The geOLOgy Of The Bras d’Or Lakes, nOva scOTIa*

The geOLOgy Of The Bras d’Or Lakes, nOva scOTIa*

J.SHAW1,D.J.W.PIPER,andR.B.TAYLORGeological Survey of Canada Atlantic

Bedford Institute of OceanographyBox 1006, Dartmouth, Nova Scotia, B2Y 4A2

TheevolutionoftheBrasd’OrLakessincetheretreatofthelasticesheetsc.15ka(thousandsofradiocarbonyearsbeforepresent,wherepresentisdefinedas1950)isinferredfrommultibeambathymetry,seismicreflectionprofiles,andsedimentcores.ThethicknessofstratifiedsedimentintheLakesoverlyingglacialtillshowsthattherewasastep-likeretreatoficetowardsalateicecentreinthewesternpartoftheBrasd’OrLakes.Asiceretreated,alakeformedintheareaofthemodernBrasd’OrLakesandprobablydrainedthroughLittleBrasd’OrChannel.Iceretreatandsealevelchangeonthecontinentalshelfoffsouth-easternCapeBretonareinferredfrommultibeambathymetrythatshowsproglacialsubaerialriverchannelsandsuggeststhatsealevelwasperhaps50mlowerthanpresentabout15ka.Relativesealevelappearstohaverisensubsequently,sothatmarineconditionsexistedinBrasd’OrLakesbasinat10to9ka.Sealevelmayhaverisento-15m(belowmodernsealevel)beforefallingagainintheearlyHolocene.ThisfallingearlyHolocenerelativesealevelresultedinthecreationoffreshwaterlakes,withaprominenterosionsurfaceat-25mmarkingthelakelevelinsomeareas.RisingsealevelthenresultedinareturntomarineconditionsintheLakesby4to5ka.

L’évolutiondeslacsBrasd’Ordepuisleretraitdesdernièresnappesglaciairesilya15000ansserévèleparlabathymétriemultifaisceaux,lesprofilsderéflexionsismiqueetlescarottesdesédiments.L’épaisseurdessédimentsstratifiésdansletillsus-jacentdeslacsdémontrequ’ilyaeuunretraitenescaliersdesglacesversuncentrefini-glaciairesituédanslapartieoccidentaledeslacsBrasd’Or.Leseaux,libéréeslorsduretraitdesglaces,s’échappèrentprobablementvialecanalduLittleBrasd’OrpourformerunlacdanslelitactueldeslacsBrasd’Or.Leretraitdesglacesetleschangementsduniveaudelamersurlaplateformecontinentaleausud-estdeCap-Bretonsontmisenévidenceparlabathymétriemultifaisceaux,quimontredeslitsderivièresub-aériensproglaciairesetindiquequeleniveaudelamersetrouvaitpeut-êtreà50mplusbasqu’aujourd’huiilyaenviron15millesd’années.Lahausseduniveaudelamerdepuiscetteépoqueaprovoquél’inondationdeslacsBrasd’Orancestrauxilyade9à10millesd’années,etleniveaudeseauxauraitatteint-15mavantdechuteraudébutdel’Holocène.Cettechuterelativedudébutdel’Holocènearésultéenlacréationdelacspourunesecondefois,avecuneimportantesurfaced’érosionà-25mquimarqueleniveaudeseauxdanscertaineszones.Ceslacsontétéfinalementinondésparlamerilyade4à5millesd’années.

Introduction

TheBrasd’OrLakesareaseriesoflow-salinitylakesfillingdeepdepressionsincentralCapeBretonIsland(Fig1).ThelakesystemconsistsofBrasd’OrLakeinthesouth(Fig2),whichincludesWestBay,EastBay,DenysBasin,andSt.Peter’sInlet.BarraStraitprovidestheconnectiontothenorthernpartoftheLakes,whichincludetheGreatBrasd’OrChannel,St.Andrew’sChannel,St.Patrick’sChannel,LittleBrasd’OrChannel,andWhycocomaghBay.TheBrasd’OrLakesareconnectedtotheAtlanticOceanatthreelocations:GreatBrasd’OrChannel,withaminimumdepthofabout8m;LittleBrasd’OrChannel,a6-m-deepsinuousestuary;andthecanalandboatlocksatSt.Peter’sInlet.Inthispaper,wepresentandsynthesizeinformationonthegeologicalhistoryofthe

Lakes,focusingonthepasttentofifteenthousandyears,duringwhichperiodthelast

PROC.N.S.INST.SCI.(2002)Volume42,Part1,pp.127-147.

*GeologicalSurveyofCanadacontribution2001110.1Authortowhomcorrespondenceshouldbeaddressed

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Fig 1 MapofCapeBreton,showinglocationoftheBrasd’OrLakesandthebedrockgeologyofCapeBretonIsland,NovaScotia(simplifiedfromKeppie,2000).

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Fig 2 Mapshowingship’stracksforcruise96-155(CCGSHart)andtracksandcoresitesforcruise85-036(CSSDawson).Cores16and18(discussedinthetext)arenumbered.Themapalsoshowslocationsofseismicprofilesillustratedinfigures.ThebathymetriccontoursarefromCanadianHydrographicServicefieldsheetssummarizedinCanadianHydrographicService(1990,1991,1993).

