IDAHO GEOLOGICAL SURVEY
MOSCOW-BOISE-POCATELLO
DIGITAL WEB MAP 137
PHILLIPS AND WELHAN
WWW.IDAHOGEOLOGY.ORG
GEOLOGIC MAP OF THE MENAN BUTTES QUADRANGLE,
JEFFERSON and MADISON COUNTIES, IDAHO
CORRELATION OF MAP UNITS
Alluvial Units
Artificial
Unit
Mass Movement
Units
Eolian Units
Units
m
Volcanic
Units
Qls
Qa
Qes
Qas
HOLOCENE
Qc
11.43 ka*
LATE
Qt
William M. Phillips and John A. Welhan
Qto
PLEISTOCENE
Qblg
?
Qg
Menan Buttes
Volcanic Complex
2011
QUATERNARY
Qtcb
Qtca
Qtu
Qtn
Qtns
Qtna
Qts
Qtss
Qtsa
Qtc
126 ka*
?
MIDDLE
Qto
Qa
PLEISTOCENE
Qblg
Qblg
INTRODUCTION
Qa
Older terrace alluvium of Egin terrace (late? to middle? Pleistocene)—
Medium sand and pebbly sand forming extensive fill terrace. Sand largely
obsidian but mixed with quartz and rhyolitic lithics near surface. Pebbles
consist of subrounded to rounded quartzite, obsidian, rhyolitic tuff, and
basalt (Kuntz, 1979). Generally lies beneath basalt of Little Grassy Butte
(Qblg). However, about 3 m (10 ft) of coarse fluvial sand correlated with
Egin terrace overlies Qblg in a now-closed landfill in sec. 26, T. 6 N., R. 38
E. (G. Embree in Ferdock, 1987, p. 45). Also, reported to be both under and
on top of Qblg in the Juniper Buttes area to the northeast (Kuntz , 1979). The
surface northeast of North Menan Butte in sec. 35, T. 6 N., R. 38 E. mapped
as Qes may be a partially dissected portion of Egin Bench covered by Holocene eolian sand and colluvium. Age of unit uncertain. May partially
record high discharge glacial outburst flooding in the headwaters of the
Henrys Fork during Bull Lake glaciation at ~140 ka (W. Scott quoted in
Allison, 2001, p. 18). Water wells indicate minimum thickness of 37 m (120 ft)
in map; thickness is 13-30 m (43-98 ft) in Juniper Buttes area (Kuntz, 1979).
Qto
Qto
Qa
This map depicts bedrock and surficial geological units in the Menan Butte
quadrangle. The area sits on the edge of the eastern Snake River Plain, a
major crustal downwarp associated with the Yellowstone hotspot. Late
Miocene–Pliocene rhyolitic volcanic rocks of the Heise Volcanic Field were
erupted in this portion of the Snake River Plain between 6.62–4.45 Ma as
the hotspot passed beneath the region (Morgan and McIntosh, 2005). At
2.06 Ma, when the hotspot was located near its present position, the Huckleberry Ridge Tuff was erupted from the Henrys Fork Caldera (Christiansen,
2001). In the Menan Buttes quadrangle, both the Heise rhyolites and the
Huckleberry Ridge Tuff are covered by Snake River alluvium and basaltic
lava flows. The map lies at the junction of the two major tributaries of the
Snake River. On the north, the Henrys Fork drains highlands largely underlain by rhyolitic caldera deposits of the Island Park-Yellowstone area. These
rocks tend to break down relatively quickly to form sandy alluvium rich in
obsidian. To the east, the South Fork originates in the Grand Teton area
underlain by Precambrian, Paleozoic, and Mezosoic rocks. These rocks
produce the diverse clast lithologies found in the South Fork. During the
course of at least two glaciations at ~140 ka and 25-13 ka (Licciardi and
Pierce, 2008), the Snake River transported enormous quantities of gravel
onto the Snake River Plain (Scott, 1982). In the map, the two streams filled
a subsiding basin with hundreds of feet of alluvium. When basaltic magma
erupted into this basin, steam explosions occurred that mixed quenched
magma with gravel and sand to form the tuff cones of Menan Buttes. These
structures are unusual for the Snake River Plain and are among the largest
tuff cones in the world (Ferdock, 1987). Following the formation of the tuff
cones, a voluminous lava flow erupted from Little Grassy Butte about 24 km
(15 mi) northwest of the map. This flow impinged onto the floodplain of the
Henrys Fork, causing the stream to move eastward. During the Holocene,
sand carried by northeast-directed winds was trapped in the craters of
Menan Buttes, and formed small dunes and sand sheets on the irregular
topography of the Little Grassy Butte lavas and on Egin Bench. The Henrys
Fork and South Fork (including small splays such as Texas Slough and
Bannock Jim Spring Slough) reworked alluvium and deposited new
sediments. On June 5, 1976, much of the area flooded when the Teton Dam
failed catastrophically (Thomas and others; 1976). This flood was about 100
times larger than any historic Snake River flood; hence it provides perspective on the effects of exceptionally large prehistoric events (Scott, 1977).
