Sandstone xenoliths included in an alkali-basalt lava were discovered in the Toge area, western Mineoka Belt, Boso Peninsula, central Japan. The sandstone is composed of plagioclase, quartz, felsic volcanic rock fragments, and datolite of secondary replacement origin, and is thought to be identical to the Kamitsuka or Atagoyama Formation, Mineoka Group. The host alkali-basalt and associated picrite basalt show geochemical characteristics of within-plate type basalts, and are correlatable to the alkali-basalt of the same chemistry at Hegurinaka 5km SW which indicates 19.62±0.90 Ma Ar-Ar age. The occurrence of the sandstone implies that the alkali-basalts in the Mineoka Belt erupted near the convergent margin in the early Miocene time.

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The Shimanto Supergroup in the Inomisaki-Nakamura area, western Shikoku, is favorable for the examination of geohistory around the boundary between the Cretaceous and the Paleogene, because the Maastrichtian-Paleocene strata, which are not exposed in most of the Shimanto Belt, occur as good outcrops in this area. Geological survey of the area and radiolarian fossils extracted from black or gray shale, acidic tuff, red shale, green shale, siliceous shale and rubbles of shale led the authors to the following conclusions. 1) The studied area can be divided into the following twelve tectonostratigraphic units on the basis of lithofacies and depositional ages inferred from radiolarian assemblages : Nonokawa Fm. (early Campanian), Oyu Melange (late Campanian), Nakamura Fm. (late Campanian-early Maastrichtian), Warabioka Fm. (Santonian-early Campanian), Ukibuchi Fm. (late Campanian-) Maastrichtian), Ishimiji Fm. (Early Paleocene), Ida Fm. (Late Paleocene), Kamochi Fm. (Early-Middle Eocene), Inomisaki Fm. (Middle Eocene), Tanokuchi Fm. (Middle-Late Eocene), Hiromi Complex (Middle Eocene), and Arimisaki Fm. (Late Oligocene-Early Miocene). 2) Depositional ages of the tectonostratigraphic units become younger southward except for the Warabioka Formation which is presumed to be slope basin deposits. It is possible that the Warabioka Formation is made up of megablocks of slope failure origin. 3) The chronological relationships among black (gray) shale, red shale, green shale and siliceous shale are suggestive of the subduction of a very young oceanic plate during the late Campanian to Early-Middle Eocene. 4) Pebbly shales with large rubbles of sandstone and shale of slope-failure origin begin to increase after the Late Campanian. The ages of shale rubbles in pebbly shale generally become younger corresponding to the depositional age of the host strata.

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Time-series sediment traps were deployed at two water depths, about 1, 000m and 3, 000m, in the western equatorial Pacific during 1999, which revealed seasonal changes of diatom fluxes. Two types of oceanographic conditions were investigated : Sites MT1, MT2 and MT3 were located in the Western Pacific Warm Pool (WPWP) ; and Site MT5 was located in the Equatorial Up-welling Region (EUR). Annual mean fluxes of total diatoms were high at Sites MT1 and MT5, and the lowest at Site MT2. The taxonomic composition of the diatoms was different between the two regions (the WPWP and the EUR). Pennate diatoms dominated the assemblages at Sites MT1, MT2 and MT3 in the WPWP. However, at Site MT5 in the EUR, the average percent occurrence of pennate diatoms was lower than that in the WPWP and the contribution of centric diatoms was higher than the latter. Nitzschia bicapitata was the dominant species in the WPWP, while Rhizosolenia bergonii was the dominant species in the EUR. Such a different composition of the diatom taxa is attributed to difference in environmental conditions, especially nutritional conditions between the WPWP and the EUR. At all sites, the flux levels of diatom taxa showed distinct seasonal patterns, while the relative abundances of the diatom assemblages did not show such a seasonality. Thus, the taxonomic composition of the equatorial diatom assemblages are considered to be stable with changes of source of nutrient supply. In the western equatorial Pacific, the seasonal changes of diatom fluxes reflect the seasonal change of nutrient concentrations, while the relative abundances of the diatom assemblages reflect the hydrographic and nutritional conditions of the surface waters.

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The Kashio Tectonic Belt in the Ichinose region consists of mylonite and Awasawa metamorphic rocks, which is situated between the Ryoke Belt and the Sanbagawa Metamorphic Belt. The geology of the Ichinose region is elucidated through the studies on litbology, grain size of quartz aggregates and metamorphic minerals. The mylonite belt is divided into zones F, M and C with increasing quartz grain size. Epidote recrystallizes in zones M and F. K-feldspar in pelitic rocks is rare in zone F, but is common in zone C. Phengite contents of muscovite crystals tend to increase from zone C to zone F. These petrological features suggest a decrease in metamorphic temperature from zone C to zone F. In the northern part of the Ichinose region, mylonite foliation strikes N 30°-50°E except for the southernmost region where the foliation strikes N 15 E. Mylonite foliation in the southern part of the Ichinose region strikes N 30°E which is parallel to compositional layering and the boundaries between zones C, M and F. The complex internal structure of the Kashio Tectonic Belt is evidence of a conspicuous deformation of the Kashio Tectonic Belt after the formation of mylonites in the Cretaceous time. This deformation took place during the formation of the Median Tectonic Line (MTL) in the Ichinose region, judging from the widespread occurrence of weakly metamorphosed rocks near the faults in the Kashio Tectonic Belt and the MTL. The deformation of the Kashio Tectonic Belt can be explained in terms of the rotation and northward horizontal shift of the Akaishi Mountains at about 16-15 Ma.

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Marine diatoms were examined for the 245 meter-thick Middle Miocene sequence (the Haraichi and Itahana Formations) exposed along the Ayukawa River in the Fujioka area, Gunma Prefecture, central Japan. The sequence is correlative with the Denticulopsis praedimorpha Zone (NPD 5B) and the Thalassiosira yabei Zone (NPD 5C) of Yanagisawa and Akiba (1998). The last common occurrence (LCO) of D. praedimorpha (D55), a biohorizon which defines the NPD 5B/5C boundary, is recognized between 1 and 3 meters above the Baba Tuff Bed in the upper Haraichi Formation. The numerical age of the biohorizon D55 can be regarded as same as the ⁴0Ar-³9Ar age of the Baba Tuff Bed (11.28±0.08Ma), because of a high accumulation rate (ca. 35-39cm/1, 000yr.). This age is about 0.2Ma younger than the magnetostratigraphically estimated age for D55 at ODP Site 887 and 884 in the high latitude North Pacific. The direct relationship between the D55 biohorizon with high-resolution radio-metric age should give important constraints on refining a global geologic time scale for the Miocene interval.

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Stratigraphic horizon of the Km3 tephra layer intercalated in the lower part of the Kitaura Formation, Akita Prefecture, Japan, is located above the calcareous nannofossil datum plane 11 (1.65Ma). The Km3 tephra layer, approximately 1m thick, is mainly composed of volcanic glass shards with minor amounts of plagioclase, quartz and biotite. On the basis of the stratigraphic position, lithofacies, petrographic characteristics and chemical composition of glass shards, the Km3 tephra layer is correlated to the Om-SK110 Tephra (1.65Ma). By this result, it is clarified that the Km3 tephra layer was transported more than 450km away from its probable volcanic center situated in the Hida Mountain Ranges, central Honshu Island.

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[in Japanese]

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