Geologic units in Lincoln county, Montana

Additional scientific data in this geographic area

Ravalli Group: St. Regis, Revett, and Burke Formations; or Empire and Grinnell Formations; or Empire and Spokane Formations (Middle Proterozoic) at surface, covers 24 % 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).

Piegan Group: Helena and Wallace Formations (Middle Proterozoic) at surface, covers 21 % 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).

Upper Prichard or Appekunny Formation (Middle Proterozoic) at surface, covers 13 % 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).

Lower Missoula Group: Shepard and Snowslip Formations; or Shepard Formation and unresolved Snowslip Formation equivalent (Middle Proterozoic) at surface, covers 11 % 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).

Upper Missoula Group: McNamara, Bonner, and Mount Shields Formations; locally includes lower Libby Formation in northwestern Montana (Middle Proterozoic) at surface, covers 11 % 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).

Glacial deposit (Quaternary) at surface, covers 9 % of this area

Dominantly till, outwash, and local glacial lake deposits. Shown only in western and south-central Montana. Laurentide glacial deposits are indicated with a map pattern.

Lower Prichard Formation (Middle Proterozoic) at surface, covers 8 % 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.

Alluvium (Quaternary) at surface, covers 0.6 % of this area

Gravel, sand, silt, and clay deposits of stream and river channels, and floodplains.

Granitic rock (Cretaceous) at surface, covers 0.6 % of this area

Granitic rock

Mafic intrusive rock (Middle Proterozoic) at surface, covers 0.5 % of this area

Mafic intrusive rock

Garnet Range Formation: locally includes upper Libby Formation and Pilcher Formation in northwestern Montana (Middle Proterozoic) at surface, covers 0.3 % of this area

Dark green, micaceous, fine-grained quartzite with subordinate argillite interbeds. Thickness as much as 1,200 m (3,937 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). Pilcher Formation: Light gray and red cross-bedded, feldspathic, fine grained quartzite. Garnet Range Formation: Dark green, micaceous, fine-grained quartzite with subordinate argillite interbeds. Thickness as much as 1,200 m (3,937 ft).

Purcell Lava (Middle Proterozoic) at surface, covers 0.3 % of this area

Black to blackish green alkaline basalt flows.

Red Lion through Flathead Formation: Red Lion, Pilgrim, Park, Meagher, Wolsey, and Flathead Formations; or Red Lion, Hasmark, Silver Hill, and Flathead Formations (Cambrian) at surface, covers 0.3 % of this area

Red Lion Formation: Gray limestone with siliceous lamina and black calcareous shale interbedded with thin limestone. Shallow marine. Thickness as much as 115 m (377 ft). Pilgrim Formation: Gray, commonly mottled limestone that may contain intraformational limestone conglomerate. Shallow marine. Thickness as much as 180 m (590 ft). Park Formation: Grayish green, fissile, micaceous shale with a few thin beds of calcareous sandstone and thin, gray limestone. Local arkose beds. Offshore marine. Thickness as much as 310 m (1,017 ft). Meagher Formation: Gray to bluish gray limestone and dolomitic limestone, locally mottled and with intraformational conglomerate beds. Shallow marine. Thickness as much as 300 m (984 ft). Wolsey Formation (CAw): Dark green and purplish gray fissile, micaceous shale, thin glauconitic limestone beds and thin, fine-grained sandstone beds. Shallow marine. Thickness as much as 145 m (476 ft). Flathead Formation (CAf): Pinkish gray to light gray sandstone or quartzite. Locally very glauconitic, pebbly, arkosic, or iron-stained. Marine shoreface. Thickness as much as 100 m (328 ft). Hasmark Formation: Light gray or bluish gray limestone and dark-gray calcareous shale. Shallow marine. Thickness as much as 560 m (1,823 ft). Silver Hill Formation: Brown, green, and very light gray banded calcareous shale interbedded with laminated limestone. Shallow marine. Thickness as much as 120 m (394 ft).

Syenite (Cretaceous) at surface, covers 0.1 % of this area

Syenite

Lacustrine deposit (Quaternary) at surface, covers 0.1 % of this area

Light brown to brown, well-sorted, unconsolidated, laminated sand, silt, and clay. Lima Reservoir and Red Rocks Lake area, southwestern Montana.

Pyroxenite (Cretaceous) at surface, covers 0.1 % of this area

Pyroxenite

Prichard Formation. (Mesoproterozoic) at surface, covers < 0.1 % 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).

