Seafloor seepage in the Black Sea: Mud volcanoes, seeps and diapiric structures imaged by acoustic methods

Abstract

The Black Sea is the worldwide largest anoxic basin with thick gas rich sediments facilitating seafloor seepage, which is widely distributed along the continental slopes of the Black Sea. In two areas, the Sorokin Trough off Crimea and at the continental slope off Batumi (Georgia), the distribution, structure and evolution of two different vent systems and their relation to fluid migration pathways and gas/gas hydrate occurrences have been investigated by means of high resolution multichannel seismic data. In both regions, the distribution of seafloor seepage is controlled by fluid migration along permeable pathways associated with diapiric uplift in the subsurface. In the Sorokin Trough, seepage is expressed by intensive material upflow and the formation of mud volcanoes; offshore Batumi, gas seeps with low material flux dominate. In the Sorokin Trough, most mud volcanoes are associated with near-subsurface mud diapirs. The great morphological variability of the mud volcanoes reflects different driving mechanisms, which depend on the availability of permeable fluid migration pathways and the depositional environment. A 3D seismic dataset images the detailed three-dimensional structure of the Sevastopol mud volcano and the spatial relationships to the subsurface structures. Gas/fluid migration along faults developed during the diapiric growth highly affects the formation of the mud volcano. Bright Spots in the direct vicinity of the mud volcano at relatively constant depth corresponding to the depth of the base of the gas hydrate stability zone suggest that gas hydrates are present locally where gas/fluid flow occurs, i.e. near the mud volcano.Offshore Batumi, the diapiric ridges control the development of a complex canyon-ridge system, as the diapirs guide the trend of the canyons by creating pathways preferably used by turbidity currents.