Early Paleozoic Continental Shelf to Basin Transition, Northern Virginia – Strasberg to Riverton, Virginia, July 13, 1989, VT221

Published by the American Geophysical Union as part of the Field Trip Guidebooks Series, Volume 221.

The Ordovician transition from a passive-margin, as interpreted from the Beekmantown Group rocks. to foreland-basin deposition behind an active-margin. as interpreted from New Market Limestone through Martinsburg Formation rocks. can be observed in four exposed sections in northern Virginia (Figure 1). A major regional unconformity at the top of Beekmantown Group marks the transition from carbonate deposition on a passive-margin shelf beneath the unconformity to homoclinal ramp to foreland-basin deposition associated wi.th a convergent margin (Mussman and Read. 1986). The unconformity probably is related to island arc-continent or microplate continent collision and uplift during the Early Middle Ordovician (Whitrockian 7) (Shanmugam and Lash. 1982) coupled with eustatic sea level lowering. This collision can be considered an early phase of the Taconic Orogeny of Rodgers (1971). Preunconformity depocenters. one to the north of the field trip area in Maryland and Pennsylvania and one to the south in southwestern Virginia and Tennessee, may have influenced the development of the unconformity and the depositional fabric and thickness of Middle Ordovician deposits (Colton. 1970; Read. 1980; Mussman and Read. 1986).