Qa
Qa
Qa
Qblg
Qa
Qa
Qa
Qa
Qblg
Qa
Qa
Qa
Qa
Qa
Qto
Qblg
Qls
Qa
Qa
Qes
The map is based upon compilation and consultation of master thesis
studies (Ferdock, 1987; Allison, 2001; Creighton, 1982), county soil surveys
(Noe, 1981; Jorgensen, 1979), regional geologic mapping (Scott, 1982),
domestic water well logs (available from Idaho Department of Water
Resources at http://www.idwr.idaho.gov/apps/appswell/searchWC.asp), and
field work conducted in 2007.
Qa
Qblg
Qa
Qto
Qblg
A
Colluvium (Holocene-late Pleistocene)—Massive, semi-indurated, brown to
tan, sand, cobbles, and pebbles in a clayey matrix. Composed of
sideromelane sand and rounded tuff fragments. Thickness 0.5-2.5 m (1.6 8.6 ft). Best developed on flanks of the Menan Buttes. Includes alluvial
fans composed of bedded silt and sand on northeast sides of North and
South Menan Butte.
Qls
Landslide (Holocene-late Pleistocene)—Rotational slump of older terrace
alluvium of Egin (unit Qto) onto the floodplain of Henrys Fork. Scarp has
been modified by road construction.
Artificial fill (Holocene)—Landfill (garbage dump).
Qa
ALLUVIAL UNITS
m
Qa
Alluvium of active channels and floodplain of the Snake River and main
tributary streams (Holocene)—Sand, gravel, and sandy silt. On Henrys
Fork and South Teton River, sand consists of quartz, black obsidian and
rhyolitic lithic grains while gravel clasts consist of rhyolite, basalt, and lesser
quartzite, sandstone, and granitic cobbles. Thickness <10 m (33 ft). On
South Fork of Snake River, dominated by hard, well-rounded quartzite
cobbles with lesser sandstone, basalt and limestone. Forms small islands
and bar-tops exposed at low water levels. Subject to flooding and high
water tables during spring and early summer. Parent material for poorly
drained, channeled Haplaquolls soils (Noe, 1981; Jorgensen, 1979).
Qa
Qt
Qa
m
Qa
Qes
Qa
Qa
Qtns
Qa
Qa
Qas
Alluvium of side streams (Holocene)—Gravel, sand, and sandy silt. Forms
islands, and bar-tops and beaches exposed at low water levels; also consists
of deposits in numerous relic channels. Thickness <10 m (33 ft). Side
streams of the South Fork of the Snake River are dominated by quartzite
cobbles and lesser sandstone, metamorphic and granitic rocks, while side
streams of Henrys Fork contain obsidian sands and rhyolite cobbles as well
as lesser quartzite, sandstone, and granitic rocks. Subject to flooding and
high water tables during spring and early summer.
Qt
Terrace alluvium of the Snake River and tributary streams (late Pleistocene)—
Sand and gravel similar in clast composition to unit Qa; forms fill terraces
separated by 1.5-3 m (5-10 ft) scarps from flood plain and active channels
of the Henrys Fork and South Fork of Snake River. Terrace riser height generally increases to the north along the Henrys Fork. Terrace surfaces have
gentle northwest slope indicating source of most terrace alluvium is South
Fork of Snake River. Thickness uncertain because unit cannot be distinguished from older or younger alluvial units in water well logs. Minimum
thickness about 10 m (33 ft). Unit interpreted to have been deposited during
period of waning discharge and stream incision during termination of
Pinedale glaciation at ~13-14 ka. Parent material for the Blackfoot,
Labenzo, Heiseton, and Harston soils (Noe, 1981; Jorgensen, 1979).
