Ammonites, taphonomic cycles and stratigraphic cycles in...

Ammonites,taphonomiccyclesandstratigraphiccyclesin carbonateepicontinentalplatforms

Ammonites,ciclos tafonómicosy ciclosestratigráficosenplataformas

epicontinentalescarbonáticas

Sixto FERNÁNDEZ-LóPEZ

Depto. y UEI de Paleontología,FacultaddeCienciasGeológicas(UCM), eInstituto de Geología Económica (CSIC-UCM). 28040-Madrid,

RESUMEN

Algunasvariacionesde los caracterestafonómicosy la distribuciónde las asoctactonesregistradasde ammonitesen lasplataformasepiconti-nentalescarbonáticaspermitendistinguir secuenciastafonómicaselemen-tales,tafosecuencias,taforregistros, tafociclos,megatafosecuencíasy su-pertafociclosque son el resultadode cambiosrelativosdel nivel del mar.La identificaciónde estosciclos tafonómicoses de máximaimportanciaparainterpretarlos ciclosestratigráficosen las plataformasepicontinenta-les mesozoicas,en especialcuandono hayevidenciasde las varíacionesdela líneadecostaperolos sedimentosfosilíferosdeplataformaexternaestánampliamentedesarrollados.

Duranteel JurásicoMedio se desarrollaroncincociclos ambientalesde profundización/somerizaciónen las plataformasCastellana,Aragone-say de Tortosa:cuatrociclos de profundizaciónmedia(duranteel Aale-nienseinferior, el Aaleniensemedio-Bajocienseinferior, el Bathonienscyel Calloviense)y un ciclo de profundizaciónavanzada(duranteel Bajo-cíensesuperior).Los taforregistrosde ammonitesde la BiozonaOpali-num representanlos dítimos términosde unamegatafosecuenciade so-

Cucader,íos de Geología Ibérica, nY 23, 95-136.Servicio de Publicaciones. Universidad Complutense, Madrid. 1997.

96 Sixto Fernández-López

merizacióniniciadaduranteel Toarciensesuperior.La máximaprofun-dizacióny el máximoascensorelativodel nivel del mar se alcanzarondu-ranteel Hiocrón Niortense(Bajociensesuperior).Los sedimentosde laSubcronozonaPolygyralis correspondena un pico transgresivode se-gundoorden.Los episodiosde máximasomerizaciónduranteel JurásicoMedio, que representanlos máximos descensosrelativosdel nivel delmar, correspondenal Aaleniensemedio(Biocrón Murchisonae)y al Ca-lloviense superior (Biocrón Lamberti). Los biocronosMurchisonaeyLamberti forman dospicos regresivosde segundoorden.Lasasocíacío-nesregistradasde animonitescomprendidasentreestosdosbiocronosre-presentanun supertafociclode segundoorden en la CuencaIbérica. Lossedimentosdel JurásicoMedio comprendidosentreestasdos disconti-nuidadesestratigráficas,desdeel Aaleniensemedio hastael Calloviensesuperior, también representanun supercicloestratigráficode segundoordenen laCuencaIbérica.

Palabras clave: fosilización,bioestratinomía.Ibsildiagénesistemprana,tafonomíaaplicada,estratigraffasecuencial,estratigrafíagenética,plata-formascarbonáticas,transgresión,regresión,ciclosambientales,paleoba-timetría,JurásicoMedio, PlataformaAragonesa,PlataformaCastellana,PlataformadeTortosa,CuencaIbérica.

ABSTRACT

Sornevariationsof the taphonomicfeaturesandthe distribution of thesuccessiverecordedassociationsof ammonitesin the carbonateepiconti-nentalplatforms enableto distinguishelementarytaphonomicsequences,taphosequences,taphorecords,taphocycles,megataphosequencesandsu-pertaphocyclesresultingfrom relativesea-levelchanges.The identificationof such taphonomiccycles is of utmostimportancein interpretingthestratigraphiccyclesof Mesozoicepicontinentalplatforms, when no cvi-denceof coastalonlap is preservedbut fossiliferous sedimentsof outerplatformarewidely developed.

Five environmentalcycles of deepening/shallowingwere developedin the Castiliane,AragoneseandTortosaplatformsduring the MiddleJu-rassic: four cyclesof mediandeepening(earlyAalenian,middle Aalenianearly Bajocian,BathonianandCallovian)andonecycle of advaneeddee-pening(lateBajocian).Ammonitetaphorecordsof the Opalinum Biozonerepresentthe last terms of a shallowingupwardsmegataphosequence

Amrnonites, taphonomiccyc/esanástratigraphiccvclesitt.. 97

which beganin the lateToarcian.The maximumdeepeningandthe maxí-mum relativesea-levelrisewerereachedduring the NiortenseBiochron(lateBajocian).Sedimentsof the PolygyralisSubchronozonebelongsto asecond-orderpeak transgression.Foremostshallowing episodesduringthe Middle Jurassic,representingpeakregressionof secondorder,corres-pond to middle Aalenian (MurchisonaeBiochron) and late Callovian(Lamberti Biochron). MurchisonaeandLambertibiochronsconformtwosecond-orderpeakregressions.Recordedassociationsof ammonitescom-prisedbetweenthesetwo biochronsrepresenta supertaphocycleof se-condorder in the IberianBasin.Sedimentsof the Middle Jurassiccompri-sedbetweenthesetwo stratigraphicdiscontinuities,from the middleAalenianuntil the upperCallovian, alsorepresenta stratigraphicsupercy-cíeof secondorder in the IberianBasin.

Key words: fossilization,biostratinomy,early fossildiagenesis,a-pplied taphonomy,sequencestratigraphy,geneticstratigraphy,carbonateplatforms,transgression,regression,environmentalcycles,palaeobathy-metry,Midóle Jurassic,AragonesePlafform,CastilianePhttform,TortosaPlatform,IberianBasin.

INTRODUCTION

SomepreservationalcharactersandtLie distributionof the ammonitesarethe result,andenablethe interpretation,of differentsedimentaryenvi-ronmentsin the Mesozoicepicontinentalplatforms (Fernández-López,1997a).The recordedassociationsof ammonitesformedin distalanddeepenvironmentsof the platform showdistinctivecharacterswith respecttothoseformedin proximalandshallowenvironments.Somesignificantfea-turesof the sedimentarybasins,suchas the variationsin ifie degreeof com-municationof the sedimentaryenvironments,aswell as the differencesinthe degreeof turbulence,in the rateof sedimentationandin the rate of se-dimentaccumulation,canbe estimatedon the basisof thechangesin thestateof preservationof the ammonites.

The main purposeof this work is to showsomepreservationalfeaturesof the ammoniteswhich enablethe interpretationof relativechangesof sealevel, of differentorderof magnitudeandcyclical, occurredin carbonateepicontinentalplatforms.Thesetaphonomicdataareof stratigraphicinte-restsincetheyprovidean independenttestof the cyclesdistinguishedin se-quencestratigraphyandgeneticstratigraphy.


STRATIGRAPHICCYCLES AND TAPHONOMIC CYCLES

Stratigraphiccyclesresultfrom cydllcal environmentalmodifications(e.g., eustatic,climatie and/ortectoniemodifications).In the stratigraphicalrecordit is possibleto distinguishstratigraphicsequencesandcyclesofdi-fferentorder,due to relativesea-levelchanges(Fig. 1). Analogously,asa re-sult from suchcyclical environmenta~modifications,the successiverecor-dedassociationsof a certainregionor sedimentarybasin canshowcyclicalvariationsin their secondarycharacters,resultingfrom taphonomicaltera-tion. A taphonomiccyclecomprisestwo or moresuccessiverecorded-asso-ciationsshowingcyclical variationsin their secondarycharacters,asa re-sult from an environmentalcyele.

Elementarystratigraphicsequences,parasequences,setsof parasequen-ces, taphofacies,depositionalsystems,systemstracts,depositionalsequen-cesof third order,transgressive/regressivecyclesof third orderandmega-sequencesor supercyelesof secondorder aregenetic terms of sequencestratigraphy,comprisingrock bodiesof the stratigraphicalrecord (Fig. 1).Elementarytaphonomicsequences,taphosequenees,setsof taphosequences,deepening/shallowingtaphorecords,deepening/shallowingtaphocycles,me-gataphosequencesandsupertaphocycles,aswill be exposedin the followingparagraphs,aregenetictermsof taphonomycomprisingpreservedelements,taphonicpopulationsor preservedassociationsof the fossil record.

Theidentificationof taphonomicsequencesandcyclesenablethe testof genetiedifferencesbetweenthe fossil recordandthe stratigraphicalre-cord.The stratigraphicalrecordandthe fossil recordaredifferent in natu-re, and they can be dissociatedandstudiedseparately,as it hasbeenre-peatedlyshown(Fernández-López,1984, 1986, 1987b, 1991, 1997b).Theyalsocontaindifferent information,andthe fossil recordmay supplyrelevantdataon sedimentaryenvironmentsandprocesseswhich haveleftno tracesin thestratigraphicalrecord(Fernández-López& Gómez,1990b;Fernández-López,1991, 1995). In fact, a single stratigraphiclevel canendosea setof successiverecorded-associationscomposingdifferent ta-phorecords.This setof recordedassociationsforming a condensedasso-ciationcanevencorrespondto a time interval without stratigraphicalrecord(Fernández-López,1985b, 1986, 1987b, 1991, 1995).

Relationshipsbetweenthedifferentcyclical processesthathavecondi-tionedthe continuity/discontinuityof the stratigraphicalrecordor of thefossil recordin carbonateepicontinentalplatformscanbe testedon theba-sis of the relativedurationof suchprocesses(Fig. 1). Thecalibration bet-weenstratigraphiccyclesandtaphonomiccycleshasbeenaccomplishedon

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Fig. 1,—Temporal resolution of stratigrapbie and tapbonomic eycles oí different order, as welL a> ofsorne processes nentioned in text.Hg. 1,—Solución temporal de los ciclos estratigráficos y de los ciclos tafonómicos de diferente orden,así como de algunris procesos mencionados en el texto.

thebasisof thegeochronologicalandgeochronometricdatapublishedbyGradsteinci al., (1994),Callomon(1995),0dm, Galbrun& Renard(1995)andGradsteinandOgg(1996).

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ELEMENTARY SEQUENCES

Elementarysequencesare stratigraphiccyclesof sixth-orderandrhythms,of centimetricormillimetric thickness,representinga cyclicity lo-wer than20.000years.Elementarysequencesdevelopedin carbonateepi-continentalplatformsresultfrom gradualvariationsin the turbulenceandintherateof sedimentation(Fig. 2).

