| Summary: | This part of DS 781 presents data for the geologic and geomorphic map of the Offshore of Scott Creek map area, California. The vector data file is included in "Geology_OffshoreScottCreek.zip," which is accessible from http://dx.doi.org/10.5066/F7CJ8BJW. The offshore part of the map area lies southwest of the southwest flank of the Santa Cruz Mountains, extending from the shoreline to water depths of about 60 to 75 m on the gently dipping (about 0.6° to 0.8°) continental shelf. The shelf is underlain by Neogene bedrock and a variably thick (as much as 24 m) late Quaternary sediment cover. Sea level has risen 120 to 130 m over about the last 21,000 years (for example, Stanford and others, 2011), 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). The Offshore of Scott Creek map area is now an open-ocean shelf that is subjected to full, and sometimes severe, wave energy. Shelf morphology and geology are also affected by local faulting, folding, uplift, and possibly subsidence. The offshore of Scott Creek map area straddles the right-lateral San Gregorio Fault Zone, an important structure in the distributed transform boundary between the North American and Pacific plates (see, for example, Dickinson and others, 2005). Regionally, this fault is part of a system that occurs predominantly in the offshore for about 400 km from Point Conception in the south (where it is known as the Hosgri fault; Johnson and Watt, 2012) to Bolinas and Point Reyes in the north (Bruns and others, 2002; Ryan and others, 2008). The San Gregorio Fault in the map area is part of a 90-km-long offshore segment that extends from Point Sur on the south, across outer Monterey Bay to Point Año Nuevo (just one kilometer north of the map area) on the north; Weber and Lajoie, 1980; Brabb, 1997; Wagner and others, 2002). Offshore parts of this fault system are identified on seismic-reflection data based on abrupt truncation or warping of reflections and (or) juxtaposition of reflection panels with different seismic parameters. In this map area, the San Gregorio Fault forms a distributed about 2-km-wide shear zone that includes two main faults. The nearshore eastern part of the zone, which includes the Coastways Fault, partly coincides with a prominent bathymetric lineament on the outer flank of nearshore bedrock outcrops between Waddell Creek and Davenport. The western part of the zone, which includes the Frijoles Fault, cuts across the flat, sediment-covered shelf. Cumulative lateral slip on San Gregorio Fault Zone in this region is thought to range from 4 to 10 mm/yr (Weber, 1994). McCulloch (1987) considered the San Gregorio Fault Zone the eastern margin of the Outer Santa Cruz Basin (fig. 8-1). Farther offshore, outside California's State Waters but within the map area, this basin is cut by the northwest-trending Ascension Fault (Greene and others, 2002; U.S. Geological Survey and California Geological Survey, 2010). Emergent marine terraces on the flanks of the Santa Cruz Mountains between Santa Cruz and Point Año Nuevo (fig. 1-1) are as high as 240 m with estimated uplift rates that range from about 0.2 m/year (for example, Bradley and Griggs, 1976; Lajoie and others, 1991) to as much as 1.1 mm/yr (for example, Perg and others, 2001). This uplift has been attributed to a combination of (1) advection of crust around a bend in the San Andreas Fault, and (2) uplift on the northeast (landward) side of a steep-northeast dipping offshore San Gregorio fault (Anderson, 1990; Anderson and Menking, 1994). The uplifted region in this tectonic model includes the nearshore and shelf of the Offshore of Scott Creek map area, but considerable shore-normal uplift gradients are associated with both processes and offshore uplift rates are not well constrained. Map unit polygons were digitized over underlying 2-meter base layers developed from multibeam bathymetry and backscatter data (see "Bathymetry--Offshore of Scott Creek Map Area, California" and "Backscatter--Offshore of Scott Creek Map Area, California"). The bathymetry and backscatter data were collected between 2006 and 2010. References Cited Brabb, E.E., 1997, Geologic Map of Santa Cruz County, California: A digital database, US Geological Survey Open-File Report 97-489, 1:62,500. Bruns, T.R., Cooper, A.K., Carlson, P.R., and McCulloch, D.S., 2002, Structure of the submerged San Andreas and San Gregorio Fault zones in the Gulf of Farallones as inferred from high-resolution seismic-reflection data, in Parsons, T. (ed.), Crustal structure of the coastal and marine San Francisco Bay region, California: U.S. Geological Survey Professional Paper 1658, p. 77-117. Catuneanu, O., 2006, Principles of Sequence Stratigraphy: Amsterdam, Elsevier, 375 p. Dickinson, W.R., Ducea, M., Rosenberg, L.I., Greene, H.G., Graham, S.A., Clark, J.C., Weber, G.E., Kidder, S., Ernst, W.G., and Brabb, E.E., 2005, Net dextral slip, Neogene San Gregorio-Hosgri fault zone, coastal California - Geologic evidence and tectonic implications: Geological Society of America Special Paper 391, 43 p. Greene, H.G., Maher, N.M., and Paull, C.K., 2002, Physiography of the Monterey Bay National Marine Sanctuary and implications about continental margin development: Marine Geology, v. 181, p. 55-82. 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., doi:10.1130/GES00830.1. LaJoie, K.R., Ponti, D.J., Powell, C.L., II, Mathieson, S.A., and Sarna-Wojcicki, 1991, Emergent marine strandlines and associated sediments, coastal California; A record of Quaternary sea-level fluvtuations, vertical tectonic movements, climatic changes, and coastal processes, in Morrison, R.B., ed., Quaternary non-glacial geology, conterminous United States: Geological Society of America, Geology of North America, v. K-2, p. 190-214. 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. Perg, L.A., Anderson, R.S., and Finkel, R.C., 2001, Use of a new 10Be and 26Al inventory to data marine terraces, Santa Cruz, California, USA: Geology, v. 29, p. 879-882. Ryan, H.F., Parsons, T., and Sliter, R.W., 2008. Vertical tectonic deformation associated with the San Andreas fault zone offshore of San Francisco, California: Tectonphysics, v. 429, p. 209-224. Stanford, J.D., Hemingway, R., Rohling, E.J., Challenor, P.G., Medina-Elizalde, M., and Lester, A.J., 2011, Sea-level probability for the last deglaciation - A statistical analysis of far-field records: Global and Planetary Change, v. 79, p. 193-203. Weber, G.E., and LaJoie, K.R., 1980, Map of Quaternary faulting along the San Gregorio fault zone, San Mateo and Santa Cruz counties, California: US Geological Survey Open-File Report 80-907. Weber, G.E., 1994, Late Pleistocene slip rates on the San Gregorio Fault Zone at Point Año Nuevo, San Mateo County, California, in Lettis, W.R., ed., Field trip guidebook to transpressional deformation in the San Francisco Bay region: Friends of the Pleistocene, Pacific Southwest Cell. Wagner, D.L., Greene, H.G., Saucedo, G.J., and Pridmore, C.L., 2002, Geologic Map of the Monterey 30' x 60' quadrangle and adjacent areas, California: California Geological Survey Regional Geologic Map Series, scale 1:100,000.
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