Base level
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In geology and geomorphology a base level is the lower limit for the vertical position of an erosion process.cite-ref-goudie2004-1-0[1] The modern term was introduced by John Wesley Powell in 1875.cite-ref-goudie2004-1-1[1] The term was subsequently appropriated by William Morris Davis who used it in his cycle of erosion theory.cite-ref-goudie2004-1-2[1]cite-ref-orme2007-2-0[2] The "ultimate base level" is the surface that results from horizontal projection of the sea level under landmasses (the geoid).cite-ref-goudie2004-1-3[1] It is to this base level that topography tends to approach due to erosion, eventually forming a peneplain close to the end of a cycle of erosion.cite-ref-phillips-3-0[3]cite-ref-4[4]cite-ref-greenetal2013-5-0[5]cite-ref-karnaetal2014-6-0[6]
There are also lesser structural base levels where erosion is delayed by resistant rocks.cite-ref-goudie2004-1-4[1] Examples of this include karst regions underlain by insoluble rock.cite-ref-ford2004-7-0[7] Base levels may be local when large landmasses are far from the sea or disconnected from it, as in the case of endorheic basins.cite-ref-goudie2004-1-5[1] An example of this is the Messinian salinity crisis, in which the Mediterranean Sea dried up making the base level drop more than 1000 m below sea level.cite-ref-8[8]cite-ref-goudie2005-9-0[9]
The height of a base level also influences the position of deltas and river terraces.cite-ref-goudie2004-1-6[1] Together with river discharge and sediment flux the position of the base level influences the gradient, width and bed conditions in rivers.cite-ref-whipple2004-10-0[10] A relative drop in base level can trigger re-adjustments in river profiles including knickpoint migration and abandonment of terraces leaving them "hanging".cite-ref-spotila2004-11-0[11] Base level fall is also known to result in progradation of deltas and river sediment at lakes or sea.cite-ref-kossetal1994-12-0[12] If the base level falls below the continental shelf, rivers may form a plain of braided rivers until headward erosion penetrates enough inland from the shelfbreak.cite-ref-kossetal1994-12-1[12]
When base levels are stable or rising rivers may aggrade.cite-ref-kossetal1994-12-2[12] Rising base levels may also drown the lower courses of rivers creating rias. This happened in the Nile during the Zanclean flood when its lower course became, in a relatively short time, a large estuary extending up to 900 km inland from the Mediterranean coast.cite-ref-goudie2005-9-1[9]
Base level change may be related to the following factors:
1. Sea level changecite-ref-goudie2004-1-7[1]
2. Tectonic movementcite-ref-goudie2004-1-8[1]
3. River capturecite-ref-goudie2004-1-9[1]
4. Extensive sedimentationcite-ref-13[13]
References
cite-note-goudie2004-11. ↑ citerefgoudie2004Goudie, A.S. (2004). "Base level". In Goudie, A.S. (ed.). Encyclopedia of Geomorphology. Routledge. p. 62.
cite-note-phillips-33. ↑ Phillips, Jonathan D. (2002), "Erosion, isostatic response, and the missing peneplains", Geomorphology, Vol. 45, No. 3-4. Elsevier Archived 2010-01-24 at the Wayback Machine, 15 June 2002, pp. 225-241. doi:10.1016/S0169-555X(01)00156-8.
cite-note-44. ↑ Chorley, R.J. (1973). The History and Study of Landforms or The Development of Geomorphology. Vol. Two: The Life and Work of William Morris Davis, Methuen.
cite-note-greenetal2013-55. ↑ citerefgreenlidmar-bergstr-mjapsenbonow2013Green, Paul F.; Lidmar-Bergström, Karna; Japsen, Peter; Bonow, Johan M.; Chalmers, James A. (2013). "Stratigraphic landscape analysis, thermochronology and the episodic development of elevated, passive continental margins". Geological Survey of Denmark and Greenland Bulletin. 30: 18. doi:10.34194/geusb.v30.4673.
cite-note-karnaetal2014-66. ↑ citereflidmar-bergstr-mbonowjapsen2013Lidmar-Bergström, Karna; Bonow, Johan M.; Japsen, Peter (2013). "Stratigraphic Landscape Analysis and geomorphological paradigms: Scandinavia as an example of Phanerozoic uplift and subsidence". Global and Planetary Change. 100: 153–171. Bibcode:2013GPC...100..153L. doi:10.1016/j.gloplacha.2012.10.015.
cite-note-ford2004-77. ↑ citerefford2004Ford, Derek C. (2004). "Cave". In Goudie, A.S. (ed.). Encyclopedia of Geomorphology. Routledge. pp. 124–128.
cite-note-88. ↑ citereffairbridgefinkl-jr-1980Fairbridge, Rhodes W.; Finkl Jr., Charles W. (1980). "Cratonic erosion unconformities and peneplains". The Journal of Geology. 88 (1): 69–86. Bibcode:1980JG.....88...69F. doi:10.1086/628474. S2CID 129231129.
cite-note-goudie2005-99. ↑ citerefgoudie2005Goudie, A.S. (2005). "The drainage of Africa since the Cretaceous". Geomorphology. 67 (3–4): 437–456. Bibcode:2005Geomo..67..437G. doi:10.1016/j.geomorph.2004.11.008.
cite-note-whipple2004-1010. ↑ citerefwhipple2004Whipple, Kelin X. (2004). "Bedrock channel". In Goudie, A.S. (ed.). Encyclopedia of Geomorphology. Routledge. pp. 81–82.
cite-note-spotila2004-1111. ↑ citerefspotila2004Spotila, James A. (2004). "Crustal deformation". In Goudie, A.S. (ed.). Encyclopedia of Geomorphology. Routledge. pp. 201–203.
cite-note-kossetal1994-1212. ↑ citerefkossethridgeschumm1994Koss, John E.; Ethridge, Frank G.; Schumm, S.A. (1994). "An Experimental Study of the Effects of Base-Level Change on Fluvial, Coastal Plain and Shelf Systems". Journal of Sedimentary Research. 64B (2): 90–98. doi:10.1306/D4267F64-2B26-11D7-8648000102C1865D.
cite-note-1313. ↑ citerefbabaultvan-den-driesschebonnetcastelltort2005Babault, Julien; Van Den Driessche, Jean; Bonnet, Stephanie; Castelltort, Sébastien; Crave, Alain (2005). "Origin of the highly elevated Pyrenean peneplain". Tectonics. 24 (2): n/a. Bibcode:2005Tecto..24.2010B. doi:10.1029/2004TC001697.