Geology of the Grand Teton area Totally Explained

Everything about Geology Of The Grand Teton Area totally explained

The geology of the Grand Teton area consists of some of oldest rocks and one of the youngest mountain ranges in North America. The Teton Range, mostly located in Grand Teton National Park, started to grow some 9 million years ago. An older feature, Jackson Hole, is a basin that sits aside the range. The 2,500 million year old metamorphic rocks that make up the east face of the Tetons are marine in origin and include some volcanic deposits. These same rocks are today buried deep inside Jackson Hole. Paleozoic rocks were deposited in warm shallow seas while Mesozoic deposition transitioned back and forth from marine to non-marine sediments with the Cretaceous Seaway periodically covering the area late in that era.
70 million years ago, the Laramide orogeny started to uplift western North America, erasing the seaway and creating highlands. The first part of the Teton Range was thus formed in the Eocene epoch. Large volcanic eruptions from in the Yellowstone-Absaroka area to the north left thick volcanic deposits. A series of glaciations in the Pleistocene epoch saw the introduction of large glaciers in the Teton and surrounding ranges, which at times formed part of the Canadian Ice Sheet. Moraines left by less severe ice ages impounded several lakes, including Jackson Lake.

Precambrian deposition, metamorphism, and intrusion

Perhaps 3,000 million years ago in Precambrian time, sand, limey ooze, silt and clay were deposited in a marine trough (accurate dating isn't possible, due to subsequent partial recrystaliztion of the resulting rock). Interbeded between these layers were volcanic deposits, probably from an island arc. These sediments were later lithified into sandstones, limestones, and various shales. These rocks were 5 to 10 miles (8 to 16 km) below the surface when orogenies (mountain-building episodes) around 2,800 to 2,700 million years ago intensely folded and metamorphosed them, creating alternating light and dark banded gneiss and schist. Today these rocks dominate the Teton Range with good examples easily viewable in Death Canyon and other canyons in the Teton Range. The green to black serpentine created was used by Native Americans to make bowls. Sometime around 2,500 million years ago, blobs of magma intruded into the older rock, forming plutons of granitic rock. Extensive exposures of this rock are found in the central part of the range. About 1,300 to 1,400 million years ago in Late Precambrian, 5 to 200 foot (1.5 to 60 m) thick black diabase dikes intruded as well, forming the prominent vertical dikes seen today on the faces of Mount Moran and Middle Teton (the dike on Mount Moran is 150 feet, 46 m wide). Some of the large dikes can be seen from the Jenny Lake and String Lake areas.
More than 700 million years elapsed between intrusion of the black dikes and deposition of the first Paleozic sedimentary rocks. The Precambrian rocks were uplifted during this gap in the geologic record known as an unconformity; exposed to erosion they were gradually worn to a nearly featureless plain, perhaps somewhat resembling the vast flat areas in which similar Precambrian rocks are now exposed in central and eastern Canada. At the close of Precambrian time, about 600 million years ago, the plain slowly subsided and the site of the future Teton Range disappeared beneath shallow seas that were to wash across it intermittently for the next 500 million years.

On the edge of a shallow seaway

Early in Cambrian time a shallow seaway, called the Cordilleran trough, extended from southern California northeastward across Nevada into Utah and Idaho. The vast gently rolling plain on Precambrian rocks to the east was drained by sluggish westward-flowing rivers that carried sand and mud into the sea. The site of the Teton Range was part of this plain. Slow subsidence of the land caused the sea to spread gradually eastward during Middle Cambrian time flooding the Precambrian plain. Sand accumulated along the beaches just as it does today. As the sea moved still farther east, mud was deposited on the now-submerged beach sand. In the Teton area, the oldest sand deposit is the 175 to 200 feet (53 to 60 m) thick Flathead Sandstone.
Regional uplift in latest Cretaceous time caused the seaway to retreat and transformed the Grand Teton area into a low-lying coastal plain that was frequented by dinosaurs (a fossilized Triceratops was found east of the park near Togwotee Pass). Coalbeds were eventually created from the swamps and bogs left behind after the last stand of the seaway retreated. Coal outcrops can be found near abandoned mines in and outside of the eastern margin of the park. Outcrops of older Mesozoic-aged formations can be found north, east, and south of the park.

Sundance Sea covers older deposits

Most of the basal part of the Mesozoic sequence consists of the more than 1,000 feet thick, soft, bright-red, and Triassic-aged rocks known as the Chugwater Formation. The distribution of Mud cracks, fossilized reptiles and amphibians suggest deposition in a tidal flat environment with a sea several kilometers southwest of Jackson Hole. Evaporite deposits of a few beds of white gypsum (calcium sulfate) were likely formed after shallow bodies of salt water were cut off from the sea. A small amount of iron oxide creates the red color and the formation erodes into colorful hills east and south of the park.
As the Triassic gave way to the Jurassic, wind spread salmon-red colored sand across the red beds of the Chugwater Formation to form the Nugget Sandstone. The Nugget in turn was buried by the deposits of thin red shale and thick gypsum of the Gypsum Spring Formation. Later, a warm, muddy, shallow sea with abundant marine mollusks called the Sundance Sea started to spread from Alaska south to Wyoming. More than 500 feet of soft gray fossil-rich shale and thin beds of limestone and sandstones were deposited. After the sea withdrew, the Jurassic and Lower Cretaceous-aged Morrison and Cloverly Formations were laid down on low-lying tropicaly humid flood plains. These formations erode into colorful badlands of red, pink, purple, and green claystones and mudstones, and yellow to buff sandstones. Large and small dinosaurs roamed the abundant vegetation and swamps.

