Abstract: Buried Channel System of South San Francisco Bay, Clues to Late Holcene Sedimentary Processes and Environmental Fluctuations

Paul R. Carlson, Roberto J. Anima, Gary M. Mann, Michael S. Marlow, Lynn Ingram

High-resolution acoustic profiles collected in 1992-94 reveal a complex series of incised and filled channels beneath south San Francisco Bay. The channel system extends southeast from near Hunters Point for about 5 km along the northeast side of San Bruno Shoal. The channels parallel the trend of the regional strike of Franciscan rocks found on the adjacent peninsula and also the trend of a regional aeromagnetic anomaly, suggesting deep-seated structural influence. The channel width varies from 225 to 1000 m (average ~450 m), the channel relief ranges from 3 to 10 m, and the thalweg gradient is about 0.2°. The channels are filled with laminated sand and mud, bioturbated sandy mud, and thin shell lag that varies from flat lying to steeply dipping prograding reflection units. A pr nounced unconformity truncates the channel walls and fill. Bioturbated sandy mud, overlying the unconformity, thickens from about 1.5 m in the south, to a maximum of 4 m near the northern end of the channel sequence. Three C¹⁴ dates from a 4.5 m long core that penetrated into channel fill, provide an age for the unconformity of less than 2000 yBP. Channel-fill sediment below the unconformity is more than 6300 yBP, suggesting a pronounced hiatus associated with this time of intense erosion. These channel profiles and radiometrically-dated cores may provide insite into environmental change and time of the most recent sealevel rise and flooding of south San Francisco Bay.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California