FORAMINIFERAL BIOSTRATIGRAPHY AND PALEOECOLOGY OF THE TURONIAN-CONIACIAN AUSTIN CHALK IN WEST TEXAS

Despite the placement of a proposed Global Stratotype Section and Point at Wagon Mound, New Mexico, Turonian-Coniacian rocks in the Western Interior United States have been the subject of relatively little micropaleontological study. A 30-meter section of the Turonian-Coniacian Austin Chalk is exposed in a road cut along US Highway 90 in an area of generally low topographic relief in western Val Verde County, Texas. This is one of the thickest exposures of Austin Chalk in this part of west Texas and preserves large-scale sedimentary features, including several channels (which trend to the southeast; i.e., offshore). The Austin here is comprised mainly of micritic packstone or grainstone, interbedded with thin (<1-4 cm) clay beds. These clay interbeds change in color from black/dark gray in the lowest part of the section to greenish gray to tan then back to greenish gray and finally black/dark gray. The indurated limestones that characterize the Austin here were treated with an 80% solution of acetic acid for three hours to free foraminifera from the rock matrix. Samples from the clay interbeds were soaked overnight in borax and then washed over a sieve, and are all barren of both foraminifera and nannofossils. Nannofossils were recovered from limestone throughout the section, but were so poorly preserved as to prevent specific – and, in many cases, generic – classification. Foraminifer preservation decreases upward through the section, likely due to diagenetic effects of centuries of rain water filtering down from the ground surface. We report the planktic and benthic foraminiferal assemblages and tentatively identify proposed macrofossil markers around the Turonian-Coniacian boundary

Planktic foraminifera only yield one primary marker: the entire section is above the lowest occurrence of Dicarinella concavata (late Turonian) and below the lowest occurrence of Dicarinella asymetrica (earliest Santonian). Our tentative macrofossil identifications and correlation to Austin Chalk outcrops in the Dallas area suggest that lower Austin Chalk here is Turonian in age, with the base of the Coniacian about 10 m above the contact with the upper Eagle Ford Shale. A diverse benthic assemblage dominated by low oxygen-tolerant, infaunal taxa (buliminids) is present throughout the Austin Chalk.