Postglacial deposits are formed in relating to the sea level fluctuations of Postglacial epoch. Generally they can be divided into four members, that is, the lower sand and gravel bed, the middle mud bed, the upper sand bed and the uppermost terrigenous bed. Postglacial deposits form an alluvial plain which often coinsides with a sinking area, for instance, Toyama, Osaka basin etc. They are the upper part deposits in these structual basins. Basin buildig and land subsidence are discused as examples of the land movement in Postglacial epoch. Basin building does not finish in early Pleistocene epoch but it is continueing in Holocene as Toyama basin. The land subsidence is divided into natural one and artificial one. It consists largely of compaction of Postglacial deposits and partly of crustal subsidence. The compaction of Postglacial deposits has an intimate relationship with the thickness of the mud bed and the water quantity pumped out from the deposits. The annual amount of the natural subsidence including of the compaction of sediments and land movement is only of millimetre order and generally less than 5mm, on the other hand the artificial one is centimetre order (table 2). The results of recent survey show that the pumping qantity from the diluvium deposits is larger than that from Postglacial deposits, and the land subsidence takes place diluvium deposits as well as Postglacial ones. Therefore we have to investigate not only the subsidence of Postglacial deposits but also that of the Quaternary structual basin.
It is current opinions in Japan that the Alluvial or Holocene marine deposits distributed in the coastal regions are represented by younger deposits than the Würm maximum. In this paper, the writers made a discussion on the lithological facies and subdivision of the marine deposits, based on the results of study in Tokyo and western Kyushu, as follows:- (1) In the lowland area of Tokyo, the so-called Alluvial deposits are subdivided into three lithological units, namely I, II, and III formaions as shown in Fig. 1. Among these, the third one is correlated with the Pleistocene volcanic ash, as judged from the mineralogical analysis and radiocarbon dating. (2) In the Ariake and Shiranui Bay areas, western Kyushu, the Alluvial deposits are subdivided into the Ariake and the Shimabarakaiwan formations respectively. (3) From the results of the analyses on micro-fossil fauna, mineral composition, chemical components and engineering properties of the announced formations (Figs. 2 and 3), it would be emphasized that the so-called Alluvial marine deposits may be subdivided into two major lithological or stratigraphical units. The interrelation between the units is considered to be unconformable which might have resulted from the lowering of sea level.
Geological research on the subsurface alluvial deposits would be effectively promoted in combination with the investigation of civil engineering properties of these deposits. Judging from the soil technological investigation, the civil engineering properties of clayey deposits are largely controlled by both the inter particle forces acting among the clay particles and by the soil structure (arrangement of the clay particles) in the deposits. The inter particle forces and the soil structure are affected by the geological factors including the material factors and the environmental factors of the deposits. Two groups in the civil engineering properties are recognized: primary and secondary. The primary properties are concerned with the material factors of the deposits, such as clay content, composition of clay materials, and nature of pore fluid. The secondary properties are concerned mainly with the environmental factors of the deposits, such as sedimentary environment, post depositional overburden pressure, and geological history of the deposits. These secondary properties are important to reveal the geological evolution of the deposits. In this article the author discussed the bearing of indices of civil engineering properties on the geological factors, and he presented the methods in order to detect the geological factors which influenced the properties of the deposits. In the above methods, there have been used the correlation diagrams among the indices of civil engineering properties and the geological factors. Further detailed studies on this subject are under consideration.
