Geologic Map of the Ennis 30′ × 60′ Quadrangle, Madison and Gallatin Counties, Montana, and Park County, Wyoming

U.S. DEPARTMENT OF THE INTERIOR GEOLOGIC INVESTIGATIONS SERIES I–2690 U.S. GEOLOGICAL SURVEY

GEOLOGIC MAP OF THE ENNIS 30′ × 60′ QUADRANGLE, MADISON AND GALLATIN COUNTIES, MONTANA, AND PARK COUNTY, WYOMING

By Karl S. Kellogg and Van S. Williams

2000

DESCRIPTION OF MAP UNITS Qt Talus deposits (Holocene and upper Pleistocene?)—Angular and Qal Alluvium (Holocene)—Unconsolidated silt- subangular cobbles and boulders at to boulder-size, moderately sorted to base of steep valley walls or cliffs. well-sorted sediments in modern Boulders generally as large as 2 m, flood plains, including overbank although in places as large as 10 m. deposits. Larger clasts are moderate- Locally includes minor alluvial ly rounded to well rounded. Maxi- deposits and rock-glacier deposits. mum thickness greater than 5 m Maximum thickness greater than 20 Qs Swamp deposits (Holocene)—Black to m dark-brown, organic-rich clay, silt, Ql Landslide deposits (Holocene and upper and sand in grassy or willow-covered, Pleistocene)—Ranges from chaoti- flat outcops. Mostly represents cally oriented debris to almost intact eutrophication of shallow lakes slump blocks of bedrock. About 10 Qcl Colluvium and loess (Holocene and upper m to greater than 50 m thick Pleistocene)—Unconsolidated to Qr Rock glacier deposits (Holocene and slightly indurated, mostly massive, upper Pleistocene?)—Hummocky, dark-brown to light-gray-brown lobate deposits of angular boulders deposits that mantle gently to having a frontal slope near the angle moderately sloping surfaces; of repose; locally active. In places, sediment types are intermixed by grade into and include some talus downslope movement. Colluvium deposits (unit Qt). About 20 m thick contains cobbles and pebbles derived Qf Fan deposits (Holocene and upper Pleis- from weathering of bedrock; loess is tocene)—Moderately well sorted very fine grained sand, silt, and minor pebble- to boulder-size gravel in fan- clay. Commonly contains poorly to shaped deposits at base of mountain moderately developed soil profile in fronts. Includes piedmont-slope upper part. Includes alluvium in deposits and debris-flow deposits small channels and sheetwash on Qfc Fan deposits of the Cedar Creek alluvial steeper hillsides. Some small areas fan (Holocene and upper Pleis- (as in Tobacco Root Mountains) tocene)—Moderately well sorted underlain by till in valleys above cobble and boulder gravel that forms about 2,000 m elevation. Unmapped a prominent alluvial fan at mouth of in many areas, particularly where Cedar Creek in Madison Valley. deposit is thin and forms discon- Clasts rounded to sub-rounded. tinuous veneer. Maximum thickness Surface is characterized by braided probably less than 10 m stream pattern. Locally mantled by

1 loess less than 1 m thick. Fan formed support cultivation. Mostly less than mainly during Pinedale (about 20–14 10 m thick ka) and Bull Lake (about 140–100 ka) Qgc Terrace-gravel deposits of the Cameron glaciations; deposits of both ages bench (Pleistocene)—Moderately exposed at surface (Ritter and others, sorted, moderately rounded to well- 1990); minor proximal fan develop- rounded sand and gravel underlying ment during Holocene. Modern topographically highest (and oldest) channel of Cedar Creek incised about terrace deposits in Madison Valley. 10 m into fan deposits near mouth of Formed during Bull Lake glaciation creek, indicating that proximal fan (about 140–100 ka) (Schneider and deposits are presently eroding. As Ritter, 1987). Mantled by less than 2 much as 150 m thick. Unit shown m of loess in most places separately from other fan deposits QTbf Basin-fill deposit (Pleistocene and due to its prominent size and detailed Pliocene?)—Interbedded, moderately studies (Ritter and others, 1990) sorted, poorly consolidated to Qti Till (upper Pleistocene)—Unsorted, un- unconsolidated, well-bedded to stratified, unconsolidated, subangular massive silt, sand, and well-rounded to subrounded boulders in an cobbles that are exposed in slopes unsorted matrix as fine as silt. Most adjacent to, and below, terraces in till deposited during Pinedale (about Madison Valley. Exposed basin-fill 20–14 ka) and Bull Lake (about 140– deposits are interpreted to be no older 100 ka) glaciations. Pinedale-age till than Pliocene, although they overlie preserved in hummocky deposits that buried basin-fill deposits, as thick as contain numerous closed depressions 4,500 m, that may be as old as and have a thin or non-existent soil Eocene in the Madison Valley profile; deposits of Bull Lake–age till (Rasmussen and Fields, 1983) have more rounded topography, are Tht Huckleberry Ridge Tuff (Pliocene)— more dissected, and generally exhibit Light-brown to gray, massive welded a well-developed soil profile. As tuff that contains sparse phenocrysts much as about 50 m thick of sanidine, quartz, and plagioclase. Matrix is mostly devitrified glass Qd Diamicton (upper? Pleistocene)— shards, opaque minerals, and Unsorted, unstratified, non- aphanitic minerals. Contains sparse consolidated deposit composed of pumice fragments. Lithophysae and angular to subangular clasts of vesicles not observed in quadrangle. Archean gneiss as large as 2 m Forms prominent cap to many ridges across. Believed to be mostly debris- and buttes. Age of unit is 2.0 Ma, flow deposits, but may include some and source is from Yellowstone till and landslide deposits. Mapped caldera (Christiansen and Blank, only in Jack Creek area. Maximum 1972), the northern edge of which is thickness greater than 20 m about 30 km south of quadrangle’s Qg Terrace-gravel deposits (upper southeastern corner. Thickness Pleistocene)—Moderately sorted, increases to south, and maximum moderately rounded to well-rounded thickness in quadrangle is about 50 m sand and gravel. Underlies about 12 Tls Old landslide deposits (Pliocene and recognized terrace surfaces in Miocene)—Mapped at several Madison Valley. Bearzi (1987) localities in northern Madison River recognized 11 geomorphic surfaces Valley where significant erosion and along Jack Creek. In Madison dissection has transpired since land- Valley, east of Madison River, slide events (Kellogg, 1992, 1993b) includes all but the highest (and Ts Limestone, sandstone, conglomerate, and oldest) terrace-gravel deposit, which ash deposits (Miocene?)—White, is shown separately (unit Qgc). light gray, and light-brownish-gray, Man-tled by less than 2 m of loess at moderately indurated to well- most places, although loess on many indurated, locally vuggy limestone, higher surfaces is thick enough to limy sandstone, ash-bearing

