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Layer: Geology_OffshoreTomalesPoint (ID: 3)

Name: Geology_OffshoreTomalesPoint

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Description: This part of DS 781 presents data for the geologic and geomorphic map of the Offshore of Tomales Point map area, California. The vector data file is included in "Geology_OffshoreTomalesPoint.zip," which is accessible from http://pubs.usgs.gov/ds/781/OffshoreTomalesPoint/data_catalog_OffshoreTomalesPoint.html. The morphology and the geology of the offshore part of the Offshore of Tomales Point map area result from the interplay between tectonics, sea-level rise, local sedimentary processes, and oceanography. The map area is cut by the northwest-trending San Andreas Fault, the right-lateral transform boundary between the North American and Pacific tectonic plates. The San Andreas strikes through Tomales Bay, the northern part of a linear valley that extends from Bolinas through Olema Valley to Bodega Bay, separating mainland California from the Point Reyes Peninsula. Onshore investigations indicate that this section of the San Andreas Fault has an estimated slip rate of about 17 to 25 mm/yr. The devastating Great 1906 California earthquake (M 7.8) is thought to have nucleated on the San Andreas Fault about 50 kilometers south of this map area offshore of San Francisco, with the rupture extending northward through the Offshore of Tomales Point map area to the south flank of Cape Mendocino. The Point Reyes Peninsula is bounded to the south and west in the offshore by the north- and east-dipping Point Reyes Thrust Fault, which lies about 20 km west of Tomales Point. Granitic basement rocks are offset about 1.4 km on this thrust fault offshore of Point Reyes, and this uplift combined with west-side-up offset on the San Andreas Fault resulted in uplift of the Point Reyes Peninsula, including Tomales Point and the adjacent continental shelf. Grove and others (2010) reported uplift rates of as much as 1 mm/yr for the south flank of the Point Reyes Peninsula based on marine terraces, but reported no datable terrace surfaces that could constrain uplift for the flight of 4-5 terraces exposed farther north along Tomales Point. Because of this Quaternary uplift and relative lack of sediment supply from coastal watersheds, there is extensive rugged, rocky seafloor beneath the continental shelf in the Offshore of Tomales Point map area. Granitic rocks (unit Kg) on the seafloor are mapped on the basis of massive character, roughness, extensive fractures, and high backscatter (see Backscattter A to D--Offshore of Tomales Point, California, DS 781, for more information). Neogene sedimentary rocks (units Tl and Tu) commonly form distinctive "ribs," created by differential seafloor erosion of dipping beds of variable resistance. The more massive offshore outcrops of unit Tu in the southern part of the map area are inferred to represent more uniform lithologies. Slopes on the granitic seafloor (generally 1 to 1.3 degrees) are greater than those over sedimentary rock (generally about 0.5 to 0.6 degrees). Sediment-covered areas occur in gently sloping (less than about 0.6 degrees) mid-shelf environments west and north of Tomales Point, and at the mouth of Tomales Bay. Sediment supply is local, limited to erosion from local coastal bluffs and dunes, small coastal watersheds, and sediment flux out of the mouth of Tomales Bay. Shelf morphology and evolution largely reflects eustacy; sea level has risen about 125 to 130 m over about the last 21,000 years, leading to broadening of the continental shelf, progressive eastward migration of the shoreline and wave-cut platform, and associated transgressive erosion and deposition. Given present exposure to high wave energy, modern nearshore to mid-shelf sediments are mostly sand (unit Qms) and a mix of sand, gravel, and cobbles (units Qmsc and Qmsd). These sediments are distributed between rocky outcrops at water depths of as much as 65 m (see below). The more coarse-grained sands and gravels (units Qmsc and Qmsd) are primarily recognized on the basis of bathymetry and high backscatter. Unit Qmsd forms erosional lags in scoured depressions that are bounded by relatively sharp contacts with bedrock or sharp to diffuse contacts with units Qms and Qmsc. These scoured depressions are typically a few tens of centimeters deep and range in size from a few 10's of sq m to more than one sq km. Similar unit Qmsd scour depressions are common along this stretch of the California coast where surficial offshore sandy sediment is relatively thin (thus unable to fill the depressions) due to both lack of sediment supply and to erosion and transport of sediment during large northwest winter swells. Such features have been referred to as rippled-scour depressions or sorted bedforms. Although the general areas in which both unit Qmsd scour depressions and surrounding mobile sand sheets occur are not likely to change substantially, the boundaries of the individual Qmsd depressions are likely ephemeral, changing seasonally and during significant storm events. Unit Qmsf consists primarily of mud and muddy sand and is commonly extensively bioturbated. The location of the inboard contact at water depths of about 65 m is based on meager sediment sampling and photographic data and the inference that if must lie offshore of the outer boundary of coarse-grained units Qmsd and Qmsc. This is notably deeper than the inner contact of unit Qmsf offshore of the nearby Russian River (about 50 m) which could may reflect both increased wave energy and significantly decreased supply of muddy sediment. There are two areas of high-backscatter, rough seafloor at water depths of 65 to 70 m west of northern Tomales Point. These areas are notable in that each includes several small (less than about 20,000 sq m), randomly distributed to northwest-trending, irregular "mounds," with as much as 1 m of positive relief above the seafloor (unit Qsr). Seismic-reflection data (see field activity S-15-10-NC) reveal this lumpy material rests on several meters of latest Pleistoce to Holocene sediment and is thus not bedrock outcrop. Rather, it seems likely that this material is marine debris, possibly derived from one (or more) of the more than 60 shipwrecks that have occurred 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". Units Qsw, Qstb, Qdtb, and Qsdtb comprise sediments in Tomales Bay. Anima and others (2008) conducted a high-resolution bathymetric survey of Tomales Bay and noted that strong tidal currents at the mouth of the bay had created a large field of sandwaves, dunes, and flats (unit Qsw). Unit Qkdtb is a small subaqueous sandy delta deposited at the mouth of Keys Creek, the largest coastal watershed draining into this northern part of Tomales Bay. Unit Qstb occurs south of units Qsw and Qdtb, and comprises largely flat seafloor underlain by mixed sand and silt. Unit Qdtb consists of depressions within the sedimentary fill of Tomales Bay. These depressions commonly occur directly offshore of coastal promontories, cover as much as 74,000 sq m, and are as deep as 9 m. Map unit polygons were digitized over underlying 2-meter base layers developed from multibeam bathymetry and backscatter data (see Bathymetry--Offshore of Tomales Point, California and Backscattter A to D--Offshore of Tomales Point, California, DS 781). The bathymetry and backscatter data were collected between 2006 and 2010.

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