This part of DS 781 presents data for the geologic and geomorphic map of the Offshore of Point Reyes map area, California. The vector data file is included in "Geology_OffshorePointReyes.zip," which is accessible from http://pubs.usgs.gov/ds/781/OffshorePointReyes/data_catalog_OffshorePointReyes.html
Marine geology and geomorphology was mapped in the Offshore of Point Reyes map area from approximate Mean High Water (MHW) to the 3-nautical-mile limit of Californiaâ€™s State Waters. MHW is defined at an elevation of 1.46 m above the North American Vertical Datum of 1988 (NAVD 88) (Weber and others, 2005). Offshore geologic units were delineated on the basis of integrated analyses of adjacent onshore geology with multibeam bathymetry and backscatter imagery, seafloor-sediment and rock samples (Reid and others, 2006), digital camera and video imagery, and high-resolution seismic-reflection profiles.
The onshore bedrock mapping was compiled from Galloway (1977), Clark and Brabb (1997), and Wagner and Gutierrez (2010). Quaternary mapping was compiled from Witter and others (2006) and Wagner and Gutierrez (2010), with unit contacts modified based on analysis of 2012 LiDAR imagery; and additional Quaternary mapping by M.W. Manson.
The morphology and the geology of the Offshore of Point Reyes map area result from the interplay between tectonics, sea-level rise, local sedimentary processes, and oceanography. The Point Reyes Fault Zone runs through the map area and is an offshore curvilinear reverse Fault Zone (Hoskins and Griffiths, 1971; McCulloch, 1987; Heck and others, 1990; Stozek, 2012) that likely connects with the western San Gregorio fault further to the south (Ryan and others, 2008), making it part of the San Andreas Fault System. The Point Reyes Fault Zone is characterized by a 5 to 11 km-wide zone that is associated with two main fault structures, the Point Reyes Fault and the Western Point Reyes Fault (fig. 1).
Tectonic influences impacting shelf morphology and geology are related to local faulting, folding, uplift, and subsidence. Granitic basement rocks are offset about 1.4 km on the Point Reyes thrust fault offshore of the Point Reyes headland (McCulloch, 1987), and this uplift combined with west-side-up offset of the San Andreas Fault (Grove and Niemi, 2005) resulted in uplift of the Point Reyes Peninsula, including the adjacent Bodega and Tomales shelf. The Western Point Reyes Fault is defined by a broad anticlinal structure visible in both industry and high-resolution seismic datasets and exhibits that same sense of vergence as the Point Reyes Fault. The deformation associated with north-side-up motion across the Point Reyes Fault Zone has resulted in a distinct bathymetric gradient across the Point Reyes Fault, with a shallow bedrock platform to the north and east, and a deeper bedrock platform to the south.
Late Pleistocene uplift of marine terraces on the southern Point Reyes Peninsula suggests active deformation west of the San Andreas Fault (Grove and others, 2010) on offshore structures. The Point Reyes Fault and related structures may be responsible for this recent uplift of the Point Reyes Peninsula, however, the distribution and age control of Pleistocene strata in the Offshore of Point Reyes map area is not well constrained and therefore it is difficult to directly link the uplift onshore with the offshore Point Reyes Fault structures. Pervasive stratal thinning within inferred uppermost Pliocene and Pleistocene (post-Purisima) units above the Western Point Reyes Fault anticline suggests Quaternary active shortening above a curvilinear northeast to north-dipping Point Reyes Fault zone. Lack of clear deformation within the uppermost Pleistocene and Holocene unit suggests activity along the Point Reyes Fault zone has diminished or slowed since 21,000 years ago. In this map area the cumulative (post-Miocene) slip-rate on the Point Reyes Fault Zone is poorly constrained, but is estimated to be 0.3 mm/yr based on vertical offset of granitic basement rocks (McCulloch, 1987; Wills and others, 2008).
With the exception of the bathymetric gradient across the Point Reyes Fault, the offshore part of this map area is largely characterized by a relatively flat (<0.8Â°) bedrock platform. The continental shelf is quite wide in this area, with the shelfbreak located west of the Farallon high , about 35 km offshore. Sea level has risen about 125 to 130 m over about the last 21,000 years (for example, Lambeck and Chappell, 2001; Peltier and Fairbanks, 2005), leading to broadening of the continental shelf, progressive eastward migration of the shoreline and wave-cut platform, and associated transgressive erosion and deposition (for example, Catuneanu, 2006). Land-derived sediment was carried into this dynamic setting, and then subjected to full Pacific Ocean wave energy and strong currents before deposition or offshore transport.
