Benthic Foraminiferal Biofacies in Stevens Sandstone: Relationships to Water-Mass Oxygen Levels in Late Miocene San Joaquin Basin, California: ABSTRACT

Abstract

The Stevens sandstone is an extensive and complex sequence of late Miocene turbidite sandstone and mudstone within the Monterey Formation of the San Joaquin basin. To date, the paleoenvironmental analysis of benthic foraminifera in such facies is limited largely to general inferences of paleobathymetry. A different approach uses multivariate analytical methods to classify biofacies and interpret them with respect to modern ecologic concepts derived from studies of Holocene faunas in the southern California borderland. Cluster and factor analysis help define 4 recurrent biofacies in the Coles Levee area: an agglutinated species biofacies (ASB), Uvigerina subperegrina biofacies (USB), Bolivina vaughani biofacies (BVB), and mixed calcareous biofacies (MCB). Ordination (princ pal components) plots of environmentally significant species indicate that the biofacies reflect a gradient in oxygen concentration of late Miocene water masses. The BVB and MCB represent the highest oxygen levels, the USB low but not dysaerobic levels, and the ASB the lowest oxygen concentrations. Ordination also shows that downslope transport of faunas and carbonate dissolution are also important in forming Stevens biofacies. Stratigraphic distribution of biofacies defines systematic shifts in oxygen concentration, probably linked to climate. These late Miocene biofacies variations were previously attributed to paleobathymetric changes. The distribution of species not restricted to the defined biofacies, plus the paleoenvironmental analysis presented here, argues against paleobathymetr as a complete explanation. This analytical approach shows the potential for greatly increasing our understanding of foraminiferal biofacies in submarine-fan environments. End_of_Article - Last_Page 277------------