THE APPALACHIAN THRUST BELT AND HARPERSVILLE TRANSVERSE ZONE IN THE HARPERSVILLE 7.5-MINUTE QUADRANGLE, ALABAMA

The Harpersville 7.5-minute quadrangle (HQ) is in the trailing part of the Appalachian sedimentary thrust belt in Alabama along the Harpersville transverse zone (HTZ). All regional structures in the HQ exhibit along-strike changes at the HTZ.

On the northwest, thin imbricate thrust sheets from an upper-level detachment above the regional stiff layer (Cambrian–Ordovician Knox Group massive carbonate) constitute the Coosa deformed belt (CDB). At the HTZ, the Ordovician–Mississippian upper-level stiff layer in the frontal CDB Yellowleaf thrust sheet plunges southwest beneath disharmonically deformed Mississippian shale-sandstone. In the HQ, thrust faults of the intermediate CDB truncate the Yellowleaf thrust sheet, curve from regional northeast strike to southerly strike, and end in south-plunging lateral-ramp anticlines.

South of the imbricate thrust sheets, two large-scale anticlines in Cambrian–Mississippian strata overlie ramps from the regional décollement up to the CDB detachment: northeast-trending up-to-northwest Steam Plant frontal-ramp anticline and northwest-trending up-to-northeast Fourmile Creek lateral-ramp anticline at the HTZ. Smaller, tighter folds overprint the long-wavelength folds, balancing shortening of the CDB imbricate thrust faults.

The Pell City fault curves from regional northeast strike to southerly strike at the HTZ, terminating the thrust sheet at an oblique footwall lateral ramp that truncates the CDB and Steam Plant anticline. The Pell City fault cuts up section southwestward along strike from lower Knox Group to upper Knox at a hanging-wall lateral ramp. North-northwest-striking upright isoclinal folds preserve Middle Ordovician Athens Shale in the Pell City thrust sheet, indicating lateral contraction as the hanging-wall lateral ramp was translated over the footwall lateral ramp along the HTZ.

On the south, the Wilsonville thrust fault truncates the folds in the Pell City thrust sheet, the Pell City fault, and the Steam Plant and Fourmile Creek anticlines. The Wilsonville thrust sheet ends along strike in both directions within the HTZ.

In the southeast corner of the HQ, the Talladega fault at the leading edge of the Talladega slate belt truncates the Wilsonville thrust sheet. The Talladega fault is at the leading edge of a sub-regional salient within the HTZ.