Southeastern Section - 67th Annual Meeting - 2018

Paper No. 32-1
Presentation Time: 1:30 PM


SOUTHWORTH, Scott, U.S. Geological Survey, MS 926A National Center, Reston, VA 20192

Interesting geomorphic features in the central and southern Appalachians from the Plateau east to the Piedmont reflects strong relations of geomorphology controlled by lithology and tectonism. The features are largely related to gravity processes associated with uplift, faults, and seismicity, as well as erosion associated with extreme climates. Ages of zircon and apatite in bedrock and sediments, pollen in lignite, and periglacial solifluction lobes require that some features be as old as Late Cretaceous. The presence of bauxite, kaolinite, manganese, and lignite in alluvial fans and karst depressions reflect weathering in extreme warm and wet climates.

Geomorphic features related to tectonics include: 1) domes of differential uplift across Carboniferous-Permian faults, 2) the uplifted bedrock plateau beveled by the New River, 3) antecedent creeks containing large potholes in highlands of granitic rocks on the New River plateau, 4) closed basins occupied by underfit streams on the plateau, 5) formation and retreat of the Blue Ridge escarpment on the east side of the New River plateau. If the escarpment retreated from the border fault of the Triassic Dan River basin, then part of the plateau could be of Mesozoic age, 6) uplift associated with a concentration of Eocene igneous rocks that persists as a drainage divide in the Plateau/Valley and Ridge, 7) headwater expansion of antecedent drainage systems in the Miocene across a “Potomac River plateau” into the Great Valley drainage system, and 8) rock slides along a 78 km-long bedrock dip slope that were triggered in the Cenozoic by shaking in the historically active Giles County seismic zone.

Geomorphic features requiring localized extreme rainfall events include: 1) denuded basins with as much as 600 m relief along a 34 km long bedrock dip slope at 1250 m altitude in the Valley and Ridge, 2) west-facing coves in the Blue Ridge and Valley and Ridge with as much as 1120 m relief mantled by sediment derived from a bedrock escarpment, 3) alluvial fan deposits along lower slopes are not associated with modern drainage, 4) westward-retreat of the southeast-facing Blue Ridge escarpment by debris flows, 5) channels cut 30 m into carbonate rocks that are filled with alluvium, and 6) channels cut 60 m into crystalline rocks below the ~37 ka bedrock strath terrace of Great Falls of the Potomac.