Diagenetic Response of Coeval Platform and Slope Carbonates to Sea-Level Changes: Lower Permian, Midland Basin, Texas

S. J. Mazzullo

Lower Permian (Leonardian) strata in the northern Midland Basin are coeval platform and slope carbonates deposited during repeated sea-level fluctuations which exposed platform but not slope strata. The disparate diagenetic responses of these carbonates to eustatic changes are dominantly related to the low magnitudes and short durations of lowstand phases. initial dolomitization of inner-platform carbonates occurred directly along sequence boundaries, and was affected by marine diagenetic fluids diluted by meteoric influx. These dolomites are fine crystalline (5-20 µm), with a mean ^dgr¹⁸O composition (+0.6^pmil) enriched by 3.3^pmil relative to extant sea water, and with a mean ⁸⁷Sr/⁸⁶Sr value (0.70756) similar to that of these fluid . Early recrystallization, with attendant slight depletion in mean ^dgr¹⁸O (-0.25^pmil) and decrease in Sr, occurred in shallow-buried deposits bathed in Leonardian sea water, in part, during ensuing highstands. Platform-margin strata were not often subaerially exposed during lowstands, and were not dolomitized syndepositionally.

Slope carbonates, derived from the platform margin, were originally composed of mixtures of aragonite and calcite. Despite repeated eustatic changes, these deposits were not exposed to meteoric or mixed meteoric fluids. Instead, circulation of marine fluids through shallow-buried slope deposits during both lowstand and highstand first resulted in aragonite dissolution mediated by organic matter reactions, and/or by concurrent cementation by high-Mg calcite (mean ^dgr¹⁸O = -3.1^pmil). While still in contact with circulating marine fluids during the Permian, proximal slope deposits then were pervasively dolomitized in rock-dominated systems; mean ^dgr¹⁸O compositions of the dolomites is -2.25^pmil. Distal slope deposits were not dolomitized, presumably because of l mited rates of fluid circulation, and hence, limited Mg advection through the sediments.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995