Initial Publication Date: January 13, 2025

Geologic History of the Beartooth Mountains

David. W. Mogk, Professor Emeritus, Department of Earth Sciences, Montana State University

(Download the PDF (Acrobat (PDF) 26.2MB Dec31 24) to your mobile device or print this file if you want to have access to information on this webpage while you are in the field where there is no cell or wi-fi access. The geologic features and events described in this webpage provide a more complete description of the geology of each area identified in the Field Guide to the Beartooth Mountains.

Overview

The geologic history of the Beartooth Mountains is preserved in rocks, landforms, and fossils that span 4.0 billion years (Ga) of Earth history. This section provides a timeline of the major geologic events that have occurred in the geologic history of the Beartooth Mountains. Information is presented on when major geologic events happened. Explanations are also provided that describe what evidence is used, and how to interpret geologic relations and data. It's time to start thinking as a geoscientist!

To start, a geologic index map provides a guide to the key geologic features explored in this field guide. J. Tuzo Wilson (1936) subdivided the Beartooth Mountains into 4 distinct structural blocks: the Beartooth Plateau, North Snowy Block, South Snowy Block, and Stillwater Block (described in more detail below). Each of these structural blocks are colored on the figure below and represent different types of Archean (older than 2.5 billion year) rocks. Range bounding faults are delineated showing Laramide-style high angle, basement-cored, reverse faults with sawtooth symbols, and active normal faults with hachure symbols in Paradise Valley. The stippled pattern indicates the extensive Eocene (~50 million-year-old) Absaroka Volcanic Sequence. Geologic features of note described in this field guide, starting at the top of the map and progressing clockwise, include: Suce Creek Fault (SCF). Stillwater Complex (SC), Stillwater Contact Aureole (SCA) Stillwater Mine (SM), Beartooth Front (BF), Rock Creek (RC), Eastern Beartooth high-grade metamorphic rocks (EBT), Long Lake Magmatic Complex (LL), Great Unconformity (GU), Beartooth Butte (BB), Heart Mountain Detachment Fault (HMD), New World Mining District (NW), Gardiner-Spanish Peaks Fault (GSPF), Jardine Metasedimentary Sequence (JMS), Jardine Mine (JM), Hepburn Mesa (HM), Deep Creek-Luckock Park Fault (DC-LPF), Paradise Valley Quaternary deposits, and Pine Creek Nappe (PCN).

The International Union of Geological Sciences International Chronostratigraphic Chart (Acrobat (PDF) 321kB Nov4 24) is provided for reference to the Eons, Eras, and Periods defined for geologic time. The concept of "deep time" is one of the most profound and fundamental contributions of geologic thought to human understanding of our place in the universe. Throughout this report, geologic ages are reported as: billions of years (gigaannum, Ga), millions of years (megaannum, Ma), and thousands of years (kiloannum, Ka). We'll start our exploration of Beartooth geologic history in the Eoarchean (4.0-3.6 billion years [Ga] ago) and follow the geologic timeline through the Holocene and the epic floods that occurred in the region in 2022.

In the Beginning, There Were Zircons--Eoarchean Geology, The Geologic Record From 4.0-3.6 Ga

The Beartooth Mountains have preserved some of the oldest Earth materials in the world, dating back to 4.0 Ga. It is extremely difficult to see through the veils of such great expanses of geologic time as the Earth is a dynamic planet that is constantly recycling materials in tectonic processes, (mountain-building), surficial processes (water, wind, glaciers...), and a probable global meteorite impact event (the Late Heavy Bombardment event, 4.1-3.8 Ga ago) that also pulverized the Moon to create the Lunar Highlands cratered topography. No rocks have been preserved in the Beartooth Mountains in the age range 4.0-3.6 Ga, although the world's oldest silicic igneous rock unit is the Acasta Gneiss of the Slave Province of the Canadian Shield, which has been dated at 4.03 Ga. However, rocks of the Beartooth Mountains contain populations of the mineral zircon that did form in episodes of crust formation during this time interval.

The Oldest Zircons of the Eastern Beartooth Mountains

Numerous quartzite (metamorphosed sandstone) rock units are exposed in the Hellroaring Plateau, Quad Creek, and Line Creek areas of the eastern Beartooth Mountains.  Detrital zircons analyzed from these quartzites reveal some key indicators of the genesis and evolution of this ancient continental crust in the time interval 4.0-3.6 Ga:

  • The oldest zircons in this area date back to ~4.0 Ga. 
  • There are a number of small but significant continental crust-forming events that occurred at ~3.9, 3.7 and 3.6-3.65 Ga.
  • Lu-Hf isotopic systematics of the oldest zircons provided the evidence for Mueller and Wooden (2012) and Mueller et al., (2014) to develop a model of the earliest crustal formation that involved a) early separation of mafic (basaltic) magmas from the mantle to form a stack of crustal layers akin to modern day oceanic plateaus. Partial melting of some of these basaltic layers, fluxed by the presence of water, generated more felsic (siliceous) magmas that are host to the ancient zircon crystals. These are some of the zircons that were weathered, deposited and made available for recovery by sampling the younger quartzites. The εHf values of the zircons indicate that this material came from recycling of existing rocks (the basalts) with little additional juvenile material from the mantle.  The tectonic environment  has been interpreted as a "plume-dominated" magmatic system (Mueller et al., 2014) over a hot spot, similar to modern day Iceland or Hawaii.
  • Zircons collected from quartzites in the Quad Creek area, Beartooth Mountains, were also used in a compilation study of Archean zircons from around the world by Valley et al., 2005, (4.4 billion years of crustal maturation: oxygen isotope ratios of magmatic zircon. Contributions to Mineralogy and Petrology, 150, pp.561-580) to report: "The mildly evolved, higher Archean values (6.5-7.5 o/oo) are interpreted to result from exchange of protoliths with surface waters at low temperature followed by melting or contamination to create mildly elevated magmas that host the zircons".  The crust at the surface was relatively cool, and free water was at the surface possibly in ocean basins at this early stage of Earth history! 

The Oldest Rocks in the Beartooth Mountains, Paleoarchean Rocks 3.6-3.0 Ga

The oldest silicic continental rocks in the Beartooth Mountains occur in the Quad Creek, Hellroaring Plateau, Wyoming Creek, Line Creek Plateau, and Christmas Lake areas. These are mostly "gray gneisses" of tonalitic composition--a plutonic rock that has high Si content (SiO2>65 wt %) and high aluminum and sodium content. These ancient gneisses are tectonically intercalated (faulted along shear zones) with metasedimentary and meta-igneous rocks, and occur as inclusions or pendants up to km-scale engulfed by voluminous younger 2.8 Ga magmatic rock.  The figure on the right is a U-Pb Concordia plot of zircon analyses from a "gray gneiss" in the Quad Creek area that has a magmatic age of 3.49 Ga, and partially reset age of ~2.8 Ga that corresponds with the age of the surrounding granitic rocks.  Key points about these ancient gneisses in the Beartooth Mountains:

  • Crust-forming events of this oldest felsic (high silica) continental crust occurred episodically over a time span from 3.6-3.2 Ga, a time span of 400 million years!  The oldest rocks that are currently preserved in the Beartooth Mountains started to form at ~3.6 Ga. 
  • Small increments of felsic magmas were added sequentially to thicken the continental crust in this time interval to form what is recognized as the thick, tectonically stable "Wyoming Craton".  (see also: Frost et al 2023 Creating Continents Continents_GSATG541A.pdf (Acrobat (PDF) 1.8MB Nov7 24) 'Frost et al., 2023, Creating Continents: Archean Cratons Tell the Story'] and Frost and Mueller, 2024, Archean Cratons: Time Capsules of Early Earth (Acrobat (PDF) 3.6MB Nov7 24).
  • A major crust-forming event occurred from 3.3-3.2 Ga.  This is recognized through the large number of gneisses that have been dated in this age range, and in the detrital zircon record from quartzites in the area that have received zircons from rocks of this age.
  • The tonalitic magmas formed by melting older mafic (rocks of basaltic composition) at high pressure and temperature and under hydrous conditions (i.e. water is present in the system).  Experimental studies of melting basalt have confirmed that the first melt that is produced is of tonalitic composition.
  • Trace element compositional variation diagrams indicate that the composition of these ancient tonalites is similar to those produced in a modern-day volcanic continental arc setting (like the Cascades of NW USA or the Andes of South America).
  • Lu-Hf isotopic data indicate that the magmas produced in this time interval have increasingly positive εHf values, which indicates the addition of more juvenile material from the mantle.  This is typically done through subduction-related processes.
  • All of this evidence has led Mueller et al., (2012) to hypothesize a "plume to plate" transition. The significance of these observations is that this appears to be the first operation of what is understood as modern-day plate tectonics, at least in the Beartooth Mountains and Wyoming Craton!

