Great Unconformity

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.

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.

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.

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

[14][11] Alternatively, it has been proposed that multiple smaller events, such as the formation and breakup of Rodinia, created many unconformities worldwide.

Geologic stratigraphic column of strata exposed in and near the Grand Canyon
Powell's Unconformity viewed from Lipan Point on the South Rim. Rocks of the Unkar Group of the Grand Canyon Supergroup are truncated at the base of the Tonto Group
Powell's Unconformity seen from Hopi Point on the South Rim. Steeply foliated and veined schists of the Vishnu Basement Rocks truncated at the base of the Tonto Group
An exposure of Powell's Great Unconformity, west of Montezuma, New Mexico