Palaeogeographic and stratigraphic distribution of mid ...hera.ugr.es/doi/14976432.pdfpaléogéographie et la stratigraphie. Nous présentons ici la répartition des foraminifères

Palaeogeographic and stratigraphic distribution of mid-late Oxfordianforaminiferal assemblages in the Prebetic Zone

(Betic Cordillera, Southern Spain)

Distribution paléogéographique et stratigraphique des associationsdes foraminifères de l’Oxfordien moyen-supérieur de la Zone Prébétique

(Cordillère bétique, sud-est Espagne)

Federico Olóriz *, Matías Reolid, Francisco J. Rodríguez-Tovar

Dept. Estratigrafía y Paleontología, Facultad de Ciencias, Universidad de Granada, Campus Fuentenueva, 18071, Granada, Spain

Received 29 August 2002; accepted 14 March 2003

Abstract

Middle-Upper Oxfordian assemblages of foraminifera in the Prebetic Zone (Betic Cordillera, SE Spain) were analysed at the genus levelto determine their composition, relative abundance, diversity, and dominance, as well as the size of the specimens. A relationship has beenestablished between lithofacies, palaeogeography and composition of foraminiferal assemblages, the former two also determining thestratigraphic record of these microfossil assemblages. Two assemblages of foraminifera serve to identify relatively distal and proximal areasin the Prebetic shelf. The distal assemblage is characterized by higher diversity, specimens of greater size, and more abundant planktic andagglutinated forms. Benthic forms include Ophthalmidium, Epistomina and colonies of encrusting foraminifera. The proximal assemblageshows lower diversity, lower abundance of planktic forms, Epistomina and encrusting nubeculariids, and a greater abundance of spirillinidsand Reofax. On the whole, planktic foraminifera decrease upwards in the studied succession, which, together with decreasing nodularity,could be related to system tract conditions previously proposed for Oxfordian deposits in the southern palaeomargin of Iberia.

© 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

Résumé

L’étude des associations de foraminifères de la Zone Prébétique (Cordillère Bétique, Sud-Est de l’Espagne) a consisté en l’analyse de lacomposition, de l’abondance, de la dominance, de la diversité et de la taille des foraminifères. Les résultats diffèrent selon les lithofaciès, lapaléogéographie et la stratigraphie. Nous présentons ici la répartition des foraminifères en deux assemblages caractéristiques du secteur distalet du secteur proximal de la plate-forme. Le premier est caractérisé par une diversité élevée, une grande taille des individus et par la dominancedes foraminifères planctoniques, des agglutinants, des Ophthalmidium, des Epistomina et des foraminifères encroûtants. L’assemblageproximal est surtout caractérisé par une proportion moins grande des foraminifères planctoniques et des nubéculaires encroûtants, parl’absence d’Epistomina et par une grande proportion de spirillines et de Reofax. Ainsi, les associations varient en fonction d’un gradientdistal-proximal. L’évolution stratigraphique des assemblages indique une diminution progressive des foraminifères planctoniques. Ceci peutêtre interprété, compte tenu de l’aspect noduleux des lithofaciès, comme résultant de variations du niveau marin relatif, comme cela avait déjàété proposé dans des travaux antérieurs sur les sédiments de l’Oxfordien dans la plate-forme du SE de l’Ibérie.

© 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

Keywords: Foraminiferal assemblages; Palaeoecology; Lithofacies; Palaeogeography; Sequence stratigraphy; Mid-late Oxfordian; Prebetic

Mots clés : Foraminifères ; Paléoécologie ; Lithofaciès ; Paléogéographie ; Stratigraphie séquentielle ; Oxfordien moyen-supérieur ; Prébétique

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

Geobios 36 (2003) 733–747

www.elsevier.com/locate/geobio

© 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.doi:10.1016/j.geobios.2003.03.006

1. Introduction

Oxfordian deposits register the first phase of pelagic-hemipelagic sedimentation in the Prebetic Zone after devel-opment of a broad generalized carbonate shelf environmentduring the Early-Middle Jurassic. Study of the Oxfordian isof great relevance in determining the structuring of the sub-Iberian palaeomargin, the development of the epicontinentalshelf and the evolution of its palaeoenvironmental conditionsduring the Late Jurassic. However, few studies focused onthese materials have been carried out, and most of these havebeen of a regional scope. Only recently have updated reportsbeen published on specific aspects of Oxfordian fossil assem-blages concerning sponge bioherms (Acosta et al., 1988),ammonite biostratigraphy (Olóriz et al., 1999) and macroin-vertebrate taphonomy (Olóriz et al., 2002a, b).

Analysis of microfacies and foraminiferal assemblages iscrucial to characterize palaeoenvironmental conditions in theepicontinental shelf that developed during the Late Jurassic.Early papers on these topics focused on the biozonation ofthe uppermost Jurassic-Lower Cretaceous interval (García-Hernández, 1978, 1981; García-Hernández and López-Garrido, 1979). Subsequently, García-Hernández et al.(1981) began to study Oxfordian lithofacies and microfacies,which have been characterized in detail in recent papers(Olóriz et al., 2002a).

The present study comprises the first detailed analysis ofthe composition of foraminiferal assemblages, registered in aproximal-distal gradient along the epicontinental shelf thatdeveloped on the southeastern margin of Iberia during theMiddle-Late Oxfordian (Antecedens-Bifurcatus Chron-zones). The integrated analysis of these assemblages (com-position, abundance and diversity) and their relation withlithofacies identified will facilitate their interpretation in thecontext of the environmental evolution that occurred on thisshelf environment.

2. Geological setting

The studied outcrops are located in the Prebetic Zone(Fig. 1), the outermost and most northerly part of the BeticCordillera (SE Spain), which is divided into internal andexternal sectors (Jerez-Mir, 1973). The Internal Prebetic isstructured in folds and presents a thicker and more completeMesozoic cover, with a predominance of marine deposits.The External Prebetic is structured in tectonic sheets and itsMesozoic cover is thinner, records numerous stratigraphicgaps, and shows a predominance of Jurassic with respect toCretaceous deposits and an abundance of shallow facies oflagunal-marine type with continental intercalations. Fromthe palaeogeographic standpoint, the Prebetic Zone repre-sents part of the epicontinental marine shelf system thatdeveloped on the S-SE margin of Iberia during the Mesozoic,with the Algarve shelf (S. Portugal) being the westernequivalent. The External and Internal Prebetic correspond,respectively, to the relatively proximal and distal palaeogeo-

graphic domains. Upper Jurassic outcrops in the Prebetic areusually grouped into two sectors, central and eastern ones(Olóriz et al., 1992), the former corresponding to Sierra deCazorla and Sierra de Segura, and the latter corresponding tothe outcrops further to the east. The western sector is onlyknown through data from subsurface geology.

