Evidence for the Lake Chippewa Extreme Low-Water Stage in Lake Michigan

Abstract

The established geological history of the Great Lakes region recognizes four prominent lake stages in postglacial Lake Michigan. During this time various lacustrine sediment layers (members) were deposited in the Lake Michigan basin creating the Lake Michigan Formation. Each member (Waukegan, Lake Forest, Winnetka, and Sheboygan) is correlated to a different water level stage. The lowest water level ever recorded in Lake Michigan, the Lake Chippewa Stage, occurred during the deposition of either the Lake Forest or Winnetka Members. The occurrence of this low-water stage has not been well documented because of insufficient evidence from deep water sediments. However, in this study fossil invertebrate data of shallow water clams and snails are given. The fossils are presently found in deep water sediments which identify the layer forming the lowest water stage, Lake Chippewa, in Lake Michigan. It appears the Lake Chippewa low-water stage occurred during the deposition of the Winnetka Member. The Winnetka Member contained a high concentration of Pisidium clam shells and ostracod valves. In addition, this member contained characteristically shallow-water snails of the genera Gyraulus, Valvata, Amnicola, and Lymnaea. Previous paleomagnetic studies of the upper Winnetka Member indicate this deposition and the Lake Chippewa low-water stage occurred between 6,370 and 9,700 years ago.