--> Abstract: The Geology of Offshore Point Arena Basin, Northern California, by S. S. Foland; #90935 (1998).

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Abstract: The Geology of Offshore Point Arena Basin, Northern California

FOLAND, SARA S., Amoco Production Company, Denver, CO and Dept. Earth and Marine Sciences, University of California, Santa Cruz, CA

The offshore Point Arena basin lies 250 km north of San Francisco, California and covers 8500 sq km on the outer continental shelf. It is located on the Pacific Plate immediately south of the Mendocino triple junction. The southern basin edge is upturned and crops out west of the San Andreas fault system in the Gualala area. Three offshore wells drililed in the mid-1960's penetrate the complete basinal section of Paleogene and Neogene rocks.

The Point Arena basin has a complex tectonic history. Two distinct phases of basin development are recorded in the offshore sedimentary section. The Paleogene section was deposited in a large forearc basin that extended along the entire coast of North America prior to 29 Mya. A 20 my erosional hiatus separates middle Eocene forearc sediments from the overlying Neogene section. The Iversen Basalt (23.8 my-age) represents the convergence of the Farallon-Pacific spreading ridge with the Farallon- North America subduction zone and the change from convergent to transform plate boundary. The Neogene section was deposited in this transform setting following Mendocino triple junction formation and initiation of the ancestral San Andreas fault system. Neogene rocks contain a shallowing-upward sequence that includes the Miocene, Monterey-equivalent Point Arena formation.

The Point Arena basin contains a major, northwest-trending, seismically-defined structural boundary. Rocks on either side of this structural boundary display distinct seismic stratigraphic character. Potential field data and offshore well samples indicate that the basin is floored by two distinct basement types (Mesozoic Salinian granites and Jurassic Franciscan metasediments). The boundary between the two basement rock types is parallel to the seismic structural anomaly. The juxtaposition of these two basement types supports the two-phase tectonic history interpreted from the overlying sedimentary section.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California