Foraminiferal Life and Residue Assemblages from Cretaceous Slope Deposits

Abstract

Distinctive foraminiferal assemblages and dissolution patterns are associated with Cretaceous clastic deep-water sediments along the eastern North Pacific continental margin. These sediments were deposited in bathyal, low-oxygen chemically reducing environments, as is evidenced by their foraminiferal composition, lithology, sedimentary constituents, organic-carbon content, and associated organisms. In species composition and morphology, the foraminiferal assemblages closely resemble modern faunas from low-oxygen environments on the continental slope and in deep-water basins along the eastern North Pacific Ocean. The Cretaceous life assemblage is characterized by species of Praebulimina , the subfamily Chilostomellinae, and several agglutinated and nodosariid genera, among others. In southern California this assemblage is restricted to the laminated mudstone facies of the Upper Cretaceous Point Loma Formation, which exhibits limited bioturbation and contains large amounts of organic detritus and pyrite. By comparison with modern depositional environments, bottom conditions probably were oxygen deficient but not anaerobic. Differential preservation of the foraminiferal fauna in the Point Loma Formation resulted from diagenic dissolution in the chemically reducing sediments. The subsequent residue assemblage is enriched in resistant forms such as thick-walled, compact calcareous and agglutinated benthic species; it is impoverished in planktonic and less resistant benthic species. Selected species are ranked according to their susceptibility to dissolution by noting their successive stage of preservation. These data illustrate the complex interactions between environmental and diagenetic processes that strongly influence interpretations of paleontologic age and environment of deposition.