OPTICAL AGES FROM PROGLACIAL LAKE SEDIMENTS IN SOUTHERN INDIANA: NEW AGE CONSTRAINT ON THE ILLINOIAN GLACIAL LIMIT

A classic glacial geologic concept is the Illinoian limit in mid-continental North America. This limit may incorporate glacial limits of various ages, which are by definition older than the last glacial maximum. The presence of proglacial lacustrine deposits in Mill Creek Valley in central Indiana, near the mapped Illinoian limit provides sediments amenable for optical dating, and thus testing of the age of the Illinoian limit. The complex glacial history of the area is reflected also in the deeply-incised, steep-walled, and underfit streams with common mid-drainage waterfalls on Devore Ridge, the southern flank of the Valley. Cut bank exposures along Mill Creek show a glacial diamicton subjacent to a suite of proglacial lacustrine sediments. These sediments have been previously associated with the Illinoian Glacial Lake Quincy, the stratigraphically lowest recognized proglacial lake within the Valley. This proglacial lake was formed with 5-10 km northward retreat from the Illinoian limit, depositing a variety of proglacial lacustrine sediments. Optical dating was confined to fine-grained and rhythmically-deposited proglacial lake sediments, reflecting suspension settling in an ice-front distal environment, providing sufficient solar resetting. These sediments yielded four optical ages in stratigraphic order of 156.1 ± 11.3 ka, >128.0 ± 9.2, 162.3 ± 11.8, and 136.9 ± 14.5 ka, which is consistent with a marine oxygen isotope stage-6 age for this Illinoian limit. Proglacial lake formation is generally coincident with Loveland Loess deposition ca. 180-130 ka and with July insolation minimums for 65°N to 40°N latitude, when the ice sheet is inferred to be near its maximum extent. The abundance of inferred proglacial lake outlets and the succession of lacustrine sediments over a glacial diamicton indicates a relatively stable Illinoian limit, oscillating 10-30 km within the, mapped maximum limit of the ice sheet.