OBJECTID,DescriptionOfMapUnits_ID,MapUnit,Label,Name,FullName,Age,Description,GeneralLithology,GeneralLithologyConfidence,HierarchyKey,Symbol,AreaFillRGB,DescriptionSourceID
1,DMU01,Qa,Qa,Alluvium,Holocene alluvium,Holocene,"Composite unit of unconsolidated cobble, gravel, sand, silt, and clay. Formed as alluvial deposits in valleys of active streams and in alluvial fans. Includes reworked moraine and outwash deposits. Thickness variable, generally less than 5 m (16 ft) thick.","Alluvial sediment, mostly coarse-grained",certain,1-01,Qa,"255,253,237",DAS10
2,DMU02,Qls,Qls,Landslide deposits,Holocene and Pleistocene landslide deposits,Holocene and Pleistocene,"Debris from landslides. Most common in areas underlain by Eocene volcanic rocks (Tc) and Eocene to Miocene sedimentary deposits (Tsl, Tsu, and Tm). Thickness variable, some deposits as much as 20 m (66 ft) thick.","Debris flows, landslides, and other localized mass-movement sediment
Debris flows, landslides, and other localized mass-movement sediment
Debris flows, landslides, and other localized mass-movement sediment",certain,1-02,Qls,"255,225,0",DAS10
3,DMU03,Qg,Qg,Glacial deposits,Holocene and Pleistocene glacial deposits,Holocene and Pleistocene,"Unconsolidated boulder, cobble, and sand deposits in glacial moraines and in extensive glacial outwash deposits. Locally includes glacial deposits reworked by active streams. Thickness variable, generally 5–20 m (16–66 ft) thick.","Ice-contact and ice-marginal sediment, mostly coarse-grained",certain,1-03,Qg,"255,230,148

",DAS10
4,DMU04,Qt,Qt,Terrace deposits,Holocene and Pleistocene terrace deposits,Holocene and Pleistocene,"Unconsolidated deposits perched in terraces above modern streams and forming ridge-capping gravels. Unconsolidated boulder to pebble conglomerate interbedded with sand and mud. Accumulated as stream, alluvial fan, and colluvial debris. Locally includes lesser landslide and glacial outwash deposits. Thickness variable, generally less than 20 m (66 ft) thick.","Alluvial sediment, mostly coarse-grained",certain,1-04,Qt,"255,228,107",DAS10
5,DMU05,Tm,Tm,Mudstone deposits,Middle Miocene mudstone deposits,Middle Miocene,"Thin- to thick-bedded, brownish mudstone, siltstone, and sandstone containing thin pebble-conglomerate lenses (M’Gonigle, 1994; VanDenburg, 1997). Contains beds with distinctive horse, camel, and rhinoceros fossil assemblages (M’Gonigle, 1994). Preserved near Bannock Pass in southern Horse Prairie basin, a feature that extends across the Montana-Idaho border. Named “mudstone unit” by M’Gonigle (1994) and referred to as “sedimentary rocks of Bannock Pass” by VanDenburg and others (1998). Commonly overlain by ridge-capping gravels (Qt). Angular unconformity at base, above the older basin-fill units (Tsu, Tsl, and Tc). Probably age-equivalent to strata along west side of central Beaverhead Mountains south of Yearian Creek. As much as 230 m (755 ft) thick but poorly exposed.","Mostly mudstone, interbedded with other sedimentary rocks which locally may include coarser grained clastics (sandstone, conglomerate), carbonates, and/or coal",certain,2-01,Tm,"254,208,158",DAS10
6,DMU06,Tsu,Tsu,"Basin-fill deposits, upper","Lower Miocene to Oligocene basin-fill deposits, upper",Lower Miocene to Oligocene,"Basin-fill deposits exhibiting distinctly diverse characteristics dependent on structural setting. Unit present in Horse Prairie basin on east side of central Beaverhead Mountains. Named “sedimentary rocks of Everson Creek” and correlated with units on west side of Horse Prairie basin by VanDenburg and others (1998). In Horse Prairie basin, unit consists of interbedded shale, siltstone, sandstone, and metasandstone- and siltite-pebble conglomerate. Conglomerate localized in paleovalleys. Local thin lacustrine limestone lenses (VanDenburg, 1997). Sediments are locally tuffaceous. Angular unconformities at top and base of unit. Along west side of the Beaverhead Mountains between Sandy and Kenney Creeks, unit is coarser than equivalent rocks on east side of central Beaverhead Mountains. Unit composed of cobble to boulder conglomerates (Blankenau, 1999). Deposited in active range-front-fault environment. During deposition, Horse Prairie basin was relatively quiescent, and these deposits exhibit less influence of syndepositional faulting than age-equivalent deposit on western flank of range. As much as 2,000 m (6,562 ft) thick in Horse Prairie basin (M’Gonigle, 1994; VanDenburg, 1997).",Conglomerate Conglomerate,certain,2-02,Tsu,"255,223,182",DAS10
