EVOLUTION OF SMALL COASTAL DUNE LAKES ALONG THE SOUTHEASTERN SHORE OF LAKE MICHIGAN: CLUES FROM MINERALOGY

The mineralogy of sediment cores from 3 small lakes along the eastern margin of the Lake Michigan coastal dune complexes between Holland and Saugatuck, Michigan was examined by x-ray powder diffraction. Radiocarbon ages suggest that sedimentation began during the Nipissing Transgression as stream valleys were flooded and later dammed by the coastal dunes. Mixtures of quartz and calcite are found at the base of cores from 2 lakes, and probably represent sedimentation in shallow alkaline wetlands. Sand dominates the lower portions of the cores and occurs in thin layers in the upper portions. The coarser fraction (> 5 microns) is dominated by quartz and feldspar which also dominate the sand in the adjacent dunes. The finer fraction (< 5 microns) is dominated by quartz, illite, and muscovite. The sand-rich sediments represent periods in which input from the dunes was greater than production of sediment within the lakes. Sapropels dominate the upper portion of all cores. Upon LOI, iron-rich sapropels form magnetic red peds composed of anhydrite, hematite, and subordinate magnetite. Samples in which the organic portions are removed by centrifuging rather than oxidation contain pyrite without hematite, anhydrite or magnetite. This suggests that these minerals form by oxidation of iron, sulfur, and calcium-bearing sediments during LOI. To test this possibility, a mixture of calcite and pyrite was heated at 600^o C; anhydrite and hematite formed after 4 hours. Iron-rich sapropels represent periods in which reducing conditions prevailed at the lake bottoms resulting in the precipitation of iron sulfides, perhaps due to sulfate-reducing bacteria. X-ray diffraction scans of iron-poor sapropels show quartz and whewellite (CaC₂O₄H₂O) peaks superimposed on a broad hump. The hump is replaced with cristobalite peaks upon heating to 1200^o C, indicating that it is due to amorphous silica. Many plants produce calcium oxalate, and x-ray scans of aquatic macrophytes (Brasenia schreberi and Nymphaea odorata) collected from these lakes have whewellite peaks. Iron-poor sapropels are dominated by solids formed by aquatic plants under conditions less reducing than those responsible for the iron-rich sapropels.