Tectonic History and Hiking Tour of the Northern Seven Devils Mountains, West-Central Idaho

45 Tectonic History and Hiking Tour of the Northern Seven Devils Mountains, West-Central Idaho

This extinct volcanic terrain was formed by major is- MYBP ERA PERIOD EPOCH EVENT land-arc eruptions in the ancestral Pacific Ocean, plate tec- Holo. 0.01 Quater- tonic activity, and recent alpine glaciation. It is accessed by Pleisto- ALPINE nary cene GLACIATION Forest Service Road #517 off US-HWY 95, one mile south 2 of Riggins, Idaho. Located between the Snake and Salmon Pliocene NORMAL

5 FAULTING

c i

Rivers, the northern Seven Devils rise above 9300 feet and o Miocene z comprise the westernmost mountain chain in the central 23 o ERUPTION Tertiary Oligo- OF FLOOD

Rocky Mountains. Glimpses into Hells Canyon, the deepest cene

n e 34 C BASALTS gorge in North America, and an occasional mountain goat Eocene sighting add to this dynamic landscape. 56 WALLOWA Paleo- TERRANE cene 66 ACCRETION Creta- Tectonic History c

ceous i 145 o Island-arc volcanism is represented in the northern Sev- z

Jurassic o en Devils Mountains by a thick sequence of metamorphosed s

202 e

lava flows of Middle and Upper Triassic age (Fig. 1). Mas- M Triassic Daonella 251 sive iron- and magnesium-rich volcanic rocks dominate this Wild Sheep Creek Fm. 1 Permian part of the Wallowa terrane , an ‘exotic’ oceanic assemblage 299 (Vallier, 1998) now residing in northeastern Oregon and western Idaho. Carbon- c iferous 359 i

These rocks were once part of an island chain that existed o Devonian Figure 1. Geologic far off the coast of ancient North America, similar to today’s 416 time scale showing

Mariana Islands in the northwestern Pacific Ocean (Vallier, Silurian age of volcanic rocks

z o e l a 444 P 1998). During the Late Jurassic to Early Cretaceous interval, Ordo- and history of events. vician Ages given in millions rocks of the Seven Devils region were accreted to the west- 488 Cam- of years before present ern edge of North America, which existed at that time ~10 (MYBP). After Walker 542 brian miles east of Riggins, Idaho (Fleck and Criss, 2004). This Precambrian and Geissman (2009). slow process of terrane accretion (and continental growth) shifted the Pacific coastline westward towards its present location. In west-central Idaho, the collisional event is re- Hiking Tour corded across a broad zone of deformation known locally Stop #1: Heavens Gate Vista (8429′). From the parking as the Salmon River suture (Blake et al., 2009). The island area, follow the footpath ~¼ mile east up onto the ridgeline arc-continent suturing event was followed by vertical uplift for spectacular panoramic views into Oregon, Washington, along steep northerly trending normal faults which offset and Montana. At the forest fire lookout (built ca. 1978), Miocene flood basalts. Geologically speaking, these- rela signposts inform of the surrounding peaks, rivers, and can- tively young faults have played an important role in raising yons. Below the lookout are moderately inclined, olive- the local mountains to their current elevations (Fig. 2). green metasedimentary rocks containing rounded pebbles, 1 In this context, “terrane” refers to a displaced crustal fragment with a geo- cobbles, and boulders. These mixed volcanic ‘clasts’ were logic history that differs from surrounding areas. likely sourced from lava flows similar to those exposed in

