IAPETAN RIFTING, THRUST ARCHITECTURE, AND MUSHWADS: NEW DATA FROM SECARB-USA EXPLORATION IN THE COOSA DEFORMED BELT, SOUTHERN APPALACHIANS, ALABAMA

New seismic and well data from exploration for CO₂ storage in the Appalachian thrust belt provides a new view of the Coosa Deformed Belt. These data provide insight into Iapetan rifting, thrust architecture, and the development of giant deformed shale masses that Bill Thomas called mushwads. Indeed, these were three of Bill’s most treasured topics, and these new data bring fond memories of his research and collaborations to the fore.

These data are from the heart of the Coosa Deformed Belt, which is an interior structural complex of the sedimentary Appalachians. The well was drilled and the seismic was shot and depth converted near the southwest margin of the Vandiver mushwad. The seismic profile shows the southeastern margin of the Birmingham Graben, which contains a 5,000 ft-thick section of Cambrian-age strata that includes the Shady Dolomite, the Rome Formation, and the Conasauga Formation. Southeast of the graben, a thin veneer of Conasauga strata overlies crystalline basement. The northern part of the graben has been structurally inverted, which has had a major impact on thrust-belt architecture.

The Cambrian-Ordovician carbonate section is detached near the top of the Conasauga section, and a back-thrusted fault-bend fold is imaged above the inverted graben segment. The fault-bend fold is developed above a ramp that diverges from the master decollement and an upper-level detachment that is in the Knox Group carbonates. Above the fault-bend fold, a forward thrust panel is imaged above a thrust ramp that extends into Floyd-Parkwood shale in the interior of the Vandiver mushwad. Hanging-wall cutoffs are well imaged in the seismic profile, and the leading part of the thrust has been folded with the northeast limb of the fault-bend fold. Together, the fault-bend fold and forward thrust constitute an unusual triangle zone that was localized above the inversion structure.

Exploratory drilling is set to continue northwest of the fault-bend fold in a widespread, gently dipping panel of Cambrian-Ordovician carbonate. Without the legacy of Bill’s conceptual framework, interpretation of these data would be extremely difficult. Indeed, his legacy will influence geological thinking in the region as long as mankind remains interested in rocks.