GALLATIN NATIONAL FOREST DESCRIPTION OF MAP UNITS BRIDGER RANGE Qs Surficial deposits, undivided (Quaternary)--Alluvium, colluvium, talus, landslide deposits, rock glaciers, and glacial and glaciofluvial deposits. Only selected major areas, generally exposed for at least 1 mi in a single direction, are shown Tfl Fort Logan Formation (lower Miocene)--Sandy and conglomeratic tuffaceous siltstone with interbedded volcanic ash, freshwater gastropod limestone, and fossiliferous lenses of conglomerate and gravel. Coarse fractions mostly derived from Proterozoic Belt Supergroup and Paleozoic limestone. Shown only in extreme northeast corner of Bridger Range. Exposures more than 100 ft thick; actual thickness not measured (McGrew, 1977) Ti Intrusive rocks, undivided (Eocene)--Dacite, diorite, diabase, and basalt dikes and sills. Diorite dike in northern part of Bridger Range is olive-gray, coarsely crystalline, pyroxene phenocrysts (Skipp and Peterson, 1965); composite dike in southern part of range may be similar in composition (McMannis, 1955). Sills differentiated into upper syenite layer and lower layer of biotite-augite diorite. Thickness 80-200 ft (McMannis, 1955) TKf Fort Union Formation (Paleocene and Upper Cretaceous)-- Cliff-forming, massive, nonmarine conglomerate, sandstone, and siltstone; conglomerate clasts derived from pre-Cretaceous rocks. Contains fossil spores, plants, wood, freshwater mollusks, and vertebrates. About 6,600 ft thick (Roberts, 1964b, c; 1972) Klv Livingston Group (Upper Cretaceous)--In descending order, includes Hoppers, Billman Creek, and Sedan Formations. Sedan Formation correlates with Miner Creek and Cokedale Formations, which are exposed in southernmost part of Bridger Range and in a narrow band extending to the east past Cokeville and Livingston (Skipp and McGrew, 1977). Livingston Group is volcaniclastic conglomerate, sandstone, and mudstone; and volcanic flows, sills, tuff, and breccia (Roberts, 1972). Hoppers Formation is purple-gray to gray-green, epiclastic volcanic sandstone, siltstone, mudstone, and conglomerate; 2,400 ft thick (Skipp, 1977). Billman Creek Formation is grayish-red, grayish-green, and gray volcanic mudstone and siltstone interbedded with minor volcanic sandstone, conglomerate, and vitric tuff. Includes freshwater gastropods and dinosaur bones (Skipp, 1977); 2,500 - 3,000 ft thick (Roberts, 1972; Skipp, 1977). Sedan Formation is primarily greenish-gray, epiclastic volcanic sandstone, mudstone, and conglomerate interbedded with mudflow conglomerate, welded tuff, devitrified silicified vitric tuff, bentonite, and lignitic coal (Skipp and McGrew, 1977). Freshwater mollusks, wood, and dinosaur bones present in upper part of Sedan Formation. Lower part contains ironstone nodules, magnetite-rich beds, volcanic granule-and-pebble conglomerate, and carbonaceous plant material. Lignite coal locally present at base. Sedan Formation about 3,000 - 4,500 ft (Skipp and McGrew, 1977) Ke Eagle Sandstone (Upper Cretaceous)--Upper part is gray, thin- to thick-bedded, very fine grained to conglomeratic, largely calcareous, crossbedded sandstone; calcareous concretions and intercalated coal and shale. Lower part is massive, indurated, ledge-forming, crossbedded sandstone and greenish- gray and grayish-orange volcanic sandstone and conglomerate; intercalated coal, siltstone, and shale. Thickness 150-300 ft (Roberts, 1964a; Skipp, 1977; Skipp and Hepp, 1968; Skipp and Peterson, 1965) Ku Upper Cretaceous sedimentary rocks, undivided-- In descending order, includes Telegraph Creek Formation, Cody Shale, and Frontier Formation. Telegraph Creek Formation is predominantly gray, biotitic, silty shale that contains thin interbeds of