Much of the inner shelf bedrock platform is composed of Tertiary marine sedimentary rocks, which are underlain by Salinian granitic and metamorphic basement rocks, including the Late Cretaceous porphyritic granite (unit Kgg), which outcrops on the seafloor south of the Point Reyes headland. Unit Kgg appears complexly fractured, similar to onshore exposures, with a distinct massive, bulbous texture in multibeam imagery. The Tertiary strata overlying the granite form the core of the Point Reyes syncline (Weaver, 1949) and include the early Eocene Point Reyes Conglomerate (unit Tpr), mid- to late Miocene Monterey Formation (unit Tm), late Miocene Santa Margarita Formation (unit Tsm), late Miocene Santa Cruz Mudstone (unit Tsc), and late Miocene to early Pliocene Purisima Formation (unit Tp). The Point Reyes Conglomerate is exposed on the seafloor adjacent to onshore outcrops on the Point Reyes headland and has a distinct massive texture with some bedding planes visible, but the strata are highly fractured. Based on stratigraphic correlations from seismic reflection data and onshore wells, combined with multibeam imagery, we infer rocks of the early Eocene Point Reyes Conglomerate extend at least 6 km northwest from onshore exposures at Point Reyes headland. The absence of unit Tsc in onshore wells (Clark and Brabb, 1997) suggests these rocks are unlikely to occur within the Tertiary section of this map area, north of the Point Reyes Fault. In this map area, unit Tu represents seafloor outcrops of a middle Miocene to upper Pliocene sequence overlying unit Tpr, that may include units Tm, Tsm, and Tp. Seafloor exposures of unit Tu are characterized by distinct rhythmic bedding where beds are dipping and by a mottled texture where those beds become flat-lying.
Modern nearshore sediments are mostly sand (unit Qms and Qsw) and a mix of sand, gravel, and cobbles (units Qmsc and Qmsd). The more coarse-grained sands and gravels (units Qmsc and Qmsd) are primarily recognized on the basis of bathymetry and high backscatter. The emergent bedrock platform north and west of the Point Reyes headland is heavily scoured, resulting in large areas of unit Qmsc and associated Qmsd. Both Qmsc and Qmsd typically have abrupt landward contacts with bedrock and form irregular to lenticular exposures that are commonly elongate in the shore-normal direction. Contacts between units Qmsc and Qms are typically gradational. Unit Qmsd forms erosional lags in scoured depressions that are bounded by relatively sharp and less commonly diffuse contacts with unit Qms horizontal sand sheets. These depressions are typically a few tens of centimeters deep and range in size from a few 10's of meters to more than 1 km2.
There is an area of high-backscatter, and rough seafloor southeast of the Point Reyes headland that is notable in that it includes several small, irregular "lumps", with as much as 1 m of positive relief above the seafloor (unit Qsr). Unit Qsr occurs in water depths between 50 and 60 meters, with individual lumps randomly distributed to west-trending. This area on seismic-reflection data shows this lumpy material rests on several meters of latest Pleistocene to Holocene sediment and is thus not bedrock outcrop. Rather, it seems likely that this lumpy material is marine debris, possibly derived from one (or more) of the more than 60 shipwrecks offshore of the Point Reyes Peninsula between 1849 and 1940 (National Park Service, 2012). It is also conceivable that this lumpy terrane consists of biological "hardgrounds". Video transect data crossing unit Qsr near the Point Reyes headland was of insufficient quality to distinguish between these above alternatives.
A transition to more fine-grained marine sediments (unit Qmsf) occurs around 50â€“60 m depth within most of the map area, however, directly south and east of Drakes Estero, backscatter and seafloor sediment samples (Chin and others, 1997) suggest fine-grained sediments extend into water depths as shallow as 30 m. Unit Qmsf is commonly extensively bioturbated and consists primarily of mud and muddy sand. These fine-grained sediments are inferred to have been derived from the Drakes Estero estuary or from the San Francisco Bay to the south, via predominantly northwest flow at the seafloor (Noble and Gelfenbaum, 1990).
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Map political location: Marin County, California
Compilation scale: 1:24,000
Base maps used are hillshades generated from IfSAR, LiDAR, and multibeam
mapping both onshore and offshore.