The Dolomia Principale carbonate platform in the eastern Southern Alps (NE Italy, W Slovenia): the depositional system of its early stages

The Dolomia Principale (DPR) is one of the best known Upper Triassic stratigraphic units of the Southern Alps. It is related to a wide carbonate platform, outcropping in the Southern Apennines, Dinarids chain, Australpine (Hauptdolomit Fm.) and Hungary. Margin-to-slope facies are known from the Norian, facing both intraplatform basins and open pelagic areas. However, data about the late Carnian start-up stage are limited to northeastern Italy, and little is known of the early depositional system. The Tuvalian succession was investigated by studying several geological sections along the western Julian Alps. Amulti-disciplinary approach was adopted, involving macro- and micro-facies analysis, sequence- stratigraphy and biostratigraphy (mostly ammonoids and conodonts). The lower Tuvalian was characterized by low-relief mixed terrigenous/carbonate systems (Tor Fm.), emplaced on an almost flat paleotopography since the late Julian and showing few lateral paleoenvironmental changes in a W-E direction and a transgressive- regressive trend. Mixed systems graded in a widespread high-energy inner ramp environment characterized by dominant carbonate sedimentation (amalgamated grainstone bodies). Diagenetic processes originated a dolomitized lithosome (Portella Fm.) extending over a wide area with uniform thickness (ca. 15- 20 m), and showing only local variation to peritidal settings. The top of the unit could be considered as a surface on which disparate upper Tuvalian environments were established: while northeastern areas show evidence of pure basinal sedimentation with anoxic episodes (Carnitza Fm). The southwestern sector of the Julian Alps was patterned by shallow marginal marine, mixed terrigenous/carbonate deposits, attributable to low-energy restricted (sometimes evaporitic) environments (Travenanzes Fm. and Monticello Mb-DPR). These features are related to the emplacement of a platform margin belt (DPR) approximately oriented WNW-ESE. Depositional geometries are partially preserved and the shelf-break profile denotes the evolution of a first stage marked by the margin platform onset followed by increase of the slope angle and deepening of a starved basin. The lower, main aggradational stacking pattern, is followed by an upper strongly progradational stage, dated to the uppermost Tuvalian (Spinosus Zone). Micro-facies analysis of upper slope-to-margin debris occurring in proximal breccia and calcarenite layers reveals a microbial-dominated carbonate factory. Together with microbial crusts, encrusting calcareous sponges, Tubiphytes and other Microproblematica organisms represent the main components. Moreover, an inner margin, relatively shallower and sheltered, has been described with oncoidal- bioclastic facies and microbialite layers. This area is laterally interdigitating with protected zones in which peritidal sedimentation prevails. Connection with the innermost area, subjected to southern siliciclastic input and mixed sedimentation (at least for the first aggradational stage), is missed due to Alpine tectonic displacements and elisions. Margin to slope features strictly recall typical steep slope carbonate platforms of the Mesozoic, with a dominant M-type carbonate factory. However, the whole depositional system of the early stage is quite different, northern carbonate construction was attached to a southern terrigenous coastline and alluvial system. Peritidal carbonate sedimentation was extended to the whole ESA only in the late Carnian