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Geologic units in Missoula county, Montana
[Additional scientific data in this geographic area]
- Boulder batholith and broadly related stocks (Phanerozoic | Cenozoic | Tertiary Cretaceous) at surface, covers 0.8 % 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.
- Idaho batholith and associated masses (Phanerozoic | Mesozoic Cenozoic | Cretaceous-Late Tertiary) at surface, covers 3 % 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.
- Glacial lake deposits (Phanerozoic | Cenozoic | Quaternary) at surface, covers 0.2 % of this area
Glacial lake deposits: mainly silt; believed to have been deposited in lakes formed behind temporary dams of ice or morainal deposits
- Grinnell argillite (Proterozoic | Mesoproterozoic) at surface, covers 0.2 % of this area
Grinnell argillite: reddish-purple and green argillite, quartzitic argillite, and some quartzite; generally contains some feldspar and carbonates.
- Piegan group (Proterozoic | Mesoproterozoic) at surface, covers 7 % of this area
Piegan group: most of the Piegan group is subdivided into formations that are in part equivalent to each other. Where correlations are relatively doubtful the group designation is retained. Carbonate-bearing rocks predominate in the group, but the proportions and character of the impurities in these rocks and the relations to non-carbonate-bearing rocks vary from place to place.
- Wallace formation (Proterozoic | Mesoproterozoic) at surface, covers 2 % 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.
- 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.
- 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.
- Missoula group (Proterozoic | Mesoproterozoic) at surface, covers 46 % 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.
- Newland limestone (Proterozoic | Mesoproterozoic) at surface, covers 0.7 % of this area
Newland limestone: dark bluish-gray argillaceous, dolomitic limestone with some argillite, locally schistose; segregation structures not conspicuous. In central and western Montana the Newland and Wallace formations have been treated as essentially synonymous terms by some authors.
- Border zone of Idaho batholith and associated masses (Proterozoic Phanerozoic | Mesozoic Cenozoic | Cretaceous-Late Tertiary) at surface, covers 0.5 % of this area
Border zone of Idaho batholith and associated masses: granitic gneiss and sedimentary rocks, mainly of Belt age, impregnated with granitic material. Because of lack of information they are not distinguished aournd some of the smaller granitic masses that have such border zones.
- 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
- Prichard formation (Proterozoic | Mesoproterozoic) at surface, covers 0.8 % of this area
Prichard formation: dark-gray, generally argillaceous rocks, locally sandy or quartzitic; locally metamorphosed to schist.
- 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.
- High-grade metasedimentary biotite schist, biotite gneiss, and calc-silicate gneiss; Middle Proterozoic northern Belt province near Idaho batholith (Middle Proterozoic) at surface, covers < 0.1 % of this area
Precambrian high-grade metamorphic rock; kyanite-sillimanite calc-silcate bearing schist and gneiss; scapolite common
- Glacial drift (Phanerozoic | Cenozoic | Quaternary) at surface, covers 8 % 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 sedimentary rocks, undifferentiated (Phanerozoic | Cenozoic | Tertiary) at surface, covers 5 % 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.
- Ravalli group (Proterozoic | Mesoproterozoic) at surface, covers 5 % 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.
- Alluvium (Phanerozoic | Cenozoic | Quaternary) at surface, covers 18 % 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.
- Intermediate intrusions; mostly Late Cretaceous dioritic to granodioritic plutons of the Idaho batholithic assemblage (Cretaceous) at surface, covers < 0.1 % of this area
Cretaceous plutons; intermediate; as granodiorite or diorite; probably includes unmapped older and younger crystalline bodies.