LATE QUATERNARY GEOMORPHOLOGY AND GEOCHRONOLOGY NEAR COVEY HILL, AND AN ICE STREAM DEGLACIATION MODEL FOR THE SOUTHERN ST-LAWRENCE VALLEY

Various geomorphological glacial features in the Covey Hill area are presented as part of a deglacial framework synthesing earlier work from adjacent sectors of southern St. Lawrence Valley. Emphasis is placed on relations between cross-cutting striae, drumlins and eskers orientations in order to reconstruct the regional glacial retreat, linking the Lake Champlain, Adirondack and southeastern Ontario regions. A new Champlain Sea fossiliferous site (Lac-des-Pins, dated at 10 070 ±70 ¹⁴C BP) contains marine vegetal (algae), animal (bryozoan, shells), terrestrial vegetal (wood, Carex, Dryas leaves) and animal (caribou) remains at 91±2 masl. Data from the Lac-Des-Pins fossiliferous site indicates a reservoir age difference of 600±150 years, calculated between three terrestrial radiocarbon dates and marine shells (Hiatella arctica) corrected age (&delta ¹³C=0‰). Chitineous bryozoa and algae show reservoir ages of 150± 150 and 350±150 years, respectively. From dates on shells found in the surrounding area, the average reservoir age is 800±200 years on Hiatella arctica, and 1200±300 years for Macoma baltica. Using the Lac-Des-Pins terrestrial radiocarbon date and another one from Hitchinbrooke, the marine level curve is then coherent with a younger Champlain Sea chronology, opening at 11200 ± 400 ¹⁴C BP. This event had been dated at 11,100 ¹⁴C years at Mont St-Hilaire in recent work. The draining of Lake Candona and the contemporaneous onset of the Champlain Sea have produced unique conditions of high volume water discharge that might have contributed to the initiation of the Younger Dryas event.