A Period of Tectonic Quiescence: 3.0-2.8 Ga

Very little magmatic or metamorphic activity has been recorded between 3.0 and 2.8 Ga in the Beartooth Mountains. This is significant because there are numerous bands of "metasupracrustal" rocks (i.e., rocks that formed at the surface of the Earth, including sedimentary sequences and extrusive igneous rocks such as basalt flows and volcanic ash deposits) in the Beartooth Mountains. These are rocks that were originally deposited as sedimentary rocks in this time interval and then were subsequently metamorphosed.  Key geologic relations include:

  • The rock assemblages include quartzites, pelitic schists, and banded iron formation units.  All of these rocks must have originally been sedimentary rocks that started off as sandstones, clay-rich shales, and iron formations. The sandstones are nearly pure quartz, which means they were either transported long distances or were at the surface for a long time to have other minerals weathered out. The quartzites were probably deposited as beach sands and the shales were probably deposited as offshore, lower energy marine deposits. Iron formations are chemical precipitates that form in the ocean. Limestones that would have been metamorphosed to marbles are conspicuously absent in the Beartooth Mountains. There are also some rocks of mafic composition that could have been formed as surficial lava flows, volcanic tuffs, or intruded as dikes or sills. The depositional environment would have been something like the "passive" margin of the US Atlantic seaboard or an internal  continental basin like the Michigan basin.
  • The depositional age is constrained between 3.0 Ga which is the age of the youngest detrital zircons in the quartzites and 2.8 Ga which is the age of the Long Lake Magmatic Complex which intrudes these metasedimentary rocks.
  • The "old gray gneisses" must have been uplifted from mid-crustal levels (about 20 -30 km depths) to the surface by this time so they would be exposed to surficial weathering, transport and deposition of the zircons in the basin.

A Second Orogenic (Mountain-Building) Event: Volcanic Arc Magmatism and Metamorphism in the Mesoarchean, 2.8 Ga 


The Beartooth Mountains have all the rock types, compositions and structures of a modern continental magmatic arc that are identical to those of the Late Jurassic 105-85 Ma) Sierra Nevada Batholith that formed in response to the subduction of the Farallon Plate as exposed in Yosemite National Park. The main Beartooth massif is made of voluminous (over 2500 km2 surface exposures of calc-alkaline magmatic rocks that include rocks of basaltic, andesitic, dacitic and granitic compositions.  These magmatic rocks are collectively referred to as the Long Lake Magmatic Complex (LLMC) Some of the key research findings about these rocks include:

2.8 Ga Magmatism

  • The timing of this magmatic event was relatively short, spanning 2.83-2.79 Ga (~40 Ma); this is consistent with the timing of modern orogenic (mountain-building) events. A very large amount of continental crust was produced in a relatively short time (Mueller et al., 2010). 
  • Trace element patterns, including high field strength element depletion, light REE enrichment, negative Eu anomalies, and depletion of heavy REEs are consistent with formation of these magmas in a subduction setting (Mueller et al., 2010, 2012).
  • The first occurrences of rocks of andesitic composition is significant because these are the dominant rock types that occur in continental volcanic arcs such as the Cascades of North America. They are interpreted as being the product of subduction processes.
  • Sm-Nd, U-Pb, and Lu-Hf systematics indicate there is a significant component of recycled older crust mixed with primitive melts from the depleted mantle (Wooden and Mueller, 1988); this type of mixing of source rocks is common in subduction zones. 
  • These rocks were intruded as numerous sheet-like bodies at mid-crustal levels (Henry et al., 1982). In detail, each rock type cross-cuts, and is cut by, rocks of different composition, essentially at the same time.  U-Pb geochronology of zircons shows that the numerous compositional varieties of magmatic rocks of the LLMC overlap in time (i.e., they do not follow a simple liquid line of descent from mafic to felsic) during the limited period of magmatism of 2.83-2.79 Ga
  • Mueller et al. (2010, 2014) interpreted the assemblage of magmatic rocks in the eastern Beartooth Mountains as the product of simultaneous melting of disparate mantle and lower crustal rocks to generate the diverse array of rocks observed.  In this model, the subducted mafic slab, subducted sediments, overlying mantle wedge, underplated mafic crust, ancient felsic gneisses and metasupracrustal rocks all melt simultaneously in a subduction environment.  The magmas that were generated are emplaced in the mid-crust at 20-30 km depth where the magmas cross-cut each other, and may undergo magma mingling and partial assimilation. Additional geochemical and isotopic evidence of magmagenesis in a continental marginal subduction environment has been presented by (Mueller et al., 1988; Mueller et al., 2008; Mueller and Wooden, 1988; Mueller et al., 1983; Wooden and Mueller, 1988).


High Grade Metamorphism and Deformation

Enclaves of high-grade metamorphic rocks are found as meter- to kilometer-scale inclusions (pendants, screens) in the vast After deposition of the supracrustal (sandstones, shales, banded iron formation, mafic igneous) rocks, this sequence of rocks was buried deep in the Earth and metamorphosed at high pressures and temperatures.  These newly metamorphosed rocks were also tectonically mixed with fragments of the older "gray gneisses" along high temperature fault zones known as ductile shear zones.  The supracrustal rocks and the "gray gneisses" formed under very different geologic environments, physical conditions and geologic times, so they must have been physically mixed at some time after their formation.  Metamorphism occurs when rocks recrystallize in response to changing physical conditions, predominantly temperature and pressure. Metamorphism may be "static" where only recrystallization occurs, or may be "dynamic" where recrystallization  occurs during deformation to produce strong preferred orientation of minerals into planar structures (alignment of platy minerals into planes to create schists; compositional layering that defines gneisses; alignment of linear minerals). Key  research findings about these metasupracrustal rocks include:

  • Metasupracrustal rocks of the Eastern Beartooth Mountains were subjected to at least one major cycle of "dynamothermal" metamorphism.
  • The metasupracrustal rocks of the Eastern Beartooth Mountains experienced "peak" metamorphism in the amphibolite to granulite facies and achieved temperatures as high as 750-800oC and pressures of 6-8 kilobars. One bar of pressure is equal to about 1 atmosphere of pressure, so this would be pressures of 6000-8000 atmospheres. One kilobar of pressure is equivalent to about 3 kilometers of burial under lithostatic load (i.e., the weight of the overlying rock column). 
  • So rocks that were originally deposited on the surface of the Earth as sandstones and shales must have then been buried as deep as 18-25 kilometers where they were transformed into quartzites and aluminous schists.
  • This type of deep burial typically requires "tectonic thickening" where large segments of crust are thrust over other crust.  Deep burial of sediments in a basin cannot possibly get surficial sediments down to the 18-25 km depths where peak metamorphism occurs. The 6-8 kilobar pressures recorded in the metasupracrustal rocks are most readily explained by stacking of thrust faults in a convergent continental margin setting.
  • After tectonic thickening, there is significant isostatic (buoyant) disequilibrium--i.e., less dense continental crust is depressed to deep crustal levels and this means that this crust becomes buoyant and must rise to higher levels, kind of like depressing an ice cube in a glass of water and then removing the confining pressure.  So, there is a later stage of decompression and cooling of these metasupracrustal rocks. This stage of decompression after tectonic thickening is common in convergent margin orogenic (mountain-building) cycles.
  • The physical conditions of metamorphism are convergent with the final crystallization temperatures of the surrounding Long Lake Magmatic Complex, indicating the metamorphism occurred during or just prior to emplacement of the surrounding magmatic rocks.
  • This type of crustal thickening and high-grade metamorphism typically occurs in a convergent margin setting, related to subduction processes. This is consistent with the interpretation that magmatism and metamorphism occurred in a convergent margin type tectonic environment.

Late Archean Accretion Events, 2.8-2.55 Ga, South and North Snowy Blocks

One of the great insights in the plate tectonic revolution was the discovery that large blocks of rocks (continental or oceanic) could be transported hundreds or thousands of miles to be docked or accreted against other continental blocks. These far-traveled blocks of rocks are known as "allochthonous" or "exotic" terranes.  Mogk (1988) described two sequences of allochthonous units along the western margin of the Beartooth Mountains in the South and North Snowy Blocks.

In the Beartooth Mountains, rocks of the South and North Snowy Blocks have preserved rock suites that have no common ancestry with the main Beartooth massif. Further, the anomalously low-grade metamorphism of rock in the South Snowy Block indicates a geologic history that is not related to the main Beartooth Block, and the degree of tectonic mixing on a km-scale in the North Snowy Block indicates that these units had experienced a metamorphic history prior to emplacement against the Beartooth Block; thus, they can be considered to be allochthonous (far-travelled, exotic and with no common ancestry) metamorphic terranes.

