Remarkable fossil plants and marine animals were found from the Oarasawa Formation distributing along the upstream of the Isawa River, southwestern part of Iwate Prefecture, near Akita-Iwate border.
The Oarasawa Formation, the lowest Neogene sequence in the Ou Backbone Mountain range area, has been correlated to the Nishioga Group or the Monzen Group of Oga Peninsula on the basis of characteristic lithofacies called “Oarasawa-Semi facies” without enough fossil evidence. The Oarasawa Formation outcropping along the Isawa River is composed mainly of subaqueous basaltic to andestic volcanic rocks and intercalates conglomerate, sandstone and siltstone layers, in which the fossil were newly found.
The plant fossils include some characteristic species of the Daijima-type fossil flora such as
Cyclobalanopsis nathorstii, Quercus miovariabilis, Liquiclamber miosinica and
Smilax trinervis being determined by Professor emeritus K. Huzioka. The fossil marine animals are not abundant and poorly preserved, but the fossils contain a few common species of the Kadonosawa fauna that is nearly equivalent of the fauna in the Nishikurosawa stage.
The fossil evidences show that the Oarasawa Formation is now placed in the Daijima-Nishikurosawa age. Therefore, by biostratigraphical and lithological facts, the Oarasawa Formation can be correlated consistently with the formations which were established or redefined recently, such as the Okachigawa Formation of Dewa mountoins, southern Akita Prefecture and the Sunakobuchi Formation distributting along the eastern margin of the Akita Oil Field.
The litho- and biostratigraphical informations obtained recently from the lower Green Tuff, indicate that the Green Tuff volcanism took place in the Daijima-Nishikurosawa age in many places of the inland region of the northeast Japan.
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