RESEARCH ARTICLE
Rhyolitic ignimbrites in the Rogerson Graben,
southern Snake River Plain volcanic province: volcanic
stratigraphy, eruption history and basin evolution
Graham D. M. Andrews & Michael J. Branney &
Bill Bonnichsen & Michael McCurry
Received: 10 February 2005 /Accepted: 8 March 2007 / Published online: 20 June 2007
# Springer-Verlag 2007
Abstract The 80 km long NNE-trending Rogerson Graben
on the southern margin of the central Snake River Plain,
Idaho, USA, hosts a rhyolitic pyroclastic succession, 200 m
thick, that records a period of successive, late-Miocene,
large-volume explosive eruptions from the Yellowstone–
Snake River Plain volcanic province, and contemporaneous
extension. The succession, here termed the Rogerson
Formation, comprises seven members (defined herein) and
records at least eight large explosive eruptions with
numerous repose periods. Five high-grade and extremely
high-grade ignimbrites are intercalated with three non-
welded ignimbrites and two volcaniclastic deposits, with
numerous repose periods (palaeosols) throughout. Two of
the ignimbrites are dominantly rheomorphic and lava-like
but contain subordinate non-welded pyroclastic layers. The
ignimbrites are typical Snake River Plain high-silica
rhyolites, with anhydrous crystal assemblages and high
inferred magmatic temperatures (≤ 1,025°C). We tentatively
infer that the Jackpot and Rabbit Springs Members may
have been emplaced from the Bruneau–Jarbidge eruptive
centre on the basis of: (1) flow lineation trends, (2) crystal
assemblage, and (3) radiometric age. We infer that the
overlying Brown’s View, Grey’s Landing, and Sand Springs
Members may have been emplaced from the Twin Falls
eruptive centre on the basis of: (1) kinematic indicators
(from the east), and (2) crystal assemblage. Furthermore,
we have established the contemporaneous evolution of the
Rogerson Graben from the emplacement of the Jackpot
Member onwards, and infer that it is similar to younger
half-graben along the southern margin of the Snake River
Plain, formed by local reactivation of Basin and Range
structures by the northeastwardly migration of the Yellow-
stone hot-spot.
Keywords Snake River Plain volcanic province .
Rheomorphic ignimbrite . Volcanic stratigraphy .
Geothermometry . Rhyolite . Basin and range .
Yellowstone hot-spot
Introduction
This paper documents and interprets a rhyolitic volcanic
succession in the Rogerson Graben of southern Idaho. It
presents a detailed lithostratigraphy through eight previous-
ly undescribed ignimbrites and associated ash-fall tuffs and
other volcaniclastic deposits. We provide geochemical data
and estimates of pre-eruptive magmatic temperatures for the
Bull Volcanol (2008) 70:269–291
DOI 10.1007/s00445-007-0139-0
This paper constitutes part of a special issue dedicated to Bill
Bonnichsen on the petrogenesis and volcanology of anorogenic
rhyolites.
Editorial responsibility: M McCurry
Electronic supplementary material The online version of this article
(doi:10.1007/s00445-007-0139-0) contains supplementary material,
which is available to authorized users.
G. D. M. Andrews : M. J. Branney
Department of Geology, University of Leicester,
Leicester LE1 7RH, UK
G. D. M. Andrews (*)
Department of Earth and Ocean Sciences,
University of British Columbia,
Vancouver, BC V6T 1Z4, Canada
e-mail: gandrews@eos.ubc.ca
B. Bonnichsen
927 East 7th Street,
Moscow, ID 83843, USA
M. McCurry
Department of Geosciences, Idaho State University,
Pocatello, ID 83209-8072, USA

units to allow correlation with existing and future strati-
graphic sections in the Yellowstone-Snake River Plain
volcanic province. The succession occupies an important
geographical and tectonic position for two reasons: (1) it is
equidistant between the Bruneau–Jarbidge and Twin Falls
eruptive centres (Pierce and Morgan 1992), and is a
potentially good area to correlate between successions
from each eruptive centre; and (2) it is the most westerly
(and possibly oldest) of nine major basins along the
southern and south-eastern margins of the Snake River
Plain (Rodgers et al. 2002).
An intriguing aspect of the Rogerson Graben succession
is the preponderance of intensely welded, high-grade
ignimbrites (five members) including two that are rheo-
morphic and lava-like (sensu Branney and Kokelaar 1992).
The Yellowstone-Snake River Plain volcanic province is
host to some of the youngest and best-exposed rheomorphic
and lava-like ignimbrites known, and has been the site of
several key studies (e.g., Bonnichsen and Citron 1982;
Ekren et al. 1984). Extremely high-grade ignimbrites are
common components of many volcanic provinces in a
diverse range of tectonic settings (e.g., Gran Canaria,
Schmincke 1974; Wall Mountain Tuff, Colorado, Chapin
and Lowell 1979; Keweenawan rift, Minnesota, Green and
Fitz 1993). Their structural characteristics (e.g., flow-
banding, flow folds, etc.) and emplacement mechanisms
remain poorly understood. Recent studies have concentrat-
ed on the timing and rate of welding (e.g., Freundt 1998),
the timing of rheomorphic flow relative to deposition,
welding and eventual cooling (e.g., Andrews 2006), and
the styles of rheomorphic deformation (e.g., Branney et al.
2004; Andrews 2006). Many rheomorphic ignimbrites
elsewhere are peralkaline (e.g., Gran Canaria, Pantelle-
ria), and therefore, had unusually low viscosities when
deposited. In contrast, those in the Yellowstone–Snake
River Plain volcanic province in general, and the Rogerson
Graben in particular, are as intensely welded and rheomor-
phic as ignimbrites from elsewhere but, they are character-
istically metaluminous and have very high eruption
temperatures (900–1,000°C; Honjo et al. 1992).
Geological setting
Voluminous bimodal volcanism has dominated the interior
northwestern United States since the mid Miocene, following
the initiation of Yellowstone hot-spot magmatism at ∼16.8Ma
(Camp and Ross 2004). The Yellowstone–Columbia River
volcanic mega-province is adjacent to and contemporaneous
with crustal extension and magmatism in the Basin and
Range province, leading to complex basin evolution along
the margins of the Snake River Plain (e.g., Rodgers et al.
2002), and the location of present-day seismicity around the
Yellowstone ‘seismic parabola’ (Anders et al. 1989).
Rhyolites of the Yellowstone–Snake River Plain volca-
nic province are time-transgressive, and successive volcanic
centres young northeastwards towards the Yellowstone
centre (Fig. 1 inset). Most of the eruptive centres are buried
and their stratigraphies are known primarily from outflow
sheets at the southern and northern margins of the Snake
River Plain. However, there are significant gaps in
stratigraphic coverage, both temporally and spatially, that
hinder better understanding of the temporal evolution of the
province.
Between 12 and 8 Ma, rhyolite volcanism was concen-
trated around the Bruneau–Jarbidge and Twin Falls areas
and the margins of the contemporaneously extending West
Snake River Plain graben (Pierce and Morgan 1992;
Perkins and Nash 2002; Fig. 1 inset). The period 11.7–
10.0 Ma is described by some workers as an ‘ignimbrite
flare-up’ period because of the huge volume of ignimbrites
erupted (Bonnichsen 2004; Bonnichsen et al. 2007).
Younger basalts largely bury these volcanic centres so their
nature and structure is poorly constrained (see Branney et
al. 2007). Eruptions during this time produced numerous
large-volume, rhyolitic ignimbrite outflow sheets (the
‘Idavada Volcanic Group’ of Malde and Powers 1962;
including the ‘Cougar Point Tuff Formation’ of Bonnichsen
and Citron 1982) exposed along the northern and southern
margins of the central Snake River Plain. At the Bruneau–
Jarbidge and Twin Falls eruptive centres, and on the
Yellowstone Plateau, the ignimbrite-forming eruptions were
typically followed by voluminous rhyolite lavas (e.g., the
Sheep Creek Rhyolite, Bonnichsen 1982b).
Analytical methods
Whole-rock analyses were undertaken with a Phillips
PW1400 X-ray fluorescence spectrometer (XRF) at the
University of Leicester. Glass and crystal compositions
(re-calculated to 100 wt.%) were analysed on a JEOL
8600 S electron microprobe (EMP) at the University of
Leicester, using a 15 kV accelerating potential, 30 nA
incident current, and 5 and 10 μm spot size respectively.
Stratigraphy—the Rogerson Formation
The Rogerson Formation (Fig. 2) is exposed throughout its
type locality, the Rogerson graben (Fig. 1), where it
consists of five extensive and gently (≤10°) dipping
rhyolite sheets, separated by four non-welded volcani-
clastic units. The base of the formation is not seen and
attempts to establish the regional-scale stratigraphic
relations are ongoing. It is probable that the formation
270 Bull Volcanol (2008) 70:269–291

