ABSTRACT: The internal architecture and timing of development were determined for 42 beach ridges within the Platte Bay Dunes National Lakeshore. The elevations of the foreshore sediments were used to determine the relative elevation of Lake Michigan when each ridge formed. To determine the age of the ridges and the timing of their development, 23 peat samples were dated from basal wetland sediments between ridges. A least squares regression was calculated through these dates to reduce scatter and to assign ages to unsampled wetlands. Its is assumed in this study that the age of the base of the wetland is a minimum age for the ridge lakeward of it. Ridges were found to have formed between 2,700 and 1,250 calendar years before present (B.P). Foreshore elevations show a 5 foot lowering of relative lake level between 2,050 and 2,500 B.P. After 2,050 B.P., lake level rose a total of 4 feet to reach a high at about 1,600 B.P. before beginning a long-term fall of 4 feet to current lake level. Between 2,700 and 1,250 B.P. individual beach ridges formed about every 29 (+/-5) years. Foreshore elevations indicate a well-defined grouping of four to five beach ridges approximately every 150 years. Two lake-level stands have been identified at about 2,300 and 1,700 B.P.. Foreshore elevations after 2,250 B.P. are near the long-term historical average of lake-level for Lake Michigan, suggesting that the Platte Lake embayment is very near the isostatic zero isoline for the Port Huron outlet for Lakes Michigan and Huron.
Baedke, S. , Thompson, T. A. 1993, Preliminary Report of Late Holocene lake-level variation in northern Lake Michigan: part 1: Indiana Geological Survey Open-File Study 93-04, 15 p., 8 fig.
Notes: Publications in the Indiana Geological Survey Open-File series have been inconsistently named using a variety of series titles including "Open-File Report," "Open-File Map," and "Open-File Study." Prior to 1994, a publication in this series was generally referred to as an "Open-File Report" (but not always). To help reduce confusion created by these inconsistencies, the IGS now refers to every publication in the Open-File series as an "Open-File Study." To be entirely correct in writing a bibliographic reference for a publication, one should use the series name and number that appears on the publication itself.
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Keywords: sedimentology, lake level, Holocene, Lake Michigan, geologic hazard
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