南部北上帯達曽部地域のペルム系層序の再検討

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A revised scheme on the Permian stratigraphy of the Tassobe district, northern part of the South Kitakami Terrane, NE Japan, is presented. The Permian is divided into the Tassobe and Sotokawame Formations in ascending order. The Tassobe Formation overlies unconformably on the Middle Carboniferous Ohkawame Formation, and consists of two members. The lower member is dominated by coarse elastics such as sandstone and conglomerate. The upper member is represented by prevalent mudstone and limestone which yields abundant fusulinid fossils. Considering with Hizume-Kesen'numa Fault (Ehiro, 1977), the Tassobe Formation appears the equivalent of the Nakadaira Formation of the Ochiai - Nakadaira district, both in lithological and biostratigraphical features. The Sotokawame Formation develops conformably on the Tassobe Formatone, and is constructed from the turbidite sequence of sandstone, mudstone, conglomerate, and the alternation of sandstone and mudstone. The Permian strata distributed here have been formerly divided into two formations, the Lower Permian Tassobe and the Middle Permian Kurnonoueyama Formations, mainly based on the difference in lithofacies (Hirokawa and Yoshida, 1956). The Kurnonoueyama Formation has been correlated with the Kanokura Formation in the Setamai district. But our re-examination has revealed the following two facts contradictive to the above stratigraphic classification and correlation. First, the Lower Permian fusulinid Pseudoschwagerina sp. was obtained from the limestone at Mt. Kurnonoueyama which is the type locality of the Kurnonoueyama Formation. Second, the lithofacies of the Tassobe Formation and the type Kurnonoueyama Formation are very similar with each other, and two formations can not be distinguished. Therefore, we concluded that the Kurnonoueyama Formation in the type locality is just equivalent to the Tassobe Formation. Consequently, the strata developed conformably on the Tassobe Formation should not be termed as Kurnonoueyama Formation, but are redefined and renamed as the Sotokawame Formation. As for the sedimentary environment, the Tassobe Formation indicates generally deepening upward sequence started from shallow marine facies. The sequence is succeeded by carbonate facies in middle part, and ended by mudstone facies. The sedimentary facies of the Sotokawame Formation suggests the hemipelagic environment. Therefore we conclude that the studied area was subsiding depressions during early to middle Permian time.
地学団体研究会の論文
1992-03-25

南部北上帯達曽部地域のペルム系層序の再検討

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