MONTANA BUREAU OF MINES AND GEOLOGY WWW.MBMG.MTECH.EDU MBMG OPEN FILE 584 IDAHO GEOLOGICAL SURVEY WWW.IDAHOGEOLOGY.ORG IGS DIGITAL WEB MAP 114

GEOLOGIC MAP OF THE GOLDSTONE PASS QUADRANGLE, LEMHI COUNTY, CORRELATION OF MAP UNITS Mass Movement IDAHO, AND BEAVERHEAD COUNTY, MONTANA Alluvial Deposits Deposits Glacial Deposits Disclaimer: This Digital Web Map is an informal report and may be ANACONDA RANGEKnown revised and formally published at a later time. Its content and format Qas Qam Qaf Qgty Holocene Qls Qmg Belt Supergroup may not conform to agency standards. Qalc rocks ana 1 2 2 2 2 Mont Jeffrey D. Lonn , Loudon R. Stanford , Russell F. Burmester , Reed S. Lewis , and Mark D. McFaddan Qgo Qgt QUATERNARY Idaho Pleistocene Goldstone AINS low high AINS Pass (elevation) quadrangle ? CENOZOIC 2009 WESTMOUNT PIONEER Qgto Type location, B Intrusive Rocks E ? Yellowjacket Fm. Salmon A V

x E R

50 H Yqcu E Tb Tdi TERTIARY Eocene LEMHI RANGEA Ta SALMON RIVER MOUNT D Qgt R Yqmc Qgto? Qgt A Qgt N Qalc G Reference E Qalc MONTANA Mesoproterozoic Strata sections, Lemhi Group Qalc Qalc Yqa? Qgo INTRODUCTION Western Central Eastern IDAHO Yqcu Sequence Sequence Sequence The Montana Bureau of Mines and Geology (MBMG) and the Idaho Qas Geological Survey (IGS) selected the Goldstone Pass 7.5' quadrangle in the Yqsc Yqpi Yqa Figure 1. Location of Goldstone Pass 7.5' quadrangle with respect to Qmg ? Qgt northern Beaverhead Mountains along the Montana-Idaho border for a t t MESOPROTEROZOIC known Belt Supergroup rocks and the reference and type sections of the Ysac Yqgr Yqcu Lemhi Group and Yellowjacket Formation. Shaded areas represent ? 1:24,000-scale collaborative mapping project because of its excellent faul faul 50 40 exposures of Mesoproterozoic sedimentary rock packages. To the east and Ysq Yqf Yqmc mountain ranges containing Mesoproterozoic sedimentary rocks. Yqcu Qgt northeast, in the West Pioneer and Anaconda Range (Figure 1), are 60 Qgto exposures of known Belt Supergroup rocks (Ruppel and others, 1993; Lonn Yqm Yqcl and McDonald, 2004a, 2004b), whereas to the southwest in the Lemhi Qgty that are more northwesterly are interpreted as having moved as thrust faults Yqmc Yqa? Range and Salmon River Mountains are the reference sections of the Lemhi 61 during Cretaceous compressional deformation, while the more easterly Yqcu 25 Group, Swauger Formation, and Yellowjacket Formation (Ross, 1934; ? trending portions are likely to have had a large component of left-lateral Ruppel, 1975). In the intervening Beaverhead Mountains, both the motion during that time. Brittle deformation probably post-dates thrusting, Qalc Qgo stratigraphic and structural interpretations have been controversial among Yqcl Conglomeratic quartzite (Mesoproterozoic)—White to light gray, poorly ? and may represent reactivation of the thrust as a normal fault. Qgt previous workers (MacKenzie, 1949; Tucker, 1975; Ruppel and others, sorted, medium- to coarse-grained, trough and planar crossbedded, Qalc 1993; Winston and others, 1999; Evans and Green, 2003; O'Neill, 2005; feldspathic quartzite. Exposed only at the western edge of the map. Base of The central domain consists of near vertical, east-facing strata of uncertain Tdi Qgo Qgt Tysdal and others, 2005; Lopez and others, 2006). The MBMG and IGS unit not exposed. Similar quartzite along strike to the northwest in the correlation (Yqpi, Yqgr, Yqf, and Yqm). This panel of central sequence strata 10 Qalc mapped the Homer Youngs Peak quadrangle to the north in 2007 (Lonn and Homer Youngs Peak quadrangle contains more potassium feldspar than roughly parallels that of the eastern domain, and it is possible that the Qalc 80 Qgto Qgty Qas others, 2008) and our collaborative team plans to continue 1:24,000-scale plagioclase (Lonn and others, 2008). Minimum thickness in that quadrangle central and eastern stratigraphic sequences are part of the same giant fold mapping southward to Lemhi Pass in an attempt to resolve some of the is 1800 m (6000 ft). and that they were originally in stratigraphic contact. Strong northwest-t- long-standing controversies. striking, southwest-dipping cleavage is developed in the central domain; Central Sequence this cleavage is approximately parallel to the northwest-striking parts of Yqcu Bedrock mapping in 2008 and 2009 by Lonn, Burmester, Lewis, and Yqmc both strands of the Beaverhead Divide fault, and is also parallel to the Qalc ? McFaddan was supplemented by earlier reconnaissance work by Lonn in Between the eastern and western strands of the Beaverhead Divide fault is mylonitic foliation associated with the western strand. The western strand of 65 2004. Quaternary deposits were mapped in 2008 by Stanford. Attitudes a near-vertical, east-facing, 5300 m (17,000 ft) thick stratigraphic sequence the Beaverhead Divide fault is a zone of 25° to 60° southwest-dipping 85 46 Yqcu from previous mapping by Anderson (1957 and unpublished mapping), of feldspathic, very fine- to medium-grained quartzite and subordinate STRAND 35 mylonitic foliation that approximately parallels the northwest to west strike 63 Yqgr Tucker (1975), and Lopez and others (2006) were used to supplement the siltite. Correlation of this sequence to units either west or east is uncertain. 