ANDTHE COMMITTEE ON GRADUATE STUDIES

Benthic foraminifera represent the most abundant and well researched shelled invertebrate group present withinmodern and Cenozoic continental shelf sediments. Ihave studied the distribution ofRecent benthic foraminifera from surficial sediments on the continental shelf off San Onofre, California. Because these protozoans are hypersensitive to variations in substrate and water mass character, their distributional patterns have the potential to define distinctive subenvironments, in turn reflecting the dynamic processes governing shelf sediment transport and deposition. Surface sediment samples from 70 box cores collected on the high energy continental shelf between Dana Point and Oceanside, California were analyzed for foraminifera with the aim of correlating distributional patterns with variations in sediment and water mass parameters. Sample sites were arranged in fourteen shorenormal transects. Water depths at collection sites ranged from 5 to 90 meters. One hundred seventy-nine species of Foraminifera, including some fossil species, were identified inthe samples analyzed. Water depths at collection sites ranged from 5 meters to 90 meters along the shelf edge. Quantitative analysis revealed five foraminiferal biofacies which define distinctive subenvironments across the San Onofre shelf including (a) the sand-rich active shoreface, (b) silt-rich inner shelf prism, (c) a mid-shelf active wedge of fine silty sand, (d) an outer shelf silt-rich sediment starved facies, and (c) a shelf-edge zone defined by abrupt increases in abundance of planktonic foraminifera. Characteristic taxa of each sub-environment are respectively (a) Elphidium microgranulosum, Gavelinopsis campanulata, and Cibicides fletcheri; (b) Buliminella elegantissima; (c) Eggerella advena, and Trochammina charlottensis, Haplophragmoides columbiensis, Sacammina atlantica, and Trochammina pacifica; (d) Bulimina denudata; and (c) Nonionella miocenica stella, Textularia sandiegoensis, Globobulimina hoeglundi, Recurvoides subglobosa, Bolivina pacifica, Bolivina acuminata, Recurvoides parkeri, Reophax excentricus, Bolivina acutula, Globobulimina pacifica, and Goesella flintii. Distributional patterns of major physical and chemical sediment parameters, including grain size, percent organic carbon, percent nitrogen, and percentage of calcium carbonate roughly parallel bathymetry, reflecting relationships between wave dynamics, sediment transport and deposition, and the production and deposition of organic matter across the shelf. Isopleths of the relative abundance of individual species, as well as species groups (e.g. biofacies), display similar patterns and highlight relationships between foraminiferal distribution, sediment character, zones of wave touchdown and transformation, and the abundance of organic matter. In particular, the inner shelf environment is characterized by high relative abundances of Buliminella elegantissima, which comprises up to 80 percent of total foraminiferal populations in samples collected between 10 to 30 meters water depth. B. elegantissima correlates significantly with mean grain size and with percent organic carbon in the sediment. The agglutinated species Eggerella advena marks the mid-shelf biofacies, and the distribution of this species can be correlated withpercent sand in the surficial sediments of the shelf. In the outer shelf, Bulimina denudata reflects areal changes in sediment parameters and characterizes the biofacies at water depths of 40 to 60 meters. Shoreparallel patterns, including simple isopleths of species abundance, characterize the major physical and foraminiferal trends across the San Onofre shelf area and emphasize the power of these analyses to assist inunderstanding both modern and ancient shelf environments.