ELEMENTARY SEQUENCES

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In outerenvironments,when decreasesin the rateof sedimentationareassociatedwith increasesin tbedegreeof turbulenceof thewaters,thepreservedassociationsof ammonitesshow gradualincreasein the con-centrationandin the degreeof taphonomicheritage,assometaphonomieprocessessuchasbiodegradation-decomposition,encrustation,sedimentaryinfilí, concretion,abrasion,bioerosion,fragmentation,reorientation,di-sarticulation,regroupingandremovalof ammoniteremainsareintensified.The winnowedhedsdescribedby Monaco (1995)wereformed in theseconditions. In confinedenvironments,in contrast,when increasesín therateof sedimentationareassociatedwith decreasesin thedegreeof turbu-lence, thesametaphonomicprocesseslead to theformation of ammoniteassociationswith decreasingvaluesof concentrationandtaphonomiche-ritage.Thefactors influencingtheseelementarysequencescanbe very di-verse. Some authorsconsiderthat tidal cycles,month]y or semiannual(equinoctial),canbe a causeof thisperiodicity (FerryandMangold, 1995)while othershave relatedthem to climatie cyclicity (Ferry, 1991; VeraTorres, 1994;Reboulet,1995; Mialí, 1995, 1997).

lnterpretingthe marl-limestonerhythmites formed in carbonateplat-formsrequiresto distinguishbetweenbackgroundand eventsedimentation(cf. Aigner, 1985; Brett and Baird, 1986; Fernández-Lópezand Gómez,1990c; GarcíaRamos,Valenzuelay Suárezde Centi, 1992; Ftirsich andOschrnann,1993;LópezMartínezandFernández-López,1993; Brett, 1995).Thecarbonatebackgroundsedimentationin theouterplatform is characte-

Fig. 2.—Examples of elementary sequences of che MiddLe Jurassic of the Iberian Range, showing tbechanges in turbulence and rate of sedimentation. During the development of parasequences, in carbo-nate outer platforín environments, die degree of turbulence increases as che rate of sedimentation de-creases. During tbe deveLopment of elementary sequences, however, tbe water turbulence increases intiuter plaiforin environments. bul decreases in confíned environments. SimuLtaneously, che rate of se-dimentation decreases in outer platform environments, but mercases in crmnt3ned environments. Simi-larly, che degree of tapbonomic beritage (estimated by dic proportion of reelaborated elements) and (bect)neentration of aínmonite reniains mercase in outer platform envirnnmencs, but che values of diese twoparameters decrease in confined environmencs,Fig. 2—EjecnpLos de secuencias elementales del Jurásico Medio de La Cordillera Ibérica, con indicaciónde las variaciones en el grado de turbulencia y en la casa de sedimentación. Durante el desarrollo de Lasparasecuencias. en Los ambientes de plataforma externa carbonática, aumenta el grado de turbulencia ydisminuye La tasa de sedimentación. Durante el desarrollo de las secuencias elementales, sin embargo,el grado de turbulencia aumenta en los ambientes abiertos, pero disminuye en los ambientes confinados.Simultáneamente, La casa de sedimentación disminuye en los ambientes abiertos, pero aumenta en losambientes confinados. Durante el desarrollo de estas secuencias elementales, el grado de herencia ta-fonómica (estimado por la proporción de elementos reelaborados) y La concentración de restos de am-monítes aumentan en Los ambientes abiertos de plataforma, pero los valores de estos dos parámetrosdisminuyen en los ambientes confinados de plataforma.


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Vg. 3.~l:requency -,r d~nd~n tapbonomic cbaracters displayed by aramonites included In tempestitesof carbonate outer platform. During tbe deveLopment of a tenípestite, in carbonate outer platform en-víronínents, che water turbulence and tbe rate of sedimentation decrease, During tbe development ofsuch tempestites, eonscquen¡ly. tbe ammonite sise, the ammonile concentration, Ihe taphononiic ben-tage and the ammonite inclination decrease.Fig. 3—Variaciones en diferentes caracteres talbnómicos de los aínínonites incluidos en cernpescitas deplataforma externa carbonática. Duraiste el desarrollo de una tempestita, en aníbientes de plataforma ex-tema, disminuye La turbulencia y la casa de sedimentación. En consecuencia, durajíte el desarrollo de es-tas tempestitas, disminuye cl tamaño de los amínonites, su concentración, su grado de herencia tafo-

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Ammonites,tapbonomiccyc/esant! stratigraphiccyclesin... 103

rizedby gradualvariationsin the rateof sedimentationwhich areinverselyproportionalto thedegreeof turbulence(Fig. 2). In contrast,in thesamecar-bonateepicontinentalplatforms,eventsof turbulence,suchasstorms,leadtothedevelopmentofdepositsformedunderconditionsof decreasingvaluesofrateof sedimentationanddegreeof turbulence(Fig. 3). Preservationalfea-turesand thedistributionof ammonitesenab]ethe identificationof suchtur-buleneeevents.Ammonitesincludedin tempestitesasbioclastsshowflning-upwardsgradingassociatedwith decreasingvaluesof taphonomicheritageand inclination.Tempestitesshowingfining-upwardsgradinganderosiveorsharpbasedo not contain ammonitesdisplaying imbricated grouping orpreferentialazimuthal-orientation(Fernández-López,1 997a).

Elemenrarytaphonomicsequencescomprisetwo or moresuccesscvere-corded-associationsshowinggradualvariationsin their taphonomiccha-racters.The successiverecordedassociationscomposingan elementarytaphonomicsequencecan occurin the samestratigraphiclevel, sincethecontinuity/discontinuityof thefossil recordis not determinedby thecon-tinuity/discontinuityof thestratigrapbicalrecord(Fernández-López,1986,1987b, 1995, figure 13).

PARASEQUENCESAND TAPHOSEQUENCES

Carbonatesedimentsof shallowepicontinentalplatformsareorganizedin shallowing-upwardssequencesorparasequences,of metric to decimetriethickness,coarseningandthickeningupwards,which representchangesinrelativedepthfrom subtidalto inter or supratidalenvironments(Fernández-López1985b, 1985c,1997a,1997b; Fernández-LópezandGómez,1990c;Fernández-Lópezand Meléndez, 1994). Theseshallowing-upwardsse-quencesor parasequencesalsorepresentcyclical variationsof fifth-order,for which a time intervalof 20.000to 100.000yearshasbeenestimated,accordingto Einsele(1992)andVera Torres(1994);orelsee. 100.000ye-ars (Reboulet, 1995); also, 10.000 to 200.000 years accordingto Mialí(1995).

As indictedin the figure 4, shallowing-upwardssequencesor parase-quencesin carbonateouter platform, forming a positive taphosequenceanddifferenttaphorecords,wereformedduring a phaseof increasingtur-bulenceanddecreasingrateof sedimentation.Recordedassociationsfoundin positive taphosequencescan be groupedin three successivetaphore-cords: a low turbulencetaphorecord(LTT), a moderateturbuleneetapho-record(MTT) anda high turbulencetaphorecord(HTT). High turbulence

104 Sixto Fernánt!ez-López

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Fig. 4.—Frequeney of different tapbonomic cbaraccers displayed by aranionites included in a sbaLíowing-upwards sequence or parasequence in carbonate outer platform, forniing a positive taphosequence anddifferent taphorecords. LTT = low turbulence tapborecorcl. MTT = moderate turbulence taphorecord.HTT = bigb ttírbulence taphorecord.Fig. 4.—variaciones en las frecuencias de diferentes caracteres taíonómicos de Los anímonites incluidosen las secuencias de somerización o parasecuencias de las plataformas externas carbonáticas, que cons-tituyen tafosecuencias positivas y diferentes ¡aforregistros. LTT = lafonegistro de baja turbulencia,Mrr = taforregistro de moderada turbulencia. HTT = caforregistro de alta turbulencia.

taphorecords(HTT> are predominantin shallowing-upwardssequencesor parasequencesdevelopedin shallow environmentsof proximal plat-forms. In contrast,low turbulencetaphorecords(LTT) aremorecommonly

Amrnonites,taphonomiccyclesant! síratigraphic cyclesin... 105

formed in shallowing-upwardssequencesor parasequencesdevelopedindeepenvironmentsofthedistal platform. In the lower portionof themostcompleteparasequences,whereaccumulatedelementsandpyritic ammo-nitesmaybe found,completeshellsaremostcommon.In this portion, ho-llow ammonites(j.c., showingno sedimentaryinfilí in thephragmocone)andhollow phragmocones(i.e., without septa)arethedominantfossils,butthey are usuallycompressedby gravitationaldiageneticcompaction.Re-sedimentedandreelaboratedelementsbecomemorecommonto theupperportionsof theseparasequences,asshellsare morecompletelyinfilí withsedimentandtendto acquirean encasedpatternof grouping.Towardsthetop of thesequence,processesof earlymineralizationaremoreintense.Re-elaboratedconcretionaryinternal moulds becomedominant.They maydisplayseveraldistinetivefeaturesof abrasion,fragmentation,disarticula-tion, reorientationandregrouping.Suchreelaboratedelementswill showno tracesofdeformationby gravitationaldiagenetiecompaction,but theymay developabrasionfacets formed beforethe final burial. Ammoniteshells andconcretionaryinternal mouldswill tendto produceimbricatepa-tternsof groupingandto showazimuthalreorientation.Theymayalsobecoveredby encrustingorganismsand biogenicboring. Siphunculartubeswill usuallybe disarticulatedas aconsequenceof intenseandlastingbios-tratinomie processesof biodegradation-decompositionand dissolution.Reelaboratedconcretionaryinternalmouldswill alsobe preferentiallydi-sarticulatedalongseptalsurfaces.In the later stage,long episodesof emer-síon anderosionfavour the formationof concretionaryinternalmouldswi-thoutsepta,asresultfrom dissolutionof theseptaandsubsequentinfilí ofthecavitieswith sedimentsduringepisodesof emersion,andconcretionaryinternal moulds with ellipsoidal abrasionfacetsand annularabrasionfu-rrows.

During the developmentof parasequences,the variations in the de-greeof removal (proportionof resedimentedplus reelaboratedelements)andtaphonomicheritage(proportionofreelaboratedelements)of theam-moniteassociationswill dependon thevariationsin the rateof sedimenta-tion andin the rateofsedimentaccumulation,ratherthanon thevariationsof turbulence.Thedegreeof removalandtlie degreeof taphonomicheri-tageof theammoniteassociationsareinverselyproportionalto the rateofsedimentationand to the rate of sedimentaccumulation.A decreaseínany or bothsedimentaryrateswill producean increasein thedegreeof ta-phonomicremovaland taphonomicheritage,leadingto thedevelopmentofa positivetaphosequence(flg. 4). Yet, an increasein the rateof sedimen-tation or in the rateof sedimentaccumulationwill producea decreaseín

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the degreeof removaland taphonomicheritageof thepreservedassocia-tion, leading to thedevelopmentof anegativetaphosequence(cf Fernán-dez-LópezandSuárez-Vega,1980;Fernández-López1985b, 1997a,1997c;Fernández-López,Gómezet Goy, 1985; Fernández-Lópezand Aurelí,1988; Fernández-López,Gómezy Ureta, 1988;Fernández-LópezandGó-mez, 1990b; Fernández-LópezandMouterde,1994).Thesenegativeandpositive taphonomicsequences,or taphosequencesof increasingor de-creasingturbulencerespectively,enablethe identificationof shallowing-up-wardssequencesand infilling sequencesof fifth-order respectively(Fer-nández-LópezandGóínez,1991;GómezandFernández-López,1994).

Taphosequencesand parasequencescan be groupedin sets, whichwould correspondto longer variations in relativedepth. Thesesets areínfluencedby thesubsidence.Theycomposestratigraphiccyclesof fourth-order, for which ameandurationof 0,08 to 0,5 m.y. hasbeenestimatedbyVail cf al. (1991);or else,0,1 to 0,5 m.y. accordingto Einsele(1992)andVeraTorres (1994);also,0,4 m. y. accordingto Reboulet(1995);0,2 to 0,5m.y. accordingto Mialí (1995).Setsof parasequencescanbe progradatio-nal, aggradationalor retrogradationail,accordingto their stackingpattern(cf. Van Wagoneret al., 1988; López Martínez and Fernández-López,1993;Homewood,1996).

