2016 Volume 125 Issue 3 Pages 325-351
The litho-biostratigraphy of Paleogene coalfields in northwestern Kyushu, Japan is reviewed on the basis of data obtained from the Miike Coalfield below the Ariake Sea. The Miike Coalfield extends over a wide area below the Ariake Sea as far as the west coast. The Paleogene stratigraphy of the coalfield is currently defined as the Ginsui Formation and the Ohmuta and Manda Groups in ascending order. The Manda Group consists of the Kattachi and Yotsuyama Formations in ascending order. The Kattachi Formation contains large quantities of glauconite in the basal part, the base of which has been defined as the base of the marine Manda Group. However, the Kattachi Formation contains coal seams and varies laterally, with glauconite content decreasing westerly. No significant facies variation with the underlying Ohmuta Group can be seen. The Kattachi Formation is, therefore, re-interpreted as the uppermost zone of the underlying coal-bearing Ohmuta Group. Glauconite sandstone of the basal part of the Yotsuyama Formation overlies underlying strata with a clear boundary throughout the coalfield. No significant facies variation exists in the Ohmuta Group. As a result, the Paleogene stratigraphy of the Miike Coalfield is revised into the Ginsui, Miike and Yotsuyama Formations in ascending order. The marine Yotsuyama Formation reaching ca. 800 m in thickness below the Ariake Sea is divided into four lithostratigraphic members: Ia (glauconite sandstone), Ib (shale), IIa (glauconite sandstone), and IIb (shale) Members in ascending order. The Yotsuyama Formation spans the late Middle Eocene (Subzone CP14a of Okada and Bukry, 1980) to the late Late Eocene (Subzone CP15b) age. Marine sequence, biostratigraphically correlative with Ia to IIb Members, is distributed in the Takashima and Amakusa Coalfields, while marine strata correlative with IIb Member are distributed in the Sakito-Mastushima and Isahaya Coalfields. These facts indicate that marine transgression occurred from the south and that a marine environment prevailed in northwestern Kyushu in the late Middle to late Late Eocene and peaked during the Yotsuyama Formation deposition.
