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gulf entrance and possible relations with large earthquakes
Iliana Aguilar, Christian Beck, Franck Audemard, Anne-Lise Develle, Mohammed Boussafir, Corina Campos, Christian Crouzet
To cite this version:
Iliana Aguilar, Christian Beck, Franck Audemard, Anne-Lise Develle, Mohammed Boussafir, et al..
Last millennium sedimentation in the Gulf of Cariaco (NE Venezuela): Evidence for morphological
changes of gulf entrance and possible relations with large earthquakes. Comptes Rendus Géoscience,
Elsevier, 2016, 348 (1), pp.70-79. �10.1016/j.crte.2015.10.001�. �insu-01240720�
Stratigraphy, Sedimentology (Palaeoenvironment)
Last millennium sedimentation in the Gulf of Cariaco (NE Venezuela): Evidence for morphological changes
of gulf entrance and possible relations with large earthquakes
Iliana Aguilar
a,b, Christian Beck
a,*, Franck Audemard
b, Anne-Lise Develle
c, Mohammed Boussafir
d, Corina Campos
e, Christian Crouzet
aaCNRSISTerre,universite´ Savoie–Mont-Blanc,73376LeBourget-du-Lac,France
bVenezuelanFoundationforSeismologicalResearch/FUNVISIS,ElLlanito,Caracas,Venezuela
cCNRSEDYTEM,universite´ Savoie-Mont-Blanc,73376LeBourget-du-Lac,France
dInstitutdesSciencesdelaTerred’Orle´ans(ISTO),UMR7327,CNRS/INSU,universite´ d’Orle´ans,BRGM,45071Orle´ans,France
eDpto.CienciasdelaTierra,Sartenejas,UniversidadSimo´nBolı´var,Caracas,Venezuela
Introduction
TherelativedisplacementbetweentheSouthAmerica andtheCaribbeanplatesismainlyaccommodatedalong the right lateral strike slip San Sebastian-El Pilar fault system, at thenorthern boundary of Venezuela (Fig. 1;
Audemardetal.,2000,2005;DeMetsetal.,2000;Pe´rez
et al., 2001; Ste´phan et al.,1990; Symithe et al., 2015;
Weberetal.,2001).Intheeasternpartofthefaultsystem, named El Pilarfaultor EPF, theplateboundary faultis underlinedbya1400m-deep,160km-long,50km-wide, pull-apart basin, theCariaco Basin (also named Cariaco Trough)(Schubert,1979,1982).Furthereast,theGulfof Cariaco is a smaller (65km-long, 15km-wide) and shallower (85m-deep) basin, connected to the Cariaco Basin.Theformerdevelopeddirectlyuponandalongthe EPF, on thenorthern side. TheEPF faultaccommodates mostoftherelativeplatemotionsthroughacombination C.R.Geoscience348(2016)70–79
ARTICLE INFO Articlehistory:
Received5July2015
Acceptedafterrevision7October2015 Availableonline30November2015 HandledbyIsabelleManighetti
Keywords:
ElPilarfault Cariaco Sedimentation Geochemistry Submarinelandslides Largeearthquakes Submarinepaleoseismology
ABSTRACT
TheCariacoBasinandtheGulfofCariacoinVenezuelaaretwomajorbasinsalongthe seismogenicElPilarrightlateralfault,amongwhichtheCariacoBasinisapull-apart.Both basinsaresitesofanoxiaandorganic-richdeposits.Toexaminewhetherthesedimentsin theGulfofCariacohaverecordedtracesofhistoricalorprehistoricalearthquakes,we extractedand analyzedtwelve 1m-long gravitycores,samplingthe last millennium sedimentation.Wefocusedonanalyzingthesedimentsourceswithdifferenttechniques (particlesizeanalysis,XRF,lossonignitiontests,magneticproperties,Rock-Evalpyrolysis,
14Cdating).Theresultsconfirmthatmajorupwellingoccursatthewesterngulfentrance andmakesdeepwaterflowingfromtheCariacoBasinintotheGulfofCariaco.Theseflows carryanorganic-richsuspendedload.Furthermore,wefoundevidenceofaparticular, widespreadfine-grainedsiliciclasticdeposit(namedSiCL3)withinthegulf,whoseage suggeststhatitlikelyformedduringthelarge1853ADearthquakethatstroketheCumana´
city.Wesuggestthattheearthquake-inducedlargesubmarinelandslidesthatmodified thetopographyofthegulf’sentrance,whichinturnpromotedupwellingandopenmarine water flows from the Cariaco Basin. The layer SiCL3 would be the sediment load remobilizedduringthischainofevents.