The present study is focused on the stratigraphic intervalranging from the Antecedens Zone (Middle Oxfordian) to theBifurcatus Zone (Upper Oxfordian). The studied successionis bounded at the base by a discontinuity at the top of Lowerand Middle Jurassic oolitic limestones and dolomites. Theupper boundary is recognized by the record of Epipeltocerasand is usually related to a slight increase in the clay content.Relatively distal areas of the shelf (Internal Prebetic) arecharacterized by lithofacies with abundant lumps and on-coids of nubeculariids, and show more condensed sectionscomposed of deposits corresponding to the lumpy lithofaciesgroup (Olóriz et al., 2002a). A high siliciclastic content isfound in the more proximal areas (External Prebetic), whichshow a predominance of marl-limestone rhythmites in thecentral sector of the External Prebetic and of spongiolithiclimestones eastwards.

3. Materials and methods

Four profiles were selected (Figs. 1 and 2), two belongingto the External Prebetic (central sector: Riogazas-Chorro II;eastern sector: Pozo Cañada) and two to the Internal Prebetic(Navalperal and Río Segura). Detailed previous studies ofthese profiles have been carried out by Olóriz et al. (1999,2002a).

Riogazas-Chorro II profile (RGCHSP): Located in thewestern part of Sierra de Cazorla (province of Jaén). Thestudied Oxfordian succession, approximately 8.4 m thick, isformed by 0.2 m of limestone with ferruginous ooids, 0.6 mof spongiolithic limestone and approximately 7.6 m of marl-limestone rhythmite, which at the base presents a microbialbuildup with sponges.

Pozo Cañada profile (PC): Located in a small ravine tothe SE of the village of Pozo Cañada (province of Albacete),and continuing along the whole northern slope of Sierra delChortal. The Oxfordian succession analysed, measuring ap-proximately 13.5 m thick, begins with a level of limestonewith ferruginous ooids, 0.1 m thick, above which are 13.4 mof spongiolithic limestone.

Navalperal profile (NV): Located in Sierra de Segura(province of Jaén), on the W slope of the Calar de Navalperal.The stratigraphic succession studied in the Oxfordian isabout 11 m thick, and mainly composed of nodular-likelimestones. The bottom 2 m are dolomitized and possess onlyfew, poorly preserved fossils. Above this level are 1.5 m oflumpy-oncolitic limestone, 2.7 m of condensed lumpy-oncolitic limestone and, finally, 4.8 m of lumpy-oncoliticlimestone.

Río Segura profile (RS): Located in Sierra de Segura(province of Albacete), between the villages of Yeste and

734 F. Olóriz et al. / Geobios 36 (2003) 733–747

Santiago de la Espada. The studied Oxfordian succession is11 m thick. The bottom 2.8 m are formed by nodular-likelimestones intercalated with well-stratified limestone beds.Following these are 1.6 m of condensed lumpy-oncolitic andmore nodular limestone with abundant ammonoids. Finally,there are 6.6 m of lumpy-oncolitic limestone in minor se-quences of decreasing nodularity.

The foraminiferal assemblages were studied by analysing90 thin sections obtained from 52 sampling stations. Westudied and compared microfossil assemblages registered indeposits with high carbonate content, such as the lithofaciesbelonging to the spongiolithic limestone and lumpy lime-stone lithofacies groups. Thus, we analysed the sessilebenthic foraminiferal assemblages together with the rest ofthe assemblage of vagile foraminifera. The former are espe-cially important in relation to sponges and biogenic encrust-ments to interpret eco-sedimentary conditions, as noted byGaillard (1983) and Olóriz et al. (2002a). The main limitationencountered with this approach was the difficulty of distin-guishing some genera of foraminifera in thin sections, as was

the case of nodosarids (e.g. Nodosaria-Dentalina andLenticulina-Astacolus) and nubeculariids (e.g. Nubecularia-Nubeculinella). We studied more than 10 600 specimenscorresponding to 42 genera, including Tubiphytes and Koski-nobullina; the systematic position of the latter two taxa hasnot been definitively established, but in many cases they havebeen interpreted as encrusting foraminifera (Werner, 1986;Leinfelder et al., 1993a; Schmid, 1995, 1996). Three compo-sitional spectra were considered, and pie-diagrams were con-structed to represent the foraminiferal assemblages (Figs. 2,3, 5 and 6):

• Spectrum A. This represents the whole assemblage andconsists of three groups of foraminifera differentiatedaccording to their lifestyle: a) planktic, b) vagile benthicand c) sessile benthic.

• Spectrum B. This represents the internal composition ofthe assemblage of vagile benthic foraminifera, and isformed by five components: a) ophthalmidiids; b) spirill-inids; c) nodosarids; d) agglutinated forms; and e) otherforaminifera, generally present as secondary components.

Fig. 1. Location and geological sketch of the Prebetic Zone (Betic Cordillera, SE Spain), with indication of the sections studied (NV = Navalperal, PC = PozoCañada, RGCHSP = Riogazas-Chorro II, RS = Río Segura).Fig. 1. Localisation géographique et cadre géologique de la Zone Prébétique (Cordillère Bétique, Sud-Est de l’Espagne), avec indication des coupes étudiées(NV = Navalperal, PC = Pozo Cañada, RGCHSP = Riogazas-Chorro II, RS = Río Segura).

