4 - Taphonomy of Fossil Macro-Invertebrate Assemblages

8/4/2019 4 - Taphonomy of Fossil Macro-Invertebrate Assemblages

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Original article

Taphonomy of fossil macro-invertebrate assemblages as a tool forecostratigraphic interpretation in Upper Jurassic shelf deposits

(Prebetic Zone, southern Spain)

Apport de la taphonomie des assemblages fossiles a macro-invertebres a

linterpretation ecostratigraphique des depots de plate-forme du

Jurassique superieur (zone Prebetique, Sud de lEspagne)

Federico Olriz a,*, Matas Reolid a,b, Francisco J. Rodrguez-Tovar a

aDepartamento Estratigrafa y Paleontologa, Facultad de Ciencias, Universidad de Granada, Fuentenueva s/n, 18002 Granada, SpainbDepartamento Geologa, Universidad de Jan, Campus Las Lagunillas 23071 Jan, Spain

Received 27 October 2005; accepted 2 March 2006

Available online 26 December 2007

Abstract

Composition and taphonomy of macro-invertebrate fossil assemblages, together with facies analysis, have been approached in order to interpretshifting paleoenvironmental conditions in the External Prebetic (S-SE Spain) during the early Late Jurassic (Middle Oxfordian). In oolitic andspongiolitic limestones, the size of fossil remains, mode of preservation, within-bed position, corrasion, fragmentation, epibiont and biogenicencrustation, disarticulation and uncoupling, allow recognition of two taphofacies, respectively. Identified ecostratigraphic events and trendsaccord with rapid flooding under high-energy conditions related to ecospace enlargement for cephalopods and then the persistence of lower energy,

long-lasting exposure of skeletals and higher sedimentary rates. The paleoenvironmental interpretation is consistent with neritic environmentsshifting from shallow carbonate to hemipelagic sedimentation and enlarging of shelf ecospace for marine invertebrates.# 2007 Elsevier Masson SAS. All rights reserved.

Rsum

Lanalyse taphonomique et la composition des associations fossiles principalement constitues par des macro-invertbrs, en combinaison avecune analyse de facis, sont utilises pour interprter des changements paloenvironnementaux au dbut du Jurassique suprieur (Oxfordien moyen)dans le Prbtique Externe (S-SE Espagne). Dans les calcaires oolitiques et spongiolitiques, la taille des fossiles, leur prservation, orientation dansles horizons sdimentaires, corrasion, fragmentation, encrotement biognique, dsarticulation et dcouplage de moules internes permettent lareconnaissance de deux taphofacis, respectivement. Lidentification dvnements et tendances costratigraphiques est en accord avec uneinondation rapide sous conditions de haute nergie et agrandissement de lespace cologique pour les cphalopodes, suivi de conditions de bassenergie persistantes avec exposition de longue dure pour les coquilles et un taux plus lev de sdimentation. Linterprtation paloenvironne-mentale est en accord avec le passage dune sdimentation carbonate environnement nritique hmiplagique accouple llargissement de

lespace cologique pour les invertbrs marins.# 2007 Elsevier Masson SAS. All rights reserved.

Keywords: Macro-invertebrate assemblages; Taphonomy; Taphofacies; Palaeoenvironment; Ecostratigraphy; Oxfordian; Prebetic Zone (SE Spain)

Mots cls : Assemblages de macro-invertbrs ; Taphonomie ; Taphofacis ; Paloenvironnement ; Ecostratigraphie ; Oxfordien ; zone Prbtique (Sud delEspagne)

http://france.elsevier.com/direct/GEOBIO

Disponible en ligne sur www.sciencedirect.com

Geobios 41 (2008) 3142

* Corresponding author.E-mail address: [email protected] (F. Olriz).

