sp.; Late Jurassic (Tithonian), based on B. 

piochii

Collected by J. Suppe, 1975; 

identified by D.L. Jones and J.W. Miller, 1975

No lithologic data 

provided

96 M6400 39.1333 -122.6128 Pelecypods: Buchia cf. B. elderensis (Anderson); 

Late Jurassic (Tithonian)

Collected by J. Suppe, 1975; 

identified by D.L. Jones and J.W. Miller, 1975

No lithologic data 

provided

97 M6401 39.1325 -122.6097 Pelecypods: Buchia piochii (Gabb) or Buchia elderensis (Anderson); Late Jurassic (Tithonian) Unknown No lithologic data provided No information98 OG81-29 39.1305 -122.9553 Radiolaria: Late Jurassic (Tithonian) to Early 

Cretaceous (early Valanginian)? 

Collected by H.N. Ohlin; identified By C.D. Blome Radiolarian chert in mélange Franciscan Com- plex, Central Belt

99 M6404 39.1297 -122.6319 Pelecypods: Buchia elderensis (Anderson) or B. 

piochii (Gabb); Late Jurassic (Tithonian)

Collected by J. Suppe, 1975; 

identified by D.L. Jones and J.W. Miller, 1975

No lithologic data 

provided

No information

100 M6402 39.1261 -122.5933 Pelecypods: Buchia piochii (Gabb) or Buchia elderensis (Anderson); Late Jurassic (Tithonian) Collected by J. Suppe, 1975; identified by D.L. Jones and 

J.W. Miller, 1975

No lithologic data 

provided

No information

101 M6403 39.1244 -122.6219 Pelecypods: Buchia sp., indeterminable; Late 

Jurassic or Early Cretaceous

Collected by J. Suppe, 1975; 

identified by D.L. Jones and J.W. Miller, 1975

No lithologic data 

provided

No information

42

Table 1.—Continued
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

102 LIS7 39.1241 -122.5927 Mollusks: Buchia sp., Late Jurassic or Early 

Cretaceous

McLaughlin and others, 1990; 

Identified by D.L Jones, J.W. Miller

Shale and sandstone

103 LIS23 39.1197 -122.5674 Mollusks: Buchia pacifica (Jeletzky), Early Cretaceous (Valanginian) McLaughlin and others, 1990; identified by D.L Jones, J.W. 

Miller

Slate and metasandstone, textural 

zones 1-2, and minor metavolcanic rocks

104 LIS14 39.1117 -122.5944 Mollusks: Buchia sp., Late Jurassic or Early 

Cretaceous

McLaughlin and others, 1990; 

identified by D.L Jones, J.W. Miller

Shale and sandstone

105 LIS15 39.1115 -122.5240 Mollusk:, Buchia piochii (Gabb), Late Jurassic 

(Tithonian)

McLaughlin and others, 1990; 

identified by D.L Jones, J.W. Miller

Slate and metasandstone, textural 

zones 1–3

106 LIS16 39.1110 -122.5226 Mollusks: Buchia elderensis (Anderson), Late 

Jurassic (Tithonian)

McLaughlin and others, 1990; 

identified by D.L Jones, J.W. Miller

Slate and metasandstone, textural 

zones 1–2

107 M6405 39.1067 -122.5867 Pelecypods: Buchia elderensis (Anderson); Late 

Jurassic (Tithonian)

Collected by J. Suppe, 1975; 

identified by D.L. Jones and J.W. Miller, 1975

No lithologic data 

provided

No information

108 M6406 39.1067 -122.5867 Pelecypods: Buchia elderensis (Anderson); Late 

Jurassic (Tithonian)

Collected by J. Suppe, 1975; 

identified by D.L. Jones and J.W. Miller, 1975

No lithologic data 

provided

No information

109 LIS13 39.1055 -122.5808 Mollusks: Buchia cf. B. piochii (Gabb); Late Jurassic (Tithonian) McLaughlin and others, 1990; identified by D.L Jones, J.W. 

Miller

Shale and sandstone

110 LIS12 39.1054 -122.5791 Mollusks: Buchia elderensis (Anderson), Late 

Jurassic (Tithonian)

McLaughlin and others, 1990; 

identified by D.L Jones, J.W. Miller

Shale and sandstone

111 LIS10 39.1041 -122.5802 Mollusks: Buchia elderensis (Anderson); B. piochii 

(Gabb), Late Jurassic (Tithonian)

McLaughlin and others, 1990; 

identified by D.L Jones, J.W. Miller

Shale, sandstone

112 LIS11 39.1038 -122.5788 Mollusks: Buchia piochii (Gabb), Late Jurassic 

(Tithonian)

McLaughlin and others, 1990; 

identified by D.L Jones, J.W. Miller

Shale and sandstone

113 LIS31 39.1008 -122.6019 Mollusks: Buchia elderensis (Anderson); B. piochii 

(Gabb), Late Jurassic (Tithonian)

McLaughlin and others, 1990; 

identified by D.L Jones, J.W. Miller

Sandstone and shale

43

Table 1.—Continued
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

114 LIS8 39.0868 -122.5814 Mollusks: Buchia sp., Late Jurassic or Early 

Cretaceous

McLaughlin and others, 1990; 

identified by D.L Jones, J.W. Miller

Mélange of sheared 

shale, sandstone, and ophiolitic rocks 

115 LIS1 39.0867 -122.5480 Radiolaria: Mirifusis sp.; Parvicingula sp., Late 

Jurassic (late Kimmeridgean)

McLaughlin and others, 1990; 

identified by C. Blome

Radiolarian chert in 

basalt 

116 LIS9 39.0856 -122.5732 Mollusks: Buchia elderensis (Anderson), Late 

Jurassic (Tithonian)

McLaughlin and others 1990; 

identified by D.L Jones, J.W. Miller

Sheared shale in mélange of sheared 

shale, sandstone, and ophiolitic rocks 

117 LIS21 39.0844 -122.5200 Mollusks: Buchia sp., Late Jurassic or Early 

Cretaceous

McLaughlin and others, 1990; 

identified by D.L Jones, J.W. Miller

Slate and metasandstone, textural 

zones 1–2

118 LIS24 39.0835 -122.5755 Mollusks: Buchia sp., Late Jurassic or Early 

Cretaceous

McLaughlin and others, 1990; 

identified by D.L Jones, J.W. Miller

Slate and metasandstone, textural 

zones 1–2

119 LIS20 39.0832 -122.5197 Cephalopod: indeterminate ammonite McLaughlin and others, 1990; 

identified by D.L Jones, J.W. Miller

Slate and metasandstone, textural 

zones 1–2

120 LIS25 39.0831 -122.5711 Mollusks: Buchia keyserlingi(?) (Lahusen), Early 

Cretaceous (probably Valanginian)

McLaughlin and others, 1990; 

identified by D.L Jones, J.W. Miller

Slate and metasandstone, textural 

zones 1–2

121 LIS19 39.0826 -122.5194 Mollusks: Buchia keyserlingi (Lahusen), Early 

Cretaceous (Valanginian)

McLaughlin and others, 1990; 

identified by D.L Jones, J.W. Miller

Slate and metasandstone, textural 

zones 1–2

122 LIS22 39.0820 -122.5093 Mollusks: Buchia pacifica (Jeletzky), Early Cretaceous (Valanginian) McLaughlin and others, 1990; identified by D.L Jones, J.W. 

