REGIONAL, STRUCTURAL CONTROLS ON THE DEVELOPMENT OF THE TACONIAN FORELAND AND FORELAND BASIN DURING MOHAWKIAN TIME, EAST-CENTRAL UNITED STATES

The Taconian orogeny was apparently a “transgressive” orogeny in that the locus of convergence shifted northeastwardly nearly parallel to the strike of the Appalachian Basin during Mohawkian (Blackriverian & Chatfieldian) time. By the end of Blackriverian time, the Blountian tectophase of the Taconian orogeny, largely centered on the Virginia Promontory, had expended itself; an apparently long, narrow foreland basin, the Sevier Basin, resulted, and most of the foreland developed as an extensive, peritidal, carbonate platform. By the Blackriverian-Chatfieldian transition, the focus of orogeny had shifted to the New York Promontory in what has been called the Taconic tectophase; the tectophase has been interpreted to represent initial westward subduction below an island-arc complex. Loading and far-field forces associated with the tectophase apparently caused the Blackriverian carbonate platform to collapse along older, Keweenawan, Grenvillian, and Iapetan, basement structures, both in areas that would become the foreland basin and the foreland. In particular, the western margin of the foreland basin in eastern Ohio, western Pennsylvania, and southwestern New York, as defined by black-shale distribution, forms a pronounced embayment into the foreland along Grenvillian and Iapetan structures. The embayment makes contact with a series of other connected structural lows on the foreland, which become the Sebree Trough. Highs adjacent to the trough formed the foundation for carbonate buildups which would become the “Trenton-age” (Chatfieldian) Galena-Trenton and Lexington carbonate platforms. Potential source rocks developed as black shales in structural lows defined by the Sebree Trough and parts of the foreland basin; potential reservoir rocks, in turn developed on the adjacent platforms. In latest Chatfieldian time, an apparent change in Taconic subduction polarity resulted in renewed structural reactivation, major regional deepening, and far-field tilting, which allowed foreland-basin clastics to flood the Sebree Trough and eventually inundate adjacent platforms. Clearly, paleogeography and paleoclimate were also influential, but it seems to have been the interplay between basement structures and far-field tectonic forces that largely determined the Mohawkian history of the Taconic foreland basin and adjacent foreland.