UPPER CARBONIFEROUS (PENNSYLVANIAN) MUDDY SHORELINES: DEPOSITIONAL ENVIRONMENTS OF THE PATOKA FORMATION IN THE SUBSURFACE OF WEBSTER COUNTY, KENTUCKY

We examined a complete core of the Upper Carboniferous (Pennsylvanian; Missourian) Patoka Formation from Webster County, Kentucky to determine Patoka depositional environments in this area. We identified lithologic units, noting bedding features and sedimentary structures, fossil content, and ichnofabric. The Patoka here includes 263 feet of dominantly shaly strata including four coarsening-upward intervals. Each of these intervals consists of dark-colored marine shale at the base, grading upward (successively) into limonitic shale, heterolithic shale-sandstone, and argillaceous to calcareous sandstone, all capped by rooted carbonaceous mudrocks and minor coal. The coarsening-upward intervals reveal initial deposition in poorly oxygenated marine to brackish water that became shallower and better circulated during shoreline progradation. The intervals of limonitic shale to heterolithic shale-sandstone represent shallow subtidal to intertidal mud banks analogous to those forming today on the northern coast of South America between the Amazon and Orinoco Rivers. Sparsely burrowed shaly intervals formed under conditions inimical for benthic infauna in upper intertidal mud flats. More intensively burrowed shaly intervals represent relatively favorable lower intertidal to subtidal conditions. Thin sandstones in the Patoka mark offshore bars and strandlines. Paleosols and coastal marshes developed on the prograding coastal plains. After each episode of progradation, a rise in relative sea level caused a return to deposition of dark marine shale.