Cuadernos de Geología Ibérico ISSN: 0378-ltiI2X[998. número 24. 69-98

Clay mineraisandgeochemicalevolutioninthe Toarcian-lowerAalenian

ofthelusitanianbasin(Portugal)

Mineralesdearcilla y evolucióngeoquímicaduranteel Toarciense-Aalenienseinferior

en la cuencalusitaniense(Portugal)

L. V. DUARTEDpto. of EarthSciencesof theUniversityof Coimbra,

3049CoimbraCodex,Portugal

AESTRACT

The Toarcian-LowerAalenianmegasequenceof the LusitanianBasinshowsquantitavelyimportantmarly-limestonesedimentation.lii this typeof se-dimentaryenvironmentsomeparameters(clay minerals,Sr, ~‘3Cand~‘~O)arevery importantas stratigraphicandpalaeo-oceanographicmarkers.The analy-sis of thesevariablesallowedthe following conclusionsto bemade:

The clay mineralassociationsshowlateralandverticalevolutionin these-ries that showstratigraphicdistributionauddepositionalcontrol; the accumu-lation of chlorite,particularlyin the sectorsin the westernpart of thebasín,in-dicatesinflux originating from the West,corroboratingthe palaeogeographicschemesacceptedin someof the infilling phases,in accordancewith the mo-deIs set by Duarte(1995b).

The evolutionaryspectrumof the geochemicalelementsshows,particularlytbr 8~3C, exemplarystratigraphicdistributionbetweenlower andmiddleToar-cian. lts evolution alongthe lengthof theseriesagreeswith the sequentialde-velopmentsof the series.The large,positiveandmoreabrupt incursionscan beseenin ibe transgressivedepositionalphases,whilst the moregradualnegativetendenciescorrespondto regressivemechanisms.

Key words:Clayminerals,carbonandoxygenisotopes,strontium,marl/li-inestonealtemations,Toarcian,Aalenian,LusitanianBasin.

69

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L. V. Duarte Clay mine,aIÁandgeoc.hemñ-alevolutionin Ihe Toa,-cian-lowerAalenian

RESUMO

O Toarciano-Aalenianoinferior mostrana Bacia Lusitanianauma sedi-menta9aoexpressivaem sedimentosmargo-calcários.Neste tipo de ambientesedimentar,algunsparámetrosrevelam-seimportantesmarcadoresestratigrá-ficos e paleoambientais.A análisedessasvariáveispermiteconcluir o seguinte:

As associagóesdosmineraisde argila apresentamuma evolugáolateralevertical na sériequeevidenciamumarepartigáoestratigráficae um controlode-posicional;a acumula~áodechlorite, cm especialnos sectoresocidentaisdaha-cia, pressupóeum acarreiode material argilosoprovenientede Oeste,corro-borandoos esquemaspaleogeográficosadmitidosparaalgumasdasfasesdeenchimentode acordocomosmodelosde Duarte(1 995b).

Oespectroevolutivodos elementosgeoquímicosmostra,emespecialparaoumareparti~áoestratigráficaexemplarao nivel do Toarcianoinferiore mé-

dio. A suaevoluyáoao longo da sérieéconcordantecomosdesenvolvimentossequenciaisdasérie.As grandesincursóespositivas,quasesempremaisbruscas,verificam-senasfasesdeposicionaistransgressivasenquantoque as tendén-cias negativas,ínaisgraduais,correspondema mecanisínosregressivos.

Palavraschave:Minerais de argila, isótoposde carbonoe oxigénio, estrón-cío, alternánciasmarga-calcário,Toarciano,Aaleniano,Bacia Lusitaniana.

RESUMEN

La megasecuenciacorrespondienteal Toarciense-Aalenienseinferior enla cuencaLusitaniensecomprendeunasucesióndilatadade materialesmargo-sosy calcáreos.En esteambientesedimentario,algunosparámetros(mineralesde arcilla; Sr, C.’3 y 0.8) se revelancomo indicadorespaleoceanográficosyambientalesdegran importancia.El análisisde dichasvariablesha permitidoextraerlas siguientesconclusiones:

Las asociacionesde mineralesde arcilla muestranunaevoluciónlateral yvertical enlas seriesquerevelaunadistribuciónestratigráficay un control so-bre la sedimentación;laacumulaciónde clorita,particularmenteenel sectoroc-cidentalde lacuenca, indicael influjo de aportesdel Oeste,lo que corroboralos esquemaspaleogeográficosaceptadosparaalgunasfasesde relleno, deacuerdoconlosmodelospropuestosporDuarte(1995b).

El espectroevolutivo de los elementosgeoquimicosmuestra,especial-menteen el casodel C.’3, unadistribuciónestratigráficamodélicaentreelTo-arcienseinferior y medio.Su evoluciónalo largode todaslas seriesconcuerdacon el desarrollosecuencialde las mismas.Las incursionesampliaspositivasmásabruptas,se registrandurantelas fasestransgresivas,mientrasque lastendenciasnegativasy másgradualesse correspondencon los mecanismosoprocesosregresivos.

Coade,nos de Geología Ibérica1998, núnicro 24, 69-98 70

L. V. Duarte Claymineralsand geochemicalevolutionin the Toarcian-lowerAalenian

Palabras clave: Mineralesde arcilla, isótoposde Oxigeno, isótoposdeCarbono,Estroncio,alternanciade margasy calizas,Toarciense,Aaleniense,CuencaLusitaniense.

INTRODUCTION

The evolutionaryhistory of the toarciansedimentsin the LusitanianBasin isdirectly relatedto the first rifting phaseof the North Atlantic (Wilson, 1988;Wilson ci al., 1989; Soaresci al., 1 993a).Thiseventincludesinfilling from theMiddle Triassic(Rochaci al., 1990)to the top of the Callovian,with sedimen-tary unconformitiesof basinalvalue(Soareseta!., 1 993a,b;Soares& Duarte,1995). In the Toarcian-LowerAalenianthereis a phaseof substantiallutitiemarly-limestoneaccumulation,with hemipelagiecharacteristies,despitethedepositionaldifferentiationoccurringin sornesectorssuchasTomarandPeni-che (Duarte, 1995b).Thesesedimentsshowsequentialorganisationsupportedby two megasequentialunits (MEl and ME2, Duarte& Soares,1993; Soares&Duarte,1995)and four depositionalsequences(mesosequences:MST), boundedby correlativeregionaluneonformities(DT) (Fig. 1). Thoseunits show,in dif-ferentiatedcontextsof a homoclinal ramp,shallowingupwardevolutions;thesecharacteristiesareparticularlyevidentin the proximal partsof the basinsuchasTomar(MSTT) and in the southwestborderatPeniche(MSTP) (Figs. 1 and2).

In this sedimentarycontext,it is importantboth to analysein detailthe la-teral andvertical behaviourof somechemicalandmineralogicalparametersaudestablishfine stratigraphybasedon its evolutionaryaspects.The typeof evo-lution, the anomaliesfound and/orthe correlationbetweensomeof thesepa-rametersmayalsocontributeto the understandingof the palaeoenvironmentalvariablesthat mayhavecontrolleddeposition.

