(8) Depositional facies containing vertebrate fossils in the Upper Cretaceous Himenoura Group on the Koshikijima Islands, Kagoshima, Kyushu, Japan

From Stratodynamics

Authors

Yuka Miyake1, Toshifumi Komatsu1, Makoto Manabe2, Ren Hirayama3 and Takanobu Tsuihiji4

1Kumamoto Univ., 2Natl. Mus. Nat. Sci., 3Waseda Univ., 4Tokyo Univ.

e-mail : y.m.ctty@gmail.com

Abstract

Several new vertebrate fossil sites were found recently in the Kashima and Taira areas, Koshiki-jima Islands, western Kagoshima Prefecture, Kyushu, Japan. In these areas, the Upper Cretaceous Campanian to Maastrichitian Himenoura Group crops out widely, and is composed mainly of non-marine and shallow marine deposits. We recognized fluvial, tidal flat, shoreface, shelf and slope facies in the Imuta and Taira formations on the basis of facies analysis. Non-marine vertebrate fossils such as teeth and bones of reptiles (including dinosaurs, crocodilians and aquatic turtles) and fish scales were found in these various depositional environments from fluvial to submarine slope.

Abundant vertebrate remains were obtained from lag deposits on the basal surface of fluvial channel, sand bar, point bar and swamp in the back marsh. The swamp and these bar deposits yielded moderately well-preserved remains composed of teeth and bones of theropods and crocodilians, isolated shells of trionychoid aquatic turtles (Adocus sp.; Carettochelyidae, gen., et sp. indet; Trionychidae, gen., et sp. indet), and a fish bone having an enameloid layer on the surface (probably an infraorbital) in the Kashima area. In the swamp deposits composed of organic-rich black mudstone and rarely containing laminated sandy layers, in-situ roots, and plants, vertebrate fossil remains form sparsely scattered micro-bone beds (40cm in thickness). In addition, poorly-preserved fragments of vertebrate remains are common in fluvial channel-fill lag deposits. Several costal fragments of turtles (e.g. Trionychidae, gen., et sp. indet) and many bone fragments were found in the tidal flat, tidal sand bar and tidal channel deposits, and co-occurred with oyster (Crassostrea) colonies preserved in-situ and brackish-water bivalves such as Crassostrea and Corbula. These vertebrate remains are characterized by poor preservations and disarticulations, and are obtained from tidal sand bar deposits (tidal bundle sandstone beds) containing some double mud drapes. Poorly-preserved costal fragments of soft-shelled turtle (Trionychidae, gen., et sp. indet) and vertebrate bones were occasionally preserved in the debris flow deposits composed of gravelly mudstone, and co-occured with shallow marine and brackish-water bivalves (e.g. Glycymeris amakusensis, Corbula ushibukensis and Crassostrea sp.), extraformational clasts and abundant rip-up mud clasts. Costal fragments of soft-shelled turtle were found in various depositional facies indicating fluvial, costal and slope environments. Such fragments from slope deposits had probably been transported to offshore environments by debris flow.