WYOMING WYOMING PARK COUNTY WYOMING PARK COUNTY

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45°15'

45°00'

45°30'

45°45'

109°00'

109°30'

109°30' 109°00'

110°00'

110°00' 

QUATERNARY

TERTIARY

Pleistocene

Holocene and

Pleistocene

Unconformity

Unconformity

UnconformityUnconformityUnconformity

Eocene

Paleocene 

Upper

Cretaceous CRETACEOUS

Lower

Cretaceous 

JURASSIC

TRIASSIC

SEDIMENTARY AND VOLCANIC ROCKS

INTRUSIVE ROCKS AND BRECCIA PIPES

(TERTIARY AND CRETACEOUS)

INTRUSIVE ROCKS (PROTEROZOIC AND ARCHEAN)

METASEDIMENTARY AND METAIGNEOUS ROCKS (ARCHEAN)

UNDIVIDED INTERMEDIATE AND MAFIC INTRUSIVE ROCKS

PERMIAN

PENNSYLVANIAN

DEVONIAN

ORDOVICIANCAMBRIANPROTEROZOIC

ARCHEAN

MISSISSIPPIAN

EOCENE OR 

PRECAMBRIAN

(age uncertain)

CORRELATION OF MAP UNITS

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Yellowstone National Park

MONTANA

INDEX TO MAPPED AREA

DESCRIPTION OF MAP UNITS

SEDIMENTARY AND VOLCANIC ROCKS

Surficial deposits (Holocene and Pleistocene)—Alluvium in stream-channel, flood-plain, and 

terrace deposits; talus, colluvium, landslide deposits, rock glaciers, and glacial deposits (till, outwash, and glacial lake deposits)

Travertine (Holocene and Pleistocene)—Exposures limited to the Gardiner area in 

southwesternmost part of study area

Lava Creek Tuff of Yellowstone Group (Pleistocene)—Rhyolitic ash-flow tuff in Gardiner area in 

southwesternmost part of study area

Basalt (Pleistocene)—Basalt flows in Gardiner area in southwesternmost part of study areaAbsaroka Volcanic Supergroup (Eocene)—Volcanic and volcaniclastic rocks including basaltic, 

andesitic, and dacitic flows and flow breccias, rhyolitic ash-flow tuff and vitrophyre, tuff breccias, lahars, agglomerates, agglutinates, conglomerate, and minor andesitic and dacitic intrusive bodies

Fort Union Formation (Paleocene and Upper Cretaceous)—Conglomerate, sandstone, and shale 

in southeast corner of study area

Andesite (Upper Cretaceous)—Lava flows in Sliderock Mountain area in northern part of study 

area

Lahars of Livingston Group (Upper Cretaceous)—Lahar deposits; mostly clast supported; minor 

intercalated ash-flow tuff, lava flows, flow breccias, and epiclastic conglomerate, sandstone, siltstone, and mudstone

Sedimentary rocks of Livingston Group (Upper Cretaceous)—Epiclastic conglomerate, 

sandstone, siltstone, and mudstone

Andesite (Upper Cretaceous)—Lava flows and flow breccias at or near base of Livingston GroupSedimentary rocks (Upper and Lower Cretaceous), undivided—Sandstone, shale, limestone, and 

conglomerate. Used in Sliderock Mountain area and along a fault in center of study area, where the stratigraphic assignment of these Cretaceous sedimentary rocks could not be determined with certainty

Sedimentary rocks (Upper Cretaceous)—Includes Eagle Sandstone, Telegraph Creek 

Formation, Cody Shale, Frontier Formation, and Montana Group. Sandstone and shale

Sedimentary rocks (Lower Cretaceous)—Includes Mowry Shale, Thermopolis Shale, Kootenai 

Formation, and Cloverly Formation. Sandstone, shale, limestone, and conglomerate

Sedimentary rocks (Jurassic and Triassic)—Includes Morrison Formation, Ellis Group, and 

Chugwater Formation. Sandstone, shale, and limestone

Undifferentiated sedimentary rocks (Paleozoic)Sedimentary rocks (Permian, Pennsylvanian, and Mississippian)—Includes Phosphoria, 

Quadrant, and Amsden Formations and Madison Group. Limestone, shale, sandstone, and dolomite

Sedimentary rocks (Devonian and Ordovician)—Includes Three Forks Shale, Jefferson 

Formation, and Bighorn Dolomite. Shale, limestone, and dolomite

Sedimentary rocks (Ordovician and Cambrian)—Includes Bighorn Dolomite, Grove Creek 

Formation, Snowy Range Formation, Pilgrim Limestone, Park Shale, Meagher Limestone, Wolsey Shale, and Flathead Sandstone. Dolomite, limestone, sandstone, siltstone, and shale. Limited to the Cooke City area in south-central part of study area

