New Potassium-Argon Ages, 

Geochemistry, and Tectonic Setting of 

Upper Cenozoic Volcanic Rocks 

Near Blackfoot, Idaho 

By KARLS. KELLOGG and RICHARD F. MARVIN 

U.S. GEOLOGICAL SURVEY BULLETIN 1806 
DEPARTMENT OF THE INTERIOR 

DONALD PAUl HODEl, Secretary 

U.S. GEOlOGICAl SURVEY 

Dallas l. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1988 

For sale by the Branch of Distribution 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center 

Box 25425 

Denver, CO 80225 

Library of Congress Cataloging in Publication Data 

Kellogg, Karl S. 

New potassium-argon ages, geochemistry, and tectonic setting of 

upper Cenozoic volcanic rocks near Blackfoot, Idaho. 

(U.S. Geological Survey bulletin ; 1806) 

Bibliography: p. 

Supt. of Docs. no.: I 19.3:1806 

1. Rocks, Igneous--Idaho--Blackfoot Region. 

Stratigraphic-Cenozoic. I. Marvin, Richard F. 

QE75.B9 no. 1806 [QE461] 557.3 s [551.7'8] 

2. Geology, 

II. Title. Ill. Series. 

87-W0366 
CONTENTS 

Abstract 1 

Introduction 1 

Acknowledgments 2 

Geologic setting 2 

Upper Tertiary units 5 

Starlight Formation 5 

Units younger than the Starlight Formation 7 

Geochronology 8 

Geochemistry 9 

Rhyolitic rocks 13 

Basaltic rocks 13 

Timing of Basin and Range faulting 14 

Buckskin dome 15 

Summary and conclusions 16 

References cited 18 

FIGURES 

1. Index map of part of southeastern Idaho 3 

2. Geologic map of study area 6 

3. Diagrammatic cross section of units in, and adjacent to, study area 7 

4. Ab-An-Or diagram for analyzed volcanic rocks 9 

5. AFM diagram for analyzed volcanic rocks 9 

6. Si02

-oxide variation diagram for rhyolitic rocks 13 

7. Zirconium-strontium-rubidium ternary diagrams for analyzed volcanic 

rocks 14 

8. Graph showing possible eastward migration of eastern boundary of Basin 

and Range province 15 

9. Bouguer gravity map of study area and surrounding region 17 

TABLES 

1. Potassium-argon dates for volcanic rocks from Buckskin Basin and Stevens 

Peak areas, Idaho 10 

2. Major-element analyses and weight-percent norms for selected samples of 

rhyolite and basalt from Buckskin Basin and Stevens Peak areas, Idaho 11 

Contents Ill 

New Potassium-Argon Ages, 

Geochemistry, and Tectonic Setting of 

Upper Cenozoic Volcanic Rocks 

Near Blackfoot, Idaho 

By KARL S. KELLOGG and RICHARD F. MARVIN 

Abstract 

The geochronology and geochemistry of volcanic units 

along the southeastern margin of the Snake River Plain south 

of Blackfoot, Idaho, document predominantly rhyolitic 

volcanism between about 10 and 4 million years ago followed 

by basaltic volcanism (flows of olivine tholeiite) between about 

4 and 2 million years ago. Eight new potassium-argon ages from 

units collected on and near Stevens Peak help to define ages 

of depositional and tectonic events in the region. 

The oldest volcanic units-below the 6.7-million-year-old 

tuff of Blacktail and overlying deformed, mostly Proterozoic 

and Paleozoic rocks of the thrust belt-are placed in the Starlight Formation. The lower member of the Starlight Formation 

is poorly exposed and composed of conglomerate, fanglomerate, gravels, tuffaceous sandstones, ash-flow tuffs, rhyolitic 

flows, and freshwater limestone. Two rhyolitic flows in the 

lower member-the rhyolite of Two-and-a-Half-Mile Creek and 

the rhyolite of Stevens Peak-have potassium-argon ages of 9.1 

and 9.8 million years, respectively. 

The middle member of the Starlight Formation in the 

Stevens Peak area is the tuff of Arbon Valley, dated at approximately 7.9 million years. The upper member is poorly exposed 

and contains gravel, air-fall ash, and one olivine basalt flow 

dated at 7.0 million years. 

The top of the Starlight Formation was highly dissected, 

and a coarse boulder conglomerate containing conspicuous 

clasts of Triassic(?) and Jurassic(?) Nugget Sandstone and large 

boulders of the tuff of Arbon Valley was deposited by steepgradient, west-flowing streams. The boulder conglomerate is 

locally overlain by the 6.7-million-year-old tuff of Blacktail, 

which constrains the age of the boulder conglomerate between 

7.0 and 6.7 million years. 

The boulder conglomerate was deposited during significant uplift to the east and documents a period of major Basin 

and Range faulting in the area. This period of active tectonism, 

between about 7.0 and 6.7 million years ago, is consistent with 

a model of younging of major Basin and Range faulting to the 

east. Some Basin and Range faults in the study area were active after 6.7 million years ago, and vertical offset across some 

faults of at least 50 meters has occurred since about 2.2 million 

years ago. 

The tuff of Kilgore overlies the tuff of Blacktail and is about 

4.3 million years old. Between about 4.3 and 3.7 million years 

ago, the volcanic activity changed markedly from predominantly rhyolitic ash-flow eruptions to olivine tholeiitic basalt flows 

(the basalt of Buckskin Basin), which are similar in composition to many of the olivine tholeiites of the Snake River 

Plain. Three potassium-argon ages from the basalt of Buckskin 

Basin are between 3.7 and 2.2 million years. 

All rhyolitic rocks analyzed in the study area are peraluminous. Zirconium-strontium-rubidium ternary diagrams clearly 

discriminate between the rhyolite of Stevens Peak and the 

rhyolite of Two-and-a-Half-Mile Creek and between the tuff of 

Arbon Valley and the tuffs of Kilgore and Blacktail. Although 

major-element analyses (in addition to petrography) suggest that 

the tuff of Blacktail may be slightly more silicic than the tuff 

of Kilgore, neither major-element analyses nor zirconiumstrontium-rubidium diagrams clearly discriminate between 

these two units. 

The Buckskin dome, a prominent domical uplift 7 kilometers in diameter with an apparently sagged and faulted 

center, is entirely composed of the basalt of Buckskin Basin 

at the surface. The uplift, which began as early as 2.5 million 

years ago, may be the result of injection of rhyolitic magma, 

although gravity data are somewhat ambiguous. 

INTRODUCTION 

Understanding of the timing of eruption and the 

distribution of rhyolitic and basaltic rocks of the eastern 

SRP (Snake River Plain) has been refined in recent years 

by detailed stratigraphic, geochemical, and radiometric 

studies (for example, Armstrong and others, 1975, 1980; 

McBroome and others, 1981; Embree and others, 1982; 

Morgan and others, 1984; Kuntz, 1978). Rhyolitic rocks 

and their interbedded sediments, although generally older 

than the basalts which characterize the SRP, are commonly elevated relative to the basalts in hills marginal to 

the plain so that the stratigraphy of the rhyolitic rocks 

and interbedded sediments is locally well exposed. This 

study describes the geologic and tectonic setting and the 

geochemistry of volcanic and sedimentary rocks in an 

area along the southeastern margin of the SRP. The timing of major Basin and Range uplift in the area is determined by dating volcanic rocks that bracket the age of 

a syntectonic boulder conglomerate. Information is also 

presented regarding the age and geologic history of a 

basaltic domical uplift that is similar in form to several 

well-known, larger domical uplifts which occur on the 

eastern SRP. 

The study area is located less than 12 km south of 

Introduction 
the town of Blackfoot and east of Fort Hall, Idaho, on 

the Fort Hall Indian Reservation (fig. 1). Stevens Peak 

is a prominent landmark in the northern part of the study 

area, and Buckskin Basin occupies the southern part of 

the area (figs. 1 and 2). The Buckskin Basin and parts 

of the Fort Hall, Moreland, and Blackfoot 7Y2-minute 

quadrangles cover the area. The topography is gently rolling. The maximum elevation, found east of Stevens Peak 

near the eastern margin of the study area, is 1734 m (5686 

ft); the minimum elevation, found along Ross Fork 

Creek, is 1372 m (4500 ft). Bedrock, which is composed 

predominantly of a bimodal assemblage of late Tertiary 

rhyolite flows, ash-flow tuffs, and olivine basalts, is mostly mantled by Quaternary loess deposits, so that outcrops 

are sparse. 

The area was first mapped by Mansfield (1920), 

who described the rhyolites and basalts on the Fort Hall 

Indian Reservation. Trimble (1976) mapped the Pocatello 

15-minute quadrangle, just to the south of the study area, 

and described a succession of upper Tertiary and Quaternary basaltic and rhyolitic volcanic rocks and coarse, 

clastic sedimentary rocks that overlie tectonically complex Precambrian and lower Paleozoic strata of the 

Idaho-Wyoming fold and thrust belt. Trimble (1982) 

mapped similar rocks just to the east of the study area 

in the Yandell Springs 15-minute quadrangle. 

Regionally, the volcanic evolution of the SRP in 

southeastern Idaho has been discussed by numerous 

authors. Most recently, McBroome and others (1981), 

Embree and others (1982), Leeman (1982a), and Morgan 

and others (1984) have discussed the Pliocene and 

Miocene rhyolite flows and ignimbrites, and Leeman 

(1982a, b), Greeley (1982), and Kuntz and others (1982) 

have discussed some of the Quaternary basalts. Armstrong and others (1975, 1980) have discussed the K-Ar 

(potsssium-argon) geochronology of many volcanic rocks 

of the eastern SRP. Allmendinger ( 1982) has inferred the 

probable timing of Basin and Range faulting in the Blackfoot Mountains, to the northeast of the study area, from 

a detailed geochronologic and stratigraphic study of ignimbrites. The present study has evolved from detailed 

mapping of the Stevens Peak and Buckskin Basin areas 

(Kellogg and Embree, 1986). 

ACKNOWLEDGMENTS 

Many of the problems described here were first addressed by D.E. Trimble, D.J. Doherty, G.F. Embree, 

L.A. Morgan, and H.J. Prostka, and discussions with 

most of these people have helped to orient the focus of 

this paper. In particular, the discussions and support of 

G.A. Desborough, in both the office and field, were 

crucial to the study. Field excursions and discussions with 

L.A. Morgan were an important factor in establishing 

much of the basic stratigraphy of the area. Special thanks 

are extended to StevenS. Oriel who introduced the first 

author to the geology of the area. The manuscript was 

improved immensely by reviews from L.A. Morgan and 

R.A. Thompson. 