0016-6995/$ see front matter # 2007 Elsevier Masson SAS. All rights reserved.

doi:10.1016/j.geobios.2006.03.003
mailto:[email protected]://dx.doi.org/10.1016/j.geobios.2006.03.003http://dx.doi.org/10.1016/j.geobios.2006.03.003mailto:[email protected]


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1. Introduction

Ecostratigraphy, or ecosystem stratigraphy, was firstdescribed as the natural development of biostratigraphy,to give paleoecological support to the temporal subdivision ofthe fossil record (Martinsson, 1973; Boucot, 1982; Sokolov andKaljo, 1986, among others), workingon stratigraphic resolutionat the orders of time intervals as fine as 105 years (Rabe andCisne, 1980; Martin, 1990). In this context, ecostratigraphicevents, determined by relatively short-time ecological changes,are especially informative (Kauffman, 1988). Thus, thepotential relationship between changes in ecospace, as multi-dimensional ecological volume and lithofacies reinforces theusefulness of ecostratigraphic interpretations for analyzingmajor traits of the geobiological evolution in a given basin.Hence, ecostratigraphic approaches could serve as a key forrefining recent innovations in basin analysis (i.e., Olriz et al.,1993, 1994, 1995, 1996, 2002b, 2004a, 2004b; Rodrguez-Tovar, 1993; Reolid, 2005). To complement ecostratigraphic

events, the recognition of ecostratigraphic trends (Olriz et al.,1995) informs about lengthy fluctuations in faunal composi-tions, and therefore about ecology at a longer term, which is ofspecial relevance for interpretation of palaeoenvironmentaldynamics as an integrated aspect of basin evolution. In theecostratigraphic procedure, the truly important aspect is theinterdisciplinary approach applied to fossils and associatedsedimentary rocks, on the basis of a bed-by-bed analysis(Boucot, 1982). In order to complement ecological anddepositional interpretations, the integration of taphonomicanalysis into the ecostratigraphic approach is crucial.

Taphonomic analyses of macro-invertebrate assemblages

have revitalize modern interpretation of both depositional andpaleoecological conditions, as well as that of ecologicaldynamics in a hierarchical frame (Speyer and Brett, 1988;Miller, 1990; Allison and Briggs, 1991; Kidwell and Bosence,1991; Parsons and Brett, 1991; Brett and Baird, 1993; Kidwell,1993; Kowalewski et al., 1994; Sageman et al., 1997; Smith andNelson, 2003). All of this is of special relevance forstratigraphic analysis within modern basin research based onoutcrops records (i.e., sequence stratigraphy and relatedapproaches, Rollins et al., 1990; Brett, 1995; Pittet et al.,2002). In the Jurassic of Iberia, ammonite taphonomy providesvaluable data to interpret the ecological and depositionalevolution in shelf environments (e.g., Fernndez-Lpez and

Melndez, 1994; Fernndez-Lpez, 1997, 2000; Olriz et al.,2002b, 2004b). The combined analysis of fossil preservationand lithofacies enables the identification of taphonic popula-tions, taphonomic trends, and taphofacies to improve ecostrati-graphic and sequence stratigraphic interpretations in UpperJurassic sections from the south-Iberian paleomargin (Olrizet al., 1996, 2002b, 2004b).

During the late-Middle to the earliest-Late Jurassic platedynamics affected the North Sea Graben, the growing centralNorth Atlantic, and regions related to the Tethys, determiningflooding events across European shelves under variable tectonicforcing (e.g., Marques et al., 1991; Norris and Hallam, 1995;

Jacquin et al., 1998; Gygi et al., 1998; Leinfelder and Wilson,

1998; Olriz et al., 2003a,b; and references therein). Insouthern Europe, Middle/Upper Jurassic boundary depositsfrom Tethyan epicontinental areas are usually related toferruginous deposits associated with discontinuity surfaces.These deposits have been registered in Iberia (the Prebetic Zoneof the Betic Cordillera; Acosta, 1989; Olriz et al., 2004b;Reolid, 2005, the Iberian Range; Aurell et al., 1994, 1999;Ramajo and Aurell, 1997; Ramajo et al., 2002, and the easternAlgarve Basin; Marques et al., 1991; Leinfelder, 1993); inFrance (the western Subalpine Basin; Dromart, 1989, CtedOr; Courville and Collin, 1997, Schaignay; Scouflaire et al.,1997, Haute-Marne; Collin and Courville, 2000, and thesoutheastern Paris Basin; Lorin et al., 2004); and in Switzerland(the Jura region; Ziegler, 1962; Gygi, 1969, 1981; Huber et al.,1987).

