Geologic units in Granite county, Montana

Additional scientific data in this geographic area

Upper Missoula Group: McNamara, Bonner, and Mount Shields Formations; locally includes lower Libby Formation in northwestern Montana (Middle Proterozoic) at surface, covers 31 % 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).

Granitic rock (Cretaceous) at surface, covers 11 % of this area

Granitic rock

Piegan Group: Helena and Wallace Formations (Middle Proterozoic) at surface, covers 9 % 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).

Lower Missoula Group: Shepard and Snowslip Formations; or Shepard Formation and unresolved Snowslip Formation equivalent (Middle Proterozoic) at surface, covers 7 % 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).

Sediment or sedimentary rock (Tertiary) at surface, covers 6 % of this area

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

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 4 % 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).

Gravel (Quaternary) at surface, covers 4 % 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.

Madison Group: Mission Canyon and Lodgepole Formations; or Castle Reef and Allan Mountain Formations (Mississippian) at surface, covers 4 % 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).

Kootenai Formation through Ellis Group: Kootenai, Morrison, and Ellis Group (Cretaceous and Jurassic) at surface, covers 3 % 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). 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). 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).

Phosphoria through Amsden Formation: Phosphoria, Quadrant, and Amsden Formations (Permian through Mississippian) at surface, covers 2 % 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. Amsden Formation (PAMa): Red shale, light gray limestone, and cherty and sandy limestone. Coastal plain or marine. Thickness as much as 180 m (590 ft).

Andesite and basalt (Tertiary) at surface, covers 2 % of this area

Andesite and basalt

Blackleaf Formation (Cretaceous) at surface, covers 2 % 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).

Upper Tertiary sediment or sedimentary rock (Tertiary) at surface, covers 2 % of this area

Conglomerate, tuffaceous sandstone and siltstone, marlstone, and equivalent sediment and ash beds. Local volcanic flows in southwestern Montana. Includes Sixmile Creek Formation. Dominantly fluvial, alluvial fan, mudflow, and palustrine. Composite thickness for Tsu and Tsm exceeds 4,880 m (16,010 ft).

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).

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

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

Garnet Range Formation: locally includes upper Libby Formation and Pilcher Formation in northwestern Montana (Middle Proterozoic) at surface, covers 2 % 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).

Missoula Group: Pilcher, Garnet Range, McNamara, Bonner, and Mount Shields Formations; and either Shepard and Snowslip Formations or Shepard Formation and unresolved Snowslip Formation equivalent (Middle Proterozoic) at surface, covers 1 % of this area

Upper Missoula Group (Ymu) - 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). 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). Lower Missoula Group (Yml) - 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).

Rhyolite (Tertiary) at surface, covers 1 % of this area

Rhyolite

Volcanic rock: locally includes Challis Volcanics in southwestern Montana (Tertiary) at surface, covers 1 % 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.

Gravel (Quaternary and Tertiary) at surface, covers 1 % 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.

Middle Tertiary sediment or sedimentary rock (Tertiary) at surface, covers 0.7 % of this area

Tuffaceous siltstone, sandstone, bentonitic mudstone, conglomerate, limestone, and equivalent sediment and ash beds. Local volcanic flows in southwestern Montana. Includes Renova Formation. Dominantly fluvial, floodplain, palustrine, lacustrine, debris flow, and eolian. Composite thickness for Tsu and Tsm exceeds 4,880 m (16,010 ft). Includes Anaconda Formation: Light gray, reddish tan, or brownish red, coarse breccia that laterally grades to conglomerate, sandstone, and siltstone. Locally intertongues with Lowland Creek Volcanics. Derived from overlying unmetamorphosed plate of metamorphic core complex. Thickness as much as much as 200 m (656 ft).

Granitic rock (Tertiary) at surface, covers 0.4 % of this area

Granitic rock

Ravalli Group: St. Regis, Revett, and Burke Formations; or Empire and Grinnell Formations; or Empire and Spokane Formations (Middle Proterozoic) at surface, covers 0.2 % 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).

Carter Creek through Coberly Formation: Carter Creek, Jens, and Coberly Formations (Cretaceous) at surface, covers < 0.1 % of this area

Carter Creek Formation: Varicolored sandstone, siltstone, and silty mudstone, with subordinate dark shale and lenses of coarse-grained conglomeratic dacitic tuff. Nearshore marine, brackish, and terrestrial. Thickness as much as 2,000 m (6,562 ft). Jens Formation: Dark gray shale, siltstone, siliceous mudstone, volcaniclastic siltstone, and minor fine-grained calcareous sandstone. Marine. Thickness as much as 505 m (1,657 ft). Coberly Formation: Gray limestone and tan, massive sandstone. Intertidal and shallow marine. Thickness as much as 215 m (705 ft).

Belt Supergroup (Middle Proterozoic) at surface, covers < 0.1 % of this area

Extremely large lumping of over 25+ formations. See individual unit descriptions. Belt Supergroup consists of: Missoula Group (Ym, Ymu, Yml), Piegan Group (Ypg, Yh), Ravalli Group (Yr, Ye, Ysp), and all the Lower Belt Formations (Ypu, Ypl, Yaw, Yla, Yg, Yn, Ynla, Ych, Yne).