Major 10.2-Ma Rhyolitic Volcanism in the 

Eastern Snake River Plain, IdahoIsotopic Age and Stratigraphic Setting of the 

Arbon Valley Tuff Member 

of the Starlight Formation 

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Major 10.2-Ma Rhyolitic Volcanism in the 

Eastern Snake River Plain, IdahoIsotopic Age and Stratigraphic Setting of the 

Arbon Valley Tuff Member 

of the Starlight Formation 

By K.S. Kellogg, S.S. Harlan, H.H. Mehnert, L.W. Snee, K.L. Pierce, 

W.R. Hackett, and D.W. Rodgers 

U.S. GEOLOGICAL SURVEY BULLETIN 2091 

UNITED STATES GOVERNMENT PRINTING OFFICE : 1994 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director 

For Sale by U.S. Geological Survey, Map Distribution 

Box 25286, MS 306, Federal Center 

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Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government 

Library of Congress Cataloging-in-Publication Data 

Major 10.2-Ma rhyolitic volcanism in the eastern Snake River Plain, Idaho : isotopic age 

and stratigraphic setting of the Arbon Valley Tuff Member of the Starlight Formation I by 

K.S. Kellogg ... [et al.]. 

p. cm.-(U.S. Geological Survey bulletin; 2091) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3:2091 

1. Volcanic ash, tuff, etc.-Idaho. 2. Geology, Stratigraphic-Miocene. 

3. Petrology-Idaho. 4. Starlight Formation (Idaho) 

I. Kellogg. Karl S. II. Title: Arbon Valley Tuff Member of the Starlight 

Formation. III. Series. 

QE75.B9 no. 2091 

[QE461] 

557.3 s-dc20 94-15631 

[552'.2'09792] CIP 
CONTENTS 

Abstract ................................................................................................................................ . 

Introduction ......................................................................................................................... . 

Description of the Arbon Valley Tuff Member.................................................................... 3 

New isotopic ages for the Arbon Valley Tuff Member........................................................ 5 

Sampling....................................................................................................................... 5 

Methodology................................................................................................................. 5 

Results of 40 Arf39 Ar Analysis . ........... .......... .... ......... .......... ............ ... ....... ....... ......... ... . 6 

Results of potassium-argon analysis............................................................................. 7 

Estimated age of the Arbon Valley Tuff Member................................................................ 10 

Stratigraphy of the southeastern margin of the Snake River Plain....................................... 11 

Geochemistry of the Arbon Valley Tuff Member................................................................ 11 

Source of the Arbon Valley Tuff Member........................................................................... 13 

Conclusions . . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . .. . .. . . . . . . .. . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . .. 16 

References Cited................................................................................................................... 16 

FIGURES 

1. Location map showing the known extent of the Arbon Valley Tuff Member and three rhyolite lava flows . ......... .. .. 2 

2. Photograph of the Arbon Valley Tuff Member in The Cove....................................................................................... 4 

3. Photograph of the base of the Arbon Valley Tuff Member in The Cove . . . . .. . . . . . . . . . . . . . . . .. . . . . .. . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . .. . . . . . 4 

4. Diagrams showing 40 Arf39 Ar age spectra for sanidine and biotite from the Arbon Valley Tuff Member................... 8 

5. 40 ArP6 Ar -39 ArP6 Ar isochron diagrams for sanidine and biotite from samples of the Arbon Valley Tuff Member . . . . 9 

6. Diagrammatic stratigraphic cross section of units along the southeastern margin of the Snake River Plain............. 12 

7. Strontium-rubidium-zirconium ternary diagram comparing samples from the Heise volcanic field with 

those from the Arbon Valley Tuff Member . . . .. . . . . . . .. . .. . . . . . . .. . . . . . . . . . .. . . . . . . . . . .. . . . . .. . . . . .. . . . . . . . . . .. . . . . . . . . . .. . . . . .. . . . .. . . . . . . . . . . .. . . . . . . . . . . 13 

8. Cross section along the resistivity sounding line shown in figure 1 ........................................................................... 16 

TABLES 

1. Measured production ratios for potassium- and calcium-derived argon isotopes for the U.S. Geological 

Survey RIGA reactor................................................................................................................................................... 5 

2. 40 Arf39 Ar age-spectrum data for the Arbon Valley Tuff Member of the Starlight Formation..................................... 6 

3. Summary of 40ArJ39 Ar dating results for the Arbon Valley Tuff Member of the Starlight Formation........................ 9 

4. Summary of potassium-argon ages for the Arbon Valley Tuff Member and several other rhyolitic units 

from the eastern Snake River Plain............................................................................................................................. 10 

5. Major-oxide chemical analysis of rhyolite of west Pocatello ..................................................................................... 13 

6. Selected trace-element abundances for samples of the Arbon Valley Tuff Member, rhyolites of 

Two-and-a-Half-Mile Creek and Stevens Peak, and tuffs of the Heise volcanic field................................................ 14 

III 

Major 10.2-Ma Rhyolitic Volcanism in the 

Eastern Snake River Plain, IdahoIsotopic Age and Stratigraphic Setting of the 

Arbon Valley Tuff Member 

of the Starlight Formation 

By K.S. Kellogg, S.S. Harlan, H.H. Mehnert, 

L.W. Snee, K.L. Pierce, W.R. Hackett, and D.W. Rodgers 

ABSTRACT 

The Arbon Valley Tuff Member of the Starlight Formation, formally named in this report, is a prominent rhyolite 

ash-flow tuff that is exposed at many localities along the 

southeastern margin of the eastern Snake River Plain. Where 

it occurs, the unit makes up the entire middle member of the 

Starlight Formation and extends from at least Rockland Valley on the southwest to the Blackfoot Mountains on the 

northeast, a distance of more than 125 km. The tuff is also 

recognized at one locality on the northwestern margin of the 

80-km-wide plain in the southern Lemhi Range. The tuff is 

crystal rich, containing quartz, sanidine, plagioclase, and 

biotite. It is petrographically and geochemically distinct 

from younger ash-flow tuffs of the eastern Snake River 

Plain. 

New 40 Arf39 Ar isotope dates from sanidine indicate that 

the Arbon Valley Tuff Member was erupted about 

10.20+0.06 Ma; these new dates resolve conflicting previously reported ages for the unit. A source for the Arbon 

Valley Tuff Member may have been a caldera near Blackfoot 

and Pocatello, as suggested by the following observations: 

(1) two rhyolite lava flows that have potassium-argon ages 

of 9.8±0.9 Ma and 9.1±0.3 Ma occur south of Blackfoot and 

may have erupted from caldera ring fractures, (2) the 

observed thickness of the Arbon Valley Tuff Member is 

greatest (about 60 meters) near Blackfoot, (3) a resistivity 

low that is at least 6 km (kilometers) deep coincides with a 

gravity low that extends about 15 km northwest from Blackfoot, and ( 4) there is a lack of basaltic volcanic vents in the 

area of the inferred caldera, possibly indicating low-density 

rock that acted as a barrier to the ascent of basaltic magma. 

The age and chemically distinct nature of the Arbon 

Valley Tuff Member support previously proposed models 

for cycles of rhyolitic magmatism in volcanic fields of the 

eastern Snake River Plain. Each cycle lasted for several million years, and magmatism migrated northeastward toward 

the Yell ow stone plateau. 

INTRODUCTION 

Evidence is accumulating that the Snake River Plain is 

underlain and partly surrounded by a sequence of rhyolitic 

ash-flow tuffs that represent the preserved deposits of large, 

caldera-forming eruptions (Prostka, 1979; Morgan and 

others, 1984; Morgan, 1988, 1992). An overall pattern of 

successively younger tuffs to the northeast toward the 

Yell ow stone Plateau may reflect the southwestward movement of the North American plate over a thermal mantle 

plume (Armstrong and others, 1975; Rodgers and others, 

1990; Pierce and Morgan, 1992). Furthermore, there is evidence that rhyolitic volcanism may occur in cycles, each 

lasting several million years (Pierce and Morgan, 1992; 

Morgan, 1992); three major rhyolitic volcanic fields that 

existed in the eastern Snake River Plain during the past 10 

m.y. (million years) are successively younger toward the 

northeast. These fields are (1) the approximately 10.2-Ma 

Picabo volcanic field (this age assignment is based almost 

entirely on isotopic dates from the Arbon Valley Tuff 

Member in this report), (2) the 6.7- 4.3-Ma Heise volcanic 

field, and (3) the 2.0-Ma to-present Yellowstone plateau volcanic field. 

