Geology and Stratigraphy of the

Challis Volcanic Group and

Related Rocks, Little Wood River Area, 

South-Central Idaho

By Richard F. Sanford

With a section on Geochronology by Lawrence W. Snee

Bulletin 2064-II

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2005

Posted online May 2005, version 1.0

This publication is only available online at:

http://pubs.usgs.gov/bul/2064/ii/

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Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Sanford, R.F., 2005, Geology and stratigraphy of the Challis Volcanic Group and related rocks, Little Wood River area, 

south-central Idaho, with a section on Geochronology by Lawrence W. Snee: U.S. Geological Survey Bulletin 2064-II, 

22 p.
iii

Contents

Abstract .......................................................................................................................................................... 1

Introduction ................................................................................................................................................... 1

Geology ........................................................................................................................................................... 2

Stratigraphy, Rock Types, and Volcanic History ............................................................................. 2

Paleozoic Sedimentary Rocks .................................................................................................. 2

Eocene Challis Volcanic Group ................................................................................................ 2

Miocene and Quaternary Sedimentary and Volcanic Rocks .............................................. 9

Age ......................................................................................................................................................... 9

Paleotopography and Structure ...................................................................................................... 10

Geochemistry of Whole Rocks and Stream Sediments .............................................................. 11

Alteration ............................................................................................................................................. 11

Economic Geology ............................................................................................................................. 13

Summary and Conclusions ........................................................................................................................ 14

Acknowledgments ...................................................................................................................................... 14

Geochronology by Lawrence W. Snee .................................................................................................... 14

References Cited ........................................................................................................................................ 15

Appendix ...................................................................................................................................................... 18

Figures

 1. Map of the Little Wood River area, south-central Idaho, showing geographic

locations and selected geologic features ............................................................................... 3

 2. Geologic map of part of the Little Wood River area, south-central Idaho ......................... 4

 3. Generalized stratigraphic sections of the Challis Volcanic Group at selected

locations in the Little Wood River area, south-central Idaho .............................................. 7

 4. Detailed stratigraphic sections of the Challis Volcanic Group in the Muldoon 

Summit–Little Wood River area, south-central Idaho ........................................................... 8

 5. Chemical variation diagram showing volcanic rocks from the Little Wood River

area, south-central Idaho ......................................................................................................... 14

Table

 1. Whole rock analyses of volcanic rocks, Little Wood River area, south-central

Idaho ............................................................................................................................................ 12
Abstract

The southwestern part of the Challis volcanic field 

occupies the valley of the Little Wood River and its tributaries 

in the Hailey and Idaho Falls 1°×2° quadrangles of southcentral Idaho. The Little Wood River area is a structurally 

controlled topographic basin that is partly filled by Eocene 

Challis Volcanic Group and younger rocks. Rock types in the 

Challis Volcanic Group of the Little Wood River area include, 

in order of decreasing abundance, andesite lava flows and 

tuff breccia, dacite lava flows and flow breccia, volcaniclastic 

sedimentary rocks, lithic tuff, nonvolcanic conglomerate, and 

rhyolite dikes.

A basal nonvolcanic conglomerate, that locally rests on 

upper Paleozoic sedimentary rocks at a regional unconformity, 

was deposited prior to eruption of volcanic rocks. Andesite was the first volcanic rock erupted and is a voluminous 

sequence as thick as 3,000 ft (1,000 m). Locally thick volcaniclastic sedimentary rocks accumulated in topographic lows. 

A sharp transition marks the beginning of dacite eruption from 

fissures and flow-dome complexes. Dacite flows and breccias 

are as thick as 2,000 ft (600 m). An upper volcaniclastic unit 

was deposited in paleotopographic lows following emplacement of the main dacite unit. Next, a widespread, distinctive, 

lithic-rich ash-flow tuff, correlated with the tuff of Stoddard 

Gulch, was deposited over much of the area. Deposition of 

the tuff was followed by eruption of thin andesite and dacite 

lava flows and deposition of conglomeratic sedimentary rocks. 

The entire sequence was then intruded by a dacite flow-dome 

complex composed of at least three separate intrusions. 

The Challis Volcanic Group in the study area is calcalkaline. Andesitic rocks are typically high-potassium basaltic 

andesite, high-potassium andesite, shoshonite, and banakite 

(latite). Dacitic rocks are high-potassium dacite and trachyte. 

Tuffs and vitrophyres range in composition from basaltic 

andesite to trachyte. The paleotopographic basin in which the 

Challis Volcanic Group accumulated was in part structurally 

controlled by preexisting west- and northwest-trending normal faults. Paleorelief was probably more than 2,000 ft (600 

m). Rocks of the Challis Volcanic Group typically dip about 

36° E., probably owing to Eocene extension and (or)

basin-and-range faulting and tilting. Some potential for

precious- and base-metal hydrothermal deposits is

associated with northeast-trending lineaments, argillic

alteration, and rhyolite dikes.

Introduction

The Challis volcanic field in central Idaho is the largest 

of several Eocene volcanic fields in the Pacific Northwest of 

the United States and Canada. Other fields include the British 

Columbia alkalic province, the Sanpoil field in northeastern 

Washington and southern British Columbia (Carlson and others, 1991), the Clarno field in north-central Oregon (Suayah 

and Rodgers, 1991), the Absaroka field in Montana and 

Wyoming (Gest and McBirney, 1979), the Montana alkalic 

province (O’Brien and others, 1991), and small scattered 

outliers (Moye and others, 1988). The Challis volcanic field 

was erupted during the Kamloops-Challis-Absaroka volcanic 

episode at 51–40 Ma (Armstrong and Ward, 1991).

Two models have been proposed to explain Eocene volcanism in the region. In one model, subduction of a very shallow 

dipping Farallon plate beneath the continent created a wide 

volcanic arc (Lipman and others, 1971, 1972; Lipman, 1980). 

In the other model, volcanic activity resulted from rifting as 

a result of collision between the Pacific and North American 

plates (Ewing, 1980; Fox, 1983; Fox and Beck, 1985). The 

later model was developed for Eocene rocks of northeastern Washington and British Columbia. A number of studies 

present data aimed at a comprehensive description of Eocene 

volcanism (Gest and McBirney, 1979; McIntyre and others, 

1982; Moye, 1984; Meen, 1987; Hackett and others, 1988; 

Moye and others, 1988; Norman, 1988; Armstrong and Ward, 

1991; Carlson and others, 1991; Dudas, 1991; Lipman and 

Glazner, 1991; Norman and Mertzman, 1991). In this study, I 

present stratigraphic, compositional, and structural data on a 

previously poorly known part of the Challis volcanic field that 

will aid in understanding the origin and evolution of Eocene 

volcanism in the Pacific Northwest. In view of the relative lack 

of attention given to intermediate-composition volcanism (Lipman and Glazner, 1991), the Little Wood River area, and its 

Geology and Stratigraphy of the Challis Volcanic Group 

and Related Rocks, Little Wood River Area,

South-Central Idaho

By Richard F. Sanford
2 Geology and Stratigraphy of South-Central Idaho

several thousands of feet of intermediate-composition volcanic 

rocks, is a rich source of information. 

The stratigraphy of the Challis Volcanic Group has been 

described for the northern part of the Challis volcanic field 

in the Challis 1°×2° quadrangle (Leonard and Marvin, 1982; 

McIntyre and others, 1982; Ekren, 1985) and for the southeastern part of the field in the Idaho Falls 1°×2° quadrangle 

(Moye and others, 1988; McDermott and Snider, 1989; Snider 

and Moye, 1989). In this report, I present data for the southwestern part of the Challis volcanic field in the Little Wood 

River area in the Hailey and Idaho Falls 1°×2° quadrangles 

(fig. 1). Most of this area was previously unmapped except for 

reconnaissance mapping at a scale of 1:100,000. Umpleby and 

others (1930) and Dover (1981) mapped the extreme northern part of the area. Nelson and Ross (1969) and Lukowitz 

(1971) mapped much of the Mackay 3NW and Mackay 3SW 

71⁄2-minute quadrangles east of long 113° W. Hall and Batchelder (1979) mapped the Baugh Creek Southwest 71⁄2-minute 

quadrangle. Sanford (1988) and Sanford and others (1989) 

mapped the Little Wood River Wilderness Study Area, which 

includes parts of the Baugh Creek and Mackay 3NW 71⁄2-minute quadrangles. W.E. Hall and J.N. Batchelder (unpublished 

field notes, 1973–1975, U.S. Geological Survey library) 

conducted reconnaissance mapping of parts of the Seamans 

Creek, Little Wood River, and Baugh Creek 71⁄2-minute quadrangles. Schmidt (1961) mapped some of the southern part of 

the area. The geology of the Little Wood River area is shown 

on compilations by Rember and Bennett (1979a, b) and Worl 

and others (1991).

Mapping for this report was done during the summers 

of 1987–88. Reconnaissance mapping was carried out in the 

Little Wood River area, and detailed mapping was performed 

at a scale of 1:24,000 in the Baugh Creek 71⁄2-minute quadrangle and adjacent areas, including parts of the Mackay 

3NW, Mackay 3SW, Little Wood River, and Seamans Creek 

71⁄2-minute quadrangles. Results of detailed mapping are 

shown in figure 2.

Geology

The Little Wood River area is a structurally controlled 

topographic basin that is partly filled by the Eocene Challis Volcanic Group and younger rocks. Elevated areas around the basin 

consist mostly of resistant siliceous sedimentary rocks of the 

Middle Pennsylvanian to Lower Permian Wood River Formation and lesser amounts of Lower to Upper Devonian Milligen 

Formation and Lower to Upper Mississippian Copper Basin 

Formation. Depositional relationships of the Challis Volcanic 

Group are complex, because of the large topographic relief on 

the pre-Challis surface and the multitude of volcanic centers and 

rock types. Rocks of the Challis Volcanic Group typically dip 

about 36° E. Units overlying the Challis Volcanic Group include 

the almost flat lying Miocene Idavada Volcanics and Quaternary 

basalts, gravels, and other unconsolidated sediments.

Stratigraphy, Rock Types, and Volcanic History

Paleozoic Sedimentary Rocks

The Devonian Milligen Formation (Umpleby and others, 

1930; Sandberg and others, 1975) consists of black, thinly 

bedded (1–4 in., 2.5–10 cm) chert and phyllitic, fissile quartzite, and thick (15–20 ft, 4.5–6 m) light-gray, sandy limestone. 

These rock types are interbedded with subordinate, brownweathering, finely laminated, dark-gray dolomite and slightly 

calcareous, gray-, brown-, and maroon-weathering siltstone 

and black argillite.

The Mississippian Copper Basin Formation (Umpleby 

and others, 1930; Paull and others, 1972; Nilsen, 1977; Paull 

and Gruber, 1977) consists of dark-gray turbidites including 

boulder to granule conglomerate, sandstone, siltstone, mudstone, and limestone in its lower part and gray, poorly sorted, 

fine to very coarse grained, medium- to thick-bedded lithic 

sandstone and dark-gray to grayish-black mudstone or argillite 

in its upper part.

The Middle and Upper Pennsylvanian and Lower 

Permian Wood River Formation (Umpleby and others, 1930; 

Bostwick, 1955; Hall and others, 1974; Mahoney and others, 

1991) consists of gray calcareous sandstone, brown-weathering sandy limestone, gray siltite, gray quartzite, bluish-gray 

limestone, and dark-gray chert. The base of the Wood River 

is characterized by quartz-pebble conglomerate. Limestone 

weathers dark brown. Crossbedding, convolute structures, and 

load casts are common in the calcareous sandstone. Locally, 

where overlain by thick sequences of Challis andesite, the 

Wood River Formation is contact metamorphosed. The hornfels is jet black, vitreous, and unusually hard; northwest of 

Muldoon Summit, the hornfels is 33 ft (10 m) thick.

