DELINEATING SUBTLE STRUCTURAL TRENDS IN THE CENTRAL APPALACHIAN BASIN, A COMPARISON OF PENNSYLVANIAN COAL BED AND MISSISSIPPIAN CARBONATE ISOPACH MAPS

In the Central Appalachian Basin, numerous studies have documented coal thickening and increased numbers of beds basinward related to regional subsidence, as well as coal thinning or truncation across regionally mapped faults and anticlines. Recent research indicates that the principle mined beds also exhibit subtle rectilinear thickness trends along northwest-southeast and northeast-southwest orientations, which are not directly related to mapped structural features. Where closely-spaced coal thickness data are available it can be shown that trends of coal thinning, on the order of 20 to 60 cm, are continuous along rectilinear trends for tens of kilometers. Regional isopach maps of the Fire Clay, Upper Elkhorn No. 3, and Lower Elkhorn coals all exhibit these trends, although each exhibits these trends in different parts of the coal field. In some areas, coal beds split into multiple benches along linear orientations or post-depositional paleochannels follow similar northwest-southeast and northeast-southwest orientations.

A recently compiled isopach map of the Mississippian “Big Lime” carbonates using subsurface well data also exhibits broadly rectilinear trends. The top of the Big Lime varies from less than 100 m to more than 1000 m beneath the Fire Clay coal bed across the Eastern Kentucky Coal Field. One prominent northeast-southwest change in Big Lime thickness can be traced for 75 km, which corresponds to trends of coal thinning in the Pond Creek and Fire Clay coals along part of its length. Another significant northwest-southeast trend of thinning that can be traced for more than 50 km in the Big Lime and corresponds to a trend of thinning and splitting in the Upper Elkhorn No. 3 coal across part of its length. Many smaller rectilinear trends in the coal isopachs correspond to crudely rectilinear thickness patterns in the deeper Big Lime. The similarity of trends suggests that they are related to deep-seated fault blocks or fractures above deep-seated fault blocks. One potentially useful result of this comparison is that coal bed isopachs might prove a useful tool in delineating subtle subsurface fault and fracture trends; especially since it is easier to obtain very closely spaced coal-bed thickness data in many parts of eastern Kentucky, than it is to obtain deeper oil and gas well data.