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 Ordovician to Lower Sil urian) of the central and southern Appalachians are commonly considere d as a widespread, homogeneous, and uniform quartzarenite blanket depo sited in the foreland area of the Taconic orogen. Comparison of the st ratigraphy in southwestern Virginia with that of extreme southwestern Virginia/eastern Tennessee indicates the presence of two distinct lith osomes of regional extent. The ''White Sandstone Lithosome'' (composed of ''lower'' and ''upper'' Tuscarora Sandstone, Poor Valley Ridge San dstone Member of the Clinch Sandstone, and some bedsets within the Roc kwood Formation) in extreme southwestern Virginia/ eastern Tennessee h as markedly,different stratigraphic relationships to the ''Heterolithi c Lithosome'' (composed of Rose Hill Formation, Rockwood Formation, an d Hagan Shale Member of the Clinch Sandstone), when compared to the st ratigraphic relationships of the two lithosomes in southwestern Virgin ia. Contrary to the long-held view, the ''White Sandstone Lithosome'' is divisible into several distinct genetic components, each of which h as its origin tied to different developmental stages of the Taconic or ogen 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 f or the quartz-rich detritus derived from the orogen. During a cessatio n of compressional tectonism, flexural isostatic uplift of the foredee p converted part of the foredeep from an active depositional site into an active erosional site. The extent of the denudation area (erosiona l 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 redi stributed 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 Tenne ssee and the ''Clinton'' sandstone in the subsurface of Ohio. Eustatic sealevel changes might be invoked for the origin of the isolated beds ets belonging to the ''White Sandstone Lithosome'' within the Rockwood Formation and tentatively for the origin of the Whirlpool Sandstone i n the subsurface of Ohio and West Virginia. A new location for the lit hostratigraphic boundary between the Tuscarora and Clinch Sandstones w ithin the Valley and Ridge is proposed, based on observations on the ' 'White Sandstone Lithosome.'' Flexural isostatic uplift of part of a f oredeep 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 axi s of a foredeep or transversely across the peripheral bulge.