Miocene-Pliocene microfossil biostratigraphy and paleoenvironment in the Fukaura district, Aomori Prefecture, northern Japan

Abstract

Radiolarian and foraminiferal biostratigraphy was investigated in the Fukaura district of western Aomori Prefecture, northern Japan, where the Miocene-Pliocene chronostratigraphy is well established. Based on the newly obtained data and reinterpretation of previous studies, we determined that the Odoji Formation ranges in age from the Middle to Late Miocene Epoch, the Akaishi Formation ranges from Late Miocene to Early Pliocene, and the Maido Formation is Pliocene. Our biostratigraphic and chronostratigraphic results indicate that the Miocene/Pliocene boundary exists in the Akaishi Formation. Benthic foraminiferal assemblages of the Odoji Formation contain abundant and diverse calcareous taxa indicative of warm, mesotrophic and oxic marine conditions. In contrast, the upper part of the Odoji Formation yields less diversified assemblages, which are mainly composed of infaunal agglutinated foraminifera, Martinottiella communis. These assemblages dominated by M. communis suggest high organic matter flux to the sea floor closely relating to the high primary production of the surface water, and/or oxygen-depleted conditions. The stratigraphic horizon of these faunal changes is possible to be correlated with the Foram. Sharp Line (FSL). The upper part of the Akaishi Formation contains two assemblages one dominated by M. communis as upper part of the Odoji Formation, and the other dominated by Miliammina echigoensis, which suggests oxic bottom water conditions with high primary production. The lower part of the Maido Formation contains only Globobulimina sp., which also suggests a depositional environment associated with high primary production and/or oxygen depletion. Consequently, condition with high organic matter flux caused by high primary production in surface water is inferred to have persisted from the time of the FSL to Ca. 3 Ma, when a diverse foraminiferal assemblage returned.