2004 Volume 58 Issue 1 Pages 25-36
The authors have investigated undescribed sedimentary facies of new outcrops in the southern Inashiki Upland. These facies are divided into 5 facies: Cross-stratified coarse-medium sand, Sandy silt, Clay, Microinterbeds of sand and silt and Bioturbated medium sand. Since Sandy silt contains "Kojimiso" pumice, these facies belong to the Kamiiwahashi Formation. Cross-stratified coarse-medium sand, Sandy silt and Clay are considered to be the channel/point bar, flood plain deposits respectively which were formed in the meandering river system during a low stand stage. Microinterbeds of sand and silt are considered to be the tidal flat deposits. Bioturbated medium sand includes abundant molluscan fossils (Mactra chinensis, Nitidotellina hokkaidoensis) and foraminifera fossils (Pseudononion japonicum, Elphidium kusiroense). These fossils indicate that Bioturbated medium sand was formed in the upper sublittoral zone (inner sublittoral zone) of an inner bay. Microinterbeds of sand and silt and Bioturbated medium sand were formed in the drowned valley system during a transgressive stage. Based on these results, the authors recognized that the sedimentary facies of the Kamiiwahashi Formation in the southern Inashiki Upland changed from meandering river system, through drowned valley system, to strandplain system.