A veneer of Pliocene to Holocene olivine basalt, as 

thick as 1-2 km, mantles and obscures both the rhyolites 

under the plain and their inferred source calderas (Doherty 

and others, 1979; Embree and others, 1982). Only on the 
2 

4ZO 

30 ' 

RHYOLITIC VOLCANISM IN THE SNAKE RIVER PLAIN, IDAHO 

11ZOOO ' 

50 KILOMETERS 

EXPlANATION 

M --M' 

c 

10.4 ± 0.9 

• 

Rhyolite flow 

Arbon Valley Tuff 

Member of the 

Starlight Formation-Contact 

dashed where 

uncertain 

Resistivity Sounding line-From 

Zody and Stanley 

(1973); see 

figure 8 

Suggested caldera 

margin 

Sample siteShowing 

40Arj39Ar 

or potassium age 

(Ma). Query after 

letter ind icates 

uncertainty that 

dated unit is 

Arbon Va lley Tuff 

Member . Sites J 

and K are in 

rhyolite flows 

Figure 1. Location map showing the known extent of the Arbon Valley Tuff Member and three rhyolite lave flows in Idaho. New isotopic 

ages (for samples sites A-E) and previously reported ages (for samples sites F-K) are shown and reported in tables 3 and 4. Sources of mapping used in this compilation are Trimble (1976), Trimble and Carr (1976), Hladky and others (1991), and unpublished data (K.S. Kellogg, 

1987). 
DESCRIPTION OF THE ARBON VALLEY TUFF MEMBER 3 

Yell ow stone plateau, at the northeastern end of the Snake 

River Plain immediately northeast of the area of this report, 

are both the rhyolites and structural features of the parent 

calderas clearly exposed. This exposure is due to the young 

age (2 Ma or less) of the caldera-forming events and to the 

lack of an obscuring cover of basaltic rocks. 

Early Pliocene and late Miocene caldera-forming events 

associated with the formation of the Snake River Plain are 

best recognized by their outflow tuffs exposed in the hills 

marginal to the plain (Leeman, 1982; Morgan and others, 

1984; Kellogg and Marvin, 1988; Morgan, 1992). Determining the ages, location, timing, and character of the calderas 

hidden under the plain has met with limited success. Some 

constraints on the approximate locations of the source 

calderas is provided by petrographic, geochemical, isotopic, 

geophysical, and paleomagnetic studies (Morgan, 1988, 

1992). In addition, the location of calderas hidden beneath 

basaltic flows of the plain may be indicated by areas that lack 

basaltic vents (Kuntz and others, 1982); such areas may be 

underlain by low -density rock that acted as a barrier to the 

ascent and eruption of basaltic magma. 

This study is concerned primarily with the isotopic 

dating of the Arbon Valley Tuff Member of the Starlight 

Formation, its stratigraphic setting along the southeastern 

margin of the Snake River Plain, and a brief chemical and 

petrographic description of the unit. Additional isotopic dates 

from several overlying rhyolitic units are also given, and their 

stratigraphic significance is discussed. Anticipated future 

studies will describe the various facies of the Arbon Valley 

Tuff Member and characterize the chemical and petrographic 

zonation within the unit; such studies will help to constrain 

more accurately the location of the source region. 

Rocks assigned herein to the Arbon Valley Tuff Member 

have previously been described from their extensive 

exposures along the southeastern margin of the Snake River 

Plain (Carr and Trimble, 1963; Trimble and Carr, 1976; 

Trimble, 1976, 1982; Kellogg and Embree, 1986; Kellogg 

and Marvin, 1988; Kellogg and others, 1989). The total 

known and inferred extent of the Arbon Valley Tuff Member 

is shown on figure 1. 

The Arbon Valley Tuff Member was originally named 

the middle, vitric-crystal tuff member of the Starlight Formation (Carr and Trimble, 1963) and was subsequently named 

the tuff of Arbon Valley by Trimble and Carr (1976) for 

exposures in the area of Arbon Valley in southeastern Idaho. 

The middle member of the Starlight Formation was described 

as a sequence of "ash-flow tuffs and minor bedded air-fall 

tuff," although the tuff of Arbon Valley "at many places * * * 

makes up the entire middle member" (Trimble and Carr, 

1976, p. 35). Tuff beds that "probably are about the same age 

and chemical composition as the tuff of Arbon Valley" are 

included in their middle member (Trimble and Carr, 1976, 

p. 38), although in such places it is not clear to us where the 

top or base of the middle member occurs. Partly for this 

reason, we redefine the middle member in the next section 

and propose the formal name "Arbon Valley Tuff Member of 

the Starlight Formation." 

The Arbon Valley Tuff Member is a distinctive, crystalrich, conspicuously biotite-bearing ash-flow tuff that crops 

out extensively along the southeastern margin of the Snake 

River Plain (fig. 1). It has also recently been recognized at one 

locality (sample siteD, fig. 1) along the northern margin of 

the plain (Kellogg and others, 1989). The unit is the oldest 

dated ignimbrite in the eastern Snake River Plain and is 

similar in thickness, extent, and physical characteristics to 

other major tuffs with known or suspected caldera sources, 

such as the 2.0-Ma Huckleberry Ridge tuff (Christiansen and 

Blank, 1972) and the tuffs of the Heise volcanic field (Morgan 

and others, 1984). 

Until now, the age of the Arbon Valley Tuff Member had 

been equivocal, although it was generally considered to be 

7.9±0.1 Ma (Trimble and Carr, 1976; Trimble, 1976; Kellogg 

and Marvin, 1988), based on one potassium-argon feldspar 

(type undefined) age from the northern Deep Creek Mountains (locality G, fig. 1; Armstrong and others, 1975). Another 

sample ("crystal tuff of Cosgrove Rd."; locality F, fig. 1), 

collected near Blackfoot was thought to be Arbon Valley Tuff 

Member, but it yielded a potassium-argon age of 1 0.6±0.1 Ma 

on "feldspar" (Armstrong and others, 1975). This date was 

suspected to be too old due to contamination by excess 40 Ar. 

These disparate dates (table 4) led us to recollect the Arbon 

Valley Tuff Member from known, well-studied outcrops for 

isotopic dating. Two samples were collected from Rockland 

Valley, from a section described by Trimble and Carr (1976), 

one sample was collected from a well-mapped area about 10 

km southeast of Blackfoot (Kellogg and Embree, and one 

sample was collected from a well-mapped area about 10 km 

southeast of Blackfoot (Kellogg and Embree, 1986). 

DESCRIPTION OF THE ARBON 

VALLEY TUFF MEMBER 

We herein formally propose the name Arbon Valley Tuff 

Member of the Starlight Formation. We also define the Arbon 

Valley Tuff Member as comprising the entire middle member 

of the Starlight Formation, a designation that we feel is less 

ambiguous than that used by Trimble and Carr (1976). The 

complete exposed sections of the tuff are not in Arbon Valley 

but are in The Cove (figs. 1 and 2), about 25 km east of Blackfoot. A type section is proposed in SE1

A sec. 18, T. 2 S., R. 38 

E. Incomplete reference sections of the Arbon Valley Tuff 

Member were described by Trimble (1976, p. 37). 

At the type section, the Arbon Valley Tuff Member 

consists of a lower, poorly to moderately welded, poorly 
4 RHYOLITIC VOLCANISM IN THE SNAKE RIVER PLAIN, IDAHO 

Figure 2. Arbon Valley Tuff Member in The Cove, northeast of 

Blackfoot, Idaho (SE',4 sec. 18, T. 2 S., R. 38 E.). The top of the unit 

is shown by the dashed line; the base is mostly obscured by talus. 

Rocks above and below the Arbon Valley Tuff Member are largely 

tuffaceous sandstone interbedded with a few thin basalt flows. 

Vertical relief, from lowest trees to right skyline, is about 210m. 

