Stratigraphy and Geological Structure of the Usuki Area, Oita Prefecture, Kyushu (1)

Abstract

1) The Usuki area in eastern Kyushu belongs to the so-called Chichibu Terrain of the Outer Zone of Southwest Japan., This mountainous area comprises rocks of the Chichibu supergroup, largely of the Permian period and less so of the late Mesozoic., They are arranged in a number of parallel or nearly parallel belts of NEE-SWW trend and separated by tectonic lines., 2) From paleontological and other evidences, the respective ages of the formations are as follows., (Fossils listed in Japanese text) Mesozoic formations: Shinkai formation., ., ., ., ., ., ., ., ., ., ., ., ? ., ., ., ., ., ., ., ., ., ., ., ., Upper Jurassic Yamabu formation., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., Kochian (Kl., Lowest Cretaceous) Haidateyama formation., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., Miyakoan (K3., Upper part of Lower Cretaceous) Tano formation ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., Gyliakian (K4., Lower part of Upper Cretaceous) Palaeozoic formations: Those in the Chinnanzan tectonic belt., ., ., ., ., ., ., ., ., ., ., ., Partly Lower Permian Those in the Tsukumi tectonic belt., ., ., ., ., ., ., ., ., ., ., ., ., ., ., Lower-Middle Permian Those in the Meiji tectonic belt., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., Lower-Middle Permian Katauchi formation., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., ., Visean (Lower Carboniferous) ? Those in the Usuki, Nakano and Kashimine tectonic belts., ., ., ., ., ., ., ., ., undetermined 3) Each of the tectonic lines which separates the formations into the tectonic belts is not a simple fault but a narrow zone containing several faults accompanied with a sheared zone., Peculiar igneous and metamorphic rocks squeezed out along these tectonic lines are found frequently., They are leucocratic granite, schistose diorite-gabbro, quartz-porphyry, porphy-rites, hornblende-bearing biotite granite, gneiss and serpentinite., The structural features are similar to those in the Yatsushiro-Kuma area., (See T., MATSUMOTO and K., KANMERA, 1949)., 4) The Paleozoic rocks are generally intensely folded and locally metamorphosed, while the Mesozoic rocks are moderately folded., 5) It is noteworthy that an important tectonic epoch towards the mid-Cretaceous is indicated by the unconformity between the Haidateyama (K3) and the Yamabu (K7) and the Paleozoic formations., The post-Cretaceous (or at least post-Gyliakian) crustal movements also play an important role in the development of the geological structure of the Usuki Area.,