Great Unconformity

Of the many unconformities (gaps) observed in geological strata, the term **Great Unconformity** is frequently applied to either

* the unconformity observed by James Hutton in 1787 at Siccar Point in Scotland
* that observed by John Wesley Powell in the Grand Canyon 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 sufficiently long to raise great mountains and then erode them away.

## Background

Unconformities in general tend to reflect long-term changes in the pattern of the accumulation of sedimentary or igneous strata 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 ongoing Himalayan orogeny, the older Laramide orogeny of the Rocky Mountains, or much older Appalachian (Alleghanian) 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 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.

## Powell’s Unconformity, Grand Canyon

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 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 to 1.6 billion years of time. (See also geological timescale.)

A prominent exposure of Powell's Great Unconformity occurs in Frenchman Mountain in Nevada. 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 very 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 often part of geological fieldtrips. There is a gap of about 1.2 billion years where 550 million year old strata of the Tapeats Sandstone rests on 1.7 billion year old Vishnu Basement Rocks.

## As a widespread phenomenon

More broadly, the term "Great Unconformity" has also been used to refer to the anomalous concentration of unconformities, particularly including basement nonconformites, below the base of the Cambrian. Charles Walcott was among the first to note this phenomenon, remarking in 1910:

> I do not know of a case of proven conformity between Cambrian and pre-Cambrian Algonkian rocks on the North American continent. In all localities where the contact is sufficiently extensive, or where fossils have been found in the basal Cambrian beds or above the basal conglomerate and coarser sandstones, an unconformity has been found to exist. Stated in another way, the pre-Cambrian land surface was formed of sedimentary, eruptive, and crystalline rocks that did not in any known instance immediately precede in deposition or origin the Cambrian sediments. Everywhere there is a stratigraphic and time break between the known pre-Cambrian rocks and Cambrian sediments of the North American continent.
>
> — Charles D Walcott, "Abrupt Appearance of the Cambrian Fauna on the North American Continent", *Cambrian Geology and Paleontology* (1910)
>

A potential link has been proposed between such sub-Cambrian unconformities and glacial erosion during the Neoproterozoic Snowball Earth glaciations.

## Unconformity-related deposits

Unconformity-type uranium deposits host high grades relative to other uranium deposits and include some of the largest and richest deposits known. They occur in close proximity to unconformities between relatively quartz-rich sandstones comprising the basal portion of relatively undeformed sedimentary basins and deformed metamorphic basement rocks. These sedimentary basins are typically of Proterozoic age, however some Phanerozoic examples exist.

Phanerozoic unconformity-related deposits occur in Proterozoic metasediments below an unconformity at the base of overlying Phanerozoic sandstone. These deposits are small and low-grade (Bertholene and Aveyron deposits, in France).

The two most significant areas for this style of deposit are currently the Athabasca Basin in Saskatchewan, Canada, and the McArthur Basin in the Northern Territory, Australia.

### Athabasca Basin

Main articles: Athabasca Basin
The Athabasca basin, a historical fluvial siliciclastic basin with sediments from the Hudsonian mountains with the occasional rare marine sequence. The Athabasca basin was formed during the Statherian or Paleohelikian 1.7 to 1.6 billion years ago when coarse fluvial and marine clastic sediments were laid down containing gold, copper, lead, zinc, and uranium oxides. The highest grade uranium deposits in the world are found at the unconformity between these clastic layers and the Precambrian bedrock. The Athabasca Sand Hills protected by *The Athabasca Sand Dunes Provincial Park* are unique feature of the Canadian Shield. The hills are located in northern Saskatchewan and border Lake Athabasca, which straddles the Alberta and Saskatchewan border.

The highest grade uranium deposits are found in the Athabasca Basin in Canada, including the two largest high grade uranium deposits in the world, Cigar Lake with 217 million pounds (99,000 t) U3O8 at an average grade of 18% and McArthur River with 324 million pounds (147,000 t) U3O8 at an average grade of 17%. These deposits occur below, across and immediately above the unconformity. Additionally, another high grade discovery is in the development stage at Patterson Lake (Triple R deposit) with an estimated mineral resource identified as; "Indicated Mineral Resources" estimated to total 2,291,000 tons at an average grade of 1.58% U3O8 containing 79,610,000 pounds of U3O8. "Inferred Mineral Resources" are estimated to total 901,000 tons at an average grade of 1.30% U3O8 containing 25,884,000 pounds of U3O8. http://www.fissionuranium.com/\_resources/reports/RPA\_Fission\_U\_Patterson\_Lake\_South\_Technical\_Report\_FINAL\_Feb\_2015.pdf