Palaeogeographic and stratigraphic distribution of mid-late Oxfordian foraminiferal assemblages in the Prebetic Zone (Betic Cordillera, Southern Spain)

Embed Size (px)

Text of Palaeogeographic and stratigraphic distribution of mid-late Oxfordian foraminiferal assemblages in...

  • 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

    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 ditions scientifiques et mdicales Elsevier SAS. All rights reserved.

    Rsum

    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: foloriz@goliat.ugr.es (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

  • 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 (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

    734 F. Olriz et al. / Geobios 36 (2003) 733747

  • 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

Recommended

View more >