Unconformities - Geology - Cliffs Notes

An unconformity is a contact between two rock units in which the upper unit is usually much younger than the lower unit. Unconformities are typically buried ... MyPreferences MyReadingList Geology ! Home StudyGuides Geology Unconformities AllSubjects IntroductiontoPhysicalGeology TheEarth'sExterior GeologicTime TheEarthToday HistoryofPhysicalGeology TheEarth'sOrigin TheEarth'sStructure TheEarth'sComponents MineralProperties TheRockCycle ChemicalComposition MineralsandRocks IgneousRocks ExtrusiveRockTypes RockTextures IntrusiveRockTypes IntrusiveStructures HowDifferentMagmasForm IgneousRocksandPlateTectonics MagmaticDifferentiation VolcanoesandLavas SedimentaryRocks ClasticSedimentaryRocks ChemicalSedimentaryRocks OrganicSedimentaryRocks SedimentaryFeatures SedimentaryEnvironments HowSedimentaryRocksForm MetamorphicRocks FactorsControllingMetamorphism TypesofMetamorphism MetamorphicRockTypes HydrothermalRocks MetamorphismandPlateTectonics MetamorphismDefined GeologicStructures InterpretingStructures MappingintheField Folding Fracturing Unconformities GeologicStructuresDefined TectonicForces WeatheringandErosion ProcessesofMechanicalWeathering ProcessesofChemicalWeathering Soil Weathering MassWasting PreventionofMassWasting IntroductiontoMassWasting Mass‐WastingControls TypesofMassWasting RunningWater StreamDynamics StreamErosion SedimentLoad StreamDeposition StreamValleys RegionalErosion TypesofWaterFlow GlaciersandGlaciation HowGlaciersDevelop GlacierMovement GlacialErosion GlacialLandforms GlacialDeposits GlaciersinthePast IntroductiontoGlaciation TypesofGlaciers NorthAmericanGlaciation Groundwater Permeability TheWaterTable StreamsandSprings EffectsofGroundwaterFlow GroundwaterPollution GeothermalEnergy GroundwaterandInfiltration Porosity TheShorelines Beaches ShorelineFeatures Tides Waves DesertsandWinds DesertFeatures TheEffectsofWind DistributionandCausesofDeserts Earthquakes MonitoringEarthquakes EffectsofEarthquakes EarthquakesandPlateTectonics ControlandPrediction HowEarthquakesForm SeismicWaves InsidetheEarth TheCrust TheMantle IsostaticEquilibrium TheCore MagneticFields GeophysicsDefined Gravity SeismicWaves:MethodsofDetection TheStructureoftheEarth GeothermalGradients TheOceanFloor ContinentalMargins OceanFloorSediments ActiveContinentalMargins PassiveContinentalMargins Reefs InvestigativeTechnologies MidoceanicRidges OceanicCrust PlateTectonics HowPlatesMove TypesofPlateBoundaries WhyPlatesMove MantlePlumes Pangaea EarlyEvidenceforPlateTectonics PaleomagneticEvidence SeaFloorEvidence Mountain‐Building FeaturesofMountainBelts TypesofMountains HowMountainsForm HowContinentsForm IntroductiontoMountains GeologicTime GeologicCorrelations AbsoluteAge ASummaryofEarth'sHistory GeologicTimeDefined RelativeTime EarthResourcesWeDependOn MetallicDeposits EnergyResources NonmetallicResources RecyclingandConservation ResourcesandReserves OurSolarSystem Earth'sMoon Mars JupiterandSaturn IntroductiontotheSolarSystem Mercury Venus Unconformities An unconformityisacontactbetweentworockunitsinwhichtheupperunitisusuallymuchyoungerthanthelowerunit.Unconformitiesaretypicallyburiederosionalsurfacesthatcanrepresentabreakinthegeologicrecordofhundredsofmillionsofyearsormore.Forexample,thecontactbetweena400‐million‐year‐oldsandstonethatwasdepositedbyarisingseaonaweatheredbedrocksurfacethatis600millionyearsoldisanunconformitythatrepresentsa