Geology and geomorphology--Offshore of Point Conception Map Area, California

Dates

Publication Date: 2017
Start Date: 2008
End Date: 2010

Citation

Golden, N.E., 2013, California State Waters Map Series Data Catalog: U.S. Geological Survey Data Series, https://pubs.usgs.gov/ds/781/.

Summary

This part of DS 781 presents data for the geologic and geomorphic map of the Offshore of Point Conception Map Area, California. The vector data file is included in "Geology_OffshorePointConception.zip," which is accessible from https://doi.org/10.5066/F7QN64XQ. Marine geology and geomorphology were mapped in the Offshore of Point Conception map area from the shoreline to the 3-nautical-mile limit of California's State Waters. The location of the shoreline, whi ch is from the National Oceanic and Atmospheric Administration's (NOAA's) Shoreline Data Explorer, is based on their analysis of lidar data (National Geodetic Survey, 2016). Offshore geologic units were delineated on the basis of integrated analyses of adjacent onshore geology [...]

Summary

This part of DS 781 presents data for the geologic and geomorphic map of the Offshore of Point Conception Map Area, California. The vector data file is included in "Geology_OffshorePointConception.zip," which is accessible from https://doi.org/10.5066/F7QN64XQ.

Marine geology and geomorphology were mapped in the Offshore of Point Conception map area from the shoreline to the 3-nautical-mile limit of California's State Waters. The location of the shoreline, whi ch is from the National Oceanic and Atmospheric Administration's (NOAA's) Shoreline Data Explorer, is based on their analysis of lidar data (National Geodetic Survey, 2016). Offshore geologic units were delineated on the basis of integrated analyses of adjacent onshore geology with multibeam bathymetry and backscatter imagery, seafloor-sediment and rock samples (Reid and others, 2006), digital camera and video imagery (Golden and Cochrane, 2013), and high-resolution seismic-reflection profiles. Bathymetric lidar and aerial photographs taken in multiple years were used to map the nearshore area (0 to 10 m water depth) and to link the offshore and onshore geology. The onshore geology was compiled from Dibblee (1988a, b); some Quaternary units were modified by C.W. Davenport on the basis of analysis of 2009 lidar imagery. Unit ages reflect local stratigraphic relations.

The offshore portion of the Point Conception map area largely consists of a gently offshore-dipping shelf, underlain by Neogene bedrock and Holocene sediments derived primarily from relatively small coastal watersheds draining the Santa Ynez Mountains. The dip of the shelf gradually changes from about 1.1 degrees south and east of Point Conception, to about 0.7 on the broader shelf west of Point Conception. Holocene nearshore and shelf deposits are primarily sand (unit Qms). There is a distinct field (about 1.15 km2) of northeast-striking (~60 degree azimuth) low-relief sand ridges (unit Qmsr) on the mid-shelf about 1.5 to 3.0 km south of Point Conception at water depths of 65 to 80 m; the crests of the sand ridges are as high as 60 cm and as long as 750 m long, and are spaced at intervals of about 125 to 200 m. More coarse-grained deposits (units Qmsc, Qmsd) are mapped on the basis of high backscatter and in some cases moderate seafloor relief. Unit Qmsc, ranging from coarse sand and gravel to possibly boulders, is found as (a) a southwest-striking lense at depths of 15 to 40 m, about 650 m south of Point Conception, and (b) along the margins of seafloor bedrock outcrops at depths of about 30 to 50 m between Point Conception and Point Arguello. Unit Qmsd, inferred to consist of coarse sand and possibly gravel, is found as single depressions or in groups of depressions interspersed with elevated shelf sediments (unit Qms), adjacent to seafloor bedrock outcrops at water depths of about 15 to 40 m. Areas in which unit Qmsd is found are not likely to change substantially, but the boundaries of unit(s) and locations of individual depressions (and intervening flat sheets) likely are ephemeral, changing during significant storm events. More fine-grained sediments - very fine sand, silt and clay (unit Qmsf), are mapped between depths of about 80 m and the shelf break, which is present only in the southwestern-most part of the mapped area at a depth of about 100 m. The boundary between units Qms and Qmsf is based on observations and extrapolation from sparse sediment sampling (for example, Reid and others, 2006) and camera ground-truth surveying (Golden and Cochrane, 2013). The unit Qms-Qmsf boundary should be considered transitional and approximate, and expected to shift as a result of seasonal- to annual- to decadal-scale cycles in wave climate, sediment supply, and sediment transport. Fine-grained deposits similar to unit Qmsf also occur below the shelfbreak on the upper slope at water depths greater than 100 m, where they are broken out as a separate unit (unit Qmsl) based on their location and geomorphology. The shelfbreak and upper slope are incised by a 600-m-wide and 20- to 30-m-deep south-facing trough, probably filled with sand and mud (unit Qmcr), that represents one of the five heads of Arguello submarine canyon (Greene et al., 1991). Bedrock exposures along the shoreline both north and east of Point Conception consist primarily of marine shale, claystone, and diatomite of the Pliocene and upper Miocene Sisquoc Formation (unit Tsq). Adjacent Tsq seafloor outrops commonly have a ribbed appearance, reflecting differential erosion of variably resistant interbeds. Unit Tsq overlies the Miocene Monterey Formation (Tm), which crops out on the seafloor in the nearshore on the west flank of Government Point, and in nearshore to inner shelf outcrops in the northwest part of the map area. Because of the relative lack of sampling and distinctive seafloor morphology, some seafloor bedrock outcrops can't be confidently assigned to either unit Tm or unit Tsq, and these areas are mapped as undivided Tertiary bedrock (unit Tbu).

