--> Abstract: Sandy Turbidite Distribution Patterns and Estimated Flow Velocities in Santa Monica Basin, Southern California Continental Borderland, by P. J. Hogan, B. W. Romans, S. Varnell, and A. Broughton; #90088 (2009)

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Sandy Turbidite Distribution Patterns and Estimated Flow Velocities in Santa Monica Basin, Southern California Continental Borderland

P. J. Hogan1, B. W. Romans2, S. Varnell1, and A. Broughton3
1Fugro West, Ventura, CA, [email protected], [email protected]
2Chevron ETC, San Ramon, CA, [email protected]
3Fugro GeoConsulting, Houston, TX, [email protected]

New geotechnical and geophysical data combined with published geochronologic and high-resolution seismic-reflection data offshore Southern California allow for an improved understanding of source-to-sink sediment delivery pathways and mass movement processes in Santa Monica Basin (SMB).

Mass movement processes in the enclosed SMB are dominated by sand-rich turbidity currents, which episodically deliver large volumes of sand via Hueneme Submarine Canyon. El Niño-Southern Oscillation (ENSO)-driven turbidity currents deposit thin silt-rich turbidites in SMB. Less frequent, possibly seismically-triggered, large, sand-rich turbidity currents have occurred repeatedly in the middle to late Holocene. Measured thicknesses of individual silt to medium-grained sand turbidite units range from 1 to 3 meters on the distal basin plain.

High-resolution geophysical profiles allow basin-wide correlation of large turbidite units. Radiocarbon age dates from ODP Site 1015 provide good chronostratigraphic control (Normark et al., 2006; Romans et al., 2009). New MCPT (Mini-Cone Penetration Test) and high-resolution seismic data collected in SMB document late Holocene turbidite bed geometry.

Turbidity currents in Santa Monica Basin have increased in frequency in the late Holocene. Gorsline (1996) reported several thin (<8 cm thick) basin-wide turbidites emplaced in SMB in the last 500 years. Numerous thicker, sand-rich turbidites are present, with a recurrence interval ranging from 300 to 360 years. The most recent large turbidite was emplaced ~700 yrBP, and the preceding event occurred ~1,650 yrBP.

Grain size analyses were performed on samples from several late Quaternary turbidites in ODP 1015. Turbidity current flow velocities are estimated from new relationships relating maximum grain size in a turbidite to flow velocity (Stow, 2008). Data used to generate the relationships include: (1) observational studies, and (2) theoretical studies based on interpretation of grain size data, sedimentary structures, and flow criteria.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009