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,[1] or that observed by John Wesley Powell in the Grand Canyon in 1869.[2] 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 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.

Hutton's Unconformity

Hutton's Unconformity at Siccar Point, in county of Berwickshire on the east coast of Scotland, is an angular unconformity that consists of gently dipping, reddish, Upper Devonian and Lower Carboniferous breccias, sandstones, and conglomerates of the Old Red Sandstone overlying deeply eroded, near-vertical, greyish, Silurian (Llandovery) greywackes and shales. The Llandovery greywackes and graptolite-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 lithified, folded, and uplifted; later deeply eroded and weathered subaerially; and finally buried by river and stream sediments.[1][3][4]

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. This and other unconformities provided evidence for Hutton's ideas about the recycling of geological materials and for unconformities representing very large time periods. He argued that these concepts pointed to the great antiquity of the Earth and the vastness of the geological time-scale.[1][5]

Powell’s Unconformity, 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

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.[6][7][8]

Powell's Great Unconformity is part of a continent-wide unconformity that extends across Laurentia, the ancient core of North America. It was first recognized twelve years before Powell's expedition by John Newberry in New Mexico, during the Ives expedition of 1857–1858. However, the disruption of the American Civil War kept Newberry's work from becoming widely known.[9] This Great Unconformity 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.[10][11]

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.[7][11] (See also geological timescale.)

An exposure of Powell's Great Unconformity, west of Montezuma, New Mexico

Frenchman Mountain, Nevada

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 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 is 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 (1700 million) year old Vishnu Basement Rocks.[12][13][14]

As a widespread phenomenon

The term "Great Unconformity" has also been used to refer to the anomalous concentration of unconformities, including basement nonconformities, below the base of the Cambrian.[15] Charles Walcott was among the first to note this phenomenon, remarking in 1910:[16]

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.[17][18] Alternatively, it has been proposed that multiple smaller events, such as the formation and breakup of Rodinia, created many unconformities worldwide.[19][20][21][22] Evidence indicates that the Pikes Peak unconformity was formed before the Snowball Earth glaciations.[23]

Possible causes of the Great Unconformity

There is currently no widely accepted explanation for the Great Unconformity among geoscientists. There are hypotheses that have been proposed; it is widely accepted that there was a combination of more than one event which may have caused such an extensive phenomenon. One example is a large glaciation event which took place during the Neoproterozoic, starting around 720 million years ago.[18][24][25] This is also when a significant glaciation event known as 'Snowball Earth' occurred.[18] Snowball Earth covered almost the entire planet with ice. The areas that underwent glaciation were approximately those where the Great Unconformity is located today. When glaciers move, they drag and erode sediment away from the underlying rock. This would explain how a large section of rock was taken away from widespread areas around the same time.