overlies rhyolite ignimbrites of the Cougar Point Tuff
Formation (12.7–10.5 Ma; Cathey and Nash 2004), and
possibly, inliers of upper Palaeozoic, Mesozoic, and
Palaeogene sedimentary and volcanic rocks as reported
from the base of Cassia Mountains succession (Williams
et al. 1999). The top of the formation is being denuded
as part of the present-day topographic surface, other than
where it has been buried by basalt lavas (∼ 6.8 Ma;
Bonnichsen and Godchaux 2002) at the northern end of
the graben (Fig. 1).
Jackpot Member
At its type locality at Salmon Falls Creek, 4.5 km south of
Jackpot, Nevada (Figs. 1 and 3) the Jackpot Member is ≥75 m
thick, although its base is not exposed. It outcrops extensively
(>300 km2) in the south of the Rogerson Graben, where it is
unconformably overlain by a thick, bedded volcaniclastic
deposit and the Rabbit Springs Member. It is readily
distinguished from other members by its stratigraphic
position, considerable thickness, lava-like appearance, inter-
Fig. 1 Geological map of the
Rogerson Graben and surround-
ing areas (Twin Falls County,
Idaho and Elko County,
Nevada), showing the present
distribution of the Rogerson
Formation, at localities named in
the text. Selected UTM coordi-
nates for zone 11T shown. Inset:
Location of the Rogerson Graben
in relation to southern Idaho and
inferred, successive eruptive
centres (Bruneau–Jarbidge—
B–J; Twin Falls—TF;
Yellowstone—Y) that young
to the NNE
Bull Volcanol (2008) 70:269–291 271

nal stratigraphy, and the unique occurrence of myrmekitic
intergrowths around sanidine crystals in thin section (Fig. 4a).
We subdivide the Jackpot Member into seven informal
sub-units (‘Jackpot 1 to 7’, Fig. 3) that form prominent cliffs,
based on different columnar joint patterns, lithophysae-rich
layers, and topographic benches between each cliff. ‘Jackpot
1 to 4’ (Fig. 4b) are welded, generally devitrified (lithoidal),
lava-like, and lack eutaxitic fabrics. ‘Jackpot 1 and 2’ have
well developed, closely spaced, subhorizontal joints (sheet-
joints of Bonnichsen 1982b), columnar joints and prominent
upper lithophysae-rich layers that lack joints. ‘Jackpot 3’
lacks columnar joints and lithophysae, and sheet-joint
surfaces have a linear fabric defined by orientated prolate
vesicles and are deformed by open, upright folds. ‘Jackpot 4’
is a lava-like, sheet-jointed rhyolite similar to ‘Jackpot 1,’
with some poorly developed columnar joints, and isolated
lithophysae throughout its upper half especially concentrated
in a 2 m-thick zone at the top.
‘Jackpot 5’ is 26–30 m thick, lava-like and flow-folded,
with well developed columnar jointing, and well developed
sheet-jointing in lower and central parts (Fig. 4b, c). The
top 15 m show N or NE-trending upright, open folds on a
scale of 1 m–10 m with NW–SE elongation lineations
(stretched vesicle; Fig. 1). Prolate vesicles have axial ratios
of ≤10:1 and trend parallel to the lineation. The uppermost
4 m are a perlitic vitrophyre with abundant lithophysae
partly filled with pistachio-green chalcedony.
‘Jackpot 6’ (type locality: Sweetwater Lane, 2 km south
of Jackpot, Fig. 1) is less intensely welded, more obviously
pyroclastic than the other Jackpot sub-units and is charac-
terized by the presence of abundant, equant obsidian lapilli
(1–2 cm). It conformably overlies the upper vitrophyre of
‘Jackpot 5.’ A strongly welded 20 cm-thick base grades up
into a layer of weakly welded obsidian lapilli-tuff (≤2 m).
This layer is truncated by an erosion surface overlain by
lenses of cross-bedded, non-welded lapilli-tuff of similar
composition. This in turn grades up into a 1.5 m thick layer
of obsidian-bearing lapilli-tuff containing lenses of accre-
tionary lapilli. The top of this layer is not seen.
‘Jackpot 7’ is an 8 m thick columnar-jointed, flow-
banded and dominantly lava-like sheet, similar to the non-
lithophysal parts of ‘Jackpot 1’ (Fig. 3). It is exposed
Fig. 2 General vertical section through the Rogerson Formation in
the Rogerson Graben. Soils, erosion surfaces, and unconformities are
marked. Details of lithofacies, welding grade, type locality, minimum
volume, and crystal assemblage are given on the right. plag
plagioclase, sa sanidine, pgt pigeonite, aug augite, qtz quartz, mt
magnetite, il ilmentie, zr zircon, ap apatite
272 Bull Volcanol (2008) 70:269–291

widely across the southern Rogerson Graben, where it
forms a prominent cap at the top of the Jackpot Member
(Fig. 4c). It has well developed sheet-joints and is
thoroughly de-vitrified. Folds and vitrophyres have not
been seen. Ragged quartz and sanidine concentrations,
possibly crystal-bearing fiamme (≤8 cm long), occur in a
75 cm thick layer, 1.5 m from the base.
Undifferentiated volcaniclastic units
Undifferentiated, bedded volcaniclastic deposits occur at
two stratigraphic levels: (1) unconformably overlying the
Jackpot Member; and (2) discordantly overlying the Rabbit
Springs Member (Fig. 2). Both units consist of 2–10 cm
thick layers of silt and sand-sized particles, with rare lenses
Fig. 3 Graphic log of the Jackpot Member, based upon sections at Salmon Falls Creek and Sweetwater Lane. Abbreviations on key follow
Branney and Kokelaar 2002. Inset shows the outcrop of the member
Bull Volcanol (2008) 70:269–291 273