Focused on characterizing Middle-Upper Jurassic boundarydeposits in Western Europe, Norris and Hallam (1995) relatedferruginous-oolitic deposition resting on shallow carbonates toresult from the onset of wide-regional transgression, which has

been largely recognized in Spain following a non-sequencestratigraphic interval embracing the Callovian/Oxfordianboundary (e.g., Bulard et al., 1979; Ramon et al., 1992; Aurellet al., 2002, 2003; Barn et al., 2004; Martn-Algarra and Vera,2004). Across southern Iberia (the Algarve Basin included), theinner Prerift in Morocco, and the Lusitanian Basin, Marqueset al. (1991) interpreted a complex unconformity DIIIembracing Middle/Upper Jurassic boundary local depositsand widespread non-sequences, which relate to their CallovianOxfordian Crisis identified in Terthyan and Atlantic areas.Courville and Collin (2002) interpreted lowermost Oxfordiandeposition in the Paris Basin to result in complex sequences.

Vera et al. (2004) recognized the character of complexdiscontinuity to the top of the Middle Jurassic, with potentialover-imposition of at least three late Bathonian discontinuities,underlying oldest Late Jurassic deposition in pelagic swells ofthe South-Iberian paleomargin.

Within the Mesozoic epicontinental shelf-system developedin the south-Iberian paleomargin, the External Prebeticprovides a record of mid-shelf Upper Jurassic deposits(spongiolithic limestones and marl-limestone rhythmites)overlying inner-shelf LowerMiddle Jurassic sediments (ooliticand dolomitic limestones). Occasionally, the local record offerruginous oolitic limestones overlying the stratigraphicdiscontinuity on top of the Middle Jurassic carbonates (Olriz

et al., 2004a, 2004b) represents deposition during transitionsphases to mid-shelf environments and then hemipelagicconditions in the area.

The aim of this paper is to interpret the course of ecologicaland depositional changes related to switching from carbonate tocarbonate-siliciclastic shelf occurred during the early LateJurassic in a mid-shelf setting in the External Prebetic that firstreceived ferruginous-oolitic and then spongiolithic carbonatedeposition. Hence, to propose a local scenario for the initialflooding of the Middle Jurassic carbonate platform based on thecomparative analysis of composition and taphonomy of macro-invertebrate fossil assemblages recorded in the ferruginous

oolitic limestones and the overlying spongiolithic limestones.

F. Olriz et al. / Geobios 41 (2008) 314232


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2. Geological setting

The studied outcrop (Fig. 1) is located in the ExternalPrebetic that is the outermost, landward fringe of the BeticCordillera (southeastern Iberia). Paleogeographically, thePrebetic represents part of the epicontinental system developedin the south-Iberian margin during the Jurassic (see Olrizet al., 2002a for extended treatment). The preserved ExternalPrebetic constituted a carbonate inner-shelf environment duringthe EarlyMiddle Jurassic, and a carbonate-siliciclastic mid-shelf environment during the Late Jurassic (Middle Oxfordianto EarlyMiddle Kimmeridgian, three-fold division).

Jurassic deposits in the External Prebetic correspond to theChorro Formation and the Lorente Formation (Pendas, 1971).

The Chorro Formation consists of ca. 400 m thick LowerJurassic dolomitized limestones, and 50 m of Middle Jurassicoolitic limestones with megaripples and oncoliticbioclasticfacies. This formation represents a shallow environmentundergoing carbonate sedimentation and short episodes ofcontinental deposition; its upper part shows oolitic bars.

