ORIGIN OF FINER GRAINED (<150 MICRON) DUNE SAND ALONG THE SOUTHEASTERN COAST OF LAKE MICHIGAN: THE GLACIAL CONNECTION

The origin of sand in the dunes along the southeastern coast of Lake Michigan is being investigated by a combination of textural analysis, mineralogical analysis and trace element analysis of individual quartz grains with microbeam Proton Induced X-Ray Emission (micro-PIXE). Earlier investigations showed that while the finer grained (< 150 micron) portion of the dune sand was enriched in opaque iron and iron-titanium oxides compared to the coarser grained portion (40 versus 0.3 volume % ) a similar enrichment in opaque grains did not occur in sand separated from till in deposits from glaciers that contributed sand to Lake Michigan. This suggests that the smaller less-dense grains were winnowed out during post glacial fluvial transport to the lake, longshore transport along the shore, and aeolian transport to the dunes. Petrologic examination of 24 samples from an outwash plain associated with the Kalamazoo Moraine shows an enrichment of opaque grains in the 125 sieve fraction (4.6%) compared to the 250 micron fraction (1.2%) indicating that this winnowing most likely began during early post glacial transport. Increases in the abundances of opaque grains in Lake Michigan coastal sands need not indicate increasing proximity to a glacial source as has been assumed by some workers.

Trace amounts of three elements have been detected by micro- PIXE analyses of quartz grains from dunes, till and outwash: Fe (detection limit 10 ppm), Ti (detection limit 20 ppm) and Ca (detection limit 50 ppm). Preliminary analyses indicate that quartz grains can be classified into distinct groups based on the relative abundances of these elements. Relative proportions of these groups may provide information on the sources of quartz in sand samples. One group contains no detectable trace-elements. In the analyses to date, this group is significantly more abundant in quartz from the 250 micron sieve fraction (60% of grains), than the 125 micron fraction (20% of grains). This suggests that the quartz in the finer grained samples are not entirely fragments produced by impact and abrasion of larger grains during transport but also include input from a distinctly different, finer grained, population.