The place of tubular concretions in hydrocarbon co

Theplaceoftubularconcretionsinhydrocarboncoldseepsystems:LateMioceneUrenuiFormation,TaranakiBasin,NewZealandStephanieL.NymanandCampbellS.NelsonABSTRACTThelateMioceneUrenuiFormationcroppingoutontheeast-ernmarginofTaranakiBasin,currentlyNewZealand’sonlycommerciallyexploitedhydrocarbonbasin,isdominatedbysiliciclasticslopemudstone(5%carbonate)thatlocallyin-cludesavarietyoftubularconcretions(50–85%carbonate).Dominatedbycommonlylargepipeandbulbousvarieties,asmuchas0.5tomorethan10m(1.6to33ft)longand5to70cm(2to28in.)across,theseconcretionsformedfromtheprecipitationoffine-grainedcalciteordolomitecementwithinsiliciclasticmudintheshallowsubsurface(likely100mburi-al).Mostsupportacentralconduitthatmaybeemptyorfilledwithsedimentormultiplegenerationsofpost-shallow-burialcarbonatecements.Depletedd13Cvaluesoftheconcretionarycalcitecements(−40to−25‰PDB)indicatethattheircarbonwassourcedfromoxidizedmethane.Incontrast,enrichedd13Cvaluesoftheconcretionarydolomitecements(−10to+11‰PDB)in-dicatethattheircarbonwasderivedfrommethanogenicCO2and/orextensivemethaneoxidation.Therangeofd18Ovaluesfortheconcretionarycementtypes(−2to+5‰PDB)supportsprecipitationfromfluidsbothdepletedandenrichedwithre-specttoNewZealandMiocenemarinewaters,whicharesug-gestedtoreflectepisodesofgashydrateformationanddis-sociation,respectively.Thetubularcarbonateconcretionsareinterpretedtomarkthesubsea-floorpathwaysoffocusedmethaneinfusedupwardAUTHORSStephanieL.Nyman
DepartmentofEarthandOceanSciences,UniversityofWaikato,PrivateBag3105,Hamilton3240,NewZealand;[e-mail]dhermaster’sdegreefromtheUniversityofTexasinDallasin2003andherPh.D.fromtheUniversityofWaikatoin2009.Herresearchinterestsareinhydro-carbonseeps,especiallyincarbonateminerali-zationasrecordsofhydrocarbonmigration.Presently,sheisageologistinthepetroleumindustry.CampbellS.Nelson
DepartmentofEarthandOceanSciences,UniversityofWaikato,PrivateBag3105,Hamilton3240,[e-mail]’sresearchinterestsareinsedi-mentaryandmarinegeology,especiallythestudyofcool-waterlimestonesandthehistoryofenvironmentalandclimatechangeintheSouthwestPacificregionduringtheCenozoic.HeisapastpresidentoftheGeosciencesSocietyofNewZealandandwaselectedafellowoftheRoyalSocietyofNewZealandin1994.ACKNOWLEDGEMENTSWethankPeterKingandGregBrowne(GNS,LowerHutt),PeterKamp,BrianRicketts,SteveHood,MikePearson,andChrisHendy(Uni-versityofWaikato),KathyCampbell(UniversityofAuckland),andDaveFrancis(GeologicalResearchLtd.,LowerHutt)forfieldassistanceorgeologicinformationforthisstudy.SteveCooke(UniversityofWaikato)supplementedearlierstableisotopeanalysesreportedbyFranziskaSchellenberg(formerlyUniversityofTubingen,Germany),andboththesesourcesofisotopedataaregratefullyacknowledged.Weappreci-atethelogisticalandfinancialsupportforthisstudyfromtheDepartmentofEarthandOceanSciences,UniversityofWaikato,andotherfundingassistancefromtheRoyalSocietyofNewZealandMarsdenFund(06-UOA-082).TheAAPGEditorthanksthefollowingreviewersfortheirworkonthispaper:CharlesT.FeazelandRichardB.Koepnick.Copyright©2011.TheAmericanAssociationofPetroleumGeologists.Allrightsreserved.ManuscriptreceivedFebruary9,2010;provisionalacceptanceApril26,2010;revisedmanuscriptreceivedJuly20,2010;finalacceptanceJanuary19,2011.DOI:10.1306/01191110017AAPGBulletin,v.95,no.9(September2011),pp.1495–15241495fluidescape,andassuch,theyrepresentapartoftheshallowsubsurfaceplumbingsystemofacoldseepsystemdevelopedonthecontinentalmarginboundingtheTaranakiBasininthelateMiocene.Thismarginhasbeenstronglyinfluencedbyperi-odicmovementsalongthemajorTaranakifaultthrustsystem,whichcoincidentallydirectlyun-derliestheUrenuiconcretionoccurrencesandwasprobablythemostlikelyavenueforactivefluidmigrationwithinthehydrocarbonseepsystem.Thetubularconcretionsexperiencedperiodicintervalsofexhumationandburialcausedbymarginfailurebecauseofgreaterslopeinstabilityfrominjectedfluidsandresultedinconglomeraticchannel-filldepositswithintheUrenuiFormationthatincludemass-transportedtubularconcretions.INTRODUCTIONColdseepsaresitesofhydrocarbonescapeattheseafloorthatarecommonlymarkedbyavarietyofmorphological,geologic,andbiologicalfeatures,includingpockmarks,mudvolcanoes,methane-derivedauthigeniccarbonate(MDAC)deposits,anddistinctiveassemblagesofchemosymbioticin-vertebrates(Barryetal.,1996;LewisandMarshall,1996;Greinertetal.,2001;JuddandHovland,2007).Theyarewidelyreportedinthemodernoffshore,frombothpassiveandactivecontinentalmargins(Figure1)(Judd,2003;Campbell,2006),wheretheoriginoftheascendinghydrocarbonsmaybefromthedegradationofshallowburiedorganicmatter,thethermalmaturationofdeeplyburiedkerogen,orthedissociationofclathratehy-drates(RobertsandAharon,1994).TheMDACformingasthesea-floorexpressionofcoldseepsistypicallypatchilydistributedbutmorphologicallyextremelydiverse,rangingfromconcretednodules,slabs,andchimneys,tosmallbiogenicmounds,tomuchlargerbiogenicbuildups(RobertsandAharon,1994;Diaz-del-Ríoetal.,2003;Gayetal.,2006;JuddandHovland,2007).Manyancientexamplesofthesesea-floorcoldseepcarbona