Calcisponge-microbialite reef facies, Middle Permian (lower Guadalupian), northwest shelf margin of Permian Basin, New Mexico

Exceptional mimetic preservation of reef fabrics in a dolostone subsurface core from Vacuum field, Lea County, New Mexico, along the northwest shelf margin of the Permian Basin reveals the significant microbialite role in a calcisponge-microbialite reef of the upper San Andres Formation (lower Guadalupian [middle Permian]). The shelf-margin reef facies, which comprises more than 85% of the 175-ft (53.34-m) cored section, is bordered shelfward by crestal shelf-margin shoal facies and seaward by basinal sandstones of the Delaware Mountain Group. The thick shelf-margin reef located within 500 ft (152.4 m) of basinal sandstone facies indicates a relatively steep reefal shelf margin. Microbialite encrustations are the most important binding agent in the reef, and their major contribution to the cohesion and volume of the reef has significant implications for the growth and character of other middle Permian shelf-margin reefs. The reef facies is composed of (1) a calcisponge-dominated reef framework (Guadalupia, Lemonea, Amblysiphonella, Discosiphonella [Cystaulete], and Cystothalamia); (2) a binding biota of ubiquitous micritic, laminated, and thrombolitic microbialite, moderately common Tubiphytes (= Shamovella), and sparse Archaeolithoporella and bryozoans; (3) sparse botryoids of syndepositional radial fibrous cements; (4) sparse reef dweller biota of gastropods, brachiopods, and crinoids; and (5) reef cavities lined by banded isopachous layers of marine phreatic radiaxial cements, partly filled by geopetal sediments, and finally filled by anhydrite. The paragenetic sequence is (1) reef growth, (2) framework cavities lined by marine phreatic radiaxial cements and partially filled by geopetal sediments, (3) partial aragonite dissolution and karsting, (4) rapid reflux dolomitization, (5) localized fracturing and spalling of cavity walls before and during emplacement of anhydrite, and (6) burial and pressure solution. Patchy intraskeletal skelmoldic porosity is mostly within calcisponges. Karstic fractures and cavities in the upper reef are filled with breccia clasts, fusulinid packstone-grainstones, dark argillaceous dolomudstones, and massive anhydrite.