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.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.