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外文期刊>American Journal of Science
>THE TACONIC FOREDEEP AS SEDIMENT SINK AND SEDIMENT EXPORTER: IMPLICATIONS FOR THE ORIGIN OF THE WHITE QUARTZARENITE BLANKET (UPPER ORDOVICIAN-LOWER SILURIAN) OF THE CENTRAL AND SOUTHERN APPALACHIANS
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THE TACONIC FOREDEEP AS SEDIMENT SINK AND SEDIMENT EXPORTER: IMPLICATIONS FOR THE ORIGIN OF THE WHITE QUARTZARENITE BLANKET (UPPER ORDOVICIAN-LOWER SILURIAN) OF THE CENTRAL AND SOUTHERN APPALACHIANS
The Tuscarora and Clinch Sandstones (uppermost Or-dovician to Lower Silurian) of the central and southern Appalachians are commonly considered as a widespread, homogeneous, and uniform quartzarenite blanket deposited in the foreland area of the Taconic orogen. Comparison of the stratigraphy in southwestern Virginia with that of extreme southwestern Virginia/eastern Tennessee indicates the presence of two distinct lithosomes of regional extent. The "White Sandstone Lithosome" (composed of "lower" and "upper" Tuscarora Sandstone, Poor Valley Ridge Sandstone Member of the Clinch Sandstone, and some bedsets within the Rockwood Formation) in extreme southwestern Virginia/eastern Tennessee has markedly different strati-graphic relationships to the "Heterolithic Lithosome" (composed of Rose Hill Formation, Rockwood Formation, and Hagan Shale Member of the Clinch Sandstone), when compared to the stratigraphic relationships of the two lithosomes in southwestern Virginia. Contrary to the long-held view, the "White Sandstone Lithosome" is divisible into several distinct genetic components, each of which has its origin tied to different developmental stages of the Taconic orogen and to different eustatic sealevel events. "Lower" and "upper" Tuscarora Sandstone can be related to active compressional tectonism and foredeep subsidence, when the foredeep acted as a sediment sink for the quartz-rich detritus derived from the orogen. During a cessation of compressional tectonism, flexural isostatic uplift of the foredeep converted part of the foredeep from an active depositional site into an active erosional site. The extent of the denudation area (erosional site) is thought to be marked today by the extent of the "Tuscarora unconformity" within the central Appalachians. Quartz-rich erosional detritus was shed radially away from the denudation area and was redistributed to that part of the foredeep not affected by flexural uplift and eventually carried out of the foredeep (fill area). Redistributed detritus from the active foredeep furnished the material for the Poor Valley Ridge Sandstone of extreme southwestern Virginia/eastern Tennessee and the "Clinton" sandstone in the subsurface of Ohio. Eustatic sealevel changes might be invoked for the origin of the isolated bedsets belonging to the "White Sandstone Lithosome" within the Rockwood Formation and tentatively for the origin of the Whirlpool Sandstone in the subsurface of Ohio and West Virginia. A new location for the lithostrati-graphic boundary between the Tuscarora and Clinch Sandstones within the Valley and Ridge is proposed, based on observations on the "White Sandstone Lithosome." Flexural isostatic uplift of part of a foredeep provides a possible tectonic stimulus for sediment export out of a foredeep. It complements sediment export out of shallow-marine or terrestrial foredeeps through sediment bypassing, either along the axis of a foredeep or transversely across the peripheral bulge.
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