New insights on Illinoian deglaciation from deposits of Glacial Lake Quincy, central Indiana

The deposits of Glacial Lake Quincy overlie a diamicton associated with the classically defined Illinoian limit in central Indiana. This lake covered at least 180 km2 with a depth of > 20 m and developed when the Illinoian ice sheet retreated 15 km from the maximum limit, causing lake impoundment against Devore Ridge. Overflow from Glacial Lake Quincy eroded across the ridge forming a number of steeped-walled outlets. A section along Mill Creek exposes a sedimentologic sequence associated with Glacial Lake Quincy from a subglacial diamicton to ice-proximal to ice-distal glacial lacustrine sediments. We report new optical ages by multiple aliquot regenerative dose procedure for the fine-grained rhythmically bedded sediments presumed to represent the lowest energy depositional facies, dominated by suspension settling, which maximized sunlight exposure. In turn, optical ages were determined on the fine-grained (4-11 μm) polymineral and quartz fractions under infrared and blue excitation, which yielded statistically similar ages. Optical ages span from ca. 170 to 108 ka, with the average of 16 optical ages indicating deglaciation at ca. 135 ka, generally coincident with Marine Oxygen Isotope Stage 6-to-5 transition and rise in global sea level.