52

5 75

58

32

13

42 85

10 10

20

12

17

18

23

15 20 24

80

48

70 45

27

18

40

67

73

85

Qa

Ynu

Ynl

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Ynl

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BLACK

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VOLCANO

VALLEY

U.S. Department of the Interior

U.S. Geological Survey

Pamphlet to accompany

Scientific Investigations Map 3379

Geologic Map of the Strawberry Butte 

7.5’ Quadrangle Meagher County, Montana

Geologic Map of the Strawberry Butte 

7.5’ Quadrangle, Meagher County, Montana

By Mitchell W. Reynolds and Theodore R. Brandt

Pamphlet to accompany

Scientific Investigations Map 3379

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

RYAN K. ZINKE, Secretary

U.S. Geological Survey

William H. Werkheiser, Acting Director

U.S. Geological Survey, Reston, Virginia: 2017

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

resources, natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–ASK–USGS.

For an overview of USGS information products, including maps, imagery, and publications, 

visit https://store.usgs.gov.

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U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Reynolds, M.W., and Brandt, T.R., 2017, Geologic map of the Strawberry Butte 7.5’ quadrangle, Meagher County, 

Montana: U.S. Geological Survey Scientific Investigations Map 3379, pamphlet, 16 p., 2 sheets, scale 1:24,000, 

https://doi.org/10.3133/sim3379.

ScienceBase link: Reynolds, M.W., and Brandt, T.R., 2017, Data release for the geologic map of the Strawberry Butte 

7.5’ quadrangle, Meagher County, Montana: U.S. Geological Survey data release, https://doi.org/10.5066/F73J3B61.

ISSN 2329-132X (online)
iii

Contents

Abstract...........................................................................................................................................................1

Geologic Setting.............................................................................................................................................1

Stratigraphy ....................................................................................................................................................2

Structure..........................................................................................................................................................9

Summary........................................................................................................................................................11

Acknowledgments.......................................................................................................................................11

Description of Map Units............................................................................................................................12

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

Figures

1. Generalized physiography of west-central Montana, showing location of the 

Strawberry Butte and adjacent 7.5 ́ quadrangles referred to in the text and 

annotated with names of selected topographic and cultural features ...............................2

2. Shaded relief map of the Strawberry Butte 7.5 ́ quadrangle and adjacent areas 

showing features referred to in the accompanying pamphlet..............................................3

3. Tectonic map of west-central Montana showing principal faults, major tectonic 

elements, intrusive and extrusive igneous rock bodies, Tertiary strata and major 

areas of Quaternary sediments..................................................................................................4

4. Photographs of rock fragments, pebbles, cobbles, and small boulders 

interbedded with medium- to coarse-grained sandstone of the Middle Cambrian 

Flathead Sandstone. A, Angular to subrounded clasts of Mesoproterozoic Neihart 

Quartzite and Paleoproterozoic vein quartz, granite gneiss, some orthoclase feldspar, 

and rare clasts of biotite gneiss in the Flathead Sandstone. B, Pebbles, cobbles and 

small boulders of Mesoproterozoic Neihart Quartzite, Spokane Formation and rare 

Greyson Formation together with fragments of vein quartz, orthoclase feldspar 

and granite gneiss derived from the Sheep Creek granite gneiss widely exposed 

across the northern part of the quadrangle ..................................................................................6

5. Photograph of fractured hematite-stained strata in the upper part of the Newland 

Formation have been excavated on the west flank of Iron Butte and in the past 

shipped to Three Forks, Montana, for use as flux in the production of cement ................9

Conversion Factors

U.S. Customary Units to International System

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

International System to U.S. Customary Units

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

centimeter (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Geologic Map of the Strawberry Butte 7.5’ Quadrangle, 

Meagher County, Montana

By Mitchell W. Reynolds and Theodore R. Brandt

Abstract

The 7.5′ Strawberry Butte quadrangle in Meagher County, 

Montana, near the southwest margin of the Little Belt Mountains, 

encompasses two sharply different geologic terranes. The 

northern three-quarters of the quadrangle are underlain mainly by 

Paleoproterozoic granite gneiss, across which Middle Cambrian 

sedimentary rocks rest unconformably. An ancestral valley of 

probable late Eocene age, eroded northwest across the granite 

gneiss terrane, is filled with Oligocene basalt and overlying 

Miocene and Oligocene sandstone, siltstone, tuffaceous siltstone, 

and conglomerate. The southern quarter of the quadrangle is 

underlain principally by deformed Mesoproterozoic sedimentary 

rocks of the Newland Formation, which are intruded by Eocene 

biotite hornblende dacite dikes. In this southern terrane, Tertiary 

strata are exposed only in a limited area near the southeast margin 

of the quadrangle. The distinct terranes are juxtaposed along the 

Volcano Valley fault zone—a zone of recurrent crustal movement 

beginning possibly in Mesoproterozoic time and certainly established from Neoproterozoic–Early Cambrian to late Tertiary time. 

Movement along the fault zone has included normal faulting, the 

southern terrane faulted down relative to the northern terrane, 

some reverse faulting as the southern terrane later moved up 

against the northern terrane, and lateral movement during which 

the southern terrane likely moved west relative to the northern 

terrane. Near the eastern margin of the quadrangle, the Newland 

Formation is locally the host of stratabound sulfide mineralization adjacent to the fault zone; west along the fault zone across 

the remainder of the quadrangle are significant areas and bands 

of hematite and iron-silicate mineral concentrations related to 

apparent alteration of iron sulfides. The map defines the distribution of a variety of surficial deposits, including the distribution of 

hematite-rich colluvium and iron-silicate boulders. The southeast 

corner of the quadrangle is the site of active exploration and 

potential development for copper from the sulfide-bearing strata 

of the Newland Formation.

Geologic Setting

The Strawberry Butte 7.5′ quadrangle lies near the southwest edge of the Little Belt Mountains about 24 kilometers (km) 

north of White Sulphur Springs, the seat of Meagher County, 

Montana (fig. 1). As a broad northwest-trending mountain range 

in west-central Montana, the Little Belt Mountains are bounded 

on the west by the canyon of the Smith River, merge on the northeast with the Judith River basin, and are bounded on the south by 

the North Forks of the Smith and the Musselshell Rivers. Sheep 

Creek and Black Butte Creek are the principal drainages that flow 

northwest and west across the quadrangle toward the Smith River 

(fig. 2). The southeast quarter of the quadrangle is of special 

interest to economic geologists as a result of the identification of 

apparent stratabound sulfide mineralization in Mesoproterozoic 

sedimentary rocks (Zieg and others, 2004; 2013). This quadrangle 

map, a part of the U.S. Geological Survey (USGS) legacy project, 

was produced during geologic mapping of the White Sulphur 

Springs 30′ × 60′ quadrangle (Reynolds and Brandt, 2006) before 

the intense latest subsurface exploration in the area. The map 

does not reflect the latest described subsurface extent of reported 

mineralization in a limited area west and south of Strawberry 

Butte (Zieg and others, 2013). The map depicts in detail the areal 

setting of the prospective area of mineralization, contributes to an 

understanding of the complex geologic history of the area, and 

provides baseline geologic information with which to compare 

possible changes to the area as a result of mineral exploration and 

potential development activities.

The geology of the Strawberry Butte quadrangle has 

two contrasting parts separated by the major Volcano Valley 

fault (Weed and Pirsson, 1900, p. 305) (see Volcano Valley 

fault zone in fig. 3). The southern quarter of the quadrangle, 

south of the fault, is underlain by strata of Mesoproterozoic 

age which are intruded by sills and dikes of Eocene age, 

and is mantled along the eastern edge by sedimentary rocks 

of Oligocene and younger age. By contrast, the northern 

three-quarters of the quadrangle consists of Paleoproterozoic 

metamorphic rocks overlain unconformably by the Middle 

Cambrian Flathead Sandstone that, in turn, is overlain along 

the southwestern margin of the quadrangle by Middle and 

Upper Cambrian strata. Oligocene basalt flows and Oligocene 

and Miocene sedimentary rocks are preserved in ancestral valleys of Sheep Creek and Black Butte Creek eroded across the 

metamorphic rocks and Middle Cambrian terranes, respectively, in the northern part of the quadrangle. A small erosional 

remnant of Oligocene basalt rests on Middle Cambrian strata 

and on a thin remnant of Eocene biotite hornblende monzonite 

above Allan Park in the northeast corner of the quadrangle 

(unsurveyed, T. 13 N., R. 7 E. [center, PB40—Protracted 

Block, see Strawberry Butte basemap]).
2 Geologic Map of the Strawberry Butte 7.5-Minute Quadrangle, Meagher County, Montana

Stratigraphy

The stratigraphic succession across the northern threequarters of the quadrangle is simple: Granite gneiss (Xgg) 

that includes small areas of biotite gneiss and local inclusions of metadiorite (amphibolite) (Xgbg) forms the basement of the northern area. Vogl and others (2004, p. 20, 30) 

termed the granite gneiss the Sheep Creek intrusive complex 

and provided a generalized description of a complex that 

includes older, more strongly foliated granite gneiss, and a 

younger weakly to moderately foliated granite gneiss (both 

apparent sequences termed leucogranite by Vogl and others, 

2004, p. 30). Those authors determined a 207Pb/206Pb age of 

1,817±11 million years ago (Ma) for the Sheep Creek complex and an upper intercept age of 1,851±11 Ma on amphibolite within the less-foliated granite gneiss of the complex 

(Paleoproterozoic; Vogl and others, 2004, p. 30). The youngest 

granite gneiss remains undated but is clearly part of the 

Paleoproterozoic succession. Within the quadrangle the granite 

gneiss is generally weakly foliated, but in some outcrops is 

moderately to strongly foliated. Foliation commonly dips 

steeply north or south and generally strikes eastward. Fractures are common, but tracing faults through the complex is 

difficult. Thin fault-bounded selvages of biotite gneiss (Xgbg) 

structurally interleaved with the granite gneiss are present 

along the south edge of the Volcano Valley fault zone. At the 

surface the selvages range from 0 to about 200 meters (m) 

wide and 600 to 3,900 m long.

