IDAHO GEOLOGICAL SURVEY DIGITAL WEB MAP 154 MOSCOW-BOISE-POCATELLO IDAHOGEOLOGY.ORG LEWIS AND OTHERS EOLOGIC AP OF THE ALMON UADRANGLE, EMHI OUNTY, DAHO G M S Q L C I CORRELATION OF MAP UNITS Reed S. Lewis, Kurt L. Othberg, Russell F. Burmester, Loudon R. Stanford, Mark D. McFaddan, and Jeffrey D. Lonn Artificial Glacial Mass Movement Alluvial Deposits Deposits Deposits Deposits 2012 m Qlsa Qas Qam Qaf Holocene Qtg1 Qgt INTRUSIVE ROCKS Qls Qtg2 Qafo Tdi Diorite dike (Eocene?)—Single dark gray fine-grained mafic dike in the west- Qtg3 40 Pleistocene central part of the map. Ysq Qaf QUATERNARY Qtg Qtg5 Qam 1 Ygr Ysq Ysq Qtg4 Megacrystic granite (Mesoproterozoic)—Light gray to pink, medium- to 8 Tcc ?? coarse-grained, megacrystic, slightly peraluminous granite. CIPW norm from single chemical analysis (sample from Bird Creek quadrangle; Evans 25 Tkg Qls? Qtg5 15 Qam and Zartman, 1990) is 39 percent quartz, 32 percent alkali feldspar, and 29 B R Qgt U percent plagioclase. Biotite is the only mafic mineral. Microcline mega- S H crysts typically range from 3 to 8 cm in length. Minor aplite most common Y CENOZOIC near intrusive contacts. Rock is locally mylonitized. Unit occurs in a single GULCH Intrusive Volcanic Sedimentary Qas body (Diamond Creek pluton; Evans and Zartman, 1990). This is the 75 Rocks Rocks Rocks Qaf easternmost and perhaps least deformed and shallowest exposure of similar Ysq Qtg F 1 Oligocene rock that extends west and northwest to Elk City and continues to near 15 A U TscTcc Tkg Moscow, Idaho. Outcrops weather to rounded shapes, producing coarse 45 L 45 T 15 60 grus with whole microcline megacrysts. Boulders form lag deposits in some 70 saddles and are incorporated in proximal Tkg strata. The boulders can be Qls Qtg2 TERTIARY traced at least 65 km (40 mi) to the southeast (Janecke and others, 2000). 50 Tbo Tccg 50 Qaf U-Pb zircon analyses yield an age of about 1,370 Ma (Evans and Zartman, Tkg Eocene Ygr 35 1990; Doughty and Chamberlain, 1996). Rb-Sr systematics of samples 60 mostly from Deriar and Fenster creeks were interpreted to reflect Sr loss due 40 60 60 Tcc 65 35 Ysq to heating about 100 Ma (Evans and Zartman, 1990). Qaf Qtg4 Tcty 45 Qam Tdi 50 Qtg3 50 Ysq 45 Ysq Tlm MESOPROTEROZOIC STRATA 35 Qas Ysq Qtg4 Qtg3 Ygr DIAMOND 33 Metasedimentary Rocks Metasedimentary rocks of Mesoproterozoic age are exposed in the west 40 part of the quadrangle. These had been assigned to the Gunsight Formation MESOPROTEROZOIC 35 Tkg Ysq by Evans and Green (2003), but have much lower quartzite content than 35 Gunsight Formation mapped in the Lemhi Range farther south (McBean, 20 CREEK 45 1983; Othberg and others, 2011) or rocks correlated with the Gunsight 30 25 Qtg4 Formation in the Beaverhead Mountains to the northeast (e.g., Burm- 30 70 50 75 ester and others, 2011). 25 Qam 35 PLUTON 70 Ysq Siltite, quartzite, and argillite (Mesoproterozoic)—Laminated siltite, intervals 35 Ysq Qam 40 of quartzite, and minor argillite. Siltite very dark green in 1-3 dm thick 80 GRAVEL TERRACE DEPOSITS 16 Qtg4 tabular beds. Lamination typically planar, but difficult to see in siltite 37 Qtg1 25 Qaf Gravel deposits of Holocene to Pleistocene alluvial terraces are composed because of its dark color. Quartzite feldspathic, very fine-grained and dark Tcc of moderately sorted and clast-supported sandy gravel. Clasts vary from gray to fine-grained and lighter gray, with color tone likely reflecting biotite Qgt 10 15 45 subangular to rounded pebbles, cobbles, and few boulders at mountain content. Some fine-grained quartzite may be recrystallized siltite. Some 35 46 Qam Tsc 45 17 50 front, to subrounded to rounded pebbles and cobbles near the confluence parting surfaces on thinner beds have abundant very fine white mica, more 30 Ysq? 60 Qtg4 with the Salmon River. Clasts primarily quartzite and siltite from the likely metamorphic than detrital. Coarser beds as thick as 1 m in sets 5 to 35 A' adjacent mountains. Terrace deposits form a relatively thin (3-9 m; 10-30 ft) 15 m thick may be at bottoms of thinning and fining upward sequences. cap over a streamcut bedrock surface. Several levels of terraces and terrace They constitute 5-10 percent of the section and show soft sediment defor- Qlsa INTRODUCTION Qls remnants are preserved 3-110 m (10-360 ft) above the modern streams. mation, convolute lamination, loads and truncation, rare ripple drift and 30 Qas Tcc These record long-term episodic incision of the Salmon basin, which was climbing ripple cross lamination, but most lack dark (hematite) mineral laminations typical of the Gunsight Formation. More commonly, bed tops Ysq Tkg The geologic map of the Salmon quadrangle shows rock units exposed at probably driven by glacial climate during the Pleistocene. Terrace gravels commonly are capped by and interfinger with alluvial-fan deposits (Qaf have low-angle cross lamination or grade to siltite above planar-laminated Tkg 45 Qtg the surface or underlying a thin surficial cover of soil and colluvium. 