[S-GL18] Geologic structure and tectonic history of East Asia and Japanese Islands

[SGL18-P01] Stratigraphy and geological structure of forearc basin fill on the central Boso Peninsula: 1:50,000 Quadrangle Series, Otaki District

*Masayuki Utsunomiya¹, Taku Komatsubara¹, Shuichi Tokuhashi, Terumasa Nakajima (1.Geological Survey of Japan, AIST)

Keywords:Forearc basin, Neogene, Quaternary, Quadrangle Series of geologic map

Forearc basin fills, called the Awa Group (also known as the Miura Group; Miocene to Pliocene) and the Kazusa Group (Pliocene to Pleistocene), which are exposed on the Boso Peninsula. Because many biostratigraphic datum planes, paleomagnetic reversal events, and other chronostratigraphic tools are available for these deposits, they provide the “type stratigraphy” of other equivalent sedimentary sequences on the Japanese islands and in the northwest Pacific. The developmental process of various sedimentary systems from coastal to deep-sea areas and tectonic evolution have been reconstructed based on structural geological studies. Otaki district occupies almost the center of the Boso Peninsula where the middle to upper part of the Awa Group and Kazusa Group are distributed in the area. The middle to upper part of the Awa Group is composed of the Amatsu Formation (mainly mudstone), the Kiyosumi Formation (mainly sandstone-dominated sand/mud alternation and mudstone-dominated sand/mud alternation), and the Anno Formation (mainly sandy mudstone and muddy sandstone). The Awa Group is overlain by the Kurotaki Formation of the Kazusa Group (tuffaceous sandstone and muddy sandstone) and sometimes interbedded with mass-transport deposits composed of blocks of the Anno Formation. The Kazusa Group consists of the Kurotaki Formation, Kiwada Formation (mainly mudstone), Otadai Formation (mainly sand and mud alternation), Umegase Formation (mainly sandstone dominated sand and mud alternation), Kokumoto Formation (mainly mudstone layers, sandy mudstone layers and muddy sandstone layers), Ichijiku Formation (sandstone with cross bedding), Chonan Formation (mainly mudstone, sandy mudstone), Mandano Formation (sandstone and conglomerate with cross bedding), and the Kasamori Formation (mudstone and sandy mudstone). In this presentation, we report on the stratigraphic and structural geological results obtained during the survey for the 1:50,000 scale geological map of the Otaki district.