Abstract

Large-scale recumbent folds are recognized in the Coosa Valley in Shelby and Talladega Counties, Alabama, on the basis of map pattern, structural data gathered in the field, and top sense derived from stratigraphic sequence and primary sedimentary features. Recumbent folds at two structural levels deform rocks ranging in age from Ordovician to Mississippian.

The structure evolved in three stages. Westward-directed recumbent folding about north-trending axes in Early Mississippian time was followed by refolding about upright northeast-trending axes in Pennsylvanian or later time. During or after the late folding, the area was arched by a broad, gently southeast-plunging cross anticline over which northeast structural axes were locally rotated to north-south and east-west trends.

Recumbent folding was accompanied by soft-sediment deformation within the Mississippian Floyd Shale, which in Shelby County is interpreted as flysch that was deposited during this folding. The Floyd grades upward and laterally into marine molasse (Parkwood Formation) and later into nonmarine molasse (Pottsville Formation) of Pennsylvanian age. Mississippian Floyd Shale is the youngest unit involved in the recumbent folding.

The Alabama nappes probably were formed by gravitational sliding of cover rocks toward the west or southwest into the Floyd flysch basin.

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