RECOGNITION OF LACUSTRINE SEQUENCES IN NORTHEAST ILLINOIS AND THEIR IMPLICATIONS FOR INTERPRETATIONS OF FLUCTUATIONS OF THE LAKE MICHGIAN LOBE MARGIN: HAVE WE OVERLOOKED ALLOSTRATIGRAPHY?

Lake County, Illinois is underlain by two thick glacigenic sequences that record the last retreat and readvance of the Lake Michigan lobe ice margin in and out of the Lake Michigan basin during the lobe’s last occupation of northeast Illinois between about 18,300 and 16,700 cal yr BP (15000 and 13500 ¹⁴C yr BP). The lower and older sequence (sequence 1) is dominated by lake sediment and is characteristically coarser than the overlying sequence (sequence 2). Sequence 1 was deposited during ice-margin retreat and is progressively younger lakeward. Sequence 2 was deposited during ice-margin advance and is a finer-textured sequence, the bottom of which is dominated by lake sediment. The sequence aggrades upward to terrestrial deposits that include silty clay interbedded tills and debris flows of the Wadsworth Formation (Oak Creek Formation), meltwater stream sediment and shallow lake sediment. The contact between the two sequences is typically sharp, and occurs at 660 to 690 feet a.m.s.l.

Parts of both sequences are interpreted to have been deposited in so-called Glacial Lake Milwaukee. Sequence 1 can be considered an offlap and is interpreted to be comprised of coalesced deltas or subaqueous fans, ranging in thickness from 0 to 150 ft (45m) and extends from the Fox River lowland in the west to the area of the eastern edge of the Valparaiso moraine to the east. The eastern, ice-proximal slope of the Valparaiso moraine coincides with the eastern most buried ice-contact face of sequence 1. However, the sequence is completely buried and has no surface expression. Sequence 2, which forms the Valparaiso, Tinley, and Lake Border moraines, can be considered an onlap, both overlies and insets sequence 1, and ranges in thickness from 0 to 200 ft (60m).

Differentiation of these sequences and recognition of their internal facies is based upon recognition of bounding surfaces, making them allostratigraphic units. The contact between sequence 1 and sequence 2 is typically within lake sediment and recognized by a contrast in depositional environments. The sediments at top of sequence 1 are typical of an ice-proximal lake environment and the sediments at the bottom of sequence 2 are typical of an ice-distal lake environment. Coarse facies of sequence 1 are used as an aquifer, so facies distribution has become a focus of three-dimensional mapping efforts.