Fire
and Ice - Paradise Valley Geology 101
John Gillespie & Whitney Tilt
First
Fire
Strata volcanoes erupted frequently 65-35
million years ago in Paleocene through Eocene times (Figure 1), leaving
deposits of andesitic lava, lahar debris flows and petrified forests – forming
the mountains of the Absaroka Volcanic Supergroup. These volcanoes, part of a “ring of fire”, were
caused by the subduction of the Pacific Plate under the North American Plate
and a contemporaneous uplift called the Laramide Orogeny. Two northwest
trending belts of eruptive centers, termed the Eastern Absaroka Belt and the
Western Absaroka Belt, include well known peaks like Sunlight, Washburn,
Electric, Emigrant and Hyalite. The ancestral
Yellowstone River drained the mountainous topography. Paradise Valley formed
around during this time.
35-25 million years ago during the
Oligocene epoch the valley filled with sediments from the eroding mountains and
the valley collected the fluvial deposits of the Renova Formation – much of
which is now eroded.
From 25 to 8 million years ago during
Miocene epoch tuffaceous siltstones like those at Hepburn’s Mesa were deposited
in shallow alkaline lakes and mudflats much like those seen at the Salton Sea
today.
First
Hints of More Fire
Much of the volcanic ash within Hepburn’s
Mesa siltstone originates from volcanic eruptions tied to the first arrival of
the North American Plate over the mantle hot spot that dominates the active
volcano at Yellowstone today. The track of the heat is along the Snake River
Plain where the sheet-like edge of the North American Plate moves to the
west-southwest at an inch or two a year over the relatively stationary propane
torch of the hot spot, allowing super-heated magma to penetrate through the
crust along its route. The 16.5 million year old white tuffaceous siltstone at
Hepburn’s Mesa coincides with air-fall ash from the first eruption of the hot
spot – the McDermitt caldera, which occurred soon after the deposition of the
Columbia flood basalts in Washington State. The McDermitt ash-fall caused the
death and preservation of a rich assemblage of Miocene mammal life at Hepburn
Mesa – including Merychippus – the first equine to exhibit the head of today’s
horses. Another of these ash-falls, tied to the Bruneau-Jarbridge caldera, ~12
million years ago, resulted in similar death and preservation at Ashfall Fossil
Beds State Park in Nebraska where excellently preserved Teleoceras, a
hippo-like ancestor of the rhinoceros, are stacked in layers of ash-rich
siltstone.
Coming after the Miocene lakes the
Yellowstone River dominates the return from lacustrine deposition to fluvial
deposition. At Hepburn’s Mesa and at
many places in Paradise valley, a coarsely-sorted, mixed-grain, mixed-color
conglomerate, deposited in the river bottom, can be observed immediately above
white tuffaceous siltstone.
Red
Hot Fire: Arrival of Yellowstone over the Mantle Hot Spot
2.2 million years ago, the waters of the
Yellowstone River had formidable competition.
A lava flow, a precursor to the Huckleberry Ridge super eruption –one of
the most cataclysmic volcanic eruptions in history, gravitated to the
topographic low occupied by the river.
In a boiling battle with sound and light, hiss and sizzle beyond
comprehension like can be seen where lava from Pu’u O’o meets the ocean at Kilauea today, the
lava bested the water as is documented by the thick black deposition of basalt
that lies immediately above the river bed conglomerate. The basalt originates from a local vent and
can be traced down flow to a site where it pinches out just north of
Emigrant. The 2.2 million year
radiometric age of this basalt matches that of Junction Butte in the national
park, predating the 2.1 million year Huckleberry Ridge super eruption by
100,000 years. Incongruently, there are
glacial striations scratching the top of the basalt and a thin profile of
sediment above the basalt is glacial till with glacial erratic boulders – hints
of the ice that followed the fire.
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Figure 1.
History of the Yellowstone Hotspot – creating the Snake River Plain
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A
Geological Collision
Paradise Valley lies between the Great
Plains and the edge of the Yellowstone volcanic plateau. According to the
Montana-Yellowstone Geologic Field Guide Database, the valley shares basement
rocks with the continental interior; Paleozoic and Mesozoic lithologies with
the western interior, compressive tectonics with the Fold and Thrust Belt to
the west; Basin and Range with the west and south; and Cenozoic volcanism with
much of the surrounding region. The database describes the area as a “microcosm
of the evolution of the geological understanding of the American West.”
