Table 1 Geologic Sequence in the Miyakoiima-hokubu district

Stratigraphy Thickness (m) Lithic character Holocene Sand dune deposits 1～3 Calcareous sand Soil 1～2 Clay and sandy clay Ikema Terrace Sediment 0.5～1 Clay, sand and limestone gravel Ōnokoshi Soil 2～10 Brown to red clay Ryūkyū Group Shimojishima Limestone 2～5 Coral boulder calcirudite and foraminifera calcarenite Pleistocene Hirara Limestone 2～8 Algal ball calcarenite and loose cabonate clastics sandstone Tomori Limestone 10～30 Alternation of algal calcarenite and foraminifera calcarenite Late Miocene Shimajiri Group Ōkamishima Formation Pinfudake Mudstone 150 + Siltstone with fine sandstone and tuff Nanseien Sandstone 50～70 Sandstone with siltstone and white tuff Nanseien Mudstone 200～300 Siltstone with sandstone and bentonic tuff Shimajiri Sandstone 30～50 Medium sandstone Shimajiri Mudstone 300～500 Alternation of siltstone and sandstone Ōkamishima Sandstone 200～300 Pebbly sandstone and coarse sandstone Ōkamishima Mudstone 10～ Alternation of siltstone and sandy mudstone