2 sandstone, pebbly sandstone, and flow is just north of Virginia City and pebble conglomerate. Interlayered overlies Archean basement; (2) ash deposits are common, especially 34.4±3.0 Ma (whole rock); and (3) in association with limestone. 32.7±1.4 Ma (whole rock) (Marvin Limestone may be either lacustrine and others, 1974). Unit greater than deposits or carbonate paleosols 200 m thick (Hanneman and others, 1991). Tvr Rhyolite flows (Oligocene or Mapped along Madison River, east of Eocene)—Dark-gray to black Ennis Lake, and in Spanish Creek aphanitic rhyolite flows north and basin. One maximum 40Ar/39Ar age northwest of Ennis Lake (Kellogg, on sanidine from just east of Ennis 1993b; Vitaliano and Cordua, 1979). Lake in Madison Valley (NE¼, Sec. Cut by many faults; local steep dips. 6, T. 5 S, R. 1 E.), is 16.2±0.19 Ma Correlated with volcanic rocks of (K.K. Kellogg and S.S. Harlan, Virginia City volcanic field unpub. data, 1990). Deposits in Tvt Felsic tuff (Eocene)—Pale-gray to white Spanish Creek basin are undated, but rhyolitic air-fall ash, tuffaceous probably Miocene. Maximum sandstone, and sparse gravel lenses. thickness greater than 1,000 m Poorly exposed in most places and Tl Freshwater limestone (Miocene)—Light- occurs locally at base of sequence of brownish-gray to yellowish-gray, volcanic rocks of Virginia City locally vuggy, well-bedded, fine- volcanic field. Large areas mapped grained limestone. Similar to thin by Hadley (1969b) as felsic tuff are limestone beds associated with ash- underlain mostly by chaotic, locally bearing sediments in unit Ts, tuffaceous landslide deposits. although much thicker, so is con- Thickness as great as about 40 m sidered to be Miocene in age; Tg Volcaniclastic sandstone and gravel maximum thickness greater than 20 deposits (Eocene)—Coarse-grained, m. Mapped along west side of well-bedded, cross-bedded volcani- Madison Valley (Hadley, 1969a,b) clastic sandstone, pebbly sandstone, Volcanic rocks of Virginia City volcanic and pebble and cobble conglomerate field (Oligocene and Eocene) that in places underlie the rocks of the Tvd Diatreme (Oligocene?)—Probably Virginia City volcanic field (Hadley, basaltic, contains fragments of clay 1969b, 1980) and basalt (Wier, 1982). Intrudes Volcanic rocks of undetermined affinity both volcanic rocks and Archean (Oligocene and (or) Eocene) gneiss. One oval outcrop mapped, Tv Andesite and basalt flows—In Madison about 200 m in diameter, just south of Valley in southern part of quadrangle Virginia City (Hadley, 1969b); may be outlier of Tvv Andesite and basalt flows (Oligocene volcanic rocks of Virginia City and Eocene)—Mostly dense to volcanic field scoriaceous basalt and andesite flows, Ti Dacite porphyry sills and stocks volcanic agglomerates, sandy tuff, (Eocene)—Light- to medium-gray and interbedded basaltic cinder and brownish-gray, dense, porphyritic deposits. Flows are black and dark dacite. Phenocrysts are euhedral brown and contain small phenocrysts plagioclase, hornblende, and biotite. of calcic plagioclase, olivine, augite, Petrographically very similar to opaque minerals, and, in places, dacite porphyry of Fan and Lone potassium feldspar and biotite Mountains, but at least some stocks (Hadley, 1980). At least one silicic intrudes Eocene volcanic rocks; other (latitic?) plagioclase-pyrric flow nearby rocks of similar composition occurs near base of unit, 4 km intrude Cambrian strata and Archean southeast of Virginia City. Three gneiss. One potassium-argon biotite localities gave the following date of 49.5±1.5 Ma (Chadwick, potassium-argon (K-Ar) dates: (1) 1969) 49.3±2.5 and 51.1±1.9 Ma (biotite) and 51.0±3.8 Ma (plagioclase); this

3 Absaroka Volcanic Supergroup (Eocene) Basal Daly Creek Member of Tai Intrusive rocks—Dark-gray, very fine Sepulcher Formation commonly grained basalt or andesite intrusive mapped on basis of lighter color and breccia containing sparse, small preponderance of alluvial-facies phenocrysts of plagioclase. Forms material. However, except for a two small necks in upper Porcupine gradual increase upward in per- Creek, east of Gallatin River Valley. centage of flows, no distinct Intrudes Eocene volcanic rocks and difference was noted between lower Archean gneiss. Not included in and upper parts of Sepulcher Absaroka Volcanic Supergroup by Formation Simons and others (1985), but unit Tc Basal conglomerate and siltstone (lower probably formed feeder vents for Eocene)—Cobbles and boulders younger rocks of Supergroup predominantly of Archean rocks, with Tam Mt. Wallace Formation—Shown where subordinate clasts of Paleozoic proportion of mostly andesitic and carbonate and Tertiary volcanic basaltic flows predominate over rocks, mapped near Garnet Mountain volcaniclastic rocks, as mapped by in northeastern part of quadrangle the U.S. Geological Survey (1972). (McMannis and Chadwick, 1964). Compositions similar to those in Conglomerate is locally overlain by a Sepulcher Formation, described thin, discontinuous siltstone of late below. Conformably overlies rocks of early Eocene age. Not overlain by Sepulcher Formation volcanic rocks in quadrangle, but Tas Sepulcher Formation—Mafic and similar rocks in adjacent areas occur intermediate-composition at the base of volcanic sequences of volcaniclastic rocks, flow breccias, the Absaroka Volcanic Supergroup and lava flows. Contains sparse beds TKf Intrusive felsite (lower Tertiary or Upper of light-reddish-brown welded tuffs. Cretaceous)—Greenish-gray to Includes Fortress Mountain and Daly pinkish-gray, sparsely porphyritic, Creek Members (Smedes and massive, siliceous dacite or rhyolite. Prostka, 1972), and Hyalite Peak Most phenocrysts consist of as much Volcanics (Chadwick, 1969; Hiza, as 15 percent white, strongly saussur- 1994). Most of unit composed of tized plagioclase crystals as long as 3 dark-grayish-brown, laharic breccias, mm. Rock commonly stained black volcaniclastic conglomerate, and by manganese oxide. Mainly forms light- to medium-gray volcaniclastic thin sills, dikes, and irregular pods sandstone. Both laharic deposits intruding exclusively Archean rocks. (vent facies) and conglomeratic Described by Kellogg (1993b). Unit deposits (alluvial facies) are greater undated, but shares compositional than 100 m thick in places, crudely and textural similarities with both stratified, and well indurated. Laharic Upper Cretaceous dacite porphyry of deposits composed of unsorted, Fan and Lone Mountains and Eocene angular to subrounded volcanic debris rhyolite and rhyodacite plugs near containing clasts as large as 3 m Norris (Kellogg, 1994, 1995), about across. Both lahars and conglom- 10–15 km north of quadrangle erates contain much silicified wood, TKg Gabbro sills (Eocene to Late commonly as large trunks as thick as Cretaceous)—Black, medium- 3 m, in growth positions. grained plagioclase-clinopyroxene Lavas are mostly medium-gray to gabbro that typically weathers into dark-gray, fine-grained to sparsely spheroidal blocks. In places porphyritic pyroxene or olivine- completely weathered into grussy pyroxene andesite and basalt. Flows orange-brown soil. Several larger having compositions as silicic as sills contain irregular dikes, as wide latite or trachyandesite occur in upper as 2 m, of fine-grained, pinkish-gray, part of section (as on Steamboat inequigranular clinopyroxene-biotite Mountain). Proportion of flows syenite, in which the clinopyroxene appears to increase upward in section. forms conspicuous rods as long as