Across the northern three-quarters of the quadrangle, 

the Middle Cambrian Flathead Sandstone (_f) rests unconformably on the Paleoproterozoic metamorphic rocks; 

Mesoproterozoic metasedimentary rocks (Ynu, Ynl), present south of the Volcano Valley fault zone, are absent beneath 

the unconformity north of the fault within the quadrangle. 

Stratigraphically throughout the Flathead Sandstone across 

Figure 1. Generalized physiography of west-central Montana showing location of the Strawberry Butte and adjacent 7.5 ́ quadrangles 

referred to in the text and annotated with names of selected topographic and cultural features; orange shading indicates highlands and 

green indicates lowlands.

Niehart

White Sulphur

Springs

111°30’ 111°00’ 110°30’

47°00’

46°30’

Adel

Mountains

Castle

Mountains

Elkhorn

Mountains

Spokane Hills

Dry Range

Helena

Valley

Missouri

Park

Hills

Little Belt Mountains

Smith River

Valley

Townsend

Valley

Sheep

Mountain

Strawberry

Butte

Moose

Mountain

River

Smith River

North Fork Smith River

South Fork Smith River

Judith River

Big

Belt

Mountains

Canyon Ferry Lake

15

15

20 MILES

20 KILOMETERS

Missouri River

0

0

5

5

10

10

QUADRANGLE LOCATION

MONTANA

Judith River

basin
Stratigraphy 3

Figure 2. Shaded relief map of the Strawberry Butte 7.5 ́ quadrangle (red rectangle) and adjacent areas showing features 

referred to in the text. Selected streams, blue; roads and vehicle trails, black and gray lines respectively; quadrangle 

names, green; Lewis and Clark National Forest exterior boundary, dashed and orange lines. There are private inholdings 

within the boundaries of the National Forest shown on this map.

Creek

Black

Butte Creek Black

Butte

Strawberry

Butte

Coxcombe

Butte

Moose

Mountain

Allan

Park

Sheep

Creek

Bar

Green

Mountain

Williams

Mountain

Monument

Peak

Iron

Mountain

Belt Park

Quartzite Ridge

Buckingham

Reservoir

Newlan Creek

Reservoir

Sheep Creek

Moose Creek

 Creek

Sheep

Newlan 

Creek

Newlan

Creek

Tenderfoot

Eagle

Smith

River

Rabbit

Creek

Adams

Creek

Creek

Pole Creek

CASCADE COUNTY

MEAGHER COUNTY

Rimrock Ridge

89

89

89

360

259

NATIONAL FOREST

LEWIS AND CLARK

Creek

Lake

Charcoal Gulch

Sheep

Mountain

MONUMENT

PEAK

BUBBLING

SPRINGS

BELT PARK

BUTTE

SHEEP

MTN

STRAWBERRY

BUTTE

MOOSE

MTN

WHITETAIL

RESERVOIR COXCOMBE

BUTTE

CHARCOAL

GULCH

MONUMENT

PEAK

BELT PARK

BUTTE

STRAWBERRY

BUTTE

WHITETAIL

RESERVOIR

CHARCOAL

GULCH

BUBBLING

SPRINGS

SHEEP

MTN

STRAWBERRY

BUTTE

MOOSE

MTN

WHITETAIL

RESERVOIR COXCOMBE

BUTTE

CHARCOAL

GULCH

111°7'30" 110°52'30" 110°45'

47°

46°52'30"

111°

46°45'

46°37'30"

U.S. Geological Survey National Elevation Dataset (NED)

10-meter resolution elevation data, accessed on 07-25-2016

0 1 2 3 4

0 1 2 3 4

5 MILES

5 KILOMETERS

1:200,000
4 Geologic Map of the Strawberry Butte 7.5-Minute Quadrangle, Meagher County, Montana

Figure 3. Tectonic map of west-central Montana showing principal faults, major tectonic elements, intrusive and extrusive igneous rock bodies, Tertiary strata and major areas of Quaternary sediments. Thrust sheets and igneous units listed in descending succession of inferred increasing age within tectonic elements (for example, autochthons and parautochthons). Red rectangular outline shows the location of the Strawberry Butte 7.5 ́ quadrangle (modified from Reynolds and Brandt, 2005, fig. 2, sheet 1).

111°00' 110°00' 110°30' 111°22'

47°00'

46°45'

46°30'

0 10 KILOMETERS 2 4 6 80 2 4 6 8 10 MILES

Area ofFigure 3

MONTANA

L B

MOORS

A KC CANYON

LEWIS AND CLARK TECTONIC ZONE

THRUST

VOLCANO

VOLCANO

VALLEY

FAULT

MOUNTAIN

R

R

R

C

C C

C

C

C

FAULT

CARR

WOODS

CREEK

RIDGE

SMITH VALLEY

EAST SIDE FAULT

LOST FORK 

PLUTON

CREEK

FAULT

NORTH

FAULT

LACCOLITH

ZONE

ZONE

MOUNTAIN

SHEEP

MOUNTAIN

FLANK

VALLEY

LOGAN

FAULT

CAMUS CREEK FAULT

BIG BALD

MOUNTAIN

LACCOLITH

BOULDER

BALDY

PLUTON

FORT

PLUTON

BASIN

THRUST

FAULT

FAULT

CREEK

TENDERFOOT

NEVADA CREEK

FAULT

DECOLLEMENT 

FAULT

FAULT

ZONE

RANGE

FAULT

DRYWOLF

LACCOLITH

WAITE

ZONE

MOOSE

ZONE

CREEK

FAULT

CASTLE

MOUNTAINS

PLUTON

ZONE

CREEK

FAULT

PARK

QUARTZITE

WESTERN END

ANCESTRAL CENTRAL MONTANA

UPLIFT

DRY

FAULT

HOOVER

WhiteSulphurSprings

Utica

Neihart
Stratigraphy 5

the quadrangle are granules and pebbles of feldspar, quartz, 

and pegmatitic quartz, derived from the granite gneiss (Xgg), 

and common clasts of the Mesoproterozoic Neihart Quartzite 

(fig. 4A). Other clasts that range in size from pebbles to small 

boulders are present in paleochannels scattered at all stratigraphic levels within the Flathead (fig. 4B). These clasts were 

derived from Paleoproterozoic metamorphic rocks and from 

Mesoproterozoic strata including the Neihart Quartzite, and 

Newland, Greyson, and Spokane Formations belonging to the 

Belt Supergroup. The erosionally-truncated edges of these 

Belt rocks are present beneath the Flathead Sandstone about 

12 to 25 km north-northeast and northeast of the Strawberry 

Butte quadrangle (Keefer, 1972; Reynolds and Brandt, 2006). 

Metasedimentary clasts derived from the Belt Supergroup are 

generally subrounded to rounded, and clasts derived from the 

Neihart Quartzite and Paleoproterozoic metamorphic rocks are 

angular to subrounded, locally rounded (fig. 4B). Relief on the 

unconformity at the base of the Flathead is locally as much as 

about 11 m. Rare in the lower part of the Flathead, but common 

to very abundant in the upper part are trace fossils including 

Thallasinoides, Scolithus, Arenicolites, Chondrites, and other 

annelid(?) burrows.

Middle Cambrian strata of the Wolsey Formation, (_w), 

Meagher Limestone (_m), and lower part of the Park Shale 

(_p) rest conformably on the Flathead Sandstone north of the 

Volcano Valley fault zone west of Black Butte Creek and at 

the eastern edge of the quadrangle north of Sheep and Newlan 

Creeks. An erosional remnant of the Wolsey Formation, resting 

on Flathead Sandstone, is preserved on the ridge above Allan 

Park in the northeast corner of the quadrangle. Structural relief 

between this high outcrop of the Wolsey and outcrops of the formation adjacent to the Volcano Valley fault zone is about 560 m.

