GEOLOGIC MAPPING IN THE CENTRAL CALIFORNIA COASTAL ZONE: INTEGRATING GEOLOGY, GEOPHYSICS, AND GEOMORPHOLOGY

Integrated onshore-offshore geologic maps are being used by scientists, resource managers, and private industry for assessment of regional tectonics, marine and water resources, and seismic hazards in the central California coastal zone. We have combined existing geologic and geophysical data with recently acquired (2006 to 2009) high-resolution seismic-reflection, marine magnetic, aeromagnetic, gravity, and multibeam bathymetry data to produce a preliminary seamless onshore-offshore geologic map of the area between Cayucos and Pismo Beach, extending offshore across the continental shelf to the upper slope. The map area lies within the Los Osos/Santa Maria structural domain, part of the broader San Andreas transform margin. This area is characterized by a series of northwest-trending, fault-bounded, basins and ranges that extend offshore where they are truncated by the northwest-striking Hosgri Fault.

Gravity and magnetic data allow seamless onshore to offshore mapping of magnetic serpentinite and ultramafic rocks within the Franciscan Complex and the Coast Range Ophiolite. Nonmagnetic Neogene sedimentary units can similarly be traced into the offshore based on their distinctive bedding and geophysical character. Offshore Holocene sedimentary units (as thick as 40 m) are defined based on seismic-reflection, geomorphology, and sampling data from the usSEABED bottom sampling project, include shelf sands, shelf and slope muds, and coarse sand to gravel found in ridge scour depressions. We combine seismic-reflection, gravity, and magnetic data to constrain the location, geometry, and connectivity of active faults, including the Hosgri, Los Osos, and Pecho faults. In addition, multibeam bathymetry data provide very detailed morphologic and textural information that can be used to assess the continuity of seafloor faults and features identified in the subsurface using geophysical data.