View to the west. Photograph by K.S. Kellogg. 

sorted, pumiceous, massive, sparsely crystal bearing (about 

2-3 percent crystals as large as 1 mm (millimeter) of broken 

feldspar and quartz), very pale tan, tuff that grades upward 

into a pale-tan, planar-bedded, moderately welded tuff 

(fig. 3). The massive pumiceous tuff is about 15 to 20m 

thick, locally contains tree casts, and is interpreted as an airfall deposit; the overlying planar-bedded tuff is about 1.5 m 

thick and is interpreted as a pyroclastic-surge deposit. The 

interpreted surge deposit is disconformably overlain by a 

massive, more strongly welded, highly crystal rich, lightyellowish-gray ash-flow tuff that is about 40 m thick; the 

base of the upper ash-flow tuff is shown in figure 3. 

In the upper ash-flow zone, the tuff contains abundant 

inflated to collapsed pumice fragments, and crystals 

compose as much as 35 percent of rock volume. Of the crystals, about 50 percent are stubby, partly resorbed and broken 

quartz as large as 3 mm, 30 percent are plagioclase as large 

as 2 mm, 10 percent are sanidine as large as 2 mm, as much 

as 5 percent are conspicuous biotite flakes as large as 3 mm, 

and about 2 percent are clinopyroxene as large as about 

2 mm. The matrix is entirely devitrified, although Trimble 

and Carr (1976) described the equivalent unit in the north 

end of the Deep Creek Mountains as vitric in part. An uppermost vapor-phase or lithophysal zone has not been observed, 

although at most localities the upper part of the unit is eroded 

or poorly exposed. The extremely crystal rich nature of the 

ash-flow zone and the presence of abundant stubby quartz 

and biotite crystals are unique to the Arbon Valley Tuff 

Figure 3. The base of the Arbon Valley Tuff Member in The 

Cove, northeast of Blackfoot, Idaho, showing a moderately welded, massive, pumiceous deposit interpreted as air-fall ash grading 

upward into a cross-bedded unit interpreted as a pyroclastic-surge 

deposit that is, in turn, overlain with a sharp disconformity by massive, strongly welded tuff. Hammer shows scale. View to the north. 

Photograph by K.S. Kellogg. 

Member and make it readily distinguishable from other rhyolitic tuffs of the Snake River Plain. 

At sample site C (fig. 1 ), 8 km southeast of Blackfoot, 

the thickness of the Arbon Valley Tuff Member is about 60 

m, similar to the thickness at The Cove. In Rockland Valley 

and at the northern end of the Deep Creek Mountains, the 

Arbon Valley Tuff Member is at least 25 m thick (Trimble 

and Carr, 1976). Near Blackfoot, the tuff appears to be a 

single cooling unit, although at some localities at the northern 

end of the Deep Creek Mountains, at least two cooling units 

exist (Trimble and Carr, 1976). At all localities, total crystal 

content appears to increase upward. Magnetic polarity, as 
NEW ISOTOPIC AGES FOR THE ARBON VALLEY TUFF MEMBER 5 

Table 1. Measured production ratios for potassium and calcium-derived argon isotopes for the U.S. Geological Survey TRIGA reactor, 

Denver, Colo. 

[Determination of production ratios is based on irradiated salts K2S04 for potassium-derived argon and CaF2 for calcium-derived argon] 

Irradiation e1Arf39Ar)K esArf39Ar)K (

40Arf39Ar)K 

package No. xl04 x10-2 x1o-2 

(irradiation time) 

DD26 

(30 hours) --------- 1.1 1.31 8.78 

DOS 

(25 hours)--------- 1.10 1.31 .77 

determined by a hand-held flux-gate magnetometer, is 

normal (Kellogg and Marvin, 1988 ). 

NEW ISOTOPIC AGES FOR THE 

ARBON VALLEY TUFF MEMBER 

SAMPLING 

Four samples of the Arbon Valley Tuff Member were 

collected for potassium-argon and 40 Arf39 Ar age determinations. One sample (FH87BB01) was collected about 8 km 

southeast of Blackfoot (sample site C, fig. 1) and two others 

(TSMA-1 and TSMA-2) were collected about 90 km southwest of Blackfoot in Rockland Valley, on and near a small 

hill called Mollys Nipple (sample sites A and B, fig. 1). 

Arbon Valley Tuff Member has been tentatively identified at one locality (sample site D, fig. 1) on the northern 

margin of the Snake River Plain, in the southern Lemhi 

Range. Sample DR724D3, collected at this locality, gave a 40Arf39

Ar date on sanidine of 10.30±0.04 (Kellogg and 

others, 1989), a date that is slightly reinterpreted in this 

report. 

METHODOLOGY 

Sanidine and biotite mineral separates and neutron flux 

monitors for 40 ArP9 Ar dating were wrapped in aluminum foil 

and irradiated for either 20 or 25 hours in the U.S. Geological 

Survey TRIGA reactor, Denver, Colo. (samples TSMA-1 

and -2) or in the Oregon State University reactor, Corvallis 

(sample DR724D3). Variations in neutron fluence within 

the irradiated packages were monitored by placing 6 to 10 

standards along the length of each vial; the monitor used in 

this study was hornblende MMhb-1, which has an assigned 

e9Arf37Ar)Ca e6Arf31Ar)Ca (38Aff37 Ar)Ca 

x1o-3 x104 x1o-5 

0.66 2.69 3.50 

.64 2.63 6.35 

potassium-argon age of 520.4 Ma (Alexander and others, 

1978; Dalrymple and others, 1981; Sampson and Alexander, 

1987). Corrections for interfering isotopes were made by 

measuring the argon isotopes of K2S04 and CaF2 irradiated 

in each package; calculated production ratios are summarized in table 1. With the exception of biotite from sample 

TSMA-2, which was fused in a single step, each sample was 

heated in a double vacuum furnace for 20 minutes in a series 

of 7 to 12 steps to a maximum temperature of 1 ,450°C. Five 

naturally occurring and radiogenic isotopes of argon (

40 Ar, 39 Ar, 38 Ar, 37 Ar, and 36 Ar) were measured at each step, and 

isotopic compositions were corrected for mass discrimination. All samples were analyzed using a Mass AnalyzerProducts MAP 215 rare-gas mass spectrometer operated in 

the static mode. Apparent ages were calculated using the 

decay constants recommended by Steiger and Jager (1977). 

Age plateaus, where appropriate, were determined using the 

critical value test of Dalrymple and Lanphere (1969). A 

detailed description of the analytical procedures used in this 

study is in appendix 1 of Tysdal and others ( 1990). 40 Arf39 Ar 

age-spectrum data are reported in table 2; estimated 

analytical uncertainty for the apparent age from each heating 

step is reported as ±1cr. However, to facilitate comparison 

with potassium-argon dates, the estimated analytical error in 

the summary of 40Arf39Ar plateau and isochron dates (table 

3) are reported as ±2cr. 

Potassium analyses for potassium-argon dating were 

performed by E.H. Brandt using a lithium metaborate flux 

fusion-flame photometry method (lngamells, 1970). Argon 

extraction and purification techniques used in this study are 

similar to those described by Dalrymple and Lanphere 

( 1969). Argon isotopes were analyzed using standard isotope 

dilution procedures using a 60°-sector, 15.2-cm (centimeter)-radius Nier-type mass spectrometer operated in the 

static mode. The estimated analytical uncertainty for the 

calculated potassium-argon age is reported as ±2cr. 
6 RHYOLITIC VOLCANISM IN THE SNAKE RIVER PLAIN, IDAHO 

Table 2 (below and facing page). 40 ArP9 Ar age-spectrum data for the Arbon Valley Tuff Member of the Starlight Formation, eastern 

Snake River Plain, Idaho. 