fine-grained sandstone, limestone, and altered crystal vitric tuff. Upper part contains large calcareous cannonball concretions; lower part contains pelecypods and ammonites. About 285-400 ft thick (Roberts, 1964a; Skipp, 1977; Skipp and Hepp, 1968; Skipp and Peterson, 1965). Cody Shale is dark-gray shale and mudstone that contain thin interbeds of gray-green, glauconitic, calcareous to argillaceous, crosslaminated and rippled, fossiliferous (ammonites) sandstone and siltstone; ironstone nodules and cone-in-cone structures occur locally. About 600-1,100 ft thick (McGrew, 1977; Skipp, 1977; Skipp and Hepp, 1968; Skipp and Peterson, 1965). Frontier Formation is mostly gray, green, or brown, thin- to medium-bedded, fine- to coarse-grained, calcareous to argillaceous, locally crosslaminated, ridge-forming sandstone; light-gray quartz, feldspar, and rock fragments contrast with dark-gray chert grains and give the sandstone a characteristic "salt-and-pepper" appearance. Contains interbeds of mudstone, siltstone, shale, quartzite, chert-pebble conglomerate, siliceous limestone, and fossil oyster banks; mudstone and siltstone intervals form valleys. About 500-700 ft thick (McGrew, 1977; Skipp, 1977; Skipp and Peterson, 1965) Kul Upper and Lower Cretaceous sedimentary rocks, undivided--In descending order, includes Mowry Shale (Upper Cretaceous) and Thermopolis Shale and Kootenai Formation (Lower Cretaceous). Mowry and Thermopolis Shales are dark-gray shaly mudstone and porcellanite interbedded with greenish-gray, medium-grained to very coarse grained, feldspathic, calcareous and argillaceous, thin-bedded sandstone; basal grayish-orange, ledge-forming quartzitic sandstone. Shales total about 600 ft thick (McGrew, 1977; Roberts, 1964a; Skipp, 1977; Skipp and Hepp, 1968). Kootenai Formation is red, gray, and purple hematitic mudstone interbedded with yellowish-gray and grayish-purple, quartzose, medium- to coarse-grained sandstone. Gray, nodular, gastropod-bearing, freshwater limestone common in upper part (about 100 ft below top) and just above basal ledge- and talus-forming, salt-and-pepper sandstone. Lenses of granule- to pebble-size chert conglomerate in basal sandstone. About 200-400 ft thick (McGrew, 1977; Skipp, 1977) Ju Jurassic sedimentary rocks, undivided--Includes Morrison Formation (Upper Jurassic) and underlying Ellis Group (Upper and Middle Jurassic). Ellis Group comprises Swift, Rierdon, and Sawtooth Formations; near Livingston the Sawtooth's lithic equivalent is the Piper Formation. Morrison Formation is red, gray, and purple mudstone and hackly shale intercalated with light gray, yellowish-brown, and yellowish-orange siltstone and sandstone, some conglomeratic; calcareous, quartzose, commonly crossbedded. Black shale near top; gray freshwater limestone in beds and nodules; thick red mudstone at base. About 200-400 ft thick (McGrew, 1977; Roberts, 1964a; Skipp and Hepp, 1968; Skipp and Peterson, 1965). Ellis Group is light-gray to light-brown, quartzose, calcareous, commonly iron stained sandstone and siltstone, interbedded with gray, fossiliferous, ledge-forming oolitic or sandy limestone and chert-pebble conglomerate. Basal bed grades from chert breccia or chert-pebble conglomerate on the west to thin gray mudstone to fossiliferous limestone on the east. Thickness ranges from 2 to 213 ft (McGrew, 1977; Skipp and Hepp, 1968; Skipp and Peterson, 1965) PMu Permian, Pennsylvanian, and Mississippian sedimentary rocks, undivided--In descending order, includes Phosphoria (Permian), Quadrant (Pennsylvanian), and Amsden (Lower Pennsylvanian and Upper Mississippian) Formations. Phosphoria Formation (McKelvey and others, 1959) (probably better correlated with Shedhorn