Birmingham anticlinorium in the Appalachian fold-thrust belt, basement fault system, synsedimentary structure, and thrust ramp

Abstract

Abstract At Birmingham, in the central part of the Appalachian foldthrust belt in Alabama, the Birmingham anticlinorium is associated with a large-scale ramp on the regional decollement and includes the Opossum Valley and Jones Valley thrust sheets (Figs. 1,2, and 3). The southeast limb of the asymmetric anticlinorium in the hanging wall of the Jones Valley fault dips gently southeast into the Cahaba synclinorium (Locality 42); the northwest limb in the footwall of the Opossum Valley fault is overturned and dips steeply southeast (Locality 43) (Fig. 3). The youngest rocks in the anticlinorium are Early Pennsylvanian, indicating thrusting during the Alleghanian orogeny. The Middle Cambrian Conasauga Formation is the oldest unit in the hanging walls of the Opossum Valley and Jones Valley thrust faults, thus indicating the stratigraphic level of the regional decollement (Fig. 3). The Appalachian fold-thrust belt in Alabama may be divided into three domains (Thomas, 1982). The Birmingham anticlinorium is the most northwesterly structure in the central domain, which is characterized by large-scale thrust ramps and associated folds having structural relief of more than 15,000 ft (4,500 m). In contrast, the northwestern domain is characterized by broad, flat-bottomed synclines and narrow, asymmetric anticline having relief of less than 10,000 ft (3,000 m). The southeastern domain is characterized by broad, low-angle, multiplelevel thrust sheets. Amplitude of structures within the central domain, as indicated by dip angles on fold limbs and by preserved thickness of Paleozoic rocks in the synclines, defines a minimum depth to basement of approximately 23,000 ft