Subaerial Paleokarst in the Upper Pennington Formation (Upper Mississippian) Limestones at Leatherwood Ford, Big South Fork, Tennessee

The uppermost Pennington Formation (Mississippian - latest Chesterian) consists of limestone deposits, which, in some areas, contain evidence of paleokarst. A 100 m long continuous outcrop of the upper Pennington Formation was studied at Big South Fork National River and Recreation Area, in Scott County, Tennessee, using both field and petrographic methods, in order to test the hypothesis that the paleokarst is of subaerial origin.

The upper Pennington limestone section was subdivided into 4 depositional units, interpreted as recording sea-level change through the end of the Mississippian. Although there was an overall regression toward the end of the Mississippian, changes from unit 1 - a brecciated dolomicrite that reflects minor exposure, to unit 2 - a skeletal packstone that reflects a flooding surface, indicate a slight transgression that began lagoon development in this area. Unit 3 - a peloidal wackestone to packstone, reflects the growth and the entrenchment of the lagoon, whereas unit 4 - a peloidal-skeletal grainstone, reflects the demise of the lagoon due to regression, after which the upper Pennington limestone was subaerially exposed.

Paleokarst features associated with this period of subaerial exposure were mapped and catalogued according to three scales of features: macroscale, mesoscale, and microscale. Macroscale refers to major features expressed as paleotopographic variation along the exposure surface, such as paleo-dolines, the largest of which has a width of approximately 30m, and a depth of 4m. Other features include a shallow and flat-floored kamenitza with a width of 7m and a depth of only 0.5m. Mesoscale features include mantling limestone breccia-conglomerate, red and green claystone paleosols, limestone breccia clasts, and iron oxide and oxy-hydroxide crusts. Microscale features include meteoric cements, grain micritization, reddening of the limestone, and extensive pore development.

Five phases of paleokarst are recognized: Phase 1 included the initial development of paleokarst features, particularly at the microscale. Phase 2 was the development of the macroscale paleo-dolines and paleo-kamenitzas. Phase 3 occurred as soil and vegetation developed on the karst plain. Phase 4 was marked by pulses of early Pennsylvanian sedimentation, in which paleo-channels developed, and mantling deposits, including a residual breccia-conglomerate, began to drape exposed limestone, and collapse at the paleoexposure surface occurred. Finally, phase 5 involved the movement of reducing fluids along the unconformity and the precipitation of burial cements.