GENESIS AND MINIMUM AGES OF ICE-MARGINAL LAKES AND ICE-WALLED LAKES PLAINS ON THE VALPARAISO MORAINIC SYSTEM IN WILL COUNTY, ILLINOIS. (USA)

In Will County, Illinois, we have identified three types of lakes intimately related to the ablation of dead-ice permafrost of the last glaciation: ice-walled lake plains, an ice-marginal lake, and a lake formed by a collapsed tunnel valley. The ice-walled lake plains occur on ground moraine between the Rockdale and Manhattan Moraines. Both moraines were eroded by the ca. 19,000 cal yr BP Kankakee Torrent. Regional data indicate that the moraines formed between about 22,500 and 20,500 cal yr BP. We obtained seven sediment cores from an ice-walled lake, and washed more than 20 m of core that yielded enough material for one radiocarbon age: 19,170 +/- 130 cal yr BP. As a minimum age for the Manhattan Moraine, the age is consistent with the regional data. A second lake type, interpreted as a lake marginal to the Westmont Moraine of the Valparaiso Morainic System, is named Glacial Lake Pine. On the east side of the lake deposit is a long, curving ridge with geomorphic characteristics of an ice-walled lake. We interpret this feature to be an ice-walled lake that was overridden by ice that formed the Westmont Moraine. Like the previous example, the lake provided little material for radiocarbon dating, but has yielded two ages: 18,490 ± 85 and 18,700 +- 60 cal yr BP. These ages are consistent with the Valparaiso Morainic System post-dating the Kankakee Torrent. The third lake type is unusually long and narrow (4 km by 1.5 km), and inset in the glacial drift, including the Westmont Moraine. The orientation, size, and form of Eagle Lake compared to the surrounding landscape indicates an origin through collapse of a tunnel valley. The basin contains at least 19 m of fossiliferous sediment. Radiocarbon ages from near the base and top of the sampled succession date from 16,250 ± 80 to 3,150 ± 50 cal yr BP. The discrepancy in ages between Eagle Lake and Glacial Lake Pine, about 2,000 years, indicates that collapse of the tunnel valley associated with Eagle Lake involved melting of deeply buried ice. Glacial Lake Pine, on the other hand, formed due to melting of near-surface dead-ice permafrost shortly after formation of the Westmont Moraine.