thumbnail

California State Waters Map Series — Offshore of Fort Ross, California

Open-File Report 2015-1211
By: , , , , , , , , , , , , , , , and 
Edited by: Samuel Y. Johnson and Susan A. Cochran
https://doi.org/10.3133/ofr20151211

Links

  • Document: Pamphlet (pdf)
  • Sheets:
    • Sheet 1 (pdf) Colored Shaded-Relief Bathymetry, Offshore of Fort Ross Map Area, California By Peter Dartnell and Rikk G. Kvitek
    • Sheet 2 (pdf) Shaded-Relief Bathymetry, Offshore of Fort Ross Map Area, California By Peter Dartnell and Rikk G. Kvitek
    • Sheet 3 (pdf) Acoustic Backscatter, Offshore of Fort Ross Map Area, California By Peter Dartnell, Mercedes D. Erdey, and Rikk G. Kvitek
    • Sheet 4 (pdf) Data Integration and Visualization, Offshore of Fort Ross Map Area, California By Peter Dartnell
    • Sheet 5 (pdf) Seafloor Character, Offshore of Fort Ross Map Area, California By Mercedes D. Erdey and Guy R. Cochrane
    • Sheet 6 (pdf) Ground-Truth Studies, Offshore of Fort Ross Map Area, California By Nadine E. Golden, Guy R. Cochrane, and Lisa M. Krigsman
    • Sheet 7 (pdf) Potential Marine Benthic Habitats, Offshore of Fort Ross Map Area, California By Bryan E. Dieter, H. Gary Greene, Charles A. Endris, Mercedes D. Erdey, and Erik N. Lowe
    • Sheet 8 (pdf) Seismic-Reflection Profiles, Offshore of Fort Ross Map Area, California by Samuel Y. Johnson, Ray W. Sliter, Stephen R. Hartwell, and John L. Chin
    • Sheet 9 (pdf) Local (Offshore of Fort Ross Map Area) and Regional (Offshore from Salt Point to Drakes Bay) Shallow-Subsurface Geology and Structure, California By Samuel Y. Johnson, Stephen R. Hartwell, Janet T. Watt, and Ray W. Sliter
    • Sheet 10 (pdf) Offshore and Onshore Geology and Geomorphology, Offshore of Fort Ross Map Area, California By Samuel Y. Johnson, Stephen R. Hartwell, and Michael W. Manson
  • Related Works:
  • Dataset: Data Catalog (html) The GIS data layers for this map are accessible from “Data Catalog—Offshore Fort Ross, California,” which is part of California State Waters Map Series Data Catalog. Each GIS data file is listed with a brief description, a small image, and links to the metadata files and the downloadable data files.
  • Metadata: Metadata (html)
  • NGMDB Index Page: National Geologic Map Database Index Page
  • Download citation as: RIS | Dublin Core

Introduction

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 Fort Ross map area is located in northern California, on the Pacific coast of Sonoma County, about 90 km north of San Francisco and 60 km south of Point Arena. The onshore part of the map area is largely undeveloped, used primarily for grazing and recreation; the small town of Jenner (population, 136), located at the mouth of the Russian River, is the largest cultural center. The coast and shoreline are rugged and scenic, characterized by rocky promontories, kelp-rich coves, and nearshore rocks and sea stacks. U.S. Highway 1 extends along the coast through the map area, crossing the Russian River and passing through Sonoma Coast State Park and Fort Ross State Historic Park.

The Offshore of Fort Ross map area is cut by the northwest-striking San Andreas Fault, the right-lateral transform boundary between the North American and Pacific plates. The fault intersects the shoreline a few kilometers south of Fort Ross at Timber Gulch, and it juxtaposes Jurassic, Cretaceous, Paleocene, and Eocene rocks of the Franciscan Complex to the northeast and Tertiary sedimentary rocks to the southwest. In this area, the San Andreas Fault has an estimated slip rate of 17 to 24 mm/yr. The devastating great 1906 California earthquake (M7.8) is thought to have nucleated on the San Andreas Fault offshore of San Francisco, about 90 km to the south, with the rupture extending northward through the Offshore of Fort Ross map area to the south flank of Cape Mendocino. Approximately 3.6 m of lateral offset occurred at Timber Gulch during this event.

