K-Ar dating was performed for volcanic rocks from the early Miocene Odose Formation, Fukaura-Ajigasawa area, Aomori Prefecture in northeast Japan to determine the age of the early Miocene transgression, prior to the Nishikurosawa transgression of these area, and to make clear an spatial distribution of marine sediments (>20 Ma) found in the Oga Peninsula, Akita Prefecture.
The Odose Formation is divided into three members: the Kiyotakizawa Andesite, the Azumagawa Rhyolite and the Kohamadatezawa Andesite Members in ascending order in the Ajigasawa area (Hirayama and Uemura, 1985), which are corresponded to the lower, middle(Sawabe volcanic rock) and upper part in the Fukaura area (Moritani, 1968).
Preliminary facies analysis of the Sawabe volcanic rock and the upper part of the Kiyotakizawa Andesite Member of the Odose Formation indicates that these volcanic sequences were deposited in the near shore environment, evidenced by the presence of sandstone with hammocky cross stratification and intrusion of spiracle into aa lava flows.
A high alumina olivine pyroxene andesite of the Sawabe volcanic rock has an age of 20.0±1.0 Ma which coincide with the age of 20.6±2.0 Ma reported by Fukudome
et al. (1990), and an alkali olivine basalt from the upper part of the Kiyotakizawa Andesite Member shows 19.4±1.1 Ma.
Because marine molluscan fossils were found by Hirayama and Uemura (1985) from the middle part of the Kiyotakizawa Andesite Member, the age from the upper part of the member constrains the upper limit a marine sediment age by early Miocene transgression in Ajigasawa area. Furthermore, the result of the present K-Ar age study indicates that the marine sediments in the area are well correlate with the sediments in the Oga Peninsula.
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