|Mt. Simon Formation, Michigan Basin and Ohio Area
According to Vugrinovich (1986), the "Upper Cambrian silty dolomites, dolomitic sandstones and shales" function as an aquitard. We interpret this to mean that they are a regional aquitard. The isopach map of Catacosinos and others (1986) (7top seal thickness) does show one area in south-central Michigan where the Eau Claire thins to a thickness of zero.
Catacosinos, P. A., and Daniels, P. A., Jr., 1986, Stratigraphy of middle Proterozoic to Middle Ordovician formations of the Michigan Basin, in Catacosinos, P. A., and Daniels, P. A., Jr., eds., Early sedimentary evolution of the Michigan Basin: Geological Society of America, Special Paper 256, p. 53-71.
Vugrinovich, Raymond, 1986, Patterns of regional subsurface fluid movement in the Michigan Basin: Michigan Department of Natural Resources, Geological Survey Division, Open File Report 86-6.