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Geologic units in Judith Basin county, Montana
[Additional scientific data in this geographic area]
- Ordovician, undifferentiated (Phanerozoic | Paleozoic | Ordovician) at surface, covers < 0.1 % of this area
Ordovician, undifferentiated: Mainly Bighorn dolomite; near Idaho, Kinnikinic quartzite.
- Colorado shale (Phanerozoic | Mesozoic | Cretaceous-Early Cretaceous-Middle(?) Cretaceous-Late) at surface, covers 23 % 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.
- Devonian, undifferentiated (Phanerozoic | Paleozoic | Devonian) at surface, covers 4 % of this area
Devonian, undifferentiated: comprises Three Forks formation consisting of carbonaceous and calcareous shale with some sandstone and limestone, Jefferson limestone, and unnamed units of Devonian age.
- Piegan group (Proterozoic | Mesoproterozoic) at surface, covers 0.8 % 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.
- Cambrian, undifferentiated (Phanerozoic | Paleozoic | Cambrian) at surface, covers 5 % 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.
- Tertiary volcanic rocks (Phanerozoic | Cenozoic | Tertiary) at surface, covers 0.7 % 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.
- Eagle sandstone (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 0.9 % 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.
- Pre-Belt gneiss, schist, and related rocks (Archean Proterozoic(?) preCambrian-Proterozoic(?) preCambrian(?) Phanerozoic | Paleozoic(?) Mesozoic(?) Cenozoic | Cambrian(?) Ordovician(?) Silurian(?) Devonian(?) Carboniferous(?) Permian(?) Triassic(?) Jurassic(?) Cretaceous(?) Tertiary) at surface, covers 0.1 % of this area
Pre-Belt gneiss, schist, and related rocks: comprises all the rocks older than the Belt series except the Stillwater complex. These include the Cherry Creek group, consisting of interlaminated gneiss, schist, marble, and quartzite; and the Pony series of Tansley and others, consisting of gneiss and schist of both sedimentary and igneous origin. Recent work has resulted in the distinction of small masses of granitic and injected rocks now regarded as resulting from intrusion in Cretaceous or Tertiary time. Other such masses may have escaped recognition.
- Mississippian, undifferentiated (Phanerozoic | Paleozoic | Carboniferous Mississippian) at surface, covers 22 % 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.
- Telegraph Creek formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 1.0 % of this area
Telegraph Creek formation: buff mainly soft, fissile sandy shale with subordinate amounts of concretionary sandstone.
- 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 < 0.1 % of this area
Claggett formation: chiefly dark-gray shale with iron-stained concretions; locally sandstone present; numerous bentonite beds near base.
- Jurassic, undifferentiated (Phanerozoic | Mesozoic | Jurassic) at surface, covers 4 % 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 8 % 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.
- Terrace deposits (Phanerozoic | Cenozoic | Tertiary Quaternary) at surface, covers 21 % of this area
Terrace deposits: gravel, sand, and silt of terrace remnants.
- Tertiary coarse-grained intrusive rocks (Phanerozoic | Cenozoic | Tertiary) at surface, covers 3 % 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.
- 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.