Facies Trends, San Pedro Bay, California

Facies trends were developed for San Pedro Bay, California, based upon 44 well-distributed stations. The area of investigation extends from the shore out to depths of 176 m, a distance of about 20 km, and spans a distance along the shelf of about 25 km. Sediments of the bay are mostly detrital sands along the shore line, silty sands on the shelf, and silts and silty sands on the slope off the edge of the shelf; residual and relict sediments (sands and silty sands) occupy a central region, contributing to more complex sedimentary patterns. Values for calcium carbonate and nitrogen are highest along the outer edge of the shelf and in the upper bathyal zone. Polychaetes dominate the megafauna; foraminifera dominate the microfauna. Forammiferal abundance and species diversity are greatest in outer areas of the shelf and in the upper bathyal zone; live/dead ratios are highest in central and inner shelf areas; live foraminifera are about three times as abundant in the upper bathyal zone as on the shelf. Calcareous perforate foraminifera dominate both dead and live assemblages throughout the area. Planktonic/benthic ratios are highest in the upper bathyal region; values for planktonic specimens per gram increase 20–100 times from the shelf into the bathyal zone. In the upper 10 m of the water column, phosphate and plankton cells per liter attain highest values in the central and inner shelf areas; however, in the lower 10 m of the water column, there is a general increase in phosphate values away from shore. Benthic foraminiferal populations may be grouped into inner shelf Buliminella elegantissima (d'Orbigny), central and outer shelf Nonionella-Trochammina, and upper bathyal Bolivina acuminata Natland groups. Hopkinsina pacifica Cushman is diagnostic of the general harbor region; Bulimina marginata denudata Cushman and Parker occurs in both the harbor and in the outer shelf and upper bathyal zones. The edge of the shelf, a structural trend, is defined by the boundary zone between the Nonionella and Bolivina acuminata Natland faunal groups. Similar structural trends may be indicated in the California marine Miocene by boundary zones between analogous faunal groups