735F. Olóriz et al. / Geobios 36 (2003) 733–747

Fig. 2. Lithological columns for sections studied in the Internal (NV, RS) and the External Prebetic (central sector: RGCHSP; eastern sector: PC) showinglithofacies distribution and biostratigraphy. Composition of foraminiferal assemblages: A (total assemblage), B (vagile benthics), C (sessile benthics). Changesin the abundance of encrusting nubeculariids and diversity of foraminifera in the succession studied.Fig. 2. Logs lithologiques des coupes étudiées dans le Prébétique Interne (NV, RS) et Externe (secteur central : RGCHSP ; secteur oriental : PC) montrant larépartition des lithofaciès et la biostratigraphie. Composition des associations de foraminifères : A (ensemble de la faune), B (benthiques vagiles), C (bentiquessessiles). Changements dans l’abondance des nubécularidés encroûtants et diversité des foraminifères dans la succession étudiée.


• Spectrum C. This represents the internal composition ofthe assemblage of sessile benthic foraminifera, and isformed by four components: a) siliceous agglutinatedforaminifera; b) Bullopora; c) Tubiphytes; and d) un-specified sessile foraminifera. Colonial encrusting fora-minifera were not included, due to the difficulty ofdistinguishing the number of individuals in a colony onthe basis of thin sections (e.g. Nubecularia, Nubecu-linella and Koskinobullina). A separate analysis wasperformed for nubeculariids because of their great im-portance in both biogenic encrustments and oncoids.

Other palaeoecological parameters studied that character-ize foraminiferal assemblages were: a) abundance (number

of specimens recorded per unit of surface area in thin sec-tion); b) diversity, both in number of genera and the alphaindex (the a-diversity of Fisher et al., 1943); c) dominance, atthe genus level; and d) specimen size. Following palaeoeco-logical interpretations made of shell morphology (Bernhard,1986; Corliss, 1991; Nagy, 1992; Tyszka, 1994; Nagy et al.,1995), for the assemblages of benthic foraminifera recordedwe established three groups, which are related to their micro-habitat depth within the substrate:

• Epifaunal foraminifera: living on the sediment surfaceor in the topmost centimetre (Corliss, 1991). This groupincludes all the sessile foraminifera, as well as ophthal-midiids, miliolids, spiral agglutinated foraminifera (Am-modiscoides, Glomospira and Trochammina) and spiralcalcareous foraminifera (spirillinids, Epistomina, Tro-cholina, etc), except Lenticulina.

• Shallow infaunal foraminifera: living in the sediment ata depth of less than 2 cm (Corliss, 1991), and formed byelongated agglutinated foraminifera, predominantlyuniserial ones and those which may be streptospiral orplanispiral in their initial states (Ammobaculites, Ammo-marginulina and Haplophragmium), as well as by elon-gated calcareous foraminifera (nodosarids such as No-dosaria, Dentalina and Planularia).

• Ubiquitous foraminifera: represented both by Lenticu-lina, which tolerates a wide range of microhabitats, fromepifaunal to deep infaunal (Tyszka, 1994), and by unis-erial (Reophax) and biserial (undifferentiated textular-ids) agglutinated foraminifera, the latter varying be-tween shallow and deep infaunal microhabitat.

Finally, analysis of fragmentation was performed in orderto test whether the foraminiferal assemblages registered cor-responded to a greater or lesser degree to palaeobiocenosis,or whether distortions had occurred due to post-mortemtransportation. According to Parson and Brett (1991), thefragmentation threshold for each specimen is variable, and soit is preferable to use a single species or genus found in all theenvironments registered. In the present case, a preliminaryanalysis enabled us to determine that fragmentation wasrestricted almost exclusively to the genus Lenticulina. In theremaining groups, no more than 5% of specimens were foundto be fragmented. Thus, Lenticulina was chosen for thetaphonomic study, being a ubiquitous organism that ispresent in all the profiles and one that is interpreted as beingendobenthic and epibenthic (i.e. ubiquitous as consideredabove).

4. Results

In general terms, and after excluding the foraminiferalassemblage recorded in the ferruginous oolitic limestonelithofacies due to its local record, the average foraminiferalassemblage in the Prebetic domain corresponding to theTransversarium? and Bifurcatus Zones is abundant and di-versified, dominated by vagile benthic foraminifera (58%),followed by planktic foraminifera (30%) and sessile benthic

Fig. 3. Composition of foraminiferal assemblages according to lithofacies.Pie-diagrams as in Figure 2.Fig. 3. Composition des associations de foraminifères par rapport au litho-faciès. Diagrammes circulaires comme dans la Figure 2.


foraminifera (12%). The assemblage of vagile benthic fora-minifera mainly consists of agglutinated forms (36%) andspirillinids (28%). Nodosarids and porcelaneous foramin-ifera are normally present as minor components. Plankticforaminifera are represented exclusively by the genus Globu-ligerina. The assemblage of sessile benthic foraminifera isdominated by nubeculariids. Non-colonial sessile benthicforaminifera are dominated by siliceous agglutinated fora-minifera (44%), followed by Tubiphytes (28%), while Bul-lopora and other undetermined foraminifera comprise therest of the assemblage. A detailed analysis of foraminiferalassemblages in the four profiles selected (Fig. 2) revealedimportant differences that might be related to the lithofacies(Figs. 3 and 4), palaeogeography (Figs. 5 and 6) and stratig-raphy.

4.1. Foraminiferal assemblages and lithofacies

Detailed analysis of the different groups of foraminiferareveals the close relationship with lithofacies (Fig. 2), interms of composition, abundance and diversity of the assem-blages registered. According to lithofacies, the followingfeatures can be distinguished (Fig. 3):

4.1.1. Ferruginous oolitic limestoneThis limestone level is 10 cm thick, with irregular mor-

phology and ferruginous surfaces that bound small rockybodies. It is found exclusively in the External Prebetic(Fig. 2) and possibly belongs to the Antecedens Zone(Acosta, 1989). Its fabric varies from grain-supported tomatrix-supported, and shows a packstone-wackestone tex-

Fig. 4. Relative content in significant genera of foraminifera (>1%) in the assemblages registered, and their diversity according to lithofacies.Fig. 4. Contenu relatif de genres significatifs de foraminifères (>1%) dans les associations enregistrées, avec leur diversité par rapport au lithofaciès.


ture containing abundant ferruginous ooids, peloids, quartzand bioclasts, mainly fragments of crinoids and foraminifera.

