Download Geologic Map of the Baker Quadrangle, Lemhi County, Idaho: DWM

IDAHO GEOLOGICAL SURVEY
MOSCOW-BOISE-POCATELLO
DIGITAL WEB MAP 141
OTHBERG AND OTHERS
WWW.IDAHOGEOLOGY.ORG
GEOLOGIC MAP OF THE BAKER QUADRANGLE, LEMHI COUNTY, IDAHO
CORRELATION OF MAP UNITS
Artificial Deposits
Glacial Deposits
m
Kurt L. Othberg, Reed S. Lewis, Loudon R. Stanford, Russell F. Burmester, and Mark D. McFaddan
Qlsa
Qam
Qas
2011
Qgt
Twc
Qtg 4
Twc
Qtg 2
Twc
15
Tkg
25
Qtg 3
m
Qtg 3
Qafo
Twc
Qas
Qtg 3
15
Qaf
Qam
Qafo
45
50
Tkg
Qtg 2
40
20
Qlsa
18
Tkg
Qafo
Qaf
m
m
SIN
30
ME
NT
15
Qas
Qaf
Qtg 4
18
52
20
30
Qtg 2
Qtg 4
The map is the result of our research and field work in 2009 and 2010, and
previous research in the region by Anderson (1957 and unpublished
mapping), Tucker (1975), Harrison (1985), and Blankenau (1999). Many
concepts for geologic units were developed while mapping the adjacent
Goldstone Mountain quadrangle (Lewis and others, 2011), part of a
1:24,000-scale collaborative mapping project started in 2007 by the Idaho
Geological Survey and the Montana Bureau of Mines and Geology.
Attitudes from previous mapping by Anderson (1957 and unpublished
mapping), Tucker (1975), and Blankenau (1999) were used to supplement
the structural data collected by the authors. Soils information is from Hipple
and others (2006).
16
Tkg
Qas
Qafo
Qaf
Qtg 2
Qas
Qaf
Qafo
15
15
20
20
Qtg1
Qaf
Tmc
Qtg 4
15
Twc
Qtg 2
Qaf
15
25
Qaf
Qtg 6
14
24
21
Tkg
Qtg 5
Qaf
Qlsa
Qas
Tsc
Twc
Qaf
22
Qls
Qafo
Tkg
Qtg 2
Qaf
Qas
Qtg1
20
16
Tcty
25
15
Qaf
19
19
24
Qas
45
Qaf
Qtg1
Ti
26
80
Ysq
Tlm
Ttcr?
24
Qtg1
20
14
35
Qafo
Qtg 3
Qtg 3
30
15
15
Qls
20
Tmcb
40
40
Qaf
Tmc
Qaf
Tlm
20
37
40
Qtg1
Qas
Qtg 2
15
Tmcb
25
21
Qaf
Tsc
Tmc
Qtg1
Qaf
Qtg 3
Tqs 2
63
Tsc
Qaf
Qlsa
Qtg1
35
Qaf
A'
Qafo
Qtg 3
Tldac
Tqt
LT
Qalc
35
Qaf
Qas
15
Tsc
Tlm
Tlm
Qaf
30
15
Qtg 2
Tkg
G
Qtg 2
1
0.5
3000
4000
5000
6000
0
1
7000
LD
PA STO
SS N
E
GO
GO
M LDS
OU T
NT ON
AI E
N
2000
MILE
OU SAL
NT
AI
N
1000
FEET
KILOMETER
IDAHO
Contour interval 40 feet
Supplementary contour interval 20 feet
QUADRANGLE LOCATION
AG
CR EN
EE CY
K
UTM Grid and
1989 Magnetic North
Declination at Center of Map
0
1
M
1000
0
PO
PE ISO
AK N
0.5
H
SP AN
RI NO
NG N
EA
SA ST
LM OF
ON
SCALE 1:24,000
BO
Tcty
MN
1
Qafo
Field work conducted 2009 - 2010.
This geologic map was funded in part by the U.S.
Geological Survey National Cooperative Geologic
Mapping Program,
USGS Award No. G10AC00223.
Digital cartography by Collette Gantenbein 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.
ADJOINING QUADRANGLES
A’
5,000
Qaf
Fault G
Tkg
Qas
Fine-grained alluvial and lacustrine deposits (Holocene to Pleistocene)—Silt,
sand, and minor gravel deposited in closed depressions. These sediments
are 1-12 m (3-40 ft) thick.
Qaf
Alluvial-fan and debris-flow deposits (Holocene to late Pleistocene)—West
slope of Beaverhead Mountains steep valley-side alluvial and debris-flow
fans. Angular to subrounded, poorly sorted, matrix-supported pebble to
boulder gravel in a sand, silt, and clay matrix. In the foothill slopes alluvial
fans are primarily composed of sand, silt, and clay with common matrixsupported gravel clasts. Commonly grades into, interfingers with, and caps
side-stream alluvium (Qas). Thickness of deposits varies greatly, ranging
from 1 to 24 m (3 to 80 ft). Soils vary from weakly developed to moderately
developed.
