Abstract
The Pliocene Ohta Tephra Bed is distributed over a distance of more than 150km in central Japan. Several sedimentary facies can be recognized in this tephra bed. In terms of content, volcanic glass and pumice predominate (more than 99%). The former is roughly smaller than 0 phi, and the latter larger than 0 phi. Volcanic glass mean specific gravity of 2.25 is higher than pumice that of 1. 15. Particle falling velocity of the Ohta tephra can be described by:
Cd=26+24/Re, W=[-9η+{81η2+156ρa3(σ-ρ)g}0.5]/39ρa
where σ and ρ are the particle and fluid densities respectively, g is the acceleration due to gravity, η is fluid dynamic viscosity, Re is grain Reynolds number, a is the particle radium, Cd is non-dimensional drag coefficient, and W is the falling velocity. This formula means the transition to turbulence is promoted at a lower Reynolds number than for the corresponding smooth spherical particles. More poorly sorting of this fluvial channel and bar facies can be explained by the above formula, which suggests that more varied grain in diameter can be deposited under a small falling velocity range. The association of sedimentary facies combined with semi-quantitative analysis for dispersive and hydraulic equivalence of volcanic glass and pumice particles reveals that the deposition of hyperconcentrated flood flow in this study is seem to be influenced by hydraulic equivalence. Grains of some channel fill facies are in hydraulic equivalence under tractive flow. However, grains of some channel fill facies are more closely in dispersive pressures, suggesting small grain flow occurred on a foreset surface of channel bar.
