| Popis: |
Understanding the timing and rate of ice retreat in the Great Lakes Region is critical to understanding any relationship between ice lobes and climate during the Late Wisconsinan time period. The purpose of this study is to constrain the age of the Sturgis Moraine, an end moraine of the Saginaw Lobe in south-central Michigan using minimum-limiting radiocarbon and OSL ages. Previous correlations between till stratigraphy and end moraines suggest that the Sturgis Moraine formed between 15,500 and 16,100 14C yrs BP, but with little direct supporting chronologic data. To date the Sturgis Moraine, Livingstone sediment cores were collected from three scour lakes within tunnel channels at the distal side of the moraine. Scour rather than kettle lakes are chosen to minimize any organic accumulation lag from meltout of buried ice. Basal ages from gravelly sand in these cores are 13,700±60, 13,750±80, and 13,300±60 14C yrs BP. These ages are similar to basal ages from Clear Lake (13,300±300 14C yrs BP), a kettle lake on the distal side of the Valparaiso Moraine of the Lake Michigan Lobe, and the Hyre (13,690±50 14C yrs BP), Kenan (13,880±70 14C yrs BP), and Pyles (13,510±160 14C yrs BP) sites from kettle lakes on the distal side of the Fort Wayne Moraine of the Huron Erie Lobe. The minimum-limiting ages for the Sturgis Moraine presented in this study suggest a younger ice margin and smaller Saginaw re-entrant than previously envisioned. OSL dated sand dunes from within an outwash valley distal of the Saginaw Moraine (14.3±0.6 and 14.1±0.5 ka) and from dunes that migrated out of the valley bottom (12.6±0.4, 12.3±0.4, 12.4±0.5 and 12.0±0.4 ka) appear to record two separate aeolian activation periods. These ages agree with other sand dune chronologies from northwest Indiana and northwest Ohio suggesting regional variation in climate at these times. The radiocarbon and OSL ages from this study provide a best limiting minimum age of 13,750 ± 80 14C yrs BP (16,320-16,930 cal yrs BP) for deglaciation at the Sturgis Moraine. |