GEOLOGIC SETTING 

The eastern Snake River Plain is largely characterized by voluminous olivine tholeiitic basalt as young as 

Holocene in age (Leeman, 1982b; Kuntz and others, 

1982). Despite this widespread veneer of basalt, it has long 

been recognized that mostly older rhyolitic flows, ashflow tuffs, pumice- and ash-fall deposits, and intercalated 

volcanogenic sedimentary rocks underlie the surface exposures on the SRP and are, in fact, volumetrically dominant, as indicated by deep drilling (Doherty and others, 

1979; McBroome, 1981). The buried rhyolitic ash-flow 

tuffs, lava flows, and interbedded sediments are probably 

all derived from numerous overlapping calderas located 

beneath the basalts of the SRP (Prostka, 1979; Prostka 

and others, 1979; Doherty and others, 1979; McBroome 

and others, 1981; Morgan and others, 1984). The locus 

of rhyolitic volcanism has shifted northeastward along 

the axis of the SRP at a rate of approximately 3.5 em 

per year (Armstrong and others, 1975); ignimbrites in the 

vicinity of Twin Falls, Idaho, are about 9-10 Ma 

(Williams and others, 1982) and those of the Yellowstone 

Plateau at the northeastern end of the SRP are as young 

as 0.6 Ma (Christiansen and Blank, 1972). At any one 

place along the plain, the cessation of rhyolitic volcanism 

was followed by basaltic outpourings, producing the striking bimodal assemblage of rocks. 

Most of the exposed rhyolitic rocks are topographically elevated relative to the younger basalts, cropping 

out in hills along the southeastern and northwestern 

margins of the eastern SRP (Mansfield, 1920; Mansfield 

and Ross, 1935; Skipp and others, 1979; McBroome, 

1981; Trimble, 1982; Allmendinger, 1982). Relative uplift 

of the hills marginal to the SRP occurred during Neogene 

and Quaternary Basin and Range faulting, although it 

should be noted that hills marginal to the plain were probably not uplifted in an absolute sense. A seismicrefraction study of the eastern SRP and its margin indicates that the SRP is a large downwarped feature 

(Sparlin and others, 1982). Along the southeastern margin 

of the plain, rocks of the Cordilleran thrust belt might 

extend locally for several tens of kilometers under the 

volcanics of the plain. In contrast, the seismic-refraction 

study suggests that volcanic rocks along the northern 

margin of the plain might be in fault contact with the 

rocks of the Cordilleran thrust belt and might not be 

underlain by pre-Tertiary rocks (Sparlin and others, 1982; 

Pankratz and Ackermann, 1982). 

2 K-Ar Ages, Geochemistry, and Tectonic Setting, Upper Cenozoic Volcanic Rocks, Blackfoot, Idaho 
I 

,--r_J 

I 

Lemhi 

Range 

/ Idaho National 

, J / Testing Laboratory 

/ 

/ 

East Butte 

Middre Butte 

Big Southern Butte 

Cedar Butte 

0 

L. 

l 

--, 

I 

I 

I 

I 

I 

I 

J 

~ 

10 20 30 KILOMETERS 

Figure 1. Index map of a part of southeastern Idaho showing major physiographic features adjacent to study area, 

which is enlarged in figu re 2. Site W-1 is locality at which Walcott Tuff was sampled . 

Geologic Setting 3 
In the Blackfoot Mountains about 25 km northeast 

of Stevens Peak, down warping has occurred along a complex network of high-angle normal faults, the oldest of 

which are north and east trending; younger faults of 

smaller displacement are northwest and northeast trending (Allmendinger, 1982). Allmendinger inferred that 

Basin and Range faulting in the Blackfoot Mountains occurred primarily between about 5.9 and 4.7 Ma and that 

Basin and Range faulting has shifted eastward in time 

since its initiation in north-central Nevada about 17 Ma. 

The stratigraphy of the various upper Tertiary, 

largely rhyolitic volcanic and sedimentary units along the 

southeastern margin of the SRP is complicated by the active tectonism of the region; major changes in both facies 

and thickness over small distances are common. A diverse 

assemblage of rhyolitic ash-flow tuffs, rhyolitic flo~s, 

basaltic flows, volcanogenic sandstone, conglomerate, 

sedimentary breccia of probable mudflow origin, and 

freshwater limestone were originally defined by Mansfield 

(1927) as the Salt Lake Formation. 

Attempts have been made in the last few years to 

subdivide the heterogeneous and widespread Salt Lake 

Formation into localized and stratigraphically distinct 

units. Carr and Trimble (1963) reassigned the lower part 

of the Salt Lake Formation in the vicinity of American 

Falls Reservoir to their Starlight Formation and did not 

use the term Salt Lake Formation in this area, although 

they recognize that Salt Lake Formation might be a useful 

term in other, less volcanogenic areas (D.E. Trimble, written commun., 1986). On the basis of fossil evidence, 

Trimble and Carr (1976) reported the Starlight Formation is early to middle Pliocene in age, although Late 

Miocene ages (to be discussed) for both the overlying 

Walcott Tuff and the tuff of Arbon Valley, the middle 

member of the Starlight Formation, necessitate that the 

Starlight Formation be upper Miocene. 

The chronology of major ash flows along both 

margins of the SRP has been discussed by McBroome and 

others (1981), Embree and others (1982), and Morgan and 

others (1984). Morgan and others (1984) simplified the 

stratigraphic terminology that contained many local unit 

names (their terminology will be used in this paper). Four 

major ash-flow tuffs recognized in the general study area 

are, from oldest to youngest, as follows: 

1. Tuff of Arbon Valley: This unit was first 

described by Carr and Trimble (1963) as the 

middle member of the Starlight Formation, 

and in most places it makes up the entire middle member. This compound cooling unit has 

been mapped as far southwest as the Rockland 

quadrangle {Trimble and Carr, 1976) and is exposed as far northeast as the Blackfoot River 

in the Blackfoot Mountains (Embree and 

others, 1982). A K-Ar age of 7.9±0.1 Ma 

(recalculated using the decay constants of 

Steiger and Jager, 1977) was determined for the 

tuff of Arbon Valley by Armstrong and others 

(1975). 

2. Tuff of Blacktail as described by Morgan and 

others (1984); also called the tuff of Spring 

Creek by Embree and others (1982): This major ash-flow deposit of the eastern SRP has 

been associated with a caldera with eruptive 

vents on both sides of the plain (Morgan and 

others, 1984). This unit crops out at least as 

far southwest as Stevens Peak. Zircon fissiontrack ages of 6.5 ±0.3 and 6.6±0.7 Ma have 

been determined for the tuff of Blacktail 

(Morgan and others, 1984). 

3. Walcott Tuff: This unit crops out prominently in the northern part of the Rockland 

quadrangle (Trimble and Carr, 1976), almost 

50 km southwest of the study area. Trimble 

( 1982) reported an isolated outcrop of W aicott 

Tuff in the Yandell Springs quadrangle, immediately east of the study area, although 

paleomagnetic evidence suggests that this rock 

is the tuff of Kilgore (L.A. Morgan, written 

commun., 1986). K-Ar ages for the Walcott 

Tuff (corrected for new decay constants of 

Steiger and Jager, 1977) range from 6.3 ±0.3 

Ma (Marvin and others, 1970) to 6.9±0.4 Ma 

{Trimble and Carr, 1976). Armstrong and 

others (1975) reported an age (also corrected) 

of 6.5 ±0.3 Ma for the Walcott Tuff. 

4. Tuff of Kilgore: This name was applied first 

to the area along the northern margin of the 

plain by McBroome and others (1981), who 

correlated the tuff of Kilgore with the tuff of 

Heise (Prostka and Embree, 1978) along the 

southeastern margin of the plain. Morgan and 

others (1984) later renamed the tuff of Heise 

the tuff of Kilgore. This widespread unit is 

prominently exposed in hills along the northeastern and southeastern margins of the SRP 

and has been recognized as far southwest as 

Stevens Peak. Armstrong and others ( 197 5) 

reported a whole-rock K-Ar age of 4.2±0.1 

Ma (age recalculated with decay constants of 

Steiger and Jager, 1977), and Morgan and 

others (1984) listed two feldspar K-Ar ages of 

4.3±0.3 and 4.8±0.3 Ma and four zircon 

fission-track ages of 4.1-4.4 Ma (maximum 

uncertainty of 0.6 Ma). From these ages, an 

average age of about 4.3 Ma can be established 

for the tuff of Kilgore. 

Basalts of the SRP-Yellowstone Plateau province 

are predominantly olivine tholeiite (Leeman, 1982b). 

These basalts are as young as Holocene in age in the 

vicinity of Craters of the Moon National Monument 

4 K-Ar Ages, Geochemistry, and Tectonic Setting, Upper Cenozoic Volcanic Rocks, Blackfoot, Idaho 
(Kuntz and others, 1982), about 90 km west-northwest 

of the study area, and Pleistocene in age along the 

Blackfoot River (Armstrong and others, 1975), 10 km 

northeast of the study area. In the Buckskin Basin area, 

the basalts are late Pliocene in age (this study). 

Several domical or piston-shaped uplifts of basalt, 

cored by Quaternary rhyolitic intrusive bodies, occur 

along the axis of the SRP. The most prominent of these 

uplifts is Big Southern Butte (Spear and King, 1982), 

which is about 50 km west of Blackfoot (fig. 1) and is 

cored by 0.3-Ma rhyolite (Armstrong and others, 1975). 

Rhyolite in the core of East Butte, northeast of Big 

Southern Butte, has a K-Ar age of 0.6 Ma (Armstrong 

and others, 1975). Middle Butte, another prominent 

basalt-mantled dome located between Big Southern Butte 

and East Butte, does not have an exposed rhyolitic core, 

but aeromagnetic and gravity data indicate that it is also 

cored by rhyolite (Kuntz, 1978). A prominent but smaller 

uplifted circular feature in the study area, named 

Buckskin dome (fig. 2), shares many physical characteristics with these dome-shaped features of the SRP. 

Large regions of the SRP and the adjacent hills, 

including the study area, are mantled by thick loess and 

dune deposits mostly of Pleistocene age (Lewis and 

Fosberg, 1982). 

