Features from the Field: Angular Unconformities - EGU Blogs

An angular unconformity is an erosional surface that truncates older, tilted sedimentary layers and that is overlain by younger layers, ...  Search all blogs    Search this blog only  Search all blogs    Search this blog only EGUBlogs»Divisions»TectonicsandStructuralGeology»FeaturesfromtheField:AngularUnconformities TheworldwidefamousangularunconformityatSiccarPoint,betweenverticallayersofSiluriangraywacke(below)andgentlydippingDevonianlayersoftheOldRedSandstoneFm.(above).Photo©DaveSouzaviaWikimedia.commons Anangularunconformityisanerosionalsurfacethattruncatesolder,tiltedsedimentarylayersandthatisoverlainbyyoungerlayers,orientedparalleltotheunconformity.Thediscoveryandinterpretationofangularunconformities,likethefamousHutton’sunconformityatSiccarPoint,Scotland,markedaparadigmshiftinthegeologicaltheoriesofthe18thcentury. Atthattime,thetheoryofNeptunism,proposingrocksweretheresultofcrystallizationfromaprimordialocean,wascontraposedtoPlutonism:theideathatrocksoriginatedfromthecrystallizationofmagmaandsubsequentweathering,erosion,anddepositionassedimentarylayers.NeptunisttheoriesbelievedEarthformedafteracatastrophe,likethemythicalflooddescribedintheGenesis.Ontheotherhand,plutonistsbelievedEarthhadbeen,andwasbeingshapedbythesameprocessesthatoccurtoday(UniformitarianPrinciple). In1787-1788,JamesHutton,oneofthefathersofmoderngeology,examinedaseriesofangularunconformitiesinScotland,includingthatexposedatSiccarPoint.ThepresenceofsedimentaryfeatureslikeripplemarksintheverticallayersbeneaththeunconformityconvincedHuttonthattheverticallayerswereoriginallylyinghorizontallyontheseafloorandthatwerelatertilted,eroded,andagainsubmerged,withthedepositionofyoungerlayersontopoftheunconformity.In1788,Huttonpublishedabookwherehesummarizedhisobservationsas“wefindnovestigeofabeginning,noprospectofanend”.Withthosesimplewords,Huttonwasproposinganewandrevolutionaryidea–thattheworldwasnottheresultofacreationoracatastrophicevent. AngularunconformityatPraiadoTelheiro,Algarve,Portugal.Here,verticallayersofCarboniferoussandstoneandblackshalearetruncatedbyhorizontallayersofTriassicredsandstone.DeformationoftheCarboniferouslayersoccurredduringtheVariscanOrogeny.Photo©AndréCortesãoviaImaggeo. TheoriginalideasofHuttononangularunconformitiesdidnotchangemuchduringthelast250years,buttodaywehaveabetterunderstandingoftheprocessesthatshapetheEarth’sdeepinteriorandofhowthesespectacularfeaturesoriginate.Angularunconformitiesformwhenoriginal,horizontallayersaredeformed,exposedatthesurface,erodedaway,andthenoverlainbyfreshlydepositedlayers.Thiscycleusuallycoincideswithaperiodinwhichdepositioninabasinisinterruptedbyadeformationevent,liketheformationofamountainchain,andrequiresseveralmillionsofyearstooccur. Hutton’sunconformity,forinstance,markstheclosureofaPaleozoicocean,theIapetusOcean,andtheCaledonianOrogeny.Thetiltedlayersareroughly435millionyearsold,whilethesub-horizontallayers,whichdepositedafterthemountainswereerodedaway,are370millionyearsold.Amere65millionyears(whichisthetimethatseparatesusfromthedinosaurs–justtosay)ismissingfromthegeologicalrecord. AngularunconformityattheGrandCanyon,betweentheCambrianTontoGroup(above)andthePrecambrianGrandCanyonSupergroup(below).Thetimegaphereisatleast200millionyears.Photo©JamesSt.JohnviaFlickr. Angularunconformities,likeallotherunconformities,representmajorgapsinthesedimentaryrecord(i.e.aperiodoferosionandnodeposition)andtheirstudyisofgreatimportancetoreconstructmajortectoniceventsthatoccurredinthepast.Someunconformitiesmaybealsousefulforwater,oil,andmineralexploration,asthesesurfacesmaymarkhorizonsofincreasedordecreasedporosity,hencecontrolthemigrationoffluidsintherockmass. Referencesandfurtherreading. Hutton,J.(1788).TheoryoftheEarth;oraninvestigationoftheLawsobservableintheComposition,Dissolution,andRestorationofLandupontheGlobe.TransactionsoftheRoyalSocietyofEdinburgh.RoyalSocietyofEdinburgh1(Part2),209-304. ShanmugamG.(1988)Origin,Recognition,andImportanceofErosionalUnconformitiesinSedimentaryBasins.In:KleinspehnK.L.,PaolaC.(eds)NewPerspectivesinBasinAnalysis.FrontiersinSedimentaryGeology.Springer,NewYork,NY.https://doi.org/10.1007/978-1-4612-3788-4_5 VideobyBritishGeologicalSurvey–SiccarPoint:theBirthplaceofModernGeology. angularunconformitydeformationGeologysedimentarygeologysedimentarystructuresstratigraphyStructuralGeologyTectonicsunconformity BySamuelePapeschiSamuelePapeschiisapostdocatJAMSTEC(Kochi,Japan).Asaresearcher,hestudiesdeformationinrockscombiningfieldworkwiththeanalysisofrocksundertheopticalandelectronmicroscopes.Heconstantlyhearsthecallofthemountains,wherehelovestohikeandwork,takingsnapshotsofimpressiveFeaturesfromtheField. MindsoverMethods:Reconstructingoceanslosttosubduction March12,2018 TSMust-Read–Riba(1976)SyntectonicunconformitiesoftheAltoCardener,SpanishPyrenees:ageneticinterpretation December14,2020 Featuresfromthefield:crenulationcleavage April27,2020 FeaturesfromtheField:StretchingLineations June21,2021 3Comments GrahamCunningham October21,2020,8:14am SiccarPointtellsustheEarthisdynamic.Darwintolduslifeisdynamic.TwogreatestcontributionsofEarthscience? Reply RichardArrthurBScFGS October21,2020,9:50am anerosionalsurfacethattruncatesolder,tiltedsedimentarylayersandthatisoverlainbyyoungerlayers,orientedparalleltotheunconformity MyIrespectfullysuggestyoucorrectthisgrammaticallyandgeologicallytosay…. anerosionalsurfacethattruncatesolder,tiltedsedimentarylayers‘whicharethen’overlainbyyoungerlayers.Iftheseyoungerlayersweredepositeduponalevelunconformitysurfacetheybecomeparalleltothatsurfaceandthisiscalledanoverstepunconformity.Iftheyoungerlayersweredepositedonatiltedsurface.progressivelayerscanextendoverthemarginofanyprecedinglayersandthisiscalledanOverlapuncomformity Reply SamuelePapeschi October22,2020,8:18am DearRichard, thankyouforyourcomment.Asauthor,Itrytokeepthetextassimpleaspossible,sothatitisaccessibletoeveryone.Iwilltakeyourcommentintoaccountandtrytoimprovethetextinthatpart.Thankyouverymuch. 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