RADIOCARBON AGE-BASED PHASES OF THE LAKE MICHIGAN LOBE, ILLINOIS, USA

We have culled 881 finite radiocarbon ages determined from within Illinois to revise bounding ages of phases for the Lake Michigan lobe. The dataset contains 592 ages determined using standard benzene synthesis-liquid scintillation, including “legacy” ages determined in the early 1950’s at the inception of the radiometric radiocarbon dating method. In addition, the dataset includes 266 radiocarbon ages determined by accelerator mass spectrometry (AMS). We analyzed the dataset based on context, precision, and accuracy to vet minimum or maximum age estimates of diachronic phases. The entire dataset will be made available in a forthcoming chapter in GSA Special Paper 530.

The last glaciation in Illinois is marked by a local maximum margin in northeastern Illinois of the Marengo Phase (modal probability, 28,000 cal yr B.P.), and the statewide glacial maximum of the Shelby Phase (24,200 cal yr B.P. From about that point, the Lake Michigan lobe entered an overall retreat mode with readvances at about 22,200 and 21,100 cal yr B.P., the minimum ages of the Marseilles and Minooka subphases, respectively, of the Livingston Phase, and 20,500 cal yr B.P. of the Woodstock Phase. The latter age is also the conservative estimate of the onset of the lacustrine Milwaukee Phase. This phase ended as the Lake Michigan lobe made its final advance and retreat into Illinois during the Crown Point Phase (18,490 to about 16,500 cal yr B.P.). Deposits of the late Crown Point Phase interfinger with proglacial lacustrine deposits of the Glenwood Phase (16,900 – 15,000 cal yr B.P.).

The probability density of the entire dataset (excluding ages we deem questionable) reveals pdf maxima doublets related to organics buried by rapidly accumulating silty sediment typically in loess/paleosol successions and slackwater lake environments. The older peaks are typically of material occurring in dense accumulations of organic carbon (stumps, wood fragments, peat), whereas the younger peaks are ages of smaller organics, and likely represent collection bias of event horizons. Some, if not most, low pdf values are related to retreat of the Lake Michigan lobe into its basin forming a trap for fines, resulting in slowdown or cessation of loess deposition along the Illinois River and other outwash conduits.