SrRATIÚRAPHIC SECTIoNs

Thereis quite agood understandingof thecarbonatefaciesthat characteri-se the Toarcian-LowerAalenianseries in alt the Lusitanian Basin (Duarte,1 995b; 1997). Hemipelagiesedimentationdominatesa Largepart of the basin,with abundantammonoids,sometimeswith very rhythmicmarí/limestoneal-ternations.The Tomar and Penicheregionscontrastwith this faciessetting,showingassociationswith characteristicof a shallowingrampwith strongbent-hie bioclastiedeposition(Duarte, 1995b, 1997) andanouterto middlesubma-nnefan with calciturbidites(Wright & Wilson, 1984) respectively(Fig. 3).

In orderto demonstratethe depositionalvariability in the basinduring thistime period, six sectionswerechosen(Coimbra,Figueirada Foz, Rabayal,Porto de Mós, Tomar and Peniche),alí of them referencedin the biostrati-graphicscaleof ELMI eta!. (1989)(Fig. 2).

71Coadernos de Geología Ibérica

[998,oú¡ncro24. 69-98

Riostratigraphy

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L. V. Duarte Clay mineralsandgeochemicalevolutionin the Toarcian-lowerAalenian

FIG. 1 —Sequentialchanof the Toarcian-LowerAalenian fromthe LusitanianBasin: megasequentialunities (Soares& Duarte, [995),unconfonwitiesanddepositionalsequences(Duarte.1995b).FIG. 1—CuadrosecuencialdelToarcicnse-Aalenieí,seinferiorparala CuencaLusitaniense:Unidadesmegasecuenciales,segúnSoaresy Duarte (1995);disconformidadesy secuenciasdeposicionales.segúnDuarte(1995b).

Fío. 2—Locationmapof thestudiedsectionsandlateraland verticalrepartitionsof thedepwútionaíse-quences(horizontalnot tu seale). 1- Marí/limestonedecimetricaltemationswith small brachiopods:2-Calc¡lutitic to calcarenitiefaciesaltematedwith marísin centimetrieseale:3- Metric[o decimetrielutitiemarí/limestonealternations;4- Decimetriemarí/limestonealleniationswith bioconstruclions;5- Marísandmarly-limestoneswith brachiopods;6- Marís andmicritie limestoneswith bioconstructions;7- Bio-clastielimestones:8- Lenticularsiliciclastiefacies;9- Ooiitic andpeloidalfacies; lO - Oolitic andsandylimestonesfacies.I’iG. 2—Mapade situaciónde los perfilesestudiadosy reparticiónhorizontal y verticalde lassecuencias

72Cuadernos de Geología Ibé,ica[998, número24,69-98

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L. V. Duarte Clay minerals aná geochemical evolution in the Toarcian—It> wer Aalenian

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FIG. 3—Fac¡esandpalcogeographicmap duringM5T3 plenase(Bilrons-Bonarellii zones)(Duarte,1995b). 1 - Mariy-limestonefacies; 2- Bioclastie facies;3- Detrital andresedimeotedoolit¡c facies;4 - Emergedblock; 5 - ilcsperic Massif(submergedduring MST3); 6 - Thickness;7 - Palaeocurrents.F[G. 3—Faciesy mapapaleogeográficodurante]a faseM5T3 (cronozonasBifrons-Bonarellii; Duarte,1995b). 1-Faciesde calizasmargosas;2-Faciesbioclásticas;3-Faciesoolíticasresedimentadasy de-trincas;4-Bloque emergido;5- MacizoHespérico<sumergidodurantela taseMST3); 6- Potcnci¿~,7-Paleocorríentes.

ANALYTICAL METHOD5

The sedimentaryparametersin this studyincludeclay mineraisin marlyfa-cíes and sornechemicalcomponentsof the carbonatefraction suchas stron-tium andthe carbonaud oxygenstableisotopes.The samplingtook into con-sideration the previouscharacterisationof the Iithofaciesandtheir

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c:uadernos de Geología Ibéru a[998. número24, 69-98 74

L V. Duarte Clay mineralsandgeochemicalevolutionin theToarcian-lowerAalenian

mesosequentialsetting(videDuarte,1995b).Carewas takento makethepro-cesssystematic,andat timesevenrandom,to bestillustratethe sedimentaryvariability of the series(Tables1 to 9 and Figs. 4 to 7). The stableisotopeswereonly analysedin the Rabayal,FigueiradaFoz andPortodeMós sections(Figs. 8 and9).

Clay Minerais: The <lay fraction of man facieswere analysedby X-raydiffraction (XRD) usingaPhillips PW1840diffractometerwith CuK radiation.The methodof mineralogicalpreparationand determinationwas defined inLapa& Reis(1977). Semi-quantitativeevaluationsarebasedon the calculationof theheightsandareasof theregisteredpeaks,a methodwhich is describedinThorezeta!. (1970) (in Lapa& Reis, 1977). Vermiculite andinterstratifiedmi-neralswerenot includedin this processdueto the difficulty in obtainingwelldefinedpeaksfor them.The determinationof the crystallinity of illite usingtheKubler index (Kubler, 1964),was also carriedout in the characterisationof theclay-mineralassemblages.

Strontium:The chemicalanalyse(about400 samples)werecari-iedout un-der the carbonatephasevia atomieabsorptionspectrometry.

Carbonand Oxygen Isotopes:The analytieal processof 70 samplesconsistedof the removalof carbondioxide via the solubility of the cabonatephaseusing 100%phosphoricacid(MeRea,1950; Craig, 1957). The final iso-topie compositionwas obtainedwith theuseof a massspectrometer,with thecalculatedvaluesbeing in relation to POE international standard(Craig,1957).

CLAY MINERALS

The aparentrhythmicity demonstratedby the marí/limestonelithofaciesdoesnol agreewith the associationsof the clay minerals.Tbis study of ap-proximatelyfour hundredsamplesshowsthat thereis aclearstratigraphicdi-víston of themineralsin relationto the mesosequentialunits established(Figs.

1 and2), with simpleandcomplexassociationsof illite, kaolinite, smectite,ch-lorite, varioustypesof mixed layersandrare vermiculite. In this context,la-teral variation of the mineralogicalassociationscan be seenin determinedstra-tigraphic positions.This fact, altloughdifficu]t to interpretduefo te exclusiveactionof diagenesis,contributesdecisively in palaeogeographicreconstitu-tions.

Theresultsobtainedallow the discussionto be gearedin two different di-rections:astratigraphic(sequential)one,anda palaeogeographicone.In the lat-ter, the variationsof clay minerals(both horizontalandvertical) revealthat thebasin sufferedoppositedetritic influencesfrom east andwest (discussioninDuarte, 1 995b).Thepossibledetritie origin of the mineralscan serveas an tm-portantcriterion in the understandingof thetypeof influx originatingfrom thelandbordersof te basinvia te extensivetluctuationsin sea-level.

