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

Geologic units in Stillwater county, Montana

[Additional scientific data in this geographic area]

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

Judith River formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 12 % 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

Eagle sandstone (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 7 % 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

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

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 4 % 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

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

Permian, undifferentiated: chert, sandstone, limestone, quartzite, and shale with rock phosphate mostly at base; mainly Phosphoria formation

Lithology: fine-grained mixed clastic; carbonate; chert; phosphorite; black shale; oil shale

Cretaceous volcanic rocks (Phanerozoic | Mesozoic Cenozoic | Cretaceous-Late Tertiary) at surface, covers 5 % of this area

Cretaceous volcanic rocks: flows and pyroclastic rocks, mainly of intermediate composition with subordinate amounts of interbedded sedimentary rocks. Some of the rocks have been regarded as belonging to the Livingston formation, but this usage is not adopted here. Where data are inadequate for separation, some Tertiary volcanic rocks may be mapped with the Cretaceous volcanic rocks.

Lithology: volcanic rock (aphanitic); fine-grained mixed clastic

Montana group, undifferentiated (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 0.2 % 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

Hell Creek formation (Phanerozoic | Mesozoic Cenozoic | Cretaceous-Late Tertiary | Paleocene) at surface, covers 13 % 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

Lennep sandstone (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 3 % of this area

Lennep sandstone: mainly dark-brown andesitic sandstone with intercalated shale; locally contains thin coal beds.

Lithology: sandstone; mudstone; coal

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

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

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

Triassic, undifferentiated: conglomerate, sandstone, shale, and impure limestone belonging to the Dinwoody and Thaynes formations and other units of Triassic age, and the Chugwater of Triassic and Permian age.

Lithology: fine-grained mixed clastic; limestone; medium-grained mixed clastic

Fort Union formation (Phanerozoic | Cenozoic | Tertiary Cretaceous-Late | Paleocene) at surface, covers 15 % 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

Tertiary coarse-grained intrusive rocks (Phanerozoic | Cenozoic | Tertiary) at surface, covers < 0.1 % 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 3 % of this area

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

Lithology: alluvial terrace

Alluvium (Phanerozoic | Cenozoic | Quaternary) at surface, covers 8 % 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

Mississippian, undifferentiated (Phanerozoic | Paleozoic | Carboniferous Mississippian) at surface, covers < 0.1 % 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

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 12 % 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)

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.

Lithology: mudstone; sandstone

Stillwater complex (Archean Proterozoic(?) preCambrian-Proterozoic(?) preCambrian(?) Phanerozoic | Paleozoic(?) Mesozoic(?) Cenozoic | Cambrian(?) Ordovician(?) Silurian(?) Devonian(?) Carboniferous(?) Permian(?) Triassic(?) Jurassic(?) Cretaceous(?) Tertiary) at surface, covers 0.7 % of this area

Stillwater complex: consists chiefly of ultrabasic intrusive rocks and metamorphic rocks not yet named or subdivided.

Lithology: anorthosite; gabbroid; ultramafic intrusive rock; quartz monzonite

Bearpaw shale (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 10 % 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 6 % 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

Pennsylvanian, undifferentiated (Phanerozoic | Paleozoic | Carboniferous Pennsylvanian) at surface, covers < 0.1 % 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.

Lithology: sandstone; mudstone; carbonate; conglomerate; chert