7,DMU07,Tsl,Tsl,"Basin-fill deposits, lower","Oligocene to Upper Eocene basin-fill deposits, lower",Oligocene to Upper Eocene,"Composite unit of sandstone, conglomerate, limestone, and shale. Boulders (less than about 0.5 m in diameter), cobbles, and pebbles in conglomerate mainly from Mesoproterozoic metasedimentary rocks of the locally exposed Apple Creek (Yac), Gunsight (Yg), and Swauger (Ys) Formations; deposits in upper Pattee Creek and Gould Basin contain cobbles to boulders of Mesoproterozoic granite (as much as 2 m in diameter). Most conglomeratic deposits contain minor volcanic clasts; some sandstones and shales are tuffaceous. are fine grained to aphanitic, generally laminated, and platy. Small gastropod fossils are common (Flores and M’Gonigle, 1991). Formed in fresh-water lakes and hot springs.  Top and base of unit are angular unconformities. Within the section, angular unconformities and lenticular layers are also common (Lucchitta, 1966; VanDenburg and others, 1998). These are the “Medicine Lodge beds” of Scholten and others (1955) and M’Gonigle and Hait (1997) and also called “sediments of Bear Creek” by VanDenburg and others (1998). Deposited during early phases of extensional faulting that led to local structurally controlled paleovalleys and to present basin and range topography (VanDenburg and others, 1998; Janecke and others, 2000). The very large boulders of Mesoproterozoic augen gneiss in upper Pattee Creek and Gould Basin were interpreted as evidence of major river drainages in which the boulders traveled about 65 km from locations where Mesoproterozoic granite is presently exposed (Janecke and others, 2000). However, the present mapping determined that the Cow Creek thrust fault is concealed beneath the Gould Basin boulder occurrence and, because this fault carries the granite (Lund and others, 2003a, b), it is possible that granite is also in the buried upper thrust plate. Thus, there may have been a more proximal source for the granite boulders prior to development of the present Gould Basin topography. As thick as 1,900 m (6,234 ft) (M’Gonigle and Hait, 1997; VanDenburg and others, 1998).",Clastic sedimentary rock,certain,2-03,Tsl,"255,234,204
",DAS10
8,DMU08,Tc,Tc,"Challis Volcanic Group, undivided","Eocene Challis Volcanic Group, undivided",Eocene,"Composite acidic- to intermediate-composition tuff and flow units of the Challis Volcanic Group. Regionally, extrusive units mostly erupted about 51–45 Ma, although minor activity persisted to about 39 Ma. There are four informal members in the map area, which are not mapped separately in this study. The youngest part of the volcanic succession is tannish to greenish, quartz-sanidine welded tuffs with interlayered dark dacite lava flows and conglomerate. These overlie the regionally recognized greenish-white, crystal-vitric, biotite-bearing, “rhyolite tuff of Curtis ranch”. In the tuff of Curtis ranch, crystal fragments are predominantly quartz and plagioclase.  Some exposures contain abundant volcanic lithic fragments and upper part contains flow-breccia layers. Tuffs throughout upper part of the volcanic succession are compacted and locally welded. Upper part of the volcanic succession is as much as 400 m (1,312 ft) thick where most completely preserved. The middle part of the volcanic succession consists of thick andesitic, basaltic, and rhyodacitic flow and flow-breccia units with several thin interlayered rhyolite tuffs. The most widespread unit in the middle part of the volcanic section contains dark olive-green, to gray, to reddish-brown, aphanitic to fine-grained, compound basalt to rhyodacite flows. These flows are commonly porphyryitic. Augite phenocrysts are common and biotite phenocrysts are also present. Matrix contains mostly plagioclase but potassium feldspar is in some flows and some flow units have relatively high potassium contents. Flows have basal flow breccias and vesicular-textured tops. These basalt-rhyodacite flows are the most widespread volcanic rocks in the map area. These middle-unit flows form the base of the Challis Volcanic Group in the fault block south of the Lemhi Pass normal fault. The thin rhyolites interlayered in the middle unit are composed of light-colored, crystal, vitric, or lithic tuffs. Middle unit is as much as 1,100 m (3,608 ft) thick. The oldest part of the volcanic succession is composed of interlayered tuff and conglomerate units, including basal