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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

2006

Vol. 4, no. 1

212-213

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.