秋田油田新第三系・第四系層序に関する最近の変革

Abstract

Neogene stratigraphy of the Akita oil fields (s.l.) was first established by OHASHI (in TOYAMA, 1925) in the Oga Peninsula, and later by OMURA (1928, 1930) in and around Akita City, which is an important oil-producing area in the region. The oil-bearing marine strata in the Akita City area belong to the Onnagawa, Funakawa, Tentokuji and Sasaoka Formations, the type locality of the former two is in the Oga Peninsula. The Tentokuji and Sasaoka Formations were designated in the Akita City area and have been correlated with the Kitaura and Wakimoto Formations of the Oga Peninsula. The stratigraphic succession in the Akita City area has long been regarded as the standard succession not only for the Akita oil fields but also for the Neogene sequences along the Japan Sea coastal regions.Recent studies in magnetostratigraphy and in biostratigraphy based on diatoms, calcareous nannoplankton and planktonic foraminifera indicate that the correlation, so far established between the successions in the Oga Peninsula and Akita City area, is not valid. Based on these studies, the Onnagawa Formation of the Akita City area is correlated with the lower to middle part of the same formation of the Oga Peninsula, and the Funakawa Formation of the Akita City area is correlated with the upper part of the Onnagawa to the basal part of the Funakawa Formation of the Peninsula. The Tentokuji and Sasaoka Formations are correlated with most of the Funakawa Formation of the Peninsula, unlike the usual practice of correlating them with the Kitaura and Wakimoto Formations. It is after the deposition of the Sasaoka Formation that the Early Pleistocene Kitaura and Wakimoto Formations were deposited in a marine basin, which existed as a remnant of the earlier one, to the west of the present coast line of Akita-Noshiro.The Kitayuri thrust system along the present coast line seems responsible for the difference in the sedimentary environment of the two areas since the time of deposition of the upper part of the Onnagawa Formation in the Oga Peninsula.