Three Forks through Maywood Formation: Three Forks, Jefferson, and Maywood Formations or Palliser and Alexo Formations, and Fairholme Group in northwestern Montana near Canadian border (Mississippian and Devonian) at surface, covers < 0.1 % of this area

Three Forks Formation (MDt): Sappington Member (upper part of formation):yellowish orange and yellowish gray siltstone. Trident Member (middle part of formation): greenish gray and light olive gray, fossiliferous, calcareous shale that contains interbeds and nodules of fossiliferous, argillaceous limestone. Logan Gulch Member (lower part of formation): yellowish gray and grayish red, argillaceous limestone breccia and shale breccia that may include dolomitic siltstone. Marine to restricted marine with evaporite basins. Thickness as much as 185 m (607 ft). Jefferson Formation (Dj): Birdbear Member (upper part of formation): light to medium gray, sucrosic dolomite. Lower part of formation: grayish black, commonly petroliferous dolomite or limestone that may be interbedded with light gray quartzite. Marine. Thickness as much as 520 m (1,706 ft). Maywood Formation: Thin-bedded dolomitic limestone and dolomite interbedded with calcareous shale. Shallow marine. Thickness as much as 120 m (394 ft). In northwestern Montana near Canadian border - Palliser Formation: Gray and greenish gray limestone and dolomitic limestone with locally abundant black chert. Pinkish gray, very thinly laminated, calcareous mudstone at base. Marine. Thickness as much as 300 m (984 ft). Alexo Formation: Pale yellowish orange, medium grained, calcareous siltstone with calcareous mudstone at the base. Shallow marine. Thickness as much as 595 m (1,952 ft). Fairholme Group: Brownish black, bioturbated, calcareous mudstone, gray algal limestone and dolomite, and pale yellowish orange, medium-grained, calcareous sandstone at the base. Thickness as much as 550 m(1,805 ft).

Glacial deposits. (Pleistocene) at surface, covers < 0.1 % 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).

Devils Glen through Flathead Formation: Devils Glen, Switchback, Steamboat, Pentagon, Pagoda, Dearborn, Damnation or Elko, Gordon, and Flathead Formations (Cambrian) at surface, covers < 0.1 % of this area

Devils Glen Formation: Light gray dolomite with subordinate algal limestone and shale. Shallow marine. Thickness as much as 200 m (656ft). Switchback Formation: Green and gray, locally calcareous, fissile shale or mudstone, interbedded with thin beds of gray, flaggy limestone or dolomite. Shallow marine. Thickness as much as 95 m (312 ft). Steamboat Formation: Tan, thick-bedded, hard limestone with thin layers of green fissile shale and nodular shaly limestone. Shallow marine. Thickness as much as much as 90 m (295 ft). Pentagon Formation: Black fissile shale and brownish gray, platy limestone beds. Laterally replaced by Pagoda Formation. Shallow marine. Thickness as much as 88 m (289 ft). Pagoda Formation: Green shale interbedded with maroon calcareous mudstone and gray limestone, and lower fissile black shale and thin limestone-pebble conglomerate. Shallow marine. Thickness as much as 120 m (394 ft). Dearborn Formation: Upper brownish gray to gray limestone and lower interbedded green shale, shale-pebble conglomerate, and calcareous sandstone. Shallow marine shelf. Thickness as much as 115 m (377 ft). Damnation Formation: Brownish gray or bluish gray fossiliferous, locally oolitic limestone and subordinate dark gray shale. Shallow marine. Thickness as much as 70 m (230 ft). Elko Formation: Light brownish gray dolomite and subordinate mudstone. Thickness as much as 220 m (722 ft). Gordon Formation: Upper part of formation: dark green to dark brown, fissile shale with thin beds of sandstone and gray, glauconitic limestone. Lower part of formation: ferruginous sandstone, sandy shale, and grayish green shale. Shallow marine. Thickness as much as 90 m (295 ft). Flathead Formation (CAf): Pinkish gray to light gray sandstone or quartzite. Locally very glauconitic, pebbly, arkosic, or iron-stained. Marine shoreface. Thickness as much as 100 m (328 ft).

Ravalli Group. (Mesoproterozoic) at surface, covers < 0.1 % 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).

Piegan Group. (Mesoproterozoic) at surface, covers < 0.1 % 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 < 0.1 % 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).