| Summary: | This part of DS 781 presents data for the geologic and geomorphic map of the Offshore Aptos map area, California. The vector data file is included in "Geology_OffshoreAptos.zip," which is accessible from http://dx.doi.org/10.5066/F7K35RQB. Most of the offshore occupies very gently dipping (about 0.1° to 0.4°) continental shelf, extending from the nearshore to water depths of about 70 m. In the southwestern part of the map, the shelf is incised by the north-trending head of Soquel Canyon, which has a maximum depth of 260 m on the south edge of the map. The shelf is underlain by late 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 transgressive erosion and deposition. Sea-level rise was apparently not steady during this period, leading to development of shoreline angles and adjacent submerged wave-cut platforms and risers (Kern, 1977). These features commonly are commonly removed by erosion or buried by shelf sediment, however their original morphology is at least partly preserved along the rim of upper Soquel Canyon. Geologic map units include three wave-cut platforms (units Qwp1, Qwp2, Qwp3) and risers (units Qwpr1, Qwpr2, Qwpr3), separated by shoreline angles at depths of approximately 96 to 100 m, 108 m, and 120 to 125 m. The deepest paleoshoreline (about 120 m deep) approximately corresponds to sea level during the final stages of the last sea-level lowstand (Stanford and others, 2011). Submergence during sea-level rise also cut off the direct connection between Soquel Canyon and coastal watersheds, rendering the submarine canyon relatively inactive. Although slightly sheltered in Monterey Bay, the Offshore of Aptos map area is now subjected to significant wave energy and strong currents. Shelf morphology and geology are also affected by local faulting, folding, and uplift. The shelf in the Offshore of Aptos map area is cut by a diffuse zone of northwest-striking, steeply dipping to vertical faults mapped with high-resolution, seismic-reflection profiles. Faults are mapped on the basis of abrupt truncation or warping of reflections and (or) juxtaposition of reflection panels with different seismic parameters. Seismic profiles traversing this diffuse zone cross as many as 13 faults over a distance of 8 km. Mapped fault lengths in this diffuse zone are typically 2 to 7 km, and the strike of these offshore faults rotates from about 325° to 350° from southwest to northeast. Faults in this diffuse zone cut through Neogene bedrock and locally appear to disrupt overlying latest Quaternary sediments, and the presence of warped reflections along some fault strands suggests there may be both vertical and strike-slip offsets. This broad, distributed zone of deformation resembles the northwest-trending Monterey Bay Fault Zone (Greene, 1977, 1990), which occurs about 10 km farther west in outer Monterey Bay and similarly lacks a lengthy continuous "master fault." Deformation in both the Monterey Bay Fault zone and the diffuse zone of faults in the Offshore of Aptos map area is attributable to its location in the 40-km-wide, northward-narrowing structural zone between two major, right-lateral, strike-slip faults, the San Andreas Fault to the east and the offshore San Gregorio Fault to the west (McCulloch, 1987; Brabb, 1997; Wagner and others, 2002; Dickinson and others, 2005). Emergent late Pleistocene marine terraces on the south flank of the Santa Cruz Mountains in and north of northeastern Monterey Bay are as high as 125 m. Anderson and Menking (1994) report a 50- to 60-m elevation for the shoreline angle tied to the lowest emergent terrace (assigned to oxygen isotope stage 5c or 5e) in the Aptos vicinity, suggesting an uplift rate of about 0.4 to 0.6 mm/yr. Anderson (1990) and Anderson and Menking (1994) attributed this uplift to advection of crust around a bend in the San Andreas Fault, which lies 13 km northeast of the Aptos shoreline. The uplifted region in this tectonic model would include the nearshore and shelf of northeastern Monterey Bay, but there are considerable shore-normal uplift gradients and offshore uplift rates are not constrained. From La Selva Beach west to the western edge of the map area, the upper Miocene and Pliocene Purisima Formation (unit Tp; Powell and others, 2007) forms discontinuous outcrops that extend from coastal bluffs into the offshore to depths as great as 25 m. The seafloor outcrops are most prominent offshore of Soquel Point and have relatively low relief, probably in large part due to low structural dips. The Purisima Formation also forms outcrops in the steep walls of the head of Soquel Canyon. Other "hard bottom" in the map area