South of the Volcano Valley fault zone, the Strawberry 

Butte quadrangle is underlain by metasedimentary rocks of the 

Mesoproterozoic Newland Formation. In general, Newland 

Formation strata are folded and dip gently south from the 

Volcano Valley fault zone, but are folded to dip generally north 

into the fault from near Strawberry Butte (SW1⁄4 sec. 19 and 

NW1⁄4 sec. 30, T. 12 N., R. 6 E.) on the east to the southwest 

edge of the quadrangle. As mapped, the upper part of the formation (Ynu) consists of interbedded limestone, cherty limestone, 

and dolomitic argillite and argillite. Limestone intervals that 

range from 2 to 12 m thick are generally thin bedded and display internal flat to wavy laminae and local disrupted laminae. 

Several beds of arkosic arenite, 0.2–1 m thick, are present in the 

lower part of this limestone-bearing succession. The lower half 

of the upper part grades downward into dominantly dolomitic 

argillite, argillite, minor silty argillite, minor siltite, and scattered very thin interbeds of silty limestone and limestone. 

These rock types locally include very thin intervals of strongly 

contorted and broken beds of calcareous and dolomitic silty 

argillite and silty limestone. Strata of the lower half are typically intruded by Eocene biotite hornblende dacite sills (:bhd) 

in secs. 25, 26, 35 and 36, T. 12 N., R. 6 E., and secs. 1 and 2, 

T. 11 N., R. 6 E. The upper part of the formation grades downward through about 150 m into the lower part of the formation.

Figure 3. Tectonic map of west-central Montana showing 

principal faults, major tectonic elements, intrusive and extrusive 

igneous rock bodies, Tertiary strata and major areas of Quaternary 

sediments. Thrust sheets and igneous units listed in descending 

succession of inferred increasing age within tectonic elements 

(for example, autochthons and parautochthons). Red rectangular 

outline shows the location of the Strawberry Butte 7.5 ́ quadrangle 

(modified from Reynolds and Brandt, 2005, figure 2, sheet 1).

—Continued

Mesoproterozoic metasedimentary rocks of the autochthon

Mesozoic sedimentary rocks of the autochthon

Paleozoic sedimentary rocks of the autochthon; locally includes

 Eocene sills and dikes

Mesoproterozoic metasedimentary rocks of the paratochthon

Lewis and Clark tectonic zone—Boundaries inferred

Stratigraphic boundary

Major high-angle normal fault—Bar and ball on downthrown block

Major fault with complex displacement—Including normal,

 strike-slip and reverse movement through geologic time

Major high-angle reverse fault—R on upthrown block

Edge of major thrust sheet—Sawteeth on upper plate

Edge of decollement surface at base of frontal fault-and-fold belt

Fault—Dotted where concealed by Cenozoic units or water

Neoarchean metamorphic rocks of the autochthon

Paleoproterozoic metamorphic rocks of the autochthon

R

C

Quaternary and Pliocene(?) sediment in veneers on older

 rocks—Locally includes Miocene and Oligocene

 sedimentary rocks in valley of Smith River

Miocene and Oligocene sedimentary rocks—Locally includes

 Eocene(?) sedimentary rocks at base

Oligocene basalt

Oligocene(?) and Eocene rhyolite with breccia blocks of

 Paleozoic sedimentary rocks

Eocene intrusive rocks—In plutons, laccoliths, and large sills of

 the autochthon and parautochthon

Eocene and Cretaceous intrusive rocks penetrating the Moors

 Mountain thrust sheet

Paleozoic sedimentary rocks of the frontal fold-and-fault belt

Neoproterozoic diorite sill in Moors Mountain thrust sheet

Mesoproterozoic metasedimentary rocks of the Moors

 Mountain thrust sheet

Mesozoic sedimentary rocks of the paratochthon—Become

 contiguous with the autochthon in T8N, R19E

Paleozoic sedimentary rocks of the paratochthon—Become

 contiguous with the autochthon in T8N, R19E

EXPLANATION

 [Thrust sheets and igneous units listed in descending succession of

inferred increasing age; some Quaternary and Tertiary deposits are not shown]
6 Geologic map of the Strawberry Butte 7.5-minute quadrangle, Meagher County, Montana

Figure 4. Rock fragments, pebbles, cobbles, and small boulders interbedded with medium- to coarse-grained 

sandstone of the Middle Cambrian Flathead Sandstone. A, Angular to subrounded clasts of Mesoproterozoic 

Neihart Quartzite and Paleoproterozoic vein quartz, granite gneiss, some orthoclase feldspar, and rare 

clasts of biotite gneiss in the Flathead Sandstone, NW1⁄4 SW1⁄4 NE1⁄4 sec. 15, T. 12 N., R. 6 E. Pencil for scale 

is 14 centimeters (cm) long. B, Pebbles, cobbles and small boulders of Mesoproterozoic Neihart Quartzite, 

Spokane Formation and rare Greyson Formation together with fragments of vein quartz, orthoclase feldspar 

and granite gneiss derived from the Sheep Creek granite gneiss widely exposed across the northern part of the 

quadrangle. SW1⁄4 SW1⁄4 SW1⁄4 sec. 15, T. 12 N., R. 6 E. Pocket knife for scale is 9 cm long.

A

B
Stratigraphy 7

The lower part of the Newland Formation (Ynl) consists 

of laminated to very thin bedded dark gray to black dolomitic 

argillite and silty argillite with scattered very thin pale-brown 

weathering interbeds of dark gray limestone, generally 

1–4 centimeters (cm) thick. This lower part of the formation 

includes very thin to thin beds of contorted and broken argillite and dolomitic silty argillite. Several thin intervals of silty, 

argillitic dolostone are present in the Little Sheep Creek area. 

The lowest part of the formation is exposed at Strawberry 

Butte in secs. 19 and 30, T. 12 N., R.7 E., where it contains 

common disseminated sulfide minerals, and in the core of a 

broad anticline in the southeast corner of the quadrangle.

South, southwest, and west of the quadrangle a succession of Belt Supergroup rocks belonging to the Greyson 

and Spokane Formations conformably overlies the Newland 

Formation (Reynolds and Brandt, 2005, 2006). In turn, this 

succession of Belt Supergroup rocks is overlain unconformably by the Middle Cambrian Flathead Sandstone. The contrast of rocks below the Flathead north of the Volcano Valley 

fault zone and south of the fault zone documents faulting that 

is up on the north side of the fault zone and down on the south 

side, sometime during the time interval of Neoproterozoic 

through Early Cambrian (sections A–A′, B–B′, and C–C′). That 

contrast, with as much as 2,600 m of Mesoproterozoic rock, 

mostly very fine-grained strata on the south block, abruptly 

truncated against the Volcano Valley fault but absent by erosion across most of the upthrown north block, documents that 

the Mesoproterozoic Belt basin must have extended north 

well beyond the fault and likely north of the area of the Little 

Belt Mountains. The present north margin of Belt rocks is a 

structural boundary. Thus, the northern and eastern extent of 

the Belt Supergroup in the area is not a sedimentologic embayment, but rather is a structural salient bounded on the north by 

a major fault zone. 

Xenoliths in Eocene dikes intruding the Newland 

Formation in the central southern part of the Strawberry Butte 

area provide evidence of the substrate below the Newland 

south of the Volcano Valley fault zone. Xenoliths include 

fragments of the lower part of the Newland, the Neihart, and 

Paleoproterozoic quartz-biotite gneiss, biotite schist, and 

quartz-feldspar fragments derived from the granite gneiss. 

Clearly, before the Middle Cambrian Flathead Sandstone 

was deposited across the entire area, the Mesoproterozoic 

Neihart Quartzite, possibly the Chamberlain Formation, and 

Paleoproterozoic metamorphic rocks were present beneath the 

Newland on the south side of the Volcano Valley fault zone; 

however, all but the metamorphic rocks had been removed 

by erosion from the upthrown block on the north side of the 

fault zone. Debris from erosion of the area north of the fault 

zone was carried southward and incorporated in the Flathead 

Sandstone widely across south-central Montana. Strata of Late 

Cambrian through Cretaceous age, present in areas adjacent 

to the south, northwest, and east (Reynolds and Brandt, 2005, 

2007), are absent in the Strawberry Butte quadrangle.

Tertiary igneous rocks are present in three forms in 

the quadrangle. The oldest igneous rock, biotite hornblende 

dacite (:bhd) of Eocene age, described below, constitutes 

thin sills and dikes exposed across the southern margin of 

the quadrangle. An erosional remnant of younger medium to 

coarsely crystalline biotite hornblende monzonite (:bhm) 

crops out in the northeast corner of the map area (unsurveyed, T. 13 N., R. 5 E. [center, sec. PB40]). That rock is 

likely a remnant of a coarse-grained sill that extends south 

from the Woods Mountain laccolith north-northwest of the 

quadrangle ([Reynolds and Brandt, 2005] which has an age of 

53.07±0.28 Ma [L.W. Snee and D.P. Miggins, USGS, written 

commun., 2002]). The youngest igneous rocks are Oligocene 

trachybasalt (:b) exposed in the valley of Sheep Creek across 

the center of the quadrangle.

