Link and Janecke -- Geology of East-Central Idaho: Roadlogs 295 Geology of East-Central Idaho: Geologic Roadlogs for the Big and Little Lost River, Lemhi, and Salmon River Valleys Paul Karl Link Department of Geology, Idaho State University, Pocatello ID 83209 Susanne U. Janecke Department of Geology, Utah State University, Logan UT 84322 INTRODUCTION As part of this introduction we present several summary fig- This article contains a discussion of the regional geology and ures. First is a regional geographic map (Fig. 1), and a simplified historical geography of east-central Idaho, north of the Snake River geologic map on the same base (Fig. 2). The Proterozoic and Plain and southeast of the Salmon River, and four roadlogs for Paleozoic stratigraphic correlation diagram (Fig. 3) also shows the main highways in this huge (15,000-km2) area. The roadlogs thrust fault relations. Figure 4 shows major structures and basins progress from south to north and west to east. They include: of the Trans-Challis fault zone and Figure 5 shows the Challis 1) Big Lost River Valley, Volcanic field. Photographs in the road log section illustrate many 2) Little Lost River-Pahsimeroi Valley, of the field localities. In the introductory summary we cite (out- 3) Birch Creek-Lemhi Valley, and of-sequence with their position in the text) several photographs 4) Stanley to Salmon along the Salmon River. in which specific stratigraphic features are present. This article covers much of the same country described by East-central Idaho contains diverse and economically impor- Ross (1963) in the first geologic roadlog of east-central Idaho, tant, mineral deposits. Specific areas of mineralization are dis- and is a companion to field trip descriptions of south-central Idaho cussed in the roadlogs. More general discussions include Hall to the west, from Arco to Ketchum (Link et al., 1988), and of the (1985), Ruppel and Lopez (1988), Fisher and Johnson (1995a), central Lost River Range (Janecke, 1995a). With those papers, it and Worl et al., (1995), and papers therein. completes an updated set of geologic descriptions of east- and south-central Idaho. Our intention is to provide more precise and GEOLOGIC HISTORY OF EASTERN IDAHO fully referenced information than available in the Roadside Ge- Summary ology of Idaho (Alt and Hyndman, 1989). Our intended audience East-central Idaho (Figs. 1 & 2) lies within the Cordilleran includes all manner of geologists and those interested in the in- fold and thrust belt, and in the Basin-and-Range province. Rocks teraction of Idaho geology, geography and history. and structures within this region reflect a long and complex his- Regional geologic map references for the present discussions tory of deformation. Strata were deposited here in the include Challis National Forest (Wilson and Skipp, 1994), Idaho Mesoproterozoic Belt intracratonic rift basin, and episodically in National Engineering Laboratory (Kuntz et al., 1994); Challis 1° the late Neoproterozoic and Paleozoic Cordilleran miogeocline. x 2° sheet (Fisher et al., 1992); Dillon 1° x 2° sheet (Ruppel et al., The Early Neoproterozoic Beaverhead Impact caused shock de- 1993); Hailey and western Idaho Falls 1° x 2° sheets (Worl et al., formation and may have controlled subsequent tectonic instabil- 1991); and the surficial map of the eastern Snake River Plain ity in the southern Beaverhead and Lemhi Mountains. During the (Scott, 1982). One of our agendas is to promote use of those maps. Paleozoic, marginal basins and uplifts formed, rather than the Without a geologic map, many of our descriptions lose context. regionally extensive Paleozoic passive-margin succession that is Another important standard reference are stratigraphic correla- present south of the Snake River Plain. Paleozoic tion charts for Idaho (Isaacson et al., 1983 and Ballard et al., tectonostratigraphic events include: transpressional latest Devo- 1983). nian and Mississippian Antler deformation, Early Mississippian Link, P.K. and Janecke, S.U., 1999, Geology of East-Central Idaho: Geologic Roadlogs for the Big and Little Lost River, Lemhi, and Salmon River Valleys, in Hughes, S.S., and Thackray, G.D., eds., Guidebook to the Geology of Eastern Idaho: Pocatello, Idaho Museum of Natural History, p. 295-334. 