Erosional history of the New River, southern Appalachians, Virginia

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Virginia Polytechnic Institute and State University

Much of the bedrock surface of the Southern Appalachian Valley and Ridge Province is covered by a veneer of surficiaI deposits classified as alluvium, colluvium, and residuum. In this investigation the surficial geology of a 60-km2 area of the New River drainage system was mapped at a scale of 1:24,000. The area is located within the Eggleston and Newport 7 1/2-minute topographic quadrangles in Giles County, southwest Virginia.

The data derived from mapping the surficial geology (particularly alluvial deposits), in conjunction with other field observations and with heavy-mineral analyses, are interpreted in terms of provenance, depositional environment (or accumulation), and preservation of the surficial deposits. In addition, these data are applied to an interpretation of the evolution of the New River drainage in the Valley and Ridge Province during the latter half of the Neogene.

The areal distribution of the surficial materials indicates that in a humid, temperate climate deposits of surficial materials tend to be preserved if they overlie carbonate bedrock which weathers chemically but tend to be eroded from shale and sandstone bedrock which weathers mechanically. In carbonate terrains where surface runoff is minimal, surficial materials are let down in place by solution and have been accumulating in a piecemeal fashion over a time period which in some areas may include all of Cenozoic time.

Analysis of the transparent heavy-mineral assemblages contained in the modern alluvium and older alluvial deposits of the area indicates that radiation-damaged zircon (intermediate and metamict) is unstable under conditions of subaerial weathering. Earlier workers have suggested that zircon is dissolved by acid ground water. This study supports these earlier suggestions and further demonstrates in a semiquantitative manner that the solution rate of radiation-damaged zircon may be a linear function of time as measured against either tourmaline er normal zircon. The estimated period of time over which the solution rate of zircon appears . to be lil1ear is on the order of 10 m.y.

The areal distribution and lithology of alluvial deposits provide evidence which can be used to reconstruct the late Cenozoic evolution of part of the New River drainage system within the Valley and Ridge Province. These data, in conjunction with assumptions involving lithologic and structural variations within the stratigraphic section which has been removed by erosion, suggest that the James and Roanoke Rivers have captured three northeastern tributaries of the New River during the latter half of the Neogene. Within this time period no evidence was found of major changes in the course of the New River itself (except for meander loops) between Radford and Narrows.