Fig. 4 a Lower part of the Jackpot Member at Salmon Falls Creek,
showing subdivisions into ‘Jackpot 1–5’ by topographic benches and
variations in columnar-jointing patterns. b Stratigraphy approximately
5 km east of Backwaters (Fig. 1), showing units dipping gently
northwards. Note the thin and uniform nature of the welded Rabbit
Springs, Brown’s View and Grey’s Landing Members, producing
prominent topographic benches. c Base of Rabbit Springs Member at
Backwaters, showing increasing fusing intensity in originally non-
welded volcaniclastic deposits (meTv massive eutaxitic vitric tuff; sLTv
stratified vitric lapilli-tuff; mLTv massive vitric lapilli-tuff; xsT cross-
stratified tuff; Branney and Kokelaar 2002). d Photomicrograph (PPL)
of the Rabbit Springs Ignimbrite basal vitrophyre showing welded Y-
shaped shards, strongly flattened around the corner of a plagioclase
crystal. Note the axiolitic devitrification textures developed in the
largest shards. e Photomicrograph (PPL) of Brown’s View Ignimbrite
274 Bull Volcanol (2008) 70:269–291

of accretionary lapilli or ash-pellets, some rare rounded
pumice lapilli, and intercalated thin palaeosols containing
calcified rootlets. Individual layers are laterally continuous
for ≥5 m; however, scour-structures, ripple laminations,
low-angle cross-stratification (cross-set height ≤10 cm),
normal graded bedding, and de-watering structures are
common. Both units are typically buried by modern
slopewash, and are locally bioturbated, making further
examination difficult.
The lower bedded volcaniclastic unit unconformably over-
lies a buried landscape developed in Jackpot sub-units 5, 6,
and 7 (Fig. 2), where the Jackpot Member has been extended
on normal faults to produce several 25–30 m deep, NNE-
trending, km-scale graben. This is best demonstrated imme-
diately to the east of the Backwaters area (Figs. 1 and 4b).
In turn, the lower bedded volcaniclastics are conformably
overlain by the Rabbit Springs Member, and the topmost
1 m is fused against the basal vitrophyre of the later unit
(Fig. 4b, c).
Rabbit Springs Member
The Rabbit Springs Member (Fig. 5) uncomformably over-
lies horsts developed in the Jackpot Member and conform-
ably overlies undifferentiated volcaniclastic deposits
(Figs. 2, 4b, c). It is an 8–12 m thick sheet with a minimum
extent of 200 km2, exposed across the central and eastern
portion of the Rogerson Graben, where it is the lowest of
several members that thin to a feather-edge and pinch out
eastwards against the Shoshone Hills graben-margin (Fig. 1).
In the Backwaters area it is disrupted by a pair of contem-
poraneous normal faults (the member thickening from 9 to
12 m) that deform the basal but not upper vitrophyre.
In the type locality of Backwaters, Idaho (Fig. 1) it is an
11 m thick tuff, with a lithoidal centre, and spherulitic and
lithophysal upper and lower vitrophyres (Fig. 5). The basal
vitrophyre is massive perlite, with a eutaxitic fabric of
flattened Y-shaped ash shards, some of which wrap
crystals (Fig. 4d). It is devoid of pumice lapilli and is
characterized by an abundance of spherulites and litho-
physae (≤8 cm across) some of which are filled with white
chalcedony. The central lithoidal zone is thoroughly
devitrified, with closely spaced sheet-joints, and relic
pseudomorphs after spherulites within the matrix. This
unit has yielded an 40Ar–39Ar date of 10.37 Ma±0.13
(Bonnichsen et al. 2007).
Undifferentiated volcaniclastic deposits
The upper bedded volcaniclastic unit discordantly overlies
the Rabbit Springs Member and thins rapidly to the eastern
margin of the graben; however, the contact is not exposed.
The sedimentary characteristics of the unit are nearly
identical to the lower volcaniclastics (see above); however,
at least three thick (≥1 m) palaeosols (containing calcified
rootlets) are developed within the unit. Internal angular
unconformities are recorded by planar erosion surfaces
cutting palaeosols, and subsequent discordant burial by
later volcaniclastics. This is best demonstrated immediately
to the south of the Backwaters area (Fig. 1). The upper
bedded volcaniclastics are conformably overlain by the
Brown’s View Member, although the contact is not exposed
(Fig. 2).
Brown’s View Member
The Brown’s View Member (type locality: Backwaters,
Figs. 1 and 6) is a 4–8 m thick, ≥400 km2 sheet that is
exposed intermittently across the central Rogerson Graben,
where it conformably overlies undifferentiated volcaniclas-
tic deposits. It is the oldest member exposed outside the
Graben to the northwest (Fig. 1), where it onlaps and
oversteps older lava-like rhyolites of the Brown’s Bench
massif (10.22 Ma±0.09 (40Ar–39Ar); Bonnichsen and
Godchaux 2002).
A massive, chocolate-brown lithoidal centre (∼ 1.5 m
thick) separates two sparsely spherulitic vitrophyres (≤3 m
thick; Fig. 6). Both vitrophyres exhibit a ≤ mm-scale
eutaxitic fabric (Fig. 4e), cuspate shards, and crystal and
lithic fragments. There are no pumice lapilli or fiamme. The
massive upper vitrophyre contains isolated lithophysae (5–
8 cm diameter) and passes up into a thin orange palaeosol
(∼20 cm thick), with angular obsidian lapilli and small,
calcified rootlets. In contrast to the Rabbit Springs Member,
the Brown’s View Member contains less lithophysae and
spherulites, and it lacks sheet-joints.
Backwaters member
The Backwaters Member (type locality: Backwaters, Fig. 1)
is 8–15 m thick. It likely conformably overlies the Browns
View Member (contact not well-exposed) across the centre
and north of Rogerson Graben (Fig. 1). It comprises at least
two units (A and B on Fig. 7) separated by a palaeosol and
erosion surface. Unit A is a ≥8 m thick layer of non-welded,
massive tuff, containing lenses of sub-rounded to angular
pumice lapilli and lithics supported by a tuff matrix, and is
overlain by a 2 m thick palaeosol. The base of this unit is
not exposed. Unit B is a 3–6 m thick, massive tuff
containing lenses of sub-angular pumice lapilli and abundant
calcified rootlets. It has an erosional base and the upper 4 m
is host to a bioturbated palaeosol. Grass imprints are
preserved on the upper surface of the palaeosol. The lower
2 m have been fused to black vitrophyre and the palaeosol
baked, likely due to heat conducted downwards from the
overlying Grey’s Landing ignimbrite (Fig. 8a).
Bull Volcanol (2008) 70:269–291 275

Grey’s Landing Member
The Grey’s Landing Member (type locality: Grey’s Land-
ing, Idaho, Fig. 1) is a 5–65 m thick, rhyolitic sheet
comprising a stratified ashfall deposit and a lava-like
ignimbrite with a lower vitrophyre, a thick lithoidal centre,
and a thin upper vitrophyre locally overlain by a non-
welded top (Figs. 7 and 8b). The upper vitrophyre is eroded
at the Type Section but is exposed elsewhere (e.g., Cedar
Creek Reservoir, Fig. 1). In sections less than 5 m thick the
entire member is vitric.
The Grey’s Landing Member unconformably overlies the
Backwaters Member both within and outside the Rogerson
Graben, and its full areal extent is estimated to be ≥400 km2
(Fig. 1). It has a wedge-shaped form within the Rogerson
Graben that thins to a feather-edge towards the east (Fig. 8c)
and south. Outside the graben it onlaps and oversteps older
lava-like rhyolites of the Brown’s Bench massif, and drapes
the palaeo-fault scarp of the Brown’s Bench Fault (Cricket
Creek; Fig. 1). This unit has yielded an K–Ar date of 7.62
Ma±0.4 (Hart and Aronson 1983). It is overlain by late-
Miocene and early-Pliocene basalt lavas in the north of the
Fig. 5 Graphic log of the Rabbit Springs Member, from the Backwaters section. Inset shows the outcrop of the Member
276 Bull Volcanol (2008) 70:269–291

graben and in the adjoining Snake River Plain (Bonnichsen
and Godchaux 2002, Fig. 2).
An equally extensive, parallel-stratified ash layer, ≤1.5 m
thick, drapes the upper palaeosol of the Backwaters
Member (Figs. 7, 8d, e). Along Highway 93, (Fig. 2) the
stratified ash sits on a ∼2 m thick bedded tephra succession
in a local depression in the Backwaters Member. The
stratified ash becomes progressively more fused and
compacted under thicker ignimbrite (Fig. 8d, e).
The basal vitrophyre (Figs. 7, 8d, e) is 1–3 m thick,
massive and spherulitic (2–4 cm diameter), and conform-
ably overlies the underlying stratified ash. Extensive,
horizontal sheets (>5 m long, ∼6 cm thick) of devitrified
rhyolite are found within the vitrophyre (Fig. 8d), with a
lineation (∼ E–W) developed on their surfaces (Fig. 1).
Rare lithophysae (<8 cm diameter) are concentrated in
graben-flank sections. Vitroclastic textures are preserved in
strain-shadows around rotated crystals, in an otherwise
flow-banded and flow-folded, glassy matrix (Fig. 9d).
A red–brown lithoidal central zone is pervasively flow-
banded and flow-folded, with dm- to 10 m scale flow-folds
(Fig. 10a–c), including sheath folds (e.g., Branney et al.
2004) of colour banding (Fig. 9b-d). Flow-folding scale and
style define two deformation domains (Fig. 8c); a lower
Fig. 6 Graphic log of the Brown’s View Member from the Backwaters section. Inset shows outcrop of the Member
Bull Volcanol (2008) 70:269–291 277

