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

Geologic units in Sanders county, Montana

[Additional scientific data in this geographic area]

Prichard formation (Proterozoic | Mesoproterozoic) at surface, covers 19 % of this area

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

Lithology: meta-argillite; quartzite; carbonate; chert

Diabase, metagabbro, and diorite and related rocks (Proterozoic) at surface, covers 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

Ravalli group (Proterozoic | Mesoproterozoic) at surface, covers 34 % 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

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

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

Intermediate Precambrian sediments, gray interlaminated siltite and argillite with minor quartzite of northern Idaho.

Lithology: siltstone; meta-argillite; quartzite

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.

Lithology: alluvium; glacial drift; carbonate

Tertiary sedimentary rocks, undifferentiated (Phanerozoic | Cenozoic | Tertiary) at surface, covers 0.2 % 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

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

Glacial lake deposits (Phanerozoic | Cenozoic | Quaternary) at surface, covers 4 % of this area

Glacial lake deposits: mainly silt; believed to have been deposited in lakes formed behind temporary dams of ice or morainal deposits

Lithology: glaciolacustrine

Idaho batholith and associated masses (Phanerozoic | Mesozoic Cenozoic | Cretaceous-Late Tertiary) at surface, covers 0.4 % 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

Argillite, siltstone, quartzite, and dolostone; Middle Proterozoic Missoula Group; northern Belt province (Middle Middle Proterozoic) at surface, covers < 0.1 % of this area

Intermediate Precambrian sediments, light-colored siltite overlying multicolored fine-grained detrital beds of northern Idaho.

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

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

Piegan group (Proterozoic | Mesoproterozoic) at surface, covers < 0.1 % 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.

Lithology: carbonate; meta-argillite; quartzite

Wallace formation (Proterozoic | Mesoproterozoic) at surface, covers 20 % 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

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

Missoula group (Proterozoic | Mesoproterozoic) at surface, covers 2 % 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