The Daiichi-Kashima Seamount located at the junction of the Japan and Izu-Bonin Trenches is topped by a flat summit at the depth of about 4,000m, showing clear geomophological characteristic of a guyot. Two limestone formations are distinguished on and around the Daiichi-Kashima Seamount, namely the upper Limestone and the Lower Limestone Formations. The Upper Limestone Formation constructing the summit of seamount is considered to have been a barrier-reef type carbonate bank on the basis of lithology and acoustic stratigraphy. The lithological and paleontological studies of this Formation indicate its deposition to be under shallow-reef environments ranging from buildup to shelf lagoon during the Barremian to Cenomanian. The Lower Limestone Formation is found on the saddle topped by a terrace of about 5,200m depth and also on the Hirata depression, both located to the west of seamount. This Formation comprises debris-flow deposits, pelagic limy mudstone and shallow-water carbonate rock of talus origin, all of which are assignable to deep-water carbonates deposited during the time of Upper Limestone Formation deposition around the saddle and lower slope of the Daiichi-Kashima Seamount. The talus and debris-flow deposits of the Lower Limestone Formation are considered to have been derived from the Upper Limestone Formation.
The world-wide distribution of carbonate platforms constructed and drowned during the Middle Cretaceous suggests the eustatic rise of sea-level caused by regional uplift and volcanic activity in the oceanic basin. The thickness of Middle Cretaceous shallow-water sediments suggests that about 900m rise of sea-level took place during the Middle Cretaceous. According to Hoshino's hypothesis the depth of the top of Daiichi-Kashima Seamount represents the location of the Middle Cretaceous sea-level. The present study shows that the sea-level was about 4500m lower than the present in the Barremian to early Aptian and about 3,600m lower in the early Cenomanian.
In consideration of the relationships between the Cretaceous sea-level and the geological structure of studied area, the geohistory of the Daiichi-Kashima Seamount can be summarized as follows: The formation of Daiichi-Kashima Seamount was initiated on a volcanic island during the pre-Barremian. During the Barremian to early Cenomanian, a carbonate bank has been developed on this island due to the general eustatic rise of sea-level. During the reef growth a huge amount of carbonate debris was derived from the shallow bank and deposited on the saddle and lower slope of the Daiichi-Kashima Seamount. By the end of early Cenomanian, the bank carbonates have attained a thickness of about 900m. During the late Cenomanian to Turonian the carbonate bank was drowned as a result to rapid rise of sea-level. Since then the sea-level has continued to rise and reached the present level.