Geologic units in Shoshone county, Idaho

Additional scientific data in this geographic area

Piegan Group. (Mesoproterozoic) at surface, covers 31 % of this area

Calcareous and dolomitic siltite, quartzite, and subordinate argillite of the Helena and Wallace formations in northern Idaho; south of St. Joe River, correlative rocks include garnet grade feldspathic quartzite and calc-silicate gneiss. (Mesoproterozoic Belt Supergroup).

Lower Missoula Group. (Mesoproterozoic) at surface, covers 20 % of this area

Siltite, argillite, and dolomitic siltite in northern Idaho; includes Snowslip and Shepard formations, rocks previously mapped as upper member of Wallace Formation, and correlative garnet-grade phyllite and schist south of St. Joe River. Also includes schist, micaceous quartzite, and calc-silicate rocks in lower part of Meadow Creek metamorphic sequence east of Elk City, and argillite, siltite, phyllite, and calc-silicate rocks south of Lost Trail Pass. (Mesoproterozoic Belt Supergroup).

Ravalli Group. (Mesoproterozoic) at surface, covers 15 % of this area

Feldspathic quartzite and subordinate siltite and argillite of the Burke, Revett, and St. Regis formations in northern Idaho; south of St. Joe River, includes correlative garnet-grade quartzite and schist. (Mesoproterozoic Belt Supergroup).

Upper Missoula Group. (Mesoproterozoic) at surface, covers 11 % of this area

Feldspathic quartzite and subordinate siltite and argillite. Includes Mount Shields, Bonner, Striped Peak, and Libby formations in northern Idaho and possibly correlative quartzite north of Salmon and in the Meadow Creek metamorphic sequence east of Elk City. (Mesoproterozoic Belt Supergroup).

Metamorphic rocks. (Paleoproterozoic and Archean) at surface, covers 9 % of this area

Pend Oreille granodiorite gneiss near Priest River (~2650 Ma); schist, gneiss, and subordinate quartzite along North Fork Clearwater River; Kelly Creek granite gneiss northeast of Pierce (~2670 Ma); orthogneiss in Pioneer Mountains (~2600 Ma and 695 Ma); granite gneiss, granite, schist, and amphibolite of Green Creek complex in Albion Mountains (~2600 Ma); granite gneiss in Beaverhead Mountains east of Leadore (~2450 Ma); and marble, quartzite, schist, and amphibolite of uncertain age near Henrys Lake. (Mesoproterozoic to Archean Basement Rocks).

Prichard Formation. (Mesoproterozoic) at surface, covers 7 % of this area

Siltite, quartzite, and argillite in northern Idaho; deposited in deep water and contains voluminous ~1470 Ma mafic sills. Includes siltite, quartzite, fine-grained schist, and carbonate rocks structurally above anorthosite near Boehls Butte, northeast of Elk River. (Mesoproterozoic Belt Supergroup).

Challis intrusive rocks. (Eocene) at surface, covers 2 % of this area

Shallow roots of Challis volcanic field. Older suite of granodiorite and quartz monzodiorite and subordinate diorite, granite, and subvolcanic dacite; includes Jackson Peak, Beaver Creek, Marsh Creek, and Summit Creek stocks (49-45 Ma). Younger suite of granite and minor syenite and subvolcanic rhyolite; includes Sawtooth, Casto, Bungalow, and Lolo Hot Springs plutons (47-43 Ma). (Eocene Challis Magmatic Complex and Related Sedimentary Rocks).

Anorthosite. (Paleoproterozoic) at surface, covers 1 % of this area

Metamorphosed anorthosite at Boehls Butte northeast of Elk River (~1787 Ma). Unit is in structural contact with overlying rocks of the Prichard Formation. (Mesoproterozoic to Archean Basement Rocks).

Orthogneiss. (Paleoproterozoic) at surface, covers 0.8 % of this area

Biotite tonalite gneiss and hornblende-biotite tonalite gneiss (~1870 Ma) north of North Fork of Clearwater River, and south of Coeur d'Alene. (Mesoproterozoic to Archean Basement Rocks).

