GLACIAL LAKE AGASSIZ - ITS NORTHWEST MAXIMUM EXTENT AND OUTLET IN SASKATCHEWAN (EMERSON PHASE)

Six different lines of evidence support the hypothesis that glacial La ke Agassiz expanded an additional 70,000 km(2) over that previously ma pped in northwestern Saskatchewan and that the lake discharged out the northwestern (Clearwater) outlet, then through glacial Lake McConnell and Mackenzie River to the Arctic Ocean. Elevations of formerly unmap ped (1) strandlines and (2) glaciolacustrine sediments between the pre viously mapped northwest limit of Lake Agassiz and the Clearwater-lowe r Athabasca spillway indicate that water extended 170 km farther north west. Recently, mapped strandlines at elevations up to 60 m above the previously mapped extent of Lake Agassiz can be traced along (3) isoba ses to the mouth of the spillway. Based upon (4) six radiocarbon dates recovered from spillway flood deposits in the Athabasca River valley and its late Pleistocene delta, the (5) Clearwater spillway was cut at 9.9 ka BP. This date is synchronous with the initiation of the Emerso n Phase (9.9 ka BP) in the southern Lake Agassiz basin and (6) coincid es with the position of the Laurentide Ice Sheet at the Cree Lake Mora ine (10 ka BP) along the northern margin of the lake. Following closur e of the eastern outlets at the onset of the Emerson Phase, Lake Agass iz transgressed toward the northwest into the deglaciated and isostati cally depressed glacial foreland in the Churchill River valley to an e levation of 490 m, the pre-flood elevation of the Churchill-Mackenzie drainage divide at the head of the Clearwater-lower Athabasca spillway . The Beaver River Moraine (an earthen drainage divide) was breached, resulting in lowering Lake Agassiz 52 m to a stable elevation at 438 m . The lake discharged 21,000 km(3) of water into the Arctic Ocean that raised global sea level by 6 cm.