4 1 cm. Syenite, in turn, is intruded by percent green hornblende, 10–20 thin aplitic dikes. Unit crops out near percent microcline (slight develop- Meadow Village at Big Sky and ment of braid perthite) commonly as intrudes Upper and Lower Cretaceous phenocrysts as long as 3 cm, 0–20 rocks percent quartz, 5 percent biotite, 0–1 Kd Dacite porphyry of Fan and Lone percent clinopyroxene cores in some Mountains (Late Cretaceous)— hornblende, trace to 3 percent Gray to greenish-gray porphyritic magnetite, and traces of apatite and dacite that weathers to a very light conspicuous sphene and zircon; gray or tan. Euhedral to subhedral hypidiomorphic texture. Includes phenocrysts compose 30–50 percent minor dark-gray hornblende diorite of the rock; phenocrysts are 70–90 and granodiorite along border and in percent zoned plagioclase crystals satellitic stocks east of main batho-

(An25–35) as long as 1 cm, trace to 15 lith. Weathers light gray, in rounded percent green hornblende crystals as tors or flat grussy outcrops. K-Ar age long as 6 mm, 0–15 percent biotite of batholith is 71–74 Ma (Vitaliano flakes as long as 3 mm, and 0–10 and Cordua, 1979) percent equant quartz crystals as long Ks Sphinx Conglomerate (Upper Creta- as 5 mm. Mafic minerals are com- ceous)—Reddish-orange con- monly altered to chlorite and epidote; glomerate with sandstone matrix plagioclase is sericitized. Matrix is cemented by calcite and hematite. very fine grained and dense, and Contains well-rounded cobbles and contains about 5 percent opaque boulders derived from Paleozoic and minerals. Fine-grained chill zone Mesozoic strata shed from thrust extends inward several centimeters plates to west; represents an un- from contacts. Commonly contains roofing sequence, the clasts generally mafic autoliths as large as about 0.5 increasing in age upwards (DeCelles m. Forms sills, some greater than 80 and others, 1987). Lower contact is m thick, in two intrusive centers conformable with underlying Living- underlying Fan Mountain and Lone ston Formation. Well exposed only Mountain. The sills are thought to on Sphinx Mountain and The Helmet. form a “Christmas-tree laccolith” Minimum thickness about 610 m complex underlying both mountains Kb Beaverhead Group (Upper Cretaceous)— (Swanson, 1950), although Kellogg Conglomerate and interbedded (1992) found no evidence for a limestone containing well-rounded central “trunk.” Late Cretaceous beds clasts of Archean gneiss and Belt dip away from the central peaks at Supergroup quartzite. Mapped by both Fan and Lone Mountains. Hadley (1969b) in Gravelly Range as Intrudes rocks that range in age from two separate units (Sphinx Con- Middle Cambrian to Late Cretaceous. glomerate and Tertiary gravel), but Where less than about 20 m thick, both units were reinterpreted on fossil unit is indicated on map by a single and stratigraphic evidence as Beaver- line. Potassium-argon (K-Ar) and head Group rocks of Cretaceous age 40Ar/39Ar ages from hornblende are (J.M. O’Neill, oral commun., 1997) about 68–69 Ma (Tysdal and others, Kl Livingston Formation, undivided (Upper 1986; K.S. Kellogg and S.S. Harlan, Cretaceous)—Livingston Formation unpub. data, 1994) exposed on west side of Madison Kgd Granodiorite of Tobacco Root batholith Range, near Sphinx Mountain, and (Late Cretaceous)—Gray, coarse- east of Gallatin River, south of the grained, inequigranular to por- Spanish Peaks fault. Does not crop phyritic, massive hornblende-biotite out west of Madison River valley. granodiorite, monzogranite, and Undivided unit shown only in monzodiorite (classification of southeastern part of quadrangle. Streckeisen, 1976). Typically Description after Tysdal (1990) contains 50 percent normally zoned Klu Upper member—Cobble and boulder

oligoclase (An25–An28), 15–25 conglomerate composed of well-

5 rounded volcanic clasts in matrix of Ktc Telegraph Creek Formation (Upper coarse-grained volcaniclastic sand- Cretaceous)—Light-gray to dark- stone; upper part is predominantly brown, thin-bedded to medium- sandstone. Lower contact is con- bedded, feldspathic, calcite-cemented formable. Thickness as much as 180 sandstone and interbedded dark-gray m siltstone and mudstone. Middle Klm Middle member—Brown, maroon, and marker sequence is a 20-m-thick gray dacite to basalt flows, autoclastic white tuffaceous siltstone and sand- breccia, tuff breccia, and welded tuff; stone. Contains thin flaggy sandstone minor interlayered volcaniclastic beds in lower part. Conformably sandstone. Lower contact is con- overlies Cody Shale. Does not crop formable. Thickness estimated 300– out west of Madison River valley. 450 m Thickness about 200 m Kll Lower member—Complexly inter- Kc Cody Shale (Upper Cretaceous)—Upper tonguing units of dark-green to dark- part consists of thin-bedded black brown, medium- to coarse-grained, fissile shale that is interbedded with locally pebbly volcaniclastic sand- minor amounts of thin-bedded, stone, olivine basalt, mudflow brown, commonly bioturbaded breccia, volcaniclastic conglomerate, calcareous, fine-grained sandstone. and mudstone. Conformable with Lower part consists of black, fissile underlying Everts(?) Formation. shale and minor siltstone that Thickness ranges from about 60 to weathers dark gray. Does not crop 210 m) out west of Madison River valley. Ku Everts Formation, Virgelle Sandstone, Conformably overlies Frontier Telegraph Creek Formation, Cody Formation. Thickness about 300 m Shale, Frontier Formation, and Kf Frontier Formation (Upper Creta- Mowry Shale, undivided (Upper ceous)—Mostly a sequence of alter- Cretaceous)—May locally include nating black shale and light-gray to Muddy Sandstone and Thermopolis yellowish-tan, thin-bedded to very Shale. Mapped in area of Cretaceous thick bedded, cross-bedded sand- outcrops east of Gallatin River stone. In the Madison Range, sand- Kev Everts Formation and Virgelle Sandstone, stone ledges are as thick as 3 m; ratio undivided (Upper Cretaceous) of sandstone to shale is about 1:3. Everts Formation—Thin- to medium- Shale is locally carbonaceous or bedded, light-gray to dark-gray, coaly. Shale sequences are as thick poorly to moderately sorted, quartz- as about 20 m and commonly contain rich sandstone; intercalated with thin- equally spaced 5- to 10-cm-thick bedded, greenish-gray to dark-gray sandstone beds, spaced 15–20 cm mudstone and siltstone. Contains a apart. Contains several white porcel- few thin beds of dense limestone, lanite beds; one prominent porcel- porcellanite, and coal. About equal lanite bed in the Madison Range, quantities of sandstone and finer about 6 m thick, is about 15 m above grained sedimentary rocks. Does not base of sequence and displays well- crop out west of Madison River developed ball-and-pillow structures. valley. Conformably overlies Conformable with underlying Mowry Virgelle Sandstone. About 425 m Shale. Formation thickens greatly to thick the west; thickness in Madison Range Virgelle Sandstone—Thin- to thick- about 140–180 m while that in Gra- bedded, medium- to coarse-grained, velly Range is greater than 1,500 m light-brown to yellowish-brown, Kmo Mowry Shale (Upper Cretaceous)— trough-crossbedded quartz sandstone Mostly brownish-gray and greenish- that forms prominent white- gray tuffaceous mudstone and shale weathering ledges. Conformably and, in upper part, contains abundant overlies Telegraph Creek Formation. thin sandstone beds. Lower Vaughn Does not crop out west of Madison Member consists of conspicuous Valley. About 25–50 m thick gray, green, yellow, brown, orange,

