MONTANA BUREAU OF MINES AND GEOLOGY WWW.MBMG.MTECH.EDU MBMG OPEN FILE 604 IDAHO GEOLOGICAL SURVEY WWW.IDAHOGEOLOGY.ORG IGS DIGITAL WEB MAP 134

GEOLOGIC MAP OF THE GOLDSTONE MOUNTAIN QUADRANGLE, LEMHI COUNTY, CORRELATION OF MAP UNITS Glacial Deposits Mass Movement Alluvial Deposits IDAHO, AND BEAVERHEAD COUNTY, MONTANA Deposits Qgty Qmg Qas Holocene Qtg1 Qaf 1 1 1 1 2 1 Qgt Qms Qls Reed S. Lewis , Kurt L. Othberg , Loudon R. Stanford , Russell F. Burmester , Jeffrey D. Lonn , and Mark D. McFaddan Qgo Qafo QUATERNARY Pleistocene ANACONDA RANGEKnown Qgoo Qgto Belt Supergroup 2011 Qtg3 rocks ana Mont Qtgh Idaho AINS 46 AINS 40 40 Goldstone 20 44 Sedimentary Deposits Mountain Ysq 18 WESTMOUNT PIONEER 40 Qmg quadrangle 10 17 Qgt Type location, B 30 30 E 25 42 40 Oligocene Yellowjacket Fm. A 30 Salmon ? Ysq Ysq V 20 TkgTsc x E 55 10 late R Qmg 80 H 29 50 15 55 E 35 45 Volcanic Rocks Intrusive Rocks LEMHI RANGEA Ysq 78 SALMON RIVER MOUNT D 35 Qgty R 28 A 35 55 CENOZOIC N G 66 Qgty middle E 30 Reference MONTANA 65 Qgt Tqs2 sections, 11 Qmg 29 30 Lemhi Group Ttcr Qmg Qgty 35 Qgty 70 IDAHO 40 40 Tlm TERTIARY 32 Figure 1. Location of Goldstone Mountain 7.5' quadrangle with respect to 45 22 Eocene 20 Qgty 20 Tgt known Belt Supergroup rocks and the reference and type sections of the 15 Tr Tpd Tmd Tdi Lemhi Group and Yellowjacket Formation. Shaded areas represent 20 Tldac 30 Qgt Qgt Qgty 50 mountain ranges containing Mesoproterozoic sedimentary rocks. 40 Tmd Qgty Ysq Qgt 40 15 Tcg2 60 Ysq 12 27 20 12 Tmd 40 Tqs1 45 31 Qgty 23 Qgt Tqt 25 Ysq 60 40 30 60 25 8 Tcg 60 1 early Qmg 35 50 A’ 40 10 Qgty Metasedimentary Rocks 50 50 15 10 Ysq 15 25 MESOPROTEROZOIC

25 20 Qgo 20 Ysq 25 20 15

40 16 15 Qgto Till deposits older than the last glacial maximum (Pleistocene)—Poorly sorted Ysq Siltite, quartzite, and argillite (Mesoproterozoic)—Dominated by laminated REFERENCES Qafo 50 INTRODUCTION 15 bouldery till. Mostly subangular to subrounded clasts. Found only by Sandy siltite and argillite, the unit contains coarser and thicker intervals. Finer Anderson, A.L., 1957, Geology and mineral resources of the Baker quadrangle, Creek at mountain front. Early Pinedale Glaciation or older. Soils moder- grained intervals have graded couplets and couples of dark siltite and darker, 10 23 Lemhi County, Idaho: Idaho Bureau of Mines and Geology Pamphlet 112, 40 50 The Idaho Geological Survey (IGS) and the Montana Bureau of Mines and ately well developed. Thickness 15 m (50 ft). gray to green argillite. Up to about one third of these intervals is white Qgt 45 71 p. 40 15 Geology (MBMG) selected the Goldstone Mountain 7.5' quadrangle in the (weathering) very fine-grained feldspathic quartzite that gives the rock a 15 Qgo Blankenau, J.J., 1999, Cenozoic structural and stratigraphic evolution of the Qas 45 50 central Beaverhead Mountains along the Montana-Idaho border for a Outwash gravels of last local glacial maximum (Pleistocene)—Subrounded to candy striped appearance. These quartzite layers commonly are discontinu- 45 20 southeastern Salmon basin, east-central Idaho: Utah State University M.S. 25 30 1:24,000-scale collaborative mapping project as part of a larger project to rounded, well sorted sandy cobble to boulder gravel. Primarily Pinedale ous and preserve hummocky and ripple drift cross laminations, some of thesis, 143 p., 3 plates. 50 understand the structural and stratigraphic relationships between two equivalent; traceable to terminal moraine deposits of Qgt. Forms outwash which are distorted in loads, and rarely have lag bases that have more and 25 60 18 20 Evans, K.V., and G.N. Green, 2003, Geologic map of the Salmon National 35 Ysq 45 20 25 different Mesoproterozoic sedimentary rock packages. To the east and terrace 12 m (40 ft) above Sandy Creek. Soils weakly developed. Thickness coarser quartz. Thick siltite beds have more diffuse multi-mm to cm lamina- 30 3-9 m (10-30 ft). Probably overlies stream-cut surface on bedrock. Forest and vicinity, east-central Idaho: U.S. Geological Survey Geologic 20 northeast, in the West Pioneer and Pintler Mountains (Figure 1), are tions that are planar, gently undulating (hummocky), disturbed by soft 45 25 Ysq 55 30 Investigations Series Map I-2765, 19 p., scale 1:100,000. 