Great Unconformity
Encyclopedia
Of the many unconformities
(gaps) observed in geological strata, the term Great Unconformity is most notably applied to either the unconformity observed by James Hutton
in 1787 at Siccar Point
in Scotland, or that observed by John Wesley Powell
in the Grand Canyon
in 1869. Both instances are notable for demonstrating that the contacts between sedimentary formations of greatly different age could represent epochs sufficiently long to raise great mountains and then erode them away.
Hutton's Unconformity
at Siccar Point
(and other localities) reflects a gap of about 80 million years between certain red sandstones
deposited sometime in the Devonion Period (approximately 350 to 416 million years ago) over marine sediments laid down in the Silurian Period (approximately 416 to 444 million years ago).
Powell's unconformity in the Grand Canyon
is between the Tapeats Sandstone of Middle or Lower Cambrian
Age, about 500 million years ago, and igneous and metamorphic rock of the Statherian
Period of the Paleoproterozoic
Eon, around 1700 million years old, give or take a few hundred million years, with a gap of over a billion years. (See also geological timescale.)
Unconformities in general tend to reflect long-term patterns of sediments or igneous flows collecting in low-lying areas (often ocean basins, such as the Gulf of Mexico
or the North Sea
, but also Bangladesh
and much of Brazil
), then being uplifted and eroded (such as the on-going Himalayan orogeny, the older Laramide orogeny
of the Rocky Mountains
, or much older Appalachian orogeny and Ouachita
orogenies), then subsequently subsiding, eventually to be buried under younger sediments. The intervening periods of tectonic uplift
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 the supercontinent cycle
, the periodic merger of all the continents into one approximately every 500 million years. Below Powell's unconformity the Grand Canyon Supergroup of rocks (formed at an earlier age) was uplifted, tilted, and then eroded during the Grenville orogeny, when the Rodinia
supercontinent came together (with the Australian subcontinent juxtaposed to what's now Arizona). When Rodinia separated (and Australia drifted off) the Tapeats Sandstone (above the unconformity) collected in a basin or shallow sea stretching across much of what is now the southwestern United States. (The process was more complex than related here; see the references for details.)
Unconformity
An unconformity is a buried erosion 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 of time before deposition of the younger, but the term is used to describe...
(gaps) observed in geological strata, the term Great Unconformity is most notably applied to either the unconformity observed by James Hutton
James Hutton
James Hutton was a Scottish physician, geologist, naturalist, chemical manufacturer and experimental agriculturalist. He is considered the father of modern geology...
in 1787 at Siccar Point
Siccar Point
Siccar Point is a rocky promontory in the county of Berwickshire on the east coast of Scotland.It is famous in the history of geology for Hutton's Unconformity found in 1788, which James Hutton regarded as conclusive proof of his uniformitarian theory of geological development.-History:Siccar...
in Scotland, or that observed by John Wesley Powell
John Wesley Powell
John Wesley Powell was a U.S. soldier, geologist, explorer of the American West, and director of major scientific and cultural institutions...
in the Grand Canyon
Grand Canyon
The Grand Canyon is a steep-sided canyon carved by the Colorado River in the United States in the state of Arizona. It is largely contained within the Grand Canyon National Park, the 15th national park in the United States...
in 1869. Both instances are notable for demonstrating that the contacts between sedimentary formations of greatly different age could represent epochs sufficiently long to raise great mountains and then erode them away.
Hutton's Unconformity
Hutton's Unconformity
Hutton's Unconformity is any of various famous geological sites in Scotland. These are places identified by 18th-century Scottish geologist James Hutton as an unconformity, which provided evidence for his Plutonist theories of uniformitarianism and about the age of the Earth.-Theory of rock...
at Siccar Point
Siccar Point
Siccar Point is a rocky promontory in the county of Berwickshire on the east coast of Scotland.It is famous in the history of geology for Hutton's Unconformity found in 1788, which James Hutton regarded as conclusive proof of his uniformitarian theory of geological development.-History:Siccar...