While you may or may not understand the
above, overly-technical description, here is a partial list of some of the
geological highlights in the Paradise Valley. All are easily found and observed:
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Landforms:
Gallatin Range, Beartooth Range, Hepburn Mesa, Yankee Jim Canyon
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Metamorphic
basement rocks of the North Snowy Block, near Pine Creek
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Paleozoic,
Mesozoic, and Cenozoic sedimentary rocks
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Hepburn’s
Mesa Formation basalts
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Point
of Rocks volcanic/intrusive center (Absaroka Volcanic Field)
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Travertine
deposits at Gardiner and the Liberty Cap at Mammoth
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Glacial
deposits (Wisconsinan/Pinedale age) – more below
-
Devils
Slide (Figure 2)
-
Giant
Ripples flood deposits at Corwin Springs
-
Rangefront
alluvial fans
-
Chico
and La Duke Hot Springs hydrothermal features
-
Nelson
and Armstrong Spring Creeks
Mountain
Formation
Beginning some 65 million years ago,
during the early Cenozoic, the Yellowstone region underwent periods of uplift
and folding that alternated with periods of quiet and erosion. Volcanic
activity, accompanied by uplift and folding, showered the landscape with
volcanic ash, dust, silt and sand. Deluges of coarser debris flowed down the
volcano flanks in lahars which became the volcanic breccias. Periods of quiet
allowed semi-tropical plants and animals to flourish and nearly 200 fossil species
have been identified in the Absarokas, perhaps the most diverse petrified
forest in the world. These include
unexpected species like breadfruit, persimmon, magnolia and mangrove. Resumed volcanism buried or transported entire
forests into valleys as witnessed at Spirit Lake below Mount St. Helens and
other places around the globe.
The Beartooth Plateau, forming the eastern
boundary of Paradise Valley and consisting mainly of Precambrian basement rocks
with a local veneer of Eocene Absaroka volcanic rocks, pushed its way upward
through sedimentary rock during the Laramide orogeny (~70-45 million years
ago). It contains some of the oldest
exposed rock on Earth. The Quad Creek Quartzite on Beartooth Pass has been
radiometrically dated as 3.95 billion years old. Yankee Jim Canyon provides
exposures of foliated crystalline metamorphic rock - granitic gneiss, micaceous
schist, and pegmatite dikes formed miles below the surface.
Cinnabar Mountain, home to the Triassic Chugwater
red bed of the Devil’s Slide, provides a view of remnants of the hanging wall
of the Gardiner thrust fault exposing nearly vertical beds of Paleozoic and Mesozoic
rock (Figure 2) and provides “hieroglyph-like” evidence of ~500 million years
of Earth history.
The Gallatin Range, forming the western
boundary of Paradise Valley, is generally lower in elevation and less vertical
as compared to the Beartooths. Now at
~10,000 elevation, many of the peaks are comprised of Paleozoic carbonates that
were originally deposited in epieric oceans.
Paradise Valley is in a downthrown position relative to the Beartooths
as a result of Basin & Range normal faulting that arrived about the same
time as the Yellowstone Hot Spot and they were buried by younger volcanic rock
after faulting occurred. Paradise Valley
is the easternmost expression of the Basin and Range province. The extensional
tectonics, often described as a rift, are represented by the presence of scarps
along the Deep Creek-Luccock Park-Emigrant fault.
As uplift continued across the region, the
mountains and valleys grew in elevation and cooled. Increasing snowfall failed
to melt as rapidly as it fell. The Ice Ages arrived.
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Figure 2.
Devils Slide, Corwin Springs. Vertical bands with older (Paleozoic) rocks on
right and younger (Mesozoic) rocks on left. 1-
Pinedale glacial till; 2-Devonian-Mississippian undifferentiated limestone;
3-Triassic rock; 4-Cretaceous undifferentiated rock.
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Ice
Age Cometh
Beginning about two and a half million
years ago, yesterday in geologic time, the Pleistocene epoch in the northern
hemisphere has been characterized marked by advances and retreats of
continental ice sheets. The Yellowstone region underwent a series of glacial
stages. The earliest glaciations were largely
bulldozed away by subsequent ones, removing evidence of their distribution.
