Polyphase Deformation of Lateral Ramps in Appalachian Foreland Fold and Thrust Belt and Talladega Slate Belt in Alabama

W. Edward Osborne, Gregory M. Guthrie

The foreland fold and thrust belt (FFTB) and Talladega Slate belt (TSB) in Alabama have undergone a complex deformation history due to foreland-directed thrust-sheet emplacement. In the FFTB and TSB near the coastal plain overlap, surface geology indicates that west-northwest to east-southeast-oriented lateral ramps (LR) have a polyphase deformation history. In the TSB, mylonites developed in a ductile environment terminate laterally at zones of brittle deformation. Lateral stratigraphic changes from Lower Cambrian to Lower Devonian(?) metasedimentary rocks provide additional evidence for a first-generation LR related to emplacement of the Talladega-Cartersville thrust sheet. Later generation LR structures are related to emplacement of underlying thrust sheets in the FFTB. Northwest of the TSB, a duplexlike structure at the approximate structural level of the Pell City thrust sheet changes to a regional thrust sheet northeast of a proposed lateral ramp. In the overlying Talladega sheet, this generation is represented by brittle, high-angle faults. Northwest of the Pell City thrust sheet(?), the structural style of the Coosa deformed belt changes from a wide zone of complex imbricate thrusts to a region of complex folding. Northwest of the Coosa deformed belt, stratigraphic and structural characteristics of the Helena thrust sheet (a target of current petroleum exploration) indicate a down-to-the-northeast LR displacement. This displacement is recognized by a change in stra igraphic level of surface structures from predominantly Cambrian and Ordovician in the southwest to Pennsylvanian in the northeast, and by a change in structural style from high-level imbricate thrusts to a major synclinorium (Coosa synclinorium) in the northeast. Movements along these late-stage LR are reflected in the TSB by west-northwest to east-southeast-oriented cross folds with a down-to-the-northeast displacement. Thus, in this region of the Appalachian orogene, lateral ramps in younger thrust sheets deform older thrust sheets. The constant sense of displacement and position through time in subjacent LR indicate that position of LR may be controlled by down-to-the-northeast basement offsets.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.