Qafo
Older alluvial deposits (Pleistocene)—Angular to subrounded, poorly sorted,
matrix-supported pebble to boulder gravel in a matrix of sand, silt, and clay.
Thickness highly varied, ranging from 1 to 15 m (3 to 50 ft). Soils moderately
developed to well developed.
7,000
Ttcr
Qtg1 Qaf
Tmc
FEET
Yg
Tmc
Tcty
Tlm
Ysq
Tqt
Tldac
Tgt
Tldac
Tsc
t
Faul
C
Gravel deposits of Pleistocene alluvial terraces are composed of moderately
sorted and clast-supported sandy gravel. Clasts vary from subangular to
rounded pebbles, cobbles, and few boulders at mountain front, to
subrounded to rounded pebbles and cobbles near the confluence with the
Lemhi River. Clasts primarily subrounded to well-rounded pebbles,
cobbles, and boulders of quartzite and siltite from the adjacent mountains.
Terrace deposits form a relatively thin (3-9 m; 10-30 ft) cap over a streamcut bedrock surface. Several levels of terraces and terrace remnants are
preserved 3-195 m (10-640 ft) above the present-day streams. These record
long-term episodic incision of the Salmon basin, which was probably
driven by glacial climate during the Pleistocene. Near the Beaverhead
Mountain front and adjacent to hills of Tertiary sediments, terrace gravels
commonly are capped by and interfinger with alluvial-fan deposits (Qaf
and Qafo), which are included in the terrace unit locally.
5,000
Tlm
FEET
Tcty
4,000
bedding
Ysq
Tqt
3,000
Tlm
2,000
Qtg1
Published and sold by the Idaho Geological Survey
University of Idaho, Moscow, Idaho 83844-3014
Tsc
Gravel of first terrace (Holocene to Late Pleistocene)—Forms terrace 3-9 m
(10-30 ft) above modern streams. Soils weakly developed. Includes giantboulder debris flow that forms the mouth of Pratt Creek. This deposit with
boulders up to several meters across was probably derived from a rock
avalanche in the steep-sided glacial valley up Pratt Creek.
Wimpey Creek formation (Oligocene and Eocene)—Vitric siltstone, bentonite,
mudstone, and carbonaceous shale with interbeds of conglomerate and
sandstone. Colors vary from white to greenish gray and pinkish brown.
Locally stained and cemented with iron and manganese. Mostly
distinguished by poorly consolidated fissile carbonaceous shale and
bentonitic beds. Bed thicknesses range from a few millimeters to a few
meters. Depositional environments vary from proximal mixed-load
streams, to sandy mixed-load streams, to flood-basin swamps and ponds on
the basin floor (Harrison, 1985). Toward the basin detachment fault the unit
grades and interfingers with the conglomerate-rich Kriley Gulch formation
(Tkg). Also grades and interfingers basinward with the Salmon City
formation (Tsc) and the Mulkey Creek formation (Tmc). Gradational
contact with these units estimated from geomorphic expression of lithology.
Forms gently sloping, low-relief, and "badlands" topography that is prone to
erosion and landsliding when wet.
Salmon City formation (Oligocene and Eocene)—Fine- to coarse-grained,
moderate to well-sorted vitric quartz arenites interbedded with vitric
siltstone, shale, and minor conglomerate. Outcrops generally buff colored.
Bed thicknesses range from 50 cm to 3 m. Depositional environments
predominantly distal sandy stream and distal shallow braided stream
deposits in basin-axis longitudinal stream system (Harrison, 1985). Includes
portions of Blankenau’s (1999) Tertiary sandstone and the middle
conglomerate of the sedimentary rocks of Tendoy. Depending on location
within the depositional basin, the unit grades into and interfingers with the
Wimpey Creek formation (Twc), the Mulkey Creek formation (Tmc), and the
Kriley Gulch formation (Tkg). Gradational contact with these units
estimated from geomorphic expression of lithology. Forms smooth,
relatively steep slopes with intermixed ledges formed by thin beds of
cemented conglomerate.
Tmc
Mulkey Creek formation (Oligocene and Eocene)—Thin- to medium-bedded
vitric to siliceous siltstone and finely laminated shale. Includes minor beds
of carbonaceous shale with well-preserved plant remains representative of
the Haynes Creek Flora (Axelrod, 1998). The unit coarsens upward into
common vitric arenite and quartz arenite beds. Sample of thin tuff interbed
dated at 31 Ma (Axelrod, 1998). Deposited in shallow lake at southern end
of actively subsiding basin. Over time, the lake filled giving way to flood
basin and longitudinal stream environments. Includes portions of
Blankenau’s (1999) shale facies of the sedimentary rocks of Tendoy.
