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Geologic History of the Wood River Valley Region, by James McMindes
General Geologic History
Ancient Idaho: Archean and Proterozoic
The Paleozoic
The Mesozoic
The Cenozoic
References
General Geologic History
 The
Wood River Valley is a scenic valley located in south-central Idaho. Many
people know this region as the Sun Valley area, home of the first destination
ski resort in America. Its’ geology is quite diverse, spanning in time for
two billion years from the early Proterozoic to the present. These include
multiple mountain building episodes, passive continental margin buildup,
extensional tectonics, volcanism, glaciation, mineralization, and river
processes. The valley itself is a product of a thinning continental crust,
which happened in the recent geologic past (Miocene). The Pioneer and Smoky
Mountains border the Wood River Valley on the east and west, respectively.
The Smoky Mountains are composed of Paleozoic sediments of the Sun Valley
Group, which are intruded by Cretaceous-aged rocks of the Idaho batholith.
The Pioneer Mountains are made up of Paleozoic sedimentary rocks, Eocene
volcanics, and rocks of the Pioneer Mountain Metamorphic Core Complex. The
uplifting of both these mountain ranges via Miocene extensional tectonics
resulted in fault blocking and the formation of a down-dropped block that
is the Wood River Valley. Geologists refer to this type of downward moving
piece of continental crust as a graben. Likewise, the bordering uplifted blocks
are called horsts. Quaternary volcanism and alpine glaciation created the
source material for a large alluvial fan, which is found at the far southern
part of the valley at the intersection with the Snake River Plain. Today
the topography of the valley is fairly flat, being filled with Quaternary
alluvium from the Big Wood River.
The diverse geologic history that is displayed in this part of central Idaho can be seen in the variation of rock types and ages. The oldest rocks are found in the Pioneer Metamorphic Core Complex (PMCC), which makes up the nucleus of the Pioneer Mountains. These rocks include calc-silicates, quartzite, and schist that have been age-dated to the early Proterozoic (»2 Ga). Paleozoic sedimentary rocks are located in both the Smoky and Pioneer mountains, structurally overlying the older units. The Paleozoic rocks include sandstone, shale, and limestone of Ordovician through Permian age. Cretaceous plutonic rocks (granodiorite) are positioned within the PMCC along with Eocene-aged quartz monzodiorite. The granodiorite are part of the massive Atlanta Lobe of the Idaho batholith. These batholithic rocks are found extensively throughout the Smoky Mountains where they intrude into the older Paleozoic sedimentary deposits.
What caused the current state of affairs in Wood River Valley area geology? Worldwide and regional geologic events over the past two billion years have left a mark on this area. As stated above, the earliest forming rocks were from the early part of the Proterozoic Eon and are exposed as part of the Pioneer Metamorphic Core Complex. Because these early rocks make up the ancient crystalline core of the continent they are referred to as the basement.
There have been three periods of mountain building, called orogenies, that have effected the Wood River Valley region. The evolution of the western continent during the Paleozoic included two orogenic events, although only one affected central Idaho. The first orogenic phase, the Antler orogeny, occurred during the middle Paleozoic and is characterized by the placing, through thrusting, of oceanic sedimentary rocks (Roberts Mountain allochthon) over passive continental margin rocks. The second period of mountain building happened during the late Cretaceous, and was called the Sevier orogeny. Both of these mountain building events were created by compressional tectonics, and resulted in a shortened and thickened continental crust. The Sevier was distinguished by the intrusion volumous amounts of granitic rock into the continental basement and overlying Paleozoic stack. The latest episode of mountain building started during the Miocene, and may or may not be going on today. This, unlike the first two, involves extensional tectonics, or a thinning of the continental crust. This is the part of the Basin and Range province, which extends from northern Mexico north into Canada. The main structure of this province is a series of northerly trending, alternating fault blocked basins (grabens) and ranges (horsts). This episode is responsible for the rise of the mountains that can be seen in the area today (i.e. Pioneers and Smokys) and the down-dropped block that is the Wood River Valley.
