SOME PROBLEMS ON THE FORMATION OF TOKI SURFACE (EARLY PLEISTOCENE) IN TONO DISTRICT AND NORTHWESTERN MIKAWA PLATEAU, CENTRAL JAPAN

Abstract

In Tono District and around Mt. Sanage, there extend widely hilly uplands composed of gravelly layers so called Toki Gravel Beds and Mizuno Member of Yadagawa Formation. Their summit plane shows notable accordance and is called as Toki Surface and Fujioka Surface (Machida et al., 1962: Kiso, 1963: Kaizuka et al., 1964) . In Mashizume and Yamaoka Basins located between Byobusan and Enasan Faults, Tazawa Gravel Beds are deposited as a thick layer (Itoigawa, 1955). These gravel beds are geographically separated each other by high fault scarps (see Figs. 2 and 3). Although they have been tentatively correlated each other, their correlation was based on qualitative inference by comparison with their fades. The purpose of our study is to make clear the stratigraphic relationships between them on a quantitative basis, to infer the supply route of sediments, and furthermore, to discuss the crustal movement after the formation of Toki Surface.
Because it is very difficult to make exact correlation between gravel layers, the following idea and methods were applied in this study. If both Toki and Fujioka Surfaces are correlated each other, the areal changing pattern of their facies such as lithologic composition, grain size and fabric orientation of gravel should show gradual change from the upper reach to the lower, regardess of the faults. If not correlated, in reverse, they should make discontinuity. The authors, then, selected many outcrops close to the summit plane of hills and measured (1) lithologic compositions, (2) maximum grain size of Nohi rhyolite and chart gravel which occupy greater part of component gravel, and (3) fabric orientations of gravel. Spatial distributions of litholgic composition of the two kinds of gravel (see Fig. 4) show that component proportions of Nohi rhyolite gravel, generally speaking, reduce gradually from the north to the south, but those of chart gravel increase southward. Their spatial pattern shows gradual change without any discontinuity by the faults. Areal distribution of maximum grain size of Nohi rhyolite gravel (Fig. 5) shows approximately the size reduction from the north to the south and we cannot recognize an abrupt change along the faults. These facts, therefore, imply that the gravel beds in the northern areas are correlated to those in the southern areas, and both of them are the same stratigraphic bed layers deposited at the nearly same time. Paleocurrents infering from fabric orientations shows roughly southward orientations. This fact supports the above interpretation.
As alluvial bed gravel is deposited rotating around its long axis, the long axis of graveldeposited widely on the top of hills and are correlated to Toki Gravel Beds stratigraphically.
Toki Surface has been defined as depositional surface of Toki Gravel Beds. But there extends widely erosional low-relief surfaces such as the northern part of Kiso River and the southern areas of Enasan Fault. In these areas, we can find gravel beds which cover the top of hills as a veneer in some places or which do not in other places. As the ero=sional surface of Toki changes continuously to the depositional surface and both erosional and deposional surface contact with each other at same level, the Toki Surface has probably been formed by lateral planation at the last stage of deposition of Toki Gravel Beds.