Of the many
unconformities (gaps) observed in
geological strata, the term Great Unconformity is frequently applied to either the unconformity observed by
James Hutton
James Hutton (; 3 June O.S.172614 June 1726 New Style. – 26 March 1797) was a Scottish geologist, agriculturalist, chemical manufacturer, naturalist and physician. Often referred to as the father of modern geology, he played a key role ...
in 1787 at
Siccar Point in
Scotland
Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to ...
,
[Rance, H (1999]
''Historical Geology: The Present is the Key to the Past.''
QCC Press, New York or that observed by
John Wesley Powell
John Wesley Powell (March 24, 1834 – September 23, 1902) was an American geologist, U.S. Army soldier, explorer of the American West, professor at Illinois Wesleyan University, and director of major scientific and cultural institutions. H ...
in the
Grand Canyon
The Grand Canyon (, yuf-x-yav, Wi:kaʼi:la, , Southern Paiute language: Paxa’uipi, ) 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 m ...
in 1869. Both instances are exceptional examples of where the contacts between sedimentary strata and either sedimentary or crystalline strata of greatly different ages, origins, and structure represent periods 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 geochro ...
sufficiently long to raise great mountains and then erode them away.
Background
Unconformities tend to reflect long-term changes in the pattern of the accumulation of sedimentary or
igneous
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
strata in low-lying areas (often ocean basins, such as the
Gulf of Mexico
The Gulf of Mexico ( es, Golfo de México) is an ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of the United ...
or the
North Sea
The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian ...
, but also
Bangladesh
Bangladesh (}, ), officially the People's Republic of Bangladesh, is a country in South Asia. It is the eighth-most populous country in the world, with a population exceeding 165 million people in an area of . Bangladesh is among the mo ...
and much of
Brazil
Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
), then being uplifted and eroded (such as the ongoing
Himalayan orogeny, the older
Laramide orogeny
The Laramide orogeny was a time period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the ...
of the
Rocky Mountains
The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in straight-line distance from the northernmost part of western Canada, to New Mexico ...
, or much older
Appalachian (Alleghanian) and
Ouachita orogenies), then subsequently subsiding, eventually to be buried under younger sediments. The intervening periods of
tectonic uplift
Tectonic uplift is the geologic uplift of Earth's surface that is attributed to plate tectonics. While isostatic response is important, an increase in the mean elevation of a region can only occur in response to tectonic processes of crustal th ...
are generally periods of mountain building, often due to the collision of tectonic plates. The "great" unconformities of regional or continental scale (in both geography and chronology) are associated with either global changes in eustatic sea level or the
supercontinent cycle, the periodic merger of all the continents into one approximately every 500 million years.
Hutton's Unconformity
Hutton's Unconformity
Hutton's Unconformity is a name given to various notable geological sites in Scotland identified by the 18th-century Scottish geologist James Hutton as places where the junction between two types of rock formations can be seen. This geological ph ...
at
Siccar Point, in county of
Berwickshire
Berwickshire ( gd, Siorrachd Bhearaig) is a historic county, registration county and lieutenancy area in south-eastern Scotland, on the English border. Berwickshire County Council existed from 1890 until 1975, when the area became part of th ...
on the east coast of
Scotland
Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to ...
, is an
angular unconformity that consists of gently dipping, reddish, Upper
Devonian
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wh ...
and Lower
Carboniferous
The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferou ...
breccia
Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.
The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...
s,
sandstone
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicat ...
s, and
conglomerate
Conglomerate or conglomeration may refer to:
* Conglomerate (company)
* Conglomerate (geology)
* Conglomerate (mathematics)
In popular culture:
* The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes
** ...
s of the
Old Red Sandstone
The Old Red Sandstone is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the northeastern seaboard of North America. It also exte ...
overlying deeply eroded, near-vertical, greyish,
Silurian
The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleoz ...
(
Llandovery
Llandovery (; cy, Llanymddyfri ) is a market town and community in Carmarthenshire, Wales. It lies on the River Tywi and at the junction of the A40 and A483 roads, about north-east of Carmarthen, north of Swansea and west of Brecon.
Hi ...
)
greywacke
Greywacke or graywacke (German ''grauwacke'', signifying a grey, earthy rock) is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lit ...
s and
shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...
s. The Llandovery greywackes and
graptolite-bearing shales of the
Gala Group were deposited by turbidity currents in a deep sea environment about 425 million years ago. The overlying Devonian strata were deposited by rivers and streams about 345 million years ago. Thus, this unconformity reflects a gap of about 80 million years during which deep sea sediments were
lithified,
folded, and
uplifted; later deeply eroded and weathered subaerially; and finally buried by river and stream sediments.
Exposures of the unconformity at Siccar Point, provided James Hutton, accompanied by
John Playfair
John Playfair FRSE, FRS (10 March 1748 – 20 July 1819) was a Church of Scotland minister, remembered as a scientist and mathematician, and a professor of natural philosophy at the University of Edinburgh. He is best known for his book ''Illu ...
and
Sir James Hall
Sir James Hall of Dunglass, 4th Baronet FRS FRSE (17 January 1761 – 23 June 1832) was a Scottish geologist and geophysicist. He was a Member of Parliament for St. Michael's borough (Mitchell, Cornwall) 1807–1812.
Education
Hall was born at Du ...
