| Summary: | This part of DS 781 presents data for the geologic and geomorphic map of the Offshore Santa Cruz map area, California. The vector data file is included in "Geology_OffshoreSantaCruz.zip," which is accessible from http://dx.doi.org/10.5066/F7TM785G. The offshore part of the map area lies south and southwest of the southwest flank of the Santa Cruz Mountains, on the open Pacific Coast and in northwestern Monterey Bay. This offshore area extends from the shoreline across the gently dipping (about 0.7° to 0.8°) continental shelf to water depths of about 75 to 90 m at the outer limit of California's State Waters. The shelf is underlain by Neogene bedrock and a variably thick (as much as 32 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 Santa Cruz map area is now subjected to full, and sometimes severe, wave energy and strong currents. Shelf morphology and geology are also affected by local faulting, folding, and uplift. The western part of the offshore map area is cut by the northern part of the diffuse, northwest-striking, about 5-km-wide Monterey Bay Fault Zone (Greene, 1990). Mapping, based on seismic-reflection profiles, reveals that the zone can include as many as ten or more vertical to steeply dipping strands, which range in length from about 1 to 9 km in the map area. Greene (1990) suggested the fault zone may have both vertical and strike-slip offset based on the presence of warped reflections along some fault strands. Fault-related deformation clearly affects Neogene bedrock, but faults in this zone do not appear to offset Quaternary deposits. The Monterey Bay Fault Zone lies subparallel to the active San Gregorio Fault Zone (McCulloch, 1987; Dickinson and others, 2005), which extends through the southwest corner of the map area (outside California's State Waters) and has an estimated 156 km of right-lateral offset. The northwest-trending, strike-slip deformation associated with the San Gregorio and Monterey Bay Fault Zones appears to largely postdate deformation along the north-trending Ben Lomond Fault. Stanley and McCaffrey (1983) used gravity anomalies to extend mapping of the Ben Lomond fault for 3 km from bedrock exposures in the Santa Cruz Mountains beneath emergent marine terrace deposits to the shoreline about 930 m west of Point Santa Cruz. This onshore-offshore geologic map shows the fault extending an additional 4 km to the south in the offshore, based on interpretation of high-resolution bathymetry and seismic-reflection data. Emergent marine terraces on the flanks of the Santa Cruz Mountains in and north of Santa Cruz are as high as 240 m with estimated uplift rates that range from about 0.2 mm/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 Santa Cruz map area, but probable shore-normal uplift gradients are associated with both processes and offshore uplift rates are not well constrained. From the northern edge of the map area south to western Santa Cruz (about 1400 m west of Point Santa Cruz (fig. 1-2), the upper Miocene Santa Cruz Mudstone (unit Tsc) forms continuous outcrops that extend from coastal bluffs into the offshore to depths as great as 35 m. To the southeast, similar continuous onshore-to-offshore outcrops of the younger (Pliocene and late Miocene) Purisima Formation (unit Tp; Powell and others, 2007) extend southwest from bluffs at Point Santa Cruz and east of the mouth of the San Lorenzo River. The Santa Cruz Mudstone (Tsc) seafloor outcrops are characterized by differentially eroded layers (harder and softer interbeds) that are folded and densely fractured, creating a relatively "shattered" appearance on shaded relief maps. The adjacent seafloor outcrops of the Purisima Formation (Tp) are similarly folded but have less distinct and more diffuse bedding surfaces (in part due to lower dips) and are notably less fractured, and thus have a distinctly different seafloor geomorphic expression. Modern nearshore and inner- to mid-shelf sediments are mostly sand (unit Qms) and a mix of sand and gravel (units Qmsc and Qmsd). In addition to its presence on the broad shelf, unit Qms notably occurs in well-defined paleochannels that cut through nearshore bedrock exposures at the mouths of several small coastal watersheds, including Liddell Creek, Laguna Creek, Yellow Bank Creek, Majors Creek, Baldwin Creek, Wilder Creek, and Moore Creek. These distinct channels extend to water depths of 20 to 30 meters and formed by subaerial erosion during sea-level lowstands (Anima and others, 2002). The more coarse-grained sands and gravels (units Qmsc and Qmsd) are primarily recognized on the basis of bathymetry and high backscatter. Unit Qmsc mainly occurs adjacent to bedrock in water depths less than 35 m. Unit Qmsd forms erosional lags in scoured depressions at water depths ranging from about 15 to 35 m. These Qmsd depressions are typically irregular to lenticular; a few tens of centimeters deep; range in size from a few 10's to as much as about 550,000 m2; and are either bounded by relatively sharp and less commonly diffuse contacts with unit Qms sands, or by abrupt contacts with bedrock on the margins of lowstand paleochannels (see above). Qmsd depressions are most abundant in a northeast-trending zone between bedrock outcrops offshore of Point Santa Cruz. Such scour depressions are common along this stretch of the California coast (see, for example, Cacchione and others, 1984; Hallenbeck and others, 2012; Davis and others, 2013) where surficial offshore sandy