| Summary: | Buchanan County, located in southwest Virginia at the eastern margin of the Appalachian Plateau, is underlain by gently dipping Pennsylvanian formations. The lowest Pennsylvanian Lee Formation is exposed in an area of only 0.3 of a square mile, but information on its character and thickness has been obtained from wells drilled primarily for natural gas. Two members or the Lee are recognized intormally. The lower member containing the Pocahontas coals and distinguished by a predominance of conglomerate and orthoquartzitic sandstone may be the approximate equivalent or the Pocahontas Group of West Virginia. The upper member is characterized by “dirty” sandstone. siltstone, shale, and coal and a lack of conglomerate and orthoquartzitic sandstone.
Subsurface data indicate that rocks of Devonian and Early and Middle Mississippian age were deposited in a stable shelf environment, an environment dissimilar to that of the Lee Formation consisting predominantly of continental beds. Published regional surface data supported by 11thologic analysis and isopach mapping of the present report indicate that post-Mississippian pre-Pennsylvanian time in Buchanan County was a time of emergence and erosion. During this time, basinlike topographic depressions were developed in southeastern and northwestern Buchanan County separated by a low northeast-southwest trending ridge in central Buchanan County coincident with the present axis of the Grapevine Branch Anticline.
Initial subsidence of the post-Mississippian surface probably accompanied a rising Blue Ridge igneous and metamorphic terrain, the probable source of the Lee elastics, southeast of Buchanan County. Initial subsidence invited a southeastward transgression or the Lee sea which is believed to have advanced as far southeast as central Buchanan County where it probably was halted initially by the low topographic ridge coincident with the present-day Grapevine Branch Anticline. Subsequent southeast advances were impeded by the rapid filling of southeastern Buchanan County with continental sediments, and the southeastern margin of the marine environment remained essentially static throughout lower member time. Inference of a marine environment and its extent is based on the marine aspect of the orthoquartzite as contrasted with the continental aspect of the “dirty” sandstones and associated shales of the formation.
Isopach maps suggest that deposition in Buchanan County during Lee time was controlled predominantly by the character of local subsidence. It is interred from the maps that during lower member time Buchanan County subsided essentially as two blocks. Southeastern Buchanan County subsided differentially toward the southeast and the block was hinged along a line coincident with the present-day Grapevine Branch Anticline in the central part of the county. Simultaneously, northwestern Buchanan County foundered almost vertically with only a slight component of southeastward tilt. As a result, continental deposition was localized in southeastern Buchanan County while marine conditions prevailed in northwestern Buchanan County.
Failure of the central Buchanan County hinge at the end of lower member time was followed by general differential subsidence during which the county acted as a single unit and subsided toward the southeast in a manner analogous to the subsidence of southeastern Buchanan County during lower member time. A new hinge-line which functioned throughout upper member time is believed to have been located along the northwestern border of the county as indicated by the present distribution of orthoquartzite in the upper member. === M.S.
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