Schist and slate of graywacke-mudstone protolith.
Tonalite to leucodiorite (~1,792, 1,793, 1,800 Ma).
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.
Granite, reddish, variably porphyritic, massive; includes the St. Cloud Red (~1,779 Ma), Foley (~1,774, 1,779 Ma), Pierz (~1,779 Ma), and Pease granites, and Richmond charnockitic granite (~1,772 Ma). An unnamed granite in south-central Minnesota is slightly older at ~1,792 Ma.
Interlayered volcanic and sedimentary rocks.
Granitoid intrusions, variably magnetic. Includes the Ann Lake granite (~1,784 Ma) and Glendorado pluton (~1,788 Ma).
Gray granodiorite to granite intrusions. Includes the Reformatory (~1,783 Ma), Freedhem (~1,775, 1,776 Ma), Isle (~1,779 Ma), and Warman (~1,787 Ma) intrusions.
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).
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).
Tonalite. Locally migmatitic, variably magnetic; contains abundant biotite schist paleosome of graywacke protolith (presumably derived from the Little Falls Formation, unit Pls).
Granitic to granodioritic orthogneiss. Includes the McGrath Gneiss (~2,752 ± 15, 2,557 Ma) and unnamed units.
Schist and tonalitic to granodioritic paragneiss. Typically adjacent to large batholithic complexes.
Granitic intrusion of unknown age. Low gravity and magnetic expression.
Granitic orthogneiss and migmatite. Geophysical map patterns imply this unit intruded other gneisses.
Gabbro, pyroxenite, diorite, and lamprophyre. Includes the Watab quartz diorite (~1,780 Ma), St. Wendell quartz gabbro, and an unnamed diorite (~1,786 Ma).
Volcaniclastic rocks of felsic to intermediate composition.
Includes conglomerate and siliceous mudstone.
Dioritic to granodioritic intrusion of uncertain age. Moderate gravity and magnetic expression.
Granodiorite, variably foliated. Includes the Bradbury Creek (~1,877 to 1,857 Ma) and Philbrook (~1,854 Ma) intrusions.
Granitic rocks, undifferentiated. Largely inferred from geophysical maps.
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.