Neogene strata are deformed in a fold belt that, on a regional scale, is located within the large, west-striking, south-dipping homocline that extends from the south flank of the Santa Ynez Mountains into the offshore to the south and west. Nearby regional cross sections (Redin et al., 2005) and industry seismic-reflection profiles (sheet 7, fig. 10; Sorlien and Nicholson, 2015) show that the homocline has formed above the blind Pitas Point-North Channel fault system, which extends westward in the offshore for more than 100 km from Pitas Point (10 km northwest of Ventura) to beyond Point Conception. South of Point Conception, the tip of the fault system is inferred to be buried beneath the slope to a depth of about 2 to 3 km below sea level, about 5 to 6 km offshore. In the offshore part of the map area, closely-spaced seismic-reflection profiles image many shallow, west-northwest striking folds that have variable geometry, length, amplitude, continuity, and wavelength. The Government Point syncline is the most prominent fold; the axis of this structure has been mapped onshore at Point Conception (Dibblee, 1988a) and extends into the offshore both to the west and east with a cumulative length of more than 22 km. Local thickening of post-LGM sediment along the axis of the Government Point syncline could reflect syntectonic sedimentation. Onland at Point Conception, Rockwell et al. (1992) also document active folding of late Pleistocene onland terraces in the Government Point syncline. This regionally extensive syncline and many shorter, west-trending folds and faults are probably rooted in blind thrusts and backthrusts in the hanging wall above the Pitas Point-North Channel fault system.

The Santa Barbara Channel is a mature hydrocarbon province (Kunitomi et al., 1998), well known for hydrocarbon-induced seafloor topography (Keller et al., 2007). Shelf and upper slope geomorphic features associated with petroleum seepage are prevalent near Point Conception (Draut et al., 2009; Lorenson et al., 2009, 2014;), and include fields of dense pockmarks (unit Qmp) and asphalt mounds (unit Qas). In the Offshore of Point Conception map area, pockmarks are predominantly grouped or, less commonly, solitary; are circular to elliptical; range in size from 50 to 150 m along their long axis; typically are 20 to 40 cm deep; and commonly have a central cone as much as 150 cm high. Grouped pockmarks cover about 4.2 km of the offshore map area. The largest mapped asphalt mounds cover 675,856 m and 168,950 m, and have as much as about 12 m of relief above the seafloor. Draut et al. (2009) noted that such large mounds likely consist of a mix of tar and sediment, and Lorenson et al. (2014) calculated areas and volumes of asphalt in the Point Conception and larger Santa Barbara Channel area. The asphalt mounds in the Offshore of Point Conception map area are mainly found on the south flank of the Point Conception anticline and cover about 2.6 square km.

Pipelines through State Waters that are associated with petroleum platforms (located farther offshore in federal waters) are mapped as unit af. The remains of two large (about 75-m and 60-m long) ships, mapped as unit ass, are located about 550 m northeast of Government Point at 10 m water depth, and about 5.8 km east of Government Point at a depth of 22 m.

References Cited:

Dibblee, T.W., 1988a, Geologic Map of the Lompoc Hills and Point Conception Quadrangles, Santa Barbara County, California, edited by H.E. Ehrenspeck (1988) and J.A. Minch (2009): Dibblee Geological Foundation Map DF-18, scale 1:24,000.

Dibblee, T.W., 1988b, Geologic Map of the Point Arguello and Tranquillon Mountain Quadrangles, Santa Barbara County, California, edited by H.E. Ehrenspeck (1988) and J.A. Minch (2009): Dibblee Geological Foundation Map DF-19, scale 1:24,000.

Draut, A.E., Hart, P.E., Lorenson, T.D., Ryan, H.F., Wong, F.L., Sliter, R.W., and Conrad, J.E., 2009, Late Pleistocene to Holocene sedimentation and hydrocarbon seeps on the continental shelf of a steep, tectonically active margin, southern California, USA: Marine Geophysical Research, p. 193 206, doi:10.1007/s11001-009-9076-y.

