TIMING AND EXTENT OF PLIO-PLEISTOCENE GLACIATIONS IN NORTHWESTERN CANADA AND EAST-CENTRAL ALASKA

Northwestern Canada and eastern Alaska are recognized as having one of the oldest known continental glacial records (late Pliocene) preserved in stratigraphic sections. These include the individual and complex records of Cordilleran, montane and continental glaciations. Regional scale glaciations (Cordilleran and continental) started in northwestern Canada and east-central Alaska between 2.9 and 2.6 million years ago. Overall, two Cordilleran glaciations and two plateau ice caps (Horton Ice Cap) developed in late Pliocene (Gauss and Matuyama Chron). During the early Pleistocene, three Cordilleran glaciations occurred, while one to five continental glaciations (Keewatin Ice Sheet and Horton Ice Cap) are inferred from the Banks Island stratigraphic record (late Matuyama Chron). Three mid-Pleistocene glaciations are recorded for the Cordilleran (including the Reid Glaciation) as well as three continental (Keewatin Ice Sheet and Horton Ice Cap) events (early Brunhes Chron). During the late Pleistocene (late Brunhes) a well defined, extensive continental ice sheet (Keewatin) covered western and northwestern Canada, while in the Yukon Cordillera and Yukon-Tanana Uplands, two glaciations (early-late Pleistocene Eagle Glaciation, and late Pleistocene McConnell Glaciation) are recognized. Successive Cordilleran glaciations diminished in size, while continental glaciations increased. The moisture source for the Cordilleran ice was largely the Pacific Ocean, however, for the Horton Ice Cap, an open Arctic Ocean may have been a significant moisture source. The role of tectonics in the development of the two major physiographic barriers (Wrangell/St Elias Mountains and the Continental Divide (Mackenzie/Selwyn Mountains) appears to have been an important controlling variable in moisture distribution in northwest Canada and east-central Alaska. The timing and interplay of tectonic uplift versus erosion of these barriers has very much controlled the growth, thickness and decay of ice masses in the interior of Yukon and valleys east of the continental divide.