UPPER-LEVEL MUSHWAD (DUCTILE DUPLEX) IN THE SOUTHERN APPALACHIAN THRUST BELT

In the southern Appalachian thrust belt, the regional décollement is in Cambrian shale; and an overlying, massive, Cambrian-Ordovician, carbonate stiff layer controls the geometry of large-scale thrust sheets. A mushwad (ductile duplex) of Cambrian shale along a sub-décollement basement fault system elevates and deforms the geometry of an alignment of frontal ramps (Jones Valley-Opossum Valley and Big Canoe Valley). In the trailing thrust sheet (Helena) behind the mushwad, a trailing brittle duplex of the stiff layer transmits shortening to an upper-level, brittle duplex (Coosa deformed belt) in Ordovician-Mississippian strata at the top of the stiff layer. Above the upper-level, brittle duplex, the shortening is accommodated within a thick, Mississippian-Pennsylvanian, shale-sandstone succession in the Coosa synclinorium, where folds in Pennsylvanian (lower Pottsville Formation) sandstones are disharmonic with and detached from the regional stiff layer in the Helena thrust sheet. The amplitude and wave length of the disharmonic folds indicate an upper-level detachment and mushwad in the Mississippian Floyd Shale, in the stratigraphically lower part of the Mississippian-Pennsylvanian clastic succession above the regional stiff layer. The upper detachment level corresponds to the roof of the upper-level, brittle duplex in the Coosa deformed belt.

Northeastward along strike, the disharmonic folds and upper-level mushwad end where the Coosa synclinorium plunges northeastward over a footwall lateral ramp into a regional depression, and displacement on the upper-level mushwad is transferred across strike to a trailing frontal ramp (Eden fault). The northeast end of the mushwad corresponds in location to an along-strike stratigraphic change in the lower part of the Floyd Shale. A brittle unit, consisting of the Mississippian Hartselle Sandstone and Bangor Limestone (cherty limestone), is >130 m thick to the northeast; the brittle unit grades out southwestward into a monotonous succession of shales in the lower Floyd Shale, which constitutes a thick ductile unit for localizing the upper-level mushwad.