ß2015Acade´miedessciences.PublishedbyElsevierMassonSAS.Allrightsreserved.
* Correspondingauthor.
E-mailaddress:beck.christian7@gmail.com(C.Beck).
ContentslistsavailableatScienceDirect
Comptes Rendus Geoscience
ww w . sci e nc e di r e ct . com
http://dx.doi.org/10.1016/j.crte.2015.10.001
1631-0713/ß2015Acade´miedessciences.PublishedbyElsevierMassonSAS.Allrightsreserved.
ofcreep(60%)andco-seismicoffsets(40%)(Audemard, 2007;Jouanneetal.,2011;Reinozaetal.,2015).TheEPF faulthasinparticularproducedlargeearthquakes(i.e.with estimatedMSKintensitiesofIX/X;Audemard,2007)over thelastcenturies;mosthaveseriouslyaffectedthecityof Cumana´ (locationinFig.2).Inanattempttoestimatethe seismogenicpotentialoftheEPFfault,manystudieshave beenconductedinthelastdecades,mostwereledbythe Venezuelan Foundationfor Seismological Investigations.
SincethemainsectionoftheEPFfault(named‘‘VE-13b’’by Audemard, 2007) is offshore (Fig. 2), multiple high- resolution seismicreflection datahavebeenacquiredto analyze the fault and to search for information on its historical and prehistorical ruptures (Audemard et al., 2007;VanDaeleetal.,2011).Subaqueouspaleoseismology has also been conducted, using approaches previously developed along other seismogenic faults in Venezuela (Carrilloetal.,2008).Thesestudiesanalyzethesedimen- tarycontentinshort‘‘gravitycores’’(1mlong)soasto searchandidentifysedimentarychangesthatmighthave resultedfromlargeearthquakemotions,asitwasobserved rightafterthemostrecentlargeearthquakeinthearea(Mw
6.9 Cariaco earthquake, Lorenzoni et al., 2012; Thunell etal.,1999).
The present paper focuses on this question, and explores the possible sedimentary signature of large historical earthquakes intheGulf of Cariacoand in the easternpartoftheCariacoBasin.Theeasternregionofthe Cariacobasinisinterestingbecauseitmarkstheentrance totheGulfofCariaco,anditisthesiteofasteep,400m- high,north–south-trendingescarpmentat thewestern edgeoftheManzanares Riverdelta(MRdinFig.2). The MRddeltaiswheretheCumana´ cityhasdeveloped.This delta is crosscut by theEPF fault.Therefore, we might expectthatlargehistoricalandprehistoricalearthquakes ontheEPFfaultintheareaofCumana´ havedisturbedthe nearbyeastern partof the Cariaco Basin,with possible consequences on the sedimentation within the Gulf of Cariaco.
To examine these hypotheses, we analyze the sedi- mentary content of 12 cores extracted in the Gulf of Cariaco.We focuson determiningthesediment sources (based on mineralogy and geochemistry) and their distributionpattern.Previousresultshavesuggestedthat upwelling at the basin–gulf junction significantly con- trolledthesediment sources(Gade,1961; Okuda,1981, 1982; Okuda et al., 1974, 1978). In turn, part of the upwellingmighthavebeenenhancedbyhistoricallarge Fig.1.TectonicsettingoftheGulfofCariaco.ActivefaultsfromAudemardetal.(2000,2005),DeMetsetal.(2000),Pe´rezetal.(2001),Weberetal.(2001), Symitheetal.(2015).
Fig.2. (Coloronline).GeomorphicsettingoftheGulfofCariacoandofitsconnectionwiththeCariacoBasin.SS:SalazarSill;CAB:CerroAbajoBasin;GD:
GuaracayalDeep;MRd:ManzanaresRiverdelta;CH:Caiguı¨re Hills.BathymetrysimplifiedfromCaraballo(1982a).Shaded-reliefmapfromGarrityetal.
(2004).
earthquakedisruptions of thebasin–gulf junction mor- phologyandtopography.
