Offshore Geology and Geomorphology from 

Point Piedras Blancas to Pismo Beach, 

San Luis Obispo County, California

By Janet T. Watt, Samuel Y. Johnson, Stephen R. Hartwell, and Michelle Roberts

Pamphlet to accompany

Scientific Investigations Map 3327

2015

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015 

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Suggested citation:

Watt, J.T., Johnson, S.Y., Hartwell, S.R., and Roberts, M., 2015, Offshore geology and geomorphology from Point 

Piedras Blancas to Pismo Beach, San Luis Obispo County, California: U.S. Geological Survey Scientific 

Investigations Map 3327, pamphlet 6 p., 6 sheets, scale 1:35,000, http://dx.doi.org/10.3133/sim3327. 

ISSN 2329-132X (online)
iii

Contents

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

Map Units ...................................................................................................................................................................1

Faults and Folds.........................................................................................................................................................2

Regional and Local Tectonics.....................................................................................................................................3

Acknowledgments..........................................................................................................................................................4

References Cited ...........................................................................................................................................................4

Additional References for Onshore Geologic Mapping ..................................................................................................6
1

Offshore Geology and Geomorphology from Point Piedras 

Blancas to Pismo Beach, San Luis Obispo County, 

California

By Janet T. Watt, Samuel Y. Johnson, Steven R. Hartwell, and Michelle Roberts

Introduction

Marine geology and geomorphology were mapped along the continental shelf and upper slope 

between Point Piedras Blancas and Pismo Beach, California. The map area is divided into the following 

three (smaller) map areas, listed from north to south: San Simeon, Morro Bay, and Point San Luis (see 

index map on sheets 1, 3, 5). Each smaller map area consists of a geologic map (sheets 1, 3, 5) and the 

corresponding geophysical data that support the geologic mapping. Each geophysical data sheet includes 

shaded-relief multibeam bathymetry (fig. 1 on sheets 2, 4, 6), seismic-reflection-survey tracklines (fig. 2 

on sheets 2, 4, 6), and residual magnetic anomalies (fig. 3 on sheets 2, 4, 6), as well as a smaller version 

of the geologic map (fig. 4 on sheets 2, 4, 6) for reference. Although the geologic maps are presented 

here at 1:35,000 scale, map interpretation was conducted at scales of between 1:6,000 and 1:12,000. 

Map Units

Offshore geologic units were delineated on the basis of integrated analysis of adjacent onshore 

geology (see “Additional References for Onshore Geologic Mapping” list), seafloor-sediment and rock 

samples (Reid and others, 2006; Pacific Gas and Electric Company, 2011), multibeam bathymetry and 

backscatter imagery, magnetic data (Langenheim and others, 2009, 2012; Sliter and others, 2009), and 

high-resolution seismic-reflection profiles (Sliter and others, 2009). To maintain onshore-to-offshore 

consistency, map-unit symbols for the Mesozoic and Tertiary units reflect the published onshoremapping usage. If no onshore-equivalent unit exists, as is the case with the Quaternary units, the mapunit symbols are made up of letters that indicate the following: the Period, usually followed by “m” for 

marine sediment; the predominant sediment type; and, finally, a geomorphic descriptor, if applicable. 

Nearshore and inner shelf deposits are predominantly sand (Qms), as determined by seafloor 

sampling (Reid and others, 2006). These sandy-shelf areas locally include coarser grained units such as 

channel deposits (Qmc), rills (Qmr), and sorted bedforms (Qmss; Trembanis and Hume, 2011). Unit 

Qmss typically exists as erosional lags in scour depressions whose contacts with horizontal sand sheets 

of unit Qms usually are relatively sharp but, less commonly, are diffuse. These scour depressions, 

which generally form irregular to lenticular exposures that have abrupt landward contacts with bedrock, 

typically are a few tens of centimeters deep and range in size from a few tens of meters to more than 1 

km2

. Such scour depressions are common along this stretch of the California coast (see, for example, 

Cacchione and others, 1984; Hallenbeck and others, 2012), where offshore sandy sediment is relatively 

thin (and, thus, is unable to fill the depressions) owing to low sediment supply and also to erosion and 

transport of sediment during large northwest winter swells. 

Inner shelf deposits (predominantly sand) grade to finer grained (mud and sand) outer shelf 

(Qmsf) and slope (Qmsl) deposits at depths greater than 70 m. The boundary between units Qms and 

Qmsf was determined on the basis of seafloor-sediment samples (Reid and others, 2006), as well as

interpretation of multibeam backscatter imagery. This boundary likely shifts seaward or landward as a 

result of seasonal- to annual- to decadal-scale cycles in sediment supply, sediment transport, and wave 
2

climate. These shelf and slope deposits locally contain pockmarks (Qmp), hummocky relief (Qmsh), 

and landslide deposits (Qls). Landslide deposits may represent various forms of submarine sediment 

instabilities, including slumps, slides, and collapse depressions. 