5 exposures of known Belt Supergroup rocks (Ruppel and others, 1993; sediment deformation, or deformed into similar folds where cleavage devel- 15 Qgoo Harrison, S.L., 1985, Sedimentology of Tertiary sedimentary rocks near Salmon, 22 Outwash gravels older than last local glacial maximum (Pleistocene)— opment is extreme. Intervals contain sets of several to tens of dm to m thick 45 Qgto Lonn and McDonald, 2004a, 2004b), whereas to the southwest, in the Qafo 20 Subrounded to rounded, well-sorted sandy cobble to boulder gravel. Forms Idaho: University of Montana Ph.D. dissertation, 175 p. 10 white very fine- to fine-grained feldspathic quartzite beds with bases loaded 30 35 20 Lemhi Range and Salmon River Mountains, are the reference sections of 35 outwash terrace 18-24 m (60-80 ft) above Sandy Creek. Traceable to old Hipple, Karl, Karen Langersmith, Rulon Winward, Dal Ames, and Bradley 40 the Lemhi Group, Swauger Formation, and Yellowjacket Formation (Ross, into dark argillite tops of underlying beds. Thick beds typically have bedding moraine deposits (Qgto). Early Pinedale or older. Shows evidence of relict Duncan, 2006, Soil survey of Custer-Lemhi area, Idaho, parts of Blaine, Ysq 1934; Ruppel, 1975). In the intervening Beaverhead Mountains, both the defined by dark mm-scale laminations. Most laminations are planar, but Qafo 35 35 meltwater channels. Soils moderately well developed. Thickness 3-12 m Custer, and Lemhi counties: United States Department of Agriculture, stratigraphic and structural interpretations have been controversial among load and convolute lamination and dm thick stacks of cm-scale ripple cross (10-40 ft). Probably overlies stream-cut surface on bedrock. Probably Natural Resources Conservation Service, 1270 pages, soil maps at Tkg? previous workers (MacKenzie, 1949; Tucker, 1975; Ruppel and others, lamination are common. Attempts to map coarse intervals across the terrane 15 age-equivalent to gravel of third terrace (Qtg ). http://websoilsurvey.nrcs.usda.gov/app/. Qgoo 30 1993; Winston and others, 1999; Evans and Green, 2003; O'Neill, 2005; 3 were unsuccessful. This possibly indicates that they are channels, not sheets, 15 Janecke, S.U., J.J. Blankenau, C.J. VanDenburg, and B.S. Van Gosen, 2001, Map Tysdal and others, 2005; and O’Neill and others, 2007). In 2008 the but structural dissection of the stratigraphy probably contributed to their 55 of normal faults and extensional folds in the Tendoy Mountains and Beaver- Qgo 40 MBMG and IGS mapped the Goldstone Pass quadrangle to the north discontinuity. Thickness uncertain because of deformation, but a minimum Qgto 55 TERTIARY SEDIMENTARY DEPOSITS OF head Range, southwest Montana and eastern Idaho: U.S. Geological Qas 40 (Lonn and others, 2009) and the Kitty Creek quadrangle to the east (Lewis of 2400 m (8000 ft) likely. Tentatively correlated with the type Inyo Creek 45 45 Survey Miscellaneous Field Studies Map MF-2362, scale 1:100,000. 40 and others, 2009a) and our collaborative team plans to continue THE SALMON BASIN Formation of the Lemhi Group (Ruppel, 1975) and rocks below the Inyo 65 55 Lane, E.W., 1947, Report of the subcommittee on sediment terminology: Trans- 40 25 22 1:24,000-scale mapping along the Beaverhead Range in an attempt to Creek that are not exposed in the Lemhi Range. Shown as Apple Creek 20 45 actions of the American Geophysical Union, v. 28, no. 6, p. 936-938. resolve some of the long-standing controversies. Tkg Kriley Gulch formation (Oligocene and Eocene)—Matrix- and clast-supported Formation on the Salmon National Forest map (Evans and Green, 2003). 