--> --> Abstract: Stepwise Termination of the Permian Reef Complex, Permian Basin: Implications for Structural and Depositional Patterns of Saline Giants, by Charles Kerans, Chris Zahm, Joseph El-Azzi, and Nathan Jones; #90124 (2011)

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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Stepwise Termination of the Permian Reef Complex, Permian Basin: Implications for Structural and Depositional Patterns of Saline Giants

Charles Kerans1; Chris Zahm2; Joseph El-Azzi2; Nathan Jones1

(1) Dept. of Geological Sciences, University of Texas at Austin, Austin, TX.

(2) Bureau of Economic Geology, University of Texas at Austin, Austin, TX.

Carbonate-rimmed evaporite-filled basins are a common configuration in major oil and gas reservoir systems such as in the Paradox, Permian, Michigan, and Pre-Caspian basins. These petroleum systems are generally well-understood, but the driving force for the carbonate-to-evaporite transition is not. The Late Guadalupian Capitan Reef and its equivalent shelf (Tansill) and basinal (Lamar, post-Lamar siltstone) strata provides a unique depositional and structural record of cessation and collapse of this long-lived Permian carbonate platform and subsequent basinal evaporite filling.

Mapping of the Tansill-Capitan-Lamar system using photomosaic and airborne lidar was conducted in Dark, Walnut, and Rattlesnake Canyons, with reconnaissance mapping of Slaughter and McKittrick Canyons. This provides basic data on shelf-slope facies composition and architecture as well as location, spacing and timing of syndepositional fractures and collapse scars. The transition from a healthy Capitan platform to Castille evaporite deposition includes: (1) a change in shelf-margin trajectory from low-angle progradation to retrogradation and aggradation; (2) evolution from a healthy carbonate shelf with a well-developed periplatform mud apron (Lower Tansill-Capitan-Lamar) to a microbially-dominated shelf and shelf-margin with only slope breccias and no mud export to form a periplatform blanket (Upper Tansill-Capitan-post-Lamar siltstone); (3) oversteepening of the shelf-margin reef, collapse events, and the only well-developed suite of syndepositional marine-cemented fracture systems known from the Capitanian system; (4) a complete turn-over of outer shelf fauna with the replacement of the suite of larger fusuline forams by smaller more diverse forms, and a proliferation of microbial carbonates, and (5) deposition of a clast-rich onlapping debris wedge restricted to the proximal slope and complete starvation on basin floor prior to onset of evaporite deposition.

This record favors a Late Capitanian sea-level rise that drove shelf-margin aggradation, rimming, and isolation of the basin rather than the oft-speculated eustatic fall. Restriction led to the evolution of a microbially-dominated margin with little or no sediment export to the slope and basin and eventual oversteepening and failure of the margin. A similar aggradational stacking of shelf-margin reefs is also observed immediately prior to onset of Messinian evaporites in the Mediterranean and possibly in the Pre-Caspian.