Abstract

Lower Permian platform-interior (peritidal and shallow subtidal) deposits, platform-margin reefs and grainstones, and re-sedimented periplatform carbonates are pervasively dolomitized in and around the Midland Basin. Peritidal and shallow subtidal dolomites are near-stoichiometric and fine to medium crystalline, with mean δ¹⁸O composition (0.6 ‰ and -0.25 ‰, respectively) enriched relative to presumed early Permian seawater. The range of their mean δ¹³C compositions (2.9-3.2 ‰) is depleted relative to early Permian seawater. Dolomite Strontium isotopic ratios (0.70739-0.70756) record a Permian seawater signal. The “Abo-type” platform-margin dolomites are coarsely crystalline and also nearly stoichiometric. Mean δ¹⁸O composition of these dolomites (-0.22 ‰) also is enriched, and their mean δ¹³C composition (1.81 ‰) also is depleted, relative to early Permian seawater. These data suggest that the platform-interior and platform-margin deposits underwent syndepositional dolomitization by interaction with interstitial hypersaline marine fluids in open diagenetic systems. Carbon isotopic composition suggests that dolomitization was promoted by active bacterial sulfate reduction. Correlation of increasing dolomite crystal size, progressively more depleted δ¹⁸O compositions, and decreasing Sr concentrations indicate varying degrees of later neomorphic alteration of the newly-formed dolomites, perhaps as a consequence of interaction with fluids of mixed marine-meteoric composition.

Periplatform dolomites are medium to coarsely crystalline and near-stoichiometric. The range of their mean δ¹⁸O composition (-2.6 ‰ to -3.96 ‰) is depleted relative to early Permian seawater, but the most enriched δ¹⁸O compositions in this sample set are within that of modern seawater. The range of mean δ¹³C compositions (1.55-3.05 ‰) are depleted to varying degrees relative to early Permian seawater. Mean Sr isotopic composition of the dolomites (-0.7078) also records a Permian seawater signal. Oxygen and Sr isotopic compositions suggest that dolomitization occurred either syndepositionally during the early Permian in semi-closed, marine interstitial fluid systems modified by bacterial sulfate reduction, or later (late Permian), in which case the dolomites are of late diagenetic, post-depositional origin. Progressively more depleted δ¹⁸O compositions and decreasing Sr concentrations indicate varying degrees of later neomorphic alteration of the newly-formed dolomites without evidence of meteoric involvement.

If precipitation of periplatform dolomites occurred syndepositionally, then the platform and proximal basin deposits were probably dolomitized at the same time. If the periplatform dolomites are late diagenetic (i.e., post-depositional), then syndepositional dolomitization may have only occurred on the platform. Regardless, pervasive dolomitization appears to have been promoted by fluid circulation and active bacterial sulfate reduction through and within the sediments. Bacterial sulfate reduction acts to overcome critical kinetic barriers to dolomite nucleation. Dolomites in Holocene peritidal and shallow subtidal sediments in northern Belize may provide a modern analog to inferred widespread syndepositional dolomitization of some ancient platform deposits.