45 45 Lewis, R.S., R.F. Burmester, L.R. Stanford, J.D. Lonn, M.D. McFaddan, and K.L. 30 30 pebble to boulder conglomerate interbedded with lithic sandstone and Qgoo 27 Tr 20 Othberg, 2009a, Geologic map of the Kitty Creek quadrangle, Lemhi Qtg1 Bedrock mapping in 2009 by Lewis, Burmester, Lonn, and McFaddan minor tuff. Forms steep slopes with coarse gravelly soils and resistant ridges 40 55 County, Idaho and Beaverhead County, Montana: Idaho Geological Survey Qafo 50 followed reconnaissance work by Lonn, Burmester, and Lewis in 2006- capped with common lag pebbles and cobbles, and infrequent lag boulders Qgo 30 30 25 2008. Mapping in 2009 and 2010 of Tertiary strata was by Lewis, Stanford, as long as to 3 m (10 ft). Outcrops of cemented beds are common lower in Digital Web Map 112 and Montana Bureau of Mines and Geology Open- 15 15 Othberg, and McFaddan, and of Quaternary deposits by Othberg and the section but rare higher in the section. Exposures along Kenney Creek File Report 582, scale 1:24,000. 15 Qas Ysq Stanford. Attitudes from previous mapping by Anderson (1957 and unpub- described by Tucker and Birdseye (1989) show primarily matrix-rich, Lewis, R.S., K.L. Othberg, R.F. Burmester, J.D. Lonn, L.R. Stanford, and M.D. poorly sorted conglomerate with highly angular clasts typical of proximal McFaddan, 2009b, Geologic map of the Bohannon Spring quadrangle, 60 lished mapping) and Tucker (1975) were used to supplement the structural 15 35 25 Lemhi County, Idaho and Beaverhead County, Montana: Idaho Geological 35 data collected by the authors. Mapping and unit descriptions by Blankenau facies in alluvial fans that are dominated by debris-flow deposition. Volca- (1999) were used extensively to supplement our work in the Tertiary units in nic clasts are common near the base, but are absent near the top where Survey Digital Web Map 113 and Montana Bureau of Mines and Geology Open-File Report 583, scale 1:24,000. 40 the southern part of the map. Soils information is from Hipple and others quartzite and siltite dominate. Granite cobbles and boulders are present in SYMBOLS 22 Lonn, J.D., and Catherine McDonald, 2004a, Geologic map of the Kelly Lake 35 15 (2006). the middle part of the section, and are likely from a Mesoproterozoic 7.5' quadrangle, western Montana: Montana Bureau of Mines and Geology 40 20 pluton northwest of Salmon (Evans and Green, 2003). Along Kenney Creek, 25 40 Open-File Report 500, 15 p., scale 1:24,000. 40 Blankenau (1999) identified a basal matrix-poor breccia composed of Contact: dashed where approximately located. 25 poorly sorted angular quartzite clasts. Higher in the section, more rounded, Lonn, J.D., and Catherine McDonald, 2004b, Geologic map of the Dickie Hills 75 10 30 DESCRIPTION OF MAP UNITS Thrust fault: teeth on upper plate; dashed where approximately 7.5' quadrangle, southwestern Montana: Montana Bureau of Mines and 50 clast-supported clasts in the conglomerate indicate increasing braided- 50 stream deposition in a more distal part of the fan complex. Harrison (1985) located; dotted where concealed. Geology Open-File Report 501, 14 p., scale 1:24,000. 30 25 Lonn, J.D., L.R. Stanford, R.S. Lewis, R.F. Burmester, and M.D. McFaddan, 35 15 Grain size classification of unconsolidated and consolidated sediment is included the Kenney Creek rocks in her Kriley Gulch formation, and noted 30 Ysq Detachment fault: hachures on upper plate; dashed where approxi- 2009, Geologic map of the Goldstone Pass quadrangle, Lemhi County, 34 based on the Wentworth scale (Lane, 1947). Bedding thicknesses and a similar upward transition to better sorted and clast-supported sediment. mately located; dotted where concealed. Idaho, and Beaverhead County, Montana: Idaho Geological Survey Digital lamination type are after McKee and Weir (1953), and Winston (1986). The unit grades into the Salmon City formation (Tsc); both are informal 30 Web Map 114 and Montana Bureau of Mines and Geology Open-File 40 Distances and bed thicknesses are given in metric units. Formation lithostratigraphic units named and described by Harrison (1985). Tucker Normal fault: ball and bar on downthrown side; dashed where 25 10 Report 584, scale 1:24,000. 18 thickness and elevation are listed in both meters and feet. Multiple litholo- and Birdseye (1989) included the Kenney Creek rocks in the Challis Volca- approximately located; dotted where concealed. 