Miller

Slate and metasandstone, textural 

zones 1–2

123 LIS26 39.0815 -122.5706 Mollusks: Turbo sp.; Buchia sp., probably B. Keyserlingi (Lahusen); Early Cretaceous (probably 

Valanginian)

McLaughlin and others, 1990; 

identified by D.L Jones, J.W. Miller

Slate and metasandstone, textural 

zones 1–2, and minor metavolcanic rocks

124 LIS32 39.0788 -122.5383 Mollusk: Inoceramus sp., probably Cretaceous 

(maybe Late Cretaceous?)

McLaughlin and others, 1990; 

identified by D.L Jones, J.W. Miller

Sheared argillite and 

serpentinite in fault zone

125 LIS18 39.0783 -122.5170 Mollusks: Buchia sp., probably B. pacifica (Jeletzky), probably Early Cretaceous (Valanginian) McLaughlin and others, 1990; identified by D.L Jones, J.W. 

Miller

Slate and metasandstone, textural 

zones 1–2

44

Table 1.—Continued
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

126 LIS17 39.0774 -122.5169 Mollusks: Buchia pacifica (Jeletzky), Early Cretaceous (Valanginian) McLaughlin and others, 1990; identified by D.L Jones, J.W. 

Miller

Slate and metasandstone, textural 

zones 1–2

127 LIS28 39.0743 -122.5121 Mollusks: Buchia sp., possibly B. uncitoides (Pavlow), Late Jurassic or Early Cretaceous. If B. 

uncitoides, then Early Cretaceous (Valanginian)

McLaughlin and others, 1990; 

identified by D.L Jones, J.W. Miller

Float; derived 

from slate and metasandstone, textural zones 1–2

128 LIS33 39.0697 -122.5273 Mollusks: Buchia elderensis (Anderson), Late 

Jurassic (Tithonian)

McLaughlin and others, 1990; 

identified by D.L Jones, J.W. Miller

Sheared shale in mélange of sheared 

shale, sandstone, and ophiolitic rocks 

129 LIS41 39.0622 -122.4420 Radiolaria: Parvicingula altissima; P. hsuii sp.; 

P. turrita; Mirifusus baileyi sp., M.(?) mediodilitata; Pedobursa sp.; Saitoum pagei sp., Eucyrtidium(?) ptyctum; Archaeodictyomitra rigida sp.; Late Jurassic (Tithonian, subzone 2B of Pessagno, 1977) 

Identified by E.A. Pessagno (Pessagno, 1977; loc. 

NSF997-A,B,C); cited in McLaughlin and others, 1990 

Radiolarian chert in 

basalt 

130 LIS27 39.0581 -122.4835 Mollusks: Buchia pacifica (Jeletzky), Early Cretaceous (Valanginian) McLaughlin and others, 1990; identified by D.L Jones, J.W. 

Miller

Shale and sandstone

131 LIS30 39.0570 -122.4937 Mollusks: Buchia crassicolis (Lahusen) sensu 

lata., Late Cretaceous (Valanginian)

McLaughlin and others, 1990; 

identified by D.L Jones, J.W. Miller

Sandstone and shale

132 kiel-bear.ck 39.0521 -122.4058 Bivalves: probable Buchia crassicollis solida, 

“Modiola” major; Gastropods: Lithomphalus enderlini, Paskentana paskentaensis, fissurellid limpet; Serpulid worm tubes; solemyids; arcoids; Early Cretaceous, probably Valanginian

Univ. of California collections 

UCMP A-7308 , UCMP D-2725; Location examined by K. Campbell, 1996, as reported in Kiel and others, 2008

Thin-bedded siltstone turbidites 

with in situ carbonate pods (0.1–1.5 m diam.); interpreted as fossil coldseep deposits

Lower Great Valley 

sequence

133 MG81-79 39.0486 -122.7364 Radiolaria: Late Jurassic (Tithonian) to Early 

Cretaceous (Valanginian)

Collected by R.J. McLaughlin; 

identified By C.D. Blome

Metachert, associated with slaty 

metasandstone +metavolcanic rocks

Franciscan Complex, Eastern 

Belt, (Yolla Bolly terrane)

45

Table 1.—Continued
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

134 P4 39.0424 -122.4204 Brachiopods: Peregrinella whitneyi Gabb; Mollusks: Modiola major; Pecten complexicosta; 

Lucina colusaensis; Turbo wilburensis; Turbo colusaensis; Turbo? Humerosus, Lower Cretaceous, late Berriassian-early Hauterivian, based on P. whitneyi

J.E. Lawton, 1956; age based on 

Kiel and others, 2014

Detrital serpentinite

135 P1 39.0331 -122.3893 Brachiopods: Peregrinella sp.; Lower Cretaceous, 

late Berriasian-early Hauterivian

J.E. Lawton, 1956; age based on 

Kiel and others, 2014

Detrital serpentinite

136 LIS38 39.0331 -122.4156 Brachiopods: Peregrinella whitneyi Gabb; Early 

Cretaceous, late Berriassian-early Hauterivian

McLaughlin and others, 1990; 

identified by D.L. Jones, J.W. Miller

Cold-seep-related 

limestone lens in detrital serpentinite

137 LIS37 39.0322 -122.4162 Brachiopods: Peregrinella whitneyi Gabb; Early 

Cretaceous, late Berriassian-early Hauterivian

McLaughlin and others, 1990; 