DEPOSITIONALSEQUENCESAND DEPOSITIONALSYSTEMS

Depositionalsequencesof third orderrepresentrelativesea-levelchan-geswith an estimatedcyclicity of one m.y. (with arangeof 0,5 to 5 m.y.accordingto Vail et al., 1991; Gonnin ti. aL, 1992, 1993; Jaequinet aL,1992; 0,5 to 3 m.y. accordingto Einsele,1992; Gracianskyet al. 1993;VeraTorres, 1994;JaequinandVail, 1995; 1 m.y. in FeIs, 1995; 1 to 10m.y. accordingto Mialí, 1995; 1,5 to 2,5 m.y. in Reboulet,1995; 1 toS m.y.in Rousselle,1997 ). Depositionalsequencesarestratigraphicsequencesofgreatermagnitudethanparasequencesor setsof parasequences.

A cycle of relativesea-levelchange,asrecognizedin sequencestrati-graphy,comprisesfrom a relativesea-levelfail up to the following relativesea-levelfalí (Vail cfal., 1987; Wilgus etat, 1988;Ferry, 1991; Rioult cfal., 1991; Homewoodcf al., 1992; Burchetteand Wright, 1992; Gra-cianskyet al., 1993; Brett, 1995;JaequinandVail, 1995).Relativesea-le-vel changesdeterminerelativechangesof thepotentialof accommodationand, in carbonateplatforms,relativechangesin theproductionrateof se-diments.In carbonateepicontinentalplatforms,themaximumproduction

A,nmonites, taphonomic cvclesant! stratigraphiccyc/esin... 107

of sedimentaryparticlestakesplaceto a depthof abouta tenof metersun-dergoinga quick falí thereafter(Wilson, 1975;James,1983;Wilgus cfal.,1988; Einsele, 1992; Handford & Loucks, 1993; Wright and Burchette,996). in carbonateepicontinentalplatforms,relativechangesof thepo-

tential of accommodationandthevariationsin the rateof relativesea-levelchangeare two fundamentalconceptsnot only to interpretsequencesandsedimentarycyclesof thestratigraphicalrecordbut alsoto understandthecyclical andsequentialcharacterofthe fossil record.

The curverepresentingthe relativechangesof the potentialof acco-mmodationcanbe subdividedin fourpartsto differentiatefour successivephasesduring the developmentof the third-orderdepositionalsequences(Fig. 5). According to the termsproposedby Jaequin& Vail (1995),fourdistinctivedepositionalsystemsor systemstractsare developedin thesefour phasesof relativesea-levelchange:1) lower lowstandsystemstract(LLST); 2) upperlowstandsystemstract (ULST); 3) backsteppinghigh-standsystemstract (BHST or TST); 4) foresteppinghighstandsystemstract (FHST or HST). The whole of thesedepositionalsystemscorres-pond to a depositionalsequenceofthird ordenEachdepositionalsequencerepresentsa cycle of eustaticchange,andit is boundedby discontinuitiesproducedby relativesea-levelfalís.

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Fig. 5—Dítterencdeposítional syt-tems and suecessive phases which can be distinguisbed during che de-vclopment of a depositional sequenee of tbird order. LLST = lower Lowstand systems tract. UL5T = up-per lowstand systems craet. BL-JST or T5T = baekstepping highstand (or transgressive) sysiems tract.FHST nr HST = lorestepping highstand systems tract.Iig. 5—Diltrentes sistemas deposicionales y ¡ases sucesivas durante el desarrollo de una secuencia de-posicional de tercer orden - LL5T = sistema deposicional inferior de bajo nivel. LJL5T sistema de—posicional superior de bajo tu vel - BHSL t ¡ST sistema deposicional retrogradacional de alto nivel -FHST o 1-151 = sisiema deposicional progradacional de alto nivel.

lOS Sixto Fernández-López

Depositionalsystemsor systemstractsof lowstandarenot usuallyre-presentedin carbonateepicontinentalplatforms.In thesedepositionalsi-ttings, eachthird-orderdepositionalsequencecomprisestransgressivede-posits,of moreor lesslimited thickness,correspondingmainly to highstanddeposits(BaumandVail, 1988;Brett, 1995; MiaU, 1995). In carbonateepi-continentalplatforms,themaximumrateof carbonateproductiontakespla-ce during the developmentof the highstandprogradationaldepositionalsystems.Thehighstandretrogradationalor transgressivedepositionalsys-temsaccumulatein thedeepestor protectedareasandareboundedatthetop by adiscontinuitywhich representsthemaximumflooding surface.

Deposits forming the transgressiveor retrogradationaldepositionalsystemstypically composethinning- and fining-upwardssedimentaryse-quencescontainingcondensedrecordedassociations.Theyarealsoclassi-cally knownto be formedby condensedsediments.Yet, thedegreeof se-dimentarycondensationmay be very different accordingto the relativedepthof theplatform: condensedsedimentsaredepositedin distal anddeepareaswhile expandedsedimentsdeveloplaterally, in proximal andshallowarcas(Fernández-LópezandGóínez,1991;GómezandFernández-López,1992, 1994).In bothextremeconditions,thediscontinuitybelow thesede-posits containingcondensedassociationscorrespondsto the top of theunderlying depositionalsequenceand representsthe boundarybetweenthetwo successivedepositionalsequences(cf. Arnott, 1995).

Depositionalsystemsquickly thin out towardsthe proximal areas,butconcretionaryinternal mouldsof ammoni tes associatedwith nondeposi-tional hiatusescanbe incorporatedas reelaboratedelementsin theulteriorhighstandsystemstract, giving rise to condensedassociationswith com-pleteconcretionaryinternal moulds.This would explain thehighdegreeoftaphonomiccondensationreachedby recordedassociationsof ammonitesin shallow carbonateepicontinentalplatforms,evenwhen suchassociationsare included in mudstonesediínentsdepositedduring episodesof maxi-mum rate of sedimentation.Interpretingtheseprocessesof taphonomiccondensation,in carbonateepicontinentalplatforms,requirestaking intoaccountthe relativedurationof suchtaphonomicprocessesasaccumula-tion, resedimentationandreelaboration(Fig. 1). Accumulationofammo-nite shellson thedeepseabasinsmay be practicallyinstantaneousafter thedeathof the organisms.l-Iowever, in carbonateepicontinentalplatforms,accu-mulationof theammoniteshellscantakeplaceseveralmonthsafterthede-ath (Fernández-López,1987a, 1997a).Resedimentedshellsmay stayon thedepositionalsurfacefor severaltensof years,beforebeingburied.RecIa-boratedinternal mouldsof ammonitesmaystay on thedepositionalsurfa-

Ammonites, taphonomic cycles ant! .stratigraphiccyclesin... 109

ceafterbeingexhumedandbeforethefinal buflal for morethantenmillionyears,as for manyammonitefossilssubmittedto reelaborationprocessesinthe Castiliane,Aragoneseand Tortosaplatformsthrougbthe Middle Ju-rassxc(Fernández-Lópezcf al., 1996). The highestvaluesof taphonomiccondensationin ammoniteassociationsaredisplayedin shallowepiconti-nentalplatformsratherthanin deepseaenvironments.However,thedegreeof taphonomicheritage(estimatedby the ratio of reelaboratedelementsinthe whole assemblage)canreach100%in both cases(Fernández-López,1997a).

CONDENSED SECTIONS PROXIMAL• DISTALSEDIMENTS expanded - condensedTAPHONOMIC CONDFNSATiON high - lowTAPHONOMIC HERITAGE high - moderateDEGREE OF PACKING Iow - highSTRATIGRAPHIC PERSISTENCE 0W - highTAPHONIC POPULATION OF TYPE 1 absent - presentHOLLOW AMMONITES abundant - soarceREELABORATED AMMONITES rotinded - angularBIOGENIOBORINOS abundaní - soarcePYRITIC INTERNAL MOULDS soarce - CommonSTRATIGRAPHIC GAPS common - soarce

LabIo 1 —Difíerenlisíl eIíaracters of tbe ct,ndensed sections forníed in depositional sequences of ¡bird nr-<ter in proximal or distal arcas.Tabla ¡—Caracteres diferenciales de Las secciones condensadas íormadas de secuencias deposicionalesde tercer orden en áreas proximales o distales.

Condensedsectionsformed in depositionalsequencesof third ordershowdifferent charactersin proximal areasin relationto distal areas(Ta-ble 1). Thedegreeof taphonomiccondensationin ammoniterecordedasso-ciationsreachesthehighestvaluesin shallowepicontinentalplatforms,notin deepbasins,though thedegreeoftaphonomicheritage(j.c., the ratio ofreelaboratedelementsto the whole of recordedelements)can, in bothca-ses,reach100% (Fernández-López,1997).Thedegreeof packingof am-moniteremains(estimatedby thedifferencebetweenthe numberofspeci-mensandthenumberof fossiliferousleveissubdividedby thenumberoffossiliferouslevels)andthe stratigraphicalpersistence(proportionof fo-ssiliferouslevels)displaysmaller valuesin proximal than in distal areas.


Condensedsectionsfrom distal areasusuallycontain taphonicpopulationof type 1 (Fernández-López,1997a).In suchareas,phragmoconesarenormally filled by sediment,andconcretionaryinternalmouldsdisplaydisarticulationsurfacesand fractureswith acote margins.Pyritic am-monitesarecommonin certaindistal areas.On theotherhand,in proxi-mal areas,taphonicpopulationsareusuallyof type2 or 3, thoseof type1 beingnot represented.Hollow ammonites(Le., shellsshowingno se-dimentaryintilí in thephragmocone)are abundant,reelaboratedinternalmouldsshowhigh valuesof roundnessandsphericityaswell ascommonbiogenieborings,audpyritic ammonitesaresearce(Fernández-López,1997).Stratigraphicsuccessionsin shallowepicontinentalplatlbrmsareusually more incompletethan thoseformed in deepbasins(cf Sadíer,1981; Sehindel, 1982; McKinney, 1985; Tipper, 1987; Kowalewaki,1996). However,despitethe abundanceand wide rangeof biostrati-graphicgapsin suchsequences,registratiegapsare usually not so im-portantin condensedsectionsof shal[ow platforms,the registraticsuc-cessionbeing usually more than the correspondingbiostratigraphicsuccessíon.

TRANSGRESSIVE/REGRESSIVESEDIMENTARY CYCLES

Third-ordertransgressive/regressivesedimentarycyclesrepresentlong-term variations of the relativesea-level,of severalmillion years(from0,5 to 5 m.y. accordingto Vail ci al. 1991;Jaequinci al., 1992; 0,5 to 3m.y., with an averagedurationof 0,8 m.y. for the Mesozoic,accordingtoBinsele, 1992; 1-loínewoodcf al., 1992; Gracianskyel al. 1993; Vera To-rres,1994; Jacquinand Vail, 1995; 1,5 to 2,5 m.y. in Reboulet,1995; 1 to10 m.y. accordingto Mialí, 1995).

