STRATIGRAPHIC EVOLUTION OF THE SANTA MARIA BASIN, OFFSHORE CALIFORNIA

Interpretive stratigraphic columns based on detailed descriptions of 73 well logs were used to understand the relative effects of active tectonics on Mesozoic to Cenozoic stratigraphy in the Santa Maria Basin (SMB), offshore California. The study confirms that the Hosgri-Purisima-Lompoc Fault system plays a major role in the regional Cenozoic tectonics and sedimentations. A working model is that the offshore SMB initially formed as an Oligocene forearc basin that evolved into a rift basin during early-middle Miocene, with basin-bounding faults reactivated by Pliocene-Quaternary transpression.

Using selected well data, 73 stratigraphic columns were constructed from mud and electric logs, lithologic summaries, and paleontological information. Mud logs provided detailed information for lithologic analyses. Formation boundaries were determined by correlating electric and mud logs, and micropaleontological reports. Micropaleontologic interpretations were based upon nanofossil zonations for 50 wells. Summaries included geologic ages, lithologic subdivisions, depositional environments, and microfossil determinations. Stratigraphic columns were integrated into a tectonic and sedimentation model that was constructed into isochore thickness maps for the Monterey, Sisquoc, and Foxen Formations and Quaternary deposits. Isochore thickness and fault maps were used to assess the activity on structural highs and lows, and to locate faults along linear trends of tightly-spaced contours.

Results of this investigation show that the offshore SMB subsided rapidly from early 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 system and the thickest accumulations of Monterey, Sisquoc, and Foxen Formations formed in two depocenters. The two depocenters migrated and evolved through the history of the basin, contributing to significant lateral variability in the thickness of stratigraphic units. The southern depocenter was located west of the Hosgri Fault zone, and southwest of Point Arguello. The northern depocenter formed off Point Sal.