Abstract
Thick piles of basalt of middle Miocene Aosawa Formation are spread extensively in the underground of the Yurihara Oil and Gas Field, back-arc side of the NE Japan arc. Another nearly correlative basalt (Taikura Basalt Member of Hatamura Formation) is exposed locally on a hilly land to the east of the oil and gas field. These two basalts show some significant difference in thickness, lithofacies, environment of eruption and petrological characteristics.
The basalt of the Aosawa Formation erupted on the subsiding submarine basin and consists of relatively undifferentiated (total FeO/MgO=0.9-1.4) tholeiitic basalt. It is similar in mode of occurrence and petrological characteristics to other correlative basalts within the Akita-Yamagata oil field basin in the back-arc region of the NE Japan arc. On the other side, the Taikura Basalt Member was accumulated on land or in shallow water, and is slightly differentiated and rich in TiO2, K2O and light REE compared with the basalt of the Aosawa Formation. Both the basalts have some affinities in mode of occurrence and petrological features for the tholeiitic and alkalic basalt in a rift zone such as the Rio Grande rift. Both the basalts were probably formed in a back-arc rift zone during middle Miocene age.
