FOLAND, SARA S., Farallon Energy Group Ltd., Denver, CO; NORMAN MAHER, Monterey Bay Aquarium Research Institute, Moss Landing, CA; and JANET W. YUN, Earth Sciences Department, University of California, Santa Cruz, CA

Abstract: Pockmarks Along the California Continental Margin: Implications for Fluid Flow

Pockmarks observed along the continental margin of California give new insight to processes of fluid flow. Pockmarks are circular to elongate depressions that are associated with fluid expulsion. We document the presence of pockmarks from Pt. Conception northward to the Eel River Basin. We find that pockmark size varies from giant (>250 m in diameter) to small (10 m in diameter), and morphologies vary from conical to sediment-draped and in-filled. Additionally, we observe that pockmarks may be randomly distributed, clustered, or occur along linear trends.

Offshore central California, pockmarks are imaged on the shelf and upper slope, including a 560 km² pockmark field containing about 1,500 individual depressions between water depths 900 m to 1200 m. Pockmarks range from 130 m to 260 m in diameter and are 8-12 m deep. Pockmark densities range from 1.5/ km² in the north increasing to 6/ km² in the south. The southern extent of the pockmark field is unknown but almost certainly extends beyond the southern edge of the surveyed area.

A single field of pockmarks is identified in the northwestern portion of the offshore Point Arena Basin. Individual pockmarks are in excess of 200 m in diameter. The pockmarks occur in linear trends oriented north-northwest. The pockmark trends occur over subsurface faults identified from multichannel seismic reflection surveys. These subsurface faults cut Miocene-age reflectors.

In the offshore Eel River Basin, nearly 4,000 pockmarks 10-25 m in diameter occur on the upper continental slope in an area about 2100 km{2}. Over 80 percent of the pockmarks are located between 400 to 600 m water depths. These small pockmarks are in contrast to four giant (200 m diameter) pockmarks imaged near the headscarp of a submarine slide. The observed bimodal distribution of pockmark size suggests different fluid expulsion mechanisms.

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