Open-File Report 2015–1114
In 2007, the California Ocean Protection Council initiated the California Seafloor Mapping Program (CSMP), designed to create a comprehensive seafloor map of high-resolution bathymetry, marine benthic habitats, and geology within the 3-nautical-mile limit of California’s State Waters. The CSMP approach is to create highly detailed seafloor maps through collection, integration, interpretation, and visualization of swath sonar data, acoustic backscatter, seafloor video, seafloor photography, high-resolution seismic-reflection profiles, and bottom-sediment sampling data. The map products display seafloor morphology and character, identify potential marine benthic habitats, and illustrate both the surficial seafloor geology and shallow subsurface geology.
The Offshore of Point Reyes map area is located in northern California, about 40 km north of San Francisco and about 50 km south of Fort Ross. The map area is in the northern part of the Gulf of the Farallones National Marine Sanctuary, and it includes all or parts of three California Marine Protected Areas. The largely undeveloped onshore area, which is part of the Point Reyes National Seashore, occupies much of the southwestern part of the triangular Point Reyes peninsula. The coast and shoreline are scenic and diverse, ranging from the rolling hills and secluded sandy beaches that surround Drakes Bay and the estuaries of Drakes Estero and Estero de Limantour, to the steep, rocky granitic promontories at Point Reyes headland, up to the long, exposed windswept Point Reyes Beach. Upland areas are used primarily for grazing and recreational hiking. Several decades of oyster farming in Drakes Estero ceased in 2014.
The map area occupies a dynamic tectonic setting. The northwest-striking San Andreas Fault Zone, which lies just 3 km northeast of the map area, forms the northeast boundary of the Point Reyes peninsula. The east-west-striking Point Reyes Fault Zone extends across the continental shelf in the southern part of the map area. Tectonic influences that impact the shelf morphology and geology in the map area are related to these faults and associated folding, uplift, and subsidence. Offshore of the Point Reyes headland, granitic basement rocks are offset vertically about 1.4 km along the Point Reyes Fault Zone; this uplift, combined with west-side-up offset on the San Andreas Fault, has resulted in uplift of the Point Reyes peninsula and the adjacent shelf. Deformation associated with north-side-up motion across the Point Reyes Fault Zone has resulted in a distinct bathymetric gradient across the Point Reyes Fault Zone: an emergent or shallow bedrock platform is present to the north and east of the fault zone, and a deeper, submerged bedrock platform lies to the south. Cumulative post-Miocene slip rate on the Point Reyes Fault Zone in the map area is poorly constrained, but it is estimated to be 0.3 mm/yr on the basis of vertical offset of granitic basement rocks.
The seafloor in the map area generally extends from the shoreline to water depths of about 40 to 60 m, except for the area south of the Point Reyes headland where water depths reach 60 to 70 m. This difference is the result of a distinct bathymetric gradient south and west of the Point Reyes headland, which is related to north-side-up motion along the Point Reyes Fault Zone. The bedrock platform in the nearshore and inner shelf to midshelf areas (50 to 60 m depth) north of the Point Reyes headland is relatively flat (less than 0.8°) and is overlain by sand-sized to coarser grained sediment. Finer grained sediments are found in water depths greater than 60 m south of the Point Reyes headland and also extend into shallower (less than 40 m) water within Drakes Bay. Surficial and shallow sediments were deposited in the last about 21,000 years during the approximately 125-m sea-level rise that followed the last major lowstand associated with the Last Glacial Maximum, at which time the entire Offshore of Point Reyes map area was emergent and the shoreline was about 30 km south and west of the present-day shoreline.
Circulation over the continental shelf in the map area is dominated by the southward-flowing California Current, the eastern limb of the North Pacific Gyre. Associated upwelling brings cool, nutrient-rich waters to the surface, resulting in high biological productivity. The current flow generally is southeastward during the spring and summer; however, during the fall and winter, the otherwise persistent northwest winds are sometimes weak or absent, causing the California Current to move farther offshore and the Davidson Current, a weaker, northward-flowing countercurrent, to become active.
Sediment transport in the map area largely is controlled by surface waves and tidal currents in the nearshore and, at depths greater than 20 to 30 m, by tidal and subtidal currents. In the map area, nearshore littoral drift of sand and coarse sediment is to the south, owing to the dominant west-northwest swell direction. Scour from large waves and tidal currents removes and redistributes sediment over large areas of the inner shelf. Farther offshore, bottom currents generally flow to the northwest, distributing finer grained sediment accordingly.
Seafloor habitats in the Offshore of Point Reyes map area range from unconsolidated continental-shelf sediment to hard substrate, and they also include an area of sediment-covered hard mounds that may be marine debris. Rocky-shelf outcrops and rubble are considered to be promising potential habitats for rockfish and lingcod, both of which are recreationally and commercially important species. Dynamic bedforms, such as the sand waves in northwestern Drakes Bay, are considered to be potential foraging habitat for juvenile lingcod and possibly migratory fishes, as well as for forage fish such as Pacific sand lance.
Chapters in the Pamphlet
Chapter 1. Introduction By Janet T. Watt
Chapter 2. Bathymetry and Backscatter-Intensity Maps of the Offshore of Point Reyes Map Area (Sheets 1, 2, and 3) By Peter Dartnell and Rikk G. Kvitek
Chapter 3. Data Integration and Visualization for the Offshore of Point Reyes Map Area (Sheet 4) By Peter Dartnell
Chapter 4. Seafloor-Character Map of the Offshore of Point Reyes Map Area (Sheet 5) By Mercedes D. Erdey and Guy R. Cochrane
Chapter 5. Ground-Truth Studies for the Offshore of Point Reyes Map Area (Sheet 6) By Nadine E. Golden and Guy R. Cochrane
Chapter 6. Potential Marine Benthic Habitats of the Offshore of Point Reyes Map Area (Sheet 7) By H. Gary Greene, Charles A. Endris, and Bryan E. Dieter
Chapter 7. Subsurface Geology and Structure of the Offshore of Point Reyes Map Area and the Salt Point to Bolinas Region (Sheets 8 and 9) By Janet T. Watt, Samuel Y. Johnson, and Stephen R. Hartwell
Chapter 8. Geologic and Geomorphic Map of the Offshore of Point Reyes Map Area (Sheet 10) By Janet T. Watt, Michael W. Manson, and H. Gary Greene
Also of Interest
Open-File Report 2015–1068, California State Waters Map Series—Offshore of San Francisco, California, by Guy R. Cochrane and others.
Open-File Report 2015–1041, California State Waters Map Series—Drakes Bay and Vicinity, California, by Janet T. Watt and others.
Open-File Report 2015–1088, California State Waters Map Series—Offshore of Tomales Point, California, by Samuel Y. Johnson and others.
Open-File Report 2015–1098, California State Waters Map Series—Offshore of Salt Point, California, by Samuel Y. Johnson and others.
First posted June 29, 2015
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Watt, J.T., Dartnell, P., Golden, N.E., Greene, H.G., Erdey, M.D., Cochrane, G.R., Johnson, S.Y., Hartwell, S.R., Kvitek, R.G., Manson, M.W., Endris, C.A., Dieter, B.E., Sliter, R.W., Krigsman, L.M., Lowe, E.N., and Chin, J.L. (J.T. Watt and S.A. Cochran, eds.), 2015, California State Waters Map Series—Offshore of Point Reyes, California: U.S. Geological Survey Open-File Report 2015–1114, pamphlet 39 p., 10 sheets, scale 1:24,000, https://dx.doi.org/10.3133/ofr20151114.
ISSN 2331-1258 (online)