In outcrops throughout northeast Texas, the Austin Group consists of interbedded thin to very thick-bedded (0.3-1.5 m) chalk with thin intervening calcareous claystone ("marl") layers. Both chalk and claystone are moderately to intensely bioturbated, and multiple generations of crosscutting burrows are common. Austin trace fossils occur as endogenic full-relief individuals filled with chalk, clay, or iron oxides. The abundance and diversity of ichnofossils within the Austin are in distinct contrast to the paucity of other megafossils, except large, locally conspicuous inoceramids with oyster epiliths.

Variations in ichnofossil content, quality of burrow preservation, and petrographic character allow definition of three Austin substrate types. (1) Softground--vaguely mottled, argillaceous, foraminiferal biocalcilutite containing poorly preserved Planolites, Thalassinoides, and Chondrites. Burrowing is intense. Chondrites typically infests fillings of other ichnofossils. Inoceramids occur as widely scattered epifauna. (2) Bioclastic lenses--coarse-grained inoceramid biocalcirudite with interstitial chalk matrix. This lithology forms broad, low-relief channels; dense networks of Thalassinoides occur on lower surfaces. (3) Firmground--phosphatic, glauconitic, foraminiferal biocalcilutite containing well-preserved Rhizocorallium jenense and Pseudobilobites. South of Dallas, the disconfo mable Austin-Taylor contact is a Rhizocorallium-dominated firmground omission surface.

The Austin contains a shallow marine (middle to inner shelf) ichno-assemblage. A vertical increase in the ratio of suspension to deposit-feeding burrows suggest that the Austin is a shallowing-upward sequence.

Trace fossils impart textural heterogeneities to chalk which can either enhance or degrade reservoir quality and can also complicate wireline log interpretations and well completion procedures.

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