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Mineral Resources > Online Spatial Data > Geology > by state > Montana Geology

Geologic units in Fallon county, Montana

Fort Union formation (Phanerozoic | Cenozoic | Tertiary Cretaceous-Late | Paleocene) at surface, covers 67 % of this area

Fort Union formation: Clay shale, siltstone, and sandstone; local lenses of impure limestone, and numerous lignitic beds; contains Tertiary plant and animal fossils but no dinosaurs; base generally placed at the lowest of the succession of lignite beds within it; includes the Tongue River member, Lebo shale member, and Tullock member.

Lithology: shale; siltstone; sandstone; coal; limestone

Alluvium (Phanerozoic | Cenozoic | Quaternary) at surface, covers 5 % of this area

Alluvium: mainly valley fill consisting of silt, sand, and gravel; includes some terrace deposits and glacial drift of Pleistocene age in some areas; locally includes hot spring tufa. The older part of the alluvium, where present, is probably of Pliocene age.

Lithology: alluvium; glacial drift; carbonate

Ludlow Formation (Phanerozoic | Cenozoic | Tertiary | Paleocene ) at surface, covers < 0.1 % of this area

White, tan, yellow, and gray, cross-bedded, fine- to medium-grained,silty sandstone interbedded with locally bentonitic, gray siltstone, claystone, and sandy to silty claystone. Characterized by uranium-bearing lignite beds and "clinker" beds fromed by burning coalseams. Thickness up to 420 ft (128 m).

Lithology: sandstone; siltstone; claystone

Fox Hills Formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers < 0.1 % of this area

Olive-brown sand, shale, and sandstone; marine shoreline and offshore sediment; as thick as 120 meters (400 feet).

Lithology: sand; shale; sandstone

Alluvium (Phanerozoic | Cenozoic | Quaternary) at surface, covers < 0.1 % of this area

Clay to boulder-size clasts with locally abundant organic material. Thickness up to 75 ft (23m).

Lithology: clay or mud; silt; sand; gravel

Hell Creek Formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers < 0.1 % of this area

Tan to brown, light- to dark-gray, "somber beds" of shale. Interbedded with brown to red carbonaceous shale, gray and brown bentonitic silty shale, and gray, brown and yellow siltstone, sandstone, and claystone-pebble conglomerate. Thickness 260-600 ft (79-183 m).

Lithology: shale; siltstone; sandstone; conglomerate

Ludlow Formation (Phanerozoic | Cenozoic | Tertiary | Paleocene) at surface, covers < 0.1 % of this area

Gray-brown and yellow-brown silt, sand, clay and sandstone, and lignite; river, lake, and swamp sediment; as thick as 100 meters (300 feet).

Lithology: silt; sand; clay or mud; sandstone; coal

Pierre Formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers < 0.1 % of this area

Dark-gray shale; marine offshore sediment; maximum outcrop thickness is a few hundred meters.

Lithology: shale

Terrace Deposits (Phanerozoic | Cenozoic | Quaternary) at surface, covers < 0.1 % of this area

Clay to boulder-size clasts deposited as pediments, paleochannels, and terrace fills of former flood plains. Thickness up to 75 ft (23m).

Lithology: clay or mud; silt; sand; gravel

Fox Hill sandstone (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 4 % of this area

Fox Hills sandstone: Typically shaly sandstone grading upward into massive brownish sandstone with white sandstone of the Colgate member locally at top.

Lithology: sandstone; mudstone

Bullion Creek Formation (Phanerozoic | Cenozoic | Tertiary | Paleocene) at surface, covers < 0.1 % of this area

Yellow-brown silt, sand, clay, sandstone, and lignite; river, lake, and swamp sediment; as thick as 200 meters (600 feet).

Lithology: silt; sand; clay or mud; sandstone; coal

Pierre shale (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 11 % of this area

Pierre shale: dark-gray clay shale with calcareous and ferruginous concretions and sandy members.

Lithology: shale; sandstone; bentonite

Hell Creek formation (Phanerozoic | Mesozoic Cenozoic | Cretaceous-Late Tertiary | Paleocene) at surface, covers 13 % of this area

Hell Creek formation: somber-gray sandstone and greenish shaly clay and mudstone containing dinosaur bones; a few thin lignite and subbituminous coal beds.

Lithology: sandstone; mudstone; coal

Slope Formation (Phanerozoic | Cenozoic | Tertiary | Paleocene) at surface, covers < 0.1 % of this area

Gray-brown and yellow-brown silt, sand, clay, sandstone, and lignite; river, lake, and swamp sediment; as thick as 100 meters (300 feet).

Lithology: silt; sand; clay or mud; sandstone; coal

Hell Creek Formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers < 0.1 % of this area

Gray sand, silt, clay, and sandstone; river sediment; as thick as 150 meters (500 feet).

Lithology: sand; silt; clay or mud; sandstone