In 1936 OMURA distinguished two tectonic movements, the Dewa disturbance during the Latest Miocene and the Earliest Pliocene and the Oil-field disturbance of the Latest Pliocene, in the inner oil-fields of Northeast Honshu, Japan. He considered the Dewa disturbance is characterized by an epeirogenic movement and oil-traps were completed by the orogenic Oil-field disturbance.
In this oppotunity, I explained my personal consideration on the Dewa disturbance. This disturbance is an orogenic movement in the Katsurane substage (Earliest Pliocene), being associated with the geanticlinal upwarping since the Later Funakawan stage (Latest Miocene). Oil-bearing formations in Akita oil-fields, such as the Onnagawa, the Funakawa and the Katsurane, were strongly folded and faulted, with N-S trend, by this disturbance, then oil-traps were formed in anticlinal structures. Some weaker tectonic activities followed in the Later and the Latest Pliocene, but they were rather remote from oil-accumulation.
The sedimentation of the orogenic phase of the Dewa disturbance is represented by the Katsurane formation which is characterized by alternation facies of abnormal rudaceous sediments and mudstone. The remarkable differentiation of the Akita sedimentary basin, in which upheaval belts and sinking zones appeared here and there, were caused by the Dewa disturbance. Some large faults had controlled, with long-ranged activity, thrust upheaval in one side of the fault and reverse sinking in another side, through the Tentokuji and the Sasaoka stages (Later Pliocene). In these minor basins, younger formations, such as the Tentokuji and the Sasaoka, formed gentle structure, but the underlying oil-bearing formations were severely folded and faulted.