Recognition of Paleo-Oxygenation Fluctuations in California Neogene Basins
Charles E. Savrda, David J. Bottjer
Application of a model developed from the study of analogous contemporary basins of the California continental borderland margin facilitates detailed reconstruction of temporal fluctuations in bottom-water oxygenation during deposition of fine-grained siliceous strata in Miocene-Pliocene basins of California. This model, most effectively employed in detailed vertical sequence analyses, involves the synthesis of the following criteria into a trace-fossil tiering framework: (1) general sedimentary fabric, (2) trace-fossil assemblage composition, (3) burrow size parameters (diameter and vertical extent), and (4) cross-cutting relationships.
Stratigraphic sections of the Monterey and Sisquoc Formations are characterized by alternating intervals of laminated and bioturbated beds. Laminated strata indicate periods of anoxic conditions, whereas bioturbated intervals reflect episodes of improved oxygenation. Bioturbated intervals contain one or more of three recurring oxygen-related ichnofacies. In order of decreasing frequency of occurrence and increasing oxygenation, these are the Chondrites, Planolites, and Thallassinoides ichnofacies. Constructed oxygenation curves for sections examined indicate that levels of oxygenation during deposition of these strata were generally low and that temporal fluctuations in redox conditions were generally gradual and of low magnitude. Rare high-magnitude oxygenation events recorded by the Thalassinoides ichnofacies appear to have been associated with depositional hiatuses and the development of hardgrounds.
More extensive application of the model to key stratigraphic sections that are well known biostratigraphically may provide the basis for an oxygen-related cyclostratigraphy that, in turn, may provide tools for correlation and improved understanding of temporal changes in paleo-oceanographic and paleo-ecologic conditions within and between individual Neogene basins.
AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.