Transgressive/regressivesedimentarycyclesof third order compriseoneor more depositionalsequencesof third order. Six successivephasescanbedistinguishedwithin thedeepening/shallowingenvironmentalcyclesof third-orderin carbonateepicontinentalplatforms,on the basisof there-lative changesin thepotential ofaccommodationandin theproductionrateof sediments(Fig. 6):

1) lneipientdeepeningphase:openmarineenvironmentsarerestric-ted to the distal or more depressedareasof the platform, and the acco-mmodationspaceandthe productionrateof carbonatebegin to increase,after aphaseof emersionandregionalerosion.

Amnwnites, taphonomiccyc-lesant! .s-tratigraphiccyclesin... III

OEEPENING/SHALLOWING TRANSGRESSIVE/REGRESSIVEENVIRONMENTAL OVOLE SEDIMENTARV CYCLEOF THIRD ORDER OF THIRD aRDER

1Emersíon and regional eros,onAdvancedshallowing 4Incipiení shaLlow¡ngMedian shallowing_____Median deepening .. 2incípíent deepening

Erosion and regional emersion 1

RELATIVE OHANGES RELATIVE CHANGES RELAilVE CHANGES RELATIVE CHANGESOF SEA LEVEL OF SEA LEVEL. OF SEA LEVEL OF POTENTIAL

deepening - shal¡owing lransgression - re9ression rise . fail OF ACCOMMODATION____ ~-e--— ......~high - Iow

4- -4

Fig. 6—Relative changes of Ibe potencial of accommodation (¿e., subsidence and eustasy) and relativechanges of sea Level, cwo fundamental concepts of sequence scratigraphy, have conditioned che deve-Lopment of transgressive/regressive sedimentary eyeles and environmeatal cycíes of deepening/sha-Ilowing in carbonate epicontinental platfornís. The curve representing relative ebanges of sea level do-ring tite deveLopment of transgressive/regressive sedimentary cyeíes of third order may be subdividedinto four parts Lo differentiace four successive phases: 1) rapid faIl, 2) increasing rate of risc, 3) rapidrise. 4) deereasing rate of te. From che pbase of incipient deepening until that of advanced shaílowing,síx successive pbases can be discinguished in an environmental cycíe of deepening/shalLowing.Fig. 6—Los cambios relativos del potencial de acomodación (Le., subsidencia y custasia) y los cambiosrelativos del nivel del mar, dos conceptos fundamentales de Estratigrafía Secuencial, han condiciona-do el desarrollo de ciclos sedimentarios transgresivo/regresivos y de ciclos ambientales de profundi--zaeión/somerízacíon en Las plataformas epiconcinentales carbonáticas, La curva que representan loscambios relativos del nivel del mar durante el desarrollo de Los ciclos sedimentarios transgresivo/re-gresivos de tercer orden suele ser subdividida en cuatro partes para diferenciarcuatro fases s-uees,vas:1) descenso rápido. 2) tasa de ascenso creciente. 3) ascenso rápido. 4) Lasa de ascenso decreciente, Des-

de La fase de profundización incipiente hasta la de somerización avanzada, seis fases sucesivas puedenser distinguidas en un ciclo ambiental de profundización/somerización.

2) Mediandeepeningphase:openmarine environmentsexpandto-wards the proximalandhigh areasof the outerplatform.

3) Advanceddeepeningphase:theproductionrateof sedimentsis in-sufficient respectto the accommodationspacethat is generatedin theplatform. As a consequence,thedepositionalenvironmentsretrograde.

4) Incipient shallowingphase:the productionrate of sedimentsin-creasemore quickly than the potentialof accommodationandthe rateofsedimentationincreasesin the deepestareas.

5) Medianshallowingphase:openmarineenvironmentsconstraintpro-gressivelytowardsmoredistalanddepressedareasof theouterplatform.

1

¡12 Sixto Fernández-López

6) Advancedshaflowingphase:openmarineenvironmentsonlyoccupythe most distalor depressedarcasof theplatform. Thepotentialof accommodationand theproductionrateof carbonatedecrease.Pro-ximal or high areasof the platform aresubmittedto emersionand ero-sion processes.

Transgressive/regressivesedimentarycycles of third order compriserelativeand cyclical variations of sealevel. Yet, environmentaldeepe-ning/shallowingcyclesof third-order,also involving cyclical variationsofthepotentialof accommodationof sediments,canexclusivelyresultfromcyclical variationsof the subsidence.On theotherhaud,a high productionof sedimentscancompletelyinfilí thespaceofaccommodationgeneratedby a relativerise of sealevel, resultinga shallowingphaseevenduring atransgressivephase(Wilgus ci al., 1988;JaequinaudMalI, 1995).Traus-gressíveor regressivephasescannotbe simply identitiedby indicationsofdeepeningor shallowing.It is necessaryto consideras well the time-spacerelationshipsbetweenthedifferentenvironmentsdeveloped.Openmarineenvironmentsoccupymorecoastalandproximal areasof thcplatformsdu-ring the transgressivephases,andmore externalanddistal areasduringtheregressivephases.Consequently,the transgressive/regressiveenviron-mentalcyclescan be inferredon thebasisof the time-spacerelationshipsbetweenihe different envíronmentswhich composethe deepening/sha-llowing environmentalcycles.Identification of deepening¡shallowingcy-cies is of utmostimportancein i¶flerpreting dic transgressivekegressivecyclesin carbonateepicontinentalplatforms,whereno evidenceof coastalonlapis preservedbut fossiliferoussedimentsof outerplatform arewidelydeveloped,as it occursoften in theEuropeanplatformsduring the MiddleJií

In carbonateepleontlnentalphtfori. ~ relativo chanuesofsQajoyel VOz

sulting from custatiemovementsand subsidencealsolcd to particularta-phonotrticcycles: theso-calleddeepening/shallowingíaphocvcles.Recordedassociationsofammonitesgeneratedin different phasesof theseenviron-mentaldeepening/shallowingcyclesshowdistinctivesecondarycharactersand composeseparatetaphorecords(cf. Fernández-López,1987b; 1995).Recordedassociationsof ammonitescomprisingan ideal deepening/sha-llowing taphocyclein carbonateepicontinentalplatformscan be grotipedwith taphonomiccriteriain taphorecordsof different category:incipient-dee-pening,mean-deepeningandadvanced-deepeningtaphorecords,aswell asincipicnt-shallowing,mean-shallowingand advanccd-shallowingtapho-records.

Arnmonites,taphonomiccyclesant! stratigraphiccyclesIn... 113

BOUNDARIES BETWEEN DEEPFNING/SHALLOWINGTAPI-IOCYCLES

The first stepto identify the successivedeepening/shallowingta-phocyclesin carbonateepicontinentalplatformsis to identify the wide-spreadregistraticgaps,i.e. gapsof the fossilrecordshowinga regionalextent,andaffectingthedeepestareasofdic outerplatform (Fig. 9). As it hasbeenrepeatedlyshown (Fernández-López,1984, 1985b, 1986, 1987b, 1991,1995, 1997;Fernández-López& Gómez,1990b;Aurelí, Meléndez& Me-léndez,1993)reelaboratedfossilsas witnessesof non-depositionordenu-dation eventsoftheseafloor are valuableinstrumentsto recognizeandes-timate the geochronologicrangeof stratigraphicgaps.Such episodesareusually Ioundin carbonatesequences.On theotherhand, registraticgapsmay indicateintervals of emersionand regional erosion.They may bequite common in carbonateepicontinentalplatforms. Registraticgapsidentified by meansof ammoniteshavegenerallysmallergeochronologicalamplitudethanthecontemporarystratigraphicgaps,and theyenableto as-certain, with greaterprecision,the episodesof regionalemersionin theplatform.

The rateof sedimentationin outerplatform arcasfalís down andsomeformersubtidal environmentscan beemergedduring a phaseof rapidre-lative sea-levelfalí. At thesametime, theprocessesof sedimentationandaccumulationof ammoniteshellsin the moreproximal areasof theplat-form are interrupted.Undersuchconditions,both the sedimentsand thepreservedelementsundergosomespecificmodificationsbecauseof sub-aerialexposureandintenseearlydiagenesis.The taphonomicmodificationsusually involve reelaborationprocesses,abrasionanddissolution.The rateof sedimentationfalís downto zeroin emergedareas,leadingto thedeve-lopmentof hardgroundsandrockgrounds.Theseconditionstook placeindic Castiliane,Aragoneseand Tohosaplattbrmsduring dic MurchisonaeBiochron (Aalenian), the ParkinsoniBiochron (Bajocian), the DiscusBiochron(Bathonian)andduring theLamberti andMariaebiochrons(Ca-llovian/Oxfordian),giving rise to stratigraphicgapsof regional extensioninthe IberianBasin (Fernández-López,1980, 1985a, 1985b, 1985c;AureIl,1990; Fernández-Lópezand Gómez,1990a, 1990b, 1991; Meléndezetal.. 1990;Aurelí. Fernández-López& Meléndez, 1995;Ramajoand Me-léndez, 1996).

Processesof biodegradation-decompositionanddissolutionaffeetingthe scarceammoniteremainswhich occasionallyarrive to theemergedarcasoftheouterplatform, for exampleduring stormevents,arevery in-tenseandrapid.Empty ammoniteshells,devoidof soft-partsin the body


chamber,arequickly dissolvedin theseareas.Partial internalmouldsofthe body chamber(i.e., hollow ammonites)can be exceptionallyfor-med,indicatingveryhigh rateof sedimentaccumulation.Such sedimen-tary conditions favour taphonomicreelaboration,so the reelaboratedconcretionaryinternal mouldsof ammonitesmay becomeabundantorevendominant in the recorded-associationsformed in theseenvíron-ments.Thesereelaboratedelementswill show some specific featuressuchastracesofdissolutionanda complex infilling, resultingfrom muí-tiple episodesof sedimentaryinfdl separatedby episodesof cementation.Someof thesesedimentsinfilling the shells are siliciclastic, and affectnon-displacedelements;hence,they aresiliciclastie pseudomorphs.Con-cretionaryinternal mouldsareusually regrouped,imbricatedor vertica-lized during the reelaborationprocesses.Concretionarymouldswithoutseptacan be formed(Fernández-López,1 997a,figures 11-12).TheycanaNoacquirerolí andtruncationalabrasionfacets,as well asdisarticulationsurfacesandfractures.

Once the successiveregistratiegapsof regionalextensionhavebeenidentified, the time spanrepresentedby condensedor by expandedsec-tions,aswell as the intervalswith condensedassociationsmaybe estima-ted (Fig. 9). A valuableeriterion to interpretthe environmentaldeepe-ning/shallowingcyclesis thecharacterandgeographicdistributionof thedifferent taphonicpopulationsof ammonites.Condensedassociationscon-taining greaternumberof taphonicpopulationsof type 1 (TP-1) representadvanceddeepeningenvironments.In contrast,condensedassociationsiricluding only taphonicpopulationsoftype 3 (TP-3)representincipiení dee-peningor advancedshallowingenvironments.

Oneor more deepeningtaphorecordscan be developedduring the dee-peningphasesin carbonateepicontinentalplatforms,accordingto theen-vironmentalconditionsandon the rateofrelativesea-levelchanges.Dee-peningtaphorecordsdevelopedin the most distal areasof the platformusually show morediversefeaturesthan thoseformed in the shallowestmostandproximal areas(Fig. 7). They may rangefrom incipient deepe-ning to advanceddeepeningtaphorecordsin themost distal areasof theplatform. However,taphorecordsdevelopedsimultaneouslyin theproximalandshallowarcasof thecarbonateepicontinentalplatformswill only ran-ge from mediandeepeningto advanceddeepening.It meansthat taphore-cordsof incipient deepeningphaseswill not bepresentin shallow0V pro-ximal areasof carbonateepicontinentalplatforms.