MOSCOW OFFICE BOISE SATELLITE POCATELLO SATELLITE MORRILL HALL, THIRD FLOOR IDAHO WATER CENTER, SUITE 242B PHYSICAL SCIENCE, ROOM 201B UNIVERSITY OF IDAHO 322 E. FRONT STREET IDAHO STATE UNIVERSITY PO BOX 443014 BOISE, ID 83702-7359 MS 8071 MOSCOW, ID 83844-3014 208-332-4420 Fax 208-332-4400 POCATELLO, ID 83209-8071 I DAHO 208-885-7991 Fax 208-885-5826 208-282-4254 Fax 208-282-4414 GEOLOGICAL SURVEY www.idahogeology.org email: [email protected] the Upper Cannon Lake area (Fig. 2). Many clasts display Stop #2: Windy Saddle (7606′). Through this narrow symmetrical cigar (stretched) or pancake (flattened) shapes wind gap runs the shallow northeast dipping Heavens Gate acquired during metamorphism and deformation related fault (Gray and Oldow, 2005), which separates the deformed to island arc-continent collision. Strong linear and planar rocks viewed at Stop #1 and basaltic island-arc lava flows deformation fabrics characterize western portions of the comprising the northern Seven Devils Mountains (Fig. Salmon River suture zone (Gray et al., 2012) 2). Only remnants of the fault are preserved here; deeply

o DRY DIGGINS 116 32’ k. HELLS CANYON E. C HEAVENS #3 OVERLOOK F #1 A’ 30’’ o GATE VISTA eep rk BERNARD h S S LAKES k 33 ST heep

