KIRKLAND, BRENDA, University of Texas at Austin, Austin, TX, and SUSAN LONGACRE and EMILY STOUDT, Texaco, Inc., Houston Research Center, Houston, TX

ABSTRACT: Permian Reef Geology Trail Guide, McKittrick Canyon, Guadalupe Mountains National Park, West Texas-Reef Facies

The massive Capitan or reef facies is a prominent cliff-forming unit along the Permian Reef Geology Trail. The presence of the

fusulinid Polydiexodina through much of the reef facies indicates that the Capitan Formation along most of the trail is age equivalent to the middle and upper Yates Formation, hence older and stratigraphically below the Tansill-equivalent reef facies frequently visited in Walnut and Dark canyons.

Modern reefs contain four dominant components: (1) a diverse frame-building and binding community, (2) marine cement, (3) internal sediment that, along with marine cement, fills framework void spaces, and (4) bioeroding organisms. Similar distinctive elements are present in the Yates-equivalent Capitan Formation, except that evidence of bioerosion is rare. Frame-building organisms are dominated, not by corals as in modern reefs, but by calcareous sponges and bryozoans. A number of binding organisms are also present. Volumetrically, the most important of these encrusters is the red alga Archeolithoporella, which is associated with precipitation of prodigious amounts of marine botryoidal aragonite cement; a similar association of red algae and aragonite cement exists in modern reefs. Ot er prominent binders/encrusters include the problematic organism Tubiphytes and various bryozoans. In addition to marine botryoidal aragonite cement, other marine cements include isopachous rims of Mg-calcite. Spectacular examples of layered, skeletal-derived, and pelletal internal sediment fill framework voids and fractures throughout the Capitan reef along the Permian Reef Geology Trail.

Carbonate cements are prominent features of the Capitan Formation. They record changing fluid chemistries from marine to meteoric to burial conditions. The most prominent cements include the following, listed in order from first to latest formed: (1) marine botryoidal aragonite fans, (2) marine isopachous fibrous calcite crusts, (3) marine inclusion-rich prismatic calcite crusts, (4) dolomite crystals precipitated on marine cements and calcspars, (5) fine to medium crystalline, luminescent meteoric calcspar, (6) very coarsely crystalline, cloudy, luminescent burial calcspar, and (7) medium to coarsely crystalline, dully luminescent burial calcspar.

AAPG Search and Discovery Article #90994©1993 AAPG Southwest Section Meeting, Fort Worth, Texas, February 21-23, 1993.