Geologic and Geophysical Maps of the Eastern ThreeFourths of the Cambria 30 ́ x 60 ́ Quadrangle, Central 

California Coast Ranges 

Pamphlet to accompany 

Scientific Investigations Map 3287 

U.S. Department of the Interior

U.S. Geological Survey 

2014 
This page is intentionally left blank
Contents 

Contents ........................................................................................................................................................................... ii 

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

Interactive PDF ............................................................................................................................................................ 2 

Stratigraphy ..................................................................................................................................................................... 5 

Basement Complexes ................................................................................................................................................. 5 

Salinian Complex ..................................................................................................................................................... 5 

Great Valley Complex ............................................................................................................................................ 10 

Franciscan Complex .............................................................................................................................................. 11 

Early Quaternary and Tertiary Strata ......................................................................................................................... 12 

Piedras Blancas Block ........................................................................................................................................... 12 

Santa Lucia Range Block ....................................................................................................................................... 12 

San Antonio Block .................................................................................................................................................. 12 

Quaternary Surficial Deposits .................................................................................................................................... 13 

Paleontology ................................................................................................................................................................. 13 

Radiometric Ages .......................................................................................................................................................... 14 

Geophysical Data .......................................................................................................................................................... 14 

Gravity Anomalies ..................................................................................................................................................... 18 

Magnetic Anomalies .................................................................................................................................................. 19 

Structure ........................................................................................................................................................................ 20 

Structural History ........................................................................................................................................................... 25 

Description of Map Units ............................................................................................................................................... 28 

Quaternary surficial deposits ..................................................................................................................................... 28 

Early Quaternary and Tertiary strata ......................................................................................................................... 31 

Piedras Blancas Block ........................................................................................................................................... 31 

Santa Lucia Range Block ....................................................................................................................................... 32 

San Antonio Block .................................................................................................................................................. 34 

Basement complexes ................................................................................................................................................ 35 

Salinian complex .................................................................................................................................................... 35 

Franciscan Complex .............................................................................................................................................. 36 

Cambria terrane ................................................................................................................................................. 37 

Unnamed Disrupted Terrane(s) ......................................................................................................................... 37 

Great Valley complex ............................................................................................................................................. 38 

Great Valley Sequence ...................................................................................................................................... 38 

Coast Range ophiolite ........................................................................................................................................ 38 

References Cited ........................................................................................................................................................... 39 

ii 
Figures 

1.	 Shaded relief map of the onshore part of the Cambria 30 ́ x 60 ́ quadrangle, annotated with geographic 

names used in the text....................................................................................................................................3 

2.	 Index map of structural blocks and named faults in the map area..................................................................4 

3.	 Index map of tectonostratigraphic elements in the map area..........................................................................6 

4.	 Maps and cross-sections illustrating the tectonostratigraphic setting and history of the study area...............7 

5.	 Gravity and aeromagnetic maps of the study area........................................................................................15 

6.	 Cross section of the map area and an adjacent area to the east, through Point Piedras Blancas and San 

Antonio Reservoir, with gravity and magnetic model.....................................................................................21
 

7.	 Index map of microseismicity in the map area, color-coded by depth...........................................................24
 

Tables 

1.	 Specifications of aeromagnetic surveys within the Cambria quadrangle......................................................18
 

iii 
 

 

 

Geologic and Geophysical Maps of the Eastern ThreeFourths of the Cambria 30 ́ x 60 ́ Quadrangle, Central 

California Coast Ranges 

By R.W.Graymer, V.E. Langenheim, M.A. Roberts, and Kristin McDougall 

Introduction 

This report contains geologic, gravity anomaly, and aeromagnetic anomaly maps of the 

eastern three-fourths of the 1:100,000-scale Cambria quadrangle and the associated geologic and 

geophysical databases (ArcMap databases), as well as complete descriptions of the geologic map 

units and the structural relations in the mapped area. A cross section is based on both the 

geologic map and potential-field geophysical data. 

The geologic map is a significantly modified version of an unpublished geologic map 

compiled by V.M. Seiders in the early 1990s. Three factors led to the decision to remap the area 

rather than publish this existing compilation. First, almost 20 years of new geologic information 

has been gathered for the map area. Second, tectonic models for the California Coast Ranges 

have changed since the 1990s, and these new models have a strong influence on the depiction of 

through-going faults, volcanic fields, folds, and other mapped features. Third, areally consistent 

and continuous potential-field datasets make it possible to map concealed faults and rock bodies 

in a way that was not possible before. Changes to the Seiders geologic map include the 

following: 

1.	 Updated depiction of the active San Simeon Fault Zone (considered to be part of the 

major Hosgri Fault Zone) and adjacent geologic map units 

2.	 Mapping of Quaternary surficial deposits throughout the onshore map area 

3.	 Detailed mapping in the Williams Hill area in the northeast corner of the map 

4.	 Extensive revisions based on new mapping in the Cambria area in the southeast corner of 

the map 

5.	 Revised stratigraphic depiction based on subdivision of structural blocks and Mesozoic 

basement terranes 

6. Revised interpretations based on potential-field geophysical data. 

There are also smaller additions and revisions throughout the map area. 

The Cambria 30 ́ x 60 ́ quadrangle comprises southwestern Monterey County and 

northwestern San Luis Obispo County. The land area includes rugged mountains of the Santa 

Lucia Range extending from the northwest to the southeast part of the map; the southern part of 

the Big Sur coast in the northwest; broad marine terraces along the southwest coast; and broad 

1 
valleys, rolling hills, and modest mountains in the northeast (fig. 1). Most of the area is sparsely 

inhabited, including the Hunter Liggett Military Reservation, Los Padres National Forest 

(including a number of designated wilderness areas), and a very large tract belonging to the W.R. 

Hearst estate. Public roads and trails are limited, and access is therefore difficult. The 

unincorporated town of Cambria (population ~6,500) along the southern coast is the only sizable 

community. The primary point of public interest in the map area is Hearst San Simeon State 

Historical Monument, centered around Hearst Castle, the mega-mansion built by Julia Morgan 

for media magnate William Randolph Hearst on a ridge overlooking the Pacific Ocean (a ridge 

created by hanging-wall uplift on the active Oceanic Fault, fig. 2). 

Geologic interest focused on the Cambria region in 2003, when the Oceanic Fault and 

related blind faults ruptured to generate the M6.6 San Simeon earthquake (a somewhat confusing 

name because the earthquake did not occur on the San Simeon Fault). Two lives were lost, 47 

people were injured, and almost 500 homes and commercial structures were damaged in San 

Luis Obispo and Santa Barbara Counties (California Seismic Safety Commission, 2004). The 

Oceanic Fault is one of a suite of poorly understood faults that splay from the major strike-slip 

Hosgri Fault. We undertook regional geologic and geophysical mapping after the earthquake 

both to serve as a basic data set for analyses of the earthquake and associated faults and to glean 

insights into the fault system from the new observations. 

Interactive PDF 

The maps are presented as an interactive, multilayer PDF, rather than more traditional 

pre-formatted map-sheet PDFs. Various geologic, geophysical, paleontological, and base map 

elements are placed on separate layers, which allows the user to combine elements interactively 

to create map views beyond the traditional map sheets. For example, fault lines, traditionally part 

of the geologic map sheet, can be overlain on gravity data to examine the role of the faults as 

boundaries of bodies with differing density. A layered PDF also allows the user to select 

simplified views by excluding some of the elements of a traditional map sheet, such as focusing 

on geologic unit distribution by omitting structural data layers. In addition, a layered PDF allows 

the user to select either a traditional topographic map base or a topographic shaded-relief base 

(or no base at all). Four traditional map sheets (geologic map, gravity map, aeromagnetic map, 

paleontological locality map) are easily compiled by choosing the associated data layers or by 

choosing the desired map under Bookmarks. 

2 
 

 

 

Figure 1. Shaded relief map of the onshore part of the Cambria 30 ́ x 60 ́ quadrangle, annotated with geographic 

names used in the text. Also shown is the 2003 M6.6 San Simeon earthquake epicenter. The main faults are shown 

as black lines. 

3 
 

 

 

 

 

Figure 2. Index map of structural blocks and named faults in the map area. The complex Coast Ranges Fault is represented by 

gray lines. The newly mapped blind-fault reach of the Jolon Fault is shown as purple. Block-bounding faults are shown as 

orange. Faults of the San Gregorio-Hosgri Fault System are labeled SG-HFS. Also shown are traces of some offshore faults 

derived from bathymetric geomorphology as depicted on the U.S. Geological Survey topographic quadrangle. Reverse and thrust 

faults are shown with teeth on the hanging-wall side; right-lateral faults are shown by arrow pairs. 

4 
 

Stratigraphy 

The stratigraphy1 of the California Coast Ranges consists of three amalgamated Mesozoic 

and Paleogene basement complexes (Franciscan, Great Valley, Salinian) overlain by Tertiary and 

early Quaternary strata and Quaternary surficial deposits (fig. 3). Tertiary and early Quaternary 

strata differ across the map area, reflecting large-scale Tertiary and Quaternary offset on major 

faults as well as topographic separation of depositional basins, which necessitates subdivision of 

the Tertiary and Quaternary stratigraphy into three assemblages limited to major structural 

blocks (Piedras Blancas, Santa Lucia Range, San Antonio, see fig. 2). 

Basement Complexes 

Paleocene and Mesozoic rocks in the map area can be divided into three very distinctive 

rock suites that reflect the complex history of rock formation and accretion at the subduction 

margin of western North America throughout the Cretaceous and early Tertiary. Each basement 

complex represents a different part of the arc-forearc-accretionary prism subduction-margin 

system. 

Salinian Complex 

The Salinian complex is made up of Late Cretaceous plutonic rocks and older 

metamorphic wallrock, locally overlain unconformably by latest Cretaceous marine sedimentary 

rocks. Plutonic and metamorphic rocks represent magma chambers that underlay the Late 

Cretaceous continental-margin arc, as well as their host rocks. North of the map area, these rocks 

overlie Late Cretaceous pelitic schist along a folded thrust fault (fig. 4A; Ducea and others, 

2009). The original position of the arc is known to have been south of the Sierra Nevada, 

because restoration of Tertiary San Andreas Fault slip places a distinctive gabbroic body (Logan 

Gabbro), now present in the southern San Francisco Bay region, with a correlative gabbroic body 

in the San Emigdio Mountains (fig. 4B). However, beyond that constraint, the original position is 

not known. Various workers have proposed competing hypotheses (for example Dickinson, 

1983; Hall, 1991), but multiple lines of evidence strongly suggest that Salinia is equivalent to the 

San Emigdio, Mojave, and eastern Transverse Ranges blocks east of the San Andreas Fault (as 

summarized by Powell, 1993; fig. 4B). 

1

 The U.S. Geological Survey standard for geologic maps is to use chronostratigraphic 

nomenclature (for example, lower Quaternary, Upper Cretaceous). However, in the California 

Coast Ranges, including the map area, deposition during periods of tectonic uplift, as well as 

tectonic juxtaposition of different stratigraphic sequences and interleaved terranes, has in many 

places resulted in older units being above younger units, even without overturning or structural 

repetition. As a result, we find the use of chronostratigraphic terms to be potentially confusing 

(such as middle Pleistocene marine terrace deposits everywhere above Upper Pleistocene marine 

terrace deposits) and instead use geochronologic nomenclature (for example, early Quaternary, 

Late Cretaceous), which is unambiguous, throughout. 

5 
 

 

 

Figure 3. Index map of tectonostratigraphic elements in the map area, showing the distribution of Quaternary surficial 

deposits, Pleistocene and Tertiary strata, and early Tertiary and Mesozoic basement complexes. See figure 2 for 

structural block and fault labels. 

6 
 

A
 

Figure 4–See caption, p. 10
 

7
 
 

B
 

Figure 4–See caption, p. 10
 

8
 
 

C 

Figure 4–See caption, p. 10 

9
 
 

 

 

 

 

 

 

Figure 4. Maps and cross sections illustrating the tectonostratigraphic setting and history of the study area. A. 

