Coal in the Michigan basin is in thin, relatively discontinuous, laterally variable seams of non-coking, high volatile B and C bituminous rank, with a range of 10,300 to 12,300 Btu.

The known coalfields range in size from about 100 to 1,500 acres (40 to 60 ha.), mostly in less than 250-acre (100 ha.) areas. Known remaining reserves are generally in beds less than 3 ft (0.914 m) thick. Ash and sulfur content are lower in the northeastern part of the basin, 3 to 9% and 1 to 3%, respectively, in the thicker and more continuous seams which have been mined, but to the south and west these qualities deteriorate to undesirable levels. However, it is possible the rank and quality both might be substantially improved by more modern cleaning technologies than were available at the time of the earlier mining when most of the analyses available were made.

The coal-bearing strata are of Early and lower Middle Pennsylvanian age as determined by marine invertebrate and plant fossil paleontologic techniques. This sequence ranges in thickness from the eroded edges basinward to as much as 650 ft (198.12 m) but the thickness is exceedingly variable owing to its deposition on an erosional surface of several hundred feet of relief which was developed entirely on rocks of varying levels of Mississippian age, and to the post-Middle Pennsylvanian erosion surface. The overlying strata in the center of the basin are consolidated shales, siltstones, and gypsiferous deposits of the "Redbeds" sequence of Late Jurassic age. Peripherally to this thin veneer above the Pennsylvanian in the central part, 300 to 600 ft (92 to 183 m) of unconsolidated Pleisto ene gravels, silts, clays, and peat buries the Pennsylvanian over most of the areal extent. The relief on this pre-Jurassic plus pre-Pleistocene surface exceeds 200 ft (61 m) in many places and erosional channels appear to have removed the Pennsylvanian strata including the coals in a number of places.

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