[

40 ArR, abundance of radiogenic 40 Ar reported in volts of signal on a Faraday detector; 39 Ar K• abundance of radiogenic (potassium-derived) 39 Ar reported in 

volts of signal; F, ratio of 40ArR to 39ArK after correction for mass discrimination and interfering isotopes; 39Arf37 Ar, ratio of39ArK to 37Arca (this value can 

be converted to the approximate potassium/calcium by multiplying by 0.5); percent 40 ArR and percent 39 Ar, percent of radiogenic 40 Ar and percent of total 39 Ar released in each temperature step; 40Arf36 Ara, measured atmospheric argon ratio used for mass discrimination at time of analysis; J value, neutron flux 

parameter; P, temperature steps used in the calculation of plateau or weighted mean ages; mg, milligram; lcr, one standard deviation] 

Temperature 40AfR 39AfK F 39 Arf1Ar 40ArR 39Ar Apparent 

{oC} {Eercent} {Eercent} age {Ma at Icr} 

Sample TSMA-1/DD26, sanidine 

750 ------------- 0.72977 0.80135 0.911 17.71 45.8 3.1 10.33±0.08 

925P------------ 2.58593 2.89676 .893 36.80 92.2 11.3 10.13±0.04 

1 ,050P ------- 5.44387 6.10064 .891 56.57 96.3 23.8 10.1I±0.04 

I, I25P --------- 5.7I608 6.39102 .894 72.25 97.3 25.0 10.I5±0.04 

I , I 7 5P --------- 7.50813 8.35358 .899 8I.28 97.8 32.6 I0.20±0.03 

1,200----------- .77473 .85292 .908 59.I5 95.6 3.3 10.3I±0.08 

1 '4 50----------- .20098 .18972 1.059 27.13 81.9 .7 I2.01±0.26 

Total gas------ 0.897 10.I8±0.04 

Sample TSMA-1/DD26, biotite 

600 ------------- 0.02222 0.01671 1.330 

700 ------------- .066I7 .08806 .751 

750 ------------- .24334 .26574 .916 

800P ------------ .81191 .88915 .913 

85 0 p ------------ I.24399 1.36917 .909 

900 p ------------ I.24933 1.36676 .914 

9 SOP------------ 1.39505 1.52089 .917 

1 ,OOOP --------- 1.27934 1.39266 .919 

1, 0 SOP --------- 1.99034 2.17140 .917 

1,1 OOP --------- 1.94584 2.11503 .920 

1,250----------- .12297 .12735 .966 

1, 150P --------- .73930 .80460 .919 

Total gas ------ 0.916 

Potassium -argon ages and analytical data for the Arbon 

Valley Tuff Member and several other rhyolitic units from 

the eastern Snake River Plain are given in table 4. 

RESULTS OF 40AR/39AR ANALYSIS 

The samples from Rockland Valley (TSMA-1 and -2; 

sample sites A and B, fig. 1) yield largely concordant 40Ar/ 39 Ar age spectra (figs. 4A-4D; table 2). Sanidine age spectra 

from the two samples are similar and are concordant at intermediate temperature steps but also show a tendency toward 

older apparent ages at higher temperatures. Sanidine from 

sample TSMA-1 gives similar plateau and isochron dates 

13.61 9.3 0.1 15.04±1.11 

26.40 9.8 .7 8.52±0.35 

107.43 35.3 2.2 10.37±0.20 

242.90 60.5 7.3 10.34±0.05 

396.49 77.5 11.3 10.29±0.05 

430.42 81.1 11.3 10.35±0.05 

231.31 72.2 12.5 10.39±0.05 

30.89 55.4 11.5 10.41±0.05 

26.08 47.1 17.9 10.38±0.05 

116.80 56.7 17.4 10.42±0.05 

8.61 72.5 1.1 10.94±0.34 

48.21 66.4 6.6 10.41±0.08 

10.38±0.06 

(fig. SA) of 10.15±0.08 Ma and 10.20±0.08 Ma, respectively. The isochron from this sample yields an initial 40Ar/ 36 Ar intercept of 282±54 that is identical within analytical 

uncertainty to the atmospheric value of 295.5. Sanidine 

from sample TSMA-2 gives a plateau age of 10.22±0.06 Ma 

(fig. 4C), an isochron age of 10.21±0.08 Ma, and an initial 40ArP6Ar value of 307±11 (fig. 5C). The age spectra for 

biotite from sample TSMA-1 is concordant over 95 percent 

of the 39 Ar released and gives a plateau age of 10.38±0.04 

Ma (fig. 4A). The isochron age from this sample is 

10.35±0.04 Ma, with an initial 40Arf39 Ar value of 297±1 that 

is essentially identical to atmospheric composition. A totalfusion 40ArP9 Ar date from biotite from sample TSMA-2, 

largely analogous to the conventional potassium-argon dates 
NEW ISOTOPIC AGES FOR THE ARBON VALLEY TUFF MEMBER 7 

Temperature 40ArR 39ArK F 39Arl7Ar 40ArR 39Ar Apparent 

{oC} {~rcenQ {percent} age {Ma at lo} 

Sample TSMA-2/DD26, sanidine 

750 ------------ 1.05434 1.18772 0.888 9.81 50.7 4.8 10.04±0.03 

925P---------- 2.98041 3.31239 .900 10.24 86.1 13.4 10.18±0.03 

1 ,050P --------- 5.33740 5.91695 .902 19.76 88.3 23.9 10.20±0.04 

1, 125P --------- 5.22751 5.77619 .905 37.79 92.2 23.3 10.24±0.03 

1,175P----- 6.25376 6.90363 .906 54.08 95.0 27.9 10.24±0.03 

1,225----------- .84547 .92310 .916 19.22 83.7 3.7 10.36±0.04 

1 '4 50 ----------- .69711 .74707 .933 17.85 75.6 3.0 10.55±0.05 

Total gas------ 0.904 10.23±0.03 

Sample TSMA-2/DD26, biotite 

1 ,250 -------- 0.93755 1.01232 0.926 113.12 52.6 100.0 10.34±0.05 

Sample DR724D3/D05, sanidine 

600 ------------- 0.16304 0.18443 0.884 14.34 31.1 1.3 11.50±0.66 

700 ------------- .40978 .53065 .772 27.37 79.2 3.7 10.05±0.12 

800 p ------------ .80322 1.02427 .784 37.06 89.9 7.1 10.21±0.06 

850 p ------------ .96063 1.22677 .783 44.22 91.7 8.6 10.19±0.03 

900 p ------------ 1.39954 1.79025 .782 51.75 94.5 12.5 10.18±0.05 

9 50 p ------------ 1.25230 1.56660 .799 58.55 97.7 10.9 10.40±0.06 

1 ,OOOP --------- 2.25125 2.86723 .785 67.12 96.2 20.0 10.22±0.04 

1 ,050P --------- 3.49659 4.39068 .796 73.09 97.8 30.6 10.37±0 .. 05 

1 '150 ----------- .42797 .51600 .829 45.93 50.9 3.6 10.79±0.14 

1 ,2 50 ----------- .20634 .23540 .877 48.05 27.3 1.6 11.41±0.48 

Total gas ------ 0.793 10.33±0.07 

SAMPLE DATA 

1. TSMA-1/DD26: sanidine; 177.3 mg; measured 40 Arf36Ara=298.9; J value= 0.006308±0.25 percent (1cr). 

2. TSMA-1/DD26: biotite; 103.0 mg; measured 40Arf36Ara = 298.9; J value= 0.006297±0.25 percent (1cr). 

3. TSMA-2/DD26: sanidine; 199.4 mg; measured 40Arf36Ara = 298.9; J value 0.006287±0.25 percent (1cr). 

4. TSMA-2/DD26: biotite; 11.5 mg; measured 40Arf36Ara = 298.9; J value= 0.006205±0.25percent (1cr). 

5. DR724D3/D05: sanidine; 81.2 mg; measured 40Arf36Ara = 298.9; J value= 0.007236±0.25 percent (lcr). 

reported below, gives an apparent age of 10.34±0.10 Ma. A 

summary of the plateau and isochron dating results is given 

in table 3. 

Both the biotite 40 ArP9 Ar apparent ages from TSMA -1 

and TSMA-2 are slightly older than those from the 

coexisting sanidines. This apparent discordance may reflect 

either 40 Ar loss from the sanidines or contamination of the 

biotites by excess 40 Ar. The sanidine age spectra show no 

evidence of apparent 40 Ar loss nor does the biotite initial 40ArJ36

Ar value from sample TSMA-1 suggest the presence 

of excess 40 Ar (fig. 5B). Because several recent 40 Arf39 Ar 

studies indicate that sanidine from ignimbrites typically 

yields reliable and reproducible dates, whereas biotites tend 

to be less dependable (for example, Mcintosh and others, 

1990), we consider the plateau ages derived from the sanidines to best represent the age of the Arbon Valley Tuff 

Member at this locality. Calculation of a weighted mean for 

the two sanidines following Taylor (1982) results in an 

apparent age of 10.19±0.04 Ma. 

