Stratigraphy and conodont and ammonoid ages of Upper Triassic Nakijin Formation in Hedomisaki area, Okinawa-jima, Japan

Abstract

We present a lithostratigraphic, biostratigraphic (conodont and ammonoid), and petrographic study of Upper Triassic bedded limestones (Nakijin Formation) from Hedomisaki, northern Okinawa Island, Japan. The Hedomisaki limestones comprise two formations: Upper Triassic bedded limestone of the Nakijin Formation and massive limestone of the Hedomisaki Formation of unknown age. The Nakijin Formation is divided into three members: (1) lower = dark gray micritic limestone (ca. 110 m thick); (2) middle = clastic limestone (ca. 180 m thick); and (3) upper = clastic limestone with slump beds (ca. 110 m thick). Biostratigraphic datums based on conodont and ammonoid fossils show that the Nakijin Formation was deposited in the early Carnian through the middle Norian. Microfacies in the Nakijin Formation are classified into four major facies: (1) radiolarian limestone–mudstone; (2) thin-shelled bivalve mudstone–wackestone; (3) litho–bioclastic packstone; and (4) peloidal–bioclastic packstone. Facies 1 and 2 are dominated by planktonic biota, indicating a deep-water pelagic environment. The clastic limestones (3, 4) in the middle to upper member of the Nakijin Formation were likely deposited in a shallow-water environment on and around an oceanic seamount in an open ocean setting.