Interfingers laterally with the Salmon City formation (Tsc), and grades
upward into and interfingers with the Wimpey Creek formation (Twc).
Gradational contact with these units estimated from geomorphic
expression of lithology. Forms badland topography and bright white and
buff outcrops. Surface commonly displays finely spaced mudcracks or
“popcorn” weathering.
Tmcb
Intercalated basalt (Oligocene and Eocene)—Thin flow or sill of basalt or
andesite, either interbedded with siltstone and shale or intruded into the
sedimentary rocks along bedding.
Tkg
GRAVEL TERRACE DEPOSITS
Fault C
Lemhi
River
Qam
Qaf
4,000
2,000
Qalc
6,000
Qls
Tlm
Twc
Made ground (Holocene)—Artificial fills and gold placer tailings. Fills
primarily dams for ponds constructed from local earth materials. Tailings
from hydraulic mining form “alluvial” fans deposited onto the flood plain of
Bohannon Creek.
Side-stream alluvium (Holocene)—Pebble to boulder sandy gravel in stream
valleys emanating from mountain fronts. Pebble to cobble sandy gravel
from mountain front to confluences with the Lemhi River. Subrounded to
well-rounded, moderately sorted and stratified pebble to boulder sandy
gravel. Gravel clasts primarily quartzite and siltite. Silty and clayey in
foothill stream valleys and localized bottomlands where sheet wash and
colluvium are included. Angular to subrounded, poorly sorted, moderately
stratified pebbly to cobbly sandy silt; includes minor sheet wash, colluvium, and fan deposits. Soils undeveloped to weakly developed. Deposits
are 1-6 m (3-20 ft) thick.
Yg?
Tgt
3,000
Bearing and plunge of asymmetrical small fold showing
clockwise rotation viewed down plunge.
Qas
30
A
Tqt
Strike and dip of flow foliation.
Main-stream alluvium (Holocene)—Well-rounded, moderately sorted and
stratified pebble to boulder sandy gravel of the Lemhi River. Gravel clasts
mostly quartzite, siltite, and volcanic rocks. Includes flood-plain areas of
silt, clay, and sand. Deposits are 3-9 m (10-30 ft) thick. Weakly developed
soils.
Tsc
Qaf
R
Tlm
Qaf
Qls
Strike and dip of crenulation cleavage.
Qam
Qafo
33
Qtg 3
KE
Qls
25
Qam
30
BA
Qls
Qaf
T
Qas
Qaf
Yg?
Janecke and Blankenau (2003) interpreted the Salmon basin as one of
several supradetachment basins that formed in east-central Idaho and
western Montana between 46 and 31 Ma (late middle Eocene to early
Oligocene). Blankenau (1999) studied the structure and stratigraphy in the
southern portion of the Salmon basin. He described and mapped basin
sedimentary rocks and interpreted their stratigraphic and structural
relationships in the basin. Previously, sedimentary rocks of the Salmon
basin were described, subdivided, and mapped by Anderson (1957) and
Tucker (1975). Harrison (1985) studied the sedimentology of the basinfilling sediments, identified a series of gradational facies, and described and
sketch-mapped several lithostratigraphic units. In contrast to Anderson's
ideas, she demonstrated that the basin sediments are conformable, and
their lithologic distribution resulted from depositional environments that
varied by proximity to the active, basin-bounding fault. The sediments were
deposited in alluvial-fan, braided-stream, mixed-channel and flood-plain,
and lake environments in a downfaulted subsiding basin. Harrison (1985)
defined laterally gradational and interfingering, coarser and finer grained
lithostratigraphic units. The units are local and informal: Harrison’s
“Formation” is changed to “formation,” and her Mulkey Creek “Member” is
changed to “formation” because the map distribution suggests equal status
with the other units. Blankenau (1999) mapped the sedimentary rocks south
of Haynes and Sandy creeks. His lithologic units and described facies help
define the ancestral basin’s tectonic setting and resulting depositional
environments. Several local informal units from his map are included here
in the following basin-wide lithostratigraphic map units. Most units are
semi-consolidated; cementation is restricted to thin beds of sandstone and
conglomerate, which are not laterally extensive. As a result, outcrops are
rare and many slopes are covered with thin sheet wash and colluvium.