UPPER TERTIARY UNITS 

Starlight Formation 

In the study area, the Starlight Formation overlies 

pre-Tertiary rocks of the fold and thrust belt and everywhere underlies either the Walcott Tuff or the tuff of 

Blacktail. Eocene hornblende andesite, which underlies 

the Starlight Formation about 12 km east of the study 

area (Trimble, 1982), has not been observed. In most 

places, the top of the Starlight Formation is not clearly 

defined largely because of the extensive mantle of eolian 

deposits. 

The Starlight Formation is divided into lower, middle, and upper members (fig. 3). The lower member is 

very poorly exposed in the study area and includes several 

outcrops of vuggy, black-stained travertine as thick as 

3 m in the eastern part of the area; travertine as thick 

as about 50 m crops out several kilometers east of the 

study area (Trimble, 1982). Two rhyolite flows-the 

rhyolite of Two-and-a-Half-Mile Creek and the rhyolite 

of Stevens Peak -are placed in the lower part of the lower 

member. Other lithologies noted by Trimble (1976, 1982) 

in adjacent areas in the lower member, such as greenishgray to light-tan rhyolitic tuff, tuffaceous sandstone and 

marl, and basalt and basaltic tuff, are not exposed in the 

study area. The total thickness of the lower member is 

unknown, although Trimble ( 1982) stated that it is greater 

than 244 m just east of the study area. 

Several small outcrops of diamictite, composed of 

moderately rounded clasts of Precambrian and Paleozoic 

quartzite as long as 2 m, crop out in the southeastern part 

of the study area and are tentatively placed in the lower 

member. No matrix material is exposed; the boulders are 

loosely embedded in loess. These rocks occur stratigraphically a few meters above Paleozoic rocks (Ordovician 

Swan Peak Quartzite), although they could be part of the 

boulder conglomerate that overlies the Starlight Formation in other areas. 

The rhyolites of Two-and-a-Half-Mile Creek and 

Stevens Peak are added to the lower member of the 

Starlight Formation in this report and are the oldest dated 

rocks in the study area. The rhyolite of Two-and-a-HalfMile Creek in the southern part of the area, named for 

a creek just south of the area of figure 2 (Kellogg and 

Embree, 1986), was originally thought by Trimble (1976), 

who did not give it a formal name, to be younger than 

the Starlight Formation. A biotite K-Ar age of 9.1 ±0.3 

Ma (table 1) indicates that the rhyolite of Two-and-aHalf-Mile Creek predates the tuff of Arbon Valley, the 

middle member of the Starlight Formation. The other 

flow, the rhyolite of Stevens Peak, has a reported K-Ar 

age of 9.8 ±0.9 Ma (Karlo and Jorgenson, 1979; mineral 

separate unspecified). 

The two rhyolite flows are petrographically distinct 

from each other; small, reddish-brown biotite phenocrysts 

and as much as 10 percent quartz phenocrysts (as long 

as 2 mm) characterize the rhyolite of Two-and-a-HalfMile Creek, whereas biotite and quartz were not observed 

in the rhyolite of Stevens Peak. In both units, the attitude 

of flow foliation is highly variable, reflecting the highly 

viscous nature of the magma. High viscosity is also suggested by the approximately 250 m of relief for each unit, 

although erosion and thick loess largely obscure the 

original morphology. 

The rhyolite of Stevens Peak is almost entirely 

devitrified, whereas numerous brown glass stringers, 

typically 1 em thick but as thick as several meters, define 

flow foliation in the rhyolite of Two-and-a-Half-Mile 

Creek. Small folds in flow foliation in the rhyolite of 

Two-and-a-Half-Mile Creek plunge 5-30° to the west. 

The middle member of the Starlight Formation is 

represented by the tuff of Arbon Valley, which Trimble 

and Carr (1976) designated as the lower unit of the middle member, although in the study area, the tuff makes 

up the entire middle member. The_ tuff of Arbon Valley 

is a white to tan, pumiceous, crystal-rich (as much as 

about 30 percent total phenocrysts), rhyolite ash-flow 

tuff. The phenocrysts are as long as 3 mm and consist 

of quartz (15 percent), plagioclase (10 percent), sanidine 

(5 percent), and about 1 percent each clinopyroxene and 

conspicuous biotite. Pumiceous or densely welded tuff 

clasts as long as 3 em are common. As much as 25 m of 

the lower part of the tuff is white, poorly welded, and 

Upper Tertiary Units 5 
0 2 4 KILOMETERS 

Qe 

Ttw?~ 

Tss 

17 ___l_ 

3-

Qe 

Alluvium 

Eolean deposits; loess and dune sand 

Olivine tholeitte basalt 

Tuff of Kilgore 

Walcott Tuff 

Tuff of Blacktail 

Boulder conglomerate 

Upper member (fsu) 

Basalt now (fsb) 

Tuff of Arbon Valley 

Lower member 

Travertine (fslt) j 

Rhyolite of Two-and-a-haH-Mile Creek (Tst) f; 

VJ 

Rhyolite of Stevens Peak (fss) 

Contact 

Fault -Dotted where concealed; ball and bar on 

downthrown side 

Strike and dip 

Direction and plunge of crenulation lineation in 

rhyolite flow 

Figure 2. Geologic map of study area (simplified from Kellogg and Embree, 1986). Labeled sites refer to dated localities 

(table 1 ). 

6 K-Ar Ages, Geochemistry, and Tectonic Setting, Upper Cenozoic Volcanic Rocks, Blackfoot, Idaho 
c 

0 

:; 

E 

& 

4.3Ma 

6.4Ma 

6.7 Ma* 

7.0 Ma* 

.E 7.9 Ma 

g 

(ii 

en 

l9.1 Ma* 

Tuff of Kilgore 

2.2 Ma* 

3.7 Ma* 

9.8Ma 

EXPLANATION 

I I 

f-----11-=----1 Limestone, dolomite 

0 0 D 0 o 

0 o o Conglomerate, diamictite 

ftl+tl,-'rY-r'rti Basalt 

Air-fall ash and ashy sandstone 

Ash-flow tuff 

Rhyolite flow 

Figure 3. Diagrammatic cross section of units in, and adjacent to, study area. Thicknesses are approximate and are based 

on this study and Trimble (1982). New ages followed by asterisk. 

pumiceous and locally contains tree casts. The total thickness of the tuff of Arbon Valley is as much as 60 m. 

Magnetic polarity, as determined using a portable fluxgate magnetometer, is normal. Armstrong and others 

(1975) reported a K-Ar age of 7.9 Ma (recalculated using 

the decay constants of Steiger and Jager, 1977) for the 

tuff of Arbon Valley from a locality southwest of 

Pocatello. 

The upper member of the Starlight Formation consists of gravel and ash- and pumice-fall deposits and, 

about 20 m below the base of the tuff of Blacktail, a 5-mthick, fine-grained olivine basalt flow. The uniform 

thickness of the flow over an area of at least 3 km2 suggests that the topography was relatively low at the time 

of eruption, a situation which prevailed during deposition of most of the Starlight Formation (D.E. Trimble, 

oral commun., 1986). A K-Ar whole-rock age of7.0±0.3 

Ma was obtained from the basalt (table 1). 

Over large areas, the upper member is entirely 

eroded, and the resistant top of the welded section of the 

tuff of Arbon Valley is the uppermost exposed unit of 

the Starlight Formation. 

Units Younger Than the Starlight Formation 

A coarse boulder conglomerate locally overlies the 

Starlight Formation in the eastern part of the study area 

and in places is directly on the eroded top of the tuff of 

Arbon Valley. The conglomerate resembles the diamictite that is placed tentatively in the lower member of the 

Starlight Formation, although it contains locally large (as 

much as 5 m in diameter) boulders of the tuff of Arbon 

Valley and well-sorted clasts (as much as 1.2 min length) 

of white Ordovician Swan Peak Quartzite and tan 

Triassic(?) and Jurassic(?) Nugget Sandstone. Kellogg and 

Embree (1986) placed this unit in the upper member of 

the Starlight Formation, although reexamination of outcrops in the Yandell Springs quadrangle established, as 

Trimble (1982) originally maintained, that the boulder 

Upper Tertiary Units 7 
conglomerate was deposited on a highly eroded and irregular surface cut into the Starlight Formation and is, 

therefore, a separate unit. 

At least two major ash-flow tuffs of the eastern 

SRP are exposed on or near Stevens Peak. The tuff of 

Blacktail and the tuff of Kilgore are both mapped in this 

area, although paleomagnetic evidence suggests that some 

outcrops near Stevens Peak mapped as tuff of Blacktail 

might be the Walcott Tuff (L.A. Morgan, oral commun., 

1986). Both the tuff of Blacktail and the tuff of Kilgore 

overlie the boulder conglomerate. The tuff of Blacktail, 

which overlies the Starlight Formation on Stevens Peak, 

is a light-gray, dense to glassy ash-flow tuff containing 

about 5 percent phenocrysts (as long as 2 mm) of plagioclase and quartz and sparse sanidine, clinopyroxene, and 

opaque minerals. Where observed, the basal vitrophyre 

is about 50 em thick. Plagioclase and sanidine separates 

yielded K-Ar ages of 6.6±0.3 and 6.7±0.2 Ma, respectively (table 1), in agreement with zircon fission-track ages 

of 6.5-6.6 Ma (Morgan and others, 1984). The magnetic 

polarity of the tuff of Blacktail is normal, as determined 

using a portable fluxgate magnetometer. 

A small outcrop of tuff questionably correlated 

with the Walcott Tuff crops out in the southern part of 

the study area. The tuff is purplish gray, densely welded, 

and devitrified and contains approximately equal 

amounts of plagioclase phenocrysts as long as 1.5 mm 

and smaller sanidine phenocrysts; trace amounts of 

quartz, clinopyroxene, and magnetite are also present. 

A whole-rock K-Ar age of 6.4±0.2 Ma was determined 

for this unit in the study (table 1), an age that falls within 

the range of corrected published ages of 6.3 ± 0.3 Ma and 

6.9±0.4 Ma (Marvin and others, 1970; Trimble and Carr, 

1976) for the Walcott Tuff. 