conglomerate. Greenish-white, unwelded rhyolite vitric tuff locally forms the top of the basal part of the volcanic succession in the study area. This vitric tuff bears minor tuffaceous and crystal fragments and is as much as 125 m (410 ft) thick. The bulk of the basal unit is light pinkish-gray, poorly welded to unwelded rhyolite tuff. The distinctive feature of the basal tuff is abundant lithic fragments, especially clasts of the fine-grained metasandstone and siltite country-rock, but also vitric, crystal, and tuffaceous clasts. The fragments are generally less than 1 centimeter (cm) in diameter and subangular to subrounded. Conglomerate, predominantly bearing metasedimentary country-rock pebbles to cobbles is locally interbedded between the two rhyolitic units. Basal unit of the volcanic succession is as much as 500–650 m thick.","Extrusive igneous material
Extrusive igneous material",certain,3-01,Tc,"252,234,204",DAS10
9,DMU09,Tr,Tr,Rhyolite dike,Eocene rhyolite dike,Eocene,"Medium-gray, aphanitic matrix, fine-grained, feldspar-phenocrystic rhyolite dikes.","Fine-grained, felsic-composition intrusive igneous rock",certain,4-01,Tr,"243,112,33",DAS10
10,DMU10,Tg,Tg,Granite,Eocene granite,Eocene,"Light gray, hornblende-biotite granite and porphyritic granite. Also, fine-grained, hornblende-biotite granodiorite in margin of granite south of Little Eightmile Creek and in other small exposures. Intruded into reactivated, steep, transpressional parts of fault zones and ductilely deformed rocks along and near Little Eightmile Creek. The largest exposure, south of the mouth of Little Eightmile Creek, is surrounded by an about 200-m (656 ft)-wide contact metamorphic hornfels zone in the Apple Creek Formation.","Coarse-grained, felsic-composition intrusive igneous rock
Fine-grained, mafic-composition intrusive igneous rock",certain,4-02,Tg,"243,113,96",DAS10
11,DMU11,Td,Td,Mafic dike ,Eocene mafic dike,Eocene,"Black to dark-gray, fine-grained, mafic intrusive rock in thin dikes. Reported compositions include diorite and basalt. Commonly near the faulted contact between Apple Creek and Gunsight Formations and in fold hinges and fracture zones (for example, Copper Queen mine area). Also several small bodies in exposures of Challis Volcanic Group.","Fine-grained, mafic-composition intrusive igneous rock
Coarse-grained, felsic-composition intrusive igneous rock",certain,4-03,Td,"246,150,121",DAS10
12,DMU12,Trd,^d,Dinwoody Formation,Lower Triassic Dinwoody Formation,Lower Triassic,"Yellow-tan, fissile, silty shale and interbedded yellowish-weathering, dark-gray limestone. Poorly exposed. Base unconformable. About 300 m (984 ft) thick.","Marine sediment, mostly fine-grained",certain,5-01,Trd,"153,212,192",DAS10
13,DMU13,Pp,Pp,Phosphoria Formation,Permian Phosphoria Formation,Permian,"Black chert, gray carbonate rocks, and bluish siltstone preserved in the southern range crest in the map area. Upper part of preserved Phosphoria Formation is bedded black chert with thin beds of dolostone and limestone. Chert beds exhibit characteristic bluish-white phosphatic spots on surface. Chert is about 50 m (164 ft) thick and equivalent to “Tosi Chert Member” of McKelvey and others (1959). Lower part of formation is dark-gray dolostone and minor limestone containing dark-gray-to-black, ropy chert nodules, mapped as “Phosphoria and Park City Formations, undivided” by Lucchitta (1966). About 280 m (919 ft) thick. Lowest exposed beds are bluish siltstone with millimeter-scale, orange-weathering fragments. Lowest beds less than 30 m (98 ft) thick. These rocks were deposited in the western extent of the Phosphoria epicontinental basin. Lower contact with the Snaky Canyon Formation (PMs) is gradational. Where structural complexity makes it difficult to assign these beds to appropriate formation because lowest beds are not present, some exposures of Phosphoria Formation may be combined with the upper Juniper Gulch Member of the Snaky Canyon Formation. Upper contact is erosional. Locally, unit is unconformably overlain by Lower Triassic Dinwoody Formation (Trd) or Challis Volcanic Group (Tc). Total thickness is about 260 m (853 ft) (Lucchitta, 1966). Rocks of the Phosphoria epicontinental basin have potential for phosphate resources as well as byproduct U, F, Cd Cr, Ni, Mo, Ag, rare earth elements, and black-shale vanadiumV resources (Oberlindacher and Hovland, 1979). Base of the unit is exposed in many bulldozer-cut prospects made during past uranium exploration. Phosphate in the Phosphoria Formation forms pelletal phosphorite interbedded with organic-matter-enriched mudstone and siltstone, limestone, dolomite, and chert. Potential economic interest is restricted to shales in the Phosphoria Formation, as described by Oberlindacher and Hovland (1979). Total thickness is about 260 m (853 ft) (Lucchitta, 1966).","Marine sediment, mostly fine-grained",certain,5-02,Pp,"171,225,250",DAS10
