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Geologic units in Treasure county, Montana
[Additional scientific data in this geographic area]
- Hell Creek formation (Phanerozoic | Mesozoic Cenozoic | Cretaceous-Late Tertiary | Paleocene) at surface, covers 37 % 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.
- Judith River formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 0.6 % 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.
- Bearpaw shale (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 22 % 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.
- Fort Union formation (Phanerozoic | Cenozoic | Tertiary Cretaceous-Late | Paleocene) at surface, covers 29 % 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.
- Terrace deposits (Phanerozoic | Cenozoic | Tertiary Quaternary) at surface, covers < 0.1 % of this area
Terrace deposits: gravel, sand, and silt of terrace remnants.
- Alluvium (Phanerozoic | Cenozoic | Quaternary) at surface, covers 9 % 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.