The tuff of Kilgore immediately overlies the tuff 

of Blacktail on and near Stevens Peak and is mostly a 

gray, pinkish-gray, and tan, dense, very fine grained to 

glassy ash-flow tuff containing about 2 percent plagioclase (An20 by microscopic analysis) phenocrysts (as long 

as 2 mm) and trace amounts of sanidine, augite, and 

opaque minerals. On the pointed summit of Stevens Peak, 

a well-exposed section of flat-lying tuff of Kilgore consists of a basal brown ash greater than 3 m thick that 

grades upward into a 0.5-m-thick black vitrophyre in turn 

overlain by a prominent gray to mottled-gray and orange 

3-m-thick spherulitic zone capped by a 6-m-thick lithophysal zone. The tuff of Kilgore is undated in the study 

area but has been dated elsewhere by both the K-Ar 

whole-rock and fission-track methods at about 4.3 Ma 

(Armstrong and others, 1975; Morgan and others, 1984). 

The youngest volcanic rocks in the study area are 

a sequence of olivine-augite basalt flows, which are 

similar to most basalts of the SRP (Leeman, 1982b) (figs. 

4 and 5), and here are informally called the basalt of 

Buckskin Basin. These basalts define a circular pattern 

of low hills to the west of Stevens Peak and also crop 

out along Ross Fork Creek. Cuttings from numerous 

water wells indicate that the basalt also underlies several 

tens of meters of loess and water-lain sediment within 

Buckskin Basin (Norman Bird, U.S. Bureau of Indian 

Affairs, Fort Hall, Idaho, written commun., 1984). The 

basalt of Buckskin Basin ranges in age from 2.2 to 3. 7 

Ma (table 1). 

The basalt of Buckskin Basin is gray to black, fine 

to medium grained, and diktytaxitic to subophitic. Flow 

tops are highly vesicular, and basal zones are typically 

rubbly and oxidized. Individual flows are between 1 and 

10m thick. The matrix of subophitic basalt is composed 

of about 70 percent plagioclase (approximately An70 by 

microscopic analysis) laths, about 20 percent olivine, and 

typically 5 percent but as much as 30 percent augite. The 

rock contains sparse phenocrysts of plagioclase as long 

as 3 mm and olivine. 

Light-tan, massive, silty loess and light-tan, finegrained, stabilized dune sand deposits mantle most preQuaternary rocks in the area. Quaternary alluvium 

underlies the flood plains of the Blackfoot River and Ross 

Fork Creek. 

GEOCHRONOLOGY 

K-Ar age determinations were made in the U.S. 

Geological Survey laboratories in Denver, Colo. Potassium analyses were performed by E.H. Brandt using a 

lithium metaborate flux fusion-flame photometry method 

(lngamells, 1970). Argon-extraction and -purification 

techniques were similar to those· described by Dalrymple 

and Lanphere (1969). Argon composition was determined 

by H.H. Mehnert using standard isotope-dilution procedures with a 60°-sector, 15.2-cm-radius Nier-type mass 

spectrometer operating in the dynamic mode. The 

estimated analytic uncertainty for a calculated K-Ar age 

is reported as 2a. K-Ar ages and analytic data for samples 

collected at seven sites (A through G, fig. 2) are given 

in table 1. 

Three isotopic ages were obtained at site A for the 

rhyolite ofTwo-and-a-Half-Mile Creek (table 1): 9.1 ±0.3 

Ma (biotite), 8.1 ± 0.2 Ma (potassium feldspar), and 

12.5 ± 2.3 Ma (plagioclase). In view of our experience 

with discordant ages, we think that the 9.1-Ma age from 

biotite is the most reliable. It has frequently been observed 

that potassium feldspars do not quantitatively retain 

radiogenic argon and, therefore, commonly have ages 

which are too young; thus, we feel that the 8.1-Ma 

potassium-feldspar age is too young. The 12.5-Ma age 

from plagioclase is the least reliable of the three ages 

because of the very low potassium content; the uncertainty arising from the potassium content is indicated by the 

large analytic error attached to the isotopic age. 

8 K-Ar Ages, Geochemistry, and Tectonic Setting, Upper Cenozoic Volcanic Rocks, Blackfoot, Idaho 
Ab 

An 

Field for 

Columbia River 

Basalt Group 

Or 

Figure 4. Ab-An-Or diagram for analyzed volcanic rocks 

shown in table 2. Ab, normative albite+ 5/3 (normative 

nephaline); An, normative anorthite; and Or, normative orthoclase. Open circles, rhyolites; closed circles, basalts. X 

represents average value for 78 samples of olivine tholeiitic 

basalt from Snake River Plain (Leeman, 1982a). Fields for 

Columbia River Basalt Group and "average" calc-alkaline 

volcanic rocks are from Irvine and Baragar (1971). 

F 

A M 

Figure 5. AFM diagram for analyzed volcanic rocks shown 

in table 2. A, K20+ Na20; F, total Fe as Fe2

0 3

; and M, MgO 

(all values in weight percent). Open circles, rhyolites; closed 

circles, basalts. X represents average value of 78 samples of 

olivine tholeiitic basalt from the Snake River Plain (Leeman, 

1982b). 

Two samples of the tuff of Blacktail were collected 

(sites B and C) for isotopic dating. Plagioclase from a 

sample of basal vitrophyre from site B has an age of 

6.6±0.3 Ma. Both the potassium feldspar and plagioclase 

from the devitrified upper third of the tuff at site C have 

ages of 6.7 Ma (uncertainties are ±0.2 and ±0.3 Ma, 

respectively). These K-Ar mineral ages are in agreement 

with fission-track zircon ages of 6.5 ± 0.3 and 6.6 ± 0. 7 

Ma from samples collectea elsewhere from the tuff of 

Blacktail (Morgan and others, 1984). The age of the tuff 

of Blacktail is therefore well constra~ned at 6.6-6. 7 Ma. 

A specimen of an ash-flow tuff at site G has a K-Ar 

whole-rock age of 6.4 ± 0.2 Ma. This tuff is tentatively 

correlated with the Walcott Tuff. The tuff of Kilgore was 

not collected for dating purposes because its age is already 

well established at 4.3 Ma (Armstrong and others, 1975; 

Morgan and others, 1984). 

A basalt specimen from the basalt flow in the upper member of the Starlight Formation at site D has a 

K-Ar whole-rock age of 7.0 Ma, an age consistent with 

the 7.7-Ma age of the underlying tuff of Arbon Valley 

(middle member of the Starlight Formation). The age of 

the basalt flow is also consistent with the age of 6.6-6. 7 

Ma for the overlying tuff of Blacktail. 

Two of the dated samples of basalt of Buckskin 

Basin were collected in a gully on the northwestern side 

of the Buckskin dome (site E) where six basalt flows dip 

gently to the northwest. Sample BB704A, from the top 

flow, has a K-Ar plagioclase age of 2.5 Ma; sample 

BB699A, from the bottom flow, has a K-Ar whole-rock 

age of 2.2 Ma. These ages are not in accord with the 

relative stratigraphic order of the flows, although the 

uncertainty in the ages is large. However, the ages do indicate that these flows are Late Pliocene in age. 

A third olivine basalt (sample BB608A) was collected in a quarry adjacent to Ross Fork Creek in the 

southwestern part of the study area (site Fin fig. 2). The 

flow is greater than 3 m thick and has a horizontal attitude. A plagioclase concentrate has an age of 3. 7 Ma, 

significantly older than the basalts on the northwestern 

side of Buckskin dome. Basaltic volcanism in Buckskin 

Basin thus encompassed a time span of more than a 

million years. 

GEOCHEMISTRY 

Whole-rock major-element analyses were performed at the U.S. Geological Survey's Branch of 

Analytical Chemistry on 16 rocks that are believed to 

represent the compositional variation of the volcanic 

rocks in the study area (table 2 and figs. 4, 5, and 6). 

These rocks include 12 samples of the rhyolitic rocks and 

4 samples of the basalts. Selected trace elements on 67 

rhyolite samples and 28 basalt samples were determined 

Geochemistry 9 
... 

= 

~ 

I 

)o 

... 

)o 

~ C'1 

~ :T 

~ 

3 

c;;· 

~ AI 

:I 

Q. 

a 

0 

:I 

;::;· 

! 

:;· 

~ 

c 

"'CI 

"'CI 

~ 

("') 

~ 

:I 

0 

N 

0 

;::;· 

<:: 

0 

;:;-

AI 

:I 

;::;· 

~ 

~ ;II:"" 

~ 

Di = 

1"'1 

;II:"" 

~ ~ AI 

:T 

0 

Table 1. Potassium-argon dates for volcanic rocks from Buckskin Basin and Stevens Peak areas, Idaho 

[B, biotite; KF, potassium feldspar; PI, plagioslase; WR, whole rock. Constants used: A.f =0.581 X w- 10 /yr; A.f3 = 4.962 X w-10 /yr; atomic abundance of 40J(;K = 1.167 X 10-4mole/mole) 

Description of 

unit sampled 

Basalt of Buckskin Basin 

Sample number 

(site from fig. 2) 

Uppermost of six •••••••••• BB704A (E) 

exposed flows. 

Lowermost of six •••••••••• BB699A (E) 

exposed flows. 

From quarry adjacent •••••• BB608A (F) 

to railroad. 

Tuff of Blacktail 

From porphyritic •••••••••• BF748A (B) 

vitrophyre. 

0.5 m above vitrophyre •••• BF752A (C) 

0.5 m above vitrophyre •••• BF752A (C) 

Wolcott(?) Tuff ••••••••••••• BB712A (G) 

Basalt of upper part of ••••• BF694A (D) 

Starlight Formation. 

Rhyolite of Two-and-a-•••••• BB731A (A) 

Hal £-Mile Creek 

(from porphyritic, BB731A (A) 

spherulitic vitrophyre). 

BB731A (A) 

Latitude 

Longitude 

43°07'52"N. 

ll2°22'10"W. 

43°07'52"N. 

ll2 ° 2 2 ' 1 0 "W. 

43°01 '57"N. 

ll2°20 '25"W. 

43°08'44"N. 

ll2°17 '27"W. 

43°08'38"N. 

ll2°16 '07"W. 

43°08'38"N. 

ll2°16 '07"W. 

43°00' 5 7"'N. 

ll2°19 '40"W. 

43°08'07"N. 

ll 2 °1 7 '3 6 "W. 

43°00'19"N. 

ll 2 °N. ' 2 2 "W. 

43°00'19"N. 

ll2°22 '22 "W 

43°00'19"N. 

ll2°22 '22"W. 

