Mapping depositional systems from the East Balkan thrust belt to the southwest Black Sea – provenance and reservoir quality implications at the transition between Peri- and Para-Tethys

The East Balkan thrust belt is a north vergent retro-arc belt that formed during the closure of the Vardar Ocean and subsequent post-collisional convergence. Compressional deformation began in the late Campanian as earlier half-grabens were inverted. Thrust propagation resulted in the progressive cannibalisation and northward migration of the foreland, and thrust-sheet-basin development. Siliciclastic depositional systems were shed northward and then eastward from the active thrust belt into the southwest Black Sea. A coarsening-upward progression from thin-bedded, low-density turbidite to thick-bedded to massive, high-density turbidite and mass flow deposition, along with a northward migration of siliciclastic facies, is evident at outcrop. Carbonate deposition, along with quartz-rich siliciclastic deposition during low stands, characterised the more northerly, passive, Moesian Platform stratigraphy. Petrographic and provenance data including detrital zircon U-Pb dating, can clearly discriminate between these two systems and validate our outcrop observations.

Late Middle to Late Eocene half-graben formation and the shallowing upward of the foreland basin succession signals the beginning of orogenic collapsed and rebound in the East Balkans. This, when combined with drops in base-level, culminated in a major mid Oligocene to Early Miocene hiatus onshore, and canyon formation and large-scale turbidite progradation offshore. These turbidite systems form actual and potential reservoir targets in the southwest Black Sea. Basal canyon fill facies comprise thick, structureless, pebbly sandstones and interbedded sandstones, siltstones and mudstones, which are interpreted as high-density turbidites and debrites, and canyon levee or intra channel low-density turbidites, respectively. Compositional data demonstrate a clear link between well sample material and onshore depositional analogues. Furthermore, the compilation of a circum-Black Sea basement and detrital zircon U-Pb age database has enabled us to highlight specific source regions likely to fall within the catchment of these depositional systems. Very limited porosity data from offshore Oligocene sandstones suggest that they have good reservoir quality.