EMPLACEMENT, BURIAL HISTORY, AND THERMAL MATURATION OF GIANT DEFORMED SHALE MASSES IN THE CONASAUGA FORMATION, SOUTHERN APPALACHIAN THRUST BELT

The Conasauga Formation in the Southern Appalachian thrust belt in Alabama contains giant carbonate-bearing shale duplexes in a belt spanning about 120 miles in length and nearly 30 miles in width. Thickness of these shale masses locally exceeds 14,000 feet, indicating a four-fold thickening of the original Cambrian shale section. Analysis of the burial and thermal history provides critical insight into the timing of emplacement of the duplexes. Thermal maturity data from the Black Warrior Basin. Cahaba synclinorium, and Coosa synclinorium constrained models of coal-bearing Pennsylvanian strata, and data from the Big Canoe Creek shale gas field constrained models of the Conasauga shale masses.

A series of 1-D burial history models based on wells in and on the flanks of the shale masses indicate an initial phase of decelerating subsidence associated with Iapetan rifting and passive margin development. Subsidence accelerated during the Alleghanian orogeny, and strata reached maximum burial near the end of the Pennsylvanian. Post-orogenic uplift occurred until the Late Cretaceous, when Paleozoic strata were reburied below the Gulf of Mexico coastal plain.

Thermal history models indicate that Conasauga strata approached the oil window during passive margin development and that major maturation occurred in the Alleghanian foreland near maximum burial. Analysis of the relationship of thermal maturity data to structural geometry indicates that maturation is largely post-kinematic. Accordingly, thrusting and emplacement of the deformed shale masses occurred early in the structural evolution of the southern Appalachian thrust belt, perhaps as early as Moscovian time. Vitrinite reflectance gradients indicate that the interior thrust belt structures were buried 1-2 km deeper than the adjacent Black Warrior Basin. Formation of the duplexes partially offset accommodation of the synorogenic basin fill. Thermal conductivity of carbonate-rich pre-orogenic strata, including Conasauga shale, is much higher than that of the shale-dominated synorogenic strata. Evidence for widespread pressure solution further suggests that the shale masses were sites of thermal advection during thrusting.