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icesheetsretreatedfromCapeBretonIsland.OurinformationdealingwiththemarinegeologyoftheBrasd’OrLakescomesprimarilyfromseismicsurveysandsedimentcorescollectedfromCSSDawsonin1985(cruise85-036)andtheCCGSJ.L.Hartin1996(cruise96-155),togetherwithinformationfrommultibeambathymetrysurveysonthecontinentalshelfoffsoutheasternCapeBretonIsland.

Bedrock geology Thelakesandlakebasinsarelocalisedover‘weakrocklowlands’(seesummaryinGrant,1994).Widespreadriftingandregionaltectonicplatemove-mentsinsouthernCapeBretonIsland,attheendoftheDevonianperiodsome360

Fig 3 GeologicaltimelineshowingevolutionofthebedrockgeologyandthephasesofglaciationaroundtheBrasd’OrLakes.(ModifiedfromRoland1982andGrant1994).

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131BRAS D'OR LAKES GEOLOGY

millionyearsago,formedaseriesofsmall,fault-boundedbasinsbetweenhighlandsofresistantcrystallinerocks(Calder,1998).Gravelandsandweredepositedasalluvialfansattheedgesofbasinsandinriversystemsandlakesonthebasinfloors.Lithifi-cationofthesesedimentsproducedtheresistantconglomeratesandsandstonesoftheHortonGroup(Fig1).Circa340millionyearsagotheseafloodedmanylowareasinAtlanticCanada,includingthebasinsofsouthernCapeBretonIsland,depositingathicksequenceoffinegrainedmudsandevaporites,includinggypsum,anhydrite,andsalt(Fig3).TheWindsorGrouprocks,astheyareknown,underliemostoftheBrasd’OrLakes.Youngercoal-bearingrocksoftheSydneyBasinweresubsequentlydepositedovermuchofsouthernCapeBretonIsland.CapeBretonIslandhasbeenslowlyupliftedinthepast230millionyears,largely

asaresultofopeningofthemodernAtlanticOcean.MostoftheoriginalsedimentaryrocksofCarboniferousagehavebeenremovedbygradualerosionoverthisperiod(RyanandZentilli,1993),butHortonandWindsorGrouprocksarestillpreservedintheiroriginalbasinsaroundtheBrasd’OrLakes.TheWindsorGrouprocksarepar-ticularlysoftandthuseasilyeroded.ThiswasparticularlythecaseduringtheTertiary,whenriversdeepenedtheircoursesandBrasd’OrLakeandGreatBrasd’OrChannelwerecarvedoutbetweentheuplandsofmoreresistantcrystallinerocks.TheresistantblocksformplateauswithelevationsthatincreasefromsoutheasttonorthwestacrossCapeBretonIsland.OutcropsofWindsorrocksareobservedtodayscatteredthroughtheBrasd’OrLakes

basin,someasdistinctivewhiteshorecliffs.Thedissolutionofevaporitesproducesdistinctivekarst topography in theBrasd’OrLakesbasin. Forexample,sinkholesoccurincoastalareasnearBarraStrait,andarealsovisibleinunpublishedmultibeambathymetryimageryfromthere.Thepresenceofthesedepositsmayalsoberesponsi-blefortheextremelydeepwatersobservedinSt.Andrew’sChannel.Boehner(1985)discoveredthroughdrillingonBoularderieIslandthatthedeepareascoincidewithprojectedoutcropsofrocksaltatthebaseoftheWindsorGroup.Thesedepositswouldhavedissolvedquicklyiffoldingandfracturingallowedgroundwatertocontactthesalt.Fulldevelopmentofthedeep-waterdepressionsmayhaveoccurredinrecentgeologictimes;thehistoryofthesefeaturesisnotclear.

Regional Quaternary geology Twotothreemillionyearsago,sealevelinsouthernCapeBretonIslandwasprobably25to50mhigherthannow,whichwouldhavefloodedtheBrasd’OrLakesbasin(Roland,1982);RolandalsosuggestedthatthecoastalshelvesstretchingnorthwardfromSt.Ann’sBayalongtheeasterncoastofCapeBretonIsland,andfromGreatBrasd’OrChanneltoNyanzaBay,aretheprod-uctsofthesehigherseas.TheclimateprogressivelydeterioratedduringthefollowingQuaternaryperiod,withhighlandglaciersfirstformingperhaps1.6millionyearsagoandperiods ofwidespread continental glaciation starting about 0.5million yearsago(Piperetal.,1994).Betweenglacialperiods,eachofwhichlastedmanytensofthousandsofyears,wereinterglacialperiodswithclimatessimilartotoday.Duringthelastinterglacialperiod,theSangamonInterglacialabout125,000yearsBP,sealevelwas2-7mhigher,formingafossilintertidalwavecutbenchalongmanyAtlanticshoresandthenorthshoreofEastBay(Grant,1994).Thelocationoftheinterglacialpaleoshorelinewouldnothavedifferedmuchfromthepresentshorealongsteeperuplandareas,butwouldhavereachedseveralkilometresinlandalonglowlandareas.LakeAinslieBasin,MargareeValleyandpossiblyMabouEstuary(Fig1)werearmsofthesea,whichmayhaveconnectedtotheBrasd’OrBasin(Grant1994).TheBrasd’OrBasinwouldhavebeenlargerastheborderinglowlandswerefloodedandtheadjacentuplands,suchastheBoisdaleHills(Fig2),mayhavebeenislands.Twoadditional

