Geologic units in Cass county, Minnesota

Slate and graywacke (Paleoproterozoic) at surface, covers 21 % of this area

Slate and graywacke. Includes the Rove (~1,836 to 1,777 Ma), Virginia (~1,832 Ma near base), and Thompson Formations (youngest zircons in the Thompson Formation ~1,790 Ma).

Mafic metavolcanic rocks (Neoarchean) at surface, covers 21 % of this area

Mafic metavolcanic rocks. Includes minor volcaniclastic and hypabyssal intrusive rocks metamorphosed to lower greenschist to lower amphibolite facies; includes the Ely Greenstone (~2,722 Ma).

Foliated to gneissic tonalite, granodiorite, and diorite (Neoarchean) at surface, covers 18 % of this area

Foliated to gneissic tonalite, granodiorite, and diorite. Includes the Lookout Mountain tonalite (~2,718 Ma) of the Giants Range batholith and other intrusions within batholithic complexes defined by a low-to-moderate gravity signature with magnetic foliation apparent from aeromagnetic maps.

Granitic intrusion (Neoarchean) at surface, covers 11 % of this area

Granitic intrusion. Includes the Sacred Heart (~2592, 2,603 Ma) and Ortonville granites, the Shannon Lake Granite (~2,674), and other intrusions having low gravity and magnetic signatures.

Calc-alkalic volcanic and volcaniclastic rocks (Neoarchean) at surface, covers 7 % of this area

Calc-alkalic volcanic and volcaniclastic rocks.

Schist of sedimentary protolith (Neoarchean) at surface, covers 5 % of this area

Schist of sedimentary protolith. Metamorphosed to upper greenschist to amphibolite facies.

Granite to granodiorite (Neoarchean) at surface, covers 5 % of this area

Granite to granodiorite. Variably magnetic.

Coleraine Formation (Late Cenomanian) at surface, covers 4 % of this area

Conglomerate, lignite, sandstone, and shale; shallow marine and non-marine. Late Cenomanian age is inferred. Despite exposures in mines on the Mesabi Iron Range, the position of much of the unit contact is highly speculative.

Volcaniclastic rocks of felsic to intermediate composition (Neoarchean) at surface, covers 2 % of this area

Volcaniclastic rocks of felsic to intermediate composition.

Volcanogenic graywacke and mudstone (Neoarchean) at surface, covers 2 % of this area

Volcanogenic graywacke and mudstone. Includes the Lake Vermilion Formation.

Mille Lacs and North Range Groups and equivalent sedimentary rocks (Paleoproterozoic) at surface, covers 1 % of this area

Includes the Mahnomen and Rabbit Lake Formations and Trout Lake marble. Unit descriptions from GEOLEX - Mahnomen Formation: Upper member described as gray to light-gray siltstone, argillite, and fine-grained quartzose sandstone containing irregularly distributed beds and lenses of iron-formation and allied anomalously iron-rich sedimentary rocks; base of unit defined arbitrarily as the top of the lowermost mappable iron-formation layer of the middle, iron-formation member. Middle, iron-formation member--interlayered cherty and slaty iron-formation that is similar lithologically to the smaller iron-formation lenses within the upper and lower members; contains rocks formerly assigned to the Trommald Formation and the Emily Member of the Rabbit Lake Formation (Morey, 1978). Lower member--predominantly siltstone, fine- to medium-grained quartzose graywacke, and argillite; also includes minor quartz arenite and irregularly distributed layers and lenses of anomalously iron-rich sedimentary rocks. Rabbit Lake Formaiton: gray to very dark-gray slate, siltstone, and very fine-grained graywacke; includes irregularly distributed layers and lenses of carbonaceous slate and also carbonate- and sulfide-facies iron-formation, chiefly in the lower part of the unit.

Pokegama Quartzite (Paleoproterozoic) at surface, covers 0.9 % of this area

Includes conglomerate and siliceous mudstone.

Undifferentiated (Cenomanian to Campanian) at surface, covers 0.5 % of this area

Conglomerate, sandstone, mudstone, shale, marlstone, siltstone, and minor lignite, deposited in marine and non-marine settings; likely Cenomanian to Campanian age. Unit outline is the product of contouring the stratigraphic top and base, from which an isopach grid was created. Because the distribution is patchy, unit boundaries were drawn from the gridded data to represent locations where more than 25 feet (8 meters) of thickness occurs. As a result, many areas outside of the unit boundaries may be overlain by thin Cretaceous strata and the unit is depicted without a contact line.

Virginia Formation (Paleoproterozoic) at surface, covers 0.3 % of this area

Slate and graywacke with thin carbonate layers.

Granitoid intrusion (Neoarchean) at surface, covers 0.3 % of this area

Granitoid intrusion. Constrained solely by low gravity and magnetic signatures.

Dioritic to granodioritic intrusion of uncertain age (Archean or Paleoproterozoic) at surface, covers 0.2 % of this area

Dioritic to granodioritic intrusion of uncertain age. Moderate gravity and magnetic expression.

Schist and tonalitic to granodioritic paragneiss (Neoarchean) at surface, covers 0.1 % of this area

Schist and tonalitic to granodioritic paragneiss. Typically adjacent to large batholithic complexes.

Mafic intrusions including pyroxenite, peridotite, gabbro, and lamprophyre (Paleoproterozoic) at surface, covers < 0.1 % of this area

Mafic intrusions including pyroxenite, peridotite, gabbro, and lamprophyre. Defined largely by magnetic signature. One intrusion in Morrison County is ~1,791 Ma, and lithologically similar intrusions cut the Foley Granite (~1,774 Ma).

Gabbro, peridotite, pyroxenite, lamprophyre, and metamorphic equivalents (Neoarchean) at surface, covers < 0.1 % of this area

Gabbro, peridotite, pyroxenite, lamprophyre, and metamorphic equivalents. Includes the Oaks intrusion (~2,671 Ma) in the Wabigoon subprovince, and a ~2,639 Ma lamprophyre in the western Wawa subprovince; locally defined by variably high gravity and magnetic signatures.

Magnetic intrusions, undifferentiated (Archean) at surface, covers < 0.1 % of this area

Magnetic intrusions, undifferentiated. Typically too small to ascertain gravity expression.

Gunflint (~1,878 Ma), Biwabik, and Emily Iron Formations (Paleoproterozoic) at surface, covers < 0.1 % of this area

Locally includes thin basal sandstone and conglomerate. Capped by an irregular layer of brecciated iron-formation and ejecta derived from the 1,850 Ma Sudbury meteorite impact.