Abstract

An exceptionally well-preserved calcis-ponge-microbial boundstone reef occurs in a dolostone core from the Upper San Andres Formation (Guadalupian, Middle Permian), East Vacuum Grayburg San Andres Unit, Lea County, New Mexico. The shelf-margin reef is bounded shelfward by crestal shelf-margin grainstone shoal facies and seaward by basinal Delaware sandstones. The main reefal buildup is >108 ft thick (base of reef not cored), and is overlain by three thinner sequences of inter-bedded doloboundstone, fusulinid dolograin-stones, skeletal dolowackestone-packstones, and reef talus dolorudstones. A karstic fracture extends for 30 feet down through the upper interbedded sequence and is filled with breccia clasts, fusulinid packstones, geopetal internal sediments, and massive anhydrite.

The main components of the reef boundstone facies are: 1) a skeletal framework dominated by calcareous sponges; 2) a binding biota of thrombolitic microbial encrustations and masses, moderately common Tubiphytes, sparse fistuliporid bryozoans, and rare Archeolithoporella red algae; 3) sparse to moderately common small botryoids of syndepositional originally-aragonitic radial fibrous cements; 4) a sparse associated reef dweller biota composed mostly of gastropods, brachiopods, and crinoids; and 5) reef constructional cavities that are partly filled by geopetal sediments, lined by thick isopachous layers of radiaxial cements, and filled by anhydrite. The reef framework is dominated by the calcisponge Guadalupia and the closely-related Lemonea. Massive to tabular growth forms of Guadalupia and Lemonea occur throughout the reef, but branching growth forms become increasing dominant downward through the lower half of the reef. Associated smaller calcisponges include common Amblysiphonella and Discosiphonella (Cystauletes), and sparse Cystothalamia. Discospihonella (Cystauletes), and other smaller calcisponges, sometimes occur growing downward from the roofs of boundstone cavities. Thrombolitic microbial encrustations and masses are by far the most important binding agent in the reef and contributed significantly to both the cohesion and mass of the buildup.

The exceptional preservation of the reefal fabrics in a dolostone core is the result of the fine-crystallinity of the dolomite, which is thought to have resulted from very early and rapid dolomitization by refluxing fluids supersaturated with Mg. There are also later-diagenetic sparse void-filling medium- to coarse-crystalline dolomite cements, rare calcite cements, and sporadic very small amounts of pore-lining bitumen. The abundant reef framework cavities are up to several cm across, and commonly contain geopetal deposits of laminated dolomitic mud, silt, and very finegrained peloidal packstone. Following the geopetal deposits, the cavities are typically lined by isopachous rims of layered radiaxial cements that were marine phreatic in origin. Finally, the cavities were filled by medium-crystalline anhydrite precipitated during burial diagenesis. Reef facies porosity is patchy in distribution and composed of intraskeletal skelmoldic porosity within the calcisponges, other skelmoldic porosity, intercrystalline dolomite porosity, some microporosity, and very sparse open fractures. The patchy distribution of porosity is increased by pore-filling anhydrite, which preferentially fills larger cavities and skelmolds.

The main reef core is relatively thick for San Andres shelf-margin complexes, and it probably grew aggradationally through the Guad 9 sequence. The stratigraphic position of the reef immediately adacent to the basinal Delaware sands is reminiscient of the San Andres sequences described by Sonnenfeld and Cross (1993) from Last Chance Canyon in the Guadalupe Mountains of New Mexico.