14,DMU14,PMs,PMs,"Snaky Canyon Formation, undivided","Lower Permian to Upper Mississippian Snaky Canyon Formation, undivided",Lower Permian to Upper Mississippian,"Upper and lower carbonate members and a middle sandstone member comprise the Snaky Canyon Formation. The Juniper Gulch Member, upper member of the Snaky Canyon, is medium-bluish-gray, sandy dolostone with interbedded quartz sandstone layers. Amount of interbedded sandstone decreases upward, grading to predominantly light- to medium-gray, cherty dolostone. The underlying Gallagher Peak Sandstone Member, the middle member, is similar to the older Upper Mississippian Bluebird Mountain Formation (Mb) but with larger proportion of calcareous cement. The basal Bloom Member is medium- to dark-gray, cherty limestone with common chert nodules and interbedded thin quartz sandstone similar to sandstone of Bluebird Mountain Formation. Lower contact is abrupt, at base of lowest carbonate beds; upper contact is gradational. Different facies in thrust duplex and Rocky Canyon subdomains of southeastern part of map. Rocks in the Rocky Canyon subdomain contain more sandstone and quartzite, in comparison to rocks in the thrust duplex subdomain, and may be more properly mapped as the Quadrant Formation, as used by Lucchitta (1966), now considered an eastern, cratonal equivalent facies (Skipp, Hoggan, and others, 1979). Type section for formation is in southern Beaverhead Mountains, where total thickness is 1,350 m (4,429 ft) (Skipp, Hoggan, and others, 1979). In the study area, the Snaky Canyon Formation totals about 560 m (1,837 ft) thick (reinterpreted from Lucchitta, 1966).","Marine sediment, mostly fine-grained",certain,5-03,PMs,"145,201,237",DAS10
15,DMU15,Mb,Mb,Bluebird Mountain Formation,Upper Mississippian Bluebird Mountain Formation,Upper Mississippian,"Light-brown weathering, very fine grained, light-gray, quartzitic sandstone and lesser quartzite, calcareous sandstone, and sandy limestone. Unfossiliferous. Lower contact is gradational, at base of first carbonate-cemented quartz sandstone; upper contact is abrupt. Quartz sands derived from inner craton (Skipp, Hoggan, and others, 1979). Only recognized in small area on west side of Railroad Canyon, directly overlying type section of the Railroad Canyon Formation. 105 m (344 ft) thick (Skipp, Hoggan, and others, 1979).","Marine sediment, mostly fine-grained",certain,5-04,Mb,"155,165,198",DAS10
16,DMU16,Mr,Mr,Railroad Canyon Formation,Upper Mississippian Railroad Canyon Formation,Upper Mississippian,"Pink-, yellow-, orange-weathering, dark-gray shale, mudstone, silty limestone, and calcareous siltstone. Strata are thin bedded to fissile and flaggy. Dark shale and siltstone have a musty odor. Lower and upper contacts are conformable and relatively abrupt. Unit is poorly exposed and the surface expression is as swales and valleys, in which springs and slumps are preferentially located. Basal strata are siltstone and mudstone of shallow-water origin, overlain by deep-subtidal shale and by cycles of deep- to intermediate-subtidal calcareous siltstones (Batt and others, 2007). Formation mapped previously as Big Snowy Group (Lucchitta, 1966) and Big Snowy Formation (Skipp, 1984). Unit occupies a narrow facies belt formed in an intraramp basin known from the Beaverhead Mountains study area (Skipp, Sando, and Hall, 1979; Batt and others, 2007). Considered facies equivalent to the South Creek, Surrett Canyon, and Arco Hills Formations of the foredeep carbonate bank sequence of Idaho and the Big Snowy Formation of the cratonal section of Montana (Skipp, Sando, and Hall, 1979; Wardlaw and Pecora, 1985). Shale in unit commonly acts as locus for faults that in many places excise all or part of formation. Slivers of unit Mr are preferentially silicified along fault zones; narrow zones of silicified siltstone are common indicators of faults in southern part of map area. Railroad Canyon Formation is 205 m (673 ft) thick at type section on west side of upper Railroad Canyon (Wardlaw and Pecora, 1985).","Marine sediment, mostly fine-grained",certain,5-05,Mr,"177,188,210",DAS10