Sandstone in this area) consists of a thin (no thicknesses given-- presumably not more than several feet thick) section of phosphorite, phosphatic mudstone, carbonaceous mudstone, and chert (McKelvey and others, 1959); mapped with Quadrant Formation by Roberts (1964a, 1972). Quadrant Formation is well-sorted calcareous quartzite to quartzose sandstone; light-gray dolomite in lower part; lower part forms ledges. Thickness 0 to 150 ft (McGrew, 1977; Roberts, 1964a; Skipp and Hepp, 1968). Amdsen Formation is light-gray, weathering to white, thin-to thick-bedded dolomite. Interbeds of pale-grayish-orange sandstone and siltstone in upper part. Massive red sandstone, gray sandstone, siltstone and mudstone, and red claystone interbedded with gray argillaceous limestone in lower part; dolomite-limestone breccia at base. About 250-600 ft thick (McGrew, 1977; Skipp and Hepp, 1968) Mm Madison Group (Mississippian)--Includes Mission Canyon Limestone and underlying Lodgepole Limestone. Mission Canyon Limestone is gray, aphanitic to medium-crystalline, cherty, massive, cliff-forming limestone and dolomite. Red-siltstone-filled cavernous zones, limestone breccia beds, and gray dolomite locally in upper part. About 700-1,200 ft thick (McGrew, 1977; Roberts, 1964a; Skipp, 1977; Skipp and Hepp, 1968; Skipp and Peterson, 1965). Lodgepole Limestone is gray, thin- to medium-bedded, fossiliferous, locally cherty limestone and dolomite; weathers into platy blocks. Red and yellow silty limestone interbeds; thin dark-gray shale at base. Forms step-like ledges. About 500-750 ft thick (McGrew, 1977; Roberts, 1964a; Skipp, 1977; Skipp and Hepp, 1968; Skipp and Peterson, 1965) MDt Three Forks Formation (Lower Mississippian and Upper Devonian) and Jefferson Formation (Upper Devonian), undivided--Three Forks Formation is yellow calcareous siltstone, impure gray fossiliferous limestone, and dark-gray and green mudstone and shale. Thin beds of dolomite in upper part; olive-gray and reddish-brown calcareous, fossiliferous shale in lower part. Thickness 150-200 ft (McGrew, 1977; Roberts, 1964a; Skipp and Peterson, 1965). Underlying Jefferson Formation is brownish- to olive-gray, thick-bedded to massive, sparsely fossiliferous, fetid dolomite and interbedded gray limestone; local stromatolites. About 400-600 ft thick (McGrew, 1977; Roberts, 1964a; Skipp and Peterson, 1965) Cu Cambrian sedimentary rocks, undivided--In descending order, includes Snowy Range Formation (and laterally equivalent Red Lion Formation) and Pilgrim Limestone (Upper Cambrian); and Park Shale, Meagher Limestone, Wolsey Shale, and Flathead Sandstone (Middle Cambrian). Locally may include Devonian Maywood Formation (McGrew, 1977), a red and yellow calcareous siltstone and gray dolomitic limestone (Skipp and Peterson, 1965). Snowy Range Formation is mainly limestone and shale distinguished by rounded limestone pebbles in the Grove Creek Limestone Member; underlain by thick red, gray, and green shale, gray limestone, and siltstone-pebble conglomerate of undivided lower members; bed of columnar limestone locally in lower part of formation (Roberts, 1964a; Skipp and Peterson, 1965). Combined thickness of Maywood and Snowy Range Formations 100-200 ft (Skipp and Peterson, 1965). Pilgrim Limestone is gray, thin-bedded to massive limestone and dolomite; forms cliffs. Oolites, grayish-orange and yellow mottling, and flat-pebble conglomerates are common. Interbeds of greenish-gray shale in lower part; glauconitic and fossiliferous at base. Thickness 350-500 ft (McGrew, 1977; Roberts, 1964a; Skipp and Peterson, 1965). Park Shale is grayish-green to multicolored clay shale; thin beds of argillaceous limestone, siltstone, and sandstone; sparse