Offshore geology and geomorphology -- Estero Bay map area [sheet 3]

Title:: Offshore geology and geomorphology -- Estero Bay map area [sheet 3]
Abstract:: This part of SIM 3327 presents data for geologic units on the offshore geologic and geomorphic map of the Estero Bay map area, in the vicinity of Morro Bay, California. The vector data file is included in "Geology_ EsteroBay.zip," which is accessible from <https://pubs.usgs.gov/sim/3327/data/sim3327_data_catalog.html>.
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 onshore-mapping usage. If no onshore-equivalent unit exists, as is the case with the Quaternary units, the map-unit 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 are usually 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 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 high-relief 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. 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 high-resolution 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.
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 seismic-reflection 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 high-resolution 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 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 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.
Reference cited:
Cacchione, D.A., Drake, D.E., Grant, W.D., and Tate, G.B., 1984. Rippled scour depressions of the inner continental shelf off central California: Journal of Sedimentary Petrology, v 54, p. 1280-1291.
Gornitz, V., 2009, Sea level change, post-glacial, in Gornitz, V., ed., Encyclopedia of paleoclimatology and ancient environments: Springer, Encyclopedia of Earth Sciences Series, p. 887–893.
Hanson, K.L., Lettis, W.R., 1994, Estimated Pleistocene slip rate for the San Simeon fault zone, south-central coastal California, in Seismotectonics of the Central California Coast Ranges, Alterman, I.B., McMullen, R.B., Cluff, L.S., and Slemmons, D.B. (Eds.): Geological Society of America Special Paper 292, p. 133-150.
Hanson, K.L., Lettis, W.R., McLaren, M.K., Savage, W.U., and Hall, N.T., 2004, Style and rate of quaternary deformation of the Hosgri fault zone, offshore south-central California, in Evolution of sedimentary basins/offshore oil and gas investigations- Santa Maria Province, Keller, M.A. (Ed.): U.S. Geological Survey Bulletin 1995-BB, 37 p.
Hardebeck, J.L., 2010, Seismotectonics and fault structure of the California Central Coast: Bulletin of the Seismological Society of America, v. 100, p. 1031-1050.
Johnson, S.Y., and Watt, J.T., 2012, Influence of fault trend, bends, and convergence on shallow structure and geomorphology of the Hosgri strike-slip fault, offshore central California: Geosphere, v. 8, no. 6, 25 p.
Lambeck, K., and Chappell, J., 2001, Sea level change through the last glacial cycle: Science, v. 292, p. 679–686, doi: 10.1126/science.1059549.
Langenheim, V.E., R.C. Jachens, and K. Moussaoui (2009). Aeromagnetic survey map of the central California Coast Ranges, USGS Open File Rept. 2009-1044, <https://pubs.usgs.gov/of/2009/1044/>.
Lettis, W.R., Hanson, K.L., Unruh, J.R., McLaren, M., and Savage, W.U., 2004, Quaternary tectonic setting of South-Central Coastal California: in, Evolution of sedimentary basins/offshore oil and gas investigations-Santa Maria Province, Keller, M.A. (Ed.): U.S. Geological Survey Bulletin 1995-AA, 24 p.
McCulloch, D.S., 1987, Regional geology and hydrocarbon potential of offshore central California, in Scholl, D.W., Grantz, A., and Vedder, J.G., eds., Geology and Resource Potential of the Continental Margin of Western North America and Adjacent Oceans -- Beaufort Sea to Baja California: Houston, Texas, Circum-Pacific Council for Energy and Mineral Resources, Earth Science Series, v. 6., p. 353-401.
Pacific Gas and Electric Company, 2011, Report on the analysis of the Shoreline Fault Zone, central coastal California, Report to the U.S. Nuclear Regulatory Commission, <http://www.pge.com/myhome/edusafety/systemworks/dcpp/shorelinereport/>.
Petersen, M.D., Frankel, A.D., Harmsen, S.C., Mueller, C.S., Haller, K.M., Wheeler, R.L., Wesson, R.L., Zeng, ., Boyd, O.S., Perkins, D.M., Luco, N., Field, E.H., Wills, C.J., and Rukstales, K.S., 2008, Documentation for the 2008 update of the United States National Seismic Hazard Maps: U.S. Geological Survey Open-File Report 2008-1128, 61 p.
Reid, J.A., Reid, J.M., Jenkins, C.J., Zimmerman, M., Williams, S.J., and Field, M.E., 2006, usSEABED: Pacific Coast (California Oregon, Washington) offshore surficial-sediment data release: U.S. Geological Survey Data Series 182, <https://pubs.usgs.gov/ds/2006/182/>.
Sliter, R.W., Triezenberg, P.J., Hart, P.E., Watt, J.T., Johnson, S.Y., and Scheirer, D.S., 2009, High resolution seismic-reflection and marine magnetic data along the Hosgri fault zone, central California: U.S. Geological Survey 2009-1100, v. 1.1, <https://pubs.usgs.gov/of/2009/1100/>.
Trembanis, A.C., and Hume, T.M., 2011, Sorted bedforms on the inner shelf off northeastern New Zealand—Spatiotemporal relationships and potential paleo-environmental implications: Geo-Marine Letters, v. 31, p. 203–214.
Supplemental_Information:: Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Although this Federal Geographic Data Committee-compliant metadata file is intended to document the data set in nonproprietary form, as well as in ArcInfo format, this metadata file may include some ArcInfo-specific terminology.

