2012 Volume 71 Issue 1 Pages 3-13
This study presents that the grain-size distribution obtained by the hydrometer method is affected by diatom frustules in bulk sediments finer than 6 phi in median size. This fact implies that detrital grains need to be extracted from bulk muddy sediments to clarify sedimentary processes of detrital particles. This paper reveals that detrital grain size distribution of the surface sediments and sedimentary processes in Lake Kizaki, central Japan. Detrital grains were extracted by removing diatom frustules composed of amorphous opal, and were measured by laser grain size analyzer. Surface sediments can be divided into four groups. The Group I and II sediments are classified into clayey silt, and distribute in the offshore zone. The Group III sediments are of sandy silt or sand-silt-clay, and form transitional zone between the offshore and following nearshore. The Group IV sediments are of silty sand or sand, and their distributions almost correspond to nearshore zone. Boundaries between the Group III and Group IV cross the counter line toward offshore at the river mouth, implying that flood event plays an important role of sedimentation in Lake Kizaki. Sandy and gravelly sediments of the nearshore zone can be regarded as residual sediments by wave action. Sediments composed of transitional zone transported by waves and spread from the river mouth during flooding event and normal periods transported toward offshore, and then settle down quickly according to Stokes’ law. Moreover, fine sediments composed of the offshore zone remained suspended and appeared to be controlled by surface current.
