九州四万十帯日向層群の赤・緑色珪質泥岩とデュープレックス

The Paleogene Hyuga Group of the Aradani Area in East Kyushu is regarded as an accretionary complex in the Shimanto Terrane. The Group was formerly inferred to form a melange including various sized blocks of sandstones, red and green siliceous mudstones, and basaltic volcanic rocks. The siliceous mudstones are not blocks in the melange, but occur as a low-angled thrust sheet of 40m thick.
The Hyuga Group of the Aradani Area consists of four tectonic units; (1) lower chaotic unit, (2) siliceous mudstone unit, (3) mudstone unit, and (4) upper chaotic unit in ascending order. Each unit except for the siliceous mudstone unit forms a duplex structure, in which strata are imbricated, dipping 30° to 50° Each duplex is formed by mono-lithologic strata. The fundamental structure of the Paleogene Shimanto Terrane in Kyushu is characterized by low-angled nappe structures with stacked duplexes, in which duplexes lie one above the other, similarly to those of the Cretaceous Uchinohae Formation in South Kyushu.
Original succession of the Hyuga Group is (1) basaltic volcanic rocks, (2) red and green siliceous mudstones of Middle Eocene, (3) black mudstones of Late Eocene, (4) sandstones and alternating beds of sandstone and mudstone of Late Eocene to Early Oligocene. Chaotic beds of melange facies originate from all above-mentioned strata. The siliceous mudstones, which are considered to be hemipelagic sediments, were subducted, and then formed a thrust sheet during underplating. The siliceous mudstones were probably situated in the decollement horizon, but did not flow as a lubricant layer.