fossiliferous limestone and limestone-pebble conglomerate in upper part. Nonresistant; forms valleys. Thickness 100-200 ft (McGrew, 1977; Roberts, 1964a; Skipp and Peterson, 1965). Meagher (pronounced "mar") Limestone is thin- to medium-bedded, mottled grayish-orange and medium-gray, fine-grained limestone and yellowish-gray dolomite; locally oolitic; crumpled bedding. Thickness 300-600 ft (McGrew, 1977; Roberts, 1964a; Skipp and Peterson, 1965). Wolsey Shale is olive- to brownish-gray micaceous clay shale; contains gray impure limestone in upper part and fine-grained, thin-bedded, glauconitic sandstone in lower part. Abundant trace fossils ("worm cast" markings). Forms valleys. Thickness 100-500 ft (Roberts, 1964a; Skipp and Peterson, 1965). Flathead Sandstone is yellowish-gray to reddish-gray, fine- to coarse-grained, thin- to thick-bedded, locally crossbedded quartzite and quartz sandstone. Locally conglomeratic in lower part. Very resistant; forms ridges. Thickness 0-300 ft (McGrew, 1977; Roberts, 1964a; Skipp and Peterson, 1965) Ym Middle Proterozoic metasedimentary rocks, undifferentiated--Includes the Spokane and underlying LaHood (formerly North Boulder Group) Formations of the Belt Supergroup. Spokane Formation is light-grayish-green argillite, gray to yellowish-gray quartzite and sandstone, and minor thin-bedded grayish-red limestone; rippled beds are common. Thickness 2,200-3,200 ft (McGrew, 1977). LaHood Formation is dark-greenish-gray, medium-grained to very coarse grained to conglomeratic, micaceous, feldspathic sandstone that contains interbedded dark-gray limestone and impure aphanitic laminated calcareous argillite; sandstone forms about 30 percent of sequence and is most common in upper part; limestone, with cone-in-cone structures, and argillite dominate lower part. More than 2,500 ft thick (Skipp and Peterson, 1965) Am Metamorphic rocks (Archean)--Primarily gneiss. Includes schist, granite, quartzite, pegmatite, amphibolite, and mafic intrusive rocks (Roberts, 1964a) CRAZY MOUNTAINS Qs Surficial deposits, undivided (Holocene)--Alluvium, talus, rock glaciers, and glacial deposits. Only selected major areas, generally exposed for at least 1 mi in a single direction, are shown Ta Alkalic intrusive rocks (Eocene)--Mafic alkalic rocks (including malignite, nepheline syenite, analcite syenite, and theralite), trachyte porphyry, and related intrusive rocks. Typically these rocks are sodium rich, silica undersaturated, and strongly alkaline (Harlan, 1986). No plagioclase feldspar and calcic amphibole (Dudas, 1990). Includes dikes, sills, laccoliths, and small stocks Ti Intrusive rocks, undivided (Eocene)--Diorite, diabase, and fine-grained rocks ranging from basalt to rhyolite; includes dikes and sills. Commonly porphyritic; phenocryst composition ranges from 5 to 50 percent. Transitional between alkaline and calc-alkaline compositions. Probably genetically related to quartz monzodiorite phase of Big Timber stock (Tqm) (du Bray and others, 1993). Probably includes alkalic bodies (Ta) on west flank of Crazy Mountains and north of Shields River Big Timber stock (Eocene) Tqm Quartz monzodiorite phase--Quartz monzonite to granodiorite. Locally contains inclusions of diorite and gabbro (Tdg) along contact. Forms an irregular-shaped mass at center of Big Timber stock (du Bray and others, 1993). 40Ar/39Ar ages on biotite: 49.23(+/-)0.1 and 49.33(+/-)0.12 Ma (S.S. Harlan, USGS, unpub. data, 1996) Tdg Diorite and gabbro phase--Granular diorite and gabbro form main part of Big Timber stock. Locally contains stoped blocks of pyroxene-rich gabbro. Coarser grained and darker in color than quartz monzodiorite phase (Tqm) (du Bray and others, 1993). 