LITHOLOGIC VARIABILITY AND PROVENANCE OF PLEISTOCENE BEDLOAD MATERIAL IN THE MEREDOSIA CHANNEL OF THE UPPER MISSISSIPPI RIVER

Pleistocene alluvial deposits of the Upper Mississippi Valley are well known for their lithologic variety, and thus, this colorful “river rock” is widely used as landscaping gravel. Few studies have focused on provenance or downstream variations in lithologies of the bedload sediment in the Upper Mississippi River system. This study analyzed pebble fractions of the Henry Formation sampled from two closely-spaced dredge sites along the upstream edge of the Meredosia Channel of the ancient Mississippi River in northwestern Illinois. The goal of this project is to develop a better understanding of the controls on lithologic variability between different bedload size fractions. Seven lithological categories were identified in pebble counts including: carbonates, sandstone, quartzite, mafic igneous, intermediate igneous, silicic igneous, and lastly miscellaneous quartz and chert. Grain size fractions for this study included coarse to very coarse pebbles (-5 to -7 Φ), coarse pebbles (-4.9 to -4Φ), and medium pebbles (-3.9 to -3 Φ). Relative percentages of different lithologies vary significantly between the two sample sites, even within respective grain size fractions. In particular, carbonate clasts dominate all pebble fractions at the upstream sample site, suggesting contributions from Silurian bedrock, exposed along the eastern bluffs in a reach 2-17 km upstream. Southward flowing glacial meltwaters undercut these NE-SW trending bluffs between the Cattail and Meredosia Channels of the ancient Mississippi River, supplying carbonate bedload material. The downstream sample site is about 4 km farther away from the bluffs and is situated closer to the middle of the valley. The smaller percentages of carbonate clasts here reflect rapid physical weathering and breakdown of carbonate clasts with downstream transport. Other common lithologies include silicic and mafic igneous clasts, which were transported a minimum distance of ~610 km and ~580 km from their respective upstream bedrock sources. Mafic and silicic components vary little between both sites for size fractions -3.9 to -3Φ and -4.9 to 4Φ, likely reflecting the lithologies’ durability in the fluvial system.