Locally poorly drained with water levels <1.5 m (<5 ft) from surface, as
indicated by soils with aquic textures.
Qa
Qtn
32
Qc
16
2
Qtn
14
Qtn
Qa
13
20
13
Qtn
Qg
20
Qtn
Qtns
11
Qtns
Qtn
Qtn
Qc
34
15
2
Qc
32
Qtns
Qtn
Qa
Qt
Qas
Qa
Qas
Qt
Qas
Qa
Qtna
Qtns
Qtn
Qes
Qas
Qas
Qa
Qt
3
Qtn
Qtns
Qtns
Qas
13
Qtna
21
Qtns
Qas
Qa
6
Qtn
Qtna
Qtn
Qa
Qas
Qas
Qa
Qtn
15
Qc
Qc
Qt
4
35
Qblg
Qtns
Qtn
Qas
Qa
Qa
23
Qt
Qa
Qa
Qa
13
Qtns
31
Qtns
Qtna
Qa
Qa
5
Qas
Qa
12
Qa
12
Qtna
Qtn
Qtn
Qtn
Qtn
Qtns
Qtn
Qtns
Qtn
Qtn
Qc
4
5
10
Qtu Qtn
Qtn
Qtu
Qtu
Qtc
Qa
15
Qa
Qts
Qts
Qtn
Qtca
8
Qc
Qc
21
31
Qa
17
11
Qa
Qts
C
22
Qes
12
Qa
Qa
Qa
Qt
Qc
Qa
Qa
33
Qa
Qa
Qa
Qt
Qt
Qc
Tuff of North Menan Butte (late Pleistocene)—Indurated, gray-green to
brown, poorly sorted, massive to thin bedded, palgonitic, lapilli tuff to fine
tuff. Average grain sizes of tuff decrease from medium to coarse ash on vent
interior, to fine ash in distal deposits. Accidental lithics composed of basalt
are rare; quartzite and sandstone also decrease in grain size from proximal
to distal deposits. The concentration of accidental lithics on North Menan
Butte is the lowest in the Menan Volcanic Complex. Thin sections of tuff
average 56.5 percent angular sideromelane, 27.6 percent pore space, 8.5
percent tachylite, 2.6 percent phenocrysts (1.1 percent plagioclase and 1.5
percent olivine), 4.3 percent palagonite, and 0.5 percent accidental
material. Along the crater rim and more rarely along slopes, cobble- to
boulder-sized accidental basalt clasts are concentrated by erosional
processes that remove surrounding tuff. Many of the larger accidental clasts
display ventifacting. Where the tuff is well exposed and undisturbed by
redeposition, reverse and normally graded beds are present. Most beds are
planar, with rare mantle and cross-bedding structures, and lobate beds of
unarmored vesicular lapilla interbedded with fine tuff. Armored and accre-
Qtn
Qtn
Qa
Qas
Qes
AC KN OW L E D G M E N TS
We thank the landowners in the area for access to their property. K.
Othberg (IGS) and C. Kersey (University of Idaho) assisted with paleomagnetic sample collection and analysis. G. Embree (BYU-Idaho) led several
helpful field trips to the Menan Buttes Volcanic Complex.
5,600
South Menan Butte
Qtn
Qa
Qtn
Qtc
Qts
Qtsa
sand and gravel
gravel and sand
4,600
clay
basalt
sand and gravel
?
?
basalt
?
clay
gravel and sand
4,400
gravel and sand
?
basalt
?
basalt of vent
(projected)
basalt of vent
(projected)
basalt of vent
clay
?
4,200
?
4,000
4x vertical exageration
Qas
Qa
R
KE
PA
R
O
AN
G
UR
XB
RE
M
BU EN
TT AN
ES
FEET
RI
RI
IS
W
IDAHO
Contour interval 10 feet
E
KILOMETER
Y
1
7000
GB
0
6000
RI
5000
PL
M
LA ARK
KE E
NE T
4000
E
3000
VI
0.5
2000
MILE
LL
1000
1
D
PA EE
RK R
S
0
QUADRANGLE
LOCATION
ADJOINING QUADRANGLES
B’
6,000
6,000
5,800
5,600
Qtn
5,400
5,400
FEET
Qg
n
D
l
α95
4,600
clay
07P020
Qblg
43.82689 -112.04051
7
334.6
67.9
2.6
Strike and dip of tuff.