Value	Definition
af	Artificial fill
Jo?	Coast Range ophiolite (uncertain)
KJf	Franciscan Complex
Ksi	Unnamed sandstone and interbedded shale
TKu	Bedrock, undivided
Qls	Landslide deposits
Qmc	Marine channel deposits
Qmp?	Marine pockmarks (uncertain)
Qms	Marine nearshore and shelf deposits
Qms/Jo?	Thin (0-4 m) ephemeral layer of marine sediment overlying Coast Range ophiolite (uncertain)
Qms/KJf	Thin (0-4 m) ephemeral layer of marine sediment overlying Franciscan Complex
Qms/Ksi	Thin (0-4 m) ephemeral layer of marine sediment overlying unnamed sandstone and interbedded shale
Qms/TKu	Thin (0-4 m) ephemeral layer of marine sediment overlying bedrock, undivided
Qms/Tmpm	Thin (0-4 m) ephemeral layer of marine sediment overlying Pismo Foramtion, Miguelito Member
Qms/Tus	Thin (0-4 m) ephemeral layer of marine sediment overlying sedimentary bedrock
Qmsc	Coarse-grained marine nearshore and shelf deposits
Qmsf	Fine-grained marine nearshore and shelf deposits
Qmsh	Marine shelf hummocky deposits
Qmsl	Marine slope deposits
Qmss	Marine shelf sorted bedforms
TKu	Bedrock, undivided
Tmpm	Pismo Formation, Miguelito Member
Tus	Sedimentary bedrock

Value

Definition

Artificial fill

Jo?

Coast Range ophiolite (uncertain)

KJf

Franciscan Complex

Ksi

Unnamed sandstone and interbedded shale

TKu

Bedrock, undivided

Qls

Landslide deposits

Qmc

Marine channel deposits

Qmp?

Marine pockmarks (uncertain)

Qms

Marine nearshore and shelf deposits

Qms/Jo?

Thin (0-4 m) ephemeral layer of marine sediment overlying Coast Range ophiolite (uncertain)

Qms/KJf

Thin (0-4 m) ephemeral layer of marine sediment overlying Franciscan Complex

Qms/Ksi

Thin (0-4 m) ephemeral layer of marine sediment overlying unnamed sandstone and interbedded shale

Qms/TKu

Thin (0-4 m) ephemeral layer of marine sediment overlying bedrock, undivided

Qms/Tmpm

Thin (0-4 m) ephemeral layer of marine sediment overlying Pismo Foramtion, Miguelito Member

Qms/Tus

Thin (0-4 m) ephemeral layer of marine sediment overlying sedimentary bedrock

Qmsc

Coarse-grained marine nearshore and shelf deposits

Qmsf

Fine-grained marine nearshore and shelf deposits

Qmsh

Marine shelf hummocky deposits

Qmsl

Marine slope deposits

Qmss

Marine shelf sorted bedforms

TKu

Bedrock, undivided

Tmpm

Pismo Formation, Miguelito Member

Tus

Sedimentary bedrock

Offshore geology and geomorphology -- Estero Bay map area [sheet 3]

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