Hydrogeologic framework of the Midwestern Basins and Arches region in parts of Indiana, Ohio, Michigan, and Illinois

The rocks that make up the regional Silurian and Devonian carbonate aquifer System in Ohio, Indiana, and adjacent states, consists of limestones, dolomites, varied amounts of terrigenous clastics, and evaporates. The carbonate-rock aquifer system is underlain by Ordovician shales and interbedded carbonate rocks that are virtually impermeable and regionally extensive. These units collectively form a barrier to ground-water flow that effectively limits the transfer of significant quantities of water through the base of the Silurian and Devonian carbonate aquifer. Where the shales and siltstones that make up the upper confining unit are present, the carbonate-rock aquifer is confined above by Devonian and Mississippian shales and siltstones. Where the confining unit above the aquifer has been eroded, the overlying glacial sediments partly confine the carbonate-rock.