South Snowy Block

The South Snowy block contains a distinct metasedimentary sequence (Jardine Metasedimentary Sequence; JMS) that was originally mapped with accompanying petrologic and geochemical data by Casella et al. (1982). The JMS is unique in the Beartooth Mountains due to anomalously low-grade metamorphism compared to the main Beartooth Block. Exposures of the Jardine Metasedimentary Sequence extend from near Gardiner, MT at the confluence of Bear Creek and the Yellowstone River and continue east to the area near Tower Junction in Yellowstone National Park at Garnet Hill and Junction Butte. Excellent exposures can be accessed via trails traversing through the Black Canyon of the Yellowstone River. A summary of the geology of the South Snowy Block was published by Mogk et al., (2012), Origins of a Continent (Acrobat (PDF) 2MB Nov13 24), that was the product of a NSF-sponsored Research Experiences for Undergraduates project. Key geologic relations of the South Snowy Block include:

  • The rock types include varieties of phyllites, biotite schists, quartzites, rare metaconglomerates and banded iron formations.
  • Primary sedimentary structures, such as compositional layering and graded bedding, low angle cross stratification, rip-up clasts and channel scour deposits, are commonly preserved. 
  • These relations indicate that sedimentary deposition occurred as turbidites--submarine debris flows that shed off a continental shelf and produce gradational bedding due to settling of increasingly finer sediments from turbidity currents as the energy regime decreases. These are often referred to as "Bouma Sequences". Protoliths of these rocks are interpreted as graywackes and mudstones deposited in a submarine fan setting.
  • Metamorphism of this sequence is anomalously low grade compared to most Archean metasedimentary sequences in the Wyoming Province and produced phyllitic schists in the west (near Gardiner, MT) and andalusite-staurolite-garnet biotite schists to the west near Tower Junction in Yellowstone National Park (Osborne et al., 2011). This mineral assemblage and related mineral geothermobarometers restrict peak metamorphic conditions are 3.8 Kbar and 550oC. This discontinuity in metamorphic grade is a key indicator that the JMS is "exotic" or out of place with respect to the rest of the Beartooth Mountains. 

  • The contact between the low-grade rocks of the JMS and the crystalline rocks that are similar to the LLMC in the Slough Creek area of Yellowstone National Park is a major ductile shear zone originally mapped by Casella et al., (1982). Indicators of fault movement direction (kinematic indicators) demonstrate that the  high-grade crystalline rocks were thrust over the lower-grade JMS metasediments. Geobaromety of the crystalline rocks records crystallization pressures of ~8 kbar, and the metamorphism of the JMS is <3.8 Kbar; this means that there is an uplift (or "throw") across this shear zone of at least 12 km displacement of the crystalline rocks over the JMS.

 

  • An early stage of isoclinal folding is attributed to soft sediment deposition in a turbidite setting (Fereday et al., 2011) and two stages of open and crenulation folding have been defined (Jablinski and Holst, 1992), creating a "dome and basin" fold interference pattern (Ramsey, Type 1).

  • Detrital zircon U-Pb data from a quartzite near Jardine have a maximum frequency at ~3.2 and 3.0 Ga with a subsidiary concentration at 3.5 Ga and a minimum age of 2.9 Ga, a pattern not observed in any of the other quartzites across the Beartooth Mountains and Wyoming Province. And, there are no zircons of age 2.8 Ga, so the Late Archean magmatic rocks of the LLMC can't be a source of these sediments.

  • Two epizonal (emplaced within a few kilometers of the surface), bulbous plutons, the Crevice Stock and Hellroaring Stock, intrude the Jardine metasedimentary rocks (Brookins, 1968; Montgomery and Lytwyn, 1984; Philbrick et al., 2011). Many of these magmatic rocks have peraluminous affinities (i.e., an aluminum-rich source area such as melting a pelitic schist), have an estimated depth of crystallization of 10-12 km (3-4 Kbar), and have primary U-Pb zircon ages of ~2.8 Ga--the same age as the main LLMC Beartooth magmatic event.
  • The age of juxtaposition of the JMS against the LLMC is bracketed by the age of the youngest detrital zircon in the JMS which is 2.9 Ga and represents the youngest age of deposition of the sediments and 2.8 Ga, which is the age of the cross-cutting Crevice and Hellroaring Plutons (Mueller et al., 2014a; Philbrick et al., 2011).
  • Consequently, The Jardine metasedimentary sequence is interpreted as exotic to the Beartooth Mountains and to blocks of basement rocks in surrounding ranges based on its unique detrital zircon pattern and anomalously low metamorphic grade (Mogk, 1988). The crosscutting Crevice and Hellroaring Plutons share geochemical affinities with the LLMC of the Beartooth Mountains (Mueller et al., 2014). These plutons place a minimum age of ~2.8 Ga for the tectonic emplacement of the Jardine metasediments against the Beartooth Plateau block.
  • In addition, the western margin of the South Snowy Block is delineated by the high-grade Snowy Shear Zone which is well-exposed in Yankee Jim Canyon at the south end of Paradise Valley north of Gardiner, MT (see: Erslev, 1992; Webber et al., 2019)

  • Jardine Mine- Mineral Hill MIne.  Hargrave et al., (2000) report:  "The Jardine and Crevice mining districts are located on the western edge of the Absaroka Range, in the Bear Creek drainage just north of Yellowstone National Park. These adjacent districts are geologically similar and are generally discussed as one (Seager, 1944; Reed, 1950; Johnson and others, 1993). Placer gold was discovered in the area around 1862 and during the next decade lode deposits were found on Mineral Hill and Crevice (Crevasse) Mountain. The gold-arsenic-tungsten lode deposits occur as replacement veins in Precambrian schist and quartzite (Elliot and others, 1983). Most of the mining activity has been near Mineral Hill in the Jardine mining district (also referred to as the Sheepeater or Bear Gulch district). The mines consisted of more than 30 adits, approximately 7 miles of underground workings, and a number of open cuts (Reed, 1950). Mines were active in both districts until around 1942, although most of the production was from the Jardine Mine. Recorded production through 1949 totaled at least 201,000 ounces gold; 12,615,131 pounds arsenic trioxide; 766,122 pounds tungsten; 36,000 ounces silver, and minor amounts of copper and lead (Johnson and others, 1993). In 1979, a new phase of gold exploration began at the Jardine mine and production resumed in 1989. From 1989 through 1996, an additional 273,441 ounces of gold was recovered from the mine (R.B. McCulloch, personal commun., 2000). The mine was considered active until 2000 at which time the operator (TVX Gold, Inc.) halted production with the intention of quitting its position at Jardine." The banded iron formations are host to the gold mineralization of the Jardine Mine (Hallager, 1984; Smith, 1996). Mineralization is concentrated in the structural domes where mineralizing fluids ponded below the impermeable iron formations. Read an interesting historical account of Jardine Gold Mining  on the Edge of Yellowstone by Robert Goss (2020) and from Western Mining History  Mineral Hill Mine.

North Snowy Block

Reid et al., (1975) produced a geologic map that defined the basic map units in the North Snowy Block, and Mogk et al., (1988) reinterpreted these rock units as a zone of major tectonic dislocation and mixing. Key geologic relations are:

  • A diverse collection of rock units are juxtaposed on high-grade shear zones including from east to west: the Mount Cowen Augen Gneiss (with large microcline potassium feldspar crystals),  Davis Creek Schist (phyllites in the greenschist facies), a TTG gneiss unit that has been ductilely deformed and metamorphosed in the lower amphibolite facies, the Pine Creek Nappe which is an isoclinally folded (parallel limbs of folds), Alpine-style thrust unit with an amphibolite core and symmetrically distributed George Lake Marble and Jewel Quartzite units, an overlying suite of high-grade gneisses that have been partially melted, and a granitic sill that is a "stitching pluton" between the Davis Creek Schist and TTG gneiss.
  • These units each have unique geologic histories with respect to their processes and environments of formation, metamorphic grade and deformational histories. In general, the metamorphic grade increases  towards higher structural levels, requiring a tectonic inversion of higher grade rocks over lower grade rocks by faulting. These units are mostly tectonically mixed along high-grade ductile shear zones.
  • The age of juxtaposition is anchored by injections of the 2. 55 Ga granitic sill between the Davis Creek Schist and TTG gneiss.
  • The most significant of the units in the North Snowy Block is the Pine Creek Nappe.  This is a km-scale isoclinal fold structure that has been emplaced as a thrust-nappe. The lower limb is strongly attenuated with respect to the top limb, indicating significant tectonic thinning, and the bounding quartzite has strongly developed grain size reduction (mylonitization) indicating deformation under ductile conditions. The thrust fault contact in the image below (left) between the overlying Pine Creek Nappe and the underlying TTG gneiss is traced along the tree line on the cliff. This structure has all the attributes of modern-day thrust nappes in tectonic settings such as the collision of the African and Eurasian Plates to form the Alps.  The importance of this structure is that it is one of the oldest examples in the world of horizontal tectonics due to convergent tectonics.


Stillwater Complex: a 2.712 Ga Layered Mafic-Ultramafic Complex

The Stillwater Complex is a Late Archean layered mafic-ultramafic magmatic body that is exposed along the northern front of the Beartooth Mountains for over 60 km with a thickness of up to 5500 meters. Geophysical evidence indicates that the bulk of the complex extends several kms north of the Beartooth Mountains in the subsurface (Finn et al., 2020).  Detailed field guides of key locations in the Stillwater Complex have been compiled by Czamanske and Zientek (1985). Very accessible and easy to read overviews of the Stillwater Complex are provided by McCallum (1988 (Acrobat (PDF) 22.7MB Nov19 24)) and McCallum (2002) (Acrobat (PDF) 797kB Nov19 24) for the On the Cutting Edge 2003 Teaching Petrology workshop.

