i

The Cedars Ultramafic Mass, 

Sonoma County, California

By M. Clark Blake, Jr., Edgar H. Bailey, and Carl M. Wentworth

Open-File Report 2012–1164

Version 1.2, April 2016

U.S. Department of the Interior 

U.S. Geological Survey 
ii

U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

First release: 2012

Revised: August 2012 (ver.1.1)

Revised: April 2016 (ver. 1.2)

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Suggested citation:

Blake, M.C., Jr., Bailey, E.H., and Wentworth, C.M., 2016, The Cedars untramafic mass, Sonoma County, 

California (ver. 1.2, April 2016): U.S. Geological Survey Open-File Report, 2012–1164, 13 p., 1 plate. 

(Available at http://pubs.usgs.gov/of/2012/1164/.)

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report.
iii

Contents

Abstract...............................................................................................................................................................1

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

Regional Geologic Framework............................................................................................................................5

Description of the Peridotite................................................................................................................................8

Peridotite Chemistry............................................................................................................................................9

Plate Tectonic Setting....................................................................................................................................... 10

Conclusions ...................................................................................................................................................... 11

Acknowledgments............................................................................................................................................. 12

References Cited .............................................................................................................................................. 12

Figures

1. Photograph of the interior of the Cedars mass in the upper Austin Creek drainage. ......................................2

2. Index map showing the Cedars ultramafic mass and its geologic context. .....................................................3

3. Photograph of typical sheared serpentinite, Cazadero area. ..........................................................................5

4. Photograph of prominent mineral foliation weathered out on a joint surface in hartzburgite in the Cedars 

ultramafic mass.................................................................................................................................................6

5. Map showing folded foliation within the Cedars ultramafic mass, form lines drawn to generalize the mapped 

strikes, and bedding attitudes in the surrounding Franciscan Complex greywackes.........................................7

6. Map showing mineral lineations within the Cedars ultramafic mass and form lines from fig. 2. ......................8

7. Original compilation of attitudes and lineations by E.H. Bailey on 7.5-minute topographic maps spliced at 

their common inner corner. A full-scale copy of this map compilation is separately included with this report as 

of2012-1164_plate.pdf (at http://pubs.usgs.gov/of/2012/1164/) ...................................................................... 11

Table

1. Chemical analyses of hartzburgite from the Cedars peridotite (PCC‐1a, b) and from the Burro Mountain 

peridotite of the Southern California Coast Ranges (4BU‐66)......................................................................... 10
The Cedars Ultramafic Mass, Sonoma County, 

California

By M. Clark Blake, Jr., Edgar H. Bailey*, and Carl M. Wentworth

Abstract

The Cedars ultramafic mass is a mantle fragment that consists of partially serpentinized 

spinel harzburgite and dunite. Compositional layering and a chromite lineation define a penetrative 

metamorphic foliation that almost certainly formed in the upper mantle. Although detailed 

petrofabric and mineral chemistry are presently lacking, it seems reasonable that the Cedars 

peridotite represents a slice of mantle tectonite that once formed the base of the Coast Range 

ophiolite, and not an abyssal peridotite tectonically emplaced within the Franciscan accretionary 

prism.

Introduction

In 1955, Edgar Bailey, a geologist with the U.S. Geological Survey (USGS) in Menlo Park, 

California, flew over the Cedars area of western Sonoma County and, from its color and distinctive 

vegetation, recognized that it was underlain by a mass of ultramafic rock (fig. 1). Bailey had worked 

for many years in Franciscan and associated rocks in the Coast Ranges studying mercury deposits, 

including the New Almaden area in Santa Clara County (Bailey and Everhart, 1964). He later 

visited the Cedars area and decided to map the geology of the enclosing Skaggs Springs 15' 

quadrangle (fig. 2), to determine how the ultramafic body formed and got to its present position. As 

part of the justification for this work, he undoubtedly mentioned that both chromite and magnesite 

deposits occur in these ultramafic rocks, particularly at the Layton Mine and Red Slide claims, 

respectively (Dow and Thayer, 1946; Bradley, 1925), although it does not appear that any further 

research was done on either of these commodities in the Cedars mass.

We describe some of the history of that early work and later regional studies that helped to 

define the geologic setting of the Cedars ultramafic mass, present previously unpublished structural 

data for the mass, and include some other relevant observations and data. We do not, however, 

present a full modern analysis of the mass and its petrologic and structural history, which would be 

a much larger undertaking.

