ABSTRACT: Slope Facies along the Permian Reef Geology Trail--McKittrick Canyon, Guadalupe Mountains, Texas

BEBOUT, DON G., Bureau of Economic Geology, University of Texas at Austin, Austin, TX, DENISE MRUK, Marathon Oil, Midland, TX, and ALTON BROWN, ARCO Oil and Gas, Plano, TX

The Permian Reef Geology Trail, McKittrick Canyon, Guadalupe Mountains National Park, provides one of the best exposures of the slope sediments equivalent to the upper Guadalupian Capitan reef. Two major depositional styles are represented by the slope sediments along the trail. Lower slope cohesive debris flows, which dip into the basin at 15 to 20 degrees are equivalent to the Tansill-age reef on the shelf and post-Lamar sediments in the basin. Upper slope carbonates and sandstones, which dip into the basin at 20 to 35 degrees, are equivalent to the Yates-age reef on the shelf and Lamar Limestone in the basin.

The Tansill-equivalent lower slope is represented predominantly by massive megabreccia made up of large blocks, up to 15 ft across, of Archaeolithoporella-sponge boundstone. Matrix between the blocks is sparse and consists of poorly sorted, silty, dolomitic skeletal wackestone and packstone. Locally preserved at the base of these debris-flow deposits are remnants of burrowed wackestone of the upper part of the Lamar Limestone. Associated with the megabreccias are beds of skeletal packstone and grainstone that were deposited by high-density turbidity currents and grain flows.

The Yates-equivalent upper-slope facies are diverse and include interbedded autochthonous slope wackestones to packstones with fenestrate and ramose byrozoans, sponges, brachiopods, and crinoids; siliciclastic beds of fine-grained sandstone with dolomite matrix; steeply dipping, fusulinid-rich beds; breccias of locally derived slope material; and reef blocks. The blocks are comparable in fabric and faunas to those of the Yates-age reef exposed along the trail above.

In addition to differences in position along their respective depositional profiles, changes in slope facies also reflect changes in growth styles of the shelf margin from more progradational (Yates) to more aggradational (Tansill). The siliciclastic sandstone beds just above the uppermost occurrence of the fusulinid Polydiexodina on the slope are thought to correlate with a significant siliciclastic bypass surface resting immediately on the last occurrence of Polydiexodina (base of Yates C) on the shelf. This proposed correlation provides the best opportunity for reconstructing the depositional profile of the Capitan shelf margin.

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