4,200
Strike and dip of overturned tuff.
07P021
Qblg
43.81417 -112.00854
8
322.5
64.9
4.0
basalt
basalt
basalt of vent
4,000
C
C''
5,400
C'
slumped vent
deposits
5,000
40
5,200
profile at
maximum
extent
Qtca
Qtc
N
5,400
Central Menan Butte
5,200
189.1
Modified from Ferdock (1987).
4x vertical exaggeration.
Extent of 1976 Teton Dam flood (Thomas, Ray, and Harenberg, 1976).
20
basalt
sand and gravel
Crater rim.
N
5,000
Qtc
4,000
4,800
clay
gravel and sand
Qtc
sand and gravel
4,600
4,600
gravel and sand
basalt
n = number of oriented cores.
D = site mean declination of characteristic remnant magnetism.
I = site mean inclination of characteristic remnant magnetism.
α95 = confidence limit for the mean direction at the 95% level.
k = precision parameter.
N = normal polarity.
4,400
sand and gravel
basalt
basalt
4,400
clay
basalt
basalt
gravel and sand
basalt
sand and gravel
4,200
4,400
4,200
basalt of vent
4,000
Published and sold by the Idaho Geological Survey
University of Idaho, Moscow, Idaho 83844-3014
4x vertical exageration
4,600
basalt
basalt
basalt
clay
sand and gravel
clay
sand and gravel
Landslide block: slump blocks of tuff (Qtn and Qts) on flanks of North
and South Menan Buttes.
540.9
Qtn
sand and gravel
sand and gravel
sand and gravel
Horizontal tuff.
Demag
level (mT)
Qblg
4,000
gravel and sand
gravel and sand
4,400
Polarity
k
Qes
Qtn
FEET
Latitude Longitude
5,000
Qtna
Modified from Ferdock (1987).
4,000
FEET
Unit
5,200
4,200
4,000
FEET
5,000
Table 1. Paleomagnetic data for basalt of Little Grassy Butte.
Samples are from the Deer Parks quadrangle.
Site
number*
slump folds
Qtn
Normal fault: ball and bar on downthrown side.
12
Qtna
Qes
4,800
Contact: dashed where approximately located.
3
5,800
5,600
5,200
SYMBOLS
profile at
maximum
extent
North Menan Butte
Qa
Noe, H.R., 1981, Soil survey of Madison County area, Idaho: U.S. Department
of Agriculture, Soil Conservation Service, 128 p., 29 map plates, scale
1:24,000.
Phillips, W.M., T.M. Rittenour, and Glenn Hoffmann, 2009, OSL chronology of
late Pleistocene glacial outwash and loess deposits near Idaho Falls, Idaho:
Geological Society of America Abstracts with Programs, v. 41, p. 12.
Rittenour, Tammy, and H.R. Pearce, 2009, Drought and dune activity in the
Idaho Falls dune field, Snake River Plain, southeastern Idaho: Geological
Society of America Abstracts with Programs, v. 41, no. 7, p. 619.
Scott, W.E., 1982, Surficial geologic map of the eastern Snake River Plain and
adjacent areas, 111º to 115º W., Idaho and Wyoming: U.S. Geological
Survey Miscellaneous Investigation Series Map I-1372, scale 1:250,000.
Scott, W.E., 1977, Geologic effects of flooding from Teton Dam failure, southeastern Idaho: U.S. Geological Survey Open-File Report 77-507, 11 p., 1
plate, scale 1:48,000.
Thomas, C.A., H.A. Ray, and W.A. Harenberg, 1976, Teton dam flood of June
1976, Menan Buttes quadrangle, Idaho: U.S. Geological Survey Hydrologic Investigation HA-570.
Modified from Ferdock (1987).
B
Field work conducted 2007.
This geologic map was funded in part by the U.S. Geological Survey
National Cooperative Geologic Mapping Program,
USGS Award No. 07HQAG0070.
Digital cartography by Collette Gantenbein,Theresa A. Taylor,
Loudon R. Stanford, and Jane S. Freed at the Idaho Geological Survey’s
Digital Mapping Lab.