1. Geologicalsettingandsedimentaryenvironmentof theGulfofCariaco
1.1. TectonicsettingandcharacteristicsoftheGulfofCariaco catchmentarea
TheoffshoretraceoftheEPFintheGulfofCariacohas beenidentifiedinhigh-resolutionseismicdata(Audemard etal.,2007;VanDaeleetal.,2011;Fig.2).Thefaulttrace appearssegmented,withtwolargerelayzonesbetweenits longestsegments:theGaracayalpull-apartor‘‘Garacayal deep’’ (GD, Fig. 2), and theCaiguı¨re Hills push-up (CH, Fig.2). The entrance of thegulf is thesite of thelarge ManzanaresRiverdelta.
Accordingtohistoricalarchives(inAltezOrtegaetal., 2004; Audemard, 2007; Grases, 1990), four major earthquakes with associated tsunami hit the City of Cumana´ in1530,1853,1929and1997.Reportsindicate that4–6m-and6m-highrun-uphittheCumana´ coast duringthe1853and1929events,respectively.Tracesof Holocenepaleo-tsunamishavealsobeenfoundincoastal lagoonsintheManzanaresRiverdelta(Lealetal.,2014).
RightaftertheMw6.91997Cariacoearthquake,signifi- cant and long-lasting sediment resuspensions were observedintheeasternCariacoBasin,andwereattributed toearthquake-inducedsubmarinelandslides,mostprob- ablyonthewesternslopeoftheManzanaresRiverdelta (Lorenzonietal.,2012;Thunelletal.,1999).Furthermore, inhabitants from both northeastern and southeastern coastalareasreported a loweringoftheseasurfaceby several meters, but no subsequent inland overflowing.
Theseobservationssuggestthatatsunamiresultedfrom the1997 earthquake,whichinducedeast–westmoving waves.The tsunamimay haveenhanced the observed sedimentresuspensions.
The reliefs that bound the gulf’s catchment area (Supplementary Material S1) are 400m a.s.l. in the ArayaPeninsulaand800ma.s.l.inthenorthernflankof theEasternInteriorRange.Nomajortributaryisdirectly flooding intothe gulf. On the southern coast, only few smallandshortgulliescarrytemporaryminorterrigenous sedimentswithinthegulf.Tothenorth,thecoastisarid and its sedimentary contribution in the gulf is almost negligible.Tothewest,theManzanaresRiverhasawide, elevated, humid catchment area within the Eastern InteriorRange,butitdoesnotdirectlyflowintothegulf.
Anartificialchannelhasbeenduginthe1970stomitigate theRioManzanaresfloods(Grases,1979,1990),butthis channeldoesnotflowintothegulf(Fig.2).Thegeological substratumofthegulf’swatershed(S1)showsacontrast betweentwogroupsoflithologies:mica-schists,quartzi- tes,andgreenschisttothenorth,andclaystones,siltstones, andsandstonestothesouth.Thegulfsubstratumisthus dominatedbysiliciclasticrocks.Plio-Pleistocenelimesto- nes,calcareoussandstonesandmarlsexistlocallyatthe northern entrance of the gulf where they provide an additional minor carbonatecontribution (Macsotay and Moore,1974).
1.2. Bathymetryofthegulfandofitswesternentrance
ThefloormorphologyoftheGulfofCariacoshowsfour singularities(Fig.2)(Caraballo,1982a,b;Maloney,1966;
MorelockandMaloney,1972;Morelocketal.,1972):
theGuaracayalbasin(GDinFig.2),anelongatedpull- apart90mdeepalongtheEPF,inthesouthofthegulf (Audemardetal.,2007);
thelargeCerroAbajoBasin(CAB),70mdeep,which occupiesmostofthegulf;
tothenorthwest,theSalazarSill(SS),a5km-wide,40-to 50m-deep, irregular channel, which links the Cerro AbajoBasintothewesternentranceofthegulf;
atthewesterntipofthegulf,theManzanaressubmarine canyon,3km northwestoftheRioManzanaresdelta andthecityofCumana´.Thiscanyonaccountsfora1350- mdepthdifferencebetweenthewesternmouthofthe gulf and the deepest part of the Cariaco Basin. The eastern flank of the canyon, which bounds the Rio Manzanaresdelta,isverysteep(upto208C).
