Abstract The Istria ‘Depression’ or sub-basin of offshore Romania lies at the intersection of the trans-European Tornquist–Teisseyre ‘Zone’ and the Black Sea back-arc basin, just outboard of the East Carpathian orogenic welt. Its Late Mesozoic–Cenozoic succession records an extraordinary polyphase history of subsidence and sedimentation, interrupted by several quite spectacular second-/third-order erosional unconformities, reflecting the interplay between these tectonic domains. The unconformities divide the succession into a number of stratigraphic sequences. The sub-basin first developed as a transtensional rift in the Triassic–Early Jurassic, evolving into a narrow oceanized trough in the later Jurassic. This was tilted west during the Early Cretaceous, and the residual Late Jurassic topography was filled and buried by a west-facing clastic–evaporite wedge. Following Late Aptian–Albian(?) rifting, post-rift subsidence and spreading in the Western Black Sea imposed a strong easterly tilt, encouraging the partial evacuation of its Early Cretaceous sedimentary fill by gravity-driven mass wastage. The incised valley topography was subsequently infilled and buried during the later Cretaceous and Early Cenozoic. During the mid-Late Cenozoic, the Black Sea Basin experienced intermittent periods of partial to complete isolation from the world ocean and significant base-level drawdown. The first major sea-level fall occurred in the Eocene when the Istria ‘Depression’ was deeply incised, to be healed by Oligocene shales during the subsequent rise. Yet another period of drawdown and exposure occurred in the mid-Miocene, with extensive shelf-margin mass wastage and erosion, followed by re-flooding and deposition of a transgressive backstepping sequence in the middle-late Miocene. Messinian drawdown in the Mediterranean caused a further period of isolation and falling base level. The shelf margin was again exposed, and experienced widespread mass wastage and slumping. Rising sea level eroded the earlier slumped sequence and the margin was healed by a lowstand prograding wedge in the late Miocene–early Pliocene. This was followed by shelf sedimentation in the Plio-Pleistocene periodically interrupted by canyon-incision events, testifying to continued climatically or tectonically imposed base-level fluctuations. Several direct and indirect tectonic factors were responsible for valley/canyon incision within the Istria Depression and erosion of the Romanian Black Sea shelf margin. These include: (1) the local structural framework; (2) direct tectonic uplift and tilting; and (3) more indirect tectonically imposed isolation encouraging significant base-level falls.

Abstract It is generally believed that the western part of the Black Sea opened during the Early Cretaceous. However, recent data and interpretation from the Turkish margin suggest rifting continued into the Coniacian or Santonian. In this review, the evidence related to the Black Sea rifting on the conjugate Romanian margin is reassessed. Our integrated interpretation of this region, supported by outcrop observations, core and detrital zircon data, suggests that rifting started during the Aptian and continued intermittently until the mid-Turonian in two distinct stages. These stages are bounded by significant unconformities and reflect the progressive widening of the rift system. The first synrift stage started in the Aptian with the deposition of fluvial and lacustrine clastic successions, and locally marine carbonates in semi-isolated depocentres. These sinks began to coalesce during the latest Aptian–Albian with shallow-marine transgression from the east, and deposition of coastal swamp, deltaic and littoral facies. The second phase of rifting during the Cenomanian was marked by transgressive shallow-marine deposits overstepping the earlier Albian depocentres. Continental break-up followed in the mid-Turonian associated with regional uplift and erosion of the basin margin and the local deposition of fluvial conglomerates.

Abstract The Oligocene–Early Miocene Maykop depositional system of the Western Black Sea Basin is investigated in terms of sediment supply and provenance. Potential sediment source regions and conduits for sediment supply into the deep-water portion of the basin are evaluated based on the tectonic history and framework of the region, and are supported by observations from published well, reflection seismic and isopach data. The outcrop geology of the present-day land areas adjacent to the basin is used as a guide to the likely provenance and, hence, quality of potential siliciclastic reservoirs. Reservoir presence and reservoir quality are key subsurface risks for exploration in deep-water plays involving Maykop turbidite sandstones and charge from the well-known Maykop organic carbon-rich mudstones that are widespread across the basin. Sediments sourced from the NE Moesian Platform and Dobrogea, channelled into the offshore Black Sea via the Histria Trough, are considered moderate risk in terms of primary reservoir quality, as evidenced by thick packages of fine-grained sediment. In contrast, sediments derived from the southern Strandja Massif fed into the Burgas Basin, and potentially into the deeper-water Turkish Black Sea, are relatively low risk in terms of reservoir quality, given the abundance of acidic intrusions within the massif. Sediment derived from parts of the northern Strandja Massif, especially the volcaniclastics of the Srednogornie region, are likely to have poorer reservoir quality characteristics. Sediments derived from the granitic Bolu Massif within the Pontides might be of good reservoir quality but are likely to be ponded behind the offshore Kozlu Ridge. An important sediment source-to-sink system was derived from the Balkanides and entered the deeper-water western Black Sea via the Kamchia Trough. The present-day Kamchia river is a relatively minor sediment supplier to the Black Sea, but the palaeo-Kamchia river of the Oligocene–Early Miocene would have exploited a much greater drainage area consisting of an axial trunk stream, occupying the newly formed Kamchia Foredeep to the north of the Balkanides, and transverse rivers sourcing sediment from the granitic and gneissic bodies of the Balkan Mountains and from Early Cretaceous and Palaeogene sandstones. These would provide reasonable reservoir quality, and it is estimated from reference source-to-sink relationships that offshore sediment flux via this system was probably at least eight times greater than at present. Known shelf-edge canyons in offshore Bulgaria facilitated this sediment reaching the deep water offshore, where a sedimentary fan with a length in excess of 150 km is likely to have developed. This suggests that the potential is good for encountering good-quality reservoir sands in the Maykop succession deep water of the western Black Sea, and this aspect of regional play risk could be of less concern than was previously considered.

The Moesian Platform is a crustal block within southern Europe, located beyond the southwestern margin of the East European craton. Along this margin lie terranes that were accreted to Baltica as part of Far Eastern Avalonia during Late Ordovician–Early Devonian time and terranes that already formed part of Cambrian Baltica, displaced as proximal terranes together with Far East Avalonian terranes. The tectonic history and crustal affinity of the Moesian Platform, however, remain poorly understood. A review of available tectonostratigraphic, paleontological, and geochronologic data suggests that the Moesian Platform comprises four distinct terranes, two with Baltican and two with Avalonian affinities. A fifth terrane, North Dobrogea, lies between the Moesian Platform and the East European craton and records Variscan (Carboniferous) accretion. This accretionary record leads to the paradox that the youngest accreted crust (North Dobrogea) lies closest to the craton, whereas the earlier accreted crust and crust derived from the craton itself are now located more externally. A review of terranes along the southwestern margin of the East European craton, between the North Sea and the Black Sea, suggests that a dextral strike-slip dominated the southwestern Baltican margin during Late Ordovician–Early Devonian accretion of Far Eastern Avalonia, much as is the case in western North America today. Variscan indentation of the Bohemian Massif led to escape-displacement of some Caledonian terranes, and strike-slip displacement during the Mesozoic opening of Mediterranean-style oceanic basins led to the current juxtaposition of Moesian terranes, inverted with respect to their accretionary history.