Foraminiferal biostratigraphic and paleoceanographic analysis of the Eagle Ford, Austin, and lower Taylor Groups (Middle Cenomanian through Lower Campanian) of central Texas.

摘要

The foraminifera of the Eagle Ford, Austin, and lower Taylor Groups in central Texas were analyzed to recognize discrete biofacies and to interpret paleoceanographic conditions for this portion of the Upper Cretaceous. One hundred twelve samples, prepared and analyzed for foraminiferal content, were obtained at regular intervals from a composite stratigraphic section compiled from outcrop sections, several pre-existing cores, and from cores collected to cover otherwise unexposed portions of the section. From the greater than 75,000 specimens identified in this study, 161 benthic and 36 planktic species were described and illustrated. Important insights gained from the identification process include the observation that most individuals are relatively small (125 μm) and that very few biostratigraphically useful species are found within the >63 μm size fraction examined here. The species concepts and stratigraphic ranges of many taxa, developed from larger size fractions, require emendation.; Multivariate analysis of benthic foraminiferal population structure by cluster, principal component, and canonical variate analyses, revealed the presence of 17 significant biofacies groupings. These groups separate the units of the Eagle Ford Group, isolate two portions of the Taylor, and divide the Austin Group into three main portions (an upper, middle, and lower), comprised of a total of 11 subgroups. At least four significant sources of variation are present in the data, which appear to relate to changes in relative sea level, nutrient levels, oxygen availability, and changes in large-scale productivity of the chalk sea.; Foraminiferal populations from the Eagle Ford Group record the influence of a northern Boreal water mass and an associated exclusively agglutinated assemblage in the middle Cenomanian. These waters interacted with a southern Tethyan water mass along a fluctuating oceanographic front, which periodically advanced and retreated across the study location. During the late Cenomanian, the Boreal water mass, and associated microfauna, was displaced by a southern Tethyan water mass, which was marked by the presence of a layer of anoxic water of variable depth. This anoxic zone traversed Texas before its recognition within the Western Interior, verifying a southern source for at least the initial advent of anoxia in the central Interior Seaway.