LAKE-LEVEL HISTORY OF LAKE-MICHIGAN FOR THE PAST 12,000 YEARS - THE RECORD FROM DEEP LACUSTRINE SEDIMENTS

Collection and analysis of an extensive set of seismic-reflection prof iles and cores from southern Lake Michigan have provided new data that document the history of the lake basin for the past 12,000 years. Ana lyses of the seismic data, together with radiocarbon dating, magnetic, sedimentologic, isotopic, and paleontologic studies of core samples, have allowed us to reconstruct lake-level changes during this recent p art of the lake's history. The post-glacial history of lake-level chan ges in the Lake Michigan basin begins about 11.2 ka with the fall from the high Calumet level, caused by the retreat of the Two Rivers glaci er, which had blocked the northern outlet of the lake. This lake-level fall was temporarily reversed by a major influx of water from glacial Lake Agassiz (about 10.6 ka), during which deposition of the distinct ive gray Wilmette Bed of the Lake Michigan Formation interrupted depos ition of red glaciolacustrine sediment. Lake level then continued to f all, culminating in the opening of the North Bay outlet at about 10.3 ka. During the resulting Chippewa low phase, lake level was about 80 m lower than it is today in the southern basin of Lake Michigan. The ri se of the early Holocene lake level, controlled primarily by isostatic rebound of the North Bay outlet, resulted in a prominent, planar, tra nsgressive unconformity that eroded most of the shoreline features bel ow present lake level. Superimposed on this overall rise in lake level , a second influx of water from Lake Agassiz temporarily raised lake l evels an unknown amount about 9.1 ka. At about 7 ka, lake level may ha ve fallen below the level of the outlet because of sharply drier clima te. Sometime between 6 and 5 ka, the character of the lake changed dra matically, probably due mostly to climatic causes, becoming highly und ersaturated with respect to calcium carbonate and returning primary co ntrol of lake level to the isostatically rising North Bay outlet. Post -Nipissing (about 5 ka) lake level has fallen about 6 m due to erosion of the Port Huron outlet, a trend around which occurred relatively sm all (+/- approximately 2 m), short-term fluctuations controlled mainly by climatic changes. These cyclic fluctuations are reflected in the s edimentological and sediment-magnetic properties of the sediments.