Facies Description and High-Frequency Cyclicity of the Upper Glen Rose and Edwards Formations, Lower Cretaceous of Austin, Texas

C. Kerans, J. A. Bellian, C. Calkins, M. H. Davis, E. Frost, B. Garcia-Fresca, J. Kimball, S. M. Fullmer, and R. G. Loucks
Department of Geological Sciences, The University of Texas at Austin, Austin, Texas

Comanche Shelf carbonates of Albian age are superbly exposed throughout the Texas Hill Country with some of the best exposures along Loop 360 in Austin, Texas. Individual roadcuts provide one-dimensional stacking patterns for facies analysis and cycle hierarchy for stratigraphy. This study presents an initial review of the upper Glen Rose through upper Edwards formation from these exposures. Stacking patterns, and the evolution of the sequence, confirm earlier models of Albian platform and platform-margin accumulation. Strong progradation of the upper Glen Rose and retrogradation to slight progradation of the Walnut and Edwards is suggested. In addition a higher-frequency base level fluctuation within each of these composite sequences provides constraints for understanding facies architecture of the hydrocarbon pay intervals close to the Stuart City reef margin. A similar advantage applies to the matrix porosity elements of the large Edwards formation aquifer.

A composite section of stratal exposures yields a 130-meter section that consists of haloturbated breccia, algal laminites, cross-stratified skeletal intraclast grainstones, skeletal grainstones, monopleurid bafflestones, skeletal wackestones and packstones, burrowed wackestones and packstones, argillaceous wackestones, marl, and calcareous silts. These facies were grouped as higher frequency cycles that compose parts of two composite sequences, the upper Glen Rose and the Walnut-Edwards. Both the Glen Rose and Walnut-Edwards formations high frequency sequences are typical greenhouse symmetrical systems with master flooding surfaces characterized by bioturbated marls, early sea level high stands that are characterized by mud-dominated skeletal facies, and late high stands that are dominated by peritidal cycles with abundant early dolomite.

AAPG Search and Discovery Article #90032©2004 GCAGS 54th Annual Convention, San Antonio, Texas, October 10-12, 2004