2021 Revised NPS Geologic Stratigraphic Column of the Grand Canyon

The term Great Unconformity is frequently applied to the

unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

observed by John Wesley Powell in the

Grand Canyon The Grand Canyon is a steep-sided canyon carved by the Colorado River in Arizona, United States. The Grand Canyon is long, up to wide and attains a depth of over a mile (). The canyon and adjacent rim are contained within Grand Canyon Nati ...

in 1869. It is an exceptional example of relatively young sedimentary rock

strata In geology and related fields, a stratum (: strata) is a layer of Rock (geology), rock or sediment characterized by certain Lithology, lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by v ...

overlying much older sedimentary or crystalline strata. The intervening period of

geologic time The geologic time scale or geological time scale (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochronolo ...

is sufficiently long to raise the earlier rock into mountains which are then eroded away.

Powell's Unconformity, Grand Canyon

The Great Unconformity of Powell in the

is a regional unconformity that separates the

Tonto Group The Tonto Group is a name for an assemblage of related sedimentary strata, collectively known by geologists as a '' Group'', that comprises the basal sequence Paleozoic strata exposed in the sides of the Grand Canyon. As currently defined, the T ...

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 Vishnu Basement Rocks is the name recommended for all Early Proterozoic crystalline rocks (metamorphic and igneous) exposed in the Grand Canyon region. They form the crystalline basement rocks that underlie the Bass Limestone of the Unkar Gr ...

. 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 Laurentia or the North American craton is a large continental craton that forms the Geology of North America, ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of ...

, the ancient core of North America. It was first recognized twelve years before Powell's expedition by John Newberry in

New Mexico New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...

, during the Ives expedition of 1857–1858. However, the disruption of the

American Civil War The American Civil War (April 12, 1861May 26, 1865; also known by Names of the American Civil War, other names) was a civil war in the United States between the Union (American Civil War), Union ("the North") and the Confederate States of A ...

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 The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

–Early-

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

Sauk sequence. The submergence of Laurentia ended a lengthy period of widespread continental denudation that exhumed and deeply eroded

Precambrian The Precambrian ( ; or pre-Cambrian, sometimes abbreviated pC, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of t ...

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. (See also

geological timescale The geologic time scale or geological time scale (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochronolo ...

Frenchman Mountain, Nevada

A prominent exposure of Powell's Great Unconformity occurs in Frenchman Mountain in

Nevada Nevada ( ; ) is a landlocked state in the Western United States. It borders Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. Nevada is the seventh-most extensive, th ...

. Frenchman Mountain exposes a sequence of

Phanerozoic The Phanerozoic is the current and the latest of the four eon (geology), geologic eons in the Earth's geologic time scale, covering the time period from 538.8 million years ago to the present. It is the eon during which abundant animal and ...

strata equivalent to those found in the Grand Canyon. At the base of this sequence, the Great Unconformity, with the Tapeats 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-old

of the Tapeats Sandstone rests on 1.7-billion-year-old Vishnu Basement Rocks.

Possible causes of the Great Unconformity

There is currently no widely accepted explanation for the Great

Unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

among

geoscientists A geologist is a scientist who studies the structure, composition, and History of Earth, history of Earth. Geologists incorporate techniques from physics, chemistry, biology, mathematics, and geography to perform research in the Field research, ...

. There are hypotheses that have been proposed; it is widely accepted that there was a combination of more than one event which may have caused such an extensive phenomenon. One example is a large glaciation event which took place during the

Neoproterozoic The Neoproterozoic Era is the last of the three geologic eras of the Proterozoic geologic eon, eon, spanning from 1 billion to 538.8 million years ago, and is the last era of the Precambrian "supereon". It is preceded by the Mesoproterozoic era an ...

, starting around 720 million years ago. This is also when a significant glaciation event known as

Snowball Earth The Snowball Earth is a historical geology, geohistorical hypothesis that proposes that during one or more of Earth's greenhouse and icehouse Earth, icehouse climates, the planet's planetary surface, surface became nearly entirely freezing, fr ...

occurred. 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. A potential link has been proposed between such sub-Cambrian unconformities and

glacial erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

during the

Snowball Earth

glaciation A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate be ...

s. Alternatively, it has been proposed that multiple smaller events, such as the formation and breakup of

Rodinia Rodinia (from the Russian родина, ''rodina'', meaning "motherland, birthplace") was a Mesoproterozoic and Neoproterozoic supercontinent that assembled 1.26–0.90 billion years ago (Ga) and broke up 750–633 million years ago (Ma). wer ...

, created many unconformities worldwide.

References

External links

; 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 * {{cite web , url=https://www.bbc.com/future/article/20210901-the-strange-race-to-track-down-a-missing-billion-years , title=The strange race to track down a missing billion years , work=BBC , first=Zaria , last=Gorvett , date=1 September 2021 * 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

last accessed September 22, 2013. * Share, J. (2012a) [http://written-in-stone-seen-through-my-lens.blogspot.com/2012/02/great-unconformity-of-grand-canyon-part.html ''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.

The Unconformity in Wyoming
Rock formations of Arizona Rock formations of Nevada Rock formations of Scotland Geology of Arizona Geology of Nevada Geology of Scotland Unconformities