72 Qgty of the zone. Because the western strand Beaverhead Divide fault and the data collected by the authors. Most of this sequence is similar to the western sequence described below, Qgto Qalc Freeman thrust of the Homer Youngs Peak quadrangle (Lonn and others, ? Qgt but the lowest unit (Yqm, medium-grained quartzite) resembles rocks of the 27 34 2008) merge just northwest of the Goldstone Pass quadrangle, the western Yqmc eastern sequence. We divided the central sequence into four informal units Yqmc Qalc 83 Yqcu DESCRIPTION OF MAP UNITS strand here represents the combined displacement on both faults. It Yqcl 46 57 N ? based on grain size, color, and sedimentary structures. separates units Yqm and Yqf of the central sequence from Ysac of the STER Grain size classification of unconsolidated and consolidated sediment is EA Yqpi western sequence. This ductile shear zone contains mafic sills (Tdi) that Qgty BEAVERHEAD based on the Wentworth scale (Lane, 1947). Bedding thicknesses and Pink quartzite (Mesoproterozoic)—Medium- to fine-grained, well-sorted exhibit foliation parallel to that of the shear zone. Lineation within these 30 30 A' lamination type are after McKee and Weir (1953), and Winston (1986). feldspathic quartzite that has a pinkish cast on fresh surfaces. Typically 37 37 flat-laminated m to dm beds with little siltite or argillite. Includes more mud sills plunges to the southwest with s-c fabric indicating top to the west 45 Qgt Distances and bed thicknesses are given in abbreviation of metric units Yqf 47 normal motion. Like the sub-parallel eastern strand, this northwest-striking 65 40 (e.g., dm=decimeter). Formation thickness and elevation is listed in both chips and muscovitic parting surfaces than underlying unit. One quartzite sample contains about 40 percent plagioclase and no potassium feldspar. fault jogs eastward in the Goldstone Pass quadrangle. The portions that Qgty Tdi Qgo meters and feet. Multiple lithologies within a rock unit description are listed Qgty The plagioclase in that sample is highly recrystallized. Top of unit not strike northwest are interpreted as having moved as thrust faults during Qgo in order of decreasing abundance. Yqm exposed, but thickness at least 900 m (3000 ft). Cretaceous compressional deformation, while the east-west striking Qalc portions are thought to have had a large component of left-lateral motion. 20 ALLUVIAL DEPOSITS Yqgr Green quartzite (Mesoproterozoic)—Pale green, fine-grained feldspathic 45 Qaf Qmg Both strands of the Beaverhead Divide fault appear to turn southward just 52 quartzite. Typically m- to dm-thick beds of flat-laminated to non-laminated 44 Yqa Qas Side-stream alluvium (Holocene)—Rounded to subrounded cobble to boulder east of the quadrangle and become the Bloody Dick Creek fault zone of the quartzite and varied dark green siltite, most as graded tops to thick quartzite Qgty Qmg Qas gravel and sand. Mostly derived from re-worked till and outwash gravels. Kitty Creek quadrangle (Lewis and others, 2009). Qalc Qmg DIV beds. Rare red argillite skins in upper part. Commonly weathers white but 70 ID Thickness 1-3 m (3-10 ft). E 20 has surface stain of rusty red. Includes rare mud chips and muscovitic The western domain is complexly folded and faulted. Two ill-defined thrust Qgty Qgt Qgty 20 Qgt? Qam parting surfaces. Ten quartzite samples contain about 30-50 percent faults were mapped. The northeastern of the two that passes through Yqm Tdi Main-stream alluvium (Holocene)—Rounded to subrounded cobble to plagioclase and 0-10 percent potassium feldspar. Some of the potassium Cowbone Lake is characterized by strong foliation. Rocks in its hanging Qgty 80 boulder gravel and sand. Mostly derived from re-worked till and outwash 75 20 feldspar is patchy and probably secondary. One quartzite sample from the wall are tightly folded and locally overturned. The other fault southwest of Yqgr gravels Thickness 1-3 m (3-10 ft). 