296 Guidebook to the Geology of Eastern Idaho 114° Salmon R. 93 Salmon R. Bitterroot Range 0 5 10 20 miles Leesburg Salmon River Mountains 0 5 10 20 Salmon kilometers 93 28 Lemhi R. Blackbird Mt. reek Baker Road log #3 Middle Fork Salmon R. Lemhi Pass 45° Yellowjacket anther C P Tendoy Lemhi Middle Pass Fork Moyer Peak Montana Peak Salmon75 R. Beaverhead Fort Bannock Lemhi PassBannock Pass Lemhi Range anyon Mountains Hayden Salmon River Mountains Creek 29 ailroad C R Ellis Leadore May Lemhi R. Pahsimeroi R. Road log #2 Pahsimeroi Mountains 113° Grouse Peak Gilmore Patterson Challis Gilmore Medicine Lodge Cr. G Summit Road Log # 4 randview Big Creek 75 C Big 28 B Thompson Ck Bayhorse a Little LostSawm R Yankee Fork n Windy eaverhead M yo Goldburg n Peak Lone Pine ill Canyon Nicholia Peak 93 Summit 21 Malm Cr. Pass Gulch . Sunbeam Clayton Salmon R. Donkey Lemhi Range ountains Spar Willow Hills White Cloud Canyon Creek Lone Pine Stanley Doublespring Summit Pass Bell Mt. Warm Birch Creek Sawtooth Spring Peaks Lost River Range Creek n yo an ll C Hawley Mt. Diamond Peak Sku 75 Sawtooth Valley Borah Peak Blue Dome Chilly Leatherman Peak Borah Little Peak Mt. McCaleb Pass Creek 44° Summit Range Horst Lost East Fork Salmon River Mackay Lost River Range Saddle Mt. Trail Creek Rd. Mackay River Res. 22 Big Lost R. White Knob Big Lost R.Leslie Boulder Mountains Smoky Arco Pass Mountains Howe Mountains Pioneer Antelope Creek Hurst Cyn. Mountains 26 Arco Hills Arco 20 Road log #1 93 Big Lost R. 115° 114° 113° Figure 1. Map showing geography of east-central Idaho, with some of the localities mentioned in this article. Route of segments 1,2,3, and 4 are shown. Figure 2 is at the same scale. faulted foreland-basin deposition east of the Antler belt, and in- tion formed in normal-fault bounded marginal-basins. The Early version tectonics during the Pennsylvanian and Permian Ances- Mississippian Madison Group carbonate bank did not prograde tral Rockies orogeny. Large carbonate bank systems were present west into east-central Idaho. in Silurian, Late Devonian, and Late Mississippian time. In the Deformation and intrusion of the Mesozoic Cordilleran oro- central-Idaho black-shale mineral belt, syngenetic sedimentary- genic belt produced regional northeast-vergent thrust faults, nu- exhalative base-metal deposits of the Devonian Milligen Forma- merous folds (Ross, 1947), and, in the western part of the area, Link and Janecke -- Geology of East-Central Idaho: Roadlogs 297 Yag Yb Kib Salmon R. Yag . R 0 5 10 20 n miles o Yag Leesburg m 0 5 10 20 a l S Salmon kilometers k r Lemhi R. o Ti Salmon Basin F Detatchment e Blackbird Mt. l Lemhi d Fault d i M Valley Yb Lemhi Pass 45° Yellowjacket Yb Panther Creek Lemhi Pass Fault Middle Tendoy Fork Moyer Peak Thunder Peak Yb Agency-Yearian Mountain TMCC Salmon R. Fault Cauldron Complex Ti LM Pzs Bannock Pass Panther Creek Poison Yb Half Graben Pzs Montana Creek Allison Ck. Fault Thrust Yb Idaho Yb MG Yb Ellis Pzs Pzc Twin E Pzc Leadore L Peaks Lemhi Lemhi R. Beaverhead Caldera Grouse Pahsimeroi R. Pluton Peak Pahsimeroi Patterson BM Medicine Challis Pzc Lodge Cr. Fault MT C Yb Hawley Creek Trans Challis Fault System M ID Little Lost R. Thrust Goldburg Yankee Fork Valley Pzc Lemhi Beaverhead Fault Lone Pine Lost River Fault Peak G Pzc Lemhi WS Pzc Yb Valley Salmon R. N S Stanley Doublespring Pass Sawtooth Fault Kib Pzc Pzc WC TS Birch Creek Hawley Mt. Diamond Peak Borah Peak Little Chilly Pioneer Thrust Pzc Kib Lost Copper Pass Creek ° Pzc Pzc BD 44 Pzbs Summit Ti Borah River Basin Peak MK Mackay FS Horst Saddle Mt. Valley Thrust Pzc Big Lost R. Big Lost R.Leslie Fault Pzc Pzc Big Lost Arco Eocene Challis volcanic Pass H Group and QT sediments Howe Pzbs PC Ti Intrusive rocks WPz R. Pzc Valley Kib Idaho Batholith Pioneer Pzc Mountains A Pzbs Black shale assemblage Core Complex Pzc Pzc Arco Pzc Carbonate assemblage Big Lost R. Pzs Shelf assemblage Pioneer Thrust Yag Augen gneiss Normal fault Og Beaverhead pluton Thrust fault Pzc Yb Belt Supergroup Detachment fault Eastern Snake WPz Pioneer Core Complex Fault-segment boundary River Plain 115° 114° 113° Figure 2. Regional geological map of pre-Tertiary rocks of east-central Idaho. Map compiled from Bond (1978) and modified after Wilson and Skipp (1994), Schmidt et al. (1994) and Janecke et al. (1997; 1998). Segment boundaries on normal faults are indicated by heavy arrows and are named as follows (from Janecke, 1993 and references therein): Lost River fault; A=Arco; PC=Pass Creek; MK=Mackay; TS=Thousand Springs; WS=Warm Spring; C=Challis; Lemhi Fault, H=Howe; FS=Fallert Springs; S=Sawmill Gulch; G=Goldburg; P=Patterson; M=May; Beaverhead fault, BD=Blue Dome; N=Nicholia; BM=Baldy Mountain; L=Leadore; MG=Mollie Gulch; LM=Lemhi. 298 Guidebook to the Geology of Eastern Idaho Figure 3. Stratigraphic units present in several thrust sheets and mountain ranges of east-central Idaho, after Skipp (1988a), Wilson and Skipp (1994), Link et al. (1995; 1996). the extensive, mainly Late Cretaceous, Atlanta lobe of the Idaho and is actively extending along a system of dominantly north- batholith. Extension along several sets of normal faults began northwest-striking normal faults. The magnitude 7.3 Borah Peak before Middle Eocene Challis volcanism, exhumed the Pioneer earthquake in 1983 provoked interest in the neotectonics and earth- metamorphic core complex, and produced numerous Tertiary half- quake hazards of the area.