SORTING STRATIGRAPHIC SPAGHETTI IN THE CONTEXT OF GEOLOGIC MAPPING: EXAMPLES FROM MAPPING IN THE MISSISSIPPIAN GREENBRIER GROUP AND BLUEFIELD FORMATION OF THE CENTRAL APPALACHIAN BASIN

Compilation of geologic maps covering large areas is confounded by discontinuities in geologic contacts across boundaries of states, counties, or quadrangles. Resolving these issues requires synthesis of existing data along with employment of modern precision mapping techniques to verify contact locations and stratigraphic spatial relationships. Recent USGS mapping in the Appalachian Basin of Virginia and West Virginia provides examples of the need for close examination of previous work before reusing existing geologic contacts.

Monroe County, West Virginia, encompasses the type areas for several formations in the Mississippian Greenbrier Group and units in the overlying Bluefield Formation of the Mauch Chunk Group. These lithostratigraphic units were established in a multi-county geologic report by D. Reger (1926). This stratigraphy has been referenced in many succeeding geological maps, theses, and topical studies, not only in Monroe County, but in other areas of the Appalachian Basin stretching from Tennessee to Pennsylvania. During our ongoing mapping in Monroe County we discovered errors and inconsistencies in identification and correlation of the reference sections for several key lithostratigraphic units. These findings are verified via examination of the rocks and recognition of the stratigraphic sequences at multiple localities in the field, and by analysis of high-resolution lidar terrain models. For example: rocks exposed in the type area and the prime reference section of the well-known Alderson Limestone, at the top of the Greenbrier Group, are in fact rocks of the Glen Ray Limestone that occur well above the base of the overlying Bluefield Group. In another case, rocks exposed at the type locality of the Greenville Shale which occurs below the Alderson Limestone, are actually outcrops of the Lillydale Shale of the basal Bluefield Formation. These inconsistencies confuse geologists who use these sections as comparative references for work outside of Monroe County, and throw doubt on the validity of the placement of some of the contacts within the original Monroe County map. Most of these contacts are followed in the current West Virginia State geologic map and may have influence the placement of contacts on adjoining maps of Virginia.