65 Yqpi Yqa uppermost part of the unit contains about 16 percent plagioclase and 15 Cowbone Lake is characterized by both ductile and brittle deformation. Qgty Qalc 80 Fine-grained deposits in glaciated uplands (Holocene-Pleistocene)—Silt and percent potassium feldspar. Appears to grade upward into Yqpi with Mylonite and chloritic shears are present in the saddle south of Cowbone Qgty Qgty 89 50 25 sand deposited behind end moraines and in glacially scoured depressions. Lake and mylonite is present on the ridge near the west map boundary. 30 Qalc increase in grain size through a zone of alternating colors. Grades Qgty 50 80 70 10 Thickness 1-4 m (3-12 ft). Yqf 17 Yqa downward into Yqf with a decrease in siltite and an increase in dark Northeast-facing strata in its hanging wall are overturned or steeply north- Qmg east dipping. An asymmetric fold in it indicates some top to the west motion Yqgr Qaf Alluvial and debris-flow fan gravels (Holocene)—Angular to subangular poorly laminations that define trough cross beds and decrease in ripple cross 89 Qgt so it is probable that it, like the western strand of the Beaverhead Divide 48 sorted boulder gravels and sand. Found on steep valley walls. Thickness lamination. Thickness approximately 2200 m (7000 ft). Yqm Qgty 48 fault, was reactivated during Eocene extension. FA 1-10 m (3-30 ft). U Yqf Fine-grained quartzite (Mesoproterozoic)—Pale green to gray, fine- to very 33 30 LT Qgo Qam 80 85 Qalc fine-grained, medium- to thick-bedded light-weathering feldspathic 23 50 MASS MOVEMENT DEPOSITS Qgo quartzite and minor darker siltite and argillite. Many beds appear to have 41 40 SYMBOLS Tdi Qgty Qls Landslide deposits (Holocene)—Angular unsorted sandy boulder gravel. Thick- about 30 percent feldspar; one quartzite sample contains about 18 percent Qmg Yqf ness less than 12 m (40 ft). plagioclase and 7 percent potassium feldspar. Bedding defined by dark Contact: dashed where approximately located. Yqf laminations of specular hematite in discontinuous, non uniform 25 25 80 Oblique thrust fault: teeth on upper plate; arrows indicate 20 Qmg Mass movement and glacial deposits undifferentiated (Holocene)—Angular laminations up to 2 mm thick. Despite well-developed cleavage or Tdi Qgo direction of motion; dashed where approximate; dotted 10 unsorted to poorly sorted boulder to large boulder gravels. Mostly ramparts foliation, the dark laminations define large, low-angle trough cross Qgty 55 where concealed. 30 Tdi 33 and slumps derived from moraine remnants and frost-wedged debris. Yqsc 45 Yqpi bedding. High-angle planar, ripple, and climbing ripple cross lamination as 40 Found high on glaciated valley walls. Includes some alluvial-fan gravel and Thrust fault: teeth on upper plate; dashed where approximately Yqm well as flat laminations are less common. Some steep laminations (30-60 35 Yqgr 30 60 young glacial and periglacial deposits. Thickness less than 18 m (60 ft). degree) truncate underlying laminations, defining loads that apparently located; dotted where concealed. 65 Qaf Qgto? grew during deposition. Argillite is present as thin layers or skins (some Reactivated thrust fault: teeth on upper plate; bar and ball on Qgty Qgt 85 Qgt GLACIAL DEPOSITS discontinuous) on quartzite parting surfaces, rarely as light-colored mud downthrown side on reactivated fault segments; dashed 40 Yqm 45 50 chips, and as graded tops of darker siltite and argillite couples within the 30 Qalc Qgty where approximately located; dotted where concealed. Qgt Young glacial and periglacial deposits (Holocene?)—Poorly sorted angular to unit. The lower contact with Yqm is obscured by strong northwest-striking, 45 35 Qgt subangular boulder gravel and till. Sandy boulder till on some ramparts and Normal fault: ball and bar on downthrown side; dashed where 25 Qgty southwest-dipping cleavage developed at an acute angle to bedding, but moraines. Includes pro-talus ramparts, inactive rock glaciers, and moraines 20 35 Yqf appears to grade down section into Yqm. An alternate explanation is approximately located; dotted where concealed. Ysac 85 of the Little Ice Age and older(?) deposits in cirques and northeast-facing 50 Tdi that the contact is a fault. Yqf is lithologically similar to the Yqff unit of the Yqf protected areas above 2500 m (8200 ft). Lichens common on all but young- Anticline axial trace, approximatel y located; dotted where Qgt western sequence exposed to the northwest in the Homer Youngs Peak and concealed; arrow indicates plunge direction. 30 Qalc 30 est (uppermost) deposits. MacKenzie (1949) classified the larger deposits as Qgty Bohannon Spring quadrangles that is tentatively correlated with the Yqsc 40 40 Ysac 25 Qgo rock glaciers, but well-developed lateral moraines of some deposits Syncline axial trace, dashed where approximatel y located; 32 30 25 Gunsight Formation of the Lemhi Group (Lonn and others, 2008; Lewis and 40 indicate a glacial component to their origin. Lateral moraines found in the others, 2009). Some folding in this unit makes thickness estimates dotted where concealed; arrow indicates plunge direction. 