75 Cuadernos de Geología Ibérica

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L. V Duarte Clay minc’rals and geochemical evolution in the Toarcian-lower Aalenian

FIG. 4—Evolurion of clay mineraisandgeochemicaldatain theFigtíeiradaFo,. regioíl (seelegendin Fig. 5).FIG. 4.—Evoluciónde los datosgeoquimicosy de los mineralesde arcillaen la regióndeFigueiradaFos (y. la leyenda,en la Hg. 5).

CucIde rnos de Geología tbéríco[998.númcru 24, 69-98 76

L. V. Duarle Clay mineralsandgeochemicalevolutionin the Toarcian-lowerAalenian

Fír; 5.—Evolutionof claymineralsandgeochemicaldatain thePortode Más region.Fw;. 5.—Evoluciónde los datosgeoquimicosy de losmineralesde arcilla en la regiónde Portode Más.

Cuadernos de Geología Ibérica[995, número24, 69-98

LI~ Ehuta Kaolínite Smectite Chíerite Not Observad

77

L V. Duarte Clay mineralsandgeochemicalevolutionin theToa,c.ían-lowerAalenian

FIG. 6—Evolutionolclay mineralsandgeochemicaldatain theTomarregion(seelegendin Fig. 5).Fíc;. 6—Evoluciónde losdatosgeoquímicosy de los mineralesde arcilla en la regiónde Tomar(y. laleyenda,en la Fig. 5).

RESULIS AND DísCussíoN

Illite: This mineral is presentin alí mineralogicalassociations.Its predo-minanceis soevidentthatthe towestvalues,with the exceptionof’ MST2A andthe baseof MST2B, arealwaysaboye55%.The amountsof illite in thesetwoexceptions,whichare alwaysassociatedwith quite importantvaluesof smec-

Coade,nos cíe Geología Ibérica[998. número 24. 69-98 78

L. V. Dua,-te Clay minerals and geochemical evolulion in the Toarcian-lower Aalenian

Fío. 7—Evolutionof claymineralsandgeochemicaldatain thePenicheregion(seelegendin Fig. 5).Fío 7—Evoluciónde los datosgeuquimicosy de los mineralesdearcillaen ta regiónde Peniche(y. laleyenda,en la Hg. 5).

Coadernos de Geología Ibérica79 [998, número 24, 6998

L. Y Duarte Clay mineíaLt and geochemical evolution in the Toa,cian-lower Aalenian

% illite % Kaolinite % Smec-tite % Chlorite Sr (ppm)

RabaíalPortoMósCoimbraE. FozTomarPeniche

72-lOO70-9155-8660-9443-8365-97

0-176-253-197-227-214-30

o0-3

2-38o

6-40o

0-lI 268-3730-4 290-311

0 406-7740-33 330-510

0 590-5 358-396

TÁní» t—Clay n3ineraisand Strontiuíndistributionin MST1. MSTTI andMSTPITABLA [—Mineralesde laarcilla y distribucióndel Estroncioen MSTI,MSTTI y MSTPT

Sections % Illite % Kaolinite % Sinectite % Chlo,ite Sr (ppm)

Raba9alPortoMósCoimbíaM. VelhoCantanhedeF. FozTomarPeniche

88-9788-9632-657-51

32-6560-9356-Sl51-Sl

4-II3-126-3010-2810-217-398-3019-41

0-3o

0-3229-8223-480-9

0-19o

0 181-4780 309-5570 627-1.032O —

o181-972

19071-461

Oo

2-12

TA[311¿ 2—Clay mineralsaí,d Strontiumdistribuhion in MST2A, baseof MSTT2 and MSTP2A.TAnIA 2.—Mineralesde Ja arcilíay disíribucióndel Estroncioen MST2A, basede MSTT2y MSTP2A.

tite, illite-smectitemixed-layersaudkaolinite, sporadicallyreachvaluesof lessthan 10%.Theselow valuesaredtíe to antagonisticbehaviourbetweenillite andsmectite.

lis higliesí valueis foundfrom ihe top of MST3 (generallyaboye75%;Ta-bIes 1 to 6) to the top of MST4II, becomingat times an exclusive mineral,equallingthe randornly interstratifiedillite-smectite. This tendencyseemstocontinuepastthe upperboundaryfor that unit (ME2), remainingin the sedi-mentsof the middle andupperAalenian(MF).

The stateof degradationof úhis mineral is a characteristiewhich reínainspracticallyconstaníin the wholeseries.showingvcry variablecrystallinityva-lues (between0.5 and 1.1). However, thereare sornesignsthat indicatedepo-sitional variability.This is the casein MSTP2A andMST2B of the westernre-gíons(Figueirada Foz and5. Pedrode Moel), whercthe detritic contributionanddic sedimeníarycontexííakeon auniqueappeararwe,audwherecrystalli-nity leveN mayreachvaluesof 0.5 and 0.6, relatively highcr thanvaluesobtai-

Sec.tions

c.er,de,,,,>.sdc Geología Ibéil> o[998 naneen,24 69-98 80

L. V. Duarle Clay mineralsandgeochemicalevolutionin theToarcian-lowerAalenian

% Illite % Kaolinite % Smectite % Chíarite Sr (ppm)

AlvaiázereRaba~alPortoMósCoimbraM. VelhoII Foz5. P. MoelTomarPeniche

76-9286-9667-8941-9129-4743-7245-479-7857-84

7-244-142-334-375-2411-1814-237-3312-39

0-9o

0-190-4037-5114-4412-360-840-8

0-80-7O

0-11Oo

5-17o

194-339125-36188-308

134-727257-321127-197

0-12 241-412

TÁmE 3—CIaymineralsandStrontiumdistributionin MST2B, MSTT2 andMSTP2B.LABIA 3—Mineralesde la arcilla y distribucióndel Estroncioen MST2B,MSTT2y MSTP2B

Sections

AlvaiázereRaba~alPortoMósCoimbraF. FozTornarPeniche

% Illite % Kaolinite

83-1007 1-95

46-10029-lOO43-9479-89

35-lOO

0-95-240-40-163-390-100-17

% Smectite % Chlorite Sr<ppm)

0-7 0-lI 206-2100-5 0-7 209-353

0-54 0 211-2890-71 0 275-3810-38 0-9 230-3344-17 0 45-2050-47 0-2 232-423

TABLE 4—ClaymineralsandStrontiumdistributionin MSTS,MSTT3andMSTP3.TABLA 4.—Mineralesde la arcilla y distribucióndel Estroncioen MST3, MSTT3 y MSTP3

nedin the othersectors.this evidenceseemsto be linked to detritic influx orí-ginating from the west borderof the basin ratherthan a possiblediageneticoverprint,hypotheticallyresponsiblefor the increasein the illite crystallinity.Therefore, this mineral was probably inherited from the surroundingconti-nentallandmasses.Its almosíexclusiveoccurrencefrom the top of MST3 (topof Gradatazone)is certainlyrelatedto the regressivedevelopmentof the sedi-mentation,relatedto lowstandof sea-level,as is shownin DUARTE (1995b)(Fig. lO).

