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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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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
- 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.
- 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.
- 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.
- 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.
- 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.
- 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