4th International Submarine Canyon Symposium (INCISE2018)

5-7 November 2018, Shenzhen, CHINA

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This is the detail of Morphology and sedimentation of Sakawa fan delta slope, Sagami Bay, Japan

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Author
Saiko Sugisaki
Co-Author(s)
Taku Ajioka, Hajime Katayama and Ken Ikehara
Abstarct Title
Morphology and sedimentation of Sakawa fan delta slope, Sagami Bay, Japan
Abstarct Body
Izu Peninsula on the Philippine Plate has collided to Honshu island on the Amur and Okhotsk Plates from south, and the plate boundary of the Philippine Plate locates at both sides of the peninsula along the axes of Suruga (at west) and Sagami (at east) Bays. The collision has created uplift of young mountains, and has maintained large coarse sediment supply and large difference in water depth in the bays. Sakawa fan delta slope is a steep slope with its average gradient of 5–6˚ in the northwestern corner of Sagami Bay. We obtained a new swath bathymetry and backscatter data with surface sediment samples from 20 sites from the Sakawa fan delta slope and its surroundings. Numerous shallow channels were characteristics of the slope. Most of the origin of the channels locate offshore of modern river mouths, and the upper reaches of the channels incise the upper slope. Width of the channel is very narrow at the uppermost slope, but becomes wider in downslope direction. Slope gradient changes systematically from steep to gentle in downslope direction (upper slope: >15˚, middle slope: 5–10˚, lower slope: 2–5˚). The strongest backscatter intensity occurs at the middle slope, and the intensity becomes weaker in the lower slope. Relatively thick hemipelagic mud was collected from the middle slope, and some sand layers were obtained from the lower slope. However, the previous studies indicated the occurrence of thick gravelly sand at the middle slope. Some sand layers at the lower slope contained plant debris. Upward coarsening reverse grading and internal erosional surfaces in the sand layers suggest that at least some sand layers were flood origin. Density bottom currents with sands and gravels have confined in the incised channel at the upper slope. Coarse-grained sand and gravel deposits at the middle slope where the incised channels become unclear, but fine-sand and mud transports further downslope and deposits at the lower slope where the velocity of density currents are decreased. Repeated occurrence of flood-related density bottom currents through shallow channels had transported sand and gravels, and had formed sand layers at the lower slope.
INCISE Theme
Session 1: Canyon processes in space and time (formation, evolution, circulation)
Presentation Type
Poster Presentation
Keywords
fan delta slope, morphology, surface sediment, flood deposit

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