identified by D.L. Jones, J.W. Miller

Cold-seep-related 

limestone lens in detrital serpentinite

138 P3 39.0316 -122.4063 Brachiopods: Peregrinella sp.; Lower Cretaceous, 

late Berriasian-early Hauterivian

J.E. Lawton, 1956; age based on 

Kiel and others, 2014

Detrital serpentinite

139 LIS36 39.0307 -122.4292 Brachiopods: Peregrinella whitneyi Gabb; Early 

Cretaceous, late Berriassian-early Hauterivian

McLaughlin and others, 1990; 

identified by D.L. Jones, J.W. Miller

Detrital serpentinite

140 P2 39.0307 -122.3875 Brachiopods: Peregrinella sp.; Lower Cretaceous, 

late Berriasian-early Hauterivian

J.E. Lawton, 1956; age based on 

Kiel and others, 2014

Detrital serpentinite

141 MG81-96 39.0307 -122.6525 Radiolaria: Late Jurassic (Tithonian) Collected by R.J. McLaughlin; 

identified by C.D. Blome

Metachert, associated with slaty 

metasandstone +metavolcanic rocks

142 M5929 39.0292 -122.4836 Pelecypods: Buchia piochii or B. uncitoides; Late 

Jurassic (Tithonian or Berriasian)

Collected by J.C. Maxwell, 

1972; identified by D.L. Jones, 1972

Shale and minor 

sandstone

Lower Great Valley 

sequence

143 M5931 39.0275 -122.4911 Pelecypods: Buchia piochii; Late Jurassic (Tithonian) Collected by J.C. Maxwell, 1972; identified by D.L. 

Jones, 1972

Shale and minor 

sandstone

Lower Great Valley 

sequence

144 kiel-wlbr 39.0274 -122.4258 Bivalves: “Modiola” major, Solemya stantoni, 

“Lucina” colusaensis, Pecten complexicosta; Gastropods: Paskentana paskentaensis, Retiskenea? tuberculata; Brachiopods: Peregrinella whitneyi; Early Cretaceous (Hauterivian).

Collected by K. Campbell. 

Reported in Campbell and Bottjer, 1995; Campbell and others, 2008; Kiel and others, 2008

Fibrous cemented 

micritic carbonate, within sedimentary serpentinite; interpreted as fossil cold-seep deposits

Sedimentary serpentinite in lower 

Great Valley sequence

46

Table 1.—Continued
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

145 LIS29 39.0265 -122.4285 Brachiopods: Peregrinella whitneyi Gabb; Lower 

Cretaceous, late Berriassian-early Hauterivian

McLaughlin and others, 1990; 

identified by D.L. Jones, J.W. Miller

Shale, sandstone, 

and detrital serpentinite

146 LIS39 39.0261 -122.3891 Brachiopods: Peregrinella whitneyi Gabb; Early 

Cretaceous, late Berriassian-early Hauterivian

McLaughlin and others, 1990; 

identified by D.L. Jones, J.W. Miller

Cold-seep-related 

limestone lens in detrital serpentinite

147 M5928 39.0258 -122.4844 Pelecypods: probably Buchia piochii; Late Jurassic 

(Tithonian)

Collected by J.C. Maxwell, 

1972; identified by D.L. Jones, 1972

Shale and minor 

sandstone

Lower Great Valley 

sequence

148 M5932 39.0231 -122.4819 Pelecypod: probably Buchia piochii; Late Jurassic 

(Tithonian) 

Collected by J.C. Maxwell, 

1972; identified by D.L. Jones, 1972

Shale and minor 

sandstone

Lower Great Valley 

sequence

149 LIS4 39.0231 -122.4744 Mollusks: Buchia cf. pacifica (Jeletzky); Early 

Cretaceous (Valanginian)

McLaughlin and others, 1990; 

identified by D.L. Jones, J.W. Miller

Sandstone and shale 

with carbonate lenses

150 LIS6 39.0228 -122.4834 Mollusks: Buchia elderensis (Anderson), Late 

Jurassic (Tithonian)

McLaughlin and others, 1990; 

identified by D.L. Jones, J.W. Miller

Isoclinally folded 

shale in ophiolitic mélange

151 FL5 39.0224 -122.3895 Brachiopods: Peregrinella sp.; unidentified 

macrofossils; unidentified foraminifers; Lower Cretaceous

J.E. Lawton, 1956 Detrital serpentinite

152 LIS5 39.0218 -122.4823 Mollusks: Buchia elderensis (Anderson); B. piochii 

(Gabb), Late Jurassic (Tithonian)

McLaughlin and others, 1990; 

identified by D.L. Jones, J.W. Miller

Isoclinally folded 

shale in ophiolitic mélange

153 M5933 39.0181 -122.4906 Pelecypods: Buchia piochii; Late Jurassic (Tithonian) Collected by J.C. Maxwell, 1972; identified by D.L. 

Jones, 1972

Shale and minor 

sandstone

Lower Great Valley 

sequence

154 M5936 39.0167 -122.4733 Pelecypods: Buchia fischeriana; Late Jurassic 

(Tithonian)

Collected by J.C. Maxwell, 

1972; identified by D.L. Jones, 1972

Shale and minor 

sandstone

Lower Great Valley 

sequence

155 LIS34 39.0143 -122.4787 Mollusks: Buchia piochii (Gabb); Late Jurassic 

(Tithonian)

McLaughlin and others, 1990; 

identified by D.L. Jones, J.W. Miller 

Shale and sandstone

156 LIS2 39.0138 -122.4899 Mollusks: Turbo sp.; Buchia piochii ( Gabb); B. 

elderensis (Anderson), Late Jurassic (Tithonian)

McLaughlin and others, 1990; 

identified by D.L. Jones, J.W. Miller

Mélange of sheared 

shale, sandstone, and ophiolitic rocks

47

Table 1.—Continued
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

157 LIS35 39.0133 -122.4743 Mollusks: Buchia piochii (Gabb) McLaughlin and others, 1990; 

identified by D.L. Jones, 

Shale and sandstone

158 LIS40 39.0131 -122.3826 Brachiopods: Peregrinella whitneyi Gabb; Early 

Cretaceous, late Berriassian-early Hauterivian

McLaughlin and others, 1990; 

identified by D.L. Jones, J.W. Miller

Cold-seep-related 

limestone lens in detrital serpentinite

159 M5937 39.0111 -122.4903 Pelecypod: Buchia piochii; Late Jurassic (Tithonian) Collected by J.C. Maxwell, 1972; identified by D.L. 

