Dolomitization of lower Permian platform facies, Wichita Formation, north platform, Midland basin, Texas

Abstract

Three specific occurrences, or types, of dolostone are recognized in regionally widespread platform dolostones in the Wichita Formation (Permian, Lower Leonardian) in the northern Midland basin of Texas: (1) non-porous dolostones composing supratidal-intertidal facies: rocks deposited and inherently overprinted by peritidal diagenetic features in this environment, (2) porous dolostones composing subtidal-intertidal facies: rocks deposited originally in subtidal environments and subsequently exposed and overprinted diagenetically on intertidal flats; and (3) porous subtidal dolostones: rocks deposited originally in subtidal environments and which were never exposed syndepositionally to peritidal diagenetic overprinting. Fine crystalline (5–40 μm) dolomites in all three dolostone types have mean δ18O composition of 0.6‰ PDB, but are enriched in δ18O by as much as 3.3‰ relative to estimated lower Leonardian sea water. These δ18O-enriched, fine crystalline dolomites are preserved mainly in the supratidal-intertidal dolostones, whereas more coarsely crystalline (> 40 μm, mean =78 μm) dolomites occur in the subtidal and subtidal-intertidal dolostones. Coarsely crystalline dolomites have δ18O compositions that are slightly lighter (mean=−0.25‰) than the fine crystalline dolomites, but are still enriched by a mean of 1.6‰ relative to Leonardian sea water. All dolomites are near-stoichiometric, with87Sr/86Sr within the range of values of lower Permian sea water. Coarse crystalline dolomites are more depleted in Sr, Mn, and Fe than fine crystalline dolomites although, regardless of crystal size, all dolomites contain less than 125 ppm Sr and less than 100 ppm Mn.