Distribution and Origin of the Neogene Carbonate Rocks in the Akita Oil Fields, Japan

Abstract

Since 1967, the writers have studied the stratigraphical and geographical distributions of carbonate rocks of the Funakawa, Onnagawa and Nishikurosawa formations of the Neogene in the Akita oil fields, Japan. They considered the origin of these carbonate rocks from the geological and mineralogical points of view. The results are summarized as follows:
1. Stratigraphically, these carbonate rocks are generally distributed in the following formations;
uppermost horizon of Nishikurosawa formation
uppermost horizon of Ormagawa formation
lower horizon of lower Funakawa formation
uppermost horizon of upper Funakawa formation
Geographically, the carbonate rocks are remarkably distributed in the Onnagawa and Nishikurosawa formations along the southern coast of Oga peninsula, in the Nishikurosawa formation of the eastern part of Kameda-machi and in the Onnagawa formation of the Southern part of Yajima-machi.
2. The detrital substances derived from the inorganic and organic matters are the main source materials for these carbonate rocks, while the chemical deposits from the sea water occupy only a part of the carbonate rocks. Depositional environments of these rocks are ranged from the inner-neritic to bathyal conditions.
The supply of both calcium and magnesium is necessary for the formation of carbonate rocks from the source materials because of the poorness of these elements in them. Concerning the carrier waters of calcium and magnesium and the geologic stages of supply of these elements, it is considered that the supply of calcium and magnesium from the salt water originated from the earlier lithification during the period of syn-diagenesis is more effective to the transformation of source materials than the supply of these elements from the water originated from the ana-diagenesis, epi-diagenesis and hydrothermal alteration. Also, the additional supply of these elements from the volcanic detritus in the sea may sometimes be very important.
In the studied area, the formation of chemical deposits is very rare, and genetically, this formation is mainly controlled by the following two factors; one is the abnormal increase in magnesiun and calcium contents of the sea water caused by the supply of large amount of volcanic detritus in the sea, and the other is the temperature rise of the sea water caused by the same phenomenon.