Evolution of a Mixed Carbonate-Siliciclastic Platform: Lower Permian, Northern Midland Basin, Texas
MAZZULLO, S. J., and W. E. HIPKE, Wichita State University, Wichita, KS
Lower Permian (Wolfcampian-lower Leonardian) rocks in the northern Midland basin consist of a thick sequence of carbonates, evaporites, shales, and sandstones. The stratigraphic arrangements of these contrasting lithotopes and corresponding facies changed through time as a function of the interplay between sea level fluctuations and different mechanisms of siliciclastic sediment input into the depositional systems. Wolfcampian and basal lower Leonardian (Wichita-Albany) systems are rimmed carbonate platforms whose depositional geometries were modified to various extents by erosion during sea level lowstands of relatively long duration and considerable magnitude. Siliciclastics, predominantly shales and fine siltstones, were derived from the north and east and are restricted in their o currence to basinal settings. In contrast, lower Clear Fork Systems, particularly in the Tubb Member, are rimmed platforms consisting of interbedded carbonates, evaporites, and siltstones. Coeval basinal facies are represented by carbonate detritus, shales, and siltstones (Dean Formation). These rocks were deposited during more frequent sea level fluctuations of short duration and low magnitude. The Tubb Member is characterized stratigraphically by seven repetitive cycles of siltstones and carbonates, each of approximately 16,000 yr duration (fourth-order Milankovitch type). The siltstones were derived from the northwest, and were deposited on the platform during sea level lowstands, probably as eolian sheet sands and desert wadis. Characteristic geometries of such deposits, however, wer strongly modified during subsequent platform transgressions. Intervening carbonates represent sea level highstand phases. Maximum periods of platform accretion during these cycles coincided with highstands, although basinward progradation of the platforms was helped by the development during lowstands of siliciclastic foundations in proximal basinal settings.
AAPG Search and Discovery Article #91011©1991 AAPG Southwest Section Meeting, Abilene, Texas, February 9-12, 1991 (2009)