1.3. UpwellingontheeasternslopeoftheCariacoBasinand anoxiaintheGulfofCariaco
Prior works have suggested a strong anoxia in the CariacoBasinandintheGulfofCariaco(BonillaandHui- Lin,1979;Caraballo,1982a;Miro´ Orell,1974).Inthebasin, theanoxiaestablishedaftertheLastGlacialMaximum,due to an increase in thesurface productivity(Lyons et al., 2003). Hydrologicaland geochemicalinvestigationsalso evidencedpresentmassivewaterexchangesandseasonal strong anoxiain thedeepestpartoftheGulf ofCariaco (Gade,1961;Okuda,1981,1982;Okudaetal.,1974,1978;
Richards,1960).Itissuggestedthattheseexchangesand anoxiaresultfromacyclicpatternoftradewindsand a thermal stratification of isohaline waters (Fig. 3). The seasonalregime alongthenorthernVenezuelancoastis controlledbythemigrationoftheInter-TropicalConver- gence Zone (ITCZ) (Haug et al., 2001). Two different regimes are evidenced along the eastern slope of the Cariaco Basin and inside the gulf (Fig. 3): a strong upwellingperiod(A)fromFebruarytoJune,duringwhich flows of cold water are moving upward, and a weak upwellingperiod(B)betweenJulyandDecember.Upwel- ledcoldanddensesubsurfacewatersflowfromtheCariaco Basinintothegulf;theybypassthe50m-deepSalazarsill, andthenmovealongthebottomofthegulf(fordensity reasons) towardsits innermostparts(Caraballo,1982a).
Thiscoldflowmergeswiththeoverallgulfwaterwhenit reachestheeasternpartofthegulf.Duringthesephasesof cold-water renewal in the gulf, the oxygen content is increased.Bycontrast,duringtheperiodsofmoderateto nullrenewalofcoldbottomwaterinthegulf,theoxygen contentdecreasesby0.2–0.5ml/l(Okuda,1982).
BonillaandHui-Lin(1979)havesuggestedthatthehigh concentrationinorganicmatterfoundinthesedimentsof thegulfmayresultfrombothahighprimaryproductivity and the upwelling process. Interflows rich in organic suspendedloadshaveactuallybeenobservedtoenterthe I.Aguilaretal./C.R.Geoscience348(2016)70–79
72
gulf(Okudaetal.,1974,1978).Upwellingprocessesthus significantlyaffectthesedimentarycontentintheGulfof Cariaco. While these upwelling processes are mainly controlled by climatic changes, they might also occur followingasignificantchangeinthesubmarinemorphol- ogyofthegulfentrance(especiallytheManzanarescanyon andtheSalazarSill).Wewillpayaparticularattentionto thislaterpossibilityinthefollowing.
2. Coreacquisitionandanalyses
We extractedand analyzedsedimentarycoresinthe Gulf of Cariaco to search for possible changes in the sedimentarycontentthat mightattest tomorphological and topographic changes in the basin–gulf system.The sedimentarysignatureofmorphologicalchangesinsmall- sized basins(lacustrineormarine) hasbeenrevealedin priorworks,basedoncoredata(Avs¸ar,2013;Avs¸aretal., 2015;Beck,2009;Becketal.,2007;Camposetal.,2013;
Carrilloetal.,2008).Thecoreswerecollectedonboardthe R/VGUAIQUERIII,operatedbytheEasternOceanographic Institute(InstitutoOceanogra´ficodeOriente,Universidad deOriente,Cumana´).Weusedagravitypistoncorerbased on theclassical BENTHOS design, with a 6-cm internal diameter.Thecoreshaveanaveragelengthof1m.
Thecoringsites(locationinFig.2)wereselectedwith twomaintargets:1)todefinethedepositionalprocesses and sediment sources in theGulf of Cariaco,and 2)to identifythepossibleimpactsofEPFfaultlargeearthquakes onthesedimentation.Fourcoresaredistributedalongan east–west trendacross theGuaracaya´ldepression. Four othercoreswereextracted5kmtothenorth,intheCerro Abajodepression.Athirdsetofcoreswasextractedsoasto completeanorth–southlinethatwouldintersecttheeast–
westsection.Inthefollowing,wepresentonlytheresults from thecoresCariac-09-05, Cariac-09-06,Cariac-09-15 andCariac09-18(Fig.2).Fig.4showstheirpositionalonga north–south3.5kHzseismicreflectionprofile(VanDaele etal.,2011).
Laboratoryanalysesencompassdifferentnondestruc- tivemeasurementsandimaging,anddifferentgeochemi- cal, mineralogical, and microscopic observations. The proceduresaredescribedinSupplementaryMaterialS2.
