SILURIAN BASIN-REBOUND UNCONFORMITY IN PENNSYLVANIA AND ITS RELATIONSHIP TO THE "CLASSICAL" TACONIC UNCONFORMITY

In SW and central Virginia a disconformity occurs at the base and/or within the Tuscarora Ss. In this area, the Upper Tuscarora Ss. overlies unconformably Martinsburg Fm., Lower Tuscarora Ss. or Juniata Fm. This Tuscarora unconformity is interpreted as a basin-rebound unconformity, developed during a lull in convergent tectonism that led to isostatic uplift of the Taconic orogen and its adjacent foredeep. In New York, New Jersey and eastern Pennsylvania the "classical" Taconic unconformity displays an angular unconformable relationship between deep-marine foredeep deposits and overlying Silurian deposits. The tilted deep-marine deposits indicate deformation of the foredeep fill and sequential incorporation of these deposits into the growing Taconic orogen. To the SW of the area of angular unconformity, at Swatara Gap, Pennsylvania, conglomeratic Tuscarora Ss. overlies disconformably Martinsburg Fm. Intervening Juniata Fm. and Bald Eagle Ss. apparently were eroded away prior to deposition of the Tuscarora Ss. Preliminary field investigations indicate that this disconformity can be traced farther to the SW, where Tuscarora Ss. (with a thin gravelly bed) overlies Juniata Fm. or lower portions of the Tuscarora Ss. This disconformity occupies the same stratigraphic position as the Tuscarora unconformity in central Virginia and is interpreted as a basin-rebound unconformity. At Susquehanna Gap (Harrisburg, Pennsylvania), immediately to the SW of Swatara Gap, this disconformity occurs between conglomeratic Tuscarora Ss. and a subjacent unnamed stratigraphic unit ('Juniata/Bald Eagle equivalent'). The contact between this 'Juniata/Bald Eagle equivalent' and the subjacent Martinsburg Fm. is also disconformable. This lower (and older) unconformity is interpreted to reflect more distal effects of the foredeep deformation that led to tilting of the Martinsburg Fm. beds farther to the NE. Apparently, the amount of later isostatic basin rebound during the Taconic orogeny was not sufficient to cause amalgamation of the unconformities resulting in the preservation of the two temporally and causally different unconformities. Farther to the NE, however, both later rebound unconformity and earlier foredeep-deformation unconformity are probably amalgamated to form the 'classical' Taconic unconformity.