Geologic units in Flathead county, Montana

Additional scientific data in this geographic area

Piegan Group: Helena and Wallace Formations (Middle Proterozoic) at surface, covers 24 % 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 17 % 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).

Upper Missoula Group: McNamara, Bonner, and Mount Shields Formations; locally includes lower Libby Formation in northwestern Montana (Middle Proterozoic) at surface, covers 14 % 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 10 % 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).

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.

Upper Prichard or Appekunny Formation (Middle Proterozoic) at surface, covers 5 % 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).

Kishenehn Formation (Tertiary) at surface, covers 4 % of this area

Light to dark bluish gray, locally sandy clay with interbeds of light gray, nodular sandstone, conglomerate, and coal. Dominantly fluvial, palustrine, and lacustrine. Exposed thickness 230 m (755 ft).

Alluvium (Quaternary) at surface, covers 3 % of this area

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

Devils Glen through Flathead Formation: Devils Glen, Switchback, Steamboat, Pentagon, Pagoda, Dearborn, Damnation or Elko, Gordon, and Flathead Formations (Cambrian) at surface, covers 3 % 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).

Gravel (Quaternary) at surface, covers 2 % of this area

Variable deposits that range from pebble to boulder size and include sand, silt, and clay. Dominantly alluvial terrace, abandoned channel and floodplain, remnant alluvial fan, and local glacial outwash.

Glacial lake deposit (Quaternary) at surface, covers 2 % of this area

Light brown laminated silt, fine-grained sand, and clay. Shown only in northwestern and central-western Montana.

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 2 % 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).

Kootenai Formation (Cretaceous) at surface, covers 1 % of this area

Kootenai Formation (Kk, Kku, Kkm, Kkl): Red, maroon, and olive gray mudstone, tan or gray siltstone, calcareous concretions, limestone beds, and several prominent sandstone beds that include the Greybull Member (top of the formation in south-central Montana) and the Sunburst Member (middle to lower part of the formation in northwest-central Montana), both dominantly quartzose sandstone. Basal Cutbank or Pryor Conglomerate. Alluvial plain with local marine influence in the north. Thickness as much as 335 m (1,099 ft).

Madison Group: Mission Canyon and Lodgepole Formations; or Castle Reef and Allan Mountain Formations (Mississippian) at surface, covers 1 % of this area

Mission Canyon Formation (Mmc): Gray, massive limestone with chert beds and nodules, and solution breccia zones. Shallow marine. Thickness as much as 520 m (1,706 ft). Lodgepole Formation (Ml): Woodhurst Member (upper part of formation): light gray, well-bedded limestone, typically with much dark chert, interbedded with thinner calcareous mudstone beds. Paine Member (middle part of formation):dark gray, thin-bedded, silty or fossiliferous limestone. Cottonwood Canyon Member (lower part of formation): black shale with basal conglomeratic lag deposit. Shallow marine. Thickness as much as 305 m (1000 ft). In Northwest Montana - Castle Reef Formation: Medium to light gray, thick-bedded limestone or dolomite. Sun River Member (upper part of formation): light gray dolomite with thick fossiliferous lenses. Shallow marine. Thickness as much as 300 m (984 ft). Allan Mountain Formation: Dark gray, thinly bedded limestone with thin mudstone and shale partings, and nodular chert. Shallow marine. Thickness as much as 200 m (656 ft).

Mount Pablo Formation through Ellis Group: Mount Pablo, Morrison, Swift, Rierdon, and Sawtooth Formations (Cretaceous and Jurassic) at surface, covers 0.6 % of this area

Mount Pablo Formation: Reddish gray and grayish green mudstone and dark gray shale interbedded with gray limestone and sandstone. Gray basal conglomerate or conglomeratic sandstone. Alluvial plain. Thickness as much as 30 m (98 ft). Morrison Formation (Jm): Green, gray, or red mudstone and marlstone with subordinate limestone and sandstone beds. Upper part Neocomian age in central Montana and contains carbonaceous black shale and coal. Fluvial, paludal, and lacustrine. Thickness as much as 180 m (590 ft). Ellis Group (Je) - Swift Formation (Jsw): Orangish brown, glauconitic, flaggy-bedded, commonly fossiliferous, fine-grained sandstone or sandy coquina with subordinate dark gray shale interbeds; chert pebbles common. In west-central and northwestern Montana, a dark gray, noncalcareous, micaceous shale forms the lower part of the formation, commonly with a basal chert-pebble conglomerate or conglomeratic sandstone as much as 3 m (10 ft) thick. Shallow marine. Thickness as much as 70 m (230 ft). Rierdon Formation (Jr): Gray, locally fossiliferous limestone that may contain floating grains of quartz sand, interbedded with greenish gray limy shale. Lagoonal and marine shelf. Thickness as much as 105 m (344 ft). Sawtooth Formation: Western Montana: dark gray, platy to shaly, dense limestone with local basal conglomerate. Central Montana: upper calcareous siltstone, middle dark gray shale with thin limestone interbeds, and lower fine-grained sandstone. Three local members Bowes, Firemoon, and Tampico, in descending order. Bowes Member: dark gray to medium gray, calcareous mudstone, limestone, and quartzose sandstone. Firemoon Member: dark to medium gray, limestone and calcareous mudstone. Tampico Member: very light gray, well-sorted quartz sandstone and siltstone, and chert-pebble conglomerate. Shallow marine. Thickness as much as 205 m (673 ft).

