FORMATION OF EROSIONAL VALLEYS IN THE VICINITY OF MT. DAISEN, WESTERN JAPAN

Abstract

Volcano Daisen is surrounded by many uplands which are composed of pyroclastic fluvial gravels derived from lava and pyroclastic flows. They are classified into the Higher Terraces I, II, III, Middle Terraces I, II, III, Lower Terraces, deposition surface of the Old Pyroclastic Flow, Nawa Pumice Flow and New Pyroclastic Flow (Fig. 1). Formative ages of these uplands range in time from the Middle or Late Pleistocene to the Post-Glacial Epoch. They are covered with falling ash layers. Their thickness reflects the formative ages. The ash layers are called D 1, D 2, ……, D 23 from top to bottom. Age of the horizon of D 12 was dated by 14C method to be 30200±3500 and that of D 3 was to be 17200±400 and 18200±740 years P. P. (Fig. 2). The uplands are dissected by valleys in various degree. Observation of clino-unconformities in falling ash layers on slopes of these valleys indicates relative ages when the slopes were formed (Fig. 3). Figure 4 was made by collecting data around Mt. Daisen. It shows that formative ages of the valley slopes are concentrated at several horizons in the columnar section. It is not probable that such concentrations were formed by several discontinued ash falling. Therefore, the valley slopes can be considered to have been formed in definite periods, which are called the (1)st, (2)nd, ……, (5)th period in order of formation. The and period began with the stage of D 17, which was considerably changed into red soils. The (4)rd period began 30000 years ago and the (5)rd period 17000-13000 years ago. It cannot be simply concluded that the climate in these periods was cold or warm. Next, the valley slopes are composed of steep slopes, more than 20° and gentle ones, less than 20°. The steep ones have resulted from slumping and gully erosion, which make valleys larger and deeper. The gentle ones were made by some agencies which make valleys shallower and gentler (Fig. 5) . Distribution of the steep and gentle slopes are as follows. On the uplands of the younger stage, the valley slopes are almost composed of the steep slopes. On those of the middle stage, the gentle slopes appear on the upper reaches and top of valley walls on the middle and lower reaches, and the steep slopes are predominant along the bottom of valleys on the middle and lower reaches. On the old uplands, the gentle slopes are predominant along the whole course of valleys. Regularity of distribution like this sugg-ests that each valley is developed through the process similar to the above. On this point, it is observed as follows. Valleys on the Lower Terraces cut D 1 and are accordant with the present sea level. Therefore, they have been formed since the stage when sea level approached the present heisht. Valleys on the New Pyroclastic Flow cut D 3, 2 and 1. The gentle slopes begin to be formed at the heads of some valleys. Valleys on the uplands of the middle stage, for instance, those on the Nawa Pumice Flow, were formed in the (4)th and (5)th period. In the (4)th period, valley systems showed roughly the same appearance as present. They were rejuvenated by slumping and head-ward erosion in the (5)th period. Examples of valles on the old uplands of the old Pyroclastic Flow are shown in Figure 6. The figure shows that valleys had been developed in roughly the same appearance as the present one in the (3)rd period on the lower reaches. Headward and lateral erosion revived in the (4)th period on the upper and middle reaches. On Higher Terrace I on the left bank of the Hino River, on an upland of the old Pyroclastic Flow at the northwestern foot and so on, valleys of the gentle slopes were formed in or before the (3)rd period, since when the valley slopes were scarcely rejuvenated. Generally speaking, most valleys on the old uplands, especially those deeper than 15m, were formed in or before the 3rd period.