Water Resources Collections and Archives

This study seeks to examine patterns of longterm sediment movement along a portion of the California coast centering around the mouth of San Francisco Bay. Naturally-occurring heavy minerals were used to trace the influence of the several sources of sediments. Surface samples were collected from beaches and from t~e 8djacent portion of the shelf under less than 130 feet of water. The samples obtained were analyzed mechanically.and petrographically. Six petrographic provinces were differentiated on the basis of physical and mineralogical properties.

It was found that sands south and west of the Golden Gate in less than 60 to 100 feet of water reflect the mineralogy of San Francisco Bay sedim~nts, a"d samples from the mollusk-rich Bolinas Bay and adjacent areas to the north and west contained large amounts of aragonite. Sediments in 60 to 100 feet of water west of the Golden Gate are unusually high in hornblende and sediments in more than 100 feet of water are somewhat higher in minerals of the Franciscan Formation than 3ed~ments closer to the coast.

It is concluded from this information that the San Francisco Bay Ear and adjacent sediments south and west of the Golden ~ate have been derived principally from San Fran~isco Bay, gnd that sediments in the Bolinas fay area are derived in large part from the decomposition of shells of modern marine organisms. The areas in 60 to 100 feet of water and greater than 100 feet of water do not appear to have any modern sources of sediment and are interpreted as relicts of features developed during lower stands of sea level.

Similarities between sediments in more than 100 feet of water in this area and sediments in the same environment to the north of the area studied suggest a less complicated distribution of sediments and perhaps extensive longshore transport of sediments during this lower stand of sea level. The distribution of recent sediments near the coast in the area studied indicates that longshore transport is now only of limited, local importance.