Upper Permian Capitan Reef: Revision of Outcrop Model

P. M. Harris, R. A. Garber, G. A. Grover

A depositional and diagenetic model for the Capitan reef complex (Late Permian, Guadalupian age) has evolved during more than 50 years of outcrop studies in the Guadalupe Mountains of west Texas and New Mexico. The model relates the shelf margin (Capitan Limestone) with equivalent shelf (in ascending order, Seven Rivers, Yates, and Tansill Formations) and basin (Bell Canyon Formation) strata. It has proved to be important in relating hydrocarbon distribution in shelf and basin strata in the Permian basin and has been important as an analog in numerous other basins. Detailed study of the northern rim of the Delaware basin, centering on a 4,800-ft core, has caused us to reevaluate the outcrop-defined depositional model for the Capitan shelf margin along the following themes

Geologic evolution:
Progradation of the margin was not uniform throughout deposition of the Capitan as is portrayed in most reconstructions. Outcrop mapping and log correlations, in fact, show that 75% of the total basinward progradation of the Capitan occurred during deposition of the Seven Rivers Formation. This maximum progradation corresponds to back-reef carbonates largely devoid of siliciclastics, thick carbonate debris beds on the slope and basin edge, and thick siliciclastics deposited in the basin.

Depositional facies and diagenesis:
The pisolitic shoal complex, the predominant feature marking the highest part of the shelf margin, was more laterally extensive than known from outcrop. Siliciclastic sands are continuous throughout the back reef having formed across tidal flat, lagoon, and pisolite shoal settings. These sands at times extended through the reef, en route to the deep basin, by filling gulleys or small channels. Unlike the outcrop, the lower Capitan (equivalent to the Seven Rivers Formation) in core is entirely dolomitized, contains abundant biota, and commonly has boundstone textures. Magnesite is pervasive in parts of the Yates and Tansill shelf facies, where it mimics dolomite.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.