EVIDENCE FOR HUNDREDS OF METERS OF EROSION DURING THE EARLY-MIDDLE JURASSIC OF MICHIGAN

During the Middle-Late Jurassic redbed sands and muds, limestones and evaporites were deposited in valleys in a hilly landscape on Pennsylvanian sedimentary rocks (mostly shales) of Michigan's Lower Peninsula. Thermal-maturation indicators of Michigan Basin Paleozoic sedimentary organic matter and Jurassic pollen and spores indicate some sort of heating of Paleozoic strata after the Pennsylvanian and before the Middle-Late Jurassic. Most interpretations of Michigan Basin thermal indicators suggest that hundreds of meters (but probably less than a kilometer) of sediment were deposited in the Lower Peninsula during the Permian and/or Triassic, and that any such Permo-Triassic sedimentary rocks were eroded completely away by the Middle-Late Jurassic. (Deposition of clastic sedimentary rocks derived from the ancestral Appalachians likely continued for some time after the time represented by the youngest Paleozoic (Pennsylvanian) sedimentary rocks of the Michigan Basin.) Post-Jurassic sedimentary rocks were probably also deposited, but at no time after the Jurassic did sediments accumulate to great enough thicknesses to thermally modify fossil pollen and spores in the Middle-Late Jurassic redbeds.

Hundreds of meters of Ordovician and Devonian limestones had not yet been eroded from the western Upper Peninsula when Jurassic kimberlites were active there. Recently documented thermal indicators from Ordovician strata on the Kankakee Arch, the La Salle Anticline in the Illinois Basin, and the Appalachian Basin indicate relatively rapid Middle Jurassic exhumation across much of the southern Great Lakes region. Combined with the observed thermal immaturity of thermal indicators in Michigan Basin Jurassic redbeds, these observations constrain the erosion of hundreds of meters of Paleozoic through Early Jurassic sedimentary rocks to the Middle Jurassic. The precise chronologic and causal relationships between thermal indicators in Paleozoic strata, Permo-Triassic sedimentation, Jurassic erosion and Middle-Late Jurassic deposition, displacement of strata in regional impact structures, activity of kimberlites and cryptoexplosion structures, and other cratonic processes in and around the Michigan, Illinois and Appalachian basins remain to be established.