MELTWATER SOURCE FOR THE KANKAKEE TORRENT

The Kankakee Torrent (KT) was a cataclysmic meltwater flood that scoured the landscape west of Kankakee, Illinois, just prior to 18.91 ± 0.40 ka BP, as it flowed into Glacial Lake Wauponsee, which then burst through its moraine dam and drained down the Illinois River Valley. The ages are based on radiocarbon assay of tundra plant fossils preserved in the base of a post-flood scour channel just downstream of the moraine. The source of the meltwater that caused these events has often been attributed to an embayment along the margin of the ice sheet in southern Michigan bounded by the Lake Michigan, Saginaw, and Huron-Erie Lobes. Two OSL ages from the Kalamazoo Morainic System of the Lake Michigan Lobe (19.8 ± 1.3 ka; 17.7 ± 1.5 ka; mean = 18.75 ka) were obtained from lake sediment deposited on stagnant ice adjacent to the ice margin of the Crown Point Phase advance. Four OSL dates were obtained from sandy sediment in collapsed morainal deposits from the Kalamazoo Moraine of the Saginaw Lobe (17.1 ± 2.1 ka; 18.9 ± 1.1 ka; 19.9 ± 2.1 ka; 18.6 ± 2.1 ka; mean = 18.63 ka) suggesting that the Kalamazoo margins of the Lake Michigan and Saginaw Lobes were synchronous at ~19 ka BP and could therefore have supplied meltwater which flowed southward out of the embayment into the Kankakee lowlands in Indiana and eventually into Illinois, where it generated the KT. Although previously suggested, these dates provide the first temporal evidence for the age and contemporaneity of the two margins. Possible sources of meltwater within the embayment include Glacial Lake Plainwell, a lake impounded along the Lake Michigan Lobe that drained southward into the broad lowland of the embayment, as well as networks of large tunnel valleys terminating at the margins of both lobes. Cross-cutting relationships indicate that meltwater emanating from tunnel valleys in the Huron-Erie Lobe drained across the area somewhat later than the other two lobes and may have postdated the KT. New data from Illinois shows that KT-scoured Livingston Phase ground moraine deposits have a minimum age of 19.1 ± 0.1 ka BP. Earliest Valparaiso Morainic System deposits, possibly eroded by the KT, have a minimum age of about 18.7 ± 0.1 ka BP.