GEOLOGIC MAPPING OF THE BRONSON NORTH AND BRONSON SOUTH 7.5 MINUTE QUADRANGLES, BRANCH COUNTY, MICHIGAN

This study contributes to a better understanding of the glacial history of southwest Michigan through newly completed surficial geologic mapping within the Bronson North and Bronson South quadrangles in Branch County, Michigan. This project was supported by the USGS EDMAP program in conjunction with the Michigan Geological Survey during the summer of 2017. The purpose of this project was to characterize the near-surface sediments and bedrock topography of the quads. I achieved this using 185 hand-auger borings, 103 grain size analysis tests, 215 passive seismic depth-to-bedrock measurements, and 3.4km of ground penetrating radar transects. Overall, there is 350 feet of bedrock relief from west to east across the quadrangles, including a northwest trending bedrock cuesta in the underlying Mississippian Coldwater Shale. This bedrock scarp underlies an area of streamlined drumlins in the east, which become diffuse southwest of the scarp. These drumlins are composed of diamicton above 900ft in elevation in the northern quadrangle with coarse-grained outwash underlying swales between drumlins. The southern quadrangle is predominantly composed of coarse-grained outwash with a glaciofluvial sediment upland in the southeast corner of the quad. Surficial sediments in this upland were deposited within a braided stream system during deglaciation. This upland exhibits hummocky topography formed during marginal stagnation of the Saginaw Lobe as it paused or readvanced during its final retreat. Glaciofluvial sediments within these quadrangles were deposited by meltwater streams emanating from the Huron-Erie and Saginaw Lobes of the Laurentide Ice Sheet during their final retreat around ~19,000 years ago.