Kaolin¡tc: With the exceptionof the presenceof mixed-layers,whoseidentificationandevaluationis alwaysdifficult, kaolinite is the secondmostre-presentedtypeof clay mineral in the series.Parallelto the expansiveminerals,

Sections

81C uade,nos de Geología Ibérica

[998. número 24. 69-98

L. V. Duarte Clay minerals and geochemical evolution in the Toarc-ian-lowerAalenian

Sections % Illite % Kaolinite % Smectite % Chlorite Sr <ppm)

Alvaiázere 88-100 0-12 0 0 —

Raba9al 93-lOO 0-5 0 0-2 226-373Coimbra 100 0 0 0 195-324Cantanhede 83-100 0-13 0-15 0-8 290-292F.Foz 69-91 4-25 0 0-18 244-370Peniche 53-100 0 0-47 0 133-322

TABLE 5—Clay minetalsandStrootiundistributionin MST4A andMSTP4A.TABLA 5.—Mineralesde la arcilla y distribucióndel Estroncioen MST4A y MSTP4A.

Sec.tion.s % Ilíhe % Kaolinite % Smectite % Chlorite Sr(ppm)

Alvaiázere 95-100 0 0-5 0 —

Raba~al 92-lOO 0-6 0 0 207-385PortoMós 77-100 0-10 0-8 0-23 189-298Coimbra 94-100 0 0 0-6 248-343Cantanhede 85-100 0-13 0-7 0 309-324E. Foz 57-90 6-25 0 4-19 288-413

TABLE 6—Clay ínineralsandStrontiumdistributionin MST4B.TABLA 6—Mineralesde [a arcilla y distribuc,óndel Estroncioen MST4B

its highestrepresentationis in MST2A (rnaximumof 39% in Figueirada Foz;Table 2, Fig. 4), in MSTP2A (19 to 41% with an averageof 33.4%;Table 2,Fig. 7) and in the basesof MST2B (maximumof 37% in Coimbra;Table 3)and MSTP2B(maximumof 40%,with an averageof 28%; Table3). It beco-mesrarerfrom MST3 onwards,andevendisappearsin the regionsof Coimbraand Portode Mós (Tables4 to 6; Fig. 5). In MST4A, it is practicallyabsent,ex-cept for the regionsof CantanhedeandFigueirada Foz, occasíonallypresentwith valuesunder 12%. In MST4B,this tendencycontinues,beingintensifiedin the differentiatedsectorin the northwestpart of the basin.In TomarandPe-niche (Figs. 6 and7), this mineraldisappearsearlierIban in the otherregions(top of MSTT3 andbaseof MSTP3).This couldbe an indicationof an everin-creasingcontinental landmassinfluenceon deposition(marineenvironmentbe-coming progressivelyshallower).

The abundanceof kaolinite in the jurassicsedimentsis, accordingtoChamley(1989),an indicatorof landmasseswith ahot climateandconstanthumidity The absenceof this mineral,or its reduction,seenfrom the Llpper

Cuadernos cje Geología Ibé,ica[998.udencro 24, 69-98 82

L. y Dua,-te Clay mineralsandgeochemicalevolutionin theToarcian-lowerAalenian

Toarcianupwardscanbe justified by a lowering of sealevel and the deve-lopmentof more arid conditions.Jtsenrichmentin the distalsectorof the ba-sin (Figueirada Foz),throughoutthe units MST4A andMST4B, maybe theresultof two mechanisms:diageneticalterationof the otherclay mineralspre-sent(illite andchlorite) or the detritic influx from thewest,as is thecasewithchlorite.

lf one can assumea strong relationshipwith the climate, then the presen-ce/absenceof kaolinitemaybe associatedmorewith thedegreeof drainageon tecontinentalareasthanthe transgressiveor regressivecharacterof the sedimenta-tion. It may evenbepossiblethat a periodof morearid conditionsmay be res-ponsiblefor the practicaldisappearanceof kaolinite in the marineenvironment.

Sniectite:Smectiteandrandomlyillite-smectitemixed layersshowan ap-parentlymorehomogeneousstratigraphicdistribution,with theselast explainedín somecases,as theresultof diagenesis(degradation).

With high valuesin MSTT1 (andin MST1 in Coimbra;Table 1), MST2A(23 to 48% in Cantanhede;28 to 82% in Montemor;Table 2), baseof MST2B(37 to 51% in Montemor;Table 3), baseof MST3 (<71% in Coimbra,< 54%in PortodeMós; Table 4, Fig. 5) and in MSTP3(Fig. 7),smectiteshowsquiteopen basalreflections,denotinglow crystallinity values.Thehorizontaldistri-bution of this mineral is quite variable, with extremelyobjectivezonation inMST2A andthe baseof MST2B (Serpentinuszone)in theregion from Mon-termor-o-Velhoto Cantanhedeandin MST3 (Bifrons-Gradatazones)in the re-gionsof CoimbraandPortode Mós.

Therehasbeenalot of discussionabouttheorigin of theseminerals.Theideadefendedby Chamleyesa!. (1992), that clay mineralogybasedon tectonicinstability, seemsto coincideparticularlywell with the occurrenceof this mi-neralin MST2A. This unit, characterisedby depositionin shallowcarbonateramp with increasedenergyconditions(Duarte& Soares,1993), is sympto-matieof the intensityof mechanicalerosion,which accordingto Chamleyeta!.(1992),mayhavecontributedto thedismantlingof smectiteresultingfrom pe-dologicalactivity atthe surface.The priveligeddistribution of this mineral inMST2A in Montemor region,or rather alongthe Arunca-Montemoralign-ment,maybe directly linked to the meridional fracturationshown in Soaresetal. (1993a).

With the exceptionof MST2A, its highestlevel of occurrenceappearsintransgressive(flooding) conditionsdemonstratedin the basesof the mesose-quencesand, in panicularin the casesof MST2B, MSTP3andMST3; the lat-ter, in sectorsas far away,in palaeogeographicalterms, as CoimbraandPortodeMós (Fig. 3), wheresmectiteis exclusivelyassociatedwith illite. In fact, itseemsthat its highestvalueshavenothingto do with anytype of diageneticevolutionbetweenthe baseandtop of theseries,in spiteof the factthat locally,difractogramsshowa very degradedassociationof smectiteand illite.

Owing to goodstratigraphicdistributionof thismineral, it is possiblethatthe role of diagenesisis well expressedin mineralogicalassociationswith ran-

83 Cuade,nos de Geología Ibérica[998, nómcro 24. 69-98

L. V. Duarte Claymineralisandgeochemicalevolutionin theioarcian-lower Aalenian

domly illite-smectite mixedlayers.Theseclay mineralscan be the resultof theintermediatetransformationof smectitein illite (is strongthe antagonismbet-ween thesetwo minerals).