Ancient Idaho: Archean and Proterozoic
There is no record on earth of the first 700 million or so years of geologic
history. The earliest known rocks can be divided into two types, greenstone
and gneiss belt rocks. The earliest dated rocks in Idaho were formed during
the Archean (3.9 to 2.5 Ga), and can be found in the uplifted and exposed
core complexes of the Priest River and Albion Mountains. Many geologists
believe that approximately 70% of the earth’s continental crust was created
by the end of the Archean. The first crust was most likely the result of
oceanic crust being subducted under other piece of oceanic crust, such as
what is happening today as the Caribbean Plate overrides the subducting
North American Plate. This results in island arcs like the windward islands
of the Caribbean. It is believed that during the Archean the pieces of this
juvenile continental crust moved from one place to another, eventually accreting
with other pieces of primordial crust forming the first supercontinent.
At the suture between these accreted continental masses were compressionally
produced mountain belts.
As mentioned above, Archean rocks can be divided into two broad groups that comprise distinct types. Although there are petrologic differences, the rocks were most likely formed during the same geologic event, which was the collision of blocks of continental crust. The first group is the rock of the greenstone belts. These range from basalt to rhyolite and associated sedimentary rocks. Today, these belts are found in tightly folded, elongate, linear bands. Greenstone belts get their name from the green minerals that resulted from the hydration of basalt. The parental minerals olivine, pyroxene, and plagioclase change to epidote, chlorite, and actinolite as pore space water reacts with the original minerals at temperatures and pressures common in low-grade metamorphism. Some of these basalts where formed from underwater eruptions, given their pillow structures and the chemical similarity to the rocks that are formed at mid-ocean ridges today (MORBs). There are some greenstone basalts that show affinities to the rocks that are formed in hotspots, such as the Hawaiian/Empress Island chain. There are also greenstone basalts that are comparable to rocks formed as a result of the subduction of oceanic crust (calc-alkaline). Associated with the basalt are other volcanic rocks such as andesite and rhyolite that are more felsic, thus more evolved geochemically. These last two rocks are not nearly as volumous as the basalt and are usually found only locally within the greenstone belts. The volcanic eruptions that formed the andesite and rhyolite were more explosive than the effusive basaltic eruptions, evident from the high amount of angular inclusions (xenoliths). Also established within the greenstone belts are sedimentary rocks such as greywacke, conglomerate, and shale, which have provenance with the andesite and rhyolite. But, what kind of geologic event produced the greenstone belts is still being debated. Although the stratigraphic succession of rocks from basal marine basalts through upper andesite/rhyolite deposits and the similarities with contemporary subduction-related rocks, lead many to believe that greenstones were formed in convergent margins. Could these have been very ancient equivalents to modern day island arcs such as what is found in the Caribbean?
The second distinct group of rocks, which are found within the Archean provinces, are the rocks of the gneiss belts. Gneiss is a variety of rock that foliates into alternating bands of dark (mafic) and light (felsic) minerals due to the high temperatures and pressures that are found in regional metamorphism. The original, or parental rock, of the gneisses could have been sedimentary such as the greywacke, conglomerate, and shale of the greenstone belts. The gneisses may also have been derived from greenstone igneous rocks. The latter rocks being originally formed either as partial melts of the continental sedimentary rocks (greywacke, conglomerate, etc.) or from melting of the upper mantle. Evidence of a continental source comes form the extensive zones of migmatite that is found within the gneiss belts. Support of a mantle origin comes form chemical affinities with rocks of the upper mantle.
The Archean rocks found in Idaho are part of an age date region called the Wyoming Province. Although these rocks most likely exist in the deep basement of much of the state, only a small amount is uncovered at the surface. This exposure being the result of the very interesting, albeit enigmatic, geologic structure mentioned previously called a metamorphic core complex. These are relatively small, domal-shaped areas that were uplifted and exposed during times of crustal extension. In the case of the Idaho core complexes the extension took place during the Eocene and Oligocene. This was a time that the crust was most likely rebounding after the period of Sevier compression, which resulted in thrusting and substantial plutonism. The metamorphic core complexes will be addressed later, particularly the specific one that occurs in the Pioneer Mountains. Although this complex does not expose Archean rocks like two of the other four complexes in Idaho, it does have continental basement from the Paleoproterozoic.