Reviewed by J.D. Kauffman, Idaho Geological Survey.
Map version 10-11-2011.
PDF (Acrobat Reader) map may be viewed online at
www.idahogeology.org.
5,000
4,000
4,000
0.5
1
5,200
Qes
Central Menan Butte
Qa
Qa
0
?
5,400
slumped vent
deposits
Qts
4,200
REFERENCES
Allison, R.R., 2001, Climatic, volcanic, and tectonic infuences on late Pleistocene sedimentation along the Snake River and in Market Lake: Bonneville,
Jefferson, and Madiison counties, Idaho: Idaho State University M.S. thesis,
153 p.
Christiansen, R.L., 2001, The Quaternary and Pliocene Yellowstone Plateau
volcanic field of Wyoming, Idaho, and Montana: U.S. Geological Survey
Professional Paper 729-G, p. G1-G145.
Creighton, D.N., 1982, The geology of the Menan Complex, a group of
phreastmagmatic constructs in the eastern Snake River Plain, Idaho: The
State University of New York, University at Buffalo M.S. thesis, 76 p.
Ferdock, G.C., 1987, Geology of the Menan Volcanic Complex and related
volcanic features, northeastern Snake River Plain, Idaho: Idaho State
University M.S. thesis, 171 p., geologic map and cross-sections, scale
1:12,000.
Forman, S.L., and J. Pierson, 2003, Formation of linear and parabolic dunes on
the eastern Snake River Plain, Idaho in the nineteeth century: Geomorphology, v. 56, no. 1-2, p. 189-200.
Gaylord, D.R., J.J. Coughlin, A.J. Coleman, M.R. Sweeney, and R.H. Rutford,
2000, Holocene sand dune activity and paleoclimates from Sand Creek, St.
Anthony dune field, Idaho: Geological Soceity of America Abstracts with
Programs, v. 32, no. 5, p. 10.
Jorgensen, Wendell, 1979, Soil survey of Jefferson County, Idaho: U.S. Department of Agriculture, Soil Conservation Service, 219 p., 66 map plates,
scale 1:20,000.
Kuntz, M.A., 1979, Geologic map of the Juniper Buttes area, eastern Snake
River Plain, Idaho: U.S. Geological Survey Miscellaneous Investigations
Map I-1115, scale 1:48,000.
Licciardi, J.M., and K.L. Pierce, 2008, Cosmogenic exposure-age chronologies
of Pinedale and Bull Lake glaciations in greater Yellowstone and the Teton
Range, USA: Quaternary Science Reviews, v. 27, p. 814-831.
Morgan, L.A., and W.C. McIntosh, 2005, Timing and development of the Heise
volcanic field, Snake River Plain, Idaho, western USA: Geological Society
of America Bulletin, v. 117, no. 3/4, p. 288-306.
Tuff of Menan Buttes, undivided (late Pleistocene)—Indurated, gray-green
to brown, poorly sorted, massive to thin bedded, palagonitic, lapilli to fine
tuff. Cannot be reliably correlated with eruptive source.
Qtu
Qt
Qt
Qa
1000
UTM Grid and
1979 Magnetic North
Declination at Center of Map
Agglutinate spatter of Center Menan Butte (late Pleistocene)—Poorly
exposed, broad, circular mound, 250 m (830 ft) in diameter, 11 m (36 ft)
high. Composed of oxidized, welded, scoriaceous spatter bombs and
pillow lava fragments. Interpreted to be remnants of late-stage spatter
ramparts and pillows formed when magma erupted into small lake occupying Center Menan Butte.
basalt of vent
Qtn
4,400
Qa
1
Qblg
sand and gravel
Qa
Qa
Qg
Qa
Qa
Qas
Qblg
4,600
Qa
SCALE 1:24,000
17
Qtca
Qtna
Qa
LE
0 39
Explosion breccia of Center Menan Butte (late Pleistocene)— Unconsolidated lapilli and block-sized, angular clasts of dense black, and red/black
scoriaceous basalt fragments, 1-6 cm thick. Shown as stipple pattern where
found only within colluvium. Interpreted to be remnants of lava lake
destroyed by explosion.
