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

Geologic units in Chouteau county, Montana

[Additional scientific data in this geographic area]

Montana group, undifferentiated (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers < 0.1 % of this area

Montana group, undifferentiated: mainly sandstone and shale, soft and in part poorly consolidated; some coal; shown undivided only in areas where detailed data are lacking and in these areas some rocks not of Montana age may have been included. In particular, rocks designated Km in southwestern Montana may include some Tertiary strata.

Lithology: fine-grained mixed clastic; coal

Wasatch formation (Phanerozoic | Cenozoic | Tertiary) at surface, covers 0.2 % of this area

Wasatch formation: Light-colored massive sandstone; drab-colored shale and coal in southeastern Montana; and variegated, dominantly red beds of clay and sandstone in north-central Montana.

Lithology: sandstone; shale; coal; claystone

Hell Creek formation (Phanerozoic | Mesozoic Cenozoic | Cretaceous-Late Tertiary | Paleocene) at surface, covers 0.3 % 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

Colorado shale (Phanerozoic | Mesozoic | Cretaceous-Early Cretaceous-Middle(?) Cretaceous-Late) at surface, covers 40 % 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

Eagle sandstone (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 18 % 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

Tertiary volcanic rocks (Phanerozoic | Cenozoic | Tertiary) at surface, covers 4 % of this area

Tertiary volcanic rocks: Flows and associated pyroclastic deposits, with subordinate amounts of intercalated sedimentary beds and lignite. The volcanic material is mostly latite, quartz latite, and andesite but includes some rhyolite and basalt. The distinction between Tertiary and pre-Tertiary volcanic rocks was not made in some of the reports used in the complilation. Hence in the less well-known areas some pre-Tertiary volcanic rocks may be included.

Lithology: volcanic rock (aphanitic); mixed clastic/volcanic; coal

Judith River formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 10 % 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

Claggett formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 19 % 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

Bearpaw shale (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 0.7 % 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

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

Telegraph Creek formation: buff mainly soft, fissile sandy shale with subordinate amounts of concretionary sandstone.

Lithology: mudstone; sandstone

Eagle sandstone (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 3 % 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. Gray to buff massive cliff-forming sandstone with iron stained concretions in the upper part. In western Montana the Virgelle is a formation and in central Montana it is the basal member of the Eagle sandstone.

Lithology: sandstone; siltstone

Fort Union formation (Phanerozoic | Cenozoic | Tertiary Cretaceous-Late | Paleocene) at surface, covers 0.3 % 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

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

playa

Lithology: playa

Tertiary coarse-grained intrusive rocks (Phanerozoic | Cenozoic | Tertiary) at surface, covers 0.6 % of this area

Tertiary coarse-grained rocks: Some of these rocks have the composition of quartz diorite, monzonite, and similar rocks but most are alkalic syenite, leucite-rich rocks, and other alkalic varieties. Some of the rocks that are associated areally with the Cretaceous volcanic rocks may be Cretaceous in age. Includes the larger intrusive masses in the eastern part of western Montana and the western part of eastern Montana, most of which have domed upper surfaces. Many of these masses have been regarded as laccoliths, but generally without adequate proof. The Boulder batholith and related stocks are mapped separately.

Lithology: alkalic intrusive rock

Terrace deposits (Phanerozoic | Cenozoic | Tertiary Quaternary) at surface, covers 0.4 % of this area

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

Lithology: alluvial terrace

Alluvium (Phanerozoic | Cenozoic | Quaternary) at surface, covers 2 % 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

Kootenai formation and associated rocks (Phanerozoic | Mesozoic | Jurassic Cretaceous-Early) at surface, covers 0.2 % 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