Effects of the S-P Event (Sinemurian-Pliensbachian, Early Jurassic) on the Trento Platform: evidences from the Lombardian basin and similarities with the Carnian Pluvial Event (Upper Triassic)

In the Early Jurassic, the paleogeography of the western Tethys was that of a series of fault-bounded highs and lows. On one of these highs, the Trento Platform, now exposed in the Southern Alps, the shelf carbonate succession of the Calcari Grigi Group deposited from the Hettangian to the late Pliensbachian. The Trento Platform records a first deepening of the sedimentary environments in correspondence of a δ13C negative perturbation, the so-called “arnioceras event”, occurred in the middle part of the Sinemurian (Masetti et al., 2016). After that, the depositional system in the inner platform switches from a peritidal flat (Monte Zugna and Loppio Oolitic Limestone Formations) to a lagoon (Rotzo Formation). This change occurs around the Sinemurian-Pliensbachian boundary when a major negative δ13C shift recorded globally by marine carbonates and wood, named S-P Event, is documented (Korte and Hesselbo, 2011). In this contribution a new δ13C isotope record from the deep-water sediments deposited in the Lombardian basin, whose age is constrained with nannoplankton biostratigraphy, is presented. Quantitative petrography (modal analysis) has been carried on calciturbidites. A prominent negative carbonate carbon-isotope shift that can be referred to the S-P Event is highlighted. Across the δ13C spike, a decrease of the material exported from the platform and a change in the composition of calciturbidites, from microoncoids-dominated to skeletal grains-dominated, are observed. On the platform, the first part of the S-P Event is elided by a subaerial exposure surface, followed by the deposition of marly limestones that testify for an increase of terrigenous input into the lagoon, and by the onset of meso-eutrophic conditions. We interpret this evolution as the expression of a crisis in the carbonate production of the Trento Platform across the S-P Event, coupled with a climate change towards more humid conditions. The sedimentary evidences recorded in correspondence of the S- P carbon-cycle perturbation on the Trento Platform are also compared to those documented at the Carnian Pluvial Event (early Late Triassic) in the Dolomites, with whom they share close similarities.