Eocene Challis Volcanic Group

In the Little Wood River area the Challis Volcanic Group 

includes, in order of decreasing abundance, andesite lava flows 

and tuff breccia, dacite lava flows and flow breccia, volcaniclastic sedimentary rocks, lithic tuff, nonvolcanic conglomerate, and rhyolite dikes. These rock types comprise 10 map 

units: from oldest to youngest, basal conglomerate, basal 

andesite, lower volcaniclastic sedimentary rocks, andesite 

lava flows and tuff breccia, dacite lava flows and flow breccia, upper volcaniclastic sedimentary rocks, lithic-rich tuff, 

upper mixed unit, dacite intrusion–flow-dome complex, and 

rhyolite dikes (figs. 2–4, app.). In the northeastern part of the 

study area, east of the Little Wood River, the section consists 

of conspicuously stratified andesite flows and tuff breccia. In 

the northeastern part of the study area, west of the Little Wood 

River, the section consists of dacite flows and flow breccias 

that have almost no apparent stratification. In the central part 

of the study area, large areas are dominated by unstratified 

dacite flows and flow breccias, particularly in the Baugh Creek 

area. Between Muldoon Summit and the Little Wood River, 

the section consists of stratified flows and breccia comprising 
Geology 3

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Figure 1. Little Wood River area, south-central Idaho, showing geographic locations and selected 

geologic features. Areas of detailed mapping (fig. 2) outlined.
4 Geology and Stratigraphy of South-Central Idaho

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Figure 2. Geologic map of part of the Little Wood River area, south-central Idaho. A, Baugh Creek area; B, Copper Creek area. 

Explanation is on page 5. Stratigraphic sections labeled A through F shown in figure 3.
Geology 5

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6 Geology and Stratigraphy of South-Central Idaho

approximately equal volumes of andesite and dacite. Because 

this area includes the most complete and representative section, a detailed section was measured (fig. 4, app.).

The basal conglomerate rests unconformably on the 

Wood River Formation and crops out in a narrow band south 

of Elk Mountain (fig. 2A). The conglomerate probably correlates with the informal basal Smiley Creek conglomerate of 

Paull (1974), which has been described from many places elsewhere in the Challis volcanic field (Nelson and Ross, 1968, 

1969; Dover, 1969, 1983; Paull, 1974; McIntyre and others, 

1982; Fisher and others, 1983; Burton and Blakey, 1988). In 

the Little Wood River area, the conglomerate is steeply dipping and exhibits an angular unconformity with the underlying Wood River Formation. Thickness varies from 0 to 150 ft 

(0–50 m). The conglomerate is massive and has little internal 

sedimentary structure. It is light gray and consists of gray 

quartzite pebbles 1–10 cm in diameter in a white fine-grained 

matrix. Unlike other locations in the Challis volcanic field, no 

volcanic clasts were observed in the Little Wood River area. 

The closest suitable source for the conglomerate is a paleohigh 

north of the outcrop where resistant beds of the Wood River 

Formation today form a topographic high in the area of Elk 

Mountain; however, more distant sources are possible. Lack of 

volcanic detritus in the conglomerate in the Little Wood River 

area indicates that volcanic activity had not yet begun, at least 

not in the source area for the conglomerate. Dips of bedding of 

as much as 60° indicate postdepositional tilting.

An early phase of intermediate-composition volcanism is 

recognized throughout the Challis volcanic field as the first of 

three main phases of volcanism (McIntyre and others, 1982; 

Moye and others, 1988). In the Little Wood River area, this 

phase is represented by large volumes of andesite and later 

dacite lava flows, ash-flow tuff, and tuff breccia (fig. 4, app.).

Andesite is a part of three units that are locally distinguishable: a basal andesite, a main andesite, and an upper 

mixed unit (fig. 4, app.). The basal andesite is separated from 

the main andesite by a volcaniclastic layer. The main andesite 

is separated from the upper mixed unit by an upper volcaniclastic unit and a tuff unit. In most of the study area, where the 

intervening volcaniclastic and tuff units are absent, the andesite is mapped as a single, undivided andesite unit (fig. 2). A 

thick andesite unit at the base of the Challis Volcanic Group is 

typical of the entire volcanic field (Leonard and Marvin, 1982; 

McIntyre and others, 1982; Ekren, 1985; Moye and others, 

1988; McDermott and Snider, 1989; Snider and Moye, 1989).

The basal andesite crops out near the head of Seamans 

Creek west of Muldoon Summit (fig. 2A). It consists of lava 

flows deposited unconformably on the Wood River Formation. Thickness varies from 0 to 500 ft (0–170 m). It weathers 

brown and is black, dark gray, and greenish brown on fresh 

surfaces. Pyroxene and hornblende phenocrysts as long as 0.25 

mm are common to abundant, and plagioclase phenocrysts, as 

long as 1 mm, are rare to common. Pits on weathered surfaces 

of the andesite may indicate dissolved olivine.

The lower volcaniclastic sedimentary unit crops out in the 

vicinity of Muldoon Summit (fig. 2A). Both lower and upper 

volcaniclastic units are generally poorly exposed, form gentle 

slopes, and are associated with landslides. The lower sedimentary unit is best exposed along road cuts. It consists of very 

immature, epiclastic, volcaniclastic sedimentary rocks that are 

buff, white, and green, massively bedded to finely laminated, 

poorly consolidated sandstone, siltstone, pebble conglomerate, 

and claystone. Graded bedding is common. Clasts are angular to subrounded and include detrital plagioclase, sanidine, 

biotite, and hornblende grains, volcanic clasts, chert clasts, 

and carbonized wood and leaf fragments. The fine horizontal 

laminations, graded bedding, pebbles, and well-preserved 

plant remains indicate deposition in moderate- to low-energy 

streams and lakes. Plagioclase and mafic minerals in sandstone locally impart a salt-and-pepper texture. Rock fragments 

have a maximum diameter of 2 cm. Clasts are typically matrix 

supported. The detrital grains consist of minerals characteristic of phenocrysts in rocks of the Challis Volcanic Group and 

indicate erosion and reworking of volcanic rocks.

The main andesite makes up about 60–80 percent of the 

preserved Challis Volcanic Group in the Little Wood River area 

and is more than 3,000 ft (1,000 m) thick in the vicinity of Garfield Mountain (figs. 3, 4). Although vents were not recognized, 

the thickest accumulation of andesite probably indicates the 

primary source area. The andesite typically forms well-stratified deposits consisting of lava flows and tuff breccia. Volcaniclastic sedimentary rocks identical to the lower and upper sedimentary units are commonly interbedded with andesite flows 

and breccia, indicating that volcanic material was transported 

as lava flows, ash-flow tuff, and stream sediment. Stream-transported volcaniclastic detritus and plant matter were reworked 

and deposited in stream channels and small lakes. Andesite lava 

flows are massive and fractured and tend to form resistant cliffs 

and knobs, whereas andesite tuff breccia tends to form gentle 

slopes having a distinctive dark-greenish-brown color. Flows 

are locally vesicular, especially at flow tops. Some vesicles 

are filled with calcite, quartz, chalcedony, epidote(?), or, in at 

least one case, amethyst. Both andesite flows and tuff breccia are greenish brown, gray, and buff gray on fresh surfaces 

and weather reddish brown and brownish gray. Locally, a red 

weathering rind as thick as 1 mm is present. Tuff breccia is 

monolithic to heterolithic and contains subangular fragments of 

porphyritic and nonporphyritic andesite, dacite, and vitric tuff. 

Andesite flows and fragments of flows contained in tuff breccia 

are porphyritic, glomeroporphyritic, and aphanitic and contain 

phenocrysts of plagioclase, pyroxene, hornblende, biotite, olivine(?), and opaque iron oxide minerals in a finely crystalline 

groundmass. Phenocrysts range in size from 0.5 to 5 mm and in 

abundance from 0 to 60 percent.

A sharp transition marks the change from andesite to 

dacite eruption (figs. 3, 4). Locally, volcaniclastic sedimentary 

rocks indicate an erosional unconformity at this transition. The 

main dacite unit erupted from fissures and formed flow-dome 

complexes mainly west of the Little Wood River. These 

complexes are as thick as 2,000 ft (600 m). Dacite flows and 

flow breccias filled paleotopographic lows and blanketed the 

older andesite. The viscous flows built up coalescing domes 
that created a complex topographic surface. Dacite lava flows 

and flow breccia are distinguishable locally but grade into one 

another over relatively short distances (feet). Dacite flows and 

breccia commonly form steep slopes, cliffs, and knobs. They 

typically are massive, and crude columnar jointing is present locally. Blocks of andesite and dacite in the breccia are as 

much as 12 ft (4 m) in diameter. Weathered and unweathered 

surfaces are purplish, reddish, and greenish gray. Textures are 

porphyritic to glomeroporphyritic, and phenocrysts include 

plagioclase, biotite, hornblende, and pyroxene. Plagioclase 

crystals are commonly 3–5 mm in size and constitute 10–40 

percent of the rock. Biotite commonly forms conspicuous 

books, locally altered to a bronzy color, that are 2–3 mm in 

size and commonly aligned parallel with foliation. Groundmass is white, light gray, and dark gray and aphanitic.

An upper volcaniclastic sedimentary unit was deposited 

in paleotopographic depressions following emplacement of the 

main dacite unit. The upper sedimentary unit crops out mainly 

between Cold Spring Creek and Hailey Creek and in the 

Lake Hills. The best exposures are in gullies and landslide 

scarps and range from 0 to 160 ft thick (0 to 48 m). As in the 

lower sedimentary unit, clastic material consists of reworked 

volcanic rock and coalified plant debris. The unit consists of 

very immature, epiclastic, volcaniclastic sedimentary rocks 

that are buff, white, gray, green, and pink, massively bedded 

to finely laminated, poorly consolidated sandstone, siltstone, 

pebble conglomerate, boulder conglomerate, and claystone. 

Graded bedding is common, and crossbedding, cut-and-fill 

structures, and synsedimentary slump features are locally 

present. Horizontal laminations, graded bedding, planar 

crossbedding, channel scours, transported boulders, carbonized wood fragments, and well-preserved leaf impressions 

indicate transport and deposition in low- to high-energy lakes 

and streams. Clasts are angular to subrounded and consist of 

detrital plagioclase, sanidine, biotite, and hornblende grains, 

volcanic clasts, glassy lithic fragments, and carbonized wood 

and leaf fragments. Plagioclase and mafic minerals in sandstone locally impart a salt-and-pepper texture. Rock fragments 

have a maximum diameter of 3 ft (1 m). Clasts are typically 

matrix supported. The suite of detrital grains indicates erosion 

and reworking of volcanic rocks. 

