Geology and geomorphology--Offshore of Pacifica map area, California

Title:: Geology and geomorphology--Offshore of Pacifica map area, California
Abstract:: This part of DS 781 presents data for the geologic and geomorphic map of the Offshore of Pacifica map area, California. The vector data file is included in "Geology_OffshorePacifica.zip," which is accessible from <https://pubs.usgs.gov/ds/781/OffshorePacifica/data_catalog_OffshorePacifica.html>.
The continental shelf within California's State waters in the Pacifica area is shallow (water depths of 0 to about 40 m) and flat continental shelf with a very gentle (less than 0.5 degrees) offshore dip. The morphology and geology of this shelf result from the interplay between local tectonics, sea-level rise, sedimentary processes, and oceanography. Tectonic influences are related to local faulting and uplift (see below). Sea level has risen about 125 to 130 m over the last about 21,000 years (for example, Lambeck and Chappel, 2001; Gornitz, 2009), leading to progressive eastward migration (a few tens of km) of the shoreline and wave-cut platform, and associated transgressive erosion and deposition (for example, Catuneanu, 2006). The Offshore of Pacifica map area is now mainly an open shelf that is subjected to full, and sometimes severe, Pacific Ocean wave energy and strong currents. Most of the offshore map area is covered by marine sediments; artificial fill (unit af) occurs only at the site of the Pacifica Pier. Given their relatively shallow depths and exposure to high wave energy, modern shelf deposits are mostly sand (unit Qms). More coarse-grained sands and gravels (units Qmss and Qmsc) are primarily recognized on the basis of bathymetry and high backscatter (Bathymetry--Offshore of Pacifica map area, California, and Backscatter--Offshore of Pacifica map area, California). Unit Qmsc occurs as nearshore bars (less than 12 m water depth) for about two kilometers north of Mussel Rock and more locally offshore Pacifica, and in two isolated patches farther offshore at about 25 m water depth. Unit Qmss forms erosional lags in features known as "rippled scour depressions" (for example, Cacchione and others, 1984) or "sorted bedforms" (for example, Trembanis and Hume, 2011), at water depths of about 15 to 25 m, in contact with offshore bedrock uplifts and unit Qms. Such features are common along this stretch of the California coast where offshore sandy sediment can be relatively thin (thus unable to fill the depressions) due to both lack of sediment supply from rivers and to significant sediment erosion and offshore transport during large winter storms. Although the general areas in which both unit Qmss scour depressions and unit Qmsc bars occur are not likely to change substantially, the boundaries of the unit(s) are likely ephemeral, changing seasonally and during significant storm events. Areas where shelf sediments form thin (less than 2 m) veneers over low relief bedrock of the Franciscan Complex (see below) occur in the northern half of the map and are mapped as unit Qms/KJf. This hybrid unit is recognized and delineated based on the combination of flat relief, continuity with moderate to high relief onshore or offshore bedrock outcrops, high-resolution seismic-reflection data, and in some cases moderate to high backscatter. The thin sediment layer is regarded as ephemeral and dynamic, and may or may not be present at a specific location based on storms, seasonal to annual patterns of sediment movement, or longer-term climate cycles. In a nearby, similarly high-energy setting, Storlazzi and others (2011) have described seasonal burial and exhumation of submerged bedrock in northern Monterey Bay. Offshore bedrock exposed at the seafloor is mapped as Jurassic and Cretaceous Franciscan Complex, undivided (unit KJf); Cretaceous granite (unit Kgr); Tertiary and (or) Cretaceous rock, undivided (unit TKu); unnamed sansdstone, shale and conglomerate of Paleocene age (unit Tss); and the Upper Miocene and Pliocene Purisima Formation (unit Tp). These units are delineated by extending outcrops and trends from mapped onshore geology and from their distinctive surface textures as revealed by high-resolution bathymetry (Bathymetry--Offshore of Pacifica map area, California). Purisima Formation outcrops in the southernmost part of the offshore map area form distinctive "ribs," caused by differential erosion of variably resistant, interbedded lithologies (for example, sandstone and mudstone). In contrast, granitic rocks have a densely cross-fractured, rough surface texture, and both the Franciscan Complex and the unnamed Paleocene sedimentary unit have a more masssive, irregular, and smoother surface texture. Purisima Formation outcrops occur in water as deep as 35 m, whereas other bedrock units occur in shallower (less than 20 m) water depths, most commonly adjacent to coastal points underlain by bedrock (for example, Pedro Point and Montara Point). Map unit polygons were digitized over underlying 2-meter base layers developed from multibeam bathymetry and backscatter data. The bathymetry and backscatter data were collected between 2006 and 2010.
References 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.
Catuneanu, O., 2006, Principles of Sequence Stratigraphy: Amsterdam, Elsevier, 375 p.
Gornitz, V., 2009, Sea level change, post-glacial, in Gornitz, V., ed., Encyclopedia of Paleoclimatology and Ancient Environments: Encyclopedia of Earth Sciences Series. Springer, pp. 887-893.
Lambeck, K., and Chappell, J., 2001, Sea level change through the last glacial cycle: Science, v. 292, p. 679-686.
Trembanis, A.C., and Hume, T.M., 2011, Sorted bedforms on the inner shelf off northeastern New Zealand: spatiotemporal relationships and potential paleo-evironmental implications: Geo-Marine Letters, v. 31, p. 203-214.
Supplemental_Information:: Map political location: San Mateo County, California Compilation scale: 1:24,000 Base maps used are hillshades generated from IfSAR, LiDAR, and multibeam mapping both onshore and offshore (Bathymetry--Offshore of Pacifica map area, California, DS 781, for more information).

Value	Definition
af	Artificial fill and anthropogenic material
Qms	Marine nearshore and shelf deposits
Qmss	Marine shelf scour depressions
Qmsc	Coarse-grained marine nearshore and shelf deposits
Qms/KJf	Sediment-covered Franciscan complex
Tp	Purisima Formation
Tss	Unnamed sandstone, shale, and congomerate
TKu	Undifferentiated sedimentary deposits
Kgr	Granite
Kgr?	Granite (questionable)
KJf	Franciscan complex

Value

Definition

Artificial fill and anthropogenic material

Qms

Marine nearshore and shelf deposits

Qmss

Marine shelf scour depressions

Qmsc

Coarse-grained marine nearshore and shelf deposits

Qms/KJf

Sediment-covered Franciscan complex

Purisima Formation

Tss

Unnamed sandstone, shale, and congomerate

TKu

Undifferentiated sedimentary deposits

Kgr

Granite

Kgr?

Granite (questionable)

KJf

Franciscan complex

Geology and geomorphology--Offshore of Pacifica map area, California

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