Jones, 1972

Shale and minor 

sandstone

Lower Great Valley 

sequence

160 M5935 39.0097 -122.4814 Pelecypods: Buchia fischeriana; Late Jurassic 

(Tithonian)

Collected by J.C. Maxwell, 

1972; identified by D.L. Jones, 1972

Shale and minor 

sandstone

Lower Great Valley 

sequence

161 LIS3 39.0094 -122.4914 Mollusks: Buchia elderensis (Anderson); B. piochii 

(Gabb), Late Jurassic (Tithonian)

McLaughlin and others, 1990; 

identified by D.L. Jones, J.W. Miller

Mélange of sheared 

shale, sandstone, and ophiolitic rocks

162 MG81-28 38.9995 -122.7049 Radiolaria: Early Cretaceous (Probably Aptian/

Albian)

Collected by R.J. McLaughlin; 

identified by C.D. Blome

Metachert, associated with slaty 

metasandstone +metavolcanic rocks

Franciscan Complex, Eastern 

Belt (Yolla Bolly terrane)

164 P5 38.9973 -122.3917 Brachiopods: Peregrinella sp.; Lower Cretaceous, 

late Berriasian-early Hauterivian

J.E. Lawton, 1956; age based on 

Kiel and others, 2014

Detrital serpentinite

165 P6 38.9953 -122.3935 Brachiopods: Peregrinella sp.; Lower Cretaceous, 

late Berriasian-early Hauterivian

J.E. Lawton, 1956; age based on 

Kiel and others, 2014

Detrital serpentinite

166 MG81-20 38.9945 -122.6773 Radiolaria: Late Jurassic (Tithonian) to Early 

Cretaceous (Valanginian)

Collected by R.J. McLaughlin; 

identified by C.D. Blome

Metachert, associated with slaty 

metasandstone +metavolcanic rocks

Franciscan Complex, Eastern 

Belt (Yolla Bolly terrane)

167 MG81-31 38.9937 -122.6983 Radiolaria: Late Jurassic (Tithonian) to Early 

Cretaceous (Valanginian)

Collected by R.J. McLaughlin; 

identified by C.D. Blome

Metachert, associated with slaty 

metasandstone +metavolcanic rocks

48

Table 1.—Continued
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

168 JL-11-4 38.9303 -122.5692 Unidentified ammonite, Lower Cretaceous Swe, 1968 Dominantly 

feldspathic, brownish- to tan-weathering sandstone plus conglomerate

169 F-23-3 38.9252 -122.5073 B. piochii; Late Jurassic, Tithonian Swe, 1968; identified by Win 

Swe

Dominantly mudstone, minor 

lithic greywacke, thin-bedded chert associated with pillow basalt in lowest beds

170 S-30-1 38.9224 -122.5517 Dinoflagellates: Late Lower-Early Upper Cretaceous, Albian-Cenomanian? Swe, 1968; identified by W.R. Evitt Feldspathic brown- ish- to tan-weathering sandstone 

with conglomerate lenses

171 MG81-230 38.9216 -122.5418 Cephalopods: Baculites sp.; a ribbed Ammonite sp; 

Late Cretaceous, possibly as young as Campanian

Collected by R.J. McLaughlin, 

1981; identified by D.L. Jones, personal commun., 8/11/81

Lithic quartzose 

wacke with gray carbonate concrets

Great Valley sequence

172 N-3-F 38.9183 -122.5184 B. piochii; Late Jurassic, Tithonian Swe, 1968; identified by D.L. 

Jones

Dominantly mudstone, minor lithic 

greywacke

173 FL3 38.9074 -122.3084 Foraminifers: Globotruncana (Rotudina) stephani

(Gandolfi); Globigerina infracretacea Glaessner; Globigerina sp.; Dentalina sp.; Cibicides sp.; Marsonella oxygona (Reuss); Bathysiphon sp.; Upper Cretaceous (middle-upper Cenomanian);

J.E. Lawton, 1956; identified by 

K. Kupper

Mudstone and clay 

shale

174 MG81-113 38.9073 -122.6389 Radiolaria: Late Cretaceous (Coniacian/Santonian) Collected by R.J. McLaughlin; 

identified by C.D. Blome

Sheared silty 

mudstone with carbonate concretions in fault zone

175 A-24-16 38.9073 -122.6127 Spores, dinoflagellates; Lower Cretaceous (may 

include Upper Jurassic?)

Swe, 1968; identified by W.R. 

Evitt 

Conglomerate, sandstone, interbedded 

mudstone

49

Table 1.—Continued
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

176 kiel-rcky.ck 38.8991 -122.4765 Bivalves: Pecten complexicosta, Astarte trapezoidealis, Buchia sp., possibly B. pacifica or B. 

crassicollis; Gastropods: Atresius liratus, Amberleya morganensis, Lithomphalus enderlini; Early Cretaceous (Valanginian).

Described in Kiel and Campbell, 

2005; earlier by Gabb, 1869, and Stanton, 1895

Isolated white 

carbonate lenses; interpreted as fossil cold-seep deposits

Lower Great Valley 

sequence, Blue Ridge Member of lower Crack Canyon Formation

177 F-3-3 38.8985 -122.6691 Dinoflagellates; uppermost Lower to lowermost 

Upper Cretaceous, Albian-Cenomanian

Swe, 1968; identified by W.R. 

Evitt 

Dominantly mudstone

178 MG80-150 38.8961 -122.5512 Mollusk: Nucula sp.; indet. Gastropod; tube, unknown origin; age indet. Collected by J. Donnelly-Nolan and F. Goff; identified by 

D.L. Jones and J.W. Miller

Carbonate concretion in arkosic 

wacke, float near spring. In situ wacke sandstone nearby

Great Valley sequence

179 FL7 38.8954 -122.4806 Mollusks: Pecten complexicosta Gabb; Turbo 

morganensis Stanton; Astarte(?) trapezoidalis Stanton; Atresius liratus Gabb(?); Buchia sp. (inflated, unnamed); Lower Cretaceous, Valanginian, based on “Turbo” morganensis 

J.E. Lawton, 1956; age based on 

Kiel and others, 2008

Limestone lens near 

base of Crack Canyon Formation

180 F-14 38.8944 -122.5317 B. uncitoides; Lower Cretaceous, Berriasian Swe, 1968; identified by D.L. 

Jones

Dominantly mudstone, minor lithic 

greywacke

181 N-26-5 38.8928 -122.5619 Unidentified pelecypods, Upper Cretaceous Swe, 1968 Conglomerate, sandstone, interbedded 

mudstone

182 S-29-10 38.8883 -122.6220 Pollen, dinoflagellates; Upper Cretaceous Swe, 1968; identified by W.R. 

Evitt 

Massive lithofeldspathic, brownish-weathering 

sandstone

183 A-23 38.8856 -122.4342 Mollusks: Buchia crassicolis; Pecten complexicosta Gabb; Lower Cretaceous, Valanginian; J.E. Lawton, 1956; age based on Jones and others, 1969 Sandstone, grit, peb- ble conglomerate, 

and limestone

184 F-22-3 38.8812 -122.5895 Spores, dinoflagellates; Lower Cretaceous (lower 

part?)

