Geomorphic Development of the Ena Basin, Gifu Prefecture

Abstract

The Ena Basin is located along the western margin of the central mountains, bordered by the bold Adera fault-scarp. Along the southern margin, the basin is separated by the Byobusan fault-scarp from the Mikawa highland, which descends abruptly from the central mountains with rather a small relief and similar altitude of 500-700 meters. On the other hand, the northern and western margins of the basin are obscure. The basin floor penetrates northward into the mountain-land along the valleys; in the west of the basin there extend vast flat-topped hills of about 400 meters. Therefore, the basin opens to the west. The Kiso River is cutting a deep gorge at the northern margin of the basin, but the floor of the basin is a hillland of rather low relief, composed of Toki gravel beds and partly of granitic rocks. It is concluded that the basin has been formed by down warping to the east with faulting NWN SES and ENE-WSW during and after the deposition of the Toki gravel bed, from the fact that the Kiso River cuts down a deep gorge in an antecedent course, and that the Toki gravel bed shows similar vertical changes of gravel size and contains lignite seams at almost the same altitude at every place. The uplands in the basin are subdivided into several terraces as follows (Fig, 5): 1: Alluvial plain 2-4: They are terraces and most of them have charcters of rock bench except the elevated fan along the Nakatsu River. It is very difficult to correlate each other for their irregular distribution and the absence of the key bed or gravel. 5 (Akatsuchidaira): They are higher elevated fans by the Nakatsu River and others, whose deposits were supplied from the Byobusan fault-scarp. 6: They are the erosion surfaces by the Kiso River which consist of only the Toki gravel beds; the Akatsuchidaira beds deposited on this surface. 7: This is pediment-like surface consisting of the Toki gravel bed and granitic rocks, and their features are very complicated. They were formed during and after the time of deposition from the Byobusan fault-scarp, but the foot of the scarp and the Akatsuchidaira have been disconnected by erosion since then.