Geologic units in Otter Tail county, Minnesota

Mafic metavolcanic rocks (Neoarchean) at surface, covers 26 % 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).

Granite to granodiorite (Neoarchean) at surface, covers 17 % of this area

Granite to granodiorite. Variably magnetic.

Foliated to gneissic tonalite, granodiorite, and diorite (Neoarchean) at surface, covers 16 % 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.

Undifferentiated (Cenomanian to Campanian) at surface, covers 13 % 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.

Schist of sedimentary protolith (Neoarchean) at surface, covers 8 % of this area

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

Volcanogenic graywacke and mudstone (Neoarchean) at surface, covers 5 % of this area

Volcanogenic graywacke and mudstone. Includes the Lake Vermilion Formation.

Granitic intrusion (Neoarchean) at surface, covers 3 % 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.

Granitoid intrusion (Neoarchean) at surface, covers 3 % of this area

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

Volcaniclastic rocks of felsic to intermediate composition (Neoarchean) at surface, covers 3 % of this area

Volcaniclastic rocks of felsic to intermediate composition.

Slate and graywacke (Paleoproterozoic) at surface, covers 2 % 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).

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

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.

Granodiorite to hornblende diorite, locally magmatically foliated (Neoarchean) at surface, covers 0.6 % 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.

Magnetic intrusions, undifferentiated (Archean) at surface, covers 0.5 % of this area

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

Granitic intrusion of unknown age (Archean or Paleoproterozoic) at surface, covers 0.3 % of this area

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

Mafic intrusions including pyroxenite, peridotite, gabbro, and lamprophyre (Paleoproterozoic) at surface, covers 0.2 % 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).

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.