The term Great Unconformity is frequently applied to the unconformity observed by John Wesley Powell in the Grand Canyon in 1869.[1] It is an exceptional example of relatively young sedimentary rock strata overlying much older sedimentary or crystalline strata. The intervening period of geologic time is sufficiently long to raise the earlier rock into mountains which are then eroded away.
Powell's Unconformity, Grand Canyon
edit
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.[2][3][4]
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.[5] 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.[6][7]
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.[3][7] (See also geological timescale.)
Frenchman Mountain, Nevada
editA 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.[8][9][10]
Possible causes of the Great Unconformity
editThere 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.[11][12][13] This is also when a significant glaciation event known as Snowball Earth occurred.[11] Snowball Earth periods 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.[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.[15][16][17][18]
See also
edit- Geology of the Grand Canyon area (with time scale)
- List of orogenies
- Orogeny (mountain building)
References
edit- ^ 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.
- ^ 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.
- ^ a b 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.
- ^ "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.
- ^ 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.
- ^ 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.
- ^ a b 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.
- ^ 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.
- ^ Rowland, S (nd) Frenchman Mountain Great Unconformity site. Department of Geoscience, University of Nevada, Las Vegas, Nevada.
- ^ 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.
- ^ a b c 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.
- ^ 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.
- ^ College, Dartmouth (2022-01-26). "New Research Strengthens Link Between Glaciers and Earth's Puzzling "Great Unconformity"". SciTechDaily. Retrieved 2022-02-19.
- ^ White, W. A. (1973) "Deep Erosion by Infracambrian Ice Sheets". Geological Society of America Bulletin, v. 84, pp. 1841–1844.
- ^ "Big chunks of history (and rock) are missing in North America, study says". CNN. April 27, 2020.
- ^ Joel, L. (5 February 2018). "Erasing a Billion Years of Geologic Time Across the Globe". Eos.org.
- ^ 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.
- ^ 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.
External links
edit- Hutton's Unconformity
- Anonymous (2003) Siccar Point Field Excursion Preview. School of GeoSciences, University of Edinburgh, Edinburgh, Scotland. last accessed September 22, 2013.
- Moore, R (2009) Siccar Point. Reports of the National Center for Science Education. 29(1):26. last accessed September 22, 2013.
- Rowan, C (2011) The making of an angular unconformity: Hutton’s unconformity at Siccar Point. Highly Allochthonous. last accessed September 22, 2013.
- Powell's Unconformity
- Gorvett, Zaria (1 September 2021). "The strange race to track down a missing billion years". BBC.
- Abbott, W (2001) Revisiting the Grand Canyon – Through the Eyes of Seismic Sequence Stratigraphy. AAPG Datapages / Search and Discovery, American Association of Petroleum Geologist, Tulsa, Oklahoma. last accessed September 22, 2013.
- Brandriss, M. (2004) Angular unconformity between Proterozoic and Cambrian rocks, Grand Canyon, Arizona. GeoDIL, A Geoscience Digital Image Library, University of North Dakota, Grand Forks, North Dakota. last accessed September 22, 2013.
- Share, J. (2012a) The Great Unconformity of the Grand Canyon and the Late Proterozoic-Cambrian Time Interval: Part I – Defining It. last accessed September 22, 2013.
- Share, J. (2012a) The Great Unconformity and the Late Proterozoic-Cambrian Time Interval: Part II – The Rifting of Rodinia and the "Snowball Earth" Glaciations That Followed. last accessed September 22, 2013.