6 pink, and cream-colored bentonitic ledge-forming salt-and-pepper sand- mudstone, porcellanite, siltstone, and stone that contains a basal chert- minor interbedded quartz sandstone, pebble conglomerate as thick as 1 m. although strikingly varicolored units Lower contact is unconformable. To- are missing in northern Madison tal thickness in the northern Madison Range. Poorly exposed in most Range is about 90 m (Kellogg, 1992, places. Unconformable with under- 1993a); in the Gravelly Range it is as lying Muddy Formation. Thickness thick as 170 m (Hadley, 1980), and in 90–180 m the southeastern part of the map area Kmt Muddy Sandstone and Thermopolis it as much as about 130 m thick Shale, undivided (Lower Cre- Ju Morrison Formation (Upper Jurassic), taceous) Ellis Group (Upper and Middle Muddy Sandstone—Thin- to medium- Jurassic), and Woodside Siltstone bedded, medium- to coarse-grained, and Dinwoody Formation (Lower brown to brownish-gray, poorly to Triassic), undivided moderately indurated, clayey, ledge- Jm Morrison Formation (Upper Jurassic)— forming salt-and-pepper sandstone; Upper 20–30 m is mostly black and locally contains mud chips as long as locally purple shale that contains 1 cm. In northern Madison Range, minor intercalated thin- to medium- formation typically exposed as upper bedded, rusty-brown and gray quartz and lower sandstone sequence sandstone that is lensoidal in places. interrupted by central, poorly exposed Lower, thicker part of the Morrison is shaly sequence. Thickness varies composed of thin-bedded, gray, widely; in the northern Madison yellow, orange, red, and green shale Range it is about 20–45 m thick; and siltstone interbedded with lesser farther south in the Madison Range it amounts of gray quartz arenite and is as great as 107 m (Tysdal, 1990); thin-bedded, brown limestone. in the Gravelly Range it is about 15 m Uncon-formable with underlying unit. thick (Hadley, 1969b, 1980) About 75–100 m thick Thermopolis Shale—Composed of an upper sequence (three-quarters of the J ed Swift Sandstone, Rierdon Limestone, and total thickness) that is black to dark- Sawtooth Formation of Ellis Group gray, locally carbonaceous, fissile (Upper and Middle Jurassic), shale and poorly indurated, thin- Woodside Siltstone (Lower bedded, silty brown sandstone. Triassic), and Dinwoody Formation Lower one-quarter of unit is thin- to (Lower Triassic), undivided—In medium-bedded, fine-grained, white most places this combined unit is to tan quartz arenite that contains poorly exposed. Total thickness black shale interbeds; ripple marks about 185 m and Liesegang bands are common in Swift Sandstone—Brown, medium- to sandstone beds. Unconformable coarse-grained glauconitic sandstone, contact with underlying Kootenai locally containing abundant chert Formation. Thickness throughout pebbles, and minor olive-green shale; quadrangle about 70–80 m glauconite commonly weathered to Kk Kootenai Formation (Lower Creta- orange limonitic clots. More cal- ceous)—Upper 10–15 m is medium- careous in eastern part of map area bedded light-gray, micritic, oolitic (Tysdal, 1990). Unconformably limestone that contains abundant overlies Rierdon Limestone. Less gastropod fossils in uppermost part. than 12 m thick Middle part is variegated red, purple, Rierdon Limestone—Thin- to thick- yellow, and gray shale, mudstone, bedded, yellowish-gray to brownish- siltstone, sandstone, and locally gray, fine-grained, oolitic limestone; nodular freshwater limestone. For- contains a few limy siltstone inter- mation typically weathers to a reddish beds. Bivalves Camptonectes sp. and soil. Lower 10–40 m is medium- to Gryphaea sp. locally abundant. coarse-grained, gray, well-indurated, Forms prominent cliffs near Shell

7 Creek in western Madison Range. rubbly appearance. Conformably Thickness about 8–30 m overlies Quadrant Sandstone. Sawtooth Formation—Interbedded Formation about 35–70 m thick, limestone and shale; weathers though is about 50 m thick in yellowish brown and is very poorly northern Madison Range exposed. Sawtooth unconformably Mqa Quadrant Sandstone (Pennsylvanian), overlies Dinwoody Formation. Amsden Group (Lower Penn- Thickness 25–55 m sylvanian and Upper Miss- Woodside Siltstone—Brick-red to issippian), and Snowcrest Range orange-red, thin-bedded siltstone and Group (Upper Mississippian), mudstone, interbedded with gypsum undivided and scattered thin beds of gray lime- Quadrant Sandstone—Medium- to stone. Mostly poorly exposed, thick-bedded, white to yellowish-tan, although forms prominent red soil. well-sorted, fine- to medium-grained, Thickness increases markedly to dolomite-cemented quartz arenite; southeast; nonexistent north of cross-beds common. Lower half and Porcupine Creek in Gallatin Range at least upper 15 m of formation (Simons and others, 1985) and only a contain a few medium-bedded, light- “feather edge” may exist in the yellowish-tan dolostone beds. Crops northernmost Gravelly Range out prominently. Conformable con- (Hadley, 1980). Unit previously tact with underlying Amsden Group. mapped as Chugwater Formation in Thickness about 75 m southeastern part of quadrangle Amsden Group—Varies widely, both in (Simons and others, 1985). Thick- stratigraphy and thickness. In central ness 0 to about 220 m (Tysdal, 1990) and southern Madison Range, and in Dinwoody Formation—Brown silty Gravelly Range east of Greenhorn sandstone, siltstone, and thin-bedded, thrust, unit consists of about 12–50 m brown limestone. Small brachiopods of brick-red, reddish-brown, and locally abundant. Weathers light pink calcareous siltstone, silty shale, yellowish brown and is poorly shale, sandstone, and light-gray exposed. Dinwoody unconformably limestone and yellowish-gray dolo- overlies Shedhorn Sandstone. mite. Along western Madison Range, Thickness ranges between about 20 upper part contains medium- to thin- and 80 m bedded, gray to grayish-tan dolo- Pmu Shedhorn Sandstone (Lower Permian), stone, locally quartzitic, that grades Quadrant Sandstone downward into gray limestone beds (Pennsylvanian), and Amsden (correlated by Kellogg, 1992, with Group (Lower Pennsylvanian and Middle and Lower Pennsylvanian Upper Mississippian), undivided— Devils Pocket Formation and Alaska Mapped in Garnet Mountain quad- Bench Limestone of Wardlaw and rangle by McMannis and Chadwick Pecora, 1985); this sequence overlies (1964), who used the term “Phos- maroon siltstone and shale that phoria Formation” instead of contain a few thin beds of gray “Shedhorn Sandstone.” Units limestone and dolostone (correlated described below with Lower Pennsylvanian Tyler Ps Shedhorn Sandstone (Lower Permian)— Formation of Wardlaw and Pecora, Mostly medium- to massive-bedded, 1985). Total thickness of Amsden gray, fine- to coarse-grained, very Group in western Madison Range is well indurated quartz-rich sandstone 88 m that contains white cherty stringers Snowcrest Range Group—Mapped and nodules. Thin, shaly phosphorite only in western Madison Range beds in middle of formation are rarely (Kellogg, 1992), although rocks of exposed. Dolomitic near base. In the group may have been either Gravelly Range, chert is locally unrecognized elsewhere or placed in massive and filled with voids, so that Amsden Group. Upper 57 m of chert outcrops commonly have a Snowcrest Range Group (Upper