Bailey’s proposed project, Coast Range Ultramafics, was approved by the USGS in 1956, 

and in August of that year, Clark Blake was hired as his assistant. Field mapping was started soon 

after and Blake continued in that role until 1960, when he left to begin graduate studies, although he 

continued to work on the project on a part-time basis. Bailey hired several other field assistants 

during the project, including Carl Wentworth in 1961. 

_____________________

* Although Edgar Bailey died in 1983, we include him as an author here because of his leading role 

in the original mapping and study of the Cedars ultramafic mass and its geologic context. 
2

As part of the field investigation, the Cedars ultramafic mass (listed as the Cazadero 

ultramafic mass in some earlier reports) was mapped in considerable detail and numerous samples 

were collected for petrographic, geochemical, and paleomagnetic studies (Blake, 1963). These 

studies determined that the Cedars mass was in fault contact with surrounding rocks of the 

Franciscan Complex, a eugeosynclinal assemblage of greywacke-type sandstone, greenstone 

(altered basalt), radiolarian chert, and minor foraminiferal limestone (fig. 2). Previously, such Coast 

Range ultramafic masses were thought to be igneous intrusions (see, for example, Tallioferro, 1943, 

p. 202–206). 

As the work proceeded, Bailey led a number of informal geologic field trips to the area that 

stimulated interest by other geologists and led to several specialized projects. These included a 

study by Coleman and Lee (1962, 1963) of the glaucophane-bearing Franciscan metamorphic rocks 

Figure 1. Photograph of the interior of the Cedars mass in the upper Austin Creek drainage. 

Note typical rusty color of weathered peridotite and sparse vegetation. Trees are Sargent 

Cypress (Hesperocyparis sargentii), endemic to harzburgite and dunite. Glossy gray-green 

rock low on the near slope is serpentinite on fractures exposed at an old chromite mine. Photo 

by J.W. Shervais, 2011.
3

Figure 2. Index map showing the Cedars ultramafic mass and its geologic context. Area is the 

Skaggs 15-minute quadrangle, with its four 7.5-minute quadrangles labeled. Principal roads are 

gray, streams blue; principal towns shown by black dots. Mine symbol, Layton chromite mine; 

RS, Red Slide claims; location of sample 63-75 shown by circled X.
4

that crop out near Cazadero. Their discovery here of metamorphic aragonite clearly demonstrated 

that glaucophane-bearing metamorphic rocks (blueschists) formed under very high pressures and 

relatively low temperatures, conditions that are now recognized to be restricted to subduction zones.

Also important were a geochemical investigation of the serpentine springs within the Cedars 

mass (Barnes and O’Neal, 1966), which indicated that serpentinization of the ultramafic rocks was 

ongoing under surface conditions, and a gravity study of the ultramafic mass that was made in the 

early 1960s but published later (Thompson and Robinson, 1975). More recently, the high-pH 

springs within the Cedars mass described by Barnes and O’Neal (1966) have been found to contain 

microbial colonies that, together with their serpentine host, may provide a model for the 

development of early life (R.G. Coleman, written commun., 2012; Sleep and others, 2004, 2011).

In 1964, Bailey and coauthors published an extensive report dealing with the Franciscan 

rocks throughout California and their relation to coeval sedimentary rocks along the west side of the 

Great Valley (Bailey, Irwin, and Jones, 1964). That report included a cross section that ran 

westward from the Great Valley through the Cazadero area to the ocean. In that cross section, the 

deformed and metamorphosed Franciscan rocks were shown to be overlain, along a folded thrust 

fault, by little-deformed, late-Mesozoic rocks of the Great Valley sequence (Irwin, 1964). Critically, 

a sheet of serpentinized ultramafic rock was shown to be emplaced along that thrust fault.

In 1969, Bailey and others proposed that some of the ultramafic and mafic rocks within the 

Coast Ranges were part of the oceanic basement to the late Mesozoic Great Valley sequence, which 

they called the Coast Range ophiolite, rather than being part of the Franciscan Complex, as 

previously thought. These newly assigned rocks included some in the northeast corner of the map 

area near Healdsburg and another small patch near Cazadero (Bailey and others, 1970). The Cedars 

ultramafic mass, together with several other ultramafic bodies in the Coast Ranges, were not then 

assigned to the Coast Range ophiolite because they did not occur in association with other ophiolitic 

lithologies (gabbro and basalt). In fact, some studies elsewhere in the Coast Ranges led to the 

proposal that ultramafic masses like the Cedars represent fragments of abyssal peridotite derived 

from oceanic fracture zones and tectonically emplaced into the Franciscan accretionary prism 

during subduction (Loney and others, 1971; Coleman, 2000).

