Hyperpycnites Deposited 700 km Away from River Mouths in the Central Japan Sea

摘要

Relatively little is known about the characteristics of deposits and potential runout distances of hyperpycnal currents. This study describes and discusses evidence of sediment deposition from sustained (long-duration), quasi-steady turbidity currents in the distal part of the Toyama deep-sea channel, which extends ca. 700 km from river mouths in the central Japan Sea. The study is based on gravity cores and airgun seismic reflection profiles obtained from the channel distal reaches. The silty turbidite beds of the channel's terminal fan show rhythmic layering that indicates sustained turbidity currents with distinct flow-strength fluctuations. Some of the rhythmite beds show a fining-upward internal trend (net flow-strength waning), whereas others show an upward coarsening (net flow-strength waxing) followed by fining. Seismic reflection profiles from the channel levees show large bedforms of climbing-dune type, attributed to the spillover of thick sustained turbidity currents. The deposition of the terminal fan rhythmites and accretionary levee bedforms is attributed to turbidity currents generated by hyperpycnal river effluent. A quantitative assessment of sediment concentrations in the coastal rivers indicates that their effluents could become hyperpycnal nearly every year or during every major seasonal flood. The density of a river-generated underflow would increase by the entrainment of saline water and seafloor sediment on the steep slope of the Toyama Bay, resulting in robust, long-runout turbidity currents. The estimated flow velocities of these currents were around 0.3 m/s, and their recurrence period for the last 1000 years was of the order of 70 years. The estimated duration of hyperpycnal flows required for the deposition of rhythmite beds 700 km away from the river mouth is of the order of several days to 3–4 weeks. The study provides new insights into the recognition and classification of hyperpycnites in distal zones of deep-marine turbiditic systems.