Shallowing taphorecordscompriserecordedassociationsyoungcrthan those of deepeningtaphorecordsof the saínecycle. Shallowing

Ammonites, taphonornic cycles ant!stratigraphic cyclesiii... 115

SEDIMENTARYPALAEOENVIRONMENTS

PROXIMAL PLATFOPM 1CONFINEO ENVIRONMENTS THRESHOLO OUTEA ENVLRONMENTS

50km E. E- E+ E. E-

DEEPENINGISHALLOWING TAPHOCYCLE

Emersion and erosionAdvanced shallowingMedian shallowing ________________________________

incipient shallowing _________________________________Advanced deepeningMedian deepeningIncipient deepeningErosion and emersion

Hg 7 —Taphorecoíds of ammonites developed in carbonate epicontinental platformsduring an enví-ronmental deepening/shaLíowing cycLe show more diverse characters in distal and depressed arcas chanin proximal and shaLLow arcas. Taphorecords of incipient deepening and tose of advanced shaLLowingwere onLy tLevelopecL in clistal and deep areas of ¡líe pLatforrn. lOT = ineipient-deepening taphorecord.MDT = median-deepening taphorecord. ADr = advanced-dcepening taphorecord. NT = incipienl-sha-Llowing taphorecord. MST = median-shaLltíwing taphoíectírd. A5r = advanced-shallowing taphorecord.I;ig. 7—Lis taforregistros de ainmonites desarrollados en las plataformas epicontinentales carbonáticasdtírante un ciclo ambiental de profundización/somerización presentan caracteres más variados en lasáreas distales y depíimidas que en las áreas proximales y someras. Los taforregistros de profundizaciónincipiente y Los de somerización avanzada sólo se desarrollaron en las áreas distales y profundas de Lasplatalormas caibonáticas. LOT = taforregistro de profiíndi-zación incipiente. MDT = taforregistro de pro—fundización media. ADT — taforregistro de profcíndización avanzada, NT = talbrregistro de somerí-zacion incipiente. MSr = taforregistro de soníerización media. A5T = laforregistro de somerizaciónavanzada.

taphorecordsreachgreatestdevelopmentin the mostdistal areasof theplatform, wheretherecordof thewholeenvironmentalchangefrom in-cipient to advancedshallowingtaphorecordsis morecomplete.Accor-dingly, in the proximal and shallow areasof the carbonateplatformsfor thesametime span,only incipient to meanshallowingtaphorecordsare developed.

1

DISTAL PLATFO~A

- — ~- -REGISTRATIC GAP

AST:

MST___________IST

ADT

MDT¡ DT

REGISTRATIC GAP

lIé Sixto Fernánt!ez-López

INCIPIENT-DEEPENINC TAPHORECORDS

lncipient-deepeningtaphorecordsrepresentthe first phaseof thedeepe-ning/shallowingtaphocycles.They.aregeneratedin the distal or more de-pressedareasof theouterplatform, whenthespaceaccommodationandtheproductionof carbonatebegin to increase,aftera phaseof emersionandre-gional erosion.Sedimentsarequickly cemented,and firmgrounds,showingtracesof dessicationandlocal weathering,are dominant.Ratesof sedimen-tatíonareusually very low, but ratesof sedimentaccumulationmay be veryvariable.At the sametime, stratigraphicand registraticgapsarestill deve-loped in the proximal areasof the outerplatform. Suchpalaeogeographicconditionswere developedin the Castiliane,Aragoneseand TortosapIal-tbrms during the Bradfordensis, Concavum, Discites, Laeviuscula and Pro-pinquansbiochrones(Aalenian-Bajocian;Fernández-López.1985c; Fer-nández-Lópezand Gómez, 1990a, 1990b, 1991; Fernández-Lópezu al.,1996),as well asduring theZigzagBiochron(early Bathonian;Fernández-López,Meléndezy Suárez-Vega,1978;Fernández-LópezandAurelí, 1988;Fernández-Lópezeta!., 1988, 1996;Lardiés, 1990)andduring theBullatusBiochron(earlyCallovian; Sequeirosand Meléndez,1987; AureIl, Meléndezy Meléndez,1990;Fernández-López& Meléndez,1995b, 1996).

After a phaseof regionalemersionandwhenthe rateof relativesea-le-vel falí in the outerplatform becomesmínímal or zero, the shellsof am-monitescanstill be preservedasaccumulatedelementsin themostdistal ordepresscdareas,but they will tendto bequickly resedimented(Fig. 8). Theimportationof shells by nekroplankticdrift is usually low, althoughtheycan locally form high concentrations.Taphonic populationsof type 3,composedof resedimentedor reelaboratedelements,aredominant.The de-greeof packing(estimatedby the difttrence betweenthe numberof spe-c¶mensandthe numberof fossiliferouslevelswith respectto the numberoffossiliferouslevels) in thepreservedassociationsof ammonitesmay alsobevcry váriable.Ammonite associationsshow very low valuesof stratigra-phic persistence(proportionof stratigraphiclevelswith ammonites)andsearce

Hg - 8.—Charaeteristi e feat ores of the different taphoreeords of att moni les compos i ng a deepening/sh a—llowing taphocycLe in carbonate epicontinenczíl pLatforms. IDT = incipient-deepening taphoreeord.MDT = median-deepening tapborecord. ADT = advanced-deepening tapborecord. ¡ST = ineipient-sha-LLowing Laphorecord. MST = median-shalLowing tapLiorecord. AST = advanced-shaLLowing taphoreeord.Hg. 8—Caracteres de Los diferentes taforregisíros de anímonites que componen un tafociclo de pro—fondización/somerización en Las plataformas epiconcinentales carbonáticas. IDT = taforregisíro deprofundización incipiente. M DT = taforregistro dc profcíndización media. ADí = taforregistrode prolundi zación avanzada. ísr = tafon-eg istro de someriMci ón incipiente - MST = caforregi síro de so—merización media. A.ST r ¡aforregistrí de símeri¡aeión avanzada,

Ammonites, íaplwno¡nic en lesand strat¡graphic úyúJt’sin. J 17

DEEPENING/SHALLOWINO TAPHOCYCLE¡>4 CARBONATE EPICONTINENTAL PLATFORUS

o ¡Dr •PCT’ADT.IST •MST•ABT o

MECMANISMS OF TAPHONOMIC ALTERATION and resulta:ELOO EGRAflATION-D ECOM POS¡TIONBody chamberswithsoft-partsSheils with peñostraeum —~rn~—— —ct~~=~————Siphijneijlar tobes wilh connectiríg ringo —.n~——ENC R U SIATiONIntrathalamous enerustingo -~—-—- —~mnflExtralhaiamuo eneruotingoSirornatolile aminae ~

SEDIMENTARY LNF¡LLPhragnioconeo wiLh sedimenlary mliiiSiliciciastie pseudoniorphsSINSEDIMENTARY MINERAIIZATIONCaleerecus concretionary interna] rnouids -n~——- —~~j —~~~nrPhosphalic concretioííaiy ititertal meuldo —~t~-— —~

Gleucontio concretionary infernal mouids- ~Pyritie infernal meuldoSilleilied coneretionary niernal mouido ....~—..ABRAS ¡ONInfernal metido asti I,víral.onaj faceto ~- _________

Internet meuldo with mli faceta ______ —.~bienio] metido with eiiipsoidal faceto - E~— —o-- -

Interno] mooldswith annularttírrov¿s c~=—BLOEROSIONInterna] metido with biogenie bodngs ______ —~

S¡NSEOIMENTARY DISSOLUTLONSIíeJIs without septe (hoilow phragínoconeo> - - - - —~mIl~——- -~--

PeuiootracawithootseptaneitberweLiConcrelionaiy internal meuldo ~ñtlioutsepte an=~.—TAPHONOMIC DISTORTiONSheio wílh oponed fractures rmr~-.—-- ~ ~

SheIls with elosed fracturesComuplele ohelloliTomplele phragrnoeones ~- —~z~FragmenLaíy infernal metido ~ -~

Meuldo with d~conUnuoos cornpaetion -~t~---e-- ----~I~n~--—Meuldo with continuouo compaction —e-. -~—--i-Ioflow amnionileo (ptiragmoconeo without sedimenlary infílí) ~REORLENTATIONSheils wiiii azirnuthai reorienlation -t~———-——-—-~IV —=~~~IilInternet neuldo cita aziníulhat reorientation ~ —~=no ________________Verlicaishello CE~=—.---- -_______Vertical concreliooary interne] meuldo ~DISARTICULATIONDisarticulated aptycf~usShells wilhout aptyehus - -

DiosílicuLated sinhuncular tubosDisarticulated Internet metido mm~— —~

DISPERSALTaptionie populationo oftype 1 - - --

Taphonicpopulationooftypo2Taphonic popuiationo of¡vro - _____________

REGROIJP ¡NGEncasedotíelIs -um~n——-- SU

lntricatedohols ~ ~ ~

inibrlcated intemal moulds -ínm~~——— _____________

RE MOVALAccumuleted elemento - —~3~—-

Resodimented elementoReelaborated elemento ~

—

t tg Sixto Feruóndez-L1ipez

lateral continuity.Biostratinomicprocessesof biodegradation-decompositionareintense.Ammoniteshellsusuallylacksoft-partsandaptychusin the bodychamber,as well asperiostracumor connectingrings. Siphunculartubestendto be disarticulated.Among the resedimentodandburied shells,the phrag-moconeswithout sedimentaryinfilí are common, indicatingvery rapidpro-cessesof sedimentaryinfilí andhigh rateof sedimentaccumulation.Hollowammonitesmaintaintheir original volume,showingno tracesof distortionbygravitational compaction,as a result from rapid early cementation.Shellscanalso showmicrobial laminae,developedduring theresedimentation,andskeletalremainsof very diversoencrustingorganisms,indicativeof lasting andintenseprocessesof nokroplankticdrift. Resedimentedshellsandreelaboratedinternalmoulds may be overgrownby calcareousstromatolitic laminae,andsometimesby phosphaticand/orferruoínouscoatings.Reolaboratedinternalmouldsmay displayroil and truncationalabrasionfacets;moreseldom,asso-ciated with hard andhomogeneoussubstrates,they may show ellipsoidalabrasionfacetsor annularfurrows.Preservedelementsareusuallyreoriented~mdregrouped,fonning encasedor imbricatedpatterns,evenwhen theyarein-cludedin sedimentsof channeledfaciesor depositedby eventsof turbulence.In suchenvironmentalconditionspositive taphosequencesmay be formed.Condensedassociationsincluded in the baseof the parasequencesusuallyshow high valuesof taphonomiccondensation,andsomepreservedelementsdisplaytracesof subaerialexposure,erosionandtransportation.IheserecIa-boratedelementscan be ovenolder in age thantheunderlyingparasequence.