Conglomerate beds weather to a gravelly soil mantle. Where bedding
attitudes are nearly horizontal, cemented sandstone and conglomerate
beds form low, flat benches that can be misinterpreted as Quaternary
terraces.
arrow
ALLUVIAL DEPOSITS
Qas
Tmc
Qls
15
66
Qtg 3
FA
UL
Qaf
Qls
TERTIARY SEDIMENTARY DEPOSITS AND
VOLCANIC ROCKS OF THE SALMON BASIN
28
Qtg 3
Qaf
45
Tldac
Tsc
33
15
20
16.5o
Outwash gravels of last local glacial maximum (Pleistocene)—Subrounded to
rounded, well-sorted sandy cobble to boulder gravel. Forms outwash
terrace 12 m (40 ft) above Sandy Creek. Soils weakly developed. Primarily
Pinedale equivalent. Thickness 3-9 m (10-30 ft). Probably overlies streamcut surface on bedrock.
ARTIFICIAL DEPOSITS
26
Qtg 2
10
OY
30
Qaf
ND
Yg?
Qaf
Tqs 2?
20
TE
Qalc
Qafo
m
35
GN
Qgo
28
Qas
Qaf
15
6,000
24
Qtg 3
Tkg
Tcty?
1o 54
located;
Mineral modifiers are listed in order of increasing abundance. Grain size
classification of unconsolidated and consolidated sediment is based on the
Wentworth scale (Lane, 1947). Distances and bed thicknesses are given in
abbreviation of metric units (e.g., cm=centimeter). Formation thickness and
elevation are listed in both meters and feet. Multiple lithologies within a
rock unit description are listed in order of decreasing abundance.
Qtg 3
Qtg 2
20
Yg
7,000
25
Tmc
15
Qalc
Base digitally scanned from 24,000-scale USGS
film separates, 1989.
Shaded elevation from 10 m DEM.
Topography compiled from aerial photographs
taken 1985. Field checked 1986. Map edited
1989.
Projection: Idaho coordinate system, central
zone (Transverse Mercator). 1927 North
American Datum.
10,000-foot grid ticks based on Idaho coordinate
system, central zone.
1000-meter Universal Transverse Mercator grid
ticks, zone 12.
Till deposits of last local glacial maximum (Pleistocene)—Poorly sorted sandy
to clayey boulder till. Clasts subangular to subrounded. Forms end
moraines. Late Pinedale Glaciation equivalent. Soils weakly developed.
Thickness as much as 20 m (65 ft).
DESCRIPTION OF MAP UNITS
15
A
Qgt
43
Qtg 3 Qafo
Qafo
Qaf
Qls
25
Landslide deposits (Holocene to Pleistocene)—Unstratified, poorly sorted silty
clay and gravelly silty clay. Deposited by slumps, slides, and debris flows
that primarily occur in Tertiary sediments. For some, map shows the
landslide scarp and the headwall (steep area adjacent to and below the
landslide scarp) from which material broke away (see Symbols).
Qaf
Qtg 4
Qls
Qls
30
Qls
Tgt
C
Tkg
50
50
Deposits of active landslides (late Holocene)—Unstratified, poorly sorted silty
clay and gravelly silty clay. Deposited by slumps, slides, and debris flows
from slope failures in Tertiary sediments. Directly related to and formed
after development of water ditches and irrigation.
Extent of gravel pit excavations.
FAU
40
Qls
Qlsa
Extent of placer-pit excavations; exposes map unit Twc.
Qaf
Qtg1
Qaf
INTRUSIVE ROCKS
Ti
Kriley Gulch formation (Oligocene and Eocene)—Matrix-poor breccia,
matrix-supported conglomerate, and clast-supported conglomerate;
includes interbeds of ashes, vitric siltstone and sandstone. Silica and
hematite cement are common near the basin detachment fault. Clast sizes
commonly pebbles and cobbles, but large boulders locally occur as lag
deposits from weathered and eroded beds. Beds are predominantly breccia
and matrix-supported conglomerate lower in the unit, but transition
upward to better sorted and cross-stratified clast-supported conglomerate.
East of the Lemhi River, clast compositions are primarily Mesoproterozoic
quartzite, siltite, and argillite derived from the adjacent Beaverhead
Mountains. West of the Lemhi River, lower part of unit is predominantly
debris-flow breccia with a high percentage of volcanic clasts derived from
the Challis Volcanic Group. Depositional environments vary from proximal
fan and fan head at the base of the unit to mid fan and proximal braided
stream in the upper part (Harrison, 1985). Includes portions of Blankenau’s
(1999) middle conglomerate of the sedimentary rocks of Tendoy, and the
conglomerate of Kenney Creek of the sedimentary rocks of Sacagawea.
Percentage of fine-grained beds increases laterally, and depending on
location within the depositional basin, the unit grades and interfingers with
the Salmon City formation (Tsc), Wimpey Creek formation (Twc), and the
Mulkey Creek formation (Tmc). Gradational contact with these units
estimated from geomorphic expression of lithology. Forms steep slopes
with coarse gravelly soils and resistant ridges capped with common lag
pebbles and cobbles, and occasional lag boulders.
