New Discoveries Revealed by Seafloor Mapping on the Monterey Bay Continental Shelf

Abstract

In order to understand the links between the seafloor geology and biological habitats of Monterey Bay National Marine Sanctuary, the US Geological Survey has acoustically swath-mapped the Monterey Bay Shelf from the nearshore out to 150 m depth. We have mapped bedrock outcrops, coarse sand deposits, lineations related to the tectonic effects of active faults, and hard-grounds, perhaps related to seepage of sub-bottom waters and the resultant chemical precipitations on the seafloor. Most of the inner-shelf bedrock exposures can be correlated to exposures that occur in the coastal cliffs and roadcuts from northwest of Santa Cruz along route 1 to south of Monterey. These seafloor outcrops are erosional remnants that rise above the modern sediments of the continental shelf and must have stood well above the surrounding coastal plain during the time of lowered sealevel 12,000 years ago. A large percentage of the outer-shelf seafloor consists of outcrops of the Purisima Formation, an upper Miocene to Pliocene sedimentary sequence exposed along seacliffs and uplifted marine terraces above Santa Cruz and Watsonville. Large patches of medium to coarse sands are common on the inner to mid shelf. These sands may be the remnants of a 10,000-yr-old transgressive lag deposit, seen in windows through the thin surficial modern sediment. The medium to coarse sands are found within flat-floored 1-m-deep troughs in the form of large 1-m wavelength sand waves, that are kept mobilized seasonally by the large winter storm waves. This seasonal mobilization is believed to inhibit the deposition of finer sediments. Granitic rocks that make up the Monterey Peninsula are also seen as acoustically-distinctive outcrops on the continental shelf around the Peninsula out to depths of from 80 to 100 meters.

This abstract is from Monterey Bay National Marine Sanctuary Symposium, Sanctuary Currents '99, Climate Change and the Sanctuary, Research Symposium, 1999.