The orientations of 4,787 joints were measured at 43 separate locations in the area from Charlevoix, Michigan, to Alpena, Michigan, and north to the tip of the lower peninsula of Michigan. The sample localities are all in Devonian carbonate rocks and shales of the Michigan Basin. Nearly all of the joints are vertical. At least one major joint set is identifiable at each location, and four major sets are present at more than half of the locations. At 40 of the 43 sample localities, there is a joint set with a strike of about 54 degrees (N54°E); at 35 locations, there is a set striking about 133 degrees (N47°W). At 32 locations, there is a set with a strike of about 92 degrees (N88°W), and a set with a strike of about 2 degrees (NO2°E) is present at 31 sample localities. Mean orientation of any of the four joint sets does vary from location to location but is relatively consistent over the entire area for each of the sets. Fluctuations in mean orientation of any of the joint sets or the presence or absence of the joint sets is independent of formation, lithology, and distance between sample stations.
Joint set orientation is independent of the regional strike around the Michigan Basin, which varies from 54 degrees (N54°E) to 114 degrees (N66°W) within the study area. The joint pattern does not appear to be related to structural trends previously postulated to exist in the Precambrian basement of the area. Two of the joint sets may be related to major low-amplitude folds in the Paleozoic strata of the Michigan Basin. In situ stress measurements show the orientation of maximum horizontal compressive stress to be compatible with these joint sets and the folds.