, the clearest example of an unconformable relationship between two sets of sedimentary strata that involved a complex geological history. The clear truncation of near-vertical Silurian sedimentary strata by well-bedded conglomerates and sandstones belonging to the Upper Old Red Sandstone allowed Hutton to demonstrate the existence of significant breaks in the geological record, in this case a break separating strata that were then called ''alpine schistus'' and ''secondary strata.'' This and other unconformities provided evidence for Hutton's ideas about the ''recycling'' of geological materials and for unconformities representing very large time periods. He argued that these concepts pointed to the great antiquity of the Earth and the vastness of the geological time-scale.
Powell’s Unconformity, Grand Canyon
The Great Unconformity of Powell in the
Grand Canyon
The Grand Canyon (, yuf-x-yav, Wi:kaʼi:la, , Southern Paiute language: Paxa’uipi, ) 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 m ...
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 ...
from the underlying, faulted and tilted sedimentary rocks of the
Grand Canyon Supergroup
The Grand Canyon Supergroup is a Mesoproterozoic to a Neoproterozoic sequence of sedimentary strata, partially exposed in the eastern Grand Canyon of Arizona. This group comprises the Unkar Group, Nankoweap Formation, Chuar Group and the S ...
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
Laurentia or the North American Craton is a large continental craton that forms the 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 North America, althoug ...
, the ancient core of North America. It was first recognized twelve years before Powell's expedition by
John Newberry in
New Mexico
)
, population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano)
, seat = Santa Fe, New Mexico, Santa Fe
, LargestCity = Albuquerque, New Mexico, Albuquerque
, LargestMetro = Albuquerque metropolitan area, Tiguex
, Offi ...
, during the Ives expedition of 1857–1858. However, the disruption of the
American Civil War
The American Civil War (April 12, 1861 – May 26, 1865; also known by Names of the American Civil War, other names) was a civil war in the United States. It was fought between the Union (American Civil War), Union ("the North") and t ...
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 Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...
-Early
Ordovician
The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya.
T ...
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
The Sixtymile Formation is a very thin accumulation of sandstone, siltstone, and breccia underlying the Tapeats Sandstone that is exposed in only four places in the Chuar Valley. These exposures occur atop Nankoweap Butte and within Awatubi and ...
, of the Grand Canyon Supergroup. At the base of the Grand Canyon Supergroup, where it truncates the
Bass Formation
The Bass Formation, also known as the Bass Limestone, is a Mesoproterozoic rock formation that outcrops in the eastern Grand Canyon, Coconino County, Arizona. The Bass Formation erodes as either cliffs or stair-stepped cliffs. In the case of t ...
, 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 to 1.6 billion years.
(See also
geological timescale.)
Frenchman Mountain, Nevada
A prominent exposure of Powell's Great Unconformity occurs in
Frenchman Mountain in
Nevada
Nevada ( ; ) is a U.S. state, state in the Western United States, Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. N ...
. Frenchman Mountain exposes a sequence of Phanerozoic 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
strata
In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as e ...
of the Tapeats Sandstone rests on 1.7 billion (1700 million) year old Vishnu Basement Rocks.
As a widespread phenomenon
The term "Great Unconformity" has also been used to refer to the anomalous concentration of unconformities, including basement nonconformities, below the base of the Cambrian.
Charles Walcott was among the first to note this phenomenon, remarking in 1910:
A potential link has been proposed between such sub-Cambrian unconformities and
glacial erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is dist ...
during the
Neoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago.
It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is prec ...
Snowball Earth
The Snowball Earth hypothesis proposes that, during one or more of Earth's icehouse climates, the planet's surface became entirely or nearly entirely frozen. It is believed that this occurred sometime before 650 M.Y.A. (million years ago) du ...
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 bet ...
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 and broke up 750–633 million years ago.
were prob ...
, created many unconformities worldwide. Evidence indicates that the
Pikes Peak
Pikes Peak is the highest summit of the southern Front Range of the Rocky Mountains, in North America. The ultra-prominent fourteener is located in Pike National Forest, west of downtown Colorado Springs, Colorado. The town of Manitou S ...
unconformity was formed before the Snowball Earth glaciations.
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. There are theories 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 unit of geologic time from 1 billion to 538.8 million years ago.
It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is prec ...
, starting around 720 million years ago.
This is also when a significant glaciation event known as '
Snowball Earth
The Snowball Earth hypothesis proposes that, during one or more of Earth's icehouse climates, the planet's surface became entirely or nearly entirely frozen. It is believed that this occurred sometime before 650 M.Y.A. (million years ago) du ...
' occurred.
Snowball Earth
The Snowball Earth hypothesis proposes that, during one or more of Earth's icehouse climates, the planet's surface became entirely or nearly entirely frozen. It is believed that this occurred sometime before 650 M.Y.A. (million years ago) du ...
covered almost the entire planet with ice. The areas that underwent glaciation were approximately those where 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 ...
is located today. When
glacier
A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such a ...
s 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.
See also
*
Geology of the Grand Canyon area (with time scale)
*
List of orogenies
*
Orogeny
Orogeny is a mountain building process. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. An '' orogenic belt'' or ''orogen'' develops as the compressed plate crumples and is uplifted ...
(mountain building)
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. (2102a
last accessed September 22, 2013.
* Share, J. (2102a) [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