Geochemical indications of provenance change as recorded in Miocene shales: opening of the Japan Sea, San'in region, southwest Japan

Abstract

Shales from the San'in Region, Southwest Japan, exhibit geochemical patterns that record provenance change during the opening of the Japan Sea. Four formations, the Koura, Josoji, Ushikiri and Furue, are described. The basal Koura Formation is non-marine, while those overlying are marine. The Koura Formation shows variation from basic to silicic composition, due to fluvial transportation of source rocks with lower CIA (chemical index of alteration) than those of the Josoji Formation. The source material of the Koura Formation is mainly felsic (possibly plutonic) rocks, as shown by Cr Zr - Y Ni systematics, and similar to Ebi granitic rocks of the Hida continental massif in SiO 2 TiO 2 - Al 2O 3 TiO 2 ratios. The Josoji Formation is enriched in SiO 2 from high biogenic productivity. Ti Zr and Cr Zr - Y Ni ratios of the Josoji Formation suggest that basalt to andesite were significant lithologies in the source region. Provenance change from the Koura to the Josoji Formations was possibly related to the transgression caused by the first opening of the Japan Sea, where sediments were derived from deeply weathered material in the wider source areas. The Ushikiri Formation shows mainly intermediate composition, and is characterized by relatively higher Ca and Sr. This is indicative of uplift of the Paleogene igneous rocks in the source areas, with intermediate volcanic activity, probably related to the second opening event. The Furue Formation has similar composition to the Ushikiri Formation, but with comparatively higher CIA, suggesting another episode of deeper source area weathering, and well mixed source material. The results are consistent with published geological interpretations of the area, and show that the geochemistry of shales from active margins is a useful tool for examining the development of such margins. Locally derived shales retain source composition variability and signatures, while those deposited in deeper basins show more consistent chemistry closer to PAAS composition.