This dataset captures in digital form previously published maps showing the inferred extent and subsurface elevation of several Paleozoic consolidated rock units in the northern Midwest of the United States. Data released here were published as figures within a U.S. Geological Survey (USGS) Water Mission Area report documenting results from a study of the regional hydrogeology and ground-water quality of the Cambrian-Ordovician aquifer system in the northern Midwest. This study was part of the USGS national Regional Aquifer-System Analysis (RASA) Program. Sandstone and carbonate rocks of Cambrian and Ordovician age compose much of the sedimentary rocks overlying the Precambrian basement in the northern Midwest and form the major aquifer system of that area, named the Cambrian-Ordovician aquifer system by the USGS RASA Program. This aquifer system underlies about 161,000 mi2 (square miles) in northern Illinois, northwestern Indiana, Iowa, southeastern Minnesota, northern Missouri, and Wisconsin.
The published USGS RASA investigation defined the subsurface extent and altitude of the top of eight stratigraphic intervals within the lower Paleozoic consolidated rocks, typically including a major named formation or group plus its regional stratigraphic equivalents. Major mapped intervals include, from lowest to highest, the Upper Cambrian Mount Simon Sandstone, Eau Claire Formation, Ironton and Galesville Sandstones, and St. Lawrence and Franconia Formations, Lower Ordovician Prairie du Chien Group, Middle Ordovician St. Peter Sandstone and Galena Dolomite, and the Upper Ordovician Maquoketa Shale. The USGS RASA study also included maps of the contoured top of Precambrian rocks, top of combined Middle Devonian through Silurian rocks, and top of combined Pennsylvanian, Mississippian, and Upper Devonian rocks; these maps were also digitized and are included in this data release. This dataset includes vector structure contour data digitized from page-sized figures in USGS Professional Paper 1405-B (Young, 1992). Some maps from this report had previously been digitized by a USGS saline groundwater assessment project and released as digital datasets on the USGS Water Mission Area’s National Spatial Data Infrastructure (NSDI) node (U.S. Geological Survey, 2015). For consistency and completeness, those data have been reformatted, attributed, and assembled with additional data digitized from the source RASA report for this data release. The dataset includes a geographic information system geodatabase that contain digital structure contour data as polyline feature classes for all of the geologic units contoured in USGS Professional Paper 1405-B (Young, 1992). Vector data are attributed according to the USGS National Cooperative Geologic Mapping Program’s GeMS digital geologic map schema. The geodatabase includes non-spatial tables that describe the sources of geologic information, a glossary of terms, a description of units, and a geomaterials dictionary. Also included is a Data Dictionary that duplicates the Entity and Attribute information contained in the metadata file. To maximize usability, spatial data are also distributed as shapefiles and tabular data are distributed as ascii text files in comma separated values (CSV) format.