 Biotite hornblende dacite sills and dikes (:bhd) intrude 

beds of the lower part of the upper Newland Formation in the 

southern part of the quadrangle, their northern extent limited 

by the Black Butte Creek fault. The thin dacite sheets are 

intruded nearly parallel to bedding in the Newland Formation 

but locally cross laminae and beds of the Newland at low 

angles. The sills and dikes extend generally north from an 

apparent intrusive center on the north flank of Coxcombe 

Butte (fig. 2), about 1.3 miles south (secs. 2 and 11, T. 11 N., 

R. 6 E.) of the south edge of the Strawberry Butte quadrangle. 

At their northern end, the sills and dikes seem to be rotated 

structurally east adjacent to, and terminate against, the Black 

Butte Creek fault. Only one sill, too thin to map, was observed 

in the upper Newland north of the Black Butte Creek fault. 

Sills range from about 4 to 12 m thick, and have metamorphosed beds of the Newland Formation into which they are 

intruded a distance of about 5 to 20 cm adjacent to the sills. 

The biotite hornblende dacite intrusive rocks are fine 

to medium crystalline, generally holocrystalline, and locally 

porphyritic. Phenocrysts include plagioclase 2.5–4 millimeters 

(mm) across and locally common zoned crystals 5–7 mm 

across, rarely as large as 2 cm long and 0.8 cm wide. Locally, 

hornblende crystals and clots of crystals are as large as 6–8 mm 

long and 2–4 mm wide, with rare clots as much as 1.7 cm 

across. Biotite crystals are euhedral to subhedral, 1–3 mm 

across. Quartz crystals commonly display resorbed margins and 

comprise 3–5 percent of the rock. The biotite hornblende sills 

locally contain tabular fragments of quartz-biotite schist as large 

as 2 cm by 3 cm, rare fragments of granite gneiss as large as 

1.5–2 cm across, and at several localities, quartzite fragments 

0.6–1 cm across, likely derived from Neihart Quartzite below, 

and feldspar phenocrysts as large as 8 mm across, derived from 

granite gneiss basement below. Sills within and immediately 

south of the map area have 40Ar/39Ar isotopic ages ranging from 

57.04±0.21 to 54.61±0.27 Ma (L.N. Snee and D.P. Miggins, 

USGS, written commun., 2009).
8 Geologic Map of the Strawberry Butte 7.5-Minute Quadrangle, Meagher County, Montana

Andesitic basalt flows of Oligocene age (:b, 32.04±0.10 

to 31.71±0.24 Ma [L.N. Snee and D.P. Miggins, USGS, written commun., 2001]) are banked against Paleoproterozoic 

granite gneiss and associated crystalline rocks in an ancestral 

valley of Sheep Creek from the junction of Moose Creek 

and Sheep Creek (sec. 13, T. 12 N., R.6 E) northwest to Pole 

Creek at the west edge of the quadrangle (fig. 2); south-center 

sec. 32, T. 13 N, R. 6 E.). Oligocene sedimentary rocks overlie 

the basalt unconformably so that along part of the trace of 

basalt outcrops, the sedimentary rocks rest unconformably 

on the Paleoproterozoic metamorphic rocks. However, in 

NW1⁄4 sec. 3 and SE1⁄4 NE1⁄4 sec. 4, T. 12 N., R. 6 E., basalt 

rests on part of the Oligocene sedimentary sequence and is 

depositionally overlapped by the younger part of the sedimentary sequence. The basalt is microcrystalline with scattered 

phenocrysts of olivine and rare pyroxene. Locally the basalt 

is highly vesicular with white quartz and calcite filling some 

vesicles. Commonly only single flows are evident, although 

as many as 3 flows, each with a basal flow breccia, constitute 

the sequence between Squaw and Pole Creeks (NW1⁄4 sec. 3 

and SE1⁄4 NE1⁄4 sec. 4, T. 12 N., R. 6 E.) and on Sheep Creek 

north of its junction with Moose Creek (SE1⁄4 sec. 12 and 

NE1⁄4 NW1⁄4 sec. 13, T. 12 N., R. 6 E.). The exposed thickness of the basalt ranges from a wedge edge to about 75 m. 

The basalt accumulated in a northwest-trending valley eroded 

by ancestral Sheep Creek; that valley extended beyond the 

Strawberry Butte quadrangle across the area now drained by 

Jack Creek and upper Eagle Creek in quadrangles adjacent on 

the west (Reynolds and Brandt, 2005). From the junction of 

Moose and Sheep Creeks to Pole Creek, the base of the lowest 

basalt flows declines in elevation northwest about 90 m toward 

Smith River. We interpret that the ancestral valley was eroded 

likely during late Eocene time inasmuch as the Little Belt 

Mountains were the locus of laccolithic and tabular intrusions 

intruded at depth during early and early–middle Eocene (for 

example, Woods Mountain laccolith about 7–12 km north and 

northwest of the valley [fig.2]; 53.07±0.28 Ma [L.N. Snee and 

D.P. Miggins, USGS, written commun., 2004; Reynolds and 

Brandt, 2005]), but those intrusive rocks are now exposed at 

the surface as much as 245 m above early Oligocene basalt at 

the bottom of the ancestral valley. The absence of an evident 

single source for the basalt, such as a volcanic cone or dome, 

suggests that fractures or a fault, subsequently covered by the 

basalt, were the likely orifices for flows along Sheep Creek 

and between Squaw and Pole Creeks (section A–A′). An isolated outlier of basalt caps the ridge above Allan Park (unsurveyed, T. 13 N., R. 7 E. [center, PB40]) at an elevation of 

2,297–2,320 m (7,510–7,615 ft), about 620 m above outcrops 

along Sheep Creek.

Sedimentary and volcaniclastic rocks of Oligocene and 

Miocene age (N:s) are present in the drainage of Sheep 

Creek in the southeastern part of the quadrangle (secs., 19, 20, 

and 29, T. 12 N., R. 7 E.), and from Moose Creek (secs. 12 

and 13, T. 12 N., R. 6 E.) northwest beyond Pole Creek 

(diagonally across the northern part of T. 12 N., R. 6 E., into 

the southeast part of T. 13 N., R. 6 E.). These rocks rest unconformably on, and locally interfinger with, the Oligocene basalt 

(:b). The rocks include tuffaceous siltstone, commonly very 

pale orange to grayish orange pink, silty and sandy claystone, 

pebble conglomerate, and silty sandstone. Commonly near the 

base of the unit are lenses and beds of dark brown basaltic ash 

that weather to dark brown soil; higher in the sequence are 

discontinuous exposures of a vitric pumice tuff interval that 

we have traced discontinuously from near the Smith River, 

about 13 km to the west into the Strawberry Butte quadrangle. 

Clasts in the conglomerate beds range from sand-size to 

boulders as large as 2 m across with the boulders concentrated 

mainly at or near the base of the unit. Clasts are subrounded to 

rounded and consist of Flathead Sandstone, Paleoproterozoic 

granite gneiss and feldspar grains derived from the gneiss, 

Upper Cambrian and abundant Devonian and Mississippian 

carbonate rocks, and rare Mesoproterozoic calcareous argillite 

and siltite fragments. Clasts derived from the Eocene intrusive rocks in nearby areas are present but rare. Notably absent 

among clasts of all sizes are fragments of gossan, including 

ferruginous silica, which are now scattered across surface 

areas on and adjacent to altered Newland Formation strata 

exposed by erosion within and south of the Volcano Valley 

fault zone. Near the upper part of the unit are scattered to common fragments and pebbles of basalt, similar to the unit that 

underlies the sedimentary succession. Lenses of claystone and 

silty claystone might be shallow lake deposits, and we conjecture that most of the sandy and clayey siltstone beds are likely 

of windblown origin. Generally the beds are poorly exposed 

but can be excavated at shallow depths and are locally exposed 

both along U.S. Forest Service road 119 and in gullies eroded 

across the unit.

In addition to thin deposits of alluvium in active stream 

channels and several young gravel deposits on terraces 

above the active creeks, two groups of upper Tertiary(?) or 

Quaternary deposits were mapped in detail. The first group 

consists of scattered to common cobbles and boulders of 

Paleozoic sedimentary rocks distributed in patches and linear 

clusters across bedrock surfaces. In several areas such as 

on the flanks of Black Butte Creek (secs. 20, 21, 27, 28, 29, 

T. 12 N., R. 6 E.) the boulders are so abundant that the deposits nearly resemble outcrops of the principal Paleozoic bedrock 

unit from which the boulders were derived. In particular, 

clasts of Devonian Jefferson Formation as large as 6 × 3 × 2 m 

(above ground surface) constitute the principal component of 

the boulder deposits. The mantle of boulders contains fewer 

and smaller clasts of Mississippian Mission Canyon and 

Lodgepole Limestones, and Cambrian Pilgrim Limestone, and 

sparse, but locally common Cambrian Flathead Sandstone. 

Most clasts are 0.6–1 m across. The concentration of the boulders on the middle reaches of Black Butte Creek and Copper 

Creek suggest that those drainages were common tracks for 

floods or debris flows that transported the boulders west and 

north from outcrops of those units in the Green Mountain area 

about 9–12 km east-northeast of the deposits (fig. 2). Common 
Structure 9

angular fragments of Cambrian Flathead have been shed on 

hillsides radially south, west, and north from outcrops of the 

formation on Black Butte.