 

The Stillwater Complex crystallized over a ∼3 million-year interval from 2712 Ma to 2709 Ma (Wall et al. 2018). Magmatic layering was originally sub-horizontal, and sedimentary structures such as graded and cross-bedding are locally preserved in the igneous rocks (Page 1977). Only the southern edge of the Stillwater Complex is exposed in steep to near vertical layering due to faulting along the Mill Creek Stillwater Fault Zone and related Laramide structures (Page 1979; Geraghty 2013). The complex is divided into lithologic units (Zientek et al. 1985) that include a) a zone of diabase and norite sills and dikes, b) a chilled basal zone, c) an ultramafic zone; d) lower, middle and upper banded zones that include troctolites (olivine-plagioclase), norites (orthopyroxene-plagioclase), gabbronorites (orthopyroxene-clinopyroxene-plagioclase), gabbros (clinopyroxene-plagioclase) and anorthositic gabbros (plagioclase>clinopyroxene); e) anorthosites; and f) an underlying contact metamorphic aureole that includes metapelitic hornfels (cordierite-hypersthene and cordierite-anthophyllite assemblages; Labotka and Kath, 2001), quartzite, and banded iron formation (Page 1977; Page and Zientek1985). The magmatic rocks preserve remarkable igneous structures on the outcrop scale such as cm- to meter-scale rhythmic compositional layering, cross-cutting pegmatitic bodies, cm-scale "oikocrysts" (poikilitic pyroxenes with inclusions of older magmatic phases; "oik" = Greek for egg), and "inch-scale layering" (Boudreau 1995) which formed by an Ostwald ripening process in a fluid rich environment that preferentially melted plagioclase to produce enriched, residual pyroxene-rich layers.  On the microscopic scale, spectacular igneous textures are well-preserved and reveal magmatic cumulus and post-cumulus processes (Jackson 1961; McCallum et al. 1980; Raedeke and McCallum 1984; Zientek et al. 1985).


Layered mafic-ultramafic igneous bodies such as the Bushveld Complex in South Africa, Sudbury Complex in Ontario and many others are known to be hosts to a wide array of mineral deposits for Cu, Ni, Cr, and platinum group elements (PGEs, Pt, Pd, Ir). Mineral exploration in the Stillwater Complex has included magmatic Ni-Cu sulfides, chromite deposits in the ultramafic zone, and Pt-Pd deposits in the banded zone (Page et al. 1985a; Boudreau et al. 2020). Ore-forming processes have been interpreted as direct precipitation of magmatic sulfide crystals to produce ore minerals chalcopyrite (CuFeS2) and pentlandite (FeNi)9S8 or precipitating from infiltrating magmatic fluids. Cu-Ni sulfide mineralization occurs in the sill/dike complex immediately subjacent to the main Stillwater magmatic body (Page 1979; Page et al. 1985b), but was never economically mined. Chromite deposits occur in the ultramafic zone, and these were mined in the Benbow region to support the World War II war effort (Lipin et al. 1985) and in the Mountain View section in the early 1960's (Page et al. 1985c). Platinum group element exploration began in the late 1960's in search of deposits similar to the Merensky Reef in the Bushveld Complex, South Africa.  Discovery of the platinum-bearing horizon, now known as the J-M (Johns-Manville) reef occurred in 1973, and mining commenced in 1985 by the Stillwater Mining Company, a joint venture of Johns-Manville, Anaconda and Chevron companies. The mine is now operated by Sibanye-Stillwater Company, with mining operations at portals on the main Stillwater River and the East Boulder River. The Stillwater Complex produces ~10% of the domestic demand for platinum and palladium.  The Stillwater Complex has proven reserves of ~20,000,000 oz (Boudreau et al. 2020), and in 2020 produced ~18,000 kg of platinum and palladium with a market value of ~$1.1 billion (U.S. Geological Survey, National Minerals Information Center 2021).


Proterozoic Geologic Events: 2.5 Ga- 538 Ma

Very little tectonic activity was experienced in the Beartooth Mountains during the Proterozoic Era, from ~2500-600 Ma. By the end of the Archean, thick (>30-40 km), stable continental crust was established that led to cratonization of the Wyoming Province. For more information on the formation of the Wyoming Craton see: a) Mueller, P. A., and Carol D. Frost (2006), The Wyoming Province: a distinctive Archean craton in Laurentian North America, b) Frost et al., (2023) Creating Cratons: Archean Cratons Tell the Story, c) Mogk et al., 2023,  Crustal genesis and evolution of the Archean Wyoming Province: Continental growth through vertical magmatic and horizontal tectonic processes ,and d) and for general information on creating cratons Frost and Mueller (2024). Archean cratons: time capsules of the early Earth. Elements, 20(3), pp.162-167. Some key geologic relations involving the Wyoming Craton include:

  1. The Proterozoic rock record is almost completely missing in the Beartooth Mountains from ~2500 to ~600 Ma.  Other than a few mafic dikes (described below), there is no record of new magmatic additions to the crust in this time interval, and there is no record of deposition of Proterozoic sedimentary rocks (the Mesoproterozoic Belt Supergroup is not recognized in this area).
  2. The Wyoming Craton is surrounded by Paleoproterozoic orogenic belts in the time range of 19-1.6 Ga including: a) the Great Falls Tectonic Zone (GFTZ) on the northwest margin, b) the Cheyenne Belt (CB) on the south and c) the Trans-Hudson Orogen (THO) on the east. Each of these orogenic belts formed as the result of tectonic collisions between continental blocks. The GFTZ formed in response to the collision between the Medicine Hat Block and Hearne Block with the Wyoming Craton; the Cheyenne Belt formed through accretion of volcanic arcs of the 1.9-1.6 G Yavapai-Colorado Terranes; and the THO formed in response to collision between the Superior Craton (Canadian Shield) and the Wyoming Craton.  Kevin Burke, a planetary geologist from NASA's Lunar Planetary Institute, famously branded the Wyoming Craton as "Nuclear North America" as it provided the "seed" for growth of the continent through various tectonic events and mechanisms through geologic time.
  3. The tectonic juxtaposition of all these diverse crustal units is related to a cycle of "supercontinent assembly", and resulted in the formation of the Laurentia Supercontinent ~1.9-1.7 Ga ago.  This stage of continental assembly and growth is thoroughly discussed in Geological Society of America Memoir 220, Whitmeyer et al., 2023, Laurentia: Turning Points in the Evolution of a Continent. and chapters in that book by Whitmeyer et al., 2023, The tectonic evolution of Laurentia and the North American continent: New datasets, insights, and models and Harms and Baldwin, 2023, Paleoproterozoic geology of SW Montana: Implications for the paleogeography of the Wyoming craton and for the consolidation of Laurentia.  Tectonic events along the GFTZ include development of the Little Belt Magmatic arc at about 1. 86 Ga (Mueller et al., 2002, Paleoproterozoic crust within the Great Falls tectonic zone; implications for the assembly of southern Laurentia and evidence from crustal xenoliths in the GFTZ (Gifford et al., 2014, Precambrian crustal evolution in the Great Falls Tectonic zone: insights from xenoliths from the Montana Alkali Province; Gifford et al., 2018, Extending the realm of Archean crust in the Great Falls tectonic zone: Evidence from the Little Rocky Mountains, Montana. The somewhat younger "Big Sky Orogeny" at ~1.77 Ga has been described by Harms et al., 2004, Advances in the geology of the Tobacco Root Mountains, Montana, and their implications for the history of the northern Wyoming Province, Harms et al., 2022, The Paleoproterozoic of Montana, Harms and Baldwin, 2022, Paleoproterozoic geology of SW Montana: Implications for the paleogeography of the Wyoming craton and for the consolidation of Laurentia  and supported by isotopic ages of high grade metamorphism (Mueller et al., 2005, Paleoproterozoic Metamorphism in the Northern Wyoming Province: Implications for the Assembly of Laurentia).
  4. The Beartooth Mountains are interior to the margins of the Wyoming Craton and largely did not experience the magmatism, high-grade metamorphism, or deformation associated with these Paleoproterozoic events. 
    • However, conventional K-Ar age dates have been locally reset due to this tectonothermal metamorphism along the western margin of the Beartooth Mountains to ages of about 1800-1600 Ma.  But, the main Beartooth Massif has largely escaped resetting by these Paleoproterozoic events, with K-Ar ages remaining intact at about 2500 Ma.
  5. Mafic Dike Emplacement: Numerous stages of emplacement of mafic dikes have been recorded in the Beartooth Mountains. These were first documented by Prinz (1964, 1965).  Mueller and Rogers (1973) attempted to characterize the different suites of dikes based on differences in whole rock elemental compositions (primarily decrease in MgO and increase in TiO2 and K2O with time). Mafic rocks are notoriously difficult to date precisely due to the absence of zircons (mafic rocks are Si undersaturated, so the mineral zircon won't crystallize).  However, using Rb/Sr and K/Ar methods, they determined the ages of the dikes ranged from  2750 to 740 million years (Ma). Baadsgard and Mueller (1971) also used Rb/Sr and K/Ar methods to identify three stages of dikes: 2550, 1300 and 740 Ma (noting that the K-Ar results were suspect due to chemical resetting). Rowan and Mueller (1971) dated two dikes in the Gardner Lake area and report an age of 2750 Ma for a dike that is metamorphosed in the amphibolite facies but did not experience penetrative deformation and folding, and the younger dike was emplaced between 1800-1600 Ma. The oldest generation of mafic rocks have undergone a static hydrothermal metamorphism in the greenschist to lower amphibolite facies (epidote-amphibolite), but most of the dikes retain their igneous textures (diabasic texture, with well-defined plagioclase laths surrounded by a matrix of anhedral clinopyroxenes). A special type of mafic dike, colloquially referred to as the "leopard rock" by Prinz (1964) is a plagioclase cumulate rock with plagioclase crystals growing numerous centimeters in diameter and comprising over 50% of the rock. Harlan (2003, Cutting Edge Teaching Petrology Workshop) reports: "The age of the leopard dikes is not known with certainty, but mineral and whole-rock samples from a leopard rock dike in the northern Bighorn Mountains gave a Rb-Sr isochron of 2200 ± 35 Ma (Stueber et al., 1976)".  More recent use of the more precise 40Ar/39Ar dating method by Harlan et al., (1997) report an age of 774 +/-4 Ma for the Christmas Lake dike. MacKinder et al., (2019) correlate the Christmas Lake dike with the "Gunbarrell Large Igneous Province", which is a plume-generated dike system that is expressed from the Yukon to Wyoming. These dikes may be part of a giant radiating dike swarm related to an ancient mantle plume and may record the initial breakup and rifting of the supercontinent Rodinia (Park et al., 1995; Harlan et al., 1997. A Review of the Mafic Dikes of the Beartooth Mountains (Microsoft Word 4.6MB Nov19 24) was prepared by Steve Harlan, USGS, for the 2003 On the Cutting Edge Teaching Petrology Workshop.
  6. It's interesting to note that there is virtually no contribution of the dominant 2800 Ma LLMC f the Beartooth Mountains to the detrital zircon population in the adjacent LaHood and Neihart Formations of the lower Mesoproterozoic Belt Basin with depositional ages starting at about 1500 Ma (see: Mueller et al., U-Pb ages of zircons from the Lower Belt Supergroup and proximal crystalline basement: Implications for the early evolution of the Belt Basin); i.e., the crystalline rocks of the Beartooth Mountains do not appear to be exposed at the surface of the Earth at ~1500 Ma.
  7. An overview of the Proterozoic geologic history of Montana can be found in Harms and Baldwin, 2022, Paleoproterozoic of Montana .  Mesoproterozoic rocks of the Belt Basin are not present in the immediate vicinity of Beartooth Country.