08RB057 75 largest deposits are tree covered on distal slopes. Today, deposits appear 40 20 30 problematic, but it is roughly 900 m (3000 ft) thick. 35 30 20 60 Qalc 65 inactive; debris-covered ice is found only in protected areas above young- Strike and dip of bedding. Ta 35 25 40 30 80 est ramparts or moraines at or above 2800 m (9300 ft). Thickness up to 25 Yqm Medium-grained quartzite (Mesoproterozoic)—White, medium-grained 25 Tdi Qmg 45 Strike of vertical bedding. 25 Yqgr m (80 ft). quartzite in upward-fining sequence. Upper part is 15-30 cm thick 40 36 50 15 70 65 20 25 65 55 flat-laminated quartzite beds interbedded with 7- to 10-cm-thick green Strike and dip of bedding; ball indicates bedding known to be 25 40 40 Qam Qgt Glacial till of last local glacial maximum (Pinedale) (Pleistocene)—Poorly 30 40 argillite and siltite. Fewer and more diffuse heavy mineral laminations, upright. 45 35 45 sorted sandy to clayey boulder till. Clasts subangular to subrounded. Also 45 20 45 30 35 more argillite, and thinner beds than the overlying Yqf unit. Lower part is 50 25 23 40 Yqf 85 60 includes younger till deposited near or just below cirque floors up to 2600 Strike and dip of bedding, strike variable. 10 36 50 more thickly bedded white quartzite. Two quartzite samples contain about 30 Ta 65 60 m (8600 ft). Includes end moraine, recessional moraine, and some 45 Yqpi 14 percent plagioclase and 10-11 percent potassium feldspar. Base of unit 75 Strike and dip of bedding known to be overturned. 70 40 Qmg outwash. Thickness up to 120 m (400 ft). 50 60 Yqgr not exposed, but thickness at least 600 m (2000 ft). Approximately 1200 m Ta Ysac Yqgr 80 Strike and dip of bedding interpreted to be overturned. Qgto Older till deposits of the last glacial maximum (Pleistocene)—Poorly sorted (4000 ft) is present on the Bohannon Spring quadrangle to the west. Qgt 30 60 Horizontal bedding. 20 Tdi bouldery till. Mostly subangular to subrounded clasts. Remnant till present 45 15 Qgty on interfluve surfaces 80-180 m (260-600 ft) above more recently glaciated Western Sequence 65 24 Strike and dip of foliation. 25 35 valley floors in the Berry Creek and Pioneer Creek drainages. Also found 65 West of the western strand of the Beaverhead Divide fault is a folded 20 2025 Tdi down stream from terminal moraine deposits of Qgt in the Bloody Dick Strike and dip of mylonitic foliation. 35 65 Ysac sequence consisting of siltite, fine- to very fine-grained feldspathic 55 Creek and Big Hole River drainages. Thickness highly varied, up to 50 m Qgt 40 Qgo quartzite, and argillite. Carbonate cement is present locally. We correlate Strike and dip of foliation where present with bedding or ? (160 ft). 55 Qgty this sequence with the Lemhi Group of the Belt Supergroup because of layering. 40 Qgo Yqsc 10 40 Qgo Glacial outwash gravels of last local glacial maximum (Pinedale) similarities to known Lemhi Group rocks south of Salmon. This correlation 60 Yqf 50 Strike and dip of cleavage. 30 (Pleistocene)—Subrounded to rounded, sorted sandy cobble to boulder is in agreement with Evans and Green (2003), although we have been 40 30 gravel. Deposit on terrace 1.5 m (5 ft) above Qam. Thickness at least 2 m (6 conservative by applying lithologic unit assignments and only offer Strike of vertical joint. ? 80 Qgt WESTERN DIVIDE ft). tentative correlations to specific Lemhi Group formations. To the west in the ERHEAD 30 20 BEAV Bohannon Spring quadrangle we divided this sequence into six informal Bearing and plunge of mylonitic lineation. 20 STRAND 35 35 INTRUSIVE ROCKS units based on grain size, presence of carbonate, and sedimentary 15 Qgto Bearing and plunge of mineral lineation. Ysq 85 Qgt FA structures (Lewis and others, 2009). Only the three lowest of those units are 45 33 42 UL T Tb Basalt dike (Tertiary)—Single fine-grained mafic dike that appears to be less 5 Bearing and plunge of asymmetrica l small fold showing 45 exposed in the Goldstone Pass quadrangle. 20 35 altered and thus younger than the Tdi unit. Exposed on the ridge southeast counterclockwise rotation viewed down plunge. 50 75 Yqsc 80 Ysac Qmg of Cowbone Lake where it both cuts bedding and locally branches parallel Carbonate-bearing quartzite and siltite (Mesoproterozoic)—Light gray to Qgty Qmg 5 Qgt 40 to it. uncommon pale pink very fine-grained to rarely medium-grained quartzite Bearing and plunge of asymmetrica l small fold showing 55 Qmg and subordinate siltite. Orange to white weathering. Locally contains clockwise rotation viewed down plunge. 