Geologic units in Todd county, Minnesota

Slate and graywacke (Paleoproterozoic) at surface, covers 44 % 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).

Schist and tonalitic to granodioritic paragneiss (Neoarchean) at surface, covers 16 % of this area

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

Foliated to gneissic tonalite and granodiorite (Mesoarchean to Paleoarchean) at surface, covers 10 % of this area

Foliated to gneissic tonalite and granodiorite.

Volcaniclastic rocks of felsic to intermediate composition (Neoarchean) at surface, covers 8 % of this area

Volcaniclastic rocks of felsic to intermediate composition.

Granitic intrusion of unknown age (Archean or Paleoproterozoic) at surface, covers 7 % of this area

Granitic intrusion of unknown age. Low gravity and magnetic expression.

Mafic metavolcanic rocks (Neoarchean) at surface, covers 5 % 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).

Schist of sedimentary protolith (Neoarchean) at surface, covers 3 % of this area

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

Granitic orthogneiss and migmatite (Mesoarchean to Paleoarchean) at surface, covers 2 % of this area

Granitic orthogneiss and migmatite. Geophysical map patterns imply this unit intruded other gneisses.

Dioritic to granodioritic intrusion of uncertain age (Archean or Paleoproterozoic) at surface, covers 1 % of this area

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

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

Schist and slate of graywacke-mudstone protolith.

Gabbro, peridotite, pyroxenite, lamprophyre, and metamorphic equivalents (Neoarchean) at surface, covers 1.0 % 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.

Foliated to gneissic tonalite, granodiorite, and diorite (Neoarchean) at surface, covers 0.7 % 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.

Mafic intrusions including pyroxenite, peridotite, gabbro, and lamprophyre (Paleoproterozoic) at surface, covers 0.3 % 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).

Granodiorite (Paleoproterozoic) at surface, covers 0.2 % of this area

Granodiorite, variably foliated. Includes the Bradbury Creek (~1,877 to 1,857 Ma) and Philbrook (~1,854 Ma) intrusions.

Mille Lacs and North Range Groups and equivalent sedimentary rocks (Paleoproterozoic) at surface, covers 0.1 % 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.

Iron-formation (Neoarchean) at surface, covers < 0.1 % of this area

Iron-formation. Includes the Soudan Iron Formation and many unnamed units.

Granodiorite to hornblende diorite, locally magmatically foliated (Neoarchean) at surface, covers < 0.1 % of this area

Granodiorite to hornblende diorite, locally magmatically foliated. Includes the Britt Granodiorite (~2,685 Ma) and other intrusions having moderate gravity and magnetic signatures.