The Early Triassic evolution of a terrigenous carbonate ramp after the P/T mass extinction
Early Triassic subsidence rates were between 50-100 m/Myr, with a comparatively uniform distribution through time and space. Sedimentation was able to compensate this rate and therefore the region stayed near sea-level into the early Middle Triassic, recording major bio-evolution and palaeo-environmental change. The Permo-Triassic Boundary is marked by the most severe mass extinction of the entire Phanerozoic, very well documented by the classical outcrop of the region (e.g. Tesero, Bulla). The extinction event dramatically impacted on the biological communities, largely affecting the carbonate production dynamics. In the Dolomites, carbonates continued nevertheless to accumulate across the boundary at similar rates, preventing any significant chronological gap from being developed. The Early Triassic (Induan and Olenekian) is recorded by loose carbonate-terrigenous, storm dominated ramp deposits (Werfen Fm, ranging from 300 to 500 m, where uncut by the Anisian erosion), punctuated by peritidal and emersion episodes, under arid conditions (e.g. Andraz Mb). The ramp system dips toward eastern areas. During the earliest Triassic, transgressive oolites (Tesero Mb) gave way to loose micritic muds (Mazzìn Mb), with strongly oligotypic faunae. Bioturbation too was scanty. Mollusc diversification then gradually increased (Siusi-Seiser Mb), and ooids reappeared (e.g. Gastropoden Oolit and Cencenighe Mb.s). During the Early Triassic, the carbonate fraction was always dominated by loose calcareous muds and mollusc bioclasts, ce-mentation was scanty and bioconstructions absent. Throughout the Early Triassic, carbonate dominated phases alternated with episodes of increased terrigenous input, often associated with transgressive phases (e.g. micaceous silt-rich Campil Mb, Olenekian).