LATE-STAGE-5 GLACIO-ISOSTATIC SEA IN THE ST-LAWRENCE VALLEY, CANADA AND UNITED-STATES

Although post-glacial marine sediments of late Wisconsinan and early H olocene age are common in eastern Canada and the northeastern United S tates, remnants of older Pleistocene marine sediments are scarce. A fo ssiliferous marine clay that predates the classical Wisconsinan was re cently discovered in the St. Lawrence Valley. A dominantly estuarine e nvironment is inferred from the geochemistry of the shells (delta(18)O = -7.1) and from benthic foraminifer and ostracode assemblages, The c lay indicates-a marine invasion (Cartier Sea) shallower and probably s horter than that during the upper late Wisconsinan Champlain Sea episo de (12,000-9,500 yr B.P.). The pollen content shows that regional vege tation during the marine episode began as open tundra, then became a B etula and Alnus; crispa forest, reached a climatic optimum with Quercu s, Corylus, and Abies, and concluded as a Pinus/Picea boreal forest. A corrected infrared stimulated luminescence age of 98,000 +/- 9000 yr is compatible with the epimerization ratio of shells. The Cartier Sea resulted from a post-glacial glacio-isostatic marine invasion in the S t. Lawrence lowlands. It probably occurred during late stage 5 and is tentatively assigned to the transition of oxygen isotope substages 5b/ 5a. This marine episode dates to stage 5 of the preceding continental glacier which extended to middle latitudes in NE America. (C) 1996 Uni versity of Washington.