Geology 7

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Figure 3. Generalized stratigraphic sections of the Challis Volcanic Group at selected locations in the Little Wood 

River area, south-central Idaho. Lines of stratigraphic sections are shown in figure 2A, except for stratigraphic section 

G which was measured in the Lake Hills (fig. 1).
8 Geology and Stratigraphy of South-Central Idaho

Next, a thin, distinctive, lithic-rich composite, welded to 

unwelded ash-flow tuff was deposited, probably over much of 

the study area (fig. 4). Because of erosion, only small remnants of the tuff remain. West of the Little Wood River, the tuff 

consists of three emplacement units, the bottom two of which 

formed one cooling unit (app.). The lithic-rich tuff forms a distinctive marker horizon that typically forms low rubbly ledges 

and cuestas where it crops out in three places west of the Little 

Wood River, in several places on Garfield Mountain, and in the 

Lake Hills. It is lithologically very similar to and probably correlates with the tuff of Stoddard Gulch in the southeastern part 

of the Challis volcanic field, which has been dated at 44.3±2.7 

Ma using K-Ar methods on hornblende (Skipp, 1988, 1989) 

and at 47.3±0.15 Ma using Ar-Ar methods on biotite (F.J. 

Moye, unpub. data, 1989). The tuff in the Little Wood River 

area represents the interval of explosive volcanism that characterizes the middle phase of Challis volcanism throughout the 

volcanic field as identified by Moye and others (1988). In the 

Little Wood River area, the tuff is thickest in the Lake Hills 

and thinner both west and north, ranging from 0 to more than 

100 ft (30 m) in thickness. Lithic-rich tuff is reddish brown 

where devitrified and black and perlitic where still vitrified; 

it commonly has a white or green altered matrix. The lithicrich tuff is unwelded to densely welded and vitric-, crystal-, 

and pumice-rich. Lithic material constitutes 10–20 percent of 

the rock and phenocrysts 10–25 percent. Phenocrysts include 

plagioclase averaging 2 mm in diameter and conspicuous 

euhedral books of biotite averaging 2 mm in diameter. Biotite 

is aligned parallel with foliation. Lithic material ranges from 2 

mm to 25 cm in size and includes andesite, dacite, and rhyolite 

clasts. Pumice fragments are 5–15 cm long and moderately to 

extremely flattened. 

Above the ash-flow tuff is a sequence of mixed thin 

andesite flows separated by volcaniclastic conglomerate that 

forms the upper mixed unit (fig. 4, app.). Clasts of rhyolite 

indicate that some rhyolite was emplaced prior to deposition 

of the upper mixed unit. Rare clasts of Paleozoic quartzite 

indicate that some Paleozoic rocks were being eroded, but 

the predominance of volcanic clasts indicates that the Challis 

Volcanic Group covered most of the area.

A period of dacite intrusion and flow-dome formation followed deposition of the andesite and conglomerate of the upper 

mixed unit. Late dacite flow-dome emplacement corresponds 

to the third and final phase of Challis volcanism as identified 

by Moye and others (1988). Crosscutting relations are clearly 

visible for a distance of 2 mi (3.2 km) from the top of the measured section near the junction of the Little Wood River and 

Muldoon Creek south toward the Little Wood River reservoir. 

The intrusive contact extends about 800 ft (244 m) vertically. 

The intrusive mass itself consists of at least three crosscutting 

emplacement units with different proportions of hornblende 

and biotite and different abundance and size of plagioclase phenocrysts. Part of the mass may be extrusive, but it was clearly 

emplaced near the surface late in the volcanic history of the 

area. Because the intrusions were emplaced in layered volcanic and volcaniclastic rocks, the intrusive relations are clearly 

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Figure 4. Detailed stratigraphic sections of the Challis Volcanic Group in the Muldoon Summit–Little Wood River area, 

south-central Idaho. See appendix for detailed log.
visible. Undoubtedly other flow-dome complexes are present in 

the area, but, where dacite has intruded dacite and wellstratified rocks are absent, relations are less clear.

Rhyolite dikes cut andesite in the vicinity of Garfield 

Mountain and Copper Creek. Most of the dikes are in a swarm 

west of the junction of Copper and Muldoon Creeks, but 

isolated dikes are present farther north. The dikes are almost 

vertical and trend east-northeast to north-northeast. The 

rhyolite is white and contains euhedral fresh sanidine, quartz, 

and sparse biotite in a devitrified, fine-grained, white matrix. 

The only firm age constraint is that the dikes are younger than 

the main andesite, which they cut. If the rhyolite dikes formed 

contemporaneous with rhyolite that was eroded and deposited 

as detrital clasts in conglomerate of the upper mixed unit, then 

the rhyolite dikes would predate the upper mixed unit. Thus, 

the rhyolite dikes probably are younger than the lower part of 

the main andesite unit and older than the upper mixed unit. 

If the rhyolite dikes are unrelated to the rhyolite clasts in the 

upper mixed unit, then the dikes could be Miocene or younger.

Miocene and Quaternary Sedimentary

and Volcanic Rocks

Miocene Idavada Volcanics consisting of rhyolite ashflow tuffs were deposited unconformably on Challis andesite, dacite, and lithic-rich welded tuff. Idavada flows are flat 

lying and as a unit are as thick as 700 ft (200 m). Densely 

welded zones in the Idavada Volcanics are black and vitreous 

on fresh surfaces. Nonwelded zones are brown and locally 

highly vesicular and exhibit buff, punky weathering. Surfaces 

between flows are marked locally by pebble conglomerate 

horizons. The tuffs contain phenocrysts of sanidine, plagioclase, iron-titanium oxide minerals, and pyroxene. Lithic fragments and pumice are common but not abundant.

At least two major sets of Quaternary basalt flows were 

erupted in the valley of Muldoon Creek. A higher, older, 

more dissected set of flows caps mesas on both sides of Little 

Fish Creek in the Lake Hills at elevations of 6,000–6,200 ft 

(1,829–1,890 m). A lower, younger, less dissected set of flows 

was erupted at an elevation of at least 5,700 ft (1,738 m) and 

flowed down the Muldoon Creek valley for about 5 mi (8 km) 

and then down the Little Wood River valley for another 5 mi (8 

km) to an elevation of approximately 5,000 ft (1,524 m). The 

upper ends of each flow are covered by Quaternary morainal, 

terrace, and outwash deposits. The topographic expression of 

both sets of basalt flows suggest that each consists of at least 

two individual flows. Flow tops are vesicular, and the top 

surface typically is covered by a mantle of loess more than 

6 ft (2 m) thick in places. Dissection by Muldoon Creek and 

the Little Wood River reveals much of the upper part of the 

flows along canyon walls. The edges of the flows have fields 

of boulders that streamed off the flows and into the canyon. 

Blocks in the boulder field have significant lichen cover. Fresh 

and weathered surfaces are medium to dark gray. The loess 

cover, boulder field, and lichen cover suggest a pre-Wisconsin 

(pre-Pinedale) age (K.L. Pierce, oral commun., 1988). Phenocrysts in the basalt include plagioclase, pyroxene, and olivine.

Pleistocene Bull Lake(?) terrace deposits consist of 

unconsolidated cobble, gravel, and sand deposits, 80–200 ft 

(24–60 m) above valley bottoms along the Little Wood River, 

Baugh Creek, and Muldoon Creek. Probably contemporaneous morainal and outwash deposits along Muldoon Creek are 

as thick as 140 ft (42 m). The terrace deposits can be traced 

along the Little Wood River valley downstream to the basalt. 

On the east side of the valley, just north of the basalt, the 

sediments have the appearance of beach deposits. The similar 

elevation of the upper surfaces of the terrace deposits, beachlike deposits, and basalt flows suggests that a lake may have 

formed behind a dam of basalt flows that came down Muldoon 

Creek (Sanford, 1988). In the valley of Muldoon Creek, large 

moraines accumulated on the younger, lower basalt; thus, 

the younger basalt flows predate the older gravel deposits. 

The large volume of the morainal deposits suggests that the 

younger flows were contemporaneous with the higher terrace 

gravels in the Little Wood River valley. The extensive erosion of the higher terrace deposits, the relation of terrace and 

morainal deposits to the basalt flows, and the large original 

volume of the older terrace and morainal deposits suggest a 

Bull Lake age for the higher terrace gravels and moraines.

Pleistocene and Holocene, Pinedale(?) and younger 

alluvium consists of unconsolidated alluvial terrace deposits 

of silt, sand, and gravel that form flat valley bottoms. Streams 

have eroded only about the top 3 ft (1 m) of these deposits. 

Compared to the highly eroded, higher terraces, these terrace 

deposits are virtually intact.

Quaternary landslide deposits are slump deposits mainly 

on steep slopes underlain by bleached volcaniclastic sedimentary rocks, tuffs, and tuff breccias. The largest such deposit 

is in the Lake Hills. It is 11,000 ft (3,300 m) long and 2,500–

5,000 ft (750–1,500 m) wide, and descends from an elevation 

of 6,400 ft (1,920 m) at the head of the scarp to 5,000 ft (1,500 

m) at the distal end. The fresh appearance of the slide, its lack 

of internal drainage, and its superposition on the youngest 

stream terrace in the Little Wood River valley suggest that the 

slide occurred within the past several thousand years (R.H. 

Morris, U.S. Geological Survey, written commun., 1972).

Age

The age of the units in the study area can be estimated by 

correlation with dated rocks elsewhere in the Challis volcanic field (see section, “Geochronology”). The whole eruptive 

cycle lasted from 51 to 44 Ma (McIntyre and others, 1982; 

Moye and others, 1988, 1989). Dominantly effusive eruptions 

of intermediate-composition volcanic rocks were between 

51 and 48 Ma. Intermediate-composition volcanism began at 

52.03±0.32, 50.92±0.22, and 48.17±0.18 Ma in the southwestern part of the field near Ketchum (F.J. Moye and others, written 

commun., 1991), at 49.4±0.7 Ma in the southernmost part of 

the field (Betty Skipp, personal commun., in Moye and others, 

Geology 9
10 Geology and Stratigraphy of South-Central Idaho

1988), and at 48.8±0.17 Ma in the southeast part of the Challis 

volcanic field (F.J. Moye and others, written commun., 1991). 

These dates suggest an eastward progression for the onset of 

volcanism in the southern Challis volcanic field (F.J. Moye and 

others, written commun., 1991). In the Little Wood River area, 

this early phase of volcanism is represented by the andesite and 

dacite that make up about 90 percent of the section.

Thereafter, at about 48.5–47 Ma, explosive silicic volcanism was common. The change from intermediate to silicic 

composition coincides with a change in stress from northeast-southwest extension along northwest-trending structures 

to northwest-southeast extension along northeast-trending 

structures (Moye and others, 1988, 1989). The tuff of Stoddard Gulch, which probably correlates with the lithic-rich tuff 

of this report, was first dated by K-Ar methods on hornblende at 44.3±2.7 Ma (Skipp, 1988, 1989) and later dated 

at 47.37±0.15 Ma by Ar-Ar techniques on single crystals of 

biotite (F.J. Moye and others, written commun., 1991).

The final stage of volcanism is characterized by dacitic 

to rhyolitic domes and flows emplaced at 48–45 Ma; a few 

intrusions are as young as 40 Ma (F.J. Moye and others,written 

commun., 1991). These intrusive rocks correlate with large 

hypabyssal intrusions such as the granite of the Sawtooth 

batholith, the Casto pluton, and the Mackay stock. This phase 

of eruption probably corresponds to emplacement of the dacite 

flow-dome complex along the Little Wood River and the rhyolite dike swarm and isolated dikes in the Muldoon area.

