Geologic age and depositional environments of the uppermost part of the Middle Miocene Iriomote Formation in the Iriomote-jima Island, Okinawa, southwest Japan

Abstract

Geologic age and depositional environments of the uppermost part of the Middle Miocene Iriomote Formation, which is widely distributed in the Iriomote-jima Island, Okinawa, southwest Japan, were examined. Based on depositional facies analysis, four depositional cycles are recognized in the study interval. Calcareous nannofossil biostratigraphic study was tried to determine geologic age. Well-preserved calcareous nannofossils were found only from the infills of the trace fossil Thalassinoides suevicus piping down from transgressive lag deposits of the third depositional cycles.Calcareous nannofossil biostratigraphic data strongly suggest that the uppermost interval of the Iriomote Formation has deposited during the period between . Ma and . Ma, early Middle Miocene (calcareous nannofossil zone: CN ). The following scenario can be considered to explain the fact that well-preserved calcareous nannofossils were found from the T. suevicus fills: ( ) Due to selective cementation of the burrow boundaries, infilled nannofossils were also protected from chemical dissolution during diagenesis, ( ) The sediments containing calcareous nannofossils in the infills were filled down from the sea-floor through the burrow opening during transgressive to regressive stages, and ( ) The burrows penetrating deep below the seafloor were protected from the subsequent physical and biological destructions. Biostratigraphic data obtained from the burrow fills can provide very informative data to determine the geologic age.