Upper Jurassic (Middle Oxfordian to Lower Kimmeridgian)deposits correspond to the Lorente Formation, and constitute thefirst interval of pelagichemipelagic sedimentation in thesoutheastern epicontinental system occurring in Iberia duringthe Mesozoic. The comparatively most proximal, landwardpreserved sectors of the platform (External Prebetic) are mainlyrepresented in the Lorente Formation by a 70100 m thicksuccession made of spongiolithic limestones and marl-limestone

Fig. 1. Location and geological sketch of the Betic Cordillera (A) and central sector of External Prebetic (B). Synthetic lithological succession for Jurassic rocks inSierra de Cazorla according to Garca-Hernndez and Lpez-Garrido (1988) (C).Fig. 1. Situations gographique et gologique de la Chane Btique (A) et de la partie centrale du Prbtique Externe (B). Succession lithologique synthtique des

roches jurassiques dans la Sierra de Cazorla, daprs Garca-Hernndez et Lpez-Garrido (1988) (C).

F. Olriz et al. / Geobios 41 (2008) 3142 33


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rhythmites (Rodrguez-Tovar, 1993; Olriz et al., 2003a,b;Reolid, 2005).

The Riogazas-Chorro section (RGCH) crops out in theSierra de Cazorla area (central sector of the External Prebetic;Fig. 1). The RGCH section is located in the El Chorro sheet(Foucault, 1971), and includes the contact between the ChorroFormation and the Lorente Formation. The studied section(Fig. 1) is 1.1 m thick and shows, from bottom to top, one orlocally two limestone beds containing ferruginous ooids andpisoids (Middle Oxfordian Plicatilis Zone), three limestonebeds (% 0.7 m total) characterised by a high content of spongeremains (Middle Oxfordian Transversarium Zone), and thenseveral meters of alternating marls and limestones (UpperOxfordian Bifurcatus Zone to Lower Kimmeridgian PlatynotaZone).

3. Methodology

Sedimentological and paleontological analyses were per-

formed separately for each of the two studied facies:ferruginous oolitic limestones and spongiolithic limestones.Outcrop observations were complemented by microscopicanalysis on polished slabs and thin sections, under transmitted-light microscope and binocular magnifying glass, for char-acterizing microfacies.

Macro-invertebrate assemblages were studied following theecostratigraphic procedure used previously (Olriz et al., 1993,1994, 1995). Remains of fossil macro-invertebrates (includingfragments and complete specimens) were collected bed-by-bedtogether with preliminary taphonomic observations in theRGCH section. To avoid counting problems, sponge remains

were excluded from the analysis (because of the difficulty intheir extraction and subsequent evaluation). Other remains thatare normally abundant and disarticulated (e.g., crinoids) wereexcluded because of the impossibility of determining thenumber of individuals. Thus, sponges and crinoids wereanalysed only qualitatively.

Laboratory processing of samples yielded a total of 775specimens of macro-invertebrates, which were characterized asfollows. Three compositional spectra were considered andrepresented in pie-diagrams: spectrum A as a general picturerepresenting the relative abundance in ammonoids, belemnoidsand benthos; spectrum B reflecting the internal composition ofammonoids assemblages (perisphinctoids, Sowerbyceras,

Phylloceratina and Lytoceratina, haploceratids and othersAmmonitina; Olriz et al., 1995); and spectrum C applied toshow the internal composition of benthic assemblages(brachiopods, bivalves, regular echinoids, irregular echinoidsand others; mainly gastropods and ahermatypic corals).

In addition, taphonomic features were characterized indetail, including: mode of preservation, size of skeletalremains, within-bed position, corrasion (sensu BrettandBaird,1986), fragmentation, epibiont and biogenic encrustation,disarticulation and uncoupling (sensu Olriz et al., 2002b,2002c, 2004b). The preservation mode refers to both, therelative occurrence of inner moulds with and/or without

neomorphic shell and the type of sediments that filled up

ammonite, bivalve, brachiopod and gastropod carcasses. Thesize of skeletal remains has been controlled to recognize itspotential relationship with their variable completeness. Sizequantification was made taking into account maximum lengthsto allow identification of size intervals. For fragmentation andcorrasion, the fragmentation index (Fi)andthecorrasionindex(Ci) sensu Olriz et al. (2002b, 2002c, 2004b) were used. TheCi informs about the degree of corrasion of a group of samples,calculated as the mean value obtained by summing theproducts of the number of samples (n) presenting differentdegrees of corrasion [high corrasion degree (HCD, > 60%worn) 100; medium corrasion degree (MCD, 1060%worn) 50; low corrasion degree (LCD, < 10% worn) 1],1], and dividing the resulting value by the total number of

samples considered (N), including those presenting no signs ofcorrasion.

