BURIAL HISTORY AND THERMAL MATURATION OF GIGANTIC DEFORMED SHALE MASSES IN THE CONASAUGA FORMATION (CAMBRIAN), SOUTHERN APPALACHIAN THRUST BELT

The Conasauga Formation in the Southern Appalachian thrust belt in Alabama contains giant deformed shale masses in a belt spanning about 120 miles in length and nearly 30 miles in width. Thickness of these 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 and the hydrocarbon potential of the shale masses.

A series of 1-D burial history models based on wells in and on the flanks of the shale masses indicates an initial phase of decelerating subsidence associated with Iapetan rifting and the establishment of a passive margin along the southeastern flank of the Alabama promontory. Subsidence accelerated during the Carboniferous in response to the Alleghanian orogeny, and maximum burial is thought to have been reached during Permian time. Post-orogenic uplift and unroofing occurred until the Late Cretaceous, when some of the shale masses were reburied below the Gulf of Mexico coastal plain.

Lopatin models indicate that Conasauga strata may have entered the oil window during passive margin development, but major maturation is thought to have occurred near maximum burial during the Late Paleozoic. This is when Conasauga strata entered the dry gas window. Analysis of the relationship of thermal maturity data to structural geometry indicates that maturation is largely post-kinematic. Accordingly, emplacement of the deformed shale masses is thought to have occurred early in the structural evolution of the southern Appalachian thrust belt.