Fig. 7 Graphic log of the Backwaters and Grey’s Landing Members based upon sections at Backwater, Grey’s Landing and Salmon Dam. Inset
shows outcrop of the Grey’s Landing Member
278 Bull Volcanol (2008) 70:269–291

‘flat’ domain of cm- to m-scale, recumbent isoclinal folds,
and an upper ‘steep’ domain in which isoclinal folds are
refolded by m- to 10 m scale upright folds. A penetrative
lineation (Fig. 10a) is developed on some sheet-joint
surfaces, especially in the less folded parts, and the
orientation of lineations and parallel fold hinges varies
with increasing height, as reported by Branney et al. (2004).
‘Dimple joints’ (Fig. 10b; Bonnichsen 1982b), dominate
the lower 10 m where sheet joints and flow-folds are
absent. Vesicles (0.2–20 cm diameter) occur in the upper
20 m of the lithoidal zone. Some have risen buoyantly,
intruding and deforming the flow-banding (Fig. 10c).
A dark grey, upper vitrophyre is flow-banded and flow-
folded. It is perlitic and locally spherulitic (≤3 cm diameter)
and lithophysal (≤4 cm diameter). It is best exposed at
Salmon Dam and Cedar Creek Reservoir (Fig. 1); however,
it has been removed at the type locality. It locally shows
autobrecciation, with jigsaw-fit blocks ≤0.5 m in diameter.
At Salmon Dam and Cedar Creek Reservoir a massive to
faintly stratified, aphyric, orange, sand-sized ash is pre-
served within 10 m scale synforms developed in the upper
vitrophyre. Typically the contact between welded vitro-
phyre and non-welded tephra is a breccia in which angular
clasts (≤10 cm diameter) of non-welded tephra are
supported by a matrix of non-welded and partly fused
orange ash, which is progressively more fused towards the
vitrophyre.
Coyote Creek Member
The Coyote Creek Member (type locality: road-cut along
Highway 93, 10 km south of Rogerson, Idaho, Fig. 1) is a
4–5 m thick, buff-brown, non-welded, massive, rhyolitic
volcaniclastic layer that conformably overlies the Grey’s
Fig. 8 Sections though the
Grey’s Landing Member a Bak-
ing and fusing zonation with the
Backwaters Member, Back-
waters. b 55 m-thick Grey’s
Landing Member dominated by
massive, lithoidal centre, sitting
on baked Backwaters Member
palaeosol at Grey’s Landing.
c Cross section through the
Grey’s Landing ignimbrite from
Salmon Falls Creek Canyon
(west) to Highway 93 (east),
showing wedge-shaped profile
and relative thickness of the
‘steep’ and ‘flat’ zones. d Detail
of the Grey’s Landing Member
base where it is ∼60 m-thick,
showing the basal vitrophyre
lying on fused and stratified,
ashfall tuff, in turn lying on
baked Backwaters Member
palaeosol, Backwaters. e Detail
of the base of the Grey’s Landing
Member where it is ∼12 m-thick,
showing limited fusing of the
underlying stratified tuff. Only
the upper 30 cm of the stratified
tuff are fused, Highway 93. mLT
massive lapilli-tuff, mLTv
massive vitric lapilli-tuff, rheomT
rheomorphic tuff, rheomTv
rheomorphic vitric tuff, //sTv
parallel stratified vitric tuff
(Branney and Kokelaar 2002)
Bull Volcanol (2008) 70:269–291 279

Landing Member (Fig. 2), although it is poorly exposed
and has a limited areal extent (∼4 km2). It is composed of
poorly sorted silt-and sand-grade material, mainly glass
shards. The upper surface is a thick palaeosol (≤1.5 m),
and the whole deposit is strongly bioturbated. Irregularly
shaped ashy concretions (∼5 cm diameter) are common.
Sand Springs Member
The Sand Springs Member (type locality: road-cut along
Highway 93, 10 km south of Rogerson, Figs. 1 and 2) is a
≥8 m thick, dark-grey rhyolitic ignimbrite that conform-
ably overlies the soil at the top of the Coyote Canyon
a b
c d
Fig. 9 Grey’s Landing Member
a rotated and mantled, euhedral
plagioclase crystal and associat-
ed <mm-scale folds from the
basal vitrophyre, Grey’s Land-
ing, section parallel to stretching
direction (PPL). Note the flow
banding and ‘mylonitic appear-
ance.’ b Sketch of dm-scale,
sheath-like, intrafolial, similar-
style isoclinal folds, typical of
the Grey’s Landing ignimbrite
‘flat zone’; Grey’s Landing,
viewed parallel to stretching
direction. c Sketch of m-scale,
complexly refolded folds in the
‘steep zone,’ Cedar Creek Res-
ervoir, viewed perpendicular to
transport direction. d Complex
cm-scale folding, refolding and
attenuation of flow banding,
Salmon Dam, oblique to
stretching direction
Fig. 10 a Lineation developed
on sheet-joint surface, Grey’s
Landing ignimbrite Salmon
Dam (pen 12 cm). b Dimple-
joints on surface of a loose
block, Grey’s Landing ignim-
brite Cedar Creek Reservoir
(hammer shaft 40 cm). c Sketch
of 15 cm diameter, spherical
vesicle in banded, devitrified
Grey’s Landing ignimbrite, Ce-
dar Creek Reservoir (pen
12 cm). Note how the vesicle
has intruded upwards (‘diapir-
ic’), and broken through some
banding. d Photomicrograph
(PPL) of the Sand Springs
Member showing eutaxitic, fine-
tuff matrix surrounding abun-
dant crystals and < mm-scale
lithics
280 Bull Volcanol (2008) 70:269–291

