Depositional environments and taphonomy of the bone-bearing beds of the Lower Cretaceous Kuwajima Formation, Tetori Group, Japan

Abstract

Thebone-bearing beds of the Lower Cretaceous Kuwajima Formation (Tetori Group) are described. Three facies of bone-bearing beds (Facies I: carbonaceous sandstones; Facies II: dark grey fine-grained silty sandstones; Facies III: dark greenish-grey mudstones) are present in inter-channel deposits that originated on a floodplain. The grain size of the sediments, and plant and molluscan fossils occurring in each bone-bearing bed, indicate that Facies I was deposited in a peat marsh, Facies II in a shallow lake, and Facies III in a vegetated swamp. Isolated small bones and teeth are the most abundant vertebrate fossils. Common elements in Facies II are aquatic vertebrates such as fishes and turtles. Facies III is characterized by the occurrence of terrestrial lizards, tritylodontid synapsids and mammals. Vertebrate fossil assemblages in Facies II and III are not mixed with each other even though they both represent parautochthonous assemblages. In contrast, Facies I is allochthonous, and is composed mostly of reworked sediments from Facies II. Depositional environments of the bone-bearing beds are strongly correlated with the composition of their fossil assemblages, indicating that different facies preserve the original faunal differences that existed between the shallow lake and vegetated swamp environments.