THE GEOLOGIC SETTING FOR NEW DETRITAL ZIRCON DATA OF PALEOGENE-NEOGENE CONGLOMERATES ON THE CANADIAN PLAINS

The Cenozoic of Western Canada and northern United States was marked by a change from compressional to extensional tectonics. The result was regional uplift and magmatic events. The uplift resulted in a major unconformity and deposition of extensive regional sheets of gravel and sand, of which only isolated remnants remain. These units are the Eocene to Miocene Cypress Hills Formation, the Miocene Wood Mountain Formation, Miocene Flaxville Formation and preglacial Souris River gravels, All four stratigraphic units consist of gravel and sand with lesser amounts of clay. The formations were largely deposited as laterally continuous sheets of braided river gravels, with some occurrences of meandering-river sedimentation. The sediment

for the Cypress Hills and Wood Mountain formations indicate that regional paleoslope dipped towards the north–northeast. The modern prairie landscape of western Canada began to evolve with the deposition of these gravels during the Eocene with creation of a basin-wide unconformity followed by deposition of an extensive braidplain system that was subsequently uplifted, incised, and molded into its present form. The continental drainage divide shifted north approximately 600 to 800km since the Oligocene. Likely, the landscape of southern Alberta, with its major uplands along the drainage divide, had already developed much of its relief by the Late Miocene. This paper sets the geological background for new detrital zircon data to be presented in an accompanying paper.