Ci nHCD 100 nMCD 50 nLCD 1

N

The Fi indicates the degree of fragmentation among a set ofindividuals, and is calculated as the mean value obtained bysumming the products of the number of individuals (n)presenting different degrees of fragmentation [high fragmenta-tion degree (HFD, remains are little representative of originaldimensions and shape) 100; medium fragmentation degree(MFD, fragmentation affect appreciably the shape andsize) 50; low fragmentation degree (LFD, fragmentationthat does not affect significantly the shape and size) 1], anddividing the resulting value by the total number of samplesconsidered (N), including those with no fragmentation.

Fi

nHFD 100 nMFD 50 nLFD 1

N

Uncoupling refers to disaggregation in internal moulds ofembedded ammonoids, in relation to location of septa andwhorl overlapping (sensu Olriz et al., 2002b, 2002c, 2004b;phragmocone disarticulation sensu Fernndez-Lpez andMelndez, 2004).

The combined analysis of lithofacies, composition andtaphonomic features in macro-invertebrate assemblages willallow to identify taphofacies to be used as a standard tool inreconstructing ancient depositional environments (e.g., Brettand Baird, 1986; Speyer and Brett, 1986, 1988; Kowalewskiet al., 1994; Olriz et al., 2002b, 2002c; Zuschin et al., 2003).

4. Material and database

4.1. Ferruginous oolitic limestone

Ferruginous oolitic deposits in the Prebetic Zone show thin,local deposition related to the Middle/Upper Jurassic boundary.A planar, ferruginous surface, or a thin ferruginous crust,separates Upper Jurassic ferruginous oolitic limestones fromMiddle Jurassic carbonates. Generally, ferruginous depositspinch-out and their thicknesses appear to be related to thenumber of beds (usually one or two). In the RGCH section, this

lithofacies consists of two beds showing thickness variation



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between 10 and 40 cm and a decreasing upwards abundance inferruginous ooids (Fig. 2). The lower boundary is irregular andlocally related to the record of a ferruginous crust. The

ferruginous oolitic limestone shows a grain-supported bioclas-tic fabric (packstone) (Olriz et al., 2004b; Reolid, 2005) withabundant peloids (30%), ferruginous ooids and pisoids (26%),and bioclasts (26%). Other grains, including quartz, aresecondary. Among bioclasts, echinoderm remains, molluscfragments (mainly ammonoids) and foraminifera dominate.Planktic foraminifera are dominant (82%), while benthicforaminifera are scarcer (mainly spirillinids, nodosariids,agglutinated forms, and Epistomina). Notable is the combinedrecord of Epistomina and Trocholina, as well as the highabundance of thick-shelled, great-size, fragmented specimensofLenticulina (72% of the record of this genus; Olriz et al.,

2004b; Reolid, 2005).

4.1.1. Fossil assemblages of macro-invertebrates

A total of 600 specimens, including fragments, wereanalyzed from the ferruginous oolitic limestone lithofacies.

The macro-invertebrate fossil assemblage (Fig. 2) is composedof ammonoids (70%), belemnoids (21%), and benthics (9%).Among ammonoids, phylloceratids are dominant (28%, withgenus Sowerbyceras representing 11% of ammonoids),followed by perisphinctoids (14%). Dominant benthics aregastropods, ahermatypic corals and bivalves. Belemnoids areespecially frequent in the lower part of the lithofacies overlyingthe ferruginous crust, but their record decreases upward.

4.1.2. Taphonomic analysis

Taphonomic analysis (Olriz et al., 2004b) revealed ahigh abundance of specimens per rock volume (Fig. 3), the

frequent preservation of neomorphized carbonate shells,

Fig. 2. Stratigraphic column for the Riogazas-Chorro section showing composition and mean values of macro-invertebrate fossil assemblages and taphonomicfeatures, both of these gathered from the ferruginous oolitic limestone (lower pie-diagrams) and the spongiolithic limestone (upper pie-diagrams).Fig. 2. Colonne stratigraphique de la section de Riogazas-Chorro indiquant la composition et les valeurs moyennes des assemblages fossiles macro-invertbrs,ainsi que les caractristiques taphonomiques.



