A Mass Movement Model for the Santa Maria and Santa Lucia Basins, Offshore California

Saenz, Joseph M.¹, Frank E. Denison², Thomas J. O'Neil³, Craig K. Ogawa⁴ (1) Naval Facilities Engineering Service Center, Port Hueneme, CA (2) Frank Denison Consultant, Westlake Village, CA (3) Oxnard College, Oxnard, CA (4) Minerals Management Service, Camarillo, CA

In the offshore Santa Maria and Santa Lucia Basins (SMB-SLB) portions of the continental shelf and slope margin have been unstable for the past 2 million years. This instability was caused either by rapid sediment loading during Plio-Pleistocene low sea level stand(s) or by seismic activity refracting from underlying Mesozoic and Cenozoic strata. The instability is consistent with soft-sediment deformation and small slumps in Holocene-Pleistocene strata of the offshore SMB-SLB study area. Stability of the offshore SMB-SLB continental shelf and slope margin has increased since Quaternary age due to a decrease in sedimentation rates.

We predict that sedimentation rates and seismic activity are the dominant factors for slope stability in the offshore SMB-SLB area. In the location of instability, rapid and asymmetric loading of water-saturated sediments with high permeability and porosity extends seaward where sedimentation rates are low. Coupled with a tectonic and sedimentation model and seismic events, geophysical data shows several west-trending submarine landslides that bisect a series of long northwest trending synclines and anticlines in the study area. We postulate that earthquake energy is amplified in these synclinal structures, interpreted using geophysical data, as evidenced by seismic activity (e.g.: November 4, 1927 M 7.3 Lompoc Earthquake), and fluid migration in seafloor features (gas-vent craters, pockmarks, buried channels, and seafloor rubble and hummocks). The analysis demonstrates how a relationship between sedimentation rates, fluid migration, and amplification of seismic energy within large synclinal structures all affect the distribution and size of these mass movement deposits.