Peritidal carbonate platform onset and widening: sedimentological and isotopic evidences (Dolomia Principale, Upper Triassic)

The Late Triassic Dolomia Principale formation (DP; upper Carnian - Norian - ?Rhaetian) represents the largest epicontinental carbonate platform of the western Tethys, with a size comparable to today´s Great Barrier Reef in Australia. The DP was deposited during a time span of ca 20 Myr, with thicknesses of 500 to 4000 m. The platform interior facies are represented by a thick succession of peritidal carbonates. Dolomitic beds are organized in meter-scale shallowing-upward cycles, characterized by subtidal facies (rich in marine fauna), grading upward into intertidal–supratidal facies with planar stromatolites. The platform margin has been documented in the Northern Julian Alps, near Tarvisio, where a prograding upper Carnian platform–basin transition is preserved. The margin mainly consists of serpulids, microbialites and marine phreatic cements and separates the DP platform interior facies from the clinostratified breccia and megabreccia slope facies. The coastal area at the back of this huge peritidal carbonate platform is recorded by the siliciclastic-carbonate deposits of the Travenanzes Formation (TVZ; upper Carnian p.p.) in the Dolomites, which is about 200 m thick. This coastal system is characterized by dryland river systems, mainly drying out before the shoreline, and by coastal mudflats and sabkhas. Calcic and vertic paleosols indicate an arid/semi-arid climate. Seaward of the shoreline, a carbonate tidal-flat developed. Meter-scale shallowing-upward peritidal cycles are equivalent to those of the adjacent DP, except for thin dark clay intercalations representing sporadic fluvial sedimentsupply to the shallow-marine system. The depositional architecture of the TVZ exhibits an overall transgressive pattern, with a strong retrogradation of the shoreline related to the low sediment supply of continental origin to the carbonate lagoon. Equally, the progradation of the platform margin is related to the high carbonate production on the whole carbonate system. The progradational geometry of the platform margin, together with the retrogradational architecture of the coastal area at its back, result in a gradual widening of the platform lagoon system. The stable isotopic composition of dolomite in the TVZ was analyzed along three stratigraphic sections. Oxygen and carbon isotope values show no correlation (R2 < 0.001), and a wide range of δ13C values (from ca -6‰ to +4‰) is observed, compared with a narrow range in δ18O (from ca 0‰ to +3‰). δ13C is strongly facies controlled, suggesting the absence of a burial diagenetic overprint. δ18O shows a positive upward trend, from 0–1‰ to 2–3‰, in the three sections analyzed. δ18O values are consistent with dolomite precipitation from Carnian sea-water. The widening of the platform gradually limited the exchange with open-marine waters in the restricted lagoon. The gradual δ18O enrichment observed up-section could be explained by progressively longer residence times of seawater on the platform and consequent