Deposits of iron gossan, iron-stained bedrock, and colluvial fragments of the gossan constitute the second mapped 

group of Tertiary(?) and Quaternary age deposits. Bedded iron 

gossan that dips north is present nearly continuously from the 

NE1⁄4 sec. 34, T. 12 N., R. 6 E., east-northeast across sec. 26, to 

the center of sec. 27, T. 12 N., R. 6 E., between the Black Butte 

Creek fault on the south and limestone beds of the Newland 

Formation on the south side of the Volcano Valley fault. No 

major cross faults seem to disrupt the continuity of either the 

limestone beds or the bedded gossan. At Iron Butte, E1⁄2 sec. 34, 

T. 12 N., R. 6 E., bedded iron gossan dips west at about 12o–20o, 

shallow enough to have been mined and transported in the past 

to near Three Forks, Montana to serve as flux in production of 

cement (fig. 5). Similar but more local gossan beds are present in the southern third of sec. 24, T. 12 N., R. 6 E. adjacent 

to the Volcano Valley fault and discontinuously east of Little 

Sheep Creek in SE1⁄4 sec. 31 and SW1⁄4 sec. 32, T. 12 N., R. 7 E. 

Mapped separately adjacent to these areas are dolomitic argillite beds stained moderate red and grayish red by iron oxides 

derived from the gossans. These stained beds might locally 

be indicative of iron-bearing sulfides at depth in their vicinity, 

such as in the N1⁄4, sec. 33, T. 12 N., R. 6 E., and in secs. 31 and 

32, T. 12 N., R. 7 E., as well as in sec. 25 T. 12 N, R. 6 E., and 

sec. 30, T. 12 N., R. 7 E. Areas across which bedrock is mantled 

with colluvial boulders and fragments of iron gossan are shown 

on the map as potentially useful indicators of upslope sites of 

iron gossan or iron sulfide beds.

Structure

Contrasting geologic structures in the Strawberry Butte 

quadrangle result from the juxtaposition across the Volcano 

Valley fault zone of complex geologic structure within the 

Lewis and Clark tectonic zone on the south and structures 

of the continental autochthon north of the fault zone (fig. 3). 

The zone is a fundamental break in the earth’s crust in westcentral Montana. The Volcano Valley fault zone arcs from 

west to north to southeast across the southern part of the 

Strawberry Butte quadrangle. Structural relations along and 

across the fault zone are complex with regard to rock types 

involved, geometry of the faults, and ages of movement along 

the zone. The northernmost principal fault bounding the 

fault zone generally separates Mesoproterozoic sedimentary 

rocks (Newland Formation of the Belt Supergroup) on the 

south from Paleoproterozoic granite gneiss and biotite gneiss 

overlain unconformably by Paleozoic rocks on the north side 

of the fault zone. The dip of the principal fault varies along its 

strike: along its western trace in the quadrangle, the principal 

fault varies from nearly vertical to approximately 75o–80o 

south and southeast. Along a short trace of the fault north of 

Strawberry Butte (SW1⁄4 sec. 19, T. 12 N., R. 7 E.,) the apparent dip of the fault reverses so that the fault dips north and 

displaces the Cambrian Flathead Sandstone over the lower 

part of the Newland Formation at the surface and at shallow 

depths. The dip of that segment of the fault likely reverses 

with depth to again dip steeply south and southwest. We suggest that, at the maximum curvature of the fault zone (sec. 24 

Figure 5. Fractured hematite-stained strata in the upper part of the Newland Formation have been 

excavated on the west flank of Iron Butte and in the past shipped to Three Forks, Montana, for use 

as flux in the production of cement. Lenses and thin irregular beds of gossan are interbedded in the 

altered rocks of the Newland Formation here and along the trace of the Volcano Valley fault zone.
10 Geologic Map of the Strawberry Butte 7.5-Minute Quadrangle, Meagher County, Montana

T. 12 N., R. 6 E., and sec. 19, T. 12 N., R 7 E.), the zone 

intersects at depth in the crust with the major east-west fault 

zone that extends yet farther east along Sheep Creek (Sheep 

Creek fault zone) and beyond as high-angle fault zones (North 

Flank and Waite Creek fault zones bounding the north flank of 

the eastern part of the Little Belt Mountains [fig. 3; Reynolds 

and Brandt, 2007]). The southeast striking segment of the 

Volcano Valley fault zone continues farther southeast and east 

as an oblique-slip fault, where it separates parautochthonous 

Mesoproterozoic rocks of the Helena structural salient from 

younger rocks of the autochthon on the north-northeast 

(fig. 3).

Narrow wedges of Paleoproterozoic granite gneiss and 

biotite gneiss are present within the fault zone along the south 

side of the principal fault and are separated by another major 

fault bounding the south margin of the zone from continuously 

exposed Mesoproterozoic sedimentary rocks on the south. 

The lengths of the wedges vary from west to east from about 

3.9 km (wedge extends west into the adjacent Sheep Mountain 

quadrangle) to 0.62 km to 2.1 km; they pinch and swell in 

width from about 300 m to 25 m to 200 m from west to east 

along strike. Rocks along the fault margins of the wedges are 

sheared and locally mylonized, but rocks within the wedges 

generally maintain their coarser gneissic texture. The strike 

wedges along the trace of the fault zone suggest a component 

of east-northeast horizontal shear together with vertical shear 

during deformation along the zone in order to generate the 

narrow wedges of the Paleoproterozoic rocks. The net oblique 

movement likely translated into dominantly reverse movement 

in the southeast quarter of the quadrangle.

In the Sheep Mountain quadrangle, immediately adjacent 

on the west to the Strawberry Butte quadrangle, very thin 

tectonic wedges, here termed selvages, of Neihart Quartzite 

are faulted between the Paleoproterozoic gneissic rocks along 

the Volcano Valley fault zone and the Mesoproterozoic strata 

south of the fault zone (Reynolds and Brandt, 2005). Those 

quartzite selvages provide evidence of major vertical movement along the zone, inasmuch as the Neihart Quartzite generally lies as much as 1,050 m stratigraphically beneath strata of 

the Newland Formation exposed there at the surface.

A major fault zone, here called the Black Butte Creek 

fault zone, diverges from the Volcano Valley fault zone in 

the eastern edge of the adjacent Sheep Mountain quadrangle 

(NW1⁄4 sec. 32, T. 12 N., R. 6 E. [Reynolds and Brandt, 2005]). 

As the Black Butte Creek fault diverges from the Volcano 

Valley fault zone, the trace of Black Butte Creek fault also 

arcs across the Strawberry Butte quadrangle. Limestone units 

of the upper part of the Newland Formation between the two 

faults are folded anticlinally at the west end of the enclosed 

block, but from near Copper Creek to the western edge of 

NW1⁄4 sec. 25, T. 12 N., R. 6 E., the limestone units and preserved iron-silica gossan units dip north. Those strata between 

the principal faults in sec. 24 and 25, T. 12 N., R. 6 E., are 

folded in a west-trending syncline. No evidence of cross faults 

exists in the north dipping beds between the faults from the 

NE1⁄4 sec. 34, T. 12 N., R. 6 E. and SE1⁄4 sec. 27, T. 12 N., 

R. 6 E., as far east as the NW1⁄4 sec. 25, T. 12 N., R. 6 E.

Beds of the Newland Formation and Eocene dikes and 

sills south of the Black Butte Creek fault strike north at moderate to high angles into the fault. In the southeast corner of 

the quadrangle, the eastward curvature of the dikes and sills 

adjacent to the Black Butte Creek fault suggest a component 

of lateral slip, south side moving west along the fault. Apparent stratigraphic separation on the fault increases from the 

west edge of the quadrangle where displacement is within 

anticlinally folded limestone units of the upper part of the 

Newland Formation to the southeast corner where the upper 

Newland on the east-northeast side of the fault is juxtaposed 

against the lowest part of the formation on the southwest side. 

Dolomitic argillite and siltite beds southwest of the Black 

Butte fault are folded in a northwest-plunging anticline that 

terminates against the fault in the SE1⁄4 sec. 25, T. 12 N., 

R. 6 E. The stratigraphic separation thus suggests a rotational 

component of oblique slip along the fault, south side west and 

progressively up eastward.

