MAPPING MINNESOTA’S PRECAMBRIAN BEDROCK

Minnesota’s Precambrian bedrock ranges in age and rock type from ca 3.5 Ga (Paleoarchean) gneisses of the Minnesota River Valley subprovince of Superior Province, to ca 1.1 Ga (Mesoproterozoic) volcanic, intrusive, and sedimentary rocks of the Midcontinent Rift. This presentation explores the creation and utility of Precambrian bedrock geologic maps. It provides a progress report on mapping in the state, and speculates about what’s next. Historically, mapping Precambrian rock in Minnesota was conducted largely in pursuit of metallic mineral deposits and the geologic settings that host them. Although mineral potential remains an important consideration, today’s maps address a broader array of potential issues that include hydrogeologic properties. Mapping relies on a variety of data sources, most of which are digital. The maps incorporate bedrock outcrops, drilling records, cuttings and core, aeromagnetic and gravity maps and models, down-hole geophysical logs, lidar topographic imagery, minerals exploration records, structural data, and previously published and unpublished works. These data sets are interpreted in the context of contemporary orogenic, rift, and depositional models, and temporally pinned by high-resolution geochronologic data. The majority of mapping is funded by programs of the Minnesota Legislature and USGS. Both funding sources contribute to production of stand-alone maps at scales of 1:24,000 to 1:100,000, and map packages known as County Geologic Atlases. The latter include database maps, bedrock and surficial sediment maps, bedrock topographic and depth to bedrock maps, geologic cross-sections, 3D grids of surfaces, and the extensive digital data sets used to create them. In addition, we’re currently using geophysical methods to model Precambrian surfaces beneath “removable layers” and infer map units in the underlying basement. These “layers” are largely relict foreland and successor basins, and their floors are major unconformities that represent temporal hiatuses that convey significant shifts in crustal rheology. The geometry of these surfaces says something about the tectonic forces that shaped them, and they may have potential for placer gold, uranium, and other mineral deposits