| Summary: | This part of DS 781 presents data for the geologic and geomorphic map of Monterey Canyon and Vicinity, California. The vector data file is included in "Geology_MontereyCanyon.zip," which is accessible from http://dx.doi.org/10.5066/F7XD0ZQ4. The offshore part of the Monterey Canyon and Vicinity map area contains two geomorphic regionsâ(1) the continental shelf, and (2) Monterey Canyon and its tributaries (the Monterey Canyon "system"), including Soquel Canyon. The continental shelf in the Monterey Canyon and Vicinity map area is relatively flat and characterized by a variably thick (and locally absent) cover of uppermost Pleistocene and Holocene sediment that overlies Neogene bedrock and Pleistocene paleo-channel and canyon fill (unit Qcf). Inner-shelf and nearshore deposits are mostly sand (unit Qms), and are thickest (as much as 32 m) in a shore-parallel bar offshore that extends to the mouth of the Salinas River (sheet 9). Slope failures off of the west flank of this delta-mouth bar have resulted in three west-trending elongate sandy lobes (unit Qmsl); individual lobes are as much as 3,000-m long and 800-m wide, have as much as 150 cm of relief above the surrounding smooth seafloor, and are commonly transitional to upslope chutes. Unit Qmsf lies offshore of unit Qms in the mid shelf, consists primarily of mud and muddy sand, and is commonly extensively bioturbated. Sediment cover typically thins in the offshore direction and toward Monterey Canyon (sheet 9); Pleistocene paleo-channel and canyon fill (unit Qcf) and the upper Miocene and Pliocene Purisima Formation (unit Tp; Powell and others, 2007) are exposed on the outer shelf and along the rims of the modern Monterey Canyon system. Both the Purisima Formation (Tp) and Pleistocene paleo-channel and canyon fill (Qcf) 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 Qms/Tp or Qms/Qcf, 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. Sea level has risen about 125 to 130 meters 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. Sea-level rise was apparently not steady, leading to development of pairs of shoreline angles and adjacent submerged wave-cut platforms (Kern, 1977) during pulses of relative stability. Latest Pleistocene paleoshorelines are best preserved along the flanks of Soquel Canyon, where three sets of wave cut platforms (units Qwp1, Qwp2, Qwp3) and paired risers (units Qwpr1, Qwpr2, Qwpr3) are bounded by shoreline angles at water depths of about 120 to 125 meters, about 108 meters, and about 96 to 100 meters. Within the Monterey Canyon system, geologic and geomorphic units are delineated and characterized on the basis of multibeam bathymetry (sheet 1), backscatter (sheet 3), published samples and descriptions of geology within the canyon system (e.g., Greene, 1977; Barry and others, 1996; Stakes and others, 1999; Wagner and others, 2002; Paull and others, 2005a, 2005b, 2010), and, where available, seismic-reflection profiles (sheet 8) and video observations (sheet 6). Major geologic and geomorphic components within the canyons include the canyon-head region, canyon axial channels, canyon walls, and canyon benches and platforms. The canyon-head region of Monterey Canyon includes sandy channel fill (unit Qchc; Paull and others, 2005a) and inter-channel sediment-draped ridges (unit Qchr) inferred to have formed largely by erosion of the canyon head into older canyon and (or) channel fill. There is a geomorphic gradational transition down-canyon from canyon-head channel fill (unit Qchc) to proximal active axial channel fill (unit Qcpcf), and both channel-fill units include dynamic crescent-shaped sandy bedforms (Paull and others, 2005a, 2010; Smith and others, 2005; Xu and others, 2008). Beyond the canyon head region, the axial channel of Monterey Canyon forms a sinuous ribbon of coarse-grained deposits (unit Qccf3), sloping about 3.5° to the western edge of the map area (Paull and others, 2005a, 2010; Greene and others, 2002; Xu and others, 2008). Xu and others (2002, 2008, 2013) and Paull and others (2010) have documented recent sediment-gravity flows down the Monterey Canyon axial channel, indicating that it is an active conduit of sediment transport. Map units adjacent to the axial channel include canyon walls, benches, and landslides. Canyon walls (unit Qmscw) that are relatively smooth are generally covered by muddy Quaternary sediments (Paull and others, 2005a, 2010), whereas steeper and rougher segments of canyon walls commonly contain exposures of bedrock or incised Pleistocene paleo-channel and canyon fill (unit Qcfcw). Purisima Formation outcrops occur in the upper canyon walls (unit Tpcw). Older, underlying bedrock units (Greene, 1977; Barry and others, 1996; Stakes and others, 1999; Wagner and others, 2002) are exposed at greater depths along canyon walls. These older units include the Miocene Monterey Formation (unit Tmcw), Miocene and Oligocene sandstone (unit Tscw), Tertiary volcanics intrusive into sedimentary bedrock and Cretaceous granodiorite (unit Tvcw), and Cretaceous granodiorite (unit Kgcw). Exposures of these bedrock units and incised Pleistocene paleo-channel and canyon fill outcrops in the canyon walls (unit Qcfcw) are inferred to result from a combination of erosion by dense sediment flows down the axial channel and continuing landslide failure of the canyon walls. Relatively flat areas immediately adjacent to the axial channel or within the canyon walls are mapped as inner benches (unit Qcb2) and outer benches (unit Qcb1), respectively (bench