Geologic units in Yellow Medicine county, Minnesota

Undifferentiated (Cenomanian to Campanian) at surface, covers 47 % 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.

Granitic orthogneiss and migmatite (Mesoarchean to Paleoarchean) at surface, covers 22 % of this area

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

Granitic intrusion (Neoarchean) at surface, covers 9 % 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.

Leucogranite (Neoarchean) at surface, covers 8 % of this area

Leucogranite. Occurs along the Yellow Medicine Shear Zone and elsewhere, primarily in batholithic settings.

Mafic volcanic and hypabyssal intrusive rocks of uncertain age (Archean or Paleoproterozoic) at surface, covers 6 % of this area

Mafic volcanic and hypabyssal intrusive rocks of uncertain age. Adjacent to and within the Yellow Medicine shear zone.

Amphibolitic to dioritic gneiss (Mesoarchean to Paleoarchean) at surface, covers 4 % of this area

Amphibolitic to dioritic gneiss.

Mylonite, varied protolith (Archean) at surface, covers 2 % of this area

Mylonite, varied protolith. Age of shearing unknown and likely protracted.

Gabbroic to dioritic intrusion (Archean or Paleoproterozoic) at surface, covers 0.6 % of this area

Gabbroic to dioritic intrusion. High to moderate gravity and magnetic signature. Includes Providence and Cottonwood intrusions in southwestern Minnesota.

Granitoid gneiss with dioritic to amphibolitic enclaves (Mesoarchean to Paleoarchean) at surface, covers 0.2 % of this area

Granitoid gneiss with dioritic to amphibolitic enclaves. Produces moderately high and varied gravity and magnetic signatures.

Pierre Shale (Upper Cretaceous) at surface, covers 0.1 % of this area

Blue-gray to dark-gray, fissile to blocky shale with persistent beds of bentonite, black organic shale, and light-brown chalky shale. Contains minor sandstone, conglomerate, and abundant carbonate and ferruginous concretions. Thickness up to 1,000 ft (305 m).

Granitic to granodioritic orthogneiss (Neoarchean) at surface, covers < 0.1 % of this area

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

Carlile Shale (Upper Cretaceous) at surface, covers < 0.1 % of this area

Dark-gray to black, silty to sandy shale with several zones of septarian, fossiliferous, carbonate concretions. Contains up to three sandstone units in the upper portion of the formation and sandy calcareous marl at the base. Thickness up to 330 ft (100 m).

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).

Niobrara Formation (Upper Cretaceous) at surface, covers < 0.1 % of this area

White to dark-gray argillaceous chalk, marl, and shale. Weathers yellow to orange. Contains thin, laterally continuous bentonite beds, chalky carbonaceous shale, minor sand, and small concretions. Thickness up to 150 ft (46 m).