This part of DS 781 presents data for the geologic and geomorphic map of the Offshore of Bodega Head map area, California. The vector data file is included in "Geology_OffshoreBodegaHead.zip," which is accessible from http://pubs.usgs.gov/ds/781/OffshoreBodegaHead/data_catalog_OffshoreBodegaHead.html
The morphology and the geology of the Offshore of Bodega Head map area result from the interplay between tectonics, sea-level rise, local sedimentary processes, and oceanography. The Offshore of Bodega Head map area is cut by the northwest-trending San Andreas Fault, the right-lateral transform boundary between the North American and Pacific tectonic plates. From southeast to northwest, this fault extends through Bodega Bay and Bodega Harbor, crosses the Bodega Head isthmus at the mouth of Salmon Creek, and extends in the offshore for about 20 km before passing onland at Fort Ross, about 12 km north of the Offshore of Bodega Head map area. The San Andreas Fault juxtaposes Cretaceous granitic rock on the southwest with the Jurassic, Cretaceous, and early Tertiary Franciscan Complex on the northeast. Uplift of the granitic rock (unit Kg) on the west side of the San Andreas Fault has created extensive, rugged, rocky seafloor, centered offshore Bodega Head and extending northwest for about 15 km, from the western flank of Bodega Bay to the shelf offshore of the mouth of Salmon Creek. At its south and north ends this rocky seafloor extends to water depths of about 40 m and 50 m, respectively where it is onlapped by young sediment (see below); offshore of Bodega Head, the rocky seafloor extends to water depths of 80 m. Northeast of the San Andreas Fault, offshore rocky outcrops of the Franciscan complex (units Kfs, Kjfss, fsr) occur only in the nearshore (water depths less than 15 m).
Sediment-covered areas of the offshore part of the map occur in gently sloping (less than about 1 degree) nearshore, inner-, and mid-shelf environments. Sediment supply to the shelf north and west of Bodega Head is predominantly from coastal watersheds including the Russian River (4.5 km north of the map area) and Salmon Creek. Sediment supply to Bodega Bay and the shelf in the southern part of the map area is from small coastal watersheds and estuaries such as Estero Americano and Estero de San Antonio, and most likely from sediment flux out of the mouth of Tomales Bay, located two kilometers south of the map area. Shelf morphology and evolution largely reflects eustacy; 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.
Given present exposure to high wave energy, modern nearshore to inner-shelf sediments are mostly sand (unit Qms) 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. Unit Qmsc occurs in two areas: (1) as a linear nearshore bar (water depth less than 10 m) offshore of Salmon Creek - the eastern contact of this unit is queried because of the lack of data in the surf zone; and (2) as a west-trending bar at the north end of Bodega Bay. Unit Qmsd forms erosional lags in scoured depressions that are bounded by relatively sharp contacts with bedrock or sharp to diffuse contacts with unit Qms. 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 (see, for example, Cacchione and others, 1984; Hallenbeck and others, 2012) 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" (see, for example, Cacchione and others, 1984) or "sorted bedforms" (see, for example, Goff and others, 2005; Trembanis and Hume, 2011). 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 lies offshore of unit Qms, consists primarily of mud and muddy sand and is commonly extensively bioturbated. The water depth of the transition from sand-dominated marine sediment (unit Qms) to mud-dominated marine sediment (Qmsf) occurs at depths of about 45 to 50 m except offshore of Bodega Head where seafloor bedrock outcrops extend to depths of 80 m at the outer limit of California's State Waters.
The smooth seafloor in the northern part of the map area between water depths of 40 m and 70 m notably includes an about 3-km-wide field of elongate, shore-normal, paired sediment lobes and chutes (unit Qmsl). Individual lobes within the field are as much as 1,000-m long and 150-m wide, and have as much as 2 m of relief above the surrounding smooth seafloor. This sediment-lobe field lies just one kilometer west of the San Andreas Fault and we infer that large earthquakes on this structure generated strong ground motions and slope failures, mobilizing small sediment flows that moved down the gentle slope and were deposited as lobes. Unit Qmsl also occurs as a 250-m-wide field of four discrete lobes and paired arcuate, low-relief scours on the south flank of a west-trending bar in northern Bodega Bay.
Map unit polygons were digitized over underlying 2-meter base layers developed from multibeam bathymetry and backscatter data (see Bathymetry--Offshore Bodega Head, California and Backscattter A to C--Offshore Bodega Head, California, DS 781, for more information). The bathymetry and backscatter data were collected between 2007 and 2010.
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. 1,280-1,291.
Goff, J.A., Mayer, L.A., Traykovski, P., Buynevich, I., Wilkens, R., Raymond, R., Glang, G., Evans, R.L., Olson, H., and Jenkins, C., 2005, Detailed investigations of sorted bedforms or "rippled scour depressions", within the Martha's Vineyard Coastal Observatory, Massachusetts: Continental Shelf Research, v. 25, p. 461-484.
Hallenbeck, T.R., Kvitek, R.G., and Lindholm, J., 2012, Rippled scour depressions add ecologically significant heterogeneity to soft-bottom habitats on the continental shelf: Marine Ecology Progress Series, v. 468, p. 119-133.
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.
Peltier, W.R., and Fairbanks, R.G., 2006, Global glacial ice volume and Last Glacial Maximum duration from an extended Barbados sea level record: Quaternary Science Reviews, v. 25, p. 3,322-3,337.
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.
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 (see Bathymetry Hillshade--Offshore Bodega Head, California, California, DS 781, for more information).