TECTONO-EUSTATIC MODIFICATION OF THE EARLY TRENTON SHELF: EVIDENCE FOR SHELF REORGANIZATION DURING THE ONSET OF TACONIC TECTONISM (VERMONTIAN TECTOPHASE)

Upper Ordovician (Mohawkian) successions of the Mohawk Valley, New York State, exhibit an abrupt transition from a passive continental margin to a tectonically active, rapidly subsiding foreland basin. Recent work in the upper Black River to lower Trenton demonstrate uplift, rather than subsidence, as a predominant feature of the early Trenton shelf. Several third-order sequences can be correlated across the Ontario-to-New York platform, indicating strong eustatic control. Even so, progressive lateral changes within sequences are tectonically -related. Eustatic and tectonic modifications of the early Trenton shelf can be summarized as follows: 1) Uppermost Turinian Lowville Fm. facies are similar across the entire shelf, suggesting a nearly planar, aggraded shelf. 2) The upper Lowville-Watertown Limestone contact (basal Chatfieldian) is marked by an erosional/karstic unconformity (SB) with maximal truncation near Middleville, NY, delineating a transient topographic high in the central Mohawk Valley region. 3) Subsequent Watertown-Selby Ls. (TST) exhibit rapid lateral transitions from peritidal micrites into massively-bedded, offshore wackestones and packstones, northward into eastern Ontario, Canada. Conversely, the overlying Napanee Formation (HST) facies are similar across the same region. 4) The second Chatfieldian sequence is marked at its base by severe truncation in the central Mohawk Valley, as evidenced by erosional clasts of Lowville Ls. and even Grenville basement, but becomes conformable to the NW. 5) The Kings Falls (TST) and lower Sugar River (HST), thicken and grade into deeper water facies northward. 6) Higher Sugar River (HST) deposits maintain greater thickness to the north, but also demonstrate rapid lateral SE facies change across the former Mid-Mohawk Valley high into dark gray Flat Creek Shale in eastern New York State. Thus, the Trenton Shelf developed a temporary positive feature in the Mohawk Valley, and a depocenter to the NW in the Black River Valley to southeastern Ontario region. This high subsided abruptly in the mid-Chatfieldian, coincident with migration of the foreland basin axis into eastern New York State.