This lithofacies is characterized by a great abundance offoraminifera, especially planktic foraminifera (81%), in con-trast to the scarcity of encrusting benthic foraminifera.Among the vagile benthic foraminifera, there are similarquantities of spirillinids, nodosarids, agglutinated and otherforaminifera, with a lesser abundance of ophthalmidiids. Thenodosarids belong mainly to the genus Lenticulina, showinga thick, large shell (average 270 µm) and frequent fragmen-

tation (72% of the specimens). There is a notably high pro-portion of Epistomina and Trocholina, which are more abun-dant than in other lithofacies, and the absence ofnubeculariids is significant. This lithofacies contains a totalof 14 genera, of which only 10 exceeded 1% of relativeabundance (Fig. 4), and there was a strong predominance ofplanktic foraminifera. The a-diversity value of Fisher et al.(1943) for the assemblage of benthic foraminifera was 2.7.The benthos is dominated by epifauna, mainly representedby planispiral calcareous foraminifera.

Fig. 5. Composition of foraminiferal assemblages in pie-diagrams as in Figure 2. Palaeogeographic distribution and size changes in selected genera and/orgroups of foraminifera, as well as foraminiferal diversity, across a proximal (External Prebetic) – distal (Internal Prebetic) transect in the Prebetic Zone. Legendas in Figure 2.Fig. 5. Composition des associations de foraminifères dans les diagrammes circulaires comme dans la Figure 2. Distribution paléogéographique et changementsde taille dans des genres et/ou groupes de foraminifères représentatifs, ainsi que la diversité des foraminifères, à travers un transect allant d’un domaine proximal(Prébétique Externe) à un domaine distal (Prébétique Interne) dans la Zone Prébétique. Même légende que la Figure 2.


4.1.2. Condensed and bioclasts-rich lumpy-oncoliticlimestone

This lithologic term refers to limestones of nodular, lumpyappearance, with indistinct, irregular stratification surfacesand a large quantity of ammonoids. This lithofacies is foundonly in the Internal Prebetic, in the lower part of the BifurcatusZone (Olóriz et al., 2002a) (Fig. 2). The fabric is intermediatebetween grain-supported and matrix-supported and its com-ponents show random distribution. The texture is packstone-wackestone with lumps and oncoids of nubeculariids. Thislithology shows a high proportion of skeletal grains.

The abundance of foraminifera is high, the assemblagebeing mainly composed of Globuligerina (45%), vagilebenthic foraminifera, and to a lesser extent, sessile benthicforaminifera. The vagile benthic foraminifera are mainlyagglutinated forms, followed by ophthalmidiids and spirill-inids. The sessile foraminifera are predominantly siliceousagglutinated and Bullopora. Nubeculariids are very abundantand constitute a fundamental part of the oncoids. The pres-ence of fragmented shells is limited exclusively to specimensof thick-shelled Lenticulina (20%). A total of 23 genera areidentified, of which 13 are present exceeding 1%, with a clear

Fig. 6. Composition of foraminiferal assemblages according to palaeogeography and the stratigraphic interval and horizons recognized, from the AntecedensZone (Acosta, 1989) to the Bifurcatus Zone in the Prebetic. Pie-diagrams as in Figure 2.Fig. 6. Composition des associations de foraminifères en fonction de la paléogéographie, de l’intervalle stratigraphique et des horizons reconnus, depuis la Zoneà Antecedens (Acosta, 1989) jusqu’à la Zone à Bifurcatus dans le Prébétique. Diagrammes circulaires comme dans la Figure 2.


predominance of Globuligerina (Figs. 3 and 4). Thea-diversity value for the assemblage of benthic foraminiferais 4.6. The microbenthos is mainly comprised of Ophthalm-idium, Spirillina, Bullopora, Tubiphytes and Epistomina,which indicates a predominance of epifaunal microhabitats.

4.1.3. Lumpy-oncolitic limestoneThis lithofacies, recorded exclusively in the Transver-

sarium? and Bifurcatus Zones in the Internal Prebetic(Fig. 2), is composed of limestones of nodular-like appear-ance. The fabric is grain- and matrix-supported, with noparticular distribution of components. The texture comprisespackstone and wackestone in which the most common grainsare lumps, oncoids of nubeculariids, peloids and bioclasts.

The abundance of foraminifera varies greatly, being high-est in the lumpy-oncolitic limestone that precedes the con-densed lithofacies. The foraminiferal assemblage is domi-nated by vagile benthic foraminifera (58%), followed byplanktic foraminifera, and finally, sessile benthic foramin-ifera (Fig. 3). The assemblage of vagile benthic foraminiferais dominated by agglutinated forms (Ammobaculites andReophax). The assemblage of sessile benthic foraminifera ismainly comprised of Tubiphytes and siliceous agglutinatedforms. Nubeculariids are very abundant. There is a low pro-portion of fragmented specimens of Lenticulina (23%). Inthis lithofacies, we registered a total of 28 genera, of which11 exceeded 1% of relative abundance, with a predominanceof Globuligerina (Fig. 4). The a-diversity value for the as-semblage of benthic foraminifera is 4.3. The main micro-habitat in the microbenthos corresponds to vagile epifauna.Increased occurrence of sessile epifauna as well as of shallowinfaunal foraminifera, mainly Ammobaculites, has been reg-istered in the lumpy-oncolitic limestone recorded in the up-per part of the Bifurcatus Zone.

4.1.4. Spongiolithic limestoneThis lithofacies is composed of well-stratified limestones

characterized by a great abundance of sponges, which com-prise a fundamental part of the rock. The lithofacies onlyappears in the External Prebetic, in the Transversarium? andBifurcatus Zones (Fig. 2). The fabric varies from grain- tomatrix-supported, with a packstone-wackestone textureshowing bioclasts (e.g. foraminifera, sponge spicules, frag-ments of echinoderms), lumps, tuberoids and oncoids ofnubeculariids.