North of the Volcano Valley fault zone, offset of the 

unconformity between the base of the Flathead Sandstone and 

the underlying Paleoproterozoic crystalline rocks demonstrates 

vertical structural relief of about 300–350 m between outcrops 

of the base of the Flathead against the fault zone to the base of 

the Flathead above Allan Park (T. 13 N., R 7. E. [secs. PB40 

and 41]; section C–C′). General south dips of 5o to 10o from 

the high outcrops at Allan Park to the gentle synclinal axis in 

the Flathead in secs. 22, 23, and 24, T. 12 N., R 6 E., define 

the upward arch of the Paleoproterozoic core of the Little Belt 

Mountains (sections A–A′ and C–C′). The south limb of the 

gentle arch declines south across the northern two-thirds of 

the quadrangle. That limb is broken by east-trending normal 

faults which displace the south flank of the arch downward 

nearly 650 m from outcrops up-plunge in the Moose Mountain 

quadrangle adjacent on the east to the Strawberry Butte 

quadrangle. We infer that a buried normal fault zone follows 

the general west-northwest trend of Sheep Creek as far as 

Pole Creek (sections A–A′ and B–B′). Oligocene strata and 

Oligocene basalt cover the trace of the inferred fault. If so, 

those units document the age of faulting and, particularly, the 

time of erosion of the valley of Sheep Creek as pre-Oligocene, 

likely middle and late Eocene. Similarly, from the narrows on 

Sheep Creek near Strawberry Butte east across the adjacent 

Moose Mountain quadrangle, Oligocene and Miocene strata 

and Oligocene basalt are present in the lower part of the valley 

of Sheep Creek (Reynolds and Brandt, 2007). Erosion of the 

valley predated the Oligocene units, and the linearity of the 

valley further suggests that the valley might have originated 

by erosion along a fault zone active during late Eocene time 

that subsequently served as conduits for the basalt to reach 

the surface.
Acknowledgments 11

Arching along the axis of the western part of the Little Belt 

Mountains accompanied by normal faulting along the south 

flank (sections A–A′; C–C′) seems to have been active as late as 

Pliocene time. The arching might have resulted in the southward migration and superposition of meanders of Sheep Creek 

onto the Paleoproterozoic granite gneiss core of the quadrangle. 

Across the center of the quadrangle normal faults have the 

structural effect of displacing the Flathead Sandstone downward 

toward the Volcano Valley fault zone. At the southwest edge 

of the quadrangle, west-trending normal faults offset middle 

and upper Cambrian rocks adjacent to the fault zone on the 

north. These normal faults terminate in deformed shale beds of 

the Wolsey Formation which are, in turn, truncated by northnorthwest trending normal faults that terminate south against the 

Volcano Valley fault zone and merge north along Copper Creek 

in secs. 21 and 28, T. 12 N., R. 6 E. We have not found evidence 

of northeast- or north-trending faults in the quadrangle.

The geologic structure at Black Butte (N1⁄2 sec. 26, 

T. 12 N., R. 6 E.) is anomalous (section B–B′). Aprons of colluvial fragments of Flathead Sandstone with rare fragments of 

Meagher Limestone conceal bedrock beneath the northeast, 

north, and west flanks of the Butte. Silica-cemented quartz 

arenite that supports and is exposed on the Butte is highly 

sheared and brecciated throughout. In much of the exposed area 

of bedrock, iron has been leached from the sandstone so that its 

color is very light gray, nearly white. Nevertheless, segments of 

the sheared rock locally contain granules and pebbles of quartzite and rare burrows(?) similar to the nearby unsheared Flathead 

Sandstone. Slickensides on rock slices exposed on the Butte are 

commonly oriented about 260o with dips of 5o southwest and 

98o with dips 10o to 50o east, generally parallel to the principal 

fault of the Volcano Valley fault zone; other slickensides dip 60o 

to 75o south, down the apparent dip of the fault zone and yet 

others dip only a few degrees south. The structure is interpreted 

here as tectonically shingled segments of Flathead Sandstone, 

interleaved along the Volcano Valley fault zone by recurrent 

reverse movement during Neoproterozoic to Middle Cambrian 

time and normal movement along high-angle shears during middle Eocene to possibly late Pliocene time. Shearing and leaching 

of iron and silica cementation might have resulted from thermal 

effects of an intrusion at depth, suggested also by the unique 

radial topographic expression of the Flathead Sandstone only in 

the area of Black Butte.

Summary

The Strawberry Butte quadrangle provides a discontinuous record of recurrent tectonism in this segment of 

west-central Montana. North and northeast of the Strawberry 

Butte quadrangle, Mesoproterozoic sedimentary rocks rest 

unconformably on the metamorphosed Paleoproterozoic 

rocks (Keefer, 1972). Also, xenoliths of Paleoproterozoic 

and late Archean metamorphic rocks in biotite hornblende 

dacite intruded into the Mesoproterozoic Newland Formation 

suggest that the Mesoproterozoic Belt succession in the 

southern part of the quadrangle rests unconformably on older 

Paleoproterozoic and Archean metamorphic rocks exposed 

by deformation, uplift, and erosion during late Paleoproterozoic or early Mesoproterozoic time, before deposition of the 

Belt Supergroup strata. A major episode of uplift, principally 

on the north side of the Volcano Valley fault zone, occurred 

between early Neoproterozoic and Middle Cambrian time. 

That movement is represented by the unconformity between 

the Middle Cambrian Flathead Sandstone that rests on 

Paleoproterozoic metamorphic rock north of the fault zone and 

on Mesoproterozoic sedimentary strata on the south side of the 

Volcano Valley fault zone. During that time interval, vertical 

movement on the north side of the fault zone in the quadrangle 

was likely 1,600–1,750 m with an unknown, but significant 

component of horizontal movement.

A major intrusive episode is represented by both the biotite 

hornblende monzonite in the Allan Park area that is an outlier of 

extensive igneous intrusion northwest of the quadrangle (Woods 

Mountain laccolith, dated as 53.07±0.38 to 50.83±30 Ma 

[Reynolds and Brandt, 2005]) and the biotite hornblende 

dacite dikes south of the Black Butte Creek fault. We infer 

that horizontal and vertical fault movement on the Black Butte 

Creek fault occurred during the late early and middle Eocene 

(52–45 Ma). The valleys of Moose and Sheep creeks, possibly 

along a middle to late Eocene fault, were eroded prior to extrusion of basalt (32.04–31.71 Ma) in the valleys. We infer that 

the youngest phase of deformation was gentle arching of the 

northern two-thirds of the quadrangle likely during Miocene and 

possibly Pliocene time. Our inference is based on the superposition of Sheep Creek meanders across both Miocene-Oligocene 

sedimentary rocks and basalt in the northwest part of the quadrangle and across Paleoproterozoic metamorphic rocks in the 

central western part of the quadrangle.

Acknowledgments

Access to private lands in the Strawberry Butte quadrangle was graciously provided by local ranchers and land 

owners. The U.S. Forest Service, Lewis and Clark National 

Forest, through the White Sulphur Springs District Office, 

facilitated access to the public lands in the quadrangle. 

David J. Lidke and Scott C. Lundstrom (USGS) through their 

technical reviews improved the presentation of the map and 

text. Jeremy Havens drafted figure 1 and, through successive 

reviews, patiently digitized the geologic sections and correlation of map units diagram. Julie A. Herrick (USGS) reviewed 

geologic names and terminology used in the report, skillfully 

enhanced figure 2, and served as a thoughtful and careful editor. Figure 3 was prepared by Springfield and Springfield, Inc., 

Fort Collins, Colorado.
12 Geologic Map of the Strawberry Butte 7.5-Minute Quadrangle, Meagher County, Montana

Description of Map Units

[Terminology for carbonate rocks from Folk, 1962]

Qa Alluvium (Holocene)—Unconsolidated stratified to poorly stratified sand, gravel, silt, and 

clay, deposited primarily by streams; locally includes some colluvium and some gravel 

on low terraces in stream valleys. Thickness as much as 18 m

Qac Alluvium and colluvium, undivided (Holocene)—Unconsolidated stratified to poorly 

stratified rock fragments, gravel, sand, and silt deposited by gravity and stream transport 

down hillsides and rills east of Black Butte in 1⁄4NE 1⁄4SE 1⁄4NE sec. 26, T. 12 N., R 6 E., 

and along valley walls of Moose Creek in west-center sec. 7, T. 12 N., R. 7 E.; thin local 

deposits are not shown. Thickness as much as 14 m

Qc Colluvium (Holocene)—Unconsolidated generally unstratified rock fragments, gravel, 

some interstitial silt deposited by gravity on hillsides; mapped locally west-southwest 

of Black Butte in NE1⁄4 SE1⁄4 sec. 27, T. 12 N., R 6 E.; very thin local deposits are not 

shown. Thickness as much as 7 m

Ql Landslide deposits (Holocene and Pleistocene)—Angular blocks and fragments of basalt 

(:b), other rock debris, and soil that have moved down slope on shale substrate in 

N1⁄2 SE1⁄4 sec. PB40 (unsurveyed), T. 13 N., R. 7 E.; generally chaotic and unstratified 

fragments. Thickness as much as 12 m

Qt Terrace gravel (Holocene? and Pleistocene)—Gravel, sand, and some silt, generally 

unconsolidated, locally stratified and poorly stratified; clasts rounded to subrounded; 

contains angular colluvial rock fragments derived from adjacent hillside; mantles 

smooth surface inclined gently north at elevations about 3–8 m above recent stream 

channel of Copper Creek in sec. 28, T, 12 N., R. 6 E., and mantles a surface inclined 

gently east at elevations about 6–10 m above Sheep Creek in SE1⁄4 SE1⁄4 sec. 13, 

T. 12 N., R. 6 E. Thickness as much as 5 m

Qoa Old alluvium (Holocene? or Pleistocene)—Stratified to poorly stratified sand, gravel, silt, 

and clay; unconsolidated to weakly and locally moderately consolidated; deposits generally form abandoned braided alluvial fans where major streams emerged from areas of 

confining bedrock slopes; also forms rounded natural benches adjacent to, and dissected 

by, braided modern stream channels. Deposited primarily by sheetflood. Mapped at 

the east edge of the quadrangle in secs. 19, 20, 29, and 30, T. 6 N., R. 7 E. Thickness 