Sedimentary rocks (Cambrian)—Includes Grove Creek Formation, Snowy Range Formation, 

Pilgrim Limestone, Park Shale, Meagher Limestone, Wolsey Shale, and Flathead Sandstone. Limestone, sandstone, siltstone, and shale

INTRUSIVE ROCKS AND BRECCIA PIPES (TERTIARY AND CRETACEOUS)

Intermediate and felsic intrusive rocks (Tertiary or Late Cretaceous)—Laccoliths, plugs, dikes, 

sills, and irregular-shaped bodies of fine-grained and porphyritic rhyolite, dacite, quartz latite, andesite, and diorite

Intermediate and felsic rocks (Eocene)—Dikes, sills, and irregular-shaped bodies. Andesite, quartz 

latite, dacite, and rhyolite; commonly porphyritic

Dacite porphyry (Eocene)—Stock, laccoliths, sills, dikes (not shown on map), plugs, and irregularshaped bodies. Includes some andesitic and rhyolitic rocksGranodiorite porphyry of Emigrant Peak area (Eocene)—Stock near head of Emigrant Creek on 

south flank of Emigrant Peak in western part of study area

Diorite porphyry and basaltic andesite (Eocene)—Small stock (or plug) at head of Mill Creek in 

western part of study area, and plugs northwest of Cooke City

Monzodiorite of Independence district area (Eocene)—Stock, at Monument Peak in central part 

of study area, composed of multiple phases of andesitic intrusive breccia, monzodiorite, quartz monzodiorite, and quartz monzonite; cut by minor granite dikes

Dacite porphyry of Lulu Pass (Eocene)—Laccolith and dikes (not shown on map) about 3 mi (5 

km) north-northwest of Cooke City

Breccia of Homestake mine area (Eocene)—Explosion-collapse breccia pipe, about 2.2 mi (3.5 

km) north of Cooke City, composed of blocks of Cambrian sedimentary rocks, porphyritic intrusive rocks, volcanic rocks, and Precambrian gneissic rocks in chloritic matrix; altered and mineralized; intruded by later dikes of rhyolite and quartz latite porphyry

Breccia of Alice E mine area (Eocene)—Collapse(?) breccia, about 1 mi (1.6 km) north of Cooke 

City, composed of blocks of Cambrian sedimentary rocks in a pulverized rock-flour matrix; locally altered and mineralized; intruded by later dikes of quartz latite porphyry

Dacite porphyry of Henderson Mountain (Eocene)—Stock and dike (not shown on map) about 1 

mi (1.6 km) north of Cooke City

Dacite porphyry of Homestake mine area (Eocene)—Stock, sills, dikes (not shown on map), and 

irregular-shaped bodies, about 2 mi (3.2 km) north of Cooke City; contains abundant rounded, commonly embayed quartz phenocrysts; commonly altered and locally mineralized

Complex of Fisher Mountain area (Eocene)—Stock(?) composed of aphanitic and fine-grained 

porphyritic dacite, rhyolite, and quartz latite and felsite breccia, about 2.8 mi (4.5 km) north of Cooke City; intruded by quartz-rich dacite and porphyritic and aphanitic dikes (not shown on map); strongly feldspathized, silicified, sericitized and (or) argillitized, and pyritized

Diorite of Scotch Bonnet Mountain (Eocene)—Stock and irregular-shaped intrusive bodies about 

3.3 mi (5.3 km) north of Cooke City

Andesite and trachyandesite porphyry (Eocene)—Sills in Cooke City areaAndesite (Late Cretaceous)—Dikes in Sliderock Mountain area in northern part of study areaDiorite of Sliderock Mountain (Late Cretaceous)—Stock; variable grain size and textures may 

indicate multiple phases; locally altered

Rhyolite and quartz latite porphyry (Late Cretaceous)—Sills, dikes (not shown on map), and 

irregular-shaped bodies in Goose Lake area, about 6.5 mi (10.5 km) north- northeast of Cooke City

Syenite of Goose Lake (Late Cretaceous)—Stock, about 7 mi (11.3 km) north- northeast of 

Cooke City, composed of multiple phases of syenite and monzonite of variable grain sizes and textures; locally altered and mineralized

INTRUSIVE ROCKS (PROTEROZOIC AND ARCHEAN)

Mafic intrusive rocks (Proterozoic and Archean)—Includes sills, dikes, stock, and irregular-shaped 

bodies of alkali olivine dolerite, metadolerite, metanorite, metagabbro, and quartz dolerite, and dikes of uncertain or unknown affinities

Hornblende quartz diorite (Archean)—Small bodies associated with quartz monzonite (Agr) south 

of eastern exposures of the Stillwater Complex (Asw units)

Granitic intrusive rocks (Archean)—Includes stocks and irregular-shaped bodies of fine-, medium-, 

and coarse-grained quartz monzonite and aplite south of eastern exposures of the Stillwater Complex (Asw units). Also includes stocks and bodies of granite in the Gardiner area in southwesternmost part of study area

