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Aptychi from the Berriasian/Valanginian (France and Spain) : new stratigraphical and morphological details

Vašíček Z., Janssen N. M .M., Klein J.

Annales Societatis Geologorum Poloniae

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2016

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Vol. 86, No. 3

265--272

EN

Late Berriasian–earliest Valanginian lamellaptychi (Cephalopoda) are described in detail for the first time. With one exception, all species belong to early representatives of Mortilletilamellaptychus. The majority of the well dated aptychi come from the cephalopod-rich sediments of the Vocontian Basin in south-eastern France; others are from south-eastern Spain. Thorolamellaptychus anglesensis sp. nov. is introduced.

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Oxfordian carbon-isotope stratigraphy in the Subbetic, Southern Spain

Bruchez S., Caracuel J., O'Dogherty L., Sandoval J.

Volumina Jurassica

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2006

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Vol. 4, no. 1

150-151

EN

We present a detailed carbon-isotope stratigraphy for two well ammonite-dated sections of Sierra de Lugar, (Province of Murcia) and Cortijo Cardador, (Sierra Gorda, Province of Granada). Both are located in the Subbetic palaeogeographic domain, and they represent the more characteristic and complete Oxfordian sections for two epioceanic swells of the southern Iberia palaeomargin (the External and Internal Subbetic, respectively). The sections analyzed are two of the better outcrops showing Oxfordian deposits with ammonites in epioceanic rosso ammonitico facies of the Betic Cordillera. As usually recognized in nodular limestone facies, condensations and hiatuses are common, but the well outcropping and record in these sections allow an accurate ammonite biochronostratigraphy (Caracuel et al. 2000). The Oxfordian in the Lugar section is 9.65 meters thick, mainly marly rosso ammonitico facies. The following ammonite zones were identified: Renggeri Zone (Lower Oxfodian), upper Plicatilis (Antecedens Subzone) and Riazi (or Transversarium) zones (Middle Oxfordian), and Bifurcatus, Bimammatum and Planula zones (Upper Oxfordian). The Cortijo Cardador section is more condensed (2.50 m thick), calcareous wackestones dominate over marly levels. The Oxfordian succession begins with well bedded and nodular limestone of the Transversarium Zone which overlies Middle Callovian condensed limestones. The same ammonite zones recognized in the Lugar section are well identified in the Cortijo Cardador with the exception of the Lower Oxfordian, allowing a detailed biostratigraphic correlation. The ä13C curve is very detailed and representative for the Lower Oxfordian (Renggeri Zone) to Lower Kimmeridgian (Platynota Zone) interval. The curve shows a marked trend towards positive values (inherited since the Callovian, especially from the Middle Callovian, Anceps Zone) throughout the Lower Oxfordian, and displays a relative maximum (around 3.4-3.7‰) in the lowermost part of the Riazi Zone (approximately at the Plicatilis-Riazi zones boundary). The values decrease through the rest of the Middle Oxfordian (upper part of the Riazi Zone). Some fluctuations are recorded in the Bifurcatus and Bimammatum zones, but the general trend is towards lighter isoptopic values, to reach a relative minimum (around 2.4-2.5‰) at the Bimammatum-Planula zones boundary. Analysis of faunal turnover reveals a casual link between the global carbon cycle and the ammonite evolution. We found a good correlation of extinctions with ä13C minima, and radiations with ä13C increases. Such correlation supports the idea that perturbations-variations in the global carbon-cycle reflect rapid palaeoenvironmental changes. Hence, our study reveals how major faunal turnover in the marine realm, as shown by ammonite evolution, can be used as a proxy to identify major palaeoenvironmental crises of their ecosystems. An increase in radiolarian content occurs at the Plicatilis-Riazi zones, recording at some place of the Internal Subbetic red siliceous limestones and radiolarites within the rosso ammonitico facies. The discrete occurrence of siliceous facies within the condensed ammonitico rosso facies is probably related to strength pelagic productivity at this time, allowing the deposition of siliceous sediments even in epioceanic swells of the Tethys (Subbetic, Trento Plateau, etc.). The correlations with previous isotopic curves in other Tethyan areas are also analysed and discussed.

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Aalenian carbon-isotope stratigraphy: a tool for the ammonite radiolarian and nannofossil zonation calibrations in the Western Tethys

Sandoval J., Aguado R., Bartolini A., Bill M., Bruchez S., O'Dogherty L., Perilli N.

Volumina Jurassica

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2006

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Vol. 4, no. 1

212-213

EN

Here we report a detailed carbon-isotope stratigraphy for the Aalenian (Middle Jurassic) pelagic carbonates in the Betic Cordillera (southern Spain), which represents an ideal region to directly tie the stable carbon-isotope curves to ammonite zones and subzones, and thereby for the first time achieve an accurate chronostratigraphic calibration. For this purpose we have selected two sections (Agua Larga in the province of Granada and Cerro de Mahoma in the province of Murcia) which represent basinal deposits of the southern Iberian palaeomargin. In these sections ammonite are common and relatively wellpreserved. Biostratigraphic analyses enable the recognition of the uppermost Toarcian (Aalensis Zone), the complete Aalenian (Opalinum, Murchisonae, Bradfordensis and Concavum zones) and the lowermost Bajocian (Discites Zone). Calcareous nannofossils and radiolarians (at discrete beds of the Upper Aalenian - Lower Bajocian) are also common in these Median Subbetic hemipelagic sections. The Subbetic Aalenian is characterized by a monotonous and rhythmical alternance of marlstones and marls in continuous sedimentation throughout the analysed interval. We present a ?13C curve very detailed (bed by bed) for the uppermost Toarcian - lowermost Bajocian interval. The curve shows a relative minimum (around 1‰) in the Upper Toarcian, a weak positive shift (around 2‰) in Lower Aalenian (Comptum Subzone), a decreasing values (newly around 1‰) in the Middle Aalenian (Bradfordensis Zone), a positive peak of 2.7‰ in the Upper Aalenian (Concavum Zone and Subzone) and a new relative minimum (1.5-1.7‰) at Aalenian/Bajocian boundary. A good correlation among the isotope curve and the different biostratigraphic zonations is accurately presented. We explore the powerfulness of this well-calibrated ?13C curve as a tool for stratigraphic correlation. In this sense, the biochronostratigraphic position of the radiolarian UAzones 1-2 (mainly based on Italian sections with scarce record of ammonites, Baumgartner et al. 1995) are here redefined by means of the isotope reference curve of the Subbetic. We present the results of a biostratigraphic study of nannofossils assemblages throughout the studied interval. In addition, on the base of a semiquantitative study, we have selected the most abundant taxa (>10%) from the whole assemblage to perform a multivariate analysis (principal components): Biscutum dubium, B. intermedium, Crepidolithus crassus, Carinolithus superbus, Discorhabdus striatus, Schizosphaerella spp., Watznaueria contracta and the genus Lotharingius. This multivariate analysis reveals a strong correlation between the abundances of oligotrophic (C. crassus and Schizosphaerella spp.) and eutrophic (B. dubium and B. intermedium) taxa and the fluctuation of the C degree curve.