Material 

dated 

Pl 

WR 

Pl 

Pl 

KF 

Pl 

WR 

WR 

B 

KF 

Pl 

40Ar 

K2o% (1 o1 0 mole/ gram) 40Ar% Age (Ma) ±2CT 

0.20 0.007183 6 2.5±0.5 

0.39 0.01224 4 2.2±0.2 

0.19 0.009918 6 3.7±0.7 

0.885 0.08410 36 6.6±0.3 

7.11 0.6834 82 6.7±0.2 

0.68 0.06531 12 6.7±0.3 

5.065 0.4681 71 6.4±0.2 

0.625 0.06319 26 7.0±0.3 

8.22 1.074 46 9.1±0.3 

10.68 1.243 86 8.1±0. 2 

0.17 0.03058 37 12.5±2.3 
C') 

~ :r 

~ 

3 

=.· 

~ 

Table 2. Major-element analyses and weight-percent norms for selected samples of rhyolite and basalt from Buckskin Basin and Stevens Peak areas, Idaho 

Rhyolite of 

Stevens Peak 

Sample No. 

Latitude 

Longitude 

Major elements 

Sio2 •••••••••••• 

A1 2o3 ••••••••••• 

FeO •••••••• • • • • • 

Fe 2o3 ••••••••••• 

MgO •••••••• • • • •. 

CaO ••••••••••••• 

Na 2o •••••••••••• K20 ••••••••••••• 

Tio2 •••••••••••• 

P2o5 •••••••••••• 

MnO ••••••••••••• 

H2o+ •••••••••••• 

H2o-........... . co2 ••••••••••••• 

Total ••••••••••• 

BF691B 

43°08'10"N. 

112°18'00"W. 

74.5 

12.4 

0.23 

1.55 

0.42 

1.22 

3.50 

4.74 

0.48 

0.05 

0.07 

0.19 

0.26 

0.27 

99.88 

Normative Minerals 

(volatile free). 

Q ••••••••••••••• 

c ...•......••.•. 

OR •••••••••••••• 

AB •••••••••••••• 

AN. ••••••••••••• 

DI •••••••••••••• 

wo •••••••••••••• 

HY. ••••••• •••••• 

OL •••••••••••••• 

MT •••••••••••••• 

IL •••••••••••••• 

TN •••••••••••••• 

RU •••••••••• •••• 

AP •••••••••••••• 

Al/(Na+K+Ca)2 ••• 

33.44 

o.oo 

28.17 

29.78 

4.15 

0.86 

0.65 

0.64 

0.00 

o.oo 

0.64 

0.36 

o.oo 

0.12 

0.946 

BF624B 

43°08'34"N. 

112°17'42"W. 

73.6 

12.8 

0.43 

1.35 

0.29 

0.82 

3.56 

4.60 

0.48 

0.10 

0.07 

0.03 

0.17 

0.02 

98.32 

34.02 

o. 73 

27.70 

30.70 

3.48 

0.00 

o.oo 

0.74 

o.oo 

0.23 

0.93 

0.00 

o.oo 

0.24 

1.039 

1Potassium-argon age given in table 1. 

2Molar ratio. 

Rhyolite of 

Two-and-a-HalfMile Creek 

BB731B1 

43°00'19"N. 

112°22'22"W. 

74.0 

12.3 

0.36 

0.78 

0.16 

0.74 

2.67 

5.21 

0.13 

0.03 

0.01 

2.60 

0.08 

(0.01 

99.07 

38.17 

1.03 

31.94 

23.43 

3.60 

o.oo 

o.oo 

0.41 

0.00 

0.85 

0.26 

o.oo 

0.00 

0.07 

1.081 

BB718B 

43°00'47"N. 

ll2°20'42"W. 

74.6 

12.4 

0.28 

0.86 

0.16 

1. 73 

2.89 

5.07 

0.13 

0.03 

0.01 

0.19 

0.21 

0.70 

99.26 

35.47 

0.00 

30.30 

24.73 

5.96 

0.87 

0.59 

0.00 

0.00 

0.56 

0.25 

0.00 

o.oo 

0.07 

0.926 

BB671B 

43°07'10"N. 

112°15'47"W. 

72.8 

13.0 

0.21 

1.00 

0.24 

0.95 

2.79 

5.13 

0.13 

0.03 

0.01 

1.30 

0.61 

<O.Ol 

98.20 

35.96 

1.25 

31.48 

24.51 

4.69 

0.00 

o.oo 

0.62 

o.oo 

0.35 

0.26 

0.00 

0.00 

0.07 

1. 095 

Tuff of 

Arbon Valley 

BB667B 

43°07'23"N. 

112°16'10"W. 

72.0 

13.1 

0.99 

0.82 

0.33 

1.76 

2.64 

4.24 

0.30 

0.08 

0.03 

1.54 

0.32 

<0.01 

98.15 

37.58 

1.28 

26.02 

23.02 

8.53 

o.oo 

o.oo 

1.58 

0.00 

1.23 

0.59 

0.00 

0.00 

0.20 

1.083 

BB670B 

43°07'08"N. 

112°15'49"W. 

72.0 

13.0 

0.16 

0.86 

0.27 

0.53 

2.75 

5.18 

0.05 

0.03 

0.05 

2.34 

0.69 

<O.Ol 

97.92 

36.60 

2.08 

32.26 

24.52 

2.56 

0.00 

o.oo 

0.71 

o.oo 

0.56 

0.10 

o.oo 

o.oo 

0.07 

1.172 

3Fe 2o3 in excess of Ti02+1.5 is converted to FeO for normative calculations, following Irvine and Baragar (1971). 

Tuff of 

Black tail 

BF707B 

43°08'08"N. 

112°15'20"W. 

75.6 

12.8 

0.25 

0.38 

0.17 

0.79 

3.31 

5.13 

0.29 

0.03 

0.01 

0.25 

0.23 

<O.Ol 

99.24 

35.29 

0.44 

30.69 

28.36 

3. 77 

0.00 

0.00 

0.43 

0.00 

0.00 

0.56 

o.oo 

0.00 

0.07 

1. 030 

BF752B1 

43°08'38"N. 

112°16 '07"W. 

75.5 

12.4 

0.17 

0.82 

0.14 

0.54 

3.19 

5.15 

0.23 

0.03 

0.01 

0.39 

0.36 

<O.Ol 

98.93 

36.62 

0.68 

30.99 

27.49 

2.53 

0.00 

0.00 

0.36 

o.oo 

o.oo 

0.39 

0.00 

0.03 

0.07 

1. 051 
... 

~ Table 2. Major-element analyses and weight-percent norms for selected samples of rhyolite and basalt from Buckskin Basin and Stevens Peak areas, 

Idaho-Continued 

~ 

I 

~ Tuff of Walcott(?) Walcott Tuff Basalt of upper Basalt of 

> QQ Kilgore Tuff from site W-1 part, Starlight Buckskin Basin t!> 

~ in figure 1 Formation 

C) 

BB712B1 BF694B1 BB608B1 BF699B1 BF704B1 t!> Sample No. BF750B BF641B BK753B 0 

n Latitude 43°08'48"N, 43°08'11"N, 43°00' 5 7"N. 42°41 '29"N, 43°08'07"N. 43°01 '47"N. 43°07'52"N, 43°07'52"N, :r 

t!> Longitude 112°17 '28"W. 112°17 '37"W, 112°19 '40"W. 112°57 '37"W, 112°17 '36"W, 112°20 '25"W, 11 2 ° 2 2 ' 1 0 "W. 112°22'10"W, 3 

:a-

~ Major elements 

Ill Sio 2 •••••••••••• 71.3 72.1 75.9 73.6 48.5 46.2 45.9 46.2 :I 

c. Al 2o3 ••••••••••• ll.9 12.3 12.2 11.8 p.8 14.7 15.0 14.7 

-1 FeO ••••••••••••• 0.62 0.72 0.46 0.49 9.28 12.1 10.7 12.5 a Fe 2o3 ••••••••••• 0.74 0.54 0.96 o. 77 35.09 3.00 3.31 2.21 0 MgO •••••••• ••••. 0.23 0.26 0.17 0.22 6.13 5.75 7.52 7.72 :I 

;::;· 

Ill cao ••••••••••••• 2.15 0.68 0.46 0.54 9.92 9.98 10.5 9.95 ! Na 2o •••••••••••• 2.78 2.78 3.30 2.89 2.58 2.52 2.46 2.44 

:;· 

~ K20 ••••••••••••• 5.20 5.29 5.13 5.46 0.65 0.66 0.42 0.41 

c: 

Ti02 •••••••••••• 0.23 0.22 0.25 0.21 2.40 3.26 2.73 3.13 

'C P2o5 •••••••••••• 0.03 0.10 0.03 0.03 0.31 1.04 0.50 0.55 'C 

~ MnO ••••••••••••• 0.03 0.02 0.01 0.03 0.20 0.21 0.21 0.22 

l""l H2o+, ••••••••••• 2.82 2. 72 0.30 2.89 0.37 0.36 0.25 0.25 t!> 

:I H2o-•••••••••••• 0.39 0.41 0.13 0.17 0.36 0.18 0.14 0.09 0 

N co2 ••••••••••••• 1.15 0.05 <0.01 (0.01 <0.01 0.04 0.17 0.01 0 

;::;· 

< Total ••••••••••• 99.57 98.19 99.30 99.10 99.59 100.00 99.81 100.38 0 

;::;-

Ill Normative Minerals :I 

;::;· (volatile free). 

;:11:1 Q ••••••••••••••• 32.73 35.64 36.33 35.92 0.97 0.00 o.oo 0.00 0 

n c. 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.00 1.06 0.46 0.24 0.00 0.00 0.00 o.oo ;Ill:" 

~ OR •••••••••••••• 31.89 32.88 30.66 33.59 3.89 3. 92 2.50 2.42 

= AB •••••••••••••• 24.41 24.74 28.24 25.46 22.12 21.44 20.94 20.64 

07' AN •••••••••••••• 4.81 2.86 2.11 2.58 24.47 26.99 28.81 27.94 n 

;Ill:" 

Q DI •••••••••••••• 1.68 0.00 o.oo 0.00 19.16 13.21 16.58 14.66 

~ wo •••••••••••••• 1.66 o.oo 0.00 o.oo o.oo o.oo 0.00 o.oo 

c: HY. •••• ••••••• •• o.oo 1.26 0.43 0.57 18.31 17.74 9.53 11.70 Ill OL ••••••••••• ,,, 0.00 o.oo o.oo o.oo 0.00 3.64 10.27 12.22 :r 

0 MT •••••••••••••• 1.11 0.82 0.80 1.11 5.73 4.37 4.83 3.20 

IL ••••••••• , •• ,. 0.45 0.44 o.oo 0.42 4.62 6.23 5.22 5.94 

TN •••••••••••••• o.oo 0.00 o.oo 0.00 0.00 0.00 0.00 o.oo 

RU •••• ••••, •••,, o.oo o.oo 0.00 0.00 o.oo 0.00 0.00 0.00 

AP ••• ,,,,,,,,,,, 0.07 0.25 0.07 0.07 0.74 2.48 1.19 1.34 

Al/(Na+K+Ca) 2 ••• 0.844 1.066 1.033 1.013 0.601 0.639 0.636 0.652 
by X-ray fluorescence analysis. The trace elements deemed 

most useful in differentiating the various units are Zr 

(zirconium), Sr (strontium), and Rb (rubidium) (fig. 7). 