17,DMU17,Ms,Ms,Scott Peak Formation,Middle Mississippian Scott Peak Formation,Middle Mississippian,"Medium-gray, medium- to thick-bedded, mostly pure limestone. Much of unit contains orange-weathering, wispy and bedded, dark-gray chert. Beds are fossiliferous, with corals and brachiopods predominant; encrinite layers are common but not diagnostic. Thin-bedded strata were described in the upper part by Lucchitta (1966). Typically forms cliffy exposures. Intensely cleaved as the result of deformation associated with thrust faulting. Brecciated and recemented by calcite near low-angle normal faults. Previously mapped as Madison Limestone following cratonal basin terminology (Lucchitta, 1966) prior to more recent facies interpretations that the unit in the central Beaverhead Mountains was deposited in a miogeoclinal setting (Skipp, Sando, and Hall, 1979). Although commonly thinned by low-angle normal faults, unit may be as much as 700 m (2,296 ft) thick (Skipp and others, 1984).",Carbonate sediment,certain,5-06,Ms,"201,208,218",DAS10
18,DMU18,Mm,Mm,Middle Canyon Formation,Middle and Lower Mississippian Middle Canyon Formation,Middle and Lower Mississippian,"Silty limestone with shale partings and thinly interbedded shales. Limestones are in thin to medium beds that originated as carbonate mudstone (Skipp, Sando, and Hall, 1979). Base is exposed on the south side of Goat Mountain and, there, determined to be an erosional unconformity. Unit formed in lower portions of prograding carbonate-bank complex (Huh, 1967). Also preserved in Thompson Gulch thrust plate where unit was transformed into thinly laminated, fine-grained, dark-gray phyllitic marble and calcareous phyllite. Previously mapped as Lodgepole Formation following cratonal basin terminology (Lucchitta, 1966) but herein included in shelf (miogeoclinal) succession (Skipp, Sando, and Hall, 1979). About 250 m (820 ft) thick but complicated by isoclinal folding and thrust faulting.",Carbonate sediment,certain,5-07,Mm,"152,181,209",DAS10
19,DMU19,Dj,Dj,Jefferson Formation,Upper Devonian Jefferson Formation,Upper Devonian,"Primarily dark-gray dolostone with lesser amounts of limestone, limestone evaporite-solution breccia, and sandy dolostone. Tectonic brecciation of unit in most locations makes identification of bedding difficult. Base of unit is in angular unconformity with underlying Mesoproterozoic, Ordovician, and Silurian rocks. Ruppel and Lopez (1988) reported a maximum thickness of about 825 m (2,707 ft) in Lemhi Range to the west where formation includes Middle and Upper Devonian members older than rocks deposited in this map area; unit abruptly thins from Lemhi Range toward Beaverhead Mountains but thickens eastward into southwestern Montana (Sloss, 1954; Scholten, 1957; Scholten and Hait, 1962; Grader and Dehler, 1998). Jefferson Formation in Lemhi Range and Beaverhead Mountains was deposited in miogeoclinal setting but was deposited in the cratonal basin east of the study area. Preserved in lower thrust slivers of Baby Joe imbricate-thrust system along western range front. About 60 m (197 ft) thick in study area where only upper part of formation is present.",Carbonate sediment,certain,5-08,Dj,"177,154,202",DAS10
20,DMU20,SOs,SOs,Saturday Mountain Formation,Lower Silurian to Middle Ordovician Saturday Mountain Formation,Lower Silurian to Middle Ordovician,"Upper part is medium-dark-gray to medium-gray, finely crystalline dolostone that weathers to distinctly mottled lighter gray colors. Black chert nodules are also common as well as numerous beds with abundant fossils and fossil fragments. This grades downward into yellowish-gray to medium-gray, finely crystalline dolostone characterized by a network of irregular, interlaced, hairlike wisps and veinlets of white dolostone and locally by abundant black chert nodules. Most exposures in Beaverhead Mountains are the yellowish-gray lower dolostone. Subdivided into several members to the west in the Lemhi Range (Ruppel and Lopez, 1988). The two rock types that are in the Beaverhead Mountains, are probably equivalent to lower part of formation as mapped in the Lemhi Range. Top of formation is an erosional unconformity. About 20 m (66 ft) thick.","Marine sediment, mostly fine-grained",certain,5-09,SOs,"199,178,214",DAS10
