ABSTRACT: The Birmingham Anticlinorium, Alabama: Recognizing a Blind Duplex and Defining its Internal Geometry

Daniel J. Patterson, Richard H. Groshong, Jr.

Duplex systems of fault-bend and fault-propagation folds are recognized as major structural styles in fold-thrust belts though the traps are widely exploited only in the Canadian foreland. A major barrier to exploration is defining duplex geometry with sufficient accuracy. Rules of section balancing are used to develop criteria for the recognition of a duplex and approximating its internal geometry. The Birmingham anticlinorium, located close to the leading edge of the Appalachian fold-thrust belt, is an example of a large-scale blind duplex that consists of fault-bend-fold horses or fault-propagation folds. Fault-bend folding requires approximately 35 km of displacement in the foreland, whereas fault-propagation folding allows thrust-related shortening to die out in fold within the duplex. The duplex consists of the Cambrian Conasauga Formation and possibly the underlying Rome Formation. The roof thrust is near the base of the Cambrian-Ordovician Knox Group. Duplex character is determined from limb lengths and dip variations, Conasauga outcrops, and SCAT analysis of wells on the flanks of the anticlinorium and along strike. The approximate area of the duplex remains constant along strike, but duplex height ranges from about three to six times ramp height. Structural relief may be accounted for by overlapping fault-bend-fold horses or by fault-propagation folds. Area balance based on stratigraphic thickness of units involved in the duplex, cross-sectional area under closure, and back limb length indicate approximately 10 fault-bend-fold

horses are needed to fill the structural relief of the anticlinorium. The number of fault-propagation folds needed to fill the area under closure is determined to be approximately 30, based on ramp height and fault spacing required to attain observed structural relief. The folds do not have conventional fault-propagation fold geometry because they do not propagate above the Conasauga into the stiff beam formed by the Knox. Geometries are documented with sequentially restored area-balanced cross sections.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990