Rhyolitic Rocks 

The rhyolitic rocks are similar in major-element 

composition as shown on the normative feldspar ternary 

diagram (fig. 4) and the AFM diagram (fig. 5). They are 

average to mildly potassic calc-alkaline rhyolites, as defined by Irvine and Baragar (1971), and most contain normative corundum (table 2), indicating that they generally 

are mildly peraluminous (that is, the molar ratio 

Al/(Na+K+Ca)>1), which is common for intracratonic 

rhyolitic rocks. The rhyolitic rocks are also strongly 

subalkaline (fig. 6). 

Because the rhyolitic rocks erupted from several 

widespread sources along the Snake River Plain over 

about a 5-Ma period, the major-element chemical 

analyses do not define a consistent differentiation trend 

(fig. 6), although rocks from the same unit are generally 

grouped together. Both the tuff of Kilgore and the tuff 

of Arbon Valley have silica contents of less than 73 percent, but the tuff of Blacktail has a silica content greater 

than 75 percent. The tuff in the southern part of the study 

area (fig. 2) is similar in its major-element chemistry to 

the samples of the tuff of Blacktail but for reasons 

(discussed earlier) other than geochemistry is correlated 

with the Walcott Tuff. The rhyolites of Two-and-a-HalfMile Creek and Stevens Peak have similar silica contents 

of about 74 percent. 

Some scatter in both the major- and trace-element 

composition of the tuffs might be due to (1) vertical 

chemical zonation, for which samples were not specifically collected (with one exception noted later) and (2) some 

small, exotic component incorporated during transport. 

Despite this scatter, the geochemical data suggest that a 

discrimination of most units can be made. 

Sample BB670B (table 2) is from the basal ash of 

the tuff of Arbon Valley and has a slightly higher content of normative corundum and a smaller normative 

anorthite content than the two samples (BB671B and 

BB667B) from the overlying densely welded section. 

These content differences imply that the early eruptive 

phases of the tuff of Arbon Valley are more evolved 

(more peraluminous and contain more sodic feldspars) 

than the later phases, a fact in agreement with Hildreth's 

(1979) model for zoned magma chambers underlying 

calderas (that is, early eruptions tap the higher, more 

evolved portions of the magma chamber). 

The Zr-Sr-Rb geochemistry of the rhyolitic rocks 

(fig. 7) demonstrates that some units can be discriminated 

on the basis of trace elements. The groupings are tightest 

for the two rhyolite flows (fig. 7A) rather than for the 

I I I I 

0.4 - • -

0 0.3 01 

:::t 0.2 

- 0 • - _o Cb 0 + -

0.1 • • 1::.1::. ... - I I I I 

.-

2.0 - I I I I - 0 

I") 

• • ~ Q) 

lJ... 1.0 

0 + ... 0 0 

- 0 • • 1::. -

1::. 

_l_ I I I 

13.0 - 8 I I I 

0 - • 1::. 

I") 

0 

N 

< 12.0 

•• A. 0 • ... - - ':) + I I I I _A 

I I + I I 1::. • 

ON 8.0 

~ 0 

-o\ • • 0 - 0 0 • • 

Alkaline-subalkaline line of Irvine and 

N 

0 Baragar (1971 ); 11 percent alkalies z 7.0 - at 71 percent Si02 - q I I I 

71 72 73 74 75 76 

Si02, WEIGHT PERCENT 

Figure 6. Si02-oxide variation diagram for rhyolitic rocks. 

Fe20 3 is total iron calculated as Fe20 . Open circles, tuff of 

Kilgore; open triangles, tuff of Blac~tail; closed triangles, 

suspected Walcott Tuff collected at site G (fig. 2); crosses, 

Walcott Tuff collected at site W-1 (fig. 1 ); open squares, tuff 

of Arbon Valley; closed circles, rhyolite of Two-and-a-Half-Mile 

Creek; and closed squares, rhyolite of Stevens Peak. Values 

are taken from table 2. 

ash-flow tuffs (fig. 7B), a situation that is most probably 

the result of some contamination of the ash-flow tuffs 

by exotic material during transport. The rhyolite of 

Stevens Peak is enriched in Zr relative to Rb and Sr as 

compared to the rhyolite of Two-and-a-Half-Mile Creek. 

The tuff of Arbon Valley is enriched in Sr relative to Zr 

and Rb as compared to the tuffs of Kilgore and Blacktail. 

The fields for the Kilgore and Blacktail tuffs overlap, 

although the field for the tuff of Kilgore extends farther 

toward the Zr corner. Although differences in silica content (fig. 6) appear to be more useful in discriminating 

these two tuffs than differences in Zr, Sr, and Rb, chemical 

data from elsewhere along the margin of the SRP show 

that the silica content of these two units has considerable 

overlap (L.A. Morgan, written commun., 1986). 

Basaltic Rocks 

All analyzed samples of basalt of Buckskin Basin 

are olivine tholeiites (fig. 5) and are slightly depleted in 

magnesium and enriched in potassium and iron relative 

Geochemistry 13 
Zr 

Sr 

Rhyolite of 

Two-and-a-HalfMile Creek 

Rhyolite of 

Stei Peak~ 

Sr 

Sr 

Zr Rb 

Figure 7. Zirconium-strontium-rubidium ternary diagrams for (A) closed circles, rhyolite of Two-and-a-Half-Mile 

Creek; and closed squares, rhyolite of Stevens Peak; (B) open squares, tuff of Arbon Valley; open triangles, tuff of 

Blacktail; and open circles, tuff of Kilgore; and (C) open pentagons, the basalt of Buckskin Basin; and closed pentagons, basalt of upper member of Starlight Formation. 

to average SRP basalt (Leeman, 1982b) (figs. 4 and 5). 

Figures 4 and 5 do not show significant differences between the basalt of Buckskin Basin and the one sample 

of basalt in the upper member of the Starlight Formation, although the latter basalt is depleted in alumina 

relative to the basalt of Buckskin Basin and contains no 

normative olivine (table 2); however, olivine is an observed modal phase. 

TIMING OF BASIN AND RANGE FAULTING 

North-northwest-trending high-angle faults related 

to Basin and Range extension are well developed in the 

study area and outline fault blocks in which strata dip 

as much as 30°, generally east-northeast. The largest apparent throws of about 120 m occur on faults cutting the 

tuff of Arbon Valley in the northeastern part of the map 

area. 

The timing of maximum Basin and Range activity 

in the Stevens Peak area can be determined by bracketing 

the age of the boulder conglomerate that overlies the 

Starlight Formation. The boulder conglomerate contains 

large clasts of Triassic(?) and Jurassic(?) Nugget Sandstone, among other lithologies, which must have come 

from at least 10 km to the east (Trimble, 1982). Currently, several drainage divides separate outcrops of Nugget 

Sandstone and deposits of the boulder conglomerate. 

Displacements across prominent Basin and Range faults 

cutting Paleozoic and Mesozoic strata at least 5 km to 

the east of the study area are of the order of 1 ,000 m or 

more (Hladky, 1986; Kellogg and others, 1986; Trimble, 

1982), and it seems reasonable that erosion of the 

Starlight Formation and deposition of the boulder conglomerate were controlled. by high-energy, west-flowing 

streams or possibly debris flows during periods of 

relatively large movements along these Basin and Range 

faults. 

14 K-Ar Ages, Geochemistry, and Tectonic Setting, Upper Cenozoic Volcanic Rocks, Blackfoot, Idaho 
The age of the boulder conglomerate is constrained 

by the dated volcanic rocks in the study area. Trimble 

(1982) recognized that the boulder conglomerate, which 

he called "quartzite gravel:' overlies eroded Starlight Formation and is overlain by welded ash-flow tuffs, now 

known to be the tuff of Blacktail. The basalt in the upper member of the Starlight Formation is 7.0±0.3 Ma, 

but the tuff of Blacktail is 6. 7 ± 0.2 Ma. Therefore, a very 

large component of Basin and Range faulting occurred 

between about 7.0 and 6.7 Ma. This age is in agreement 

with Allmendinger's (1982) evidence that Basin and Range 

faulting is shifting eastward in time (fig. 8) and supports 

the contention that the rate of easterly migration is slowing. Initial easterly migration of the eastern edge of Basin 

and Range deformation, starting 17 Ma in northern 

Nevada, was about 6.5 em/yr. Over the past 7 .0-6. 7 Ma, 

however, this easterly migration east of Stevens Peak has 

slowed to about 1. 7 em/yr. Allmendinger (1982) suggested 

that this slowing might be due to the migration of Basin 

and Range extension eastward from crust which was initially thinned during Late Proterozoic rifting into increasingly thickened crust to the east. 

Evidence exists that at least some Basin and Range 

faulting occurred in the area during the deposition of the 

Starlight Formation. Pebble, cobble, and minor boulder 

conglomerates occur in the lower member of the Starlight 

Formation in the Yandell Springs quadrangle, immediately east of the study area (Hladky, 1986; Trimble, 1982), 

indicating that some uplift associated with early phases 

of Basin and Range movement occurred as long ago as 

about 9 Ma. The Starlight Formation was probably 

deposited in north-trending, fault-bounded valleys, 

although the predominantly fine-grained nature of the 

Starlight deposits and the continuity and constant thickness of beds over distances of at least several kilometers 

support the contention that mostly low topography existed during Starlight deposition. 

