The Great Unconformity is one of geology’s great mysteries. It is a gap of missing a 2 billion-year from the Geological record, where Tapeats Sandstone overlying gently dipping beds of Precambrian-age sedimentary rocks of the Grand Canyon Supergroup The contact between the Precambrian igneous crystalline rock (2.6 to 2.8 billion years old) and overlying Cambrian Flathead Sandstone (520 million years old) is Missing and exposed as a Angular unconformity. The place is a Very scenic walk or bike ride up Shoshone Canyon. The unconformity is known as Great Unconformity because it was thought to be a particularly large gap, maybe a global gap.
Scientists have developed several hypotheses to explain how, and when, this staggering amount of material may have been eroded. A Team of researchers studying the unconformity that occurs on the Ozark Plateau in the United States has found chemical evidence in rocks. It is suggested that the Great unconformity began forming toward the end of the Precambrian, between about 850 and 680 million years ago. This is caused due to the global tectonic uplift associated with the breakup of the ancient supercontinent Rodinia.
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Geology of The Great Unconformity
The Great Unconformity was first observed by John Wesley Powell during his 1869 exploration of the Grand Canyon. The Tapeats Sandstone of the Tonto group (525 million-year-old) lies on the metamorphic rock of the Vishnu basement (1.740 billion years), a time gap of over 1.2 billion years.
This “missing time” between the ancient earth and the Phanerozoic Eon ( 542 million years) is found worldwide. In Shoshone Canyon, it can best be observed just before Tunnel 1. Here the Cross-bedded Cambrian Flathead Sandstone (520 million years) rests on Precambrian igneous (2.65-2.6 billion years) and metamorphic crystalline rock (2.80-2.75 billion years). The time gap of from 2.08 to 2.28 billion years is about twice as long as that seen in the Grand Canyon and almost half the age of the earth.
The Origin of “The Great Unconformity” is one of the major debatable things in geology. Why is there such a long gap in the rock record that occurred worldwide during this specific time?
Rocks found immediately below “The Great Unconformity” are igneous and metamorphic and crystallized at great depth and pressure. Rocks above “The Great Unconformity” are sedimentary in origin, contain fossils, and were deposited on the surface of the earth. The Study of the exposed surface suggests a Significant uplift and erosion, perhaps 3 to 5km of the Precambrian igneous rocks eroded before the overlying sedimentary rocks were deposited. The Great Unconformity rocks –The formation exposed in The Great Unconformity is Coarsely crystalline pink granite and black gneiss below the contact and buff to tan-colored coarse-grained sandstone above the contact.
Currently, there is no widely accepted explanation for the Great Unconformity among geoscientists. It is widely accepted that there was a combination of more than one event which may have caused such an extensive phenomenon.
There are two competing theories about the origin of Great Unconformity-
1. Uplift and erosion occurred due to episodic plate tectonic events where continents collided, merged, and separated.
Wyoming represents the oldest rocks of continental crust in the Rocky Mountain Region and some of the first to crystallize as the molten earth cooled. The crystalline rock in Shoshone Canyon is part of the Archean Wyoming craton, an ancient land mass that underlies most of Wyoming and parts of the adjacent states. The Wyoming cratonic core segments assembled through collision driven by plate tectonics in the middle Archean (3.2-2.8 billion years). They joined with the other subprovince to create a microcontinent 2.65 billion years ago. It remained an independent “micro-continent” for at least 750 million years. An Archean-Phanerozoic unconformity, a time gap when amazing things occurred in earth’s crustal evolution. Beginning about 1.9 billion years ago, Wyoming amalgamated with other Archean crustal blocks to establish the core of the North American continent (Laurentia).
The Grenville Orogeny occurred 1.3 billion years ago when the Eurasian and African continents collided with North America to form the Appalachian Mountains and the first “supercontinent” Rodinia (Motherland). Wyoming was located at the center of the resulting land mass that stretched pole to pole. Several significant events in earth’s evolution occurred during the existence of Rodinia. The breakup of Rodinia, beginning about 750 million years ago, left the Wyoming craton on the southwestern edge of North America at a tectonically passive margin. A shallow sea encroached across this margin onto the Wyoming craton 230 million years later depositing the cross-bedded Cambrian Flathead Sandstone (520 million years) that rests on the Precambrian rocks in the Shoshone Canyon.
2. The massive worldwide glaciation event took place during the Neoproterozoic, starting around 720 million years ago. This is also when a significant glaciation event known as ‘Snowball Earth’ occurred. Snowball Earth covered almost the entire planet with ice. The areas that underwent glaciation where the Great Unconformity is located today. When glaciers move, they weathered and erode sediment away from the underlying rock. This would explain how a large section of rock was eroded away from widespread areas around the same time.
Things to do in Great Unconformity
The most scenic walk or bike ride found close to Cody on the Bureau of Reclamation road up the Shoshone Canyon toward the dam. visitors can Start at the Hayden Bridge parking area, it is an out-and-back 6.4-kilometer total walk. Elevation gain is only about 60 meters. Hikers can follow the Shoshone River on pavement the whole way. For rock climbers, there are a number of bolted climbing routes once you reach the granite.
Photo-geowyo.com/
Directions to The Unconformity
Visitors can Start at the intersection of US 14-16-20 (Sheridan Ave) and US 14A (16th St) in downtown Cody and then head west toward Yellowstone on US 14-16-20. Travel through town, 0.8 miles Buffalo Bill Center of the West, 2.8 miles Old Trail Town, 3.2 miles Rodeo Grounds-Stampede Park, 5.1 miles Hayden Bridge-Bureau of Reclamation Road, 5.7 miles bridge over Shoshone, 6.4 miles pull-off and park on right side of the highway just before the first short tunnel, cross highway to the south side and touch The Great Unconformity.