MrsoíAN-DEEPENING TAPHORECOI4DS

Median-deepeningtaphorecordsareformedin the outorplatform underoxygenatedmarinewaterconditionsandwhenthe rateof relativesea-levelrise is sufficiently slow as to allow an increasein the productionof carbo-nate. Such conditionspromotethe developmentof bioconstructionsandhinder the processof lateral accretion.Thisdeepeningphasetendsto coin-cide with the aggradationphaseof sedimentarycyclesof third order(cfJacquinand Vail, 1995). The rateof sedimentaccumulationis usuallyhigh, althoughthe rateof sedimentationcanbe very variablein thediffe-rent areas.Accordingto themorphologyof theseafloor, theonterplatformcanhayorestrictedconditionsin the proximal areasor generalizedopen-seaconditions.Environmentalconditionswill generallybe characterizedby alowcr turbulencethan during thephaseof initial deepening,the influenceof theswell and tho stormson theseafioor beingalso lower Thesepalaco-

A,nmonites,taphonomiccyclesant! stratigraphic cyclesin... 119

geographicconditionsweredevelopedin the Castiliane,AragoneseandTortosaplatformsduring the HumphriesianumandNiortensebiochrones(Bajocian; Fernández-López,1 985c; Fernández-Lópezand Mouterde,1985; Fernández-Lópezef al., 1996),as well asduring the ProgracilisBiochron(Hathonian;Fernández-Lópezcfal., 1978;Lardiés,1990)andtheGracilisBiochron (early Callovian;SequcirosandMeléndez,1987).

A dcpthof severaltensof metersis reachedin thedistal areasduringthisphaseof slow relativesea-levelrisc. Ammoniteassociationswill spreadthroughoutthe most proximal and shallow arcasof the outer platform.The importationof shellsby nekroplankíicdrift is high, as a result from themercasein thespaceof accommodation.Taphonicpopulationsof types 3and 2 aredominaníamongtheseassociations.Patternsof shell distributionareusually ofgroupedtype,andthedegreeof packingaswell asthe stra-tigraphicpersistenceofthe recordedassociationsof ammonitesarevery va-riable in thedifferentareas.Thedegreeoftaphonomicheritageandthede-gree of removal displayedby thc successivcammoniteassociationsdecreasequickly whenthe rateof productionof sedimentsexceedsthe rateof relative risc of sea level. However, theseindicesdo not show mea-ningful variationsif the rate of productionof sedimentsmatchesthe rateofrelativesea-levelrisc. Positivetaphosequencesbecomemuchmoreclearerwhenthc rateofsedimentationdecreases.In theproximal arcas,coincidingwith the boundaries of the parasequences,condensedassociationssho-wing tracesof subaerialexposurecanstill be gencratedduring this phase.

In outer environments,the rate of sedimentaccumulationis usuallyhigh, but the rate of sedimentationmay be very variable in the differentarcas.Accumulatedshellsarescarce,the resedimentedshellsbeingdomi-nant.Locally, coinciding with the boundariesof the successiveparase-quences,reelaboratedinternalmouldswith rolí andtruncationalabrasionfacetsareformed.Biostratinomicproccssesof biodegradation-decompo-sition are less intensetbanin the phasesof incipient or advanceddeepe-ning. Ammonite sbells usually lack soft-partsand aptychusin dic bodychamber,but theycanmaintainthe periostracumandthc connectingringsduring the burial. Hollow ammonitesand partial internal moulds of thebody chamber,indicativeof avery high rateof sedimentaccumulation,areabundant.Resedimentedshellsmaybe overgrownby intrathalamousandextrathalamous,encrustingorganisms.Reclaboratedinternalmoulds areusuallycoveredby stromatolitic laminae.Concretionaryinternal mouldscan be carbonated,phosphaticor glauconitie.Silicified internal moulds,asso-ciatedwith sedimentsdepositedin conditionsof high rateof sedimcntaccumulationbut low rateof sedimentation,maylocally becommon.

1


Restrictedenvironmentsmay becommonin theproximal arcasduringthis phase.The rateof sedimentationandtherateof sedimentaccumulationare both generallyhigh in theseprotectedcnvironments.Reelaboratedlbssils areherevery searceandlimited to thc boundariesbetweensucce-ssíveparasequences.Accumulatedshellsmaybecomequite commonin dicarcasof lower turbulence.Biostratinomieprocessesof biodegradation-decompositioncan be retardedby the deficiencyin shortageof oxygen.Ammonite shells usually lack soft-partsin the body chamber,but theycan still maintainthe periostracumand theconnectingrings. Siphunculartubesareusually articulated.Among the recordedelcmcnts,partial internalmoulds of the body chamberand hollow ammonites,indicative of veryhigh rateof sedimentaccumulation,aredominant.Resedimentedshellsandreelaboratedmoulds are usually coveredby stromatoliticlaminae.Con-cretionaryinternal ínoulds areusuallycarbonated.In themostconfineden-vironments,pyritic internal moulds and shellswithout septa(i.e., hollowphragmocones)canbe formedby earlydissolutionnearthewatcr/sedimentinterface.

ADVANCED-DEEPENJNG TAPHORECORDS

Advanced-deepeningtaphorecordsdevelopedin carbonateepiconti-nental platformscharacterizethc last stadiumsof thc deepeningphases.They acquire the maximumdevelopínentin the most distal and deepcstarcasof the outerplatform, whereoxygenatedenvironmentsprevail but theproductionrateof sedimentsis insufficicnt in relation to the accommoda-tion spacegenerated.Consequently,the ¡ate of sedimentaccuniulationdecreasesandthe depositionalenvironmentsretrograde.The rate of sedi-mentationand the rate of sedimentaccumulationare both usually low.Parasequencesshow a Iess distinetive developmentthan in the previousphases.Thresholdsanddepressedarcascanbe distinguished,but theydonot show importantbrcaksor abrupttransitions.Lateral variationsof thcenvironmentalconditionsaregenerallygradual.The seafloor in the outcrplatform remainsbelow the fair weatherand the storm wave baselevelsduring thesemaximumdeepeningepisodes.Such palaeogeographiccon-ditions werereachedin thc Castiliane,Aragoneseand Tortosaplatformsduring the NiortenseBiochron (PolygyralisSubchronozone,late Bajo-cian; Fernández-López,1 985c;Fernández-Lópezcf al., 1996).

The onterplatfonnconstitutesa carbonaterampwhosemaximumdepthcan vary from severaltensto a hundredmetersduring this last deepening

Ammonites,taphonomiccyclesant! stratigraphiccycles¿u... 121

phase.The final episodesof this decpcningphasecorrcspondto thc max¡-mumdeepeningstadiumsin thc platform.Thoughthc concentrationofam-moniteshellstendsto be lessimportantthanin theprcviousphase,taphonicpopulationsof type 1, 2 and 3 may be produced.The occurrenceof taphonicpopulationsof type 1 is indicativeofautochthonousbiogenicproduction,sho-wing no signsof sortingby nekroplankticdrift. Ammoniteassociationsshowrelatively Iow valuesof packing,but relatively high valuesof stratigraphicpersistence.Biostratinomicprocessesof biodegradation-decompositionareintense.Ammoniteshellsusuallylosethe soft-parts,aptychusandperiostra-cum beforethe burial. Siphunculartubesareusuallydisarticulated.Remainsof intrathalamousor extrathalamous,encrustingorganismsdevelopprefe-rentiallyon the shellsof taphonicpopulationsof type2 or 3. Thc degreeofremovalandthe degreeof taphonomicheritageacquirerelativelyhigh values.Completeconcretionaryinterna! mouldsof thebodychamberandphragmo-cone,indicativeof low rate of sedimentaccumulation,are abundant.Remo-bilization leveisarecommon,oftenenclosingphosphaticor carbonatedcon-cretionary moulds. Pyritic internal moulds are formed only locally.Reelaboratedinternal moulds show usually disarticulationsurfaceswithsharpmargins. Fragmentaryinterna! mouldscanaÑo occur, but bearingnosignsof rounding,encrustationor biocrosion,becauseof the low degreeofturbulencenearthe watcr/scdimentsurface.Howcver, concrctionaryinternalmouldsand shellscan locally beregroupcdand imbricatedor azimuthallyre-oriented.Episodeswith predominanceof backgroundsedimentationgiverisc to coarsening-upwardsrecordedassociationsincludedin fine grainedse-dimcntssbowingno evidenceof basaldiscontinuity.In contrast,eventsof tur-bulence lead to fining-upwardsrecordedassociationsincluded in coarserdepositswith an erosivebase(Fernández-López,1997a, figure 15).

INCIPWNT-SHALLOWING TAPHORECORDS

Lncipient-shallowingtaphorecordsrcpresentthe first stadiumof thesballowingphasesdeveloped in carbonateepicontinentai platforrns. Josuchgreatdepthconditions,theseafloor oftheouterplatformremainbe-low the fair weatherandthc stormwavebaselevels.The first dcpositshavewide geographicaldistributionandareaggradationalor scarcelyprograda-tional. Thcincreaseofterrigcnoussupply leadsto a correlativequick increaseof the sedimentationrate,resultingin thc developmentof marly, expandedsuccessions.Thc rateof sedimentationis high in the decpestarcas,but therateof sedimentaccumulationcanbe very variable.Suchconditionswere

122 Sixto Fernánt!ez-López

developedin thc Castiliane,AragoneseandTortosaplatforms during theHumphriesianum,NiortenseandGarantianabiochrones(Blagdeni,Bacula-ta, Dichotoma and Subgarantisubchronozones,Bajocian;Fernández-Ló-pez, 1983, 1985c,1986, 1987a;Fernández-LópezandMouterde,1985;Fer-nández-Lópezcfal., 1996;Fernández-LópezandMeléndez,1995a,1996).

The totallydelugedouterplatform, with predominanceof softgrounds,constitutesa homogeneouscarbonaterampthat tendsto be deeperthanse-yeraltcnsof meters,with relativelystableenvironmentsduring thephaseof incipient shallowing.Taphonicpopulationsof type 1, 2 and 3, with re-latively high valuesof packingandstratigraphicpersistence,areformed.Theoccurrenceof taphonicpopulationsof type 1 is indicativeof autoch-thonousbiogcnicproduction,showingno signsof sortingby nckroplankticdrift. In thearcasof greaterdepth,thesuccessiverecordedassociationscanbe expandedassociations,their stratigraphicallysuccessivcelementsre-prescntingcontemporaryorganisms.Biostratinomieprocessesof biode-gradation-decompositioncan be inhibited by rapid burial. Buried shcllsusually lack sedimentaryinfilí in thephragmoconeandarepreservedasho-llow ammonites,still maintainingthe soft-parts,periostracumand co-nnectingrings. Siphunculartubesare usuallyarticulated.Remajusof in-trathalamousor extrathalamous,encrustingorganismsareonly developedon somepreservedelementsof taphonicpopulationsof type2 or 3. In thetaphonicpopulationsof type 1, preservedelementslack usuallyencrustingorganismsor stromatoliticlaminaein thcsedistal arcasof theplatform. In-ternal mouldscanbe carbonatedorpyritic. Carbonateconcretionaryinter-nal moulds areonly formed,during theearly diagenesis,in arcasof mini-mumsedimentationrate.Reelaboratedelcmentsarevery scarceoc absent.Accumulatedsbellsmay be locally common.Thedegreeof removalcanbevariable,but thedegreeof taphonomicheritagewill be from vcry low tozero.Signsof abrasionand biocrosionon shellsandinterna! moulds arevery searee.‘[he septaandwall of theshell candisappearby earlydisso-lution, while theperiostracummaystill remain,giving risc to compressedelementsshowingcontinuousdeformationby gravitationaldiageneticcompaction.Disarticulatedaptychii nvay be locally common. Ammoniteshellswill nortnallyappeardispersedin the sediment,showingno patternof imbricatedor encasedregrouping.Taphosequencesand successiveas-sociationsof ammoniteswill not display shawchangesin preservation.