Also in 1969, a new geologic mapping project by the USGS was begun in the north San 

Francisco Bay area that included the Cedars area. This new work incorporated the original mapping 

of Bailey and others, modified it to better fit with plate tectonics and several other important 

concepts that had recently been developed, and enlarged upon it to cover all of Sonoma County and 

parts of adjacent counties (Blake and others, 1971).

In a volume of collected papers dedicated to E.H. Bailey that was issued soon after his death 

in 1983, the older geologic mapping was further modified by combining a number of fault-bounded 

units having similar stratigraphy, age, and metamorphic and structural history (Blake and others, 

1984; Murchey and Jones, 1984; Wahrhaftig, 1984). Thus, most of the Franciscan Complex was 

shown to consist of tectonostratigraphic terranes characterized by basal basalt (greenstone), 

depositionally overlain by radiolarian chert or foraminiferal limestone that in turn was overlain by 

greywacke. Based on geochemical and biostratigraphic studies, it was proposed that the basalt had 

formed at a mid-ocean ridge and was then carried across the ocean by sea-floor spreading, during 

which time it was covered by radiolarian or calcareous ooze. The rocks finally arrived at a tectonic 

trench where the oceanic rocks were covered by continent-derived, clastic sedimentary rocks 

(turbidites) and then subducted to different depths, as indicated by their metamorphic mineral 

assemblages. 
5

In 2002, Blake and others (2002) further refined these ideas for the western Sonoma County 

region and included a structural cross section that runs through the Cedars ultramafic mass. This 

shows the ultramafic mass to be thrust over the Coastal belt Franciscan along its western margin, 

but at its eastern margin the thrust fault is cut by a high-angle fault that places mélange against the 

ultramafic mass (see fig. 2).

Regional Geologic Framework

Figure 2 shows the Cedars ultramafic mass and the surrounding terranes and mélange of the 

Franciscan Complex. Also shown is the outline of the original Skaggs Springs 15' quadrangle and 

the four 7.5' quadrangles that compose it. The Cedars mass consists of relatively intact, partially 

serpentinized peridotite with a sheared serpentinite margin (fig. 3). The surrounding Franciscan 

terranes range in age from Middle Eocene (Wheatfield Fork) to Jurassic (Cazadero) and consist 

primarily of greywacke that has been subjected to a wide range of metamorphic conditions that 

extends from zeolite to blueschist facies. 

Figure 3. Photograph of typical sheared serpentinite, Cazadero area. Rock hammer for scale. 

Photo by C.M. Wentworth, 1961.
6

As mentioned earlier, the Cedars ultramafic mass is in fault contact on its northern and 

western sides with sedimentary rocks of the Coastal belt Franciscan, now locally differentiated into 

the Wheatfield Fork terrane (McLaughlin and others, 2009). Unlike the interior of the mass, which 

is largely intact and only partly serpentinized, the margins have been extensively sheared and 

altered to serpentine minerals. Locally, the greywacke adjacent to the serpentinite margin on the 

north and west contains a thin alteration zone up to a meter or so wide where the greywacke is 

largely replaced by whitish prehnite — another example of the rodingites described by Coleman 

(1967). The conditions under which this prehnite formed are probably limited by those for Coastal 

belt metamorphism described by Ernst and McLaughlin — less than 5–8 km and a temperature of 

200–300° C — but the actual physical conditions involved were probably much lower. The contact 

Figure 4. Photograph of prominent mineral foliation weathered out on a joint surface in 

hartzburgite in the Cedars ultramafic mass. M.C. Blake for scale. Photo by E.H. Bailey, 1963.
7

has not been greatly modified since the prehnite zone formed. The gravity study mentioned earlier 

showed that the ultramafic mass does not extend to great depth, and modeled it as a thin, dishshaped body lying on the Franciscan Complex (Thompson and Robinson, 1975, fig. 9). Similarly, 

recent magnetic modeling indicates that the mass is quite thin, with a thickness perhaps little more 

than the present topographic relief (R.J. Jachens, oral commun., 2012).

Figure 5. Map showing folded foliation within the Cedars ultramafic mass, form lines drawn to 

generalize the mapped strikes, and bedding attitudes in the surrounding Franciscan Complex 

greywackes. See figure 7 for original compilation of foliation attitudes.