40 Arf39 Ar dating of sanidine from sample DR724D3, 

collected north of the Snake River Plain (sample siteD, fig. 

1) gave a somewhat discordant, U-shaped age spectra (fig. 

4D). Although the age spectra do not satisfy the requirement 

for a plateau date, a weighted mean age of 10.20±0.06 Ma 

can be calculated for the intermediate temperature steps 

(table 3), using the criteria of Dalrymple and Lanphere 

(1969). The U-shaped spectra suggest the presence of excess 40 Ar, and an isochron from this sample yields an initial 40 Ar/ 36 Ar value of 309±3 (fig. 5D) that is significantly greater 

than the atmospheric value of 295.5. Consequently, the 
8 RHYOLITIC VOLCANISM IN THE SNAKE RIVER PLAIN, IDAHO 

Table 3. Summary of 40 Arf39 Ar dating results for the Arbon Valley Tuff Member of the Starlight Formation, Snake River Plain, Idaho. 

(39Ar, percent of total potassium-derived 39Ar, and (steps), number of steps used in the calculation of the plateau age; ¥JAr/36Ar,, initial 40Ar/36Arratio determined 

from the isochron analysis; BT, biotite; S, sanidine; --,no data] 

Sample No. Sample site Latitude, Mineral Fusion age Plateau results Isochron results 

(fig. 1) longitude dated (Ma) 39 Ar (steps) Age(Ma) 40ArJ36Ari Age (Ma) 

TSMA-1 --------- A 42°31'10"N., s 92.7 (4 of7) 10.15± 0.08 282±54 10.20± 0.08 

112°51 '47"W. 

TSMA-1---------- A 42°31'10"N., BT 95.8 (8 of 12) 10.38±0.10 297±1 10.35 ± 0.04 

112°51'47"W. 

TSMA-2---------- B 42°31 '20"N., s 88.5 (4 of7) 10.22± 0.06 307± 11 10.21 ± 0.08 

112°51'39"W. 

TSMA-2 ---------- B 42°31'20"N., BT 10.34 ± 0.10 

112°51 '39"W. 

DR724D-3 ------- D 43°49'00"N., s 

112°51 '30"W. 

isochron age of 10.22±0.06 Ma probably best represents the 

age of the sample, although the dates calculated by either 

method are statistically indistinguishable at the 95-percent 

confidence level. The apparent age from this sample is in 

excellent agreement with the 40 Arf39 Ar dates from sanidines 

from the southeastern part of the Snake River Plain. 

RESULTS OF POTASSIUM-ARGON ANALYSIS 

The analytical results of potassium-argon analysis of 

three samples of the Arbon Valley Tuff Member are given in 

table 4. Sample FH87BB01, collected near Blackfoot 

(sample site C, fig. 1) gave concordant potassium-argon 

dates on biotite, sanidine, and plagioclase; the mean of three 

determinations is 10.2±0.9 Ma. Sample TSMA-1 from Rockland Valley gave a potassium-argon biotite date with very 

large uncertainty (9.7±3.4 Ma) due to a low percentage (6.0 

percent) of radiogenic argon. The potassium-argon biotite 

age from TSMA-2 is 10.4±0.6 Ma. 

Several other tuffs from the southeastern margin of the 

Snake River Plain that were dated by the potassium-argon 

method may also be Arbon Valley Tuff Member. 

Hutsinpiller and Parry (1985) reported a date of 10.7±0.5 Ma 

(mineral separate unspecified) on a well-exposed but 

uncorrelated tuff (sample site H, fig. 1; table 4) that is within 

the known range of outcrop of the Arbon Valley Tuff 

Member. At the north end of the Sublette Range (sample site 

I, fig. 1 ), a biotite-bearing "vitric tuff' from the base of a 

89.8 (6 of 10) 10.28± 0.10 309±3 10.22± 0.06 

section of rhyolite yielded potassium-argon ages of 

11.1±0.2 Ma for biotite and 9.7±0.14 Ma for feldspar (type 

unspecified) (Armstrong, 1975; table 4). The suggested 

correlation of this unit with the Arbon Valley Tuff Member 

is based not only on the similarity of the ages, but also on the 

observation that very few other ignimbrites in the eastern 

Snake River Plain are biotite bearing. 

Until the identity of the dated units can be verified, the 

areas of outcrop for the Arbon Valley Tuff Member shown 

surrounding sample sites H and I on figure 1 are considered 

tentative. 

ESTIMATED AGE OF THE ARBON 

VALLEY TUFF MEMBER 

The 40 ArP9 Ar dates obtained from sanidines from the 

northern and southeastern margins of the Snake River Plain 

(sample sites A and D, fig. 1; table 3) are identical at the 

95-percent confidence level and are concordant with the 

potassium-argon age determinations (table 4). Combining 

the two sanidine 40 ArP9 Ar plateau dates from the southeastern margin of the Snake River Plain with the isochron date 

from the Lemhi Range locality on the northern margin of the 

plain, we obtained a weighted mean date of 10.20±0.06 Ma, 

which we consider best represents the age of emplacement of 

the Arbon Valley Tuff Member of the Star light Formation. 
STRATIGRAPHY OF THE SOUTHEASTERN MARGIN OF THE SNAKE RIVER PLAIN 9 

Table4. Summary of potassium-argon ages for the Arbon Valley Tuff Member of the Starlight Formation and several other rhyolitic units 

from the eastern Snake River Plain, Idaho. 

[BT, biotite; S, sanidine; P, plagioclase; FS, feldspar (type unspecified); WR, whole rock. Constants used: A.E=0.581x10-10/yr; A.13=4.962xl0- 10/yr; atomic 

abundance of 40K/K=1.67x10-4

mole/mole. Dates for sample sites C and E were previously reported in Kellogg and others (1989) without analytical details. 

Ages of Armstrong and others ( 1975) are corrected for new decay constants of Steiger and Jager (1977). Leaders(--), no data or data given in original reference 

Sample Sample Latitude, Material ~0 40Ar 40Ar Unit sampled 

No. site (fig. 1) longitude dated (percent) (1010 mole/gram) (percent) Age (Ma) ±2cr 

New ages: 

Arbon Valley Tuff Member-------- TSMA-1 A 

Arbon Valley TuffMember ------------- TSMA-1 A 

Arbon Valley Tuff Member------------- TSMA-2 B 

Arbon Valley Tuff Member------------- TSMA-2 B 

Arbon Valley Tuff Member------------- FH87BB01 c 

Arbon Valley Tuff Member------------- FH87BB01 c 

Arbon Valley Tuff Member------------- FH87BB01 c 

Rhyolite of west Pocatello --------------- FH86-2685B E 

Rhyolite of west Pocatello --------------- FH86-2685B E 

Previously reported ages: 

Tuff of Arbon Valley1------------------------ F 

("crystal tuff, Congrove Rd") 

Tuff of Arbon Valley1 -------------------- G 

Tuff of Arbon Valley?2 ------------------- H 

Tuff of Arbon Valley?1 -------------------------

("vitric ash flow" at base of section) -

Tuff of Arbon Valley?1 -------------------

("vitric ash flow" at base of section) 

Rhyolite ofTwo-and-a-Half-Mile Creek3 ------- J 

Rhyolite of Stevens Peak4 --------------- ------- K 

1 Annstrong and others ( 1975 ). 2Hutsinpiller and Parry (1985); analyzed mineral not specified. 3Kellogg and Marvin (1988). 4Karlo and Jorgenson (1979); analyzed mineral not specified. 

STRATIGRAPHY OF THE 

SOUTHEASTERN MARGIN OF THE 

SNAKE RIVER PLAIN 

42°31'10"N., 

Ilr51'47"W. 

42°31'10"N., 

112°51'47"W. 

42°31'20"N., 

112°51'39"W. 

42°31'20"N., 

1l2°51'39"W. 

43°07'15"N., 

112°15'36"W. 

43°07'15"N., 

112°15'36"W. 

43°07'15"N., 

112°15'36"W. 

42°54'20"N., 

112°30'38"W. 