Overview map of tectonostratigraphic elements in the northern and central California Coast Ranges; B. Index map of 

California showing the main strands of the San Andreas Fault System (black) and the Nacimiento Fault (green), 

along with a number of geographic provinces, mountain ranges, and rock bodies offset by fault slip mentioned in the 

text; C. Cartoon tectonostratigraphic cross sections illustrating various models of Coast Range ophiolite (CRO) 

formation (Dickinson and others, 1996; Blake and others, 2002). 

What is clear, however, is that the Salinian arc basement was rapidly unroofed in latest 

Cretaceous time, bringing the plutonic and metamorphic rocks now in the map area to the surface 

as the depositional base for a thick sequence of Maastrichtian (latest Cretaceous) marine strata 

(in the map area, made up of the El Piojo, Shut-in, Italian Flat, and Steve Creek Formations). 

This depositional environment continued at least into the Paleocene (as shown in the map area by 

the overlying Piedras Altas Formation). 

Great Valley Complex 

The Great Valley complex in the Cambria quadrangle is composed of three parts: the 

basal Jurassic Coast Range ophiolite, unconformably overlain by Late Jurassic and Early 

Cretaceous Toro Formation, in turn unconformably overlain by the Cretaceous Atascadero 

Formation. The basal ophiolite in the Cambria area has been included in the Coast Range 

ophiolite, which also makes up the base of the Great Valley complex present east of the San 

Andreas Fault (fig. 4A). However, restoration of Neogene offset on the San Andreas Fault places 

the Coast Range ophiolite in the map area 300 km or more southeast of the correlated ophiolite 

east of the San Andreas Fault. The Cambria region ophiolite may also have been affected by 

large-scale offset related to transport of the Salinian Complex. Therefore the Coast Range 

ophiolite in the map area may not be genetically related to the Coast Range ophiolite to the east. 

Nevertheless, we summarize four genetic models for the Coast Range ophiolite here, even 

though much of the underpinning of these models is based on observations of the unit east of the 

San Andreas Fault. We note that the ophiolite bodies in the two regions are the same age and 

same geochemical composition, so the genetic setting of the two may be similar. Furthermore, 

some of the data incorporated into the genetic models does come from the ophiolite in or near the 

map area. At the very least, the models outlined below are plausible models for the ophiolite in 

the map area. 

The Coast Range ophiolite formed between about 173 and 164 Ma (Blake and others, 

1992; Hagstrum, 1997; Hopson and others, 1997), but the tectonic setting of ophiolite formation 

is controversial. Dickinson and others (1996) summarize three different models: accreted 

ophiolite formed in the back-arc of an east-facing ocean-island arc, far-travelled mid-ocean ridge 

ophiolite, and autochthonous ophiolite formed in the continental-margin forearc (fig. 4C). More 

recently, Graymer and his coauthors in Blake and others (2002) proposed a fourth model that 

took into account additional data, including the nature of coeval rocks in the Sierra Nevada that 

reflect the tectonic setting of ophiolite formation, as well as the age of the oldest Franciscan 

Complex metamorphism. In that model, the ophiolite formed by rifting in the forearc basin of an 

island arc that then accreted to the North American margin in Late Jurassic (Kimmeridgian, ~153 

Ma) time (fig. 4C; see Graymer, 2005, for details of the timing of accretion). 

The ophiolite is unconformably overlain by late Jurassic and Cretaceous marine 

sedimentary rocks of the Great Valley sequence, represented in the map area by the Toro 

Formation and overlying Atascadero Formation. In the central California Great Valley, these 

10 
strata formed in the forearc of the North American continental margin volcanic arc (Dickinson, 

1971; Hamilton, 1978) after accretion of the Jurassic island arc and Coast Range ophiolite (Blake 

and others, 2002; Graymer, 2005). The Toro and Atascadero Formations were deposited far 

south of the present-day Great Valley, but lithologic and petrologic similarity between the Great 

Valley sequence to the north and in the map area argues for a similar depositional environment. 

Franciscan Complex 

The rocks of the Franciscan Complex are mostly derived from Jurassic to Cretaceous 

oceanic crust and pelagic deposits overlain by Upper Jurassic to lower Tertiary turbidites. These 

rocks were variably metamorphosed, depending on the depth of subduction prior to unroofing, 

from high-grade glaucophane schist to incipient prehnite-pumpellyite facies. In the northern 

Coast Ranges (fig. 4A), the Franciscan Complex is made up of three structural belts: an Eastern 

Belt composed of Jurassic and Cretaceous coherent medium- to low-grade blueschist 

metamorphic rocks (lawsonite-albite and lawsonite-jadeite-quartz sub-facies); a Central Belt 

composed of slabs of coherent Jurassic and Cretaceous low-grade metavolcanics and 

metasediments (prehnite-pumpellyite facies) interleaved within mélange; and a Coastal Belt 

composed mostly of Late Cretaceous and Tertiary, low-grade metasediments (laumontite facies). 

The position of these belts reflects the order of accretion and depth of subducition of Franciscan 

Complex terranes. The earliest-accreted Eastern Belt rocks are also farthest subducted and most 

metamorphosed, whereas the last-accreted Coastal Belt rocks are least subducted and 

metamorphosed. The belts are further divided into tectonostratigraphic terranes, large faultbounded blocks with an internally consistent stratigraphy and metamorphic grade (for example, 

Blake and others, 1982). Mélange is not considered part of any terrane, but rather a tectonic 

mixture of rocks from multiple terranes (Blake and others, 2002). 

Franciscan Complex rocks in the Cambria region are part of the Nacimiento Belt (fig. 

4A). They can be roughly subdivided into three main lithologic categories: (1) coherent and 

nearly coherent bodies that retain recognizable sedimentary layers and contain Cretaceous 

microfossils (Hsu, 1969; Smith, 1978; Seiders, 1989a); (2) broken formation in which original 

bedding is entirely or almost entirely disrupted, but rock types are largely unmixed and exotic 

blocks (serpentinite, high-grade metamorphic blocks) are missing; and (3) mélange composed of 

sheared sedimentary matrix containing blocks of sandstone and metasandstone, chert and 

metachert, basalt and greenstone, serpentinite, and blueschist. This suite of rock types most 

resembles the Central Belt; rocks like those of the other belts are not found in the map area. The 

similarity of lithology, as well as age, suggests that Franciscan Complex rocks of the Nacimiento 

Belt are correlative with the Central Belt. The relatively coherent Late Cretaceous 

metasedimentary rocks near Cambria can be assigned to a terrane, herein called the Cambria 

terrane. As mentioned above, mélange is not considered part of any terrane, and in the map area 

the broken formation presently lacks the needed lithologic and stratigraphic data to determine 

whether it should be considered part of the Cambria terrane or some other terrane(s). 

A model for the tectonic history of all Franciscan Complex terranes follows: oceanic 

rocks (including mid-ocean-ridge basalts and seamounts); overlain by pelagic and detrital 

sediments; brought into proximity with North America by convergent plate motion; and more or 

less subducted, accreted, and moved northward to their current position on the continent by 

transform offset along the continental margin. Earlier workers (Howell and others, 1977) have 

11 
 

suggested that the Cambria terrane was deposited as a trench basin, formed within the 

accretionary prism and subsequently structurally interleaved with older accreted rocks. 

Early Quaternary and Tertiary Strata 

Early Quaternary and Tertiary strata in the Cambria region are divided into three distinct 

assemblages bounded by major faults (fig. 2): the Piedras Blancas block, the Santa Lucia Range 

block, and the San Antonio block. These assemblages represent geographically distinct 

depocenters of strata, juxtaposed by large-scale offset on their bounding faults. Topographic 

separation of the Santa Lucia Range and San Antonio blocks in Pliocene and younger time has 

also resulted in deposition of different rock types. 

Piedras Blancas Block 

The Piedras Blancas block lies west of the San Simeon Fault along the coast in the westcentral part of the onshore map area. Amalgamated Franciscan Complex and Great Valley 

complex basements are overlain unconformably by early Miocene to Pliocene strata. These rocks 

probably reflect a depocenter once located relatively about 90 km south of their present location, 

brought to their present position relative to rocks farther east by right-lateral offset on the 

Hosgri-San Simeon Fault (Hall, 1975; Jachens and others, 2009; Langenheim and others, 2013). 

Santa Lucia Range Block 

The Santa Lucia Range block lies between the San Simeon and Nacimiento Faults. Like 

the Piedras Blancas block it is composed of amalgamated Franciscan Complex and Great Valley 

complex basements, here overlain unconformably by Oligocene to Pliocene strata. The Tertiary 

strata differ from the Piedras Blancas strata in the presence of Oligocene and Miocene volcanic 

units, along with other differences discussed below in the Description of Map Units. The bulk of 

the Tertiary rocks in this block crop out south of the Oceanic Fault, probably due to relatively 

greater erosion in the more uplifted northern hanging wall of this reverse fault. 

San Antonio Block 

The San Antonio block lies between the Nacimiento and Rinconada Faults. Unlike the 

other blocks, the basement of this block is composed of Salinian complex rocks. Also unlike the 

other blocks, there is not a universal unconformity between Mesozoic and Cenozoic units, but in 

places a continuous latest Cretaceous to Paleocene stratigraphy is preserved. Instead, there is a 

block-wide unconformity between late Oligocene and early Miocene strata and underlying 

Paleogene strata. Oligocene and Miocene strata in this block have been given the same names as 

roughly coeval strata in the Santa Lucia Range block, but, as described below, the rock types and 

depositional environments of the units in the two blocks are distinct, so presumption of original 

continuity or proximity is ill advised. 

12 
Quaternary Surficial Deposits 

Quaternary surficial deposits in the map area are mostly undivided. The exceptions are 

beach sand and marine terraces along the Pacific coast, river terraces along the San Antonio 

River and other major drainages, and older alluvial fan deposits in Lockwood Valley 

A series of discrete marine terrace surfaces were adapted from a Quaternary map of the 

San Simeon region produced by Weber (1983). The locations of a few of these surfaces in the 

vicinity of San Simeon Point were later revised by Hanson and others (1994). Both sources 

originated as paper maps, which were scanned, georeferenced, and later digitized in ArcGIS. To 

better fit the 1:100,000 scale, some of the smallest terrace remnants were not included, and other 

contacts were modified as needed. The names and age designations of marine terraces are based 

on each surface’s relative elevation above mean sea level and degree of erosional modification. 

Correlation of these terraces is derived from the estimated local tectonic uplift rates as discussed 

by Weber (1983). 

Paleontology 

Many different kinds of fossils have proved invaluable to understanding the geology of 

the map area: microfossils, including radiolaria, foraminifers, and pollen in the Franciscan 

Complex; mollusks in the Great Valley and Salinian complex sedimentary rocks as well as the 

overlying Tertiary strata; and foraminifers and diatoms in the Monterey Formation. A complete 

listing of the fossils in the map area is beyond the scope of this report, but we make extensive 

reference to fossil data in the discussions below. 

A majority of the microfossil data used in this study are derived from samples in the U. S. 

Geological Survey collections currently housed in the Micropaleontology Laboratory in 

Flagstaff, Arizona, and Chevron Oil Company slides borrowed from the California Academy of 

Sciences in San Francisco, California, and reported in Malmblorg and others (2008). 

Assemblages were reviewed for consistency with modern biostratigraphic understanding and 

revisions were made as appropriate. Age interpretations use the correlation of the benthic 

foraminiferal stages and zones to the international time scale as discussed in McDougall (2008). 

Cenozoic environmental interpretations are based on an overview of California benthic 

foraminifers by Ingle (1980), Ingle and Keller (1980), and Blake (1981, 1991) and studies of 

cosmopolitan benthic foraminifers by Douglas (1981) and van Morkhoven and others (1986). We 

have intentionally not used all of the foraminiferal data in the published record for the map area 

in the discussion that follows. The data we chose not to use falls into one of three categories: 

1.	 Locality descriptions of some samples that were inadequate and that could not be 

located with any certainty or that plotted on the “wrong” side of a contact. We 

interpret this to indicate that either the locality or the contact are slightly misplaced, 

but it is beyond the scope of this study to try to reoccupy the localities to determine 

whether the locality or contact need to be relocated. 