Clilorite: This mineral tendsto occurcompletelyrandomlyin the wholese-ries, with values rarely reaching10%.Thereareexceptionsin the sectionssi-tuatedin Ihe mostwesternpartsof te basin,suchasMSTL in FigneiradaFoz(< 33%; Table 1, Fig. 4), MST2B in 5. Pedrode Moel (5 to 17%; Table 3 ),MSTP2A (< 12%; Table 2) andMSTP2B (< 12%; Table 3) in the region ofPeniche(Fig. 7). The contentof this mineral in theclay assemblagesfrom thetop of MST3 arepractically constantin FiguciradaFoz (<18% in MST4A; la-ble 5; < 19% in MST4B; Table 6). Its absencein determinedstratigraphiccompartmentsof the seriessucli as MST2A (Table2), baseof MST3 andthewholeTomarsection(Fig. 6) is alsoimportant.

Not beingagoodstratigraphicmarker,the occurrenceof chlorite seemstobe closely relatedto the geographiclocationof the sectionsin the basin,there-fore being a good palaeogeographicindicator. Its frequent associationwithsilty micaceousminerals,presentin the marísof the manyunits in the Figuei-rada Foz to Penichecoast,allows us to consideradetritic origin from the Westborderof the basin.Assumingthis, chlorite appearsas a mineraldirectly inhe-rited from the igneousandmetamorphicrocksof the Berlengahorst, a blockwhich was probablyemergedduring the UpperToarcian(Wright & Wilson,1984; Duarte, 1 995b). Its abruptreductionro the eastof FigueiradaFoz, par-ticularly in the caseof MST4B, may be in agreementwith Zimmermann(1982)who statesthat thismineralhasa terrigenousorigin with depositionaf-tera lessimportant transportphase.

Verniiculite: In this study, vermiculiteappearscasually in mineralogicalassociations,andit is not possibleto relateit to anytypeof stratigraphicposi-tion and/or geographiclocation. Consideringthat its presenceis associatedwíth sectionsalong the lengtbof ¡0w slopingvertices,it is possiblethat it is re-latedto morerecentsuperficialpedologicalalterations.This idea is corrobora-tedby Chamleyeta!. (1992,p. 116), that explainsthe vermiculitie stateasre-sult of smectitealterationdueto Mg2* fixation.

STRONTIUM DISTRIBUTION

RESULIS AND DísCussíoN

Thevaluesof Sr, whenanalysedvertically (Figs. 4 to 7), do not showanykind of tendency,but demonstratestratigraphiczonationpractically concor-dantwith the mesosequentialunits (Tables1 to 6). This fact aloneaparentlydemonstratesa weak alteration in diagenetieprocesses.The variability ofthis elementin the toarcianlithofaciesallows us to drawthe following con-clusions:

Caade,nos de (;eología ¡hética[998, número 24 69-98 84

L. V Duarte Clay mineralsand geochemicalevolutionin the Toa,-cian-lowerAalenian

— The main anomalies,with valuesof over 1000 ppm, sornewith morethan4% of thetotal rock, canbe seenin particularcontexts,beingassociatedwith celestitenodules(top of MST2B, in Raba9al),encrinitic facies(MST3, inCoimbra,Rabayl,Alvaiázereand Porto de Mós) and lateral faciesof mud-rnounds(MST4B, in Rabagal).

— Isolating the lithological contextsmentionedaboye,thehighestvaluesofSr in the carbonatephasecan be observedin MST2A (Coimbra-Figueirada Fozregion;Table 2). The high valuesobservedin limestonelithology, on averagehigherthan600ppm, standout in comparisonto theothervaluesobservedin theunitsaboyeandbelow it. It is alsopossibleto establishchemicalzonationin thewholeToarcian seriesof Tomarsection(with valuessystematicallybelow200ppm; Tables 1 to 4; Fig. 6). MST2B seemsto shownegativeevolution,a ten-dency which is particularlyevidentin RabagalandPortode Mós (Fig. 5).

Valuesof this elementmaybe conditionedby numerousfactorsincludingdiageneticones.Accordingto Renar(1984), theevolutionof tbk elementinpelagicenvironmentsseemsto be, amongother reasons,associatedwithtransgression-regressioncycles.However,theanalysisof its behaviourin itsassocíationwith the variationof sea-levelin the Portugueseseriesneedsto becarriedout with sorneprudence.The apparentantagonismin the valuesofstrontium,observedin the microsparitsof the units MST2A (on average600ppm) andMST2B (120 to 250 ppm), discardthe possibilityof diageneticef-fects andjustify a primary origin. However, the well-fundamentedtheoryput forward by Renard(1984,p. 357) that the synthesisedcarbonatesduringthe transgressivephasesenrichedin strontiumrelativeto thoseproducedin re-gressivephasescannotbe appliedto the toarcianseriesof the Lusitanianba-sin. In fact, consideringthe particulartypeof sedimentationin MST2A (videDuarte, 1995b),wherebenthicmacrofaunais absentandthe level of 6’3C isvery low (aswiIl be demonstratedfurtheron), it is possiblethat this elementmay havea continentallandmassorigin, linked to evaporitesin the drainagearca. This link with thesalinity of the watermayjustify the strongincreaseinbenthic faunain the baseof MST2B (vide Duarte, 1 995b) wherethe valuesare particularlylow (on average200-250ppm). The samerelationshipbet-weenbenthic faunaandSr is visible forexample,in MSTT2wherenot onlyis thereabig percentageof organisms,but also10w valuesof that elementun-der 200 ppm) areevident(Fig. 6).

STABLE ISOTOPES

Ihe stableisotopestudiesin the carbonatesedimentshavebeenwidely usedin the stratigraphicdefinition of the oíd sedimentaryrecordandas importantin-dicatorsin palaeoclimaticreconstructions.It is known that their variations,particularly in pelagicenvironments,witness the main chemicalchangesoc-

Cuadernos de Geología Ibé,ica85 [998 número 24. 69-93

L V Duarte Clay mineralsandgeochemicalevolutionin theToarcian-lowerAalenian

Sampies &3C 318Q

251 Z6 MD 1,43 —3,79

251 Z12 MSTI 1,84 —3,28

251 Z13 2,07 —3.82

251 ZíSa 1,73 —3,58251 Zl5d MST2A 1,08 —3,52251 FCI5 1,20 —3,14251 Zl5g 1,97 —2,77

251 Z19 2,20 —3,9225 1 Z22 4,07 —3,8025 1 Z32 3,72 —3,74251 Z40 3,07 —3,89251 Z44 MST2B 2,73 —3.80251 Z46 2,59 —3,79251 Z48 2,09 —3,52251 Z49 1,87 —3,45251 ZSOs 1,71 —3,19

251Z51 2,52 —3,80251 ZS4s 2,49 —3,03251 ZSS MST3 1,99 —2,82251 ZS9b 1,98 —3,07251 Z64 1,70 —3,03251 ZGS 1,77 —2,57

251 Z66a 1,55 —2,68251 Z66d MST4A 1,79 —2,48251 Z68c 1,92 —2,72251 Z69b 2,19 —2,08

251 Z70s2-l 1,89 —2,5925 1 Z70s2-2 1 .89 —4,70251 Z75C MST4B 1,95 —2,51251 Z9() 1,87 —2,30

251 Z94 MF 2,08 —2,66251 ZIOS 2,22 —265

TABIE 7—Isotopiecompositionsuf thesaínples.studiedfrom Raba~alsection.TABLA 7—Composicionesisotópicasde las muestrasestudiadasde [acolumnade Raba~al.