Paleotopography and Structure

The Challis Volcanic Group in the Little Wood River valley unconformably overlies Paleozoic rocks and fills preexisting paleotopographic lows. Most contacts between the Challis 

Volcanic Group and Paleozoic rocks show depositional onlap 

of the Challis Volcanic Group. Paleorelief probably was more 

than 2,000 ft (600 m). Over large areas, the Challis Volcanic 

Group has been eroded preferentially relative to the underlying 

siliciclastic Paleozoic rocks. This erosion has exhumed the preChallis paleotopography, which roughly resembles the modern 

topography. The pre-Challis paleotopography was controlled by 

variations in erosional resistance of the underlying Wood River 

Formation. Modern topographic highs are commonly related 

to resistant siliceous units within the Wood River Formation. 

The Challis Volcanic Group wraps around these highs, and the 

irregular contact relationships show that the pre-Challis paleotopographic slopes resemble the modern slopes. For example, 

along the western contact of the Challis Volcanic Group in 

the vicinity of Seamans Creek, volcanic rocks have filled 

west-trending valleys approximately in the same locations as 

valleys today. An elongate exposure of andesitic volcanic rocks 

surrounded by Wood River Formation, east of the Little Wood 

River in the central part of the study area, is the remnant of a 

south-trending pre-Challis paleovalley dissected at 90° angles 

by two west-flowing modern streams. Perhaps this was the valley of the Little Wood River or a similarly oriented predecessor 

during pre-Challis time. Onlap of the Challis Volcanic Group 

onto Paleozoic rocks is also present on the long west-trending 

contact south of Bell Mountain and on the long west-northwest 

contact north of Baugh Creek. In the extreme northeastern part 

of the study area, a north-northwest-trending paleovalley filled 

with andesite and bounded on both sides by Paleozoic rocks is 

clearly visible from points along the ridge to the south.

Pre-Challis paleotopography was controlled in part by 

pre-Challis west- and northwest-trending extensional faults. 

Early- or pre-Eocene northwest-trending structures controlled 

the early phase of Challis volcanism elsewhere in the Challis 

volcanic field (McIntyre and others, 1982; Moye and others, 

1988, 1989), and early Tertiary extension is recognized in Late 

Proterozoic and early Paleozoic rocks of the Pioneer Mountains 

metamorphic core complex to the north (Wust, 1986; O’Neill 

and Pavlis, 1988). In the Little Wood River area, the influence 

of pre-Challis faulting can best be seen south and southwest 

of Bell Mountain. The long, straight, west-trending contact 

between the Challis Volcanic Group and the Wood River Formation is clearly a depositional contact; however, marker beds 

in the Wood River Formation on the east side of the Big Wood 

River valley show that structural blocks have been dropped 

down to the south along multiple, parallel, west-trending faults. 

It may be inferred that these west-trending faults continue to 

the east under the Challis Volcanic Group and that motion on 

them was in part responsible for pre-Challis uplift north of the 

inferred fault. The depositional contact indicates that significant erosion took place on the inferred fault scarp prior to 

Challis volcanism. A similar long, straight, depositional contact 

having a west-northwest trend is present along Baugh Creek 

and extends south of Elk Mountain. Based on similarity with 

the inferred pre-Challis fault south of Bell Mountain, a covered 

fault may be postulated north of Baugh Creek and south of Elk 

Mountain. Faulting in Paleozoic rocks to the northwest would 

show if this fault actually exists.

High-angle, northwest-trending, post-Challis normal 

faults are present as straight contacts between Challis Volcanic 

Group and Paleozoic rocks. The long, straight, northwesttrending contact bounding the Challis Volcanic Group on the 

northeast of the study area in the vicinity of Copper Creek 

may be a post-Challis normal fault. The similarly oriented, 

short, normal fault near Muldoon Summit clearly displaces the 

Challis Volcanic Group and postdates volcanism. The age and 

orientation suggest basin-and-range faulting. The similarity 

in orientation between pre- and post-Challis faults suggests 

that post-Challis faults may have been active prior to Eocene 

volcanism.

The Challis Volcanic Group has a consistent tilt toward the 

east at an average orientation of N. 8° E., 36° E. The regional 

tilt is clearly visible in thick sequences dominated by layered 

andesite flows and breccias, for example, between Muldoon 

Summit and the Little Wood River and in the northeastern part 

of the study area in the vicinity of Garfield Mountain. Strikes 

and dips were determined (1) from the map patterns of interbedded lava flows and sedimentary rocks covering large areas, 

(2) from outcrop measurements of volcaniclastic sedimentary 
rocks, and (3) from outcrop measurements of lava flow layering and foliation in ash-flow tuffs. Except for dips of flow 

layering in dacitic lavas, which are highly variable, dip values 

from all three sources generally agree. About one-third of the 

40 outcrop measurements are from sedimentary rocks that 

were almost flat lying when deposited; thus, the tilt is mainly 

postdepositional. Further, much of the regional tilt must be 

structural because, although many eruptive centers are inferred, 

the regional tilt has one consistent, eastward direction. This 

tilt is more likely related to northwest-trending, presumably 

basin-and-range faults rather than to large stratovolcanoes. 

Similarly, consistent northward tilt in the southeastern Challis volcanic field to the east is also interpreted as the result of 

postdepositional tilting and faulting (F.J. Moye and others, 

written commun., 1994). Local variations are probably due to 

depositional tilt and local, postdepositional tectonic activity. 

It is unlikely that the entire area is made up of a single homoclinal sequence. Because a 36° dip for 12 mi (20 km) across 

the study area would imply an unreasonably large vertical 

displacement, several undetected faults probably separate the 

region into structural blocks. A likely place for one such fault is 

along the Little Wood River. Although it is unlikely that major 

faults would be overlooked in well-stratified rocks, there may 

be undetected faults in large areas of dacite flows, domes, and 

flow breccias.

Northeast-trending lineaments are conspicuous on aerial 

photographs and topographic maps. They are most commonly 

shown by straight sections of stream channels, colinear small 

tributary valleys, and vegetation changes. No displacements 

are evident; however, because the lineaments are typically in 

valley bottoms, detailed relations are not visible. The orientation suggests a relationship to similarly oriented Eocene 

extension zones exhibited by dike swarms and isolated dikes 

in the Muldoon area (fig. 2B), by dike swarms associated with 

the Mackay stock (F.J. Moye and others, written commun., 

1994), by a major horst block extending to the Lost River 

Range (F.J. Moye and others, written commun., 1994), and by 

the Trans-Challis fault system in the Challis 1°×2° quadrangle 

(Bennett, 1986; Kiilsgaard and others, 1986). The lineaments 

may be fracture zones related to incipient rifting.

Geochemistry of Whole Rocks and

Stream Sediments

Representative volcanic whole rocks were analyzed by 

X-ray fluorescence for major elements (table 1; fig. 5). Analytical procedures are described in Boyd and Mertzman (1987). 

Analyzed Challis Volcanic Group rocks are all potassium 

rich. Rocks referred to in this paper as andesite are typically 

high-potassium basaltic andesite and high-potassium andesite, 

shoshonite, and banakite (latite). Rocks referred to as dacite 

are high-potassium dacite and trachyte. Tuffs and vitrophyres 

range in composition from basaltic andesite to trachyte. Some 

of the compositional variation in these rocks may be due to 

hydrothermal alteration. The tuff sample that plots as a basaltic 

andesite, for example, is visibly altered, and the high-silica 

rhyolite sample appears to have been silicified and sodium 

exchanged for potassium. The Challis Volcanic Group is calcalkaline and has an alkali-lime index of 58.1, according to the 

classification of Peacock (1931). The range of compositions in 

the study area is similar to that reported for the Challis volcanic 

field to the north and east (McIntyre and others, 1982; Moye 

and others, 1988; Norman and Mertzman, 1991).

Miocene Idavada Volcanics are rhyolite and are compositionally similar to rhyolite of the Snake River Plain (Leeman, 

1982). Quaternary basalt does not follow the high-potassium 

trend of the Eocene and Miocene volcanic rocks.

Stream-sediment analyses for trace elements by inductively coupled argon plasma methods are reported for samples 

collected in the part of the study area west of long 113° W. in 

Smith (1995) and in the Little Wood River Wilderness Study 

Area in Sanford and others (1989). Stream sediment anomalies in Ag, As, Ba, Bi, Cr, Cu, La, Mo, Ni, Pb, Sb, V, and Zn 

are probably related to background levels in black shale and 

to minor hydrothermal mineralization in the Milligen and 

Wood River Formations (Sanford and others, 1989). Streams 

draining terrain underlain by Challis Volcanic Group are commonly anomalous in copper and chromium and less commonly 

anomalous in zinc. These elements are probably anomalous 

because intermediate-composition rocks are typically enriched 

in these elements relative to more felsic rocks used to define 

the threshold values. Lack of associated anomalies in elements 

such as silver, lead, or zinc suggests that the copper anomalies are not due to epithermal mineralization. Chromium is 

likely to have been derived from andesite, which has typical 

chromium contents of 200–400 ppm (Norman and Mertzman, 

1991), rather than from shale, which has typical chromium 

contents of 100 ppm (Vine and Tourtelot, 1970). No stream 

sediments or rock chips from east of long 113° W. were 

analyzed; thus any possible mineralization associated with the 

Blackspar mine (fig. 2B) was not identified.

Alteration

All signs of alteration were examined for relevance to 

economic levels of mineralization. Aerial photographs reveal 

patches of white rocks, and Landsat imagery shows areas 

of green alteration. The white alteration is probably due to 

bleaching and argillization of volcaniclastic sedimentary 

rocks, and clay minerals suggest alteration by shallow, acidic, 

oxidizing ground water; however, no evidence of silicification, 

veining, or mineralization was observed.

Celadonite imparts a distinctive light-apple to dark-green 

color and alters both andesite and dacite. Celadonite alteration 

is common in outcrop, and larger areas of celadonite alteration 

show up on Landsat imagery with band 2. Celadonite alteration is most intense in dacite flows and flow breccias along the 

