Wikipedia

Base level

Ingeologyandgeomorphologyabase levelis the lower limit for anerosion process.[1]The modern term was introduced byJohn Wesley Powellin 1875.[1]The term was subsequently appropriated byWilliam Morris Daviswho used it in hiscycle of erosiontheory.[1][2]The "ultimate base level" is the surface that results from projection of thesea levelunder landmasses.[1]It is to this base level that topography tends to approach due to erosion, eventually forming apeneplainclose to the end of a cycle of erosion.[3][4][5][6]

Aerial picture of theEbroriver as it reaches theMediterranean Seaby theEbro Delta

There are also lesserstructuralbase levels where erosion is delayed by resistant rocks.[1]Examples of this includekarstregions underlain by insoluble rock.[7]Base levels may be local when large landmasses are far from the sea or disconnected from it, as in the case ofendorheic basins.[1]An example of this is theMessinian salinity crisis,in which theMediterranean Seadried up making the base level drop more than 1000 m below sea level.[8][9]

The height of a base level also influences the position ofdeltasandriver terraces.[1]Together withriver dischargeandsediment fluxthe position of the base level influences thegradient,width andbedconditions in rivers.[10]A relative drop in base level can trigger re-adjustments inriver profilesincludingknickpointmigration and abandonment of terraces leaving them "hanging".[11]Base level fall is also known to result inprogradationof deltas and river sediment at lakes or sea.[12]If the base level falls below thecontinental shelf,rivers may form a plain ofbraided riversuntilheadward erosionpenetrates enough inland from theshelfbreak.[12]

When base levels are stable or rising rivers mayaggrade.[12]Rising base levels may also drown the lower courses of rivers creatingrias.This happened in theNileduring theZanclean floodwhen its lower course became, in a relatively short time, a large estuary extending up to 900 km inland from the Mediterranean coast.[9]

Base level change may be related to the following factors:

  1. Sea level change[1]
  2. Tectonic movement[1]
  3. River capture[1]
  4. Extensive sedimentation[13]

References

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  1. ^abcdefghijGoudie, A.S.(2004). "Base level". In Goudie, A.S. (ed.).Encyclopedia of Geomorphology.Routledge. p. 62.
  2. ^Orme, Anthony R. (2007). "The Rise and Fall of the Davisian Cycle of Erosion: Prelude, Fugue, Coda, and Sequel".Physical Geography.28(6): 474–506.Bibcode:2007PhGeo..28..474O.doi:10.2747/0272-3646.28.6.474.S2CID128907423.
  3. ^Phillips, Jonathan D. (2002),"Erosion, isostatic response, and the missing peneplains",Geomorphology,Vol. 45, No. 3-4.ElsevierArchived2010-01-24 at theWayback Machine,15 June 2002, pp. 225-241.doi:10.1016/S0169-555X(01)00156-8.
  4. ^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.
  5. ^Green, 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.
  6. ^Lidmar-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.
  7. ^Ford, Derek C. (2004). "Cave". InGoudie, A.S.(ed.).Encyclopedia of Geomorphology.Routledge. pp. 124–128.
  8. ^Fairbridge, 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.S2CID129231129.
  9. ^abGoudie, 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.
  10. ^Whipple, Kelin X. (2004). "Bedrock channel". InGoudie, A.S.(ed.).Encyclopedia of Geomorphology.Routledge. pp. 81–82.
  11. ^Spotila, James A. (2004). "Crustal deformation". InGoudie, A.S.(ed.).Encyclopedia of Geomorphology.Routledge. pp. 201–203.
  12. ^abcKoss, 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.
  13. ^Babault, 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.
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