Mississippian and younger geologic units form a regional system of aquifers and confining units in the central Lower Peninsula of Michigan. The area of the regional aquifer system is about 22,000 square miles. The aquifer system consists of three bedrock aquifers, which are separated by confining units. Bedrock aquifers and confining units are overlain by surficial glaciofluvial aquifers, which are complexly intercalated with confining beds composed of glacial till and fine-grained lacustrine deposits.

Geophysical and geologic logs were used to characterize the hydrogeologic framework of this regional aquifer system and to delineate and map boundaries of aquifers and confining units. Geophysical logs and water-quality data were used to delineate the base of freshwater within the aquifer system and to determine geologic controls on the distribution of freshwater in the aquifer-system units.

Pleistocene glaciofluvial deposits are the largest reservoir of fresh ground water in the mapped region, and the thickness of this aquifer unit exceeds 900 feet in some areas. The Saginaw aquifer, the composite of sandstones of Pennsylvanian age, typically ranges in thickness from 100 to 350 feet in areas where this unit is used for water supply. In the western part of the aquifer system, the Saginaw aquifer is separated from glacial deposits by 100 to 150 feet of Jurassic "red beds." "Red beds" are a confining unit, and the Saginaw aquifer contains saline water where it is overlain by these deposits. The Saginaw confining unit, which is principally shale, separates the Saginaw aquifer from the underlying Parma-Bayport aquifer. Thickness of the Saginaw confining unit is about 50 feet in the eastern and the southern parts of the aquifer system, about 100 feet in the north, and 100 to 250 feet in the west. The Parma-Bayport aquifer, which consists mostly of permeable sandstones and carbonates, is 100 to 150 feet thick in most areas. The ParmaBayport aquifer contains freshwater only in subcrop areas where it is in direct hydraulic connection with glacial deposits. Dissolved-solids concentration of ground water increases down regional dip in the ParmaBayport aquifer, and saline water or brine is present in this aquifer where it is overlain by the Saginaw confining unit.

The Michigan confining unit, which is about 300 to 400 feet thick in most of the area mapped, is primarily interbedded shale, carbonate, and evaporite. This confining unit overlies the Marshall aquifer, which consists of one or more stratigraphically continuous sandstones of Mississippian age. Composite thickness of blanket sandstones that form the Marshall aquifer is typically 75 to 200 feet. Freshwater is present in the Marshall aquifer only in areas where it is a subcrop beneath glacial deposits. Dissolved-solids concentration of ground water in the Marshall aquifer increases down regional dip, and saline water or brine is present in this unit where it underlies beds of the Michigan confining unit. The Mississippian Coldwater Shale forms the base of the regional aquifer system.

Relief on the base of freshwater is about 600 feet. Altitudes of the base of freshwater are low (200 to 400 feet) along a 30- to 45-mile-wide north-south-trending corridor near the center of the aquifer system. The trend of this corridor corresponds to an area where thickness of the Saginaw aquifer ranges from 100 to 370 feet. In isolated areas in the northern and the western parts of the aquifer system, the altitude of the base of freshwater is below 400 feet; however, the altitude is above 400 feet in most of the mapped area. In the southern and the northern parts of the aquifer system, where the Saginaw aquifer is thin or absent, altitudes of the base of freshwater range from 700 to 800 feet and from 500 to 700 feet, respectively.

Geologic controls on the distribution of freshwater in the regional aquifer system are (1) direct hydraulic connection between sandstone aquifers and freshwater-bearing, permeable glacial deposits; (2) impedance of upward discharge of saline water from sandstones by lodgment tills with very low permeability; (3) impedance of recharge of freshwater to bedrock (or discharge of saline water from bedrock) by very low permeability Jurassic "red beds"; and (4) the presence of units characterized by very low vertical-hydraulic-conductivity, which are within and between sandstone units.