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.

Oceanographic surveys in the Black Sea during 1998, 2002, and 2004 in the framework of a French-Romanian joint project, and recently in the framework of the European project ASSEMBLAGE, complement previous seabed mapping and subsurface sampling studies undertaken in the Black Sea by various international expeditions. Until the Ryan and Pitman flood theory and prior to this project, it was proposed that the Black Sea was predominantly a fresh-water lake interrupted by possible marine invasions coincident with high sea level during the Quaternary. From the recent surveys carried out on the western part of the Black Sea it is evident that the Black Sea's lake level rose on the shelf to at least the isobath −40 to −30 m as ascertained by the landward limit of extent of the Dreissena layer characteristic of brackish to fresh-water conditions. This rise in the lake level could coincide with the answer of the Black Sea catchment's basin to the meltwater drained from the thawing of the ice cap ensuing Melt Water Pulse 1A (Bard et al., 1996). It is possible that at that time the lake level filled by fresh water reached the level of its outlet and spilled into the Mediterranean Sea. Later, in the mid-Holocene at 7.5 k.y. B.P., the onset of salt-water conditions is clearly evident in the Black Sea. From these observations Ryan et al. (1997) came to the conclusion that the Black Sea could have been filled by salt water cascading from the Mediterranean. Even though this hypothesis has been challenged (Aksu et al., 2002b , 1999b), the recent confirmation of the excellent preservation of drowned beaches, sand dunes, and soils during Ifremer (Institut français de recherche pour l'exploitation de la mer) surveys seems to support the Ryan and Pitman hypothesis (Ryan and Pitman, 1999). The multibeam echo-sounding and the seismic reflection profiles acquired on the Romanian margin during our surveys revealed wave-cut terraces at an average water depth of 100 m. More evidence of seawater penetration is marked at the Bosphorus outlet by the presence of recent canyon heads mapped during the last cruise in 2002. The cores recovered on the Romanian continental shelf penetrated an erosion surface, indicating subaerial exposure well below the level of the modern Bosphorus outlet. The 14 C ages documented a simultaneous colonization of the terrestrial surface by marine mollusks at 7.1 k.y. B.P. The most recent palynology analysis and studies of the dynocyst population (Popescu, 2004) document a real onset of fresh-water arrival during the Younger Dryas and abrupt replacement of Black Sea dynocyst by Mediterranean population, coincident with the onset of the marine mollusks.