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marineconnectionsatSt.Peter’sandthroughEastBaytoSydneymayhaveexisted,andtheLittleBrasd’OrChannelwaswider(Grant,1994).Onland,most fresherosional featuresandglacialdepositswere formedduring

thelast(Wisconsinan)glaciation,fromabout75to10ka(1ka=1000years).EightphasesofglaciationaredistinguishedbyGrant(1994)onthebasisofstudiesonland,althoughthereisdebateoverthedetailsandthetiming,particularlyof theearlierphases(e.g.,Steaetal.1998).DuringPhaseA(62to47ka)localicespreadintothelowlandsfromtheicecaponthehighlands.Thereremainslittledepositionalorerosionalevidenceofthisevent.PhaseBwasamajorglacialeventduringwhichforeignicefromthewestspread

eastwardacrossCapeBretonIsland.ThedirectionoficemovementcoincidedwiththeorientationofvalleysintheBrasd’OrBasin,facilitatingthescouringoftheweakunderlyingrocks.TheiceflowisalsothoughttohaveblockeddrainageasitadvancedintoEastBay,changingthesaltwaterarmintoaproglaciallakecalledLakeCameronabout62ka.ThetimingisbasedonpollenstudiesofthenearbyCastleBaysilts,de-posited20mabovemodernsealevel(deVernalandMott,1986;deVernaletal.1986).GlacialPhaseCinvolvedregionalicesheetsflowingsoutheastoverCapeBreton

IslandbutthereislittleevidenceofthisphaseintheBrasd’OrBasin.Itissuggestedtheregionaliceoverrodeapre-existingicemass,whichcoveredtheareaduringtheearlypartofphaseC.IndirectevidenceofclimaticwarmingandaglacialrecessionbetweenicephasesCandDisprovidedbytheoccurrenceofshellandwoodsampleswhichvariedinagefrom47to32ka,andamastodonfemursampledfromafarmalongMiddleRiverwhichyieldedagesaround32ka(Grant,1994).Glacial ice inphaseDflowednorth across the southern lowlands froman ice

centresouthofCapeBretonIsland.Theicewasresponsibleforreshapingdrumlinfieldsinthesouthernlowlandandfordepositingthedistinctivebrightredfossiliferoustillcontainingmarineshell fragments. DuringphaseDtheremayalsohavebeenicespreadingoutwardfromthehighlandsandintersectingthemainnorthwardflowofice.Grant(1994)arguedthatphaseDwasofMid-Wisconsinanage,butmorerecentlySteaetal.(1998)putastrongcasethatthecoevalnorthwardflowinNovaScotiawasLateWisconsinanage.Theimplicationisthatallthesubsequentphasesdescribedhereareyounger.PhaseEwastheestablishmentofanislandcentredicecapfrom15to13ka(Fig

4),whichisrecordedbysoutheasttrendingstriationsanddriftsouthoftheBrasd’OrLakes.Thereisalsoapatchyveneeroflocallyderivedstonytillfromthecrystallineuplands.TheageofthisphasegiveninGrant(1994)wasbasedonSteaetal.(1992)estimates.Presumablyitwasthemeltwaterfromthisicemassthatcutchannelssev-eralkilometresoffeasternCapeBretontomodernwaterdepthsofroughly40-50m.AstheinlandiceshrankintotheBrasd’OrLakesbasin,iceinPhaseFimpingedon

theGulfofSt.Lawrenceshore,dammingseveralvalleys(includingAinslieandMarga-ree)withkamemoraines(Grant1994).GrantarguedthatphaseFpredated10-11ka.PhaseGsawthecontinuedretreatofBrasd’Oriceintothebasinasrecordedby

ice-marginalmorainesandmeltwaterchannels(Fig4).ThisiceleftmorainesnorthofWhycocomaghandnorthofNyanzaBayalongthenorthernbasinandatsomestagebuiltthelargesubmergedridgeacrossSt.Ann’sBayandamoraineatNewCampbelltonalongtheGreatBrasd’OrChannel(Grant1994).Astheicemassshrankandpossiblyre-advancedintheperiod11-10ka,aseriesoficedammedlakesformedinthewest,includingDenysBasin.SurfaceorganicsdatedatanumberofsitessuggestedtoGrant

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Fig 4 ThefinalglacialeventsintheBrasd’OrLakesarea,adaptedfromFig114ofGrant(1994).PhaseEistheestablishmentofanisland-centredicecap.InphaseF,GulfofSt.Lawrenceiceimpingesonthewestcoast.PhaseGisconcentricretreatalflowandlocaladvanceoflowlandice.PhaseHrepresentsradialflowandconcentricretreatofthehighlandsremnanticecap.Areasbetweenmodernsealevelandthe-50misobathhaveanintermediateshading;theseareaswereemergentduringstageG.