42°54'20"N., 

1Ir30'38"W. 

s 10.8 1.5760 77.1 10.1± 0.3 

BT 8.74 1.2149 6.0 9.7± 3.4 

s 9.46 1.3701 47.2 10.0± 0.4 

BT 8.69 1.2885 40.2 10.4± 0.6 

s 11.45 1.234 55.1 9.4± 0.5 

BT 8.79 3.005 30.1 10.4± 0.9 

p .89 .0999 56.6 10.8± 0.7 

p 1.14 .1295 31.7 7.9± 0.7 

WR 4.73 .5377 53.1 7.9± 0.4 

FS 10.6± 0.1 

FS 7.9± 0.4 

10.7± 0.5 

BT 11.1 ± 0.2 

FS 9.7 0.14 

BT 9.1±0.3 

9.8± 0.9 

The stratigraphic position and setting of the Arbon 

Valley Tuff Member, along the southeastern margin of the 

Snake River Plain, shown diagramatically between about 

American Falls on the southwest and Blackfoot on the northeast (fig. 6), is based largely on the new dates for the Arbon 

Valley Tuff Member and is a revision of the stratigraphy 

proposed by Kellogg and Marvin (1988). The pre-Tertiary 

rocks, strongly deformed during late Mesozoic thrusting, 

extend less than several kilometers to the northwest under 
10 RHYOLITIC VOLCANISM IN THE SNAKE RIVER PLAIN, IDAHO 

15 I 15 I I 

co co 

~ ~ l _I ~ 

10.38±0.04 Ma ~ I ~ J l 10.15±0.08 Ma ui w· (..9 

<( n (..9 ~ 10 i=' - <( 10 t- - I- I- z z w w a: a: <( <( a.. a.. a.. a.. <( <( 

0 I 0 I 

0 50 100 0 50 100 

A 39ArK RELEASED, IN PERCENT B 39ArK RELEASED, IN PERCENT 

15 I 15 I 

co co ~ ~ 

~ I _I ~ L 10.28±0.10 Ma 

w' 1 0.22±0.06 Ma ui jJ f (..9 I I=::F (..9 <( = 10 - <( 10 ~ - I- I- z z w w a: a: <( <( a.. a.. a.. a.. <( <( 

0 l 0 I 

0 50 100 0 50 100 

c 39ArK RELEASED, IN PERCENT D 39ArK RELEASED, IN PERCENT 

Figure 4. 40 Arf39 Ar age spectra for sanidine and biotite from the Arbon Valley Tuff Member, Idaho. A, sanidine from sample TSMA-1; 

B, biotite from sample TSMA-1; C, sanidine from sample TSMA-2; and D, sanidine from sample DR724D3. Interpretation of these types 

of diagrams is discussed by McDougall and Harrison (1988). 

the eastern Snake River Plain (Sparlin and others, 1982). Farther to the east, about 27 km east of Blackfoot, steeply northeast dipping andesitic flows and volcanic breccias (not shown 

on fig. 6), dated about 40--47 Ma (Armstrong, 1972), overlie 

thrust-faulted Mesozoic rocks. These volcanic rocks are 

coeval with the widespread rocks of the Eocene Challis 

volcanic rocks of central Idaho (for example, Mcintyre and 

others, 1982). 

The 10.20±0.06-Ma date for the Arbon Valley Tuff 

Member is the oldest Miocene isotopic age yet recorded 

east of the Sublett Range along the southeastern margin of 

the plain (Armstrong, 1975). However, several hundred 

meters of mostly rhyolitic ash and rhyolitic water-lain sediments beneath the Arbon Valley Tuff Member (correlated 

with the lower member of the Starlight Formation of Trimble and Carr, 1976) suggest that nearby rhyolitic volcanism 

began before 10.2 Ma. The source for these older rhyolitic 

deposits is unknown but may be related in part to early 

stages of eruption of the caldera that produced the Arbon 

Valley Tuff Member. 

Two rhyolite lava flows exposed between Blackfoot and 

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

the rhyolite of Stevens Peak, give potassium-argon ages of 

9.1 and 9.8 Ma, respectively (sample sites J and K, fig. 1; 

table 4 ). As discussed in the next section, these flows are 

interpreted as resulting from ring-fracture eruptions of the 

source caldera for the Arbon Valley Tuff Member (Kellogg 

and others, 1989). A rhyolite flow ("trachyandesite porphyry" of Trimble, 1976) from the upper Starlight Formation 

is exposed in a railroad cut immediately west of Pocatello 

and was collected for isotopic dating (locality E, fig. 1). This 

unit, herein informally named the rhyolite of west Pocatello, 

is not genetically related to the Arbon Valley Tuff Member, 

but is discussed here to define better the stratigraphic setting 

of the southeastern margin of the Snake River Plain. Its 

major-element composition is similar to other rhyolites of the 
GEOCHEMISTRY OF THE ARBON VALLEY TUFF MEMBER 11 

i-

~ 

~ ..r 

<!:: "' (() 

20,000 

8,000 

6,000 

~ 4,000 

<!:: 

0 

..r 

2,000 

00 

c 

lsochron age 10.20±0.08 Ma 

40ArJ36Ari 282±54 

10,000 20,000 

39ArJ36Ar 

2,000 4,000 

39Arj36Ar 

30,000 

6,000 8,000 

2,000,----r------.-----.-----,-------,------, 

i-

~ 10,000 

<!:: 

0 

..r 

lsochron age 10.35±0.04 Ma 

40ArJ36Ari 297±1 

500 1,000 

39Arj36Ar 

lsochron age 10.22±0.06 Ma 

40Arj36Ari 309±3 

10,000 

39ArJ36Ar 

1,500 

20,000 

Figure 5. 40Arf36Ar-39Arf36Ar isochron diagrams for sanidine and biotite from samples of the Arbon Valley Tuff Member, Idaho. A, sanidine from sample TSMA-1; B, biotite from sample TSMA-1; C, sanidine from sample TSMA-2; and D, sanidine from sample DR724D3. 

Interpretation of these types of diagrams is discussed by McDougall and Harrison (1988). 

Snake River Plain (compare analysis in table 5 with those 

reported in table 4 of Kellogg and Marvin, 1988). Up to now, 

the only isotopically dated unit in the upper Starlight 

Formation is a basalt flow, which yielded a date of 7 .0±0.3 

Ma (Kellogg and Marvin, 1988). The rhyolite of west 

Pocatello is dark gray and contains conspicuous light-gray 

phenocrysts of plagioclase. The flow gave a potassium-argon 

plagioclase age of7.9±0.7 Ma and a potassium-argon wholerock age of7.9±0.4 Ma (new ages reported in table 4). 

The Starlight Formation is overlain by a coarse boulder diamictite, interpreted as deposited as debris flows 

derived from an eastern tectonic escarpment during rapid 

basin-and-range uplift (Kellogg and Marvin, 1988). The 

diamictite is, in turn, overlain by three crystal-poor rhyolitic ash-flow tuffs of the Heise volcanic field (Morgan 

and others, 1984), erupted between 7.6 and 4.3 Ma. These 

tuffs represent the final phase of rhyolitic volcanism near 

Blackfoot before an abrupt change occurred between 4.3 

and 3.7 Ma to basaltic volcanism, marked by the eruption 

of olivine basalt of predominantly tholeiitic composition 

(Kellogg and Marvin, 1988). 

GEOCHEMISTRY OF THE ARBON 

VALLEY TUFF MEMBER 

Major-element geochemical analysis of three samples 

from the basal part of the upper massive, welded zone of the 

Arbon Valley Tuff Member shows that the unit is mildly peraluminous, as are the tuffs of the Heise volcanic field 

(Kellogg and Marvin, 1988, table 4). These analyses were 

performed on whole-rock samples and may reflect some contamination acquired during transport. Perhaps partly due to 

this limitation, major-element chemical analysis is not a particularly useful method for discriminating the different 

rhyolite tuffs of the Snake River Plain (Kellogg and Marvin, 

1988). However, trace-element geochemical analysis of 11 
12 RHYOLITIC VOLCANISM IN THE SNAKE RIVER PLAIN, IDAHO 

0.1-0.4 Ma 

Younger basalt 

flows of 

Neeley Formation 

(/) 

~ c 

<1l 

·~ C) 

E 

<r: 

I 

• • • o . • -

~ 

0 • • _OQ 0 • • 0 0 • • • • 0 0• 0 • • 0 

0 

0 

:g. 

