DETAILED GEOLOGIC MAPPING OF THE OBED WILD AND SCENIC RIVER AND SURROUNDING AREA: FOX CREEK, HEBBERTSBURG, AND LANCING 7.5-MINUTE QUADRANGLES

The Alleghanian Cumberland Plateau overthrust (CPO) is the westernmost major structure in the Appalachians and is linked geometrically and kinematically to the Sequatchie anticline to the SW. Its NE boundary consists of complex, short (< 10 km) segments of thrust faults and steeply dipping, short, (< 5 km) dextral tear faults and lateral ramps separating the CPO from the undeformed Wartburg basin. The dextral Emory River (ER) fault is the main tear fault in this system. All rock units in the CPO sheet are Pennsylvanian sandstone, shale, coal, and siltstone with underlying Mississippian carbonates and shales exposed to the SW in the breached Sequatchie anticline. In the map area, the Pennsylvanian is represented by the Crab Orchard and Crooked Fork Groups, with limited exposure of upper Gizzard Group.

Detailed geologic mapping, mesostructural, and cross-section analysis in the Lancing, Hebbertsburg, and Fox Creek 7.5-minute quadrangles have provided further insight into the structure of the CPO. In addition to the existing 357 structural measurements made by previous workers, 1,623 more data stations were recorded including 1,033 (533 in Lancing, 319 in Hebbertsburg, and 181 in Fox Creek) structural measurements. Four cross sections have been constructed through this area using new structural data collected on the surface combined with well data from nearby oil and gas wells.

The Sequatchie anticline terminates in the map area, and is breached for much of its length exposing the Sequatchie Valley fault (SVF) producing the anticline as a fault bend fold. The SVF and CPO are the same structure, and the relationship between these structures is illustrated in the Lancing quadrangle, where older Pennsylvanian Sewanee conglomerate is exposed in the breached northern terminus of Sequatchie anticline SW of the ER fault. Construction of a balanced cross section in the Lancing quadrangle through the northern terminus of the Sequatchie anticline and the CPO, where it outcrops on Hatfield Mountain, requires a previously unknown subsurface fault to account for the differences in displacement between the exposed SVF and the CPO, and balance the section. This project achieved its goal of producing detailed 1:24,000-scale geologic maps of the Obed Wild and Scenic River that cross several major tectonic features.