Basinal inlets in a wide carbonate platform system: a case history from the Late Triassic of northeastern Dolomites (Southern Alps, NE Italy)

Upper Carnian to Norian successions cropping out in the Dolomites (eastern Southern Alps, ESA) are typically characterized by a thick (locally over to 1000 m) succession of peritidal cycles known as Dolomia Principale (DPR), mainly representing inner facies of one of the widest carbonate platform of the Mesozoic. Despite the Dolomites represent an area affected by minor tectonic deformation, faulting and displacements increase eastwards, resulting in the complex structural assessment of the S. Stefano di Cadore-Val Bordaglia area. Among several tectonic units, the Mt. Col Unit stands out for its peculiar Upper Triassic stratigraphic framework. Particularly attention has been paid to a Tuvalian carbonate-terrigenous basinal succession cropping out along the riverbed of the Rio di Mezzodì. Lithofacies are mainly represented by dark marl/pelite and limestone alternations, with irregular coarse to fine calcarenite intercalations. Carbonate grains of finer lithology are frequently represented by fragments of pelagic bivalves and benthic forams, whereas coarser beds commonly contain also gastropods, echinoderms and occasionally cortoids and Microproblematica-type fragments. Pyrite crystals and phosphatized shells are frequent, suggesting disoxic conditions on the sea bottom. The unit overlies a dolomitized interval consisting of bivalve and gastropod-rich packstones to grainstones, with common crustacean fecal pellets, attributable to a carbonate ramp environment (Heiligkreuz Fm.). No specific stratigraphic trend is identifiable in this basinal succession, but its top is always truncated and it is commonly flanked to massive facies of DPR. Recent and new ammonoid findings allocate the deep water series to the T. Dilleri/Subbullatus – A. Spinosus Zone, allowing good correlation with the Carnitza Fm. which crops out 90 km eastwards (Julian Alps, S. Karawanken). A georeferenced database containing all geological data available from literature about the Tuvalian substage of the ESA was created. Punctual and areal informations have been interpolated taking account of structural constraints, and a non-palinspastic paleoenvironmental map has been developed for the T. Subbullatus Zone. Results show a general SW to NE transition of environmental belts, respectively from subaerial/alluvial plain settings to shallow terrigenous- carbonate lagoons, with a narrow carbonate platform margin elongated in an approximate WNW-ESE direction, facing open pelagic environments northwards. Considering the polyphase Alpine tectonic displacement of the Mt. Col unit and its presumable northern provenance, the continuous occurrence of a W-E elongated basin could be hypothesized for the Carnic Alps, and likely for a restricted region north to the current Insubric-line position. In the wider paleogeographic scenario of the Tethyan region, a western pelagic inlet connected to the eastern Hallstatt marine domain can be depicted, separating thus the ESA from the Upper Australpine nappes, at least during late Carnian. Moving westwards, away from true open marine areas, water circulation became more and more restricted, justifying disoxic conditions inferred from sediments. Simultaneously, the terrigenous input increased in the same direction, because of the approach to inner sectors, where a direct connection of coastal carbonate-siliciclastic systems to the basin cannot be excluded. The occurrence of a sea-slice cutting the wide DPR carbonate platform environment cannot be ignored in future paleogeographic reconstruction, even if more work is needed to detect those causes that led to its onset.