Stratigraphy and Structure of Tertiary Deposits in Oga Peninsula Akita Prefeture

Abstract

Tertiary deposits of the Oga peninsula in Akita prefecture affords an almost complete record of successive sediments The Tertiary stratigraphy of the peninsula has been studied by many geologists since 1892 and considered by them as a type locality of the Akita oilfields The writer who worked the geology of the peninsula during last six years wishes to propose the following revision of stratigraphic sequence of the Tertiary system in the peninsula in view of the cycle of sedimentation in descending orderbShibikawa beds(unconformity)Wakimoto ormationupper Kitaura formationHunakawa group middle Hunakawa formationLower Minamihirasawa formationOnnagawa formationDaijima group Nishikurosawa formationDaijima formation(unconformity)Sugoroku complexIn reference to the petroleum geology the Hunakawa group is most important the Kitaura and the Minamihirasawa formations being the oil-bearing depositsThe Oga peninsula is divisible into three different blocks on the basis of geologic structures. The west block is composed of marginal deposits and is faintly folded The middle which is seperated distinctly by the Minamihirasawa fault from the west and by the Hamamaguchi Wakimoto line from the east is monoclinally structured taking NW-SE strike and NS dip In the east the heavy Hunakawa group is normarly foldedAs a result of both stratigraphical and structural considerations of Tertiary deposits in the peninsula, a hopeful oil-productive fileldprobadlly develops east of the Hamamaguchi-wakimoto line In other words the productive oil-field of Akita may extend from Yabase to the Mt Kampu district crossing the Lake Hachiro.