長崎半島東岸長崎市北浦町の上部白亜系層序の再定義とその地質年代学的意義

Abstract

We propose a new stratigraphic unit, the Nagasaki Kitaura Formation, for the Upper Cretaceous sedimentary rocks on the coast of Kitaura, Nagasaki City, Nagasaki Peninsula, northwestern Kyushu, Japan, to replace the previously named Kitaura Formation. The Nagasaki Kitaura Formation is divided into two lithostratigraphic units that are in fault contact: the Akasakino-hana Sandstone and Mudstone Member (ASMM), the lower, thin unit (>13 m thick), and the Zatobama Gravelly Sandstone and Mudstone Member (ZGSMM), the upper, thicker unit (>140 m). The ASMM consists of shallow marine deposits yielding ammonoid (Polyptychoceras obatai and cf. Phylloceras sp.) and bivalve fossils including an inoceramid (Platyceramus japonicus), and an incomplete femur of a hadrosauroid dinosaur was unearthed from the fluvial ZGSMM. Based on the biostratigraphic range of Platyceramus japonicus (late Santonian? to early Campanian), Polyptychoceras obatai (late Santonian), and the 206 Pb/ 238 U dates of the detrital zircon from the ASMM (the youngest concordant date = 83.6±5.0 Ma; the weighted mean age of the youngest date cluster = 85.74±0.75 Ma; 95% confidence level), the Nagasaki Kitaura Formation is no older than the late Santonian and potentially extends to the Campanian in age. The main body of the formation, the ZGSMM, is correlated with the lower part of the Mitsuse Formation (middle Campanian) on the western Nagasaki Peninsula. The stratigraphy of the Nagasaki Kitaura Formation can be assessed in association with the depositional environments of the lower half of the Upper Cretaceous Himenoura Group in western Kyushu.