秋田県矢島地域の女川層炭酸塩岩に関する堆積地化学的知見

抄録

The carbonate rocks are abundantly distributed in the Miocene Nadakayama (Onnagawa) Formation of the southeastern part of the Yashima district, Northeast Japan. In order to obtain some informations on the sedimentary geochemistry of the Neogene carbonate rocks, 15 carbonate rocks collected from 23 carbonate horizons found in an outcrop of the Onnagawa siliceous mudstone sequence of 45m in thickness were examined through the microscopic observation, X-ray diffractometry and chemical analysis of organic matter. Also, 12 siliceous mudstone sampled in stratigraphic sequence of the same outcrop were examined for comparison with the data of the carbonate rocks.
The carbonate rocks are generally dolomitic and occur as bed or large lense, or nodule in siliceous mudstone. Sometimes, micronodules are found in bedded carbonates. Sedimentary structures such as graded bedding and laminations, and tracefossils are occasionally observed in those carbonate rocks and also in siliceous mudstone. The results of X-ray diffractometry and chemical analysis of organic matter are summarized as follows.
The carbonate rocks are composed of dolomite, quartz, feldspar, montmorillonite, illite, chlorite and concomitant glauconite and pyrite. The relative abundance of clay minerals in clay fraction under 2am and of pigments in carbonate rocks resemble those in siliceous mudstone. Quartz and total organic carbon are more abundantly and dolomite are less contained in bedded carbonate rocks than in nodular carbonate rocks. Quartz and total organic carbon are contained in siliceous mustone three times as much as in carbonate rocks. The stratigraphic variation of the amount of quartz and of total organic carbon in carbonate rocks, however, resemble those in siliceous mudstone, and the stratigraphic variation of the amount of dolomite in carbonate rocks seems to be in inverse relation to the one of the amount of quartz in siliceous mudstone.
On the basis of those results and other informations, some considerations on the depositional environment and the formation process of these rocks were also made.