Middle–late Miocene paleoenvironment of the Japan sea inferred by sedimentological and geochemical characterization of coeval sedimentary rocks

Abstract

The middle–late Miocene siliceous mudstones and siltstones of the Akita and Aomori regions in Northern Honshu, Japan, provide an excellent geological record of the paleoenvironment of the Japan Sea. In this paper, we present results of sedimentological and geochemical analyses of the Akaishi and Odoji Formations from the Tsugaru Basin in the Aomori region and of the Onnagawa Formation from the Sugisawa area in the Akita region. Lithofacies descriptions, geochemical compositions (including major elements, total organic carbon [TOC], total nitrogen [TN], and total sulfur [TS]), and petrographic observations of the framboidal pyrite were used to investigate the origin of the detrital fraction, the paleoproductivity, and the paleoredox conditions of the bottom waters in the Japan Sea. Moreover, through interpretation of these data, the relationship between organic carbon preservation and the paleoenvironmental evolution was clarified. The data indicate that paleoweathering was moderate to high in the detrital fraction source terrain; the detrital fraction originated from a felsic source and was probably transported from Asia to the Japan Sea by monsoons and from the Japanese islands by riverine input; the high concentrations of biogenic SiO2 indicate high primary productivity in the surface water, probably induced by intense upwelling of nutrients; the Tsugaru Basin area was characterized by oxygenated bottom water, whereas the Sugisawa area was characterized by oxygenated to low-oxygenated bottom water based on the lithofacies variation and the size of the framboidal pyrite; and the organic carbon accumulation in the Odoji and Onnagawa Formations was mainly controlled by paleoproductivity and dilution from the detrital fraction rather than the paleoredox conditions. Alternatively, the organic carbon accumulation in the Akaishi Formation was likely controlled by the paleoredox conditions.