Trimble (1976) also mapped a boulder diamictite 

in the lower member of the Starlight Formation, within 

10 km south of the study area, although the tuff of Arbon 

Valley does not occur in the same area to mark the top 

of the lower member of the Starlight Formation. The 

diamictite lies directly over Proterozoic rocks of the thrust 

belt and predominantly contains blocks of Proterozoic 

and Paleozoic quartzites as large as 3 m in diameter as 

well as minor and smaller basaltic to andesitic clasts. The 

diamictite contains a tuffaceous matrix and is locally 

cemented by calcite. Some of these deposits resemble the 

boulder conglomerate of the study area (K.S. Kellogg, 

unpub. data, 1984). Although it cannot be proven at this 

time, we believe that the diamictite south of the study 

area is equivalent to the boulder conglomerate of the 

study area, which overlies the Starlight Formation. 

Maximum displacement of the 4.3-Ma tuff of 

Kilgore, 1 km north of Stevens Peak, is 60 m, down to 

the northeast. Maximum displacement of the 7. 7 -Ma tuff 

w E 20~--~----~--~--~~--~--~r---~ 

5 

I' 'North-central Nevada 

" " " " " " " 

Raft Riv:' f._, 

Mountains 1 ', 

I ' 

.1. ', 

Stevens Peak ~\ (This study) I 

Blackfoot Mountains \ 

\ 

\ 

\ 

Intermountain Seismic Belt \ 

\ 

0~---L----~--~----J---~----~--~ 

700 600 500 400 300 200 1 00 0 

DISTANCE, IN KILOMETERS 

Figure 8. Graph showing the possible eastward migration of 

eastern boundary of Basin and Range province. Positions are 

measured west of intermountain seismic belt (Smith and Sbar, 

1974), and ages are those of initial uplifts of current mountain 

ranges. Position and age of inferred major Basin and Range 

faulting in study area (Stevens Peak) is shown. Adapted from 

Allmendinger (1982). 

of Arbon Valley, near the eastern border of the study 

area, is 120 m, down to the southwest. The difference 

in the amount of displacement on faults cutting these two 

tuffs is probably due to movement during the period of 

major Basin and Range faulting 6.7-7.0 Ma. 

Some Basin and Range faults with less than 50 m 

of throw cut the basalt of Buckskin Basin, indicating that 

relatively minor Basin and Range faulting has occurred 

in the area since 2.2 Ma. The exact amount of throw is 

difficult to determine because of the extensive loess cover 

that obscures both the faults and the attitude of the flows. 

BUCKSKIN DOME 

The most prominent feature in the study area is the 

Buckskin dome, which is a circular feature 7 km in 

Buckskin Dome 15 
diameter (fig. 2). The rocks underlying the Buckskin 

dome are composed entirely of the basalt of Buckskin 

Basin. Kellogg and Embree (1986) described the structure of the Buckskin dome as follows: 

The basalt flows in the outermost arcuate hills of the 

Buckskin dome dip away from the center of the dome 

at 5° to 20°. Blocks of faulted basalt inside the Buckskin 

dome contain flows that generally are nearly horizontal. 

These orientations suggest either that the interior of the 

original dome flattened by sagging after uplift or that uplift 

was piston-like, and the strata on the periphery of the 

structure was tilted at greater angles than that in the center. 

Karlo and Jorgenson (1979) speculated that the 

Buckskin dome had been uplifted by rhyolitic magma, 

similar to that observed at Big Southern Butte and East 

Butte. According to their model, pulses of magma injection resulted in doming of the overlying basalts. Subsequent removal after uplift caused the sagging and faulting 

in the central part of the dome. We find no evidence to 

contradict this hypothesis. 

At site E (fig. 2), the attitudes on the dated basalt 

flows give a constraint on the timing of uplift of the 

Buckskin dome. An angular unconformity of about 8 ° 

separates the upper two flows from the lower four flows; 

the upper two flows dip 12 ° to the northwest and the 

lower four flows dip 20 ° to the northwest. Because the 

flows were extruded during a short time interval about 

2.3 Ma, the significant difference in the dips of the two 

upper flows compared to the lower flows strongly suggests that some rotation (and presumably uplift of the 

dome) was occurring contemporaneously with basaltic 

volcanism but that a substantial if not predominant part 

of the rotation (and uplift) occurred after about 2.3 Ma. 

Geophysical data are somewhat ambiguous regarding the existence of a buried rhyolitic body under the 

Buckskin dome. The available gravity data (fig. 9) 

(Bankey and others, 1985) suggest that rhyolitic magma 

might have intruded to the southeast and possibly under 

Buckskin dome. No gravity stations were placed on 

Buckskin dome itself, and the surrounding stations define 

a north-northwest-trending, 5- by 10-km gravity low of 

15 milligals immediately to the southeast of the dome, 

largely outside the study area. Exposures in the immediate 

area of this gravity low are of Quaternary loess and 

alluvium. The trend of the gravity anomaly and several 

other nearby anomalies is parallel to the prevalent Basin 

and Range faults in the area and could be at least partially due to these features. 

The available aeromagnetic data (Zietz and others, 

1978) indicate a northwest-trending, low-amplitude 

(about 60 gammas), magnetic high zone approximately 

coincident with the gravity low zone on the southeastern 

side of Buckskin dome. This anomaly could be due to 

either Tertiary Basin and Range valley fill (the contained 

volcanic rocks and debris would be relatively magnetic) 

or a buried rhyolitic magma body. 

Ferry Butte-a 115-m-high, 3-km-in-diameter hill 

about 5 km west of the study area (fig. I)-might also 

be a dome-shaped uplift similar to Buckskin dome. Ferry 

Butte is almost entirely mantled by loess, although a few, 

small rubbly outcrops of basalt, similar to the basalt of 

Buckskin Basin, occur on the eastern side. 

SUMMARY AND CONCLUSIONS 

The Tertiary rocks of the Buckskin Basin and 

Stevens Peak areas document a 10-Ma period of volcanic 

activity that began with deposition of the Starlight Formation, a sequence of variable thickness consisting of early 

rhyolite flows overlain by ash falls, ash-flow tuffs, 

genetically related intercalated sediments, freshwater 

limestone, and basalt flows. The Starlight Formation 

blanketed much of the eastern SRP until about 7.0 Ma. 

During deposition of the Starlight Formation, before the 

onset of major Basin and Range faulting, the topography 

along the eastern margin of the plain was relatively 

moderate compared to that which existed after 7.0 Ma. 

Two rhyolite flows placed in the lower member of 

the Starlight Formation-the rhyolite of Two-and-a-HalfMile Creek (9 .1 Ma) and the rhyolite of Stevens Peak (9 .8 

Ma)-are chemically and petrographically distinct. The 

7.9-Ma tuff of Arbon Valley makes up the middle 

member of the Starlight Formation and is the most prominently exposed unit in the study area. 

Major Basin and Range faulting, mostly to the east 

of the study area, produced west-flowing, high-gradient 

streams that deeply incised the Starlight Formation and 

deposited a coarse boulder conglomerate. The timing of 

the onset of major Basin and Range faulting is constrained by a 7 .0-Ma basalt in the upper member of the 

Starlight Formation and by the 6.7-Ma tuff of Blacktail, 

which overlies the boulder conglomerate. These ages are 

consistent with a model for easterly younging of the onset 

of Basin and Range faulting (Allmendinger, 1982). 

Although major faulting occurred between 7.0 and 6. 7 

Ma, some Basin and Range faults have undergone much 

younger activity; vertical movement of at least 50 m has 

occurred across north-northwest-trending faults, cutting 

2.2-Ma basalt of Buckskin Basin in the area studied. 

Both the 6.7-Ma tuff of Blacktail and the overlying 4.3-Ma tuff of Kilgore crop out prominently in the 

study area and had their sources on both the north and 

south margins of the eastern SRP (Morgan and others, 

1984). Additional work is needed to determine if the 

6.5-Ma Walcott Tuff crops out in the study area. 

Sometime between 4.3 and 3.7 Ma, the composition of volcanic rocks in the Stevens Peak and Buckskin 

Basin area changed abruptly and markedly from predominantly rhyolitic to predominantly basaltic. This change 

reflects the cessation of explosive rhyolitic volcanism after 

16 K-Ar Ages, Geochemistry, and Tectonic Setting, Upper Cenozoic Volcanic Rocks, Blackfoot, Idaho 
40° 

07' 

30" 

• 

• 

• • 

0 

• 

2 4 6 8 KILOMETERS 

Figure 9. Bouguer gravity map of study area and surrounding region. Contours are in milligals, with 2-milligal contour interval. 

Dots represent gravity stations. Modified from Bankey and others (1985). 

4.3 Ma along the main part of the eastern SRP and the 

onset of local basaltic fissure eruptions. The 2-Ma-andyounger ash-flow tuffs from the Island Park and 

Yellowstone Plateau areas (Christiansen and Blank, 1972) 

at the northeastern end of the SRP have not been observed in the study area. 

Summary and Conclusions 17 
The Buckskin dome is a circular domical feature 

7 km in diameter that formed after about 2.5 Ma and 

is entirely composed at the surface of basalt of Buckskin 

Basin, an olivine tholeiite similar to much of the basalt 

of the SRP. The Buckskin dome has the shape of a sagged 

domical uplift and is cut both by north-northwesttrending Basin and Range faults and by ring faults probably caused by the uplift and (or) subsequent sagging of 

the center. Gravity and magnetic data are ambiguous 

about the nature of the rocks underlying the Buckskin 

dome. The gravity data suggest that a relatively lowdensity rock might occur within 10 km to the southeast 

of the Buckskin dome and might extend under the structure itself. It is suggested that the dome was produced 

by injection and subsequent withdrawal pf rhyolitic 

magma. Ferry Butte, a prominent hill about 5 km east 

of the study area composed entirely of olivine basalt, 

might have formed in the same manner. 

REFERENCES CITED 

Allmendinger, R.W., 1982, Sequence of late Cenozoic deformation in the Blackfoot Mountains, southeastern Idaho, 

in Bonnichsen, Bill, and Breckenridge, R.M., eds., 

Cenozoic geology of Idaho: Idaho Bureau of Mines and 

Geology Bulletin 26, p. 505-516. 

Armstrong, R.L., Harakel, J.E., and Neill, W.M., 1980, K-Ar 

dating of Snake River Plain (Idaho) volcanic rocks-new 

results: Isochron/West, no. 27, p. 5-10. 

Armstrong, R.L., Leeman, W.P., and Malde, H.E., 1975, K-Ar 

dating, Quaternary and Neogene volcanic rocks of the 

Snake River Plain, Idaho: American Journal of Science, 

v. 275, no. 3, p. 225-251. 

