HIGH-RESOLUTION SEQUENCE STRATIGRAPHIC MODEL FOR SUBSURFACE MISSISSIPPIAN GREENBRIER GROUP, WEST VIRGINIA

The 50 to 500m thick mixed carbonate-siliciclastic Mississippian Greenbrier Group of West Virginia formed on the western and northern distal margins of the Appalachian foreland basin under the influence of Carboniferous glacio-eustasy during transition into icehouse conditions. The Greenbrier Group has been studied in great detail in outcrop for almost hundred years, however little is known concerning the sequence stratigraphy in the subsurface, the trends and stacking patterns of facies. We are generating a high-resolution sequence stratigraphic framework for the Greenbrier Group throughout the subsurface in West Virginia, using well data (200 wells with cuttings, wireline logs and some core). A series of detailed regional cross-sections and isopachs of time slices are being prepared to better understand these trends. Five major sequences occur composed of three to four regionally mappable high frequency sequences. Major- and high frequency sequences are composed of lowstand red beds up-dip and shallow marine sands along the ramp margin, semi-transgressive shale's, high-stand quartz peloidal grainstone (dominantly eolian), peritidal lime mudstone, peloid grainstone, ooid grainstone, skeletal grainstone, open marine skeletal wackestone/mudstone, and shaly slope mudstone. Sequence boundaries are placed below lowstand sands, red beds, caliches and/or eolianites. Thickness trends strongly reflect tectonically induced basinal subsidence. Isopachs of selected time show the effects of subtle regional tectonic features in controlling distribution of facies trends. The ramp margin in the vicinity of the basinal hinge line separates the relatively stable up-dip sections from the faster subsiding basin and localized high-energy grainstone trends, as did subtle tectonic highs. Subtle tectonic lows on the platform and ramp margin localized siliciclastics (shale's, calcareous siltstones and sandstones). Subsidence rates in the up-dip and downdip areas differ by an order of magnitude, yet the eustatic signal, was the dominant cause of the 3rd order sequences and component high frequency sequences.