The San Andreas Fault has an important influence on coastal geomorphology. The coastline in the northern part of the map area, southwest of the onshore San Andreas Fault, is characterized by steep shoreline bluffs and as many as four uplifted, relatively flat marine terraces that range in elevation from about 15 to 100 m. Northeast of the San Andreas Fault, about 12 km of coastline is marked by steep, landslide-prone cliffs that commonly are 200 to 300 m high.

The mouth of the Russian River and its estuary cut through the steep coastal topography in the southern part of the Offshore of Fort Ross map area. The Russian River drains a large watershed (3,470 km2), and it has an annual discharge of about 2 km3 (1,600,000 acre-feet) and an annual sediment load of about 900,000 metric tons. The map area is part of the Russian River littoral cell, in which the predominant longshore drift is to the south. Small pocket beaches are most common along the shoreline, but longer linear beaches are present near the mouth of the Russian River.

The seafloor in the north half of the map area is characterized by rocky outcrops of Tertiary sedimentary rocks. The rugged nearshore zone and the inner shelf area (to water depths of about 50 m) typically slopes gently seaward, whereas the smooth midshelf area within California’s State Waters (about 50 to 85 m deep) is relatively flat. In contrast, the nearshore to midshelf area in the south half of the map area, which lies directly offshore of the mouth of the Russian River, has a more uniform, relatively flat slope. Shallow-marine and shelf sediments were deposited in the last about 21,000 years during the sea-level rise that followed the Last Glacial Maximum (LGM). Sea level was about 125 m lower than present during the LGM, at which time the entire Offshore of Fort Ross map area was emergent and the shoreline was about 20 km west of its present location.

Circulation over the continental shelf in the map area (and in the broader northern California region) 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.

Throughout the year, this part of the northern California coast is exposed to four wave climate regimes: the north Pacific swell, the southern swell, northwest wind waves, and local wind waves. The north Pacific swell dominates in winter months (typically November through March). During summer months, the largest waves come from the southern swell, generated by storms in the south Pacific and offshore of Central America. Northwest wind waves affect the coast throughout the year, whereas local wind waves are most common from October to April.

Potential marine benthic habitat types in the Offshore of Fort Ross map area include unconsolidated continental-shelf sediments, mixed continental-shelf substrate, and hard continental-shelf substrate. Rocky shelf outcrops and rubble are considered the primary habitat type for rockfish and lingcod, both of which are recreationally and commercially important species.

Suggested Citation

Johnson, S.Y., Dartnell, P., Golden, N.E., Hartwell, S.R., Erdey, M.D., Greene, H.G., Cochrane, G.R., Kvitek, R.G., Manson, M.W., Endris, C.A., Dieter, B.E., Watt, J.T., Krigsman, L.M., Sliter, R.W., Lowe, E.N., and Chin, J.L. (S.Y. Johnson and S.A. Cochran, eds.), 2015, California State Waters Map Series — Offshore of Fort Ross, California: U.S. Geological Survey Open-File Report 2015–1211, pamphlet 37 p., 10 sheets, scale 1:24,000, http://dx.doi.org/10.3133/ofr20151211.

ISSN: 2331-1258 (online)

Study Area

Table of Contents

  • Chapter 1. Introduction
  • Chapter 2. Bathymetry and Backscatter-Intensity Maps of the Offshore of Fort Ross Map Area
  • Chapter 3. Data Integration and Visualization for the Offshore of Fort Ross Map Area
  • Chapter 4. Seafloor-Character Map of the Offshore of Fort Ross Map Area
  • Chapter 5. Ground-Truth Studies for the Offshore of Fort Ross Map Area
  • Chapter 6. Potential Marine Benthic Habitats of the Offshore of Fort Ross Map Area
  • Chapter 7. Subsurface Geology and Structure of the Offshore of Fort Ross Map Area and the Salt Point to Drakes Bay Region
  • Chapter 8. Geologic and Geomorphic Map of the Offshore of Fort Ross Map Area
Publication type Report
Publication Subtype USGS Numbered Series
Title California State Waters Map Series — Offshore of Fort Ross, California
Series title Open-File Report
Series number 2015-1211
DOI 10.3133/ofr20151211
Year Published 2015
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Pacific Coastal and Marine Science Center
Description Pamphlet: iv, 37 p.; 10 Sheets: 47.0 x 36.0 inches or smaller; Data Catalog; Metadata
Country United States
State California
Other Geospatial Fort Ross
Scale 24000
Online Only (Y/N) Y
Additional Online Files (Y/N) Y
Google Analytic Metrics Metrics page
Additional publication details