Paleozoic Sedimentation and The Great Unconformity

Middle Cambrian Paleogeopraphy of North America, 510 Ma

Middle Devonian Paleogeography of North America, 385 Ma
Early Missippian Paleogeography of North America, 345 Ma

Paleozoic Sedimentation

At the start of the Paleozoic Era, shallow seas covered the cratonic rocks of the Wyoming Province. In the Beartooth Mountains, these include a) the basal sedimentary unit, the Cambrian Flathead Sandstone, which is interpreted as a beach sand deposit; b) Devonian shales and limestone channel deposits in an estuarine setting of the Beartooth Butte Formation and according to Dorf (1934): "...lies on the marine Bighorn dolomite of Upper Ordovician age and is, in turn, overlain by the marine Jefferson limestone of the Middle Devonian; and c) the shallow water carbonate platform deposits of the middle Mississippian Madison Limestone formation. Sedimentation in the lower Paleozoic required relatively stable tectonic conditions, and sedimentary rocks were deposited in numerous transgressive-regressive marine successions onto the Wyoming Craton.

The Great Unconformity

The basal Paleozoic rock formation, the Flathead Sandstone, was deposited directly on the 2.8 Ga crystalline rocks of the Long Lake Magmatic Complex in a few isolated locations in the Beartooth Mountains. In the accompanying figure, the granitic gneisses of the 2800 Ma LLMC are in the foreground in the lower right part of the picture, and the layered sedimentary rocks of the 530 Ma Cambrian Flathead Sandstone are in the upper left part of the picture.  At this location, ~200 meters east of the Top of the World general store, you can literally step across almost 2300 million years of "lost" Earth history!  Beartooth Butte is a second instance of Lower Paleozoic rocks preserved on the Archean basement.

  •   Beartooth Butte is also of particular significance because it is the site of some of the oldest Devonian fish (29 species) and terrestrial plant fossils (5 species).

A somewhat different occurrence of the Great Unconformity occurs in the Picket Pin section of the Stillwater Complex where Cambrian limestones are in unconformable contact with the Upper Gabbro zone (Carlson et al. 1985).

Laramide-Style Faulting--Uplift of the Present Beartooth Mountains


Laramide-Style Faulting: Although the rocks of the Beartooth Mountains are ancient, the physiographic presence of the present day Beartooth Mountains is a relatively recent feature.  The Beartooth Mountains are one of many Archean-cored mountain ranges that formed by uplift on high-angle Laramide-style reverse faults as seen on the adjacent figure. The Beartooth Mountains are surrounded by high angle, reverse, Laramide-style faults (Foose et al. 1961) that include the Beartooth Fault separating the Beartooth massif from the Bighorn Basin near Red Lodge, MT (Wise 2000; Neeley and Erslev 2009), the Suce Creek Fault in the northwestern corner of the range (Robbins and Erslev 1986), and the Clark Fork and Gardiner-Spanish Peaks Faults on the south (Garihan et al. 1983).

Surrounding the Beartooth Mountains, Laramide faults emplace Archean crystalline basement over the overlying Paleozoic strata, which are now in a near vertical to overturned orientation. These structures create the rimming palisades of Mississippian Madison Limestone that mimic rows of "bear's teeth" for which the range is named. Foose et al., (1961) and Wise (2000) indicate a relative vertical displacement of ~6000 meters at the range front. The Paleogene (~60-52 Ma) age of uplift was determined by apatite fission track dating (Omar et al., 1994), but more recent zircon and apatite U-Th/He dating indicates uplift may have initiated at 100 Ma and continued through Quaternary basin-and-range extension (Carrapa et al. 2019; Orme 2020; Ronemus et al., 2023). Excellent exposures of these Laramide faults are indicated in Figure 1: Suce Creek Fault south of Livingston, MT; near Nye, MT along the Mill Creek-Stillwater Fault Zone; the Beartooth Fault west of Red Lodge, MT and near Line Creek Plateau; on the southern margin of the range on the Gardiner-Spanish Peaks Fault; and, at the "Devil's Slide" area north of Gardiner, MT.  Of historical interest, J. Tuzo Wilson (1936) did his dissertation on the Mill Creek-Stillwater Fault Zone that defines the boundary of the North Snowy and Stillwater Blocks against the Beartooth Block and this Precambrian structure was reactivated by Laramide-style faulting on faults such as the South Prairie Fault and Bluebird Thrust that cut the Stillwater Complex (Thacker et al., 2017). The mechanisms, locations, and timing of Laramide-style faulting provide a good case study in the evolution of geologic thought regarding the origin of contractile block uplifts (Foose et al. 1961), reactivation of Precambrian structures (e.g., Marshak et al. 2000; Thacker et al., 2017), internal deformation of hanging wall crystalline rocks (e.g., Erslev and Rogers, 1993), the role of back thrusts (Erslev 1993) and folds (Neely and Erslev, 2009), the occurrence of low-grade metamorphism and circulating fluids (e.g., Thacker et al. 2017), and the relationship of flat slab subduction and rollback of the Farallon plate to Laramide faulting to (Carrapa et al. 2019).

The Devil's Slide structure near Yankee Jim Canyon, Yellowstone River, consists of steeply dipping formations of Cambrian through Cretaceous sediments that have been tilted to near vertical orientation on the hanging wall of the Gardiner Fault.  The red units include the Amsden, Chugwater and Morrison Formations. The two prominent ridges are volcanic sills that are more resistant to weathering than the surrounding sedimentary rocks.


"Laramide Porphyry Intrusive Rocks":  The Beartooth Massif is cut by numerous late-stage igneous bodies colloquially referred to as the "Laramide Porphyry Intrusive Rocks". These were first described by Rouse et al., (1937) and the Columbia project also mapped and reported on their occurrences in numerous reports (Eckelmann and Poldervaart, 1957; Poldervaart and Bentley, 1958; Harris, 1959; Casella, 1964; Skinner, 1969).  Rock types range in composition from dacitic to rhyodacitic, and may contain plagioclase and orthoclase phenocrysts (with Carlsbad twinning) up to cm-scale and that comprise up to 50% of the rock. Peyton et al., (2012) dated one of the porphyry bodies at 98-96 Ma. Mapping by Van Gosen et al., (2000) and Lopez (2001) demonstrates that the range boundary faults cut across these porphyry bodies. The most recent reports on the Laramide Porphyry rocks are from Brailer et al., (2024) (Acrobat (PDF) 5.3MB Nov24 24) and Cunningham et al., (2024) (Acrobat (PDF) 1.1MB Nov24 24) from the 2024 Tobacco Root Geological Society field conference.

