Grain Size Distributions of Recent Alluvial Plain Deposits and Shallow Marine Sediments, Part 2 ―deltaic shallow marine sediments in the north-western Mikawa Bay, Japan―

摘要

To make clear the relationship between grain size distribution and geomorphology in deltaic shallow marine environments, the writer analyzed numerous bottom sediments by his method (Moriyama 1976). Geomorphology in the northwestern Mikawa Bay is shown in figure 1． Delta platform with about 1.5km width with extends along coast and it is occupied by tidal flat. Off the Yahagi River coast, steep fore-set slope extends south-eastward far off the fore-set slope. Statistical four moments of dredged samples (Tab. 1) and also correlation between mean and skewness (Fig. 3) indicate that most of delta platform deposits are coarser than 3.0 phi, most of bottom-set slope deposits are finer than 6.0 phi and strong minus correlation is between mean and skewness. From size frequency distridution curves in normal probability scale (Fig. 4, 5), the writer calculated mean and standard deviation of each sub-population in rifference to all samples (Fig. 6). Almost delta platform deposits are composed by two or three sub-populations and means of sand population agglomerate near 3.0 phi and 1.0 phi. Clay population have large standard deviation and their means are mostly 8-9 phi. Lack of sub-population in 4-7 phi is common to the all samples. Association of sub-populations of bottom-set-slope deposits is very complicated and their clay population shows high percentage in contrast to delta platform deposits. Lack in 5-6 phi can be seen as well as in the platform deposits. Above-mentioned facts indicate that there is a very striking contrast between coarse sands prevailing sand populations inside of fore-set slope and fine clay prevailing clay population beyond the fore-set slope. The writer considers it is occured by the following reasons; inside of the fore-set slope, the silt size grains are not produced so much, drained sediments with much sand populations from river mouths are deposited on shallow delta platform on which violent tidal current flows, and, beyond the fore-set slope, sediments containing much clay population spread out to deposit on the bottom-set slope further off the coast where week tidal current flows.