8 Mississippian Lombard Limestone) is MDtj Three Forks Formation (Lower Missis- mostly thin- to medium-bedded, gray sippian and Upper Devonian) and limestone that becomes more dolo- Jefferson Formation (Upper Devo- mitic and shaly toward base. The nian), undivided Lombard overlies Kibbey Sandstone, Three Forks Formation—Mostly thin- which consists of about 75 m of thin- bedded, yellowish-orange to to medium-bedded, yellowish-gray to yellowish-tan siltstone and silty maroon, friable dolomitic sandstone, limestone containing a few thin sandy dolostone, and siltstone interbeds of brick-red-weathering Mbs Big Snowy Group (Upper Missis- siltstone. Weathers light yellowish sippian)—Mapped only west of tan. Contains a medium-gray, Greenhorn thrust in Gravelly Range irregularly bedded, approximately 12- (Hadley, 1969b, 1980); upper, thicker m-thick, rough-weathering limestone part consists of thin- to thick-bedded, about 25 m above base of unit (Logan pale olive gray, locally very fossili- Gulch Member). Formation poorly ferous limestone and minor dark- exposed; forms slopes and swales. greenish or olive-gray mudstone and Unconformably overlies Jefferson shale. Lower 60 m or so consists of Formation. Thickness 40–60 m red and greenish-gray shale and cal- Jefferson Formation—Thin- to thick- careous sandstone and minor bedded, black, brown, dark-gray, and greenish-gray cherty limestone; light-gray, petroliferous, medium- sandstone and red color increase crystalline to coarsely crystalline downward in sequence. Detailed dolostone; colors vary considerably description of Big Snowy Group is over short stratigraphic intervals. given by Hadley (1980). Total Weathers mostly brown, and outcrops thickness 135–275 m are typically knobby and irregular; Mm Madison Group, undivided (Upper and forms conspicuous brown hoodoos. Lower Mississippian) At least upper 15 m is thick- to Mmc Mission Canyon Limestone (Upper massive-bedded, gray dolostone and Lower Mississippian)— solution breccia (Birdbear Member). Medium- to massive-bedded, light- Unconformably overlies Bighorn gray, gray, and brownish-gray, Dolomite (?) in northern Madison medium-crystalline limestone and Range. Thickness about 110 m minor dolostone. Weathers light Orm Bighorn Dolostone(?) (Ordovician), Red gray. Chert stringers and nodules are Lion Formation (Upper Cam- common; locally fossiliferous; brian), Pilgrim Dolostone (Upper solution breccias in uppermost part. Cambrian), and Park Shale Prominent ridge-forming unit. (Middle Cambrian), undivided Conformable with underlying Bighorn Dolostone(?)—Medium-gray, Lodgepole Limestone. Thickness sugary dolostone in 0.2- to 1.0-m- about 240 m thick beds; weathers very light gray. Ml Lodgepole Limestone (Lower Missis- Tentatively placed in Bighorn Dolo- sippian)—Thin- to medium-bedded, mite by Hanson (1952) along ridge in gray to brownish-gray, finely crystal- NE¼, sec. 22 and NW¼, sec. 23, T. 5 line limestone that commonly grades S., R. 1 E. in northwestern Madison into silty limestone interbeds several Range; not recognized south of this centimeters thick. Locally cherty; location. Unconformably overlies upper half is profusely fossiliferous. Red Lion Formation. Thickness about Conformable with underlying Three 3 m at location noted above Forks Formation (erroneously Red Lion Formation—Thin-bedded, described as unconformable above medium-gray to tan, siliceous dolo- the Three Forks in Kellogg, 1992, stone containing conspicuous orange- 1993a). Thickness about 180 m tan to reddish-tan, cherty stringers as thick as 2 cm; lower 7 m contains intraformational clasts as large as 5 mm. Along west side of Gallatin

9 Range, contains a lower greenish- m Meagher Limestone (Middle gray, locally quartzitic shale sequence Cambrian)—Thin- to massive- as thick as 10 m (Dry Creek Shale bedded, light-gray to brownish-gray, Member) (McMannis and Chadwick, finely crystalline limestone. Locally 1964). Unconformably overlies oolitic, especially in upper part. Pilgrim Formation. Thickness about Upper 30 m is thin-bedded, gray 40–60 m limestone that contains conspicuous Pilgrim Dolostone—Gray, light-gray, orange mottles; upper few meters and brownish-gray, medium- to contains fissile gray-green shale. massive-bedded, locally oolitic, Middle 50 m is medium- to massive- medium-crystalline dolostone. bedded limestone, locally mottled tan, Weathers light gray and contains and containing small silicic limestone irregularly shaped darker gray stringers. Lower 30 m is thin- to mottles. Conformably overlies Park medium-bedded, tan-mottled, gray Shale. Forms conspicuous crags. limestone that contains a few inter- Thicknesses about 30 m in northern calated micaceous shale beds in lower Madison Range (Kellogg, 1992), part. Forms cliffs. Conformably about 60 m in the south-central part overlies Wolsey Shale. About 100– of the quadrangle (Tysdal, 1990); as 150 m thick in Madison Range much as 120 m in the Gravelly (Tysdal, 1990; Kellogg, 1992), 100– Range, although is missing along east 110 m thick in Gravelly Range side of range (Hadley, 1969b), and as (Hadley, 1980), but only about 50 m much as 75 m in the northern Gallatin thick in northern Gallatin Range Range (McMannis and Chadwick, (McMannis and Chadwick, 1964) 1964) wf Wolsey Shale and Flathead Sandstone, Park Shale—Greenish-gray to tan, undivided (Middle Cambrian) fissile shale. Poorly exposed, slope- Wolsey Shale—Mostly thin-bedded, forming unit conformably overlies greenish-gray, olive-drab, gray, and Meagher Limestone. Thickness grayish-brown micaceous sandstone, about 30–50 m in Madison Range and siltstone, and shale. Sandstone beds Gravelly Range, although is missing are wavy and bioturbated, contain along eastern side of Gravelly Range green and gray- and green-mottled (Hadley, 1969b); thickness 50–75 m shale interbeds, and generally weather in northern Gallatin Range brown; animal trails are common; (McMannis and Chadwick, 1964) locally glauconitic. Near middle of unit is a 10- to 15-m-thick section of u Red Lion Formation and Pilgrim thin-bedded, dark-gray, brown- Dolostone, undivided (Upper weathering argillaceous limestone Cambrian)—Mapped east of interbedded with lesser amount of Gallatin River and north of Spanish sandstone and shale. Upper 5 m is Peaks fault (McMannis and interbedded wavy-laminated, thin- Chadwick, 1964) bedded, gray limestone and gray, rp Red Lion Formation (Upper Cambrian), micaceous siltstone. Conformable Pilgrim Dolostone (Upper with underlying Flathead Sandstone. Cambrian), and Park Shale Forms slopes and swales. Thickness (Middle Cambrian), undivided— 30–65 m Mapped south of Cedar Creek and Flathead Sandstone—Thin- to medium- east of Madison Valley (Tysdal, bedded, medium-to coarse-grained, 1990; Kellogg, 1992) reddish-brown, tan, and purplish-tan, mi Park Shale, Meagher Limestone, Wolsey quartz-rich, feldspathic sandstone; Formation, and Flathead locally weathers to rusty red. Two Sandstone, undivided (Middle thin zones of fine-grained, micaceous, Cambrian)—Mapped east of greenish-gray argillaceous sandstone Gallatin River and north of Spanish near top of formation. Basal part of Peaks fault (McMannis and formation contains rounded pebbles Chadwick, 1964) of metamorphic rock. Unconformably