By the end of the Archean (»2.5 Ga) plate motion and the subsequent accretion of the small pieces of continental crust, caused the formation of the earth’s first supercontinent. But as we will see throughout earth history, supercontinents don’t stay together forever. The beginning of the Proterozoic witnessed at least some of the supercontinent standing above sea level. When situations like this happens it leads into one of the general rules of geology. This states that when rocks are exposed to the atmosphere for any reasonable length of time, they will undergo weathering and erosion. At the same time there will be only superficial amounts of sediment deposition. This was certainly true of much of the late Archean and early Proterozoic. The supercontinent probably had many mountainous areas due to the associated effects of collisional tectonics and subduction-related volcanism. Contemporaneous with the eroding of the Archean/Proterozoic orogenic belts were the effects of the internal thermal forces that were working on breaking the continent apart. Continental crust is lighter and more felsic than its oceanic cousin. Due to the fact that felsic material does not conduct heat well, the internal heat from the mantle will work over time to successfully rift continents apart. Modern analogs of the rifting Proterozoic continental crust are localities such as the East African Rift and the Basin and Range. Rifted regions generally form an area of fault-blocked horsts and grabens. As the rift grows the continental margins on both side of the rift become passive, subside, and develop into continental shelves, therefore developing a diagnostic stratigraphy. This will include shelf sandstone and limestone, continental rise and slope deposits such as turbidites, and deep-sea rocks such as chert. These rocks generally overlie unconformably on the Archean and early Proterozoic basement rocks.
During the middle Proterozoic (»1.6 Ga) in the area that is now the northern Rocky Mountains of Idaho and Montana, a large roughly circular basin developed. This was a catchment for massive amounts of sediments that were being shed off of a piece of continent that has long since rifted away. Where this continental piece is today is still being debated. Some believe that it is part of Siberia, others southeastern Asia or Oceania. These clastic rocks make up what is called the Belt Supergroup and were deposited over 800 million years reaching a western depth of 12 miles. The supergroup is divided into two main sections; the lower group is called the Prichard formation and is made up of dark gray sandstone and mudstone of a deep marine origin. The second section is the Missoula group, which is composed of more colorful sandstone and mudstone, which was deposited, in a shallow marine environment. Unlike the Archean rocks, the middle to late Proterozoic Belt rocks are very well exposed in central and northern Idaho.
The Paleozoic
 During
the early through the middle part of the Paleozoic, the western United States
was a segment of a large passive margin. The beginning of the era was the
Cambrian Period, which was a time of proliferating marine life. This is
what is commonly referred to as the “Cambrian Explosion.” This was a time
of worldwide sea level rises. The flooded western continental margin, which
at the time ended in western Idaho, was a trap for vast amounts of sediments
that were shed off of the exposed parts of the continent This erosion of
the continent went on through much of the Paleozoic. These sediments are
today very well exposed in the mountains of south-central Idaho. The Paleozoic
sedimentary rocks of this area can be viewed as parts of a sequence of deposits
that were laid down prior to, during, and after the Antler Orogeny. The
first part of the sequence is from what is defined as an “oceanic assemblage”
of sediments that were deposited on a submerged edge of a continental crust.
Formations such as the Milligan are parts of this assemblage (Dm on the
USGS Hailey 1°x2° geologic quad). The deposition started in the Ordovician
and continued through the Silurian and ended in the Devonian. The western
margin continued to be passive well into the Devonian laying down the Milligan
formation. The late Devonian through early Mississippian brought a change
to this margin called the Antler Orogeny. Although the reason for this mountain
building episode is not entirely agreed upon, what is known is that the
western continental crust was shortened, most likely by compression. This
compression raised the previously deposited sedimentary rocks, such as the
Milligan formation, upwards into the Antler Highlands. It is believed by
some geologists that the Antler Orogeny was the result of the docking of
an island arc with a western dipping subduction zone. The evidence for the
Antler is seen in the Wood River Valley area by folds in the Devonian sedimentary
rocks that were caused by compression. These folded deposits that were part
of the Paleozoic continental rise and slope, were thrust up unto the continental
shelf. Another piece of evidence is found west of the trend of the highlands.
