ABSTRACT

Assemblages of cytheracean ostracodes from the Dessau, Burditt, and Sprinkle formations of the upper Austin and lower Taylor groups of Travis County, Texas, were examined in order to identify recurring species associations and the paleoenvironmental factors that control their stratigraphic distribution. From Q-mode cluster analysis, six sample-groups were identified, all of which correspond closely to observable lithofacies. Four recurrent species associations were identified from the R-mode cluster analysis and were found to correspond to four of the six sample-groups. Indices of species diversity and of its components, species richness and species equitability, show noticeable changes from one lithofacies to another, with the greatest changes occurring at the disconformable Dessau-Burditt and Burditt-Sprinkle (Austin-Taylor) contacts.

The stratigraphic distributions of cytheracean ostracode faunas in the upper Austin and lower Taylor appear to be controlled by the distribution of lithofacies. Some of the faunas, however, differ primarily in abundances of species, as opposed to containing different species. Such faunas can be differentiated by comparing values of species diversity and its components. Plotted trend-curves of the indices provide visual aids for this purpose. The parallel shapes of the trend-curves for two outcrops suggest potential use of diversity trend-curves for stratigraphic correlation, and major breaks in these curves at known disconformities show potential for detecting previously unrecognized disconformities. Appearances and disappearances of a few species, including Alatacythere cheethami, "Hazelina" austinensis, Schuleridea travisensis, and Loxoconcha retiolata, may be temporally significant.