Where an older unit (typically bedrock) is overlain by a thin (less than 2 m) Quaternary deposit, 

a composite geologic unit is mapped; the composite unit, shown with a stipple pattern on the older unit, 

is designated by a composite label indicating both the overlying sediment cover and the lower (older) 

unit, separated by a slash (/). For example, Qms/Tus indicates that a thin sheet of Qms overlies Tus. 

Such units are recognized on the basis of high backscatter, low relief, continuity with moderate- to highrelief bedrock outcrops, and (in most cases) high-resolution seismic-reflection data. The overlying 

sediment is interpreted as an ephemeral layer that may or may not be continuously present, depending 

on storms, seasonal and (or) annual patterns of sediment movement, or longer term climate cycles.

Sea level was approximately 120 to 130 m lower during the Last Glacial Maximum (about 21 ka; 

see, for example, Lambeck and Chappell, 2001; Gornitz, 2009). This approximate depth corresponds to 

the modern shelf break, a lateral change from the gently dipping (0.8° to 1.0°) outer shelf to the slightly 

more steeply dipping (about 1.5° to 2.5°) upper slope in the central and northern parts of the map area. 

South of Point San Luis in San Luis Bay (sheets 5, 6), deltaic deposits offshore of the mouth of the Santa 

Maria River (11 km south of the map area) have prograded across the shelf break and now form a 

continuous low-angle (about 0.8°) ramp that extends to water depths of more than 160 m. The shelf 

break defines the landward boundary of slope deposits (Qmsl). North of Estero Bay, the shelf break is 

characterized by a distinctly sharp slope break that is mapped as a landslide headscarp above landslide 

deposits (Qls) (sheets 1, 2). Multibeam imagery and seismic-reflection profiles across this part of the 

shelf break show evidence of slope failure, such as slumping, sliding, and soft-sediment deformation, 

along the entire length of the scarp. Notably, this shelf-break scarp corresponds to a west splay of the 

Hosgri Fault that dies out just north of the scarp, suggesting that faulting is controlling the location (and 

instability) of the shelf break in this area.

Lithologies of Tertiary and older seafloor bedrock units were determined on the basis of seafloor 

samples where available; otherwise, they were interpreted on the basis of their correlation to onshore 

geology, using their seafloor texture visible in multibeam imagery in addition to their magnetic 

character. Qms Tertiary units mapped in the offshore include rocks of the Obispo, Monterey, and Pismo 

Formations. The rhythmically bedded Monterey and Pismo Formations are characterized by their 

distinctive seafloor texture in multibeam imagery; in addition, they are relatively nonmagnetic. These 

rocks are underlain by, or are in fault contact with, Jurassic and Cretaceous basement that consists of 

rocks of either the two unnamed sandstone units (Ksl, Kslc), an unnamed graywacke unit (KJug), the 

Franciscan Complex (KJf), or the Coast Range ophiolite (Jo). These older units often are characterized 

by irregular, lumpy seafloor texture and complex magnetic signatures, owing to the highly deformed 

nature of the variably magnetic rocks.

Certain Mesozoic and Tertiary basement rocks, including ultramafic rocks of the Coast Range 

ophiolite Jo) and diabase and basalts of the Obispo Formation (Tod), are highly magnetic, making highresolution magnetic data (fig. 3 on sheets 2, 4, 6) an important mapping tool. Although metavolcanic 

rocks within the Franciscan Complex in this map area apparently are not magnetic, serpentinite is.

Serpentinite, which is derived from the Coast Range ophiolite and is associated with the Franciscan 

Complex, commonly forms elongate magnetic bodies along fault zones and, thus, can assist in fault 

mapping. 

Faults and Folds

Faults and folds shown on these maps are near-surface (upper about 200 m) structures that are 

mapped primarily on the basis of interpretation of minisparker seismic-reflection data (tracklines shown 
3

in fig. 2 on sheets 2, 4, 6; see also, Sliter and others, 2009). These fault and fold interpretations were 

further modified on the basis of multibeam imagery and magnetic data; these laterally continuous 

datasets allow for more accurate interpolation of structures between the seismic-reflection profiles. 

However, we do not infer fault activity from these data, as we have no direct age data for surficial 

sediments.

Offshore faults and folds are symbolized as follows: a solid line is used where the location is 

certain; a dashed line, where the location is inferred; and a dotted line, where the location is concealed. 

In addition, queries are added where the existence of a feature is questionable. A fault or fold is mapped 

as inferred when its location is determined by interpolating between seismic-reflection profiles, as well 

as in bedrock outcrops where a lineament exists in the multibeam imagery but where clear evidence of 

fault offset or folded strata is lacking. A fault or fold is mapped as concealed where it can be identified 

in seismic-reflection data but where no surface deformation is apparent in either multibeam imagery or 

seismic-reflection data. However, it is important to note that, in areas where multibeam imagery does 

not exist, a fault or fold mapped as concealed may, in fact, have surface expression that is at a scale 

undetectable in the seismic-reflection data alone. Therefore, it is possible that the location of a fault or 

fold mapped as concealed may be just as accurate as that of a fault or fold whose location is mapped 

with more certainty (that is, with a solid or dashed line). 