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thaticehadretreatedtotheconfinesoftheBrasd’OrLakesbyca.10ka(MottandStea,1993;Miller1997).PhaseHinvolvedalocalindependenticecaponthehighlandsthatdidnotimpacttheBrasd’OrLakesdirectlybutmayhaveproducedvalleyglacierswhichextendedsomedistancedowntheMiddleandBaddeckValleys.Thereisaproblemreconcilingthescenariooutlinedabovewiththemorerecent

workofSteaetal.(1998)andSteaandMott(1998)whodateGrant’sPhaseEat12.5–11.0ka.SectionswhereglaciolacustrineclayoverliespeatconclusivelyshowthatglaciallakedevelopmentfromGulficepostdated10.8ka,suggestingthatPhaseFcouldbeequatedwiththeYoungerDryasperiod(c.11-10ka).Infact,fortheperiod11-10ka,Gulficewasimpingingonthewestcoast,asmallcapoficeexistedinsouthwestCapeBreton,andalargerresidualcapontheCapeBretonHighlands.Itislikelytherefore,thattheeventsinGrant’sphasesE-Goccurredintheperiod12.5–10ka.PhaseH–theicecapinthehighlands–mayhavebeenlater.

Quaternary deposits of the Bras d’Or Lakes

SeismicreflectionprofilesandpistoncoresfromtheBrasd’OrLakescollectedoncruise85-036(CSSDawson,Fig2)provideinformationonthelateQuaternaryhistoryoftheLakes.ThelongestcoresfromtheLakespenetrateonlytheHolocenepostglacialsequence(last104years),sothatthecharacterofPleistoceneglacialandlateglacial

Fig 5 Linedrawingofairgunprofile(a)alongSt.Andrew’sChanneland(b)attheeasternendofWestBay,showingdistributionofbedrock(wherevisible),tillandyoungersediments.RefertoFig2(inset)forlinelocation.

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sedimentsmustbeinferredfromseismicprofilesbycomparisonwithbetter-knowndepositsonthecontinentalshelf(KingandFader,1986).ThesequenceofacousticunitsrecognizedintheBrasd’OrLakes(Lynch1995)issummarizedinTableIandinFig5.

Table I SeismostratigraphicsequenceintheBrasd’OrLakes(modifiedfromLynch,1995).

Unit4.Surficialmuds,Holoceneage. Facies1A-stratified(nearsurface) Facies1B-amorphoustransparent(St.Andrew’sChannel&St.Patrick’sChannel) Facies1C-weaklystratified

Unit3.Proglacialsediments(EmeraldSiltequivalentonScotianShelf)

Unit2.Incoherenttoweaklystratified. Facies3a-thicksequencefillingvalleys,withpositivesurfacerelief Facies3b-thindrapeovertill

Unit1 Glacialtill

Bedrock

Unit1overliesbedrockovermostoftheflooroftheBrasd’OrLakes(e.g.,Figs.5a,6d),butisabsentinnorthernGreatBrasd’OrChannel.AirgunseismicprofilesshowthatUnit1istypically30mthickincentralBrasd’OrLake(Fig5b),withgreaterthicknesseswheredrumlinsaredeveloped(Fig6c). DrumlinsarealsorecognisedinEastBay(Fig6e).InSt.Andrew’sChannel,Unit1is30-40mthickonthedeepchannelfloor(Fig5a).Unit1isinterpretedasglacialdiamict(ortill).Unit2(TableI)consistsofsedimentwithweakacousticstratificationthatforms

adrapeafewmetresthickoverpartsoftheUnit1(Fig6e)andresemblesFaciesCofEmeraldSiltdescribedbyKingandFader(1986)fromtheScotianShelf.InsomebasinsUnit2istensofmetresthick,withpositivesurfacerelief(Fig6e),andhastheacousticcharactertypicalofmuddydebrisflowdeposits.Unit2wasdepositedveryclosetoaglacialicemargin.Unit3(TableI)consistsofacousticallywell-stratifiedsedimentthattendstodrape

overpre-existingtopography(Fig6).TheacousticcharactercloselyresemblesthatoftheEmeraldSilt(FaciesAandB)oftheScotianShelf(KingandFader1986),whichisasiltymuddepositedfromproglacialsedimentplumes.IncentralBrasd’OrLake,Unit3istypicallyabout5mthickandgenerallythinstowardsshallowerwater.Thissuggestsdepositionfromproglacialplumesdistantfromthehighsedimentationratesfoundneartheicemargin,withtheupslopethinningresultingfromslightwinnowingbywavesinshallowerwater.RapidlocalthickeningisfoundnearBaddeck(Fig6b),suggestingproximitytoanicemarginhere.Unit3iseitherabsentor<2mthickinnorth-easternWestBay(Fig6c),suggesting

thattheremayhavebeenlaterstagnanticeinthisarea.Incontrast,muchthickersequencesofUnit3are found in thenorth-easternpartsofEastBay,St.Andrew’sChannel (Fig 6a) andGreat Bras d’Or Channel, indicating a prolonged period ofproglacialdeposition,presumablyduringthelatephaseGofGrant (1994).Abruptchangesinthicknessofthisunitoneithersideofabedrockridgesuggeststhatthisridgerepresentsthesiteofastableicemargin,sothatstratifiedsedimenttothenortheastwas

