Abstract
An alluvial plain develops along the head of Suruga Bay which is surrounded with Qu-aternary volcanoes such as Mts. Fuji, Ashitaka, and Hakone to the north, and volcanic groups in the Izu Peninsula to the east. This plain comprises the Tagata Plain, the Mishima Fan, the Kano River Delta, the Ukishimagahara and the Fuji River Fan. These areas have distinct nature in landforms owing to variety of the geologic and geomorphic conditions of respective drainage area (Fig. 1).
The aim of this paper is to clarify the stratigraphy and the ;distribution of the recent for-mations in each area, and to reconstruct their palaeo-sedimentary environments. This study is based on the geological discriptions of borehole logs, the observations of core samples, foraminiferal analysis of core samples and the results of radio carbon dates. Then the geo-morphic development of the region was considered on the basis of correlation of the recent formations among these sub-regions.
The Tagata Plain is a valley plain situated in the middle to lower reach of the Kano River. The recent formations in the Tagata Plain are composed of basal sand and gravel (B), lower fluvial deposits (L), middle clay (M), upper pumiceous sand and gravel (U) and top alluvial deposits (T). The basal sand and gravel is considered to represent the former river bed gravel. The middle clay deposited under the low salinity environment, and it is divided into two parts of marine clay (MC) in the lower and brackish clay (BC) in the upper. The pumice contained in the pumiceous sand and gravel (PG) was derived from the pumice fall and flow deposits ejected at about 3, 000 years ago from Kawagodaira volcano located in the upper reach of the Kano River. The Mishima Fan is situated in the lower reach of the Kise River which flows from the east foot of Mt. Fuji. The fan is consisted of the Mishima lava flowed from Fuji volcano at about 10, 000 years ago and fan gravel (FG) covering the Mishima lava. Marine sand (MS) is distributed in the south end of the fan area. The Mishima lava abuts on the foot of the Shizuura Mountains to the south, and a narrow pass was formed between the Mishima lava and the Tertiary rocks of the Shizuura Mountains (Fig, 7).
The Kano River Delta develops in the mouth of the Kano River flowing into Suruga Bay. The delta is composed of marine gravel (MG), humic clay (HC) and deltaic sand (DS) from the bottom to the top.
The Ukishimagahara is a swamp distributed along the southern foot of Fuji and Ashitaka volcanoes, and the southern side is bounded by a coastal barrier formed of sand and gravel which were mainly derived from the Fuji River. In this area, peaty clay (PC) forming the swamp was underlain by marine gravel (MG). The height of a subsurface of MG becomes deeper towards the flanks of Mt. Ashitaka.
The Fuji River Fan is distributed from the Tagonoura area to the mouth of the Fuji River.
The bases of the correlation among the sub-regions are as follows.
1. In the N-S section of the Mishima Fan (Fig, 7), marine sediments were restricted in the narrow pass between the Mishima lava and the Shizuura Mountains. This reveals that the sea water invaded into the Tagata Plain area through the narrow pass after the Mishima lava had flowed out.
2. Brackish clay (BC) in the Tagata Plain, humic clay (HC) in the Kano River Delta and peaty clay (PC) in the Ukishimagahara are inferred to be the sediments 'under the common low salinity environment during the same period (ca. 4, 500-3, 500 yr B. P.).
The coastal barrier consisted of marine sand and gravel (MG) blocked up the landward area at about 4, 500 yr B. P.
3. The supply of coarse materials had increased since 3, 000 years ago in both the Tagata Plain and the Mishima Fan regions. Pumiceous gravel (PG) was accumulated in the Tagata Plain in association with the eruption of the pumiceous fall and flow from Kawa godaira volcano at about 3, 000 yr B. P..