80 30 15 45 10 15 Ta Andesite dikes (Eocene)—Four north-northwest striking mafic dikes near the carbonate cement, or voids between silicate grains formerly occupied by 15 Bearing and plunge of small fold axis. 40 Ysac western map boundary. Euhedral plagioclase and hornblende 1 mm and carbonate, particularly near the upper part of the unit and, in places, 70 59 23 Ysac less in length and similarly small stubby pyroxene(?) crystals compose the 55 Yqsc appreciable magnetite. Two quartzite samples contained about 40-56 Vein. 45 35 bulk of the rock. 80 Qgty Yqsc percent plagioclase and no potassium feldspar. A third contained patchy Yqsc 26 10 Dike. Tdi (late?) potassium feldspar (about 4 percent) and about 38 percent Qgt 10 50 30 Diorite dikes and sills (Eocene)—Medium- to fine-grained hornblende diorite. 55 35 22 plagioclase. Thickness uncertain due to folding and erosion, but 50 40 40 70 85 51 Qgto Similar rocks to the northwest described by MacKenzie (1949) as meladior- Fault breccia. 5 66 54 14 35 Qgty Ysac approximately 700 m (2300 ft) immediately west of map. Tentatively 08RB057 40 75 Tb 28 ite composed of altered hornblende, albite, biotite, chlorite, and clinozo- 70 50 32 Date sample location and number. 20 32 isite, with andesine and orthoclase in some of the less altered rocks. Locally correlated with the Big Creek Formation of the Lemhi Group (Ruppel, 1975) Ysq 70 50 70 72 30 10 55 46 89 contains abundant magnetite. Occurs both along the western strand of the based on similarity with rocks described in its type section. 10 35 Ysq 80 Qgty 72 Beaverhead Divide fault, where it is typically foliated or has sheared Ysac 10 34 Carbonate-bearing siltite, argillite, and quartzite (Mesoproterozoic)—Laminated REFERENCES 5 40 margins and chloritized fractures, and within the country rock near that 15 10 55 70 Qgo to thinly bedded light green siltite, darker argillite, and white, 25 75 Anderson, A.L., 1957, Geology and mineral resources of the Baker quadrangle, 75 80 Qls? fault, where it is less deformed. Sample 08RB057 has U-Pb age of 46 ± 2 carbonate-bearing fine-grained feldspathic quartzite. More gray and less green 30 50 50 Lemhi County, Idaho: Idaho Bureau of Mines and Geology Pamphlet 112, 25 Ma (Richard Gaschnig, written commun., 2009). in upper part of unit. Siltite and argillite as distinct, laterally discontinuous 50 71 p. 40 85 70 75 80 80 laminae and graded couplets but deformation obscures these characteristics Anderson, A.L., 1959, Geology and mineral resources of the North Fork quad- 25 20 Qas MESOPROTEROZOIC STRATA where unit becomes a light colored phyllite. Local ptygmatically folded silt in rangle, Lemhi County, Idaho: Idaho Bureau of Mines and Geology 5 85 25 non-polygonal “crinkle cracks”. Includes 5-10 percent dm-scale or thinner 75 Low metamorphic grade metasedimentary rocks of Mesoproterozoic age Pamphlet 118, 92 p. 50 siltite and quartzite that display hummocky and thin ripple cross lamination, Evans, K.V., and G.N. Green, 2003, Geologic map of the Salmon National 15 43 Ysac underlie most of the Goldstone Pass quadrangle. These rocks have been 10 30 Ysac loads, and convolute bedding. These share many characteristics with Ysq, but Forest and vicinity, east-central Idaho: U.S. Geological Survey Geologic 80 40 85 58 Qgto variously assigned by previous workers to the Belt Supergroup, the Lemhi 45 80 Investigations Series Map I-2765, 19 p., scale 1:100,000. 55 color and grain size variation typically more diffuse. Locally, some of these 65 Group, and (or) the Yellowjacket Formation. We describe three main 40 39 72 45 layers contain MnCO , judging by solubility in weak acid and Kilroy, K.C., 1981, Ar/ Ar geochronology and geochemistry of the Carmen 80 metasedimentary rock packages in the quadrangle: 1) poorly sorted, 3 Qgt chocolate-brown weathering, but some of this may be remobilized instead of Creek stock, and related intrusions of the northern Beaverhead Mountains, Qas 25 80 40 medium- to coarse-grained quartzite northeast of the eastern strand of the Idaho-Montana: Columbus, Ohio, Ohio State University, M.S. thesis, 100 p. 45 85 original. Carbonate is most abundant in the Cowbone Lake area and may be 75 Beaverhead Divide fault (eastern sequence); 2) quartzite and siltite found Lane, E.W., 1947, Report of the subcommittee on sediment terminology: Trans- 40 more common low in the unit. Thicker skins of dark argillite have rare fluid between the eastern and western strands of the Beaverhead Divide fault actions of the American Geophysical Union, v. 28, no. 6, p. 936-938. 