Abstract: Distribution and Temporal Migration of Bank-Top Foraminifera Assemblages, Outer Southern California Borderland
Gregg H. Blake
Foraminiferal analysis of living and dead populations from samples collected in the outer borderland of southern California has demonstrated a strong correlation between the geographic distribution of benthic assemblages and geologic and oceanographic factors. Most of the samples were collected in the bank-ridge and insular-shelf environments of the Santa Rosa-Cortes Ridge.
The environments of the shallow banks and ridges are typified by low sedimentation rates, strong bottom currents, and upwelling. The substrate consists of biogenic materials, relict sands and, in the close proximity of the larger northern Channel Islands, patchy distributions of minor detrital deposits.
Four geographically restricted species assemblages and one cosmopolitan-species assemblage were defined by cluster analysis. They are referred to as: the Outer Banks Assemblage, the Insular Shelf Assemblage, the Deep Ridge and Bank Assemblage, the Slope and Basin Assemblage, and the Cosmopolitan Assemblage. The Outer Banks Assemblage is dominated by a Cassidulina-Hanzawaia community contrasted to the Insular Shelf Assemblage which consists of three benthic communities, the Nonionella-Bulimina, Buccella-Cibicides, and Cassidulina-Hanzawaia.
The Cassidulina-Hanzawaia community is the dominant faunal group of the bank-ridge environments shallower than 300 m. In nearshore samples influenced by detrital sedimentation, the dominance of the Cassidulina-Hansawaia community is reduced by the presence of the Nonionella-Bulimina and the Buccella-Cibicides communities. The present eastern boundary of the distribution of the Outer Banks Assemblage is the Santa Catalina Ridge.
Species of the Outer Banks Assemblage have very close affinities to marl assemblages associated with Pleistocene terrace deposits (i.e., Lomita Marl and the Santa Barbara Formation) along the coast of Southern California. During a time of lower sea level, species of the Outer Banks Assemblage were common constituents of faunas along the shallow parts of the inner coast. This extension of the distribution of the Outer Banks group suggests an extension of the outer-borderland shallow-bank and ridge environment by means of the modification of the nearshore coastal environment.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC