Water Resources Collections and Archives

Grain size and heavy mineral analyses of 6 cliff, 12 beach, and 44 marine sediment and rock samples from Bolinas Bay were done as a part of a study of sediment transport on the continental shelf off Central California. This work indicates:

(1) The heavy mineral assemblage is predominantly green hornblende with secondary amounts of hypersthene and augite. Glaucophane and jadeite occur in locally high concentrations near shore. (2) The pattern of distribution of the heavy minerals shows (a) a tongue of high concentrations of minerals with a granitic source extending northwest from the San Francisco Bar, (b) flanked on the north and northeast by increasing landward concentrations of Franciscan metamorphic minerals. (3) The major source of heavy minerals is the San Francisco Bar. Secondary contributions come from Bolinas Lagoon and the adjacent cliffs. (4) The circulation in the bay is primarily counterclock-wise; produced by a combination of wave refraction around Duxbury Reef and the tidal Coast Eddy Current. The tidal influence, however, of Bolinas Lagoon is restricted to about one mile from the lagoon mouth. Circulation patterns in the bay greatly influence the sediment distribution. (5) The annual sediment flux in Bolinas Bay is about 300,000 cubic yards. The bottom sediments in the bay are apparently in quasiequilibrium.