The sedimentary rocks there were formed in a depositional basin called a
foreland basin. The foreland basins are formed within a tectonically stable
area that borders an orogenic belt. Although, there were other late Paleozoic
and early Mesozoic orogenic events farther south, after the early Mississippian
this part of the western continent returned to be tectonically passive.
The Mesozoic
 During
the Cretaceous south-central Idaho went through another tectonic event.
This is called the Sevier orogeny and resulted in thrust faulting, metamorphism,
and plutonism. The magmatism of this time was extensive and happened in
a large section of the North American Cordillera. The deformation that accompanied
the Sevier orogeny can be seen in the fabrics in the Paleozoic rocks of
the Wood River Valley area. This deformation is seen as north trending,
east vergent folds, and northwest trending, southwest dipping thrust faults.
There is an elongate belt of thrust faults similar in age, called an overthrust
belt, which runs along the western Cordillera from Mexico to Alaska. Current
with the Sevier, although farther to the west and north in far western Idaho,
Oregon, and Washington was the docking of numerous exotic terranes. There
were two distinct spatial types, coastal types such as the Sierra Nevada
of California, and the inland types such as the Idaho batholith. The rocks
of these settings are distinct in both their chemistry and petrology. The
coastal type rocks have a lower Sr ratio that indicates a mantle component.
The major rock types of the coastal type are tonalite and granodiorite.
The inland type suites have a major influence from the continental crust,
having a higher Sr ratio, with rocks varieties such as mica-rich granite
and granodiorite. The cause of the creation of the Idaho batholith is conjectural,
with some geologists believing that partial melting of the continental crust
from an over-thickened crust was responsible. Where others theorize that
the cause was subduction related. Another question is how this very large
mass of granitic rock was emplaced into a compressed. The most common belief
is that the preexisting crustal rocks were incorporated into the batholith’s
magma.
The depth of crystallization of the Idaho batholith is approximately ten miles being completed by the beginning of the Cenozoic. Metamorphism from the intrusion of the batholith rocks reached deep into the crust to affect the rocks of the continental basement. This large batholith has approximately 200 by 75 miles in area and is divided into two lobes based on age and location. The older Atlanta lobe is the larger and southerly lobe of the batholith. It is also the older of the two, having been emplaced between 100 and 75 million years ago. The Bitterroot lobe is smaller and younger having been emplaced 85 to 65 million years ago. The lobes are separated by a topographic high called the Salmon River Arch (also called the Lemhi Arch). There is variation in the rock composition from west to east in both the lobes. The western section of the Atlanta is metamorphosed and folded, with the major compositions being tonalite and quartz diorite. Some geologists believing that these rocks were the result of partial melting of oceanic rocks. The eastern part of the lobe consists of granodiorite and granite. These rocks seem to have a major component from the continental crust. The rocks of the western portion of the Bitterroot lobe consist of tonalite and quartz diorite, whereas granodiorite is found farther to the east. In the Wood River Valley area the rocks of the Atlanta lobe can be found largely in the Smoky Mountains.
The Cenozoic
 During
the Eocene there was an intense and widespread, albeit geologically short-lived,
period of magmatism in parts of the western cordillera. This series of events
was composed of both plutonism and volcanism, with the extrusive deposits
in central Idaho being named the Challis volcanics. This event is similar
to the other eruptive episodes such as the one that created the Absaroka
volcanic pile of northwestern Wyoming. In fact the Challis volcanic field
is part of a belt of Eocene-aged intrusive and extrusive igneous rocks that
extend into southern British Columbia. The products of this period of magmatism
are indeed quite diverse in composition and variety of deposits, particularly
in Idaho. All of this happened from 55 Ma to 40 Ma, a fairly short period
of time.
What caused this Eocene magmatism is still open to debate. Some geologists believe it was the result of very shallow-angle subduction and imbrication of the Farallon oceanic plate beneath the North American continental plate. Others think that the magmatism was caused by a convergence and subsequent collision between the Pacific and North American plates, causing intracontinental rifting. What is known was that there was a gap of time, of about 20 million years, between the emplacement of the Idaho batholith and the Eocene magmatism. During this time there was little, if any, magmatic activity in central Idaho. Also known is that there was plate convergence and subduction going on during the late Cretaceous. The granodiorite of the Idaho batholith were created from partial melting of upper mantle rocks. The rocks of the Eocene were from the sources from the upper crust. Could a decompression melting have triggered the melting of upper crustal rocks? The temperature of melting of felsic rocks is lowered when the pressure is reduced.
