Palaeogeographic and stratigraphic distribution of mid-late Oxfordianforaminiferal assemblages in the Prebetic Zone
(Betic Cordillera, Southern Spain)Distribution palogographique et stratigraphique des associations
des foraminifres de lOxfordien moyen-suprieur de la Zone Prbtique(Cordillre btique, sud-est Espagne)
Federico Olriz *, Matas Reolid, Francisco J. Rodrguez-TovarDept. Estratigrafa y Paleontologa, Facultad de Ciencias, Universidad de Granada, Campus Fuentenueva, 18071, Granada, Spain
Received 29 August 2002; accepted 14 March 2003
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 ditions scientifiques et mdicales Elsevier SAS. All rights reserved.
Ltude des associations de foraminifres de la Zone Prbtique (Cordillre Btique, Sud-Est de lEspagne) a consist en lanalyse de lacomposition, de labondance, de la dominance, de la diversit et de la taille des foraminifres. Les rsultats diffrent selon les lithofacis, lapalogographie et la stratigraphie. Nous prsentons ici la rpartition des foraminifres en deux assemblages caractristiques du secteur distalet du secteur proximal de la plate-forme. Le premier est caractris par une diversit leve, une grande taille des individus et par la dominancedes foraminifres planctoniques, des agglutinants, des Ophthalmidium, des Epistomina et des foraminifres encrotants. Lassemblageproximal est surtout caractris par une proportion moins grande des foraminifres planctoniques et des nubculaires encrotants, parlabsence dEpistomina et par une grande proportion de spirillines et de Reofax. Ainsi, les associations varient en fonction dun gradientdistal-proximal. Lvolution stratigraphique des assemblages indique une diminution progressive des foraminifres planctoniques. Ceci peuttre interprt, compte tenu de laspect noduleux des lithofacis, comme rsultant de variations du niveau marin relatif, comme cela avait djt propos dans des travaux antrieurs sur les sdiments de lOxfordien dans la plate-forme du SE de lIbrie. 2003 ditions scientifiques et mdicales Elsevier SAS. All rights reserved.
Keywords: Foraminiferal assemblages; Palaeoecology; Lithofacies; Palaeogeography; Sequence stratigraphy; Mid-late Oxfordian; Prebetic
Mots cls : Foraminifres ; Palocologie ; Lithofacis ; Palogographie ; Stratigraphie squentielle ; Oxfordien moyen-suprieur ; Prbtique
* Corresponding author.E-mail address: firstname.lastname@example.org (F. Olriz).
Geobios 36 (2003) 733747www.elsevier.com/locate/geobio
2003 ditions scientifiques et mdicales Elsevier SAS. All rights reserved.doi:10.1016/j.geobios.2003.03.006
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 (Olriz et al., 1999) and macroin-vertebrate taphonomy (Olriz 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 (Garca-Hernndez, 1978, 1981; Garca-Hernndez and Lpez-Garrido, 1979). Subsequently, Garca-Hernndez et al.(1981) began to study Oxfordian lithofacies and microfacies,which have been characterized in detail in recent papers(Olriz 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(Olriz 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 (Olriz 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 Caada) and two to the Internal Prebetic(Navalperal and Ro Segura). Detailed previous studies ofthese profiles have been carried out by Olriz et al. (1999,2002a).
Riogazas-Chorro II profile (RGCHSP): Located in thewestern part of Sierra de Cazorla (province of Jan). 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 Caada profile (PC): Located in a small ravine tothe SE of the village of Pozo Caada (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 Jan), 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.
Ro Segura profile (RS): Located in Sierra de Segura(province of Albacete), between the villages of Yeste and
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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 Olriz 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 and
consists 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 = PozoCaada, RGCHSP = Riogazas-Chorro II, RS = Ro Segura).Fig. 1. Localisation gographique et cadre gologique de la Zone Prbtique (Cordillre Btique, Sud-Est de lEspagne), avec indication des coupes tudies(NV = Navalperal, PC = Pozo Caada, RGCHSP = Riogazas-Chorro II, RS = Ro Segura).
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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 tudies dans le Prbtique Interne (NV, RS) et Externe (secteur central : RGCHSP ; secteur oriental : PC) montrant larpartition des lithofacis et la biostratigraphie. Composition des associations de foraminifres : A (ensemble de la faune), B (benthiques vagiles), C (bentiquessessiles). Changements dans labondance des nubcularids encrotants et diversit des foraminifres dans la succession tudie.
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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 surface
or 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 (Ammobacu...