3. Resultanalysis:recentsedimentationinthecentral andsouthernGulfofCariaco
3.1. Sedimentlayeringandcomposition
Inallanalyzedcores,thesedimentsappearedhomoge- nousandorganic,madeofacontinuousfine-grainedmud (hemipelagic fraction). High-resolution X-ray pictures (SCOPIX)didnotrevealanylayering,withtheexception ofafewcoarseterrigenouslayers(leftloginFig.5).Rare macrofossils (thin-shelled bivalves, 1 to 2cm in size, Turitella-typeGastropods,andfewDentalium-typePtero- pods)werefounddispersedwithintheorganic-richmud.
Microscopicobservationsofthehemipelagicmud(smear slides and SYSMEX pictures) revealed clayey aggregates withclay-sizedopaqueandmicriticparticles.Asiliceous biogenic material was found in significant proportion, mostlyintheformofDiatomsandSilicoflagellids.Silt-size fibrousO.M.wasfoundtobeonlyaminorcomponent.
3.2. Generalchemicalevolution:XRDelementaryprofiles
Sincethesedimentaryfeaturesandchemicalcharacte- ristics are clear in core Cariac-09-06, we take it as a reference.Fig.5 showstheXRFprofilesof theelements that we consider as best attesting to the presence of terrigenoussiliciclasticinputs(Zr,Fe)orcarbonates(Ca), orforopenmarineinfluence(Br).ThesimilarityoftheAl, Si, and K profiles confirms that these three elements describe the evolution of the clay mineral content adequately. Three siliciclastic layers (noted SiCLi) are identifiedintheK,ZrandFeprofiles,whereeachformsa clear peak (i.e. increase of concentration). Pronounced variationscoincidentwiththethreepeaksareobservedin Fig.3.SeasonalthermalsituationandhydrodynamicconnectionsbetweentheGulfofCariacoandtheCariacoTrough.CompilationfromCaraballo(1982a andb),Okuda(1981,1982),andGade(1961).
the Al, Si, K (increased content) and CaCO3 (decreased content) profiles. Two of the siliciclastic layers are characterizedbycoarsegrains(Section3.5). Bycontrast, theSiCL3layerisonlydistinguishableontheXRFprofiles whereitformsapeaksimilartothoseformedbySiCL1and SiCL2(Fig.5).Thethreelayersarealsocharacterizedbya
decreaseinO.M.content(Section3.3).Furthermore,the bromine(Br)contentsystematicallydecreasesinthethree SiCL layers. However, subsequently to a Br decrease in layerSiCL3incoreCariac-09-06(Guaracayaldeep),theBr content increases progressively in cores Cariac-09-15 (centralgulf)andCariac-09-18(closetosoutherncoast).
Fig.4.(Coloronline).Locationofcoresalongavery-high-resolutionseismicsectionacrossthecentralGulfofCariaco(profilelocationinFig.2);EPFmt:El PilarFaultmainactivetrace(Audemardetal.,2007);R:minorfaults(VanDaeleetal.,2011).
Fig.5.(Coloronline).XRFscanningdatafromcoreCariac-09-06.SiCLi:siliciclasticlayers;BrprofilesforcoresCariac-09-18(closetosoutherncoast)and Cariac-09-15(Gulfcenter)areshowntodiscusscorrelationsandsignificationofpost-SiCL3increaseinBr;dottedverticallineasareferencevaluefor comparison(notethatCariac-09-18displaysasimilarBrevolution,butforlowervalues).BPagescorrespondtoprobabilisticmedianofcalibratedvalues distribution;the2sintervalisindicatedinFig.9.
I.Aguilaretal./C.R.Geoscience348(2016)70–79 74
3.3. Organicmattercontentandcharacteristics
Rock-Evalpyrolysisresultsonthethreecoresalongthe north–southprofile(Fig.4)aresummarizedinFig.6with respecttoTotalOrganicCarbon(TOC),S2peak,Hydrogen Index (HI), and Oxygen Index (OI). Diagrams labeled A represent the OI/HI plots similar to the Van Krevelen diagrams (Espitalie´ et al., 1985; Lafargue et al., 1998).
