IL SISTEMA DEPOSIZIONALE DELLA DOLOMIA PRINCIPALE NEL SUDALPINO ORIENTALE: DAL LIVELLAMENTO DELLA PALEOTOPOGRAFIA ALL’IMPOSTAZIONE DELLA PIATTAFORMA CARBONATICA (TRIASSICO SUPERIORE)

Abstract

The Dolmia Principale Formation (Main Dolomite) is one of the most known Upper Triassic unit from the Southern Alps. It is mainly related to the inner part of a wide carbonate platform, extending from the Southern Appennines to the Dinarids chain, to the Australpine and to Hungary too. Margin-to-slope facies are known from the Norian, facing both intraplatform basins and open pelagic areas. However, data about the Upper Carnian start-up stage are limited to the northeastern Friuli province, and less is known on the early depositional system. In order to increase knowledge on first evolutionary stages, the Julian- Tuvalian succession has been investigated by studying several geological sections spread in the eastern Southern Alps, from the eastern Cadore region up to northwestern Slovenia. Multi-disciplinary examinations have been carried on, involving macro- and micro-facies analysis, sequence- stratigraphy, biostratigraphy and geochemistry (whenever possible). Like in many sectors of western Tethys, the Lower Carnian of western Julian Alps was characterized by the high-relief carbonate platform demise, followed by a gradual infilling of accommodation space, due to the increased runoff and siliciclastic input (Rio del Lago Fm.) related to the Carnian Pluvial Event. This resulted in an almost flattened paleotopography, where low-relief mixed terrigenous/carbonate systems developed in the upper Julian, showing few lateral paleoenvironmental changes in a W-E direction and a transgressive- regressive trend (Tor Fm.). Similarly, the lower Tuvalian was characterized by high-energy inner ramp environments, but with a predominant carbonate sedimentation (amalgamated grainstone body). Diagenetic processes originated a dolomitized lithosome extending on a wide area with the same thickness (15- 20 m ca), and showing only local variation to peritidal settings. The top of the unit could be considered a sort of guide-level on which upper Tuvalian disparate environments were established: while northeastern areas show evidences of pure basinal sedimentation, even if anoxic episodes (Carnitza Fm.), southwestern sectors of Julian Alps are patterned by shallow marginal marine deposits, attributable to low-energy, restricted and sometimes evaporitic environments (Travenanzes Fm. and Monticello Mb.-Dolomia Principale). These features are clearly related to the emplacing of a platform margin belt (Dolomia Principale), approximately oriented in a WNW-ESE way. Depositional geometries are partially preserved and show the evolution of different phases: a first stage marked by the margin platform onset and the following increasing of slope angle and deepening of a starved basin has been recognized. The lower main aggradational stacking pattern is followed by an upper strong prograding stage, that has been dated to the uppermost Tuvalian (Spinosus ammonoid zone). Micro-facies analysis of upper slope-to-margin debris packed into proximal breccia and calcarenite layers revealed a microbial dominated carbonate factory. Together microbial crusts, also encrusting calcareous sponges, Tubiphytes and other Microproblematica organisms represent main components. Moreover, an inner margin part, relatively shallower and sheltered, has been identified, consisting in oncoidal- bioclastic facies and microbialite layers. This area is laterally interfingered with repaired zones in which peritidal sedimentation prevails. In order to provide a comparison, the eastern Julian Alps (i.e. Julian Platform auct.) depositional system has been analyzed. Despite the carbonate factory seems to be more varied, depositional geometries and other features are slightly similar and a connection to the Dolomia Principale carbonate platform cannot be excluded. Aimed to set the depositional system outlined for the northeastern Friuli in the wider Southern Alps framework, a georeferenced database has been created by entering both direct field observations and reviewed data from last two century literature. This allowed to obtain an updated structural scheme and three different scenarios for the upper Tuvalian paleoenvironmental distribution. Particularly, in southern sectors alluvial or exposed areas occur, followed in the uppermost Tuvalian by a tidal flat dominated landscape. These show a lateral transition to northeastern marginal coastal areas and to a basinal belt, elongated approximately in a WNW-ESE direction. In northernmost sectors, open pelagic sedimentation has been indeed documented also for the Norian time. The occurrence of the basin (up to the northeastern Cadore region) has been related to a western inlet connected to the more deep and oxygenated eastern Hallstatt marine domain. At the same time, the eastern Julian Alps system has been interpreted as an outer branch of the Dolomia Principale carbonate platform, settled in a more exposed position to the eastern open marine setting, allowing thus slightly different inner platform sedimentary conditions respect to the western part.