Golden, Nadine E., and Cochrane, Guy R., 2013, California Seafloor Mapping Program video and photograph portal. U.S. Geological Survey data set, http://dx.doi.org/10.5066/F7J1015K

Greene, H.G., Clarke, S.H., Jr., and Kennedy, M.P., 1991, Tectonic evolution of submarine canyons along the California continental margin, in Osborne, R.H., ed., From shoreline to abyss: Society for Sedimentary Geology Special Publication 46, p. 231-248.

Keller, E.A., Duffy, M., Kennett, J.P., and Hill, T., 2007, Tectonic geomorphology and hydrocarbon-induced topography of the mid-Channel anticline, Santa Barbara basin, California: Geomorphology, v. 89, p. 274 286.

Kunitomi, D.S., Hopps, T.E., and Galloway, J.M., eds., 1998, StrucTbure and petroleum geology, Santa Barbara Channel, California: American Association of Petroleum Geologists, Pacific Section and Coast Geological Society, Miscellaneous Publication 46, 328 p.

Lorenson, T.D, Hostettler, F.D., Rosenbauer, R.J., Peters, K.E., Kvenvolden, K.A., Dougherty, J.A., Gutmacher, C.E., Wong, F.L., and Normark, W.R., 2009, NaTbural offshore seepage and related tarball accumulation on the California coastline, Santa Barbara Channel and the southern Santa Maria Basin source identification and inventory: U.S. Geological Survey Open-File Report 2009-1225 and MMS report 2009-030, 116 p. and spreadsheets, http://pubs.usgs.gov/of/2009/1225/.

Lorenson, T.L., Wong, F.L., Dartnell, P., and Sliter, R.W., 2014, Greenhouse gases generated from the anaerobic biodegradation of naTbural offshore asphalt seepages in southern California: Geo-Marine Letters, v. 34, p. 281 295, doi: 10.1007/s00367-014-0359-1/.

National Geodetic Survey, 2016, NOAA Shoreline Data Explorer: National Oceanic and Atmospheric Administration, National Geodetic Survey data viewer and database, accessed May 1, 2016, at http://www.ngs.noaa.gov/NSDE/.

Redin, T., Forman, J., Kamerling, M., and Galloway, J., 2005, Santa Barbara Channel strucTbure and correlation sections CQmp2 to CS-42: American Association of Petroleum Geologists, Pacific Section, Publication CS 36, 12 sheets.

Reid, J.A., Reid, J.M., Jenkins, C.J., Zimmerman, M., Williams, S.J., and Field, M.E., 2006, usSEABED Pacific Coast (California, Oregon, Washington) offshore surficial-sediment data release: U.S. Geological Survey Data Series 182, available at http://pubs.usgs.gov/ds/2006/182/.

Rockwell, T. K., Nolan, J., Johnson, D.L., and Patterson, R.H., 1992, Ages and deformation of marine terraces between Point Conception and Point Conception, Western Transverse Ranges, California, in Wehmiller, J.F. and Fletcher, C. (eds.), Quaternary Coasts of the United States Marine and Lacustrine Systems: Society of Economic Paleontologists And Mineralogists, p. 333-341.

Sorlien, C.C., and Nicholson, C., 2015, Post 1-Ma deformation history of the Pitas Point-North Channel-Red Mountain fault system and associated folds in Santa Barbara channel, California: USGS NEHRP final report, Award G14AP00012, 24 p.

Contacts

Point of Contact :: U.S. Geological Survey, Pacific Coastal and Marine Science Center
Originator :: S.R. Hartwell, S.Y. Johnson, C.W. Davenport
Metadata Contact :: U.S. Geological Survey, Pacific Coastal and Marine Science Center
Distributor :: U.S. Geological Survey - ScienceBase

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Geology of offshore of Point Conception map area	https://www.sciencebase.gov/catalog/file/get/58a354e5e4b0c82512869c36?name=Geology_OffshorePointConception.jpg&allowOpen=true

Type: Publication that references this resource

Johnson and others, 2018, California State Waters Map Series--Offshore of Point Conception, California: U.S. Geological Survey Open-File Report	https://doi.org/10.3133/ofr20181024

Purpose

To expand geologic mapping to the seafloor within California's State Waters, to update coastal geologic mapping, and to contribute to a uniform regional geologic database. Additionally, to provide a geologic map for the public and geoscience community to aid in assessments and mitigation of geologic hazards in the Santa Barbara coastal region and to provide sufficient geologic information for land-use and land-management decisions both onshore and offshore.

Geology and geomorphology--Offshore of Point Conception Map Area, California

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