Middle Albian Age of the “Regional Dense” Marker Bed of the Edwards Group, Pawnee Field, South-Central Texas

Lowell E. Waite¹, Robert W. Scott², and Charles Kerans^3
1 Pioneer Natural Resources USA, Inc., Irving, Texas 75039
² Precision Stratigraphy, Cleveland, Oklahoma 74020
³ Department of Geological Sciences, University of Texas, Austin, Texas 78705

Analysis of an extensive (950 ft thick) whole diameter core from the Edwards/ Stuart City Formation in the Schroeder #1 well, Pawnee Field, Bee County, Texas, shows it to consist of a single, large scale, upward-coarsening succession represented by six main facies. The majority of rock types consist of light-colored rudist-bearing boundstones, packstones and grainstones typical of a Lower Cretaceous shelf margin complex. Diagnostic foraminifer and rudist assemblages date the succession as Middle to lowermost Upper Albian (108.0-104.5 Ma).

Embedded within the normal reef package is a distinctive, thin (12 ft-thick), darkcolored boundstone unit consisting primarily of the binding alga Lithocodium. Using available well control and based on wireline log signatures, the algal boundstone unit at Pawnee can be positively correlated to the informal “Regional Dense” marker bed of the San Marcos Platform interior. This correlation requires the marker bed to be Middle Albian (Fredericksburg) in age, and not correlative to the Kiamichi Formation (Washita) of north-central Texas as previously reported by some. The algal boundstone most likely represents the maximum flooding interval of a single, third-order depositional sequence recorded within the Edwards at the Stuart City shelf margin.

AAPG Search and Discover Article #90069©2007 GCAGS 57th Annual Convention, Corpus Christi, Texas