40Ar/39Ar ages on biotite: 49.2(+/-)0.07 and 49.33(+/-)0.11 Ma (S.S. Harlan, USGS, unpub. data, 1996) Tcl Porphyry of Campfire Lake (Eocene)--Quartz monzodiorite porphyry, compositionally similar to quartz monzodiorite phase of Big Timber stock (Tqm). Separated from main body of Big Timber stock by sedimentary rocks of Fort Union Formation (TKf). At southern end of area of outcrop, unit has a quenched appearance with plagioclase phenocrysts (2- to 5-mm-long) in an aphanitic to glassy groundmass (du Bray and others, 1993) TKf Fort Union Formation (Paleocene and Upper Cretaceous)--Siltstone, mudstone, sandstone, and pebble conglomerate. Unit is mainly hornfels in a several-mile-wide aureole of contact metamorphism adjacent to Big Timber stock (du Bray and others, 1993). Near Livingston, 6,615 ft thick (Roberts, 1972) Klv Livingston Group (Upper Cretaceous)--Volcaniclastic conglomerate, sandstone, and mudstone; tuff and volcanic breccia. Contains flows and sills (Roberts, 1972). Thickness 6,455 ft (Roberts, 1972) REFERENCES CITED du Bray, E.A., Elliott, J.E., Wilson, A.B., Van Gosen, B.S., and Rosenberg, L.A., 1993, Geologic map of the Big Timber Stock and vicinity, southern Crazy Mountains, Sweet Grass and Park Counties, south-central Montana: U.S. Geological Survey Miscellaneous Field Studies Map MF-2253, scale 1:24,000. Dudas, F.O., 1990, Petrogenesis and mantle source of igneous rocks in the Crazy Mountains, Montana: State College, Pennsylvania State University, Ph.D. dissertation, 442 p. Harlan, S.S., 1986, Timing and deformation along the leading edge of the Montana Disturbed Belt: Bozeman, Montana State University, M.S. thesis, 87 p. McGrew, L.W., 1977, Geologic map of the Sixteen quadrangle, Gallatin and Meagher Counties, Montana: U.S. Geological Survey Geologic Quadrangle Map GQ-1383, scale 1:24,000. McKelvey, V.E., and others, 1959, The Phosphoria, Park City and Shedhorn Formations in the Western Phosphate Field: U.S. Geological Survey Professional Paper 313-A, 47 p. McMannis, W.J., 1955, Geology of the Bridger Range, Montana: Geological Society of America Bulletin, v. 66, p. 1385-1430. Roberts, A.E., 1964a, Geologic map of the Fort Ellis quadrangle, Montana: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-397, scale 1:24,000. Roberts, A.E., 1964b, Geologic map of the Bozeman Pass quadrangle, Montana: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-399, scale 1:24,000. Roberts, A.E., 1964c, Geology of the Hoppers quadrangle, Montana: U.S. Geological Survey Geologic Quadrangle Map GQ-258, scale 1:24,000. Roberts, A.E., 1972, Cretaceous and early Tertiary depositional and tectonic history of the Livingston area, southwestern Montana: U.S. Geological Survey Professional Paper 526-C, 120 p. Skipp, Betty, 1977, Geologic map and cross section of the Wallrock quadrangle, Gallatin and Park Counties, Montana: U.S. Geological Survey Geologic Quadrangle Map GQ-1402, scale 1:24,000, 1 sheet. Skipp, Betty, and Hepp, Mary-Margaret, 1968, Geologic map of the Hatfield Mountain quadrangle, Gallatin County, Montana: U.S. Geological Survey Geologic Quadrangle Map GQ-729, scale 1:24,000, 1 sheet. Skipp, Betty, and McGrew, L.W., 1977, The Maudlow and Sedan Formations of the Upper Cretaceous Livingston Group on the west edge of the Crazy Mountains basin, Montana: U.S. Geological Survey Bulletin 1422-B, 68 p. Skipp, Betty, and McMannis, W.J., 1971, Geologic map of the Sedan quadrangle, Gallatin and Park Counties, Montana: U.S. Geological Survey Open-File Report 71-264, 2 sheets, scale 1:48,000. Skipp, Betty, and Peterson, A.D., 1965, Geologic map of the Maudlow quadrangle, southwestern Montana: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-452, scale 1:24,000, 2 sheets.