BUILDING A DIGITAL DATABASE FOR THE MIDCONTINENT RIFT SYSTEM

The 1.1 Ga Midcontinent Rift system (MCR) is one of the most prominent geophysical features of the North American craton, extending over 2,000 kilometers through the central U.S. and Lake Superior region into southern Ontario. This major continental rift and its related mineral deposits have been the subject of numerous studies over the past century, despite the fact that MCR-related rocks are only exposed in the Lake Superior region. Much of our knowledge of the majority of the rift, which is concealed beneath Paleozoic bedrock or thick glacial cover, comes from geophysics and relatively sparse drill hole data.

More than a decade ago the USGS initiated collaboration with the Ontario Geological Survey, the Minnesota Geological Survey, and academic partners in U.S. and Canada to compile available MCR data into a single digital source. A geologic base was compiled from 16 published maps (with scales ranging from 1:100K to 1:1M) to create a single geologic map for the entire rift in both the U.S. and Canada. MCR stratigraphy and geological attributes were standardized among all the maps.

The backbone of the dataset is a point file of ~6000 MCR rock samples (mostly plutonic, volcanic, and sedimentary) that have been analyzed for whole rock major and trace element geochemistry, stable and(or) radiogenic isotopic signatures, and(or) age. These rock sample points have related tables with location and rock type information, along with whole-rock geochemical data (and methods), isotopic data, and (or) age data, as available, as well as references. An additional dataset of ~600 mineral deposit and occurrence locations were compiled with accompanying data attributes for commodities, deposit and host rock geology, size, and development status. Locations of >1,900 MCR-related drill holes in Michigan, Minnesota, and Wisconsin were compiled into an additional dataset. A depth to Precambrian basement surface model was created by digitizing contour lines from rectified paper maps of depth to basement and converted to a surface using GIS geoprocessing tools. Compiled gravity and magnetic data for North America were clipped to the extent of the geologic map. Together, these datasets provide a foundation of digital geological, geochemical, and geophysical data to further explore the geology and mineral resources of the MCR system.