Geology of the Grand Teton area

From Wikipedia, the free encyclopedia

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 2500 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.

Contents 1 Precambrian deposition, metamorphosis, and intrusion 2 Paleozoic and Mesozoic deposition 3 Tertiary uplift and deposition 4 Quaternary volcanic deposits and ice ages 5 References

[edit] Precambrian deposition, metamorphosis, and intrusion

Perhaps 3000 million years ago in Precambrian time, sand, limey ooze, silt and clay were deposited in a marine trough (accurate dating is not 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 2800 to 2700 million years ago intensely folded and metamorphosed them, creating alternating light and dark banded gneiss and schist.^[1][2] 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 and stream-polished serpentine pebbles are locally called 'Teton jade.'

Sometime around 2500 million years ago, blobs of magma intruded into the older rock, forming plutons of granitic rock.^[3] Extensive exposures of this rock are found in the central part of the range. About 1300 to 1400 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).^[4][5] Some of the large dikes can be seen from the Jenny Lake and String Lake areas. A long period of erosion then began, creating a major gap in the geolgic record called an unconformity. The landscape was eventually reduced to a flat plain or expanse of rolling hills.^[6]

[edit] Paleozoic and Mesozoic deposition

Deposition resumed in the Cambrian period and continued through the Paleozoic era, creating nine major formations which together are 4000 feet (1200 m) thick (the only geologic period in the Paleozoic not represented is the Silurian). This unit was laid down in a shallow sea and later became a discontinuous mix of dolomite, limestone, sandstones, and shales. The layers of this unit are relatively undeformed for their age even though periodic upwarp exposed them to erosion, creating uncomformities . Fossilized brachiopods, bryozoans, corals, and trilobites are found in the carbonate rock layers with the best examples found outside the park in the Alaska Basin. The most complete examples of this unit are found to the west, north, and south of park borders.^[7]

Mesozoic deposition changed from primarily marine to a mix of marine, transitional, and continental that varied over time as crustal conditions altered the region. By the close of this era, 10,000 to 15,000 feet (3000 to 4500 m) of sediment accumulated in 15 recognized formations. The most extensive non-marine formations were deposited in the Cretaceous period when the eastern part of the Cretaceous Seaway (a warm shallow sea that periodically divided North America in that period) covered the region. Their sediment came from rock eroded from a mountain chain east of the seaway interbeded with ash from volcanos west of the seaway in the Sierran Arc (a long volcanic island chain like the modern Andes Mountains but in island form). This ash eventually became bentonite, a clay which expands in water and thus causes landslides in the park.^[8]

Regional uplift in latest Cretaceous time (see below) 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.

[edit] Tertiary uplift and deposition

The uplift responsible for erasing the Cretaceous Seaway and fusing the Sierran Arc to the rest of North America was caused by the Laramide orogeny. Starting 70 million years ago and lasting well into the first half of the Cenozoic era, the Laramide was the main mountain-building episode responsible for creating the Rocky Mountains. Compressive forces from this orogeny created north-south-trending thrust faults along with general regional uplift. Erosion of the Targhee uplift north of park borders was driven by steepened stream gradients. Gravel, quartzite cobbles, and sand from this erosion eventually became the 5000 foot (1500 m) thick Harebell Formation seen today as various conglomerates and sandstones in the northern and northeastern parts of the park.^[9]

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.^[10]

Uplift intensified and climaxed early in the Eocene epoch when large thrust and reverse faults created small mountain ranges separated by subsiding sedimentary basins. One of the reverse faults, the north-south trending 10 mile (16 km) long Buck Mountain Fault, elevated what is today the central part of the Teton Range and one of the basins created was Jackson Hole. Subsidence of Jackson Hole and other basins in the area provided a resting place for more and more sediment (subsidence kept up with deposition at Jackson Hole). Erosion of the up-faulted ranges provided much of that sediment.

Wanning of the Laramide orogeny in the Eocene coincided with volcanic eruptions from two parallel volcanic chains separated by a long valley in the Yellowstone-Absaroka area to the north. Huge volumes of volcanic material such as tuff and ash accumulated to great depth in the Grand Teton area, forming the Absaroka Volcanic Supergroup. Additional eruptions east of Jackson Hole deposited their own debris in the Oligocene and Miocene epochs.

In Miocene time about 9 million years ago, a 40 mile (64 km) long steeply east dipping normal fault system deepened the Jackson Hole basin and started to uplift the westward-tilting eastern part of the Teton Range (the youngest mountain range in the Rocky Mountains).^[11] 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 (1830 m) above the valley floor on the summit of Mount Moran. Drilling in Jackson Hole found the same formation 24,000 feet (7300 m) below the valley's surface, indicating that the two blocks have been displaced 30,000 feet (9100 m) from each other. Thus an average of one foot of movement occurred every 300 years (1 cm per year on average).^[12]

In Pliocene time, Jackson Hole's first large freshwater lake was impounded by east-west fault movement in what is today the southern end of the park. Geologists call this fault-scarp dammed body of shallow water Lake Teewinot and it persisted for around 5 million years.^[13] Fossilized clams and snails are found in the lakebed sediments. 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.^[14] Most of these units within the park are, however, buried under younger deposits.

[edit] 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 2000 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.^[15] 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.^[15] This continental-sized glacial system stripped all the soil and vegetation from countless valleys and many basins, leaving them a wasteland of bedrock strewn with boulders after the glaciers finally retreated. Parts of Jackson Hole that were not touched by the following milder glaciations still cannot support anything but the hardiest plants (smaller glaciers deposit glacial till and small rocks relatively near their source, while continental glaciers transport all but the largest fragments far away).

A less severe glaciation, known as Bull Lake, started sometime between 160 to 130 thousand years. Bull Lake helped repair some of the damage of the Buffalo event by forming smaller glaciers which deposited loose material over the bedrock. In that event, the large glacier which ran down Jackson Hole only extended just south of where Jackson, Wyoming now sits and melted about 100,000 years ago.

Then from 25,000 to 10,000 years ago the lower volume Wisconsin glaciation carved many of the glacial features seen today. Burned Ridge is made of the terminal moraine (rubble dump) of the largest of these glaciers to affect the area. Today this hummocky feature is covered with trees and other vegetation. Smaller moraines from a less severe part of the Pinedale were formed just below the base of each large valley in the Teton Range by alpine glaciers. Many of these piles of glacial rubble created depressions that in modern times are filled with a series of small lakes (Leigh, String, Jenny, Bradley, Taggart, and Phelps). Jackson Lake is the largest of these and was impounded by a recessional moraine left by the last major glacier in Jackson Hole. A collection of kettles (depressions left by melted stagnant ice blocks from a retreating glacier) south of the lake is called the Potholes. The basins that hold Two Ocean Lake and Emma Matilda Lake were created during the Bull Lake glaciation.^[16] 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.^[17] 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.

Mass wasting events such as the June 1925 Gros Ventre landslide continue to change the area. Three miles (4.8 km) outside of the current park's southeastern border, this particular slide in one minute transported several cubic miles (around 10 cubic km) of rock down Sheep Mountain and into the Gros Ventre River valley, 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 in 1927 and flooded the town of Kelly, Wyoming, killing six. Geologists think that the initial slide, which broke off a chunk of Pennsylvanian-aged Tensleep Sandstone, was caused by water-saturated rock, river undercutting, and a small earthquake.^[18]

[edit] References