Samples from five basins on the southern California continental borderland were examined for diatoms and silicoflagellates. These basins, Santa Monica, San Pedro, Santa Catalina, San Nicolas, and Tanner, represent a transect of approximately 85 nmi in a southwesterly direction across the continental shelf. The two nearshore basins, Santa Monica and San Pedro, contain the greatest species diversity and the smallest number of reworked specimens. Tanner basin, however, has the greatest abundance of diatoms and silicoflagellates in the study area, and the highest percentage of reworked specimens. The reworked diatoms are mid-Miocene to late Miocene in age. San Nicolas and Santa Catalina basin samples contained both lower species diversity and abundance totals than the other three basins. These two basins also contained a significant number of reworked diatoms.
As might be expected benthic species were common in Santa Monica and San Pedro Basins, however, a large number of benthic species were in the Tanner Basin samples, which was not expected. There was also a disproportionately low number of smaller planktonic diatoms in the sediments when compared to the very large numbers found in the phytoplankton standing crop in this area.