21,DMU21,Ok,Ok,Kinnikinic Quartzite,Middle Ordovician Kinnikinic Quartzite,Middle Ordovician,"White to light-gray, fine- to medium-grained, vitreous orthoquartzite. Unit is compositionally supermature and well sorted. Subrounded quartz grains are tightly cemented and, in many cases, are difficult to distinguish from the quartz cement. Where lower contact is not structural, unit rests unconformably on Cambrian–Ordovician Beaverhead pluton (OCg) . Although determined to have been deposited on the Beaverhead pluton in the restricted exposures of the present map area, detrital zircon analyses do not indicate grains of Late Cambrian age either in the Beaverhead Mountains (one sample from Italian Gulch in present study area) or regionally (Baar, 2009). Deposited in foreshore to shoreface environments (James and Oaks, 1977). Preserved only in Baby Joe imbricate-thrust system along western range margin. As much as 30 m (98 ft) thick.",Quartzite,certain,5-10,Ok,"149,192,194",DAS10
22,DMU22,OCd,O_d,Dolostone,Middle Ordovician? to Upper Cambrian? Dolostone,Middle Ordovician? to Upper Cambrian?,"Fine-grained, greenish-gray, fossiliferous dolostone and dolomitic limestone. Thin, light-orange wisps common. Distinctive one- to two-cm (<1 -in) rip-up clasts, indicating possible tidal reworking of mud-cracked deposits. Unit preserved between underlying quartzite-pebble conglomerate (CZc) or quartzite of Mesoproterozoic Swauger Formation (Ys) and overlying Upper Mississippian to Lower Permian Snaky Canyon Formation (PMs). Both upper and lower contacts are unconformable. Locally exposed in far eastern extent of Grizzly Hill domain, in bulldozer cuts in upper Hood Gulch and about 1 km (0.6 mi) east of there. These rocks are unlike other carbonate units to the west; they are most similar to thin Middle Cambrian shale and carbonate adjacent to the eastern edge of the map where they were probably included in Summerhouse Formation by M’Gonigle (1994). May represent a remnant of the western edge of cratonal-shelf sedimentary rocks, possibly equivalent to the Middle Cambrian Meagher Limestone in southwest Montana. About 15 m (50 ft) thickness preserved.","Mostly carbonate rock, interbedded with clastic sedimentary rock",certain,5-11,OCd,"253,225,212",DAS10
23,DMU23,Cf,_f,Flathead Quartzite,Middle Cambrian Flathead Quartzite,Middle Cambrian,"Light gray quartz sandstone to quartzite. Quartz grains rounded. Some layers contain minor iron-oxide cement, causing reddish-tan weathering. Lower beds are arkosic and contain grit (very coarse sand to fine pebble) layers. Upper beds bear scolithos tubes. Base deposited unconformably on Paleoproterozoic gneiss (Xg). Unit only on lowest thrust plate at eastern edge of study area where Mesoproterozoic strata are absent and Paleozoic rocks were deposited directly on Paleoproterozoic basement. About 7 km (4.3 mi) northwest of the northernmost exposure of Flathead Quartzite (along the eastern map margin) upper beds of quartzite are overlain by greenish, micaceous (glauconitic) shales typical of the Middle Cambrian Wolsey Shale. These characteristics led to reinterpretation of the quartzite from Lower Ordovician Summerhouse Formation of the Idaho miogeoclinal succession as interpreted by M’Gonigle (1994) to the Middle Cambrian Flathead Quartzite and Wolsey Shale of the cratonal basin succession. Thickness variable, as much as 70 m (230 ft) thick.",Fine-grained sandstone with quartzite,certain,5-12,Cf,"231,184,167",DAS10
24,DMU24,CZc,_Zc,Conglomerate,Lower Cambrian? to Neoproterozoic? Conglomerate,Lower Cambrian? to Neoproterozoic?,"Predominantly quartzite-pebble to -cobble conglomerate and quartzite. Most recognized exposures are quartzite with interbedded conglomerate. Where conglomerate is lacking, unit is difficult to distinguish from and probably included with underlying Swauger Formation (Ys). Quartzite contains bimodal grain-size distribution and is cemented by silica in lower part of unit. Conglomerate layers are as thick as 2 m (6.6 ft); cobbles derived primarily from subjacent quartzite of the Mesoproterozoic Swauger (Ys) and Gunsight (Yg) Formations; minor clasts of vein quartz and rare chips of light-colored siltite. Amount of carbonate cement increases upward. Preserved thickness is about 10 m (33 ft). Base of formation is angular unconformity over Apple Creek, Gunsight, and Swauger Formations; top of formation unconformable. Maximum thickness about 20 m (66 ft).",Conglomerate,certain,5-13,CZc,"228,147,136",DAS10