Sedimentation in Surrounding Basins: As the Beartooth Block was uplifted, eroded sediments filled the adjacent Bighorn and Crazy Mountain Basins. Numerous studies have shown the relationship of uplift of the Laramide structures of Montana and Wyoming and related sedimentation: Decelles (1986), Dutcher et al., (1986), Decelles et al. (1991 a and b), May et al., (2013), Fan and Carrapa (2014), Welch et al., (2022). Brailer 2024 showed the cobbles of the Laramide Porphyry are present in the Late Cretaceous Frontier Formation of the Bighorn Basin.  Sedimentation in these basins commenced in the Cretaceous and continued through the Paleocene.  The Livingston Group is a sequence of volcanic sedimentary rocks that were deposited in the Crazy Mountain Basin and were derived from the Cretaceous Elkhorn Mountain Volcanics to the west. This unit was originally described by Weed (1893) with further studies by Roberts (1963, 1972) and Skipp and McGrew (1977). The stratigraphic position of the Livingston Group is above the Eagle Sandstone and below the Fort Union Formation (Scarberry et al., 2021). Thacker (2024) (Acrobat (PDF) 3.2MB Nov27 24) studied the role of the Stillwater River drainage in the tiunig of trans0range drainage incision of the Beartooth Mountains. An excellent overview of Laramide Uplifts and their surrounding sedimentary basins can be found in Vuke and Metesh (2020) "Synorogenic basin deposits and associated Laramide uplifts in the Montana part of the Cordilleran foreland basin system."

The Absaroka Volcanics--A Major Eocene Volcanic Sequence (55-45 Ma)


The Absaroka Volcanic Supergroup is defined by a northwest trending series of eruptive centers extending ~250 km from south of Bozeman, Montana, through the northern part of Yellowstone National Park, crossing the southwestern corner of the Beartooth Mountains, and continuing southeasterly to near Cody, Wyoming (Chadwick 1970; Smedes and Prostka 1972). It is the largest Eocene volcanic field in the northern Rocky Mountains, covering an area of 23,000 km2, with a cumulative volume of 30,000 km3, and with thicknesses up to 1500 meters (Smedes and Prostka 1972).

Numerous volcanic structures are evident across the volcanic field, including volcanic necks and plugs, breccia pipes, dikes and sills, and hypabyssal stocks (Chadwick 1970; Smedes and Prostka 1972).  The eruptive centers are stratovolcanoes surrounded by a proximal "vent facies" of volcanic and volcaniclastic rocks, lava flows, autoclastic flow breccias, mudflows (lahars), avalanche debris, and airfall tuffs and more distil alluvial facies include well-bedded and reworked volcanigenic sedimentary rocks such as conglomerates and breccia, volcanic sandstone and siltstone, and air-fall tuff (Smedes and Prostka1972).Rocks range in composition from andesite to dacite, and Chadwick (1970) defined an eastern belt that is more potassic than the western belt which is more sodic. The Absaroka Volcanics have an eruptive history from 55-45 Ma (Feeley 2003), and Feeley and Cosca (2003) obtained a high precision 40Ar/39Ar age of 49.6-48.1 Ma for the Sunlight volcano phase of the Absaroka Volcanics. Feeley (2003) and Feeley and Cosca (1993) provide geochemical evidence that the Absaroka Volcanic rocks were produced in response to shallow subduction of the Farallon plate in the Late Cretaceous through Paleocene.  Extensive sequences of Absaroka Volcanics can be accessed in the vicinity of Cooke City, Montana, in Yellowstone National Park from the northeast entrance to Tower Junction (and Mt. Washburn), and in Paradise Valley of the Yellowstone River near Point of Rocks and also near Chico Hot springs (Chadwick 1970; Locke et al. 1985).

Petrified Forests: Specimen Ridge YNP and Tom Miner Basin  


The Petrified Forests of Yellowstone National Park are preserved in the Absaroka Volcanics, and are prominently exposed on Specimen Ridge, south of the Lamar River near Tower Junction.  These have been described in detail by Dorf (1960), Tertiary Fossil Forests of Yellowstone National Park (Acrobat (PDF) 1.6MB Nov24 24) and Dorf (1980). These fossil forests cover an area of 64 km2 to a thickness of 366 meters. Twenty-seven successive forests were formed after being sequentially wiped out by lahars and volcaniclastic fluvial deposits, followed by soil formation and rebirth of the next generation of forest. Petrified stumps still in their live upright positions are preserved throughout the area.  Over 80 species of trees and shrubs have been cataloged in these deposits. Fossilization of the forest materials occurred though circulation of mineralizing Si-rich groundwater. A modern day analogue can be found around Mt. St. Helens, Washington where trees in the "blast zone" have been buried by lahar and ashfall deposits.  Another occurrence of a petrified forest in the Absaroka Volcanics can be found in Tom Miner Basin, west of Yankee Jim Canyon of the Yellowstone River, ~20 miles north of Gardiner, MT.

Heart Mountain Detachment (HMD)


The HMD is the world's is a prime example of extensional gravity-tectonics, as opposed to orogenic (i.e., compressive) processes, as recognized as far back as Bucher (1933, 1947).  It is the world's largest landslide, displacing ~500 square miles (1300 sq. kilometers) a distances of at least 25 miles.  Movement on the detachment occurred ~49 million years ago. Detailed mapping by Pierce (1957) identified these structures as detachment thrusts or decollements that have separated on a basal shearing plane and traveled long distances by gravitational gliding. More than 50 separate blocks have been identified ranging in size from 100s of meters to ~ 10 km, and up to750 meters thick.  Heart Mountain is located in the Bighorn Basin, just north of Cody, Wyoming; this block of rock traveled almost 60 miles to the east, emplacing the lower Paleozoic carbonate rocks (Ordovician Bighorn Dolomite) over the basin fill rocks of the 55 Ma Eocene Willwood Formation. The breakaway fault located in the northwest part of Yellowstone National Park has been described by Pierce (1980). The HMD detachment surface developed in a bedding layer near the base of the Ordovician Bighorn Dolomite (Figure 8). The slip surface dips only about 2o to the southeast, but hanging wall rocks were displaced as much as 45 km into the adjacent Bighorn Basin. Hanging wall rocks include lower Paleozoic units (Ordovician Bighorn Dolomite to Mississippian Madison Limestone) and Eocene Absaroka Volcanic rocks. Members of the Eocene Cathedral Peaks Formation rest unconformably on the Mississippian Madison Limestone and are part of the allochthonous blocks.  However, younger (mid-Eocene, 49 Ma; Smede and Prostka, 1992) volcanic rocks of the Wapiti Formation were erupted soon after faulting occurred and were emplaced on tectonically exposed segments of the HMD detachment zone (Pierce and Nelson 1970). To the east, the detachment surface cuts up section onto the former land surface of rocks of Tertiary age.   An overview of the Absaroka Volcanics on the hanging wall of the HMD and the Ordovician Bighorn Dolomite can be seen from the Pilot and Index Vista on the Beartooth Highway.  A trip down Sunlight Basin (Chief Joseph Highway) to Jim Smith Creek and White Mountain (north of Dead Indian Pass) provides direct access to seeing the HMD in situ. See the description by Steven Losh, Minnesota State University-Mankato, for an excellent detailed description of the HMD and a related website on The Geology of Wyoming--Heart Mountain.

On a cultural note, Heart Mountain was the site of the infamous Heart Mountain War Reclamation Center where Japanese-Americans were interned during WWII. You might also be interested in reading the lyrical writing of Gretel Ehrlich in her novel Heart Mountain. "Ehrlich explores the twin solitudes of political exile and geographic isolation in this powerful novel—the story of Japanese Americans forced into a relocation camp—set in Wyoming during World War II." (from Goodreads). More information on the WWI Japanese American internments can be found at Wyohistory.org The Internment Camp at Heart Mountain 1942-1945 and the Densho Encyclopedia.


New World Mining District

The Absaroka Volcanic sequence is host to numerous hard rock mineral deposits in the Cooke City area and in the Emigrant Mining District (Paradise Valley) and Independence Mining District (headwater area of the Boulder River). The New World mining district, located north of Cooke City, was first described by Lovering (1929) with subsequent geologic descriptions by Elliott et al. (1992) and Johnson and Meinert (1994). Mineral deposits were discovered as early as 1869 and mining proceeded intermittently from the 1880s through the 1920s, although these operations were challenged by the high cost of logistics and difficulties in processing the polymetallic ores (Lovering 1929). Au-Cu-Ag orebodies formed as stratobound replacement/skarn deposits (Kirk and Kirk 2002). Mineralizing hydrothermal solutions emanating from Absaroka Volcanic near-surface intrusions circulated along bedding planes in the Cambrian Meagher Limestone, in associated diatremes and breccia pipes, and adjacent to high angle contacts with hydrothermally altered porphyritic dacitic stocks (Elliott et al. 1992). Gold mineralization occurs with quartz-pyrite alteration, and formed at temperatures between 240-415oC and in fluids with salinities between 3.6-11.8 wt% NaCl equivalency (Johnson and Meinert 1994). The McLaren open pit mine operated from 1933 to 1953 and produced 305,700 metric tons of ore with grades of 6.31 g/t Au, 8l91 g/t Ag, and 0.59% Cu (Elliott et al., 1992).  More recent exploration drilling by Crown Butte Mines from 1989 to1996 led to the discovery of a "porphyry-type" copper-gold mineralized zone at depth with proven ore reserves of 7.9 MM tons averaging 0.261 opt gold, 1.05 opt silver, and 0.74% copper, and totaling 2 million ounces of gold (Kirk and Kirk 2002).