Swe, 1968; identified by W.R. 

Evitt 

Dominantly mudstone, minor lithic 

greywacke

185 F-6 38.8793 -122.5256 B. pacifica; Lower Cretaceous, Valanginian Swe, 1968; identified by D.L. 

Jones

Dominantly mudstone, minor lithic 

greywacke

Table 1.—Continued

50
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

186 F-5b 38.8792 -122.5276 B. uncitoides; Lower Cretaceous, Berriasian Swe, 1968; identified by Win 

Swe

Dominantly mudstone, minor lithic 

greywacke 

187 F-11 38.8785 -122.5208 B. uncitoides; Lower Cretaceous, Berriasian Swe, 1968; identified by D.L. 

Jones

Dominantly mudstone, minor lithic 

greywacke

188 D-11-4 38.8781 -122.5928 Unidentified pelecypods and echinoderm Swe, 1968 Massive lithofeldspathic, brownish-weathering 

sandstone

189 F-10(a,b) 38.8773 -122.5316 B. piochii; Late Jurassic, Tithonian Swe, 1968; identified by D.L. 

Jones

Dominantly mudstone, minor 

lithic greywacke, thin-bedded chert associated with pillow basalt in lowest beds

190 F-10a 38.8773 -122.5316 Dinoflagellates; Upper Jurassic Swe, 1968; identified by W.R. 

Evitt 

Dominantly mudstone, minor 

lithic greywacke, thin-bedded chert associated with pillow basalt in lowest beds

999 M5171 39.9544 -123.0306 Cephalopod: Belemnite fragment; Pelecypods: 

scraps of large, coarse-ribbed Buchia, possibly B. pacifica; probably Cretaceous

Collected by J. Suppe, 1969; 

identified by D.L. Jones, 1969

No lithologic data 

provided

No information

999 M5169 39.9442 -123.0417 Pelecypod: Buchia piochii; Late Jurassic (Tithonian) Collected by J. Suppe, 1969; identified by D.L. Jones, 1969 No lithologic data provided No information999 M5168 39.8842 -123.0731 Pelecypod: Arca tehamaensis? Stanton, Amberleya dilleri Stanton; Early Cretaceous (upper 

Valanginian)

Collected by J. Suppe, 1969; 

identified by D.L. Jones, 1969

No lithologic data 

provided

No information

999 M5173 39.8081 -122.9878 Pelecypod: probably Buchia cf. B. piochii; probably Late Jurassic (Tithonian), but preservation 

poor

Collected by J. Suppe, 1969; 

identified by D.L. Jones, 1969

No lithologic data 

provided

No information

999 M5172 39.8022 -123.0389 Pelecypod: Buchia cf. B. pacifica; Early Cretaceous (Valanginian?) Collected by J. Suppe, 1969; identified by D.L. Jones, 1969 No lithologic data provided No information999 M5170 39.8006 -123.0344 Pelecypod: Buchia piochii?; Late Jurassic (Tithonian)? Collected by J. Suppe, 1969; identified by D.L. Jones, 1969 No lithologic data provided No information

Table 1.—Continued

51
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

999 M6563 39.7736 -122.7278 Pelecypods: Buchia pacifica (Jeletzky); Early 

Cretaceous (early middle Valanginian)

Collected by J. Suppe, 1977; 

identified by D.L. Jones, 1977 

Meta argillite and 

metasandstone

Franciscan Complex, Eastern 

Belt, mélange of Yolla Bolly terrane, Goat Mountain area

999 M6598 39.7628 -123.0686 Pelecypods: possibly Buchia sp., cf. B. keyserlingi

(Lahusen); possibly Early Cretaceous (latest Valanginian)?

Collected by J.C. Maxwell, 

Univ. of Texas, Austin, 1976; identified by D.L. Jones, 1977

Sheared argillite Franciscan Complex, Central Belt, 

“Poison Rock” mélange 

999 M6599 39.7625 -123.0683 Pelecypod: too poorly preserved for definitive 

ident, but may be B. keyserlingi

Collected by J.C. Maxwell, 

Univ. of Texas, Austin, 1976; identified by D.L. Jones, 1977

Sheared argillite Franciscan Complex, Central Belt, 

“Poison Rock” mélange 

999 O-8-2 38.8719 -122.5279 Dinoflagellates; Upper Jurassic Swe, 1968; identified by W.R. 

Evitt 

Ophiolitic mélange

999 A-3-8 38.8710 -122.6141 Turritella pachecoensis, Paleocene Swe, 1968; identified by Win 

Swe

Massive, structureless sandstone, 

mudstone, and conglomerate

999 O-25-5a 38.8680 -122.5619 B. pacifica; Lower Cretaceous, Valanginian Swe, 1968; identified by Win 

Swe

Conglomerate, sandstone, interbedded 

mudstone

999 D-10-3 38.8634 -122.5431 Buchia piochii, reworked in Lower Cretaceous Swe, 1968; identified by D.L. 

Jones

Sandstone, interbedded mudstone

999 D-10-3(1) 38.8634 -122.5431 Dinoflagellates; Lower Cretaceous Swe, 1968; identified by W.R. 

Evitt 

Conglomerate, sandstone, interbedded 

mudstone

999 D-10-3(2) 38.8634 -122.5431 Dinoflagellates; Upper Jurassic reworked into 

Lower Cretaceous

Swe, 1968; identified by W.R. 