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which originally were aragonitic carcasses (ammonoids andgastropods), as well as macrocrystalline calcite filling in theinnermost chambers (e.g., gastropods, ammonoids). Fossilremains size smaller than 30 mm dominates (Fig.2),especiallyin the case of ahermatypic corals (9.0 mm mean-size) andgastropods (11.3 mm mean-size). Macrofossils showing largermean-size are belemnoids (37.7 mm) and Phylloceratina

(28.1 mm). The largest belemnoids are located at the baseof the lowermost bed (Fig. 3A). The most frequent within-bedorientation of fossil remains is oblique to quasi-vertical (61%),while quasi-horizontal orientation is most common amonglarger specimens. The orientation of shell fragments is mainlyconcave up (61%). Features revealing long-lasting exposure ofskeletals on the sea-floor are rarely observed, as indicated bylow frequencies of corrasion (Ci < 2%), microbial encrusta-tion (absent), and macroscopic epibionts (rare; < 3%serpulids). In contrast, the mean value of fragmentation ishigh (Fi = 55%), especially in ammonoids (higher inperisphinctoids 67% and lower in Sowerbyceras 29%).

Evidences of uncoupling were recorded only rarely. In the

case of benthic organisms, disarticulation is variable; commonin bivalves (100%) and echinoderms (100%) and less frequentin brachiopods (25%).

4.2. Spongiolithic limestone

Spongiolithic limestones crop out in the External Prebetic

and are developed as well-stratiphied limestones, with bed-thickness ranging from 12 to 50 cm, and a high abundance ofsiliceous sponge remains (Olriz et al., 2003a,b; Reolid, 2005;Fig. 3B). In the RGCH section, this lithofacies overlies theferruginous oolitic limestones and consists of three beds 24, 20and 23 cm thick (Fig. 2). The fabric under microscope is grain-to matrix-supported, microfacies being characterised aswackestone (locally packstone) mainly composed of bioclasts(25%), oncoids (23%), peloids (18%), ooids (13%), lumps(10%) and tuberoids (5.5%), showing secondary aggregategrains and quartz. The most abundant bioclasts are echinodermremains, indeterminate mollusc fragments, bivalves and

foraminifera. Benthic foraminifera (agglutinated forms, spir-

Fig. 3. [upper left] Lower surface of ferruginous oolitic limestone bed showing abundant belemnoids (B) and ammonoids (A). [upper right] Spongiolithic limestonewith common sponge remains (Sp) and crinoid stem (Cr). Lower views showing ferruginous oolitic limestone with abundant fossil remains such as belemnite rostra(B), ammonite phragmocones with neomorphized shell-walls and septa (A), and indeterminate shells. Scale bar = 1 cm.Fig. 3. [En haut gauche] Surface infrieure dun banc calcaire oolitique ferrugineux riche en blemnites (B) et en ammonites (A). [En haut droite] Calcaire ponges (Sp) et tige de crinode (Cr). (Photos du bas) Calcaire oolitique ferrugineux avec dabondants restes de fossiles comme des rostres de blemnites (B), desphragmocnes dammonites coquilles nomorphoses (A), ainsi que des coquilles indtermines. chelle = 1 cm.



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illinids, nubeculariids, ofthalmidiids and nodosariids) aredominant (62%) while planktics are scarce.

4.2.1. Fossil assemblages of macro-invertebrates

A total of 175 fossil macro-invertebrates were analysed(Fig. 2), showing the clear dominance of ammonoids (69%),especially Sowerbyceras (44% of ammonoids). Benthicorganisms represent 19% of the macro-invertebrate assem-blages, among which infaunal forms are dominated by irregularechinoids (51% of benthics), while brachiopods (21% ofbenthics) dominates suspension-feeding epifauna, aside fromthe occurrence of abundant sponge remains and crinoids (onlyqualitatively analyzed; see above). Siliceous sponges corre-spond to Dictyida, Lychniskida and Lithistida.