The Bull Lake glaciation, ~200,000 years
ago through 130,000 years ago, concurrent with the continental Illinois glaciation,
created widespread valley glaciers including one that covered the northwest
portion of Yellowstone Park, and flowed northward down the current course of
the Yellowstone River. Overtopping lesser peaks like Cinnabar and Dome Mountain
as it flowed northward, its width varied from 3-6 miles and it height estimated
at 3,000 feet. Most of the landforms created by the Bull Lake glaciation are gouged
and overprinted by the subsequent Pindale glaciation, but some good landforms
are preserved in the Teton and West Yellowstone areas.
The region’s last glaciation, the
Pinedale, occurred concurrent with continental Wisconsin glaciation began
approximately 70,000 years ago and reached its maximum ~25,000 years ago. It left the most visible record and its
recession has been carefully studied by cosmogenic and other dating methods.
The Beartooth Mountains, capturing precipitation from the caldera-rich lowland
of the Snake River Plain, provided the incubator for the Yellowstone Outlet
Glacier. From its source high in the Beartooths near Granite Peak to its
terminus at the Eight Mile Moraine near the junction of the Yellowstone River
and Mill Creek, it was nearly 70 miles long.
Based on eroded mountain tops and
remnant horns and aretes, in some places towards its center the glacier was
approximately 4,000’ thick. Tom Miner and Big Creek were both covered by
Pinedale ice. Evidence of this glaciation can be seen in moraine remnants
around Chico Hot Springs and the ice- margin channels and terraces on the
mountain slopes between Big Creek and Fridley Creek on the west side of the
valley. As the glacier receeded, much like the collapse of Glacial Lake MIssoula,
ice dams broke in the Lamar Valley and catastrophically released massive volumes
of impounded water. Enormous floods, some with headwalls estimated at 150-300’,
ripped down the Black Canyon of the Yellowstone and deposited ripple marked
flood bars of boulders as much as 50’ high near Gardiner.
Glacial ice left its signature on the Paradise
Valley landscape in the form of moraines, glacial striations, moraines, ice
marginal channels, kettles, polished sheepback hills, till and glacial erratics,
including many along Big Creek.
In the blink of an eye (geologically
speaking), plants and animals recolonized the landscape and humans arrived to
hunt and forage the abundance of life at the ice edge. Along with sheep traps
and bison jumps, lithic quarries, stone circles, fire pits and other artifacts
of Clovis and other Paleo-Indian peoples are found in the area.
Hepburn’s
Mesa
A glimpse of the geological history of the valley is
on view at Hepburn’s Mesa (Figure 3), the bluffs dominating the east side of
the Yellowstone River, opposite Mountain Sky Guest Ranch lands. At the bottom
are the oldest rocks-light colored fine-grained sedimentary rocks formed from
sediments deposited in an ancient lake that once occupied the site. Atop these
rocks are the unconsolidated river bed gravels and conglomerates that are in
turn capped by a basaltic lava flow, and lastly by glacial till.
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Figure 3. Aerial photo of Yellowstone River and Big Creek
drainage, looking south. 1-Hepburn's Mesa; 2-Confluence of Big Creek &
Yellowstone River; 3-Glacial till; 4-Sheep Rock; 5- Mountain Sky Guest Ranch.
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The sedimentary beds contain abundant Miocene fossils,
including ancient rodents, moles, and a proto-horse called Merychippus. The
basalt lava flow erupted from a volcanic vent some 2.2 million years ago. It is
of the same age as the Junction Butte basalt in Yellowstone Park and is a precursor
lava flow to the Huckleberry Ridge super eruption at Yellowstone. Its dark coloring is the result of high
concentration of iron, including magnetite. On top of the mesa lies the
youngest material, glacial till dating to the Pleistocene age. The surface of
these deposits is hummocky and is littered with abundant glacial erratic
boulders. For generations, Native Americans drove bison off the mesa’s cliffs
and captured sheep in rock traps.
Hepburn’s Mesa is named for John Hepburn, a local
rancher, rockhound and amateur paleontologist. He arrived in the Emigrant area
in 1909 after working in Yellowstone National Park for many years. From 1935
until his death in 1959, he operated a museum displaying many of the geologic
specimens and fossils he had found in this area. The museum still stands and is
listed on the National Register of Historic Places.
