Water Resources Collections and Archives

Sediments of Monterey Bay are divisible into five heavy mineral provinces. Two of the provinces are from the Salinas and Pa,iaro rivers, the other three are not traceable to any known source. Sediments of the Salinas River have high garnet content, and the minerals glaucophane and lawsonite distinguish the Pajaro River sediments. A mineral province is restricted to beach sands along the north shore of the bay, and is carried into the bay by longshore drift from the northwest. The heavy mineral provinces do not coincide with the age differences of the sediment cover. The San Lorenzo River does not produce a detectable mineral province.

The grain size of the sediment cover decreases uniformly with water depth from the shoreline to a depth of 300 feet, then becomes coarser in a band along the edge of the continental shelf. Grain size modes correspond to conditions of wave agitation over most of the bay. Polymodal samples and samples not in agreement with this relationship can be shown to contain relict Pleistocene components. Sediment is being transported seaward from the shorelines by simple wave agitation, and deposited at various water depths according to grain size.

The sediment cover occurs in bands that are aligned subparallel to the depth contours. The outermost band on the continental shelf contains coarse sediment of Pleistocene age. The middle band is of very fine grained sediment of Holocene age. The nearshore band is of medium and coarse grained sediment probably of both Holocene and Pleistocene age. Pleistocene influences are found in this band at depths of 100 feet in the northeast corner of the bay.