2.	 Data that seems “obviously wrong” (for example, a late Miocene locality in a unit 

overlain by well-documented middle Miocene strata). We leave the detailed mapping 

of the structures required to explain such data, if in fact they are correct, to future 

workers. 

13 
3.	 Less restrictive fossil ages in a unit that has an age more constrained by other 

stratigraphic data. For example, some units in the Salinian Cretaceous may include 

only Late Cretaceous foraminifers, but the unroofing history of Salinian basement, as 

well as more robust foraminiferal data from adjacent areas, limit all Salinian 

Cretaceous strata in the map area to Maastrichtian or younger. 

Microfossil samples were examined from all three structural blocks. Although faunas are 

sparse in the Piedras Blancas and Santa Lucia Range blocks, foraminiferal faunas in the San 

Antonio block are common. The samples examined are listed in the paleontological data files 

(CambriaPaleoLocality.xlsx) along with the age and ecological interpretations. Faunal lists are 

given for selected samples (CambriaPaleoChecklist.xlsx). In general, these assemblages indicate 

that the Vaqueros Formation was deposited at upper bathyal depths (150–500 m), and deposition 

of the Monterey Formation began in the upper middle bathyal depths (500–1,500 m) then 

shallowed to upper bathyal depths (150–500 m). 

Radiometric Ages 

Four units in the map area have been analyzed using radiometric dating techniques. Ernst 

and others (2011) report U-Pb zircon ages of 26.8±0.3 and 27.4±0.4 Ma for two samples of the 

Cambria Felsite (Tc). Zircons from the Coast Range ophiolite in the Piedras Blancas block 

(Mattinson and Hopson, 2008) yielded a U-Pb age of 165.580±0.038 Ma. Detrital zircon analysis 

has yielded a maximum depositional age of ~17 Ma for the Lospe Formation in the Piedras 

Blancas block (J. Colgan, written commun., 2011), 77–90 Ma (Morisani and others, 2005) for 

some slabs of Franciscan Complex graywacke in mélange in the Santa Lucia Range block, and 

about 80–90 Ma (Jacobson and others, 2011) for Franciscan Complex graywacke from the 

Cambria terrane. 

Geophysical Data 

Gravity and magnetic data are useful for projecting the surface geology into the 

subsurface. Gravity data are processed to reflect density variations within the upper and middle 

crust and are particularly well suited for determining the shape of Cenozoic basins, because of 

the significant density contrast between dense Mesozoic basement rocks and lighter Cenozoic 

rocks. Magnetic data reflect magnetization variations within the crust and are well suited for 

mapping the distribution of rock types that contain magnetite. Both gravity and magnetic 

anomalies can be related to rock type, providing a means to map remotely some aspects of the 

geology. 

About 671 onshore gravity measurements were used to produce an isostatic gravity map 

of the quadrangle (fig. 5). Offshore, the gravity data are from a 6-km grid (Decade of North 

American Geology Project, 1987). Sources of data include the agency formerly known as 

Defense Mapping Agency, Burch and others (1971), and Pan-American Center for Earth and 

Environmental Studies (2010). Gravity measurements are non-uniformly distributed in the 

region, with an average spacing of 1 km. 

Gravity data were reduced to free-air anomalies using standard formulas (for example, 

Telford and others, 1976). Bouguer, curvature, and terrain adjustments to a radial distance of 

166.7 km were applied to the free-air anomaly at each station to determine the complete Bouguer 

14 
15 

Figure 5–See caption, p. 17 

A 
16 

Figure 5–See caption, p. 17 

B 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 5. A. Color isostatic gravity map of the eastern three-quarters of the Cambria 1:100,000-scale quadrangle. 

Light black lines, 2 mGal contours. Dotted and heavy black lines, faults. Brown dots, locations of density boundaries 

from maximum horizontal gravity gradient. White line, location of cross section shown in fig. 6. JF, Jolon Fault; NF, 

Nacimiento Fault; OF, Oceanic Fault; RF, Rinconada Fault; SSF, San Simeon Fault. Black-rimmed blue dots are 

drillholes that penetrated basement rocks at shallow depth. The green triangle is a drillhole that bottomed in Miocene 

sedimentary rocks at nearly 1.8 km depth. 1, gravity high attributed to metamorphic rocks of the Salinian complex; 2, 

easternmost extent of continuous exposure of Salinian complex rocks; 3, isolated outcrop of Salinian basement; 4, 

outcrops of latest Cretaceous sedimentary rocks. B. Color shaded-relief aeromagnetic map of the eastern threequarters of the Cambria 1:100,000-scale quadrangle. Thin black lines, 25 nT contours. Pale-gray lines, aeromagnetic 

survey boundaries. Dotted and heavy black lines, faults. Red dots, locations of magnetization boundaries from 

maximum horizontal gradient. JF, Jolon Fault; NF, Nacimiento Fault; OF, Oceanic Fault; RF, Rinconada Fault; SSF, 

San Simeon Fault. Black-rimmed blue dots are drillholes that penetrated basement rocks at relatively shallow depth. 

The green triangle is a drillhole that bottomed in Miocene sedimentary rocks at nearly 1.8 km depth. 1, outcrop of 

Lospe Formation; 2 and 6, magnetic highs attributed to concealed Salinian basement rocks; 3, northwest edge of 

magnetic rocks interpreted to be truncated by offshore faults; 4, magnetic highs coincident with outcrops of diabase 

and serpentinite; 5, truncation of magnetic rocks interpreted to be truncated by Oceanic Fault. 

anomalies at a standard reduction density of 2,670 kg/m3 (Plouff, 1977). An isostatic adjustment was 

then applied to remove the long-wavelength effect of deep crustal and (or) upper mantle masses that 

isostatically support regional topography. The isostatic adjustment assumes an Airy-Heiskanen model 

(Heiskanen and Vening-Meinesz, 1958) of isostatic compensation. Compensation is achieved by varying 

the depth of the model crust-mantle interface, using the following parameters: a sea-level crustal 

thickness of 25 km, a crust-mantle density contrast of 400 kg/m3, and a crustal density of 2,670 kg/m3 

for the topographic load. These parameters were used because (1) they produce a model crustal 

geometry that agrees with seismically determined values of crustal thickness for central California, (2) 

they are consistent with model parameters used for isostatic corrections computed for the rest of 

California (Roberts and others, 1990), and (3) changing the model parameters does not significantly 

affect the resulting isostatic anomaly (Jachens and Griscom, 1985). The resulting isostatic residual 

gravity values should reflect lateral variations of density within the middle to upper crust. Accuracy of 

the data is estimated to be on the order of 0.1 to 0.5 mGal. 

The aeromagnetic map is based on data from four surveys of varying resolution (table 1). The

onshore surveys (U.S. Geological Survey, 2005; Langenheim and others, 2009) were flown at a nominal 

height of 245–305 m (800–1,000 ft) above ground along flight lines spaced 0.54 to 0.8 km (0.33 to 0.5 

mi) apart. Accuracy of these modern surveys is 1 nanotesla or better. The offshore survey ((McCulloch 

and Chapman, 1977) was flown at a constant barometric altitude of 610 m along flight lines 1.6 km

apart. All data were adjusted to a common magnetic datum and then merged by smooth interpolation 

across a buffer zone along the survey boundaries.

To help delineate structural trends and gradients expressed in the gravity and magnetic fields, a

computer algorithm was used to locate the maximum horizontal gradient (Cordell and Grauch, 1985; 

Blakely and Simpson, 1986). Concealed basin faults beneath the valley areas were mapped using

horizontal gradients in the gravity field. We calculated magnetization boundaries on a filtered version of

the magnetic field to enhance shallow sources. First, we subtracted a numerically derived regional field 

from the actual merged data. The regional field was computed by analytically continuing the merged 

aeromagnetic data to a surface 200 m higher than that on which the measurements were made, an 

operation that tends to smooth the data by attenuating short-wavelength anomalies (Blakely, 1995). 

Second, the resulting residual aeromagnetic field was mathematically transformed into magnetic

potential anomalies (Baranov, 1957); this procedure effectively converts the magnetic field to the 

17 
 

 

 

 

 

 

 

 

equivalent gravity field that would be produced if all magnetic material were replaced by

proportionately dense material. The horizontal gradient of the magnetic potential field was then

calculated. Gradient maxima occur approximately over steeply dipping contacts that separate rocks of 

contrasting densities or magnetizations. For moderate to steep dips (45° to vertical), the horizontal 

displacement of a gradient maximum from the top edge of an offset horizontal layer is always less than

or equal to the depth to the top of the source (Grauch and Cordell, 1987). 

Table 1. Specifications of aeromagnetic surveys within the Cambria quadrangle. 

Survey Year Altitude Flightline spacing Flightline direction 

flown (km) 

Paso Robles 305 m above ground
 

(Langenheim and others, 2008 0.8 NE./SW.
 

2009)
 

Salinas Valley South (U.S. 244 m above ground
 

Geological Survey, 2005) 2002 0.54 NE./SW.
 

Carmel (this report) 2009 305 m above ground 0.8 NE./SW.
 

California Coast
 

(McCulloch and Chapman, 1976 610 m barometric 1.6 NE./SW.
 

1977)
 

Gravity Anomalies 

Gravity anomalies within the Cambria quadrangle (fig. 5A; Gravity Map, see PDF) reflect the 

density contrast between dense Mesozoic basement rocks and overlying, less dense Cenozoic 

sedimentary rocks and deposits. The lowest gravity values on land are located in the northeast corner of 

the map where Miocene and younger rocks are exposed in Jolon, Lockwood, and San Antonio Valleys. 

The highest gravity values correspond to outcrops of metamorphic rocks of the Salinian complex in the 

north-central part of the map (1 in fig. 5A). These rocks, the southern tip of an elongate northeast-tilted 

section of the Coast Ridge Belt that mostly crop out north of the quadrangle, likely represent exposures 

of the deepest and more mafic parts of the continental margin arc, with calculated seismic velocities of 

approximately 6.5 km/s (Ducea and others, 2003). The eastward decrease in gravity values measured on 

these metamorphic and plutonic rocks along the north-central edge of the quadrangle (region between 1 

and 2 in fig. 5A) may reflect, in part, the transition from dense, lower crustal rocks to more felsic, midcrustal rocks within the Salinian complex. An outcrop of Salinian basement surrounded by Miocene 

sedimentary rocks (3 in fig. 5A) contains an even tighter, 6-mGal gradient, which is likely caused by 

faults associated with the edge of a Cenozoic basin (striped region in fig. 5A). Gravity values also 

gradually decrease southeastward from Salinian basement complex outcrops to outcrops of latest 

Cretaceous sedimentary rocks (such as units Ksc, Kep, and Ku on Gravity Map (see PDF); 4 in fig. 5A) 

that overlie the basement complex. 

Gravity values also decrease very gradually to the south-southeast from the highest values on 

Salinian complex basement onto outcrops of Franciscan Complex across the Nacimiento Fault. The 

source of the gravity high across the fault was attributed tentatively by Burch and others (1971) to 

topography composed of rocks with a density of 2,800 kg/m3

; however, the gravity high persists if the 

data are reduced to a density of 2,800 kg/m3. The source of the higher values is not known but could be 

deeply buried, given the broad, nearly 20-km width of the southeast gradient of the high. The 

18 
Nacimiento Fault does not coincide with a prominent gravity gradient but rather a slight, northwesttrending gradient, where slightly denser Franciscan Complex is juxtaposed against less dense latest 

Cretaceous Salinian strata. Given that the Nacimiento Fault marks a significant seismic-velocity contrast 

but no significant gravity gradient to the southeast of the Cambria quadrangle (Howie and others, 1993), 

the absence of a strong gravity gradient coupled with a prominent seismic velocity contrast suggests that 

the fault places relatively coherent, higher-velocity Salinian complex rocks (granite and gneiss) against 

fractured, lower-velocity Franciscan Complex rocks (as exemplified by units of mélange and broken 

formation). Fractures can reduce seismic velocity significantly without a corresponding reduction in 

density (Stierman and Kovach, 1979). No active-source seismic refraction data are available for the 

Cambria quadrangle. A statewide California tomographic model (Lin and others, 2010) does not show 

much of a velocity contrast across the Nacimiento Fault in this area, but that likely reflects the low 

resolution (10–20 km grid spacing) and smoothing of the model. 

