The orientations of joints were measured at 142 locations across the northern Michigan Basin in rocks of Cambrian to Devonian age. Four major vertical joint sets were found throughout the area. The average trends of the four sets are 052°, 134°, 091°, and 001°. There are small random fluctuations in mean orientation of all the sets, but they remain remarkably consistent in orientation throughout the area, across lithologic and stratigraphic boundaries, and across a regional strike change around the Michigan Basin of about 115°.
The joint pattern does not seem to be related to structural trends in the Precambrian basement, inferred from geophysical data. The joint set at 134° has the same trend as a series of major folds in the Paleozoic rocks of the Michigan Basin, suggesting that this joint set may be related to the folding. The joint set at 052° is parallel with the maximum horizontal compressive stress that exists today throughout the mid-continent region. This joint set may be extension joints formed in response to the present stress regime.