The Shimato Terrain in the Kii Peninsula, Southwest Japan : with Special Reference to Its Geologic Developments Viewed from Coarser Clastic Sediments

Abstract

The Shimanto Supergroup in the Kii Peninsula is divided into two groups, i.e. the Hidakagawa Group in the northern part and the Muro Group in the southern part. The former is mainly Cretaceous and the latter is early Cenozoic. In this paper conglomerates and sandstones of the two groups are treated with, and several considerations on the geologic development of the Shimanto Terrain are given. In the Cretaceous the Shimanto Terrain was in a eugeosynclinal condition, while in the early Cenozoic it changed to be in a miogeosynclinal condition. The change of environment is shown in the differences of megascopic characters of clastic sediments and properties of sandstones. The provenances of the Shimanto Terrain can be generally sought to the north of the geosyncline, that is, the so-called Meso-Volcanics in the Inner Zone, various rocks in the Ryoke Metamorphic Belt, the Paleozoics in the Chichibu Terrain and the so-called Torinosu Group. After the close of the Shimanto geosyncline a marine transgression occurred again at the Middle Miocene, and formed the Kumano and Tanabe Groups. Their basins were smaller and more local in comparison with the Shimanto geosyncline. Clastic materials were supplied mainly from the Sambagawa Metamorphic Belt and the Muro Group. Orthoquartzite gravels were commonly found from the conglomerates in the southernmost district of the Muro Group. They are supermature orthoquartzites with excellent sorting and rounding. These rocks cannot be seen in the present Japanese Islands and furthermore the paleocurrent data of this district show apparently from south to north direction. Such being the case, it may be reasonably concluded that a missing land once existed to the south of the Shimanto geosyncline where nowadays is the Pacific Ocean.