An Extensive Hydrocarbon Flow on the Western Santa Barbara Shelf, California. Greene, H. G., Eichhubl, P., Maher, N. Moss Landing Marine Laboratories, 7282 Moss Landing Road, Moss Landing, CA 95039 United States An extensive hydrocarbon flow has been imaged and sampled along the western Santa Barbara shelf. Recently processed EM300 bathymetric data collected by the Monterey Bay Aquarium Research Institute (MBARI) along with in situ observations and seafloor sampling using MBARI'S ROV Ventana indicate that tar or asphaltum flowed 2.8 to 4.6 km from near the present shoreline across the mainland shelf and in places to the break in the shelf (100 m deep). The mapped part of this flow covers an area of over 7.9 km2, and probably covers twice that area. It is exposed along the eastern wall of a submarine canyon located along the western boundary of the flow. On the upper shelf, just below where the apparent terminus of the flow is located at the shelf break, distinct flow banding can be seen indicating that further down slope flow of hydrocarbons may have occurred here. The surface of the flow is covered with sediment, but a rill and lobate texture can be distinguished in the bathymetry. Many tar volcanoes exist on the northwestern part of the flow and some of the largest measure 40 m across and 2.8 m high. The western Santa Barbara hydrocarbon flow represents extensive surface venting of a subsurface petroleum reservoir. It is uncertain whether this event occurred during the Pleistocene or Holocene time. However, the presence of fresh-looking tar-patties on the surfaces of many of the tar volcanoes suggests that small-scale fluid flow may still be occurring. This feature has a distinct morphology and exhibits channels and canyons whose heads appear to represent eruptive flow centers.