Cuadernos de Geoloeóa Ibérica 86i998 número24. 69-95

L. V. Duarte Clay mineralsandgeochemicalevolutionin the Toarcian-lowerAalenian

Samples

308 PM3

308 PM7308 PMI3308 PM19

308 PM3I308 PM62308 PM72308 PM8l308 PMl 13308 PMI6O

308 PM199308 PC202308 PC206308 PC218308 PC243308 PC265308 PC288

308 PMSO9308 PC304308 PM544308 PMS92b308 PM642308 PM894

MST 1

MST2A

MST2B

MST3

MST4

~C

1,66 —4,05

0,66 —4,030,20 —4,680,89 —3,46

3,87 —3,374,27 —3,684,29 —3,833,83 —3,423,33 —3,582,79 —3,08

2,54 —3,012,46 —2,792,61 —2,862,55 —3,022,05 —2,841,92 —2,612,05 —2,35

1,481,911,932,071,751,68

—1,95—2,71—2,29—2,78—2,40—2,32

308 PM906 MF 2,18 —1,81

TABLE 8—Isotopiecoínpositionsof thesampiesstudiedfrom theP. Mós sectionTABLA 8—Composicionesisotópicasde lasmuestrasestudiadasen la columnade PMós.

curring in oceans(Seholle& Arthur, 1980; Renard& Letolle, 1983; Renaró,1984,1986).

Whenselectingsectionsfor isotopieanalysis,variousfactorsweretakeninto accountincluding, the hemipelagiecharacterof the sediments(marl/li-mestone),the existenceof a fine stratigraphyframework supportedby am-moniteseales;their geographicpositionin the basffi,continuity of observationandweakaffection to tectonieprocesses.Therefore,the TomarandPenichesectionswcre excludeddue to the fact that they show very particular sedi-

87 Cuadernos de Geología Ibé,i,a[998 número24. 69-98

L V Duarte Clay mineralsandgeochemicalevolutionin the Toa;-cian-lowerAalenian

WC 3sQ

239 VF4239 VF3O

239 BÓS

239 B138239239239

239239239239

MSTI

MST2A

B254B309B314

B400B439B455B489

239 B820239 B838

MST2B

MST3

MST4A

239 13882 MST4B

1,44 -3,162,34 -3,29

0,47 -2,540,64 -2,47

2,37 -3,233,00 -3,331,68 -3,09

2.21 -3,391 ,80 -2,831,92 -2,941,76 -3,08

1,581,77

2,06

-2,29

-2,49

-2,36

TABLE 9.—lsotopiccompositionsof the satnptesstudiedfrom tbeF. Foz section.TABLA 9—Composicionesisotópicasde las muestrasestudiadasen la columnade F Foz

mentaryconditions(Duarte,1995b, 1997). lsotopicdataarepresentedin Ta-bIes7 to 9, their stratigraphicandverticaldistributionsbeingshownin Fig. 8andFig. 9.

RESULTS

Carbon Isotopes: In the sectionsanalysed,the valuesof ~‘3Cvary betwe-en +0.2%oand -4-4.27%c,with significantoscillationsbetweensorneof the me-sosequentialunits. The isotopie variation curve of carbon (8’3C) reinforcesthe tendenciesillustrated in Duarte(1995a).As is demonstratedin Fig. 8, thelargefluctuationsoccurmainly betweenthe baseof the Lower Toarcianand theGradatazonc (MSTI, MST2A, MST2B and MST3) thus indicating its im-portanceas a stratigraphicmarker.In MST4 (UpperToarcian-LowerAalenian),the isotopicvaluesscemmorehomogeneous,still showing atendencyto gra-(lual reduction.

Samples

c~~uaderno.sde(g¿olc>,~~cJ fIcé,,, a[998 número 24 69-98 88

L V. Duarte Clay mineralsandgeochemicalevolutionin the Toarcian-lowerAalenian

Detailedanalysisshowsthat thefluctuationsin 6’3C aretendenciallyne-gative along the length of the mesosequenceswith nearly perfectgradualevolution schemesin MST2B andMST3. The moreabrupt alterations,whichalwayscorrespondto positiveoscillations,wereregisteredin the MD/MST1,MST2A/MST2B boundaries,and betweenMST2B and MST3 (particularlyin the Raba~alsection)(Fig. 8). Dueto criteria of a sedimentologicalandpa-laeoecologicalnature,thebaseof theseunits (MSTL, MST2B andMST3) isinterpretedas beingthe resultof adeepeningphasein the sedimentation,orrather,an increasein paleobathymetry.In thiscontext,this geochemicalsignseemsto clearly illustratethe changesin sealevel. 8’3C suppliesthe perfectexplanationfor the macroscopiecontrastbetweenthe MST2A andMST2Buntts. The abrupt increaseclearly shows the changein sedimentarystyleoperatingbetweenthe two sub-units,with the depositionof marí/limestonedecimetrieto rnetric alternations,rich in benthicmarinefossils in the baseofMST2B. The extentof this tendency,shownin Fig. 8, is clearlydemonstratedby the positivevariation of 3% —between+0.89%(PMI9) and+3.87%(PM3 1)— andcan be seenin the Portode Móssection(Table8). In the otherspositionsof the basin, althoughthe variations are not as marked,they alsoshow valueswhich are statistically important—from +0.64% (B138) to+2.37% (B254) in Figueira da Foz (Table 9) and from +1.97% (Zl5g) to+4.0704(Z22) in Rabagal(Table7).

In contrast,the boundarybetweenMST1 andMST2A shows the onlyabrupt reductionin the evolutionof ~~3Cin the following: from +2.07%c(Z13)to +l.08%c (Zl5d) in Raba~al(Table 7), from +1.66%(PM3) to +0.2%(PM 13) in PortodeMós (Table8) andfrom +2.34%(VF3O) to +0.47%(B68)in Figueirada Foz (Table 9). The nature of the facies and their sequentialarrangementillustratea correlativereductionof the watercolumn in MST2A(Duarte,1995b, 1997).

In the upperToarcian,the evolutiontendsto becomealittle morecornplex.In anycase,5~t tendsto diminish alongthe lengthof MST4 in ah sections,re-achingvalueslower than+2%:+1.87%(Z90 iii Rabagab,+1.68%(PM894 inPortodeMós) and+ 1.77%(B838 in Figueirada Foz) at the top of the meso-sequence.If the boundarybetweenMST4A andMST4B is not marked, intermsof significantoscillationof the carbonisotope,this is not the casebet-weenMST4B andthe baseof MF. An isotopicincreasewas observedin thismegasequentialboundary(ME2/MF), whichcontradictsthe previousnegativeevolution.In Raba9alandPortodeMós (Tables7 aid 8), wherethemodifica-tion of the faciesandtheir sequentialarrangementis morenoticeable,thevaluesincreaseto +2.22%(ZIOS) and +2.18% (PM906)respectively,levels of thebaseof MF.