Tertiary intrusion (early Oligocene?)—Dark red to gray andesitic intrusion. A
whole-rock 40Ar/39Ar age determination of 37.08 ± 0.21 Ma has an
unreliable age spectrum and is probably too old (Blankenau, 1999, sample
JB96-145). The intruded sediments may be as young as those dated at 31
Ma located 2.5 km (1.5 mi) to the southwest where probable Oligocene
volcanism is also documented by a mafic lava or sill intercalated with
fine-grained sedimentary rocks (see Tmcb description).
MESOPROTEROZOIC STRATA
Low metamorphic grade metasedimentary rocks of Mesoproterozoic age
are exposed in the northeast and southwest corners of the quadrangle.
Those in the southwest were assigned to the Gunsight Formation by Evans
and Green (2003), a correlation with which we agree. Those in the
northeast, assigned to the Gunsight and Apple Creek formations, were
shown separated by a low-angle fault (Evans and Green, 2003). We did not
find evidence of this fault, and instead of assigning Lemhi Group names,
have given the rocks there a lithologic unit assignment (Ysq), and tentative
correlations to other Lemhi Group formations, consistent with our mapping
to the north in the Bohannon Spring quadrangle (Lewis and others, 2009).
MASS MOVEMENT DEPOSITS
Headwall scarp of landslide; ticks on top of scarp.
48
Qtg 2
Qaf
Yg
Qtgh
High-elevation terrace gravel deposits (Pleistocene)—Subrounded to wellrounded pebble to boulder sandy gravel. Forms terrace remnants at least
120 m (400 ft) above modern streams. Two rounded-gravel deposits are at
225 and 415 m (740 and 1,360 ft) above the Lemhi River. Deposits are 6 m
(20 ft) thick. Soils of original terrace surface eroded away.
Tectonic breccia.
Tlm
35
Qtg 4
13
20
Qtgh
Strike and dip of bedding; ball indicates bedding known to
be upright.
Strike and dip of cleavage.
5
Ttcr
Qam
Qas
15
Qls
Gravel of sixth terrace (Early? Pleistocene to Pliocene)—Forms terrace
remnants 168-195 m (550-640 ft) above Lemhi River. Soils of original
terrace surface eroded away.
Tmc
Tlm
Yg
Qtg6
Estimated strike and dip of bedding.
30
55
45
46
Tqt
Qlsa
35
Qls
40
Qaf
44
Qaf
30
Gravel of fifth terrace (Early? Pleistocene)—Forms terrace 122-146 m (400480 ft) above the modern streams. Soils of original terrace surface
eroded away.
Strike and dip of bedding.
Strike and dip of bedding, strike variable.
Qtg 3
Qas Tkg
36
35
approximately
25
Qafo
Qtg 5
Tcty
Tlm
Qtg5
Normal fault: ball and bar on downthrown side; dashed where
approximately located; dotted where concealed; arrow
indicates dip of fault.
Syncline axial trace,
indicates plunge.
Tsc
Qtg 5
Qtg 2
Tcty
Qafo
Gravel of fourth terrace, (Middle? Pleistocene)—Forms terrace 61-110 m
(200-360 ft) above modern streams. Soils well developed.
Anticline axial trace, approximately located; arrow
indicates plunge.
Tkg
18
Qls
13
50
Qtg1
Tlm
35
10
18
Tmc
Qas
Qls
28
23
14
16
Qtg 4
Tmc
Quartzite-bearing ash-flow tuff (Eocene)—White, pinkish red, white-gray, or
pink-gray welded tuff containing abundant angular gray quartzite lithics
and less abundant smoky quartz and plagioclase (Tqt of Blankenau, 1999).
Poorly to densely welded.
Yg
Gunsight Formation (Mesoproterozoic)—Quartzite, siltite, and argillite.
Quartzite very feldspathic and dark, but weathers light gray where not iron
stained. Lowest exposures have tabular to channel-shaped beds 10 cm to
3 m thick in thinning upward stacks, separated by 1-10 m thick intervals of
more thinly bedded quartzite and siltite. Upper exposed part much thinner
bedded and more poorly exposed. Load bases more common in lower part;
bedding surfaces covered with muscovite and light-colored mud chips or
flakes more common in upper part. Atypical hornfels-like hardness, dark
color, and paucity of dark laminations that are common to Yg elsewhere
attributed to higher grade or different metamorphism here. Neither base nor
top exposed, but thickness in the type section about 30 km (20 mi) to the
south-southeast is about 1,800 m (6,000 ft; McBean, 1983). Includes two
isolated bodies east of the main mass that may be landslide blocks (see
cross section).
Ysq
Siltite, quartzite, and argillite (Mesoproterozoic)—Laminated siltite and
argillite with
coarser and thicker intervals. Lowest exposures are
decimeter-scale dark brown stained, dark gray siltite and very fine-grained
quartzite interbedded with thinner siltite and dark argillite. Grades upward
to the northeast through finer grained intervals with graded couplets and
couples of light-weathering dark siltite and darker gray to green argillite.
