16:00 〜 16:15
[HCG21-09] Numerical Experiments on Internal Delta Formed in Minibasin
キーワード：混濁流、小型海盆、深海三角州、海底扇状地、数値モデル
Submarine fan is a large topography formed on the deep-sea floor by the supply of sediment from the land to the deep-sea floor by turbidity currents. A general submarine fan model was developed, which assumes that a submarine fan is composed of a submarine canyon, a leveed channel and a lobe. The model agrees with the characteristics of submarine fans that develop in large sedimentary basin. However, some minibasins do not fit the general submarine fan model. For example, leveed channels were not observed in slope minibasins formed in active margins such as the Awa Group, and small-scale channel was recognized. In Basin 4 of the Brazos-Trinity Basin at the Gulf of Mexico, leveed channels and lobes were not observed, and deltaic topography with distributary channels was observed. Therefore, the submarine fan model cannot be applied to minibasins, and a new facies and topography model may be required for minibasins. Furthermore, it is known that ponding occurs in minibasins. Ponding is a phenomenon in which a turbidity current reaches a downstream slope of the basin, forcing the formation of a hydraulic jump and filling the basin with suspended muddy sediment. In large basins, ponding does not occur because the turbidity current dies before reaching the downstream barrier. Based on these observations of previous research, we hypothesize that ponding creates a deltaic topography in a minibasin. Ponding develops a topset with shallow distributary channels upstream of the point of the hydraulic jump and a foreset with a steeper slope is formed downstream of the region of the hydraulic jump due to rapid sedimentation. The bottomset with muddy deposit is formed at the distal region. To test the hypothesis that deltaic topography develops in minibasins by ponding, we conducted 2D numerical experiments of turbidity current. In this study, a horizontal 2D four-equation model introducing water detrainment was employed as numerical models (Parker et al., 1986; Toniolo et al., 2006) and implemented using turb2d (Naruse, 2020). The calculation conditions are the following. The calculation region had a 4.2 m width and 1.9 m length, creating an artificial mainibasin surrounded by 10% slopes. The grid size was set to 0.05 m. The inlet was set upstream of minibasin and continuous turbidity currents ran from the inlet for 86400 seconds. Flow velocity and height at inlet were 0.05 m/s, 0.05 m, respectively. Two grain-size classes, 20 micrometer and 40 micrometer, were used for suspended sediment, and the concentrations at each grain-size class were set to 0.001. Numerical calculations and flow observation showed that the bore migrated upstream after the turbidity current reached the downstream barrier. The internal hydraulic jump stabilized near the boundary between the upstream slope and the basin plain, and the current became thick rapidly at the downstream point when the hydraulic jumps occurred, indicating the occurrence of ponding. When spatial variations in bed thickness were observed, a topset with a slight gradient was observed near the hydraulic jump, a foreset with a steeper gradient than other regions was observed downstream region of the internal hydraulic jump, and there is a flat bottomset near the counter-slope. The grain-size distribution in the deposit showed that the proportion of coarse-grained sediments decreased downstream while the proportion of fine-grained sediments increased. These results are consistent with our hypothesis. However, the distributary channels in the topset were not reproduced. This may be due to the inability to reproduce the small channel width of distributary channels because of the large grid size in this study. This is the first study to form the deltaic topography in a minibasin using a numerical model. Further numerical and flume experiments will be conducted to develop a sedimentary facies model for minibasins.