In shallow arcas,successiverecordedassociationsofammonitescanbecondensed.Biostratinomicprocessesof biodegradation-decompositionaremore intensethan in deeparcas. Sedimcntaryinfilled phragmoconesandsignsof encrustationwill bemorecommon.Internalmouldscanbe both car-

Arnmonites, taphonomic evclesant! stratigraphic cyclesin... 123

bonated,phosphaticor glauconitie.Reelaboratedelementsareincreasinglyabundant,whilst accumulatedshellswill becomeprogressivelyscarce.Thcdegreeof removal and the degreeof taphonomicheritagewill be increasinglyhigh. Evidenceof abrasionandbioerosionmay be commonboth in shellsandconcretionaryinternal moulds.Preservedelementscanmaintaintheoriginalvolume and form asa resultfrom earlycementation,showingno evidenceofgravitationaldeformationby diageneticcompaction.Disarticulatedaptychiimay be common.Shellsandconcretionaryinternal mouldscandisplay im-bricatedorencasedpatterns.TaphosequenccsandsuccessiveassociationsofammoniteswiIl notgenerallydisplay shawchangesin preservation.

MEDLAN-SHALLOWING TAPHORECORDS

The rateof sedimentationin outer platform environmentsis generallylower duringthe phasesof medianshallowing thanduring thoseof mci-picnt shallowing. Early cementationprocessesmay be commonin theproximal areasof theplatform, though the¡ateof sedimentaccumuiationmaybe high in distal arcas.Depositionalsystemsshowprogradationalorretrogradationalstackingpatternsandsearcethicknesses.Carbonatebio-constructionscan be also developedduring this shallowingphasein theoxygenatedandproximal environments,whilst microfilarnentfaciestendtobe dominantin the mostdistalanddeepestarcas.Conditionsof higher tur-bulencewill resultin an mercaseof stormsandwave action.Suchpalaco-geographicconditionswere developedin theCastiliane,AragoneseandTortosaplatformsduring theOpalinumBiochron(Aa]enian;Fernández-Ló-pez,1985c;Fernández-Lópezand Gómez,1990a,1990b;Goy cf al., 1994;Fernández-Lópezcia!., 1996),during thc GarantianaBiochron andatthcbcginningof theParkinsoniBiocliron (TetragonaandAcris subchronozo-nes,late Bajocian;Fernández-López,1985c, 1987a;Fernández-LópezandMouterde, 1985; Fernández-Lópezet al., 1996), as well asduring themiddle Bathonian(Fernández-Lópezcia!., 1978;Lardiés, 1990)andtheAncepsBiochron(middle Callovian;SequcirosandMeléndez,1987;Au-relí etal., 1990, 1995;Fernández-Lópezci al., 1996).

Recordedassociationsof ammonitescan still developin everyareaoftheouterplatform during this phase,when the relativesealevel canrcachsornetensof metersof depth.Theautochthonousproductionof ammoniteshells may falí down to zero, but the importation of shells can locallygive risc to high concentrations.Taplioniepopulationsof types3 and2 aregenerallydominantin theseassociations.Pattcrnsof distributionof shclls


are usuallyregrouped.Ratesof sedimentaccumulationare high, but therateof sedimentationcan be very variablefrom some arcasto others.Concretionaryinternal mouldscanbecalcareous,phosphaticorglauconi-tic. Accumulatedelementsare virtually absent.Some associationsmayshow pattemsof regroupingandreorientation.Changesin taphosequencesare generallysharperthan in the previousphaseof incipient shallowing.

In proximal arcas.condensedassociationsarecommonin dic baseoftaphosequences,which canbe positiveor negative.Thc degreeofremovalandtaphonomicheritageis higherin the uppermostandmostproximal re-cordedassociations.Accumulatedelementare absent.Reelaboratedele-ments, showingroil and truncational facets,may be locally dominant.Biostratinomieprocessesof biodegradation-decompositionareusuallyin-tense.Ammoniteshellscommonly lack thesoft-partsandaptychusin thebody chamber,aswell as thc periostracumandconncctingrings. Amongthe recordedelements,partial internalmouldsof body chamber(i.c., ho-110w ammonites)aredominant,asa consequenceof thc high rateof sedi-mentaccumulation.Shellsandconcretionaryinternalmouldsshowabun-daníanddiverseremrnnsof encrustingorganisms,-asweII asstromatoliticlaminae.Concrctionaryinternal mouldsmaybecarbonated,phosphaticorglauconitic. Silicified concretionarymoulds can also be locally formed.

ADVANCED-SHÁLLOWING TAPHORECORDS

Advanced-shallowingtaphorecordsrepresentthe lastphaseof thedcc-pening/shallowingtaphocycles.Theyareformedin environmentscharac-terized by low rate of sedimentation.Early cementationprocesscsarecommon. Progradationalunits of sc-arcethicknessaredevelopedin distalarcasof theplatform. Theseunits towardsthe continentpineh out anddeveloperosiveunconformities.Suchpalaeogeographicconditionsweredevelopedin the Castiliane,Aragoneseand Tortosaplatfornisduring theOpalinum Biochron (early Aalcnian; Fernández-López,1985c; Fernán-dez-LópezandGómez,1990a,1990b; Goy cf al., 1994;Fernández-Lópezcf al., 1996), during theParkinsoniBiochron (Bomfordi Subchronozone,late Bajocian; Fernández-López,1985c, 1987a; Fernández-Lópezcf al.,

1996),aswell as during the Retrocostatum(Bathonian;Fernández-Lópezcf al.. 1978; Lardiés. 1990), Coronatumand Athleta biochrons(Callo-vian; Gómez,1979;Sequciros,Cariouet Meléndez,1984;Fernández-Ló-pezcf al., 1985;SequcirosandMeléndez,1987;Meléndez,1989; AureIl,1990:Aurelí cf al., 1990, 1995;Lardiés,1990;Fernández-LópezandMe-léndez, 1995a,1990b, 1996;Fernández-Lópezcivil., 1996).

Ammonites,taphonomiccyclesant! stratigraphic evelesin... 125

Whenthe relativerisc of sealevel ceasesor whenthe relativesealevelstartsto falí, thepreservedassociationsofammonitescanbe limited to themostdistal arcasof the outerplatformor wider areas,dependingon themorphologyof the sea floor. In theseammoniteassociations,taphonicpopulationsof type 3, composedeitherofresedimentedorreelaboratedele-ments,aredominant.Biostratinomieproccssesof biodegradation-decom-positionaregencrallyintenseduring this phasc.Ammonite shellslack thesoft-partsandaptyohusin thebody chamber,aswell asperiostracum.Si-phunculartubesaredisarticulated.

In distal arcas,therecordedassociationsofammonitesare dominatedby resedimentedelementswith very variablevaluesofpackingandveryhighvaluesof stratigraphicpersistence.‘[he partial internalmouldsofthebody chamber(Le., hollow ammonites),indicative of vcry rapid sedi-mentaryinfilí andhighrateof sedimentaccumulation,aredominant.1-lo-llow ammonitcsmaintainingtheir original volume and form also arecommon,asa resultfrom thelow rateof sedimentationandrapidearlyce-mentation.Ammonite shells can presentmicrobial laminae,developedduring the resedimentationprocesses,and skeletal remainsof very di-verseencrustingorganisms,indicative of intenseandlasting processesofnekroplankticdrift. Positive taphosequencesare usually well develop.Reelaboratedelementsenclosedin thecondensedassociationsfoundin thebaseof the parasequencescan display differcnt signsof subaerialexpo-sure,erosionandtransport,andthey canbe evenolderthan thepreviousparasequence.

Ammoniterecordcd-associationsof themostshallow andproximal arcasareusuallycondensedassociationsof scarcelateralcontinuity.The successi-ve recordedassociationsshow shawchangesin preservationalfeatures.Cal-careous,phosphaticor glauconitic concretionaryinternal moulds showingmultiple sedimentaryinfill phasesare dominant among the recordedele-ments.Calcareous,phosphaticand/orfcrruginous,stromatoliticlaminae,aswell as encrustingorganisms(in particular,serpulids,bryozoansand oys-ters) aregenerallydevelopedon resedimentedshellsandreelaboratedmoulds.Reclaboratedinternalmouldscanshow ¡oíl andtruncationalabrasionfacets,as well asellipsoidal abrasionfacetsor annularfurrows. Concretionaryinter-na! moulds without septaalso are formed by early dissolutionduring theprocessesof reelaboration.Preservedelementsareusuallyreorientedandre-grouped,forming encasedor imbricatedpatternsduring thc episodesof sma-ller rateof sedimentaccumulation.The recordedassociations,in turn, mayshow normal grading when thcy are included in sedimentsdepositedbyeventsof turbulence.

Sixto Fernández-López

MEGASEQUENCESAND SUPERCYCLES

Megasequencesor supercyclesof secondorder representlong-termvariations,severalnúllion years long (over 5 m.y. accordingto Vail cfa!., 1991; Rousselle,1997; over 3 m.y. accordingto iacquin and Vail,1995; 3 to 50 m.y. accordingto Einsele,1992; Gracianskycf al., 1993;VeraTorres,1994; over 2.5 m.y. in Reboulet,1995; lOto lOO m.y. accor-ding to Miau, 1995).

Five deepening/shallowingtaphocycleswere developedduring theMiddle Jurassic,in the Castiliane,Aragoneseand Tortosaplatforms

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Amnionites,taphonomiccyclesant! .stratigraphiccyclesin... 127

(Fig. 9): four median-deepcningtaphocycles(during theearlyAalenian, thcmiddle Aalenian-earlyBajocian,the Bathonianand the Callovian) andoneadvanced-deepeningtaphocycle(during the late Bajocian).

Thesucccssiverecordedassociationsof ammonitescomposingan idealdeepening/shallowingtaphocyclecan be groupedin two megafaphose-quences,eachoneof thoseshowinggradualvariationsin their secondarycharacters:a deepcningmegataphosequcnceand a shallowing megara-phosequence(Fig. 9). Eachmegataphosequencccancompriseoneorseveraltaphosequences.Theboundarybetweentheshallowingandthe deepcningmegataphosequcnces,betwecndeepening/shallowingtaphocycles,corres-pondsat mostto a registratiegapwhich is usuafly associatedwith a wi-despreadstratigraphicgapof regionalextensionin theplatform. I-{owever,an incipientor meanshallowingtaphorecordcanbe followedby a medianor advanceddeepcningtaphorecordreprcscntinga new megataphosc-quenceanda different taphocycle.Thediffcrent megataphosequencesofdic sameordershould be integratedin a megataphosequenceofhigheror-der, on the basisof theamplitudeof the registratiegapsandthe develop-mcntof thesuccessivetaphorccords,in interpretingthe relationshipsbet-ween suceessivemegataphosequencesand taphocycles,as well as toascertaindic episodesof maximumdeepening.