The coastal barriers in Japan are of three different types, formed at three different times (Fig. 4). 1) The oldest barriers, named as “Tsugaru-type”, form an extensive marine terrace having originated as ancient bar and lagoon, probably during the Shimosueyoshi (Riss-Würm interglacial) stage. 2) “Akita-type” of the ancient barriers are believed to have been formed during some transgressional (interstadial) phases in Würm stage. The top of the deposits is separated from the succeeding Holocene soft sediments. 3) The youngest barriers, called as “Kanazawa-type”, are of the Holocene transgression. All of those types is often developed along a same stretch of coastal area, where the ancient barriers are partly incised with numerous valleys, buried by the Holocene shallow marine, lagoon or fluviatile deposits. Based on the sedimentological analyses of subsurface data, it is generally established that the environment of deposition in the Japanese coastal area is changed from fresh or brackish to marine water during the early Holocene, in spite of the growth of sand spits or bars. Along the flanks of relatively large delta and fluvial fan, however, marine transgression was prevented its effect by the large supply of sediments. During the late Holocene, three series of beach ridges built by a prograding shoreline are developed on the coastal barriers. The environment in the landward area is generally changed from marine to brackish or fresh water. The cause is attributed to the intermittent emergences of shoreline. It is remarkable that these emergences can be recognized even on the relatively sinking coasts in Japan.
This original article is a preliminary report on the discussion related to the changes of the forest and climate in the Postglacial age, based on the present writer's palynological investigations about the late Quaternary deposits in Japan, especially Hokuriku region, during the last ten years. The evolution of the forest of Japan since the Late Glacial age is divided into seven phases and may be summarized as follows: (1) Cold stage (E-G phases, 12000-11000 years before the present): This stage is divided into three phases, namely G phase: Fagus, Abies and Larix; F phase: Corylus and Gramineae; and E phase: Fagus with Abies and Larix. F phase is satisfactorily correlated with the Alleröd oscillation in Europe. (2) Increasing warmth stage (D phase, 11000-8000 years before the present): This stage is trangitional age from the Late Glacial to Postglacial climatic optimum, and the dominanting pollen grains were made up of deciduous Quercus and Pinus, with Picea and Fagus as generally subordinate associates. (3) Postglacial climatic optimum (C phase, 8000-4000 years before the present): This phase is correlated with the Atlantic and the early part of Subboreal in Europe. The characteristic genera were evergreen Quercus, Alnus and Cryptomeria. At some time during the Postglacial certain warmth-living plants were more widely distributed than at present, reaching higher latitudes as well as higher altitudes, namely the forest zone was 200-300m higher and it was 2-3°C warmer than the present time in average temperature. The beginning of this phase can be tentatively correlated with the lower horizon of the first Jomonian Tado remains, being dated as about 8000 years before the present. Moreover. the maximum warmth stage may be correlated with the age of the Flandrian transgression. (4) Cool stage (B phase, 4000-1500 years before the present): The characteristic forest of this phase was the mixed flora of evergreen broad leaves-trees and Fagus crenata (submerged erect tree stumps). It was some 1°C lower than the present time in average temperature. The dominanting pollen grains are Cryptomeria, Castanea and Pinus. The decreasing warmth stage may almost correspond with the age of the miner falling of sea-level in the Hokuriku region. (5) Present climate stage (A phase): After about 1500 years before the present. These climatic changes, exclucive of (4) Cool stage, satisfactorily correspond with the results obtained by many investigaters throughout the world. The figure (Fig. 1) shown the changes of the forest and climate during Postglacial age throughout the world and Japan will help for understanding.
The radiocarbon dates so far obtained from the Jomon and Yayoi periods in Honshu and the relevant periods in Hokkaido were plotted on a graph according to the chronological order of pottery types in archaeological framework. The distribution of plotted points is in general consistent with the chronological sequence of pottery types except for the earliest Jomon period. As to this period, plotted points of Hokkaido are markedly dispersed within a wide range of time and those of Honshu are inconsistent with the chronological order of pottery types. Moreover, in the latter there is a gap of 2500 years which is too long as to be assigned to the period of eight pottery types having no available radiocarbon date as yet. In this respect, the discrepancy between the radiocarbon date and the true age which is possibly due to the climatic change in the past seems to provide an adequate interpretation.
The radiocarbon dates measured by Gakushuin University until January 1966 are arranged in order to give a chronological information on the alluvial deposits and the volcanic activities. The dates relating to the change of sea level are also listed to show the local changes along the Islands of Japan.