Abstract
The middle Jurassic to lowermost Cretaceous Somanakamura Group crops out in an area along the eastern margin of the Abukuma Mountains, Fukushima Prefecture, northeast Japan. Shallow marine carbonates, 35-meter thick, constitute the upper part of the Nakanosawa Formation, one of the constituent formations of the group. The carbonate sequence named the Koike Limestone is Tithonian in age. Stratigraphical and sedimentological investigations reveal that the carbonate sequence is divisible into five sedimentary units, each of which consists of the grainstone/packstone/wackestone with abundant oncoids and/or peloids and skeletal and non-skeletal grains with micrite coatings, that grades upward into the coral floatstone. The former is thought to have been formed in open, high-energy marine environment while the latter in restricted low-energy one. These data are taken to suggest that relative transgressions intermittently occurred five times during the deposition of the Koike Limestone. The pattern of sea-level changes inferred from the depositional sequence of the middle to upper Somanakamura Group agrees well with the long term eustatic curve derived from coastal onlap.
