Great Unconformity

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.

Examples

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 Devonian 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 (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 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.)

See also

• Unconformity
• Geology of the Grand Canyon area (with time scale)
• Orogeny (mountain building)
• List of orogenies

References

• Billingsley, George H.; Priest, Susan S.; Felger, Tracey J. (2008), "Geologic Map of the Fredonia 30´ x 60´ Quadrangle, Mohave and Coconino Counties, Northern Arizona", U.S. Geological Survey, Scientific Investigations Map 3035, Pamphlet to accompany Scientific Investigations Map 3035 

• Merten, Gretchen (January 2005), "Geology in the American Southwest", in Anderson, Michael F., A Gathering of Grand Canyon Historians: Ideas, Arguments, and First—Person Accounts: Proceedings of the Inaugural Grand Canyon History Symposium, January 2002, Grand Canyon Association 
• Rance, Hugh (1999), Historical Geology: The Present is the Key to the Past, New York, N.Y.: QCC Press 
• Timmons, Mike; Karlstrom, Karl; Dehler, Carol (Winter 1998—99), "Grand Canyon Supergroup Six Unconformities Make One ‘Great Unconformity’: A Record of Supercontinent Assembly and Disassembly", Boatman's Quarterly Review 12 (1): 26–32 
• Timmons, J. Michael; Karlstrom, Karl E.; Hiezler, Matthew T.; Bowring, Samuel A.; Gehrels, George E.; Crossey, Laura J. (November 2005), "Tectonic inferences from the ca. 1255–1100 Ma Unkar Group and Nankoweap Formation, Grand Canyon: Intracratonic deformation and basin formation during protracted Grenville orogenesis", Geological Society of America Bulletin 117 (11—12): 1573–1595, doi:10.1130/B25538.1