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probablydepositedatthesametimeastilltothesouthwest(Fig5a).Seawardofthisridge,Unit3isinterbeddedwithacousticallyincoherentwedgesofUnit2,interpret-edasproglacialdebrisflows(Fig6a).Insomeareaswhereunit3isthick,acousticpenetrationismaskedbyshallowgas.Unit3istruncatedbyawidespreaderosionsurfaceinwaterdepthsof<50m,with

strongplanationatdepthsofabout20-25m(Fig6c).AsmalldeltahasprogradedacrossthiserosionsurfacesouthofBarraStrait(Fig6d),withtopsetsindicatingawaterlevelasshallowas16.5m,apparentlyunderconditionsoffallingwaterlevel.InWestBay,asmoothplanarsurfaceatawaterdepthof21mwithanerosionalnotch(Fig6c)contrastswithrougherseabedinshallowerwater.Unit4(TableI)consistsofmudwithvariabledegreesofacousticstratification.The

unitpartiallydrapesoverexistingtopography,butistypicallytwiceasthickinbasinsasoverridges.ItsthicknessappearsstronglyinfluencedbytidalcurrentsbetweenGreatBrasd’OrChannelandBarraStrait.IncentralSt.Andrew’sChannel,a10-mthick

Fig 6 HuntecDTSboomerprofilesshowinglateglacialfeaturesoftheBrasd’OrLakes:(a)proglacialsedimentsadjacenttosouthernmarginofSt.Andrew’sChannel;(b)ice-marginsedimentsofUnit3thatthickenrapidlynorthwardtowardsBaddeck;(c)thinorabsentUnit2inWestBayandthesmootherosionsurfacesat-29mand-22m;(d)erosionsurfaceat-19manddeltaprograda-tionfrom-16m,southeastofBarraStrait;(e)erosionsurfaceandlocationofcore85036-18inEastBay(bedrockinterpretedfromairgunseismicprofile).LocationoflinesonFig2(inset).

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Fig 7 Summaryofpistoncore85-036-016Pshowinglithology,biostratigraphy(fromdeVernalandJetté1987andLortie1987),δ13C(Hillaire-Marcel1987)andinterpretedageandenvironmentalhistory.Thetreepollenspectraillustratedhere are spruce (Picea), pine (Pinus), hemlock (Tsuga) and birch (Betula).Dinoflagellatesanddiatomsareunicellularalgae.Thesettingofcore16issimilartothatofcore18,illustratedinFig6e.

debrisflowdeposit(unit2,recognizedfromitsroughsurfacereliefandacousticallyincoherentcharacter)occursatthebaseofUnit4(Fig6a).Pistoncores16and18fromEastBay(Figs2;6e)bothpenetratetheentirethickness

ofUnit4andwerestoppedbyabedofsandandgravel(clasts<1cm).Bothcoresitesarelocatedabout5mdeeperthananearbyerosionalterrace.Thebasalsandandgravelareinterpretedassedimentsweptofftheerosionalterraceinthelittoralzoneandcorrespondstoastrongreflectioninseismicreflectionprofiles(Fig6e).Overlyingsedimentconsistsprincipallyofmud,insomeplaceswithmanysiltylaminae.DetailedbiostratigraphicstudiesrevealthreedifferentdepositionalenvironmentswithinUnit4.Thebasalmetreofsedimentincore16containssparsemarinedinoflagellates(deVernalandJetté1987)andveryraremarinediatoms(Lortie1987)(Fig7).Althoughthesparsefloramightbeinterpretedasreworkedoldermaterial,themarinecharacterofthisintervalisconfirmedbyisotopicanalysisoforganiccarbon(Hillaire-Marcel,1987). It isoverlainby1mofsedimentwithmostlycoldfreshwaterdiatomsandfreshwaterdinoflagellatecysts.Thatlayerisoverlainby1.5mcontainingarichvarietyofmarinediatomsanddinoflagellatecysts,withevidenceofanupwarddecreaseinsalinityandtemperature.Thetransitionbetweenfreshwaterandmarinediatomflorasismarkedbyamixedassemblageoffreshwater,brackishwaterandmarinespecies.Nomaterialsuitableforradiocarbondatinghasbeenfoundinthecores.Correlationof the treepollenassemblages inCore16with theregionalpalynostratigraphyofLivingstone(1968)suggeststhatthebasalmarineintervaldatesfromthePiceaandBetulazoneat10to9ka,thelacustrineintervalfrom9kato4-5ka,andtheuppermarineintervalisyoungerthan4-5ka(deVernalandJetté1987).

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Fig 8 MapoftheBrasd’OrLakesshowinggeneralizedsedimentgrainsizeatsam-plingsites(modifiedfromVilks1967).

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SurficialsedimentdistributionintheBrasd’OrLakes(Fig8)wasdescribedbyVilks(1967)onthebasisofsome100grabsamples.Thisinformationisaugmentedbylaterbottomsamplesfromcruise85-036andbysidescansonardatareportedbyCooper(1993).DeeperareasoftheLakesareflooredwithmud,exceptforsandsinsomeareasflushedbytidalcurrents.MoreexposedshallowareasofthewesternpartoftheLakesarecommonlyflooredbygravellysandymud,resultingfromerosionoftill.

postglacial changes in lake level

Wehaveinterpretedtheseismicreflectionprofilesandcoresintermsofchangesinlakelevelandsalinity.Otherevidenceaugmentsthesefindings.Grant(1994)reportedsubmergedchannelsoffthenortheastcoastofCapeBreton,thelargestofwhichap-pearedtobeextensionsofAconiBrookandLittleBrasd’OrChannel.Theyreacheddepthsof40-50m,andwerethoughttobecoevalwiththelaterstagesoftheBrasd’OrIceCap.WangandPiper(1982)foundtruncatedtillsurfacesdownto-50min(i.e.belowpresentsealevel)GabarusBay,onthesouthcoastofCapeBretonIsland;belowthisdepth,tillsurfaceswereunmodified.TheyconcludedthatthemaximumearlyHoloceneloweringofrelativesealevelwas50m.However, there are some apparent contradictions in published interpretations.

