A PRELIMINARY TECTONIC AND SEDIMENTATION MODEL OF THE SANTA MARIA BASIN, OFFSHORE CALIFORNIA

The offshore Santa Maria Basin (SMB) subsided rapidly from Oligocene into late Miocene. By late Miocene, an orthogonal component of compression initiated in response to a shift in motion between the Pacific-North American plates with strike-slip motion concentrated along dextral fault systems. Controlled by active tectonism, compressional structures formed along the Hosgri Fault zone and the thickest accumulations of Monterey, Sisquoc, and Foxen Formations formed in two depocenters. These two local depocenters in the offshore SMB migrated and evolved through the history of the basin, contributing to tremendous lateral variability in the thickness of different stratigraphic units. The southern depocenter was located west of the Hosgri Fault zone, southwest of the Point Arguello, and north of the Amberjack high. The northern and smaller depocenter formed off Point Sal.

By late Pliocene, strike-slip motion became more dominant and structures began to form when tilting, uplift and deformation of the Franciscan basement caused fold growth and compressional displacement of Tertiary-Quaternary sediments west of the Hosgri Fault zone. An offshore Point Sal high developed and is marked by an abrupt thinning of the Foxen to 500 ft or less. The northern depocenter migrated northward during the Pliocene and was filled with over 10300 ft of Monterey, Sisquoc, and Foxen Formation deposits. Simultaneously, the elongated southern depocenter trending NW-SE, was filled with over 10200 ft of these deposits.

By Quaternary time, motion on the Hosgri Fault zone decreased to accommodate the relative change of the Pacific-North American plate motion. Basin-forming transtension was replaced by transpression. Uplift and subsidence of rigid blocks controlled the vertical separation of Quaternary deposits on the Hosgri Fault. The Quaternary deposits contain phosphatic oolites and glauconite grains. Both types of authigenic minerals form in sediment starved locations on the seafloor thought to be located along uplifted blocks of the Hosgri Fault system. The northern depocenter migrated eastward and the southern depocenter has been filled. A new depocenter has formed NW of Point Arguello and contains 3000 ft of Quaternary deposits. As the Amberjack high does not show thickness variations of sediments, it is interpreted to be inactive.