--> Abstract: Relative Sea Level Record of Yates and Tansill Formation Shelf Facies, Permian Reef Geology Trail: Implications for Platform Development, by C. Kerans and P. M. Harris; #91012 (1992).

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ABSTRACT: Relative Sea Level Record of Yates and Tansill Formation Shelf Facies, Permian Reef Geology Trail: Implications for Platform Development

KERANS, C., Bureau of Economic Geology, University of Texas at Austin, Austin, TX, and P. M. HARRIS, Chevron Oil Field Research Company, La Habra, CA

The Permian Reef Geology Trail, Guadalupe Mountains National Park, serves as one of the best exposed and most accessible sites for examining facies relationships at a carbonate platform margin. The highest part of the trail includes progradational outer shelf facies of the uppermost Yates Formation, and retrogradational basal Tansill Formation shelf-crest and possible outer shelf facies. Shelf strata are particularly important for understanding the evolution of the platform-to-basin system because they provide the most sensitive record of relative sea level oscillations. The shelf facies provide the best record for identifying zones or surfaces within the massive reef that should reflect important base-level changes. Integral to understanding the link between shelf and reef-margin fac es is the "fall-in bed" or deep-rimmed shelf geometry of the Capitan margin, which represents at least in part primary depositional topography.

A record of high-frequency/low-amplitude changes in relative sea level in the Yates and Tansill formations is displayed by (1) 10-30 ft thick upward-coarsening skeletal/peloidal/pisolitic wackestones-grainstones and (2) 10-20 ft thick skeletal carbonate to siliciclastic siltstone cycles. Analysis of a single cycle in a dip profile from shelf crest to reef shows a trend from cross-stratified skeletal-pisolitic grainstone near the crest to massive burrowed skeletal packstone nearest the reef. These facies changes occur within the reefward sloping profile of the individual cycles, and together these data demonstrate that the Capitan reef equivalent to the uppermost Yates Formation strata was deposited in 30-40 ft of water depth.

A more substantial relative sea level shift interpreted from the shelf strata on the trail is a fall that brings shelf-crest facies in juxtaposition with outer shelf and uppermost reef facies. The fall-in profile indicates a minimum downward facies shift of 30-40 ft. The surface formed by this shift coincides with the highest occurrence of the fusulinid Polydiexodina, which is widely distributed in the outer shelf, slope, and toe-of-slope facies, thereby providing a potential time surface for evaluating the impact of the fall in terms of offset within reef and slope facies. Yates cycles above this surface record an upward-thinning trend and increase in siliciclastics consistent with long-term progradation. Basal Tansill cycles record landward-stepping facies of a transgressive systems tract, comparable to that recorded elsewhere in the Tansill.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)