Sedimentological evolution of the Emine and Kamchia basins, eastern Bulgaria

Abstract

Mountain belts are inherently asymmetric, defined by the sense of plate subduction. The resultant orogen can be divided into peripheral and retro-arc wedges with the retro-wedge overlying the overthrust plate. Modelling suggests that retro-wedges and their neighbouring foreland basins have patterns of sedimentary architecture and sediment sourcing distinct from that of peripheral foreland basins. This study is a sedimentological and petrographical investigation into the evolution of the retro-arc wedge and foreland basin of the Hellenide/Balkan system from the Late Cretaceous to Mid-Miocene. This system is analogous in tectonic setting to the small retroforeland basins of the Apennines and Pyrenees. The east-west trending Balkans were the collisional product of the late Mesozoic/early Cenozoic Alpine Orogeny. The depocentres studied here were situated on the overthrust plate of the Eurasian-Anatolian subduction zone, north of the orogenic deformation front, the Stara Planina Frontal Thrust, and south of the stable craton, the Moesian Platform. Present day coastal exposures in Eastern Bulgaria, oriented perpendicular to the strike of the orogenic system, preserve an excellent onshore record of the transition from a tectonically inactive basin subordinate to a major back arc basin (the Late Cretaceous to End Palaeocene Emine Basin) to compressional underfilled and subsequently overfilled foreland basin (the Lower Eocene to mid-Miocene Kamchia Basin). Late Cretaceous sediment filling of the trough-like Emine Basin was in the form of deep-water turbidites (Lower Emine Formation) transported west-to-east and sourced from Cretaceous granitic basement approximately 250 km west of modern outcrops. Turbiditic deposition continued through the Palaeocene (Upper Emine Formation) but the southern basin margin began to experience compression during this time and sporadic influx of submarine fans (Kozichino Formation) sourced from the arc/incipient thrust wedge to the south of the depocentre mixed with the along-strike transported turbidites. Deposition into the Emine Basin continued to be turbiditic during the Lower Eocene (Atanas and Gebesh Formations) but there are signs of stronger basin instability and shedding of sediment off both the northern (platform) and southern (orogenic) margins from high energy deposits (Bardarevo and Meshilika Formations). The Mid-Miocene Illyrian Unconformity is a period of major tectonic compression and uplift creating structural features such as the Irakli and Obzor Synclines and mass flows sourced from recycled arc volcanics (Obzor Formation) are documented from this period. Mid and Upper Eocene deposition persisted to be easterly-transported turbidites (Dolen Chiflick Formation) but the depocentre migrated north into a shallower and geographically more restricted basin, the Kamchia Basin. The dominant control on sedimentation became increasingly eustatic rather than tectonic. By the Oligocene, sedimentation was uniformly low energy and shallow water (Ruslar Formation). The Mid-Miocene Galata Formation displays high energy, directed flow in a very narrow channel-like basin into the early Black Sea which had, by this period, begun to open through east-west extension from plate reconfiguration and orogenic collapse.