Analysis of Lower to Middle Ordovician carbonate successions and the conodonts they contain from Tennessee to New York show that (1) in the northern Great Valley, strike-parallel disconformable contacts occur between Lower and Middle Ordovician and within Middle Ordovician carbonate units, (2) apparent maximum extent of unconformities along tectonic promontories, and (3) relatively rapid subsidence and transgression at promontories. Diachronous early Paleozoic collisional events at an irregular continental margin might explain these observations. Initial Taconic collisions may have occurred at the Virginia promontory resulting in uplift and erosion of the Knox/Beekmantown shelf in Whiterockian time followed by rapid subsidence and transgression. Uplift and erosion, possib y related to continued convergence migrated southwest and at least as far northeast as Lexington, Virginia; beyond Lexington, shelf deposition continued relatively uninterrupted on the east side of the Great Valley. Collision at the New York Promontory could have produced the two pre-Blackriverian and pre-Rocklandian Middle Ordovician intervals of uplift and erosion on the Beekmantown shelf. These unconformities are greatest near Newburgh, New York, and decrease in magnitude northeastward and southwestward. West of Reading, Pennsylvania, in the Great Valley, carbonate shelf deposition remained virtually continuous. Thus between Reading and Lexington, in the Pennsylvania reentrant, continuous deposition during early Paleozoic collisions suggests that an irregular outline of the continenta margin may have controlled patterns of sedimentation during collision tectonics.

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