Evitt

Conglomerate, sandstone, interbedded 

mudstone

999 MG81-162 38.8616 -122.6694 Radiolaria: Late Jurassic (Tithonian) Collected by R.J. McLaughlin; 

identified by C.D. Blome

Radiolarian chert 

in ophiolitic mélange 

999 F-4-3 38.8611 -122.7421 Pollen, dinoflagellates; Upper Cretaceous Swe, 1968; identified by W.R. 

Evitt 

Siltstone and interbedded massive 

fine-grained white sandstone

Table 1.—Continued

52
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

999 D-17-6 38.8596 -122.6590 B. pacifica; Lower Cretaceous, Valanginian Swe, 1968; identified By Win 

Swe

Dominantly mudstone, minor lithic 

greywacke

999 N-10-2 38.8579 -122.5495 B. uncitoides; Lower Cretaceous, Berriasian Swe, 1968; identified by D.L. 

Jones

Conglomerate, sandstone, interbedded 

mudstone

999 F05 38.8570 -122.3703 Cephalopod: Belemnite, Cylindroteuthis tehamaensis, Late Jurassic (Tithonian?) Collected by D. Enderlin, 1986 Mudstone Lower Great Valley sequence999 D-17-3 38.8555 -122.6483 Pollen, dinoflagellates; Upper Cretaceous Swe, 1968; identified by W.R. 

Evitt 

Dominantly sandstone

999 F016 38.8551 -122.3674 Cephalopods: Ammonite, Phylloceras knoxvillense; Belemnite, Cylindroteuthis tehamaensis; 

Mollusks: Buchiids; Late Jurassic (Tithonian)?—Reworked?

Collected by D. Enderlin, 1987 Chert-volcanic-rich 

conglomerate and greywacke, fossils in conglomerate 

Lower Great Valley 

sequence (Knoxville Formation?)

999 F017 38.8538 -122.3632 Mollusks: Buchia piochii; Late Jurassic (Tithonian); disarticulated valves Collected by D. Enderlin Chert-volcanic-rich pebble conglomerate and 

greywacke, casts and molds

Lower Great Valley 

sequence (Knoxville Formation?)

999 F018 38.8528 -122.3637 Cephalopods: Ammonite, rostrum fragment, possibly Spiticeras sp.?; no age assigned Collected by D. Enderlin Chert-volcanic-rich conglomerate Lower Great Valley sequence (Knoxville Formation?)999 A-24-2 38.8501 -122.6042 Pollen, dinoflagellates; Upper Cretaceous (possibly 

lower Maestrichtean)

Swe, 1968; identified by W.R. 

Evitt 

Alternating sandstone, mudstone

999 F02 38.8499 -122.3573 Mollusks: Buchia piochii; Late Jurassic (Tithonian) Enderlin Fragments in grey- wacke Lower Great Valley sequence999 O-9-8 38.8499 -122.7276 Dinoflagellates, preservation poor; probably Lower 

Cretaceous or Upper Jurassic 

Swe, 1968; identified By W.R. 

Evitt 

Dominantly mudstone, minor 

lithic greywacke, thin-bedded chert associated with pillow basalt in lowest beds

999 M-7-4 38.8498 -122.6858 B. piochii; Late Jurassic, Tithonian Swe, 1968; identified by D.L. 

Jones

Dominantly mudstone, minor 

lithic greywacke, thin-bedded chert associated with pillow basalt in lowest beds

Table 1.—Continued

53
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

999 M-7-4 38.8498 -122.6858 Dinoflagellates; Upper Jurassic Swe, 1968; identified By W.R. 

Evitt 

Dominantly mudstone, minor 

lithic greywacke, thin-bedded chert associated with pillow basalt in lowest beds

999 N-10-3 38.8489 -122.5247 B. fischeriana?; Late Jurassic, Tithonian Swe, 1968; identified by D.L. 

Jones

Dominantly mudstone, minor 

lithic greywacke, thin-bedded chert assoiated with pillow basalt in lowest beds

999 F01 38.8489 -122.3565 Mollusks: Buchia piochii; Late Jurassic (Tithonian) Enderlin Fragments in grey- wacke Lower Great Valley sequence999 F025 38.8476 -122.3446 Buchiids (inflated forms, disarticulated valves) 

cobble in creek; no outcrop; Early Cretaceous?

Collected by D. Enderlin Calcareous sandstone Lower Great Valley sequence (Crack 

Canyon Formation)

999 N-10-4 38.8476 -122.5216 B. piochii; Late Jurassic, Tithonian Swe, 1968; identified by D.L. 

Jones

Dominantly mudstone, minor lithic 

greywacke 

999 F06 38.8472 -122.3487 Mollusks: Buchia pacifica?; Early Cretaceous 

(Valanginian?)

Enderlin “Marly cobble” in 

creek; no outcrop

Lower Great Valley 

sequence (Crack Canyon Formation)

999 F07 38.8472 -122.3467 Mollusks: Buchia pacifica?; Early Cretaceous 

(Valanginian?)

Enderlin Calcareous finegrained sandstone 

and siltstone

Lower Great Valley 

sequence (Crack Canyon Formation)

999 F019 38.8460 -122.3607 Mollusks: disarticulated small valves, Buchia 

piochii; Cephalopods: Ammonite rostrum fragment, Phylloceras knoxvillense; Late Jurassic (Tithonian)

Collected by D. Enderlin Hydrothermally 

bleached, locally pyritized argillic alteration

Lower Great Valley 

sequence

999 F026 38.8454 -122.3538 Mollusks: Buchia piochii, valves and fragments; 

Late Jurassic? (Tithonian)?

Collected by D. Enderlin Massive sandstone Lower Great Valley 

sequence (Knoxville Formation?)

999 F-18-16 38.8447 -122.5894 Desmophyllites diphylloides; Upper Cretaceous, 

Campanian and possibly Late Maastrichtian

Swe, 1968; identified By Win 

Swe

Alternating sandstone, mudstone

Table 1.—Continued

54
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

999 D-19-3 38.8413 -122.6466 Dinoflagellates; probably Upper Jurassic Swe, 1968; identified by W.R. 

Evitt 

Dominantly mudstone, minor 

lithic greywacke, thin-bedded chert associated with pillow basalt in lowest beds

999 F04 38.8411 -122.3491 Mollusks: Buchiid valve; Late Jurassic or Early 

Cretaceous

 Enderlin Siltstone-dominated 

turbidites, overturned, disrupted

Lower Great Valley 

sequence

999 F011 38.8404 -122.3363 Mollusks: Buchiids, small valves (to 1 cm), 

inflated form (juvenile?) ; reworked? No age assigned

Collected by D. Enderlin Marl and greywacke Lower Great Valley 

sequence (Crack Canyon Formation?)

999 F021 38.8381 -122.3435 Mollusks: Buchiids (B. piochii?), Late Jurassic? 

(Tithonian?) 

Observed by Enderlin; left in 

place

Cobble of marl Lower Great Valley 

sequence (Knoxville Formation?)

999 F010 38.8376 -122.3342 Mollusks: Buchia pacifica?; Early Cretaceous 

(Valanginian?)

