It is generally accepted that the slight unconformity or unconformable relation in the tephras indicates immediately the existence of period of erosion preceding the stabilization of the slopes, followed by the deposition of the next tephra layers. But, generally speaking, typical unconformable horizons in the Pleistocene tephra formations of the southern Kanto districts coincide with boundaries of marine sediments and also with the boundaries of the volcanic activities of both western volcanoes, Mt. Hakone and Mt. Fuji (UESUGI, 1976; KIKUCHI and KANTO QUATERNATY RESEARCH GROUP, 1977). Therefore, the authors are of the opinion that not only in the Pleistocene tephra formations but also in the Holocene tephra formation, considerable number of unconformable relations may be true unconformities.
In this paper, the unconformable relations in the Holocene tephra formation are described around the northern and eastern flanks of the Fuji volcano, located in the uppermost reaches of the two drainage basins of the Sagami and the Sakawa.
The localities of the typical outcrops are shown in Figure 1, and the schematic profiles of several typical outcrops where the unconformable relations are observed, are shown in Figure 2. Standard sequence of the Holocene tephra formation of this area is illustrated on the left column of Figure 3. On the right column, all the unconformable relations and other data are recorded.
The results are summarized as follows:
1) These unconformable relations are recognized in nine horizons as shown in Figure 3.
2) The unconformable horizons which are recognized in the both drainage basins are only three, that is, beneath the base of the Holocene tephra formation, and beneath the Yu-1 and Yu-2 tephras. In these horizons, buried soils underlain by the stabilized slope develops conspicuously and the tephra layers or lava flows overlying these soils are the greatest in scale among the Holocene volcanic activities of the Fuji volcano.
3) There exists no other unconformable horizons in the northern flank, and, on the other hand, in the eastern flank six horizons are recognized in addition. Those are beneath the S-0, S-1, S-5, S-15, S-24 and S-25 tephras.
The reason for this difference between the two drainage basins, is one of the problems for the further investigation.