60 Lopez, D.A., J.M. O'Neill, and E.T. Ruppel, 2006, Preliminary geologic map of Qtgh gies within a rock unit description are listed in order of decreasing nic Group and interpreted a western source for the deposits, noting as the Montana parts of the Goldstone Pass and Goldstone Mountain quad- 5 58 abundance. evidence the retrograde alluvial fan sequence and the diverse clast litholo- Anticline axial trace, approximately located; dotted where 40 rangles, southwestern Montana: Montana Bureau of Mines and Geology 37 10 gies that include volcanic, granitic, and rare clasts of biotite gneiss in concealed; arrow indicates plunge direction. 10 Open-File Report 536, scale 1:24,000. Tkg 30 addition to the ubiquitous quartzite. However, Harrison (1985) and Blanke- Tr Syncline axial trace, approximately located; dotted where McKee, E.D., and G.W. Weir, 1953, Terminology for stratification and cross- 32 ALLUVIAL DEPOSITS nau (1999) interpret the Kenney Creek deposits as forming in proximal fan, Tr mid fan, and proximal braided stream environments during activity along concealed; arrow indicates plunge direction. stratification in sedimentary rocks: Geological Society of America Bulletin, 20 30 20 Qas the Salmon Basin detachment. Includes portions of Blankenau’s (1999) v. 64, p. 381-390. 15 Side-stream alluvium (Holocene)—Pebble to boulder sandy gravel in stream 20 20 Strike and dip of bedding. MacKenzie, W.O., 1949, Geology and ore deposits of a section of the Beaver- Qtgh 35 valleys emanating from the Beaverhead Mountains. Silty and clayey in middle conglomerate of the sedimentary rocks of Tendoy, and the 35 head Range east of Salmon, Idaho: University of Idaho M.S. thesis, 65 p., Qafo 15 Tpd 10 foothill stream valleys and localized bottomlands where sheet wash and conglomerate of Kenney Creek of the sedimentary rocks of Sacagawea. 65 Strike and dip of bedding; ball indicates bedding known to be Qtg 5 35 20 50 scale 1:31,000. 3 20 20 40 20 colluvium are included. Soils undeveloped to weakly developed. Thick- upright. Qafo 17 Tsc Salmon City formation (Oligocene and Eocene)—Interbedded sandstone, O'Neill, J.M., 2005, Newly recognized Chief Joseph metamorphic core Qtgh 7 ness 1-6 m (3-20 ft). 50 5 15 siltstone, shale, and minor conglomerate. Sandstone is a vitric quartz Strike and dip of bedding, strike approximate. complex and younger structures in the north-central Beaverhead 45 50 42 35 20 arenite. Shale is thin-bedded tuffaceous silt to massive bentonite. Locally, Mountains-expanding the temporal and spatial limits of post contractional 50 50 10 Qaf well-cemented conglomerate beds form fins. Depositional environments Strike and dip of bedding, strike variable. extension in the northern Rocky Mountains: Geological Society of America 50 21 37 15 Alluvial-fan and debris-flow fan deposits (Holocene to late Pleistocene)— Qtg3 35 10 Angular to subrounded, poorly sorted, primarily matrix-supported pebble mostly distal sandy stream and lake shore deposits. In adjacent quadrangles Abstracts with Programs, v. 37, no. 6, p. 4. 45 Horizontal bedding. 15 15 O'Neill, J.M., E.T. Ruppel, and D.A. Lopez, 2007, Great Divide megashear, 39 to boulder gravel in a sand, silt, and clay matrix. Soils undeveloped to unit grades into and interfingers with the Kriley Gulch formation (Tkg). Ysq Montana, Idaho, and Washington—an intraplate crustal-scale shear zone Qas 5 weakly developed. Commonly grades into side-stream alluvium (Qas). Includes Blankenau’s (1999) Tertiary sandstone and shale units of the 30 Strike and dip of bedding determined from 3-point analysis on bed 48 sedimentary rocks of Tendoy. Blankenau (1999) places age between 46 recurrently active since the Mesoproterozoic: U.S. Geological Survey Qaf Qls 25 Thickness highly varied, ranging 1 to 15 m (3-50 ft). trace in Google Earth 3-D imagery. Qafo Ysq? and 31 Ma, late middle Eocene to early Oligocene. Open-File Report 2007-1280-A, p. 1-10. Tpd? Qtg 40 30 24 Strike and dip of foliation. Ross, C.P., 1934, Geology and ore deposits of the Casto quadrangle, Idaho: 40 3 Ysq Qas 20 Qafo Older alluvial-fan deposits (Pleistocene)—Angular to subrounded, poorly U.S. Geological Survey Bulletin 854, 135 p. 45 55 Strike and dip of mylonitic foliation. Qms sorted, primarily matrix-supported pebble to boulder gravel in a sand, silt, CHALLIS VOLCANIC GROUP Ruppel, E.T., 1975, Precambrian Y sedimentary rocks in east-central Idaho: U.S. Qms Tr Ysq? and clay matrix. Soils moderately to well developed. Thickness highly Geological Survey Bulletin 889-A, 23 p. 55 Strike and dip of foliation where present with bedding or layering. 