Accumulation of gas hydrates in mass transport deposits at Krishna-Godavari Basin, Bay of Bengal: Foraminiferal, sedimentologic, and seismic evidence

Gas hydrates have been reported in diverse environments ranging from marine sediments of continental margins and deep-sea fans to permafrost zones. However, they are rarely reported from mass transport deposits (MTDs). Here, we combine paleontologic, grain size, stable isotopes, two-dimensional seismic profiles, and downhole log data from the Quaternary sediments of National Gas Hydrate Program site 10, Krishna-Godavari Basin, India, to identify a mid depth MTD and accumulation of the gas hydrates within it. Gas hydrates at site 10 are present from 26 to 160 m below sea floor (mbsf), with a change in hydrate saturation at 90 mbsf. The dominance of shelfal foraminifera (Ammonia spp., Bolivinita subangularis, Loxostomum amygdalaeformis, and Quinqueloculina seminulum) in deep sea and the mixing of large rounded grains with clays confirm the existence of shelf sediments within similar to 90 and similar to 153 mbsf, interpreted as MTDs. The seismic profile also bears the signature of MTDs within this interval. The gravity-driven MTDs transported these shelf sediments to the continental slope environment and formed a different one from the topmost MTD. A higher abundance of Cibicides wuellerstorfi and the absence of Cassidulina carinata within the MTD suggest a change in the depositional environment. The usual range of delta C-13 of Bulimina marginata and a well-ventilated depositional environment indicate the formation of post depositional gas hydrates within MTDs. Mixing of sediments lowered the porosity in MTDs. Gas hydrates within this zone are nucleated within intergranular/fracture pores by entrapment of upward-migrating methane through faults/fractures from deeper successions.