<1l 

C!5 

I 

2.2 Ma 

~~~~-!:~~:} 6.2 Ma 

6.7 Ma 

Tuffs of Heise 

volcanic center 

~·. 'c:C3' ... 'd::::::::::;. ~ ~ .c::::.. 

• • ~· • • 0 • • • C::; • • ·. ·. : ,;,: : • : ;_·_· =· =· c' ::::r:::Jo:::I::LJ_LLLLLLL..LL.l_LLLLU...L.Ll-Ll..LL..l""" 

Starlight 

Formation 

METERS 

200 

100 

~ · .. ·=· · .o • • • • • • o 7.9 Ma* 

= / Rhyolite of ;/ ~ 

• • • 0 ' '}/ / /west Pocatello /'1' / 

.c::::> ...c:. =='" . ·~·.·. ·. •:·.·:: ···~·:. ~-··:·~·-· ..... :. 

<::::. = •• ·0- •- •• 0 •• 

10 20 KILOMETERS 

Upper member of 

Starlight Formation 

10.20 MA* 

} 

Lower member of 

Starlight Formation 

EXPLANATION 

Limestone, dolomite 

Conglomerate, diamictite 

Sandstone, sand 

Argillite and clay 

f---=,....,.---,=-1 

Air-fall ash and ashy sandstone L-::o==i~ 

Basalt 

Eolian deposits 

Rhyolite flow 

Ash-flow tuff 

-~~- Fault-Arrows show direction 

of movement 

Figure 6. Diagrammatic stratigraphic cross section of units along the southeastern margin of the Snake River Plain between about 

Blackfoot and American Falls, Idaho. Section is oriented parallel to the margin of the Snake River Plain. Thicknesses are approximate and 

are based on Trimble (1982), Kellogg and Marvin (1988), and results of this study. New isotopic ages from this study are noted by an 

asterisk. 

samples of the Arbon Valley Tuff Member does show 

consistent differences from the trace-element analysis of 

samples from the Heise volcanic field (fig. 7, table 6). 

Relative abundances of rubidium, strontium, and zirconium serve to distinguish the Arbon Valley Tuff Member 

from the tuffs of the Heise volcanic field (fig. 7). Samples 

from near Blackfoot provide most of the analyses for the 

Arbon Valley Tuff Member; two samples from Rockland 

Valley and one from the Lemhi Range on the northwestern 

side of the plain are also plotted. Compared to the Heise tuffs, 

the Arbon Valley Tuff Member is enriched in strontium 

relative to zirconium and rubidium. The spread of analytical 

values is due to several factors, of which the two most 

important may be chemical zonation within the Arbon Valley 

Tuff Member and contamination of the tuff by foreign 

material during transport. 

Trace-element abundances for the rhyolites of Stevens 

Peak and Two-and-a-Half-Mile Creek are distinct from the 

upper, massive welded zone of the Arbon Valley Tuff 

Member; their relationships in the rubidium-strontium-zirco-
SOURCE OF THE ARBON VALLEY TUFF MEMBER 13 

Table 5. Major-oxide chemical analysis of rhyolite of west Pocatello, Idaho. 

[Sample FH86-2685B; sample site E, fig. 1] 

Oxide Weight 

percent 

Si 0 2--------------------------------------------------------------- 73.3 

A12 03 ------------------------------------------------------------- 12.4 

Total iron (as F eO) -------------------------------------------- 2. 91 

MgO -------------------------------------------------------------- .20 

CaO --------------------------------------------------------------- . 96 

N a2 0 -------------------------------------------------------------- 3. 1 9 

K2 0 --------------------------------------------------------------- 4. 80 

Ti 0 2-------------------------------------------------------------- . 3 9 

p 2 0 5--------------------------------------------------------------- . 08 

Mn 0 -------------------------------------------------------------- . 02 

LOI (loss on ignition) --------------------------------------------'·~8~3 

T ota 1----------------------------------------------------------- 9 9. 0 8 

nium system are also shown on figure 7. Compared to the 

Arbon Valley Tuff Member, the two rhyolite flows are relatively low in strontium relative to zirconium and rubidium. 

Major-element abundances, however, are not significantly 

different between the two flows and the Arbon Valley Tuff 

Member (Kellogg and Marvin, 1988, fig. 6). The two flows 

are also distinct from each other in their trace-element (but 

not major-element) geochemistry; compared to the rhyolite 

of Two-and-a-Half-Mile Creek (as well as the Arbon Valley 

Tuff Member), the rhyolite of Stevens Peak is significantly 

enriched in zirconium relative to strontium and rubidium. 

SOURCE OF THE ARBON VALLEY 

TUFF MEMBER 

The source of the Arbon Valley Tuff Member is not 

known, although several lines of evidence suggest that it 

may have erupted from a now-buried caldera north and( or) 

west of Blackfoot. The lines of evidence suggesting a buried 

caldera are: 

1. The tuff is thickest (about 60 m) between about 

25 km east and 5 km south of Blackfoot. Preliminary observations suggest that maximum lithic-clast size also occurs in 

the region of maximum thickness, indicating a proximal 

facies. 

2. Determined ages for the Arbon Valley Tuff Member are slightly older than the ages of two rhyolite flows 

south of Blackfoot, which suggests that the flows may represent fissure eruptions from ring dikes that formed the margin 

of the caldera. However, trace-element contents of the flows 

are distinct from those of the Arbon Valley Tuff Member. 

This difference may be due to tapping of different levels of a 

zoned magma chamber. 

Sr 

~\ 

( • • \ /Arbon Valley 

*~ )( Tuff Member 

Tuffs 

volcanic 

of Heise 

field ""~\\ ~ __._ . 

Rh l

.t ®-~~--0 Rhyol1te of 

yo I e lY'--J 0 Two-and-aof ~ Half-Mile Stevens 8 Creek Peak " __ 

Figure 7. Strontium-rubidium-zirconium (Sr-Rb-Zr) ternary diagram comparing samples from tuffs of the Heise volcanic field 

(open circles; 6.7 to 4.3 Ma; Kellogg and Marvin, 1988) with those 

from the Arbon Valley Tuff Member (solid circles; 10.2 Ma), Snake 

River Plain, Idaho. Also shown are fields for the rhyolite of Stevens 

Peak and the rhyolite of Two-and-a-Half-Mile Creek (Kellogg and 

Marvin, 1988). Star with arrow indicates sample DR724D3 from 

sample site D (fig. 1 ), the only sample analyzed from the northern 

margin of the Snake River Plain. Data are given in table 6. 

3. A resistivity sounding profile across the Snake 

River Plain through Blackfoot revealed a deep, approximately 15 km wide resistivity low that extends to a depth of 

at least 6 km (fig. 8). Zohdy and Stanley (1973) suggested 

that this zone of low resistivity is a structural trough that may 

be a buried caldera. This interpretation is consistent with the 

observation that intracaldera regions are commonly hydrothermally altered (Christiansen and Blank, 1972; Lipman and 

Sawyer, 1985); the presence of hydrothermal clay substantially lowers the resistivity of a rock. 

4. The resistivity low coincides closely with an 

approximately 20 milligal Bouguer-gravity low determined 

by a detailed gravity survey along the same line of profile 

(D.R. Mabey, U.S. Geological Survey, retired, oral commun., 1991, based on unpub. data, 1973). The gravity data 

may also reflect a thick, hydrothermally altered intracaldera 

tuff. Regional Bouguer gravity data from the same area (Bankey and Kleinkopf, 1988), which did not incorporate Mabey's 

unpublished gravity profile, do not reveal an obvious circular 

or oval gravity low, although their map of the horizontal 

gravity gradient does reveal a subtle ring of high gradients 

that approximately coincides with the proposed limits of the 

caldera shown on figure 1. Mabey (1978) pointed out, 

however, that calderas are not always expressed by a gravity 

low, as demonstrated by the Henrys Fork (formerly Island 
14 RHYOLITIC VOLCANISM IN THE SNAKE RIVER PLAIN, IDAHO 