term from Paull and others, 2010 and Maier and others, 2012). Benches generally have lower slopes than surrounding canyon walls and accumulate fine-grained sediments, including muddy marine, hemipelagic, turbidite, and landslide deposits (Paull and others, 2005a, 2010). Relatively flat, smooth, sediment-covered platforms on the crests of bathymetric divides between canyon meanders are mapped as canyon platforms (unit Qmscp). Regions of the canyon walls characterized by steep, scallop-shaped scarps and paired hummocky mounds are mapped as landslides (units Qlsm). Multiple generations of landslides are mapped (units Qlsm1, Qlsm2, Qlsm3) where failure of older landslides yielded younger landslides. Paull and others (2005b) noted that landslide scarps are commonly associated with chemosynthetic biologic communities. Landslide blocks in the Monterey Canyon system (Qlsmb) are distinguished by positive relief and deflection of an axial channel. Landslide blocks are inferred to be bedrock, similar to bedrock found in adjacent canyon walls. One block in the distal portion of the Monterey Canyon system in the map area (Qlsmb1) has been previously studied, identified as composed of Cretaceous granodiorite, and informally named the âNavy Slumpâ (Greene and others, 2002; Paull et al., 2005a). Soquel Canyon is the most prominent of five mapped tributaries to Monterey Canyon. During the sea-level lowstand about 21,000 years ago, Soquel Creek fed directly into Soquel Canyon, carrying coarse-grained sediment directly to the Monterey Canyon system (see sheet 9). Sea-level rise isolated Soquel Canyon from its paired coastal watershed, and this "abandoned" tributary canyon is now being filled largely with Holocene hemipelagic sediment. The Soquel Canyon axial channel is divided into two sections based on lithology of the fill deposits. The abandoned submarine canyon axial channel fill (unit Qccf2) in upper Soquel canyon consists of fine-grained sediment. In lower Soquel Canyon adjacent to Monterey Canyon, the abandoned submarine canyon axial channel fill (unit Qccf1) contains gravel, sand, and mud (Stakes and others, 1999) that is possibly derived from Holocene and Pleistocene landslides, and may also contain bedrock exposures. Two other abandoned canyon tributaries (unit Qctf1) were likely connected to the Pajaro River during the sea-level lowstand. These two tributaries are mapped east of Soquel Canyon on the north flank of Monterey Canyon. One abandoned tributary (unit Qctf2) is mapped on the south flank of Monterey Canyon and appears to have been connected to the Salinas River during the sea-level lowstand. The shelf north and south of Monterey Canyon in the Monterey Canyon and Vicinity 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 outer 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 cross as many as 9 faults over a width of about 8 km (see, for example, fig. 7 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. Mapping fault strands in the MBFZ across the Monterey Canyon system is problematic. The combination of steep relief, increased water depths, and massive to poorly-stratified sediment fill generally results in poor quality of both high-resolution and deeper seismic-reflection profiles (sheet 8). High-resolution bathymetry does not reveal obvious tectonic landforms (such as fault lineaments or scarps) likely owing to deformation being distributed across a broad zone, minimal late Quaternary deformation, and active canyon processes. It is important to realize that the MBFZ almost certainly extends across the Monterey Canyon system with a map pattern similar to that seen on the continental shelves to the north and south, but this is not shown on the geologic maps (sheet 10) or structure maps (Maps A, B, and E on sheet 9) because conclusive evidence for the locations of individual MBFZ strands within the submarine canyon system is lacking. On a regional scale, the Monterey Bay and Vicinity map area lies in a northward-narrowing zone between the San Andreas Fault Zone (about 34 km east) and the San Gregorio Fault Zone (about 5 km west), two major structures in the right-lateral transform plate boundary between the North American and Pacific plates. Deformation associated with the MBFZ and other structures in the map area accommodates strain that results from location in this dynamic tectonic setting. 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. Brabb, E.E., 1997, Geologic Map of Santa Cruz County, California: A digital database, US Geological Survey Open-File Report 97â489, 1:62,500. 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. 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., 1977, Geology of the Monterey Bay region: U.S. Geological Survey Open-File Report 77â718, 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. 31â56. 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. 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. Kern, J.P., 1977, Origin and history of upper Pleistocene marine terraces, San Diego, California: Geological Society of America Bulletin, v. 88, p. 1,553â1,566. 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. 353â401. 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/. 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/. 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. 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. 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. 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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