Thin bedding uneven, contorted, and perhaps fluidized. Above that,
includes cosets of decimeter-scale coarser quartzite and graded beds as
thick as 1 m with quartzite bases. Orange weathering, sparsely carbonatebearing quartzite farther northeast beyond the map boundary possibly Big
Creek Formation. This is consistent with tentative correlation of Ysq with the
type Inyo Creek Formation of the Lemhi Group (Ruppel, 1975) and rocks
below the Inyo Creek that are not exposed in the Lemhi Range. Thickness
uncertain because of deformation, but a minimum of 2,400 m (8,000 ft)
likely.
GLACIAL DEPOSITS
Detachment fault: hachures on upper plate; dashed where
approximately located; dotted where concealed.
18
16
Qtg 3
20
28
Tcty?
15
Qaf
Qtg1
Qtg 5
Qafo
Qafo
Tqt
Qtg4
Twc
Qtg 4
Qafo
Qls
Gravel of third terrace (Middle? Pleistocene)—Forms terrace 37-49 m (120160 ft) above modern streams. Soils well developed.
Gradational contact between infingering units; placed along
zone of geomorphic expression of lithology.
18
Tsc
Qtg3
Contact: dashed where approximately located.
35
Qas
Qaf
Qaf 18
Qas
Qtg 3
Qafo
Tqt
SYMBOLS
Tkg
LT
Qtg 2
20
Tsc
Tcty
Gravel of second terrace (Late? Pleistocene)—Forms terrace 12-24 m (40-80 ft)
above modern streams. Soils moderately developed.
14
22
15
12
Tlm
Qaf
Qtg 2
Qtg 2
Qaf
30
Qafo
25
Qtg 3
Qtg2
13
Qtg 5
35
Qas
Tkg
Qas
Qaf
Qaf
Qaf
Qtg 2
15
Qgo
40
Qam
Qls
Qaf
Qafo
Qtg 2
30
30
Qaf
Qtg1
Qafo
Qtg1
40
The oldest rocks in the quadrangle are metasedimentary rocks of
Mesoproterozoic age that form the mountains in the northeast and
southwest parts of the quadrangle. In the northeast, their contact with
sedimentary rocks of the ancestral Salmon basin is largely along the late
Eocene-Oligocene Salmon basin detachment fault. Salmon basin sedimentary rocks vary from coarse conglomerate to shale, record a wide range
of depositional environments in the basin as it formed. Much of the basin
sedimentary section has been subsequently eroded. The oldest surficial
deposits in the quadrangle are bouldery gravels that cap an early erosion
surface on dipping sedimentary rocks. Evidence for that Pliocene or early
Pleistocene surface has been mostly removed through Pleistocene drainage
incision. The Quaternary deposits show evidence of glaciation, terracing,
incision, and landsliding. These are characteristic of Quaternary processes
that formed the thin alluvial, glacial, and mass movement deposits.
Qafo
20
Qaf
Qtgh
15
15
C
Qtg 4
FAU
20
Qafo
28
20
Tsc
Tsc
30
Qtg1
30
Qas
Dacite lava flows (Eocene)—Purple to purple-gray or dark brown dacite or
andesite lava flows containing phenocrysts of euhedral biotite, hornblende,
and plagioclase along with minor amounts of quartz and augite(?) in a fine
crystalline groundmass (Tldac of Blankenau, 1999). Near the top of the
sequence are aphanitic layers like those of the Tlm unit. Absent at surface
west of Lemhi River.
CH
30
The geologic map of the Baker quadrangle shows rock units exposed at the
surface or underlying a thin surficial cover of soil and colluvium. Thicker
surficial alluvial, glacial, and landslide deposits are shown where they form
significant mappable units. Semi-consolidated to consolidated Tertiary
sedimentary rocks form the undulating low hills and foothills between the
Lemhi River and the basin-bounding mountains.
Qas
50
15
Qls
Tldac
INTRODUCTION
TA
30
Qaf
Qtg 6
Twc
10
Green tuff (Eocene)—Green to light green biotite tuff containing abundant
euhedral biotite, smoky quartz, minor subhedral sanidine, and pumice in a
white to light green ash groundmass. Also contains chert-bearing siltstone.
(Tgt of Blankenau, 1999).