timehiatusof200millionyears.Thesedimentand/orrockthatwasdepositeddirectlyonthebedrockduringthat200‐million‐yearspanwaserodedaway,leavingthe“basement”surfaceexposed.Therearethreekindsofunconformities:disconformities,nonconformities,andangularunconformities. Disconformities.Disconformities(Figure1)areusuallyerosionalcontactsthatareparalleltothebeddingplanesoftheupperandlowerrockunits.Sincedisconformitiesarehardtorecognizeinalayeredsedimentaryrocksequence,theyareoftendiscoveredwhenthefossilsintheupperandlowerrockunitsarestudied.Agapinthefossilrecordindicatesagapinthedepositionalrecord,andthelengthoftimethedisconformityrepresentscanbecalculated.Disconformitiesareusuallyaresultoferosionbutcanoccasionallyrepresentperiodsofnondeposition. Figure1 ADisconformity Nonconformities.Anonconformity(Figure2)isthecontactthatseparatesayoungersedimentaryrockunitfromanigneousintrusiverockormetamorphicrockunit.Anonconformitysuggeststhataperiodoflong‐termuplift,weathering,anderosionoccurredtoexposetheolder,deeperrockatthesurfacebeforeitwasfinallyburiedbytheyoungerrocksaboveit.Anonconformityistheolderosionalsurfaceontheunderlyingrock. Figure2 ANonconformity Angularunconformities.Anangularunconformity(Figure)isthecontactthatseparatesayounger,gentlydippingrockunitfromolderunderlyingrocksthataretiltedordeformedlayeredrock.Thecontactismoreobviousthanadisconformitybecausetherockunitsarenotparallelandatfirstappearcross‐cutting.Angularunconformitiesgenerallyrepresentalongertimehiatusthandodisconformitiesbecausetheunderlyingrockhadusuallybeenmetamorphosed,uplifted,anderodedbeforetheupperrockunitwasdeposited. Figure3 AnAngularUnconformity Previous Fracturing Next GeologicStructuresDefined TheEarth'sExterior GeologicTime TheEarthToday HistoryofPhysicalGeology TheEarth'sOrigin TheEarth'sStructure MineralProperties TheRockCycle ChemicalComposition MineralsandRocks ExtrusiveRockTypes RockTextures IntrusiveRockTypes IntrusiveStructures HowDifferentMagmasForm IgneousRocksandPlateTectonics MagmaticDifferentiation VolcanoesandLavas ClasticSedimentaryRocks ChemicalSedimentaryRocks OrganicSedimentaryRocks SedimentaryFeatures SedimentaryEnvironments HowSedimentaryRocksForm FactorsControllingMetamorphism TypesofMetamorphism MetamorphicRockTypes HydrothermalRocks MetamorphismandPlateTectonics MetamorphismDefined InterpretingStructures MappingintheField Folding Fracturing Unconformities GeologicStructuresDefined TectonicForces ProcessesofMechanicalWeathering ProcessesofChemicalWeathering Soil Weathering PreventionofMassWasting IntroductiontoMassWasting Mass‐WastingControls TypesofMassWasting StreamDynamics StreamErosion SedimentLoad StreamDeposition StreamValleys RegionalErosion TypesofWaterFlow HowGlaciersDevelop GlacierMovement GlacialErosion GlacialLandforms GlacialDeposits GlaciersinthePast IntroductiontoGlaciation TypesofGlaciers NorthAmericanGlaciation Permeability TheWaterTable StreamsandSprings EffectsofGroundwaterFlow GroundwaterPollution GeothermalEnergy GroundwaterandInfiltration Porosity Beaches ShorelineFeatures Tides Waves DesertFeatures TheEffectsofWind DistributionandCausesofDeserts MonitoringEarthquakes EffectsofEarthquakes EarthquakesandPlateTectonics ControlandPrediction HowEarthquakesForm SeismicWaves TheCrust TheMantle IsostaticEquilibrium TheCore MagneticFields GeophysicsDefined Gravity SeismicWaves:MethodsofDetection TheStructureoftheEarth GeothermalGradients ContinentalMargins OceanFloorSediments ActiveContinentalMargins PassiveContinentalMargins Reefs InvestigativeTechnologies MidoceanicRidges OceanicCrust HowPlatesMove TypesofPlateBoundaries WhyPlatesMove MantlePlumes Pangaea EarlyEvidenceforPlateTectonics PaleomagneticEvidence SeaFloorEvidence FeaturesofMountainBelts TypesofMountains HowMountainsForm HowContinentsForm IntroductiontoMountains GeologicCorrelations AbsoluteAge ASummaryofEarth'sHistory GeologicTimeDefined RelativeTime MetallicDeposits EnergyResources NonmetallicResources RecyclingandConservation ResourcesandReserves Earth'sMoon Mars JupiterandSaturn IntroductiontotheSolarSystem Mercury Venus × BacktoTop AdamBede hasbeenaddedtoyour ReadingList! 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