Partial drowning or backstepping of the Early Norian Dachstein Carbonate Platform in the Dinarides (Poros, Montenegro)

In the Dinarides the reef rim to the open marine deep-water depositional realm (outer shelf) of the Late Triassic Dachstein Carbonate Platform is not known. On the road from Gradac to Šula near to the village Poros a more than 120 m thick far travelled and overturned Late Triassic succession of reefal to bedded siliceous limestones was studied (biostratigraphy, microfacies). The section is slightly tectonic overprinted, with slump deposits in the central and upper part.

The section starts with a roughly 20 m thick reefal to fore-reefal limestone succession with deep-water matrix in the upper part (Lacian 2 in age with following conodonts: E. rigoi, E. abneptis). Near the base the reefal limestone is think-bedded to massive (rudstones), higher up in the section various bedded. We attribute these fore-reefal limestones as part of the Late Triassic Dachstein Limestone, interestingly with a deepening upward sequence from the middle Lower Norian onwards. Around the Lacian 2-3 boundary the depositional characteristics changed relatively abrupt from reefal-rudstones to bedded siliceous limestones intercalated by few and turbidite layers containing shallow-water debris. The next, 30 m thick part of the succession consists of dm-bedded limestones with chert nodules and layers, grey limestones and reddish limestones (radiolarian-filament wackestones), in parts with slump intercalations or medium-grained microbreccias. Conodont dating show that the age this part of the section is Lacian 3 to Alaunian 1-2 in the upper part (dated by E. spatulata to E. slovakensis) probably reaching the Alaunian 3. The Alaunian 3 to Sevatian (with E. bidentata) is characterized by a thick series of slump deposits with carbonate turbidite intercalations. Upsection follow polymictic breccias (debris flows) and microbreccias (turbidites) with older open-marine hemipelagic components, as proven by conodonts. The overlying dm-bedded grey-reddish siliceous limestones with red chert nodules are Rhaetian in age dated by the appearance of M. hernsteini. Upsection 5-10 cm-bedded grey siliceous and slightly marly limestones (in a thickness of less than 20 m) follow, overlain by roughly 10 m thick dm-bedded red-grey siliceous limestones with red marl to claystone intercalations, in the lower part with slump deposits, again overlain by 5-10 cm-bedded grey siliceous and slightly marly limestones. An exact age of this part of the series could not be determined, only conodont multielements could be isolated from this part of the succession. The age is most likely Rhaetian 2-3, but earliest Jurassic for highest parts of the sequence cannot be excluded.

The higher Lacian to Late Norian part of the succession corresponds to the reef-near facies belt in open shelf position, known in the type-area in the Northern Calcareous Alps as Gosausee Limestone facies. However, the section Poros shows during the Norian a general deepening trend during the time span Lacian 3 to the end of the Rhaetian opposite of the well-known platform margin in the Northern or Southern Alps. In the Dinarides a backstepping of the reef belt in the late Early Norian result in a drowning unconformity of the Early Norian part of the long-living Dachstein Carbonate Platform.