5:15 PM - 6:30 PM
[HCG26-P03] Investigation of numerical forward model toward inverse analysis of ancient turbidities: Comparison between results of numerical simulation and grain-size analysis of the ancient turbidite bed in the Pliocene Kiyosumi Formation, Boso Peninsula, Japan
Keywords:Kiyosumi Formation, numerical simulation, turbidity currents
First, this study investigated thickness and grain-size distribution of the ancient turbidite bed in the Mio-Pliocene Kiyosumi Formation, Boso Peninsula, Japan. The Formation is composed of sand-rich alternating beds of sandstone and mudstone, which have been interpreted as deposits of the submarine fan. This study focused on the turbidite sandstone G1, which is sandwiched between two characteristic tephra and therefore it can be traced over 40 km. We measured thickness of the sandstone bed and collected samples for grain size analysis by using the settling-tube method. As a result, it was revealed that (1) bed thickness decrease downcurrent non-linearly. There are points where volume per unit area of each grain-size class decrease remarkably. (2) Locations of the points where sediment volume per unit area decrease vary depending on grain size classes. Then, we conducted numerical experiments to reproduce geometrical features of the turbidite bed described above. We employed 1D shallow water equation model with sediment conservation equations of each grain-size class and active-layer approximation of grain-size distribution of the basal surface. In this study, grain-size distribution is approximated to the two grain-size classes: medium sand (2 phi) and silt (8 phi). As a result, distribution pattern of sediment volume per unit width was well reproduced by our model. Sand-sized sediments pinched out downcurrent, whereas silt-sized sediments show continuous thickness distribution downcurrent. This result imply that the inversion model using the forward model based on shallow-water equation could be applicable for ancient turbidites that have 10s km in spatial scale.