Luisa Palamenghi

Sequence stratigraphic framework of a mixed turbidite-contourite depositional system along the NW slope of the South China Sea

Little is known about the interaction between the deep Pacific Ocean dynamics and the evolving Chinese continental margin at the time of Indonesian Gateway restriction during the late Cenozoic. A sequence stratigraphic approach, which includes characteristic alongslope variations, has been applied to the sedimentary sequences in the northwestern South China Sea (SCS) based on high-resolution multichannel seismic and bathymetric data collected during Cruise R/V SONNE 221 in May 2012. In the present study, a seismostratigraphic analysis of the Pearl River Mouth Basin sedimentary sequence was performed in the southernmost rifted block, the Yitong Ansha. In correlation with the regional lithostratigraphy, this indicates that syn-rift strata deposited from Paleogene to middle Miocene times are heavily deformed by volcanic intrusions emerging from the transitional continental crust. Two satellite basins formed and a leeward regressive-transgressive carbonate shelf ramp developed on the NE flanks of the volcanic intrusions from the middle to the late Miocene. The unconformity related to reef-drowning, interpreted as a third-order sequence boundary (1–10 Ma), is overlain by a lowstand systems tract, which developed when a downslope prograding channel-levee system merged with an upslope migrating wavy sheet. The associated mixed turbidite-contourite depositional system has persisted to the present. The interaction between turbidite and hemipelagite depositional systems, which were overprinted by the SCS Western Boundary Current regime, resulted in the formation of the Yitong Ansha Drift, a giant, elongated and detached, southward-migrating contourite drift. During deposition of the transgressive systems tract, mass transport deposits were funneled through a non-migrating debris-flow channel with levees, which subsequently filled when highstand systems tract deposition began during the early Pleistocene. Contemporaneously, an intensification of bottom current activity resulted in sediment scouring and incision of a contourite moat. Subsidence displaced the margin from subaerial exposure to upper intermediate water downward to lower intermediate water, thereby reducing the upwelling effect associated with trapped flow perturbations around the summits of the volcanic intrusions and which may have changed the flow regime at the bases of their slopes. Therefore, future studies should verify whether the intensification of bottom currents is a result of a mutual feedback between tectonics and oceanography, taking subsidence into different water masses into account, or merely a global oceanographic signal.

Sediment transport by tropical cyclones recorded in a submarine canyon off Bangladesh

Frequent cyclones originating in the Bay of Bengal landfall on the delta coast of the Ganges and Brahmaputra rivers. The cyclones are well recorded in the sediments of a canyon that is deeply incised into the shelf offshore Bangladesh. The large mud supply by the two rivers forms temporary deposits on the innermost shelf, where they are mobilized by waves and currents during the passage of cyclones. The resulting, highly concentrated fine sand-silt-clay suspension is moved by wind-induced currents and eventually plunges into the shelf canyon. These gravity flows are deposited as graded beds on the broad canyon floor. In a 362-cm-long section of a dated sediment core covering the period from 2006 to 1985, nearly all 59 graded beds can be correlated with 42 cyclones observed in that period. The threefold decrease in the sedimentation rate of the last decade compared to the period from 1994 to 1954 is due to the decreased number and power of cyclones. Compared to the sediment transfer by cyclones, the input by local sediment slumps, tidal currents, and monsoonal floods is small. Thus, cyclones dominate the mobilization and distribution of sediment on the Bangladesh shelf. This sediment dispersal mechanism is probably also typical for other shelf areas crossed by tropical cyclones.