thumb|Geologic stratigraphic column of strata exposed in and near the Grand Canyon

The Great Unconformity is the unconformity observed by John Wesley Powell in the Grand Canyon in 1869. It is an exceptional example of relatively young sedimentary rock strata overlying much older sedimentary or crystalline strata. The intervening period of geologic time is sufficiently long to raise the earlier rock into mountains which are then eroded away.

Powell's Unconformity, Grand Canyon

Powell's Unconformity viewed from [[Lipan Point on the South Rim. Rocks of the Unkar Group of the Grand Canyon Supergroup are truncated at the base of the Tonto Group.|thumb|250px]]

thumb|Powell's Unconformity seen from Hopi Point on the South Rim. Steeply [[foliation (geology)|foliated and veined schists of the Vishnu Basement Rocks truncated at the base of the Tonto Group.]]

The Great Unconformity of Powell in the Grand Canyon is a regional unconformity that separates the Tonto Group from the underlying faulted-and-tilted sedimentary rocks of the Grand Canyon Supergroup and vertically foliated metamorphic and igneous rocks of the Vishnu Basement Rocks. The unconformity between the Tonto Group and the Vishnu Basement Rocks is a nonconformity. The break between the Tonto Group and the Grand Canyon Supergroup is an angular unconformity.

Powell's Great Unconformity is part of a continent-wide unconformity that extends across Laurentia, the ancient core of North America. It was first recognized twelve years before Powell's expedition by John Newberry in New Mexico, during the Ives expedition of 1857–1858. However, the disruption of the American Civil War kept Newberry's work from becoming widely known. This Great Unconformity marks the progressive submergence of this landmass by a shallow cratonic sea and its burial by shallow marine sediments of the Cambrian–Early-Ordovician Sauk sequence. The submergence of Laurentia ended a lengthy period of widespread continental denudation that exhumed and deeply eroded Precambrian 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.

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, the Sixtymile Formation, of the Grand Canyon Supergroup. At the base of the Grand Canyon Supergroup, where it truncates the Bass 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.

Possible causes of the Great Unconformity

There is currently no widely accepted explanation for the Great Unconformity among geoscientists. 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 the Neoproterozoic, starting around 720 million years ago. This is also when a significant glaciation event known as Snowball Earth occurred.

See also

  • Geology of the Grand Canyon area (with time scale)
  • List of orogenies
  • Orogeny (mountain building)

References

; Hutton's Unconformity

  • Anonymous (2003) Siccar Point Field Excursion Preview. School of GeoSciences, University of Edinburgh, Edinburgh, Scotland. last accessed September 22, 2013.
  • Moore, R (2009) Siccar Point. Reports of the National Center for Science Education. 29(1):26. last accessed September 22, 2013.
  • Rowan, C (2011) The making of an angular unconformity: Hutton's unconformity at Siccar Point. Highly Allochthonous. last accessed September 22, 2013.

; Powell's Unconformity

  • Abbott, W (2001) Revisiting the Grand Canyon – Through the Eyes of Seismic Sequence Stratigraphy. AAPG Datapages / Search and Discovery, American Association of Petroleum Geologist, Tulsa, Oklahoma. last accessed September 22, 2013.
  • Brandriss, M. (2004) Angular unconformity between Proterozoic and Cambrian rocks, Grand Canyon, Arizona. GeoDIL, A Geoscience Digital Image Library, University of North Dakota, Grand Forks, North Dakota. last accessed September 22, 2013.
  • Share, J. (2012a) The Great Unconformity of the Grand Canyon and the Late Proterozoic-Cambrian Time Interval: Part I – Defining It. last accessed September 22, 2013.
  • Share, J. (2012a) The Great Unconformity and the Late Proterozoic-Cambrian Time Interval: Part II – The Rifting of Rodinia and the "Snowball Earth" Glaciations That Followed. last accessed September 22, 2013.

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  • The Unconformity in Wyoming