THE GREENBRIER GROUP: A STRATIGRAPHIC MODEL FOR MIDDLE MISSISSIPPIAN EUSTATIC FLUCTUATIONS AND EVIDENCE FOR THE PRESENCE OF PALEO-CARBONATE ISLANDS

The middle-Mississippian Greenbrier Limestone Group (comprised of the basal Hillsdale Limestone, Denmar Formation, Taggard Shale, Pickaway Limestone, Union Limestone, Greenville Shale, and capped by the Alderson Limestone) is the most extensive carbonate sequence in the state of West Virginia, records significant basinal/facies changes, and can be used as a proxy for eustatic change in southeastern West Virginia. In southern Pocahontas County, WV, the Greenbrier Group is about 800 feet thick and can be split into two distinguishable shallowing upward sequences. Beginning above the Hillsdale Limestone, the lowermost of these is entirely contained within the Denmar Formation (~240 feet thick), the base of which is comprised of dense, micritic limestone which grades upward into oolitic, fossiliferous beds at the top. The second sequence begins at the upper contact of the Taggard Shale (a red shale, limestone, mudstone, and paleosol zone), and includes the dense, micritic Pickaway Limestone (~180 feet thick) at the base, grading upwards into the fossiliferous and oolitic Union Limestone (~160 feet thick) above. Notably, in this region, the Greenville Shale (a black, fissile, fossiliferous shale) is absent and a repeating zone of thin, red limestones, interspersed with red shales is found at the top of the Union Limestone. Farther north, in central Pocahontas County, the Greenville exists as a 5-10-foot-thick package of finely bedded, red mudstones with columnar structures which are interpreted as stacked mud-crack sequences; like those expected in the supratidal facies of a tidal flat. The combination of highly desiccated, red shales of the Greenville to the north with the lack of Greenville, and red limestone/shale zone at the top of the Union, farther south is tentatively interpreted as a supratidal zone within a tidal flat, and a subaerially exposed carbonate island. Another red limestone zone further south may indicate the presence of a pale-carbonate island chain, however limited exposures has made this delineation difficult.