A NEW RECONSTRUCTION OF PLEISTOCENE LAKE TIGHT SHOWING GLACIAL ISOSTATIC FLEXURE AT 1.3 MA

The earliest advances of the Laurentide Ice Sheet into the midcontinent dammed and diverted north-flowing rivers, including the preglacial Teays-Mahomet River that flowed across Ohio, Indiana, and Illinois. The Teays River was dammed ca. 1.3 million years ago to form Lake Tight, a large proglacial lake that extended 200 km beyond the ice margin, covering an area of 26,000 km². Previous reconstructions of the lake have estimated elevations from purported glacial erratics, now recognized to be Devonian dropstones unrelated to modern glaciation (Ettensohn et al., 2009, AIPG Spring Field Trip Guide), and from lake sediments and possible wave-cut benches in southern Ohio. Lake elevations over most of the area remain unknown, and no previous reconstruction has accounted for crustal flexure due to glacial isostatic adjustment. We reconstructed the minimum lake elevations over the entire Lake Tight area using georeferenced soil maps (Isee Network, Soil Explorer) to determine the distribution of lacustrine-derived soils. We also used a 30-m digital elevation model to map preglacial drainages that were filled with lake sediment and either pirated or abandoned after the lake drained as an indication of lake extent. Our results show that the modern distribution of lake sediments is not horizontal, but is tilted due to glacial isostatic adjustment. The lake elevation decreases to the south by approximately 50 meters over ~120 km, consistent with rebound expected from ice loading.