DiagramslabeledBrepresentTOCvs.S2HCasproposedby LangfordandValleron(1990).Atallsites,O.M.appearsof dominantly marineorigin(typeII),and ishardly ornot altered.IncoreCariac-09-18,closetothesoutherncoast, thesamplesaredominatedbytypeIII,whichsuggestsa terrestrialoriginforO.M.IncoreCariac-09-06,twomajor groupscanbediscriminated(separatedbythereddashed lineinFig.6):partofthesampleshavetheirO.M.attesting toapurelymarineorigin,whereastheotherpartofthe sampleshavetheirO.M.attestingtoamixedmarineand terrestrial nature. In core Cariac-09-15, the diagrams revealthatO.M.isofpurelymarineorigin.Twosubsets can be discriminated however (dashed line in Fig. 6).
SimilarsubsetsarefoundincoreCariac-09-05Bandtheir valueshavebeenaddedontodiagramB(yellowdots)for comparison.Weinterpretthetwosubsetsasrevealingan O.M.contentoftwodifferentorigins:i)insituproduction, andii) inputfromtheeastern CariacoBasinlikely from upwelling.
3.4. Occurrenceofcoarserand/ormoresiliciclasticlayers (SiCL)
The observation of several split cores reveals the occurrence of two coarser siliciclasticlayers, which we namedSiCL1andSiCL2inFigs.5and7.OnX-raypictures, another siliciclastic layer is revealed (SiCL3), which appearsdifferentfromSiCL1andSiCL2.Weshowedearlier thatthethreelayersareterrigenous,andmoreprecisely siliciclastic (Section 3.3). Grain-size analysis of cores Cariac-09-18,Cariac-09-15,Cariac-09-06,andCariac-09- 05Bshowsthatasandyfractionispresentonlycloseto thesoutherncoast.TheXRFprofileofcore Cariac-09-18 (Fig.7)revealstwoclearpeakscoincidentwithanincrease Type I
Type II Type III
Type I
Type II Type III
Type I
Type II
Type III
0 100 200 300 400 500 600 700
0 50 100 150 200 250 Oxygen Index (mg CO2/g TOC)
Hydrogen Index (mg HC/g TOC)
0 100 200 300 400 500 600 700
0 50 100 150 200 250 Oxygen Index (mg CO2/g TOC)
0 100 200 300 400 500 600 700
0 50 100 150 200 250 Oxygen Index (mg CO2/g TOC)
Cariac-09-15 Cariac-09-18
Cariac-09-06
Cariac-09-05B
A
B
0 2 4 6TOC(%) 0
10 20 30 40
S2
Type I
Type II
Type II
Type II
Type III
0 2 4 6
TOC(%) 0
10 20 30 40
S2
Type I
Type III
0 2 4 6
TOC(%) 0
10 20 30 40
S2
Type I
Type III
Fig.6.(Coloronline).CharacterizationofO.M.byRock-Evalpyrolysis.GraphslabeledA)modifiedVanKrevelendiagram,graphslabeledB)S2peakvs.TOC diagram(LangfordandBlanc-Valleron,1990).Thediagramsevidenceadominanttype-II(marine)O.M.contentforcoresCariac-08-06,Cariac-08-18(close tosoutherncoast),andCariac-08-15(Gulfcenter);coreCariac-08-06isenrichedintype-III(continental)O.M.,whilecoreCariac-08-15containspurely marineO.M.,andcoreCariac-08-06includesacombinationofthetwogroupsofO.M.origin(separationbyanhorizontalreddashedline).IncoresCariac- 08-18(greendots)andCariac-08-05(yellowdots),thetype-IIO.M.samplesmaybedividedintotwosub-groups(separationbyablackdashedline).
in silt-size grains in core Cariac-09-06. The siliciclastic enrichmentintheothercores–includingthoseinthegulf center–isclayeyorclayey–silty,i.e.relatedtosuspended- loadtransportanddeposition.
Thebase-to-topevolutionofboththeSiCL1andSiCL2 layers(Fig.7)istypicalofsedimentevolutionsrelatedto floodingprocesses(Beck,2009).Therefore,SiCL1andSiCL2 do not resultfrom gravity reworkingprocesses. Rather, these layers likely arise from inputs from southern tributaries. In contrast, the SiCL3 layer appears as a widespreadinputoffine-grainedsiliciclasticmaterial.We suggestthatthiswidespread(suspended-load)inputisan allochthonous terrigenous feeding coming from the westernentranceofthegulf,likelyinducedbyupwelling.
