North-Central Section - 42nd Annual Meeting (24–25 April 2008)

Paper No. 10
Presentation Time: 8:00 AM-12:00 PM


LYLES, Kristi L., Geology, Augustana College, 639 38th Street, Rock Island, IL 61201,

The Wisconsin River between Baraboo and its convergence with the Mississippi River flows through the Driftless Area. Exposed bedrock is of the Cambrian and Ordovician Systems, which include the Tunnel City and Elk Mound Groups (sandstone), Prairie du Chien Group (dolomite), Ancell Group (orthoquartzitic sandstone), and the Sinnipee Group (dolomite). Near Baraboo, PreCambrian bedrock is exposed (Baraboo Quartzite to south, Barron Quartzite to northwest), along with glacial deposits of the the Johnstown terminal moraine system. This moraine system crosses the Baraboo Range south into the Wisconsin River valley and the area of Cross Plains. This project analyzes samples of the Wisconsin River bedload in an effort to determine to what extent bedload lithology reflects local bedrock sources. The Baraboo Quartzite and the St. Peter Sandstone of the Ancell Group are the most distinctive lithologies to trace in the bedload.

Twenty bedload samples were taken from three primary sampling sites: the area of Baraboo, Gotham, and Prairie du Chien. Individual sampling sites were chosen according to the accessibility from the shore. Samples were taken approximately 30-45 centimeters below the bed surface to minimize the interference of shore surface run-off, and were taken using a shovel by digging down into the sediment.

Sediment analysis was done on each of the twenty samples to determine grain size and mineral composition. Sediment color varied in the downstream direction from dark brown and black to an almost pure white color. . Samples collected from downstream sites contain significant, clean, quartz sand, most likely sourced from the nearby exposures of St. Peter Sandstone. This work suggests that proximal bedrock sources provide copious amounts of bedload sediment to parts of this stream, in some cases significantly diluting the amount of bedload sediment sourced from the upstream regions.