 The
styles and products of the Challis eruptions changed over the 15 million
years of activity. At the beginning the eruptions were generally more effusive
or moderately explosive. This was due mainly to the mafic material that
made up the early eruptions. The later volcanic events were more explosive
as the material became more felsic. The Challis volcanics can be found to
the north and west of the town of Ketchum and can be subdivided into four
stratigraphic packages. The basal unit consists of a conglomerate that lies
unconformably on Paleozoic sediments. It’s relationship with the overlying
volcanic rocks can be quite complex. At places it has no volcanic clasts
and is unconformable with the Challis. This is called a fanglomerate and
is believed to have originated as an alluvial fan with its derived sediments
from nearby Paleozoic rocks. The deposition of the fanglomerate is interpreted
to be pre-Challis. At other localities the matrix of the conglomerate contains
volcanic clasts all through the deposit and appears to be conformable with
the Challis. The next highest stratigraphic package and the basal unit of
the volcanics in some areas are composed of andesite lava flows and tuff
breccia. The next unit consists of dacite and rhyodacite lavas and ash flow
tuffs. This volumous package represents multiple explosive eruptions. The
last stratigraphic package of the Challis is made up of dacite domes, pyroclastic
flows, and lava flows.
Associated with the Challis episode was the emplacement of a group of plutonic rocks, which include the granite of the Sawtooth batholith, the Pioneer stock granodiorite, and the quartz monzonite of the Pioneer Metamorphic Core Complex. Contemporaneous with this magmatism was the formation of shallow-dipping, northeast trending normal faults. One of these fault sets is the Wildhorse detachment, movement on which exposed the Pioneer Metamorphic Core Complex. This interesting geologic edifice is part of a long, sinous band of Cordilleran core complexes located west of the Sevier-aged overthrust belt.
 Core
complexes are characterized by domal uplifts of basement rock with penetrative
deformation, metamorphism, and low angle normal faulting (e.g. Wildhorse
detachment fault set). These core rocks make up a lower normal fault bounded
plate, with the overlying upper plate being composed of unmetamorphosed
sediments (mostly Proterozoic and Paleozoic marine sediments). There are
approximately twenty-five core complexes located in the Cordillera. Intrusion
of granitic rocks into the core complexes is fairly common. What the relationship
between the intrusive rocks and the core complex is not fully understood.
The question remains, did plutonism cause the uplift of the dome or did
the uplift make the conditions suitable for the creation of the granitic
rocks. Like the other complexes the Pioneer has a cover of unmetamorphosed
sedimentary rocks, which are unconformably overlain by Challis volcanics.
The upper plate consists of dark sandstone and conglomerate, which are members
of the Copper Basin formation, which formed as a result of erosion of the
Antler Highlands. The core complex itself contains medium to highly metamorphosed
rocks, which can be grouped into four units. The oldest dated at 2 Ga, are
sandstone, limestone which has gone through high-grade metamorphism to become
quartzite and marble.
A large stretch of the western cordillera, from northern Mexico to central Idaho was subjected to a second phase of Cenozoic extensional tectonics. This event started 17 Ma during the Miocene when the movements of multiple plates possibly paired with back-arc spreading combined to an extensive area of crustal thinning. This thinning resulted in extended northerly trending mountain ranges divided by broad, relatively flat valleys. The mechanism for this topography is the product of fault-blocking, uplifted mountains and down dropped valleys, all bordered by high-angle normal faults. These structures can be seen in south central Idaho, including the Wood River Valley.
Further Reading and Reference:
1. Roadside Geology of Idaho by Hyndman and Alt.
2. Northwest Exposures: A Geologic Story of the Northwest by Hyndman and Alt.
3. Geologic Map of Idaho (1:500,000).
4. Northern Rocky Mountain Geologic Highway Map.
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