ORIGIN OF THE MIDDLESBORO MEMBER OF THE LEE FORMATION (LOWER PENNSYLVANIAN) IN THE CENTRAL APPALACHIAN BASIN AND ITS UNIQUE FOSSIL OCCURRENCE. Cobb, J. C., Kentucky Geological Survey, Lexington, Kentucky 40506, USA; Gastaldo, R. A., Auburn University, Alabama 36849, USA; and Chesnut, D. R. Kentucky Geological Survey, Lexington, Kentucky 40506, USA. The Middlesboro Member of the Lee Formation is an Early Pennsylvanian (Namurian) sandstone in the central Appalachian Basin in Kentucky, Tennessee, and Virginia. Field evidence from outcrops on Pine Mountain in Tennessee indicates well-defined channel and floodplain facies. The channel facies is composed of very poorly sorted lithic subarkosic sandstone in thick, irregular, and massive to cross-stratified beds. It has a scoured base and consists of a poorly developed fining-upward sequence that contains several types of stem and branch fossils. The floodplain facies is composed of shale, siltstone, coal, and underclay. It contains abundant organic debris, plant fossils, and rooting. Nearly 15 meters of the floodplain facies were eroded by the channel. Burrowing and marine fossils were not found in either facies. An extraordinary occurrence of numerous erect structures in the basal beds of the channel facies appears to be plants preserved as sandstone casts. These probable plant fossils are several meters tall and occur in groups. The structures may represent a stand of pteridosperms. Preliminary findings suggest that the erect plants are composed of aerial stems with attached lateral appendages and adventitious roots. Other explanations for these unique features, such as burrows, do not appear to fit available data. The sedimentological evidence for the origin of this sandstone indicates a bedload-dominated fluvial system of low sinuosity with subsidiary overbank and peat-swamp deposition. Although the origin of this unit has been controversial, with advocates supporting both marine and terrestrial models, the evidence from this occurrence strongly supports the terrestrial fluvial environment of deposition.