Lower Permian (Wolfcampian) Fusulinid Biostratigraphy, Carbonate Microfacies, and Stratigraphic Model, Central Basin Platform Margin, Permian Basin, Texas

Gregory P. Wahlman and Douglas R. Tasker
BP America, Houston, TX

The integration of fusulinid biostratigraphy and carbonate microfacies from several cores with well log and seismic data enabled the construction of a new Lower Permian (Wolfcampian, Hueco Group) stratigraphic model for the Central Basin Platform margin. Fusulinid biostratigraphic analyses of cores enabled recognition and mapping of Upper Pennsylvanian-Lower Permian stratigraphic units and their bounding unconformities. Detailed core descriptions and petrographic microfacies analysis defined depositional facies and sequences.

At the end of Pennsylvanian time (top of Bursum interval), sealevel lowered significantly and a terraced erosional escarpment was carved along the eastern margin of the Central Basin Platform. During subsequent earliest Permian (Wolfcampian, Nealian) sealevel rise, Lower Hueco reef-mound complexes and associated packstone-grainstone flanking facies were deposited in a bank system along the face of that Bursum escarpment. The Lower Hueco reef-mounds were constructed by a newly integrated Permian shelf-margin reef community composed of phylloid algae, calcisponges, Tubiphytes, encrusting laminar red algae, bryozoans, specialized brachiopods, and syndepositional radial fibrous cements. Reef-mound flank beds are composed of bioclastic packstones and grainstones dominated by fusulinids, and fragmented phylloid algae and Tubiphytess. Capping shoal facies are composed of similar grainstones that also contain abundant dasycladacean algal fragments. Porosity in the Lower Hueco shelf-margin bank complex was enhanced by meteoric leaching at the overlying mid-Wolfcampian unconformity. The following Late Wolfcampian (Lenoxian) transgression created less accomodation space and the overlying Upper Hueco carbonates are composed of a relatively thin sequence of well-bedded cyclic shelf-margin fusulinid-skeletal packstone-grainstone upward-shoaling facies. The post-Wolfcampian shelf-margin backstepped westward dramatically, and the Hueco platform carbonates were buried by Leonardian basinal shales and interbedded lithoclastic-fusulinid packstone debris flows.