THE PLEISTOCENE-HOLOCENE TRANSITION IN EASTERN GLACIER NATIONAL PARK AS EVIDENCED BY LAKE SEDIMENT LITHOSTRATIGRAPHY

The timing and glacial dynamics of the Younger Dryas are relatively well defined from lacustrine and geomorphic records in the western Rocky Mountains; however, climate change associated with this transition in the eastern Rockies is more poorly understood. The Many Glacier region of northeastern Glacier National Park, Montana, lacks terminal moraines to constrain the timing of glacial maxima; here we use lake sediment cores to characterize changes in climate and the periglacial environment from 13,500 to 8000 years bp. We reconstruct the advance and retreat of Grinnell Glacier using the sediments deposited by glaciofluvial processes in a chain of paternoster lakes, in particular the northern subbasin of Swiftcurrent Lake. The southern subbasin receives sediment from the Grinnell Valley, while the site in this study also captures sediment from an additional drainage system, the Swiftcurrent Valley. Using a combination of grain size, L*a*b* color space, and petrographic smear slide analyses, as well as a transition matrix calculation, we built on previous studies of cores farther upstream in the Grinnell Valley by quantitative and semi-quantitative characterization of the input of an additional valley’s sedimentation. The core’s centimeter-scale laminations reflect a dynamic sedimentation system whereby carbonate sediment eroded by the glacier was succeeded repeatedly by siliciclastic sediment eroded by the valley’s stream, indicating a period of back-and-forth glacial advance and retreat including the Younger Dryas.