The most prominent and steepest gravity gradient marks the southwest edge of a Cenozoic basin 

beneath Jolon and San Antonio Valleys in the northeast part of the quadrangle. The amplitude of the 

gravity low within the quadrangle is about 40–50 mGal; using an infinite slab approximation and 

assuming an average density contrast of 400 kg/m3

, Mesozoic basement is 2.4 km to 3.0 deep. This is 

consistent with an oil-test drill hole (triangle in fig. 5A) that bottomed in Miocene rocks at a depth of 1.8 

km close to the lowest gravity values in the basin (California Division of Oil, Gas, and Geothermal 

Resources, 1982) and with a cross section by Graham and others (1991) based on drill-hole data along 

the northern margin of the quadrangle. However, gravity values are not uniformly low in Jolon Valley. 

Gravity values in Lockwood Valley are as much as 20 mGal higher than the lowest values in the basin 

and the relative gravity high coincides with several oil test drill holes that intercepted granitic basement 

at depths between 150 m and 730 m (black-rimmed blue circles in fig. 5A). We interpret this shallow 

granite as uplifted in the hanging wall of the Jolon Fault, discussed in more detail below. The eastern 

edge of the basement high is bounded by the Rinconada Fault. 

Magnetic Anomalies 

Sources of magnetic anomalies are those rock types that contain magnetite. In the central Coast 

Ranges, these rock types are serpentinite, gabbro, and diabase of the Coast Ranges ophiolite, some 

basement rock types within the Salinian complex (such as granodiorite exposed in the La Panza Range 

southeast of the Cambria quadrangle), metavolcanic rocks of the Franciscan Complex, and locally 

Miocene mafic volcanic rocks. In general, sedimentary rocks do not produce pronounced aeromagnetic 

anomalies, but lower-amplitude magnetic highs may be associated with sedimentary strata that are rich 

in mafic/ultramafic clasts or magnetite grains. In the map area, the Lospe Formation in the Piedras 

Blancas block is locally magnetic because of ophiolitic clasts but is too thin to produce a significant 

anomaly. Where it crops out, it coincides with a local magnetic low relative to the ophiolite (1 in fig. 

5B). Mafic clasts in conglomerate may be the cause of two low-amplitude magnetic highs (2 in fig. 5B) 

in the Salinian Cretaceous strata southwest of Jolon Valley; however, the broad wavelength of these 

anomalies suggests a source within the basement. 

In the Piedras Blancas and Santa Lucia Range blocks, prominent, short-wavelength, curvilinear 

magnetic anomalies coincide well with outcrops of Coast Range ophiolite (Magnetic Map, see PDF). 

The San Simeon Fault clearly forms the eastern margin of the anomalies associated with ophiolite in the 

Piedras Blancas block. These anomalies become broader and more diffuse to the northwest, consistent 

with deeper magnetic basement offshore, but they cannot be traced west of the continental slope. The 

northwest edge of these anomalies (Magnetic Map, see PDF; 3 in fig. 5B) also coincides with a mapped 

offshore fault. 

19 
A band of short-wavelength magnetic anomalies (4 in fig. 5B) trend northwest to west-northwest 

across the Santa Lucia Range block and coincide with discontinuous outcrops of Jurassic diabase and 

serpentinite (Magnetic Map, see PDF). Note that the cross section (white line in fig. 5B) crosses this 

band of anomalies where the anomaly amplitudes are somewhat muted, indicating that the geometry of 

mafic bodies shown on the cross section (fig. 6) may be thicker and more deeply penetrating out of the 

plane of the section. Southwest of the magnetic anomalies, the magnetic field is relatively smooth, 

interrupted by a few small anomalies associated with scraps of serpentinite and an outcrop of Franciscan 

Complex metavolcanic rocks. Extensive outcrops of Franciscan Complex mélange, greywacke, and 

broken formation are characterized by a relatively smooth magnetic field. 

The southwesternmost of the Santa Lucia Range magnetic-anomaly bands appears to terminate at 

its southeast end (5 in fig. 5B) just north of the Oceanic Fault, suggesting that the fault cuts off the 

source of the anomalies. Its northwest end is likely truncated by the offshore extension of the San 

Simeon Fault. Along its northeast margin, it coincides locally with the Nacimiento Fault. 

The magnetic anomalies in the San Antonio block differ from those south of the Nacimiento 

Fault. They are broader and the sources of the anomalies are concealed beneath Cretaceous and younger 

cover. A magnetic anomaly in the northeast part of the quadrangle (6 in fig. 5B) coincides 

approximately with a gravity high, suggesting that the granitic basement is the source of the anomaly. Its 

northeast margin coincides with the Rinconada Fault and correlating these buried rocks with magnetic 

rocks exposed in the La Panza Range suggests a cumulative offset of about 38 km along the Rinconada 

Fault (Jachens and others, 2009). The offset estimate from the magnetic anomalies is supported by 

independent estimates based on geologic data of Graham (1978) and Grove (1993). 

Structure 

The map area is composed of three major structural blocks separated by two large-offset 

northwest-trending faults (figs. 2, 5A). Although broken up by other faults, including some seismogenic 

and Quaternary-active faults, the major structural blocks represent a relatively coherent 

tectonostratigraphic entity that includes the same basement and overlying stratigraphic sequence 

throughout, which differ from those of adjacent major structural blocks. The differences in composition 

of adjacent blocks suggest significant offset on the bounding faults needed to juxtapose rock bodies 

originally formed in distinct depositional environments. The Nacimiento Fault separates Salinian 

basement and overlying strata of the San Antonio block from the Franciscan Complex/Great Valley 

complex basement and overlying strata of the Santa Lucia Range block. The Hosgri-San Simeon Fault 

Zone is part of the San Andreas plate-margin fault system and separates the lithologically similar—but 

not identical—Piedras Blancas and Santa Lucia Range blocks. The Nacimiento Fault is a steeply 

northeast dipping structure (as shown by the dip of the surface trace and suggested by the aeromagnetic 

data, fig. 5B), whereas the San Simeon Fault is near vertical (as shown by elocated hypocenters, fig. 7, 

and aeromagnetic data, fig. 5B). 

20 
21 

A 

Figure 6. A. Cross section of the map area and an adjacent area to the east, through Point Piedras Blancas and San Antonio Reservoadded to show the Rinconada Fault Zone and San Antonio Fault which lie in the map area to the north of the cross-section line. Geolog

based on Durham (1974) and Graham and others (1991). CRF = Coast Ranges Fault. Undifferentiated units in the offshore subsurface 

and KJ (Cretaceous and (or) Jurassic); B. Gravity and magnetic model of cross section (bottom panel). Top and middle panels show ob

magnetic and gravity variations along the profile. D and S are density and magnetic susceptibility in kg/m3 and SI units, respectively. If 

equals zero. Note that the geometry of the Oceanic Fault is not constrained by gravity and magnetic data 
22 

B 

Figure 6.–Continued 
In addition to the block-bounding faults, the Piedras Blancas and Santa Lucia Range 

blocks include the Coast Range Fault, which forms a major basement suture between the 

parautochthonous Great Valley complex and the allochthonous Franciscan Complex. The Coast 

Range Fault has a very complex map pattern (fig. 2) resulting from its complicated structural 

history, including Mesozoic and possible Paleocene subduction and accretion emplacing the 

terranes of the Franciscan Complex beneath the Coast Range ophiolite, Paleocene(?) and Eocene 

attenuation faulting associated with thinning of the Great Valley complex and unroofing of the 

Franciscan Complex, and Oligocene and younger transpressive folding and faulting. 

All three blocks also have important Quaternary-active (seismogenic and potentially 

seismogenic) faults of the San Andreas system, including the Oceanic, Rinconada, and Jolon 

Faults, and perhaps the Cambria Fault. The Rinconada Fault in the northeast corner of the map 

area is also thought to be a major block-bounding fault with significant long-term offset 

(Dibblee, 1976; Graham, 1978), but those relations are not exhibited in the map area because 

only a few kilometers of the fault trace are found. Finally, numerous lesser faults and folds occur 

throughout the map area. 

Most faults in the map area trend north-northwest, roughly parallel to the Pacific-North 

American relative plate motion vector (DeMets and others, 2010), giving the topography, 

including the coastline, a strong north-northwest grain. This grain is cut by three northwest- to 

west-northwest-trending faults. The Oceanic Fault is known to be an active restraining step in the 

San Andreas system, transferring slip from the Hosgri-San Simeon Fault Zone to the northnorthwest-trending West Huasna Fault southeast of the map area (Hardebeck, 2010). The 

northern reach of the Nacimiento Fault, as well as a prominent but unnamed fault in the 

Franciscan Complex, have similar more westerly trends, perhaps reflecting a similar restraining 

stepover role in a previous period of faulting. Fold axes mostly trend northwest, conjugate to the 

plate boundary faults. 

Earlier workers (Durham, 1968, 1974; Page, 1970) interpreted the Jolon Fault as a major 

fault extending from its surface trace northwest of Jolon through Lockwood Valley, based on 

changes in basement depth revealed in petroleum exploration wells and changes in stratigraphy 

on either side of the fault. However, Dibblee (1976) refuted the existence of the fault beyond its 

surface trace near Jolon, citing a lack of geomorphic evidence of a fault trace and pointing out 

that the basement depth and stratigraphic data did not require a fault. Our cross section through 

Lockwood Valley (fig. 6) shows a reverse fault step in the granitic basement that explains the 

well and gravity data. Geologic map data are consistent with fault propagation folding over a 

blind fault, which would not produce the kind of geomorphic expression that Dibblee (1976) was 

expecting for a fault that ruptured the surface. Therefore, we have remapped the Jolon Fault as a 

largely blind reverse fault following the maximum horizontal gravity gradient associated with the 

basement step (fig. 2). Deposits as young as Pleistocene (unit Qoa) are uplifted around a modest 

topographic high associated with a fault propagation anticline, which we interpret as evidence for 

Quaternary deformation on the blind fault. 

We have also added a reverse fault along the base of the Monterey Formation at Williams 

Hill, because beds in the Paso Robles Formation (unit QTp) dip toward the contact in multiple 

places and the presence of older rocks up-dip from younger rocks implies a fault, and the trace of 

the contact implies a northeast dipping fault. Conversely, we have deleted a previously mapped 

23 
 

Figure 7. Index map of microseismicity in the map area, color-coded by depth. Double-difference-relocated 

epicenters from Hardebeck (2010). The 2003 M6.6 San Simeon earthquake epicenter is shown as a black square 

outline. 

strand of the Cambria Fault in the Green Valley area south of Cambria, because new road-cut 

exposures along Highway 46 clearly reveal a folded depositional contact. The Cambria Fault is 

considered potentially active because it, or a fault very near to it, was observed by Graymer to 

juxtapose basement rock with Quaternary gravels in a roadcut on the north side of Main Street in 

the Santa Rosa Creek valley within the town of Cambria. However the Cambria Fault has little 

geomorphic expression of fault activity in the Quaternary terraces, so its level of Quaternary 

activity must be low. 

24 
Structural History 

The youngest structures in the Cambria region are those related to the San Andreas Fault 

System. This fault system dominates the Neogene structural history of the map area and, indeed, 

the entire Coast Ranges. Since about 23 Ma the entire map region has been moved 300–315 km 

north relative to the Sierra Nevada/Great Valley by offset along the San Andreas Fault (Hill and 

Dibblee, 1953; Matthews, 1973; Jachens and others, 1998). At the same time, significant offset 

within and through the map area was accomodated by the Hosgri-San Simeon, Oceanic, and 

Rinconada Faults. 