Collected by D. Enderlin Conglomerate, boulder on ridgetop 

trail

Lower Great Valley 

sequence (Crack Canyon Formation)

999 F015 38.8367 -122.3381 Mollusks: valve fragments, Buchia pacifica?; 

Early Cretaceous (Valanginian?)

Collected by D. Enderlin Orange, marly greywacke Lower Great Valley sequence (Crack 

Canyon Formation)

999 F014 38.8355 -122.3274 Mollusks: Modiola sp; Turbo sp.(possibly T. 

paskentana?); Early Cretaceous (probably Hauterivian or younger?)

Collected by D. Enderlin, R.J. 

McLaughlin, and W. Elder, 2007

Boulders and 

outcrop in Foley Creek; associated with detrital serpentinite and cold-seep setting

Lower Great Valley 

sequence (transitional, Crack Canyon and Little Valley Formations)

999 F03 38.8355 -122.3471 Mollusks: Buchiid fragments; Late Jurassic or 

Early Cretaceous

Enderlin Fragments in 5-ftthick greywacke 

bed

Lower Great Valley 

sequence

999 F09 38.8345 -122.3403 Mollusks: Buchia piochii?; Cephalopod: possibly 

Cylindroteuthis tehamaensis?; Late Jurassic (Tithonian?)

Collected by D. Enderlin Marly mudstone Lower Great Valley 

sequence

Table 1.—Continued

55
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

999 F08 38.8341 -122.3371 Mollusks: Buchia pacifica?; Early Cretaceous 

(Valanginian?)

Enderlin Greywacke float; 

not abundant

Lower Great Valley 

sequence (Crack Canyon Formation)

999 F020 38.8324 -122.3423 Mollusks: Buchia piochii, disarticulated valves; 

Late Jurassic (Tithonian)

Collected by D. Enderlin Chert-volcanic-rich 

pebble conglomerate; below basal contact with basaltic andesite intrusive of Clear Lake Volcanics 

Lower Great Valley 

sequence (Knoxville Formation?)

999 F012 38.8321 -122.3233 Brachiopods: Peregrinella whitneyi in bioclastic debris; Early Cretaceous (probably late 

Berriasian-early Hauterivian)

Collected by D. Enderlin Detrital serpentinite, 

micritic mudstone, and brown sandstone; coldseep deposit

Lower Great Valley 

sequence (transitional, Crack Canyon and Little Valley Formations)

999 kiel-foley.cyn 38.8316 -122.3231 Bivalves: “Modiola” major, “inflated” Buchia sp.;

Gastropods: Paskentana paskentaensis, Brachiopods: Peregrinella whitneyi; Early Cretaceous (Valanginian-Hauterivian)

Described by D. Enderlin in 

Kiel and others, 2008

Two fault-bounded 

parallel lenses of carbonate near or in sedimentary serpentinite body within Crack Canyon Formation sedimentary section; formerly quarried for lime; interpreted as fossil cold-seep deposits

Lower Great Valley 

sequence, Crack Canyon Formation

999 F013 38.8316 -122.3228 Mollusks: Turbo sp., Modiola sp., Pecten sp.; 

Early Cretaceous (probably late Berriasian-early Hauterivian) based on F012 loc.

Collected by D. Enderlin Limestone marl and 

micritic mudstone associated with detrital serpentinite

Lower Great Valley 

sequence (transitional, Crack Canyon and Little Valley Formations)

999 Ja-16-5 38.8311 -122.6368 Pollen, dinoflagellates; Upper Cretaceous Swe, 1968; identified by W.R. 

Evitt 

Dominantly sandstone

999 D-16-5 38.8311 -122.6368 B. pacifica; Lower Cretaceous, Valanginian Swe, 1968; identified by D.L. 

Jones

Dominantly mudstone, minor lithic 

greywacke

Table 1.—Continued

56
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

999 F024 38.8308 -122.3369 Mollusks: Buchia piochii, well preserved exterior 

casts; Late Jurassic (Tithonian); 

Collected by D. Enderlin Greywacke; associated with black 

fossilized plant remains

Lower Great Valley 

sequence (Knoxville Formation?)

999 F023 38.8265 -122.3369 Mollusks: Buchia piochii, only one valve; Late 

Jurassic (Tithonian); 

Collected by D. Enderlin Gray mudstone Lower Great Valley 

sequence (Knoxville Formation?)

999 F022 38.8240 -122.3335 Mollusks: Buchia piochii, abundant molds; Late 

Jurassic (Tithonian); 

Observed by D. Enderlin Gray mudstone and 

siltstone

Lower Great Valley 

sequence (Knoxville Formation?)

999 F-4 38.8194 -122.5745 Inoceramus schmidti; Upper Cretaceous, MiddleUpper Campanian Swe, 1968; identified by D.L. Jones Alternating sand- stone, mudstone999 WS-F-4 38.8194 -122.5745 Pollen, dinoflagellates; Upper Cretaceous Swe, 1968; identified by W.R. 

Evitt 

Alternating sandstone, mudstone

999 M-6-1 38.8099 -122.6156 Nostroceras ? sp.; Upper Cretaceous, Campanian Swe, 1968; identified by D.L. 

Jones

Dominantly sandstone

999 Ja-14-8 38.8090 -122.6371 Foraminifers; Bathysiphon perampla(?); Upper 

Cretaceous

Swe, 1968; identified by Win 

Swe

Siltstone and interbedded massive 

fine-grained white sandstone

999 F-17-1 38.8000 -122.6636 Spores, dinoflagellates; probably Lower Cretaceous Swe, 1968; identified by W.R. Evitt Mudstone with thin interbedded quartz-rich 

sandstone

999 Ja-13-8 38.7968 -122.6774 Dinoflagellates; Upper Jurassic or Lower Cretaceous Swe, 1968; identified by W.R. Evitt Dominantly mud- stone, minor 

lithic greywacke, thin-bedded chert associated with pillow basalt in lowest beds

999 F-7-6 38.7918 -122.6245 Pollen, dinoflagellates; Upper Cretaceous Swe, 1968; identified by W.R. 

Evitt 

Dominantly sandstone

999 Ja-10-F 38.7870 -122.6273 Unidentified pelecypods, Upper Cretaceous Swe, 1968 Siltstone and interbedded massive 

fine-grained white sandstone

Table 1.—Continued

57
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

999 F-17-13 38.7854 -122.6505 Dinoflagellates; Lower Cretaceous (Upper 

Berriasian-Valanginian

Swe, 1968; identified by W.R. 