middle part of Baugh Creek. Lineaments that cross this area 

may be related to the celadonite alteration. The extension of 

the northwest-trending fault near Muldoon Summit is marked 

Geology 11
12 Geology and Stratigraphy of South-Central Idaho

1 2 3 4 5 6 7 8 9 10 11

7H10A 7H13 7H18 7H20 7H22A 7H59 7H22B 7H64 7H49 7H58 7H87A

SiO2 55.30 53.69 53.47 53.15 56.24 52.90 60.52 57.91 58.48 58.54 62.61

TiO2 0.81 0.85 0.97 0.80 0.77 0.98 0.70 0.65 0.93 0.99 0.59

Al2O3 13.41 13.49 12.92 13.19 17.72 11.74 16.46 14.13 14.19 14.36 14.44

Fe2O3

*1 9.40 9.05 10.26 8.84 5.85 8.96 4.46 7.08 7.78 7.84 5.25

MnO 0.15 0.14 0.16 0.15 0.05 0.14 0.04 0.10 0.10 0.13 0.09

MgO 5.93 5.71 4.60 8.97 1.83 4.72 1.33 6.34 3.64 1.72 2.83

CaO 7.57 7.74 8.70 8.21 5.74 7.67 3.68 6.37 5.05 5.69 3.75

Na2O 2.93 2.58 2.99 2.10 1.82 2.22 1.80 3.16 2.96 2.51 3.14

K2O 2.62 2.92 2.40 1.75 1.98 3.87 3.53 2.12 4.06 4.07 4.15

P2O5 0.31 0.36 0.46 0.22 0.28 0.58 0.23 0.17 0.37 0.44 0.26

LOI**2 1.36 3.67 2.88 2.29 8.39 6.62 7.84 1.62 1.86 3.37 2.84

TOTAL 99.79 100.20 99.81 99.67 100.67 100.40 100.59 99.65 99.42 99.66 99.95

12 13 14 15 16 17 18 19 20 21 22

7H35 7H109 7H110 7H21 7H22D 7H69 7H34 7H31A 7H31B 7H33 7H126

SiO2 63.41 64.83 65.80 62.92 63.26 66.17 77.00 71.64 69.55 45.84 46.25

TiO2 0.69 0.50 0.53 0.74 0.69 0.52 0.27 0.61 0.58 2.51 1.89

Al2O3 14.68 14.51 15.09 16.49 15.77 14.72 10.06 12.95 12.76 15.02 15.84

Fe2O3

*1 4.98 4.79 4.64 3.67 4.03 4.23 1.05 4.23 4.22 14.91 13.41

MnO 0.05 0.08 0.07 0.04 0.06 0.06 0.03 0.05 0.06 0.20 0.19

MgO 2.35 2.15 1.67 1.47 1.26 1.74 0.51 0.27 0.39 7.02 8.56

CaO 3.49 2.86 3.15 3.58 3.45 2.46 0.65 1.48 1.73 10.19 10.15

Na2O 3.20 3.28 3.44 3.28 2.62 2.93 0.00 3.37 2.87 2.41 2.06

K2O 4.06 3.79 3.69 4.83 5.11 4.46 7.57 4.62 4.83 0.43 0.22

P2O5 0.28 0.18 0.19 0.25 0.24 0.21 0.10 0.11 0.10 0.47 0.36

LOI**2 2.55 2.51 1.81 2.20 3.33 2.12 2.28 0.75 2.51 0.55 0.61

TOTAL 99.74 99.48 100.08 99.47 99.82 99.62 99.52 100.08 99.60 99.55 99.54

Table 1. Whole rock analyses of volcanic rocks, Little Wood River area, south-central Idaho.

[In weight percent. Analyses by X-ray fluorescence; Stanley A. Mertzman, analyst]

*1 Total Fe as Fe2O3.

**2 Loss on ignition at 950°C.
by celadonite alteration that is associated with slickensides and 

vugs filled with opalescent quartz. Thus, celadonite alteration 

probably is related partly to faults and linear features that may 

represent zones of higher permeability and fluid flow.

Economic Geology

Mineralization in Paleozoic rocks of the Little Wood 

River area has been described by Sanford and others (1989). 

The only mine workings in the Challis Volcanic Group of 

the Little Wood River area are at the Blackspar (Silver Spar) 

mine (fig. 2B). No published information is available on the 

property, and the description herein is based on observations 

by James J. Jones (consultant, written commun., 1971) and by 

members of the Hailey project. In 1923, there were more than 

4,500 ft (1,350 m) of underground workings at the Blackspar 

mine including the Slayton Tunnel, 1,700 ft (510 m) long, the 

Alexander Tunnel, 1,100 ft (330 m) long, and two shafts 200 

ft (60 m) deep and 30 ft (9 m) deep. In 1971, 1,100 ft (330 m) 

of workings was reopened and sampled. Composite samples 

of shear zone material showed no silver above 1 ppm and no 

detectable gold, lead, or zinc; the reported silver deposit at 

1,700 ft (510 m) was inaccessible and therefore not sampled. 

Thus, it is not known if the reported deposit is in rocks of 

the Challis Volcanic Group. The only known production is 

reported to be one truckload of ore of unknown grade or metal 

content. The dumps contain propylitized and silicified andesite 

that contains traces of disseminated pyrite and galena(?).

Somewhat promising locations for economically valuable minerals that have not been prospected are in areas of 

alteration and fracturing. Argillic alteration and calcite-quartz 

vug filling, indicative of hydrothermal activity, are present in 

several areas. Celadonite alteration is common and is locally 

related to faulting and lineaments. Stream sediments downstream from lineaments showed no indications of mineralization except for anomalous copper values that are probably not 

significant in terrain underlain by andesitic volcanic rocks, as 

discussed previously. Rhyolite dikes in the northeastern part of 

the area are interpreted as favorable indicators of gold-related 

mineralization; however, no evidence of silicification or 

sulfide mineralization was observed associated with argillic 

or celadonite alteration or with the rhyolite dikes. Hydrothermal alteration is common in most volcanic terrains, so that, 

without associated features suggestive of mineralization, the 

alteration and rhyolite dikes suggest only a low probability of 

economic mineralization. If mineralized rock is present, it may 

be at depth because the observed alteration features are typical 

of shallow hydrothermal and deuteric activity.

The types of mineral deposits that may be expected, if 

any are present, are indicated by deposits elsewhere in central 

Idaho. The Challis Volcanic Group is host to three types of 

deposits (Moye and others, 1989). Shallow hot-spring deposits 

are characterized by hydrothermal breccia and siliceous sinter 

at the paleosurface, extensive propylitic and argillic alteration, 

and control by north- and northeast-trending structures. Gold 

Geology 13

Sample Descriptions and Localities

 1. Challis Volcanic Group, high-potassium basaltic andesite, containing 

plagioclase and olivine(?); rust stained; Baugh Creek SW quadrangle, 

sec. 3, T. 2 N., R. 19 E.

 2. Challis Volcanic Group, high-potassium basaltic andesite, containing 

abundant pyroxene; Baugh Creek SW quadrangle, sec. 34, T. 3 N., R. 

19 E.

 3. Challis Volcanic Group, high-potassium basaltic andesite, containing 

plagioclase, pyroxene, and olivine(?); Baugh Creek SW quadrangle, sec. 

12, T. 2 N., R. 19 E.

 4. Challis Volcanic Group, high-potassium basaltic andesite, containing 

olivine(?); rust stained; Baugh Creek SW quadrangle, sec. 12, T. 2 N., 

R. 19 E.

 5. Challis Volcanic Group, high-potassium basaltic andesite, tuff, containing 

plagioclase, and quartz(?), altered; Lake Hills quadrangle, sec. 21, T. 1 

N., R. 21 E.

 6. Challis Volcanic Group, shoshonite, containing plagioclase, and pyroxene; 

Mackay 3 SW quadrangle, sec. 5, T. 2 N., R. 21 E.

 7. Challis Volcanic Group, high-potassium andesite, tuff; Lake Hills quadrangle, sec. 21, T. 1 N., R. 21 E.

 8. Challis Volcanic Group, high-potassium andesite, aphanitic, rare plagioclase; Seamans Creek quadrangle, sec. 25, T. 2 N., R. 19 E.

 9. Challis Volcanic Group, banakite, containing abundant pyroxene; Mackay 

3 SW quadrangle, sec. 29, T. 3 N., R. 21 E.

10. Challis Volcanic Group, banakite, crystal-poor, containing pyroxene; 

Mackay 3 SW quadrangle, sec. 5, T. 2 N., R. 21 E.

11. Challis Volcanic Group, banakite; Mackay 3 SW quadrangle, sec. 20, T. 3 

N., R. 21 E.

12. Challis Volcanic Group, high-potassium dacite, containing abundant 

biotite and plagioclase; Baugh Creek SW quadrangle, sec. 22, T. 2 N., 

R. 19 E.

13. Challis Volcanic Group, high-potassium dacite, containing abundant 

plagioclase, and lesser biotite, hornblende, and pyroxene; Baugh Creek 

quadrangle, sec. 14, T. 3 N., R. 20 E. 

14. Challis Volcanic Group, high-potassium dacite, containing abundant 

plagioclase and biotite, and lesser hornblende; Baugh Creek quadrangle, 

sec. 15, T. 3 N., R. 20 E.

15. Challis Volcanic Group, trachyte, welded tuff, containing abundant 

plagioclase, biotite, and fiamme; Lake Hills quadrangle, sec. 21, T. 1 N., 

R. 21 E.

16. Challis Volcanic Group, trachyte, vitrophyre, containing plagioclase, 

sanidine(?), and abundant lithic fragments; Lake Hills quadrangle, sec. 

21, T. 1 N., R. 21 E.

17. Challis Volcanic Group, trachyte, containing abundant plagioclase and 

biotite; Mackay 3 SW quadrangle, sec. 5, T. 2 N., R. 21 E.

18. Challis(?) Volcanic Group, high-silica rhyolite, tuff vitrophyre; Baugh 

Creek SW quadrangle, sec. 27, T. 3 N., R. 19 E.; mapped by Hall and 

Batchelder (1979) as Challis vitrophyre; may be Idavada Volcanics.

19. Idavada Volcanics, high-potassium rhyolite, welded tuff; Carey quadrangle, sec. 33, T. 1 S., R. 21 E.

20. Idavada Volcanics, high-potassium rhyolite, tuff vitrophyre, probably 

altered; Carey quadrangle, sec. 33, T. 1 S., R. 21 E.

21. Quaternary basalt, Lake Hills quadrangle, sec. 8, T. 1 N., R. 21 E.

22. Quaternary basalt, Baugh Creek quadrangle, sec. 15, T. 2 N., R. 20 E.
14 Geology and Stratigraphy of South-Central Idaho

and silver are present in fissures and are associated with pyrite, 

tetrahedrite, argentite, electrum, and sulfosalts. Indicator elements include arsenic, bismuth, mercury, molybdenum, and 

antimony. At intermediate depths, vein deposits are characterized by quartz-filled fissures along north- to northwest-trending structures and intersections of these structures with each 

other. Vein deposits are associated with intermediate to silicic 

intrusions, brecciation, and silicification. Ore minerals include 

pyrite, chalcopyrite, molybdenite, tetrahedrite, galena, and 

sphalerite. At the greatest depth are porphyry-type stockwork 

and contact-related deposits associated with shallow intrusions. The stockwork deposits are accompanied by widespread 

propylitic and argillic alteration and local silica flooding.

The Little Wood River area is permissive for all three 

types of deposits. The alteration, northeast lineaments, rhyolite 

dikes, and one known deposit are consistent with shallow hotspring and intermediate vein deposits. Aeromagnetic data suggest the presence of several intrusions in the northeastern part 

of the area (Mabey, 1981; Sanford and others, 1989), any one 

of which could have porphyry-stockwork or contact-related 

deposits at depth. 

Summary and Conclusions

The eruptive history of the Challis volcanic field in 

central Idaho can be divided into three parts: (1) early effusive volcanism, dominantly andesitic to dacitic, followed by 

(2) explosive dacitic to rhyolitic ash-flow tuff eruptions and 

formation of cauldron complexes, and culminating in (3) 

intrusion of late-stage dacite to rhyolite domes and plugs. In 

the Little Wood River area, the first stage represents approximately 90 percent of the section. The first stage is further 

divided into a basal andesite, a lower sedimentary unit, a 

main andesite unit, a main dacite unit, and an upper sedimentary unit. The main andesite and dacite units are the most 

widespread and are as thick as 3,000 and 2,000 ft (1,000 and 

600 m), respectively. During the first stage, thick sequences 

of sedimentary rocks were deposited in paleotopographic 

lows on and between lava flows. The second stage is represented by a thin, but widespread, welded to unwelded lithicrich ash-flow tuff that may correlate with the tuff of Stoddard 

Gulch. Interlayered andesite and conglomerate were deposited on the ash-flow tuff. Rhyolite dikes were emplaced and 

eroded late in stage two, during stage three, or later. Finally, 

in the last stage, the entire sequence was intruded by dacite 

flow-dome complexes.