MESOPROTEROZOIC
Ysq
Qafo
Tkg
Qtg1
Tsc
Tgt
Yg
BA
Tkg
Tkg
Twc
Qafo
Qtg 3
Qaf
Mafic lava flows (Eocene)—Black to dark green-gray or dark brown-black
aphanitic to porphyritic lava flows; locally vesicular (Tl of Blankenau,
1999). Includes dark brown shoshonite and medium gray to light brown
latite. Shoshonite groundmass contains small plagioclase laths, olivine, and
possibly pyroxene. A sample from a roadcut near the western map
boundary (45.0213N, 113.7420W) contained rare xenocrystic clusters as
much as 1 cm across of fine-grained olivine, and plagioclase xenocrysts as
long as 6 mm. Latite groundmass is extremely fine grained and largely
plagioclase that is aligned in a trachytic texture. A few 1 mm phenocrysts of
plagioclase are present; similarly sparse olivine phenocrysts are largely
altered to iron oxide. Interbedded with the flows are thin biotitic tuffs and
volcaniclastic sandstones. Secondary chalcedony and calcite are common.
Forms brown cobbly colluvial slopes that typically armor less resistant
sedimentary units.
Eocene
Metasedimentary
Rocks
DE
Qas
Ttcr
Qlsa
Twc
Qam
TERTIARY
Tqs 2
30
Qtg1
10
Ti
Tqt
N
Qtg 2
Tmc
Tlm
Oligocene
Tldac
15
25
Qlsa
Twc
Qtg1
Intrusive Rocks
Tlm
10
LM
O
Qlsa
Tmc
Tcty
50
Tsc
Qtg 2
Volcanic Rocks
Tmcb
60
45
SA
Qaf
20
Twc
Qaf
Qtg 2
20
Qtg 3
Qtg 4
Qtg1
Qafo
Qtg 4
Qaf
Qaf
Tuff of Curtis Ranch (Eocene)—White to greenish white tuff containing
abundant biotite and lesser amounts of smoky quartz, plagioclase, and
angular volcanic and quartzite lithics (Tcr of Blankenau, 1999). Pumice is
abundant to rare and typically concentrated at the top of the unit.
Sandstone present locally at base. Typically poorly welded, but densely
welded in places.
Tgt
Qtg 3
Qas
Ttcr
Ysq
15
Qaf
Qaf
Tkg
Tsc
40
Qlsa
Qtg1
Tmc
35
55
Qtg 2
CENOZOIC
Miocene
Twc
20
Twc
Qlsa
Younger quartz-sanidine welded tuff (Eocene)—White to pale green or
gray-green with euhedral chatoyant sanidine and smoky quartz in a fine ash
groundmass (Tqs2 of Blankenau, 1999). Degree of welding varies.
40
20
Qtg1
Pliocene
Sedimentary Deposits of the Salmon Basin
35
Qaf
Qas
20
Ysq
Qafo
Qtg 4
15
Tqs2
?
Qtg 6
Qtg 4
Twc
Qam
Qafo
Qafo
Qaf
Qtgh
?
Qtg 3
Qtg 3
Tkg
10
20
Qtg1
Qas
30
Qls
Qtg 3
Qls
30
Qls
Qtg1
Qtg 3
QUATERNARY
?
35
Twc
Qtg1
10
?
Qtg 5
Qas
Qls
Qafo
3
Qtg 4
10
30
Qafo
Younger Challis tuff and sandstone, undivided (Eocene)—Volcanic and
sedimentary rocks overlying the mafic lavas (Tlm). Includes Tuff of Curtis
Ranch (Ttcr) and overlying quartz-sanidine welded tuff (Tqs2) along
Withington Creek at the western map boundary, Tqs2 resting directly(?) on
Tlm on the north side of Haynes Creek, and lapilli-crystal-lithic tuff and
interbedded sandstone higher(?) in the section east and west of Kadletz
Creek. Ttcr and Tqs2 are described below. The lapilli-crystal-lithic tuff
contains potassium feldspar and quartz phenocrysts, and dark gray
fine-grained quartzite clasts similar to those found in Tqt. Alternative
explanation is that the Kadletz Creek area contains a remnant high with
rocks that predate the mafic lavas rather than overlie them (Tqt rather than
Tcty). Interlayered sandstone in the Kadletz Creek area contains subangular
clasts of quartz, potassium feldspar, and plagioclase.
Pleistocene
Tkg
Twc
Qtg 4
Qlsa
Qaf
Qgt
Qls
Qaf
Qtg 4
8
Qlsa
Twc
Qtg 2
55
Ysq
20
Qas
Qaf
Tcty
Holocene
Qtg 2
Qls
Qtg
Qtg 2
Qalc
Qaf
Qtg1
Qgo
CHALLIS VOLCANIC GROUP
Alluvial Deposits
Mass Movement Deposits
S TRU C TU RE
Several faults and folds traverse the quadrangle. Extensional faults in the
southeast corner of the map are discussed by Blankenau (1999) and are
shown in a simplified map by Janecke and others (2001). These faults
typically bound Tertiary units and control their distribution.
