Geologic units in Marshall county, Minnesota

Mafic metavolcanic rocks (Neoarchean) at surface, covers 22 % 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).

Undifferentiated (Cenomanian to Campanian) at surface, covers 19 % 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.

Volcaniclastic rocks of felsic to intermediate composition (Neoarchean) at surface, covers 15 % of this area

Volcaniclastic rocks of felsic to intermediate composition.

Upper Ordovician (Upper Ordovician) at surface, covers 12 % 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.

Foliated to gneissic tonalite, granodiorite, and diorite (Neoarchean) at surface, covers 8 % 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.

Granodiorite to hornblende diorite, locally magmatically foliated (Neoarchean) at surface, covers 7 % 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.

Tonalite-leucodiorite plutons (Neoarchean) at surface, covers 5 % of this area

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

Granitoid intrusion (Neoarchean) at surface, covers 4 % of this area

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

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.

Granitic intrusion (Neoarchean) at surface, covers 2 % 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.

Granite to granodiorite (Neoarchean) at surface, covers 2 % of this area

Granite to granodiorite. Variably magnetic.

Magnetic intrusions, undifferentiated (Archean) at surface, covers 1 % of this area

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

Hallock Red Beds (Jurassic) at surface, covers 0.6 % of this area

Shale, limestone, dolomite, siltstone, and sandstone.

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).

Undifferentiated Jurassic Sediments (Jurassic) at surface, covers < 0.1 % of this area

Dark-gray, greenish, and varicolored shale with local limestone (Swift and Rierdon Formation equivalents); mainly nearshore marine sediments; as thick as 180 metres (600 feet).

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).