0–10 m, but locally as much as 15 m

QNg Older gravel (Pleistocene? and Neogene?)—Gravel, sand, and some silt, unconsolidated 

to weakly consolidated; stratified; deposited dominantly by streams and as colluvium; 

clasts rounded to subrounded, subangular and locally angular; pebbles and small 

cobbles of Phanerozoic, Mesoproterozoic, and Paleoproterozoic rocks in silty or clayey 

silty matrix. Highest deposits consist of scattered pebbles in thin, patchy unconsolidated 

matrix on bedrock. Deposits at different elevations are likely of significantly different 

ages: Highest deposits (about 1,787 m, north-center sec. 25, T. 12 N., R.6 E.) might be 

as old as Miocene(?), and in NE1⁄4 NW1⁄4 sec. 30, T. 12 N., R. 7 E. might be as old as 

Pleistocene or Pliocene. Thickness as much as 12 m

N:s	 Sedimentary rocks (Miocene and Oligocene)—Shown as MIOGS in Reynolds and Brandt 

(2005, 2007). Pebble to boulder conglomerate, clayey siltstone, sandy tuffaceous siltstone, 

sandstone, vitric tuff, and local basaltic tuff; siltstone, clayey siltstone, and sandy tuffaceous 

siltstone; grayish orange pink and very pale orange, locally grayish orange pink, weathers 

same; vitric tuff very light gray to white, weathers same; basaltic tuff dusky brown and 

grayish brown, weathers same; pebble and boulder conglomerate grayish orange to pale 

yellowish brown, weathers same; conglomerate lenses contain subangular to dominantly 

well rounded granules, pebbles, and cobbles of Paleoproterozoic metamorphic rocks, 

Mesoproterozoic clasts derived from within and north of the Strawberry Butte quadrangle 

(Reynolds and Brandt, 2007), Paleozoic sedimentary rocks, rare Eocene intrusive rocks 

and, in the upper part, Oligocene basalt clasts. Separate conglomerate beds commonly 
Description of Map Units 13

have different sources of dominant clasts, including beds with dominantly resistate clasts 

including Paleoproterozoic granitic and basic metamorphic rocks, Mesoproterozoic and 

Middle Cambrian quartzite, metasiltite, siltite and siltstone derived from the north-central 

Little Belt Mountains, and beds with dominantly carbonate clasts derived from Paleozoic 

terranes southeast and east of the quadrangle. Subrounded to rounded boulders of Flathead 

Sandstone, locally as large as 2 m across, are concentrated in the lowest part of the unit 

above underlying basalt flows (:b), and are more widely scattered higher in the unit. Vitric 

tuff and tuffaceous siltstone beds contain abundant glass shards, some very fine biotite crystals and scattered sanidine(?) crystals, and contain reworked rounded sand grains derived 

from older units. Unit N:s laps unconformably onto Paleoproterozoic metamorphic rocks, 

Middle Cambrian rocks in secs. 19 and 20, T. 12 N., R. 7 E. and onto Oligocene basalt 

(:b). Thickness as much as 85 m

:b Basalt (Paleogene)—Shown as OGb in Reynolds and Brandt (2005, 2007). Trachybasalt, 

very dark gray to black, locally reddish orange; microcrystalline, generally equicrystalline; scattered phenocrysts of olivine and rare local biotite pegmatite in narrow 

fractures; locally vesicular. Unit consists of several flows, likely derived from fracture 

eruptions; local thin flow breccia between successive flows; 40Ar/39Ar isotopic ages 

range from 32.04±0.10 to 31.71±0.24 Ma (L.N. Snee and D.P. Miggins, USGS, written 

commun., 2001). Unit occupies ancestral valley of Sheep Creek in north-central part of 

the quadrangle and caps the ridge above Allan Park in NW1⁄4 SE1⁄4 sec. PB40 (unsurveyed), T.13 N., R.7 E. Thickness as much as 32 m

:bhd Biotite hornblende dacite (Paleogene)—Shown as EObhd in Reynolds and Brandt (2005, 

2007). Medium gray to medium-light-gray dacite: phenocrysts of biotite and hornblende 

in finely crystalline to aphanocrystalline groundmass of plagioclase feldspar, minor 

sanidine, rare quartz, common fine biotite, and some hornblende; some sills have clots 

of small hornblende crystals as much as 17 mm across; scattered quartz crystals as 

much as 2 mm across, many with resorbed margins; plagioclase phenocrysts as much 

as 3–4 mm across; scattered hornblende needles about 7 mm long; rock locally contains 

xenoliths, as large as 5 cm across, of Paleoproterozoic granite gneiss and biotite quartz 

gneiss and Mesoproterozoic quartzite; unit present as sills in the southern part of the 

quadrangle. Unit has 40Ar/39Ar age range from 57.04±0.21 to 54.61±0.27 Ma (L.N. Snee 

and D.P. Miggins, USGS, written commun., 2009)

:bhm Biotite hornblende monzonite (Paleogene)—Shown as EObhm in Reynolds and Brandt, 

2005, 2007 Very light gray to medium gray, weathers very light gray, light brownish gray, and locally very pale red; medium to coarsely crystalline, equicrystalline to 

porphyritic; common to abundant biotite and euhedral crystals of hornblende; alkali 

and sodic feldspar in nearly equal abundance in matrix and as phenocrysts as large as 

8 mm across; scattered quartz crystals; locally contains xenoliths of Paleoproterozoic 

and Mesoproterozoic bedrock; exposed on ridge above Allan Park in NW1⁄4 SE1⁄4 

sec. PB40, (unsurveyed), T. 13 N., R. 7 E.; the unit might be a remnant distal sill 

related to the Woods Mountain laccolith present northwest of the map area and dated as 

53.07± 0.28 Ma (Reynolds and Brandt, 2005)

_p Park Shale (Upper and Middle Cambrian)—Silty claystone and claystone, sparse very 

thin channels filled with pebbly biosparite limestone; light olive gray and light olive, 

weathers olive gray and grayish green; fissile and platy. Widely scattered discontinuous lenses, 2–4 cm thick, contain comminuted trilobite, pelmatozoan(?) and brachiopod 

fragments. Only the lowest part of the formation exposed in center sec. 29, T. 12 N., 

R. 6 E. Exposed thickness about 18 m

_m Meagher Limestone (Middle Cambrian)—Limestone, medium gray, medium dark gray, 

and medium brownish gray, weathers medium light gray with common grayish orange, 

moderate brown, and pale grayish red silty mottles, particularly in basal 10 m and in 

top 12 m. Finely crystalline limestone with channels and beds of medium- to coarselycrystalline biosparite and biopelsparite limestone. Discontinuous wavy laminae and 

beds 0.2–4 cm thick, with discontinuous silty limestone partings; weathers with knobby, 
14 Geologic Map of the Strawberry Butte 7.5-Minute Quadrangle, Meagher County, Montana

locally reticular texture especially at base and top; forms low rounded benches. Contains locally abundant small trilobite fragments, some pelmatozoan(?) plates, and sparse 

chitinous brachiopod fragments. Thickness about 15–32 m

_w Wolsey Shale (Middle Cambrian)—Silty mudstone and siltstone, light olive to light grayish green; weathers light olive gray, dusky yellow gray, and grayish red. Very thin beds 

of pale grayish red silty sandstone interbedded in olive green siltstone in basal 3 m. 

Limestone laminae and beds, 0.2–3 cm thick, are scattered throughout the siltstone in 

the upper part of the formation; limestone is light gray to light greenish-gray, weathers 

pale yellowish brown to grayish orange; trilobite biosparite; some greenish-gray siltstone chips on parting surfaces. Siltstone in lower third contains scattered Cruziana and 

annelid(?) trace fossils. Strata split shaly and platy; limestone splits platy and flaggy. 