10 overlies Archean crystalline rock. Ams Mylonitic schist—Well-foliated chlorite- Thickness variable, from 15 to 75 m actinolite schist that contains di Diabase dike (Proterozoic)—Black to abundant shear-bounded bodies of dark-greenish-gray, fine- to medium- metagabbro, metadiorite, and grained, equigranular, well-indurated amphibolite. Interpreted as sheared diabase in steep, northwest-striking amphibolite (J.M. O’Neill, oral dikes as wide as 30 m (most are commun., 1995) and may be approxi- considerably thinner). Contains mately equivalent to biotite-chlorite about 30–50 percent euhedral labra- schist (unit Abs). Includes a thin dorite, 30–60 percent augite, 0–30 staurolite-sillimanite mylonite that percent hornblende (inverted from bounds southern contact of unit augite), 0–10 percent potassium Abg Biotite gneiss, quartzite, and hornblende feldspar, 5–8 percent opaque gneiss—Approximately as mapped minerals, 1–3 percent apatite, 0–5 by Hadley (1969b); part of quartz- percent biotite, 0–3 percent quartz, feldspar gneiss and pegmatite unit of and from trace to 1 percent epidote. Vargo (1990) Emplaced during two periods: about Agg Granite, migmatite, pegmatite, and 1,455 Ma and 750–780 Ma (Harlan granitic orthogneiss—Approxi- and others, 1990) mately as mapped by Hadley (1969b), who did not describe unit in detail. LOW-GRADE METAMORPHIC AND Not known if unit is intrusive into INTRUSIVE ROCKS OF THE RUBY CREEK adjacent units or forms older AREA, EASTERN GRAVELLY RANGE basement (PROTEROZOIC OR ARCHEAN) Aif Iron-formation—Alternating layers, [Metamorphic grade of rocks in this area is as low as chlorite typically 2–10 cm thick, of reddish- facies (quartz-albite-epidote-biotite subfacies of Turner and Verhoogen, 1960), but increases in grade southward to almandine- brown to black magnetite-quartz and amphibolite facies. The Ruby Creek assemblage is bounded on the quartzite. Contains minor hydro- north by a north-dipping thrust, which places marble in the biotite and magnetite locally oxidized hanging wall above the lower grade rocks of the Ruby Creek area. to hematite and goethite. Layers are The age of the assemblage is unknown, but may be as young as Proterozoic (J.M. O’Neill, oral commun., 1995). The geology for commonly tightly folded this area is modified from Vargo (1990) and Hadley (1969a,b)] META-IGNEOUS ROCKS (ARCHEAN) Amd Metadiorite—Mapped by Hadley (1969a,b) [Igneous rock names follow Streckeisen, 1976)] but not described in detail Ass Epidote-actinolite metasediment—Green, Agp Granite porphyry of Hell Roaring metamorphosed, fine-grained clastic Creek—Pink, coarse-grained, sedimentary rocks, locally showing massive to slightly foliated biotite good cross-bedding and graded beds. monzogranite porphyry. Contains Contains quartz, plagioclase, actino- about 20 percent conspicuous lite, and epidote. Weathers to blocky euhedral to subhedral potassium- outcrops feldspar phenocrysts as long as 1.5 Abs Biotite-chlorite schist—Dark-brownish- cm; matrix contains about 30 percent gray to black, well-foliated schist quartz, 35 percent plagioclase, 15 containing quartz, plagioclase, percent potassium feldspar, and about biotite, chlorite, magnetite, and 20 percent biotite. Locally sheared, muscovite; porphyroblasts of forming well-developed pinkish-gray plagioclase abundant augen gneiss, as on Indian Ridge in the Spanish Peaks Aph Phyllite—Greenish-gray, greenish-red, Agg Granitic orthogneiss—Light-gray to light gray-red, and dark-gray phyllite pinkish gray, generally tan- containing quartz, biotite, chlorite, weathering, medium-grained, muscovite, hematite, magnetite, and hypidiomorphic, weakly to rare garnet. Contains thin layers of moderately foliated orthogneiss micaceous and actinolite-bearing generally ranging in composition quartzite and lenticular amphibolite from tonalite to monzogranite. Mafic bodies (not shown) mineral almost exclusively biotite

11 (trace to 15 percent); may contain as and dike intrusion; sills and dikes much as 5 percent almandine and, locally so closely spaced that they rarely, 5 percent hornblende, 2 form an agmatite. Cut by granitic percent augite, and 2 percent musco- orthogneiss of Summit Lake pluton vite; also contains traces of zircon, Aum Meta-ultramafic rocks—Black to dark- epidote, allanite, and opaque greenish-gray, fine- to medium- minerals. One unusual occurrence is grained, well-foliated to massive, a pluton near Summit Lake in the variably serpentinized ultramafic Spanish Peaks; the pluton is a rocks of wide-ranging composition; discordant body composed of massive includes olivine websterite, lherzolite, to weakly foliated, very light gray and olivine clinopyroxenite. Acces- biotite tonalite sory minerals include olive-green Amb Metabasite—Black (commonly speckled spinel, magnetite, and apatite. with white feldspar and pink garnet), Commonly contains secondary fine-grained, equigranular, grano- amphibole (anthophyllite or actino- blastic, weakly foliated to massive lite) serpentine, talc, dolomite, mag- hornblende-augite-almandine meta- nesite, and (or) mica. Occurs in basite. Composition variable; con- lenses, pods, and small irregularly tains 15–45 percent plagioclase shaped masses, rarely more than 10 m (mostly andesine), 10–60 percent in diameter. Probably tectonically yellowish-green to brown hornblende, incorporated into country rock 2–20 percent augite, 0–20 percent almandine, 0–5 percent reddish- GNEISSIC ROCKS OF UNKNOWN ORIGIN brown biotite, 1–3 percent opaque (ARCHEAN) minerals, and trace of apatite. In Aqf Quartzofeldspathic gneiss— some places relict porphyritic texture Mapped where is preserved as white clusters of fine- rocks are heterogeneous, generally grained plagioclase crystals as long as layered, light- to medium-gray 1 cm. Mostly occurs as sills as wide microcline-plagioclase-quartz-biotite as about 40 m concordant to foliation; gneiss. Commonly migmatitic and in some places sills show pinch-and- blastomylonitic. Some areas not swell structure and boudinage. Com- mapped in detail, such as south of monly enveloped in medium-grained Sphinx Mountain in Madison Range amphibolite margin as wide as 10 m, and east of Gallatin River in Gallatin indicating post-emplacement meta- Range, which may contain many of somatism at amphibolite grade. the other gneissic rock units Equivalent to orthoamphibolite of Agn Garnetiferous gneiss of the Tobacco Root Vitaliano and Cordua (1979) in Mountains—Includes biotite-garnet Tobacco Root Mountains. schist, sillimanite-garnet schist, garnetiferous quartzite, quartzite, Ags Hornblende-biotite granodiorite orthogneiss of Summit Lake—Light- to highly garnetiferous quartzo- medium-gray, medium-grained, feldspathic gneiss, corundum gneiss, hypidiomorphic, poorly to well gedrite schist, cummingtonite schist, foliated hornblende-biotite grano- and garnetiferous amphibolite diorite orthogneiss. Contains 30–40 (Vitaliano and Cordua, 1979). May percent plagioclase, 10–20 percent represent refractory component potassium-feldspar, 15–25 percent (“restite”) remaining after removal of quartz, 10–20 percent hornblende, partial melts and about 10 percent biotite. Most Aam Hornblende-plagioclase gneiss and rock is highly strained, forming well amphibolite—Most is gray to black, developed foliation; in such places, medium-grained, hypidiomorphic mafic minerals are concentrated in equigranular, moderately foliated to ribbony layers 2–5 mm thick. Con- well-foliated hornblende-plagioclase tains numerous, thin (3–10 mm) gneiss and amphibolite; contains as feldspar-quartz migmatitic layers. much as 5 percent quartz and traces Cut by at least three periods of sill of zircon, opaque minerals, and

