LITHOLOGIES AND PROVENANCE OF PEBBLES FOUND IN THE MORAINES OF WISCONSIN

Pebble lithologies in moraines generally reflect the relative percentages of bedrock types exposed at the earth surface along the flow paths in the up-ice direction. This study aims to quantify the fractional percentages of pebble lithologies distributed across Wisconsin by the Laurentide Ice Sheet and to compare those variations to previously mapped up-ice bedrock compositions. Samples were taken from the terminal moraines of three glacial lobes in northern Wisconsin and from the Green Bay / Lake Michigan interlobate kettle moraines. Pebbles ranging from 0.5-1.0 inches were isolated by sieving, and pebbles were sorted according to lithology. GIS software was then used to predict lithologic distributions at these sample sites based on the composition and area of up-ice bedrock and an assumption of linear ice flow. The predicted relative percentages of lithologies vary across the study area due to variations in up-ice bedrock. Large Paleozoic sedimentary basins in the east contrast with extensive PreCambrian plutonic and igneous terranes in the west. The pebble-count data from the field generally agree with predicted lithologic distributions utilizing GIS analytical techniques. Samples taken from between the GBL and LML consistently showed fewer igneous pebbles than those taken from the lobes farther west. This research adds to the growing repertoire of useful applications for GIS software by demonstrating that in this broad study area, GIS analysis is a viable way to predict compositions of glacial sedimentary deposits, assuming existing bedrock maps are accurate.