A RECORD OF MELTWATER FLOOD DISCHARGES IN GLACIAL LAKE VERMONT SEDIMENTS FROM THE LAKE CHAMPLAIN VALLEY

Before the deglaciation of the St. Lawrence Seaway, all late Pleistocene discharge from the eastern Great Lakes was directed to the North Atlantic through the Hudson Valley. This discharge was initially routed from glacial Lake Iroquois through the Mohawk Valley and into the Hudson Valley, but was later rerouted to the Hudson Valley around the northern edge of the Adirondack Uplands into glacial Lake Vermont in the Champlain Valley when lower northern routes became ice free. The rerouting event began with a large flood pulse, on the order of 100000 cms, shortly followed by a larger pulse as Lake Iroquois and Lake Vermont became confluent. The sedimentary record of these flood events is manifested in the northern end of the Champlain Valley as an erosional unconformity in older glacial lacustrine varved clays capped by coarser flood materials and then conformably draped with post-flood varved clays. In the central Champlain Valley this transition is conformable without an interruption in the varved lacustrine clays. Here there are other indicators of the floods, including significant increases in average varve thickness, ice rafted debris, and inorganic carbonate content, as well as δ¹⁸O depletion in ostracode tests. New measurements from about 250 varves at Whallonsburg, NY provide a possible correlation to the New England Varve Chronology (NEVC) in the pre-flood record; however the increased thicknesses of the flood varves drown out any regionally correlatable signal in the younger section of the sequence. The NEVC correlation suggests that the flood events began around 13,200 cal. years B.P., which is in good agreement with available radiocarbon ages. Varve counts from this and other locations in the Champlain Valley indicate that the Champlain Sea began about 200 years after the first flood pulse. That would put the rerouting of Great Lakes discharge from the Hudson Valley to the Gulf of St. Lawrence at around 13,000 cal. years B.P.