Great Unconformity

Of the many (gaps) observed in geological strata, the term Great Unconformity is frequently applied to either the unconformity observed by  in 1787 at  in Scotland, or that observed by  in the  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 or the, but also  and much of ), then being uplifted and eroded (such as the ongoing , the older  of the , or much older  and  orogenies), then subsequently subsiding, eventually to be buried under younger sediments. The intervening periods of 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, the periodic merger of all the continents into one approximately every 500 million years.

Hutton's Unconformity
at, in county of on the east coast of , is an  that consists of gently dipping, reddish, Upper  and Lower  s, s, and s of the  overlying deeply eroded, near-vertical, greyish,   s and s. The Llandovery greywackes and -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, , and ; 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 and Sir James Hall, 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. Furthermore, this and other unconformites provided evidence for Hutton's ideas about the recycling of geological materials and for unconformities representing very large periods of time. He argued that both of 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 is a regional unconformity that separates the  from the underlying, faulted and tilted sedimentary rocks of the  and vertically foliated metamorphic and igneous rocks of the. The unconformity between the Tonto Group and the Vishnu Basement Rocks is a. 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, 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 -Early. 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, of the Grand Canyon Supergroup. At the base of the Grand Canyon Supergroup, where it truncates the, 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 .)

A prominent exposure of Powell's Great Unconformity occurs in in. 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 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 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. 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

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