Stillwater Complex (Archean)—Stratiform mafic to ultramafic rocks. Divided into seven units as 

follows

Upper anorthosite, upper gabbro, and upper mixed zonesMiddle anorthosite zoneMiddle mixed, middle gabbro, and lower mixed zonesLower anorthosite zoneLower gabbro and norite zonesBronzitite zonePeridotite zone

Granitic gneiss (Archean)—Predominantly granitic gneiss and migmatite; commonly consists of 

alternating bands of more felsic and more mafic gneiss; contains inclusions of metasedimentary and metaigneous rocks

Metanorite and metagabbro (Archean)—Stock-like mass composed of metanorite and metagabbro 

cropping out in southeastern part of study area

Ultramafic rocks (Archean)—Irregular-shaped bodies and lenses of olivine-bearing rocks and 

serpentinite cropping out in southeastern part of study area

METASEDIMENTARY AND METAIGNEOUS ROCKS (ARCHEAN)

Biotite schist (Archean)—Includes minor quartzite, iron-formation, and amphiboliteAmphibolite and hornblende gneiss (Archean)—Mostly tabular and lensoid bodies enclosed in 

granitic gneiss and migmatite

Amphibolite and gneiss (Archean)—Includes trondhjemitic gneiss-amphibolite, paragneiss, and 

heterogeneous gneiss sequences, as well as trondhjemitic gneiss, tonalitic gneiss, and amphibolite; minor schist, quartzite, and iron-formation

Nappe complex (Archean)—Sheet-like, allochthonous package of rock units in northwestern part 

of study area. Includes Barney Creek Amphibolite, George Lake Marble, and Jewel Quartzite. Amphibolite, dolomitic marble, marble, and quartzite; minor schist and iron-formation

Quartzite and amphibolite (Archean)—Interlayered quartzite and orthoamphibolite; minor schistSchist and hornfels (Archean)—Metasedimentary rocks consisting predominantly of schist and 

hornfels with minor quartzite, amphibolite, and iron-formation; contact metamorphosed to hornblende-hornfels and pyroxene-hornfels facies at and near base of Stillwater Complex. Host for Homestake-type gold deposits near Jardine in southwestern part of study area

Undifferentiated metamorphic rocks (Archean)—Includes amphibolite, micaceous quartzite, and 

some gneiss. Also includes some small bodies of chromite-bearing serpentinite. Limited to southeastern part of study area

UNDIVIDED INTERMEDIATE AND MAFIC INTRUSIVE ROCKS

Dikes and sills of andesite and basalt (Eocene or Precambrian, age uncertain)ContactFault—Dashed where approximately located or inferred; dotted where concealed. Bar and ball on 

downthrown side

Thrust fault—Dashed where approximately located or inferred; dotted where concealed. Sawteeth 

on upper plate

Mylonitic shear zoneTrace of surface of tectonic denudation of Heart Mountain detachment—This surface occurs 

near Cooke City within the map area

Trace of axial plane of major anticlineTrace of axial plane of major syncline

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MISCELLANEOUS FIELD STUDIES MAP MF–2338

Version 1.0

GENERALIZED GEOLOGIC MAP OF THE ABSAROKA-BEARTOOTH STUDY AREA, SOUTH-CENTRAL MONTANA

Compiled by

B.S. Van Gosen, J.E. Elliott, E.J. LaRock, E.A. du Bray, R.R. Carlson, and M.L. Zientek

Digital database by Jeffery T. Silkwood; edited by Gregory N. Green and B.S. Van Gosen

2000

Any use of trade names in this publication is fordescriptive purposes only and does not imply endorsement by the U.S. Geological SurveyThis map was produced on request, directly fromdigital files, on an electronic plotterFor sale by U.S. Geological Survey Information ServicesBox 25286, Federal Center, Denver, CO 802251-888-ASK-USGSThis map is also available as a PDF at http://greenwood.cr.usgs.govARC/INFO coverages are available athttp://greenwood.cr.usgs.gov/pub/open-file-reports/ofr-99-0450/

Geology compiled by B.S. Van Gosen, J.E. Elliott,

R.R. Carlson, and M.L. Zientek, 1992

New mapping by J.E. Elliott, E.A. du Bray,

B.S. Van Gosen, and E.J. LaRock, 1991

Manuscript approved for publication March 14, 2000

CONTOUR INTERVAL 200 FEET

SUPPLEMENTARY CONTOUR INTERVAL 100 FEET IN BILLINGS QUADRANGLE

NATIONAL GEODETIC VERTICAL DATUM OF 1929

2 1 0 2 4 6 8 10 12 14 KILOMETERS

SCALE 1:126 720

2 1 0 468 2 10 MILES