Table 6 (below and facing page). Selected trace-element abundances for samples of the Arbon Valley Tuff Member of the Starlight 

Formation, rhyolites of Two-and-a-Half-Mile Creek and Stevens Peak, and tuffs of the Heise volcanic field, Idaho. 

[The tuff of Blue Creek was sampled southeast of Rexburg, Idaho, and does not crop out near Blackfoot. Major-element analyses for some of these samples 

are in Kellogg and Marvin (1988). Samples analyzed on a Kevex X -ray fluorescence analyzer. Analysts: K.S. Kellogg and R.A. Yeoman. Values are in parts 

per million] 

Sample 

No. Rb Sr y Zr Nb Zn 

Arbon Valley Tuff Member 

YS 621--------------- 166 46 37 125 39 64 

YS 622--------------- 127 71 23 160 25 58 

K 6 59----------------- 77 196 28 197 24 54 

BB 667------------- 84 175 17 221 20 9 

BB 671--------------- 160 54 37 134 39 15 

BB 672--------------- 103 174 22 194 23 9 

BB 675--------------- 60 183 10 160 21 26 

GO 7 6 3 -------------- 124 167 35 187 22 24 

Rhyolite of Two-and-a-Half-Mile Creek 

BB 609A ------------ 207 61 28 123 12 41 

BB 609B----------- 211 61 30 125 13 50 

FH 611------------- 181 104 25 119 13 46 

BB 715 --------------- 189 52 25 110 16 15 

BB 716--------------- 193 60 28 116 21 23 

BB 720--------------- 188 96 28 110 22 26 

BB 721--------------- 188 55 34 121 18 22 

FH 726--------------- 182 66 29 123 15 26 

FH 72 7--------------- 193 65 26 114 18 21 

BB 728--------------- 187 66 25 121 14 35 

BB 729--------------- 187 37 42 216 38 22 

FH 7 3 2 --------------- 189 53 28 113 17 30 

BB 736------------ 189 60 27 123 17 33 

BB 73 8---------------. 192 52 31 123 19 16 

BB 739--------------- 200 65 25 117 19 23 

BB 743--------------- 192 60 20 121 18 12 

Rhyolite of Stevens Peak 

BF 60 1 --------------- 157 81 64 411 40 83 

BF 602 --------------- 165 69 61 414 44 83 

BF 606 -------------- 162 96 64 477 44 87 

BF 624 -------------- 163 92 53 435 47 37 

BF 625--------------- 165 90 57 425 49 35 

BF 639--------------- 144 116 53 408 39 24 

BF 650--------------- 168 98 52 442 43 31 

BF 651------------ 159 99 51 402 45 37 

BF 690--------------- 162 84 49 413 50 47 

BF 691 --------------- 173 95 50 438 49 57 

BF 692--------------- 150 91 48 413 45 50 
SOURCE OF THE ARBON VALLEY TUFF MEMBER 15 

Sample 

No. Rb Sr y Zr Nb Zn 

Heise volcanic field 

Tuff ofBlacktail (Morgan, 1992) 

K 612------------- 195 49 34 176 37 69 

K 623---------- 204 45 40 254 33 72 

K 631--------------- 185 78 40 244 40 29 

K 632---------------- 191 67 36 234 33 28 

BF 663--------------- 208 39 26 249 41 23 

BF 664 ------------- 205 57 25 121 19 41 

BF 706 --------------- 201 53 52 239 38 14 

BF 707--------------- 194 59 31 259 44 26 

BF 71 0 --------------- 171 58 37 213 41 18 

BF 731--------------- 193 48 29 113 18 25 

BF 752--------------- 197 46 49 235 41 33 

K 7 61----------------- 197 66 42 225 40 2 

K 7 62----------------- 182 74 44 236 38 26 

Heise volcanic field 

Tuff of Blue Creek (Morgan and others, 1984) 

K 614----------------- 173 21 62 183 52 74 

K 6 3 3----------------- 174 36 55 209 42 21 

Heise volcanic field 

Tuff of Kilgore (Morgan and others, 1984) 

BF 603 ----------- 183 29 

BF 604--------------- 172 48 

K 616---------------- 170 21 

K 61 7---------------- 180 38 

K 618--------------- 148 95 

BF 620 --------------- 171 17 

BF 626--------------- 175 22 

BF 627--------------- 180 33 

BF 62 9 --------------- 170 18 

BF 630--------------- 166 42 

K 6 34---------------- 167 28 

BF 643-------------- 179 39 

BF 644 --------------- 162 24 

BF 649-------------- 164 92 

BF 709 -------------- 180 61 

YS 711------------ 199 37 

Park) caldera in the northeastern Snake River Plain (Blank 

and Gettings, 1974; Bankey and Kleinkopf, 1988). 

5. The region underlain by the proposed caldera is 

devoid of known basaltic vents, the lack of which may be due 

to the presence of low-density rhyolitic intracaldera fill that 

acted as a buttress to ascending basaltic magma (Kuntz and 

Covington, 1979; Kuntz and others, 1992). Kuntz and 

Covington (1979) suggested the name "Tabor caldera" for the 

57 

51 

61 

67 

56 

61 

46 

45 

59 

53 

49 

47 

49 

59 

54 

43 

234 42 74 

219 39 71 

328 45 87 

383 48 73 

452 42 87 

327 46 63 

223 45 33 

340 53 39 

315 51 31 

328 54 34 

216 45 31 

206 44 58 

196 44 47 

372 50 33 

208 47 32 

221 37 38 

proposed buried feature, although no outflow tuff from the 

caldera was identified. 

Although the evidence for a caldera near Blackfoot 

remains somewhat speculative, we present, as a working 

hypothesis, the location of part of a caldera rim on figure 1, 

based on the locations of the two silicic lava flows, the coincidence of the two limits of the resistivity and gravity lows, 

and the area of no known basaltic vents. 
16 RHYOLITIC VOLCANISM IN THE SNAKE RIVER PLAIN, IDAHO 

SE 

2 

NW 

METERSM 

o~.~-~~~~~~~~~~~~~~~~~~~~~.~ .. ~.~-~-~~-~ . .-+:_~:.:~:~::~:~--~·=·~~~n$MT~~ 

2000 

4000 

6000 

8000~-------------------------------------------------------------------------------------L 

10 20 30 40 KILOMETERS 

VERTICAL EXAGGERATION x 2 

EXPLANATION 

1,000-3,000 Qm [ .":: : ·. : · .. -.=[ 300-600 Q m 100-200 Om ITIIIIIJI 20-40 Q m Basement 

Figure 8. Cross section along the resistivity sounding line across the eastern Snake River Plain, Idaho (fig.l; from Zohdy and Stanley. 

1973). Points 1 and 2 are shown on both the cross section and on figure 1 for reference. Resistivity is given in ohm-meters (Q-m). 

BASEMENT is low-resistivity rock below the resolution of the resistivity method. Queries indicate resistivity boundary not resolved. 

CONCLUSIONS 

A major ash-flow eruption in the eastern Snake River 

Plain deposited the crystal-rich, biotite-bearing Arbon 

Valley Tuff Member of the Starlight Formation about 

10.20±0.06 Ma and presumably reflects the formation of a 

major caldera. Trace-element data indicate that the Arbon 

Valley Tuff Member is chemically distinct from rocks of the 

Heise volcanic field, the next younger rhyolitic volcanic 

field in the· eastern Snake River Plain, which was active 

between 7.9 and 4.3 Ma. The location of the source caldera 

for the Arbon Valley Tuff Member remains speculative, but 

a variety of field and geophysical evidence suggests that 

Blackfoot, Idaho, is above the northeastern part of the 

approximately 40 km diameter caldera. Following Kuntz and 

Covington (1979), we suggest the name "Tabor caldera" for 

this mostly buried feature. 

The chemical and petrologic character of the Arbon 

Valley Tuff Member supports the idea of chemically distinct 

magmatic cycles of rhyolitic volcanism, each of several 

million years duration (Morgan and others, 1984; Pierce and 

Morgan, 1992), that migrated northeastward along the Snake 

River Plain. Thus, the Arbon Valley Tuff Member is part of 

the speculative, approximately 10.2 Ma Picabo volcanic 

field of Pierce and Morgan (1990), which was emplaced 

several million years before the 6.7-4.3-Ma Heise volcanic 

field, which in tum was followed by the 2.0-Ma to present 

Yellowstone plateau volcanic field (Christiansen and Blank, 

1972). 

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18 RHYOLITIC VOLCANISM IN THE SNAKE RIVER PLAIN, IDAHO 

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*U.S. GOVERNMENl' PRINl'IIIX; OFFICE: 1994-673-046/86055 

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