Carbon isotope stratigraphy and ammonite biochronostratigraphy across the sinemurian-pliensbachian boundary in the western Iberian margin

Abstract

Here, we present an integrated stratigraphical study across the Sinemurian-Pliensbachian interval cropping out into the Portuguese reference section of S. Pedro de Moel (Lusitanian Basin). Characterized by marl-limestone alternations of the Agua de Madeiros Formation (subdivided into Polvoeira and Praia da Pedra Lisa members), this succession is particularly dominated by organic-rich facies (black shales) and contains a diverse fauna of benthic and nektonic macrofossils. This stratigraphic and sedimentary setting is the basis for a high-resolution carbon isotope study, constrained by new ammonite biochronostratigraphic determinations and other palaeontological data. The new ammonite collections characterize the Oxynotum and Raricostatum chronozones and the Raricostatum, Macdonnelli and Aplanatum subchronozones, and accurately identify the Sinemurian-Pliensbachian boundary in this western Iberian margin setting. Carbon isotope measurements were made from bulk carbonate (δ13Ccllb) from 351 samples, representing the Oxynotum to earliest Jamesoni (early Taylori) Chronozone intervals. The carbon isotope values exhibited a large range, varying between +2.85%c in the Oxynotum Subchronozone to very negative values observed in some limestone beds from the Raricostatum Subchronozone (lowest reaching -6.7‰), a variation clearly controlled by lithological and facies changes. Despite these strong anomalous isotopically light values (below -2‰), which are clearly associated with organic matter degradation and early diagenesis, the δ13Ccllb curve shows a long-term negative trend across the Oxynotum to the early Taylori Subchronozone interval. This tendency is reversed around 5 m above the Sinemurian-Pliensbachian boundary, and the δ13Ccllb values become positive correlative with an absence of organic matter and argillaceous sediments in the limestone beds of Praia da Pedra Lisa Member. Although the δ13C data recorded across the Sinemurian-Pliensbachian of the Lusitanian Basin had been controlled by internal depositional conditions, the general evolution of carbon isotopes agrees with the trend recognized in other basins.