VISUALIZING THE MARINE GEOLOGY OFF SOUTHERN CALIFORNIA

State-of-the-art seafloor imaging data and visualization software now allow the marine geologist to literally fly through datasets to better grasp the geomorphology of what is typically difficult to conceptualize. High-resolution multibeam (MBES) bathymetry and acoustic backscatter data from the southern California margin, collected with three different multibeam systems in three different years, have all been fused and visualized as one complex image. The multibeam data have a spatial resolutions that vary from 0.5 m to 10 m and vertical resolutions that vary from about 5 cm to a few decimeters.

The first dataset is of 30-kHz MBES bathymetry and acoustic backscatter, the second is of 95-kHz MBES data, and the third is of 300-kHz data. Together, these data show the relief of the seafloor in unprecedented detail and allows views that enhance the interpretations of processes that have formed and acted on the seafloor.

The bathymetry of Santa Monica Bay reveals a broad shelf plateau that projects seaward from an otherwise narrow shelf off the Malibu coast. The plateau is bounded by two large, simple, submarine canyons, one of which incises the present coast, and both have ample evidence of failures along their flanks. The margin of the narrow shelf is incised with numerous gullies and small canyons, in stark contrast to the large canyons bordering the plateau. Outcrops of high-backscatter material is confined to the submarine canyon walls and the center of the large plateau.

The bathymetry of San Pedro Bay is similar to the bathymetry of Santa Monica Bay in that it is composed of a broad shelf that necks down to a very narrow shelf to the south. Several large and very complex submarine canyons incise the margin of the broad shelf, one of which heads at the beach. Outcrops of high-backscatter material occur on the mid- and inner-shelf regions.

The bathymetry of the intervening area between the two bays (the Palos Verdes margin) is composed a narrow shelf with a margin that is highly dissected by large-scale failures, one of which can be followed to the related debris avalanche on the basin floor. The enhanced views of the seafloor are only the base map; the third dimension is lacking in these datasets. A thorough analysis of the areas requires precisely navigated seismic-reflection profiles and intense ground truth with samples and video/stills.