EVIDENCE FOR ONSET AND WESTWARD PROGRESSION OF TACONIAN OROGENESIS IN THE CENTRAL AND SOUTHERN CHAMPLAIN VALLEY

The key to determining the timing of the onset of Taconic orogenesis in the central and southern Champlain Valley lies in distinguishing between the primary stratigraphic features of a complex shelf margin and the tectonically induced features of the orogenic event. The primary evidence for timing is the changeover from carbonate to shale deposition. Unfortunately, a late pre-tectonic (Whiterockian, i.e., post-Beekmantown) eustatic event caused the absence of late shelf strata in several areas, misleading many previous workers into believing that orogenesis began sooner than it did.

We have recently been able to distinguish and map submarine canyons originating during this eustatic event, which has, in turn, led to the recognition of foundered portions of late Beekmantown shelf. We have also discovered evidence for several overlooked late (post-Whiterockian) shelf elements in the eastern Champlain Valley and northern Vermont Valley. Essentially, the latest carbonate shelf consisted of a series of isolated carbonate banks separated by a submarine canyon system; thick syntectonic shales were deposited within the canyons, whereas only thin shale layers were deposited atop the banks. Based on this work, we can show that the carbonate shelf throughout the central and southern Champlain Valley was active into Black River time, with the westernmost banks continuing into earliest Trenton time. There is no evidence of block faulting, karstification, or significant thickness variations within the late shelf carbonates that would suggest syntectonic depostion, but the net accumulation rate accelerated dramatically in late Chazy to early Black River time suggesting the onset of tectonic subsidence.