STRATIGRAPHIC ANALYSIS OF THE MISSISSIPPIAN FORT PAYNE FORMATION IN NORTHWEST ALABAMA AND ITS RELATIONSHIP TO THE TECTONICS OF THE BLACK WARRIOR BASIN

The Mississippian strata of northern Alabama represents deposition on a carbonate platform within the northern Black Warrior Basin, a foreland basin in Alabama and Mississippi between the Appalachian orogenic region to the southeast and the Ouachita orogenic region to the southwest. The carbonate platform was adjacent to a deep-water source of cool, oxygen-rich water that served as the source of the siliceous micrite that comprises the Fort Payne Chert, the predominate rock unit exposed along the Tennessee River in Colbert County, Alabama.

The Fort Payne Chert in the Rockpile Recreation Area in Muscle Shoals, Alabama consists of three discrete units with a notable disconformable contact between the middle unit of the Fort Payne (unit B, massive limestone with abundant chert nodules) with the underlying rhythmically deposited micrite and chert (unit A), indicating a significant erosional event. This has been interpreted as the loss of accommodation space due either to the formation of a peripheral bulge in the Black Warrior Basin (shallowing and erosion), followed by its rapid migration away from the area (deepening and deposition) during basinal evolution, or as an abrupt deepening and a subsequent scouring event associated with the deposition of the cherty middle unit (Puckett et al. 2014).

Examination of these beds in the region indicates that the disconformity is a localized scouring event on the carbonate platform rather than a broader regional phenomenon. Channel scouring occurred in the area of the deep erosional disconformity seen in the Rockpile section, but a more moderate disconformity is seen elsewhere.

While there is not enough data from the local outcrops to discern the actual location of the peripheral bulge at this time in the geologic past, the rock units, reworking of sediment, and subsequent shallowing that deposited the overlying Tuscumbia Limestone, indicate a depositional environment in the foredeep basin with an influx of cool, silica-rich water from the Ouachita Trough. The presence of the deep trough and/or the narrow foredeep basin could provide an environment for localized underwater channels that could cause the erosion and reworking of Unit A.