BASIN-REBOUND ORIGIN FOR THE TUSCARORA UNCONFORMITY IN SOUTHWESTERN VIRGINIA AND ITS BEARING ON THE NATURE OF THE TACONIC OROGENY

The Tuscarora Sandstone (Late Ordovician-EarIy Silurian) in southweste rn Virginia can be divided into two sedimentologically distinct units: a regionally extensive ''upper'' Tuscarora Sandstone and a regionally restricted ''lower'' Tuscarora Sandstone. The regionally traceable '' Tuscarora unconformity'' occurs at the base of the ''upper'' Tuscarora Sandstone and splits the Martinsburg-Shawangunk elastic wedge into tw o parts. This unconformity corresponds to the earlier identified Chero kee and Taconic Discontinuities in the central Appalachians. Based on the nature of the lacuna, regional comparisons, and paleotectonic sett ing, we conclude that the ''Tuscarora unconformity'' probably did not develop as a consequence of the end-Ordovician glaclo-eustatic event. Isostatic flexural rebound of the Taconic orogen and its adjacent fore deep, following cessation of thrusting, and erosional and tectonic thi nning of the orogen, is favored, instead, as the causal mechanism for this regional unconformity. The ''Tuscarora unconformity'' may separat e two distinct phases of thrusting and thrust-load induced foredeep su bsidence in the development of the Taconic orogen and its foreland. Th e two phases of thrusting might tentatively be correlated with two sep arate phases of terrane accretion within the central Appalachians, The Ordovician-Silurian boundary in southwestern Virginia probably falls within the lacuna associated with the ''Tuscarora unconformity.''