STRUCTURE OF THE GADSDEN MUSHWAD, APPALACHIAN THRUST BELT, ALABAMA

A 1985 exploration well drilled 2835 m of shale-dominated Cambrian Conasauga Fm below the Cambrian-Ordovician Knox Group in the Dunaway Mountain (DM) thrust sheet. A publication in 2001 coined the term “mushwad” for the tectonically thickened Conasauga Fm in the hanging wall of the Big Canoe Valley fault and beneath the composite DM, Rome, and Helena thrust sheets. Outcrop geology, one deep well, and seismic reflection profiles supported the interpretation of the tectonically thickened shale as a ductile duplex (the Gadsden mushwad) between the Big Canoe Valley floor thrust and the DM roof thrust. The DM thrust sheet, which is detached in the Conasauga Fm, is dominated structurally by the Knox Group massive carbonates, the Appalachian regional stiff layer. The out-of-sequence Rome thrust sheet of tectonically thickened Conasauga Fm forms the trailing cutoff of the DM thrust sheet; the large out-of-sequence Helena frontal ramp, dominated by the Knox Group, is the trailing cutoff of the Rome thrust sheet. In this interpretation, both the DM and Helena thrust sheets rise from the Appalachian décollement below the Knox Group. Seismic reflection profiles clearly image the Helena thrust sheet but not the underlying DM thrust sheet.

Subsequent drilling of 16 wells in the Big Canoe Creek shale-gas field provides important subsurface data. Most of the wells spudded in the Gadsden mushwad northwest of the DM thrust sheet; others drilled through the DM thrust sheet into the mushwad. One well southeast of the field spudded in the Rome thrust sheet and drilled to a depth of 2007 m, entirely in tectonically thickened Conasauga Fm. In the original interpretation, the well would have penetrated the southeast-dipping DM thrust sheet at a shallow depth below the Rome thrust sheet. Lack of the DM thrust sheet in the well shows that the thrust sheet does not extend down dip to the décollement. Exposed folds and faults, along with the now-defined short southeasterly extent, indicate that the DM thrust sheet is an isolated synclinally folded klippe of the roof thrust of the Gadsden mushwad, possibly detached from the Helena thrust sheet. The previously inferred extensive DM thrust sheet occupied a large volume below the Helena fault, a volume that is now included in the mushwad, significantly increasing the volume of the mushwad and the shale-gas resource.