The San Gregorio-Hosgri Fault System, including the San Simeon Fault Zone, is the 

main locus of San Andreas Fault System right-lateral strike slip in the map area. The amount of 

slip on this fault system over the past 30 m.y. diminishes to the south. North of the map area, the 

San Gregorio-Hosgri Fault System has accumulated about 150 km of right-lateral offset based on 

correlation of Eocene rock at Point Reyes and Point Lobos (figs. 4A, B; Clark and others, 1984). 

Within the map area, the amount of slip is reduced to about 115–130 km, based on correlation of 

rocks around Point Sur and the Cambria terrane in the map area (Graham and Dickinson, 1978) 

or rocks at Point San Luis to the south (figs. 4A, B, Dickinson and others, 2005; Jachens and 

others, 2009; Langenheim and others, 2013). Studies of thermal maturity suggest that the 

Mesozoic rocks at Point Sur and Point San Luis were subjected to higher peak temperature than 

those of the Cambria terrane (Underwood and others, 1995; Underwood and Laughland, 2001), 

supporting the Point Sur to Point San Luis offset. South of the map area, the slip is reduced to 

about 90 km, based on correlation of ophiolitic and overlying Tertiary strata around Point Sierra 

Nevada in the map area and Point Sal to the south (Hall, 1975; Jachens and others, 2009). Near 

Point Arguello, however, the slip on the fault system is thought to be near zero (Sorlien and 

others, 1999), consistent with the extent of gravity and magnetic anomalies (Langenheim and 

others, 2010; 2013). The reduction in slip within the map area from ~150 km to ~120 km has 

probably been accomplished by partition of ~30 km of slip onto the West Huasna Fault via 

oblique reverse slip on the Oceanic Fault and possibly other faults (Jachens and others, 2009; 

Langenheim and others, 2010). A similar partition of about 30 km onto interior faults and 

compression parallel to the Hosgri Fault is probably present between Point Buchon and Point Sal 

to account for the decrease from ~120 km to ~90 km. However, the mechanism to account for 

the reduction of Hosgri Fault slip from ~90 km to ~0 km between Point Sal and Point Arguello is 

at present unknown. Compressive deformation and oblique reverse slip on the intervening 

structures is not sufficient to account for more than a few kilometers (Graymer and others, 2010). 

The Oceanic Fault is the most recently seismogenic fault in the map area. It generated the 

2003 M6.6 San Simeon earthquake (see McLaren and others, 2008, for a detailed study). As 

mentioned above, the Oceanic Fault connects and transfers slip from the San Gregorio-Hosgri 

fault system to the West Huasna Fault. Its orientation relative to the Hosgri and West Huasna 

Faults results in a significant component of reverse (up to the northeast) slip. This connection has 

been proposed to account for about 30 km of apparent decrease in San Gregorio-Hosgri Fault 

System offset, but the present-day geometry of the Hosgri-San Simeon and West Huasna Faults 

(strike about N. 35° W., dip about vertical) and the Oceanic Fault (strike about N. 70° W., dip 

about 45° NE.) would translate 30 km of Hosgri-San Simeon Fault-parallel horizontal strike slip 

into roughly 25 km of Oceanic Fault-parallel horizontal strike slip, 24 km of Oceanic Faultnormal reverse slip and 17 km up to the northeast vertical throw, assuming rigid upper crustal 

blocks. A significant amount of vertical throw is indicated by the presence of coeval plutonic 

25 
rocks (unit Td) in the uplifted block and volcanic rocks (unit Tc) in the downthrown block. 

However, the stratigraphy of the block southwest of the Oceanic Fault shows that it stayed near 

sea level throughout Neogene time, so, with the footwall elevation roughly constant, reverse 

throw must have been entirely accomplished by hanging-wall uplift; 17 km is too much, because 

it would imply rocks in the northeast (hanging-wall) block were uplifted from lower crustal 

depths, and there are no metamorphic rocks of grade equivalent to that depth present in the block 

northeast of the Oceanic Fault. This means that the geometry of the faults has changed through 

time, that a major portion of the 30 km partitioned from the Hosgri-San Simeon has been taken 

up by some other fault or faults, or that a more complex model of deformation is required. 

Partition of Hosgri-San Simeon strike-slip would be accommodated with less uplift by an 

Oceanic Fault more closely subparallel to the Hosgri-San Simeon Fault and (or) with a lower dip. 

However, folds in the Neogene strata (described below) suggest that the block between the 

Hosgri-San Simeon and West Huasna Faults has narrowed by compression through the Neogene, 

so the Oceanic Fault would have been even more orthogonal to the San Gregorio-Hosgri and 

West Huasna Faults in the past. An average paleo-dip of about 16° (assuming no change in 

strike) on the Oceanic Fault would yield about 5 km of uplift, which seems reasonable when 

compared to similar Franciscan Complex lithologies in the hanging wall and footwalls, although 

the actual amount of hanging-wall uplift is unknown at this time. Alternatively, partitioning only 

~10 km of San Gregorio-Hosgri slip onto the Oceanic Fault with present day orientation would 

result in an equally reasonable ~6 km of hanging-wall uplift, but this would require ~20 km of 

San Gregorio-Hosgri slip to be taken up by faults to the south. Other faults that could have 

transferred part of the 30 km between the San Gregorio-Hosgri and West Huasna Faults include 

the Cambria Fault and, south of the map area, the Los Osos Fault. A third alternative is that the 

upper crust did not deform as rigid blocks. Down-bending of the footwall beneath the fault 

would allow some of the vertical throw to be accomplished by downward motion of the footwall 

while maintaining the portion of the footwall block south of the fault near sea level. 

The Rinconada Fault crosses the extreme northeast corner of the map area. Rosenberg 

and Clark (2009) estimate that it has about 38 km of Miocene and younger right-lateral offset 

based on correlated granitic rocks, 18 km of which is thought to have accumulated since the 

early Pliocene. This fault cuts Pleistocene alluvial gravels near Paso Robles (Rosenberg and 

Clark, 2009), east of the map area, but there is no definitive evidence of Holocene offset, and the 

fault is not seen to cut overlapping colluvium in trenches. 

Other Neogene and Quaternary structures, including the Jolon Fault, mostly 

accommodate significant southwest-northeast shortening. Out-of-plane slip on strike-slip faults 

and lateral pinch-out of various horizons within structural blocks preclude calculation of 

shortening across the whole map area, but Tertiary southwest-northeast shortening of about 21 

percent within the San Antonio block is measured using the base of the Vaqueros Sandstone in 

cross section (fig. 6). 

The Nacimiento Fault forms the boundary between Salinian complex basement on the 

northeast and Franciscan Complex/Great Valley complex composite basement on the southwest. 

At the eastern edge of the map, this fault cuts middle Miocene Monterey Shale, showing that 

some part of its activity extended into Miocene time. Microearthquakes (Hardebeck, 2010) form 

a diffuse band parallel to and slightly northeast of the mapped Nacimiento Fault for nearly a 

distance of 25 km (fig. 7). The trend of the Nacimiento seismicity is slightly more northerly than 

that of the main band of aftershocks of the 2003 San Simeon earthquake. Stress from the San 

Simeon earthquake may have triggered minor slip on a number of faults in the region northeast 

26 
of the Nacimiento Fault, including the Nacimiento Fault itself. Microearthquakes also occurred 

in this band before the 2003 mainshock. Note, however, that the southwest-dipping backthrust 

that ruptured in the 2003 earthquake cut the Nacimiento Fault (McLaren and others, 2008) where 

the Oceanic and Nacimiento Faults merge, approximately 10 km to the southeast of the Cambria 

quadrangle. 

The original subduction zone contact between the inboard (hanging wall) Great Valley 

complex and the accreting (footwall) Franciscan Complex has been overprinted everywhere in 

the map area by attenuation (normal) faults associated with the unroofing of the Franciscan 

Complex, similar to most parts of the Coast Ranges (Krueger and Jones, 1989; Harms and others, 

1992). In the map area, high-grade Franciscan Complex metamorphic rocks are juxtaposed 

against the unmetamorphosed rocks of the Great Valley complex, and the Coast Range ophiolite 

at the base of the Great Valley complex is significantly thinned. This attenuation faulting must 

have occurred after accretion of the Late Cretaceous (Campanian) Cambria terrane but prior to 

emplacement of the unnamed Oligocene(?) dacite (unit Td), which intrudes the fault contact near 

Pine Mountain east of San Simeon. In the Coast Ranges east of the San Andreas Fault, 

attenuation and unroofing of Franciscan Complex rocks was at least locally complete by 

Paleocene-Eocene time because Franciscan Complex detritus is found in Paleocene-Eocene 

strata overlying Great Valley complex strata in Rice Valley (Berkland, 1973) and New Idria 

(Dibblee and Nilsen, 1977). Pre-Oligocene Tertiary strata are not present in the part of the map 

area underlain by Franciscan Complex rocks, so the timing of attenuation and unroofing is not as 

tightly constrained. 

Salinian complex basement was rapidly uplifted in Late Cretaceous time. Radiometric 

dating of granitic rocks just north of the map region (Kistler and Champion, 2001) show that the 

basement was at depth as recently as 78.2 Ma. By the Maastrichtian (70.6–65.5 Ma) the 

basement was present at the surface and overlain by sedimentary rocks. 

The Salinas Shear Zone, which places the Salinian complex over the Cretaceous schist of 

Sierra de Salinas, does not crop out in the map area but presumably underlies all of the San 

Antonio block at significant depth. This regional thrust fault is highly deformed by folding and 

faulting but is thought to have been originally a shallowly east-dipping fault (Hall, 1991; Barth 

and others, 2003; Ducea and others, 2009). The schist has yielded detrital zircons suggesting a 

depositional age for the protolith of about 79 Ma (Ducea and others, 2009), so thrusting must 

have initiated after that time. Kidder and Ducea (2006) calculated thermobarometric 

measurements as much as 714°C and 13.7 kbar for the schist. 40Ar/39Ar dating of metamorphic 

muscovite in the schist of Sierra de Salinas suggests subsequent cooling to ~350°C by about 71 

Ma, and biotite 40Ar/39Ar dating of the schist reveals that it had been cooled to about 250°C by 

about 70 Ma (Barth and others, 2003), probably reflecting partial uplift by that time (Ducea and 

others, 2009). So peak metamorphism and maximum underthrusting must have occurred between 

79–72 Ma (Barth and others, 2003; Ducea and others, 2009). It is tempting to equate the 

underthrusting of the schist with the similarly timed uplift of Salinian complex basement, but 

Ducea and others (2009) show that similar cooling histories for the schist and granitic rocks 

suggest that much of the granite uplift postdated underthrusting and that the schist and Salinian 

complex granitic/metamorphic basement were uplifted very rapidly between about 75–68 Ma. 

The oldest structures in the map area are those related to Jurassic (?) and Cretaceous 

subduction and accretion of Franciscan Complex rocks. These structures have largely been 

reactivated, deformed, and otherwise obscured by subsequent tectonic periods, but these early 

faults may be preserved in places as mélange-block bounding faults and faults between mélange 

27 
 

and relatively coherent terranes. The interpretation of Late Cretaceous sandstone of the Cambria 

terrane as trench-basin sediments suggests that subduction was ongoing 90–80 Ma. 

DESCRIPTION OF MAP UNITS 

[BFS, benthic foraminiferal stage; CPMS, California provincial molluscan stage] 

Unit descriptions are divided into three sections. The first section describes Quaternary 

surficial deposits that are similar throughout the map area. The second section describes early 

Pleistocene and Tertiary strata and is subdivided into three sections based on areas of structural 

blocks which display a relatively consistent stratigraphy (fig. 2). As noted above, fault offset 

within the map area has juxtaposed depositional basins or parts of depositional basins that were 

once kilometers or tens of kilometers apart, therefore the stratigraphy in adjacent assemblages 

may be quite different. It is also important to note that units of similar stratigraphic position in 

various parts of the map area have been traditionally given the same unit name, despite 

differences in lithology or age. Therefore, each stratigraphic unit is described in each 

assemblage, even if a unit has the same name as one already described for another assemblage. 

