Geologic units in Polk county, Minnesota

Granodiorite to hornblende diorite, locally magmatically foliated (Neoarchean) at surface, covers 18 % 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.

Mafic metavolcanic rocks (Neoarchean) at surface, covers 17 % 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).

Granitoid intrusion (Neoarchean) at surface, covers 12 % of this area

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

Granitic intrusion (Neoarchean) at surface, covers 10 % 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.

Upper Ordovician (Upper Ordovician) at surface, covers 9 % of this area

Limestone, shaley limestone, and dolostone; includes the Maquoketa Formation, and the Stewartville, Prosser, and Cummingsville Formations of the Galena Group. Also includes limestone and dolostone of the Red River Formation, and sandstone and shale of the Winnipeg Formation in northwest Minnesota.

Schist of sedimentary protolith (Neoarchean) at surface, covers 6 % of this area

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

Mafic to ultramafic hypabyssal intrusive complexes composed of gabbro, pyroxenite, diorite, and anorthosite (Neoarchean) at surface, covers 5 % of this area

Mafic to ultramafic hypabyssal intrusive complexes composed of gabbro, pyroxenite, diorite, and anorthosite. Includes the Mentor and Deer Lake complexes, intrusions of the Newton Lake Formation, all within the Wawa subprovince; and the UBD intrusion (~2,685 to 2,695 Ma) in the Wabigoon subprovince.

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

Granite to granodiorite. Variably magnetic.

Foliated to gneissic tonalite, granodiorite, and diorite (Neoarchean) at surface, covers 5 % 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.

Biotite schist of graywacke protolith, and schist-rich migmatite (Neoarchean) at surface, covers 4 % of this area

Biotite schist of graywacke protolith, and schist-rich migmatite.

Gabbro, peridotite, pyroxenite, lamprophyre, and metamorphic equivalents (Neoarchean) at surface, covers 3 % 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.

Undifferentiated (Cenomanian to Campanian) at surface, covers 3 % 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.

Tonalite-leucodiorite plutons (Neoarchean) at surface, covers 2 % of this area

Tonalite-leucodiorite plutons. Includes the Grygla, Red Lake Falls, and other plutons having low magnetic and gravity signatures.

Volcaniclastic rocks of felsic to intermediate composition (Neoarchean) at surface, covers 0.8 % of this area

Volcaniclastic rocks of felsic to intermediate composition.

Magnetic intrusions, undifferentiated (Archean) at surface, covers 0.3 % of this area

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

Winnipeg Group (Ordovician) at surface, covers < 0.1 % of this area

Light-gray to greenish-gray shale and light-gray sandstone; marine nearshore and offshore sediment; as thick as 125 metres (400 feet).

Inyan Kara (Lower Cretaceous) at surface, covers < 0.1 % of this area

Light-gray, fine to coarse sandstone and gray shale; river, lake, and nearshore marine sediment; as thick as 135 metres (450 feet).

Precambrian Rocks (Precambrian) at surface, covers < 0.1 % of this area

Greenschists and amphibolites; banded iron formation; stretched pebble conglomerates; metabasalt; serpentinite; felsic tuff; mylonite; intermediate felsic plutonic rocks; migmatite; layered gneiss.

Red River Formation (Ordovician) at surface, covers < 0.1 % of this area

Yellowish- to brownish-gray limestone and dolomite; offshore marine sediment; as thick as 215 metres (700 feet).