La crisi sinemuriana delle piattaforme carbonatiche del Sudalpino orientale

In an undefined moment of the Sinemurian – Pliensbachian interval, a global crisis of the carbonate platforms induced the reorganization of the palaeogeography of the eastern Southern Alps (MASETTI et al., 2012). Roughly in correspondence of the Sinemurinan-Pliensbachian boundary, recent investigations have found a negative δ13C isotope anomaly attributed to a global perturbation of the carbon cycle (e.g., JENKYNS et al., 2002). The present work aimed at identifying the causes of the Southern Alps carbonate platforms’ crisis and at more accurately defining the moment in time of this event. I employed a multidisciplinary approach (facies analysis, chemostratigraphy and biostratigrapy) to investigate eight stratigraphic sections located along a West-Est oriented transect, crossing the Trento Platform, the Belluno Basin and the Friulian Platform. For the Central-Weastern sector of the Trento Platform, I have investigated the sections of Chizzola, Madonna della Corona, Rotzo, Val Gola which are characterized by the shallow water pliensbachian deposit of the Rotzo Formation (Calcari Grigi Group). For the North-Eastern sector, in which the Rotzo Fm. is absent, the sections of Foza and Col Moschin have been sampled. In these last two sections, an unconformity surface defines the upper limit of the hettangian-sinemurian peritidal facies of the M. Zugna Fm. (Calcari Grigi Group). On the top of this surface, either the drowning facies of the Fanes Piccola Encrinite unit (upper Pliensbachian) or the pelagic ones belonging to the Lower Rosso Ammonitico (upper Bajocian – lower Callovian) rest. A similar stratigraphic setting was observed in the Northern sector of the Friulian Platform, the only one where the Calcari Grigi Group crops out. In this sector, I studied the M. Cumieli section. In the Belluno Basin, located between the two above mentioned platforms, the section of M. Verzegnis (Soverzene Fm.) has been sampled. Moreover, I chose the Campanian-Lucanian Platform (Southern Apennines) in order to better characterize the geographical extension of the carbon isotope signal and to compare coeval carbonate platform deposits that doesn’t belong to the Southern Alps sector. As representative of the Campanian-Lucanian Platform, the formation called Palaeodasycladus Limestone cropping out at the Valico di Chiunzi (Lattari Mts., close to Salerno) has been investigated. In collaboration with the Earth Sciences Department of the Oxford University, analyses on the carbon and oxygen stables isotopes have been performed on about 2000 samples of powder rock (bulk rock) sampled every 20 cm along an overall stratigraphic thickness of more than 700 m. High resolution δ13C curves allowed to identify a large primary negative excursion located in the upper part of the peri-platform unit of Soverzene. The unearthing of an ammonite belonging to the Arnioceras genus allowed referring this anomaly to the boundary between Lower Sinemurian (Semicostatum zone and Turneri zone) and the upper Sinemurian (Obtusum zone). Such anomaly has been found also in the upper part of the M. Zugna Fm. (Trento Platform and Friulian Platform) and in peritidal facies of the Palaeodasycladus Limestone (Campanian-Lucanian Platform). In both cases the excursion generally corresponded to the F.O. of the foraminifera Paleomayncina termieri. The correlation of the isotope excursion has allowed to define an Arnioceras time-line across the platform and basin domains and to better calibrate the F.O. of Paleomayncina termieri. The negative excursion, in keeping with the variations in the relative sea level curve indicated in the literature (HESSELBO, 2008; HESSELBO & JENKYNS, 1998), corresponds to a transgressive phase in the platform domains induced by the crisis of the carbonate factory. This crisis determines the definitive interruption of the carbonate peritidal sedimentation existing since the upper Triassic and the consequent underfed of the Belluno Basin. From the upper Pliensbachian, a new generation of carbonate platforms got established. This is predominantly characterized by a subtidal sedimentation and initially by eutrophic conditions, in which large bivalves of the “facies a Lithiotis” accumulated (Rotzo Fm., Southern Alps; Lithiotis Member, Southern Appenine). The data suggest that the isotopic anomaly is supra-regional, as suggested by the simultaneous dismiss of coeval carbonate platforms, including that of the Corna in the Brescian Alps (SCHIROLLI, 1997), Rocca Livernà Limestone in the Ligurian Alps (DECARLIS & LUALDI, 2010), Calcare Massiccio in Umbro-Marchean-Sabina Appenine, Inici Fm. in Sicily (RONCHI et al., 2000) and Djebel Bou Dahar in the High Atlas marocain (MERINO-TOMÈ et al., 2012). The absence of reliable evidence of a significant volcanic activity in that part of the Sinemurian permitted to attribute the negative isotopic excursion to the introduction of large amounts of the lightweight 12C isotope in the ocean-atmosphere system, issued in the form of methane as a result of destabilization of gas hydrates. The most credited cause of their dissociation is the warming up of oceanic bottom water associated with changes of ocean circulation (JENKYNS, 2003 cum bibl.). It’s possible that the syn-sedimentary tectonics that was affecting the Jurassic continental margin at that period may have contributed to the destabilization of the gas hydrates, although with intensities that are difficult to estimate.