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Oxfordian carbon-isotope stratigraphy in the Subbetic, Southern Spain

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

tom Vol. 4, no. 1

150-151

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.