4.2.2. Taphonomic analysis

The taphonomic analysis (Fig. 2) reveals a high proportionof small size macrofossils (


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Fig. 4. Environmental reconstruction for Middle Jurassic oolitic limestones and the overlying deposits analyzed. (A) Oolitic limestone corresponding to MiddleJurassic oolitic bars containing terebratulids and bivalves, and local colonization by corals. ( B) Middle Oxfordian ferruginous oolitic limestone showing abundantammonoids and belemnoids, episodes of high-energy conditions and the resulting preservation context. (C) Middle Oxfordian spongiolithic limestone showingcommon benthic organisms (sponges, crinoids, echinoids, brachiopods, bivalves and serpulids), a more stable low-energy environment, lower accumulation rate, and

the resulting preservation context.



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Longer bottom exposure of skeletal remains and innermoulds (higher corrasion and colonization by epibionts).The taphonomic traits of the spongiolithic limestone arecongruent with Taphofacies II described by Olriz et al.(2002b).

These features reveal that the beginning of pelagicsedimentation over the Middle Jurassic carbonate-shelf in

the studied area, which resulted in ferruginous oolitic lime-stones, occurred in a context of ecosedimentary, environmentalturnover (Figs. 4 and 5) related to:

Rapid flooding on the shelf; Relative high water-energy; Ecospace enlargement favouring shelled cephalopods;

Short exposure time of skeletal remains (Olriz et al., 2004b); High accumulation but low sedimentation rates.

The scarcity in benthic macro-invertebrates compared withspongiolithic limestones was related to foreseeable, unfavour-able ecological conditions at the seabed, agreeing withproliferation of opportunistic organisms such as somegastropods and ahermatypic corals. The occurrence of

microfossils and lithoclasts of variable provenance (ferruginousooids, angular quartz grains, and Trocholina from emerged andnear-shore environments, and Globuligerina and Epistominafrom outer marine environments; Olriz et al., 2004b; Reolid,2005), is consistent with rapid flooding of the shelf under high-energy conditions. This interpretation is also supported byabundance of fragmented nodosariids (mainly thick-shelled

Fig. 4. Reconstitution environnementale des calcaires oolitiques du Jurassique moyen et dpts analyss. (A) Calcaire oolitique correspondant des barriresoolitiques trbratules et bivalves, localement colonises par des coraux. (B) Calcaire oolitique ferrugineux de lOxfordien moyen form par dabondantesammonites et blemnites, caractris par des pisodes de haute nergie et montrant son contexte de prservation. (C) Calcaire spongiolitique de lOxfordien moyen organismes benthiques communs (ponges, crinodes, chinodes, brachiopodes, bivalves et serpules), traduisant un milieu nergie basse, un taux daccumulation

infrieur et son contexte de prservation.

Fig. 5. Traits selected from both litho- and bioclasts and the interpreted environment, relative sea level and ecostratigraphy, all these according to ammonite

biostratigraphy. Ecostratigraphic events (EE), ecostratigraphic trend (ET), high energy (HE), low energy (LE).Fig. 5. Paramtres slectionns partir des bioclastes et lithoclastes, environnements interprts, niveau marin relatif et signals costratigraphiques. Informationcale sur lchelle biostratigraphique ammonites. Abrviations : EE : vnements costratigraphiques ; ET : tendances costratigraphiques ; HE : haute nergie ;LE : basse nergie.



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Lenticulina). Storm-dominated deposition offers an appropriatescenario for the studied ferruginous oolitic limestones, whichprobably related to wetter climate forced by the wide-regionaltransgression. Later, environmental conditions changed duringdeposition that resulted in the overlying spongiolithic lime-stones. The new environmental context was mainly related toand/or supported by:

Ecospace readjustment; Diminishing rate of accumulation in a context of higher rate

of sedimentation; Lower environmental energy; A longer exposure of skeletal remains before final burial.

