The Higata Formation, laid down in the north of the Kujukuri coastal plain, Chiba Prefecture, eastern Japan, is lithologically divided into two members of the Lower and the Upper Member. The Lower Member measures about 15-20 meters in thickness, has its upper surface about 4 meters above the sea level, and consists mainly of sandy and silty sediments which include marine molluscan shells. Abundant molluscan shells with warm water species and poorly the corals of Oulastrea crispata are contained in the upper shell bed of this member, called the Higata Shell Bed. The Upper Member measures about 2 meters in thickness and consists of peaty silt and peat in which a few ancient boats of the prehistoric men are contained. Radiocarbon datings on the molluscan shells from the Higata Shell Bed indicate ages of 5, 940±180ys. B. P. and 6, 100±190ys. B. P., and on the peat materials from. the lowermost and the middle parts of the Upper Member indicate ages for the former of 5, 470±110ys. B. P. and for the latter of 3, 660±130ys. B. P. A pollen analysis was made on samples from the columns of the four sites YK-3, 4, 6 and 7. For the studies of the paleoenvironments in the area, four pollen assemblage zones are differentiated on the basis of the prosperities of the main genera of pollen in pollen diagrams. Lithological and palynological evidences of the formation suggests a paleogeographical change at about 5, 000ys. B. P. from the bay filled with warm sea-water and its seaside salt marsh in which the halophytes grew thick, to the eutrophic lake in which the hydrophytes attached to the substrate and free-floating ones lived. The peatland has developed along the inner margin of the lake from about 4, 000ys. B. P. The forest vegetation on the hills changed at about 3, 700ys. B. P. gradually from the forests dominated by Quercus and other deciduous genera, to the forests dominated by Gryptomeria and Quercus. At about 1, 500ys. B. P., there were increases of Pinus and Gramineae, which were considered to be influences of the catastrophic destruction of the former natural forests and the agricultural activities of the prehistoric men.
In the present study, 1212 isolated fossil microtine teeth from Ando Quarry at Ofukudai karst plateau, Yamaguchi Prefecture, Japan, were examined for identifictation. Among them 280 specimens were rootless molars, and the other 932 specimens were molars with root. The former were identified as Microtus epiratticeps YOUNG 1934, which is an element of the Choukoutien fossil fauna. The taxonomic validity of M. epiratticeps YOUNG 1934, was discussed in some detail. The molars with roots were referred to Clethrionomys sp. at the present stage of study. In the genus Clethrionomys, they belong to nageri group of HINTON (1926). More precise identification is an open problem. By adding at least a new microtine fossil species of the Choukoutien fossil fauna, the present study gave more evidence to support the view already stated that the formation of the Ofukudai and the adjacent karst plateaus represents the boundary between the late-middle and early-upper Pleistocene in Japan, i. e., the age of Stegodon-Palaeoloxodon faunal transition.
This paper presents a catalog of late Pleistocene tephras in the South Kanto, showing their source volcanoes, ages, petrographic characteristics, distribution and volumes for convenient use in stratigraphic and chronological studies. Fourty-nine markers formed after the last Interglacial (ca. 130, 000-140, 000 YBP) as a result of eruptions from the Hakone, Fuji and other volcanic centers, are documented. Here, the unit of the tephra as a rule represents a deposit of one cycle of eruption, representing a brief moment in geologic time. Among the valuable characteristics for identification of the tephras are their petrographic properties. Above all, the refractive indices of certain phenocrysts (especially of orthopyroxene and hornblende) and the volcanic glass are probably the most useful. In most cases the range of petrographic characters in the marker is small. Exceptionally, however, there exist specific tephras (KlP-9-KmP-5), the petrographic features of which differ considerably between the upper and lower units of the tephra sheet. Detailed analysis of the compositional variation of the KmP-1, one of such specific markers, erupted from the Hakone volcano approximately 100, 000 YBP, possibly indicates that the magma reservoir underwent some zonation as a result of differentiation during crystallization. From the stratigraphic relationships between the marker-tephras and adjacent marine sediments, the chronology of the marine terraces and various related problems of the late Pleistocene are discussed in detail. The pumice-fall deposit TAu-12, one of the most prominent markers (dated at 140, 000-150, 000 YBP by the fission-track method), formed during the Shimosueyoshi transgressive stage, since it lies within the Shimosueyoshi marine formation at several localities. The KlP-1 and following tephras (KlP-2 to KlP-13, dated at approximately 132, 000 to 120, 000 YBP) formed during the regressive stage immediately after the highest sea level. The Shimosueyoshi rise in sea level possibly progressed rapidly in contrast with the slow fall during the regressive stage. Such changes in sea level are inferred to resemble those of the Recent deglacial hemicycle. After the Shimosueyoshi stage, the following three marine terraces were formed: the Hikihashi terrace (ca. 100, 000 YBP), the Obaradai terrace (ca. 80, 000 YBP), and the Misaki terrace (ca. 60, 000 YBP). These are easily recognizable from the tephrostratigraphy over most areas of the South Kanto.