On its south side, the serpentinized peridotite is faulted against mélange consisting of highly 

sheared greywacke and shale with many blocks and slabs of greenstone, chert, metamorphic rocks 

and serpentine. Mélange is also found along the eastern side of the ultramafic mass. The contact 

there is marked by a northwest-trending, high-angle fault that appears to have offset the ultramafic 

rocks laterally to the southeast (fig. 2). The aeromagnetic anomaly associated with the ultramafic 

rocks extends directly eastward beyond that fault, however, indicating that these magnetic rocks 

continue eastward beneath the mélange. This relation complicates the idea of simple transcurrent 

faulting (R.J. Jachens, oral commun., 2012).
8

Figure 6. Map showing mineral lineations within the Cedars ultramafic mass and form lines from 

fig. 2. See figure 7 for original compilation of lineations.

Description of the Peridotite

The rock term “peridotite” includes both harzburgite and dunite. Most of the peridotite in the 

Cedars mass, prior to serpentinization, was spinel harzburgite with subordinate dunite. The 

harzburgite is an olivine-rich orthopyroxene (enstatite)-bearing rock with accessory chromium 

spinel (chromite) and minor clinopyroxene. The dunite is almost entirely olivine with accessory 

chromium spinel. Although relatively unaltered rock is present within the mass, and the margins are 

largely serpentinite, much of the interior is also serpentinized. This is clearly indicated by a negative 

gravity anomaly (Thompson and Robinson, 1975) over a mass that has yielded a measured density 

of 3.24 on a fresh hand sample collected by R.G. Coleman (R.C. Jachens, oral commun., 2012). 

Even the standard rock sample PCC-1 (see below) contains 4.71 percent water, which implies that 

this very fresh-appearing rock contains at least 30 percent serpentine (Loney and others, 1971). 

Thus, in referring to harzburgite and dunite, we include their undeformed serpentinized equivalents 

as well.

Like many other peridotites, the Cedars mass contains a prominent compositional layering 

defined by variations in the amount of olivine and orthopyroxene (fig. 4). We measured the 
9

orientation of these layers in the field and the results are shown in fig. 5 (and see fig. 7). We found 

no evidence in the field or in limited thin-section study that the layering is due to crystal settling, as 

is commonly seen in the cumulate layers of stratiform peridotites and ophiolite sequences. Instead, 

it appears to be a metamorphic fabric. Much more detailed studies of mineral fabrics and mineral 

geochemistry in identical harzburgites and dunites in California and Oregon have demonstrated that 

such layering is related to deformation and plastic flow in the upper mantle at temperatures of 

1,000–1,200 ̊ C (Loney and others, 1971; Loney and Himmelberg, 1976). In addition, we noted in 

our field studies that there was a subtle mineral lineation, defined by aggregates of chromium spinel 

(figs. 6, 7). Similar lineations elsewhere have been interpreted to represent the trace of flow lines 

during deformation of what are now referred to as mantle tectonites (Nicholas and Boudier, 1975).

In order to enhance the mapped structures shown on figure 5, Bailey drew a number of form 

lines to help define the shape of the folded surfaces within the peridotite mass. The layering defines 

a large, moderately south-southwest plunging synformal fold with which the chromite lineations 

appear to be parallel or subparallel. The most important point that can be made about this structure 

is that it is strongly discordant to bedding attitudes in the surrounding Franciscan sedimentary rocks 

and thus formed prior to accretion and deformation of the surrounding Franciscan Complex.

Peridotite Chemistry

On May 25, 1963, Bailey and Blake collected a large sample (about 325 pounds) consisting 

of 12–14 boulders of relatively fresh harzburgite from the upper part of East Austin Creek (field 

number 63-75; see figs 2 and 7 for location). These boulders were given to the Analytical Labs of 

the U. S. Geological Survey in Menlo Park. The boulders were crushed and split into smaller 

samples that were distributed to numerous other analytical labs throughout the world. They became 

one of the USGS Standard Rock Samples, PCC-1 (splits of which are no longer available -

Govindaraju, 1994). The average chemical composition of PCC-1 determined in the 1960’s is listed 

(PCC-1-a table 1, in weight percent) as determined by 26 different labs using a variety of 

techniques, including x-ray florescence (XRF), instrumental neutron activation analysis (INAA), 

and atomic absorption (AA) (Flanagan, 1969). Also listed for comparison are a more recent analytic 

compilation for PCC-1 (PCC-1-b: Govindaraju, 1994) and an analysis of a well-studied harzburgite 

from the Burro Mountain peridotite in the southern Coast Ranges (4-BU-66: Loney and others, 

1971). The Cedars and Burro Mountain rocks are nearly identical.

