Abstract
This paper describes the sedimentological history of the bathyal slope facies deposits of the Tertiary Taishu Group in the northern part of the Tsushima Islands. The study area is underlain mainly by mudstone and alternative beds of sandstone and mudstone, and small amounts of submarine sliding deposits. Detailed mapping of this area makes it clear that the Taishu Group forms anticlinoria and synclinoria with half-wave lengths of about 0.5 km to 2 km. The axial traces trend NE and NNE. EW-trending folds had also be recognized in the northeastern part. The deposits in the Taishu Group are characterized by five large sand bodies surrounded by massive mudstone. Each sand body consists of more smaller bodies of sandstone and alternative beds of sandstone and mudstone. Comparing the stratigraphic levels of the succession in five sand bodies, five sedimentary cycles were recognized. Each sedimentary cycle exhibits coarsening- or fining-upward, which corresponds to each stage of relative sea-level change. Those stages are (a) rising to a highstand stable stage, (b) falling to a lowstand stable stage, (c) rising to a highstand stable stage, (d) falling to a lowstand stage, and (e) rising to a highstand stage, in ascending order. Likewise, sedimentary processes are considered to be as follows: In the first and second stages of (a) and (b), coarse sediment was supplied only to the east part. In the following stage (c), the supply of coarse sediment in the western part was larger than that of the eastern part. After these stages, coarse sediment was supplied all over the basin in stage (d). In the last stage (e), coarse sediment was supplied mainly to the east. Relative sea-level changes and the migrating of depocenter have played important roles in forming the stratigraphy of this area.
