秋田市北方の新第三系笹岡層の貝化石群集と堆積環境

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The northern part of Akita City is well known for its location of the type localities of the Tentokuji and Sasaoka formations which are the standard stratigraphic units of the Neogene sequence in the Akita Oil-Field. The upper Cenozoic sequence developed in the area is divided into seven stratigraphic units ; the Koarashiyama Formation, Aizen Basalt, Mizusawa Formation, Manaitayama Volcanics, Tentokuji Formation including Futatsumori Sandstone, Conglomerate and Siltstone Member, Sasaoka Formation, and Terauchi Formation in ascending order. The Koarashiyama and Mizusawa formations, and the Futatsumori Sandstone, Conglomerate and Siltstone Member of the Tentokuji Formation are newly named by the author in this study. The Koarashiyama Formation is composed of hard mudstone and corresponds to the Onnagawa Formation of Hase et al. (1970) except for its uppermost part. The Mizusawa Formation is characterized by black mudstone and is equivalent to the uppermost part of the Onnagawa Formation and the lower to middle part of the Funakawa Formation of Hase et al. (op. cit.). The Tentokuji Formation, which corresponds to the upper part of the Funakawa Formation and the lower to middle part of the Tentokuji Formation of Takayasu (1961), consists mostly of dark green siltstone and intercalates thin tuff beds. The Futatsumori Sandstone, Conglomerate and Siltstone Member, which is equivalent to the Katsurane Formation of Takayasu (op. cit.), is composed of alternating beds of coarse-grained. sandstone, conglomerate and siltstone. The Sasaoka Formation includes the upper part of the Tentokuji Formation of Takayasu (op. cit.). The brown coloured ill sorted sandstone as described under the name of the "Shibikawa Formation" by Hase et al. (op. cit.), distributed around Yomoginaizawa, Gojome Town, are included in the Sasaoka Formation in the present study. The divisions mentioned above differ from those in the previous studies in the following respects ; 1. the upper part of the Tentokuji Formation in the previous authors is here included in the Sasaoka Formation owing to the similarity of the lithofacies, and 2. the names of the Onnagawa, Funakawa and Katsurane formations hitherto used in the studied area are abandoned, because the formations cannot be continuously traced into the studied area which is far from the type localities of the mentioned three formations. The Sasaoka Formation is mainly composed of silty sandstone, fine- to medium-grained sandstone, conglomerate, and fine tuff. The sediments change upwards, in general, from fine- to coarse-grained. The lower and middle parts of the formation consist mainly of fine-grained sandstone and silty sandstone, however, at the middle of the studied area it contains intercalation of conglomerate beds. The upper part of the formation is composed mostly of fine- to medium-grained sandstone. The uppermost part of the formation at the northernmost area consists of medium- to coarse-grained sandstone. Molluscan fossils were collected from more than 100 localities. At each locality, rock sample for grain-size analysis was also collected. Then, the characteristics of sediments, the mode of occurrence of molluscan fossils, number of individuals, number of species, and combination of species were examined. Grain-size analysis was made by an automatic grain-size analyser (Niitsuma, 1971). The Sasaoka Formation can be classified into six lithologic types based on the characters of grain size frequency curves. Based on the specific composition of molluscan remains at each locality, 14 assemblages are recognized in the Sasaoka Formation. These assemblages well correspond to lithologies as follows. The Umbonium akitanum-Glycymeris yessoensis assemblage is found in a poorly sorted sandstone with conglomerate. The Macoma calcarea-Felaniella usta assemblage and Acila nakazimai-Limopsis assemblage are in sandstone of low mud contents. The Acila-Macoma calcarea assem-blage and the Turritella saishuensis-Acila nakazimai assemblage are entombed in sandstone of high mud contents. Three assemblages-the Macoma-Anadara amicula-Felaniella usta-Mizuhopecten yessoensis assemblage, Lucinoma-Paphia amabilis assemblage, and Fabulina nitidula-Callithaca adamsi-Anadara amicula assemblage-are characterized by fine-grained sandstone with mud contents of less than 40 percent. The other molluscan assemblages are in very fine- to fine-grained sandstone. These molluscan assemblages were recognized from the following stratigraphic and geographic positions (Fig. 11) : Lower part of the Sasaoka Formation ; Umbonium akitanum-Glycymeris yessoensis assemblage, Macoma calcarea-Felaniella usta assemblage and Macoma calcarea-Conchocele bisecta assemblage Middle part of the formation ; Mya cuneiformis-Clinocardium chikagawaense assemblage and Yoldia notabilis-Clinocardium assemblage (around Tomita, Gojome Town in the northernmost of the studied area), Acila-Macoma calcarea assemblage and Turritella saishuensis-Acila nakazimai assemblage (along the Babame River in the north of the area), and Acila nakazimai-Limopsis assemblage (along the Bafumi River in the center of the area) Upper part of the formation ; Glycymeris yessoensis- Yoldia notabilis assemblage (along the Babame River), Mya cuneiformis-Acila nakazimai assemblage (along the Bafumi River), and Turritella saishuensis-Limopsis crenata assemblage (around Sasaoka, Akita City). Uppermost part of the formation ; Macoma-Anadara amicula-Felaniella usta-Mizuhopecten yessoensis assemblage and Fabulina nitidula-Callithaca adamsi-Anadara amicula assemblage (around Yomoginaidai, Gojome Town), and Lucinoma-Paphia amabilis assemblage (around Sasaoka, Akita City) Judging from changes in lithofacies and molluscan assemblages, the lower part of the Sasaoka Formation was deposited in the upper neritic zone, the middle part in the lower neritic zone, and the upper part in the upper neritic and/or littoral zone.
東北大学の論文
1984-02-29

著者

島本昌憲
東北大学博物館

秋田市北方の新第三系笹岡層の貝化石群集と堆積環境

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