See also

References

  1. 1 2 3 Rance, H (1999) Historical Geology: The Present is the Key to the Past. Archived 3 December 2008 at the Wayback Machine QCC Press, New York
  2. ↑ Merten, G (2005) Geology in the American Southwest: New Processes, New Theories In MF Anderson, ed., A Gathering of Grand Canyon Historians. Proceedings of the Inaugural Grand Canyon History Symposium, January 2002. Grand Canyon Association, Grand Canyon, Arizona.
  3. ↑ Barclay, WJ, MAE Browne, AA McMillan, EA Pickett, P Stone, and PR Wilby (2005) The Old Red Sandstone of Great Britain. Geological Conservation Review Series no. 31. Joint Nature Conservation Committee, Peterborough, United Kingdom, 393 pp.
  4. ↑ Holdsworth, R. E.; Tavarnelli, E.; Clegg, P. (2002). "The nature and regional significance of structures in the Gala Group of the Southern Uplands terrane, Berwickshire coast, southeastern Scotland" (PDF). Geological Magazine. 139 (6): 707–717. Bibcode:2002GeoM..139..707H. doi:10.1017/S0016756802006854. S2CID 54197702.
  5. ↑ Jutras, Pierre; Young, Grant M.; Caldwell, W. Glen E. (2011). "Reinterpretation of James Hutton's historic discovery on the Isle of Arran as a double unconformity masked by a phreatic calcrete hardpan" (PDF). Geology. 39 (2): 147–150. Bibcode:2011Geo....39..147J. doi:10.1130/G31490.1.
  6. ↑ Billingsley, G. H. (2000). "Geologic map of the Grand Canyon 30' x 60' quadrangle, Coconino and Mohave Counties, northwestern Arizona". U.S. Geological Survey. doi:10.3133/i2688.
  7. 1 2 Timmons, M; Karlstrom, KE; Dehler, C (1998). "Grand Canyon Supergroup: Six Unconformities Make One Great Unconformity A Record of Supercontinent Assembly and Disassembly". Archived from the original on 13 July 2013.
  8. ↑ "Grand Canyon Supergroup: Six Unconformities Make One Great Unconformity A Record of Supercontinent Assembly and Disassembly" (PDF). Boatman's Quarterly Review. pp. 28–32. Archived from the original (PDF) on 28 September 2013.
  9. ↑ Kues, Barry S.; Lewis, Claudia J.; Lueth, Virgil W. (2014). A brief history of geological studies in New Mexico : with biographical profiles of notable New Mexico geologists (First ed.). New Mexico Geological Society. ISBN 978-1-58546-011-3.
  10. ↑ Peters, Shanan E.; Gaines, Robert R. (2012). "Formation of the 'Great Unconformity' as a trigger for the Cambrian explosion". Nature. 484 (7394): 363–366. Bibcode:2012Natur.484..363P. doi:10.1038/nature10969. PMID 22517163. S2CID 4423007.
  11. 1 2 Karlstrom, Karl E.; Timmons, J. Michael, eds. (2012). "Many unconformities make one 'Great Unconformity'". Grand Canyon Geology: Two Billion Years of Earth's History. Vol. 489. Boulder, Colorado: Geological Society of America. pp. 73–79. ISBN 978-0-8137-2489-8.
  12. ↑ Rowland, Stephen M. (1987). "Paleozoic stratigraphy of Frenchman Mountain, Clark County, Nevada". Cordilleran Section of the Geological Society of America. pp. 53–56. doi:10.1130/0-8137-5401-1.53. ISBN 9780813754079.
  13. ↑ Rowland, S (nd) Frenchman Mountain Great Unconformity site. Department of Geoscience, University of Nevada, Las Vegas, Nevada.
  14. ↑ Palmer, A. R. (1989). "Day 0: Early and Middle Cambrian stratigraphy of Frenchman Mountain, Nevada". Cambrian and Early Ordovician Stratigraphy and Paleontology of the Basin and Range Province, Western United States: Las Vegas, Nevada to Salt Lake City, Utah, July 1–7, 1989. pp. 14–16. doi:10.1029/FT125p0014. ISBN 0-87590-662-1.
  15. ↑ Peters, Shanan E.; Gaines, Robert R. (2012). "Formation of the 'Great Unconformity' as a trigger for the Cambrian explosion". Nature. 484 (7394): 363–366. Bibcode:2012Natur.484..363P. doi:10.1038/nature10969. PMID 22517163. S2CID 4423007.
  16. ↑ Walcott, C. D. (1910) "Abrupt appearance of the Cambrian fauna on the North American continent". Smithsonian Miscellaneous Collections 57: Cambrian Geology and Paleontology, pp. 1–16.
  17. ↑ White, W. A. (1973) "Deep Erosion by Infracambrian Ice Sheets". Geological Society of America Bulletin, v. 84, pp. 1841–1844.
  18. 1 2 3 Keller, C. Brenhin; Husson, Jon M.; Mitchell, Ross N.; Bottke, William F.; Gernon, Thomas M.; Boehnke, Patrick; Bell, Elizabeth A.; Swanson-Hysell, Nicholas L.; Peters, Shanan E. (2019-01-22). "Neoproterozoic glacial origin of the Great Unconformity". Proceedings of the National Academy of Sciences. 116 (4): 1136–1145. Bibcode:2019PNAS..116.1136B. doi:10.1073/pnas.1804350116. ISSN 0027-8424. PMC 6347685. PMID 30598437.
  19. ↑ "Big chunks of history (and rock) are missing in North America, study says". CNN. April 27, 2020.
  20. ↑ Joel, L. (5 February 2018). "Erasing a Billion Years of Geologic Time Across the Globe". Eos.org.
  21. ↑ Peak, B.A.; Flowers, R.M.; Macdonald, F.A.; Cottle, J.M. (2021-08-12). "Zircon (U-Th)/He thermochronology reveals pre-Great Unconformity paleotopography in the Grand Canyon region, USA". Geology. 49 (12): 1462–1466. Bibcode:2021Geo....49.1462P. doi:10.1130/G49116.1. ISSN 0091-7613.
  22. ↑ Boulder, University of Colorado at (2021-08-21). ""Great Unconformity" Puzzle: Geologists Dig Into Grand Canyon's Mysterious Gap in Time". SciTechDaily. Retrieved 2021-08-28.
  23. ↑ Flowers, Rebecca M.; MacDonald, Francis A.; Siddoway, Christine S.; Havranek, Rachel (2020). "Diachronous development of Great Unconformities before Neoproterozoic Snowball Earth". Proceedings of the National Academy of Sciences. 117 (19): 10172–10180. Bibcode:2020PNAS..11710172F. doi:10.1073/pnas.1913131117. PMC 7229757. PMID 32341149. S2CID 216595868.
  24. ↑ McDannell, Kalin T.; Keller, C. Brenhin; Guenthner, William R.; Zeitler, Peter K.; Shuster, David L. (2022-02-01). "Thermochronologic constraints on the origin of the Great Unconformity". Proceedings of the National Academy of Sciences. 119 (5): e2118682119. Bibcode:2022PNAS..11918682M. doi:10.1073/pnas.2118682119. ISSN 0027-8424. PMC 8812566. PMID 35078936.
  25. ↑ College, Dartmouth (2022-01-26). "New Research Strengthens Link Between Glaciers and Earth's Puzzling "Great Unconformity"". SciTechDaily. Retrieved 2022-02-19.
Hutton's Unconformity
Powell's Unconformity


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