ABSTRACT

The upper Taylor Group in Travis and Williamson Counties is lithologically monotonous, consisting of an argillaceous marl (Pecan Gap Formation) grading upward into a calcareous claystone (Bergstrom Formation), with no identifiable lithologic breaks in 142 m (466 ft) of thickness. Outcrops of more than a meter or two in thickness (3.3 to 6.6 ft) are few, and existing biostratigraphic zonations for this interval are rather generalized.

To examine the potential of Ostracoda for biostratigraphic subdivision of this interval, six outcrops totaling 64 m (210 ft) of exposed section were sampled at 1.5 m (5 ft) intervals. Fifty-four species of Ostracoda were identified and their stratigraphic ranges plotted. Species diversity and equitability trend-lines and triangular diagrams of proportional family-composition provide a basis for paleoecologic interpretation.

There are three distinct assemblages of ostracodes in these samples, and they can be assigned to interval-zones proposed by Hazel and Brouwers (1982), which had not previously been extended to Central Texas. The Pecan Gap assemblage is characterized by "Cythereis" caudata and Limburgina verricula and belongs to the Limburgina verricula Zone. The lower to upper Bergstrom assemblage is characterized by Cytheromorpha unifossula, and it contains species common to both the Limburgina verricula and the Escharacytheridea pinochii Zones. Cytherelloidea crafti and Xestoleberis ovata characterize the uppermost Bergstrom assemblage, which falls within the Escharacytheridea pinochii Zone, although the nominal species is absent.

Depositionally, the Upper Taylor is a single genetic unit. The gradational nature of faunal changes and a lack of nonrandom events in the diversity and equitability trends suggest that the first and last appearances of species should approximate evolutionary events, giving maximum chronostratigraphic significance to this biozonation scheme and offering hope for eventual refinement. The composition of these assemblages indicates deposition in a low-energy environment deeper than 75 m (246 ft).

The name "Neylandville", defined by paleontological rather than by lithological criteria, should be suppressed in Central Texas and possibly elsewhere. Instead, the name Bergstrom should encompass the entire genetic package of claystones between the Pecan Gap Formation and the Navarro Group.