Member (Fig. 11). It is restricted to the eastern margin of the
Rogerson Graben, covering ∼4 km2. The basal 7 cm
comprises a non-welded, low-angle cross-stratified tuff
(cross-set height ≤10 cm). This grades up through massive,
incipiently welded, moderately welded and then intensely
welded, eutaxitic, vitric tuff. Vitroclastic textures (Fig. 10d)
and small obsidian lapilli (0.5–1 cm diameter) are ubiquitous
throughout the deposit, becoming progressively more flat-
tened with increasing height. There are no pumice lapilli.
Geochemistry, petrology and geothermometry
We present whole-rock, glass and crystal chemical data
from the Rogerson Formation, and estimate pre-eruptive
temperatures. Our aim is to establish a geochemical data-
set to characterise the members within the Rogerson
Formation and to provide a basis for improving strati-
graphic correlation with adjacent successions (e.g., the
Cassia Mountain succession; McCurry et al. 1996) and
Fig. 11 Graphic log of the Sand Springs and Coyote Creek Members, from the Highway 93 road section. Inset shows the outcrop of the Sand
Springs Member
Bull Volcanol (2008) 70:269–291 281

distal fallout tephra deposits (e.g., Trapper Creek; Perkins
et al. 1995).
Bulk-rock chemistry
The Rogerson Formation is composed of anhydrous,
metaluminous rhyolites (68.25–75.7% SiO2), with rela-
tively high concentrations of TiO2, MgO and Fe2O3, high
Ga/Al ratios, high Fe2O3*/MgO ratios (Appendix Table 1).
Glass shard matrix chemistry
Electron microprobe spot analyses of glass shards and
welded glass matrix from non-altered vitrophyres are
presented in Appendix Table 2, however, glass analysis
was not possible in lithoidal rhyolite; furthermore, the
Jackpot Member lacks non-hydrated vitrophyre (sub-unit 5
perlitized upper vitrophyre) preventing any glass analysis.
The Rabbit Springs, Brown’s View, Grey’s Landing, and
Sand Springs Members have compositionally restricted
shard populations (71–77.5 wt.% SiO2; CaO+Na2O+K2O
∼10 wt.%), and there is no systematic variation between or
within individual members.
Crystal chemistry
Plagioclase, pyroxene, titano-magnetite, and accessory
apatite and zircon are ubiquitous throughout the Rogerson
Formation (Fig. 12). They exist as single, often broken
crystals (Fig. 9a) and as glomerocrysts of plagioclase,
pyroxene, and titano-magnetite.
Plagioclase occurs in all units and exists in three textural
forms: (1) euhedral crystals with sieve-texture (Fig. 9d); (2)
subhedral to anhedral crystals associated with glomeroc-
rysts; and (3) euhedral to subhedral, crystal fragments
(Fig. 10d). All plagioclase found in Jackpot sub-units 3 and
5 is oligoclase (An 25–35; Figs. 12 and 13a), however, the
plagioclase in ‘Jackpot 7’ and all other members in the
Rogerson Formation is andesine (An 35–An 50; Figs. 12
and 13b). Sanidine crystals (Or 50–Or 57; Fig. 13a) occur
only in the Jackpot and Rabbit Springs members. They
have two forms: (1) anhedral crystals with myrmekite rims
and (2) subhedral crystal fragments. The former are only
found in the Jackpot Member, where they are much more
abundant than the latter.
Pyroxene crystals occur in all members although those in
the Jackpot Member are partly oxidized and hydrated, and
altered to amphibole or clay. Non-zoned Ca-rich pyroxene
(augite) and Ca-poor pyroxene (pigeonite) are found in
equal abundances in all units apart from the Grey’s Landing
Member, in which augite is nearly absent from upper
sections and minor quantities of hypersthene occurs at the
base (Fig. 12). Pyroxene crystals occur in equal measure in
two forms: (1) anhedral crystals associated with glomer-
ocrysts and (2) crystal fragments. Pyroxene compositions
are presented with tie lines joining analyses from co-
existing crystals (Figs. 13c and d). Augites and pigeonites
in the Rogerson Formation exhibit a range of compositions
(Figs. 13c and d) which correspond to compositional trends
described for the Cougar Point Tuff Formation (Cathey and
Nash 2004).
Titano-magnetite and ilmenite are present in all units
other than the Grey’s Landing Member, in which ilmenite is
absent (Fig. 12). Magnetite exists as both anhedral crystals
within glomerocrysts, and as crystal fragments. Ilmenite
occurs as discrete anhedral crystals only. Subhedral and
anhedral quartz crystals are only found in the Jackpot and
Rabbit Springs members (Fig. 12) where they exist as
isolated, anhedral crystals. Anhedral apatite and zircon
crystals are abundant in all units, commonly as inclusions
in Fe–Ti oxides. Discrete apatites and zircons are only
found associated with glomerocrysts.
Geothermometry
Electron microprobe analyses of crystal rim pairs were used
to calculate mean crystal compositions (n≥25) for individ-
ual samples referenced for stratigraphic position within
each ignimbrite. Mean crystal compositions were input to
appropriate Fe–Ti oxide, two-pyroxene, and two-feldspar
geothermometers, depending on the different crystal pop-
ulations of respective members (Figs. 2 and 12). The use of
geothermometry assumes that crystal rims were in equilib-
rium with the liquid, even if crystal cores may have not
re-equilibrated.
Estimates of pre-eruptive temperature from Fe–Ti oxide
(magnetite–ilmenite) thermometry and fO2 were made
using (1) the model of Ghiorso and Sack (1991), and (2)
the QUILF 4.1 software package (Andersen et al. 1993)
applying the model of Andersen et al. (1991) and technique
of Manley and Bacon (2000). All Fe–Ti oxide pairs were
tested for Mg/Mn equilibrium following the method of
Bacon and Hirschmann (1988) before being applied to the
two geothermometers, and only samples from the Browns
View Member were found to be in equilibrium. Estimates
of pre-eruptive temperature from two-pyroxene (augite-
pigeonite pairs) thermometry were made in the Rabbit
Springs, Browns View, Grey’s Landing and Sand Springs
Members, using a pressure constant of 5 kbar (e.g., Cathey
and Nash 2004). First we used the model of Andersen et al.
(1991) through the QUILF 4.1 software package (Andersen
et al. 1993), and secondly we applied the graphical
geothermometer of Lindsley (1983) plotted onto the
pyroxene quadrilateral (Figs. 13c and d), where contours
allow visual estimation of equilibrium temperature. Esti-
mates of pre-eruptive temperature from two-feldspar
282 Bull Volcanol (2008) 70:269–291

(plagioclase-sanidine pairs) thermometry were made in the
Jackpot and Rabbit Springs Members. Using a pressure
constant of 5 kbar, we applied the geothermometers of
Ghiorso (1984) and Fuhrman and Lindsley (1988), using
the SOLVCALC 1.0 software of Wen and Nekvasil (1994).
Pre-eruptive temperatures estimated from geothermom-
etry are summarised in Fig. 12. Temperature estimates for
the Rabbit Springs, Brown’s View, Grey’s Landing and
Sand Springs Members are typically 850–1,000°C. Oxygen
fugacity estimates of −11 to −13 log units are reported from
the Brown’s View Member. The pre-eruptive temperature
of the Jackpot Member is significantly lower (≤800°C),
however, previous workers applying two-feldspar thermom-
eters to high-temperature rhyolites have urged caution,
citing concerns regarding pressure uncertainties and post-
emplacement alteration of crystals (e.g., Honjo et al. 1992;
Cathey and Nash 2004). In the absence of suitable Fe–Ti
oxide or pyroxene pairs we do not disregard these
temperature estimates but echo previous workers’ caution
in their interpretation.
Discussion
Stratigraphy of the Rogerson Graben
Jackpot member
Jackpot Member sub-units 1–5 and 7, resemble lavas
because they are massive, intensely flow-banded and
jointed (Fig. 3), and are devoid of pumice and lithic lapilli,
fiamme, and sedimentary structures. However, we interpret
them to be ignimbrites because: (1) they exhibit laterally
persistent internal stratigraphies; (2) they lack autobreccias,
either at the base and top of the member, or between sub-
units 1–5; and (3) they are too thin to be rhyolite lavas.
Laterally persistent divisions between sub-units are defined
by topographic breaks, changes in columnar joint size and
spacing, the upper vitrophyre of Jackpot 5; and lithophysae-
rich layers (Figs. 3 and 4a), which we interpret to record
breaks in deposition, rapid cooling, and volatile de-gassing
between emplacements. These divisions do not contain
Fig. 12 Comparison of estimated magmatic temperature and crystal
population with stratigraphic height in the Rogerson Formation.
Magmatic temperatures are seen to increase with height in each
member (grey arrows). Each data point represents the mean of 25 pairs
of analyses. Error bars are ± 50°C for the models of Fuhrman and
Lindsley (1988), Ghiorso (1984), Ghiorso and Sack (1991) and
Lindsley (1983). Errors bars are 2s for the model of Andersen et al.
(1993). n/d not determined
Bull Volcanol (2008) 70:269–291 283

