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

Geologic units in Mineral county, Montana

[Additional scientific data in this geographic area]

Missoula group (Proterozoic | Mesoproterozoic) at surface, covers 26 % of this area

Missoula group: chiefly red, maroon, or purple argillite; sandy or quartzitic argillite, and generally impure quartzite and limestone. The larger limestone masses are similar to the Siyeh limestone of the Piegan group. The Missoula group includes numerous named formations, most of which cannot be traced with confidence far from their type localities. Among these are the Marsh shale in the Helena region, the Striped Peak and Libby formations in northwestern Montana, five near Missoula, and others in and south of Glacier National Park.

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

Tertiary volcanic rocks (Phanerozoic | Cenozoic | Tertiary) at surface, covers < 0.1 % 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

Wallace formation (Proterozoic | Mesoproterozoic) at surface, covers 38 % of this area

Wallace formation: A heterogeneous unit that includes dark-gray argillite, arenaceous and argillaceous limestone, in part dolomitic, and gray limy quartzite, with shale and sandstone in large areas. The argillaceous and sandy or quartzitic rocks are commonly slightly calcareous. The dominantly carbonate-rich rocks contain "molar tooth" or segregation structures. Commonly characterized by thin laminae. Locally red rocks near the top may represent a transition into the Missoula group.

Lithology: meta-argillite; carbonate; quartzite

Gabbro, diorite, amphibolite, lamprophyre; Tertiary to Late Proterozoic, undivided; northern Idaho (Tertiary to Late Proterozoic) at surface, covers < 0.1 % of this area

Younger Precambrian dioritic and amphibolitic sills of northern Idaho.

Lithology: gabbro; diorite; amphibolite; lamprophyre

Siltite, argillite, dolostone, and quartzite; Middle Proterozoic Wallace Formation; northern Belt province (Early Middle Proterozoic) at surface, covers 0.1 % of this area

Intermediate Precambrian sediments, dark-colored calcareous and dolomitic argillite and siltite of northern Idaho.

Lithology: siltstone; meta-argillite; dolostone (dolomite); quartzite

Idaho batholith and associated masses (Phanerozoic | Mesozoic Cenozoic | Cretaceous-Late Tertiary) at surface, covers 1 % of this area

Idaho batholith and associated masses: faintly gneissic quartz monzonite, granodiorite, and similar rocks. The extreme eastern part of the Idaho batholith extends into Ravalli County, Mont. This and nearby masses of similar rocks are shown as associated with the Idaho batholith and designated Ki. Future studies may show that some granitoid masses further east are also allied to the Idaho batholith or that a few of the masses now grouped with that batholith are younger. Precise dating is impossible at present.

Lithology: granitoid

Schist, phyllite, quartzite, and calc-silicate rock; Middle Proterozoic metasedimentary rocks of Wallace Formation; northern Belt province (Middle Proterozoic) at surface, covers < 0.1 % of this area

Intermediate Precambrian age metasediments; generally low-to-medium grade; carbonate bearing argillite and siltite of northern Idaho.

Lithology: mica schist; phyllite; quartzite; calc-silicate rock

Argillite, siltite, and quartzite; Middle Proterozoic Ravalli Group; northern Belt Province (Middle Proterozoic) at surface, covers < 0.1 % of this area

Intermediate Precambrian sediments, multicolored siltite and argillite overlying quartzite and siltite of northern Idaho.

Lithology: meta-argillite; siltstone; quartzite

Tertiary sedimentary rocks, undifferentiated (Phanerozoic | Cenozoic | Tertiary) at surface, covers 0.9 % 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 0.3 % 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

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

Alluvium (Phanerozoic | Cenozoic | Quaternary) at surface, covers 10 % 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

Ravalli group (Proterozoic | Mesoproterozoic) at surface, covers 22 % of this area

Ravalli group: a diverse assemblage with numerous subdivisions, only a few of which have been recognized over large enough areas to be distinguished here. Near Idaho the rocks are light colored and siliceous, ranging from pure white quartzite to siliceous shale, mostly in subdued tones of gray, green, purple, and red. Near Missoula the rocks are dark-gray quartzite and quartzitic argillite. In and near Glacier National Park the Grinnell, Appekunny, and Altyn formations are distinguished.

Lithology: meta-argillite; quartzite

Diabase, metagabbro, and diorite and related rocks (Proterozoic) at surface, covers 0.1 % of this area

Diabase, metagabbro, and diorite and related rocks: dark gray fine to medium grained, mostly in dikes and sills; dominantly mafic but contain alkalai feldspar and micropegmatite in some areas.

Lithology: diorite; gabbro

Prichard formation (Proterozoic | Mesoproterozoic) at surface, covers 0.5 % of this area

Prichard formation: dark-gray, generally argillaceous rocks, locally sandy or quartzitic; locally metamorphosed to schist.

Lithology: meta-argillite; quartzite; carbonate; chert

Monzogranite, syenogranite, and syenite; Eocene phaneritic to porphro-aphanitic intrusions of the pink monzogranite suite; Challis magmatic belt (Eocene) at surface, covers < 0.1 % of this area

Eocene intrusions of predominantly felsic (pink granite) composition

Lithology: granite; quartz syenite