The lower content of quartz grains, the absence offerruginous ooids, foraminiferal assemblages showing highervalues of sessile forms and lower fragmentation ofLenticulina,absence ofTrocholina and scarcer presence of planktic forms,are consistent with more stable, relatively quiet, marine

conditions.Two main scenarios are interpreted for idealizing the

environmental change occurring in the mid-shelf ExternalPrebetic during the beginning of the Late Jurassic (MiddleOxfordian):

Sudden environmental restructuring favourable for receptionof exported materials (ferruginous oolitic limestone),including postmortem transport of macro-invertebrate car-casses in a high-energy context related to flooding events;

Later and comparatively gradual onset of a lower-energyenvironment, in a more open marine carbonate platform,

favouring less disturbed ecologic relationship between themacro-invertebrate community and its fossil record (spon-giolithic limestones).

In ecostratigraphic terms, the beginning of Late Jurassicpelagichemipelagic deposition overlying inner-shelf, LowerMiddle Jurassic carbonates, was related to a short-timeecological change, recorded as an ecostratigraphic event,reflected by the FAD of pelagic macro-invertebrates on thecarbonate shelf (macro- and micro-organisms). Afterwards, ananalogous turnover, in terms of rapid change in environmentalenergy, favoured the return to lower-energy conditions and theirpersistence throughout the rest of the Middle and the Late

Oxfordian. Hence, the possibility for recording and identifica-tion of the ecostratigraphic trend connected to lower-energyenvironments, more stable sea-bottoms, and slower burial.

6. Conclusions

In the Riogazas-Chorro section (Sierra de Cazorla, ExternalPrebetic), the analysis of Middle Oxfordian lithofacies andfossil assemblages, mainly composed by macro-invertebrates,is of value for understanding the ecological and depositionaldynamics related to a major environmental change.

Stratigraphic evolution of lithofacies (from ferruginous

oolitic limestones to spongiolithic limestones), composition of

macro-invertebrate assemblages and taphonomy (includingtaphofacies), relates to changing ecosedimentary conditionsafter initial flooding of a Middle Jurassic carbonate shelf earlyduring the Middle Oxfordian. This flooding forced bothecospace enlargement and the taphonomic process for skeletalson the seabed.

Taphofacies I identified in ferruginous oolitic limestone ischaracterized by scarce benthic forms (9% of the total macro-invertebrate assemblage vs. 70% of ammonoids and 21% ofbelemnoids) with common macrocrystalline calcite filling ofcarcasses; high fragmentation (Fi = 55%) and dominance ofbioclasts in an oblique to quasi-vertical within-bed orientation(61% of the fossil remains); and near absence of corrasion(Ci < 2%) and encrustations. This taphofacies indicatesrelative high water-energy, short exposure time of skeletonsand high accumulation but low sedimentation rates.

Taphofacies II identified in spongiolithic limestone ischaracterized by macro-invertebrate assemblages with highercontent of benthos (19% of the macro-invertebrate assemblages

vs. 69% of ammonoids), scarcity macrocrystalline calcitefilling of carcasses and fragmentation (Fi = 34%); over-dominance of bioclasts with a quasi-horizontal position(51% of the fossil remains); and higher corrasion (Ci = 24%)and colonization by epibionts and microbialites. This tapho-facies is the result of lower environmental energy, diminishingrate of accumulation in a context of higher rate of sedimentationas well as a longer exposure of skeletals on the sea-bottom.

The ecostratigraphic signature of the stratigraphic changesobserved imply ecostratigraphic events related to pulses ofhigh-energy conditions and their shift to low-energy conditionsdetermining the beginning of an ecostratigraphic trend. High-

energy conditions involved storm dominated depositioncharacterizing the analysed depocentre as a setting for thereception of both bio- and lithoclasts. Subsequent low-energyconditions started suddenly and progressively determining apersistent, calm environment throughout the analyzed Oxfor-dian, in which fossil assemblages better inform about the realecology of the seabed and the overlying water column.

Acknowledgements

This research was supported by Projects BTE3029, 1316MCyT and the EMMI Group (RNM178 Junta de Andaluca).

We are grateful to Dr. M. Kowalewski (Virginia Tech) and ananonymous reviewer for valuable suggestions and comments.

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