25,DMU25,OCg,O_g,Syenogranite,Early Ordovician to Late Cambrian syenogranite,Early Ordovician to Late Cambrian,"Reddish-brown weathering, pink to red syenogranite of Beaverhead pluton and tan-weathering leucosyenite in a small body about 4 km (2.5 mi) south of Lemhi Pass. Syenogranite of Beaverhead pluton is medium to coarse grained, equigranular to subporphyritic. It is composed of quartz, alkali feldspar, biotite, and opaque minerals and bears accessory zircon and apatite. Widespread alteration produced sericite, hematite, leucoxene, limonite, magnetite, chlorite, and quartz. Age of Beaverhead pluton is 488±5 Ma (sensitive high resolution ion microprobe [SHRIMP] U-Pb zircon dating, Lund and others, 2010). Previously, relations from exposures about 20 km (12 mi) south of the present study area were interpreted as intrusive contacts between the Beaverhead pluton and the Middle Ordovician Kinnikinic Quartzite (Scholten and Ramspott, 1968; Skipp, 1984). However, based on dates obtained from the Beaverhead pluton, the contact must be depositional because the pluton is significantly older than the Middle Ordovician Kinnikinic Quartzite (Lund and others, 2010). Thus, this pluton was exhumed by the Middle Ordovician, before deposition of the Kinnikinic Quartzite (Lund and others, 2010). Beaverhead pluton is preserved in Baby Joe imbricate-thrust system (Hawley Creek thrust sheet as shown by Lucchitta, 1966), a lower-thrust-sheet imbricate thrust sliver related to the Poison Creek thrust fault (Lund and others, 2003b) at low elevations on the western flank of the range in the southern part of the map. Leucosyenite of Lemhi Pass is fine- to medium-grained, buff-colored alkali-feldspar syenite. It consists of microcline, albite microperthite, cryptoperthite, quartz, and lesser biotite, opaque minerals, zircon, and apatite. The leucosyenite crops out in an area about 250 m long and about 75 m (820 × 246 ft) wide. It intruded Mesoproterozoic Gunsight Formation. Age of the Lemhi Pass syenite stock is about 530 Ma (Gillerman and others, 2008).","Coarse-grained, felsic-composition intrusive igneous rock",certain,6-01,OCg,"243,133,155",DAS10
26,DMU26,Ys,Ys,Swauger Formation,Mesoproterozoic Swauger Formation,Mesoproterozoic,"Tan-weathering, light-gray, medium- to coarse-grained quartzite. Quartz content typically ranges from 90 to 95 percent in well-rounded, well-sorted, glassy grains. Quartz grains are quartz cemented but detrital quartz grains remain distinguishable. Feldspar content seldom exceeds 5–10 percent. Beds are 0.5–2 m (1.6–6.6 ft) thick and commonly display trough cross laminations and lesser herringbone cross laminae. Two-dimensional ripples and subaqueous dunes are common in unit (Tysdal, 2000b). Some beds capped by thin (<0.5 cm [<0.2 in]), reddish-tan or greenish-tan siltite or argillite layers. Deposited in a tidal environment based on compositional maturity of quartzites, presence of herringbone crossbeds, subordinate current-cap deposits on some dunes, rare antidunes, and local flaser and lenticular bedding (Tysdal, 2000b). Base gradational; top eroded. Regional thickness as much as 3,100 m (10,171 ft), but only about 500 m (1,640 ft) are exposed in the map area.",Quartzite,certain,7-01,Ys,"206,175,135",DAS10
27,DMU27,Yg,Yg,Gunsight Formation,Mesoproterozoic Gunsight Formation,Mesoproterozoic,"Light- to dark-gray, very fine to medium-grained, arkosic metasandstone; about 35 percent feldspar, 50 percent quartz, and as much as 15 percent metamorphic micas. Bed thicknesses from 5 to 100 cm (2–39 in). Most common sedimentary features are parallel laminations, including common heavy-mineral lamination. Decimeter-thick trough and planar crossbeds are typical; hummocky cross-stratification locally. Mud chips locally. Unit contains finer grained sand and relatively more biotite on west side of range (Cow Creek thrust plate) and is lighter colored, containing less biotite but more distinct heavy-mineral lamination in eastern exposures. Strata in the footwall to the Cow Creek thrust fault contain two generations of metamorphic muscovite (early oriented and younger random). Deposited in fluvial environment (Tysdal, 2003). Sediment derived in part from Archean and Paleoproterozoic basement on eastern edge of map (Tysdal and others, 2005). Through most of area, lower contact is with coarse siltite unit of Apple Creek Formation (Yac) and contact is commonly angular but also contains quartz veins and local tectonic fabrics, suggesting it is a sheared unconformity. On Cabin thrust plate, lower contact is sheared and brecciated but interpreted to be an unconformable depositional contact above Paleoproterozoic gneiss (Tysdal and others, 2005) rather than a fault (contact is labeled “fault D” by M’Gonigle, 1994). Unit grades upward into Swauger Formation (Ys). Total unit thickness regionally is 4,000 m (13,123 ft; (Tysdal, 2003) but only about 1,000 m (3,280 ft) are present in central Beaverhead Mountains exposures.",Metasedimentary rock,certain,7-02,Yg,"185,152,116",DAS10