autobreccias, the presence of which is diagnostic of large-
volume silicic lavas (Henry and Wolff 1992; Bonnichsen
and Kauffman 1987; Sumner and Branney 2002), and we
know of no silicic lavas in the Snake River Plain volcanic
province that lack them. Furthermore, silicic lavas in the
region tend to be thick (≥50 to ∼300 m; Bonnichsen and
Kauffman 1987), and tend not to form distal featheredges,
as ignimbrites do (Branney et al. 2007). We tentavtively
interpret NW–SE trending lineations to record emplacement
from the Bruneau–Jarbidge area to the WNW (Fig. 1).
‘Jackpot 6’ is the least intensely welded sub-unit and its
pyroclastic origin is clearly shown by the presence of cross-
stratification, lensoidal bedding, ash shards, obsidian lapilli,
and accretionary lapilli. We interpret this sub-unit to be an
ignimbrite deposited from a pyroclastic density current that
was cooler than those that emplaced sub-units 1–5 and 7.
We suggest that the base of sub-unit 6 fused when it was
deposited on top of the still hot sub-unit 5. The absence of
weathered surfaces, soils, exotic tephra layers or sediments
and vitrophyres between sub-units suggests that it repre-
Fig. 13 Mineral compositions in the Rogerson Formation. a Ternary
feldspar compositions for the Jackpot and Rabbit Springs Members.
Tie-lines between co-existing plagioclase and sanidine crystals. b
Ternary feldspar compositions for the Browns View, Grey’s Landing
and Sand Springs Members. Note the absence of sanidine. c Pyroxene
composition quadrilateral for the Rabbit Springs Member. Jackpot
Member pyroxenes were oxidized and not analysed. Tie-lines between
co-existing clinopyroxene and pigeonite crystals. d Pyroxene compo-
sition quadrilateral for the Browns View, Grey’s Landing and Sand
Springs members. Note the absence of augite in the upper parts of the
Grey’s Landing Member. Tie-lines between co-existing clinopyroxene
and pigeonite crystals. Thermal contours from Lindsley (1983) at
5 kbar
284 Bull Volcanol (2008) 70:269–291

sents a single eruptive unit of seven rapidly emplaced
ignimbrites, which in turn, welded, cooled and devitrified
together as a simple cooling unit (Smith 1960).
Undifferentiated volcaniclastic deposits
Both the undifferentiated volcaniclastic units are interpreted
as sequences of fallout ashes and related silt- and sand-grade
epiclastic deposits. Normal graded bedding and de-watering
structures are consistent with deposition of fallout ash into
standing water and onto soft substrate, and the presence of
ripples and erosional scours suggests that currents reworked
the deposit (e.g., Nakayama and Yoshikawa 1997). The
presence of palaeosols with calcified rootlets within both
sequences suggests periods of non-deposition, plant growth,
and pedogenesis, either in shallow water or exposed at the
surface. The sources of fallout ashes are unknown; signifi-
cant thicknesses of distally and medially sourced fallout
ashes are reported from basins throughout the Snake River
Plain and adjacent regions (e.g., Perkins et al. 1995; Perkins
and Nash 2002).
The lower volcaniclastic unit fills and buries several
graben developed in the Jackpot Member. We interpret that
sustained fallout ash deposition occurred after, and possibly
during, tectonic extension of the Jackpot Member and in the
absence of contemporaneous ignimbrite emplacement. The
early Rogerson Graben, and smaller graben within it, were
probably host to small ephemeral lakes and short-lived (thin
palaeosols) dry and vegetated high-stands.
Rabbit Springs Member
The Rabbit Springs Member is a high-grade, low aspect-
ratio ignimbrite. The absence of elongation lineations, flow
folds and autobreccias suggest it is not rheomorphic. We
interpret it as a single ignimbrite emplacement unit because
of its thinness, vitroclastic textures, and lack of internal
breaks (e.g., autobreccias, intercalated fallout ashes or
palaeosols). Furthermore, we interpret it is as a simple
cooling unit because of its simple welding profile (Smith
1960). Normal faults that partition thickness variations, but
do not deform the upper vitrophyre, are interpreted to be
extensional growth faults (e.g., Childs et al. 2003) suggest-
ing ignimbrite emplacement into an actively extending
graben.
Undifferentiated volcaniclastic rocks
The upper volcaniclastic unit appears to be discordant with
the underlying Rabbit Springs Member and may be
unconformable, although an erosion surface has not been
observed. Major planar erosion surfaces, possibly flooding
surfaces, cut palaeosols within the unit, which were then
buried by onlapping and discordant (west-dipping) volcani-
clastic packages, suggesting that the half-graben was
contemporaneously extending, and that the graben-floor
was often not horizontal. The presence of thick palaeosols
suggests sustained periods of non-deposition, plant growth
and pedogenesis, between major erosive events and
subsequent deposition of fallout ash.
Brown’s View Member
The Brown’s View Member is a high-grade, low aspect-
ratio ignimbrite. We interpret the presence of ubiquitous
vitroclastic textures, the thinness of the sheet, the simple
welding profile, and the absence of breaks (e.g., intercalat-
ed palaeosols, fallout ashes, sedimentary layers, and
autobreccias) as evidence for a single emplacement unit,
and simple cooling unit (e.g., Smith 1960).
Backwaters Member
We tentatively interpret volcaniclastic units A and B of the
Backwaters Member as non-welded ignimbrites because
they are massive, poorly sorted, and contain sub-angular to
sub-rounded pumice clasts. Furthermore, they appear to
contain a substantial juvenile component where the glassy
(and therefore possibly juvenile) ash matrix of unit B has
been fused. The erosive base of unit B is also consistent
with deposition from a pyroclastic density current. How-
ever, we cannot rule out deposition from debris flows (e.g.,
Smith 1986; Palmer and Walton 1990), an interpretation
consistent with massive and poorly sorted deposits con-
taining rounded pumice lapilli. The presence of palaeosols
indicates significant pauses in deposition, representing
periods of eruptive repose.
Grey’s Landing Member
We interpret the basal stratified ash as a contemporaneous
fallout ash, based on mantling of the substrate, very good
sorting, laterally continuous lamination and thickness, as
well as the absence of erosive features and internal trunca-
tions.We infer that the layers derive from the same eruption as
that which emplaced the overlying ignimbrite because of the
absence of an intercalated palaeosol or erosion surface.
Although the Grey’s Landing Member is largely lava-
like, we interpret it to be an ignimbrite on the basis of the
vitroclastic textures in the upper and lower vitrophyres, its
low aspect ratio, and the widespread absence of a basal
autobreccia. Flow folds and elongation lineations charac-
terize rheomorphic deformation (e.g., Schmincke and
Swanson 1967; Chapin and Lowell 1979; Wolff and Wright
1981). Measurements of lineation-trends and rotation
directions in crystals (Fig. 9a) suggest that, at least initially,
Bull Volcanol (2008) 70:269–291 285

