Sedimentary rocks of Mississippian age form the lower part of a regional aquifer system in the central Lower Peninsula of Michigan. Strata of the Michigan Formation, the Marshall Sandstone, and the Coldwater Shale were subdivided into an aquifer and two confining units on the basis of hydraulic properties. The Michigan confining unit consists of shale, limestone, dolomite, gypsum, anhydrite, and discontinuous beds of sandstone and siltstone. The area! extent of this unit is approximately 17,000 square miles, and thickness typically ranges from 300 to 400 feet. The Michigan confining unit overlies the Marshall aquifer. This aquifer consists of one or more stratigraphically continuous, permeable sandstones, which are hydraulically connected at the scale of the regional aquifer system. The area! extent of this regional aquifer is approximately 22,000 square miles, and the composite thickness of permeable sandstones that form the Marshall aquifer typically ranges from 75 to 175 feet. The Marshall aquifer is freshwater bearing in areas where it is in direct hydraulic connection to Pleistocene glacial deposits. Saline water is present in this aquifer in a zone of transition from freshwater to brine. The width of this transition zone typically ranges from 10 to 30 miles, but width is as narrow as 2 to 4 miles in the northern part of the aquifer system. The Marshall aquifer contains brine in the central part of the basin (areal extent approximately 10,000 square miles), where it is more than 800 feet below land surface. The Coldwater confining unit, which underlies the Marshall aquifer, ranges from 500 to 1,300 feet in thickness. This confining unit forms the basal part of the regional aquifer system, and it consists mostly of shale.
