CONTROLS ON LOCATION AND AREAL EXTENT OF THE QUEENSTON CLASTIC WEDGE IMPOSED BY THE PRE-TACONIAN SHELF ARCHITECTURE

The Queenston clastic wedge prograded across a shale-draped carbonate shelf surface during the Late Ordovician in response to Taconian orogenesis. This wedge is well preserved in Pennsylvania and southwestern New York, but is not preserved north of the Mohawk valley. Although it has generally been assumed that the distal portions of the wedge were removed north of the Mohawk valley by subsequent erosion, we will argue that topography of the Late Ordovician surface, reflecting the structure of the underlying shelf, precluded progradation farther north than central New York.

The Cambro-Ordovician shelf was transected by a series of submarine canyons: a primary set developed in the mid-Cambrian and a secondary set developed jn the mid-Ordovician during the Knox unconformity. As a prelude to the early stages of Taconian orogenesis, marine shales were carried in from the east, filling the canyon systems and finally overtopping the adjoining shelf; the Queenston wedge developed on top of this shale. The bounds of the Queenston wedge correspond to the location of the major submarine canyons, suggesting that the shale-fill did not completely mask the underlying shelf topography. It is proposed that marine/lacustrine basins and/or rivers lay above the old canyon axes and acted as depositional boundaries and sediment sinks for the distal and lateral boundaries of the Taconian foreland wedge(s).

The most important of these barriers for the Queenston wedge was the axis of a canyon lying across the current Adirondacks; this canyon separated the shelf of the Mohawk valley and southward from the shelf of Champlain and St. Lawrence valleys. The presence of a topographic depression and accompanying sediment barrier above the Adirondack canyon axis limited the northward progradation of the Queenston wedge. Other major canyons bounded the Champlain-St. Lawrence shelf to the north and segmented the shelf of the central and southern Appalachians to the south. The width of the New York-Pennsylvania shelf segment enabled a sediment wedge to build out from the Taconian orogen, whereas the narrowness of the Champlain valley shelf segment promoted diversion of drainage to the canyon axes on either side which precluded development of a significant wedge.