A'
Qtna
4,000
Qa
Qa
GN
o
Qtcb
Menan Buttes, Undivided
5,000
Qg
Qa
Qa
MN
o
Tuff of Center Menan Butte (late Pleistocene)—Indurated gray-green to
tan, lapilli to fine, palagonitic tuff. Bedding is weakly developed, thin to
medium, and planar. The tuff grades from coarse lapilli tuff on interior of the
cone to medium tuff on the outer flanks. Tuffs on the interior of the cone are
composed of 35 percent fresh black, rounded, scoriaceous to dense lapilli;
10 percent angular to rounded, sand to boulder-sized accidental lithics of
quartzite and dense basalt, and 55 percent tan to dark gray-green ash
matrix. Accretionary lapilli and vesicles within the tuff are rare. Scattered
on the surface of the tuff beds are large accidental lithics of broken cobbles
of red, green, and white quartzite, and broken boulders of vesicular basalt.
slumped vent
deposits
A''
Base map scanned from USGS film positive, 1979.
Shaded elevation from 10 m DEM.
Topography from aerial photographs by Kelsh plotter and by
plane-table surveys 1951. Aerial photographs taken 1950.
Revisions from aerial photographs taken 1976 and other source
data. Map edited 1979. Not field checked.
Projection: Idaho coordinate system, east zone (Transverse
Mercator). 1927 North American Datum.
10,000-food grid ticks based on Idaho coordinate system, east
zone.
1000-meter Universal Transverse Mercator grid ticks, zone 11.
Qtc
A''
?
Qa
Qc
Altered tuff of South Menan Butte (late Pleistocene)—Massive, orange to
brown palagonitic tuff, similar to unit Qtna. Restricted to exposures in the
southwestern inner crater but inferred to be more common within the butte
at depth.
North Menan Butte
Qa
Qa
Qts
12
11
Qa
Qa
Qa
Qts
Qc
Qa
Qa
37
Qtss
Qa
Qts
9
Qtsa
Qc
8
5
50
Qc
Qtsa
25
Qa
Qa
Qc
17
25
Qa
3
Qc
22
11
23
Qtsa
North Menan Butte
Qa
Qts
Ash tuff of South Menan Butte (late Pleistocene)—Black, thinly laminated
to thinly bedded, cross- to planar-bedded, moderately sorted, fine to
medium sideromelane ash. Similar to unit Qtna but less common than on
North Menan Butte.
Center Menan Butte is a partially eroded and poorly exposed tuff cone
covering an area of about 1.8 km2 (1.1 mi2) and rising about 42 m (135 ft)
above the surrounding landscape. Tuffs from both North and South Menan
Buttes partially bury the cone. It has the largest rim crater diameter of any
of the Menan Butte Complex structures as well as a unique, late stage
agglutinate ring and explosion breccia inferred to mark the position of lava
erupted into a small lake.
The age of Menan Volcanic Complex is not precisely known. It probably
falls between 140 and 10 ka. The complex was erupted into water-saturated
sediments of late Pleistocene age formed from the outwash of glaciers in the
headwaters of the Henrys Fork and South Fork drainages. These glacial
deposits date to the Bull Lake and Pinedale glaciations at ~140 ka and
~22-14 ka (Licciardi and Pierce, 2008), indicating an age for the complex
of less than 140 ka. This is supported by water well logs showing that ash
deposits from the Menan Buttes Complex appear to lie upon alluvial deposits of Egin Bench (unit Qto). North of the map, the Egin Bench deposits
contain black obsidian gravels (Kuntz, 1979) thought to have formed from
glacial outburst flooding along the Henrys Fork during the Bull Lake glaciation (W. Scott quoted in Allison, 2001, p. 18). The Menan Butte deposits lie
beneath basalt lava flows erupted from Little Grassy Butte (unit Qblg),
believed to be ~10-20 ka.
Qa
Qc
22
14
6
Qtn
Qtss
5,200
Qc
22
33
Tuff of South Menan Butte (late Pleistocene)—Indurated, gray-green to
brown, poorly sorted, massive to thin bedded, palgonitic, lapilli tuff to fine
tuff. Similar to tuff of North Menan Butte except for accidental lithic
concentration. Tuff of South Menan Butte contains more than 1.5 times the
accidental lithics by volume as tuff of North Menan Butte. Accidental clasts
are most common on the rim and interior of the crater and tend to be
concentrated in thin beds with large lapilli of juvenvile vesicular basalt. The
basalt lapilli have nearly the same petrographic composition as those of
North Menan Butte. The accidental clasts range from cobble to sand size for
quartzite clasts, and boulder size (as large as 1.5 m) for vesicular basalts.