Pre-Challis west- and northwest-trending faulting and 

variable erosional resistance of Paleozoic rocks controlled the 

paleotopographic surface on which the Challis Volcanic Group 

was deposited; paleotopographic lows mostly conform to the 

present-day topographic surface. Syn- and post-Challis deformation features included northeast-trending lineaments related 

to the Trans-Challis fault system and northwest-trending basin 

and range faults, possibly reactivated, probably associated 

with eastward tilting. Some potential exists for shallow, intermediate, and deep hydrothermal deposits in areas of argillic 

alteration, northeast-trending lineaments, and rhyolite dikes.

Acknowledgments

I thank Falma J. Moye, Larry G. Snider, Betty Skipp, 

William R. Hackett, and William P. Leeman for valuable advice 

and Stanley A. Mertzman for whole-rock analyses. Falma J. 

Moye and William E. Brooks reviewed the manuscript.

Geochronology

By Lawrence W. Snee

Six samples were analyzed by 40Ar/39Ar techniques. Five 

of the six samples were collected from stratigraphic section 

A (fig. 2A), and one sample (8H138) was collected from the 

main rhyolite dike swarm south of the Blackspar mine (fig. 

2B). The data are presented herein with minimal interpretation.

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Figure 5. Chemical variation diagram showing volcanic rocks 

from Little Wood River area, south-central Idaho, using classifications of Peccerillo and Taylor (1976), Ewart (1979), and Moye and 

others (1988).
Most of the age data are consistent with stratigraphic 

relations. In general, the samples are younger toward the top. 

The different phases of volcanism (andesitic and dacitic) are 

consistent with other chronologies (F.J. Moye and others, 

written commun., 1994). Andesitic volcanism took place 

around 49.5 Ma, and dacitic volcanism took place from about 

48.2 to 47.5 Ma. The age of the rhyolite dike is particularly significant because the age of dike intrusion is poorly 

constrained by stratigraphic evidence, which allows any age 

that is post-Challis Volcanic Group, including Miocene. The 

present age data demonstrate a late Challis episode of rhyolite 

intrusion at about 45.7 Ma.

Two ages are not easily interpreted. One dacite sample 

(8H106) in the flow-dome complex yields an age of 49.4 Ma 

(undisturbed hornblende) that is significantly older than the 

47.4 Ma age for the other dacite flow-dome sample (8H105). 

Geologically, there is no apparent difference between the 

samples despite the lack of apparent disturbance; perhaps 

one sample gained or lost argon. The other puzzling sample 

(8H141) is from the lithic-crystal tuff. If the lithic-crystal tuff 

correlates with the tuff of Stoddard Gulch, it should have an 

age near 47.37±0.15 Ma (Moye and others, 1989); however, 

the measured age of 48.0±0.1 Ma is significantly different. 

Perhaps this ash-flow tuff correlates with a different, dated or 

undated tuff. Alternatively, the sample may have been thermally disturbed by overlying lavas.

References Cited

Armstrong, R.L., and Ward, Peter, 1991, Evolving geographic 

patterns of Cenozoic magmatism in the North American Cordillera—The temporal and spatial association of 

magmatism and metamorphic core complexes: Journal of 

Geophysical Research, v. 96, no. B8, p. 13201–13224.

Bennett, E.H., 1986, Relationship of the trans-Challis fault 

system in central Idaho to Eocene and Basin and Range 

extensions: Geology, v. 14, p. 481–484.

References Cited 15

Bostwick, D.A., 1955, Stratigraphy of the Wood River Formation, south-central Idaho: Journal of Paleontology, v. 29,

p. 941–951.

Boyd, F.R., and Mertzman, S.A., 1987, Composition and structure of the Kaapvaal lithosphere, southern Africa, in Mysen, 

B.O., ed., Magmatic processes—Physiochemical principles: 

Geochemical Society Special Publication 1, p. 13–24.

Burton, B.R., and Blakley, J.D., 1988, The basal Challis conglomerate, Custer County, south central Idaho—Implications 

for the initiation of Challis volcanism: Geological Society of 

America Abstracts with Programs, v. 20, p. 408–409.

Carlson, D.H., Fleck, Robert, Moye, F.J., and Fox, K.F., 

1991, Geology, geochemistry, and isotopic character of the 

Colville igneous complex, northeastern Washington: Journal 

of Geophysical Research, v. 96, no. B8. p. 13313–13333.

Dover, J.H., 1969, Bedrock geology of the Pioneer Mountains, 

Blaine and Custer Counties, central Idaho: Idaho Bureau of 

Mines and Geology Pamphlet 142, 66 p.

Dover, J.H., 1981, Geology of the Boulder-Pioneer Wilderness Study Area, Blaine and Custer Counties, Idaho: U.S. 

Geological Survey Bulletin 1497–A, p. 15–75.

Dover, J.H., 1983, Geologic map and sections of the central 

Pioneer Mountains, Blaine and Custer Counties, central 

Idaho: U.S. Geological Survey Miscellaneous Investigations 

Series Map I–1319, scale 1:48,000.

Dudas, F.O., 1991, Geochemistry of igneous rocks from the 

Crazy Mountains, Montana, and tectonic models for the 

Montana alkalic province: Journal of Geophysical Research, 

v. 96, B8, p. 13261–13278.

Ekren, E.B., 1985, Eocene cauldron-related volcanic events 

in the Challis quadrangle, in McIntyre, D.H., Symposium 

on the geology and mineral deposits of the Challis 1°×2°

quadrangle, Idaho: U.S. Geological Survey Bulletin 1658–C, 

p. 43–58.

Ewart, A., 1979, A review of the mineralogy and chemistry of 

Tertiary—Recent dacitic, latitic, rhyolitic, and related salic 

volcanic rocks, in Barker, Fred, ed., Trondhjemites, dacite, 

and related rocks: Amsterdam, Elsevier, p. 13–121.

Ewing, T.L., 1980, Paleogene tectonic evolution of the Pacific 

Northwest: Journal of Geology, v. 88, p. 619–638.

Fisher, F.S., McIntyre, D.H., and Johnson, K.M., 1983, 

Geologic map of the Challis 1°×2° quadrangle, Idaho: 

U.S. Geological Survey Open-File Report 83–523, scale 

1:250,000, 41 p.

Fox, K.F., Jr., 1983, Melanges and their bearing on Late Mesozoic and Tertiary subduction and interplate translation at 

the west edge of the North American plate: U.S. Geological 

Survey Professional Paper 1198, 40 p.

Sample 

No. Mineral Age (Ma) Height in section (ft)

Rock

description

8H121 Biotite slightly

disturbed,

≈45.6±0.1

Rhyolite dike

Sanidine 45.7±0.1

8H105 Biotite 47.5±0.1 3,320–3,430 Dacite flow dome

8H106 Biotite disturbed,

≈50±1

3,320–3,430 Dacite flow dome

Hornblende 49.4±0.1

8H141 Biotite 48.0±0.1 2,980 Lithic-crystal tuff

8H138 Biotite 48.22±0.09 2,200 Dacite lava

8H140 Hornblende 49.5±0.1 440 Andesite lava 
16 Geology and Stratigraphy of South-Central Idaho

Fox, K.F., Jr., and Beck, M.E., 1985, Paleomagnetic results for 

Eocene volcanic rocks from northeastern Washington and 

the Tertiary tectonics of the Pacific Northwest: Tectonics,

v. 4, p. 323–341.

Gest, D.E., and McBirney, A.R., 1979, Genetic relations 

of shoshonite and absarokite magmas, Absaroka Mountains, Wyoming: Journal of Volcanology and Geothermal 

Research, v. 6, p. 85–104.

Hackett, W.R., Moye, F.J., and Mertzman, S.A., 1988, Petrology of mafic to intermediate rocks from the Eocene Challis 

volcanic field, central Idaho: Geological Society of America 

Abstracts with Programs, v. 20, no. 5, p. 417.

Hall, W.E., and Batchelder, J.N., 1979, Preliminary geologic 

map of the Baugh Creek southwest 7 1⁄2-minute quadrangle: 

U.S. Geological Survey Open-File Report 79–1617.

Hall, W.E., Batchelder, John, and Douglass, R.C., 1974, 

Stratigraphic section of the Wood River Formation, Blaine 

County, Idaho: U.S. Geological Survey Journal of Research, 

v. 2, p. 89–95.

Kiilsgaard, T.H., Fisher, F.S., and Bennett, E.H., 1986, The 

trans-Challis fault system and associated precious metal 

deposits, Idaho: Economic Geology, v. 81, p. 721–724.

Leeman, W.P., 1982, Rhyolites of the Snake River Plain; 

Yellowstone Plateau Province, Idaho and Wyoming—A 

summary of petrogenic models, in Bonnichsen, Bill, and 

Breckenridge, R.M., Cenozoic geology of Idaho: Idaho 

Bureau of Mines and Geology Bulletin 26, p. 203–212.

Leonard B.F., and Marvin, R.F., 1982, Temporal evolution of 

the Thunder Mountain caldera and related features, central 

Idaho, in Bonnichsen, Bill, and Breckenridge, R.M., Cenozoic geology of Idaho: Idaho Bureau of Mines and Geology 

Bulletin 26, p. 23–42.

Lipman, P.W., 1980, Cenozoic volcanism in the western 

United States—Implications for continental tectonics, in 

Continental tectonics—Studies in geophysics: National 

Academy of Sciences, p. 161–174.

Lipman, P.W., and Glazner, A.F., 1991, Introduction to 

Middle Tertiary Cordilleran volcanism—Magma sources 

and relations to regional tectonics: Journal of Geophysical 

Research, v. 96, no. B8, p. 13193–13199.

Lipman, P.W., Prostka, H.J., and Christiansen, R.L., 1971, 

Evolving subduction zones in the western United States, as 

interpreted from igneous rocks: Science, v. 174, p. 821–825.

Lipman, P.W., Prostka, H.J., and Christiansen, R.L., 1972, 

Cenozoic volcanism and plate-tectonic evolution of the 

western United States; early and middle Cenozoic: Philosophical Transactions of the Royal Society of London, v. 

271, p. 217–248.

Lukowicz, L.J., 1971, Geology of Mackay 3 SW and west part 

of Mackay 3 SE quadrangle, Blaine County, Idaho: University of Wisconsin at Milwaukee M.S. thesis, 127 p.

Mabey, D.R., 1981, Aeromagnetic studies of the Boulder-Pioneer Wilderness Study Area, Blaine and Custer Counties, 

Idaho: U.S. Geological Survey Bulletin 1497–B, p. 81–83.

Mahoney, J.B., Link, P.K., Burton, B.R., Geslin, J.K., and 

O’Brien, J.P., 1991, Pennsylvanian and Permian Sun Valley 

Group, Wood River basin, south-central Idaho, in Cooper, 

J.D., and Stevens, C.H., eds., Paleozoic paleogeography of 

the Western United States—II: Pacific Section Society of 

Economic Paleontologists and Mineralogists, Publication 

67, p. 551–579.

McDermott, R.K., and Snider, L.G., 1989, Stratigraphy, petrology, and structure of Eocene rocks in the Navarre Creek 

area, southeastern Challis volcanic field, Idaho, in Winkler, 

G.R., Soulliere, S.J., Worl, R.G., and Johnson, K.M., eds., 

Geology and mineral deposits of the Hailey and Idaho Falls 

1°×2° quadrangles, Idaho: U.S. Geological Survey OpenFile Report 89–639, p. 117.

McIntyre, D.H., Ekren, E.B., and Hardyman, R.F., 1982, 

Stratigraphic and structural framework of the Challis Volcanics in the eastern half of the Challis 1°×2° quadrangle, 

Idaho, in Bonnichsen, Bill, and Breckenridge, R.M., Cenozoic geology of Idaho: Idaho Bureau of Mines and Geology 

Bulletin 26, p. 3–22.

