THE GEOLOGIC FRAMEWORK OF SOUTHERN LAKE-MICHIGAN

This part of the project was carried out with geophysical and geologic al sampling techniques to determine in more detail the geology of sout hwestern Lake Michigan and thereby provide the essential framework on which to base subsequent lake level, process, and sediment budget stud ies. The bathymetry of southwestern Lake Michigan is controlled by the underlying bedrock, which dips northeast toward the center of the Mic higan basin. Bedrock comprises Silurian dolomite and Devonian limeston e and shale. Quaternary sediment, 10 to 40 m thick, overlies bedrock. From Waukegan, Illinois, south to Indiana Harbor, the bottom is floore d by till, sand, pebbles, and cobbles. Sand, more common within 1 to 2 km of shore, thins lakeward to a patchy veneer. The lake floor is ero sional or nondepositional where till or gravel-cobble pavement is expo sed. In contrast, north of Waukegan and east of Indiana Harbor, fine s and covers much of the bottom and grades offshore to muddy sand, which is part of the modern, lacustrine Lake Michigan Formation. The comple x surficial bottom sediment distribution between Waukegan and Michigan City, Indiana, could be mapped in detail only where we have sidescan sonar mosaics. In those areas, the till, or coarse lag sand-gravel sur face, is covered intermittently with a layer of fine sand most often a bout 0.5 to 1.0 m thick. The sand appears to be mobile, covering and u ncovering the substrate in response to storm-driven waves and currents . Thus, sand, important for protecting the substrate from erosion and for maintaining beaches, is not abundant throughout much of the area.