Abstract

The composition, distribution, and reservoir facies characteristics of tropical mounds and reefs in the Permian Basin (west Texas and southeastern New Mexico) evolved through the Pennsylvanian and Early Permian in response to a combination of biological, ecological, and geological factors. Lower Pennsylvanian (Morrowan-Atokan) carbonate buildups were mostly broad low-relief ramp mudmounds constructed by baffling thicket communities of calcareous algae (e.g., phylloid algae, Donezella/Dvinella, Cuneiphycus, Komia). Following the widespread mid-Pennsylvanian sea-level rise, phylloid algae dominated Upper Pennsylvanian (Desmoinesian-Virgilian/“Bursumian”) shelf and shelf-margin buildups throughout the region, and continued to be common through the earliest Permian (Wolfcampian). Large well-bedded phylloid algal banks covered broad paleotopographic highs on shelves, and more localized and massive phylloid algal mounds grew along shelf and shelf-margin topographic breaks. The high reproductive and growth rates and the gregarious habit of the opportunistic phylloid algae allowed them to rapidly re-colonize and dominate shallow-water environments during the Late Pennsylvanian period of high-frequency glacioeustatic sea-level fluctuations. During that same period, in somewhat deeper-water peripheral interbank/mound areas and in upper slope settings, relatively small boundstone reefs were constructed by a thicket community of erect calcareous sponges, phylloid red algae, fenestrate and ramose bryozoans, and crinoids, which were encrusted by laminar encrusting algae and Tubiphytes.

Latest Pennsylvanian-earliest Permian tectonism, high-amplitude glacioeustatic sea-level fluctuations, and erosion accentuated shelf-to-basin topography and created an ideal geological and environmental setting for the development of shelf-margin organic buildups. In the earliest Permian (Wolfcampian), elements of the Pennsylvanian phylloid algal mound and peripheral boundstone communities underwent radiations and, together with some newly evolved elements, organized into a new Permian shelf-margin reef community. The Wolfcampian reef community consisted of a framework composed primarily of phylloid algae, calcareous sponges, fenestrate and ramose bryozoans, and sometimes specialized brachiopods, which were encrusted and bound together by Tubiphytes, laminar encrusting algae, fistuliporid bryozoans, and often abundant marine radial fibrous cements. The waning of continental glaciation, and the resultant decrease in the frequency and amplitude of glacioeustatic sea-level fluctuations, created a more stable environment and gave the new reef community the time to establish itself. That reef community persisted and radiated throughout the remainder of Permian time and played a significant role in the growth, geometry and facies relationships of shelf-margin systems, culminating in the Permian Basin with the development of the Middle Permian (Guadalupian) Capitan Reef.

Phylloid algal buildups were most commonly developed during the upper (highstand) parts of Upper Pennsylvanian cyclic depositional sequences, and therefore were often subsequently exposed subaerially. The resultant meteoric diagenesis caused dissolution and collapse of the framework of originally-aragonitic phylloid algal plates, creating a porosity system of skelmolds, vugs, and fractures. Lower Permian algal-calcisponge cement reefs developed largely during the transgressive phase of depositional cycles, and grew mainly below normal wavebase, and topographically below and seaward of crestal shelf-margin shoals. Those reefs have well-developed bioclastic packstone-grainstone flank beds, and shallow-upward into thick shelf-margin bioclastic (fusulinids, phylloid and dasycladacean algae, Tubiphytes) grainstone shoal facies that contain scattered Tubiphytes patch reefs. With the correct post-depositional and diagenetic history, Lower Permian shelf-margin reefs can develop a phylloid algal mound type of porosity system, but the flanking and capping bioclastic packstone-grainstone facies often have well-developed intergranular, intragranular, and skelmoldic porosity systems and can be volumetrically more significant.