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Layer: Geology_OffshoreMonterey (ID: 3)

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Description: This part of DS 781 presents data for the geologic and geomorphic map of the Offshore of Monterey map area, California. The vector data file is included in "Geology_OffshoreMonterey.zip," which is accessible from http://dx.doi.org/10.5066/F7F47M6T. The offshore part of the Offshore of Monterey map area contains two geomorphic regions—(1) the continental shelf, and (2) Carmel Canyon and its tributaries (part of the “Offshore Monterey system” of Greene and others (2002)). The relatively flat continental shelf in the Offshore of Monterey map area consists of exposed bedrock or bedrock overlain by only a thin (< 5 m) cover of sediment). The thickest sediment in the map area (about 16 m) occurs in a broad depression about 2 km north of the north tip of the Monterey Peninsula at water depths of about 75 to 85 m (sheet 9, Map B). Inner- to mid-shelf and nearshore deposits are mostly sand (unit Qms), with coarse sand and gravel (unit Qmsc) present in the mid-shelf about 2 km offshore of Seaside. Unit Qmsf lies offshore of unit Qms in the mid to outer shelf (water depths of 65 to 150 m), consists primarily of mud and muddy sand, and is commonly extensively bioturbated. Unit Qmsd typically is mapped as erosional lags in scour depressions that are bounded by relatively sharp or, less commonly, diffuse contacts with the horizontal sand sheets of unit Qms. These depressions typically are irregular to lenticular and a few tens of centimeters deep, and they range in size from a few tens of square meters to as much as 2,400,000 m2. They most commonly are found at water depths that range from about 15 to 90 m. 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 offshore sandy sediment can be relatively thin (and, thus, is unable to fill the depressions) owing to low sediment supply from rivers and also to significant erosion and offshore 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 Qms sand sheets are found are not likely to change substantially, the boundaries of the unit(s) likely are ephemeral, changing seasonally and during significant storm events. Sea level has risen about 125 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 associated transgressive erosion and deposition. A submerged shoreline along the flank of Carmel Canyon (water depths of 80 to 90 m) represents a relative sea-level stillstand, indicating that sea-level rise was not steady. Associated map units include a wave-cut platform (unit Qwp) and an adjacent riser (unit Qwpr). Bedrock units forming seafloor outcrops on the shelf include Cretaceous granitic rocks (unit Kgr), the Paleocene Carmelo Formation (unit Tc; Bowen, 1965), Oligocene volcanic rocks (basaltic andesite, unit Tvb), the Miocene Monterey Formation (unit Tm), the upper Miocene and Pliocene Purisima Formation (unit Tp) (Eittreim and others, 2002; Wagner and others, 2002). Unit Tu is mapped where rocks of the Carmelo, Monterey, and Purisima Formations cannot be confidently divided. Unit Kgr is notably characterized by high backscatter (sheet 3) and rough, massive, and fractured seafloor texture. In contrast the less indurated Neogene sedimentary rocks, most notably units Tm and Tp, form lower relief outcrops with common “ribbed” morphology reflecting the differential hardness (and hence erodibility) of sedimentary layers. Several of these bedrock units are in places overlain by a thin (less than 1 m?) veneer of sediment recognized on the basis of high backscatter, flat relief, continuity with moderate- to higher-relief outcrops, and (in some cases) high-resolution seismic-reflection data; these areas, which are mapped as composite units (for example, Qms/Kgr or Qms/Tp) are interpreted as ephemeral sediment layers that may or may not be continuously present, depending on storms, seasonal and (or) annual patterns of sediment movement, or longer term climate cycles. The shelf north and east of the Monterey Bay Peninsula in the Offshore of Monterey map area is cut by a diffuse zone of northwest striking, steeply dipping to vertical faults comprising the Monterey Bay Fault Zone (MBFZ). This zone, originally mapped by Greene (1977, 1990), extends about 45 km across Monterey Bay (Map E on sheet 9). Fault strands within the MBFZ are mapped with high-resolution seismic-reflection profiles (sheet 8). Seismic-reflection profiles traversing this diffuse zone in the map area cross as many as 5 faults over a width of about 4 to 5 km (see, for example, figs. 3 and 5 on sheet 8). The zone lacks a continuous “master fault,” along which deformation is concentrated. Fault length ranges up to about 20 km (based on mapping outside this map area), but most strands are only about 2- to 7-km long. Faults in this diffuse zone cut through Neogene bedrock and locally appear to minimally disrupt overlying inferred Quaternary sediments. The presence of warped reflections along some fault strands suggests that fault offsets may be both vertical and strike-slip. Specific offshore faults within the zone that are continuous with mapped onshore faults include the Navy Fault, Chupines Fault, and Ord Terrace Fault (Clark and others, 1997; Wagner and others, 2002). Carmel Canyon, a relatively straight northwest-trending arm of the Offshore Monterey system, extends through the southwestern part of the Offshore of Monterey map area. Carmel Canyon has three heads (Greene and others, 2002), two of which extend east and northeast into Carmel Bay within the map area; the third head extends southeast along the main canyon trend for about 3 km beyond the confluence with the heads in Carmel Bay. Sandy canyon-head channel deposits (unit Qchc) are mapped at the head of the northeast-trending arm of Carmel Canyon in Carmel Bay. Active Carmel Canyon channel fill is mapped as three distinct units: unit Qcpcf, characterized