Only the lower part of the formation is exposed in S1⁄2 sec. PB40 (unsurveyed) above 

Allan Park. Apparent thickness varies as a result of local internal deformation: Thickness 38–50 m

_f Flathead Sandstone (Middle Cambrian)—Sandstone, pebbly sandstone, local conglomerate; very thin sandy siltstone beds at top; dominantly pale red, locally yellowish gray, 

and very pale red; weathers pale reddish brown, pale red, locally yellowish gray, and 

light brownish gray. Sandy siltstone interbeds at top are dark grayish red and greenish gray. Sandstone: fine to coarse grained, with locally abundant pebbles as large as 

3–6 cm across; cobbles and small boulders in scattered channels in lower and middle 

parts of the formation and locally near the top. Clasts are quartz, quartzite, red and 

gray chert and siliceous siltite; minor to locally common feldspar; clasts as much as 

1 cm across; clasts are generally rounded, but shapes modified by pressure solution 

and quartz overgrowths; weakly to firmly indurated; glauconite grains scattered in 

uppermost 4 m and increase in abundance toward the top. Across the south-central part 

of the quadrangle the formation contains intervals of very coarse-grained sandstone 

with clasts dominantly of quartzite, derived likely from the Neihart Quartzite and very 

thin to thin lenticular beds of conglomerate containing clasts as large as 4 cm across in 

sequences 0.3–4 m thick, which fill channels of low relief. Conglomerate clasts consist 

of well-rounded cobbles and boulders of Mesoproterozoic Neihart Quartzite, including Neihart granule-quartz-pebble quartzite, silicified siltite and argillite likely derived 

from the Spokane Formation, common polycrystalline quartz, rounded twinned feldspar 

crystals, rare pebbles and small cobbles of granite gneiss and biotite gneiss, and light 

green to medium gray siliceous siltite and argillite derived possibly from the Greyson 

Formation. Formation displays cross-laminae to very thin cross beds, herringbone crossbedding in the lower part; thin sets of trough and ripple cross-laminae common in upper 

part in which split surfaces locally display ripple marks and some desiccation cracks; 

formation splits flaggy and slabby, locally platy; forms resistant ledges and ridges. 

Trace fossils, including Scolithus, Thallasinoides, Arenicolites, Chondrites, and other 

annelid(?) burrows, are scattered to locally very abundant in the uppermost 10–15 m, 

and present, although rare, in the lower part of the formation. Local friable glauconitic 

sandstone lenses in the top 2 m contain rare chitinous linguloid brachiopods. Formation unconformably overlies Paleoproterozoic metamorphic rocks in the quadrangle 

north of the Volcano Valley fault zone and rests unconformably on successively younger 

Mesoproterozoic metasedimentary rocks of the Belt Supergroup south and southwest 

of the quadrangle (Reynolds and Brandt, 2006). At Black Butte, N1⁄2 sec. 26, T. 12 N., 

R. 6 E., formation is strongly sheared, repeated by small faults, and recrystallized

adjacent to the Volcano Valley fault, but remnants of granule and pebble conglomerate

beds and rare trace fossils are preserved locally within sheared blocks. Thickness about

78–110 m

Belt Supergroup, part (Mesoproterozoic)

Newland Formation (Mesoproterozoic)

Ynu Newland Formation, upper part (Mesoproterozoic)—Calcareous and dolomitic 

argillite, calcareous siltite, limestone and thin arkosic sandstone intervals in upper 

part. Argillite and siltite are medium gray and medium dark gray; weather pale grayish 
Description of Map Units 15

orange and grayish orange; very thin even parallel and wavy parallel laminated locally 

producing a varve-like appearance on weathered surface; local very thin low-angle 

inclined laminae. Limestone is medium light gray and medium gray, weathers light and 

medium light gray; aphanocrystalline and very finely crystalline, locally silty; sparry 

calcite replacement locally forms some curved surfaces and reticulate patterns in limestone; scattered to common coarse calcite fills fractures; beds locally highly contorted 

and cleaved; limestone intervals 0.5–5 m thick. Arkosic sandstone is grayish orange 

pink; weathers very pale orange, and light brown; fine to medium grained, local coarse 

grains to granules; clasts of quartz, microcline feldspar, polycrystalline quartz, and rare 

laminated limestone; beds structureless to cross-laminated at low angles; local internal 

slump structures; beds 0.04–0.3 m thick. Lower exposed part of unit splits shaly and 

platy, and supports rounded slopes; upper part splits shaly and, in limestone intervals, 

platy to slabby; forms ledges on steep slopes and ridge crests. Thickness 0 to about 

2,100 m

Ynl Newland Formation, lower part (Mesoproterozoic)—Dolomitic argillite, siltite, argillite, and silty limestone and dolostone; rare very thin laminae of fine to medium-grained 

arkose. Dolomitic argillite: medium dark gray and dark gray, weathers same to medium 

light gray, locally with light brown to dark yellowish orange staining on surfaces; siltite: 

dark gray, to mainly medium dark gray, locally black and blocky, weathers same; silty 

limestone and silty dolostone medium gray and medium brownish gray, weather light 

brownish gray, light olive gray, and pale yellowish brown; dolomitic argillite and argillite very thinly laminated to dominantly laminated, with locally common thin intervals 

of slump-contorted laminae, and local interbeds of silty limestone and dolostone; black 

siltite in SE1⁄4 SW1⁄4 SW1⁄4 sec. 19, T. 12 N., R. 7 E. contains discontinuous stringers and 

laminae of chalcopyrite and pyrite, Unit splits mainly shaly and platy; silty limestone 

and silty dolomite beds split flaggy. Argillite with local siltite increase in abundance 

downward. Unit forms rounded hills or weathers to swales between Eocene biotite hornblende dacite sills (:bhd). Partial exposed thickness about 500 m

Xgg Granite gneiss (Paleoproterozoic)—Granite gneiss, pale red, moderate pinkish red, and grayish 

orange pink, weathers pale red, light brownish orange pink, to light brownish gray; mediumto coarsely-crystalline; weak to locally moderate foliation defined by alignment of biotite 

and some hornblende in coarsely crystalline quartz, alkali feldspar, minor sodic feldspar, 

local muscovite, and rare garnet. Unit includes common to locally abundant undifferentiated bodies of biotite gneiss similar to unit Xbg, below, and thin bodies of amphibolite; unit 

forms steep, rounded slopes and local cliffs. The granite gneiss complex has an approximate 

age of 1,817 Ma, and an amphibolite in the complex yielded an age of 1,851±11 Ma (Vogl 

and others, 2004, p. 20, 30)

Xgbg Granite gneiss and biotite gneiss undivided (Paleoproterozoic)—medium light gray, light 

olive gray, to medium gray, weathers same to dark greenish gray; generally medium 

crystalline, but locally coarsely crystalline; becomes finely crystalline near margins of 

principal bodies; plagioclase and orthoclase feldspar in nearly equal amounts, abundant 

hornblende and common biotite; common quartz; Dark-gray and medium-dark-gray 

biotite gneiss; grades into biotite schist; biotite with intercrystalline sodic feldspar and 

some hornblende; bodies of biotite gneiss are foliated and lineated throughout and 

locally strongly foliated and surrounded by granite gneiss; unit is generally more mafic 

than unit Xgg. Unit occurs in thin elongate wedges bounded by faults within the structural zone that constitutes the Volcano Valley fault in the quadrangle

Xbd Biotite hornblende diorite (Paleoproterozoic)—Inclusions of fine- to medium-crystalline 

metamorphosed hornblende diorite in coarsely, crystalline granite gneiss (Xgg); dark 

gray; weathers predominantly very dark gray and black; metamorphic intergrowths of 

hornblende and sodic feldspar; mapped in SE1⁄4 sec. 18, T. 12 N., R. 6 E.
16 Geologic Map of the Strawberry Butte 7.5-Minute Quadrangle, Meagher County, Montana

References Cited

Folk, R.L., 1962, Spectral subdivision of limestone types, in

Ham, W.E., ed., Classification of carbonate rocks—A symposium: American Association of Petroleum Geologists, Memoir 

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Keefer, W.R., 1972, Geologic map of the west half of 

the Neihart 15-minute quadrangle, central Montana: 

U.S. Geological Survey Miscellaneous Geologic Investigations Series Map I–726, scale 1:50,000. [Also available at 

https://pubs.er.usgs.gov/publication/i726]

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 Vogl, J.V., Foster, D.A., Mueller, P.A., Wooden, J.L., 

and Mogk, D.W., 2004, Lithology and age of pre-Belt 

Precambrian basement in the Little Belt Mountains, 

Montana—Implications for the role of the Great 

Falls Tectonic Zone in the Paleoproterozoic assembly 

of North America: Northwest Geology, v. 33, 

p. 15–34.

Weed, W.H., and Pirsson, L.V., 1900, Geology of the 

Little Belt Mountains, Montana: U.S. Geological Survey 

Annual Report 20 (1898–1899), pt. 3, p. 257–595, 

pls. 58–77. [Also available at https://pubs.usgs.gov/

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Zieg, G.A., Feeback, D.L., Vogl, J.J., Foster, D.A., Mueller, 

P.A., and Mogk, D.W., 2004, A field tour through the

Proterozoic rocks of the southern Little Belt Mountains:

Northwest Geology, v. 33, p. 100–110.

Zieg, Jerry, Scartozzi, Vince, Chutas, Nathan, Albers, Doyle, 

Gostomski, Kevin, and Jones, Jennifer, 2013, Black Butte 

Copper deposits, lower Belt Supergroup, Montana: Northwest Geology, v. 42, p. 131–148.

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Reynolds and Brandt—Geologic Map of the Strawberry Butte 7.5’ Quadrangle, Meagher County, Montana—Scientific Investigations Map 3379

ISSN 2329-132X (online)

https://doi.org/10.3133/sim3379