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Geologic units in Fergus county, Montana
[Additional scientific data in this geographic area]
- Jurassic, undifferentiated (Phanerozoic | Mesozoic | Jurassic) at surface, covers 3 % of this area
Jurassic, undifferentiated: calcareous shale and sandstone; includes the Morrison formation, the Ellis group, Sundance formation, and other rocks of Jurassic age.
- Kootenai formation and associated rocks (Phanerozoic | Mesozoic | Jurassic Cretaceous-Early) at surface, covers 7 % 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.
- Alluvium (Phanerozoic | Cenozoic | Quaternary) at surface, covers 3 % 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.
- Terrace deposits (Phanerozoic | Cenozoic | Tertiary Quaternary) at surface, covers 9 % of this area
Terrace deposits: gravel, sand, and silt of terrace remnants.
- Tertiary coarse-grained intrusive rocks (Phanerozoic | Cenozoic | Tertiary) at surface, covers 1.0 % 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.
- 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.
- Telegraph Creek formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 0.3 % of this area
Telegraph Creek formation: buff mainly soft, fissile sandy shale with subordinate amounts of concretionary sandstone.
- Mississippian, undifferentiated (Phanerozoic | Paleozoic | Carboniferous Mississippian) at surface, covers 8 % of this area
Mississippian, undifferentiated: sandstone, shale, and limestone, in part dolomitic, with chert nodules, some quartzite; includes Big Snowy group in central part of State, Madison group in central and southwestern parts; and Hannan and Brazer limestones in the northwestern part; may include small amounts of Pennsylvanian rocks in areas where stratigraphic studies are incomplete.
- Pennsylvanian, undifferentiated (Phanerozoic | Paleozoic | Carboniferous Pennsylvanian) at surface, covers 3 % of this area
Pennsylvanian, undifferentiated: in western Montana is mainly the Quadrant quartzite but includes limestone and other rocks of Pennsylvanian age so far as present data permit. Farther east other formations of Pennsylvanian or possible Pennsylvanian age are included.
- Claggett formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 8 % of this area
Claggett formation: chiefly dark-gray shale with iron-stained concretions; locally sandstone present; numerous bentonite beds near base.
- Piegan group (Proterozoic | Mesoproterozoic) at surface, covers < 0.1 % of this area
Piegan group: most of the Piegan group is subdivided into formations that are in part equivalent to each other. Where correlations are relatively doubtful the group designation is retained. Carbonate-bearing rocks predominate in the group, but the proportions and character of the impurities in these rocks and the relations to non-carbonate-bearing rocks vary from place to place.
- Judith River formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 11 % 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.
- Cambrian, undifferentiated (Phanerozoic | Paleozoic | Cambrian) at surface, covers 0.2 % of this area
Cambrian, undifferentiated: comprises Deadwood formation, in south-central Montana, and Red Lion formation, Dry Creek shale, Hasmark formation, Pilgrim limestone, Silver Hill formation, Park shale, Meagher limestone, Wolsey shale, Flathead quartzite, and other units. In a few places quartzite of Cambrian age may be mapped with the Belt series or quartzite of Belt age with the Cambrian rocks.
- Eagle sandstone (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 4 % 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.
- Colorado shale (Phanerozoic | Mesozoic | Cretaceous-Early Cretaceous-Middle(?) Cretaceous-Late) at surface, covers 18 % 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.
- Hell Creek formation (Phanerozoic | Mesozoic Cenozoic | Cretaceous-Late Tertiary | Paleocene) at surface, covers 1 % 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.