The geological history of the Istria 'depression' Romanian Black Sea shelf: tectonic controls on 2nd/3rd order sequence architecture

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 cpolyphase history of subsidence and sedimentation, interrupted by several quite spectacular 2nd /3rd order erosional unconformities, reflecting the interplay between these teetonic 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 later Jurassic. This was tilted west during the Early Cretaceous by uplift and rifting in the Western Black Sea and the residual Late Jurassic topography was filled and buried by a west-facing clastic-evaporite wedge. Late Aptian- Albian 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 buried in 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 mid-Miocene With extensive shelf margin mass wastage and erosion, followed by reflooding and deposition of a transgressive backstepping sequence in mid-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. A marine connection was re-established in late Messinian. Rising sea level eroded the earlier slumped sequence and the margin was healed by a lowstand prograding wedge in 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.