The eruptive history of the Kusatsu Shirane volcano is well described by means of 14 beds of key tephra and intercalating loess soil. Three eruptive stages are recognized. During early or middle Pleistocene the Matsuozawa volcano was formed; this is the first stage. The second stage was initiated by effusion of the Horaguchi lava, which was followed by eruptions of the Oshi pyroclastic flow, older lava flows, 3P pumice fall, and Yazawahara pyroclastic flow. Brown loess soil about 10m thick covering these deposits indicates that a dormant period of more than 100, 000 years followed this stage. The summit area upheaved about 400m or more against the foot of the volcano during this period, as is suggested by the extraordinarily steep (6.1°-3.0°) surface of the Oshi ignimbrite plateau. The third stage, which started about 14, 000 years ago, is the formation of three pyroclastic cones on the summit and contemporaneous effusion of the younger lava flows, e. g. the Kagusa lava of 7, 000 years ago and the Sessyo lava of 3, 000 years ago. In historic times, phreatic explosions have frequently occurred on the summit crater, Yugama. This means that the present belongs to the third stage. It is unlikely that eruptions of the third stage are caused by cooling of the magma chamber which was active in the second stage. The activity of the third stage seems to denote arrival of a new magma chamber at shallow depth.
A method for estimation of paleo-salinity based on diatom assemblages is presented in this paper. Diatom environmental indicators (KOSUGI, 1988b), characterized by specific ranges of salinity, were utilized to estimate paleo-salinity. As a result of comparing the estimated salinity with real salinity in present environments, both salinity were seen to correspond if the estimated salinity was recognized as conforming with indicators prescribing an appearence ratio greater than 5%. As an application of this method, the paleo-salinities at 6 localities from the Paleo Oku-Tokyo Bay were estimated. As a result, it was thought that in the earlier stage of the Jomon Transgression (Holocene Transgression) paleo-salinity rapidly increased, while at the later stage it decreased slowly. The upper limits of the marine sediments were identified as +2-3m in small valleys in sourrounding terraces and +1-2m in the Nakagawa lowland. Moreover, allochthonous cells in fossil assemblages were identified using this method. If the species groups having ranges outside the estimated range of salinity are recognized as allochthonous, more than a few allochthonous groups in a fossil assemblage are descriminated.
Sedimentary sulfur mainly of FeS2 (pyrite) was oxidized to sulfate with 30% H2O2 and the concentration of sulfate was determined by the turbidimetric method. Treatment procedures were as follows. 1) Wet sediment (approximately 0.3g) was washed twice with distilled water followed by centrifugation (3, 000rpm, 15mins), and dried at 80°C. 2) The dry sample (0.1g) was reacted with 10ml 30% H2O2 for 30mins at room temperature and then for 2mins at 90-100°C. 3) When oxidation reaction ceased (usually after 15hrs), 50ml distilled water was added and the sample filtered through a 0.45μm membrane filter to separate extract from residue. Filtrate was rinsed and brought to 100ml volume with distilled water for the turbidimetric method. The content of sedimentary sulfur determined by this method was much higher in the marine and brackish water sediments than in the freshwater sediments. The vertical variation in sulfur content was in good agreement with the succession of diatom assemblages. As a sulfur analysis indicative of paleoenvironmental changes, this method is considered useful for its simplicity and for the small amount of sample used.
The Tokusa Basin is situated at an elevation of 300m in the northeastern part of Yamaguchi Prefecture in the western Chugoku Mountains, western Japan. A 68m long core containing silt, peat, volcanic ash and clay, which accumulated in the old lake Tokusa during Pleistocene, was sampled at Tokusa-Sadayuki, Ato-cho, in the Basin. In the study reported in this paper, a part of the 68m long core: 10.6m were palynologically examined at intervals of 20cm. Three samples were measured by 14C dating method: the age of the middle samples (390-395cm) was 46, 110±1, 050y. B. P. The volcanic ash inbedded between 500 and 520cm was estimated as Aso-4 ash which accumulated in the old lake ca. 70, 000 years ago. From the results of pollen analysis from the Tokusa Basin and the Ubuka Basin (HATANAKA and MIYOSHI, 1980), seven pollen zones could be recognized; in order from the surface to the lower part, they are as follows: L zone (10, 000-15, 000y. B. P.) with subarctic pollen types (Pinus, Abies, Picea, Tsuga, Betula) P-I zone (15, 000-25, 000y. B. P.) with subarctic pollen types (Pinus, Abies, Picea, Tsuga) P-II zone (25, 000-45, 000y. B. P.) with subarctic pollen types (Pinus, Abies, Picea, Tsuga, Betula) P-III zone (45, 000-70, 000y. B. P.) with temperate pollen types (Cryptomeria) P-IV zone (70, 000-80, 000y. B. P.) with cool temperate pollen types (Fagus, Quercus, Tsuga, Picea) P-V zone (80, 000-120, 000y. B. P.) with cool temperate pollen types (Fagus, Quercus, Carpinus, Ulmus) P-VI zone (120, 000-?) with temperate pollen types (Cryptomeria, Tsuga, Pinus, Carpinus) L and P-I zones are from the Ubuka Basin, P-II and P-VI zones from the Tokusa Basin. The six pollen zones from L to P-V cover approximately the 110, 000 years of the last glacial period, while the P-VI zone is a part of the last interglacial period.