Bankey, Viki, Webring, Michael, ~Mabey, D.R., Kleinkopf, 

M.D., and Bennett, E.H., 1985, Complete Bouguer gravity 

anomaly map of Idaho: U.S. Geological Survey Miscellaneous Field Studies Map MF-1773, scale 1:500,000. 

Carr, W.J., and Trimble, D.E., 1963, Geology of the American 

Falls quadrangle, Idaho: U.S. Geological Survey Bulletin 

1121-G, 44 p. 

Christiansen, R.L., and Blank, H.R., Jr., 1972, Volcanic 

stratigraphy of the Quaternary rhyolite plateau in 

Yellowstone National Park: U.S. Geological Survey Professional Paper 729-B, 18 p. 

Dalrymple, G.H., and Lanphere, M.A., 1969, Potassium-argon 

dating-principles, techniques, and applications to geochronology: San Francisco, Calif., W.H. Freeman, 258 p. 

Doherty, D.J., McBroome, L.A., and Kuntz, M.A., 1979, 

Preliminary geological interpretation and lithologic log of 

the exploratory geothermal test well (INEL-1), Idaho National Engineering Laboratory, eastern Snake River Plain, 

Idaho: U.S. Geological Survey Open-File Report 79-1248, 

10 p. 

Embree, G.F., McBroome, L.A., and Doherty, D.J., 1982, 

Preliminary stratigraphic framework of the Pliocene and 

Miocene rhyolite, eastern Snake River Plain, Idaho, in Bonnichsen, Bill, and Breckenridge, R.M., eds., Cenozoic 

geology of Idaho: Idaho Bureau of Mines and Geology 

Bulletin 26, p. 333-344. 

Greeley, R., 1982, The style of basaltic volcanism in the eastern 

Snake River Plain, in Bonnichsen, Bill, and Breckenridge, 

R.M., eds., Cenozoic geology of Idaho: Idaho Bureau of 

Mines and Geology Bulletin 26, p. 407-422. 

Hildreth, Wes, 1979, The Bishop Tuff-evidence for the origin 

of compositional zonation in silicic magma chambers: 

Geological Society of America Special Paper 180, p. 43-75. 

Hladky, P.R., 1986, Geology of an area north of the narrows 

of Ross Fork Canyon, northernmost Portneuf Range, Fort 

Hall Indian Reservation, Bannock and Bingham Counties, 

Idaho: Pocatello, Idaho, Idaho State University, M.S. 

thesis, 109 p. 

lngamells, C.O., 1970, Lithium metaborate flux in silicate 

analysis: Analytica Chimica Acta, v. 52, p. 323-334. 

Irvine, T.N., and Baragar, W.R.A., 1971, A guide to the 

chemical classification of the common volcanic rocks: 

Canadian Journal of Earth Sciences, v. 8, p. 523-548. 

Karlo, J.P., and Jorgenson, D.B., 1979, Fault control of 

volcanic features southeast of Blackfoot, Snake River 

Plain, Idaho: Geological Society of America Abstracts with 

Programs, v. 11, no. 6, p. 276. 

Kellogg, K.S., and Embree, G.F., 1986, Geologic map of the 

Stevens Peak and Buckskin basin areas, Bingham and Bannock Counties, Idaho: U.S. Geological Survey Miscellaneous Field Studies Map MF-1854, scale 1:24,000. 

Kellogg, K.S., Hladky, P.R., and Desborough, G.A., 1986, 

Geometry and hydrocarbon appraisal of the Putnam Thrust 

adjacent to the Snake River Plain near Blackfoot, Idaho, 

in Carter, L.M.H., ed., U.S. Geological Survey Research 

on Energy Resources, 1986 Program with Abstracts: U.S. 

Geological Survey Circular 974, p. 30-31. 

Kuntz, M.A., 1978, Geology of the Arco-Big Southern Butte 

area, eastern Snake River Plain, and potential volcanic 

hazards to the Radioactive Waste Management Complex 

and other waste storage and reactor facilities at the Idaho 

National Engineering Laboratory, Idaho: U.S. Geological 

Survey Open-File Report 78-691, 70 p. 

Kuntz, M.A., Champion, D.E., Spiker, E.C., Lefebvre, R.H., 

and McBroome, L.A., 1982, The Great Rift and evolution 

of the Craters of the Moon lava field, Idaho, in Bonnichsen, Bill, and Breckenridge, R.M., eds., Cenozoic 

geology of Idaho: Idaho Bureau of Mines and Geology 

Bulletin 26, p. 423-437. 

Leeman, W.P., 1982a, Development of the Snake River PlainYellowstone Plateau province, Idaho and Wyoming: An 

overview and petrologic model, in Bonnichsen, Bill, and 

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

Idaho Bureau of Mines and Geology Bulletin 26, 

p. 155-177. 

___ 1982b, Olivine tholeiitic basalts of the Snake River Plain, 

Idaho, in Bonnichsen, Bill, and Breckenridge, R.M., eds., 

Cenozoic geology of Idaho: Idaho Bureau of Mines and 

Geology Bulletin 26, p. 181-192. 

Lewis, G.C., and Fosberg, M.A., 1982, Distribution and 

character of loess and loess soils in southeastern Idaho, 

in Bonnichsen, Bill, and Breckenridge, R.M., eds., 

Cenozoic geology of Idaho: Idaho Bureau of Mines and 

Geology Bulletin 26, p. 705-716. 

18 K-Ar Ages, Geochemistry, and Tectonic Setting, Upper Cenozoic Volcanic Rocks, Blackfoot, Idaho 
Mansfield, G.R., 1920, Geography, geology, and mineral 

resources of the Fort Hall Indian Reservation, Idaho: U.S. 

Geological Survey Bulletin 713, 152 p. 

___ 1927, Geography, geology, and mineral resources of part 

of the Fort Hall Indian Reservation, Idaho, with Descriptions of Carboniferous and Triassic fossils, by G.H. Girty: 

U.S. Geological Survey Professional Paper 152, 453 p. 

Mansfield, G.R., and Ross, C.S., 1935, Welded rhyolitic tuffs 

in southeastern Idaho: Transactions of the American 

Geophysical Union, pt. 1, p. 308-321. 

Marvin, R.F., Mehnert, H.H., and Noble, D.C., 1970, Use of 

Ar26 to evaluate the incorporation of air by ash-flows: 

Geological Society of America Bulletin, v. 8, p. 3385-3392. 

McBroome, L.A., 1981, Stratigraphy and origin of Neogene 

ash-flow tuffs on the north-central margin of the eastern 

Snake River Plain, Idaho: Boulder, Colo., University of 

Colorado, M.S. thesis, 74 p. 

McBroome, L.A., Doherty, D.J., and Embree, G.F., 1981, Correlation of major Pliocene and Miocene ash-flow sheets, 

eastern Snake River Plain, Idaho, in Tucker, T.E., ed., 

Guidebook to southwest Montana: Montana Geological Society, 1981 Field Conference and Symposium, p. 323-339. 

Morgan, L.A., Doherty, D.J., and Leeman, W.P., 1984, Ignimbrites of the eastern Snake River Plain-evidence for 

major caldera-forming eruptions: Journal of Geophysical 

Research, v. 89, no. BlO, p. 8665-8678. 

Pankratz, L.W., and Ackermann, H.D., 1982, Structure along 

the northwest edge of the Snake River Plain interpreted 

from seismic refraction: Journal of Geophysical Research, 

v. 87, no. 4, p. 2676-2682. 

Prostka, H.J ., 1979, Buried calderas of the eastern Snake River 

Plain: EOS, Proceeding of the American Geophysical 

Union, v. 60, no. 46, p. 945. 

Prostka, H.J., and Embree, G.F., 1978, Geology and geothermal resources of the Rexburg area, eastern Idaho: U.S. 

Geological Survey Open-File Report 78-1009, 15 p. 

Prostka, H.J., Embree, G.F., and Doherty, D.J., 1979, The 

Pliocene Rexburg caldera complex, southeastern Idaho: 

Geological Society of America Abstracts with Programs, 

v. 11, p. 499. 

Skipp, Betty, Prostka, H.J., and Schleicher, D.L., 1979, 

Preliminary geologic map of the Edie Ranch quadrangle, 

Clark County, Idaho, and Beaverhead County, Montana: 

U.S. Geological Survey Open-File Report 79-845, scale 

1:62,500. 

Smith, R.B., and Sbar, M.L., 1974, Contemporary tectonics 

and seismicity of the western United States with emphasis 

on the intermountain seismic belt: Geological Society of 

America Bulletin, v. 85, p. 1202-1218. 

Sparlin, M.A., Braile, I.W., and Smith, R.B., 1982, Crustal 

structure of the eastern Snake River Plain determined from 

ray trace modeling of seismic refraction data: Journal of 

Geophysical Research, v. 87, no. B4, p. 2619-2633. 

Spear, D.B., and King, J.S., 1982, The geology of Big Southern 

Butte, in Bonnichsen, Bill, and Breckenridge, R.M., eds., 

Cenozoic geology of Idaho: Idaho Bureau of Mines and 

Geology Bulletin 26, p. 395-403. 

Steiger, R.H., and Jager, E., 1977, Subcommission on geochronology: Convention on the use of decay constants in 

geo- and cosmochronology: Earth and Planetary Science 

Letters, v. 36, p. 359-362. 

Trimble, D.E., 1976, Geology of the Michaud and Pocatello 

quadrangles, Bannock and Power Counties, Idaho: U.S. 

Geological Survey Bulletin 1400, 88 p. 

___ 1982, Geologic map of the Yandell Springs quadrangle, 

Bannock and Bingham Counties, Idaho: U.S. Geological 

Survey Geologic Quadrangle Map GQ-1553, scale 

1:48,000. 

Trimble, D.E., and Carr, W.J., 1976, Geology of the Rockland 

and Arbon quadrangles, Power County, Idaho: U.S. Geological Survey Bulletin 1399, 115 p. 

Williams, P.L., Covington, H.R., and Pierce, K.L., 1982, 

Cenozoic stratigraphy and tectonic evolution of the Raft 

River Basin, in Bonnichsen, Bill, and Breckenridge, R.M., 

eds., Cenozoic geology of Idaho: Idaho Bureau of Mines 

and Geology Bulletin 26, p. 491-504. 

Zietz, Isodore, Gilbert, F.P, and Kirby, J.R., Jr., 1978 [1979], 

Aeromagnetic map of Idaho: Color coded intensities: U.S. 

Geological Survey Geophysical Investigations Map 

GP-920, scale 1:1 ,000,000. 

References Cited 19