Michigan State University Geology of the Great Lakes Lake Michigan focus David P. Lusch, Ph.D. Dept. of Geography Michigan State University

Great Lakes Watershed

Great Lakes Region Canadian Shield Michigan Sedimentary Basin Two very different geologic environments Michigan Sedimentary Basin Canadian Shield

Canadian Shield Very resistant to erosion Very old rocks (3500 - 542 million years) Very resistant to erosion Plutonic rocks (e.g., granite, diorite or gabbro) Metamorphic rocks (e.g., gneiss, schist or slate) Volcanic rocks (e.g., basalt, obsidian or scoria)

Michigan Sedimentary Basin Younger rocks (542 – 145 million years old) All sedimentary (mostly marine deposits)

Michigan Sedimentary Basin Mississippian 345 Ma Equator N Deposition in shallow seas North American Plate African

Michigan Sedimentary Basin Structural basin – like nested bowls Oldest rocks at the bottom; youngest at the top > 16,000 ft

Michigan Sedimentary Basin The Michigan Basin was inundated numerous times by oceans, which eventually filled it with sedimentary deposits. Four general sedimentary rock types fill the Michigan Basin: Sandstones Carbonates (limestone and dolostone) Shales Evaporites (halite and gypsum)

Facies Concept Lateral variations in the lithologic characteristics of a volume of sediments of the same geologic age. Off-shore non-clastic zone Near-shore clastic zone Wave energy keeps fine clastic sediments in suspension; only coarse clastic sediments settle out No wave energy - fine clastic sediments settle out no clastics so only non-clastic sediments settle out Becomes sandstone shale limestone/ dolostone

Marine transgression/regression Sea transgression (sea level rise through time) Sea regression (sea level fall through time)

Origin of the Great Lakes Relatively recent features in the landscape. Geologically ancient causes For Lake Superior - plate tectonics and rifting For the lower Great Lakes - development of the Michigan Sedimentary Basin Geologically more recent causes Glacial sculpting of bedrock, mediated by differences in their resistance to erosion Isostatic uplift of the region shifting the watershed outlet

Geologic origin of Lake Superior Mid-continent Rift system of North America A “failed” rift in the center of the North American Craton – rifting began about 1.1 billion years ago. The rift valley was flooded by oceans and filled with sandy sediments. Millions of years later, glaciers successfully eroded away the sandstones leaving the lake depression.

Origin of the 4 Lower Lake Basins

Differential erosion The various sedimentary rock types are of unequal resistance to physical erosion: Sandstones and Carbonates are stronger and tend to support highlands. Shales are weaker and tend to underlie lowlands.

Bedrock of the Great Lakes Basin

Bedrock of the Great Lakes Basin Wisconsin Dome Thornton, IL

Niagaran Escarpment Major resistant-rock (dolomite) landform in the Michigan Structural Basin

Niagaran Escarpment Scarp slope Dip slope

Niagaran Escarpment Rock Island , Wisconsin

Fayette State Park, Michigan Niagaran Escarpment Fayette State Park, Michigan

Niagaran Escarpment Dip slope Scarp slope

Niagaran Escarpment Hwy 401 Niagara Falls Lewiston, NY

Origin of the Great Lakes Relatively recent features in the landscape. Geologically ancient causes For Lake Superior - plate tectonics and rifting For the lower Great Lakes - development of the Michigan sedimentary basin Geologically recent causes Glacial sculpting of bedrock, mediated by differences in resistance to erosion Isostatic uplift of the region which shifted the watershed outlets

At its maximum, the Laurentide Ice Sheet in At its maximum, the Laurentide Ice Sheet in the Great Lakes region extended south of Michigan into southern Ohio, Indiana, and Illinois. Lobes of the ice sheet followed pre- glacial valleys and their flow regimes were influenced by resistant bedrock formations.

Glacial lobes

Lake Michigan bathymetry Whitefish Channel Whitefish Fan Chippewa Basin Two Rivers Ridge Mid-lake Plateau South Chippewa Basin

Lake Michigan bathymetry vs. bedrock type Point aux Chenes Shale Antrim Shale Ellsworth Shale Coldwater Shale Shale subcrop (dashed line inside the border)

Lake Michigan Archipelago