Upper Devonian and Mississippian rocks in the Michigan basin are analyzed by subsurface methods. Sections from one hundred sixty-eight oil wells were studied.

Isopach and lithofacies maps serve as a basis for interpreting sedimentation processes, tectonics, and environmental conditions. In order to work out a more complete picture of Upper Devonian and Mississippian sedimentation, it is necessary to subdivide the total section into operational rock units A, B, and C, which are recognized on the basis of lithologic criteria. Respective units are then interpreted in terms of tectonic and environmental conditions responsible for patterns exhibited. Integration of three arbitrary units of Upper Devonian and Mississippian sedimentation presents the regional geologic history.

Sand-shale ratio and per cent carbonate or evaporite in the total stratigraphic unit are used in the statistical analysis.

Interpretation of isopach and lithofacies patterns reveals that tectonics of the depositional area is a very important factor in controlling sedimentary facies. Tectonic conditions from shelf to open basin, to silled basin, are represented.