Upper Lochkovian to lower Famennian evolution of the Carnic Alps: perspectives from the ‘transitional facies’

The Carnic Alps correspond to the non- to low-metamorphic portion of the Variscan substratum of the Southern Alps, and display an Upper Ordovician to Upper Carboniferous succession that represents the best preserved example of Variscan succession within the Alps. The Pragian to Frasnian stratigraphic interval is characterized by the differentiation of the basin in shallow water, including carbonate buildups, and pelagic parts, and the so-called ‘transitional facies’ represent a sort of ‘connection’ between these two parts. We analyzed the depositional evolution of the basin through these facies because they 1. reflect well the depositional dynamics; 2. are datable using conodonts, unlike the shallow water units; 3. are less subject to tectonic elisions/repetitions than the basinal units. We performed geological mapping, measured stratigraphic sections of all the units and dated by conodonts all the lithostratigraphic transitions, in order to infer the lateral correlations throughout the basin, also supported by some marker beds/levels. The depositional environments of the different units have been recognized. The main aspects of the basin evolution can be summarized as follows. A transgression between the uppermost Lochkovian and the basal Pragian drowned the first patch reefs recorded in the Carnic Alps. The margin was ramp-type until the Emsian with patch reefs and shoreface deposits distally passing to tempestite and pelagic sediments. During the Emsian, a by pass slope margin separating the shallow water and the basin started to develop as shown by the increasing amount of gravitative driven deposits that persisted until the lower Famennian. In the lower Givetian the gravitative-driven deposits show their maximum progradation on top of the basinal deposits. From the Frasnian, a transgression, possibly enhanced by extensional or transtensional tectonic activity, led to progressively drowning of the carbonate buildups. The depositional evolution appears to follow the supra-regional eustatic trends until the Frasnian. From the Frasnian, the evolution differentiates from the eustatic supra-regional trends, suggesting the importance of a local tectonic control.