4 bases. Quartzite typically contains about 40-60 percent plagioclase; one Thicker surficial alluvial, glacial, and landslide deposits are shown where and Qafo), which are included in the terrace unit locally. Higher terraces 35 sample near the contact with Ygr contained interstitial potassium feldspar, Qtg they form mappable units. Semi-consolidated to consolidated Tertiary mined for gravel. 40 4 likely secondary. Argillite typically very dark, locally with white scapolite sedimentary and volcanic rocks form the undulating low hills near the 36 spots that suggest carbonate or evaporite minerals in the protolith. Except Tsc Salmon River. 25 Qtg1 Gravel of first terrace (Holocene to Late Pleistocene)—Forms terrace 3-9 m for higher apparent metamorphic grade, is similar to rocks mapped as Ysq Tdi 35 The map is the result of our research and field work in 2010 and 2011, and (10-30 ft) above the modern streams. Weakly developed soils. in the Goldstone Mountain quadrangle to the southeast (Lewis and others, 40 45 previous research by others. Many concepts for geologic units were devel- 2011). Dominance of siltite, occurrence of scapolite, and paucity of quartz- 25 45 Qtg4 Qtg Gravel of second terrace (Late? Pleistocene)—Forms terrace 12-18 m (40-60 ft) Ygr oped while mapping quadrangles in the Beaverhead Mountains to the east 2 ite favor correlation with Big Creek or possibly type Inyo Creek formations 25 above the modern streams. Moderately developed soils. Ysq (e.g., Lonn and others, 2008) during a 1:24,000-scale collaborative of the Lemhi Group (Ruppel, 1975) and rocks present below the Inyo Creek 20 Tkg Qas Qtg3 Qam in the Lemhi Range, but not described. Thickness uncertain because of 10 mapping project started in 2007 by the Idaho Geological Survey and the Qtg Gravel of third terrace (Middle? Pleistocene)—Forms terrace 30-49 m (100- 60 Tlm 3 deformation, but minimum is 1,600 m (5,300 ft) based on most continuous Qgt 25 Montana Bureau of Mines and Geology. Attitudes from previous mapping Ysq 20 Qtg 160 ft) above the modern streams. Well developed soils. 15 Qls 4 by Anderson (1956 and unpublished mapping) were used to supplement apparently homoclinal section in the southwest. 25 20 5 Qtg1 Tkg the structural data collected by the authors. Soils information is from Hipple Qtg4 Gravel of fourth terrace, (Middle? Pleistocene)—Forms terrace 61-91 m (200- Tlm and others (2006). 300 ft) above the modern streams. Well developed soils. 30 17 Tkg Qtg4 10 STRUCTURE The oldest rocks in the quadrangle are metasedimentary rocks of Mesopro- Qtg5 Gravel of fifth terrace (Early? Pleistocene)—Forms terrace 110 m (360 ft) above 40 20 Qaf terozoic age that form the mountains in the west but also underlie the the modern streams. Soils of original terrace surface eroded away. 20 12 20 Qas Qls 65 Salmon River valley. These are locally intruded by granite that is also Meso- Structures range in age from Proterozoic to recent, with most deformation 60 30 Qam Qtg4 65 60 proterozoic but slightly younger. Tertiary volcanic and sedimentary rocks of probably during Cretaceous Sevier contraction and Eocene-Oligocene 45 20 20 50 20 35 Qam the Challis Volcanic Group and Tertiary Salmon basin sedimentary rocks, Qam MASS MOVEMENT DEPOSITS extension. Proterozoic deformation may have preceded, accompanied, and 40 40 Ysq which vary from coarse conglomerate to shale, record a wide range of followed intrusion of the megacrystic granite (Ygr). Intrusion was reported 35 Qlsa Deposits of active landslides (late Holocene)—Unstratified, poorly sorted silty 20 Qas depositional environments as the basin was forming. Much of the basin to post-date regional metamorphism and folding of host strata (Evans and 45 clay and gravelly silty clay. Deposited by slumps, slides, and debris flows 10 Qls sedimentary section has been subsequently eroded. The Quaternary depos- Zartman, 1990 and references therein). Tightness of folds in Ysq compared its show evidence of glaciation, terracing, incision, and landsliding. These from slope failures in Tertiary sediments. Many are directly related to and m Qtg4 to the relative smoothness of the intrusive contact is consistent with some 55 Tkg Qafo formed after development of water ditches and irrigation. 20 Tkg are characteristic of Quaternary processes that formed the thin alluvial, folding before intrusion, although many folds may have been formed or 80 Tkg glacial, and mass movement deposits. modified during Cretaceous contraction. Migmatite along the Salmon River 35 35 25 m Qls Landslide deposits (Holocene to Pleistocene)—Unstratified, poorly sorted silty 25 Tsc to the north-northwest is the same age as the Proterozoic granite there, and 20 clay and gravelly silty clay.