A forearc basin fill of the Pleistocene Toyofusa Group in the southern part of the Boso peninsula, Japan is represented by slope to shelf sediments. The Toyofusa Group is subdivided into the Kanamaribata Formation, Ohi Formation, Higashinagata Formation, Takigawa Conglomerate, Sagashi Formation and Ikenouchi Formation in ascending order, and correlated with the middle and upper parts of the Kazusa Group. The slope to shelf sediments form a northeasterly onlapping clastic wedge and include many numbers of volcanic ash layers. Mapping of depositional associations with tracing volcanic ash time marker, together with the interpretation of depositional facies, permits identification of stratal terminations and of temporal-spatial variations of depositional environments. These enabled a precise sequence-stratigraphic analysis of the sediments of the Toyofusa Group. Seven depositional sequences have been identified in the Toyofusa Group. Each the depositional sequences show lateral terminations of depositional facies and vertical facies discontinuities at the sequence boundaries. These depositional sequences consist of complete or incomplete assemblages of fundamental components of lowstand, transgressive, and highstand systems tracts. Fission track and paleo-magnetic ages for the Toyofusa Group showed that the depositional sequences can be correlated with oxygen isotope stages of 15 to 23. By the correlations, periodic formation of depositional sequences can be attributed to glacio-eustatic sea-level changes of about 40,000 to 100,000 years.

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Volcanic ash layers of more than 250 beds intercalated in the Plio-Pleistocene Kazusa Group, Boso Peninsula, Japan were examined petrographically and lithologically. The ashes are classifiable into biotite-rich type (B-type), amphibole-rich type (A-type) and clinopyroxene-rich type (C-type). The ashes are vitric, crystalline, pumiceous and scoriaceous of various grain sizes. A particular type of ash layer tends to predominate at certain stratigraphic levels in the Kazusa Group, and it is able to establish tephrozones. Established tephrozones are Kz I, Kz II, Kz III, and Kz IV in ascending order. Kz I includes mainly C-type coarse ashes and a few A-type coarse ash layers. Kz II is characterized by B-type fine ash layers. Scoriaceous coarse ashes of C-type are dominant in Kz III. Kz IV contains many C-type pumiceous ash layers with intercalation of a few B-type ash layers. The Osaka Group in southwestern Japan and the Uonuma Group in Niigata Prefecture were deposited in the same period as the Kazusa Group. Previously established tephrozones in these sedimentary basins were compared with those in the Kazusa Group. The petrographic characteristics of the tephrozones among the separated sedimentary basins are quite different, even those of the same age. This indicates that the activities of volcanoes near the basins are largely responsible for the formation of tephrozones in each sedimentary basin. Petrographic study of tephrozones in basin-fill deposits is expected to provide general information of temporal-spatial changes in volcanic activity. Such information would be valuable for reconstruction of the volcanic activities of old and missing volcanoes in the Plio-Pleistocene.

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Extremely thick marine deposits, more than 3,000 meters, the Kazusa and the Shiousa Groups, are exposed in the Boso Peninsula, central Japan. It is generally said that the continuous tectonic subsidence of the Kanto Tectonic Basin causes such remarkable sedimentation. Especially the Shimousa Group consists of an almost continuous succession of several formations controlled by glacio-eustasy. Besides, the basin must have upheaved from deep sea during the Quaternary, because it is located on the wide uplift belt of the island arc, namely the Izu-Ogasawara island arc. Owing to interpretation of the paradox, two hypothetical ideas are proposed: one is isostatic subsidence caused by sediment loading, and the other is related to tectono-eustatic sea-level rise. Relating to the latter hypothesis, some methods to obtain eustatic sea-level changes were discussed. A relatively large-scale sea-level rise are supposed preliminarily from transition of a type of stratigraphy which is comprised of some sequenses caused by glacio-eustatic sea-level rise and changes from 'scouring and accumulation type' to 'terrace making type'. An actual tectono-eustatic sea-level curve can be obtained by subtracting amount of uplift from the apparent tectonic curve drawn by Kikuchi's method (Kikuchi 1977) based on the data of the Boso Peninsula.

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Several faults which cut Pleistocene sediments have recently been discovered in Aimi Town to the south of Yonago, Tottori Prefecture, southeast Japan. They are developed along the western margin of a plateau of Pleistocene alkaline basalt (Tsuruta basalt), which rests unconformably upon Paleogene granite. Trenching investigations show that one of these faults (Asakane Fault) has been reactivated seven times, with four of the events in the period 130,000yrs B. P. to 2500yrs B. P. Although the fault has secondary fractures which indicate lateral displacement, it cannot be determinded if the displacement is dextral or sinistral. The surface of the unconformity between the Tsuruta basalt and granite is partially inclined at a high angle, links with a fracture in granite, and is accompanied by breccia at its foot, and so thought to have been a small cliff caused by faulting. The Asakane fault is inferred to have become active just before the effusion of the Turuta basalt, because it has the same geological characteristics with steeply inclining unconformity. Moreover, it is probable that the Asakane fault is a reactivated fracture, which formed initially as a concentric fracture around a collapsed basin of early Miocene age.

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Ten species in the Nine genera of decapod crustacean fossils have been obtained from the Korematsu Formation of the early Middle Miocene Bihoku Group in the Shobara area, Hiroshima Prefecture, southwest Japan. They are grouped into three fossil assemblages on the basis of the association of several dominant species. The paleoecologic characters assigned from the occurrence of these assemblages as well as the ecological data of recent decapods is as follows: (1) Eucalliax yatsuoensis-Callianassa nishikawai assemblage: intertidal zone, sandy mud bottom. (2) Miosesarma japonicum assemblage: mud bottom of the upper part of lower sublittoral zone. (3) Carcinoplax antiqua assemblage: sand or sandy mud bottom of the upper to upper part of lower sublittoral zone. The latter two assemblages composed of sublittoral dwellers are the first report from the western part of the Setouchi Geologic Province. The crustacean fossil assemblages suggest that, the depositional environment of the Korematsu Formation has been intertidal zone of inner bay to upper part of lower sublittoral zone under warm current.

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