sediment is relatively thin (thus unable to fill the depressions) due to both low sediment supply and to erosion and transport of sediment during large northwest winter swells. Such features have been referred to as ârippled-scour depressionsâ (see, for example, Cacchione and others, 1984) or âsorted bedformsâ (see, for example, Goff and others, 2005; Trembanis and Hume, 2011). Although the general areas in which both unit Qmsd scour depressions and surrounding Qms sand sheets occur are not likely to change substantially, the boundaries of the unit(s) are likely ephemeral, changing seasonally and during significant storm events. Active sediment transport in this nearshore regime can also lead to significant ephemeral burial and exhumation of offshore Tsc and Tp bedrock reefs (Storlazzi and others, 2011). An offshore transition from unit Qms to the more fine-grained marine sediments of unit Qmsf occurs at water depths of 35 to 50 m. Unit Qmsf is commonly extensively bioturbated and consists primarily of mud and muddy sand. Edwards (2002) and Grossman and others (2006) suggested these fine-grained sediments form an extensive "mid-shelf mud belt" that was primarily sourced by the San Lorenzo River and smaller coastal watersheds. Artificial fill (unit af) is mapped in the offshore at the locations of the Santa Cruz wharf about 800 m west of the mouth of the San Lorenzo River, and at the location of a wastewater outfall pipe that cuts across the nearshore about 1,350 m west of Point Santa Cruz. Map unit polygons were digitized over underlying 2-meter base layers developed from multibeam bathymetry and backscatter data (see "Bathymetry--Offshore of Santa Cruz Map Area, California" and "Backscatter--Offshore of Santa Cruz Map Area, California"). The bathymetry and backscatter data were collected between 2006 and 2010. References Cited Anderson, R.S., 1990, Evolution of the northern Santa Cruz Mountains by advection of crust past a San Andreas Fault bend: Science, v. 249, p. 397â401. Anderson, R.S., and Menking, K.M., 1994, The Quaternary marine terraces of Santa Cruz, CaliforniaâEvidence for coseismic uplift on two faults: Geological Society of America Bulletin, v. 106, p. 649â664. Bradley, W.C., and Griggs, G.B., 1976, Form, genesis, and deformation of central California wave-cut platforms: geological Society of America Bulletin, v. 87, p. 433â449. 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. 1,280â1,291. Davis, A.C.D., Kvitek, R.G., Mueller, C.B.A., Young, M.A., Storlazzi, C.D., and Phillips, E.L., 2013, Distribution and abundance of rippled scour depressions along the California coast: Continental Shelf Research, v. 69, p. 88â100, doi:10.1016/j.csr.2013.09.010. 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. Edwards, B.D., 2002, Variations in sediment texture on the northern Monterey Bay National Marine Sanctuary continental shelf: Marine Geology, v. 181, p. 83â100. 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 ârippled scour depressions,â within the Marthaâs Vineyard Coastal Observatory, Massachusetts: Continental Shelf Research, v. 25, p. 461â484. Greene, H.G., 1990, Regional tectonics and structural evolution of the Monterey Bay region, central California, in Garrison, R.E., Greene, H.G., Hicks, K.R., Weber, G.E., and Wright, T.L., eds., Geology and tectonics of the central California coastal region, San Francisco to Monterey, Pacific Section American Association of Petroleum Geologists, Guidebook GB-67, p. 31â56. Grossman, E.E., Eittreim, S.L., Field, M.E., and Wong, F.L., 2006, Shallow stratigraphy and sedimentation history during high-frequency sea-level changes on the central California shelf: Continental Shelf Research, v. 26, p1217â1239. 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. 119â133. 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 fluctuations, 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 ocean basins - Beaufort Sea to Baja California: Circum-Pacific Council for Energy and Mineral Resources Earth Science Series, v. 6, p. 353-401. 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 deglaciationâA statistical analysis of far-field records: Global and Planetary Change, v. 79, p. 193â203. Stanley, R.G., and McCaffrey, R., 1983, Extent and history of the Ben Lomond fault, Santa Cruz County, California, in Andersen, D.W., and Rymer, M.J., eds., Tectonics and sedimentation along faults of the San Andreas system: Society of Economic Paleontologists and Mineralogists, Pacific Section Publication 30, p. 79â90. Storlazzi, C.D., Fregoso, T.A., Golden, N.E., and Finlayson, D.P., 2011, Sediment dynamics and the burial and exhumation of bedrock reefs along an emergent coastline as elucidated by repetitive sonar surveysânorthern Monterey Bay, CA: Marine Geology, v. 289, p. 46â59. 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. 879â882. Powell, Charles L., II, Barron, John A., Sarna-Wojcicki, Andrei M., Clark, Joseph C., Perry, Frank A., Brabb, Earl E., and Fleck, Robert J., 2007, Age, stratigraphy, and correlations of the late Neogene Purisima Formation, central California Coast Ranges: U.S. Geological Survey Professional Paper 1740, 32 p., available at http://pubs.usgs.gov/pp/2007/1740/. Trembanis, A.C., and Hume, T.M., 2011, Sorted bedforms on the inner shelf off northeastern New ZealandâSpatiotemporal relationships and potential paleo-environmental implications: Geo-Marine Letters, v. 31, p. 203â214.
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