Exploration ceased in 1997 when intense pressure from environmental groups raised concerns about acid rock drainage and heavy metal transport via tributary streams into Yellowstone National Park. The United States government removed the New World Mining District from mineral exploration in an agreement with Noranda-Crown Butte Mines that involved the transfer of all mineral rights to the U.S. government, a payment of $65 million for transfer of the mineral rights, and commitment of an additional $22.5 million for reclamation and remediation of historic mine sites (van Gosen, 2007). The US Forest Service has oversight of environmental remediation in cooperation with other federal and state agencies, "to assure the achievement of the highest and best water quality practicably attainable", "...to mitigate environmental impacts that are a result of historic mining" and "..."focus on stabilizing the solid mine wastes to prevent or reduce erosion onto adjacent lands or into streams" (van Gosen 2007). Historical mining activities are reported from other related mining districts situated in the Absaroka Volcanics including the Independence, Emigrant, and Sunlight Mining Districts (Parsons 1937; Wedow et al. 1975; Nelson et al. 1980; U.S. Geological Survey and U.S. Bureau of Mines 1983; Moyle and Buehler 1989; Hammarstrom et al. 1993; and, Elliott et al. 1993).   A comprehensive summary of environmental remediation activities can be found in the New World Mining District Response and Restoration Project, 2006, Project Summary.

Miocene Intermontane Basin

Miocene Intermontane Basin Lake Beds: A period of extensional tectonics created a series of grabens bounded by normal faults across SW Montana (Vuke, 2020; Thomas and Sears, 2022). The easternmost of these graben basins is located in Paradise Velley where intermontane Miocene sediments dated at 16.0-14.8 Ma underlie Hepburn's Mesa. These rocks are finely laminated mudrocks and zeolitized volcanic ash units. The depositional environment is interpreted as arid, saline lake beds. This location is known to preserve mammalian rodent, early horse, camel, and tortoise fossils (Barnosky and Labar 1989; Burbank and Barnosky, 1990).  Hepburn Mesa is located on East River Road of Paradise Valley, across the Yellowstone River from a Montana DOT rest stop, and west of Dailey Lake. The white sediments at the base of the outcrops are the Miocene lake sediments, and the overlying black rocks are the olivine-bearing Hepburn Mesa Basalts.

2.2 Ma Basalt Flows

There are three locations of Pleistocene basalt flows on the western margin of the Beartooth Mountains.  Hepburn Mesa Basalt is dated at 2.2 Ma (Smith et al., 1995; Harlan and Morgan, 2008) with 3He/4He ratios that indicate a deep mantle source as reported in Pierce et al., (2014). The 2.2 Ma age is significant because this is the age of the first major eruptive cycle of the Yellowstone Volcanics that generated the Huckleberry Ridge Tuff.   Are the two magmatic events related? No definite correlation has been made, but one model proposes that extraction of mafic melts from the mantle ponded at the base of the crust to initiate melting that produced the voluminous Huckleberry Ridge Tuff. The Hepburn Mesa Basalt may be a small sample of that mantle-derived magma that managed to escape to the surface.  A second outcrop of basalt is found to the north in Paradise Valley near the town of Emigrant, MT.  This is the former Merriman Quarry, now operated as the Black Diamond Quarry, and has been in operation since the 1880s as a main supply source for railroad track bedding for the Northern Pacific/Burlington Northern Railroad and for rip rap for river stabilization. Columnar joints are well-developed at this location. Basalt from this quarry was used to build the famous Roosevelt Arch at the North Entrance of Yellowstone Park. See the Black Diamond Quarry website  for more historical information.  A third occurrence of these basalt formations can be found at Deckard Flats, east of Gardiner MT on the road to Jardine near the Eagle Creek Campground and confluence of Bear Creek and the Yellowstone River. Here the basalt flows were emplaced unconformably on the >2.8 Ga Jardine Metasedimentary Sequence.

Quaternary Geology--The Pleistocene Ice Age and Younger (<2 Ma)

The spectacular Alpine scenery of the Beartooth Mountains is shaped primarily through glacial and fluvial processes. During the Pleistocene Ice Age, the Beartooth Mountains and adjacent Yellowstone Plateau were covered with ice caps in two major events: the Bull Lake Glaciation of 150,000-120,000 years ago and the Pinedale Glaciation of 20,000-12,000 years ago (Pierce, 1979;  Pierce et al., 2014; Liccardi and Pierce, 2018). To produce enough pressure to force the glacial ice to flow, it is estimated that there may have been over a mile of ice on the present day's surface of the Yellowstone Plateau! The Pinedale ice flows overrode the earlier Bull Lake glacial deposits in Paradise Valley and in the Red Lodge area, and ice flow during the last glacial maximum made it as far north in Paradise Valley to the 8 Mile terminal moraine near Emigrant, Montana.  A good overview of the regional glacial history is at the National Park Service site Yellowstone's Icy Past and a comprehensive description of regional glacial history is in Pierce, K.L., 1979, History and dynamics of glaciation in the northern Yellowstone Park area, US Geological Survey Professional Paper 729-F, 90 p.  


The U-shaped valley of Rock Creek south of Red Lodge Montana is a classic U-shaped glacial sculpted valley. Related glacial and periglacial features of the high Beartooth Plateau have been described by Mueller et al. (1987). These include  Pinedale and Bull Lake terraces and moraines (in the Rock Creek Valley south of Red Lodge, Montana), and hanging valleys (e.g., Hellroaring Creek). In the high Beartooth Mountains, the plateau is dissected to form "biscuit board topography" (i.e., high plateaus with scalloped cliff faces surrounded by steep cirques), with cirques, aretes and tarns creating a dramatic Alpine landscape. Periglacial features include felsenmeer (German for "sea of rocks", a surface covered with angular boulders formed by fracturing in response to mechanical weathering by frost heaving), stone stripes (freeze thaw action can sort coarser rocks into surface stripes), and nivation hollows (surface depressions formed by accelerated erosion around snow banks due to freeze thaw processes).


Perhaps the best documented glacial history in Beartooth Country is preserved in Paradise Valley, as reported by Pierce (1979).  Detailed descriptions of the glacial and other physiographic features of Paradise Valley can be accessed in the detailed road log of Locke, et al, 1995, The middle Yellowstone Valley from Livingston to Gardiner, Montana; a microcosm of Northern Rocky Mountain geology.  Key glacial features include:

  • Deposits from both the Yellowstone outlet glacier and Alpine glaciers flowing from the high Beartooth Mountains are evident. The framework for the glacial history of this area was established by Pierce (1979) and includes features formed by the Yellowstone outlet glacier such as the Pinedale terminal and recessional moraines that were deposited as much as 50 km north of Yellowstone National Park (Pierce et al. 2014). The terminal Pinedale moraine, called the Eight Mile Moraine, that formed from the Yellowstone outlet glacier forms a lobate deposit near the town of Emigrant (Pierce, 1979).
  • Glacial deposits can be dated by cosmogenic isotopic methods to obtain surface exposure ages (the time since a rock was deposited at the surface of the Earth).  Earth is always being bombarded with cosmic rays, and these high energy particles (protons and alpha particles) can interact with atoms to produce cosmogenic nuclides.  In response to this flux of cosmic rays, 16O will convert to 10Be and 28Si will convert to 26Al. As with other radiometric dating methods, the longer the exposure to cosmic rays, the larger the proportion of cosmogenic daughter products.  Sampling quartz-rich cobbles and boulders on glacial moraines will provide the cosmogenic age of deposition of the moraine. Licciardi and Pierce (2018) obtained 10Be ages of 16.5 ± 1.4 to 14.2 ± 1.2 ka for the Pinedale moraines in Paradise Valley. 
  • Remnants of Bull Lake glacial moraines  (ca. 150-140 ka) are preserved  in the northern part of Paradise Valley (Locke et al. 1995). 
  • The Pine Creek drainage is glaciated with lateral and terminal Pinedale moraines forming along the creek and into Paradise Valley. A diamicton unit (mixed, unsorted boulders, cobble, and sands in a muddy matrix) crops out on East River Road that may either be a glacial till or landslide deposit.
  • Glacial landforms in Paradise Valley on the western margin of the Beartooth Mountains include terminal and lateral moraines, glacial outwash deposits, dry abandoned channels, stagnant ice kettle topography and eskers near Pray, MT, and paired cut-and-fill terraces.
  • Flood Bars: just north of Gardiner, MT, (~4 miles north of Roosevelt Arch and the Gardiner High School), Pierce (1973) described megaripples similar to those developed in the Washington Scablands related to the Lake Missoula floods. The megaripples have wavelengths of ~10 meters and heights of 1-2 meters. The floodwaters are estimated to have been 200 feet (60 meters). Pierce (1979) interpreted these features as deposits that formed from catastrophic glacier-dammed lake outbursts. Similar flood bars are found at the mouth of Yankee Jim Canyon that may have formed both from glacial lake outbursts, and younger deposits that formed from breaching the landslide that dammed Yankee Jim Canyon (Pierce, 1979).