28,DMU28,Yac,Yac,Coarse siltite unit of Apple Creek Formation,Mesoproterozoic coarse siltite unit of Apple Creek Formation,Mesoproterozoic,"Medium- to coarse-grained siltite and fine-grained metasandstone. Color varies with metamorphic grade and structural position; light- to medium-grayish-green in lowest metamorphic-grade exposures mostly extending from range crest to Montana side of range and light to dark gray to west on Idaho side of range. Weathers dark rusty-gray to dark maroon-gray. Contains 40–60 percent quartz, 25–30 percent potassium feldspar, 5–15 percent plagioclase, about 3 percent clay matrix, and minor fine-grained, metamorphic chlorite, biotite, and (or) muscovite localized near ductile structures. Much of unit is massive, coarse siltite containing rare magnetite-bearing, heavy-mineral layers. Other common bedding type is parallel lamination, most commonly displaying erosional bases and grading upward from fine-grained, light-green or light-gray metasandstone to medium-grained darker greenish or gray siltite. Graded beds range from 1 to 100 cm thick (0.4–39 in); most about 10–25 cm (3.9–9.8 in) thick. Other sedimentary structures common in the laminated and graded beds include intrabasinal mud-chip clasts in the graded-bedding layers and more rarely climbing ripples, convolute lamination, and dish structures. Rocks metamorphosed to greenschist facies, mostly containing sparse fine-grained chlorite, muscovite, or biotite. Chlorite, where present, is generally randomly oriented. In contrast, biotite and (or) muscovite formed phyllitic fabrics in ductile deformation zones—generally either along fold hinges or near thrust faults. Additionally, second generations of fine- to medium-grained muscovite formed in secondary cleavage fabrics in the most deformed rocks. In footwall to Cow Creek thrust fault and to Lemhi Pass normal fault, upper contact with Gunsight Formation (Yg) is characterized by angular bedding relations—several locations demonstrate overturned Apple Creek Formation (Yac) below upright Gunsight Formation (Yg). In these areas, the contact zone is quartz veined and characterized by phyllitic fabrics. In upper thrust plate to Cow Creek thrust fault, the contact is more difficult to identify, less tectonized, and possibly gradational. Regionally, the contact is not overtly angular, but upper contact may have been truncated by erosion before deposition of Gunsight Formation. In the map area, the contact may represent an important angular unconformity as well as a zone of structural decoupling of undetermined origin. Apple Creek Formation is defined following studies of Tysdal. These rocks formed as below-wave-base deposits, primarily as turbidite deposits with minor debris flows. In this central Beaverhead Mountains map area, coarse siltite unit of Apple Creek Formation is uniformly thicker and coarser than described for Lemhi Range, directly to the west. Paleoproterozoic basement rocks on the east side of the Beaverhead Mountains were a significant source terrain. Total thickness exposed in map area about 2,000–2,500 m (6,560–8,200 ft). Base not exposed.",Metasedimentary rock,certain,7-03,Yac,"165,131,98",DAS10
29,DMU29,Xg,Xg,Orthogneiss,Paleoproterozoic orthogneiss,Paleoproterozoic,"Reddish-orange-weathering, light-gray to brown, medium-grained porphyroblastic granite gneiss. Medium-grained groundmass is formed of quartz and subequal amounts of plagioclase, potassium feldspar, and combined hornblende and biotite (M'Gonigle, 1994). Porphyroblasts are coarse-grained rnicrocline that originally were phenocrysts. Exhibits pervasive planar fabric and poorly formed compositional layering defined by oriented mafic minerals, elongated porphyroblasts, and groundmass quartz and feldspar. Some local mafic masses occur in the gneiss, as do crosscutting quartz veins and pegmatite dikes. Paleoproterozoic emplacement age is about 2.46 Ga (thermal ionization mass spectrometer [TIMS] U-Pb zircon, Kellogg and others, 2003).","Schist and gneiss, of sedimentary rock origin
Metaigneous rock",certain,8-01,Xg,"230,205,212",DAS10
