CUMBERLAND PLATEAU OVERTHRUST AND SEQUATCHIE ANTICLINE: GENETICALLY INDISTINGUISHABLE STRUCTURES STUDIED THROUGH DETAILED MAPPING IN LANCING, HEBBERTSBURG, AND FOX CREEK QUADRANGLES

The Alleghanian Cumberland Plateau overthrust (CPO) is the westernmost major structure in the Appalachians. Its NE boundary formed in the Cumberland Plateau in complex short (< 10 km) segments of thrust faults and steeply dipping, short, (< 5 km) dextral tear faults 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 sandstones, shales, coals, and siltstones with Mississippian carbonates and shales exposed to the SW in the breached portions of Sequatchie anticline. In the map area, the Pennsylvanian is represented primarily by the Crab Orchard and Crooked Fork Groups with very 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. Representing the westernmost Alleghanian deformation at this latitude, the CPO ramps from the weak basal detachment in the Cambrian Rome Formation up into the Pennsylvanian units, with the majority of surface expressions occurring in the shale of the Vandever Formation in the Crooked Fork Group as bedding plane faults. The CPO has directly influenced the topography of the Cumberland Plateau with repetition of units resulting in anticlines that are expressed as ridges and small mountains such as Peavine Mountain in the Hebbertsburg quadrangle, and sub-horizontal Vandever Formation shale and sandstone generally underlie the valleys.

The Sequatchie anticline has its northernmost terminus in the map area, and is breached for much of its length along the Sequatchie Valley fault (SVF) producing the anticline by rollover along this fault. 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. At this location, the ER fault transitions to the CPO as the system wraps around the northern end of the northeastward-plunging Sequatchie anticline.