12 November 2019
sequence stratigraphic architecture, systems tract, seismic facies, depositional system, submarine canyon, gas hydrate stability zone, hydrate dynamic accumulation, continental slope of the East China Sea
Many evidences for gas hydrate bearing sediments had been found in the continental slope of the East China Sea, such as bottom simulating reflections (BSRs), undersea gas springs, pyrite associated with methane leakage, mud diapirs/mud volcanos, bottom-water methane anomalies and so on. In this study, six key stratigraphic interfaces including T 0 (seafloor), T 1 (LGM, 23 kyr B.P.), T 2 (2.58 Myr), T 3 (5.33 Myr), T 4 (11.02 Myr) and T 5 (16.12 Myr) were identified, and then five third-order sequences of SQIII1 to SQIII5 were divided. However, T 5 in southern continental slope is not found, which shows that the middle-northern Okinawa Trough had begun to rift in the early Miocene, earlier than the southern segment. Four system tracts including lowstand systems tract (LST), transgressive systems tract (TST), highstand systems tract (HST) and falling stage systems tract (FSST) are further divided. The marine erosion interface of 11.02 Myr and regressive unconformity interface of 23 kyr B.P. indicate two large-scale sea level drop events in the research area. Seven typical seismic facies identified in the continental slope are continental shelf-edge deltas, littoral fluvial-delta plains, incised channels or submarine canyons, slope fans, submarine fans or coastal sandbars, littoral-neritic fine- grained sediments, mud volcanos and some other geological bodies respectively. The minimum water depth for hydrate occurrence in the Okinawa Trough is 630 m, and the thickness of gas hydrate stability zone in continental slope is between 0 and 590 m. The calculated bottom boundary of hydrate stability zone is slightly deeper than BSRs on the seismic sections. The re-depositional turbidite sand bodies, such as canyon channels, slope fans and submarine fans developed in Quaternary strata, are the predominant hydrate reservoirs. According to developing process, the dynamic accumulation of hydrate systems can be divided into three evolutionary stages including canyon erosion and hydrate stability zone migration stage, sediments destabilizing and methane leakage stage, and channel filling and hydrate re-occurrence stage.