Chronological and paleoceanographic constraints ofMiocene toPliocene ‘mud sea’ in theRyukyuIslands (southwesternJapan) based on calcareous nannofossil assemblages

Abstract

AbstractTheCenozoic sedimentary succession inOkinawa‐jima, including the upperMiocene toPleistocene siliciclastic deposits (ShimajiriGroup) and thePleistocene reef to shelf deposits (RyukyuGroup), suggests a drastic paleoceanographic change from a ‘mud sea’ to a ‘coral sea.’ To delineate the paleoceanographic evolution of the mud sea, we quantified the stratigraphic distribution of the calcareous nannofossil assemblages from theShimajiriGroup in a 2119.49 m‐deep well (NanjoR1ExploratoryWell) drilled in southernOkinawa‐jima (RyukyuIslands, southwesternJapan). Four lateMiocene andPliocene datum planes were found in the studied interval: the first occurrence ofAmaurolithusspp. (7.42 Ma), the last occurrence ofDiscoaster quinqueramus(5.59 Ma), the first occurrence ofCeratolithus rugosus(5.12 Ma), and the last occurrence ofReticulofenestra pseudoumbilicus(3.70 Ma). The calcareous nannofossil assemblages from theTomigusukuFormation and the lower part of theYonabaruFormation are characterized by a lower total number of coccoliths and abundantSphenolithus abiesthat is associated, at times, with commonDiscoasterspp. Overall, these suggest the existence of oligotrophic conditions between 5.3 and >8.3 Ma. The total number of coccoliths increased and smallReticulofenestraspp. became more common in the middle part of theYonabaru Formation, suggesting that eutrophic conditions were present between 3.5 and 5.3 Ma. The rare occurrence of calcareous nannofossils in the upper part of theYonabaruFormation indicates a return to oligotrophic conditions at 3.5 Ma. Micropaleontological evidence suggests that these oceanographic changes were likely caused by local tectonic movement (shallowing of the sedimentary basin in which theShimajiriGroup was deposited).