RADIOCARBON CHRONOLOGY SUGGESTS THAT THE WOODSTOCK PHASE ICE MARGIN (LAKE MICHIGAN LOBE) WAS INTIMATELY RELATED TO THE 18.9 KA KANKAKEE TORRENT

Continued surficial mapping and determination of ice margin chronologies utilizing the chronology archived in ice-walled lake plains suggests cause-and-effect relationships between glacial margins and large-scale flooding. In northeastern Illinois, a new suite of radiocarbon ages from an ice-walled lake plain (42.257°N, -88.414°W) on the Barlina Moraine, but proximal to the Woodstock Moraine, indicates deglaciation occurred in three pulses: 22.5-21.1 ka, 19.1-18.5 ka, and 18.0-17.4 ka (2σ errors). The middle “warm” period of 19.1-18.5 ka overlaps multiple dates from tundra plant fossils (19.0-18.8 ka) that were deposited in a lake formed in Oswego Channel immediately after Glacial Lake Wauponsee overtopped the Marseilles Morainic System during the Kankakee Torrent. There are, as of yet, no radiocarbon ages from ice-walled lake deposits or other (dead) ice-contact features associated with the Woodstock Moraine. The difficulty in finding appropriate material is associated with the coarse texture of the diamicton that forms the moraine, and its thick proglacial outwash. To date, laminated silts forming IWLPs on the Woodstock Moraine have yielded ostracode valves; evidence of fossil leaves, etc., is manifest as fine botryoidal masses of pyrite. A new campaign to find dateable organics will focus sampling the topographic lows in the ice-walled lake plains.

Proximal outwash associated with the southernmost part of the Joliet sublobe (Lake Michigan lobe) during the Woodstock Phase did not extend south of 41.5°, barely reaching the modern southern shoreline of Lake Michigan. With this configuration, the abundant meltwater indicated from the outwash deposits themselves would have contributed to the KT, as well as other torrents, such as the Fox River Torrent. The catastrophic nature of the KT in the middle and lower Illinois River valleys may be attributed to the outbursts of the surficial proglacial lakes, but the rate of discharge and volume of contributions from (stored) subglacial sources remain speculative. The long period of non-deposition in ice-walled lakes from 21.2 to 19.1 ka suggests cold conditions, the likelihood of freezing southern LIS margins, and development of high pore-water pressures, and storage of subglacial meltwater which were unleashed during the Woodstock Phase.