The subject of this paper is to establish the stratigraphy of deposits composing to Makinohara Upland, and also to discuss the landform evolution of the Makinohara Upland. The results are summarized as follows: (1). The terrace-forming deposit is divided into three beds, i.e, Furuya mud bed, Kyomatsubara sand bed and Makinohara gravel bed in ascending order. (2). The Furuya mud bed accompanied with the basal gravels is mainly composed of marine silt and clay filling the buried valleys (Fig. 10 and Tab. 1). The main part of the Furuya mud bed deposited in the narrow drowned valleys, during the early stage transgression, and covered uncomformably the Neogene semiconsolidated rocks. (3). The Kyomatsubara sand bed newly defined by this author was widely deposited in this mapping area as foreset beds at the maximum phase of the transgression (Fig. 9). (4). The Makinohara gravel bed mainly consists of widespread homogeneous fluvial gravels, the upper part of which is boulder gravel forming the alluvial fan named Makinohara. (5). Five horizons of buried topographic system are found under the Makinohara Upland; Buried surface I, II, III and IV (Figs. 8, 10) and Buried valley floor. (6). The Buried valley floor is the oldest topographic system filled with the Furuya mud bed. Two horizons of abrasion platforms (Buried surface I and II) are found cutting the slope of buried valley systems. The Buried surface I was formed just after the deposition of basal gravel of Furuya mud bed. The Buried surface II, on the Neogene strata was formed during the accumulation of upper mud part belonging to the Furuya mud bed (Fig. 10). (7). The Buried surface III covered with the Kyomatsubara sand bed is the most extensive among the three buried abrasion platform cutting the Neogene strata (Fig. 8 and 10). The level of the boundary between the Furuya mud bed and the Kyomatsubara sand bed nearly corresponds with that of the Buried surface III. (8). The Buried surface IV is the basal topography of the Makinohara gravel bed. Finally extensive rivers, laterally eroding the valley slope flew down onto the newly emerged coastal plain and slightly cut the Kyomatsubara sand bed (less than 5m) (Fig. 10). At that time, the Buried surface IV was formed as a basal topography of the Makinohara gravel bed. (9). The Makinohara Upland was constructed as alluvial fans by the Paleo-Oi River. The deposits forming the Makinohara Upland are defined to the Makinohara gravel bed.
A reversed geomagnetic polarity episode in the late Brunhes epoch has been revealed in Keno- and Kogashira pyroclastic flow deposits distributed in Kagoshima City, South Kyushu, Japan. Considering the stratigraphic relations along with the fission track age determination, this episode could be correlated with the Blake polarity event. The virtual geomagnetic pole position of the Blake event can be estimated from the thermoremanent magnetization of this terrestrial pyroclastic flow deposits. The result supports the possible hypothesis of the centered dipole reversals of the Blake event, against the alternative hypothesis of the local reversal phenomena which were caused by a certain eccentric reversed dipole (DENHAM, 1976). Taking the Blake datum level in addition to radiometric age data as markers, a reliable chronostratigraphy is introduced. Such a chronostratigraphy is plausibly consistent with both the δ18O record of a Caribbean core P6304-9 and the most remarkable sea level changes known in South Kanto District, Japan. It is suggested that the late Pleistocene stratigraphic successions in this area were mostly controlled by the latest interglacial-glacial sea level changes (Fig. 4).
In this paper, some problems of the late Quaternary geohistory of the Japanese Islands were discussed, basing upon the following results of marine investigation with topographical and sedimentological data on the straits of the Japanese Islands. 1. Charts show that wherever sizeable inland sea are separated from the ocean by narrow straits, deep hole (sea caldron) exists either in the narrows or directly adjacent them. The Korean strait (Fig. 5) is a part of the shelf, although there is a relatively deep hole (to -220m) along the west side of Tsushima Island. The depth of this deep hole attains 100m below the surrounding relict wave cut terrace (-120 to -130m). In this case, the depth of the relict wave cut terrace may reflect the sea level (about -110m) at the time of the Korean strait formation. 2. From the detailed survey of the submarine topography in the Tsugaru strait (Fig. 6), six submarine terraces were found on the sill. It is noteworthy that the depth of those terrace on both areas of the Cape Shirakami and Tappi is nearly the same (Fig. 9). This fact suggests that there has been no distinct crustal movement after the building of those terraces on the sill of the strait. Among the submarine valleys developed on the continental shelf, one of which continued to a valley on land can be detected on the eastern part of the Cape Shirakami (Fig. 7). From these circumstances of submerged terraces and valley about 80m depression of sea-level at the maximum Würm is deduced. 3. The breadth to depth ratio of the straits around the Japanese Islands (Fig. 10) seems to reflect each still standing sea-level stage, such as; -100±10, -80±5 and -45±5m, after the formation of these straits. 4. According to radiocarbon ages of shallow-water shells and peat obtained from the continental shelf and coastal plain around the Japanese islands (Fig. 12), sea-level at the maximum Würm (17, 000 to 20, 000 y.B.P) was about 80m below the present level. Therefore, those results agree with the evidence from the topography of the straits around the Japanese Islands. Fig. 11 shows the coastal geography of the Japanese Islands during the low sealevel time of 80m below the present level. 5. In most of Pleistocence, the Japanese islands was connected with the Korean Peninsula and present major islands themselves were tied to each other. It is sure that large mammals such as elephants migrated into the Japanese islands through land bridges. In the early Shimosueyoshi transgression when the sea-level was about -100m (Riss -Würm interglacial period), the Japan Sea connected with the Pacific through narrow passages located in the Korean and Tsugaru straits. At the time of maximum Würm when the sea-level was depressed to about -80m, the land bridges between Honshu and other lands were never formed. It is now believed by us that it might have been perhaps 12, 000 years ago when the sea-level rose to about -45m. This was the final stage of the land bridge in the Soya strait between Sakhalin and Hokkaido.