Western Interior Seaway expands and retracts

Brightly colored rocks continued to be deposited as the final period of the Mesozoic, the Cretaceous dawned. Another warm, shallow sea, the Western Interior Seaway, then partly and sometimes completely covered the Teton region along with most of Wyoming, About 10,000 feet of drab-colored sand, silt, and clay with some coal beds, volcanic ash layers, and minor amounts of gravel were deposited.
   The Western Interior Seaway retreated eastward from the Teton region around 85 million years ago, marked by deposition of the Bacon Ridge Sandstone. Extensive coal swamps formed along and followed the retreating seashore, leaving coal beds 5 to 10 feet thick in the Upper Cretaceous strata. Examples of these coal beds are visible in abandoned mines found in the eastern margin of the park. A modern analog of this depositional environment is the hot and humid climate of the Florida Everglades. About 5 feet of compacted plant material is needed to form 1 inch of coal.
   Fine-grained volcanic ash from volcanoes west and northwest of the Teton area was periodically deposited in in the quiet shallow water of the Western Interior Seaway throughout Cretaceous time. Ash deposited in this manor was later altered to bentonite; a type of clay used in the foundry industry and as a component of oil well drilling mud. Elk and deer in Jackson Hole use exposures of bentonite as a (bitter) salt lick. Bentonite swells when wet, which causes landslides that sometimes block access roads into Jackson Hole.
   Cretaceous-aged rocks in the Teton region form part of a huge east-thinning wedge of crust that's locally almost 2 miles thick. Most of these rocks are from debris eroded from slowly rising mountains in the west. Bentonite, crude oil and natural gas are commonly produced from the various Cretaceous formations. Enormous coal reserves, with some beds reaching 50 to 100 feet thick, are a potential vast resrouce.
   By the end of the Cretaceous, slightly more than 80 million years ago, the region's landscape was flat and monotonous; a condition that persisted during most of the Late Cretaceous.

Rocky Mountains rise

The period of uplift that resulted in the formation of the ancestral Rocky Mountains is called the Laramide orogeny. Mountains already existed west and southwest of Wyoming, with progessively older mountains (up to Jurassic age) trending west into Nevada. Latest Cretaceous time saw the formation of a low broad northwest-trending arch along the approximate area of the present Teton Range and Gros Ventre Mountains.
Part of the evidence for the first Laramide mountain building west of the Teton region is the several hundred cubic miles of quartzite boulders derived from the Targhee uplift, which was located north and west of the northern end of the present-day Teton Range. Streams carried boulders, sand, and clay from the uplift eastward and southeastward across what would become Jackson Hole. Flakes of gold and some mercury are in the resulting Harebell Formation. Two huge depositional troughs were formed in central and southern Wyoming from fine-grained debris carried farther east and southeast. Many of the larger boulders were derived from Precambrian and possibly lower Paleozoic quartzites, meaning that at least 15,000 feet of Paleozoic and Mesozoic rock must have been stripped from the Targhee uplift before the quartzites were exposed to erosion.