3.5. Chronologyandmeansedimentationrate
The different investigated parameters confirm the importanceof suspended-loadcoming fromtheCariaco Basin,including‘‘allochthonous’’O.M.attestingtoamore openmarineinfluence.Theregional
D
Rwasfounddueto the above-mentioned process. Therefore, this type of correction wasappliedtothe age–depthcurve ofcores Cariac-09-06and Cariac-09-5B (Supplementary Material S3). In both cores,a mean sedimentationrateof 1.0 to 1.2mm/yr wasdeduced. If theSiCL layers are takenas sedimentary‘‘events’’,then the‘‘normal sedimentation’’ratemightbeslightlylower.Consideringthecorrelations that we proposed between cores Cariac-09-06 and
Cariac-09-15 for theSiCL3 layer (S3),we inferthat the sedimentationratemightbelowerinthecentralgulf.
4. Discussion
Therecentdominantsedimentationinthecentraland southernpartsoftheGulfofCariaco,alongandadjacentto theEPFfaulttrace,isorganic-rich,finelyterrigenous,and poorlylayered.Biogenicandbio-inducedcomponentsare essentiallyplanktonic.Thehighparticulateorganic con- tent (up to 5% TOC or 20% O.M., Fig. 8A) appears of dominantmarineorigin(typeII),yetwithtwo different possible sources: local production, and a general input fromtheeasternCariacoBasinlikelybymeanofupwelling.
Thebrominecontentdisplaysanevolutionconsistentwith asignificantopenmarineinfluence(Thomsonetal.,2006;
Ziegleretal.,2008). Accordingtothe14Cdatingandthe chronology we have inferred (S3), the open marine influence might have started around 1850 AD. We are awarethattheBrevolutionmustbediscussedwithrespect to the O.M. content, since their combined short-term diagenesismayinduceBrchanges.Yet,theevolutionofBr andO.M.isdifferentbelowandabovethelayerSiCL3(Br vs.O.M.andBrvs.TOC,Fig.8BandFig.8C),whichsuggests that theyarenot directlylinked.We thus interpretthe post-SiCL3increaseinBrevolutionasattestingtoamore marine influence related to an increase in upwelling- driveninput(waterandsuspendedload)fromtheCariaco Basin.
Fig.7. (Coloronline).Grain-sizeevolutionincoresCariac-09-06andCariac-08-18andcharacterizationofthesiliciclasticlayers.Thebase-to-toppathsfor SiCl-1andSiCl-2indicatefloodingevents(Beck,2009);SiCl-3doesnotshowanycoarserfraction;itsterrigenous(siliciclastic)contentenrichmentisonly indicatedbychemicalparameters(XRF,Fig.6).BPagesarereportedasinFig.5.
I.Aguilaretal./C.R.Geoscience348(2016)70–79 76
Suchanupwellingincreasecouldhaveresultedfrom:i) areinforcementofthewinds,ii)amodificationofthegulf’s entrancegeometry.Paleoclimaticinvestigationsinnorth- western Venezuela(Polissaretal.,2006)indicatestrong easterlytradewindsduringtheLittleIceAge,whichended inthemiddleofthe19thcentury.Thesefindingsdonot supportthereinforcementof tradewindsinthesecond half of the 19th century. Conversely, the geometrical modification (bottommorphology,depth) of thebasin–
gulf connectionis a reasonablehypothesis knowingthe greatinstabilityofthearea.Wethusconsiderthattheopen marinesignaturerevealssignificantwaterflowsfromthe CariacoBasin,inducedbyupwellingresultingfromamajor morphologicalchangeatthebasin–gulfjunction.
Thesouthernhalfofthegulf’sentrancecorrespondsto thesteepwesternboundaryoftheRioManzanaresdelta (Fig.2).High-resolutionseismicprofilesshotinthedelta reveal the occurrence of large recent landslides (called
Mass Transport Deposits or MTDs in Fig. 9) that accumulatedatthefootofthedelta.Theirlargevolumes and their addition over time may have significantly modifiedthemorphologyandtopographyoftheconnec- tionbetweenthegulfandthebasin.Itislikelythatsomeor even most of these landslides were induced by large earthquakesontheEPFfault,asitwasobservedrightafter the1997earthquake(theharbor ofCumana´ underwent localpierscollapsesduringtheearthquake).