50 Tqs varied, ranging 1to15 m (3 to 50 ft). 2 Younger quartz-sanidine welded tuff (Eocene)—White to pale green or Ruppel, E.T., J.M. O'Neill, and D.A. Lopez, 1993, Geologic map of the Dillon Qms gray-green with euhedral chatoyant sanidine and smoky quartz in a fine Strike and dip of cleavage. 1º x 2º quadrangle, Idaho and Montana: U.S. Geological Survey Miscella- Tlm 50 ash matrix (Tqs2 of Blankenau, 1999). neous Investigations Series Map I-1803-H, scale 1:250,000. Tqt Ysq Qtg 20 Qtg Gravel of first terrace (Holocene to late Pleistocene)—Subrounded to 60 Strike and dip of joint. Tucker, D.R., 1975, Stratigraphy and structure of Precambrian Y (Belt?) 3 30 1 Ttcr Ttcr 30 well-rounded pebble to boulder sandy gravel; moderately sorted and clast Tuff of Curtis Ranch (Eocene)—White to greenish white tuff containing abun- metasedimentary and associated rocks, Goldstone Mountain quadrangle, SALLMON 25 40 dant biotite and lesser amounts of smoky quartz, plagioclase, and angular Bearing and plunge of lineation, type unknown. 20 32 supported. Forms terrace 3 m (10 ft) above Sandy Creek. Soils weakly devel- 20 Lemhi County, Idaho, and Beaverhead County, Montana: Miami Univer- oped. Thickness 3-6 m (10-20 ft). Overlies stream-cut surface on bedrock. volcanic and quartzite lithics (Tcr of Blankenau, 1999). Pumice is abundant sity, Ph.D. dissertation, 221 p., scale 1:48,000. 15 Bearing and plunge of small fold axis. Tqs Qas to rare and typically concentrated at the top of the unit. Sandstone present Tucker, D.R., and R.U. Birdseye, 1989, An Eocene synvolcanic alluvial fan 2 55 25 BASI 18 Qtg locally at base. Typically poorly welded, but densely welded in places. complex in east-central Idaho: stratigraphic relationships and structural 43 3 Gravel of third terrace (Pleistocene)—Subrounded to well-rounded pebble to 30 Bearing and plunge of mylonitic lineation. 28 N Tkg 20 boulder sandy gravel; moderately sorted and clast supported. Soil implications: The Mountain Geologist, v. 6, p. 23-30. Qas DETACHMENT 28 Tlm Qms 35 horizons well developed. Forms terrace 24-37 m (80-120 ft) above Mafic lava flows (Eocene)—Black to dark green-gray or dark brown-black 5 Bearing and plunge of asymmetrical small fold showing clockwise Tysdal, R.G., D.A. Lindsey, K.I. Lund, and G.R. Winkler, 2005, Alluvial facies, 50 35 32 Qms LEMHI Qms modern streams. Probably age-equivalent to outwash gravels older than aphanitic to porphyritic lava flows; locally vesicular. Probably andesitic to rotation viewed down plunge. paleocurrents, and source of the Mesoproterozoic Gunsight Formation, Qaf Tqt 5 basaltic composition (Tl of Blankenau, 1999). Interbedded with the flows Qms last local glacial maximum (Qgoo). Thickness 3-6 m (10-20 ft). Overlies east-central Idaho and southwestern Montana, Chapter B, in J.M. O'Neill, Tkg are thin biotitic tuffs and volcaniclastic sandstones. Secondary chalcedony 5 Bearing and plunge of asymmetrical small fold showing counter- Qas PASS stream-cut surface on bedrock. and R.G. Tysdal, eds., Stratigraphic Studies in Southwestern Montana and 25 and calcite are common. clockwise rotation viewed down plunge. Adjacent Idaho–Lower Tertiary Anaconda Conglomerate and Mesoprotero- FAU Qas LT? Qls Qtgh High-elevation terrace gravel deposits (Pleistocene)—Subrounded to well- zoic Gunsight Formation: U.S. Geological Survey Professional Paper Ysq 35 Tgt 70 Bearing and plunge of crenulation lineation. 