80 Qgty escape structures to 3 cm wide. Highly folded, but a minimum thickness of Qgty 20 Ysac? (central sequence); and 3) siltite, argillite, and fine-grained quartzite south- Lewis, R.S., R.F. Burmester, K.L. Othberg, J.D. Lonn, L.R. Stanford, and M.D. 50 25 53 25 25 800 m (2600 ft) likely. Tentative correlation with the West Fork Formation of the McFaddan, 2009, Geologic map of the Bohannon Spring quadrangle, 40 Qgt west of the western strand of the Beaverhead Divide fault (western 50 Qgt Lemhi Group (Ruppel, 1975) because it appears below Yqsc. Lemhi County, Idaho and Beaverhead County, Montana: Idaho Geological 30 Ysq sequence). Ysac? Survey Digital Web Map 113 and Montana Bureau of Mines and Geology 50 Qgty Ysq Siltite, quartzite, and argillite (Mesoproterozoic)—Laminated to thin bedded dark 27 Open File Report 583, scale 1:24,000. 10 37 Eastern sequence green siltite and dark argillite. Coarser intervals have quartzite as thin (cm) Qgt 45 Lewis, R.S., R.F. Burmester, L.R. Stanford, J.D. Lonn, M.D. McFaddan, and K.L. 24 35 35 bases of beds grading to dark siltite and darker argillite, and as thick layers 60 Qgt? 30 Northeast of the eastern strand of the Beaverhead Divide fault is an Othberg, 2009, Geologic map of the Kitty Creek quadrangle, Lemhi 60 30 commonly in groups of several beds. Siltite layers dm scale approximately County, Idaho and Beaverhead County, Montana: Idaho Geological Survey 50 10 east-facing stratigraphic sequence of poorly sorted, feldspathic, medium- to 17 40 35 35 equal in volume to cm scale siltite and argillite couplets. Thick siltite beds have Digital Web Map 112 and Montana Bureau of Mines and Geology Open 50 43 coarse-grained quartzite. This sequence is tentatively correlated with the 80 diffuse multi-mm to cm laminations that are planar or gently undulating as if Qmg Tdi Missoula Group of the Belt Supergroup because of similarities to known File Report 582, scale 1:24,000. Qgt hummocky cross laminations, in addition to soft sediment deformed Lonn, J.D., R.F. Burmester, R.S. Lewis, and L.R. Stanford, 2008, Geologic map 35 60 upper Missoula Group rocks east and northeast of the map area in the Qgty Qgty 46 laminations. Some zones characterized by graded siltite and argillite couples. of the Homer Youngs Peak quadrangle, Lemhi County, Idaho, and Beaver- 45 15 40 western Anaconda Range (Lonn and McDonald, 2004a) and West Pioneer 50 Argillite both green and dark gray. Zones with green argillite tend to have more head County, Montana: Idaho Geological Survey Digital Web Map 95 and Mountains (Ruppel and others, 1993; Lonn and McDonald, 2004b). This 55 40 mud chips and coarse quartz lag deposits a few cm thick. These alternate on a Montana Bureau of Mines and Geology Open File Report MBMG 575, 60 Ysq 35 20 Ysq correlation is in agreement with Evans and Green (2003), but conflicts with scale 1:24,000. 46 scale of 10-100 m with zones characterized by red-tan weathering slaty, very 60 Ysq 30 Tysdal and others’ (2005) assignment of it to the Gunsight Formation of the Lonn, J.D., and Catherine McDonald, 2004a, Geologic map of the Kelly Lake 58 37 dark gray argillite that locally displays pinch and swell sediment type Ysq 27 36 Lemhi Group. In the Homer Youngs Peak quadrangle, northwest of the Qgt 30 Qgty Qgty characteristics including clastic dikelets. Quartzite commonly has bedding 7.5' quadrangle, western Montana: Montana Bureau of Mines and Geology 15 Qas Goldstone Pass quadrangle, we divided this thick (6000 m; 19,000 ft) Open File Report MBMG 500, 15 p., scale 1:24,000. 89 63 Qgt defined by dark mm-scale laminations. Planar lamination most common load, sequence into four informal units based on grain size and sedimentary Lonn, J.D., and Catherine McDonald, 2004b, Geologic map of the Dickie Hills 60 40 convolute, and cm-scale ripple cross lamination in stacks to dm thickness less structures (Lonn and others, 2008). All four of those units are exposed in the 7.5' quadrangle, southwestern Montana: Montana Bureau of Mines and A common; the last are more common in this unit than any other. Nine quartzite Geology Open File Report MBMG 501, 14 p., scale 1:24,000. 1 Goldstone Pass quadrangle. Base map scanned from USGS film positive, 1997. Montana Bureau of Mines and Geology samples contain about 35-40 percent plagioclase and 0-8 percent potassium MacKenzie, W.O., 1949, Geology and ore deposits of a section of the Beaver- 2 Shaded elevation from 10 m DEM, vertically exaggerated 2x. MN Idaho Geological Survey Yqa Quartzite and argillite (Mesoproterozoic)—Pink to white, medium- to coarse-e- feldspar. Where present in these samples, potassium feldspar is patchy, head Range east of Salmon, Idaho: Moscow, Idaho, University of Idaho, YOUNGS