Fire
& Ice Talk
Fire (Volcanics).
Volcanic activity arises from the Earth’s mantle as magma pushes its way up
through fissures and other fractures in the overlying rock forming dikes,
sills, caps, and flows. Sheep Rock is a volcanic dike – a vertical shaft of
igneous rock (dacite) that has become exposed as the softer surrounding rock
has weathered away. There are several types of volcanic rock including basalt,
andesite, dacite, and rhyolite. Basalt and andesite are dark colored and
fine-grained while dacite is quartz-rich and generally lighter in color.
Rhyolite is silica rich and light-colored and it the “Yellowstone.”
Glaciers are great rivers
of ice formed by the accumulation and compaction of snow on mountains. During
their residency in the region they sculpted mountains and carved out valleys.
For thousands of years, snowfall accumulated and fed the river of ice as it
flowed downslope from the mountain tops. Glaciers are frozen conveyor belts
that take part of the surrounding land with it as it moves downslope. Rocks
became entrained in its ice. As the ice moved, these rocks scoured away more of
the underlying surface. Where the glacier’s forward-most process stopped, it
discharged its melt water, dropped its rocks, and staged a series of retreats.
Looking across the Paradise Valley an irregular
topography is evident including a series of hummocky hills, some shaped as
linear ridges, some as flat-topped headlands above the Yellowstone River, and
some as a series of gently rolling hills. These moraines and outwashes
are the remnants of long-gone glaciers.
The landform of present-day Paradise
Valley is the result of several large glaciers that flowed down the valley,
from south to north, leaving a landscape shaped by moraines and outwash. The
glaciers may be gone, but their influence is plain to see today.
Moraines are composed of
glacial till, an unsorted mixture of silt, sand, pebbles, and boulders dumped
directly from the glacial ice. Ground moraines accumulated as irregular blanket
of till deposited under the glacier while terminal, recessional, and lateral
moraines formed as retreating glaciers left a series of “windrows” at the
terminus of the glacier, and alongside the ice mass. Collectively these
moraines record the glacial record. In the Paradise Valley, terminal moraines
are evident at the outlet of Pine Creek and most of the other creeks flowing
from the Absaroka Mountains. Just south of Pine Creek on Highway 89, the road
climbs up and over the terminal moraine of the Pinedale Glacial Stage, which
reached its maximum northward extent some 16,500 years ago. To the south, the
rolling hills area evidence of the ground moraine that covers the entire valley
floor. The larger cross-valley ridges formed in front of the retreating
glaciers while the lateral moraines formed alongside the ice forms. A
definitive characteristic of a moraine is that its till is unsorted – sediment
of all different sizes is found jumbled together with no layers evident. This
characteristic helps distinguish materials deposited by glaciers from those
deposited by running water which tends to deposit different sizes of rocks in
different areas. Since the end of the last ice age, some 10,000 years ago, soil
has developed on the moraines and trees have taken root.
Outwash is well sorted
sand and gravel deposits delivered by the meltwater flowing from the end of the
glacier. Outwash deposits are flat on top as compared to the hummocks of the
moraines. Imagine 3,000 feet of ice on top of you. As the glacier flowed, its
sheer weight ground the underlying rock surface and carried the debris along.
The meltwater emerging from the foot of the glacier deposited its load of
sediment over the outwash plain, with larger boulders being deposited near the
moraines, and smaller particles travelling further downstream before being
deposited. On either side of the Yellowstone there are prominent, tall,
flat-topped benches formed by the outwash from the last glaciation. Over more
recent time, the river has cut its course down through these sediments.
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Figure 5. Pine Creek Drainage in background; Pine Creek
terminal moraine in mid-ground, and Yellowstone River in foreground. Picture
taken looking ENE from Mallard’s Rest Fish Access Site.
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Sources
and Resources
Berg, Richard B., Jeffrey D Lonn, and
William W. Locke. 1999. Geological Map of the Gardiner 30’ x 60’ Quadrangle,
South-Central Montana. Montana Bureau of Mines and Geology.
Fritz, William J. and Thomas, Robert C.
2011. Roadside Geology of the Yellowstone
Country, Second Edition Missoula:
Mountain Press Publishing Company. 149 pp.
U.S. Department of the Interior. National
Register of Historic Places Registration Form for the John Hepburn Place,
September 5, 2005.