Windermere Supergroup. (Cambrian and Neoproterozoic) at surface, covers 0.6 % of this area

Upper part consists of shallow marine and fluvial quartzose sandstone, and minor siltite, shale, and limestone of the Brigham Group of southeast Idaho, Wilbert Formation of east-central Idaho, and quartzites of unknown affinity west of Borah Peak. More highly metamorphosed equivalents are quartzite, metaconglomerate, metasiltite, calc-silicate marble, and schist of Gospel Peaks successions C and D in central Idaho; mature quartzite, biotite schist, and minor calc-silicate rocks of Syringa metamorphic sequence east of Moscow; and schist and quartzite in the Albion Range (Elba Quartzite, schist of Upper Narrows, quartzite of Yost, schist of Stevens Spring, quartzite of Clarks Basin, schist of Mahogany Peaks, and Harrison Summit Quartzite). Lower part consists of diamictite, immature sandstone, and bimodal volcanic rocks related to continental rifting. Includes Pocatello Formation (700-665 Ma), formation of Leaton Gulch near Challis, Shedroof Conglomerate in extreme northwest Idaho, and metamorphic equivalents (schist, marble, calc-silicate rocks, metaconglomerate, and ~686 Ma metavolcanic rocks) of Gospel Peaks successions A and B in central Idaho. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).

Columbia River Basalt Group. (Miocene) at surface, covers 0.5 % of this area

Large-volume lava flows of tholeiitic basalt, basaltic andesite, and subordinate andesite in western Idaho; consists of Imnaha Basalt (17.5-16.5 Ma), Grande Ronde Basalt (16.5-15.6 Ma), Wanapum Basalt (15.6-14.5 Ma), and Saddle Mountains Basalt (14.5-6 Ma). Includes porphyritic basalt and basaltic andesite in western Owyhee County. (Quaternary to Eocene Continental Volcanic and Intrusive Rocks).

Glacial deposits. (Pleistocene) at surface, covers 0.4 % of this area

Till and outwash consisting of gravel, sand, silt, and clay. Formed by valley glaciers at higher elevations and by the Cordilleran ice sheet in northern Idaho. Includes deposits of several glacial episodes. Includes rock glacier deposits and some modern alluvium derived from reworked till and outwash. (Quaternary Sediments).

Sedimentary rocks associated with flood basalts. (Miocene) at surface, covers 0.4 % of this area

Fluvial and lacustrine deposits associated with Columbia River Basalt Group and equivalent basalts (17-8 Ma); consolidated to weakly consolidated sandstone, siltstone, arkose, conglomerate, claystone, and tuffaceous sediment; subordinate intercalated basalt and rhyolitic tuff. Includes Payette and Sucker Creek formations in southwestern Idaho, sediments associated with basalt of Weiser in western Idaho, and Latah Formation in northern Idaho. Includes sedimentary rocks of uncertain origin in southwest corner of Idaho. (Quaternary to Eocene Continental Sediments and Sedimentary Rocks).

Alluvial deposits. (Quaternary) at surface, covers 0.2 % of this area

Deposits in valleys consisting of gravel, sand, and silt. Includes younger terrace deposits. May contain some glacial deposits and colluvium in uplands. (Quaternary Sediments).

Syenite and related rocks. (Cretaceous) at surface, covers 0.2 % of this area

Syenite, monzonite, quartz monzonite, and subordinate pyroxenite (115-110 Ma); includes Gem stocks north of Wallace, Gold Hill stock northeast of Potlatch, Benning Mountain stock along the Montana-Idaho border east of Sandpoint, syenite of Wall Mountain north of Bonners Ferry, monzonite of Long Canyon (~88 Ma) northwest of Bonners Ferry, and no Business Mountain pluton (uncertain age) west of Donnelly. (Paleocene and Cretaceous Idaho Batholith and Older Cretaceous and Jurassic Intrusive Rocks).

Piegan Group: Helena and Wallace Formations (Middle Proterozoic) at surface, covers 0.1 % of this area

Helena Formation (Yh): Northwestern Montana: (formerly “lower Wallace”): cycles of basal white quartzite or intraclast beds overlain by couplets of green siltite and argillite, capped by dolomite beds. Calcite pods and ribbons (molar tooth structure) common. West-central Montana: gray to dark gray limestone and dolomitic limestone with siltite partings. Thickness as much as 2,000 m (6,562). Wallace Formation: Tan-weathering, dolomitic quartzite and siltite, and black argillite with calcite ribbons (molar tooth structure) in graded pinch-and-swell couples and couplets. Thickness as much as 2,500 m (8,202 ft).