We thus interpret the SiCL3 layer as a sedimentary
‘‘event’’ related to one or to several Mass Transport Deposits on the western side of the Manzanares River delta.Thelayercouldrepresentafine-grainedsuspension astheoneobservedafterthe1997earthquake.Thelayer wouldthusindirectlytestifyforalargeearthquake.During the1853ADevent,a 4-to6m-hightsunamiwavewas reported.ThetimeofformationofSiCL3isconsistentwith theearthquaketime.Therefore,wesuggestthattheSiCL3 110
100 90 80 70 60 50 40 30 20 10 0
10 12 14 16 18 20 22
%OM(LOI550)
2 3 4 5 6
TOC(%)
OMwt.%
( L)OI550
14 16 18 20
20 30 40 50 60 70
2 3 4 5
LOG Cariac-09-06 Depth (cm) % TOC
Br(XFR 103 cnts/s) SiCL 1
SiCL 2 SiCL 3
1 2 3 A
C
Fig.8. (Coloronline).ComparisonofBrandO.M.profilesinCoreCariac-09-06.TheO.M.contentisrepresentedbyLOImeasurementsandCOTdeducedfrom Rock-Evalpyrolysis.Thepost-SiCL3BrincreaseisdisconnectedformtheO.M.evolution(A);O.M.vs.Brdiagrams(BandC)confirmthisdiscriminationfor theupper40cmofhemipelagicsediments;1)sampleswithinsiliciclasticlayers,2)samplesinorganicmudpriortoSiCL3,3)samplesinorganicmudafter SiCL3.
Fig.9. High-resolutionseismicprofileacrossGulfofCariaco’sentrance,displayingtheaccumulationofMTDs(profilelocationinFig.2).
layerwasformedasaresultofoneorseverallarge,coeval submarine landslides that were induced by intense shaking during the 1853 earthquake. The submarine landslides significantly modified the topography of the gulf’sentrance,andthesemodificationspromotedsignifi- cantwaterflowsfromtheopenmarineCariacoBasin.
Conclusions
Our results confirm that the sedimentation in the centralandthesouthernGulfofCariacois significantly fedbyupwellingatthewesternentranceofthegulf.We found evidence for a well-expressed, widespread, siliciclastic event (SiCL3), which we interpret as being thefine-grainedmaterialremobilizedduringanimpor- tantsubmarine landslidingalongtheManzanareRiver deltaforeset.Thismajorlandslidingepisodewouldhave significantlymodifiedthetopographyoftheconnection betweentheCariacoBasinandtheGulfofCariaco,and these changes would have promoted upwelling and entranceofopenmarinewaterflowsinto thegulf.We suggest that land sliding was induced by the large 1853 AD earthquake that struck the Cumana´ city.
Further work is needed to validate these hypotheses, whichconcernstheappraisalofearthquake-relatedrisks for the city and its harbor. Finally, the present investigationsshowthatearthquakerecordinsedimen- tary archivesmay bedetected throughvaried parame- ters,eachcaseneedinghoweverspecificapproaches.
Acknowledgements
This paper has been written in tribute to Jean- Franc¸ois Ste´phan, who initiated, thirty years ago, a fruitfulandstilldevelopingVenezuelan-Frenchcooper- ationin earthsciences;twoofus (CBandFA) hadthe great privilege of sharing his friendship and his permanententhusiasmduringfieldresearch.Theinves- tigations wererealized thanks toa Venezuelan-French ECOS-NortegrantNo.V10U01.I.Aguilar’sstayinISTerre LaboratoryandPhDpreparationwereachievedthanksto aPhDgrantfundedbyFundayacucho(VenezuelanPhD GrantsProgram). WealsoacknowledgeFunvisis(Vene- zuela Foundation for Seismological Investigations) and CNRS ISTerre (Earth Sciences Institute, Grenoble Alps University)foradditionalfundingoflaboratoryanalyses.
Coring campaign was performed thanks to the whole crew of R/V Guaiqueri II and Andres Lemus, head of Camudoca. We acknowledge Rachel Boscardin at ISTO Orle´ansforguiding Rock–Evalanalyses.Thispaper has beenreviewedbyKlausReicherterandAure´liaHubert- Ferrari, and by Associate Editor Isabelle Manighetti;
manythanksfortheircontributiontotheimprovement ofourinitialmanuscript.
AppendixA. Supplementarydata
Supplementarydataassociatedwiththisarticlecanbe found,intheonlineversion,athttp://dx.doi.org/10.1016/j.
crte.2015.10.001.
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