It is important to point out that the unusual chemistry of the Cedars peridotite (and its sparse 

soil) — particularly the extremely low values for calcium, potassium, and phosphorus together with 

extremely high values of magnesium, chromium, and nickel — is responsible for the unusual plant 

communities that are found on this and similar serpentinite bodies (Kruckeberg, 1984: McCarten, 

1993). In fact, the unusual plant communities of the Cedars mass have led to its designation as The 

Cedars Natural Area (Raiche, 2004).
10

Plate Tectonic Setting

In addition to structural analysis, detailed mineral chemistry has been conducted in other 

peridotite masses to determine where they formed. Recent studies (for example, Choi and others, 

2008) concluded that the mantle tectonites in all but one Coast Range ophiolite occurrence probably 

represent a supra-subduction zone setting. These authors also studied the Burro Mountain peridotite 

mass in the southern Coast Ranges and found that it too had a mineral chemistry indicating a suprasubduction zone origin, rather than having formed at a mid-ocean ridge. Until comparable studies 

are completed on the Cedars ultramafic mass, it is not possible to say what its ultimate origin was. 

Toward this end, John Shervais (written comm., 2011) has told us that he is studying a number of 

samples recently collected from the Cedars peridotite mass. Based on the nearly identical wholerock mineralogy and chemistry of the Cedars and Burro Mountain masses, however, we would 

predict that the Cedars will also prove to have a supra-subduction zone origin.

Table 1. Chemical analyses of hartzburgite from the Cedars peridotite (PCC-1a, b) and from 

the Burro Mountain peridotite of the Southern California Coast Ranges (4BU-66).

 PCC-1-a PCC-1-b 4BU-66

SiO2 41.87 41.71 41.92

Al2O3 0.85 0.675 0.75

Fe2O3 2.84 2.72 1.83

FeO 4.94 5.06 6.03

MgO 43.56 43.43 44.48

CaO 0.53 0.52 0.5

Na2O 0.05 0.03 0.02

K2O 0.01 0.007 0

H2O+ 4.71 4.71 3.52

H2O- 0.47 0.44 0.16

TiO2 0.02 0.01 0.01

P2O5 0.01 0.002 0

MnO 0.12 0.12 0.12

CO2 0.12 0.15 0.09

Cl 0.02 -- 0

F 0 0 0

S 0.01 0 0.09

Cr2O3 0.39 0.40 0.43

NiO 0.31 0.30 0.31

Sum 100.83 100.28 100.26
11

Conclusions

The Cedars peridotite mass consists largely of partially serpentinized spinel harzburgite and 

subordinate dunite. A pronounced compositional layering is defined by variations in the amount of 

olivine and orthopyroxene in the harzburgite. Also present is a mineral lineation defined by 

aggregates of chromite grains. Both of these features are considered to be the result of hightemperature ductile flow in the upper mantle. Based on water content, even the most massive 

harzburgite is partially serpentinized. Sheared serpentinite, however, is largely confined to the 

borders and within a few shear zones that transect the mass. None of the upper levels of an ophiolite 

sequence are present (gabbro, basalt), but they could have been removed by faulting. Although 

Figure 7. Original compilation of attitudes and lineations by E.H. Bailey on 7.5-minute topographic 

maps spliced at their common inner corner (see fig. 2 for quadrangle names). Outline box and 

latitude-longitude tics show relation of this map to those of figures 5 and 6. Insets at upper right, in 

Bailey’s hand, copied from back of map. Location of sample 63-75 shown by circled X. A full-scale 

copy of this map compilation is separately included with this report as of2012-1164_plate.pdf (at 

http://pubs.usgs.gov/of/2012/1164/)
12

much more detailed study is needed, we tentatively conclude that the Cedars ultramafic mass is a 

fragment of mantle tectonite that once underlay the Coast Range ophiolite.

Acknowledgments

We thank our colleagues R.C. Jachens, R.J. McLaughlin, and R.W. Graymer for helpful 

discussion and comments, and Jachens for his help with geophysical constraints on the thickness of 

the Cedars mass and offset on the adjacent northwest-trending fault. R.C. Evarts and R.G. Coleman 

provided timely and very helpful reviews of the manuscript, and J.W. Shervais shared his 

photograph of the interior of the Cedars mass (our fig. 1). We also want to acknowledge our debt to 

Edgar Bailey, who introduced us to the Franciscan and led us by example in the fine arts of field 

geology and scientific inquiry. And of course, he admonished, “Always get it right along the roads”.

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