Carbon-isotope stratigraphy and cyclostratigraphy of shallow-marine carbonates: the case of San Lorenzello, Lower Cretaceous of southern Italy

Abstract

A high-resolution (cm-scale) carbon-isotope stratigraphic study has been carried out at San Lorenzello (Matese Mountains, southern Apennines, Italy) on a shallow marine carbonate section previously correlated with coeval sections by using a combination of biostratigraphic, cyclostratigraphic, and sequence-stratigraphic criteria. This succession, deposited along the southern margin of the Alpine Tethys, is late Valanginian—early Hauterivian in age; it consists of a hierarchy of shallowing-upward cycles (elementary cycles, bundles, and superbundles) linked to Earth's orbital oscillations. We estimate that the influence of the depositional environment and diagenesis on the δ 13C signal is negligible and that the carbon-isotope composition of bulk carbonate, as well as of the individual grains, indicate a similar response to an original climate-ocean forcing. However, no clear relationship emerges between the δ 13C variations and the sea-level changes. The long-term δ 13C oscillations have been correlated with the coeval curves from the hemipelagic La Charce (Vocontian Basin, France) and from the pelagic Capriolo (southern Alps, northern Italy) sections. This comparison suggests that the C-isotope signature is well preserved at San Lorenzello, where only the upper part of the global Valanginian carbon isotope excursion is recorded in the outcrop. The available data allowed us to: (a) relate the shallow-marine stratigraphic record with the ammonite and nannofossil biozonation and the magnetostratigraphy; (b) locate, in the shallow-marine deposits studied, the Valanginian-Hauterivian boundary on the basis of the high-resolution carbon-isotope correlation with the La Charce and Capriolo sections; (c) compare the time duration of about 2.9 my, estimated by San Lorenzello orbital chronostratigraphy, with the ± 3 my reported for the same stratigraphic interval in Channell et al. (1995) and Gradstein et al. (2004).