ORIGIN OF DARK SAND ALONG THE LAKE MICHIGAN COAST

Concentrations of dark sand are common on modern surfaces and in older deposits in Lake Michigan coastal dunes. Sixteen samples of light and dark sand from 8 sites along the south eastern shore were sieved and the mineralogy of different size fractions estimated from EDS electron microprobe spectra on between 100 and 300 randomly chosen grains. Mineralogy of dark and light sand is nearly identical at the same size fraction. Larger size fractions are predominantly quartz (average 87% at 250 μm)and feldspar (11% at 250 μm). The mineralogy of smaller size fractions is more diverse (38% Fe + Fe-Ti oxides, 21% quartz, 11% carbonate, 8% garnet, 6% feldspar, 6% hornblende + pyroxene and 5% epidote at 125 μm). Dark sands were markedly finer grained (average median size 180 versus 250 μm). Textural studies of coastal dune deposits indicate that reverse size grading occurs in less than 50% of the sand overlying dark layers, suggesting that most of them do not form by avalanching. Concentrations of dark sand on the modern surface are almost invariably associated with ripples indicating that size sorting during traction may play a critical role The mineralogy suggests that coastal sands were partially derived from material carried by glaciers from the Canadian Shield. Sand size fractions were separated and sieved from 15 till samples from the Lake Michigan Glacial Lobe in southwest Michigan. Texturally sands from the tills are bimodal with a minimum at 125 μm. To date mineralogical analyses have been carried out on only 6 samples. Three samples of Saugatuck Till from the Lake Border Moraine collected 50 km apart have nearly identical mineralogy. Quartz is abundant at all sizes (82% at 250 μm, 67% at 125 μm) with feldspar and carbonate making up most of the remainder. Garnet, oxides and ferromagnesian silicates together make up less than 2%. Three samples of Muskegon Shoreline Till are also very similar to each other and dominated by quartz., but with significantly higher proportions of garnet and oxides (~5% at 125 μm) than the Saugatuck Till. This suggests that sand mineralogy can serve as a “fingerprint” for different tills. Heavy dark minerals were apparently concentrated in finer size fractions after glacial transport by the winnowing of less dense grains during fluvial,longshore and eolian transport.