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

Geologic units in Phillips county, Montana

[Additional scientific data in this geographic area]

Judith River formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 20 % 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 0.2 % 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

Cambrian, undifferentiated (Phanerozoic | Paleozoic | Cambrian) at surface, covers 0.1 % 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.

Lithology: carbonate; mudstone; sandstone; conglomerate

Eagle sandstone (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 0.2 % 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

Ordovician, undifferentiated (Phanerozoic | Paleozoic | Ordovician) at surface, covers < 0.1 % of this area

Ordovician, undifferentiated: Mainly Bighorn dolomite; near Idaho, Kinnikinic quartzite.

Lithology: dolostone (dolomite); arenite; medium-grained mixed clastic; limestone; chert

Colorado shale (Phanerozoic | Mesozoic | Cretaceous-Early Cretaceous-Middle(?) Cretaceous-Late) at surface, covers 0.7 % 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

Devonian, undifferentiated (Phanerozoic | Paleozoic | Devonian) at surface, covers 0.1 % 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.

Lithology: carbonate; fine-grained mixed clastic; conglomerate; sedimentary breccia; evaporite

Flaxville gravel (Phanerozoic | Cenozoic | Tertiary) at surface, covers 1 % of this area

Flaxville gravel: Brown, yellow, and gray gravel, sand, and silt with marl and volcanic ash locally.

Lithology: coarse-grained mixed clastic; silt; volcanic ash

Hell Creek formation (Phanerozoic | Mesozoic Cenozoic | Cretaceous-Late Tertiary | Paleocene) at surface, covers 0.9 % 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

Jurassic, undifferentiated (Phanerozoic | Mesozoic | Jurassic) at surface, covers 0.1 % of this area

Jurassic, undifferentiated: calcareous shale and sandstone; includes the Morrison formation, the Ellis group, Sundance formation, and other rocks of Jurassic age.

Lithology: fine-grained mixed clastic; limestone; evaporite

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

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

playa

Lithology: playa

Terrace deposits (Phanerozoic | Cenozoic | Tertiary Quaternary) at surface, covers 0.5 % of this area

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

Lithology: alluvial terrace

Tertiary coarse-grained intrusive rocks (Phanerozoic | Cenozoic | Tertiary) at surface, covers 0.5 % 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

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

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.

Lithology: metamorphic rock; plutonic rock (phaneritic)

Mississippian, undifferentiated (Phanerozoic | Paleozoic | Carboniferous Mississippian) at surface, covers 0.2 % 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.

Lithology: carbonate; fine-grained mixed clastic; medium-grained mixed clastic; chert

Bearpaw shale (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 57 % 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 10 % 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