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Shed Brook Discontinuity and Little Falls Gravel: Evidence for the Erie interstade in central New York

¹ Department of Geology, Tufts University, Medford, Massachusetts 02155

The Shed Brook Discontinuity in the western Mohawk Valley region of central New York is an unconformity separating lacustrine beds of the Valley Heads drift (Port Bruce stade) from pre–Valley Heads lacustrine deposits (Nissouri stade). The Little Falls Gravel is a fluvial unit that can be found draped by Valley Heads lacustrine deposits in the Mohawk Valley. The discontinuity and gravel represent subaerial erosion and braided river deposition in response to initial late Wisconsinan ice recession and lake drainage in the Mohawk Valley. These events were followed by Valley Heads lake impoundment and glacial readvances. The Shed Brook Discontinuity and Little Falls Gravel have a ¹⁴C age constrained to 17–14.1 ka on the basis of stratigraphic relationships in central New York and the correlation of paleomagnetic declination records from central New York and the ¹⁴C-calibrated New England varve chronology. The Shed Brook Discontinuity and Little Falls Gravel appear to be features equivalent in age to the Erie interstade (about 14.5–16 ka), an interval during which eastward drainage to the Mohawk Valley has been inferred for lakes in the Erie basin. Evidence for a river in the Mohawk Valley (Little Falls Gravel) indicates that it may have served as an eastern outlet for the Erie and Ontario basins, and thus allowed the first eastward diversion of meltwater away from the Mississippi Valley during the last deglaciation. Drainage down the Mohawk Valley entered Lake Albany in the Hudson Valley, which drained into Long Island Sound and not along the present Hudson River course to the New Jersey continental shelf. Meltwater drainage across New York during the latest stages of the Erie interstade may have introduced large volumes of meltwater directly to the western Atlantic, creating a situation that needs to be more fully explored in terms of its possible influence on thermohaline circulation in the North Atlantic and climatic change marked by the Port Bruce stade.

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