Abstract

The Lower Seven Rivers (Guadalupian) evaporite to carbonate facies change, commonly interpreted as a coastal sabkha/carbonate lagoon transition, represents instead an interfingering of dolomite and gypsum in a shallow subaqueous hypersaline shelf setting behind the main Capitan shelf-edge complex. Three mesosaline (35-120 per thousand ) carbonate lithofacies occur in lateral succession over a distance of 3.5 km, adjacent to the evaporites. From the evaporite margin toward the Capitan shelf crest they are: 1) evaporite-edge grapestone grainstones which are current-laminated and occur interbedded with fine-grained quartz sandstone units suggesting a moderate-energy, shallow subaqueous depositional environment; 2) skeletal-rich pellet packstone/wackestones which are commonly current-laminated or burrowed and contain a low-diversity but abundant biota of calcispheres and encrusting forams with a few molluscs and ostracods, suggesting a shallow subaqueous environment; and 3) a shallower shelf facies of pellet wackestone/mudstone characterized by increased micrite, fewer skeletal fragments, and algal stromatolites. The evaporite facies occurs as layers 1 to 7 m thick composed dominantly of mosaic gypsum deposited in a subaqueous environment (i.e., shelf basin) during times of maximum salinity, perhaps influenced partly by the evaporite-margin facies. The mosaic structure is suggested to have been formed by early diagenetic growth of sulfate nodules below the sediment-water interface. The average delta S 34 for this gypsum is +9.3 + or - 1.0 per thousand which agrees well with the average Upper Permian value of + 10 per thousand . A fourth carbonate facies of skeletal-poor pellet packstone/wackestones occurs interbedded with the evaporites. It is commonly planar laminated and contains scarce skeletal fragments similar to the skeletal-rich facies. Neither the evaporite facies nor the skeletal-poor facies within it, nor the two adjacent carbonate facies, contain sabkha features such as intertidal algal laminates, desiccation features, or subtidal to supratidal cycles capped by nodular gypsum. Post-depositional diagenesis included 1) cementation of the carbonates by CaCO 3 in two forms, including eogenetic isopachous (submarine) cement in the grapestone grainstones and eogenetic/mesogenetic equant spar cement in the grainstones and packstones; 2) eogenetic and probably syndepositional dolomitization of the carbonate rocks to a microcrystalline dolomite; 3) eogenetic insediment growth of evaporite crystals and nodules; 4) post-Permian telogenetic solution brecciation and dedolomitization; and 5) hydration and recrystallization of subsurface anhydrite to gypsum.

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