Evitt 

Mudstone with 

thin interbedded quartz-rich sandstone

999 Ja-9-12 38.7832 -122.6082 Spores, dinoflagellates; Lower Cretaceous (questionable Aptian?) Swe, 1968; identified by W.R. Evitt Mudstone with thin interbedded quartz-rich 

sandstone

999 F-8-2 38.7817 -122.6243 Cucullaea sp. and straight conical thin-shelled 

cephalopods

Swe, 1968; identified By A. 

Myra Keen

Siltstone and interbedded massive 

fine-grained white sandstone

999 F-8-2 38.7817 -122.6243 Pollen; Upper Cretaceous Swe, 1968; identified by W.R. 

Evitt 

Siltstone and interbedded massive 

fine-grained white sandstone

999 F-18-14 38.7762 -122.6247 Dinoflagellates: probably Lower Cretaceous Swe, 1968; identified by W.R. 

Evitt 

Mudstone with 

thin interbedded quartz-rich sandstone

999 F-18-15 38.7674 -122.5873 Dinoflagellates; Upper Jurassic Swe, 1968; identified by W.R. 

Evitt 

Dominantly mudstone, minor 

lithic greywacke, thin-bedded chert associated with pillow basalt in lowest beds

999 F-24-6 38.7472 -122.5523 Pollen, dinoflagellates; Upper Cretaceous Swe, 1968; identified by W.R. 

Evitt 

Dominantly sandstone

999 M6947 38.6972 -122.3689 Cephalopod: Phylloceras cf. P. contrarium (Imlay 

and Jones), Late Jurassic 

Collected by S.P. Phipps and 

submitted to USGS in 1978; identified by D.L. Jones and J.W Miller, 1979

Not given Lower Great Valley 

sequence

999 M6945 38.6931 -122.3658 Radiolaria: Late Jurassic (Tithonian) Collected by R.J. McLaughlin; 

identified by C.D. Blome

Radiolarian chert in 

ophiolitc melange 

Ophiolitic melange 

of Lower Great Valley sequence

999 M6948 38.6895 -122.3555 Radiolaria: Late Cretaceous (Coniacian/Santonian) Collected by R.J. McLaughlin; 

identified by C.D. Blome

Sheared silty 

mudstone with carbonate concretions in fault zone

Great Valley sequence

Table 1.—Continued

58
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

999 M6946 38.6846 -122.3571 Radiolaria: Late Jurassic (Tithonian) to Early 

Cretaceous (Valanginian)

Collected by R.J. McLaughlin; 

identified by C.D. Blome

Metachert, associated with slaty 

metasandstone +metavolcanic rocks

Franciscan Complex, Eastern 

Belt (Yolla Bolly terrane)

999 M6944 38.6725 -122.3408 Radiolaria: Late Jurassic (Tithonian) to Early 

Cretaceous (early Valanginian)

Collected by R.J. McLaughlin; 

identified by C.D. Blome

Radiolarian chert in 

melange

Franciscan Complex, Eastern 

Belt (Yolla Bolly terrane)

999 M6954 38.6667 -122.2779 Radiolaria: Late Jurassic (Tithonian) to Early 

Cretaceous (Valanginian)

Collected by R.J. McLaughlin; 

identified by C.D. Blome

Metachert, associated with slaty 

metasandstone +metavolcanic rocks

Franciscan Complex, Eastern 

Belt (Yolla Bolly terrane)

999 M6935 38.6653 -122.4166 Mollusks: Mytilus sp,; Indet. clams; Turbo 

paskentaensis (Stanton); Late Jurassic or Early Cretaceous 

Collected by S.P. Phipps and 

submitted to USGS in 1978; identified by D.L. Jones and J.W Miller, 1979

No lithologic data 

provided

Lower Great Valley 

sequence

999 M6937 38.6621 -122.4153 Mollusks: Buchia pacifica (Jeletzky); Early Cretaceous (Valanginian) Collected by S.P. Phipps and submitted to USGS in 1978; 

identified by D.L. Jones and J.W Miller, 1979

Not given Lower Great Valley 

sequence

999 M6938 38.6598 -122.4242 Mollusks: Buchia pacifica (Jeletzky); Early Cretaceous (Valanginian) Collected by S.P. Phipps and submitted to USGS in 1978; 

identified by D.L. Jones and J.W. Miller, 1979

No lithologic data 

provided

Lower Great Valley 

sequence

999 M6934 38.6519 -122.3943 Mollusks: Mytilus sp,; Indet. clams; Turbo 

paskentaensis (Stanton); Late Jurassic or Early Cretaceous 

Collected by S.P. Phipps and 

submitted to USGS in 1978; identified by D.L. Jones and J.W. Miller, 1979

Not given Lower Great Valley 

sequence

999 M6942 38.6510 -122.3233 Radiolaria: Late Jurassic (Tithonian) to Early 

Cretaceous (early Valanginian)? 

Collected by H.N. Ohlin; identified by C.D. Blome Radiolarian chert in melange Franciscan Com- plex, Central Belt

999 M6939 38.6316 -122.3485 Mollusks: Buchia piochii (Gabb); Late Jurassic 

(Tithonian)

Collected by S.P. Phipps and 

submitted to USGS in 1978; identified by D.L. Jones and J.W. Miller, 1979

No lithologic data 

provided

Lower Great Valley 

sequence

Table 1.—Continued

59
Map_No. ID Latitude Longitude Fossil and age Source Lithology Unit

999 M6941 38.6211 -122.2887 Mollusks: Buchia sp., Late Jurassic or Early 

Cretaceous

Collected by S.P. Phipps and 

submitted to USGS in 1978; identified by D.L. Jones and J.W. Miller, 1979

No lithologic data 

provided

Great Valley sequence

999 M6940 38.5919 -122.2590 Mollusks: Buchia piochii (Gabb); Late Jurassic 

(Tithonian)

Collected by S.P. Phipps and 

submitted to USGS in 1978; identified by D.L. Jones and J.W. Miller, 1979

No lithologic data 

provided

Lower Great Valley 

sequence

999 M6953 38.3882 -122.4033 Radiolaria: Late Jurassic (Tithonian) to Early 

Cretaceous (Valanginian)

Collected by R.J. McLaughlin; 

identified by C.D. Blome

Metachert, associated with slaty 

metasandstone +metavolcanic rocks

Franciscan Complex, Eastern 

Belt (Yolla Bolly terrane)

60

Table 1.—Continued