LandslidingandMassMovementsinGrandCanyonandTheirImplicationsforCanyonEvolutionJesseRobertson,KarlKarlstrom,PeterHuntoon,JohnWarme,RyanCrow,AndyDarling,LauraCrosseyWhatcanlandslidestellusaboutbedrockcanyonevolution?Toanswerthisquestion,weneedtounderstandwhere,when,andhowlandslidesoccuraswellastheeffectoflandslidedamsonmajorriversandcanyonwidening.Responseofcontinental-scaleriversystemstolandslidedamsdependsonstreampower,channelshape,landslidevolume,frequency,composition,andmechanism(s)ofemplacement.NewfieldandgeochronologystudiesoflandslidedepositsinGrandCanyonarebeginningtoprovidesomeanswers.Well-knownToreva-stylebedrockslumpsandslidesarepreservedinandaroundGrandCanyon.SomeexamplesnearGrandCanyonincludethoseattheVermillionandEchoCliffs(includingthereactivationofanolderlandslideonFebruary20,2013thataffectedHighway89)andCoconinoPointatGrayMountain.InGrandCanyon,notablelandslidesincludeCarbonButteineasternGrandCanyon,133-Mile,SurpriseValley,DeerCreek,205-Mile,andPeachSpringsCanyoninwesternGrandCanyon(seefigure).Otherlesswell-knowndepositscapNankoweapButteineasternGrandCanyonandFossilCreekincentralGrandCanyon.Innearlyallcases,failureoccurredonweakshalehorizonsliketheBrightAngelorHermitshales.Paradoxically,Toreva-stylelandslidesdonotoccureverywheretheselithologiesareexposed,whichsuggestsadditionalcontrols.Landslidesbetweenrivermiles(RM)130-140(includingSurpriseValley)arethedominantlandslidesinGrandCanyon.Theyrepresentthelargestvolumeoflandslidingthatrecordsmultiplegenerationsofeventsinasinglearea.Ourhypothesisisthat1)failuresarefirstlocalizedintheBrightAngelShaleorotherweakhorizonscausedbyeithera)seismictriggeringfromnearbyfaultsorb)increasedporefluidpressure,andthat2)ingeneral,majorcatastrophicslidesleadtosubsequentsmalleryetsignificantriverdammingslumpsandslides(similartorecentEchoCliffsevent).ThegoalofmyMaster’sresearchistoexaminetheinterplaybetweenthesemultipleprocessesthatmayhaveoperatedwithinthestudyareatoexplainthelocalizationoflandslides,pre-andpost-slideriverincisionratesandprocesses,andlandslidedamremoval.LandslidesbetweenRM130-140consistofseveraldistinctslidesofdiverseageandcharacter,someofwhichdammedtheColoradoRiver.Themajorslidesinclude1)133-Mile,2)SurpriseValley,CogswellButte,andsubsequentslides,3)Pancho’sRadicalRunup,and4)youngerreactivatedslidesoffCogswellButteandDeerCreek.133-Mileslidedetachedfromthesouthrim,includesTapeatsthroughRedwallstrata,anddammedtheColoradoRiverasevidencedbygravelsbothaboveandbelowslidedeposits.SurpriseValley(includingmultipleTorevablocksnorthoftheintersectionoftheThunderRiverandDeerCreekTrails)formedwhenCogswellButteslightlybackrotatedandtranslatedalongadetachmentthatunderliesitall.ThisdetachmentisbeautifullyexposednearThunderSpring.Pianoslide,asmallerslumpofftheover-steepenedsouthfaceofCogswellButte,filledaColoradoRiverpaleo-channelneartheGraniteNarrows(RM135R).Anotherslump(CogswellWest)fillstheoldestpreservedDeerCreekpaleo-channelandcanbereadilyobservedalongthepopularroutebetweenTapeatsandDeerCreeks.Pancho'sRadicalRunup,preserveddirectlyacrossfromDeerCreekFalls,involvedthrust-styleemplacementofa>1.5km-wide,>5m-thickslabofMuavthroughRedwallLimestones.Ridingona1-3mcushionofslopedebris,rivercobbles,andbrecciatedmaterial,landslidedebristraveledacrosstheriverand>200mupthesouthside.ThiseventdammedtheriverbelowDeerCreekandmayormaynothavebeendrivenbythemainSurpriseValley/CogswellButtelandslide.TheDeerCreekslidemassonriver-rightbelowDeerCreekFallsappearstobeayounger,smallerreactivationofthisinitialcatastrophicslide.ThiseventdivertedtheColoradoRivertothesouth–apaleo-riverchannelfilledwithDeerCreekslidedebrisispreservedafewmetersaboveriverlevelatRM137.2R.Thiseventdiverted

DeerCreektoitsmodernlocation,whereitthenincisedintotheTapeatsSandstone,carvingtheDeerCreekNarrowsandformingDeerCreekFalls.FieldrelationshipsandexistinggeochronologysuggestsaturationofshalestobethemaindriveroflandslidingintheSurpriseValleyarea.Differentheightsofrivergravelsburiedbeneaththedifferentslides(133-Mile,SurpriseValley,DeerCreek),insetrelationshipsofyoungerslideswithinolderones,andnoevidenceforalignmentalongafault,allargueagainsttheseismicinitiationhypothesis.Themodernpresenceofhigh-volumespringsnearSurpriseValley(i.e.Dutton,Thunder,andTapeatsSprings)suggestsgroundwatermayhaveplayedaroleinweakeningtheshalesandpromotedsliding.Additionalgeochronologyisunderwaytotestthesehypotheses;however,preliminarycosmogenicburialagedatesrecordingthetimeColoradoRivergravelsbecameburiedbeneathlandslidedebrisinpaleo-channels70and61maboveriverlevelgiveagesof940±240ka(1σ)and978±287ka(1σ),respectively.TheseagesprovidethemostdirectgeochronologytodateontheSurpriseValley-andPancho’sRadicalRunup-relatedlandslides.TheagreementofthetwodatesindifferentlocationsataboutthesameheightaboveriverlevelisinterpretedtogivetheapproximateageofboththemainSurpriseValley(CogswellButte)detachmentandPancho’sRadicalRunup.TheDeerCreekslidesmaybeasyoungasthe~21kaageonproposedlandslidedamlakedepositsatRM134.5L(Webbetal.,2005).

DigitalelevationmodelshowinglocationsofnotablelandslidesinandaroundGrandCanyon.Ingeneral,landslidesoccurredalongtherivercorridoraswellasnearmajorfaultsandfolds.MultipleeventstogetherrepresentingthelargestlandslidevolumeinGrandCanyonoccurrednearSurpriseValley.