The third section describes rocks of the Mesozoic and Paleogene basement complexes that 

underlie Tertiary strata. 

QUATERNARY SURFICIAL DEPOSITS 

Qs Beach and dune sand (Holocene)—Fine-grained, very well sorted, well-drained, 

eolian and shoreline deposits 

Qal Alluvial fan and fluvial deposits (Quaternary)—Alluvial fan deposits are brown or 

tan, medium dense to dense, gravelly sand or sandy gravel that generally 

grade upward to sandy or silty clay. Near distal fan edges, fluvial deposits are 

typically brown, medium dense sand that fines upward to sandy or silty clay. 

Floodplain deposits are medium to dark gray, dense, sandy to silty clay, 

locally with lenses of coarser material (silt, sand, and pebbles). Natural levee 

deposits consist of loose, moderately sorted to well-sorted sandy or clayey silt 

grading to sandy or silty clay. They are porous and permeable and provide 

conduits for transport of ground water. Levee deposits border stream channels, 

usually both banks, and slope away to flatter floodplains. Stream-channel 

deposits consist of poorly sorted to well-sorted sand, silt, silty sand, or sandy 

gravel with minor cobbles. Cobbles are more abundant in mountain valleys 

Qls Landslide deposits (Quaternary)—Poorly sorted clay, silt, sand, gravel, boulders, 

and rock masses. Only a few large landslides are shown. Includes all levels of 

activity (dormant to active) 

Qoa Older alluvium (Pleistocene)—Roughly horizontal beds of buff siltstone and 

claystone, buff to gray fine- to coarse-grained lithic sandstone, pebbly 

sandstone, pebbly mudstone, and pebble to cobble conglomerate. These 

deposits lie above the present alluvial level and are typically more 

consolidated and dissected by erosion 

Qt Fluvial terrace deposits (Pleistocene)—Underlies small areas on Williams Hill and 

along San Antonio River. Deposits consist of crudely bedded, clast-supported 

gravels, cobbles, and boulders with a sandy matrix. Clasts as much as 35 cm 

28 
intermediate diameter are present. Coarse sand lenses may be locally present. 

Fluvial terrace deposits rest on flat surfaces cut a few meters into bedrock. 

These surfaces are up to several tens of meters above Qal deposits. Terrace 

deposits are remnants of an older alluvial system that have been tectonically 

uplifted above present depositional levels 

Qmt Undifferentiated marine terrace desposits (Pleistocene)—Largely composed of 

unconsolidated sand and gravel. Marine terraces are geomorphically 

expressed as gently seaward sloping surfaces. Deposits include older beach 

and nearshore sediments typically capped by younger aeolian and alluvial 

sediments. Sediments are underlain by a wave-cut bedrock platform that is 

rarely exposed. Though there may be several distinct terraces within this unit, 

only the more prominent surfaces have been differentiated for this study (see 

below). These surfaces are treated as geomorphic surfaces; in many places, 

terrace-cover sediments such as sand dunes are included in the unit 

descriptions 

Qmtss San Simeon terrace (Late Pleistocene)—Lowest emergent terrace in the San 

Simeon area, consisting of a wave-cut platform and overlying sediments. 

Thermoluminescence and uranium-series dating methods reveal the terrace 

formed during a sea-level highstand at 105 ka or 80 ka (Hanson and others, 

1994). The San Simeon terrace is discontinuous north of Cambria and either 

absent or indistinguishable from the modern wave-cut platform south of 

Cambria (Weber, 1983). A lower, narrower, discontinuous step along the 

southwest side of San Simeon Point has been designated the “Point Terrace” 

by others (Hanson and others, 1994; Rockwell and others, 1987); it is not 

found elsewhere in the mapped area and appears to merge with the San 

Simeon platform 

Qmttr Tripod terrace (Late Pleistocene)—An extensive wave-cut platform and 

overlying sediments situated 3–6 m (10–20 ft) above the San Simeon terrace. 

Occasionally the step between the San Simeon and Tripod terraces is buried 

by alluvial fans or talus, giving the appearance of one broad terrace surface 

(Weber, 1983). Between Cambria and Point Estero, the terrace becomes a 

very narrow, irregular surface with sporadic old sea stacks. Weber (1983) 

suggests the Tripod terrace is a continuation of the Cayucos terrace mapped 

south of the map area. The Cayucos terrace has been dated at 120 ka using 

amino acid racemization of fossils. Assuming the terraces are correlative, the 

Tripod terrace was formed during the 120 ka sea-level highstand, when sea 

level was approximately 6 m higher than present 

Qmtos Oso terrace (Middle Pleistocene?)—Originally mapped by Weber (1983) as a 

thin, discontinuous marine-terrace surface only observed north of San Simeon 

Fault Zone. A later study (Hanson and others, 1994) identified remnants of the 

Oso terrace south of the fault. However, this correlation is tentative, as the 

Oso terrace is located in an area of complex faulting. Weber (1983) calculated 

a local tectonic uplift rate of 0.65 ft/k.y. based on the age of the Tripod 

terrace. Assuming this rate is correct, the Oso terrace would have formed 

during the 310 ka sea-level highstand 

Qmtlc La Cruz terrace (Middle Pleistocene)—This marine terrace is best developed 

29 
northeast of Point Piedras Blancas; elsewhere, it is thin and discontinuous. A 

large portion of the terrace appears to have been stripped of marine sediments 

and is now overlain by a veneer of fluvial sediments (Hanson and others, 

1994). Using the assumed local tectonic uplift rate of 0.65 ft/k.y., the La Cruz 

terrace can be correlated to a 340 ka sea-level highstand (Weber, 1983) 

Qmtcm Cambria terrace (Middle Pleistocene)—A well-developed terrace surface often 

covered by a thick package of sediments. The Cambria terrace may include 

multiple platforms, though thick aeolian cover makes distinguishing 

individual surfaces difficult (Weber, 1983). If the local tectonic uplift rates 

calculated from the age of the Tripod terrace have been constant, the Cambria 

terrace would correlate to a 540 ka or older sea-level highstand (Weber, 

1983). Dissection of the terrace is not consistent with this age, so the Cambria 

terrace is more likely correlative to a 340 ka sea-level highstand, though that 

would require a change in uplift rate to 1.1 ft/k.y. (Weber, 1983). The lower 

portion of the terrace may be correlative to the La Cruz terrace described 

above; the upper surface may merge with either Scott Rock or Del Puerto 

terraces (Weber, 1983) described below 

Qmtdp Del Puerto terrace (Middle Pleistocene)—Consists of a few scattered terrace 

remnants situated between the northern and southern strands of Oceanic Fault. 

This terrace may include two wave-cut platforms, though poor exposure 

makes this interpretation difficult to verify (Weber, 1983). It is possible that 

the lower portion of the terrace may merge with an upper surface of the 

Cambria terrace. If so, the Del Puerto terrace would have formed during the 

same 340 ka sea-level highstand as the Cambria terrace 

Qmtbb B.B. terrace (Middle Pleistocene)—A few small remnants located between the 

northern and southern strands of Oceanic Fault, near Broken Bridge Creek. 

These surfaces are 50–100 ft higher than the Del Puerto terrace. There is no 

definitive correlation of the B.B. terrace with other terraces; if a local uplift 

rate of 0.98 ft/k.y. is applied for the mid-Pleistocene, the terrace may have 

formed during the 430 ka sea-level highstand (Weber, 1983) 

Qmtsr Scott Rock terrace (Middle Pleistocene)—Preserved in scattered remnants near 

Scott Rock north of Cambria. Terrace surfaces are often covered by aeolian 

sands. South of San Simeon Creek, the Scott Rock terrace nearly merges with 

the upper part of the Cambria terrace (Weber, 1983). The age of the terrace is 

unknown; if the same uplift rate estimated for the Cambria terrace is used 

(~1.1ft/k.y.), the Scott Rock terrace may correlate to a 430–415 ka sea-level 

highstand (Weber, 1983) 

Qmtok Oak Knoll terrace (Middle Pleistocene)—Forms uplands in the Piedras Blancas 

structural block. Terrace surface is best expressed near Oak Knoll, and 

includes old sea stacks and sand dunes. Based on its elevation, it must be at 

least slightly older than the La Cruz terrace, but younger than the Cinnabar 

terrace 

Qmtcb Cinnabar terrace (Middle Pleistocene)—Scattered terrace remnants forming the 

highest uplands in Piedras Blancas structural block (Weber, 1983). 

Extensively faulted and deformed, and in several areas eroded to a bedrock 

platform covered by thin lag deposits (Weber, 1983). Using an estimated 

30 
uplift rate of 1.6 ft/k.y. (based on the best fit with the sea-level fluctuation 

curve), the Cinnabar terrace could have formed during a 435 ka sea-level 

highstand and may correlate with the Scott Rock terrace (Weber, 1983) 

Qmtvg Van Gordon terrace (Middle Pleistocene)—Scattered terrace remnants that 

manifest as topographic benches and flats south of San Simeon. Using the 

local uplift rate estimated for the Cambria terrace (~1.1ft/k.y.), the Van 

Gordon terrace would correlate to the 590–520 ka sea-level highstand (Weber, 

1983) 

EARLY QUATERNARY AND TERTIARY STRATA 

PIEDRAS BLANCAS BLOCK 

Ts Unnamed sandstone and conglomerate (Pliocene)—Gray and greenish-gray 

fossiliferous pebble to cobble conglomerate and buff-weathering greenishgray lithic sandstone. Conglomerate clasts include laminated siliceous 

mudstone, laminated chert, argillite, greenstone, greenish wacke, black chert, 

serpentinite, and vesicular andesite. Shell bits are common sand grains. 

Mollusk fossils include Pectinid bivalves, Anadara, oysters, and various 

gastropods. Hall (1976) also reports the echinoderm Dendraster, 

indeterminate bryozoans, the Scaphopod mollusk genus Dentalium, and the 

bivalve mollusk genera Nuculana and Solen. This unit was correlated with the 

Graciosa member of the Careaga Sandstone in the Santa Maria area by Hall 

(1975) based on lithologic similarity 

Tm Monterey Formation (late Miocene)—Laminated white, brown, and black chert, 

diatomite, diatomaceous shale, porcelanite, porcelaneous shale, siltstone, and 

cherty shale. Foraminifers from the unit near San Simeon Point are reported to 

be late Miocene (Malmblorg and others, 2008). 

The Monterey Formation in the Piedras Blancas block differs from that 

in the adjacent Santa Lucia Range block by the absence of significant volcanic 

rocks at the base and its younger age. It differs from that of the San Antonio 

block by its limited thickness and lack of early and middle Miocene strata at 

the base 

Tl Lospe Formation (early Miocene)—Sedimentary and minor volcanic rocks. The 

Lospe Formation in the Piedras Blancas block was mapped as Oligocene by 

Hall (1976), but correlative Lospe Formation in the Point Sal region is now 

known to be Lower Miocene based on microfossils and radiometric dating of 

interbedded volcanic rocks (Stanley and others, 1990, 1996). Preliminary 

detrital zircon data indicate that the maximum age of the Lospe Formation in 

the Piedras Blancas block is also Lower Miocene (17.9±0.9 Ma; J.P. Colgan, 

written commun., 2011). In the Piedras Blancas block this unit largely consists 

of gray, red, and green sandstone and conglomerate. Clasts are mostly 

ophiolite. 