the ignimbrite was emplaced from the east (Andrews 2006).
We interpret the upper breccia to record in situ brecciation
of welded ignimbrite in response to internal stress caused
by rheomorphic flow while the upper part of the deposit
started to cool and degas. We infer that the overlying
aphyric, orange ash is a penecontemporaneous fallout ash,
possibly co-ignimbritic, as it is partly fused against and
folded into the underlying welded ignimbrite (e.g., Walker
1983; Branney et al. 1992; Branney and Kokelaar 2002).
The origin of dimple-joints remains enigmatic; we interpret
them to be a devitrification feature because they are only
found in lithoidal rhyolite (e.g., Bonnichsen 1982b), and
suggest that each dimple may be a formed at an oblate
spherulite, originally surrounded by vitric rhyolite that has
subsequently devitrified.
We interpret the basal stratified-fall deposit, lava-like
ignimbrite and upper orange ash together as a simple
cooling unit (e.g., Smith 1960), based on fusing of the
upper and lower ash deposits by the ignimbrite and
deformation of the upper ash. Therefore, the ignimbrite
and both ashes are probably products of the same, single
eruptive event. More tentatively, we interpret the ignimbrite
to be a single flow unit based on the absence of any
evidence in the sheet for a flow hiatus, although it is
possible that such evidence would have been obscured by
rheomorphism.
Coyote Creek Member
Although somewhat enigmatic in origin, we interpret,
tentatively, the Coyote Canyon Member to be a non-welded
ignimbrite based on the presence of juvenile ash and its
massive appearance. However, once again we cannot rule
out deposition from a volcaniclastic debris flow (e.g., Smith
1986; Palmer and Walton 1990).
Sand Springs Member
The Sand Springs Member is clearly pyroclastic and
welded, based on eutaxitic vitroclastic textures and pro-
gressively flattened obsidian lapilli, and we interpret it as an
ignimbrite. It is a single emplacement unit and the simple
welding profile suggests it is a simple cooling unit.
Geochemical characteristics—petrogenesis
and geothermometry
Anhydrous, metaluminous rhyolites are typical of rhyolite
elsewhere in the Yellowstone–Snake River Plain volcanic
province (e.g., Hughes andMcCurry 2002; Cathey and Nash
2004), and are also characteristic of anorogenic A-type
granitoids in other magmatic provinces (Whalen et al.
1987). Ignimbrites of the Idavada Group (Malde and
Powers 1962) typically contain anhydrous crystal assemb-
lages of plagioclase, sanidine, augite, pigeonite, quartz, and
Fe–Ti oxides, and rarely, fayalite and orthopyroxenes (e.g.,
Bonnichsen and Citron 1982; Honjo et al. 1992). In
contrast younger ignimbrites in the eastern Snake River
Plain commonly contain Fe-rich amphibole (e.g., the Lava
Creek Tuff; Christiansen 2001) or biotite (e.g., the Arbon
Valley Tuff; Morgan and McIntosh 2005).
Those members of the Rogerson Formation that preserve
non-hydrated vitrophyre are composed of ash shards with a
restricted compositional range (73–77.5 wt.% SiO2). There is
no systematic variation in glass composition between or
within members suggesting a homogeneous and well-mixed
liquid before each eruption; moreover, these conditions
appear to have been repeated at least four times, before the
eruptions of the Rabbit Springs, Brown’s View, Grey’s
Landing and Sand Springs ignimbrites.
Crystal chemistry shows subtle variations within the
Rogerson Formation. Differences in the stratigraphic
distribution of crystal phases (e.g., abundant sanidine and
quartz; Fig. 12) and differences in crystal chemistry (e.g.,
oligoclase and andesine; Fig. 13) establish that the Jackpot
and Rabbit Springs members are significantly different
from the rest of the Rogerson Formation. Only the Grey’s
Landing Member shows any internal variation in crystal
assemblage where augite decreases with increasing height
and pigeonite remains uniformly abundant. We interpret
this to indicate subtle, vertical compositional zonation within
the Grey’s LandingMember, which in turn, may indicate that
it was erupted from a compositionally zoned magma
chamber (e.g., Hildreth 1979, 1981), with the most evolved
batch of liquid (augite = pigeonite) tapped first, followed by
progressively less evolved liquid (augite ≤ pigeonite), and
finally, the least evolved liquid (augite << pigeonite).
Pre-eruptive temperature
The extremely high-grade and rheomorphic nature, and the
lack of hydrous minerals in ignimbrites of the Rogerson
Formation, suggest high rhyolitic magmatic temperatures
(850–1,025°C). This is consistent with conclusions from
studies of other ignimbrites in the Snake River Plain
volcanic province (Ekren et al. 1984; Honjo et al. 1992;
Cathey and Nash 2004) and some large igneous provinces,
such as Etendeka-Parana (Bellieni et al. 1984; Milner et al.
1992; Kirstein et al. 2001), the Whitsunday volcanic
province, Australia (Bryan et al. 2000), the Keweenawan
rift, Minnesota (Green and Fitz 1993), and Trans-Pecos,
Texas (Henry et al. 1988).
There is a subtle increase in pre-eruptive temperature
estimate with height in each member (Fig. 12), best
displayed by estimates from the QUILF geothermometer
that displays the lowest uncertainties (≤±20°C). We
286 Bull Volcanol (2008) 70:269–291

interpret, tentatively, that this suggests compositional
zoning within the Rabbit Springs, Brown’s View, Grey’s
Landing and Sand Springs members, where each time a
cooler liquid is tapped first and followed by progressively a
hotter liquid from deeper within the magma chamber (e.g.,
Hildreth 1979, 1981).
Criteria for distinguishing lavas and lava-like ignimbrites
Investigations of lava-like rhyolites inevitably lead to dis-
cussion over emplacement mechanism: (1) as a lava, flowing
en masse from the vent; or (2) as an ignimbrite, deposited
from a pyroclastic density current. We have interpreted each
member as an ignimbrite based on several criteria: (1) unit
form and thickness, (2) textures preserved in vitrophyre, and
(3) the absence of basal autobreccias. Most rhyolite lavas in
the Snake River Plain are thick (≥50–300 m) and have aspect
ratios ∼1:102 (Branney et al. 2007). Moreover, they exhibit
widespread basal and carapace autobreccias (e.g.,
Bonnichsen 1982b; Henry and Wolff 1992; Manley 1996a),
and typically vitroclastic textures are rare and limited to
where autobreccia has been entrained into the flowing lava
(e.g., Manley 1996b). In contrast, lava-like ignimbrites are
thin (typically < 100 m thick) and have aspect ratios of ∼
1:103. Vitroclastic textures are ubiquitous, although they may
be obscured by rheomorphism and devitrification, and lower
autobreccias are very rare and localised (e.g., Sumner and
Branney 2002). Generally, the presence of abundant broken
crystals is characteristic of ignimbrites rather than of lavas
(e.g., Fisher and Schminke 1984).
Tectonic framework of the Rogerson Graben
We have begun to establish the tectonic history of the
graben using the thickness distributions of ignimbrites,
volcaniclastic sediments, and lavas; and crosscutting
relationships between stratigraphic members and faults;
back as far as the emplacement of the Jackpot Member
(>>10.37 Ma), beyond which we have no stratigraphic
or tectonic constraints. The evolution of the graben
can be divided into three phases: (1) initiation; (2)
syn-ignimbrite emplacement; and (3) post-ignimbrite
emplacement.
Several small, NNE-trending graben (≤10% E–W exten-
sion) developed in the Jackpot Member, and were infilled by
undifferentiated volcaniclastic sediments (Figs. 1 and 14).
The status of the graben-bounding Brown’s Bench fault at
this time is unknown. However, the orientation and scale of
the fault is consistent with major Basin and Range faults
further south in central Nevada, suggesting that it is an
inherited structure affecting both pre-Miocene basement
and Snake River Plain volcanics.
The extending graben-floor (NNE-trending growth faults)
was inundated by the west-thickening Rabbit Springs
Member (Fig. 14), suggesting the Rogerson Graben was an
asymmetric half-graben. Subsequent deposition of bedded
volcaniclastic sediments was interrupted with repose periods
(palaeosols), and graben-floor tilting and erosion (minor
angular unconformities). The Brown’s View, Backwaters
and Grey’s Landing members thickened westward against,
and buried, the Brown’s Bench Fault (onlapping and
overstepping older ignimbrites in the footwall; Figs. 1 and
14). The relationship between the Coyote Creek and Sand
Springs members and graben formation is unknown.
Graben development has continued since the deposition of
the Rogerson Formation, displacing late Miocene–Pliocene
basalt lavas by 40 m and an accompanying northward
propagation of the tip-point of the Brown’s Bench Fault.
In summary, the Rogerson Formation and overlying lavas
record ∼10Ma of episodic extension, graben-floor tilting, and
scarp burial, related to the formation of the Rogerson Graben.
The Rogerson Graben, therefore, shares many characteristics
(e.g., rapid in-filling by ignimbrites, long repose periods,
growth faults, prolonged extension) with basins along the
margins of the Snake River Plain (e.g., the Oakley and Raft
River basins, Idaho; Rodgers et al. 2002), and other volcanic
provinces associated with extensional tectons (e.g., exten-
sional arcs, Fackler-Adams and Busby 1998; Basin and
Range, Aguirre-Diaz and Labarthe-Hernández 2003). Basins
adjacent to the eastern Snake River Plain have been
interpreted to form through the localised reactivation of
Basin and Range normal faults by the migration of the
Yellowstone hot-spot (Anders et al. 1989; Rodgers et al.
1990, 2002), and we infer that the same scenario occurred
regarding the Rogerson Graben.
Conclusions
Eruption history
The Rogerson Formation is the product of prolonged,
explosive rhyolitic volcanism in the central Snake River
Plain. A history of ≥8 eruptions is recorded; the duration of
which is unknown but may have been c. 8–11 Ma. The
eruptive history of the Rogerson Formation, from oldest to
youngest is as follows.
1. A rapid succession of pyroclastic density currents
entered the Rogerson Graben, possibly from a source
near Bruneau–Jarbidge (lineation data; Fig. 1 inset)
and deposited a voluminous, lava-like ignimbrite
(Jackpot Member). A cooler phase of the eruption
produced non-welded, traction cross-stratified tuffs
with accretionary lapilli (‘Jackpot 6’) before a return
to higher-temperature emplacement (‘Jackpot 7’).
Bull Volcanol (2008) 70:269–291 287

