An Undescribed Fauna from the Upper Cretaceous ‘Pyroclastic Zone’ of the Austin Group at Pilot Knob, Central Texas

Linda McCall¹, James Sprinkle², Ann Molineux¹, and Christopher Garvie¹
¹Non-vertebrate Paleontology Laboratory, Texas Natural Science Center, University of Texas at Austin, 10100 Burnet Rd., Austin, Texas 78758
²Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, 1 University Station C1100, Austin, Texas 78712–0254

The 1996 construction of a drainage ditch in the former Dean Word Quarry in southeast Austin, Texas, led to the discovery of a new, unique, and undescribed fauna from the Pilot Knob ‘Pyroclastic Zone’ of the Austin Group. Most of the sparsely fossiliferous spoil piles were a random mix of different strata, impossible to interpret. However, excavation of the drainage ditch exposed a much more fossiliferous 3-ft (1-m) sequence of layers in the ditch walls. The lowermost green clay layer contained a moderate fauna. The red clay layer above it contained the most numerous and diverse fauna. The uppermost layer was a hard, yellow, clastic material and was the least diverse, containing mostly Inoceramus sp. bivalves and burrows. All specimens in this study were collected from piles of these three distinctly colored sequential layers excavated from the adjacent drainage ditch.

Calcification was the most common mode of preservation. Unlike steinkern development, typical of many bivalves and gastropods in the Austin Group, the majority of these specimens, even aragonitic ones, have been altered to calcite and still retain their exterior details and ornament.

This diminutive fauna is surprisingly diverse with over 125 taxa, including (from most to least diverse): bivalves, gastropods, crustaceans, echinoids, ammonites, worm tubes, shark teeth, fish teeth, solitary corals, sponges including borings, crinoids, bryozoans, and trace fossils. Foraminifera remain to be studied. Decapod cheliped (leg and claw) fragments were most abundant. This collection is a new and important addition to our knowledge of the ‘Pyroclastic Zone’ of the Austin Group and provides a unique record of a Texas Santonian fauna.

AAPG Search and Discovery Article #90158©2012 GCAGS and GC-SEPM 6nd Annual Convention, Austin, Texas, 21-24 October 2012