Volcaniclastic rocks such as volcanic breccia, volcanic conglomerate, volcanic sandstone, and mudstone with volcanic rock gravels are found in small bodies scattered sporadically in the Shimanto Supergroup in the east coast area of the Hata Peninsula, western Shikoku, SW Japan. The volcaniclastic rocks are contained in chaotic facies made up mainly of mudstone and sandstone of the Late Oligocene-Early Miocene (Burdigalian) Shimizu and Arimisaki Formations, and are occasionally accompanied by Middle Eocene limestone blocks. Most of the volcaniclastic rocks were originally deposited from sediment gravity flows such as debris flow and turbidity flow, although, the mechanism of their emplacement in the chaotic facies still remains unsolved. Volcanic rocks are composed of alkali basalt, trachyandesite, trachyte and comendite, which are all classified into alkaline volcanic rock series. Amygdules filled with chlorite and calcite are abundant in alkali basalt and trachyandesite, indicating that they were formed at a shallow depth. Most volcanic rocks have been subjected to some degree of alteration. Pseudomorphs of olivine are commonly found in the alkali basalts and all mafic minerals broke down to mixture of chlorite and Fe-Ti oxide minerals. The alkali basalts have geochemical affinity to within-plate basalts of seamount origin judging from some discrimination diagrams using alteration-resistant elements. All observations suggest that collision of a seamount would have occurred during the Late Oligocene-Early Miocene at the convergent margin of western Shikoku. And the fragments of it incorporated into the accretionary prism.
The purpose of this study is to examine how basin floor sedimentation responds to coastal environmental changes with relative sea-level changes. We investigated the turbidite frequency and composition of sedimentary organic matter (kerogen) in the basin floor sediments in the Kumano Trough deposited during the end of Last Glacial Maximum to post glacial ages. Composition of sedimentary organic matter changed from those rich in pollen, spores and herbaceous organic matter to those poor in these materials during the rapid transgressive stage (about 7,800 cal. yr. BP) in the submarine-fan sediments. Most of pollen, spores and herbaceous organic matter were trapped and deposited in the estuary mouth and thus the compositional changes of organic matter indicate that the bay (Ise Bay) had not been present in the upstream parts of the Kumano Trough before 7,800 cal. yr. BP. Turbidites were also deposited during the slowly transgressive stage (before 9,300 cal. yr. BP) in the submarine fan. During the slowly transgressive stage, fluvial sediments were deposited in the coastal area. The frequency of turbidity currents decreases with development of the bay (Ise Bay) during the rapid transgressive stage. The changes of sedimentary organic matter composition and turbidite frequency in the proximal environment of basin-floor fan followed by that in the distal environment. The study of turbidite frequency and organic matter composition revealed that the coastal environmental changes with relative sea-level rise affect the basin floor sedimentation.
Weathering rinds formed on terrace gravels buried during the last 300 ka were investigated to clarify the rate of chemical weathering and the process enhanced by redox reaction. The ages of terraces were estimated by tephro-stratigraphic method and 14C ages of plant tissues that were distributed in the southern part of Fukui Prefecture, central Japan. Gravels are sampled from terrace deposits that have different ages of sedimentation, 300 ka (U1), 120 ka (U2) and 50 ka (M), and the changes of physical and geochemical properties of the weathering rinds were examined. The results show that two types of weathering rinds occur. One is the 'iron-hydroxide edge developing type'; it is characterized by the enrichment of iron hydroxide formed at the front of oxidized zone in the weathering rinds. The other is the 'iron hydroxide gradually changing type'; gradual changes of iron hydroxide accumulation from gravel rims to the cores without specific enrichment at the front in the weathering rinds. The iron hydroxide accumulation observed in both types can be formed by the migration of ferrous ion under reducing subsurface conditions, and oxidized thereafter by the diffusion of oxygen from the terrace sediments. However, the detailed process of edge formation with little oxidant in the gravel matrices is still not clear. The process identified here may provide an analogue of secondary elemental migration process enhanced at any type of rock that proceeded due to water-rock interaction system in subsurface environment.
The sediments and volcanic rocks of Shitara in Aichi Prefecture have a wealth of information regarding Miocene tectonic events of southwestern Japan such as clockwise rotational motion and vertical movement. In order to place geochronological constraints on such events, we have determined zircon fission track (FT) ages for 32 rock samples collected from lower clastic sediments of the Hokusetsu Subgroup and upper volcanic rocks of the Shitara igneous complex. FT ages for felsic tuff beds intercalated in background marine clastics show late Early Miocene (~17.5 Ma) deposition of the upper part of the Hokusetsu Subgroup, which is in accordance with the biostratigraphic age assignment. On the other hand, FT ages for a thick pile of felsic lavas and pyroclastic rocks of the Shitara igneous complex show clustering around 15 Ma, indicating geologically instantaneous emplacement at that time. Combining the present results with published lithological, paleomagnetic, and structural data, we propose a model in which (1) the Hokusetsu Subgroup was deposited on the subsiding, southward-migrating southwestern Japan microplate around 17.5 Ma, just before the main phase of the clockwise rotation; (2) the clockwise rotation had ended by 15 Ma; (3) collision of the southwestern Japan microplate with the Shikoku basin and the resultant crustal uplifting occurred immediately prior to 15 Ma.
Abstract: Benthic foraminiferal (BF) assemblages in core MD01-2409 collected from a water depth of 975 m off Shimokita Peninsula were investigated to reconstruct bottom water oxygenation changes around north Japan during the last deglaciation. Abundance changes in dominant taxa indicate centennial changes in dissolved oxygen condition on the seafloor. The BF assemblages were classified into three groups: oxic, suboxic, and dysoxic taxa. The deglacial BF assemblages show dysoxic events on 100−200 year cycle.