The Lospe Formation in the Piedras Blancas block differs from that in 

the adjacent Santa Lucia Range block by its younger age, clast composition, 

and inclusion of volcanic rocks 

31 
SANTA LUCIA RANGE BLOCK 

QTcs Sedimentary rocks of Crystal Knob (Pleistocene or Pliocene)—Poorly 

consolidated nonmarine gray-green sandstone and conglomerate. Clasts 

angular to rounded, including ophiolite, Franciscan Complex rocks, and minor 

basalt of Crystal Knob. Outcrop limited to a roughly circular area about 700 m 

in diameter. Presumably deposited in a volcanic depression (Seiders, 1989a). 

The age of the unit is poorly constrained but suggested to be post-early 

Pliocene by preserved volcanic geomorphology and weak consolidation 

QTcb Basalt of Crystal Knob (Pleistocene or Pliocene)—Dark-gray olivine basalt, in 

part vesicular. Crops out as a lens within unit QTcs

Pismo Formation (early Pliocene and late Miocene) 

Tpm Mudstone member—White and lesser light-brown claystone, siltstone, sandy 

mudstone, diatomite, diatomaceous shale, porcelanite, porcelaneous shale, and 

chert 

Tps Sandstone and conglomerate member—Light-brown and orangish-brown fineto medium-grained arkose and pebble and cobble conglomerate. 

Conglomerate clasts include felsite (probably derived from unit Tc) and 

varicolored chert (probably derived from Franciscan Complex chert). Hall 

(1974) reports Mohnian or Delmontian(?) benthic foraminiferal stage (BFS) 

fossils from this unit 

Tm Monterey Formation (early and middle Miocene)—White-weathering dark-brown 

siliceous shale, porcelaneous shale, chert, claystone, siltstone, tuffaceous 

siltstone, and minor sandstone, in most places laminated. Locally contains 

orange-weathering dolomitic concretions. Flaggy outcrop common. 

Previously mapped in part as Rincon Shale (Hall, 1976). Fossils reported by 

Hall (1974) from the map area suggest Saucesian or Relizian BFS (early or 

early middle Miocene) age. More definitive foraminfera reported from this 

unit to the east (Seiders, 1982) are the same age. At its base in much of the 

map area and to the east, the Monterey Formation also includes a volcanic 

member 

Tmv Monterey Formation, volcanic member—Diabase and basalt sills and flows, 

and white to gray hard microcrystalline tuff. 

The Monterey Formation in the Santa Lucia Range block differs from 

that in the San Antonio block by its limited thickness, lack of late Miocene 

strata, and the presence of significant volcanic rocks at the base 

Tv Vaqueros Sandstone (early Miocene and late Oligocene?)—White medium- to 

coarse-grained lithic arkose. Includes shell hash locally and cobble-boulder 

conglomerate at the base. Locally glauconitic. Conglomerate clasts include 

rhyolite and dacite (probably derived from the underlying Cambria Felsite) 

and lesser varicolored chert, graywacke, and blueschist (probably derived 

from the underlying Franciscan Complex). Mega-fossils include the bivalve 

mollusk families Ostreidae and Pectinidae and indeterminate barnacles (see 

Hall, 1974, for a detailed list) reported to be Oligocene. Hall (1974) lists these 

fossils as “Zemorrian”, but that is an Oligocene benthic foraminiferal stage 

(BFS). We interpret that to mean that the mollusks should be assigned to the 

early “Vaqueros” California provincial molluscan stage (CPMS). In contrast, 

32 
Malmblorg and others (2008) report early Miocene, late Miocene, and two 

Miocene, possibly pseudosaucesian facies, foraminifer localities. Strata 

previously assigned to the “Vaqueros” CPMS vary widely in age (Hoffman 

and others, 2001; Prothero and Hoffman, 2001) but the “Vaqueros” CPMS 

appears to span late Oligocene and early Miocene (28–17 Ma; Prothero and 

Donohoo, 2001). The “early Miocene” foraminiferal age assignment implies 

the correlation with the Saucesian BFS. The “Miocene, possibly 

pseudosaucesian facies” age interpretation could not be verified but indicates 

a deepwater assemblage of Miocene age, Saucesian BFS or younger. The “late 

Miocene?” assemblage was examined and contains a single species which 

ranges from Oligocene (coeval with planktic foraminiferal zone P20) through 

Pleistocene (planktic foraminiferal zone N23). However, the age of the 

overlying Monterey Formation precludes assignment of an age younger than 

early Miocene to this unit. All together these fossil and stratigraphic data 

imply a possibly late Oligocene to early Miocene age. 

The Vaqueros Sandstone of the Santa Lucia Range block differs from 

that of the San Antonio block in the presence of Franciscan Complex and 

Cambria felsite detritus rather than Salinian complex detritus found in the 

Vaqueros Sandstone of the San Antonio block 

Tls Limestone (early Miocene or Oligocene?)—Thick-bedded, light-gray, coarsegrained, bioclastic limestone and calcareous sandstone. Locally contains 

pebbles and cobbles of felsic volcanic rocks. Contains large mollusks of 

“Vaqueros” CPMS (early Miocene and late Oligocene; Prothero and 

Donohoo, 2001), as well as early Miocene foraminifers just to the east of the 

map area at Lime Mountain (Malmblorg and others, 2008). In the map area 

this unit only has a small outcrop at the eastern edge of the map area between 

35°37'30"N and 35°45'N 

Tl Lospe Formation (Oligocene)—Maroon weathering, red, green, and greenish-gray 

lithic sandstone, mudstone, and pebble to cobble conglomerate. Clasts include 

varicolored chert and graywacke (probably derived from the underlying 

Franciscan Complex), plagioclase porphyry (probably derived from the 

underlying Cambria felsite), quartz, and lithic sandstone. Nonmarine. Note 

that the Lospe Formation here is considerably older than that in the Piedras 

Blancas block 

Tc Cambria Felsite (Oligocene)—White, welded, vesicular feldspar-quartz-pumice tuff; 

laminated, thick-bedded quartz-feldspar-biotite tuff (some euhedral biotite); 

and white and gray, hard rhyolite and dacite felsite. East of the map area 

(Cypress Mountain 7.5 ́ quadrangle), in roadcuts along Highway 46, this unit 

also includes black-weathering maroon amygdaloidal basalt with green 

serpentinite(?) xenoliths and green mineralization, white-weathering green 

tuff including lenses of silicified breccia, obsidian and flow-banded gray 

glassy rhyolite, and greenish-gray tuff breccia with boulders of red jasper. See 

Ernst and Hall (1974) for a detailed description of this unit. The felsite has 

been correlated with the Morro Rock-Islay Hill intrusive/volcanic complex 

southeast of the map area (Ernst and Hall, 1974), which has yielded K-Ar ages 

between 22.7±0.9 and 28.0±1.0 Ma (Turner, 1968; Hall and others, 1966; 

33 
Buckley, 1986) and more recently an 40Ar/39Ar age of 26.5–27 Ma (cited in 

Cole and Stanley, 1998). Ernst and others (2011) used U-Pb ages from zircons 

to date Cambria Felsite at 27 Ma. The Oligocene age is consistent with 

stratigraphic position beneath the fossiliferous Vaqueros Sandstone and 

correlation with the Oligocene rocks to the southeast 

Td Dacite (Oligocene?)—Gray to white, porphyritic-aphanitic dacite. Crops out as small 

intrusive bodies within Franciscan Complex and Coast Range ophiolite 

basement northeast of the Oceanic Fault. Tentative age based on correlation 

with the Oligocene Morro Rock-Islay Hill complex (Hall, 1976) southeast of 

the map area (see description of unit Tc above) 

SAN ANTONIO BLOCK 

QTp Paso Robles Formation (Pleistocene and (or) Pliocene)—Southwest of San 

Antonio River, this unit is made up of white-weathering, light-gray, lightbrown, and yellowish-green arkose and K-spar arkose, pebbly-cobbly 

sandstone, and pebble to cobble gravel. Clasts include granitic rocks, red to 

white rhyolite, green plagioclase porphyry, red chert, black cherty argillite, 

sandstone, laminated shale, and brown mudstone. On the northeast margin of 

Lockwood Valley, this unit is made up of angular to well-rounded pebble to 

cobble conglomerate. Clasts are almost all porcellanite and siliceous shale 

derived from Monterey Formation (Tm), which underlies the adjacent 

uplands. This unit is nonmarine in both places 

Tu Unnamed sandstone (early Pliocene and late Miocene?)—Buff-weathering lightgray sandstone. Chiefly distinguished by early Pliocene or, in places, possibly 

late Miocene mollusks, roughly equivalent to those of the Pancho Rico 

Formation east of the map area (Durham, 1974) 

Tm Monterey Formation (Miocene)—White-weathering, thin-bedded, flaggy, brown, 

gray, and black, laminated in places foraminiferal mudstone, siliceous shale, 

porcellanite, and chert. Locally, also includes minor orange dolomitic 

mudstone, fine-grained arkose, and sandy shale. Sparse mollusk fossils have 

been found. Foraminifers southwest of San Antonio River range from late 

Saucesian BFS to late Mohnian BFS (early to late Miocene), whereas those in 

the Williams Hill area are late Mohnian BFS (late Miocene) in age (refer to 

paleontological data files) 

Tv Vaqueros Sandstone (early Miocene and (or) late Oligocene)—White-, brown-, or 

gray-weathering, gray quartz sandstone and arkose. Locally includes lithic 

arkose, pebble to boulder conglomerate, shell hash, and thin beds of gray and 

brown claystone, silty claystone, and siltstone. Coarse clasts dominantly 

granitic, with lesser quartzite and andesite. “Vaqueros” CPMS (late Oligocene 

and early Miocene; Addicott, 1976) mollusk fossils are common (Loel and 

Corey, 1932; Durham, 1974). Vaqueros Sandstone strata in this block north of 

the map area are known to be Oligocene (Addicott, 1979), but Saucesian BFS 

(early Miocene) foraminifera are reported from this unit in the eastern part of 

the map area (Malmblorg and others, 2008; new data, refer to paleontological 

data files) 

34 
BASEMENT COMPLEXES 

SALINIAN COMPLEX 

Tpa Piedras Altas Formation (Paleogene)—Red, red-gray, and greenish-gray nonmarine 

sandstone, pebbly sandstone, and conglomerate. Conglomerate is poorly 

sorted, with subangular to well-rounded pebbles, cobbles, and sparse boulders; 

clasts are mainly felsic volcanic and granitic rocks and arkose. The age of this 

unit is based on its stratigraphic position conformably over the Shut-in 

Formation and unconformably beneath the Vaqueros Sandstone. We interpret 

the angular unconformity above the Salinian complex basement to represent 

considerable time and note that the Piedras Altas Formation is relatively thin; 

therefore, we suggest that the age of this unit is likely restricted to the 

Paleocene and possibly the early Eocene. This interpretation is supported by 

relations north of the map area where the equivalent angular unconformity lies 

between Paleocene and late Paleocene to Eocene strata (Graham, 1979) 

TKsi Shut-in Formation (Paleocene and latest Cretaceous, Maastrichtian)—Gray, 

marine sandstone, mudstone, and conglomerate. Clasts are felsic volcanic and 

granitic rocks. This unit has yielded Maastrichtian (Seiders, 1989a) and 

Paleocene (Durham, 1965; see also new data in paleontological data files) 

foraminifers in the map area 

TKsic Shut-in Formation, conglomerate and sandstone member—Thick-bedded 

conglomerate, sandstone, and pebbly sandstone 

Kif Italian Flat Formation (latest Cretaceous, Maastrichtian)—Thick-bedded gray 

marine sandstone, gritstone, and pebbly sandstone. Subordinate thin- to 

medium-bedded sandstone and mudstone. In the map area, has yielded 

Maastrichtian pollen and dinoflagellates (Butler, 1984) 

Ksc Steve Creek Formation (latest Cretaceous, Maastrichtian)—Thin- to thickbedded, marine, greenish-gray sandstone and olive-gray mudstone with local 

conglomerate. Sandstone includes white- and buff-weathering quartz-lithic 

and quartz-biotite-lithic wacke. This unit has previously been assigned to the 

Campanian(?), but its position between two Maastrichtian units precludes that 

interpretation 

Kscc Steve Creek Formation, conglomerate and sandstone member—Thick-bedded 

pebble to boulder conglomerate and subordinate sandstone. Clasts mainly 

felsic volcanic rocks with lesser intermediate and mafic volcanic, 

sedimentary, and granitic rocks 

Kep El Piojo Formation (latest Cretaceous, Maastrichtian)—Gray marine sandstone 

and mudstone with minor conglomerate. Sandstone chiefly thick-bedded, 

coarse-grained, brown- or white-weathering biotite and biotite-lithic arkose, 

locally pebbly or cobbly. Lesser thin-bedded, brown-weathering, fine-grained, 

laminated biotite-lithic arkose and mica mudstone and white-weathering, 

yellow-brown, muscovite-biotite siltstone. The El Piojo has yielded 

Maastrichtian, as well as long-ranging Late Cretaceous to Paleogene 

foraminifera (Seiders, 1989a; see also new data in paleontological data files) 

Kepc El Piojo Formation, conglomerate member—Thick-bedded conglomerate. 