12 apatite; locally garnetiferous. sills of metabasite. Crops out exten-

Plagioclase is typically An30 and sively in Bear Trap Canyon of Madi- weathers white. Commonly contains son River. Similar rocks, also white, migmatitic leucosomes of mapped as unit Abh, occur at several anorthosite as thick as 10 cm. Similar other less extensive localities unit in Tobacco Root Mountains interpreted to be either metamor- HIGH-GRADE TECTONITES (ARCHEAN) phosed, clay-rich dolomite or mafic- extrusive rock (Vitaliano and Cordua, At Mylonite of the Crooked Creek shear 1979). Amphibolite envelopes zone—Light- to medium-gray, around some metabasite intrusive mottled mylonite that has a well- bodies indicate at least some amphi- developed L-S tectonite fabric and a bolite was derived from intrusive fine-grained recrystallized (grano- rocks. Unit may include minor blastic) texture. Composition amounts of other Archean units variable, ranging from that of Abs Biotite schist—Black, dark-gray, and gray, quartzofeldspathic gneiss to amphi- fine- to medium-grained biotite- bolite. The mylonite equilibrated at plagioclase-quartz, ±hornblende, near-peak (lower granulite) meta- ±microcline schist. Interpreted to be morphic conditions and forms ana- sheared mafic rock that equilibrated stomosing, concordant zones in at lower amphibolite facies; not northern Madison Range (Kellogg, studied in detail. Crops out in 1993a; Kellogg and Mogk, 1991). Gallatin Canyon–Hell Roaring Lake Margins of unit are concordant with, (Spanish Peaks) area and grade, into relatively unmylo- nitized gneiss Agd Gedrite and cummingtonite gneiss— Cummingtonite gneiss is light gray, medium grained, strongly lineated, METASEDIMENTARY ROCKS (ARCHEAN) and vitreous. Contains as much as 90 Aag Aluminous gneiss and schist—Gray to percent cummingtonite, and variable dark-brownish-gray, medium- amounts of quartz, almandine, grained, inequigranular, generally muscovite, rutile, and opaque well foliated, commonly micaceous minerals. Grades into metaquartzite. gneiss and schist containing alumino- Gedrite gneiss is brown to grayish- silicate (mostly sillimanite and rarer brown, moderately well foliated kyanite). Unit contains 5–90 percent gneiss that contains as much as 70 anhedral quartz having undulatory percent clove-brown gedrite, in extinction, 0–30 percent microcline, addition to quartz, plagioclase, 0–35 percent plagioclase, 0–30 ±sillimanite, ±biotite, ±cordierite, and percent almandine, 0–20 percent traces of magnetite and rutile. Both muscovite, trace to 15 percent silli- rock types occur as small lenses and manite or kyanite, 0–10 percent concordant layers in other Archean reddish-brown biotite, 0–2 percent rocks opaque minerals, including graphite, Abh Biotite-hornblende gneiss of Beartrap and trace of zircon. Commonly rich Canyon—White, light-gray, dark- in quartz and locally grades into gray, and black, medium-grained, quartzite. Several kyanite prospects well-foliated and well-layered gneiss. are on east side of Gravelly Range, in Leucosomes contain plagioclase, sec. 6, T. 8 S., R. 1 W. (Nordstrom, quartz, biotite, ± potassium feldspar, 1947) ± garnet, and a trace of opaque Abm Biotite-muscovite gneiss—Light- to minerals. Melasomes contain biotite, medium-gray, medium-grained, hornblende, plagioclase, quartz, ± poorly foliated to well-foliated garnet, and trace of opaque minerals. quartz-feldspar gneiss that contains Contains rare quartzite layers and abundant thin schistose layers sillimanite-bearing gneiss. Common- containing both biotite and musco- ly migmatitic. Layers are typically vite. Aluminosilicate-bearing lenses 1–20 cm thick. Injected by numerous are common, and one prominent 20-

13 to 50-m-thick schistose horizon ACKNOWLEDGMENTS contains as much as 50 percent coarse-grained biotite, about 20 The authors are deeply indebted to Robert Burger, percent quartz, 0–10 percent gedrite, Alicia Grogger, Stephen Hurd, Kathy Licht, Brian 5 percent plagioclase, 5 percent Lund, John Miller, Janine Pinnow, David Taylor, and sillimanite, 3 percent kyanite (as Angela Vasquez for their assistance in the field. large blue blades), 3 percent garnet, Unpublished field mapping and insightful discussions and 3 percent muscovite. Mapped on in the field with D.R. Lageson and J.M. O’Neill are north side of Hell Roaring Creek especially appreciated. O.D. Young provided valley in Spanish Peaks valuable assistance in the preparation of the final Aq Quartzite—White, medium- to course- digital layout. R.G. Tysdal and J.M. O’Neill grained, inequigranular, moderately performed careful and greatly appreciated reviews of foliated to massive quartzite; locally the map. bright green where trace amounts of chromium-bearing mica (fuchsite) are REFERENCES CITED present. In most places unit is composed entirely of anhedral quartz Bearzi, J.P., 1987, Soil development, morphometry, grains having undulatory extinction, and scarp morphology of fluvial terraces at Jack but locally contains as much as 30 Creek, southwestern Montana: Bozeman, percent microcline, 20 percent Montana State University, M.S. thesis, 131 p. muscovite, 15 percent sillimanite, 10 Chadwick, R.A., 1969, The northern Gallatin Range, percent cummingtonite, 8 percent Montana—northwestern part of the Absaroka- almandine, 2 percent actinolite, and Gallatin volcanic field: University of Wyoming trace of zircon and opaque minerals. Contributions to Geology, v. 8, no. 2, pt. 2, p. Commonly forms prominent ridges, 150–156. especially in northwestern part of Christiansen, R.L., and Blank, H.R., Jr., 1972, Madison Range, where core of a Volcanic stratigraphy of the Quaternary rhyolite plunging, nearly isoclinal antiform is plateau in Yellowstone National Park: U.S. composed almost entirely of Geological Survey Professional Paper 729–B, p. quartzite. Unit is interlayered in most B1–B18. places with mafic amphibolite DeCelles, D.G., and 15 others, 1987, Laramide Ama Marble—White, coarse-grained, massive to thrust-generated alluvial-fan sedimentation, moderately well foliated dolomitic Sphinx conglomerate, southwestern Montana: marble that contains as much as 3 American Association of Petroleum Geologists percent quartz grains. Weathers Bulletin, v. 71, p. 135–155. orange-brown. Locally hydro- Hadley, J.B., 1969a, Geologic map of the Cameron thermally altered to commercial quadrangle, Madison County, Montana: U.S. deposits of talc on east side of Geological Survey Geologic Quadrangle Map Gravelly Range. A few thin layers of GQ–813, scale 1:62,500. calcitic marble, containing as much as ———1969b, Geologic map of the Varney 30 percent serpentine, mapped in quadrangle, Madison County, Montana: U.S. northern part of Madison Range. Geological Survey Geologic Quadrangle Map Extensive, commercially exploited GQ–814, scale 1:62,500. talc deposits occur in hydrothermally ———1980, Geology of the Varney and Cameron altered zones in large marble body on quadrangles: U.S. Geological Survey Bulletin eastern side of Gravelly Range 1459, 108 p. Aif Iron-formation—Magnetite-bearing schist Hall, W.B., 1961, Geology of part of the upper and gneiss, gruernite-magnetite Gallatin Valley of southwestern Montana: schist, and garnet-quartz rock. Laramie, University of Wyoming, Ph.D. thesis, Mapped in Tobacco Root Mountains 239 p. (Vitaliano and Cordua, 1979) Hanneman, D.L., and Wideman, C.J., 1991, Sequence stratigraphy of Cenozoic continental rocks, southwestern Montana: Geological Society of America Bulletin, v. 103, p. 1335– 1345.