There is a generally high abundance of foraminifera, thesebeing mainly vagile benthic foraminifera (64%), followed byplanktic forms and, finally, sessile benthic ones (Fig. 3). Theassemblage of vagile benthic foraminifera is dominated byspirillinids (43%), followed by agglutinated forms. The oph-thalmidiids and nodosarids are the least abundant. The sessilebenthic assemblage is mainly comprised of siliceous agglu-tinated forms. The proportions of nubeculariids are high butvariable. Fragmented specimens of Lenticulina (35%) aremainly the remains of large, thick-shelled individuals. Thislithofacies presents the largest number of genera identified

(29), of which 12 appear in a proportion exceeding 1% of therelative abundance in this assemblage (Fig. 4). There is loweroverall dominance in this lithofacies, the principal compo-nents being Spirillina, Globuligerina and Reophax (Fig. 4).Despite the high number of genera identified, their ratio tothe number of individuals recorded in the benthic foramin-iferal assemblage provides an a-diversity value of 3.8. In themicrobenthos, there was observed to be a predominance ofepifaunal microhabitats and a large proportion of foramin-ifera with a potentially deep infaunal microhabitat (Reophaxand Lenticulina).

4.1.5. Marl-limestone rhythmiteThis lithofacies is constituted of well-stratified limestones

and marls, with a predominance of limestone beds. It isrecorded in the Transversarium? and Bifurcatus Zones of theRGCHSP profile (External Prebetic) (Fig. 2). It presents agrain-supported fabric, constituted of very small homometricgrains (

lesser extent, Redmondoides and Ammodiscoides. The Inter-nal Prebetic is also characterized by the highest proportionsof ophthalmidiids and the lowest proportions of spirillinids.Nodosarids are mainly planispiral-shaped, while uniserialforms are very scarce. With respect to the assemblage ofsessile benthic foraminifera, there is predominance of sili-ceous agglutinated foraminifera and of Tubiphytes, in addi-tion to numerous colonies of nubeculariids. The proportionof fragmented foraminifera is low, with only Lenticulinapresenting higher values (22%). The Internal Prebetic has anaverage a-diversity value of 4.3.

4.2.2. External PrebeticThe assemblage is clearly dominated by vagile benthic

foraminifera, except locally in ferruginous oolitic limestonesat the base of the succession. There is a notable decrease inthe proportions of planktic foraminifera and encrusting nube-culariids in comparison with the Internal Prebetic, especiallyin the central sector of the External Prebetic in relation to thewidespread record of marl-limestone rhythmites. The domi-nant assemblage of vagile benthic foraminifera in the Exter-nal Prebetic is mainly composed of spirillinids. Agglutinatedforms present similar values in the central and eastern sectorsof the External Prebetic, but they are less numerous than inthe Internal Prebetic. In the External Prebetic, Reophax ismore abundant than Ammobaculites, both of these generashowing specimens smaller than in the Internal Prebetic. Theaverage abundance of the ophthalmidiids is clearly lowerthan that registered in the Internal Prebetic, while nodosaridsare somewhat more abundant. With respect to sessile fora-minifera, there is a clear differentiation between the easternand central sectors, which is related to the change in lithofa-cies. Eastwards in the External Prebetic, there is a predomi-nance of siliceous agglutinated foraminifera, while in thecentral sector there are abundant fragments of Tubiphytes inthe marl-limestone rhythmite. Colonial sessile foraminiferaare mainly nubeculariids, and they are much more abundantin the spongiolithic limestone and in microbial buildup withsponges. There are no colonies of nubeculariids in the marl-limestone rhythmite. Fragmentation of foraminifera isgreater than in the Internal Prebetic, although limited almostexclusively to large specimens of Lenticula. The averagea-diversity value in the External Prebetic is 3.4, being higherin the eastern sector than in the central one (a-diversity in thePC profile = 3.6, and in the RGCHSP profile = 3.3). Thisreduction in a-diversity values in the more proximal areascoincides with the decrease in the average size of the mostabundant foraminifera in the assemblages (Globuligerina,Reophax, Ammobaculites, Lenticulina and uniserial nodosa-rids), with the exception of Spirillina, which is slightlylarger.

4.3. Stratigraphic distribution

The stratigraphic variations in foraminiferal assemblagesin the four profiles studied allow us to differentiate twostratigraphic intervals (Fig. 2). These stratigraphic intervals

are preceded in the External Prebetic by a horizon comprisedof ferruginous oolitic limestone, which corresponds to theAntecedens Zone according to Acosta (1989). This horizoncontains the highest proportions of planktic foraminifera.Sessile benthic foraminifera are scarce in the central sector ofthe External Prebetic and absent from the eastern sector (PC).Epistomina in the central sector and Trocholina in the easternsector are abundant, as are the geographically unrestrictedand mainly fragmented Lenticulina (72%). The scarce sessilebenthic foraminifera mainly correspond to Bullopora tuber-culata, while nubeculariids are virtually absent. Globulige-rina presents the highest abundance in the ferruginous ooliticlimestone showing the largest size registered for this genus(170 µm). Moreover, the ferruginous oolitic limestone con-tains the greatest abundance of foraminifera, with the pre-dominance of Globuligerina, and provides highly variablea-diversity values for benthic foraminifera (3.9 in RGCHSPand 1.5 in PC).

Stratigraphic interval I: Corresponds to the Transver-sarium? Zone and/or to the lower part of the Bifurcatus Zone.It is characterized by a reduction in the abundance and size ofGlobuligerina, mainly in the External Prebetic. In the Exter-nal Prebetic the abundance of encrusting foraminifera in-creases in relation with the occurrence of both the spongi-olithic limestone (eastern sector) and the microbial buildupwith sponges (central sector). The assemblage of vagilebenthic foraminifera in the External Prebetic is dominated byspirillinids and agglutinated forms (mainly Reophax), whilein the Internal Prebetic there is a predominance of the latter(especially Ammobaculites) and/or Ophthalmidium. Thesessile benthic foraminifera in all the Prebetic mainly consistof siliceous agglutinated forms, and then Bullopora tubercu-lata. The high proportion of fragmented Tubiphytes in thecentral sector of the External Prebetic is related to the devel-opment of marl-limestone rhythmites. Colonies of nubecu-lariids are very abundant in this stratigraphic interval, exceptin marl-limestone rhythmites. Fragmentation in Lenticulinais high in the central sector of the External Prebetic (49%),but lower than in the underlying ferruginous oolitic lime-stone. Moreover, the proportion of specimens of fragmentedLenticulina in the External Prebetic (43%) is greater than inthe Internal Prebetic (22%). Nevertheless, contrary trends areobserved for stratigraphic interval I in the two domains of thePrebetic: the proportion of fragmented Lenticulina increasesupwards in the External Prebetic, but a corresponding de-crease occurs in the Internal Prebetic. In general, thea-diversity values are higher for the assemblage of benthicforaminifera in the Internal Prebetic (4.3) than in the ExternalPrebetic (3.5), where the lowest values are recorded in thecentral sector, related to the known record in marl-limestonerhythmites. Throughout stratigraphic interval I, there is atrend for diversity to decrease in the External Prebetic, and toincrease in the Internal Prebetic. The abundance of foramin-ifera is greater at the lowermost part of this interval for all theprofiles studied, although it is less accentuated in the centralsector of the External Prebetic.