12 rounded pebble to boulder sandy gravel. Forms terrace remnants at least Green tuff (Eocene)—Green to light green biotite tuff containing abundant 1700-B, p. 21-39. Qaf Ysq Tqt Ysq Tdi euhedral biotite, smoky quartz, minor subhedral sanidine, and pumice in a Ttcr 100 m (300 ft) above modern streams. Thickness unknown but probably Vein. VanDenburg, C.J., 1997, Tectonic and paleogeographic evolution of the Horse Tlm Qls Qms white to light green ash matrix. Also contains chert-bearing siltstone. (Tgt of 20 ranges 1 to 12 m (3 to 40 ft). Prairie half graben, southwest Montana: Utah State University M.S. thesis, Qtg3 10 30 10 Blankenau, 1999). Qas 20 Dike. 152 p. 10 Winston, Don, 1986, Sedimentology of the Ravalli Group, middle Belt carbon- 34 Qaf Tldac Dacite lava flows (Eocene)—Purple to purple-gray or dark brown dacite or Tqs2 Qms Location of Mesoproterozoic granite boulder. ate, and Missoula Group, Middle Proterozoic Belt Supergroup, Montana, Tdi MASS MOVEMENT DEPOSITS andesite lava flows containing phenocrysts of euhedral biotite, hornblende, Idaho and Washington, in S.M. Roberts, ed., Belt Supergroup: A Guide to and plagioclase along with minor amounts of quartz and augite(?) in a fine Fault breccia. Qmg Mass movement and glacial deposits undifferentiated (Holocene)—Angular Proterozoic Rocks of Western Montana and Adjacent Areas: Montana A 41 unsorted to poorly sorted boulder to large boulder gravels. Forms protalus crystalline groundmass (Tldac of Blankenau, 1999). Near the top of the 20 Tlm Tqt 39 Bureau of Mines and Geology Special Publication 94, p. 85-124. Ysq sequence are aphanitic layers like those of the Tlm unit. Includes tuff north- Headwall scarp of landslide; ticks on top of scarp. 5 ramparts and slumps derived from moraine remnants and frost-wedged Winston, Don, P.K. Link, and Nathan Hathaway, 1999, The Yellowjacket is not east of Sharkey Hot Springs containing abundant volcanic lithic clasts. 40 Tldac? 2 24 debris on high, glaciated valley walls. Includes some alluvial-fan gravel 30 Qtg Mylonite. the Prichard and other heresies–Belt Supergroup correlations, structure, 3 24 20 Clasts typically contain hornblende, but matrix apparently does not. 24 40 and young glacial and periglacial deposits. Thickness as much as 15 m (50 Qas Qms and paleogeography, east-central Idaho, in S.S. Hughes and G.D. Thackray, Qas Qas ft). Tcg eds., Guidebook to the Geology of Eastern Idaho: Pocatello, Idaho, Idaho Qls 2 Granite-clast conglomerate (Eocene)—Poorly sorted, well-rounded, cobble and 25 Tlm Ysq Museum of Natural History, p. 3-20. 78 Ttcr 40 Tkg 45 Qls Landslide deposits (Holocene and Pleistocene)—Unstratified, poorly sorted boulder gravel consisting of quartzite clasts and rarer (fewer?) boulders of Tldac Tgt 40 silty clay and gravelly silty clay. Deposited by slumps, slides, and debris Proterozoic porphyritic rapakivi granite (Tcg2 of Blankenau, 1999, but not Tlm 40 25 flows. Map may also show the landslide scarp and the headwall (steep previously mapped in the area immediately south of the Lemhi Pass fault). Qms STRUCTURE 10 area adjacent to and below the landslide scarp) from which material broke Tqs1 Older quartz-sanidine welded tuff (Eocene)—Tan-orange to light pink welded Qaf 30 away (see Symbols). Qas 25 tuff containing pumice fragments and euhedral to subhedral crystals of 40 Qtgh Ysq 37 20 Several faults, both contractional and extensional, and folds traverse the Qas 20 Qms Mass-movement deposits (Holocene and Pleistocene)—Angular to subangu- sanidine and smoky quartz in a fine crystalline groundmass (Tqs1 of Blanke- quadrangle. The extension faults are discussed in detail by Blankenau 29 Tsc Ysq lar, poorly sorted, silty and clayey gravel. Includes solifluction deposits, nau, 1999). Quartzite lithics are present in minor amounts. Differs from (1999) and are shown in a simplified map by Janecke and others (2001). 30 25 10 colluvium, and some alluvial-fan gravel. Tqs in that it is typically more densely welded and more tan to orange in These faults typically bound Tertiary units and controlled their distribution. Ttcr 27 30 2 50 Qas 30 Tldac? Tgt? 30 43 20 45 color. 51 30 30 Tsc 10 15 50 60 Tlm 15 33 10 PATTEE CREEK THRUST 48 51 30 33 34 10 27 Tqt Quartzite-bearing ash-flow tuff (Eocene)—White, pinkish red, white-gray, or 33 25 45 GLACIAL DEPOSITS 5 15 55 Qls 5 40 15 pink-gray welded tuff containing abundant angular gray quartzite lithics A mylonite zone exposed in the canyon of Pattee Creek is termed here the Qafo 34 25 Qgty Young glacial and periglacial deposits (Holocene?)