EITTREIM, STEPHEN L., ROBERTO J. ANIMA and ANDREW J. STEVENSON
U.S. Geological Survey, Menlo Park, CA 94025

Abstract: Seafloor Geology on the Monterey Bay Continental Shelf

A new seafloor geologic map, based on four years of acoustic swath-mapping surveys, reveals that the outer and inner shelves of Monterey Bay have rock outcrops, while the midshelf region is sediment covered. Along the tectonically uplifting coasts around Santa Cruz, the inner-shelf seafloor consists of eroded outcrops, whereas along those coasts that are subsiding or stable, such as in central Monterey Bay, the inner shelf seafloor consists only of recent sediment. Rock outcrops on the outer shelves at 90m depth or greater are in places covered with a thin veneer of recent sediment that attenuates the high acoustic reflectivity of the rock surfaces. Around the Monterey Peninsula, granitic rocks, with distinctive acoustic signature, crop out on the shelf. The sedimentary formations exposed on the northern and southern shelves, from oldest to youngest the Monterey, Santa Cruz Mudstone and Purisima Formations, respectively, can often be recognized by their acoustic signatures. The Purisima Formation, a thick bedded variable siltstone, constitutes the vast majority of outcropping bedrock on the outer shelves.

Coarse sand bodies are observed mostly in the form of 1-m deep troughs whose coarse sand floors are shaped into 1 -m length waves mobilized by winter-storm surface waves. These coarse sand bodies mostly occur on the inner shelf from 20-30-m deep, although the deepest occurs at 56 m off the southeast Monterey Bay coast in the former Fort Ord area, where the largest winter storm waves focus their energy. Areas of tectonically disturbed sediment and carbonate encrusted sediment also occur on the Monterey Shelf and produce heightened seafloor reflectivity.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California