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

Geologic units in Madison county, Montana

[Additional scientific data in this geographic area]

Clastic debris; Quaternary colluvium, fanglomerate, talus, and glacial deposits; upland-valley margins (Quaternary) at surface, covers < 0.1 % of this area

Quaternary colluvium, fanglomerate, and talus plus some glacial debris in upland valleys.

Lithology: alluvium; glacial drift; colluvium

Triassic, undifferentiated (Phanerozoic | Mesozoic | Triassic) at surface, covers 2 % 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

Alluvium (Phanerozoic | Cenozoic | Quaternary) at surface, covers 9 % 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

Jurassic, undifferentiated (Phanerozoic | Mesozoic | Jurassic) at surface, covers 0.4 % of this area

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

Lithology: fine-grained mixed clastic; limestone; evaporite

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

Tertiary sedimentary rocks, undifferentiated (Phanerozoic | Cenozoic | Tertiary) at surface, covers 22 % of this area

Tertiary sedimentary rocks, undifferentiated: clastic deposits in western Montana, mostly in valleys, and in most places not divided into formations; mostly poorly consolidated gravel, sand, silt, and clay; includes some tuffaceous material and locally lenses of lignite and bentonite; a little hot spring tufa; and in areas not yet mapped in detail, lava may be included. These rocks were in part laid down in lakes but a large part was formed in streams and alluvial fans. These rocks are Tertiary in age and as now mapped may even include some beds of Cretaceous age. Some late Tertiary terrace deposits may be included.

Lithology: medium-grained mixed clastic; coarse-grained mixed clastic; lava flow; ash-flow tuff; tuff; coal; bentonite; carbonate

Glacial drift (Phanerozoic | Cenozoic | Quaternary) at surface, covers 1 % of this area

Glacial drift: morainal and outwash plain deposits of mountain glaciers; mainly ill-sorted and poorly rounded boulders, cobbles, pebbles, and sand; may include alluvium in places

Lithology: glacial drift; alluvium

Mica schist, marble, quartzite, and amphibolite; Early Middle to Early Proterozoic metasedimentary rocks; southern Belt province (Early Middle Proterozoic to Early Proterozoic) at surface, covers < 0.1 % of this area

Older Precambrian high-grade metamorphic complex of eastern Idaho.

Lithology: mica schist; marble; quartzite; amphibolite

Pennsylvanian, undifferentiated (Phanerozoic | Paleozoic | Carboniferous Pennsylvanian) at surface, covers 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

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

Jurassic and Triassic rocks, undifferentiated: in east-central Madison County where scale did not permit differentiation on map.

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

Limestone, dolostone, shale, and arenite; Ordovician and Cambrian shallow marine inner continental-shelf deposits; southeastern Idaho (Ordovician and Cambrian) at surface, covers < 0.1 % of this area

Ordovician, commonly Lower, and Cambrian marine sediments of eastern and southern Idaho.

Lithology: limestone; dolostone (dolomite); shale; arenite

Mississippian, undifferentiated (Phanerozoic | Paleozoic | Carboniferous Mississippian) at surface, covers 4 % 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 34 % 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)

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

North Boulder group (Proterozoic | Mesoproterozoic) at surface, covers 0.5 % of this area

North Boulder group: greenish-gray coarse conglomerate at base, with arkose, conglomerate, and sandy and silty strata above. Locally includes strata resembling the Greyson and Spokane shales, thought to rest directly on the pre-Belt complex of metamorphic rocks. Some areas shown as Spokane shale, and possibly other units may include rocks of the North Boulder group. Distinguished from the vicinity of North Boulder River eastward along and north of Jefferson River and in the Bridger Range.

Lithology: meta-conglomerate; quartzite; meta-argillite; limestone

Tertiary volcanic rocks (Phanerozoic | Cenozoic | Tertiary) at surface, covers 6 % 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

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

Lithology: carbonate; mudstone; sandstone; conglomerate

Montana group, undifferentiated (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 3 % 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

Paleozoic rocks, undifferentiated (Phanerozoic | Paleozoic) at surface, covers 0.3 % of this area

Paleozoic rocks, undifferentiated: in east-central Madison County where scale did not permit differentiation on map.

Lithology: carbonate; mudstone; sandstone; chert; conglomerate

Permian, undifferentiated (Phanerozoic | Paleozoic | Permian) at surface, covers 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 0.8 % 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

Boulder batholith and broadly related stocks (Phanerozoic | Cenozoic | Tertiary Cretaceous) at surface, covers 4 % of this area

Boulder batholith and broadly related stocks: mainly quartz monzonite, but includes diorite, aplite, and other rocks. The distinctions between masses relative to the Boulder batholith and those mapped as of other ages are locally arbitrary and tentative.

Lithology: quartz monzonite; diorite; aplite

Colorado shale (Phanerozoic | Mesozoic | Cretaceous-Early Cretaceous-Middle(?) Cretaceous-Late) at surface, covers 3 % 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 2 % 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

Diorite and gabbro (Phanerozoic | Mesozoic | Cretaceous(?)) at surface, covers 1 % of this area

Diorite and gabbro: dark, moderately fine grained rocks in sills, dikes, and irregular bodies; not readily distinguished from similar rocks of other ages. Hence in the less well-known areas errors in age assignment may remain. These rocks are assigned to the Cretaceous(?).

Lithology: diorite; gabbroid