Abstract
We studied in detail the calcareous nannofossil biostratigraphy of the upper Pliocene to Quaternary sequences in the Osugozawa route of Gojome area, Aikawa and Komasugawa routes of the Oga peninsula, and two oil exploration wells of NKH-1 and 3 located in Honjo area, Akita Prefecture, Japan. The Pliocene/Pleistocene boundary redefined at the base of Gelasian Stage was identified in the lower part of the Sasaoka Formation in the Osugozawa route. The base of Calabrian stage, formerly considered as the Pliocene/Pleistocene boundary, was also identified in the lower part of the Kitaura Formation in the Oga Peninsula, based on magneto- and nannofossil bio-stratigraphy. Pliocene to Quaternary formations in the Akita area were correlated to those in the Japan Sea side area.
We also revealed the tectonic movements along the southern coast of Akita Prefecture on the basis of the nannofossils obtained from drill cuttings and the subsurface geology of the area. The results clarified that the Kita-Yuri thrust faults were active between 3.85 and 1.71 Ma. Paleodepth of western Akita Prefecture were also changed from bathyal to shallow marine environments in 2.75 Ma, a consequence of both fault activity and the climate crush associated with drastic latest Pliocene cooling.