(and other localities) reflects a gap of about 80 million years between certain red sandstones
Old Red Sandstone
The Old Red Sandstone is a British rock formation of considerable importance to early paleontology. For convenience the short version of the term, 'ORS' is often used in literature on the subject.-Sedimentology:...
deposited sometime in the Devonion Period (approximately 350 to 416 million years ago) over marine sediments laid down in the Silurian Period (approximately 416 to 444 million years ago).
Powell's unconformity in the Grand Canyon
Grand Canyon
The Grand Canyon is a steep-sided canyon carved by the Colorado River in the United States in the state of Arizona. It is largely contained within the Grand Canyon National Park, the 15th national park in the United States...
is between the Tapeats Sandstone of Middle or Lower Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
Age, about 500 million years ago, and igneous and metamorphic rock of the Statherian
Statherian
The Statherian is the final geologic period in the Paleoproterozoic Era and lasted from 1800 Mya to 1600 Mya . Instead of being based on stratigraphy, these dates are defined chronometrically....
Period of the Paleoproterozoic
Paleoproterozoic
The Paleoproterozoic is the first of the three sub-divisions of the Proterozoic occurring between . This is when the continents first stabilized...
Eon, around 1700 million years old, give or take a few hundred million years, with a gap of over a billion years. (See also geological timescale.)
Unconformities in general tend to reflect long-term patterns of sediments or igneous flows collecting in low-lying areas (often ocean basins, such as the Gulf of Mexico
Gulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...
or the North Sea
North Sea
In the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...
, but also Bangladesh
Bangladesh
Bangladesh , officially the People's Republic of Bangladesh is a sovereign state located in South Asia. It is bordered by India on all sides except for a small border with Burma to the far southeast and by the Bay of Bengal to the south...
and much of Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
), then being uplifted and eroded (such as the on-going Himalayan orogeny, the older Laramide orogeny
Laramide orogeny
The Laramide orogeny was a 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 orogeny are in dispute, as is the cause. The Laramide...
of the Rocky Mountains
Rocky Mountains
The Rocky Mountains are a major mountain range in western North America. The Rocky Mountains stretch more than from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States...
, or much older Appalachian orogeny and Ouachita
Ouachita orogeny
The Ouachita orogeny was a mountain building event that resulted in the folding and faulting of strata currently exposed in the Ouachita Mountains...
orogenies), then subsequently subsiding, eventually to be buried under younger sediments. The intervening periods of tectonic uplift
Tectonic uplift
Tectonic uplift is a geological process most often caused by plate tectonics which increases elevation. The opposite of uplift is subsidence, which results in a decrease in elevation. Uplift may be orogenic or isostatic.-Orogenic uplift:...
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 the supercontinent cycle
Supercontinent cycle
The supercontinent cycle describes the quasi-periodic aggregation and dispersal of Earth's continental crust. There are varying opinions as to whether the amount of continental crust is increasing, decreasing, or staying about the same, but it is agreed that the Earth's crust is constantly being...
, the periodic merger of all the continents into one approximately every 500 million years. Below Powell's unconformity the Grand Canyon Supergroup of rocks (formed at an earlier age) was uplifted, tilted, and then eroded during the Grenville orogeny, when the Rodinia
Rodinia
In geology, Rodinia is the name of a supercontinent, a continent which contained most or all of Earth's landmass. According to plate tectonic reconstructions, Rodinia existed between 1.1 billion and 750 million years ago, in the Neoproterozoic era...
supercontinent came together (with the Australian subcontinent juxtaposed to what's now Arizona). When Rodinia separated (and Australia drifted off) the Tapeats Sandstone (above the unconformity) collected in a basin or shallow sea stretching across much of what is now the southwestern United States. (The process was more complex than related here; see the references for details.)
See also
- UnconformityUnconformityAn unconformity is a buried erosion 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 of time before deposition of the younger, but the term is used to describe...
- Geology of the Grand Canyon areaGeology of the Grand Canyon areaThe geology of the Grand Canyon area exposes one of the most complete and studied sequences of rock on Earth. The nearly 40 major sedimentary rock layers exposed in the Grand Canyon and in the Grand Canyon National Park area range in age from about 200 million to nearly 2 billion years old...
(with time scale) - OrogenyOrogenyOrogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...
(mountain building) - List of orogenies