Petrogenetic consideration on the lower formations in the oil-field of Northeast Japan (First Report)

Abstract

As is mentioned in the previous reports, the lower parts of the Tertiary sediments in the Yokote and Shinjo basins are represented by the “green-tuff formation” which is characterized by the distinct volcanic activities in the sequense of the earlier stage of the Miocene transgression in northeastern Japan. And the upper green-tuff formation in these area are represented by the Takasegawa and Kaneyama Formation in the Yokote and Shinjo basin respectively. The petrographic observtions of the volcanics and the pyroolasts assoclng with the Takasegawa and Kaneyama Formation are given here, paying attention to the peculiarities of the volcanic activities involved in those formations both lithologically and stratigraphically. As for the volcanics, those are related with each other in the mode of occurrences as well as In the distributions as rock units, forming locally impcrtant elements of eacn formation. Several rock types are found among the volcanics as shown in Tab. 16. The lithologic characters of the pyroclastic rocks predominationg in the formations as acidic tuf fmembers are shown in Tab. 15. These pyroclasts are found to show harmonious developements with the volcanics of the acidic or intermediate rock type, and the local pecuriality of the acidic volcanic activity is shown much more distinctly.When the distributions of the vocanics are observed horizontay (in space), either the acidic or the basic rock type is found to show marked developement alternately from the northern part of the Yokote basin to the southern part of the shinjo basin, constituting several volcanic petro-provinces as shown in Fig. 2. In other words, there found local differrences in the ratio of the developements of each rock type, however, in spite of the local predominance of the distribution of some rock type, each formation may have connection with each other as a whole by involving nearly all of these rock types, On the other hand, when observed vertically (in stratigraphic successions), the acidicmernbers are found generally in the upper parts of those formatiens and the basic ones in the lower parts. This relation is commonly found as a regularity of the volcanic actions throughout the whole area. It is not always a transitional change from basic to acidic on a smooth line, however, but an alternative change comming off and on from basic to acidic which becomes acidic as a whole in the upper parts. The relations are clearly observed in Fig. 2.