ABSTRACT: Nearshore to Lower Shoreface Deposits of the "Upper" Tuscarora Sandstone of Southwestern Virginia: A Retrogradational Parasequence Intimately Related to Taconic Tectonism

DORSCH, JOACHIM, and STEVEN G. DRIESE, University of Tennessee, Knoxville, TN

The "Upper" Tuscarora Sandstone (? Late Ordovician-Early Silurian) is part of the clastic fill (Martinsburg-Shawangunk clastic wedge) of the Taconic peripheral foreland basin of the central Appalachians. It unconformably overlies the Martinsburg Formation, the Juniata Formation, and the "Lower" Tuscarora Sandstone, and thins from the distal outcrop belts in the northwest towards the proximal outcrop belts in the southeast. The "Upper" Tuscarora Sandstone onlaps onto an unconformity that developed following cessation of thrusting and isostatic rebound of the Taconic orogen and the adjacent foreland basin. The "Upper" Tuscarora Sandstone consists of a cross-stratified conglomeratic sandstone lithofacies with rare Arthrophycus, Phycodes, and Rusophycus, interpreted as fluvial deposits re orked in a nearshore environment; a cross-stratified sandstone lithofacies with abundant Arthrophycus, Monocraterion, and Skolithos, interpreted as shoreface deposits; an amalgamated hummocky cross-stratified lithofacies with abundant Skolithos, interpreted as lower shoreface deposits; and an organic carbon-rich lithofacies (mudstone to sandstone) with Arthrophycus, interpreted as representing an anoxic event during marine transgression. The stratigraphic unit is capped by a marine flooding surface (pebbles, phosphate, shell concentration) at the base of the overlying distal "shelf" deposits of the Rose Hill Formation. The "Upper" Tuscarora Sandstone represents a retrogradational parasequence formed during rapid marine transgression. This rapid transgression was accomplished by pronounce subsidence following renewed thrusting and thrust-loading in the orogen. The cratonward sloping depositional plane, established during basin rebound, was tilted back toward the orogen again during deposition of the "Upper" Tuscarora Sandstone.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)