Source Area, Depositional Environment, and Composition of Quaternary Sands, Monterey Bay, California

William D. O'Brien, Jr., William R. Dupre

A suite of 173 sand samples from the Monterey Bay region was studied, using a stepwise discriminant function analysis, to determine the role of source area and depositional environment in controlling the modal framework constituents of the sands. These medium to fine-grained sands were derived from the Salinas, Pajaro, and Carmel drainage basins, and were deposited in fluvial, nearshore marine, and eolian environments. They range in age from recent to early(?) Pleistocene.

Provenance exerts the most significant control on composition, providing an 87% assignment efficiency (independent of depositional environment); volcanic and sedimentary rock fragments were the most important variables. There was a 100% efficiency when discriminating between fluvial sands from the three drainage basins; however, the efficiencies were less strong in marine and eolian sands (90.5 and 86%, respectively). This difference is, in part, the result of modification in transit and mixing of sources.

The ability to discriminate between environments of deposition using only compositional data (independent of source) is relatively poor (57% efficiency), with monocrystalline quartz being the most important variable. If source is known, the fluvial sediments can be recognized with 100% efficiency. However, the discrimination of marine and eolian environments remains poor (60-70% efficiencies), which reflects the fact that the eolian sands were deposited in coastal dune fields derived form adjacent marine sands, with little modification by eolian processes.

No significant diagenesis affected the composition of the sands, although grain size of the sample did have an effect on analysis results. Thus, care must be taken to compare only the compositions of similar size intervals between samples.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.