Meen, J.K., 1987, Formation of shoshonite from calcalkaline 

basalt magma—Geochemical and experimental constraints 

from the type locality: Contributions to Mineralogy and 

Petrology, v. 97, p. 333–351.

Moye, F.J., 1984, Geology and petrochemistry of Tertiary 

igneous rocks in the western half of the Seventeenmile 

Mountain 15-minute quadrangle, Ferry County, Washington: Moscow, University of Idaho Ph. D. dissertation, 242 p.

Moye, F.J., Hackett, W.R., Blakley, J.D., and Snider, L.G., 

1988, Regional geologic setting and volcanic stratigraphy 

of the Challis volcanic field, central Idaho, in Link, P.K., 

and Hackett, W.R., eds., Guidebook to the geology of central and Southern Idaho: Idaho Geological Survey Bulletin 

27, p. 87–97.

Moye, F.J., Sanford, R.F., Erdman, J.A., and Johnson, K.M., 

1989, Volcanic-hosted and intrusion-related mineral deposits in the Hailey and western Idaho Falls 1°×2° quadrangle, 

Idaho, in Winkler, G.R., Soulliere, S.J., Worl, R.G., and 

Johnson, K.M., eds., Geology and mineral deposits of the 

Hailey and Idaho Falls 1°×2° quadrangles, Idaho: U.S. Geological Survey Open-File Report 89–639, p. 70–72.

Nelson, W.H., and Ross, C.P., 1968, Geology of part of the 

Alder Creek mining district, Custer County, Idaho: U.S. 

Geological Survey Bulletin 1252–A, p.1–30.
Nelson, W.H., and Ross, C.P., 1969, Geologic map of the 

Mackay quadrangle, south-central Idaho: U.S. Geological 

Survey Miscellaneous Geologic Investigations Map I–580, 

scale 1:125,000.

Nilsen, T.H., 1977, Paleogeography of Mississippian turbidites in south-central Idaho, in Stewart, J.H., Stevens, C.H., 

and Fritsche, A.E., eds., Paleozoic paleogeography of the 

Western United States: Los Angeles, Society of Economic 

Paleontologists and Mineralogists, p. 275–299.

Norman, Mark, 1988, Challis Volcanics from subcontinental lithospheric mantle?: Geological Society of America 

Abstracts with Programs, v. 20, p. 460.

Norman, M.D., and Mertzman, S.A., 1991, Petrogenesis of 

Challis Volcanics from central and southwestern Idaho—

Trace elements and Pb isotope evidence: Journal of Geophysical Research, v. 96, no. B8, p. 13279–13293.

O’Brien, H.E., Irving, A.J., and McCallum, I.S., 1991, Eocene 

potassic magmatism in the Highwood Mountains, Montana—

Petrology, geochemistry, and tectonic implications: Journal of 

Geophysical Research, v. 96, no. B8, p. 13237–13260.

O’Neill, R.L., and Pavlis, T.L., 1988, Superposition of Cenozoic extension on Mesozoic compressional structures in 

the Pioneer Mountains metamorphic core complex, central 

Idaho: Geological Society of America Bulletin, v. 100, p. 

1833–1845.

Paull, R.A., 1974, Upper Cretaceous(?) to lower Eocene postorogenic conglomerate, south-central Idaho: Geological 

Society of America Abstracts with Programs, v. 6, p. 466.

Paull, R.A., and Gruber, D.P., 1977, Little Copper Formation—New name for lowest formation of Mississippian 

Copper Basin Group, Pioneer Mountains, south-central 

Idaho: American Association of Petroleum Geologists Bulletin, v. 62, p. 256–262.

Paull, R.A., Wolbrink, M.A., Volkman, R.G., and Grover, 

R.L., 1972, Stratigraphy of Copper Basin Group, Pioneer 

Mountains, south-central Idaho: American Association of 

Petroleum Geologists Bulletin, v. 55, p. 1370–1401.

Peacock, M.A., 1931, Classification for the recognition of 

igneous rock series: Journal of Geology, v. 39, p. 54–67.

Peccerillo, Angelo, and Taylor, S.R., 1976, Geochemistry of 

Eocene calc-alkaline volcanic rocks from the Kastamonu 

area, Northern Turkey: Contributions to Mineralogy and 

Petrology, v. 58, p. 63–81.

Rember, W.C., and Bennett, E.H., 1979a, Geologic map of the 

Idaho Falls quadrangle, Idaho: Idaho Bureau of Mines and 

Geology Geologic Map Series, scale 1:250,000.

Rember, W.C., and Bennett, E.H., 1979b, Geologic map of the 

Hailey quadrangle, Idaho: Idaho Bureau of Mines and Geology Geologic Map Series, scale 1:250,000.

References Cited 17

Sandberg, C.A., Hall, W.E., Batchelder, J.N., and Axelsen, 

Claus, 1975, Stratigraphy, conodont dating, and paleotectonic interpretation of the type Milligen Formation (Devonian), Wood River area, Idaho: U.S. Geological Survey 

Journal of Research, v. 3, p. 707–720.

Sanford, R.F., 1988, Challis Volcanics, Quaternary basalts, 

and a Quaternary lake, Little Wood River area, south-central Idaho: Geological Society of America Abstracts with 

Programs, v. 20, p. 467.

Sanford, R.F., Whitney, H.A., and McCafferty, A.E., 1989, 

Mineral resources of the Little Wood River Wilderness 

Study Area, Blaine County, Idaho: U.S. Geological Survey 

Bulletin 1721–D, 15 p.

Schmidt, D.L., 1961, Quaternary geology of the Bellevue 

area in Blaine and Camas Counties, Idaho: U.S. Geological 

Survey Open-File Report 62–0120, 135 p.

Skipp, Betty, 1988, Geologic map of Mackay 4 (Grouse) NE 

quadrangle, Butte and Custer Counties, Idaho: U.S. Geological Survey Open-File Report 88–423, scale 1:24,000.

Skipp, Betty, 1989, Geologic map of Mackay 4 (Grouse) NW 

quadrangle, Butte and Custer Counties, Idaho: U.S. Geological Survey Open-File Report 89–142, scale 1:24,000.

Smith, C.L., 1995, Interpretation of the regional geochemistry 

of the Hailey 1°×2° quadrangle, south-central Idaho: U.S. 

Geological Survey Bulletin 2064–L, 22 p.

Snider, L.G., and Moye, F.J., 1989, Regional stratigraphy, 

physical volcanology, and geochemistry of the southeastern 

Challis volcanic field: U.S. Geological Survey Open-File 

Report 89–639, p. 122-127.

Suayah, I.B., and Rodgers, J.J.W., 1991, Petrology of the 

lower Tertiary Clarno Formation in north central Oregon—

The importance of magma mixing: Journal of Geophysical 

Research, v. 96, no. B8, p. 13,357–13,371.

Umpleby, J.B., Westgate, L.G., and Ross, C.P., 1930, Geology 

and ore deposits of the Wood River region, Idaho, with a 

description of the Minnie Moore and nearby mines by D.F. 

Hewett: U.S. Geological Survey Bulletin 814, 250 p.

Vine, J.D., and Tourtelot, E.B., 1970, Geochemistry of black 

shale deposits—A summary report: Economic Geology, v. 

65, p. 253–272.

Worl, R.G., Kiilsgaard, T.H., Bennett, E.H., Link, P.K., Lewis, 

R.S., Mitchell, V.E., Johnson, K.M., and Snyder, L.D., 

1991, Geologic map of the Hailey 1°×2° quadrangle, Idaho: 

U.S. Geological Survey Open-File Report, 91–340, scale 

1:250,000.

Wust, S.L., 1986, Extensional deformation with northwest 

vergence, Pioneer core complex, central Idaho: Geology, v. 

14, p. 712–714.
Dacite intrusion, probable flow-dome complex; crosscuts 

sequence below; probable fault or intrusive contact; lack of

dacite pebbles in conglomerate below suggests nondepositional

contact; plagioclase (<10 mm, average 4 mm), biotite (conspicuous, bronze-colored alteration, <3 mm), total phenocrysts,

60 percent; massive breccia; large-scale flow foliation almost

vertical, mostly steeply west dipping, perpendicular to layering, highly variable. Sample 8H145 collected at 3,420 ft

(1,040.0 m) ...................................................................................... 3,320–3,430 1,009.6–1,043.1 

Volcanic conglomerate; poorly exposed; silt-cobble size,

<20 cm, average 5–10 cm, generally smaller than lower

conglomerate unit; same clast types as lower conglomerate

unit, but conspicuously more rhyolite; thin interbeds of

stratified siltstone and sandstone .................................................... 3,300–3,320 1,003.5–1,009.6 

Andesite lava; plagioclase (3 mm average), no mafic

minerals; vugs filled with chalcedony and light-green mineral;

buff gray on fresh surface, weathers reddish-brown ....................... 3,280–3,300 997.4–1,003.5

Volcanic conglomerate; some exposure in outcrop; sandcobble size, no boulders, <30 cm; same clast types as two

lower conglomerate units; unstratified; sand-silt size matrix;

thin interbeds of silicified, white tuffaceous siltstone; marginally

matrix supported, 40 percent clasts; possible mudflow .................. 3,250–3,280 988.3–997.4 

Andesite lava; plagioclase (<3 mm, 1.5 mm average);

similar to flow below but contains slightly larger plagioclase

and has more celadonite alteration along flow partings .................. 3,220–3,250 979.2–988.3

Volcanic conglomerate; poorly exposed; sand-boulder

size, clasts <1 m, andesite, rhyolite, and vitrophyre, well

rounded; large, angular andesite blocks interpreted as lag from

overlying lava flow .......................................................................... 3,190–3,220 970.1–979.2 

Andesite lava; plagioclase (<3 mm, 1 mm average); greenishgray, aphanitic, dark-gray groundmass, weathers reddish-brown;

minor celadonite alteration; rounded, poor outcrop ........................ 3,160–3,190 961.0–970.1 

Volcanic conglomerate; poorly exposed, sand-cobble size,

<2.0 cm, well rounded; clasts include ash-flow tuff vitrophyre,

plagioclase andesite, flow-banded rhyolite, aphanitic andesite,

rare plagioclase-biotite dacite, and Paleozoic quartzite. Sample

8H144 collected at 3,130 ft (951.8 m) ............................................ 3,090–3,160 939.7–961.0 

Ash-flow tuff 3; upper cooling unit consisting of single

emplacement unit; lithic-, vitric-, crystal-rich; moderately to

densely welded at base to nonwelded at top; devitrification at

base, local pods of remnant vitrophyre of irregular shapes

Appendix

Measured section through Challis Volcanic Group—Muldoon Summit to Little Wood River, south-central Idaho.

[Stratigraphic thickness.]

