Depositional environments of the Ladinian Pelsa-Vazzoler Basin

Mt. Pelsa is a relief located near the town of Agordo (BL) in the Dolomites. Most of the mountain consists of a Ladinian (Middle Triassic) carbonate platform that experienced a tectono-magmatic event leading to the formation of an intraplatform basin, the so-called Pelsa – Vazzoler Basin. Successively, that event led to the death of many Ladinian carbonate platforms in the Dolomites area. The stratigraphic sequence of the Pelsa – Vazzoler Basin comprises an alternation of slightly dolomitized lime-mudstone, marl, and volcanic ash layers, deposited in an environment which may have temporary reached euxinic conditions. This locality is renowned because of the preservation of a highly diversified fossil biota, primarily composed of plants, invertebrates, and fishes. Ammonite biostratigraphy has allowed to date the sequence to the A. neumayri Sbz (Longobardian). Preliminary studies of the ichthyofauna have revealed that it shows many similarities with other important Middle Triassic fish assemblages from both ends of the Tethys Ocean (e.g., Monte S. Giorgio fauna in Lombardy and Switzerland, and Xingyi fauna in Southern China). Moreover, the benthic fauna, mainly represented by molluscs and gastropods, but also corals, brachiopods being scarcer, sponges, and echinoderms, underwent a complete substitution by silica, one of the first recorded since the P-T mass extinction. Despite the impressive paleontological interest, there is still a lack of research about the paleoenvironment of the basin; the studies of the feature of the sequence, during the fieldwork, led us to theorize that the Pelsa-Vazzoler basin was a intraplatform basin where the subsidence were controlled by syn-sedimentary faults (breccias) and the sedimentation was controlled by slumps from the edge of the basin (load cast structure and chaotic transport of fossils remains that successively were substituted in silica). We will present the result of facies analysis and the first result of geochemical analyses from the older part of the basin and around the fossil fish horizon (delta13C, delta18O and TOC), thin sections analyses (diagenesis events) , which contribute to a better description of the depositional environments in the Pelsa - Vazzoler Basin.