Oxygen Isotopes: The range in variation of the 6~o vanesin the threesections,from between—4.7O%oand—1.81%(Tables7 to 9). Theresultsob-tainedarenot as clearcut as thosefor carbonin termsof sedimentaryevolu-tion (Fig. 9). The clear tendencyfor a gradual increasealong the length of

89 Cuadernos-de Geología ibérica[998. número24. 69-98

L. V. Duarte Claymine,-alsandgeocheinicalevolutionin theToarcian-lowerAalenian

Sequential Analysis

Mt170 ~fl

MF

150 1—

u)

~ A

Meneghinn¡

Spee¡osurn

120 • Bonareííii

90MSTS

Gradata

Hurones

60“ej-a

ci,

30 Serpentunus

oM

MST1M D

~ • • Rabaqal

p Mós

• F. Foz

semiceíatum

Spinatum

Fío 8—¿’Cdisíribution in tlíc Toarcian-lowerAalen¡anfromthe LusitanianBasin (syntlietic sectionof Reeba~al)FIG 8—Distrihucióndcl C’ duranteel Toarciense-Aalenienseinferior cíl la CuencaLusitanieíise(sec-ción sintéticade Raba~al).

eachsection(valuesbecominglessnegative),is not a good indicatorof an ex-clusively primary geneticexplanation.Taking into accountthe thermode-pendenceof oxygen(McCrea,1950; Epsteinet al., 1953), theseresults,des-piteprecautionstakennot to includerecrystallisedlimestonelithofaciesatthesamplingphase,do not seemto showanythingmore thana diagenetieeffect.Re bestexampleof this canbe seen in Ihe almosí uniform evolutionsbel-weenMST2A andthe otherunits aboyeandbelow it (MSTI and MST2B),contraryto the abruptvariationsobservedin ~1>C.The lithification of the Ii-mestonefaciesmay be closely linked to the negativity of the &~O values(—4.68%cin Portode Mós).Evenso, the isotopievariability indicatesa seriesof small oseillations,particularly in the mesosequentialboundaries,that canbeexplainedby primary mechanisms.In somecases,the stratigraphiccom-parisonbetweenthe tendenciesobservedin eachsectioncan actuallyshowevolutionaryantagonisms.One example is the boundaryfrom MST2B toMST3, wheretherels an increasefrom —3.01%(PC199)10—239%(PC202)

90Cuadernos de c;eología Ibérico[998 número24, 69-98

L. V. Duarte Clay mine rais and geochemical evolution in Ihe Toarcian-lower Aalenian

Sequential Analysis.4 -3 -2 -1 0

Biostratig.

Mt MF Murchisonae170

150

120

- cci>2 A

• Ó~5lihúm

AalensisMeneghinniSpeciosum

BonareuIii

90MST3

Gradata

• BiIrons

60

30

~ 8u’2

SerpenI¡nus

oM

MST1MD

• RabaQal

~. Más

• F. Foz

emiceaunSpinatum

Fíe. 9~~~¿í>()distribution in theToarcian-lowerAnlenian fron~ theLusitanianBasin (syntheticsectionof Raba

9al)1w;. 9.—DistribucióndeI 0.1> duranteel Toarciense-Aalenicnseií,ferior en la CuencaLusitaniense(sec-ción sintéticadeRaba~al)

in Portode Mós (Table 8), andareductionfrom —3.19% (Z5Os) to —3.80%(Z51) in Raba9al(Table ‘7, Fig. 8). Despitethis, andtaking into considerationonly the Raba9alsection,whicb is objectively the mostcomplete,theevolu-tion of the isotopicdataseemsto closelyagree,althoughinversely,with the3’

3C tendeney:agradualincreasetowardsthe top of the mesosequenceswithreducedvariationsbetweenthe boundaríes.

DísCussíoN

Tlie Origin of 11w Variat¡onsin S’3C: In sedimentarygeologyasid parti-.cularly in the carbonatemarineenvironrnent,~>C is one of the most fre-quently usedpalaeo-oceanographicandstratigraphictools(Letolle & Renard,1980; Cavelieret a!., 1981; Renard, 1984, 1986; Shacleton& Hall, 1984;Weissert,1989; Emmanuel& Renard,1993). We know aboutits relationshipwith primaryproductivityatthe surface,with the oxidationof organicmatter in

Cuade;wos de Geo/ogía Ibérica[998,número 24. 69-9891

L. V Duarte Clay níiruerals audgeoclí.enñcalevolufion iii cheToaícian-lowerAa/mían

deeperwaters(Kroopnick, 1985)andalsoits importancein the explanationofphysiographicvariationsof carbonateplatforms(Follmi eta!., 1994).Further-more,the ideathat 813C may bea direct indicatorof palaeodepthin a pelagicenvironment(Renard& Letolle, 1983),andthereforeableto modify the eus-tatic movementcycles,is an importan¡referencein this study.

Ihe associationbetween6’3C evolutionin the toarcianmarly-limestoneofthe LusitanianBasinandthe changesin sea-levelis justified by the rneaningof the sedimentaryfaciesandtheir own vertical organisation.The variationcurvesfor the threesectionsclearly showconcordingtendenciesfor the va-rtous mesosequences.The largeabruptpositive fluctuationstend to occurinthe baseof the MST1, MST2B, MST3 andMF sequences,interpretedas fo-oding periods.

The variationsof ~13Carenormally intet-pretedas beingaconsequenceoftwo factors:the burial of organiccarbonand/orthroughthe productivityva-riations at the surfaceof the water. The link betweenthe observedvariationsand the sequentialanalysispreviouslyestablished,shouldindicatea mecha-msmof a primarynature.The possibility of carbontransfer,fixed to theor-ganiematterof marinesediments,to oceanicwaters,as a consequenceof itsdegradationandoxidation(Weissert, 1989) doesnot seemto agreewith thelow levels of organie matter presentin the sediments(datafrom Baudin,1989) anddoesnot justify the sometimesvery high valuesof 813C. Further-more,the productivity factordoesnot seemtobe sufficienttojustify íhe largenegativeincursions,as the abrupt enrichmentin 2C cannotbe duetoa simplebreak in productivity.

Oneof the most simplepossibilities,consideringthe link betweenthe large¡sotopic variationsandthe sequentialboundaries,is the influenceof continen-tal landmasson deposition,considerablyricher in 2C than marinewater.WeproveUds ideawith MST2A. Indicatingsedimentationin a shallow rampen-vironment,with stronglandmassinfluences,is in tUs sequencewherethe lo-westvaluesof 813C are observed(closeto O%c).With the exceptionof this unit,we can observein eachmesosequentialunit,a gradualevolutionof the values,with a tendencyfor ‘2C enrichment(asid subsequentreductionof &~C). The re-gressivenatureof eachmesosequenceindicatesa largercontinentallandmassinfluencein the deposition(Duarte, 1995b). On the otherhand,the positiveanomaliesobservedin the baseof MSTÍ, MST2B, MST3 andMF, resultingfrom fastertransgressiveeffects,seemto imply the opposite.

