Unconformity - Wikipedia

An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment ... Unconformity FromWikipedia,thefreeencyclopedia Jumptonavigation Jumptosearch Rocksurfaceindicatingagapinthegeologicalrecord Hutton'sUnconformityatJedburgh,Scotland,illustratedbyJohnClerkin1787andphotographedin2003. Anunconformityisaburiederosionalornon-depositionalsurfaceseparatingtworockmassesorstrataofdifferentages,indicatingthatsedimentdepositionwasnotcontinuous.Ingeneral,theolderlayerwasexposedtoerosionforanintervaloftimebeforedepositionoftheyoungerlayer,butthetermisusedtodescribeanybreakinthesedimentarygeologicrecord.Thesignificanceofangularunconformity(seebelow)wasshownbyJamesHutton,whofoundexamplesofHutton'sUnconformityatJedburghin1787andatSiccarPointin1788.[1][2] Therocksaboveanunconformityareyoungerthantherocksbeneath(unlessthesequencehasbeenoverturned).Anunconformityrepresentstimeduringwhichnosedimentswerepreservedinaregionorweresubsequentlyerodedbeforethenextdeposition.Thelocalrecordforthattimeintervalismissingandgeologistsmustuseothercluestodiscoverthatpartofthegeologichistoryofthatarea.Theintervalofgeologictimenotrepresentediscalledahiatus.Itisakindofrelativedating. Contents 1Types 1.1Disconformity 1.2Nonconformity 1.3Angularunconformity 1.4Paraconformity 1.5Buttressunconformity 1.6Blendedunconformity 2Gallery 3References 4Furtherreading Types[edit] Disconformity[edit] Disconformity Adisconformityisanunconformitybetweenparallellayersofsedimentaryrockswhichrepresentsaperiodoferosionornon-deposition.[3]Disconformitiesaremarkedbyfeaturesofsubaerialerosion.Thistypeoferosioncanleavechannelsandpaleosolsintherockrecord.[4] Nonconformity[edit] Nonconformity Anonconformityexistsbetweensedimentaryrocksandmetamorphicorigneousrockswhenthesedimentaryrockliesaboveandwasdepositedonthepre-existinganderodedmetamorphicorigneousrock.Namely,iftherockbelowthebreakisigneousorhaslostitsbeddingduetometamorphism,theplaneofjunctureisanonconformity.[5] Angularunconformity[edit] Angularunconformity Anangularunconformityisanunconformitywherehorizontallyparallelstrataofsedimentaryrockaredepositedontiltedanderodedlayers,producinganangulardiscordancewiththeoverlyinghorizontallayers.Thewholesequencemaylaterbedeformedandtiltedbyfurtherorogenicactivity.AtypicalcasehistoryispresentedbythepaleotectonicevolutionoftheBriançonnaisrealm(SwissandFrenchPrealps)duringtheJurassic.[6][7] Paraconformity[edit] Paraconformity Aparaconformityisatypeofunconformityinwhichthesedimentarylayersaboveandbelowtheunconformityareparallel,butthereisnoobviouserosionalbreakbetweenthem.Abreakinsedimentationisindicated,forexample,byfossilevidence.Itisalsocallednondepositionalunconformityorpseudoconformity.[8][9]Shortparaconformitiesarecalleddiastems. Buttressunconformity[edit] Abuttressunconformityoccurswhenyoungerbeddingisdepositedagainstolderstratathusinfluencingitsbeddingstructure.[10] Blendedunconformity[edit] Ablendedunconformityisatypeofdisconformityornonconformitywithnodistinctseparationplaneorcontact,sometimesconsistingofsoils,paleosols,orbedsofpebblesderivedfromtheunderlyingrock.[11] Gallery[edit] TheexamplesandperspectiveinthissectiondealprimarilywithEnglish-speakingcountriesanddonotrepresentaworldwideviewofthesubject.Youmayimprovethissection,discusstheissueonthetalkpage,orcreateanewsection,asappropriate.