The major discontinuitiesin the Middle Jurassiccorrespondto themiddle Aalenian(MurchisonacBiochron)andto dic late Callovian(Lam-beniBiochron).Twofurtherminor regionaldiscontinuitiescorrespondtothe late Bajocian(ParkinsoniBiochron) andto the late Bathonian(Discus

Fíg. 9.—Zonal díagram showing the maxímum thickness, che stratigrapbic and regístratie gaps, con-densed sections. expanded sections, condensed associations. types of taphonic pnpuLations, tapbore-cords. tapboeycles and megataphosequences of amníonites identified in Ihe MiddLe Jurassic of the ¡be-rian Basin (Castiliane. Aragonese and Tortosa platforms). SM = shalLowing raegataphosequence.DM = deepening megataphosequence. TP- ¡ = taphonic population of type 1. TP-2 = capbonic popu-Lation of cype 2. TP-3 = tapbonic population of type 3. IDT = ineipient-deepening taphorecord. MDT =

median-deepening taphorecord. ADT= advanced-deepening taphorecord. IST = incipient-sbaLLowingtaphorecord. MST= median-shal]owing taphorecord. AST = advanced-shalLowing taphorecord. 2ndPT = peak transgression of seeond order. 2nd PR = peak regression of second order.Fig. 9.—Esquema zonal de las potencias, lagunas, secciones condensadas, secciones expandidas, aso-ciaciones condensadas, tipos de poblaciones tafónicas. cafórregistros, tafociclos y megatafosecuencíasde anímonites identificadas en el Jurásico Medio de Las plataformas Castellana, Aragonesa y de Tortosa.SM= megalafosecuencia de somerización. DM= megatafosecuencia de profundización. TP-l = po-blación tafónica de tipo 1. TP-2 = población tafónica de tipo 2, TP-3 = población tafónica de tipo 3.IDT = caforregistro de profundización incipiente. MDr= taforregistro de profundización media.ADT = laforregistro de profundización avanzada. IST = taforregisiro de soníerización incipiente.MST= taforregistro de somerización media. A5T = taforregistro de somerízacion avanzada. 2ndPT = pico transgresivo de segundo orden. 2nd PR = pico regresivo de segundo orden,

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Biochron).Ammonitetaphorecordsof dic Opalinum Biozonerepresentthelast termsof ashallowingmegataphosequencewhich startedin the lateTo-arcian.Threedeepeningmegataphosequencesweredevelopedduring theMurchisonae-Niortensebiochrones(middle Aalenian-lateBajocian),Zig-zag-Progracilisbiochrones(Bathonian)and Bullatus-Gracilisbiochrones(Callovian).

The maximumdeepeningand the maximum relative sea-leve!riscwerereachedduringtheNiortenseBiochron(late Hajocian).Theepisodesof maximumshallowing, that representmaximumrelativesea-levelfalí,correspondto the middle Aalenian (MurchisonaeBiochron) and of thelateCallovian (Lamberti Biochron)discontinuities.The ammoniterecor-ded-associationscomprisedbetweenthesetwo biochronsrepresenta su-pertaphocycleof secondorder in the IberianBasin. The developmentofthis second-ordersupercyclewas probablyinfluencedby global eustaticchanges(cf. Hallaín, 1988; Haq cf al., 1988; Norris & Hallam, 1995).However,regionaltectonicprocessescouldcompensatetheeffectsofcus-tatie changesin sorneepicontinentalplatforms(cf. Fernández-López& Gó-mez, 1990; Underhill & Partington, 1993;Aurelí cf al., 1995; Fernández-Lópezcf al., 1996).

Taphocyclesand megataphosequencesestablishedwith ammonitesenablethe identificationof sedimentarycyclesof third-order(the so-calItransgressive/regressivecycles)andmegasequencesof secondorderres-pectively.The developmentof suchmegataphosequences,aswell as thedeveiopmentof the sedimentarycyclesof third order, dependson Ihe re-lative changesof the potential of accommodationand on the sedimentproduction.Theshallowingmegataphosequenceswill be moredistinctiveasfar astheaecommodationspacegeneratedby custatiefluctuationsandsubsidencewill be lowerandthesedimentsupplywill be higher.Thedee-peningmegataphosequences,in turn, will be moredistinctive whenboththe accommodationspaceand the sedimentproductionare higher. ‘[heidentificationof deepening/shallowingtaphocyclesis of utmostimpor-tancein interpretingthe transgressive/regressivecyclesof Mesozoicepi-continentalplatforms,whenno evidenceof thecoastalonlap can be ob-servedbut the fossiliferous sedimentsof outerplatform are widelydeveloped.

On theotherhand,as thedevelopmentof third-orderdepositionalse-quencesandsystemstractsor depositionalsystemsdependson the deve-lopmentof the third-ordertransgressive/regressivecycles,dic identificationofdeepening/shallowingtaphocyclescanbe alsousedto recognizethird-order sequencesand depositionalsystems.Median and advanced-deepe-

Arnmonites,taplionomiecyc!esant! stratigraphiccyclesin... 129

ning taphorecordsarefoundin aggradationalandretrogradationalsequen-cesrespectively.Incipientandmean-shallowingtaphorecords,in turn, arefoundin intilling sequencesandin progradationalsequencesrespcctively.In the sameway, elementarystratigraphicsequencesand parasequencescanbe identifiedanddelimitedon thebasisof taphonomiccriteria,suchaselementarytaphonomicsequencesandtaphosequences.

From thepalaeoecologicalpoint of view it is worthyof notetheslightdiachronismobservedbetweenthe deepening/shallowingtaphocyclesestablishedby ammonites,the third-ordertransgressive/regressivecyclesand the second-ordersupercyclesdevelopedin shallow platforms withrespectto the relative abundanceof ammonitetaphonicpopulationsof type 1 (Fig. 10). Themaximumabundanceof taphonicpopulationsof

DEEPENINGISHALLOWING AMMONITEENVIRONMENTAL CYCLE TAPHON~C POPULATIONS

OF TMIRD aRDER OF TYPE 1

Emersion and regional erosion

Advanced shallowing 51

Median shallowing ‘Vr!lncip¡ent shallowing

A,A4ya eddeepenjngMedian deepeningIncípiení deepeningErosion and regional emersion

RELATIVE CHANGES RELATIVE ABUNDANCEOF SEE LEVEL ¡ncreasing - decreasing

deepening - shallowing

Fig. lt)—Diachronism observed between the deepening/shaLlowing taphocycles established by ammo-nites and the relative ahiíndance of tapbonie populations of type ¡ in carbonate epicontinental plat-brms. The maximum abundance of taphonic popuLations of cype ¡ is not recorded in che sediments for-mcd during the advanced-deepening phase but in the sediments of the incipient-shallowing phase. whenmarine environmenrs in the placforíns reached the maximum values of depth and stabiLity.Fig. L0—Diacronismt, observado entre los tafocielos de profundización/somerización establecidoscon animonites y La abundancia relativa de poblaciones tafónicas de tipo 1 en plataformas epieonti-nentales earbonáticas. La máxima abundancia de poblaciones tafónicas de tipo ¡ no se encuentra en Lossedimentos orinados durante las fases de profundización avanzada sino en los sedimentos de somer’-zación incipierne formados cuando Los ambientes de plataforma alcanzan los máximc,s valores deprofundidad y estabilidad.

1


type 1 is not recordedin the sedimentstbrmedduring the advanced-dee-pcningphasebut in the sedimentsof thc incipient-shallowingphase,whenmarineenvironmentsin the platformsreachedthe maximumvalues ofdepthandstability. This fact alsoshowsthegreatercapacityof responseofthebiosedimentarysystemsto relativechangesof sealevel,with respecttothecudemiepopulationsof ammonites.

CONCLUSIONS

Recordedassociationsof ammonitesformedin carbonateepicontinen-tal platformscanbe groupedon thebasisof taphonomiccriteriain tapho-recordsofdifferent categories:incipient, meanor advanced-deepening,aswell asincipient, meanor advanced-shallowing.On thebasisof thechan-gesin thepreservationstateof dic successiverecordedassociationof am-monites, it is possibleto distinguish taphocycles,megataphosequencesandsupertaphocyclesresulting from relativechangesof sealevel. A dcc-pening/shallowingtaphocyclecomprisestwo or more successlverecordedassociationsshowingcyclicalvariationsin their secondarycbaracters,asaresultfrom a cycle of relativechangeof sealeve!. A deepening/shal!owingtaphocyclecomprisestwo megataphosequences:a deepeningand a sim-Ilowing one. Thesuccessivemegataphosequencesof thesameordershouldbe integratedin a megataphosequenceof higherorderto interpretthema-ximum deepeningepisodes.Deepening/shallowingtaphocyclesand me-gataphosequencesestablishedby meansof ammonitesenablethe identiñ-cation of depositionalsequencesof third order and stratigraphicsupercyclesof seoondorderrespectively.

The identificationof deepening/shallowingtaphocyclesis of utmostim-portancelo inrerprcdngIhe transgressive/regrcssivecyclesof Mcsozoicepicontinentalplattbrms,whenno evidenceof thecoastalonlapis observedbut the fossiliferoussedimentsof outer platform are widely developed.Using thepalaeontologicaldatain sequencestratigraphyanalysisin sha-llow marineplatformsrequirestheprior identificationof taphonomícse-quencesand cycles.The succcssiverecordedassociationscomposingse-yeral successivetaphorecordsgeneratedin different cnvironmentscanform a condensedassociationenclosedin asinglestratigraphiclevel.

Five environmentaldeepening/shallowingcyclesweredevelopedin theCastiliane,Aragoneseand Tortosaplatformsduring the Middle iurassic:four median-deepeningcycles (duringthe early Aalenian, themiddleAa-lenian-earlyBajocian,theBathonianandtheCallovian)andoneadvanced-

Arnmonite,s-,taphonomiccycles ant! s¿ratigraphiccyclesiii... 131

deepeningcycle (during the late Bajocian).Ammonitc taphorecordsof theOpalinum Biozonerepresentthe last termsof a shallowingmegataphose-quencebeginningin the late Toarcian.Threedeepeningmegataphoscquen-ces were developedduring the Murchisonae-Niortensebiochrones(Aale-nian-lateBajocian),Zigzag-Progracilisbiochrones(Bathonian)andBullatus-Gracilisbiochrones(Callovian). The maximum deepeningandthe maximum relative sea-levelrisc were reachedduring the NiortenseBiochron(late Bajocian).The sedimentsofthc PolygyralisSubchronozonebelongto a second-orderpeak transgrcssion.Episodesof maximumsha-llowing during theMiddle Jurassic,representingmaximumrelativesea-le-vel falí, correspondto thediscontinuitiesof the middle Aalenian(Murchi-sonaeBiochron)and late Callovian(LambertiBiochron).Thc MurchisonaeandLamberti biochronsconformtwo second-orderpeakrcgressions.Therecordedassociationsof ammonitescomprisedbetweendiesetwo biochronsrepresenta second-ordersupertaphocyclein the iberian Basin. The sedi-mentsof the Middle Jurassicbetweenthesetwo stratigraphicdiscontinui-des,from themiddle Aalenianuntil theupperCallovian,also representanstratigraphicsupercycleof sccondorderin the IberianBasin.

ACKNOWLEDGEMENTS

The authorwishesto expresshis acknowledgementto Drs. G. Melén-dez(Area of StratigraphyandPaleontology,Facultyof Sciences,Univ. Za-ragoza),G. PaviaaudL. Martire (Dip. Scienzedella Terra,Univ. Tormo)for their critical readingof dic manuscriptand the suggestionsreceived.This work is a contributionto thc projectPB96-0838(DGICYT-CSJC).

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Manuscrito recibido: 13-06-96Manuscrito aceptado:2 1-03-97

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