Magnetic data are particularly useful in guiding structural interpretations where massive 

Mesozoic bedrock is at or near the seafloor surface because, in these areas, seismic-reflection data often 

are poor owing to lack of penetration. Such is the case in the nearshore areas from Point Piedras Blancas 

to Shell Beach, which, with the exception of sediment-covered areas in central Estero Bay, are 

characterized by bedrock outcrops of predominantly Mesozoic basement rocks that are composed of 

variably magnetic rock types. Magnetic anomalies represent local spatial variations in the Earth’s 

magnetic field that indicate the distribution of magnetic minerals—primarily magnetite—in the 

underlying rocks. In many cases, the volume content of magnetic minerals can be related to rock type, 

and so it follows that abrupt spatial changes in the amount of magnetic minerals commonly mark 

lithologic or structural boundaries that would not be detectable in seismic-reflection data alone. For 

example, the Shoreline Fault is not detectable in seismic-reflection data but, rather, is mapped on the 

basis of correlation of magnetic-anomaly gradients, as well as on seafloor scarps visible in multibeam 

imagery. 

Regional and Local Tectonics

This map area, which encompasses part of the Los Osos structural domain, lies on the east flank 

of the offshore Santa Maria Basin (McCulloch, 1987; Lettis and others, 2004), within the broader 

Pacific–North American plate transform margin. The Los Osos domain is characterized by a series of 

onshore, northwest-trending, fault-bounded basins and ranges that extend offshore, where they are partly 

covered by marine sediment and are truncated by the northwest-striking, right-lateral Hosgri Fault. The 

Hosgri Fault is an approximately 170-km-long (or longer?) right-lateral strike-slip fault (Petersen and 

others, 2008) that has an inferred Quaternary slip rate of 1 to 3 mm/yr (Hanson and Lettis, 1994; Hanson 

and others, 2004). 

The shallow (upper 200 m) structure of the Hosgri Fault, as revealed in high-resolution seismicreflection data, is a vertical fault zone that offsets the seafloor in numerous locations and that has many

bends in strike that result in localized areas of uplift and subsidence. The map on sheet 1 shows the 

Hosgri Fault as being continuous with the onshore San Simeon Fault, and interpretation of highresolution seismic-reflection data (tracklines shown in fig. 2 on sheet 2) provides compelling evidence 

that this is the case (Johnson and Watt, 2012). This interpretation contrasts with previous interpretations 

(Pacific Gas and Electric Company, 1988; Lettis and others, 1990; Hanson and others, 2004) that 
4

showed the Hosgri Fault as ending just north of Point Estero, with slip transferring to the San Simeon 

Fault through a releasing right step. Note that previous interpretations considered the stepover and its 

associated pull-apart basin to be a barrier to earthquake rupture, and so our detailed mapping indicating 

that no Hosgri–to–San Simeon stepover exists has important implications for earthquake hazards (for a 

more detailed discussion, see Johnson and Watt, 2012). 

Both the Los Osos and Shoreline Faults, which converge with the Hosgri Fault within the map 

area, are part of a broader fault system within the central Coast Ranges. The Los Osos Fault, as imaged 

with multibeam and high-resolution seismic-reflection data, forms a west-northwest-striking zone (1 to 3 

km wide) of near-vertical faulting. Three distinct strands (north, central, and south) are evident on 

shallow seismic-reflection profiles. The Shoreline Fault, first recognized as a northwest-striking, vertical 

zone of earthquake hypocenters that parallels the coastline (Hardebeck, 2010), is expressed on the 

seafloor as a series of discontinuous, en echelon lineations and scarps visible in multibeam imagery. 

Detailed marine and airborne (helicopter) magnetic data also reveal a linear trend of magnetic 

anomalies, the edges of which are coincident with (or parallel to) the surface expression of the Shoreline 

Fault.

Acknowledgments

Funding for this work was provided by the U.S. Geological Survey (USGS) Coastal and Marine 

Geology Program, USGS Geology and Geophysics Program, and Pacific Gas and Electric Company 

(PG&E) through a Collaborative Research and Development Agreement (CRADA). We thank Roberto 

Anima, Jeff Beeson (Oregon State University), Mike Boyle, Jackson Currie, Jamie Grover, Pat Hart, 

Gerald Hatcher, Larry Kooker, Daniel Scheirer, Ray Sliter, George Tate, Peter Triezenberg, and Robert 

Wyland (all USGS, unless otherwise noted) for assistance in collection of USGS seismic-reflection and 

marine magnetic data. Thanks to Theresa Fregoso (USGS) for GIS support. We also thank Rikk Kvitek 

and the California State University, Monterey Bay, Seafloor Mapping Lab for providing high-quality 

bathymetric-mapping datasets. We would like to thank the PG&E–USGS CRADA research team, and 

Russell Graymer and Victoria Langenheim (both USGS), specifically, for their scientific collaboration 

and stimulating discussions. The insightful and constructive reviews by Jamie Conrad and Carl 

Wentworth (both USGS) greatly improved these maps.

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