Holocene--Present Day Geologic Processes

Neotectonics and Seismicity: Montana is still rocking and rolling! SW Montana is located in the Intermountain Seismic Belt, and earthquakes can happen at any time. Haller et al., (2000) have compiled Data for Quaternary faults in western Montana . The Montana Bureau of Mines and Geology has provided an online Interactive Earthquake Map that is a compilation of the distribution and magnitude of historical earthquakes in the region.

Basin and Range Style Faulting--Active Tectonics. The easternmost extension of Basin-and-Range extensional faulting is expressed as the western boundary fault of the Absaroka-Beartooth Range. The Emigrant Fault is mapped from near Suce Creek south to the Dome Mountain area in Paradise Valley; splays of this fault are recognized as the Deep Creek and Luccock Park Faults (Roberts, 1964; Personius, 1982). Bonini et al., (1972) conducted a gravity survey in Paradise Valley and reported an 18,000 foot (5500m) vertical displacement on the Deep Creek Fault. They interpreted that several thousand feet of Tertiary sediment valley fill as well as 4,000 feet of volcanic breccias are preserved in the valley. Montagne and Chadwick (1982) describe a fault scarp at Pool Creek at the north end of Paradise Valley that cuts a Bull Lake lateral moraine but is buried by a Pinedale moraine which brackets the age of movement between 150,000 and 20,000 years. An active fault that cuts alluvial fans is recognized on the flank of Dome Mountain just south of Point of Rocks in Paradise Valley.  This fault is at least 10 km (6 miles) long and is the longest segment of the Deep Creek Fault system--the fault cuts both Pinedale moraines and alluvial fans, attesting to the recent movements on this fault (Personius, 1982). The LIDAR image below clearly shows the trace of the fault scarp. LIDAR (Light Detection and Ranging) is a remote sensing method that very precisely measures and produces images of the 3-D topographic surface of the Earth.  LIDAR produces "bare Earth" images as it is capable of seeing through vegetative cover. The second LIDAR image shows a major prehistoric landslide that formed as a result of a major paleoseismic event that occurred near Yankee Jim Canyon. This massive landslide dammed the Yellowstone River and formed sandy lake deposits as far as 14 miles upstream to Gardiner, Montana (Locke et al. 1995).  This earthquake-dammed lake is analogous to the famous 1959 Hebgen Lake Earthquake that also caused a massive landslide that dammed the Madison River to form Earthquake Lake.


Regional Uplift in Response to the Yellowstone Hot Spot:  There has been considerable regional uplift due to inflation of the crust overlying the Yellowstone Hot Spot (Pierce and Morgan, 1992). This has had a profound effect on the organization of drainage basins in the area, leaving behind instances of ancestral abandoned river channels of the Yellowstone River.

Active landslide deposits, alluvial fans, debris flows, and hummocky topography attest to the continuing dynamic landscape evolution as seen throughout Paradise Valley. This is particularly evident at the unconformity between the Archean basement and the overlying Eocene Absaroka Volcanic units, where this slip surface is responsible for active landsliding and formation of hummocky topography, particularly on the west side of Paradise Valley. These landslide deposits are particularly prominent near the Rock Creek and Big Creek areas. These mass wasting processes present significant geohazards that are expected to impact the rapid human development that is in progress in this area.

Hydrology: The northern reach of Paradise Valley hosts world famous "blue ribbon" spring creek trout streams. These form through northward flow of groundwater under the regional hydraulic gradient through Cenozoic fluvial valley fill deposits (sands, conglomerates).  These aquifer deposits thin and are constricted by the narrowing of the valley at the north end of the valley, forcing upwelling of groundwater to feed the spring creeks (Locke et al. 1995). The emergent waters have constant temperatures and high nutrient value that sustain this world-class fishery. 

Hot Springs: In addition, Paradise Valley is host to Chico Hot Springs and LaDuke Hot Springs. Both hot springs are fault-controlled.  Chico Hot Springs Resort is located at the intersection of the Mill Creek-Stillwater Fault Zone and the Deep Creek Fault (Chadwick and Leonard, 1979). A deep-seated water source related to Yellowstone magmatism is ruled out by helium isotopic data (Kharaka, 1991). Chadwick and Leonard (1979) report: "...the Chico system appears to obtain its heat from deep circulation of groundwater without a magmatic component... similar to many other hot springs in the Northern Rocky Mountain region where deep circulation of groundwater in fractured crystalline rocks and/or at extensional valley bounding normal faults produces hot springs."  LaDuke hot springs is located adjacent to the Gardiner-Spanish Peaks Laramide Fault. At LaDuke Hot Springs, plans to develop geothermal energy systems on were quite controversial as there was concern that these activities might disrupt the hydrology of Mammoth Hot Springs in Yellowstone National Park. Geochemical studies of the spring waters (Kharaka et al. 1991) indicate there was low probability of interconnection with the Mammoth Hot Spring system and Sorey (1991) concluded "sustained production of this well at rates near the flow of LaDuke Hot Springs would pose no risk of adverse effects on thermal features in Yellowstone National Park". Nonetheless, concerns persisted and in 1995 the U.S. House of Representatives passed the "Old Faithful Protection Act" that would have established a 15-mile buffer zone around Yellowstone National Park to restrict development of hydrothermal resources. This legislation was not ratified by the U.S. Senate.  Nonetheless, the State of Montana and Yellowstone National Park have established plans for long-term monitoring of this hydrothermal system.  LaDuke Hot Springs has now been developed as a destination recreational resort--Yellowstone Hot Springs Resort , and Chico Hot Springs Resort has been in operation since 1900.


Jardine Travertine Deposits: Located just east of Gardiner on the road to Jardine, these travertine deposits are similar to the travertine deposits at Mammoth Hot Springs. They are outside Yellowstone National Park and have been quarried for building stone. Pierce et al., (1991) report U-Th ages of 19.57 +/- 0.12 Ka and 22.64 +/- 0.17 Ka.

Gold Mining at Emigrant Gulch: Emigrant Peak, above Chico Hot Springs, is one of many eruptive centers in the Absaroka Volcanic Province that is host to metallic deposits that have been prospected and mined. Stotelmeyer et al., (1983) report that placer gold was discovered at the mouth of Emigrant Creek. Up until 1882, lands of the Yellowstone River Valley were part of the Crow Indian Reservation, but in that year these lands were removed from the reservation, opening a gold rush into this area.  Reed (1950) reports that there was only minor production of Au from quartz vein lode deposits. Placer gold production is reported back to 1901 with minor operations through 1940, and a large bucket dredge was operated in 1941-42. Total placer production from 1901 through 1959 was 15,606 ounces (Reed, 1950). The placer dredge tailings can still be seen in the vicinity of Old Chico (this is private property).


Climate Change in Beartooth Country

The impacts of climate change are evident throughout the landscapes and ecosystems of Beartooth Country:  extreme weather events, floods, droughts, impacts on habitat (e.g., distribution of white bark pine critical to survival of grizzly bears), introduction of invasive species....See the full story in these two important climate change assessments. The recent article by Pederson et al., (2025) documents exposure of a stand of centuries old whitebark pine due to melting of perennial ice patches on the Beartooth Plateau. "In the Greater Yellowstone Ecosystem of the Rocky Mountains (United States), recent melting of an high-elevation (3,091 m asl) ice patch exposed a mature stand of whitebark pine (Pinus albicaulis) trees, located ~180 m in elevation above modern treeline, that date to the mid-Holocene (c. 5,950 to 5,440 cal y BP). Here, we used this subfossil wood record to develop tree-ring-based temperature estimates for the upper-elevation climate conditions that resulted in ancient forest establishment and growth and the subsequent regional ice-patch growth and downslope shift of treeline. Results suggest that mid-Holocene forest establishment and growth occurred under warm-season (May-Oct) mean temperatures of 6.2 °C (±0.2 °C), until a multicentury cooling anomaly suppressed temperatures below 5.8 °C, resulting in stand mortality by c. 5,440 y BP."

Epilogue: June 2022 Flood

On June 13, 2022, a warm rain-on-snow event triggered a massive melting and flooding event.  Access roads to the North and Northeast Entrances of Yellowstone National Park were wiped out. Major impacts to infrastructure were suffered, including bridge collapse and road washouts in the entire region. The towns of Gardiner and Red Lodge, Montana were inundated by floodwaters. For context, the 100-year average peak discharge of the Yellowstone River at Corwin Springs, MT (just north of Gardiner, MT and the North Entrance of YNP) is 16,000 cfs, and the record discharge was ~30,000 cfs in 1918.  In the 2022 flood event, discharge reached 49,000 cfs (!), and this event has subsequently been branded a "1000 year flood". The North (Gardiner)and Northeast (Cooke City) entrances to YNP have now been repaired and are open to tourism again, but the consequences of this catastrophic event will be felt by the impacted communities for years to come. McManamay et al., (2025) report annual business revenue losses varied by gateway community, averaging 48 % but as high as 100 %, and estimated $156 million USD losses in 2022 in visitor spending, primarily from park closures, exceeding economic losses from the COVID-19 pandemic in 2020.

Geology in Action:

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There you have it! 4 billion years of Earth history revealed in a single mountain range: the remarkable Beartooth Mountains.