Ravalli Group: St. Regis, Revett, and Burke Formations; or Empire and Grinnell Formations; or Empire and Spokane Formations (Middle Proterozoic) at surface, covers < 0.1 % of this area

St. Regis Formation: Green and purplish gray quartzite, siltite, and argillite in mud-cracked couples and couplets. Thickness as much as 900 m (2,953 ft). Revett Formation: Light gray, cross-bedded, felspathic, fine-grained quartzite. Thickness as much as 900 m (2,953 ft). Burke Formation: Gray or purple quartzite, siltite, and argillite in mud-cracked couples and couplets. Thickness as much as 1,000 m (3,281 ft). Empire Formation (Ye): Grayish green and pale olive gray argillite and siltite with subordinate thin beds of quartzite and sandy limestone. Thickness as much as 610 m. Grinnell Formation: White, cross-bedded sandstone or quartzite with red to purple siltite and argillite beds. Thickness as much as 1,160 m (3,806 ft). Spokane Formation (Ysp): Red siltite and argillite in mudcracked couplets. Thickness as much as 1,500 m (492 ft).

Granodiorite and two-mica granite. (Cretaceous) at surface, covers < 0.1 % of this area

Granodiorite and granite containing biotite, commonly with muscovite; includes bulk of Atlanta lobe (85-67 Ma) and isolated plutons in northern Idaho (107-67 Ma). (Paleocene and Cretaceous Idaho Batholith and Older Cretaceous and Jurassic Intrusive Rocks).

Upper Prichard or Appekunny Formation (Middle Proterozoic) at surface, covers < 0.1 % of this area

Upper Prichard Formation: Black and gray, pyritic siltite and argillite in planar couplets. Appekunny Formation: Gray and greenish gray argillite interbedded with light gray quartzite. Thickness as much as 1,700 m (5,577 ft).

Upper Missoula Group: McNamara, Bonner, and Mount Shields Formations; locally includes lower Libby Formation in northwestern Montana (Middle Proterozoic) at surface, covers < 0.1 % of this area

McNamara Formation: Dense green and red siltite and argillite in mudcracked couplets containing diagnostic chert beds and rip-up clasts. Thickness as much as 1,650 m (5,413 ft). Bonner Formation: Pink, cross-bedded, feldspathic, medium- to coarse grained quartzite. Thickness as much as 580 m (1,903 ft). Mount Shields Formation: Upper part: red quartzite, siltite, and argillite in mud-cracked couples and couplets with abundant salt casts. Lower part: light gray, flat-laminated, feldspathic, fine-grained quartzite. Thickness as much as 2,000 m (6,562 ft). In northwestern Montana - Libby Formation:Light to dark gray and greenish gray siltite and argillite with subordinate quartzite in mud-cracked couplets. Thickness as much as 2,300 m (7,546 ft).

Lower Missoula Group: Shepard and Snowslip Formations; or Shepard Formation and unresolved Snowslip Formation equivalent (Middle Proterozoic) at surface, covers < 0.1 % of this area

Shepard Formation: Tan-weathering, dolomitic, green siltite and argillite in couplets and microlaminae. In the west formerly considered part of the “upperWallace.” Thickness as much as 1,100 m (3,609 ft). Snowslip Formation: Green and red siltite and argillite in couplets. Western Snowslip equivalent is black siltite and argillite in couplets and microlaminae, formerly considered part of the “upper Wallace.” A new name is under consideration for the western Snowslip equivalent. Thickness as much as 1,200 m (3,937 ft).

Lower Prichard Formation (Middle Proterozoic) at surface, covers < 0.1 % of this area

Dark and light gray, pyrite- and pyrrhotite-rich siltite and argillite in planar couplets with subordinate gray, flat-laminated, fine-grained quartzite.

Diorite (Tertiary or Cretaceous) at surface, covers < 0.1 % of this area

Diorite