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

Geologic units in Petroleum county, Montana

[Additional scientific data in this geographic area]

Bearpaw shale (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 25 % of this area

Bearpaw shale: Dark-gray and brownish clay shale; thick units of nonfissile bentonitic shale; calcareous and ferruginous concretions throughout; contains some thick bentonite beds.

Lithology: shale; sandstone; bentonite

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

Claggett formation: chiefly dark-gray shale with iron-stained concretions; locally sandstone present; numerous bentonite beds near base.

Lithology: shale; sandstone; bentonite

Kootenai formation and associated rocks (Phanerozoic | Mesozoic | Jurassic Cretaceous-Early) at surface, covers < 0.1 % of this area

Kootenai formation and associated rocks: conglomerate, sandstone, shale, and mudstone; purplish and green beds are common; mainly the Kootenai; in southern Montana includes strata that have been mapped as Cloverly formation. Includes Second Cat Creek and Third Cat Creek sands of drillers in central part of State; Sunburst sand of drillers in north-central part; and Cut Bank sand of drillers in western part. As here mapped, may locally include thin units of Jurassic age.

Lithology: medium-grained mixed clastic; mudstone; limestone

Alluvium (Phanerozoic | Cenozoic | Quaternary) at surface, covers 6 % 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

Terrace deposits (Phanerozoic | Cenozoic | Tertiary Quaternary) at surface, covers 0.7 % of this area

Terrace deposits: gravel, sand, and silt of terrace remnants.

Lithology: alluvial terrace

Colorado shale (Phanerozoic | Mesozoic | Cretaceous-Early Cretaceous-Middle(?) Cretaceous-Late) at surface, covers 25 % of this area

Colorado shale: dark-gray shale and siltstone with many concretions and sandy units. Includes equivalents of Fall River, Skull Creek, Newcastle, Mowry, Belle Fourche, Greenhorn, Carlile, and Niobrara formations, and locally Telegraph Creek formation. In the less well-known areas beds of other ages may have been included.

Lithology: fine-grained mixed clastic

Hell Creek formation (Phanerozoic | Mesozoic Cenozoic | Cretaceous-Late Tertiary | Paleocene) at surface, covers 19 % 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

Judith River formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 4 % of this area

Judith River formation: light-colored sandstone at top; lower third somber-gray siltstone and sandy shale; greenish-gray clay and some lignite beds; includes the Parkman sandstone member of south-central Montana.

Lithology: sandstone; mudstone; coal

Fox Hill sandstone (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers < 0.1 % 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

Eagle sandstone (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 7 % of this area

Eagle sandstone: sandstone and shaly sandstone with lignite beds in basal part of upper unit (Keu). The Virgelle sandstone member (Kvi) at base is distinguished where possible. Near Yellowstone National Park rocks incorrectly called Laramide in early reports and now regarded as roughly equivalent to the Eagle sandstone are tentatively mapped as Eagle sandstone.

Lithology: sandstone; mudstone; coal