—Poorly sorted angular to and less abundant smoky quartz and plagioclase (Tqt of Blankenau, 1999). Pattee Creek thrust. The fault is characterized by a subhorizontal mylonite Tqt 25 55 Qtg3 FAULT C 24 Qas 2 2 subangular boulder gravel and till. Includes pro-talus ramparts, inactive Poorly to densely welded. zone a few hundred feet thick developed in both Mesoproterozoic rocks Tsc 50 Tqs Tqs 50 35 rock glaciers, and moraines deposited in cirques and northeast-facing and pegmatite. Lineations bear almost due east. This mylonitic zone shows 2 2 Tkg Tqt 25 Tcg 46 Tkg 36 1 Basal conglomerate (Eocene)—Subrounded to well-rounded cobble to tops-to-the-east kinematic indicators and is locally folded. Proterozoic 39 40 54 27 50 2 protected areas above 2,500 m (8,200 ft). Lichens common on all but 29 LEMHI P 30 52 Qaf 50 ASS 25 boulder, clast-supported conglomerate with a fine white to gray ash matrix. rocks in the footwall appear to be finer grained than those in the hanging 45 Ttcr? youngest (uppermost) deposits. MacKenzie (1949) classified the larger 36 Qaf Tkg? Ysq 20 FAULT Clasts are entirely quartzite (Tcg of Blankenau, 1999, but not previously Qtgh Qas Tqs 23 deposits as rock glaciers, but well-developed lateral moraines of some 1 wall, but less than 100 m (300 ft) of section is exposed and so they have not Qas Qas Tcg1 1 Tcg2 Tlm? 33 22 60 52 Ysq Tqt deposits indicate a glacial component to their origin. Soils undeveloped. mapped in the area immediately south of the Lemhi Pass fault). been subdivided on this map. We suspect that this structure is part of a Thickness as much as m 25 m (80 ft). major east-vergent thrust system that may daylight again on the east side of the Beaverhead Mountains as the western strand of the Bloody Dick Creek Base digitally scanned from USGS film positive, 1997. 1Idaho Geological Survey Qgt Till deposits of last local glacial maximum (Pleistocene)—Poorly sorted sandy SCALE 1:24,000 INTRUSIVE ROCKS fault in the adjacent Kitty Creek quadrangle (Lewis and others, 2009a). Shaded elevation from 10 m DEM. 2Montana Bureau of Mines and Geology 1 0.5 0 1 to clayey boulder till. Clasts subangular to subrounded. Forms end Locally within Ysq there are sheared out asymmetrical folds and stretching SELWAY Rhyolite dikes (Eocene)—Tan, iron-stained dikes with quartz phenocrysts in an MILE PASS moraines and recessional moraines. Primarily late Pinedale Glaciation Tr Topography compiled 1985. Planimetry derived from imagery SPRING Field work conducted 2006-2010. lineations on axial surface cleavage that are consistent with this motion BOHANNON GOLDSTONE MOUNTAIN taken 1995. Public Land Survey System and survey control MN FEET This geologic map was funded in part by the U.S. Geological equivalent. Includes younger till deposited in or just below cirque floors aphanitic groundmass. being distributed above the mylonite as well. current as of 1999. 1000 0 1000 2000 3000 4000 5000 6000 7000 GN Survey National Cooperative Geologic Mapping Program, up to 2,650 m (8,700 ft) and local deposits of outwash. Soils weakly devel-

Projection: Idaho coordinate system, central zone (Transverse o o USGS award No. G09AC00163. Tpd Porphyritic dacite (Eocene)—Small stock in central part of map. Contains 2-4 KILOMETER KITTY oped. Thickness as much as 36 m (120 ft). OTHER FAULTS WITHIN PROTEROZOIC ROCKS 1 49 15 BAKER CREEK GOLDSTONE Mercator). 1927 North American Datum. 1 0.5 0 1 MOUNTAIN Digital cartography by Jane S. Freed at the Idaho Geological mm phenocrysts of plagioclase, quartz, hornblende, and biotite in an 10,000-foot grid ticks based on Idaho coordinate system, Survey’s Digital Mapping Lab. aphanitic groundmass. Faults within the Ysq package are probable but difficult to document due to central zone. Contour interval 40 feet Reviewed by J.D. Kauffman, Idaho Geological Survey. the lack of stratigraphic control. Likely locations are where bedding LEMHI IDAHO PASS UTM Grid and TENDOY AGENCY Tmd Monzodiorite dikes and sills (Eocene)—Dark gray fine- to medium-grained to 1000-meter Universal Transverse Mercator grid ticks, zone 12. CREEK Map version 10-11-2011. attitudes change abruptly and cleavage is best developed, but proving that 2001 Magnetic North porphyritic hornblende monzodiorite concentrated south-southwest of Declination at Center of Map PDF (Acrobat Reader) map may be viewed online at these are faults and not just sharp