A RECORD OF LATE WISCONSINAN BREAKOUT FLOODS FROM PROGLACIAL LACUSTRINE AND MARINE SEDIMENT IN THE NORTHERN CHAMPLAIN LOWLAND

Late Wisconsinan ice recession in the Champlain Lowland was marked by at least three large proglacial lake breakout events beginning between 11,200 and 10,900 ¹⁴C years BP. The first occurred when the ice receded to a position immediately south of Covey Hill, Quebec and approximately 570 km³ of water from glacial Lake Iroquois was released into glacial Lake Vermont in the Champlain Lowland. The breakout lowered Lake Iroquois from the Main to the Frontenac level. This flood also breeched the southern dam of glacial Lake Vermont and caused the lake to drop from the Coveville to Fort Ann level, which released an additional 130 km³ of water. The second breakout occurred as the ice thinned on the northern flank of Covey Hill and approximately 2500 km³ of water was released as Lake Iroquois and Lake Vermont became confluent. The final breakout occurred when ice receded from the lower St. Lawrence Lowland resulting in the drainage of the combined lake and the incursion of the Champlain Sea. This last flood sent about 1500 km³ of fresh water to the Gulf of St. Lawrence. Analysis of cores collected near Plattsburgh, New York help constrain the timing of these floods. A massive to thinly bedded sand layer, as thick as 2 m, unconformably overlies 0.5 to 2 m of thinly bedded to laminated lacustrine clay, and is conformably overlain by 1 to 2 m of clay rhythmites (varves). The sand probably represents the larger (second) breakout, given its stratigraphic position. This implies that evidence for the first breakout flood was eroded as part of the basal unconformity during the second event. The varved clay above the sand layer contains approximately 168 couplets, which we interpret as representing the duration of the Fort Ann Stage of Lake Vermont and the period between floods 2 and 3. The varved clay is overlain by laminated to thinly bedded, fossiliferous marine clay deposited in the Champlain Sea. A 0.1 m thick layer of laminated clay and silt and fine sand partings marks the transition between the varved clay and marine facies. Wood samples recovered near the lacustrine/marine transition from two of the cores are being analyzed for AMS ¹⁴C ages.