The Journal of the Geological Society of Japan

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(1) Good displays of the Cretaceous formations are found in the Amakusa islands, Mihune district and Onogawa basin which are aligned from west to east in central Kyuyu., Of them those in the first and the third have been already reported by the writer in his previous papers., Since then he has investigated the geology of the second area, with special attention on the geologic phenomena of the Cretaceous period and the result of this survey is described in this paper., (2) The Cretaceous deposits of the district for which he proposes the name of the Mihune group are a stratigraphic unit and are divisible into three minor units as follows : a., Basal formation consists of moderately thick basal conglomerate and other coarse grained deposits 300 m., think in the northern part and of ruddaceous sandstone 50 m., thick in the southern part ; red beds are intercalated at several horizons among these deposits., b., Lower formation, 450 m., thick in the northern part and 700 m., thick in the southern part, consists mainly of sandstone and siltstone deposited in a shallow sea and contains bivalves and gastropods abundantly (see table 1); coal seams are intercalated at places., c., Upper formation, about 750 m., thick, consists of red or variegated fine grained sediments comprising dark green sandstone and siltstone, and siliceous acidic tuff is intercalated at several horizons., Plant remains are found in the sandstone at some places., (3) That the Mihune group can be correlated to the Gosyonoura group in the Amakusa islands and that both of them are middle Cretaceous (or Gyliakian in main), but not lower Cretaceous as has been regarded by previous investigators, are warranted by the palaeontological and stratigraphical evidences., (4) The group has been deposited on the basement of crystalline schists and Palaeozoic sedimentary rocks with a marked unconformity at its base., Both basement complexes are strongly folded and intruded by granitic rocks., The Palaeozoic group can be classified into two, namely, one composed of green rock, phyllite, quartzite and limestone and another of Permian slaty shale with conglomerate, sandstone and lenses of limestone., Judging from the constituents of coarse grained deposits of the Mihune group it is presumed that a part of their material was supplied from a terrain where porphyrite and its allies were extensively exposed under denudation., (5) Structure of the Cretaceous strata is "Bruchfaltung" as is indicated in geologic map and profiles., (6) Through the studies on the Cretaceous formations in central Kyusyu, the writer endeavoured to clarify the relation between two tectogenese, -early Cretaceous Oga movement in the Inner Side and middle Cretaceous Sakawa movement in the Outer Side., As a result of these studies it is determined that central Kyusyu has been a consolidated landmass and maintained a relatively stable condition during and after the time when the Sakawa movement was taking place on the Outer Side., However a certain epeirogenesis took place in this region, introducing a parageosyncline where the middle Cretaceous deposits were accumulated., The succession of these deposits seems to reveal a complete cycle of sedimentation.,

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This is a brief preliminary report with special reference to some new facts found very recently in the Palaeozoic of the Akiyosi District., I) The Palaeozoic of the Akiyosi district is divided by the late Y., OZAWA into three complexes in ascending order : 1) graywacke beds, 2) Akiyosi limestone and 3) shale, sandstone and chert, each with small lenses of limestone., All these complexes belong to the Permian and Carboniferous, according to the fossils they contain., II) The Akiyosi limestone is probably not upset as a whole as once thought by OZAWA, but in part in normal order., III) The graywacke beds and shale beds are in normal order and not upset, so far as evidences shown by cross bedding are concerned., IV) The Akiyosi limestone and graywacke beds are probably in contact by a fault with a general trend NE-SW, coinciding with one of chief tectonic lines in other districts of Tyugoku., OZAWA's well known interpretation of the geological structure of the Akiyosi district seems to me to need certain modification on the several new facts cited above.,

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