Hillaire-Marcel(1987)anddeVernalandJetté(1987),ontheonehand,requirethatrelativesealevelwashighbefore9-10ka,andthatitfellearlyintheHoloceneepoch.Ontheotherhand,Grant(1994)believedthatmeltwaterfromlate-glacialicedrainedeastacrossanemergentcoastalplainthatextendedtothemodern50misobath,thusrequiringthatrelativesealevelwaslowinlateglacialtimes.Weexaminesea-floormorphologicalevidencethathelpstoresolvethiscontradiction.

Submerged river valleys and erosion surfaces outside the Bras d’Or Lakes.MultibeambathymetrysurveysoftheseaflooroffNewWaterford(Fig1)show5steep-sided,flat-bottomedchannels,2-3mdeep,fourofwhichareillustratedinFig9.Long-profiles

Fig 9 Colouredshaded-reliefmultibeamimageofsubmergedchannels(numbered)offthecoastofCapeBretonintheNewWaterfordarea.ImagecourtesyofR.C.Courtney(GeologicalSurveyofCanada-Atlantic).LocationofchannelsshowninFig13.

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ofthechannelsareshowninFig10.Theycutacrossthegrainoftheseafloorthatisformedby2m-highridgesofgentlyfoldedCarboniferousrock.Seismicdatashowthattheycontain3-6mofsediment,anddonotextendfartheroffshoreunderaQua-ternarycover,butendat45-mdepth.Channel1(Fig13)is70m-wideandappearstobetheextensionofasmallvalley thatruns inland3.6km. Channel2 (140mwide)liesoffLinganBeachandappearstobeacontinuationofariverthatextends8kminland.Channel3maycontinueonshoreasashort(<2km)valleyatBridgeport(Fig2).Channels4-6differfromchannels1-3inthattheybeginfaroffshore,withnoconnectiontoonshorevalleys(Figs9,10).Grant (1994; 1997) mapped submerged channels extending offshore from the

PointAconiarea.RecentmultibeambathymetryshowsachanneljustnorthofPointAconithatextendsfromadepthof16mtoadepthof~40m,andseismicreflectionprofilesshowthatitcontainsupto20mofsediment.ThechannelappearstobetheoffshoreextensionofAconiBrook,ariverthanextendsupthemiddleofBoularderieIsland(Fig2).Ameanderingchanneljustsouthofthisisincisedintobedrock,withsidewalls3-4mdeep.Itextendsfrom10mto26-mdepth,andcontainsanunknownthicknessofsediment.Itsonshoreextensionisashortvalleyafewkilometreslong.NeitheroftheseistheoffshoreextensionofLittleBrasd’OrChannel.Becausesomeofthechannelsbeginoffshore,havelinearorconvexthalwegs(line

connectingthedeepestpointsalongavalley),andcutacrossCarboniferousstrata,itispossiblethattheywereinitiatedassub-glacialmeltwaterchannels.However,sub-glacialchannelselsewhereinAtlanticCanadatendtobeshorter,andnon-mean-dering.Theweightofevidencesuggeststhechannelsformedsub-aerially,probablywhenicehadretreatedtothewest,andspilledmeltwateracrossanemergentshelf,asGrant(1994)showed.However, if theydrainedanicefront, thechannelsthatbeginoffshoreimplythattheicemarginwasseawardofthepresentcoastandthatrelativesealevelwaslowbeforethestartoftheHolocene.ThechannelsoffPointAconi,asnotedabove,containupto20mofsediment.WeexpectthatasimilarburiedchannelprobablyexistsoffshorefromLittleBrasd’OrChannelwhich,withitssteepsidewalls,bearscloseresemblancetosubmergedchannelsinthePointAconiareathathavebeensurveyedwithmultibeamsystems.

Fig 10 LongprofilesofsubmergedchannelsintheNewWaterfordarea.Locationofchannels2-5showninFigure9,channels1and6inFig13.

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Fig 11 Colouredshaded-reliefmultibeamimageof seafloorshowingunmodifieddrumlins(A),rockyterrain(B)andtruncateddrumlins(C)intheIsleMadamearea.(SeeFig13forlocation).DatacourtesyofMikeLiandNedKing(Ge-ologicalSurveyofCanada-Atlantic).