Most of the accidentals show signs of breakage. The quartzite-to-basalt ratio
is about 4:1. Minor amounts of granite, gneiss, and rhyolitic tuff are also present.
FEET
B
14
Qts
Center Menan Butte
The Menan Volcanic Complex consists of phreatomagmatic tuff cones
produced by the injection of basaltic magma into alluvial sediments and
basalts of the Snake River Plain aquifer. Magma quenching and steam
explosions produced tuffs composed of basaltic glass (sidermelane and
trachylite), hydrothermally altered glass (palagonite), and phenocrysts of
plagioclase and olivine. Fragments of the sediments and basalts underlying
the cones (accidental lithics) together with bombs of quenched basalt and
clumps of pebble-sized tephra forming small balls (accretionary lapilli)
were also incorporated into the tuffs. Slumping and faulting of the cones
occurred both during and shortly after eruptions along with hydrothermal
alteration of the tuff. The volcanic edifices define a north-northwesttrending lineament that probably mirrors basalt dike orientations. Prevailing winds during the eruptions caused the cones to be elongated to the
northeast. Mapping and unit descriptions of the Menan Volcanic Complex
are taken from the detailed study of Ferdock (1987).
5,400
Qc
4
13
Qa
Qa
Qc
Qc
Basalt of Little Grassy Butte (late Pleistocene)—Gray to dark-gray, porphyritic
to nonporphyritic, tube-fed pahoehoe lava flows erupted from Little Grassy
Butte, about 24 km (15 mi) northwest of Menan Butte quadrangle (Kuntz,
1979). Consists of rare phenocrysts of olivine (1 mm) and plagioclase (3
mm) in a diktytaxitic groundmass of plagioclase, olivine, and augite crystals
(<0.5 mm). Pressure ridges and tumuli as much as 9 m (30 ft) in height with
well-preserved pahoehoe flow surfaces are surrounded by local accumulations of eolian sediment. Well-drained loam soils have formed on these
deposits (Mathon-Modkin-Bondranch complexes; Noe, 1981). Water well
logs and exposures at edges of unit indicate flow thicknesses of 2-10 m (635 ft). Paleomagnetic measurements show the unit to have normal polarity
(Table 1). Unit is undated; possibly 10-20 ka (Kuntz, 1987, quoted in
Ferdock, 1987, p. 45). The basalt overlies ash deposits of Menan Butte
Complex and both underlies and overlies alluvium of Egin Bench (unit Qto;
Kuntz, 1979).
5,600
Qas
Qa
Qc
Qc
18
South Menan Butte
A
Qc
Qc
Qc
18
Altered tuff of North Menan Butte (late Pleistocene)—Massive, brittle,
relatively featureless, orange to brown palagonitic tuff. Found on the southwestern rim and locally within the crater; interpreted to form large portion
of the cone at depth (see cross section A-A’). In contrast to the craggy, alveolar weathering of the unaltered tuff, the altered tuff weathers to smooth,
exfoliating slopes punctuated by occasional accidental basalt clasts.
Composed of about 50 percent orange-yellow palagonite, and medium
ash-sized brown sileromelane and black tachylyte coated with palagonite.
Relative to unaltered ash, pore space has been reduced to about 12
percent. Contacts between unaltered and altered ash are usually sharp. The
contacts cross bedding and locally show control by fractures.
Qa
8
7
A'
Qts
Qas
Qa
Qtcb
C'
Qtc
Qtn
Qc
Qtc
Qtc
Qtc
Qt
Qa
Qa
Qtn
Qtc Qtc
Qtc
Qtc
Qtn
4
Qas
C''
FEET
Qa
Qtna
Menan Volcanic Complex
Alluvium of Snake River outwash (late Pleistocene)—Gravel and sand
composed dominantly of very hard pink, purple and gray quartzite with
lesser rhyolite, basalt, sandstone, gneiss, and granitic rocks. Poorly exposed
in map. Exposures in nearby gravel pits indicate unit is thickly planar- to
cross-bedded, separated locally by thin, cross-bedded sand layers. Gravel is
mostly pebble- to cobble-sized, clast-supported, locally normally graded
and imbricated. Gravel framework is filled by fine to medium sand
composed of subangular black obsidian, quartzite, quartz and feldspar
crystals, muscovite, and fragments of basalt and rhyolite. Sand beds are
locally black because of high obsidian content. Water well logs suggest
minimum thickness of ~50 m (164 ft). Thickness uncertain because possible
older units cannot be reliably separated in water well logs. Unit is part of
the regional braided-stream outwash plain deposited during the Pinedale
glaciation by meltwaters from the Snake River headwaters (Scott, 1982).
