XRF ANALYSES OF CHERT PEBBLES FROM BEACHES OF SOUTHERN LAKE MICHIGAN

The composition and origin of chert on the Southern shores of Lake Michigan has been a topic of minimal discussion. While the exact origin of chert is yet to be determined, we believe that most chert pebbles were flushed out of Pleistocene glacial sediments derived from Paleozoic rock formations in the Great Lakes region. Previous studies on chert have been related to Native American Artifacts. Chert samples for this study have been collected on beaches of southern Lake Michigan, from Whihala Beach near the Illinois border to the west, and on to Long Beach, near the Michigan border on the east. This study uses a multi-technique analytical approach to sourcing chert. We will combine visual, geochemical, and petrographical methods of studying chert. Visual inspection of color and texture allowed us to distinguish nine types of chert. After processing two samples of original nine chert types with X-Ray Fluorescence (XRF) the oxides and elemental make up indicated that six distinct types of chert can be recognized. Type E chert is the most abundant on beaches, with a tan to brown color, occasionally with faint reddish spots, and a smooth waxy texture with a dull luster... Silica content in Type E chert is 95.8% and iron-oxides (1.78%) are slightly more abundant than alumina (1.68%). Type A chert is brownish-tan in color with a mottled appearance and average silica content of 96.4%. Type B chert is greyish-white with thin laminations and silica content of 97.8%. Type D chert is pale blue to grey in color, contains fossils, the least amount of silica (76.7%), and largest amount of calcite (18%) among all chert types. Binary plots of Si, Al, Ti, Sr, Mn, and Rb indicate the mixed origin of all types except D chert, which always plots away from other samples. Thin section studies and petrographical analyses of chert will provide additional information that will allow us to correlate chert pebble types with their respective source rocks.