Stratigraphic interval II: Corresponds to part of the Bifur-catus Zone (Fig. 2). The most notable feature in this intervalis the severe reduction in the abundance of Globuligerina. Inthe central sector of the External Prebetic, the occurrence ofthis genus diminishes to disappearance upwards in the Bifur-catus Zone (RGCHSP), but its last occurrence is known closeto the Transversarium/Bifurcatus boundary eastwards (PC).With respect to stratigraphic interval I, the record of vagileforaminifera shows: a) an increase in the relative abundanceof agglutinated forms (Ammobaculites and Reophax) in theInternal Prebetic (NV and RS), together with their increasedsize throughout the Prebetic; b) a reduction in the relativeabundance of Ophthalmidium in the Internal Prebetic; c) thedisappearance of Epistomina; d) a decrease in the relativeabundance of uniserial nodosarids; and e) less fragmentationof Lenticulina in the External Prebetic (36%) in contrast toslightly higher values in the Internal Prebetic (23%). In theassemblage of sessile benthic foraminifera, there is an in-creased occurrence of Tubiphytes, at the expense of siliceousagglutinated forms, in the Internal Prebetic. In qualitativeterms, nubeculariids remain the most significant encrustingforaminifera. The a-diversity values for the assemblage ofbenthic foraminifera are variable in the different profiles, andno clear trend was identified. The abundance of foraminiferais lower than in stratigraphic interval I.

5. Discussion

5.1. Foraminiferal assemblages and palaeogeography

Overall, the clear predominance of vagile benthic fora-minifera and the abundance of sessile benthic foraminifera inthe microfossil assemblage registered in the Prebetic domainare characteristic of a shelf environment. A detailed quanti-tative analysis enables two type assemblages of foraminiferato be distinguished for the Prebetic, which are closely relatedto palaeogeography (Fig. 5):

• Distal assemblage. Observed in outcrops from the Inter-nal Prebetic, and characterized by the following:C Highest proportions of Globuligerina and of encrust-

ing nubeculariids (Nubecularia-Nubeculinella).C The assemblage of vagile benthic foraminifera is

dominated by uniserial-agglutinated forms, mainlyAmmobaculites followed by Reophax, and containsfew specimens of spiral agglutinated forms (Ammodis-coides, Glomospira and Trochammina). Ophthalm-idium and Epistomina are present in highest propor-tions.

C The sessile benthic foraminifera are predominantlynubeculariids, a fundamental component of the on-coids in the lumpy lithofacies group, while the remain-ing sessile benthics are mainly constituted of siliceousagglutinated forms.

C Fragmentation is low, and mainly found in specimensof thick-shelled Lenticulina. On average, 22% of thespecimens of Lenticulina are fragmented.

C The largest average sizes of Globuligerina, Lenticulina,uniserial nodosarids, Ammobaculites and Reophax.

C Highest diversity values for assemblages of benthicforaminifera.

• Proximal assemblage. Registered in profiles from theExternal Prebetic. Distinguished from the previouslycharacterized distal assemblage by the following:C Lower proportions of Globuligerina and of encrusting

nubeculariids.C Vagile benthic foraminifera are dominated by spirill-

inids. Lower proportion of ophthalmidiids and agglu-tinated forms, the latter being less diversified andmainly constituted of Reophax. Spiral agglutinatedforms are almost non-existent. Among nodosarids,there is a higher proportion of uniserial forms.

C Sessile benthic foraminifera show the predominanceof nubeculariids, although their abundance is lowerthan in the distal assemblage (see above). The relativeabundance of non-colonial sessile benthic foramin-ifera varies according to lithology.

C Higher proportion of fragmented specimens, mainlythick forms of Lenticulina (35% in the eastern sectorand 43% in the central sector) and, to a lesser degree,of Tubiphytes in marl-limestone rhythmites.

C Smaller average size of foraminifera, except in thecase of Spirillina.

C Lower diversity values for benthic foraminifera.By distinguishing these two type assemblages, it is pos-

sible to establish a basic pattern for the distribution of fora-minifera in relation to the proximal-distal gradient (Fig. 5).In general terms, the foraminiferal assemblages confirm theincrease in distance from shore towards the Internal Prebetic,which is mainly reflected in the higher proportion of plankticforaminifera seawards. In the Internal Prebetic, particularlynoteworthy is the appearance of Epistomina, which is tradi-tionally, considered a deep benthic foraminifer found on theouter edge of the shelf (Bernier, 1984; Samson, 2001). Al-though present in scarce numbers, the distribution of Epis-tomina is very significant, and related to intervals whereGlobuligerina is more abundant.

In this context of proximal-distal differentiation, param-eters such as the variations in the sedimentation rate, silici-clastic inflows and the availability of nutrients may deter-mine local variations in foraminiferal assemblages. Thehigher proportion of mainly colonial sessile benthic foramin-ifera such as Nubecularia-Nubeculinella, found in distal ar-eas, may be explained in terms of the lower sedimentationrate. Their reduction towards more proximal areas is relatedto increased inflows of siliciclastics, this situation beingmore marked in the central sector of the External Prebetic,where marl-limestone rhythmites predominate. In this sense,the fragmentation of foraminifera constitutes a valuable toolfor interpreting possible transportation of their shells. Theincidence of fragmentation is very low among the differentgroups of foraminifera, generally less than 5% of the speci-mens, being greater among the nodosarids, especially in


Lenticulina. Fragmented Lenticulina mostly correspond tothick, sometimes ornamented forms. The proportion of Len-ticulina with fragmentation is greater towards more proximalareas, becoming more marked in the central sector, which isrelated to the presence of marl-limestone rhythmites. Thisdistribution indicates that, on the whole, Lenticulina remainsproceed from more proximal areas, which at present do notcrop out, and so the specimens of Lenticulina resistant totransportation would be observed as the only elements thatare clearly allochtonous within the foraminiferal assem-blages studied. The other groups of foraminifera identifiedpresent almost no fragmentation and could be consideredautochthonous to para-autochthonous.