PEAK JACKSON interstitial, and likely not detrital. One sample from ridge east of the West Fork Topography compiled 1985. Planimetry derived from imagery SCALE 1:24,000 LAKES Field work conducted 2008. grained, trough- and planar-crossbedded quartzite in beds 15 cm to 1 m- M.S. thesis, 65 p., scale 1:31,000. GN HOMER LOWER MINER taken 1995 and other sources. 1 0.5 0 1 thick, interbedded with 15 cm to 1 m-thick intervals of purple to green of Wimpey Creek near the southern map boundary contained only 6 percent McKee, E.D., and G.W. Weir, 1953, Terminology for stratification and cross-s- o o This geologic map was funded in part by the U.S. Geological 1 49 15 MILE Y siltite and argillite in planar beds 0.6 to 5 cm-thick. Top of unit not exposed, plagioclase and about 8 percent detrital(?) potassium feldspar. Another sample stratification in sedimentary rocks: Geological Society of America Bulletin, Projection: Idaho coordinate system, central zone (Transverse ONE A AIN Survey National Cooperative Geologic Mapping Program, T W T FEET L MONTANA ASS v. 64, p. 381-390. SE Mercator). 1927 North American Datum. P USGS Award No. 08HQAG0054 (IGS). but thickness at least 450 m (1500 ft). Very fine- to fine-grained feldspathic from the next ridge to the east contained about 30 percent plagioclase and 18 1000 0 1000 2000 3000 4000 5000 6000 7000 GOLDS BOHANNONSPRING MMOUN Lopez, D.A., J.M. O'Neill, and E.T. Ruppel, 2006, Preliminary geologic map of 10,000-foot grid ticks based on Idaho coordinate system, Digital cartography by Loudon R. Stanford and Jane S. Freed at quartzite with rare red mud chips exposed in three small road cuts west of percent potassium feldspar, some of which may have been detrital. Highly the Montana parts of the Goldstone Pass and Goldstone Mountain quad- central zone, and Montana coordinate system, south zone. UTM Grid and KILOMETER the Idaho Geological Survey’s Digital Mapping Lab. the Big Hole River tentatively included in this unit. folded and commonly cleaved to foliated, with cleavage in graded beds 1 0.5 0 1 ONE 2001 Magnetic North T Y rangles, southwestern Montana: Montana Bureau of Mines and Geology AIN 1000-meter Universal Transverse Mercator grid ticks, zone 12. T curving from more normal in coarser bases to more bedding-parallel in KITT Map version 12-9-2009. Declination at Center of Map BAKER IDAHO CREEK GOLDS Yqcu Upper coarse-grained quartzite (Mesoproterozoic)—White to light gray, Open File Report MBMG 536, scale 1:24,000. Contour interval 40 feet MOUN argillitic tops. Cleavage in finer beds is axial planar to similar folds of the siltite PDF (Acrobat Reader) map may be viewed online at poorly sorted, coarse-grained, trough and planar crossbedded feldspathic O'Neill, J.M., 2005, Newly recognized Chief Joseph metamorphic core www.idahogeology.org and www.mbmg.mtech.edu. and argillite couplets. Similar to, but overall coarser than, Ysac unit. Thickness QUADRANGLE LOCATION ADJOINING QUADRANGLES quartzite. Beds are 30-180 cm thick, separated by black argillite interbeds complex and younger structures in the north-central Beaverhead highly uncertain, but minimum thickness of 2400 m (8000 ft) likely. Tentatively Mountains-expanding the temporal and spatial limits of post contractional as much as 8 cm thick. Contains abundant granule-sized grains and sparse correlated with the type Inyo Creek Formation of the Lemhi Group (Ruppel, floating pebbles. Large black mud rip-up clasts are common. Thickness extension in the northern Rocky Mountains: Geological Society of America 1975) along with rocks below the Inyo Creek that are not exposed in the Lemhi Abstracts with Programs, v. 37, no. 6, p. 4. approximately 1500 m (5000 ft). Range. Ross, C.P., 1934, Geology and ore deposits of the Casto quadrangle, Idaho: Yqmc Multi-colored quartzite (Mesoproterozoic)—White, purple, dark gray, and U.S. Geological Survey Bulletin 854, 135 p. green, fine- to coarse-grained quartzite and siltite. Characterized by Ruppel, E.T., 1975, Precambrian Y sedimentary rocks in east-central Idaho: U.S. intervals of white to dark gray (biotite-bearing?), flat-laminated, fine- to STRUCTURE Geological Survey Bulletin 889-A, 23 p. medium-grained quartzite in beds 30-60 cm thick alternating with intervals Ruppel, E.T., J.M. O'Neill, and D.A. Lopez, 1993, Geologic map of the Dillon The most prominent structure in the map, the Beaverhead Divide fault, was 1º x 2º quadrangle, Idaho and Montana: U.S. Geological Survey Miscella- of thin-bedded quartzite, purple siltite, and black and green argillite in beds first described by MacKenzie (1949), who referred to the structure as the neous Investigations Series Map I-1803-H, scale 1:250,000. 