The stratigraphy of the Quaternary formation in the subsurface of the Uwa Basin in the western part of Ehime Prefecture is clarified by the investigation of its lithology and intercalated wide-spread tephra with drilling core samples. Uwa Formation is stratigraphically divided into the lowermost, lower, middle and upper parts. In Uwa Formation, the volcanic ashes named Uwa-1, Uwa-20, Uwa-23, Uwa-24, Uwa-26 volcanic ash can be correlated with Shikito or Yellow I, Seiganji-Toga, Hiwaki, Kobayashi-Kasamori, and Kakuto tephras, respectively. The volcanic ash in the middle terrace deposit at the southern end of the Uwa basin, named Inou volcanic ash, can be correlated with Aso-2(TL) tephra. Formation of the lowermost part of Uwa Formation started before 1.3Ma and ceased around 1.0Ma, in the Early Pleistocene. The sedimentation of the lower part began in 1.0Ma and finished by 0.33Ma. The middle part formed after 0.33 Ma. The formation age of the upper part is around 0.026 Ma, the Late Pleistocene. The most of the middle terrace deposit emerged between 0.2 Ma and 0.1Ma, at the end of Middle Pleistocene.
View full abstract