Three magnetic components were recognized in the natural remanent magnetization of the Pliocene Takafu Mudstone Member of the Shigarami Formation in the Takafu Syncline in the western part of the North Fossa Magna, central Japan, by means of progressive alternative field and thermal demagnetizations. The first component is attributed to a present-day field overprint or drilling disturbances. The second component, which constitutes the absolute majority of the natural remanent magnetization, is demagnetized at temperatures up to 100-400°C. The third component, which comprises less than 30% of the natural remanent magnetization, is demagnetized at temperatures between 300 and 600°C. The second and third components were detected at 26 sites and at 13 sites, respectively. X-ray diffraction analyses indicated that the Takafu Mudstone contains ferrimagnetic either magnetite or greigite (Fe₃S₄), or both. A sample in which only the second component was detected, contains greigite only, whereas magnetite is responsible for the third component. Microscopic observations also confirmed formation of authigenic greigite in the Takafu Mudstone. I conclude that the second component is the chemical remanent magnetization (CRM) of greigite, and the third component is the post-depositional remanent magnetization (pDRM) of magnetite. The third component display magnetic direction similar to the second component, and both components passed the fold test (bootstrap method of Tauxe and Watson, 1994). This suggests that both magnetizations were acquired before folding of the Takafu Syncline and that the Takafu Mudstone of pro-fan-delta sediments origin, recorded CRM that is more stable than pDRM within a certain geologic time.

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The volcanic, lacustrine fan-delta deposits in the Pleistocene Lower and Middle Formations of the Yachiho Group, central Japan commonly include debris-flow and hypercon-centrated-flow deposits in the upper fan-delta-plain facies. The hyperconcentrated-flow deposits are recognized by its characteristics and depositional organization with debris-flow and streamflow deposits. Hyperconcentrated-flow deposits usually coexist with cohesionless debris-flow deposits. This indicates that the generation and depositional processes of hyperconcentrated flow are closely related to cohesionless debris flow in the volcanic fan-delta setting. A depositional organization comprising cohesionless debris-flow deposits, hyperconcentrated-flow deposits, and normal streamflow deposits, in ascending order, is commonly observed. This organization indicates the longitudinal segmentation of the composite flows. Cohesionless debris flow preceded dilute hypercon-centrated flow followed by a streamflow that segregated from the initial debris flow.

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Sedimentologic and petrographic studies were conducted on ooids from allochthonous limestone bodies in the Lower Cretaceous Birafu Formation, Kochi Prefecture, Shikoku, Japan. The limestone bodies consist mainly of oolitic grainstone in which the total volume of ooids is as high as 70% in contrast with limited contents of terrigenous material (generally<20%). This suggests high-energy environments during the ooid formation and deposition. Ooid diameter varies typically between 0.1 and 0.8mm, with the extremes being 0.05 and 2.0mm, respectively. Nuclei of the ooids are constituted by lithoclasts of quartz and feldspar and bioclasts of mollusks and echinoids. The ooids exhibit a radial microfabric, although micritization of varying degree is common. Radial coatings consist of fibrous crystals 6-15μm long and 1.0-1.5μm in diameter. The ooids afford no evidence of the direct influence of micro-organisms on their formation. With the fibrous shape of the radial ooid crystals, they recall modern high-magnesian or aragonite cements typical of shallow normal marine diagenetic environments, and thus suggest an original composition of this kind. X-ray diffraction and microprobe analyses, however, consistently show MgCO₃-concentration of the ooids below 1%, which implies that mineralogy of the precursor inverted to low-magnesian calcite without loss of crystal morphology when exposed to meteoric-water diagenesis.

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A well-preserved Allodesmus (Urahoro specimen) was found from the middle Miocene Okoppezawa Formation. We studied the relationship between preservation process of the Urahoro specimen and depositional systems based on facies analysis and sequence stratigraphy. The upper part of Tokiwa, Okoppezawa and Chokubetsu formations are classified into seven sedimentary facies. The seven facies constitute two major facies successions such as, upward-coarsening and aggradational. The upward-coarsening facies succession is interpreted to have developed in response to progradation of a delta system during highstand stages of relative sea-level. The aggradational facies succession, in contrast, indicates the accumulation of turbidites on delta front or prodelta during lowstand stages. The two types of the facies successions developed repeatedly and exhibit an overall deepening pattern of section.
The fossil Allodesmus was found from shelf sediments of the upper part of the highstand systems tract just below the sequence boundary (SB1). This sequence stratigraphic position of the fossil suggests that the Allodesmus had been buried rapidly due to high sediment supply from the prograding delta system during late highstand and successive lowstand stages.

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