14 Hanson, A.M., 1952, Cambrian stratigraphy in Gallatin County, Montana: Montana Bureau of southwestern Montana: Montana Bureau of Mines and Geology Bulletin 43, 47 p. Mines and Geology Memoir 33, 46 p. Nordstrom, C.L., 1947, Geology of a kyanite deposit Harlan, S.S., Snee, L.W., and Geissman, J.W., 1990, near Ennis, Montana: Butte, Montana School of Paleomagnetic and 40Ar/ 39Ar results from mafic Mines, Senior thesis, 30 p. dikes and basement rocks, southwest Montana: Rasmussen, D.L., and Fields, R.W., 1983, Structural EOS, Transactions of the American Geophysical and depositional history, Jefferson and Madison Union, v. 71, p. 1297. basins, southwestern Montana [abs.]: American Hiza, M.M., 1994, Processes of alluvial sedimen- Association of Petroleum Geologists Bulletin, v. tation in Eocene Hyalite Peak volcanics, 67, p. 1352. Absaroka-Gallatin volcanic province, southwest Ritter, J.B., and 16 others, 1990, The Late Quaternary Montana: Bozeman, Montana State University, geology of Cedar Creek alluvial fan, Madison M.S. thesis, 163 p. River Valley, southwestern Montana, in Hall, Kellogg, K.S., 1992, Geologic map of the Fan R.D., ed., Quaternary geology of the western Mountain quadrangle, Madison County, Madison Range, Madison Valley, Tobacco Root Montana: U.S. Geological Survey Geologic Range, and Jefferson Valley; Rocky Mountain Quadrangle Map GQ–1706, scale 1:24,000. Friends of the Pleistocene Fieldtrip Guidebook: ———1993a, Geologic map of the Cherry Lake Indianapolis, Indiana University, p. 120–138. quadrangle, Madison County, Montana, U.S. Salt, K.L., 1987, Archean geology of the Spanish Geological Survey Geologic Quadrangle Map Peaks area, southwestern Montana: Bozeman, GQ–1725, scale 1:24,000. Montana State University, M.S. thesis, 81 p. ———1993b, Geologic map of the Ennis Lake Schneider, N.P., and Ritter, D.F., 1987, Late quadrangle, Madison County, Montana, U.S. Pleistocene response of the Madison River to Geological Survey Geologic Quadrangle Map regional base level change for the Madison GQ–1729, scale 1:24,000. Valley, southwest Montana [abs.]: Geological ———1994, Geologic map of the Norris quadrangle, Society of America Abstracts with Programs, v. Madison County, Montana, U.S. Geological 19, no. 5, p. 332. Survey Geologic Quadrangle Map GQ–1738, Simons, F.S., Van Loenen, R.E., and Moore, S.L., scale 1:24,000. 1985, Geologic map of the Gallatin Divide ———1995, Geologic map of the Bear Trap Creek roadless area, Gallatin and Park Counties, quadrangle, Madison County, Montana, U.S. Montana: U.S. Geological Survey Geological Survey Geologic Quadrangle Map Miscellaneous Field Studies Map MF–1569–B, GQ–1757, scale 1:24,000. scale 1:62,500. Kellogg, K.S., and Mogk, D.W., 1991, The Crooked Smedes, H.W., and Prostka, H.J., 1972, Stratigraphic Creek mylonite—A high-temperature zone of framework of the Absaroka Volcanic Supergroup ductile deformation in the Archean of the in the Yellowstone National Park region: U.S. northern Madison Range, southwestern Montana Geological Survey Professional Paper 729–C, p. [abs.]: Geological Society of America Abstracts C1–C33. with Programs, v. 23, no. 5, p. A59. Streckeisen, Albert, 1976, To each plutonic rock its Kellogg, K.S., Schmidt, C.J., and Young, S.W., 1995, proper name: Earth-Science Reviews, v. 12, p. Basement and cover-rock deformation during 1–33. Laramide contraction in the northern Madison Swanson, R.W., 1950, Geology of part of the Range (Montana) and its influence on Cenozoic Virginia City and Eldridge quadrangles, basin formation: American Association of Montana: U.S. Geological Survey Open-File Petroleum Geologists Bulletin, v. 79, p. 1117– Report 51–4, 12 p. 1137. Todd, S.G., 1969, Bedrock geology of the southern Lauer, T.C., 1967, Stratigraphy and structure of the part of Tom Minor Basin, Park and Gallatin Snowflake Ridge area, Gallatin County, Counties, Montana: Bozeman, Montana State Montana: Corvallis, Oregon State University, University, M.S. thesis, 62 p. M.S. thesis, 182 p. Turner, F.J., and Verhoogen, John, 1960, Igneous and Marvin, R.F., Wier, K.L., Mehnert, H.H., and metamorphic petrology: McGraw-Hill Book Merritt, V.M., 1974, K-Ar ages of selected Company, Inc., New York, 694 p. Tertiary igneous rocks in southwestern Montana: Tysdal, R.G., 1990, Geologic map of the Sphinx Isochron/West, no. 10, p. 17–20. Mountain quadrangle and adjacent parts of the McMannis, W.J., and Chadwick, R.A., 1964, Cameron, Cliff Lake, and Hebgen Dam Geology of the Garnet Mountain quadrangle, quadrangles, Montana: U.S. Geological Survey

15 Miscellaneous Investigations Series Map I–1815, Vitaliano, C.J., and Cordua, W.S., 1979, Geologic scale 1:62,500. map of southern Tobacco Root Mountains, Tysdal, R.G., Marvin, R.F., DeWitt, E.H., 1986, Late Madison County, Montana: Geological Society Cretaceous stratigraphy, deformation, and of America Map and Chart Series MC–31, scale intrusion in the Madison Range of southwestern 1:62,500. Montana: Geological Society of America Wardlaw, B.R., and Pecora, W.C., 1985, New Bulletin, v. 97, p. 859–868. Mississippian-Pennsylvanian stratigraphic units U.S. Geological Survey, 1972, Geologic map of in southwest Montana and adjacent Idaho: U.S. Yellowstone National Park: U.S. Geological Geological Survey Bulletin 1656, p. B1–B9. Survey Miscellaneous Field Studies Map MF– 1605–B, scale 1:96,000. Wier, K.L., 1982, Map showing geology and Vargo, A.G., 1990, Structure and petrography of the outcrops of the Virginia City and Alder pre-Beltian rocks of the north-central Gravelly quadrangles, Madison County, Montana: U.S. Range, Montana: Fort Collins, Colorado State Geological Survey Miscellaneous Field Studies University, M.S. thesis, 157 p. Map MF–1490, scale 1:12,000.