The above-described distribution of foraminiferal assem-blages for the Prebetic is coherent with the interpretationsmade of Middle Oxfordian benthic foraminifera in a shelf-basin transect in SE France (Bouhamdi, 2000; Bouhamdi etal., 2000, 2001). The latter authors related the abundance ofSpirillina on the shelf to the availability of trophic resources,fundamentally photosynthetic primary production. This con-clusion is consistent with the greater proportion of spirillin-ids observed in the lithofacies of spongiolithic limestone inthe External Prebetic, and with the higher content of nutrients(Leinfelder et al., 1993b related it to the presence of spongesin a nutrient- and organic matter-rich environment, but seeKrautter, 1998). The decreased occurrence of Spirillina as acomponent of the vagile benthos towards more distal areas iscompensated by an increase of ophthalmidiids and aggluti-nated forms, both of which are interpreted as good indicatorsof distance from the shore (Gaillard, 1983; Bouhamdi, 2000;Bouhamdi et al., 2001). Among the vagile agglutinated fora-minifera, Reophax is more abundant towards proximal areas,and Ammobaculites towards distal ones, both genera beingvery abundant on the whole shelf. According to Tyszka(1994) Reophax would be a potentially deeper infaunal fora-minifer. Thus, the distribution of infaunal agglutinated formsobserved in the Prebetic would indicate palaeogeographicdifferences in the ecological conditions of the first few cen-timetres within the substrate, which presumably would bemore unstable landwards. The External Prebetic, especiallyeastwards, would provide higher productivity (i.e. ampleavailability of nutrients) evidenced by the abundance of spir-illinids (Bouhamdi et al., 2001) and sponges (Leinfelder etal., 1993b). In this context, the presence of shallow infaunalforms (e.g. Ammobaculites, Nodosaria, Dentalina), as wellas potentially deep infaunal ones (Reophax), could be inter-preted as being related to the good oxygenation of the seaf-loor (i.e. mesothophic conditions following the model pro-posed by Jorissen et al., 1995). The reduced numbers ofspirillinids and sponges towards distal areas (Internal Preb-etic) could be indicative of a lower availability of nutrients(more oligotrophic conditions), which would mainly be con-sumed on the surface and in the first few centimetres of thesediment. This would explain the reduction in numbers of thepotentially deep infaunal genera (e.g. Reophax) with respectto the shallow infaunal ones (Ammobaculites). Greater stabil-

ity in seafloor conditions towards outermost areas is in accor-dance with the data obtained.

5.2. Palaeoenvironmental evolution

Morpho-group distribution, abundance, diversity anddominance, as well as the relative abundance of calcareouswith respect to agglutinated forms in foraminiferal assem-blages have been applied in interpretations based on se-quence stratigraphy (e.g. Cubaynes et al., 1989, 1990; Bon-net et al., 1991, 1992; Haig and Lynch, 1993; Rey et al.,1994; Hylton and Hart, 2000; Nagy et al., 2001). However,most of these interpretations were based exclusively onvariations in the assemblages of benthic foraminifera withina single lithology. The variety of lithofacies observed in theprofiles studied in the Prebetic could reflect differences in theenvironmental conditions that affected the meiofauna, whichwould make it unwise to interpret relative variations of thesea level solely on the basis of the microbenthos. For thisreason, a careful analysis has been made of the relativeabundance of planktic versus benthic foraminifera in theinterpretations of fluctuations in relative sea level, taking intoaccount the independence of planktic forms with respect tothe palaeoecological conditions at the seafloor.

The initial horizon of ferruginous oolitic limestone, be-longing to the Antecedens Zone (Acosta, 1989), is character-ized by the great abundance of foraminifera, by the predomi-nance of planktic forms and by the scarcity of benthicforaminifera, both vagile and sessile. The foraminiferal as-semblage presents a mixture of proximal (i.e. shallow) anddistal (but not necessarily deep) elements, showing highestabundances for Globuligerina, Epistomina and Trocholina(the latter being typical of high-energy, shallow environ-ments). The environmental energy must have been relativelyhigh, as shown by the abundance of nodosarids with signifi-cant fragmentation, especially specimens of thick-shelledLenticulina, which presents in the highest fragmentation.The scarcity or absence of sessile benthic foraminifera, on-coids and other biogenic encrustments could be related to thefact of rapid deposition. The ferruginous oolitic limestonehas been interpreted as the result of a transgressive pulsethroughout the Prebetic that gave rise to the start of LateJurassic hemipelagic deposition over the Early-Middle Ju-rassic carbonate shelf (García-Hernández et al., 1980, 1989;Acosta, 1989). Consequently, this could be interpreted asevidence for episodes of rapid deposition that incorporatederoded materials, such as ferruginous ooids and lateritic clayminerals (Acosta, 1989), as well as abundant angular grainsof quartz, delivered from relatively proximal areas. Both themicrofacies and the state of preservation of skeletal remainsof macroinvertebrates are congruent with deposits that oc-curred under storm conditions.

Stratigraphic interval I, corresponding to the Transver-sarium? and/or the lower part of the Bifurcatus Zones, ischaracterized by a lesser occurrence of Globuligerina withrespect to the locally underlying ferruginous oolitic lime-stone, although Globuligerina abundance remains high.