2. Several NNE–SSW trending graben developed
(Fig. 1) forming local depocentres for fallout ashes
possibly erupted from the Bruneau–Jarbidge area, and
reworked to form bedded volcaniclastic sediments.
Periods of quiescence are recorded as soils.
3. Another ignimbrite eruption, possibly near Bruneau–
Jarbidge, emplaced the welded Rabbit Springs Mem-
ber (10.37 Ma±0.13). Contemporaneous graben ex-
tension is indicated by growth faults in the ignimbrite.
4. Continued regional extension led to subsidence and
westward tilting of the Rogerson Graben floor, while
the basin accumulated non-welded rhyolitic ash and
volcaniclastic sediment with several extended periods
of soil formation.
5. The next major pyroclastic density currents to enter
the graben may have had a different magmatic source,
possibly near Twin Falls (Fig. 1 inset). It deposited a
single cooling unit, the eutaxitic Brown’s View
ignimbrite.
6. Following a repose period (soil horizon) further
pyroclastic inundations deposited two, non-welded
ignimbrites or debris-flow deposits (Backwaters Mem-
ber) separated by another significant period of repose.
7. A large-volume, compositionally zoned, rhyolitic
explosive eruption deposited a 1.5 m stratified ashfall
layer and overlying 60 m thick rheomorphic ignim-
brite (the Grey’s Landing Member). The eruption may
have been from the northeast (kinematic criteria
indicate emplacement from the east), near Twin Falls.
The rheomorphic character of the ignimbrite reflects
hot magmatic temperatures (925–1,025°C).
8. Following repose periods (palaeosols) further explo-
sive eruptions deposited the non-welded Coyote Creek
ignimbrite and the welded Sand Springs ignimbrite,
possibly also from the north.
9. Extension of the Rogerson Graben with NNE- and
NNW-trending normal faulting and the eruption of at
least three late-Miocene basalt lavas (Bonnichsen and
Godchaux 2002).
10. Salmon Falls Creek incised a ≥ 50 m deep canyon into
the basalt lavas and Grey’s Landing ignimbrite (late
Miocene, Bonnichsen and Godchaux 2002).
11. The creek was dammed by basalt lava from Salmon
Butte (Salmon Dam, Fig. 1) and then subsequently
re-incised (late Pliocene–Pleistocene, Bonnichsen
and Godchaux 2002).
Fig. 14 Schematic cross-sections through the Rogerson Graben
(Fig. 1): (top) immediately after emplacement of the Rabbit Springs
Member. Note growth fault and burial of earlier graben; (bottom)
during emplacement of the Grey’s Landing Member. Note that the
Grey’s Landing Member onlaps and oversteps pre-graben ignimbrites
and the Brown’s Bench Fault scarp
288 Bull Volcanol (2008) 70:269–291

Relation to Yellowstone–Snake River Plain volcanic
province stratigraphy
We interpret the Rogerson Formation to record deposition of
fallout ashes and ignimbrites from the Yellowstone–Snake
River Plain volcanic province into the Rogerson Graben.
During the time represented by the Rogerson Formation the
graben was actively extending and propagating northwards,
becoming a major depo-centre no later than 10.37 Ma when
the Rabbit Springs ignimbrite was emplaced. Of the several
graben and half-graben developed along the margins of the
Snake River Plain (e.g., Rodgers et al. 2002) the Rogerson
Graben is the most westerly and contains the oldest graben-
fill succession reported (Fig. 1 inset).
Further examination of adjacent successions is required
to better constrain the distributions, ages, and sources of
members within the Rogerson Formation, however, we
attempt to draw some preliminary conclusions regarding the
stratigraphic significance of the formation. The Jackpot and
Rabbit Springs (≤10.37 Ma) Members are characteristically
sanidine and quartz-phyric; the presence of these crystal
types is typical of members of the contemporaneous Cougar
Point Tuff Formation (12.7–10.4 Ma) and associated
ignimbrites and lavas erupted from the Bruneau–Jarbidge
area (e.g., Bonnichsen 1982a, b; Cathey and Nash 2004).
Moreover, NW–SE trending lineations are consistent with
emplacement from the Bruneau–Jarbidge area (Fig. 1 inset).
In contrast, the Brown’s View, Grey’s Landing (7.62 Ma)
and Sand Springs Members are devoid of sanidine and
quartz, and the Grey’s Landing ignimbrite preserves
kinematic indicators suggesting emplacement from the east;
all criteria typical of eruptions from the Twin Falls area
(10.9–8.6 Ma; McCurry et al. 1996, Wright et al. 2002).
The Rogerson Formation, therefore, may record the
gradual migration of the locus of volcanism from the
Bruneau–Jarbidge area to the Twin Falls area following
the track of the Yellowstone hotspot (Fig. 1 inset), and the
final stages of the ‘ignimbrite flare-up’ period (11.7–
10.0 Ma) of Bonnichsen (2004). Furthermore, it records
contemporaneous basin development coincident with litho-
spheric stretching around the hotspot locus (e.g., Anders
et al. 1989) and the adjacent Basin and Range province
(e.g., Rodgers et al. 1990, 2002).
Acknowledgements This work forms part of NERC-funded PhD
research by GDMA at the University of Leicester (NERC Award
NER/S/A/2001/06292). Formal reviews by Cathy Busby and an
anonymous reviewer greatly improved the manuscript; early reviews
by Steve Temperley and Richard Brown are appreciated. Thanks to
Martha Godchaux, Sarah Brown, Steve Rippington and Karen
Colclough for discussion and help in the field, and Lawrence Coogan,
Rob Wilson, Mike Norry, and Andy Saunders for assistance with
geochemical analysis. Special thanks also to Mark Ghiorso and Craig
White.
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