is mapped at the location of a wastewater outfall pipe offshore of the mouth of the Pajaro River (artificial fill; unit af). Modern nearshore and inner- to mid-shelf sediments are mostly sand (unit Qms) and a mix of sand and gravel (units Qmsc and Qmsd). There is an extensive area in northeastern Monterey Bay where unit Tp bedrock is overlain a very thin cover of Qms; such areas are mapped and labeled as composite units (Qms/Tp) and shown with a stippled pattern on the map. Storlazzi and others (2011) showed that active sediment transport in the nearshore of northern Monterey Bay can lead to significant burial and exhumation of offshore bedrock reefs, and it is likely that the sediment cover in the mapped composite areas is ephemeral and transient. 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 only adjacent to bedrock at Soquel Point in water depths less than 20 m. Unit Qmsd forms erosional lags in scoured depressions at water depths ranging from about 10 to 25 m. The Qmsd depressions have irregular to lenticular outlines; are a few tens of centimeters deep; range in size from a few 10's to as much as about 54,000 m2; and are either bounded by relatively sharp and less commonly diffuse contacts with unit Qms sands, or by abrupt contacts with seafloor bedrock outcrops. Qmsd depressions are most abundant in a northeast-trending zone between Seacliff State Beach and La Selva Beach, where they form distinct, narrow (< 50 m) linear, bands that extend about 4 km offshore. Scour depressions similar to those mapped adjacent to bedrock near Soquel Point are common along this stretch of the California coast (see, for example, Hallenbeck and others, 2012; Davis and others, 2013). 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). They form 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 ocean swells. The elongate, linear, shore-normal bands of scour depressions between Aptos and La Selva Beach are morphologically anomalous; this mode of occurrence has not been recognized elsewhere along the California coast. 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. An offshore transition from unit Qms to the more fine-grained marine sediments of unit Qmsf occurs at water depths of 25 to 30 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, Pajaro River, and smaller coastal watersheds. A 4.3 km2 zone of hummocky seafloor (Qmsh) surrounded by fine-grained Qmsf occurs in the southeastern part of the map area at 30 to 35 m water depth, about 3 to 4 km north of the head of Soquel Canyon. Bathymetric contours reveal that the hummocky zone has an embayed up-slope margin, and relief on hummocks within the zone is as much as 100 cm over 100 m. The hummocky zone probably formed by liquefaction, and associated induced ground failure forced by strong ground motions from earthquakes. The embayed upper margin of the zone also indicates some slumping, surprising given the extremely gentle dip of the shelf (about 0.2°) at this location. Earthquake sources for strong ground motions could include the distributed fault zone in northeastern Monterey Bay (several mapped faults cut the hummocky area) or the nearby San Andreas Fault (20 km to the northeast) or San Gregorio Fault (19 km to the southwest). Recent large earthquakes on the San Andreas Fault include the M 6.9 1989 Loma Prieta earthquake and the M 7.8 1906 Great California earthquake (Northern California Earthquake Data Center, 2014). Soquel Canyon is a tributary to the much larger Monterey Canyon system (Greene and others, 2002). The canyon axis plunges south about 4°; side-canyon walls generally dip about 5° but are locally as steep as 20° to 25°. Non-bedrock geologic units in Soquel Canyon are largely defined and delineated on the basis of geomorphology. Unit Qcw represents mud and sand draped over the upper submarine canyon wall. Unit Qcfa represents the mainly mud fill of the inactive axial canyon channel. 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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. Kern, J.P., 1977, Origin and history of upper Pleistocene marine terraces, San Diego, California: Geological Society of America Bulletin, v. 88, p. 1,553-1,566. 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. Northern California Earthquake Data Center, 2014, Northern California earthquake catalog: Northern California Earthquake Data Center database, accessed April 5, 2014, at http://www.ncedc.org/ncsn/. 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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. 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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