Moreover, there is an increase in the record of colonial sessilebenthic foraminifera (nubeculariids), possibly related to alower sedimentation rate. The studied area in the centralsector presents a lower increase in sessile benthic foramin-ifera, this being related to the development of the microbialbuildup with sponges. In the marl-limestone rhythmite,sessile benthic foraminifera are normally scarce, and onlyfragments of Tubiphytes appear. This fact was possiblyforced by the greater rate of siliciclastic inflows together withthe small grain size (mainly peloids), a combination whichimpeded colonization by nubeculariids and other sessileforaminifera. High incidence of fragmentation in Tubiphytesas well as in numerous specimens of Lenticulina, althoughthe latter in a lesser proportion than in the ferruginous ooliticlimestone, indicates a certain but minor degree of transporta-tion, which is coherent with the absence of tractive sedimen-tary structures. This is all concordant with the topographicdifferences interpreted by Olóriz et al. (2002a) for the Preb-etic shelf, the central sector of the External Prebetic being atopographically more depressed zone than the eastern sector,which explains its higher siliciclastic content and the occur-rence of fragmented, presumably allochtonous remains offoraminifera. The widespread development of microbial en-crustments on skeletal remains promoted an increase in thenodular appearance of the condensed and bioclasts-richlumpy-oncolitic limestone (Internal Prebetic) and, to a lesserextent, in that of the spongiolithic limestone (eastern sectorof the External Prebetic). These features are interpreted asbeing related to decreased sedimentation rate during a gener-alized increase in the distance from shore of the studiedsections in the Prebetic Zone, within the general context ofprogressing transgressive system tract conditions for thistime period (Marques et al., 1991; Olóriz et al., 2002a, b).This interpretation is compatible with the relation establishedbetween both the growth of microbial buildups and spongebioherms in the Jurassic and the record of episodes of sealevel rise at the third order (Leinfelder et al., 1994).

Stratigraphic interval II, corresponding to the upper partof the Bifurcatus Zone, is characterized by a decrease in theabundance of foraminifera, with a notable diminution inGlobuligerina, Epistomina and encrusting nubeculariids inall the sectors studied (Figs. 2 and 6). The assemblages ofvagile and sessile benthic foraminifera do not present anyimportant variations with respect to stratigraphic interval I.However, the Internal Prebetic sections studied evidence anincrease in vagile benthic agglutinated foraminifera and adecrease in sessile benthic siliceous agglutinated forms, thislatter reduction being compensated by Tubiphytes. All thelithofacies undergo a decrease in nodularity, which in theInternal Prebetic is reflected in the facies change of con-densed and bioclasts-rich lumpy-oncolitic limestones tolumpy-oncolitic limestones. The results obtained indicate adecrease in distance from shore in all the Prebetic domains,which could be interpreted as being in accordance with therecord of progressing highstand system tract conditions for

the Bifurcatus Zone in the Prebetic Zone (Marques et al.,1991; Olóriz et al., 2002a, b).

6. Conclusions

Analysis of the foraminiferal assemblages (composition,abundance, diversity and size of the specimens) observed inOxfordian profiles (Antecedens to Bifurcatus Zones) fromthe Prebetic Zone reveals their relation with lithofacies andidentifies their palaeogeographic and stratigraphic distribu-tion.

Two foraminiferal assemblages characterize relativelydistal and proximal areas in the Prebetic Zone:

• Distal Assemblage: recognized in the profiles of theInternal Prebetic. This area contains the greatest abun-dance of Globuligerina, Epistomina and encrustingnubeculariids. The vagile benthic foraminifera aredominated by uniserial-agglutinated forms, mainly Am-mobaculites, and Ophthalmidium. Diversity of benthicforaminifera is greatest and the largest average sizes ofGlobuligerina, Lenticulina, uniserial nodosarids, Am-mobaculites and Reophax are found.

• Proximal Assemblage: recognized in the profiles of theExternal Prebetic. This area contains lower proportionsof Globuligerina, Epistomina and encrusting nubecu-lariids. The vagile benthic assemblage is dominated byspirillinids, followed by uniserial-agglutinated forms,mainly Reophax. Diversity is less than in the distalassemblage, as is the size of specimens, except in thecase of Spirillina.

In the profiles studied, the variations observed in theforaminiferal assemblages within the stratigraphic intervalselected and their relationships with lithofacies have beenanalysed and interpreted in the context of sequence stratigra-phy. The oldest deposits (Antecedens Chron; Acosta, 1989),registered only in the most proximal areas (External Preb-etic), correspond to the initial flooding on the shelf, withrapid, high-energy deposits and a mixture of elements pro-ceeding from proximal (e.g. Trocholina, fragmented nodosa-rids) and distal areas (e.g. Globuligerina and Epistomina).All of this is coherent with the context of a transgressivepulse. Subsequently, two stratigraphic intervals were distin-guished. The first of these (Transversarium? and/or the lowerpart of the Bifurcatus Zones) is found throughout the domain,with high proportions of Globuligerina, increased abundanceof colonial sessile benthic foraminifera, great lithofaciesheterogeneity and an increase in nodularity in distal areas(Internal Prebetic). All these factors are considered to berelated to the development of transgressive system tract con-ditions. The second interval (Bifurcatus Zone p.p.) containslower proportions of Globuligerina and of encrusting nube-culariids, while Epistomina is absent, and fewer nodular-likedeposits are observed. These features are interpreted as beingrelated to the development of highstand system tract condi-tions.


Acknowledgements

M. Reolid is grateful to Drs. C. Gaillard and P. Hantzper-gue for research facilities provided at Université ClaudeBernard Lyon-1. The authors are indebted to J. Rey (Univer-sité Paul-Sabatier, Toulouse) and M.A. Kaminski (UniversityCollege London) who made valuable suggestions duringrevision. This research was funded by projects PB97-0803(DGICYT) and BTE2002-3029 (CICYT), and developedwithin the research program of the EMMI Group (RNM-178,Junta de Andalucía).

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Palaeogeographic and stratigraphic distribution of mid-late Oxfordian foraminiferal assemblages in the Prebetic Zone (Betic Cordillera, Southern Spain)IntroductionGeological settingMaterials and methodsResultsForaminiferal assemblages and lithofaciesFerruginous oolitic limestoneCondensed and bioclasts-rich lumpy-oncolitic limestone Lumpy-oncolitic limestoneSpongiolithic limestoneMarl-limestone rhythmite

Palaeogeographic distributionInternal Prebetic External Prebetic

Stratigraphic distribution

Discussion5.1. Foraminiferal assemblages and palaeogeographyPalaeoenvironmental evolution

Conclusions

AcknowledgementsReferences