(October2015)(Learnhowandwhentoremovethistemplatemessage) Thereisabillion-yeargapinthegeologicrecordwherethis500-million-year-olddolomitenonconformablyoverlies1.5-billion-year-oldrhyolite,nearTaumSaukHydroelectricPowerStation,Missouri. TheTaconicUnconformitynearCatskill,NewYork.ThisangularunconformityseparatestheAustinGlenFormation(Ordovician)fromtheoverlyingRondoutFormation(Silurian)andManliusFormation(Devonian).[citationneeded] EemiandisconformityinafossilcoralreefonGreatInagua,TheBahamas.Foregroundshowscoralstruncatedbyerosion;behindthegeologistisapost-erosioncoralpillarwhichgrewonthedisconformityaftersealevelroseagain. Hutton'sangularunconformityatSiccarPointwhere345-million-year-oldDevonianOldRedSandstoneoverlies425-million-year-oldSiluriangreywacke[12] DisconformitywiththeLowerCretaceousEdwardsFormationoverlyingaLowerPermianlimestone;hiatusisabout165millionyears;Texas. NonconformitybetweenthePennsylvanianFountainFormation(left)andPrecambriangneiss(right)atRedRocksPark,nearGolden,Colorado Disconformity(atthehammer)betweenunderlyingMississippianBordenFormationandoverlyingPennsylvanianSharonConglomerate,nearJackson,Ohio AngularunconformityofTriassicrocksoverlyingsteeply-tiltedCarboniferousrocksatPraiadoTelheiro,Portugal AngularunconformitybetweentheunderlyingDockumGroupandtheoverlyingExeterSandstoneatSteamboatButteinthevalleyoftheDryCimmarron,NewMexico References[edit] ^Hutton'sUnconformityArchived2015-09-24attheWaybackMachine ^KeithMontgomery(2003)."SiccarPointandTeachingtheHistoryofGeology"(PDF).UniversityofWisconsin.Retrieved2015-03-16. ^MonroeJ.S.,WicanderR.&HazlettR.W.(2007).Physicalgeology:exploringtheEarth.CengageLearning.p. 267.ISBN 9780495011484. ^"Differencebetweendisconformityandunconformity :Sedimentology-Exploration&ProductionGeology".www.epgeology.com.Retrieved2021-11-26. ^Stokes,W.Lee(1982).EssentialsofEarthHistory4thEdition.PrenticeHall,Inc.p. 65.ISBN 0-13-285890-8. ^SeptfontaineM.(1984):LeDoggerdesPréalpesmédianessuissesetfrançaises-stratigraphie,évolutionpaléogéographiqueetpaléotectonique.-Mém.Soc.Helv.Sci.Nat.,vol.97,121p.(Birkhäuseréd.) ^SeptfontaineM.(1995):LargescaleprogressiveunconformitiesinJurassicstrataofthePrealpsSouthoflakeGeneva:interpretationassynsedimentaryinversionstructures.Paleotectonicimplications.Eclogaegeol.Helv.,88:3553–576. ^CatuneanuO.(2006).PrinciplesofSequenceStratigraphy.Elsevier.p. 15.ISBN 9780080473987. ^EncyclopediaofScienceandTechnology:AnInternationalReferenceWork,Volume14.McGraw-Hill.1966.p. 192. ^GroshongR.H.(2013).3-DStructuralGeology:APracticalGuidetoSurfaceandSubsurfaceMap.SpringerScience&BusinessMedia.ISBN 9783662039120. ^NeuendorfK.K.E.(2005).GlossaryofGeology.p. 73.ISBN 9780922152766. ^CliffFord(2September2003)."SiccarPoint".FieldExcursionPreview.UniversityofEdinburghSchoolofGeoSciences.Retrieved2008-10-20. TheWikibookHistoricalGeologyhasapageonthetopicof:Unconformities WikimediaCommonshasmediarelatedtoUnconformities. Furtherreading[edit] U.S.BureauofMinesDictionaryofMining,Mineral,andRelatedTermspublishedonCD-ROMin1996. 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