1-3 cm thick. Ripple marks are common. Some bedding planes contain Miner Lakes fault. Anderson (1959) mapped its extension northwest and Tucker, D.R., 1975, Stratigraphy and structure of Precambrian Y (Belt?) small pebbles. Finer grained intervals contain some green calc-silicate Tucker (1975) extended it southeast. Ruppel and others (1993) interpreted metasedimentary and associated rocks, Goldstone Mountain quadrangle, minerals and scapolite. Outcrops have a tabular-bedded appearance. it as a major structure separating the Missoula Group to the northeast from Lemhi County, Idaho, and Beaverhead County, Montana: Oxford, Ohio, Thickness approximately 2100 m (7000 feet). the Mesoproterozoic Yellowjacket Formation and Lemhi Group to the Miami University, Ph.D. dissertation, 221 p., scale 1:48,000. southwest. Evans and Green (2003) mapped it as a thrust reactivated as a Tysdal, R.G., 2000, Stratigraphy and depositional environments of Middle A A’ Proterozoic rocks, northern part of the Lemhi Range, Lemhi County, Idaho: 10,000 10,000 normal fault, separating Missoula Group from Lemhi Group. More recently, Western Strand U.S. Geological Survey Professional Paper 1600, 40 p. Beaverhead Divide O'Neill (2005) interpreted it as a low-angle normal fault that has been Tysdal, R.G., 2002, Structural geology of the western part of the Lemhi Range, Eastern Strand rotated to vertical, with unmetamorphosed upper plate rocks now to the Pratt Fault east-central Idaho: U.S. Geological Survey Professional Paper 1659, 33 p. Darkhorse Beaverhead Divide northeast and metamorphosed lower plate rocks now to the southwest. 9,000 Creek Creek 9,000 Tysdal, R.G., 2003, Correlation, sedimentology, and structural setting, upper Ysq Qmg Fault Our mapping suggests that the Beaverhead Divide fault is a southwest-t- strata of Mesoproterozoic Apple Creek Formation and lower strata of Ysac Gunsight Formation, Lemhi Range to Salmon River Mountains, east-central Qgt Qgt Jahnke dipping zone of both ductile and brittle deformation whose activity may Creek Idaho, in R.G. Tysdal, D.A. Lindsey, and J.E. Taggart, Jr., eds., Correlation, 8,000 Qgt Yqgr 8,000 span a long time (Proterozoic? to Eocene). On the Homer Youngs Peak Ysq Qgt sedimentology, structural setting, chemical composition, and provenance quadrangle (Lonn and others, 2008), two closely spaced strands of the of selected formations in Mesoproterozoic Lemhi Group, central Idaho: Qgt Qgo Yqpi Beaverhead Divide fault were mapped. The eastern and western strands U.S. Geological Survey Professional Paper 1668-A, p. 1-22. Yqf trace southeastward into the Goldstone Pass quadrangle, where they 7,000 7,000 Tysdal, R.G., D.A. Lindsey, K.I. Lund, and G.R. Winkler, 2005, Alluvial facies, diverge and separate the map area into three major structural domains, T FEET paleocurrents, and source of the Mesoproterozoic Gunsight Formation, A Yqcu Ysac each containing a distinct stratigraphic package, here termed the eastern, east-central Idaho and southwestern Montana, Chapter B, in J.M. O'Neill, FEET Yqa central, and western sequences. The eastern domain is a thick east-facing and R.G. Tysdal, eds., Stratigraphic studies in southwestern Montana and 6,000 bedding 6,000 panel of overturned to moderately east-dipping strata (Yqa, Yqcu, Yqmc, adjacent Idaho–Lower Tertiary Anaconda Conglomerate and Mesoprotero- zoic Gunsight Formation: U.S. Geological Survey Professional Paper T A Yqcl), tentatively assigned to the Missoula Group. Lonn and others (2008) interpreted this panel as the west limb of a huge east-verging syncline 1700-B, p. 21-39. Winston, Don, 1986, Sedimentology of the Ravalli Group, middle Belt carbon- 5,000 Ysq 5,000 similar to the gigantic folds mapped by Tysdal (2002) in the Lemhi Range southwest of the map area. Cleavage in this domain is weakly developed, ate, and Missoula Group, Middle Proterozoic Belt Supergroup, Montana, Idaho and Washington, in S.M. Roberts, ed., Belt Supergroup: A Guide to but roughly parallels the strong northwest-striking, steeply southwest-t- Ysq Yqpi Proterozoic Rocks of Western Montana and Adjacent Areas: Montana Yqm dipping cleavage of the central domain. The eastern strand of the Beaver- 4,000 4,000 Bureau of Mines and Geology Special Publication 94, p. 85-124. Yqcu head Divide fault separates weakly foliated, east-facing vertical strata on Ysq Yqf Yqgr Winston, Don, P.K. Link, and Nathan Hathaway, 1999, The Yellowjacket is not Yqmc the northeast (Yqmc and Yqcu) from strongly foliated east facing rocks on the Prichard and other heresies–Belt Supergroup correlations, structure, the southwest (Yqm and Yqf). The fault zone strikes northwest to west, dips and paleogeography, east-central Idaho, in S.S. Hughes, and G.D. Thackray, 3,000 3,000 southwest to south, and is characterized by chloritic breccia containing a eds., Guidebook to the Geology of Eastern Idaho: Pocatello, Idaho, Idaho mixture of strongly foliated and non-foliated clasts. The portions of the fault Museum of Natural History, p. 3-20.

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