Contrasting Evolutionary Patterns of Lower Permian Shelf and Basinal Facies, Midland Basin, Texas
S. J. Mazzullo, A. M. Reid
The evolution of carbonate and siliciclastic shelf-to-basin depositional systems of the Lower Permian in the Midland basin was influenced by eustatic fluctuations, changing shelf-margin biota, and concurrent tectonism. The development of these systems from Wolfcampian to Leonardian time (28-m.y. duration) involved seven distinct phases that are recognized seismically as third and lesser order cycles. These phases are (1) highstand progradational shelf carbonate packages, separated by lowstand deltaic and basinal shales, deposited during relatively long-term eustatic cycles (early Wolfcampian); the component carbonate systems evolved from ramps to distally steepened ramps associated with nonframe-building algal reefs and grainstones, with little resedimented foreshelf detritus; (2) dominantly carbonate deposition during the middle and early late Wolfcampian, with construction of offlapping (but laterally juxtaposed) progradational shelves with steep platform margins deposited during a lengthy period of stillstand and/or slow submergence; dolomitized platform-margin facies are composed of marine-cemented, sponge-algal reefs and grainstones, with resedimented carbonate megabreccia to micrite channels and lobes in the contiguous shale basin; (3) shelf emergence and erosion during a major late middle(?) to late Wolfcampian lowstand contemporaneous with basinwide tectonism, with mass wastage into the basin of the terminal Wolfcampian platform-margin carbonate section; (4) regional transgression and black shale deposition followed by a repeat o Phase 2 type systems in the latest Wolfcampian to earliest Leonardian but under arid conditions; rapid vertical platform margin accretion by dolomitized, marine-cemented, sponge-algal-coral reefs and grainstones, and deposition of thick foreshelf megabreccia wedges, aprons, channels, and lobes; (5) reciprocal carbonate-siliciclastic sedimentation during short-term eustatic cycles in the early Leonardian (pre-Tubb), the precursor to Phase 6; lowstand shelf siliciclastic deposition alternating with thin sequences of highstand shelf and platform margin dolomites, and resedimented foreshelf micrites; (6) long-term lowstand and deposition of shelf and basinal siliciclastics (Tubb-Dean) in the latest early Leonardian, punctuated by only thin units of highstand shelf and platform margin dolomi es, and foreshelf micrites and intraclastic wackestones; and (7) a repeat of Phase 1 carbonate deposition and rapid shelf progradation during the middle and late Leonardian (in arid climates) with reciprocal lowstand siliciclastic shelf and basin deposition (Spraberry section). Component carbonate platform packages separated by siliciclastics and foreshelf micrites to grainstones and local megabreccias.
The evidence is inconclusive as to whether the foreshelf detritus of Phases 1, 2, and 4-7 was derived during highstand or lowstand. However, it appears certain that the dolomitization of such detritus, restricted to platform-proximal areas, was late postdepositional, possibly being related to basin dewatering. Similarly, dolomitization of shelf and platform-margin facies may be related to syn- and early postdepositional fluid flow patterns as well as to later basin dewatering.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.