Geology and Geomorphology of the Lake Michigan Coast. (Special Session)
Influence of Late Holocene Lake Level on Shoreline and Dune Distribution in the Upper Great Lakes. Todd A. Thompson, Indiana Geological Survey, Indiana University, 611 North Walnut Grove, Bloomington, IN 47405
Analysis of Glacial Sediments and Stratigraphy within Van Buren County, Michigan. Steven Beukema and Alan Kehew, Western Michigan University, Department of Geosciences, Kalamazoo, MI 49008
Recent study of late Holocene lake level in the upper Great Lakes shows that lake level was more than 4 m higher than today during the Nipissing I and II phases of ancestral Lakes Michigan, Huron, and Superior. Deposits associated with the Nipissing phases commonly occur on platforms and consist of large dunes, beach ridges, and spits. In many areas, Nipissing II phase beach ridges and spits separate smaller marginal lakes from the larger primary lake basins. After the Nipissing II phase, lake level fell 4 m in slightly over 500 years. In northern areas with high rates of isostatic rebound, the result of the 4 m fall is often expressed as a scarp. During the past 3,500 years, the upper Great Lakes experienced three long-term highs: the Algoma phase (2,300 to 3,300 cal. yr. B.P.), and two unnamed high phases (1,100 to 2,000 and 0 to 800 cal. yr. B.P.). Superimposed on these long-term trends are two quasi-periodic fluctuations having periodicities of about 160 and 33 years in duration. The approximately 30-year fluctuation is instrumental in producing individual beach ridges, whereas the 160-year fluctuation produces groups of beach ridges consisting of 4 to 6 ridges in a group. The long-term high from 1,100 to 2,000 cal. yr. B.P. contains very well defined groups that can be recognized in aerial photographs. Additionally, small parabolic dunes commonly occur along beach ridges in sites along the southern and eastern margins of the lakes. These small dunes show a systematic change in predominant wind direction from westerlies to northerlies throughout the late Holocene.
A surficial glacial landforms map of Van Buren County was recently completed as a USGS STATEMAP project (Kehew et al. 2002). Several rotasonic cores were drilled in the study area in places of interest to gain additional subsurface data. Six of these cores, distributed throughout the county, were analyzed to provide insights into the timing of glacial events and to determine depositional environments, as well as to interpret stratigraphic units that correlate across the county. Four of the cores were drilled on upland areas traditionally mapped as the Lake Border, Valparaiso, and Kendall Moraines, while two cores were drilled in lowland regions. The sequence of events interpreted is two glacial advances separated by an extensive period of glaciolacustrine and glaciofluvial activity.
Big Lake Records Preserved in a Little Lake's Sediment: An Example from Silver Lake, Michigan. Timothy G. Fisher, Indiana University Northwest, Department of Geosciences, 3400 Broadway, Gary, IN 46408; Walter L. Loope, U.S. Geological Survey; Harry M. Jol, University of Wisconsin-Eau Claire, Geography Department; William C. Pierce, University of Michigan, School of Natural Resources and the Environment
Silver Lake is a former embayment of Lake Michigan separated by a barrier-bar/dune complex. Lake sediment stratigraphy based on vibracores and a GPR transect consists of Lake Michigan Formation (LMF) sand and clay overlain by marl, peat, and organicrich mud, which records lake level fluctuations in both Michigan and Silver Lake. A hardwater corrected date of 11,310 [C.sup.14] B.P. (Bera169830) at the marl and LMF contact records the beginning of the Greatlakean substage. A 10,460[+ or -]50 (WW2787) [C.sup.14] B.P. age from a buried soil overlain by pebbly sand may be interpreted as a transgression of Lake Algonquin. In other cores, the abrupt transition to marl records a localized perched water table during the Chippewa Low. Peat deposition starting at 8,400 yr B. …