by sand crescentic bedforms, is mapped in upper Carmel Canyon; unit Qccf1 is mapped in curvilinear tributaries on the north and east flanks of Carmel Canyon and consists of gravel with minor bedrock and landslides; unit Qccf2 is mapped in primary Carmel Canyon channels, consists of sand and gravel that locally forms crescentic bedforms, and includes small bedrock outcrops and landslides. Sand and mud deposited on a bench immediately adjacent to Carmel Canyon’s axial channel is mapped as unit Qcb; this unit is present in only a small part of the map area but is more widespread in Offshore Monterey (Maier and others, 2016) and is more extensive in Carmel Canyon south of this map area. Carmel Canyon walls that are relatively smooth are generally covered by muddy Quaternary sediments (unit Qcw), whereas steeper and rougher segments of canyon walls commonly contain exposures of bedrock. Purisima Formation outcrops occur in the upper canyon walls (unit Tpcw) in the northwestern part of the map area. Older, underlying bedrock is exposed at greater depths along canyon walls (Greene, 1977; Greene and others, 1991; Wong and Eittreim, 2002). These older units include the Miocene Monterey Formation (unit Tmcw) and Cretaceous granodiorite (unit Kgcw). The relatively smooth and flat area in the southwest corner of the map area forms a bathymetric divide between Carmel Canyon to the east and Point Lobos Canyon to the west and is inferred to be covered by mud with minimal sand (unit Qmscp). Carmel Canyon is aligned with and structurally controlled by the San Gregorio fault zone (Greene and others, 1991), an important structure in the distributed transform boundary between the North American and Pacific plates (see, for example, Dickinson and others, 2005). This fault zone is part of a regional fault system that is present 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 Zone in the map area is part of a 90-km-long offshore segment that extends northward from Point Sur (about 24 km south of the map area), across outer Monterey Bay to Point Año Nuevo (51 km north of the map area) (see sheet 9; see also, Weber and Lajoie, 1980; Brabb and others, 1998; Wagner and others, 2002). High-resolution seismic-reflection data collected across the canyon do not clearly image the San Gregorio Fault Zone, due largely to significant depth and steep canyon walls. Accordingly, we have mapped the 1,000- 1,300-m-wide fault zone largely on the presence of prominent, lengthy, geomorphic lineaments (sheets 1 and 2) and both geomorphic and lithologic contrasts across the fault. Following Greene and others (2002), we note that relatively straight-sloping drainage channels and slumps lightly dissect the eastern canyon wall, which is underlain by unit Kgcw. In contrast, the western wall of Carmel Canyon is composed of steep cliffs and underlain by a mix of the Miocene Monterey Formation (unit Tmcw) and Cretaceous sedimentary rocks (Greene, 1977; Greene and others, 1991; Eittreim and others, 2002; Wong and Eittreim, 2002). Map unit polygons were digitized over underlying 2-m- and 5-m-resolution 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: 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, available at http://pubs.usgs.gov/pp/1658/. 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. 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. Clark, J.C., Dupre, W.R., and Rosenberg, L.L., 1997, Geologic map of the Monterey and Seaside 7.5–minute quadrangles, Monterey County, California–A digital database: U.S. Geological Survey Open-File Report 97-30, 2 sheets, scale 1:24,000, http://pubs.usgs.gov/of/1997/of97-030/ 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. 88–100, 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. Eittreim, S.L., Anima, R.J., and Stevenson, A.J., 2002, Seafloor geology of the Monterey Bay area continental shelf: Marine Geology, vol. 181, p. 3–34. 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, doi:10.1016/j.csr.2004.09.019. 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. Greene, H.G., Clarke, S.H. and Kennedy, M.P., 1991. Tectonic Evolution of Submarine Canyons Along the California Continental Margin. From Shoreline to Abyss, in Osborne, R.H., ed., Society for Sedimentary Geology, Special Publication No. 46, p. 231–248. 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: American Association of Petroleum Geologists, Pacific Section, Guidebook GB67, p. 31–56. Greene, H.G., 1977, Geology of the Monterey Bay region: U.S. Geological Survey Open-File Report 77–718, 347 p. 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. 31–56. 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, doi:10.3354/meps09948. 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, p. 1,632–1,656, doi:10.1130/GES00830.1. Maier, K.L., Hartwell, S.R., Johnson, S.Y., Davenport, C., and Greene, H.G., 2016, Offshore and onshore geology and geomorphology, Offshore Monterey map area, California, sheet 10 in Dartnell, P., Maier, K.L., Erdey, M.D., Dieter, B.E., Golden, N.E., Johnson, S.Y., Hartwell, S.R., Cochrane, G.R., Ritchie, A.C., Finlayson, D.P., Kvitek, R.G., Sliter, R.W., Greene, H.G., Davenport, C., Endris, C.A., and Krigsman, L.M. (P. Dartnell and S.A. Cochran, eds.), California State Waters Map Series—Offshore Monterey : U.S. Geological Survey Open-File Report 2016–XXXX, pamphlet XX p., 10 sheets, scale 1:24,000, http://dx.doi.org/10.3133/ofrXXXXXXXX. 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: Tectonophysics, v. 429, p. 209–224, doi:10.1016/j.tecto.2008.06.011. 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, doi:10.1007/s00367-010-0225-8. 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. 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: U.S. Geological Survey Open-File Report 80–907, 3 sheets, scale 1:24,000, available at http://pubs.er.usgs.gov/publication/ofr80907.

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