T140 Advances in Understanding the Laurentide Ice Sheet II

Monday, 5 November 2018
1:30 PM - 5:30 PM
Room 138-139

LAKE-LEVEL ELEVATION IN THE CHICAGO OUTLET DURING THE NIPISSING PHASE OF ANCESTRAL LAKE MICHIGAN: WENTWORTH WOODS STRANDPLAIN, ILLINOIS

Paper number: T140-7
Program: Technical Sessions
Time: 3:00 PM – 3:15 PM

Authors

Abstract

Accurate reconstructions of paleo lake level and isostatic rebound in the Great Lakes requires knowledge of the elevation of the water at each lake’s outlet. For the upper Great Lakes, four outlets were active and abandoned from the middle to late Holocene, making the development of a paleohydrograph difficult. Recent study of the Nipissing phase of the ancestral upper Great Lakes determined the elevation of the confluent peak Nipissing phase (~4,500 years ago) for the Sault (Lake Superior) and Port Huron (Lake Huron) outlets, and estimated an elevation for the Chicago outlet (Lake Michigan). We studied beach ridges in the Wentworth Woods area of the Tolleston Beach in Cook County, Illinois, to refine the estimated peak Nipissing elevation for the Chicago outlet. Thirty-one vibracores were collected that crossed 22 ridges to recover basal foreshore deposits in each ridge; these can be used for a proxy for lake level when each ridge formed. Optically stimulated luminescence samples were collected from six soil pits spaced roughly five ridges apart to produce an age model for the strandplain. Combined, these data produce a paleohydrograph that shows a peak Nipissing elevation of 182.9 m at the Chicago outlet. This elevation is 0.4 m below the elevation of the peak Nipissing at the Port Huron outlet, indicating a relative glacial isostatic adjustment rate of -0.7 cm/century for the Chicago outlet. This rate is an order of magnitude less than rebound rates calculated from lake-level gauges. Two possible reasons for this discrepancy is that the duration of lake-level gauge data is too short to recognize long-term trends or that the observed rebound between the Chicago and Port Huron outlets is a late Holocene phenomena possibly related to the dissipation of a glacial forebulge.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-318489

© Copyright 2018 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.