Geologic units in Roberts county, South Dakota

Carlile Shale (Upper Cretaceous) at surface, covers 44 % 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).

Pierre Shale (Upper Cretaceous) at surface, covers 40 % 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).

Graneros Shale (Upper Cretaceous) at surface, covers 8 % of this area

Dark-gray, noncalcareous, pyritic, poorly fossiliferous shale, with numerous sandstone layers at the base. Thickness up to 110 ft (36 m).

Niobrara Formation (Upper Cretaceous) at surface, covers 6 % 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).

Greenhorn Formation (Upper Cretaceous) at surface, covers 1 % of this area

Gray shale, mudstone, marl, calcarenite, and shaly limestone grading upward into light-gray to tan, alternating marl and thin-bedded, fossiliferous limestone. Thickness up to 40 ft (12 m).

Granite (eastern bedrock) (Upper Archean) at surface, covers 0.4 % of this area

Felsic coarse-grained granite, includes white, silty clay weathering residuum which grades downward to a pink, white, and gray siltstone.

Undifferentiated (Cenomanian to Campanian) at surface, covers 0.1 % 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.

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

Dark-gray shale; marine offshore sediment; as thick as 120 metres (400 feet).

Foliated to gneissic tonalite, granodiorite, and diorite (Neoarchean) at surface, covers < 0.1 % 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.

Granite to granodiorite (Neoarchean) at surface, covers < 0.1 % of this area

Granite to granodiorite. Variably magnetic.

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.

Granitic orthogneiss and migmatite (Mesoarchean to Paleoarchean) at surface, covers < 0.1 % of this area

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

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

Dark-gray calcareous shale; marine offshore sediment; as thick as 45 metres (150 feet).

Granitic intrusion (Neoarchean) at surface, covers < 0.1 % 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.

Mafic metavolcanic rocks (Neoarchean) at surface, covers < 0.1 % 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).

Volcaniclastic rocks of felsic to intermediate composition (Neoarchean) at surface, covers < 0.1 % of this area

Volcaniclastic rocks of felsic to intermediate composition.