140 T ws or RU F . T ws TH 45 057 W C TE 39 k.

124 S N E Heavens HELL V Gate 124 EA #4 HILL H Ridge Lily Pad 22 Pond T ws T ws(?) WINDY F #2 SR# 123 SADDLE 517 SEVEN 37 TO Basin ALPINE T ws RIGGINS Lake #5 LAKES DEVILS (~17 mi.) 7-Devils MTNS. Lake Shelf Goat 101 SPRING Lake [?] Devils Pass T ws Camp Tooth Ck. Gem (7760’) Nectar Bridge Lake DEV 23 A Mirror o RCH 45 Rock THE ILS Lake CANNON LAKES DEVILS #6 20’ Island Lake Tower of EXPLANATION 20’’ Sheep Bable T ws Appendix Lake Lake (9269’) Volcaniclastic rocks SPRING 41 SNOW Volcanic flows T ws Anticline He Devil FIELD Flow top Lake She Devil He 36 (9360’) Rock fabric Triangle Devil Upper Syncline Col of Normal fault Lake (9393’) Confusion Cannon N SPRING A 124 The Lake Trail # Purgatory 23 Ogre Thrust fault Peak Lake (9256’) Mount The Campsite Hanson Belial Goblin 0 (MI.) 1 T ws Lake (8880’) (8985’) *Base map modified after Jones (2003) – Geology by K. Gray (1997-2011). A She A’ 10k Devil DEVILS ARCH Mirror Heavens Lake Windy Gate Saddle T ws T ws(?) HEA T ws VENS GA TE THRUST 0 (ft.) Figure 2. Simplified geologic map and cross-section, northern Seven Devils Mountains. T ws : Middle to Upper Triassic Wild Sheep Creek Formation (Vallier, 1998). eroded, reddish-brown volcanic rocks near the kiosk show Further Information evidence of high-temperature fluids moving into the fault This GeoNote is intended to provide a general geologic zone (veins). Motion along the fault postdates formation of overview of the northern Seven Devils Mountains. More linear-planar fabrics (Early Cretaceous) on Heavens Gate technical reports and maps are found in the reference list Ridge and predates the eruption of Miocene basalts that below. once covered this area (Gualtieri and Simmons, 1978). Stop #3: Snake River Overlook (~8000′). From Windy Acknowledgements Saddle, descend on Trail #124, which crosses the East Fork The author thanks R.S. Lewis, J.D. Kauffman, G.W. of Sheep Creek (Fig. 2) and passes through an old burn area Grader, and A. Prazenica (‘Fishtank’) for helpful reviews. up onto the ridgeline overlooking Hells Canyon. This point is >6000 feet above the Snake River, and offers views into References the Wallowa Mountains of northeastern Oregon. On the Blake, D.E., K. D. Gray, S. Giorgis, and B. Tikoff, 2009, A tectonic transect west side of Hells Canyon, a sharp angular unconformity through the Salmon River suture zone along the Salmon River canyon separates southeast oceanic tilted strata of the Wallowa ter- in the Riggins region of west-central Idaho, in J.E. O’Connor, R.J. rane (below) and subhorizontal flood basalts (above). This Dorsey, and I.P. Madin, eds., Volcanoes to Vineyards: Geologic Field Trips through the Dynamic Landscape of the Pacific Northwest: Geo- prominent erosional surface represents a 200+ million year logical Society of America Field Guide 15, p. 345–372. time gap in local Earth history. Mountain goats are some- Fleck, R.J., and R.E. Criss, 2004, Location, age, and tectonic significance of times encountered along this winding trail segment. the western Idaho suture zone (WISZ): U.S. Geological Survey Open- File Report 2004-1039, 48 p. Stop #4: Hell Hill (~7100′). At this locality, Trail #124 Gray, K.D., A.J. Watkinson, R.M. Gaschnig, and R.M. Isakson, 2012, Age and structure of the Crevice pluton: Overlapping orogens in west-cen- skirts across a steep talus slope consisting of fractured gray- tral Idaho?: Canadian Journal of Earth Sciences, v. 49 (6), p. 709-731. ish-green volcanic rocks. Look for lighter colored blocks Gray, K.D., and J.S. Oldow, 2005, Contrasting structural histories of the of bedded limestone in the rubble, which may contain the Salmon River belt and Wallowa terrane: Implications for terrane accretion in northeastern Oregon and west-central Idaho: Geological Society ‘flat-clam’ Daonella, a shallow marine fossil species (Fig. of America Bulletin, v. 117, no. 5/6: p. 687–706. 1) used to date rocks in the Wallowa terrane. Downslope Gualtieri, J.L., and G.C. Simmons, 1978, Preliminary Geologic map of the Hells movement of mechanically weathered material has formed Canyon area, Idaho County, Idaho, and Wallowa County, Oregon: U.S. Geological Survey Open-File Report 78-805, scale 1:48,000, 2 sheets. this vast apron of loose, unconsolidated rock. From here, Jones, G.D., 2003, Hiking Idaho’s Seven Devils—The complete guide to ev- continue hiking towards the West Fork of Sheep Creek (Fig. ery trail, lake, and peak: Middleton, Idaho, CHJ Publishing, 125 p. 2). Devils Tooth and other sharp peaks rise in the distance. Vallier, T.L., 1998, Islands & rapids—A geologic story of Hells Canyon: After crossing the streambed, ascend ~1000 feet onto a Lewiston, Idaho, Confluence Press, 151 p. Walker, J.D., and J.W. Geissman, 2009, Revised geologic time scale: GSA grassy basalt plateau. Watch for Trail #123 on your left. Today, v. 19, no. 4/5, p. 60. Stop #5: Alpine Lakes (~7600′). Trail #123 branches Prepared by Keith D. Gray southeast (left) near Lily Pad Pond and passes several small Washington State University cirque basins perched below the northern Seven Devils. 2012 Volcanic bedrock hosting these lakes was scoured by mountain glaciers that existed here ~25,000-12,000 years ago; linear grooves carved into local rock surfaces (e.g., Basin Lake; Fig. 2) attest to glacial activity in this area. Perennial springs and snowfields feed into the lakes, some of which are planted seasonally with juvenile rainbow or cutthroat trout by Idaho Fish & Game (Jones, 2003).

Stop #6: Foot of the Devils (~8200′). Unobstructed views of He Devil (9393′) and She Devil (9360′), the two highest peaks of this range, compliment the magnificent geology surrounding Sheep Lake. This area exposes mas- sive volcanic rocks of the Middle and Upper Triassic Wild Sheep Creek Formation. Here, a thick sequence of basaltic lava flows and sills define the western limb of a broad anticline—in cross-section, an upright symmetric fold emerges (‘Devils Arch’, Fig. 2). Local topographic highs reflect upward arching of thick (>5000 feet) volcanic rocks and block uplift along steep northerly trending normal faults. Subparallel faults in this area are inferred to follow tributary streams of the upper Sheep Creek drainage network (West Fork), and may represent the southern continuation of a high-angle fault system identified along lower Sheep Creek (Vallier, 1998).