Abstract
Geochemical and geological studies were performed on the turbidite sequence of Anno Formation, Awa Group, Boso Peninsula, Chiba Prefecture.Based on the correlation of major, trace, and rare earth elements (REEs) contents with SiO2 content, elements are divided into positive correlation group (K, Rb, Zr, Nb, Ba, Hf, Ta, Pb, Th, U) and negative correlation group (Ti, total-Fe, Mn, Mg, Ca, P, Sc, V, Co, Cu, Zn, Sr, Y). This is considered to have been caused by an increase in the acidic component derived from the island-arc during the sea level declining period, and an increase in the basic component from fore-arc basalt during the sea level rising period. An increase in the organic carbon content and a decrease in organic carbon isotopic composition (δ13C) are recognized during the sea level declining period. This is thought to have been caused by the increase in the biogenic productivity related to the upwelling of deep seawater and the settling of organic matter-clay mineral compounds.87Sr/86Sr ratio and other geochemical data (REE pattern, La/Yb ratio) indicate that the hemipelagite is mostly composed of fore-arc basalt with variable amounts of carbonate and acidic component probably derived from the island-arc.These geochemical variations with time are consistent with the variations of δ18O of foraminiferal shell carbonate, the amount of turbidite sedimentation, and the hemipelagite thickness. Therefore, the geochemical data obtained by this study support the conclusion of Ishihara et al. (1997) who indicated that the temporal variation of turbidite in the Anno Formation is caused by the variations of seawater level.
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