FAULT "C"
The "C" fault of Blankenau (1999) is thought to cut Tertiary strata in the
southeast corner of the map. We did not see evidence for faulting in the few
places we traversed it, but exposures are poor and a large-displacement
down-to-the-west fault is required along or east of the Lemhi River to
repeat the Challis section.
FAULT “G”
The "G" fault of Blankenau (1999) is postulated to cross the southeast corner
of the map. We did not investigate the fault in detail, but have included it
here on the basis of apparent offset of Challis volcanic units along the lower
part of Kenney Creek. It may offset fault C in the same area.
SALMON BASIN DETACHMENT
The range front fault in the northeast part of the quadrangle was mapped by
Tucker (1975) and later interpreted as a low-angle detachment fault by
Blankenau (1999). It is well exposed along Kenny Creek just east of the map
where conglomerate is in contact with cataclasite formed from Proterozoic
rocks. There the detachment has a dip of less than 15 degrees. Tucker
mapped a breccia unit in the footwall of the fault along the range front that
has been depicted using symbols on the present map.
FOLDS
Several folds whose axes trend north-south to north-northeast have affected
the Tertiary strata. A detailed discussion of these and other folds southeast
of the map is given by Blankenau (1999).
RE F E RE N C E S
Anderson, A.L., 1957, Geology and mineral resources of the Baker
quadrangle, Lemhi County, Idaho: Idaho Bureau of Mines and Geology
Pamphlet 112, 71 p.
Axelrod, D.I., 1998, The Oligocene Haynes Creek flora of eastern Idaho:
University of California Publications in Geological Sciences, v. 143,
99 p.
Blankenau, J.J., 1999, Cenozoic structural and stratigraphic evolution of the
southeastern Salmon basin, east-central Idaho: Utah State University M.S.
thesis, 143 p., 3 plates.
Evans, K.V., and G.N. Green, 2003, Geologic map of the Salmon National
Forest and vicinity, east-central Idaho: U.S. Geological Survey Geologic
Investigations Series Map I-2765, 19 p., scale 1:100,000.
Harrison, S.L., 1985, Sedimentology of Tertiary sedimentary rocks near
Salmon, Idaho: University of Montana Ph.D. dissertation, 175 p.
Hipple, Karl, Karen Langersmith, Rulon Winward, Dal Ames, and Bradley
Duncan, 2006, Soil survey of Custer-Lemhi area, Idaho, parts of Blaine,
Custer, and Lemhi counties: United States Department of Agriculture,
Natural Resources Conservation Service, 1270 p., soil maps at
http://websoilsurvey.nrcs.usda.gov/app/.
Janecke, S.U. and J.J. Blankenau, 2003, Extension folds associated with
Paleogene detachment faults in SE part of the Salmon Basin, in D.R.
Lageson, and R.B. Christner, eds., 2003 Tobacco Root Geological Society
Field Conference at the Belt Symposium IV: Northwest Geology, v. 32, p. 51-73.
Janecke, S.U., J.J. Blankenau, C.J. VanDenburg, and B.S. Van Gosen, 2001,
Map of normal faults and extensional folds in the Tendoy Mountains and
Beaverhead Range, southwest Montana and eastern Idaho: U.S. Geological
Survey Miscellaneous Field Studies Map MF-2362, scale 1:100,000.
Lane, E.W., 1947, Report of the subcommittee on sediment terminology:
Transactions of the American Geophysical Union, v. 28, no. 6, p. 936-938.
Lewis, R.S., K.L. Othberg, L.R. Stanford, R.F. Burmester, J.D. Lonn, and M.D.
McFaddan, 2011, Geologic map of the Goldstone Mountain quadrangle,
Lemhi County, Idaho: Idaho Geological Survey Digital Web Map 134,
scale 1:24,000.
Lewis, R.S., K.L. Othberg, R.F. Burmester, J.D. Lonn, L.R. Stanford, and M.D.
McFaddan, 2009, Geologic map of the Bohannon Spring quadrangle,
Lemhi County, Idaho and Beaverhead County, Montana: Idaho
Geological Survey Digital Web Map 113 and Montana Bureau of Mines
and Geology Open File 583, scale 1:24,000.
McBean, A.J., II, 1983. The Proterozoic Gunsight Formation, Idaho-Montana:
Stratigraphy, sedimentology, and paleotectonic setting: The Pennsylvania
State University M.S. thesis, 235 p.
Ruppel, E.T., 1975, Precambrian Y sedimentary rocks in east-central Idaho:
U.S. Geological Survey Bulletin 889-A, 23 p.
Tucker, D.R., 1975, Stratigraphy and structure of Precambrian Y (Belt?)
metasedimentary and associated rocks, Goldstone Mountain quadrangle,
Lemhi County, Idaho, and Beaverhead County, Montana: Ohio, Miami
University Ph.D. dissertation, 221 p., scale 1:48,000.