Tayloretal.(1989)reportedchannelsincisedtoabout30mbelowsealevel,andfilledwith15mofsediment,inSt.Peter’sBay,nearIsleMadame,onthesouthwestcoastofCapeBretonIsland.Multibeamimagery(Fig11)ofashoal10kmsouthofIsleMadamerevealsdrumlinsonbothnorthandsouthsideswithNW-SEalignmentssimilartothoseonshore(Grant,1988).Theyareunmodifiedbelowdepthsof55m.Abovethisdepththeterrainisrocky,withaclusteroftruncateddrumlins(‘scars’)atadepthofabout35m.Thepostglaciallowstandwasnomorethan55mdeephere.OtherchannelsoutsidetheBrasd’OrLakesinclude8mdeepchannelsincisedtoadepthof34mbelowsealevelonPomquetBanks,inSt.GeorgesBaywestofCapeBretonIsland(Shawetal.,1995).Anunconformityisdevelopedinearlypostglacialsedimentsat-34moffthewestcoastofCapeBretonIsland(Shawetal.,1995).

Submerged river valleys and erosion surfaces within the Bras d’Or Lakes.Unpublishedresultsofa1996surveyshowthatSt.Patrick’sChannel(Fig2)contains

anetworkofsubmarinechannelsthatarecommonly100mwide.Theyareincised5-7m into sedimentswithclosely spaced,parallel, continuous,coherent internalreflections(havingtheappearanceofUnit3,TableI).Thebasesofthechannelslie17mbelowthemodernlakesurfacenortheastofLittleNarrows,andsystematicallydeepentoamaximumof29msouthwestofBaddeck.Theyareburiedby2-4mofponded,acousticallytransparentHolocenemudofUnit4,havenosurfaceexpression,andwouldthereforenotappearonamultibeamimage.UnpublishedmultibeambathymetryimageryfromDenysBasin,inthewesternpart

ofthemainlakesouthofBarraStrait,showsameanderingriverchannelthatextendseastwardtowardsthemiddleofBrasd’OrLaketoadepthofatleast12m(Paul,2001);thehydrographicchartshowsthatthischannelreachesadepthofatleast16m.Un-likethoseintheBaddeckarea,thechannelsarenotcompletelyburiedbythemud.

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TheseismicdataillustratedinFig6showthepresenceofunconformitiescutatthetopofUnit2(TableI)andinmarginaldeltaicsediments.Theyarecommonlyatdepthsof25to22m,andtestifytoasustainedlakelevelatthatdepth.Thedatafromthe1996surveyshowthattheseabedsurroundingthebasinjustnorthofCodShoals(Fig2)isflat,andconstitutesanerosionalunconformity.AcousticreflectionsofUnit3aretruncatedattheseafloortoamaximumdepthof26m.Furthermore,severalchannelsareincisedintoUnit3todepthsof26m.AmajorbreakofslopewherethetruncationzoneisjuxtaposedagainstCodShoalsisat-24m.Locally,shallowererosionalfeaturesareseenat18to20mdepth(Fig6d).

geography of the Bras d’Or Lakes in the last 15,000 years

OntheinnerScotianShelf,thereiswidespreadevidenceforhighrelativesealevelasglacialretreatoccurredbetween18and12ka.Thiswasfollowedbyapronouncedloweringofsea levelsome time in the12ka to9ka interval,and thengraduallyrisingsealeveluntilthepresentday.Thisresulted,forexample,inthecharacteristicirregular‘J’shapeofthesea-levelcurvefortheinnershelfnearHalifax(Steaetal.,1994).ThedatafromtheBrasd’OrLakessuggestthataslightlydifferentmodelisapplicabletotheevolutionoftheBrasd’OrLakes(Fig12),onewithanearlyphaseoflowrelativesealevel.In late glacial times,when south-easternCapeBreton Islandwas coveredwith

ice,valleyssuchasthoseinFig9suggestthatsealevelwasinitiallycloseto-50m.Asiceretreated,perhapstotheicemarginidentifiedinthemiddleofSt.Andrew’sChannel(Fig5a),meltwatercontinuedtodrainoutthroughchannelsacrosstheshelf,potentiallydeeplyincisingoutletssuchasLittleBrasd’OrChannel.AsiceretreatedfarthertowardsWestBay,andsealevelrose,marinewatereventuallyfloodedintotheBrasd’OrLakes.

Fig 12 ModelofchangingseaandlakelevelsintheBrasd’OrLakesregionoverthepast15000years.

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Fig 13 SuggestedgeographyoftheBrasd’OrLakesata-25mwaterlevel.(A)amajorpartoftheancientlakeswasnorthofBarraStrait;(B)LittleBrasd’OrChan-nel,formerlyanoutletforthelakes;(C)asmalllakewaslocatedjustwestofLittleNarrows;(D)locationofaformerlakeintheWhycocomagharea;(E)anarrowpartoftheancientlakesatBarraStrait;(F)awiderpartofthelakessouthofmodernBarraStrait;(G)anarrowstraitlocatedonthemodernCodShoals;(H)theancestralDenysRiverthatdrainedintotheancientlakes;(J)thelargestpartoftheancientlakesystem;(K)asillseparatingthemainlakesfromasmalllake(L)inmodernEastBay.Theboxesshowareasofmultibeambathymetrycoverage.1,2and6arechannels in theNewWaterfordareadiscussedintextandshowninFig9and10.