Tertiary uplift and deposition

The tectonic setting of western North America changed drastically as the Farallon Plate under the Pacific Ocean to the west was shallowly subducted below North American Plate. Called the Laramide orogeny, the compressive forces generated from this collision erased the Cretaceous Seaway, fused the Sierran Arc to the rest of North America and created the Rocky Mountains. This mountain-building event started in the Mesozoic 80 million years ago and lasted well into the first half of the Cenozoic era 30 million years ago.. Some 60 million years ago, these forces uplifted the low-lying coastal plain in the Teton region and created the north-south-trending thrust faults of the nearby Wyoming Overthrust Belt.. This region includes areas now occupied by the Teton Range, Gros Venture Range, Wind River Mountains and other mountain ranges to the south and east of the Tetons. A separate area of uplift called the Targhee Uplift formed north of park borders around this time.
   Subsequent erosion of the Targhee Uplift was driven by steepened stream gradients. Gravel, quartzite cobbles, and sand from this erosion eventually became the 5,000 foot (1,500 m) thick Harebell Formation seen today as various conglomerates and sandstones in the northern and northeastern parts of the park. In the Paleocene epoch large amounts of clastic sediment derived from uplifted areas covered the Harebell Formation to become the Pinyon Conglomerate. The lower members of this formation consist of coal beds and claystone with conglomerate made of quarzite from the Targhee uplift above.
   The subducting Farallon Plate was eventually completely consumed below the North American Plate, bringing an end to the Laramide orogeny. Hot and semi-plastic rock deep below western North America responded to the lack of compression beginning 30 million years ago by slowly rising; gradually pushing the overlying rock sideways both east and west.. Blocks of the brittle upper crust responded by braking along roughly parallel north-to-south trending normal faults that each have a subsiding basin on one side and a mountain range on the other. This strechting may have began to tear apart the previously-mentioned high plateau in western Wyoming around this time, but evidence from ancient sediments indicates that the Teton Fault system developed much later (see below). One block, the Jackson Hole basin, moved down while the other block, containing the westward-tilting eastern part of the Teton Range, moved up; thus creating the youngest mountain range in the Rocky Mountains. Most of the downward movement occurred right next to the fault, resuling in a 15° tilt of the Colter Formation.
   Around 10 million years ago, The resulting Teewinot Formation of lakebed sediments sits directly on the Colter and consists of limestones and claystones mixed with volcanic material and fossilized clams and snails. All told, sediments in the Tertiary period attained an aggregate thickness of around 6 miles (10 km), forming the most complete non-marine Tertiary geologic column in the United States. Most of these units within the park are, however, buried under younger deposits.
   Eventually all the Mesozoic rock from the Teton Range was stripped away and the same formations in Jackson Hole were deeply buried. A prominent outcrop of the pink-colored Flathead Sandstone exits 6,000 feet (1,830 m) above the valley floor on the summit of Mount Moran. Drilling in Jackson Hole found the same formation 24,000 feet (7,300 m) below the valley's surface, indicating that the two blocks have been displaced 30,000 feet (9,100 m) from each other. Thus an average of one foot of movement occurred every 300 years (1 cm per year on average).

Quaternary volcanic deposits and ice ages

Massive volcanic eruptions from the Yellowstone Volcano northwest of the area occurred 2.2 million, 1.3 million, and 630,000 years ago. Each catastrophic caldera-forming eruption was preceded by a long period of more conventional eruptions along even earlier volcanic episodes. One such event sent large amounts of ryrolitic lava into the northern extent of Teewinot Lake. The resulting obsidian (volcanic glass) has been potassium-argon dated to 9 million years and was used by Native Americans starting thousands of years ago to make arrowheads, knives, and spear points. The lake was dry by the time a series of enormous pyroclastic flows from the Yellowstone area buried Jackson Hole under welded tuff. Older exposures of this tuff are exposed in the Bivouac Formation at Signal Mountain and Pleistocene-aged tuffs are found capping East and West Gros Venture Buttes (both the mountain and buttes are small fault blocks). Climatic conditions in the area gradually changed through the Cenozoic as continental drift moved North America northwest from a sub-tropical to a temperate zone by the Pliocene epoch. The onset of a series of glaciations in the Pleistocene epoch saw the introduction of large glaciers in the Teton and surrounding ranges, which flowed all the way to Jackson Hole during at least three ice ages. Cascade, Garnet, Death and Granite Canyons were all carved by successive periods of glaciation.
The first and most severe of the known glacial advances in the area was caused by the Buffalo glaciation. In that event the individual alpine (mountain valley) glaciers from the Tetons' east side coalesced to form a 2,000 foot (610 m) thick apron of ice that overrode and abraded Signal Mountain and the other three buttes at the south end of Jackson Hole. Similar dramas were repeated on other ranges in the region, eventually forming part of the Canadian Ice Sheet, which at its maximum, extended into eastern Idaho. Since then humans have built a dam over Jackson Lake's outlet to increase its size for recreational purposes.
All Pinedale glaciers probably melted away soon after the start of the Holocene epoch. The dozen small cirque glaciers seen today were formed during a subsequent neoglaciation 5000 years ago. Mount Moran has five such glaciers with Triple Glaciers on the north face, Skillet Glacier on the east face, and Falling Ice Glacier on the southeast face. All the glacial action has made the peaks of the Teton Range jagged from frost-wedging. Other glaciers include Teton Glacier, below the east face of Grand Teton, Middle Teton Glacier, situated on the northeast slopes of Middle Teton, and the fast retreating Schoolroom Glacier, west of Grand Teton at Hurricane Pass. Mass wasting events such as the 1925 Gros Ventre landslide continue to change the area. On June 22 1925 an earthquake with an estimated magnitude of 4 weakened the side of a mountain located three miles (4.8 km) outside of the current park's southeastern border. The next day, 50 million cubic yards (38 million cubic meters) of water-saturated Pennsylvanian-aged Tensleep Sandstone slid 1.5 miles (2.4 km) from its source on Sheep Mountain and into the Gros Ventre River valley 2,100 feet (640 m) below, damming the river. Stressed by snow melt, the resulting 5 mile (8 km) long and 200 feet (60 m) deep lake breached the debris dam on May 18 1927 and flooded the town of Kelly, Wyoming, killing six.

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