Geologic units in Aitkin county, Minnesota

Slate and graywacke (Paleoproterozoic) at surface, covers 43 % 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).

Granitic to granodioritic orthogneiss (Neoarchean) at surface, covers 10 % of this area

Granitic to granodioritic orthogneiss. Includes the McGrath Gneiss (~2,752 ± 15, 2,557 Ma) and unnamed units.

Mille Lacs and North Range Groups and equivalent sedimentary rocks (Paleoproterozoic) at surface, covers 8 % 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.

Interlayered volcanic, volcaniclastic, sedimentary, and hypabyssal intrusive rocks (Paleoproterozoic) at surface, covers 7 % of this area

Interlayered volcanic, volcaniclastic, sedimentary, and hypabyssal intrusive rocks. Includes parts of the Mille Lacs Group, North and South Range Groups, and Glen Township Formation.

Interlayered volcanic and sedimentary rocks (Paleoproterozoic) at surface, covers 7 % of this area

Interlayered volcanic and sedimentary rocks.

Dam Lake quartzite (Paleoproterozoic) at surface, covers 5 % of this area

Dam Lake quartzite.

Little Falls Formation (Paleoproterozoic) at surface, covers 4 % of this area

Schist and slate of graywacke-mudstone protolith.

Coleraine Formation (Late Cenomanian) at surface, covers 3 % 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.

Penokean intrusion (Paleoproterozoic) at surface, covers 2 % of this area

Mille Lacs granite (~2,009 Ma).

Gray granodiorite to granite intrusions (Paleoproterozoic) at surface, covers 2 % of this area

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.

Graywacke and slate with graphitic and sulfidic zones (Paleoproterozoic) at surface, covers 2 % of this area

Graywacke and slate with graphitic and sulfidic zones.

Graywacke and slate (Paleoproterozoic) at surface, covers 1 % of this area

Graywacke and slate

Sandstone, siltstone, and local conglomerate (Mesoproterozoic) at surface, covers 1 % of this area

Sandstone, siltstone, and local conglomerate. Includes the Hinckley Sandstone and Fond du Lac (youngest detrital zircons ~1,000 Ma) and Solar Church Formations; deposition in eolian, fluvial, and lacustrine environments.

Denham Formation (Paleoproterozoic) at surface, covers 1 % of this area

Lithic sandstone (reworked saprolite), marble, and mica schist (youngest detrital zircons greater than 2,000 Ma).

Iron-formation in the Cuyuna North and South Ranges and Mille Lacs Group (Paleoproterozoic) at surface, covers 1 % of this area

Iron-formation in the Cuyuna North and South Ranges and Mille Lacs Group. Includes the Trommald Formation. Unit description from GEOLEX - Trommald Formation: interlayered cherty and slaty iron-formation, moderately deformed and weakly metamorphosed.

Sulfidic iron-formation (Paleoproterozoic) at surface, covers 0.6 % of this area

Sulfidic iron-formation.

Small mafic intrusive stocks (Mesoproterozoic) at surface, covers 0.2 % of this area

Diabase, diorite, pyroxenite, and gabbro; reversed magnetic polarity; includes the Esko and Tamarack intrusions emplaced into unit Pas (Slate and graywacke of Animikie Group).

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).

Virginia Formation (Paleoproterozoic) at surface, covers < 0.1 % of this area

Slate and graywacke with thin carbonate layers.