TECTONIC STRATIGRAPHY AND DEPOSITIONAL GEOMETRY OF MISSISSIPPIAN (OSAGEAN-MERAMECIAN) FORMATIONS OF NORTHWEST ALABAMA USING DRONE PHOTOGRAMMETRIC ANALYSIS

Mississippian strata of northwest Alabama lies 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. While several hundred kilometers north of the Appalachian fold and thrust belt proper, these formations are heavily influenced by climatic and tectonic activity occurring during this time.

The units exposed along the Tennessee River in Colbert County, Alabama are the Fort Payne and the Tuscumbia Formations, both interpreted to have formed in the deeper margins of a carbonate ramp. The Fort Payne Fm. in this section consists of three discrete units: a lower unit dominated by rhythmically bedded micritic limestone and sporadic chert, a middle unit consisting of massive limestone and abundant chert layers and nodules, and a thinly bedded limestone and chert. The overlying Tuscumbia Fm. consists of thickly bedded recrystallized limestone.

Of interest are the sedimentologic and tectonic events that led to the origin of the middle unit of the Fort Payne Formation. At this time, the region was experiencing major changes in tectonic and oceanographic settings due to the early stages of the Appalachian and Ouachita orogenies. Tectonic changes of the basin are likely recorded in basin sedimentation. The disconformity between the middle unit of the Fort Payne Chert and the underlying rhythmites indicates a major erosional event. This has been interpreted by others 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), or as an abrupt deepening and a subsequent scouring event associated with the deposition of the cherty middle unit.

Use of drone photogrammetric analysis permits an evaluation of the geometry of the erosional disconformity underlying the middle unit of the Fort Payne Formation. Examination of that contact laterally can provide additional insight into the evolution of this part of the Black Warrior Basin. To that end, this analysis incorporated high resolution UAV-based imagery of the exposures using a DJI Mini 3 Pro drone. This imagery was then used to trace contacts between lithologic units in order to better understand the processes responsible for the erosional disconformity.