MIDDLE MIOCENE-EARLY PLIOCENE PALEO-OCEANIC ENVIRONMENT OF JAPAN SEA DEDUCED FROM GEOCHEMICAL FEATURES OF SEDIMENTARY ROCKS

Abstract

Chemical and isotopic compositions (Sr isotopic ratio, major elements, trace elements, rare earth elements, total carbon, nitrogen, and sulfur contents) of rock samples collected from middle Miocene to early Pliocene sedimentary rocks on the Oga Peninsula, northern Japan, were analyzed to elucidate the paleo-oceanic environment of the Japan Sea. The rocks studied were shales from the Nishikurosawa, Onnagawa and Funakawa formations in stratigraphically ascending order. The Onnagawa sedimentary rocks in the lower (ca. 12.6–11.4 Ma), middle (ca. 10.6–9.0 Ma) and upper (ca. 8.3–7.0 Ma) horizons have high Mo/Al, Ni/Al, and Ba/Al ratios and high total organic carbon as well as high K/Ti and 87Sr/86Sr ratios and a positive Eu anomaly. These geochemical variations imply high primary productivity, and reducing conditions indicative of a deep paleo-ocean. The formation of petroleum source rocks on these horizons is attributed to increasing of nutrient delivery from the terrigenous system, which may have been induced by strong wind from Asian continent related to the uplift of Himalayan and Tibetan regions.