OSL ages between 25.2 ka and 12.6 ka (Phillips and others, 2009) are
consistent with cosmogenic surface exposure ages of Pinedale-age
moraines in the Yellowstone headwaters (Licciardi and Pierce, 2008).
Qg
Qa
Qa
Qblg
ARTIFICIAL UNIT
m
B'
Ash tuff of North Menan Butte (late Pleistocene)—Black, thinly laminated
to thinly bedded, cross- to planar-bedded, moderately sorted, fine to
medium sideromelane ash. Ash has the appearance and consistency of
sand. Locally contains small channels, stoss and lee structures, rip-up
clasts, armored lapilli, and sag structures. Also occurs as thin interbeds
within the main tuff unit and at distances well away from the vent. Total
thickness varies with position relative to the northeastward dispersal direction of tephra from the cone. At least 50 m (160 ft) thick on the northeast
flank. Composed of 59.2 percent silderomelane, 34.9 percent open spaces,
2.7 percent black tacylyte, 2.9 percent olivine and plagioclase phenocrysts,
0.4 percent accidental lithics, and trace palagonite. Interpreted to represent
dry surge eruptions. North of Menan Butte in an abandoned quarry at
SE1/4, NW1/4, sec. 34, T. 6 N., R. 38 E., about 1 m (3 ft) of planar-bedded
black tuff is exposed beneath basalt of Little Grassy Butte (unit Qblg). The
tuff is reddened for a thickness of about 30 cm by baking from the basalt. A
similar contact can be viewed at SW1/4, SW1/4, sec. 26, T. 6 E., R. 38 E.
The contact between Qblg and about 1 m (3 ft) of the black ash is also
present in the Lower Teton Observation Well #1 (IDWR Permit number
818955) drilled by the U.S. Bureau of Reclamation in SW1/4, NE1/4, sec.
25, T. 6 N., R. 38 E.
VOLCANIC ROCKS
DESCRIPTION OF MAP UNITS
Qa
Dunes and sand sheets (Holocene)—Loose, tan to brown, medium sand.
Composed of rounded quartz, obsidian, sideromelane, and basaltic tuff
grains. Forms active small dunes and sand sheets on the crater floors of the
Menan Buttes where it is also bedded with pebbly colluvium. At least 6 m
(20 ft) thick in crater of South Menan Butte. Northeast of North Menan Butte
in sec. 35, T. 6 N., R. 38 E., silty-sand covers the low-relief surface lying
topographically above the active floodplain of Henrys Fork. Parent material
for the Mathon sandy loam soil (Noe, 1981). Undated; assigned a Holocene
age based upon regional studies of sand dune activity (Gaylord and others,
2000; Forman and Pierson, 2003, Rittenour and Pearce, 2009).
Qc
Qa
Qa
Qtns
MASS MOVEMENT UNITS
SOURCE OF DATA
Qa
tionary lapilli as large as 2 cm are commonly found about midway down
outer flanks, particularly on the northwest and southeast flanks. Vesicular
lapilli of juvenile basalt are also present. Thin sections of the lapilli average
67.4 percent black opaque glass groundmass, 28.7 percent vesicles, 3.9
percent phenocrysts (2.5 percent plagioclase and 1.4 percent olivine), 0.9
percent palagonite, and 0.2 percent accidental lithics.
EOLIAN UNITS
Qa
Qa
781 ka*
*Stage boundaries of the Pleistocene from Gradstein, F.M., J.G. Ogg, A.G. Smith, Wouter Bleeker, and L.J. Lourens,
2004, A new geologic time scale, with special reference to Precambrian and Neogene: Episodes v. 27 no. 2, p. 83-100.