Clasts mostly felsic volcanic and granitic rocks. Sparse interbedded sandstone. 

35 
Crops out as discontinuous lenses in the northwest part of the formation 

Ku Unnamed strata (latest Cretaceous, Maastrichtian)—Brown and white arkose and 

lithic sandstone, pebbly to bouldery arkose and pebble to boulder 

conglomerate, and brown, gray, and dark-greenish-gray mudstone and shale. 

Unconformable contact on granitic and metamorphic basement was observed 

by us in the northern part of the map area. Mudstone in the map area locally 

includes foraminifera of long-ranging (Late Cretaceous to Eocene) arenaceous 

species (see paleontological data files). However, the age of overlying strata 

and isotopic data from the underlying basement (Mattinson, 1978; Ducea and 

others, 2009) constrain the age of the unit to Maastrichtian (latest Cretaceous) 

Km Sur series metamorphic rocks and granitic rocks, undifferentiated 

(Cretaceous)—Quartz-feldspar and hornblende gneiss, biotite schist, marble, 

and biotite quartz diorite. There are no age data from the map area for this 

unit, but Barbeau and others (2005) inferred a maximum age of protolith 

deposition of 280 Ma and 360 Ma for two Sur series samples collected just 

north of the map area in the Cone Peak 7.5 ́ quadrangle. Isotopic dates 

(Mattinson, 1978; Kistler and Champion, 2001) from a pluton (charnokitic 

tonalite of Compton, 1960) in the same structural block about 20 km north of 

the map area suggest pluton emplacement about 100 Ma (zircon U-Pb, Rb-Sr 

whole rock) and thermal overprint (Kistler and Champion, 2001) or rapid 

partial uplift (Mattinson, 1978) between about 84 and 76 Ma (hornblende and 

biotite 40Ar/39Ar, Rb-Sr). Other plutonic rocks farther north have yielded ages 

around 83.5 Ma (quartz diorite of Soberanes Point) and 117 Ma (granodioritequartz diorite of Junipero Serra Peak, Rb-Sr whole rock, Kistler and 

Champion, 2001), so we interpret the age of plutonic rocks in the map area to 

be within the span of 83.5–117 Ma 

Kmm Sur series metamorphic rocks, marble member—Blue-gray weathering, white 

crystalline marble 

FRANCISCAN COMPLEX 

Kfm Mélange (Late Cretaceous)—Sheared black and gray argillite enclosing blocks of 

graywacke, conglomerate, greenstone, diabase, chert, serpentinite, and 

glaucophane schist ranging in size from pebble to hill-sized. Sheared matrix 

has yielded Early Cretaceous (probably Albian) spore and dinoflagellate 

fossils (Page, 1970). Chert blocks have yielded Early or Middle Jurassic to 

Early Cretaceous radiolarians (Seiders, 1989a,b). Chert pebbles in 

conglomerate blocks have yielded Late Triassic to Late Jurassic radiolarians. 

Some graywacke blocks in the Santa Lucia Range block have detrital zircons 

yielding a maximum depositional age of 90–77 Ma (Morisani and others, 

2005). The mélange probably formed by tectonic interleaving and mixing of 

accreted terranes at the subduction margin along the western edge of the North 

American plate. Initiation of this mixing must postdate the Early Cretaceous 

fossils in the matrix, and mixing must have extended past the age of Late 

Cretaceous zircons and fossils in enclosed graywacke blocks and the Cambria 

terrane (see below). Note that the presence of Late Cretaceous blocks in an 

Early Cretaceous matrix precludes an olistostromal origin for the mélange in 

36 
the map area 

Cambria terrane 

Kfg Sandstone of Cambria (Late Cretaceous)—Medium- to thick-bedded, red-orange 

weathering, pink and light-gray, medium-grained arkose; yellow-brownweathering, thin-bedded to massive, in places laminated, medium- to lightgray, fine- to coarse-grained arkosic wacke; and gray biotite-lithic wacke. 

Locally, sandstone grains include much carbon debris. Sandstone beds 

separated in places by thin beds of medium-gray biotite-carboniferous 

mudstone and sandy mudstone and sheared dark-gray shale, which locally 

contain Late Cretaceous foraminfera and pollen. This unit has also yielded 

detrital zircons of about 80–90 Ma (Jacobson and others, 2011) 

Unnamed Disrupted Terrane(s) 

KJfg Sandstone (Cretaceous and Jurassic?)—Medium-bedded to very thick bedded, 

brown-weathering, greenish-gray, green, and gray biotite-lithic wacke and 

biotite arkosic wacke and attenuated thin interbeds of medium-dark-gray finegrained wacke, siltstone, claystone, and minor coal. Shale chips are common 

sand grains. Locally includes conglomerate and pebbly shale. This unit crops 

out as large fault-bounded slabs and blocks in mélange. U-Pb dates from 

detrital zircons in graywacke near San Simeon yielded maximum ages of 

deposition between 90 and 77 Ma (Late Cretaceous; Morisani and others, 

2005), younger than fossils in the mélange matrix but roughly coeval and 

perhaps equivalent to sandstone of Cambria slab, although the two units differ 

in sandstone composition. Graywacke elsewhere in the map unit is undated 

and may differ from any of the other Franciscan sandstones. Franciscan 

Complex Central Belt graywackes are known to be as old as Jurassic (Blake 

and Jones, 1974; Murchey and Jones, 1984), although the presence of Early 

Cretaceous fossils in the chert and the model of graywacke over chert 

deposition (See Basement Complexes section under Stratigraphy) makes a 

Jurassic age for any part of this unit questionable 

KJfb Broken formation (Cretaceous and Jurassic?)—Structurally disrupted graywacke 

and shale and schistose metagraywacke and slate. Locally includes minor 

blocks of chert, metachert, and basalt. In outcrop, unfoliated graywacke and 

shale are indistinguishable from unit KJfg but entirely lacking coherence at 

scales more than a few meters, resulting in a unit more resembling the block 

in matrix texture of unit Kfm but distinguished by the lack of exotic blocks 

and a paucity of chert and basalt 

KJfc Chert (Cretaceous and Jurassic)—Red and green ribbon chert with very thin shale 

partings and white and gray metachert with slate partings. Crops out as blocks 

and lenses in units Kfm and rarely in KJfb, many too small to show at map 

scale. This unit has yielded Early or Middle Jurassic to Early Cretaceous 

radiolaria in the map area (Seiders, 1989a,b) 

KJfv Mafic volcanic rocks (Cretaceous? and Jurassic)—Orange-weathering, black, 

dark-gray, and green basalt, pillow basalt, basalt breccia, basaltic tuff, and 

diabase. Basalt is locally amygduloidal. Crops out as blocks and lenses in 

units Kfm and rarely in KJfb, many too small to show at map scale. This unit 

37 
is entirely without fossil or radiometric age data. Franciscan Complex 

volcanic rocks in the Central Belt are known to be as young as Cretaceous, but 

given the Early or Middle Jurassic age of some of the chert, and the model of 

chert over basalt deposition, the possibility of Cretaceous rocks in this unit 

seems questionable 

GREAT VALLEY COMPLEX 

Great Valley Sequence 

Ka Atascadero Formation (Late Cretaceous)—Marine, medium-bedded to very thick 

bedded, greenish-gray to yellowish-brown sandstone; brown- and buffweathering, thin- and medium-bedded, medium-gray, fine-grained, biotite-rich 

lithic wacke; medium-brown, medium- to coarse-grained, biotite-lithic arkosic 

wacke; massive, medium- to coarse-grained, brown- and green-weathering, 

gray biotite-lithic sandstone; dark-brown and green biotite siltstone and shale; 

olive-gray and greenish-gray shale and mudstone. Shale and mudstone locally 

concretionary. Sandstone locally pebbly to cobbly. This unit has yielded a 

Late Cretaceous Inoceramus (Bivalvia; Mollusca) in the Pebblestone Shut-In 

quadrangle (D.L. Jones, written commun., 1985). Directly east of the map 

area, the Atascadero Formation has yielded many Late Cretaceous, possibly 

Coniacian-Maastrichtian but predominantly Campanian, fossils (Seiders, 

1982), whereas in the Atascadero area the unit has fossils as old as early Late 

Cretaceous (Cenomanian or Turonian, Hart, 1976) 

Kac Atascadero Formation, conglomerate member—Thick-bedded to very thick 

bedded pebble to cobble conglomerate. Clasts mostly felsic volcanic rocks, 

with subordinate intermediate to mafic volcanic, granitic, and (or) quartz 

sandstone clasts 

KJt Toro Formation (Early Cretaceous and Late Jurassic)—Marine, thin-bedded, 

brown-weathering, dark-brown, biotite-lithic wacke; medium- to dark-gray 

and olive-gray mudstone; fine- to medium-grained lithic arkose; coarsegrained lithic wacke; thin-bedded, light-gray-weathering, dark-brown 

mudstone and siltstone. Fine-grained strata locally include orange dolomite 

concretions. Bedding locally rhythmic. This unit contains Early Cretaceous 

(Valanginian) Buchia (Mollusca; Bivalvia) in the map area (Seiders, 1989a), 

whereas directly east of the map area it includes Latest Jurassic and Early 

Cretaceous (Tithonian to Valanginian) fossils (Seiders, 1982) 

KJtc Toro Formation, conglomerate member—Thick-bedded to very thick bedded 

pebble to boulder conglomerate. Clasts mainly chert and quartz sandstone, 

with subordinate granite and gabbro. Matrix is hard, dark-gray biotite-quartzlithic sandstone 

Coast Range ophiolite 

Jch Chert (Late Jurassic)—Thin-bedded, green and gray chert, partly tuffaceous or 

argillaceous. Locally includes grayish-green tuff and dark-gray argillite. This 

unit has yielded Late Jurassic (late Tithonian) radiolarians (Seiders, 1989b) 

Jo Undifferentiated ophiolitic rocks (Middle and Late Jurassic)—Serpentinite, 

diabase, diorite, and mafic volcanic rocks. U-Pb (zircon) analysis of rocks 

38 
from this unit in the Piedras Blancas block (Mattinson and Hopson, 2008) has 

yielded an age of 165.580±0.038 Ma 

Jv Mafic volcanic rocks—Reddish-weathering basalt, pillow basalt, basalt breccia. 

Locally includes lenses of chert that have in one place yielded Late Jurassic 

(Tithonian) radiolarians (Seiders, 1989b), indicating that the age of parts of 

the ophiolite are as young as Late Jurassic 

Jdb Diabase and diorite—Gray and greenish-gray, fine- to medium-grained 

Jgb Gabbro—Only one body near the east edge of the map area on Santa Rosa Creek 

mapped separately 

Jsp Serpentinite—Green and blue-green, massive to completely sheared serpentinite 

and serpentinized ultramafic rocks. Locally includes blue-gray serpentinite 

schist. Most ultramafic rocks are completely serpentinized (bastite), but the 

interior of the large body at Burro Mountain is relatively unaltered (Loney and 

others, 1971) 

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