Gas-Saturated Shallow Sediments in the Eastern Black Sea and Geohazard Effects

Abstract

Abstract The Black Sea, one of the largest inland seas in the world lying at the junction between Europe and Asia, is both oceanographically and geologically unique. Off the shelf, the water depth quickly plunges to an average depth of 2 km making it unusually deep for what would normally be termed a marginal sea. Methane seeps are a common feature around the basin, and mud volcanoes have been identified. It has been suggested that shelf and slope sediments of high deposition rate are methane sources, whereas the deep basin is methane sink. Slope failures and sediment instability are serious that can lead to the failure of offshore installations. In the deep basin mud volcano eruptions are almost certainly formed by breakdown of methane hydrate on a huge scale. Recent studies in marine geology indicates potential geo-resources in the Black Sea. Marine geophysical surveys were carried out in order to find out all obstacles and potential geohazards in the eastern Black Sea. Several different structures were observed in the Black Sea basin as slumps, slumping, pockmarks, faults, mud volcanoes, diapirs and dome-like structures. Structures, which contain gas hydrates are present on the profiler records as strong acoustic reflections. Introduction The Black Sea is one of the world's largest marginal sea having anoxic conditions below 80-120 m, with an area of about 432 000 km2 and a volume of 534 000 km3(Fig.1). The Black Sea is connected to the Mediterranean Sea via the Sea of Marmara and the shallow Bosphorus and Dardanelles straits. Basically, the Black Sea occupies an oval basin between the folded Alpine belts of the Caucasus and Crimea to the north and northeast and the Pontus Mountains. It was found from the interpretation of seismic reflection data1 that is quite evident that the lower part of the southern margin of Black Sea basin generally preserves the extensive tectonics associated with the rift process that generated the Black Sea. Frequently the compressive deformation was superimposed on pre-existing distensive faults, which commenced their activity in the Upper Cretaceous or at the end of the Mesozoic and continued into the Paleocene. The uplift and subsidence described is an extensive phenomenon traceable to regional tectonics of the Tethys belt2. The mechanism of the immense subsidence of the Black Sea basin has given way to the deposition of the thick sedimentary sequence. The Black Sea comprises two extensional basins formed in a back-arc setting above the northward subducting Tethys Ocean, close to the southern margin of Eurasia. Two basins coalesced late in their post-rift phases in the Pliocene, forming the present single depocentre. Off the shelf, the water depth quickly deepens down to an average depth of 2 km making it unusually deep for what would normally be termed a marginal sea. The Black Sea basin lies within the Cimmeride/Alpine fold belt on the Eurasian tectonic plate. To the south it is bordered by the Turkish micro-plate or crustal block. The mountain-building processes and their subsequent erosion around the basin have contributed to a high sediment input. Seismic studies indicate a 15 km thick blanket of sediments with unusually low seismic velocities (3.0-4.5 km/s) that reach potentially back to the Early Cretaceous (130-110 Ma). Although there is excessive supply of terrigenous sediment (exceed 100 million tones per annum) in the Black Sea, pelagic sedimentation plays the major role in the deep basin (the total abyssal sedimentation is about 10 cm/k). These sediments are rich in calcite and organic carbon3, the latter showing a high degree of preservation due to anoxia in waters below 100-150 m. P