Garnet Range Formation: locally includes upper Libby Formation and Pilcher Formation in northwestern Montana (Middle Proterozoic) at surface, covers 0.6 % 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).

Blackleaf Formation (Cretaceous) at surface, covers 0.5 % of this area

Bootlegger Member (Kbb): Dark gray shale interbedded with sandstone, siltstone, and bentonite beds. Marine. Thickness as much as 100 m (328 ft). Vaughn Member (Kbv): Black carbonaceous shale, light gray, greenish to pinkish bentonitic to tuffaceous siltstone and porcellanite, tuffaceous, arkosic sandstone, thin coal; conglomerate beds in western Montana. Fluvial and coastal delta plain. Thickness as much as 200 m (656 ft). Taft Hill Member (Kbt): Black shale, gray bentonitic siltstone, and glauconitic sandstone. Marine. Thickness as much as 180 m (590 ft). Flood Member (Kblf): Upper part: medium gray, cliff-forming fine- to medium-grained chert-rich, quartzose sandstone, middle part medium to dark gray shale or gray siltstone; lower part: yellowish brown, gray, or olive gray fine-grained sandstone and siltstone with carbonaceous laminae and thin films of coal and dark gray silty, bentonitic shale. Sandstone beds thin eastward. Quartzose basal sandstone in southwestern Montana. Nearshore marine, tidal flat, and lagoon. Thickness as much as 230 m (755 ft).

Volcanic rock: locally includes Challis Volcanics in southwestern Montana (Tertiary) at surface, covers 0.2 % of this area

Volcanic rock; In southwestern Montana - Challis Volcanics: Basaltic andesite, andesite, quartz latite, latite, rhyodacite, and rhyolite flows, and tuff, most of which is nonporphyritic and commonly spherulitic; glassy to devitrified. Subordinate water-laid tuff.

Mafic intrusive rock (Late or middle Proterozoic) at surface, covers 0.1 % of this area

Mafic intrusive rock

Sediment or sedimentary rock (Tertiary) at surface, covers 0.1 % of this area

Sediment or sedimentary rock (no unique unit description on map).

Phosphoria and Quadrant Formations (Permian and Pennsylvanian) at surface, covers 0.1 % of this area

Phosphoria Formation (Pp): Gray and tan dolomite, cherty limestone, bedded chert, siltstone, sandstone, and gray to black phosphatic and dolomitic shale. Marine shelf and offshore marine. Thickness as much as 255 m (837 ft). Quadrant Formation (PAq): Very light gray, yellowish or pinkish, well-sorted sandstone or quartzite, locally interbedded with subordinate limestone beds. Marine. Generally, thickness as much as 140 m (460 ft), but as much as 800 m (2,625 ft) in southwestern-most Montana.

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

Trachyandesite

Marias River through Kootenai Formation: Marias River, Blackleaf, and Kootenai Formations (Cretaceous) at surface, covers < 0.1 % of this area

