A revision of the Triassic sequence stratigraphic framework of the Dolomites (Italy). The impact of climate, volcanics, tectonic and changes in the carbonate factories.

Seismic-scale outcrops, the high-resolution bio-chrono-stratigraphic framework and the scarce alpine deformation render the Dolomites (Southern Alps, north-eastern Italy) an ideal reference for the Triassic global sequence stratigraphic charts (Haq et al., 1987; Gianolla & Jacquin, 1998; Hardenbol et al., 1998). However, the Triassic of the Dolomites is the result of a complex set of forcings, which determined a large variability of the carbonate sedimentary successions that includes ramps, carbonate banks, high-relief isolated or attached carbonate platforms, with different carbonate factories. The possibility to read the coeval response of carbonate and siliciclastic depositional systems to such different sedimentary forcings as sea level oscillations, climate change and volcanic activity make this region a playground to test the limits and the potentials of the sequence stratigraphic methodology. Recently, the sequence stratigraphic framework of the Dolomites was carried out according to the approaches summarized in Catuneanu et al. (2011). The revision of the depositional sequences (DSa) allowed to better define: a) the surfaces that delimit the different system tracts; b) the influence of tectonics in the timing of the DSs stacking patterns; c) the role of changing carbonate factories in the architecture of carbonate depositional systems; d) the impact of climate and/or environmental changes in the demise of carbonate systems and supply of siliciclastic sediments; e) the impact of local volcanic activity on sediment supply and carbonate production shut-off, ; and lastly g) the hierarchy of the identified depositional sequences. This led to a revised sequence stratigraphic framework (Fig. 1), characterized with respect to the original schemes (e.g., Gianolla et al., 1998) by a lower number of DSs, by the identification of FSSTs in many DSs, and by a better age control for sequence stratigraphic surfaces. One example is the impact, in the Dolomites, of the "Carnian Pluvial Event", a global episode of climate change documented worldwide at low latitudes, that involved increased rainfall and possibly global warming. This climatic event predates a drop of sea-level and caused the demise of microbial dominated high-relief carbonate platforms that dominated the region. It was followed by the coexistence of small microbial carbonate mounds and loose arenaceous carbonates. A subsequent sea level fall brought to the definitive disappearance of microbialites and shallow water carbonates switched to ramps dominated by loose carbonate sediment. The climate-induced crisis of Early Carnian shallow water carbonate systems of the Dolomites generated a geological surface similar to a drowning unconformity, although no transgression occurred. The sudden infilling of basins at the end of the Early Carnian was the result of the climatic-induced switch from high-relief carbonate systems characterized by steep slopes to a gently inclined ramp, rather than by the continuous progradation of a high- relief microbial platform. Results show that the evolution of carbonate systems of the Dolomites at the end of the Early Carnian cannot be interpreted in the light of sea level changes only, pointing out that ecological changes can induce significant modifications in depositional geometries.