--> Abstract: Ichnology and Paleosubstrates of Austin Chalk (Cretaceous) Outcrops: Southern Dallas and Ellis Counties, Texas, by W. C. Dawson and D. F. Reaser; #91004 (1991)

Datapages, Inc.Print this page

Ichnology and Paleosubstrates of Austin Chalk (Cretaceous) Outcrops: Southern Dallas and Ellis Counties, Texas

DAWSON, WILLIAM C., Texaco, Houston, TX, and DONALD F. REASER, University of Texas at Austin, Austin, TX

Ichnofossils are abundant in outcrops of the Austin Chalk near Waxahachie, Texas (designated site of the Super-Conducting Super Collider). The abundance and diversity of ichnofossils in Austin strata contrast with the paucity of other macrofossils, except large inoceramids. The lower Austin Chalk (Coniacian) disconformably overlies the Eagle Ford Shale (Turonian). Planolites, Thalassinoides, and Chondrites are conspicuous in the lower Austin. Some lower Austin strata contain well-preserved burrows having menicus fillings. However, most lower Austin ichnofossils are poorly preserved and have been compacted. The middle Austin Marl and upper Austin Chalk (Santonain) contain Planolites, Chondrites, Thalassinoides, and Pseudobilobites . Several thin, intensely burrowed, Fe-stained, horizon within the middle Austin represent omission surfaces having postomission Thalassinoides . The upper Austin disconformably underlies the Taylor Marl (Campanian). The Austin-Taylor contact is a Rhizocorallium -infested omission surface overlain by a condensed bed of phosphatic and pyritic bioclasts. Upper Austin occurrences of Rhizocorallium and Pseudobilobites are unique for North American Cretaceous chalks.

Based on cross-cutting relationships and differences in morphology, diameter, and burrow-filling sediments, numerous ichnospecies of Thalassinoides are discernable throughout the Austin. Variations in preservation quality exhibited by successive generations of ichnofossils record progressive changes in substrate consistency. Earliest formed burrows have diffuse outlines representing an initial thixotropic (softground) Austin substrate. Subsequent generations of burrows have more distinct outlines recording a gradual increase in substrate firmness. Paleo-firmgrounds are common in Austin outcrops; evidence of hardgrounds is lacking. The Thalassinoides -dominated Austin ichnoassemblage represents an inner shelf paleoenvironment.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)