Measured thickness Measured thickness

 (feet) (meters)

18 Geology and Stratigraphy of South-Central Idaho 
within devitrified material, margins between vitrified and devitrified material about 1–3 mm of purple-brown, partially devitrified material; crystals, 10–15 percent total, plagioclase (<2 mm) >

biotite (conspicuous, <2 mm); 3,020–3,090 ft (918.4–939.7 m)

lithics, 10–20 percent material, 2 mm–25 cm, conspicuous

5–10 cm, plagioclase-hornblende-pyroxene-biotite(?) andesite,

pyroxene-plagioclase andesite, aphanitic andesite, dacite(?),

common, light-gray rhyolite clasts (silicic?); pumice, 5–12 cm

long, moderately flattened, black vitrophyre in white devitrified

groundmass. Samples 8H143A and 8H143B collected at

3,050 ft (927.5 m) ........................................................................... 3,020–3,090 918.4–939.7 

Ash-flow tuff 2; upper emplacement unit in lower compound cooling unit; densely welded with compaction foliation;

forms low rubbly ledges and cuestas, laterally continuous;

lithic-, pumice-, crystal-rich, more abundant lithics and pumice

than in lower emplacement unit; poorly sorted; mostly devitrified to reddish-brown, locally black perlitic vitrophyre; phenocrysts 25 percent total, plagioclase (<2 mm) > biotite (conspicuous euhedral, <2 mm, aligned along foliation); lithics 10–20

percent, <5 cm, 1 cm average, dark-green andesite, porphyritic

andesite, light-gray dacite, angular; pumice <13 cm long,

<3 cm thick, moderately to highly flattened; locally pervasive

celadonite alteration; boundary between upper and lower

emplacement units marked by discontinuous, tan chalcedonic

quartz layer. Sample 8H142 collected at 3,010 ft (915.3 m) ......... 2,990–3,020 909.3–918.4 

Ash-flow tuff 1; lower emplacement unit in lower compound cooling unit; densely welded with compaction foliation;

forms low rubbly ledges and cuestas, laterally continuous;

lithic-, pumice-, crystal-rich, less abundant lithics and pumice

than upper emplacement unit; poorly sorted; mostly devitrified

to reddish-brown, locally black perlitic vitrophyre; phenocrysts: 25 percent total, plagioclase (<2 mm) > biotite (conspicuous euhedral, <2 mm, aligned along foliation); lithics sparse,

locally abundant, <15 mm, white porphyritic plagioclasehornblende dacite or andesite, angular; pumice common,

highly flattened, <7 cm long. Sample 8H141 collected at

2,980 ft (906.2 m); dated as 48.0±0.1 Ma

(see section, “Geochronology”) ...................................................... 2,960–2,990 900.1–909.3 

Sandstone, volcaniclastic; poorly exposed; salt-andpepper texture of plagioclase, mafic minerals, glassy lithic

material ........................................................................................... 2,800–2,960 851.5–900.1

Dacite flow top; monolithologic breccia; matrix devitrified to reddish brown and white; much celadonite alteration of

mafic minerals and glass, celadonite vug filling; phenocrysts

25 percent total, plagioclase (<3 mm, average 1 mm) > hornblende (<2 mm, average 1 mm) > biotite > pyroxene; locally

highly vesicular ............................................................................... ?–2,800 ?–851.5 

Break in section

Dacite lava; massive, less breccia than lava below (2nd

unit down); vertical jointing; phenocrysts; plagioclase (40

Appendix 19

Measured thickness Measured thickness

 (feet) (meters)
percent, < 5 mm, 3 mm average) >> biotite (bronzy alteration,

<3 mm) > hornblende; groundmass of reddish-blue-purple

alteration material; weathered surfaces gray to buff ....................... 2,480–2,800 754.2–851.5 

Dacite lava or intrusion; poorly exposed, forms grus

slopes; phenocrysts: plagioclase (40 percent, 5 mm average)

>> biotite (conspicuous, <3 mm, more abundant than in

underlying unit) > hornblende > pyroxene; groundmass white

to light gray, devitrified, and altered, pervasive vertical partings

(shears or flow foliation) about N. 10° E.; greenish gray ............... 2,320–2,480 705.5–754.2 

Dacite lava; monolithologic breccia, more massive in

center; phenocrysts: plagioclase (30 percent, 5 mm average)

>> biotite > pyroxene(?); groundmass dark-gray aphanitic,

unusually fresh and unaltered. Sample 8H138 collected at

2,200 ft (669.0 m); dated at 48.22±0.09 Ma (see section,

“Geochronology”) ........................................................................... 2,160–2,320 656.9–705.5 

Volcaniclastic sedimentary rocks; poorly exposed; silt to

sand size; thin bedded; salt-and-pepper texture, plagioclase and

mafic grains ..................................................................................... 2,100–2,160 638.6–656.9 

Andesite lava; well exposed; phenocrysts; pyroxene

(conspicuous, <3 mm) > plagioclase, no hornblende; weathered

surfaces brownish-gray without thick rust-colored rind; massive,

fractured, monolithologic breccia, locally vesicular, reddish,

oxidized and baked weathering zone at top; strike N. 40° E.,

dip 30° SE. Sample 8H137 collected at 2,050 ft (623.4 m) ........... 1,900–2,100 577.8–638.6 

Dacite lava; phenocrysts: plagioclase (40 percent, <5mm)

>> biotite (conspicuous books, <2 mm) > hornblende (<1 mm);

monolithologic breccia; blocks locally foliated; locally more

matrix less blocky material and less resistant ................................. 1,830–1,900 556.5–577.8 

Volcaniclastic sedimentary rocks; well-laminated; N. 8° E.,

34° SE.; beds 0.5–2 cm thick; grains of plagioclase, pyroxene,

lithic material, zone of euhedral pyroxene <5 mm; silt to coarse

sand size; poorly sorted, white, fine-grained layer 0.5–1 cm

thick separated by 1–2 cm thick layers of more sandy material,

rhythmically layered; pyroxene from underlying flow(s) ............... 1,760–1,830 535.2–556.5 

Andesite lava; phenocrysts, pyroxene (conspicuous,

<3 mm) > plagioclase, no hornblende; weathered surfaces

brownish-gray without thick rust-colored rind; massive,

fractured, locally breccia, locally vesicular. Sample 8H136

collected at 1,660 ft (504.8 m) ........................................................ 1,640–1,760 498.7–535.2 

Andesite lava; phenocrysts, abundant (50–60 percent

total), plagioclase (<1 mm) > pyroxene (<1 mm), no

conspicuous phenocrysts; weathered surfaces brownish-gray,

fresh surfaces gray .......................................................................... 1,580–1,640 480.5–498.7 

Andesite lava; phenocrysts, pyroxene (conspicuous,

<3 mm) > plagioclase, no hornblende; weathered surfaces

brownish-gray without thick rust-colored rind; massive,

fractured, locally breccia, locally vesicular .................................... 1,550–1,580 471.4–480.5 

20 Geology and Stratigraphy of South-Central Idaho

Measured thickness Measured thickness

 (feet) (meters)
Andesite lava; poorly exposed; probable monolithologic

breccia; plagioclase (<0.5 mm) >> pyroxene > hornblende(?),

pyroxene larger upward; fine-grained aphanitic matrix; rusty

weathered surfaces with small (1 mm) pits (weathered olivine?) ... 1,490–1,550 453.1–471.4 

Andesite lava; fine-grained: phenocryst-poor, plagioclase

(<2 mm) > pyroxene > olivine(?); dark gray with bluish sheen;

locally vesicular .............................................................................. 1,420–1,490 431.8–453.1 

Andesite lava; phenocrysts, hornblende (conspicuous,

black, <2 mm) > plagioclase (5 mm) >> pyroxene(?); gray

matrix, weathered surface light gray; strike N. 40° E., 30° SE.

Sample 8H135 collected at 1,400 ft (425.7 m) ............................... 1,380–1,420 419.7–431.8 

Volcaniclastic sedimentary rocks; poorly exposed;

heterolithologic breccia; volcanic clasts ......................................... 1,340–1,380 407.5–419.7 

Andesite lava; plagioclase (<2 mm) > pyroxene (<2 mm);

dark gray with bluish sheen on fresh surfaces ................................ 1,270–1,340 386.2–407.5 

Volcaniclastic sedimentary rocks; poorly exposed;

heterolithologic breccia; volcanic clasts ......................................... 1,220–1,270 371.0–386.2 

Andesite lava; phenocrysts, pyroxene (conspicuous,

<3 mm) > plagioclase, no hornblende; weathered surfaces

brownish-gray without thick rust-colored rind; massive,

fractured, locally breccia, locally vesicular. Sample 8H134

collected at 1,180 ft (358.8 m) ........................................................ 1,160–1,220 352.8–371.0 

Andesite lava; poorly exposed; probable monolithologic

breccia; phenocrysts, plagioclase (<0.5 mm) >> pyroxene >

hornblende(?); fine-grained aphanitic matrix; rusty weathered

surfaces with small (1 mm) pits (weathered olivine?) .................... 1,050–1,160 319.3–352.8 

Andesite lava; poorly exposed; probable monolithologic

breccia; porphyritic; phenocrysts: total, about 60 percent,

plagioclase (<4 mm) > hornblende (<2 mm) < pyroxene

(<1 mm) > biotite (<2 mm); weathered surfaces gray-brown,

little rust stain. Sample 8H133 collected at 990 ft (301.1 m) ........ 940–1,050 285.9–319.3 

Andesite lava; poorly exposed; probable monolithologic

breccia; phenocrysts, plagioclase (<0.5 mm) >> pyroxene >

hornblende(?); fine-grained aphanitic matrix; rusty weathered

surfaces with small (1 mm) pits (weathered olivine?) .................... 750–940 228.1–285.9 

Andesite lava; monolithologic breccia; phenocrysts;

hornblende (conspicuous, black, <2 mm) = plagioclase

(<3mm) >>pyroxene; pinkish-brown aphanitic groundmass;

weathered surfaces rusty-reddish-brown with 1–2 mm

weathering rind. Sample 8H132 collected at 740 ft (225.0 m).

Sample 8H139 collected at 550 ft (167.3 m) .................................. 520–750 158.1–228.1 

Andesite lava; poorly exposed; probable monolithologic

breccia; phenocrysts, plagioclase (<0.5 mm) >> pyroxene >

Appendix 21

Measured thickness Measured thickness

 (feet) (meters)
hornblende(?); fine-grained aphanitic matrix; rusty weathered

surfaces with small (1 mm) pits (weathered olivine?) .................... 460–520 139.9–158.1 

Andesite lava; monolith breccia with clasts < 60 cm;

phenocrysts, plagioclase (1–2 mm) >> hornblende >

pyroxene(?); mafic minerals all green altered; matrix

pink-buff on fresh surfaces; minor celadonite and chalcedony

veinlets. Sample 8H140 collected at 440 ft (133.8 m); dated at

49.5±0.1 Ma (see section, “Geochronology”) ................................ 420–460 127.7–139.9 

Andesite lava; poorly exposed; probable monolithologic

breccia; nonporphyritic; fresh surfaces dark-gray; weathered

surfaces rusty-red-brown with small (1 mm) pits (olivine?) ........... 230–420 69.9–127.7 

Volcaniclastic sedimentary rocks; pebble conglomerate at

base grading upward to sandstone at top; very immature,

epiclastic; clasts angular to subrounded, <2 cm diameter, gray

porphyritic andesite, black chert; groundmass white to buff,

tuffaceous, detrital plagioclase, biotite, hornblende ....................... 100–230 30.4–69.9 

Andesite lava; phenocrysts; plagioclase (<1 mm) >

pyroxene(?) = olivine(?), mafic minerals alterated; fresh

surfaces dark-gray to greenish-brown, weathered surfaces

darker brown. Sample 8H131 collected 0–50 ft (0–15.2 m) ........... 0–100 0–30.4 

Base of section. Wood River Formation; siliceous argillite,

buff-weathering, finely laminated, slaty cleavage.

22 Geology and Stratigraphy of South-Central Idaho

Measured thickness Measured thickness

 (feet) (meters)