Marias River Formation (Kmr) - Kevin Member (Kmk): Dark gray shale with calcareous concretions and numerous bentonite beds. Upper part calcareous. MacGowan Concretionary Bed (middle part of member): grayish brown concretionary dolostone and limestone with phosphatic pellets, and gray to black chert pebbles. Marine. Thickness as much as 350 m (1,148 ft). Ferdig Member (Kmf): Gray and dark gray shale with thin siltstone or sandstone beds at the top. Marine. Thickness as much as 125m (410 ft). Cone Member (Kmc): Calcareous, dark gray shale and thin limestone beds and lenses. Widespread bentonite bed and septarian concretions near base. Shallow marine. Thickness as much as 20 m (66 ft). Floweree Member (Kmfl): Dark gray, noncalcareous shale with interbeds of siltstone. Calcareous concretions and scattered granules of dark gray chert. Shallow marine. Thickness as much as 30 m (98 ft). Blackleaf Formation (Kbl) - Bootlegger Member (Kbb): Dark gray shale interbedded with sandstone, siltstone, and bentonite beds. Marine. Thickness as much as 100 m (328 ft). Vaughn Member (Kbv): Black carbonaceous shale, light gray, greenish to pinkish bentonitic to tuffaceous siltstone and porcellanite, tuffaceous, arkosic sandstone, thin coal; conglomerate beds in western Montana. Fluvial and coastal delta plain. Thickness as much as 200 m (656 ft). Taft Hill Member (Kbt): Black shale, gray bentonitic siltstone, and glauconitic sandstone. Marine. Thickness as much as 180 m (590 ft). Flood Member (Kblf): Upper part: medium gray, cliff-forming fine- to medium-grained chert-rich, quartzose sandstone, middle part medium to dark gray shale or gray siltstone; lower part: yellowish brown, gray, or olive gray fine-grained sandstone and siltstone with carbonaceous laminae and thin films of coal and dark gray silty, bentonitic shale. Sandstone beds thin eastward. Quartzose basal sandstone in southwestern Montana. Nearshore marine, tidal flat, and lagoon. Thickness as much as 230 m (755 ft). Kootenai Formation (Kk, Kku, Kkm, Kkl): Red, maroon, and olive gray mudstone, tan or gray siltstone, calcareous concretions, limestone beds, and several prominent sandstone beds that include the Greybull Member (top of the formation in south-central Montana) and the Sunburst Member (middle to lower part of the formation in northwest-central Montana), both dominantly quartzose sandstone. Basal Cutbank or Pryor Conglomerate. Alluvial plain with local marine influence in the north. Thickness as much as 335 m (1,099 ft).

Marias River Formation (Cretaceous) at surface, covers < 0.1 % of this area

Marias River Formation (Kmr) - Kevin Member (Kmk): Dark gray shale with calcareous concretions and numerous bentonite beds. Upper part calcareous. MacGowan Concretionary Bed (middle part of member): grayish brown concretionary dolostone and limestone with phosphatic pellets, and gray to black chert pebbles. Marine. Thickness as much as 350 m (1,148 ft). Ferdig Member (Kmf): Gray and dark gray shale with thin siltstone or sandstone beds at the top. Marine. Thickness as much as 125m (410 ft). Cone Member (Kmc): Calcareous, dark gray shale and thin limestone beds and lenses. Widespread bentonite bed and septarian concretions near base. Shallow marine. Thickness as much as 20 m (66 ft). Floweree Member (Kmfl): Dark gray, noncalcareous shale with interbeds of siltstone. Calcareous concretions and scattered granules of dark gray chert. Shallow marine. Thickness as much as 30 m (98 ft).

Ellis Group: Swift, Rierdon, and Piper or Sawtooth Formations; not all formations present at all locations (Jurassic) at surface, covers < 0.1 % of this area

Swift Formation (Jsw): Orangish brown, glauconitic, flaggy-bedded, commonly fossiliferous, fine-grained sandstone or sandy coquina with subordinate dark gray shale interbeds; chert pebbles common. In west-central and northwestern Montana, a dark gray, noncalcareous, micaceous shale forms the lower part of the formation, commonly with a basal chert-pebble conglomerate or conglomeratic sandstone as much as 3 m (10 ft) thick. Shallow marine. Thickness as much as 70 m (230 ft). Rierdon Formation (Jr): Gray, locally fossiliferous limestone that may contain floating grains of quartz sand, interbedded with greenish gray limy shale. Lagoonal and marine shelf. Thickness as much as 105 m (344 ft). Sawtooth Formation: Western Montana: dark gray, platy to shaly, dense limestone with local basal conglomerate. Central Montana: upper calcareous siltstone, middle dark gray shale with thin limestone interbeds, and lower fine-grained sandstone. Three local members Bowes, Firemoon, and Tampico, in descending order. Bowes Member: dark gray to medium gray, calcareous mudstone, limestone, and quartzose sandstone. Firemoon Member: dark to medium gray, limestone and calcareous mudstone. Tampico Member: very light gray, well-sorted quartz sandstone and siltstone, and chert-pebble conglomerate. Shallow marine. Thickness as much as 205 m (673 ft). Piper Formation (Jp): Upper part: red mudstone and gypsum; middle part: gray shale, limestone, and dolomite; lower part: red mudstone and gypsum. Marine and restricted coastal evaporite basins. Thickness as much as 75 m (246 ft).

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

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).