STRATIGRAPHY AND GEOCHRONOLOGY OF THE SOUTHWEST MARGIN OF THE LAURENTIDE ICE SHEET: QUATERNARY SEDIMENTARY SEQUENCES ON THE NORTHERN PLAINS, MONTANA

Quaternary sedimentary sequences on the northern Great Plains can be linked to patterns of advance and retreat of margins of the Laurentide Ice Sheet (LIS). Stratigraphic and chronologic studies (U-series, luminescence, ¹⁰Be, ¹⁴C) provide information on the extent, depositional environments, and timing along the southwest margin of the LIS (Loma and Malta sublobes, Yellowstone lobe) in Montana. Luminescence dates related to glacial Lake Great Falls at Holter Lake (near Wolf Creek) and Hower Coulee (east of Great Falls) can be compared to the ages of tephras and ¹⁰Be ages on erratics and deposits associated with glacial Lake Musselshell, and ¹⁴C measurements related to glacial Lake Glendive. At Holter Lake, luminescence measurements indicate that laminated silts were deposited around 13,500 years ago (ya) (UW355). Overlying sands date to about 9,550 ya (UW356) and are interpreted as post-glacial aeolian sediments deposited after the drainage of glacial Lake Great Falls. Near Hower Coulee, U-series measurements on shells suggest that fluvial gravels are associated with the last interglacial (OIS 5). Lake silts overlie the gravels and are buried by a diamicton interpreted as a till deposited by the LIS. The till is overlain by several lithofacies associated with a later stage of glacial lake Great Falls. Luminescence samples with measurements ranging from ~21,500-14,500 ya from the lower lake (UW468, UW469) and the upper lake (UW454, UW467) indicate these Lake Great Falls deposits are associated with OIS 2. Direct-dating of the sediments support a late-Wisconsin age for the Holter Lake and Hower Coulee stratigraphic sequences. These age estimates can be compared to ¹⁴C measurements of deposits associated with glacial Lake Glendive and the Yellowstone Lobe (Crazy Horse Till). The results imply that proglacial lakes were formed within the present-day Missouri basin by lobes of the LIS and indicate the maximum extent of the southwest margin of the Late-Wisconsin LIS.