Biostratigraphy and paleoceanographic significance of the radiolarians from the protomouth of the Gulf of California

Abstract

The presence of radiolarians in the diatomitic strata of subunit C of the Trinidad Formation (McCloy, 1984) in Baja California Sur, Mexico, is analyzed with a graphic-multivariate model in order to define a biostratigraphic frame and infer paleoceanographic events. Stichocorys peregrina and Didymocyrtis penultimus are the only index species present in the diatomites. Nevertheless, the total faunistic composition indicates that their deposition was principally promoted by upwelling processes which occurred approximately 8 to 5 million years ago, i.e., during the latest Miocene. As time elapsed in this stage, the California Current intensified until it was very evident in the protomouth of the Gulf of California. The age of the Trinidad Formation diatomites indicates that the Pacific Ocean invaded the gulf-mouth region before the tectonic processes (sea-floor accretion and spreading) were established, processes that have opened and outlined the present morphology of the Gulf of California; consequently, the basin which harbored said invasion can be called protomouth.