MarinewatersenteredtheBrasd’OrLakes,probablythroughthechannelatLittleBrasd’Or,floodingfirstthemainlakesystemandeventuallytheheadofEastBay.ThepollenstratigraphyattheheadofEastBaysuggestsanageofabout10-9kaforthisincursion.ThedeltaicdepositsinFig6dmayrepresentthelimitofmarineincursionatthistimeatabout-16m,adepthsufficienttobringseawateracrossthe-18msillthatseparatesEastBayfromtheremainderofthepaleo-lakesystem.ThiswasfollowedbyafallinrelativesealevelthateventuallycutofftheLakesfromtheseaonceagain.IntheearlyHolocene,followingthefallofrelativesealevelthatterminatedtheearly

marinephase,thestratigraphicevidencesuggestsasustainedlakelevelatabout-25m(Fig13).ThisobviouslyrequiresthatrelativesealeveloutsidetheLakeswasdeeperthan-25m,andalsothatthesillwasdeeperthantoday(-8m).Alargepartofthe

SHAW PIPER and TAYLOR144

ancientlake(Fig13,A)wasnorthofthemodernBarraStrait,anddrainedouttotheoceanviaLittleBrasd’OrChannel(B).St.Patrick’sChannelwasoccupiedbyariverthatreachedthelakesoutheastofBaddeckanddrainedtwolakesinWhycocomaghBay:alargerlakeat-12mimmediatelywestofLittleNarrows(C),andsmalllakejustsouthofWhycogomaghat-10m(D).ThelargelakewasextremelynarrowatBarraStrait(E),widenedtothesouth(F),andnarrowedagainonCodShoals(G).TheancestralDenysRiver(H)flowedeastintothelakeinthisarea.SouthofthenarrowsatCodShoal,theancientlake(J)extendedtoboththenortheastandsouthwest.ThelakewasveryconstrictedinpartsofEastBay,andprobablyendedsouthofEskasoni(K)whereasillmayhaveseparateditfromasmallerlakeattheheadofEastBay(L)atanelevationabout-18m(Thislakewasthesiteofcore16;Figs2,7).Thismid-Holocenelakewaseventuallyfloodedbytheseaasrelativesealevelroseagain.Theprincipalmodernconnectiontotheoceanisthe-8msillatGreatBrasd’OrChannel,locatedonamorainalbank(Grant1994).Whenthisconnectionflooded,itispossiblethatthesinuous,hydraulicallyinefficientchannelatLittleBrasd’Orgraduallyshoaledatitslandwardend,perhapsduetoflood-deltadeposits.DepthsforLittleBrasd’OrChannelonCHSChart4367arelessthan5.5m,althoughthereareindicationsofachannel>11mdeep,perhapstheremainsofaformerlydeeperchannel.TheLittleBrasd’Oroutletwasprobablytheoutletforthemid-Holocenelakesandmayhavebeenthepathwayfortheincursionofmarinewateratabout5to4kaassealevelroseagain.MillerandLivingstone (1993)showed that relativesea level inGreatBrasd’Or

Channelroseduringthelast4000years,reaching1.5mbelowthemodernlevelat0.95ka;therateofsealevelrisewaslessthatthatregisteredbytidegaugesintheregion(seeratesinShawetal.,1993).Itappearslikelythatcyclicfluctuationsofeustatic sea levelwere superimposedon long-termcrustal subsidence inAtlanticCanadaduringthelast3000years,andthatthemodernrapidincreasebeganc.1600AD(ShawandCeman,1999).

conclusions

ThesedimentsontheflooroftheBrasd’OrLakespreserveanimportantrecordofthecomplexsequenceofeventsthattookplaceduringtheretreatofglacialicefromCapeBretonIslandandduringchanginglakelevelsintheHoloceneepoch.Thethicknessofsedimentaboveglacialtillshowsthattherewasastep-likeretreatoficetowardsalateicecentreinthewesternpartoftheBrasd’OrLakes.Asiceretreated,a freshwater lake developedbehind a sill (probably in LittleBras d’OrChannel).Marineincursionfloodedthislakeat10to9kaandthewaterlevelmayhaverisento-16mbeforefallingagain.ThisfallingearlyHolocenerelativesealevelresultedinthecreationonceagainoffreshwaterlakes,withaprominenterosionsurfaceat-25mmarkingthelakelevel.Theselakeswerefinallyfloodedbytheseaat4to5ka.Itisevidentthat,despitetheresearchthathastakenplaceintheBrasd’OrLakes,

moreneedstobedonebeforewehaveagoodgraspofthesequenceofeventssincedeglaciation.OngoingworkbySteaandhisco-workers(Stea,2001)maysoonprovidenewinsightsintothechronologyofdeglaciation.Asignificantadvanceinunderstand-ingofrelativesea-levelandlake-levelchangesmaycomesoonerratherthanlater,formuchofthelakehasrecentlybeenmappedusingmultibeambathymetrysystems,andmoresurveyswilltakeplacein2001(Paul,2001).

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acknowledgements

MuchoftheresearchdescribedherewassupportedbytheGeologicalSurveyofCanada.ThispaperwasreviewedbyD.R.Parrott,P.J.Mudie,D.R.GrantandR.R.Stea.MikeLiandNedKingprovidedthemultibeamdataillustratedinFig11.KenPaulallowedaccesstorecentmultibeamimageryoftheBrasd’OrLakes.

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Received1March2002