folds is difficult. One such zone east of Goldstone Mountain. Hornblende acicular. Wide textural and possibly QUADRANGLE LOCATION ADJOINING QUADRANGLES www.idahogeology.org. Goldstone Mountain contains steep, tight folds and vein quartz; another compositional variation on decimeter to meter scale. Overall less mafic crosses the East Fork Kenney Creek in the center of section 18. Neither than Tdi dikes. Locally contains xenoliths of vein quartz aligned with flow juxtaposes rocks sufficiently different to map as different units, so they fabric suggesting intrusion into quartz vein system. Intrusion and (or) quartz could be minor ramps in the east-vergent thrust system. vein system may be related to local mineralization. A’ LEMHI PASS FAULT 10,000 Tdi Diorite dikes and sills (Eocene)—Medium- to fine-grained hornblende diorite. Similar rocks to the north described by MacKenzie (1949) as meladiorite The Lemhi Pass fault can be traced westward from Lemhi Pass to the south- composed of altered hornblende, albite, biotite, chlorite, and clinozoisite, ern edge of the map. VanDenburg (1997) and Blankenau (1999) interpret 9,000 with andesine and orthoclase in some of the less altered rocks. Sample the structure as a 22-24˚ south-dipping detachment fault. Blankenau specu- 08RB057 from Goldstone Pass quadrangle to the north has U-Pb age of 46 lates that the Lemhi Pass fault is offset to the north by the Salmon basin ± 2 Ma (Richard Gaschnig, written commun., 2009). Records Eocene or detachment, an interpretation tentatively adopted for this map, although later faulting where it is foliated or has sheared margins, such as north of we place the western segment slightly farther north than he did. 8,000 Wade Creek. SALMON BASIN DETACHMENT AND FAULT “C” A 7,000 7,000 MESOPROTEROZOIC STRATA The range front fault was mapped by Tucker (1975) and later interpreted as cleavage Ysq a low-angle detachment fault by Blankenau (1999). It is well exposed along Low metamorphic grade metasedimentary rocks of Mesoproterozoic age Kenny Creek where conglomerate is in contact with cataclasite formed Tqs2 Qas FEET underlie most of the Goldstone Mountain quadrangle. These rocks have from Proterozoic rocks. There the detachment has a dip of less than 15 6,000 6,000 Qms Qas bedding been variously assigned by previous workers to the Lemhi Group and (or) degrees. The fault in Kenny Creek is also described by Tucker and Birdseye Qas the Yellowjacket Formation. All are fine-grained quartzite, siltite, and (1989) who interpret the hanging-wall deposits as an Eocene synvolcanic Tkg alluvial fan complex. The “C” fault of Blankenau (1999) is postulated to Tkg Ysq argillite located southwest of the western strand of the Bloody Dick Creek 5,000 Tqt 5,000 Ttcr Tqt Ysq thrust fault (4 km (6.5 mi) east of map; Lewis and others, 2009a) and part of what cross the southwest corner of the map. The fault is poorly exposed, but is Tlm Pattee Creek we consider the “western sequence” in the Beaverhead Range. This interpreted to form the northeast margin of a second (more southwesterly) Ysq Ysq FEET Tgt sequence is here correlated with the Lemhi Group of the Belt Supergroup Tertiary basin. 4,000 Salmon Basin detachment 4,000 because of similarities to type Lemhi Group rocks in the Lemhi Range south ek thrust of Salmon. This correlation is in agreement with Evans and Green (2003), FOLDS e Cre Tpd Tqt Patte Tr although we have been conservative by applying lithologic unit assign- ments and only offer tentative correlations to specific Lemhi Group forma- Large-scale folds can only be traced for relatively short distances in this part 3,000 Ysq? 3,000 Tldac tions. To the northwest in the Bohannon Spring quadrangle (Lewis and of the Beaverhead Range. We suspect this is a result of truncation by thrust faults and associated folds that strike at an acute angle to the large-scale folds. Ysq? others, 2009b), we divided this sequence into five informal units based on Ysq? grain size, presence of carbonate, and sedimentary structures. Only the 2,000 2,000 lowest of those units is exposed in the Goldstone Mountain quadrangle.

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