For the unconformity in Scotland sometimes known as the 'Great Unconformity', seeHutton's Unconformity.
Geologic stratigraphic column of strata exposed in and near the Grand Canyon
TheGreat Unconformity is theunconformity observed byJohn Wesley Powell in theGrand Canyon in 1869.[1] It is an exceptional example of relatively young sedimentary rockstrata overlying much older sedimentary or crystalline strata. The intervening period ofgeologic time is sufficiently long to raise the earlier rock into mountains which are then eroded away.
The Great Unconformity of Powell in theGrand Canyon is a regional unconformity that separates theTonto Group from the underlying faulted-and-tilted sedimentary rocks of theGrand Canyon Supergroup and vertically foliated metamorphic and igneous rocks of theVishnu Basement Rocks. The unconformity between the Tonto Group and the Vishnu Basement Rocks is anonconformity. The break between the Tonto Group and the Grand Canyon Supergroup is an angular unconformity.[2][3][4]
Powell's Great Unconformity is part of a continent-wide unconformity that extends acrossLaurentia, the ancient core of North America. It was first recognized twelve years before Powell's expedition byJohn Newberry inNew Mexico, during the Ives expedition of 1857–1858. However, the disruption of theAmerican Civil War kept Newberry's work from becoming widely known.[5] This Great Unconformity marks the progressive submergence of this landmass by a shallow cratonic sea and its burial by shallow marine sediments of theCambrian–Early-OrdovicianSauk sequence. The submergence of Laurentia ended a lengthy period of widespread continental denudation that exhumed and deeply erodedPrecambrian rocks and exposed them to extensive physical and chemical weathering at the Earth's surface. As a result, Powell's Great Unconformity is unusual in its geographic extent and its stratigraphic significance.[6][7]
The length of time represented by Powell's Great Unconformity varies along its length. Within the Grand Canyon, the Great Unconformity represents a period of about 175 million years between the Tonto Group and the youngest subdivision, theSixtymile Formation, of the Grand Canyon Supergroup. At the base of the Grand Canyon Supergroup, where it truncates theBass Formation, the period of time represented by this angular unconformity increases to about 725 million years. Where the Tonto Group overlies the Vishnu Basement Rocks, the Great Unconformity represents a period as much as 1.2–1.6 billion years.[3][7] (See alsogeological timescale.)
An exposure of Powell's Great Unconformity, west of Montezuma, New Mexico
A prominent exposure of Powell's Great Unconformity occurs inFrenchman Mountain inNevada. Frenchman Mountain exposes a sequence ofPhanerozoic strata equivalent to those found in the Grand Canyon. At the base of this sequence, the Great Unconformity, with theTapeats Sandstone of the Tonto Group overlying the Vishnu Basement Rocks, is well-exposed in a manner that is atypical and scientifically significant in its combination of extent and accessibility. This exposure is frequently illustrated in popular and educational publications, and is often part of geological fieldtrips. There is a gap of about 1.2 billion years where 550-million-year-oldstrata of the Tapeats Sandstone rests on 1.7-billion-year-old Vishnu Basement Rocks.[8][9][10]
There is currently no widely accepted explanation for the GreatUnconformity amonggeoscientists. There are hypotheses that have been proposed; it is widely accepted that there was a combination of events which may have caused such an extensive phenomenon. One example is a large glaciation event which took place during theNeoproterozoic, starting around 720 million years ago.[11][12][13] This is also when a significant glaciation event known asSnowball Earth occurred.[11] Snowball Earth periods covered almost the entire planet with ice. The areas that underwent glaciation were approximately those where the Great Unconformity is located today. When glaciers move, they drag and erode sediment away from the underlying rock. This would explain how a large section of rock was taken away from widespread areas around the same time.[citation needed]
A potential link has been proposed between such sub-Cambrian unconformities andglacial erosion during theNeoproterozoic Snowball Earthglaciations.[14][11] Alternatively, it has been proposed that multiple smaller events, such as the formation and breakup ofRodinia, created many unconformities worldwide.[15][16][17][18]
^Kues, Barry S.; Lewis, Claudia J.; Lueth, Virgil W. (2014).A brief history of geological studies in New Mexico: with biographical profiles of notable New Mexico geologists (First ed.). New Mexico Geological Society.ISBN978-1-58546-011-3.
^abKarlstrom, Karl E.; Timmons, J. Michael, eds. (2012). "Many unconformities make one 'Great Unconformity'".Grand Canyon Geology: Two Billion Years of Earth's History. Vol. 489. Boulder, Colorado: Geological Society of America. pp. 73–79.ISBN978-0-8137-2489-8.
^Palmer, A. R. (1989). "Day 0: Early and Middle Cambrian stratigraphy of Frenchman Mountain, Nevada".Cambrian and Early Ordovician Stratigraphy and Paleontology of the Basin and Range Province, Western United States: Las Vegas, Nevada to Salt Lake City, Utah, July 1–7, 1989. pp. 14–16.doi:10.1029/FT125p0014.ISBN0-87590-662-1.
