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ABSTRACT: Trace Fossils and Paleoenvironments of Lower and Middle Austin Chalk (Upper Cretaceous), North-Central Texas

William C. Dawson, Donald F. Reaser

Outcrops of lower and middle Austin Chalk in the vicinity of Cedar Hill and Waxahachie, Texas (designated site of the Superconducting Super Collider), are profusely bioturbated. The lower Austin (Coniacian) disconformably overlies the Eagle Ford Shale (Turonian) and consists of about 60 m of medium- to thick-bedded indurated chalk with thin intervening marls. The middle Austin (Santonian) is composed of about 65 m of thick-bedded marl containing thin beds of argillaceous chalk. Both chalk and marl units are moderately to intensely bioturbated. However, ichnofossils are more conspicuous in chalks where multiple episodes of burrowing are recognizable. Early formed burrows (indistinct mottles) represent the initial softground Austin substrate. Later generations of well-prese ved burrows record a post-compaction (dewatered) firmground substrate. No hardgrounds have been recognized. The abundance and diversity of ichnofossils in Austin strata are in marked contrast to the paucity of other megafauna, except large inoceramids.

Planolites, Teichichnus, Thalassinoides, and Chondrites are the predominant ichnogenera in the lower Austin; these forms are superimposed on a background of nondiagnostic, irregularly shaped mottles. Some strata contain well-preserved J-, S-, T-, U-, Y-, and X-shaped traces having menicus fillings. These subhorizontal, endogenic, full-relief burrows are typically thrilled with dark gray (pyritic) chalk. Locally, pyrite or Fe-oxide-filled vertical tubes are preserved. Many traces have been deformed by compaction.

The middle Austin contains Planolites, Chondrites, Thalassinoides, Pseudobilobites, and both small- and large-diameter, sinuous, horizontal burrows. These ichnofossils are infilled with chalk or comminuted shells and are more resistant to weathering than the surrounding middle Austin marl. Intensely burrowed horizons occur in association with distinctive Fe-stained chalk beds (omission surfaces). Oyster-rich strata (biostromes?) are locally present, the bivalve Pycnodonte is common, and the ammonite Parapuzosia (juvenile form) occurs rarely in the middle Austin.

The Austin ichnoassemblage is equivalent to the "shelf sea" chalk trace fossil suite of Kennedy. Lower and middle Austin strata represent deposition in well-oxygenated water of normal salinity, at estimated paleodepths from 15 to 25 m (below normal wave base, inner/mid-shelf).

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990