Therefore,the resultsconfirm the ideaput forward by Letolle & Renard(1980), thatthe beliaviourof a3c in pelagiccarbonatesis an indicatorof pala-eodepth:positiveoscillationsoccurduring transgressiveevents;the oppositeoccurringduring regressivephases.

It is alsopossiblethat the differentiationof the 63C amplitudesobservedbetweenthe threesectionsmay beexplainedby kcal variationsin the physio-graphyof the seabottom.In Portode Mós it is visible via the facies(lithologi-

Cuadernos de Geología Ibérlia[998 nómcro 24. 69-98 92

L V. Duarte Clay minejal,.andgeochemicalevolutionin theToa,-cian-lowerAalenian

cal and biological ones),a sedimentologicalchangewhich is more evidentbetweenthe mesosequencesthan in the otherlocations (Raba~alandFigueirada Foz). The definition of a uniquepalaeogeographicposition in this region(DUARTE, 1995b),indicating a largertopographicplatform gradient,canalsobe explainedby thelargerverticalvariability of 513C.

DiageneticInfluence:The ideathat 813C is not verydistortedby diagene-tic processes(burial depthandrecrystallisation),its thermodependencebeinglow (Keith & Weber, 1964; Renard,1984) is clearlyconfirmedin theRabaí~alsection.Analysescarriedout in a samelevel (Table ‘7) of this section,in thecarbonatesedimentof the dominatinglithofacies (Z7052-1)andthe recrysta-llised sedimentsof a lateralsideof amud-mound(Z70S2-2),gayeexactlythesamevalue: +1.89%.Thisdoesnot occur with ~O wherethereisa reductionin the recrystallisedfacies (from —2.59%cto —4.7O%c).The valueof this samplecorrespondsto the lowest valueobtainedin the series,which showsthe effectsof thermic readjustments.

The Resultsin theEnropeanaudTethyanRealm:Iherehavebeennu-merousisotopiestudiescarriedout on oxygenasid carbonrelatedto the toarcianseries.The anoxicseriesof the lower Toarcianof Central andNorthemEurope(Kauffman, 1978; Jenkyns,1985, 1988)havesparkedmi increasedinterestinthis fleld, par¡icularly in the determinationof ~13C,both in organicandcarbo-nate composites.Analysing the information available in a carbonatepliase(Jenkyns& Clayton, 1986; Baudin, 1989; Jenkynset al., 1991; JiménezetaL, 1996), andremoving the serieslocalisedin those particulardepositionalcontexts(for examplethe ParisBasin, iii Baudin, 1989), it is possibleto showthat the highestvaluesof 53C canbe found in the Falciferumzone(generallyaboye+3%o), with a tendencyto gradually reduceup to the Bifrons zone(abont+2%c). This typeof evolutioncan be seenin alí the Lusitanianbasin see-tions, wherethe largestpositiveincursions(over+4% in RabagalandPortodeMós) are centeredin the SerpentinusZone (= Falciferum),in the alternatingmarí/limestonefaciesof the baseof MST2B (Fig. 8). This increasemay evenreach+3%, after showingvaluesvery closeto 0%in thebaseof the Serpenti-nusZone, in a sedimentarycontextof ashallow ramp(MST2A). In the restofthe column,from the Bifrons Zoneonwards,the trend is that of a gradualre-ductionof thesevalues,atype of evolutionthat is alsoobservedin the wholeTethyanRealm(Jenkyns& Clayton, 1986).Taking into considerationthe keyposition of the Lusitanian Basin in palaeogeographicaland biogeographicalterms,the 8’3C resultseonfirm an apparentlyglobal effect, at leaston the teth-yan scaleandfor the ToarcianStage.

CONCLUSIONS

The typeof analysisoutlined in this study showsthe importanceof somesedimentaryparametersas stratigraphicmarkersin the toarciancarbonatesof

93 c:,,ade,nos de Geología Ibérica[998.número24 69-98

LV. Duarte Clay mine,alsandgeochemic.aletolutionin theToarcian-lowerAalenian

the LusitanianBasin.The chanshownin Fig. 10 summarisesthe principal evo-tutionarytendenciesof tosevariables,constitutingstrongargumentsin te pa-laeoenvironmentaldiscussionof the diversesedimentaryinfilling phases(dis-cussion in Duarte,1995b).

1) Clay mineralsindicateprimary differentiationsin deposition,both inverticaland lateralterms.It is possibleto concluderhe following:

— illite is practically exclusive in the sets from MST3 onwards; its pre-senceis ubiquitousin alí mineralogicalassociations;

— kaolinite is practicallyconstantin the base,disappearingthroughoutthewhole basinat the top of the series(MST4);

— importantcontributionsof smectitein MST2A asid the baseof MST2B,that highlight its importanceas astratigraphicmarker(baseof the Ser-pentinuszone);facies with chlorite in the mineralogicalassociationsof MST2B andfrom MST4A onwardsin tite westemsectors(F. Foz and5. PedrodeN4uel); thereis also an appreciablevalueof chlorite in the lower andmiddle Toarcianof Peniche(MSTPL, MSTP2A andMSTP2B).Thisevidencesupportsthe ideathat the origin of this mineral is froísi herci-nianblockswhich wereprobably emerged,situatedin the westernbor-derof the basin.

2) The distributionof strontiumshowsgood sequentialcontrol, andcon-sequently,good stratigraphiccontrol, which is interestingbut difficult to in-terpret.High valuesobservedin MST2A mayindicateconditionsof higher sa-linity levels.This may be associatedwith the proximity of a slightly evaporiticmargin,asthe sedimentarycontextandthe associationof the faciesof that unitdemonstrateavery shallowcarbonateramp(Duarte, 1995b).

3) The palaeobathymetricevolutionof the series,judgedfrom the studyoftheseparametersand from sequentialevolution, seemsto identify itself bestwíth the behaviourof 613C. This elementshowsa closecorrelationwith the re-lative sea-levelchanges:its highestincreases,which arealmostalwaysabrupt,accompanytheflooding phasesof sedimentation;negativetendenciesdevelo-pedalongthe lengthof ihe mesosequences,indicatetypically regressíveevo-lution.

The stratigraphicdistributionof 6~& in the Toarcian-lowerAaleníanseriesof the LusitanianBasin is similar to thatobservedin othertethyanbasins.

ACKNOWLEDGEMENTS

The authorwould like to thankA F Soares(University of Coimbra) fin lengthy discussionsandconstructivecomínentsgivenin the revisionof the ínanuscriptandto two anonymousrefereesfor theircaíeful andconstructivercvicws.He would alsolike to thankCM. Zuppi andAnnick Filly of Paris-Sud

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University(Orsay),for the facilities put at bis disposalto obtain isotopicdata This studywas tinancedby JNICT mostof it havingbeendevelopedin the Departnientof EarthSciencesof theUniversiiy ofCoimbra

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