CHRONOLOGY AND EXTENT OF THE LAURENTIDE ICE SHEET IN THE ARROWHEAD REGION, NORTHEASTERN MINNESOTA, USA

The landscape of northeastern Minnesota reflects the evolution of the Koochiching, Brainerd, Rainy and Superior lobes of the Laurentide Ice Sheet (LIS) during the Late Wisconsinan glaciation. Studying this landscape and the underlying till stratigraphy fosters improved understanding of climatic conditions during the retreat of these lobes from their last glacial maximum (LGM) positions. Long-established spatiotemporal correlations linking end moraine positions with minimum limiting radiocarbon ages throughout the state indicate that the conjoined (Rainy-provenance) Rainy lobe and the (Superior-provenance) Superior lobe retreated from the St. Croix moraine by 20,500 ± 400 14C kyrs BP., to the Vermilion and Nickerson moraines, respectively, at 12,070± 170 14C kyrs BP and 10,800 ± 300 14C kyrs BP. The retreat of these northeast-sourced lobes was followed by an advance of the Keewatin-sourced Koochiching lobe, which subsequently generated the piedmont-like St. Louis sublobe and deposited the reddish fine-grained Alborn till south of the Mesabi Iron Range, which has a minimum limiting radiocarbon age of 9,720 ± 120 14C kyrs BP. Very few, and in particular maximum, age dates exist as a basis to reconstruct the activity of these lobes between these moraines. In an attempt to better constrain this activity, 19 rotary-sonic sediment cores were collected in conjunction with county-scale geologic mapping in St. Louis and Lake Counties, MN. Quaternary sediment stratigraphy was analyzed from these cores using descriptive classification developed from texture, grain counts, and geochemistry. Of particular significance is an age of 45,100 ± 1,400 14C kyrs BP obtained from wood within a thin sequence of glaciolacustrine sediment bracketed by Cromwell till of the Superior lobe and Independence till of the Brainerd lobe. This age is similar to other radiocarbon ages collected in adjacent Aitkin County, MN and serves as a maximum age constraint on the advance of the Superior lobe into northeastern Minnesota. While no organic materials were found in our regional mapping efforts that might place more accurate constraints on the timing of the latest Koochiching lobe advance, forthcoming OSL ages will aid in determining its recession from the proto-glacial Lake Agassiz basin.