The Psilonichnus Ichnofacies: An Example from the Iwaki Formation, Shiramizu Group, Joban Coal Field, Japan

The Psilonichnus Ichnofacies, as expressed in the Eocene Iwaki Formation, Shiramizu Group, Joban Coal Field, Japan, consists of suites of a single ichnospecies, namely as Psilonichnus quietis (Myo Myint 2001). These suites reflect a single faunal community, which is well distributed throughout the study area. Elements of the Psilonichnus Ichnofacies fall into three burrow types and represent ephemeral to semi-permanent dwelling structures coupled with breeding behaviors, produced by the deposit-feeding and/or scavenging trophic modes of ocypodid crabs such as Ocypode and/or Uca. The substrate consistency associated with suites of the Psilonichnus Ichnofacies indicates well-drained softgrounds or coherent stiffgrounds, reflecting periods of prolonged subaerial exposure at the high tide level. The Psilonichnus Ichnofacies of the Iwaki Formation occurs in tidal-fluvial settings with periodically brackish-water levee and interchannel environments, as well as more marine lagoonal beach and backshore environments of a probable deltaic system. The Psilonichnus Ichnofacies is erosionally overlain by tidal deposits hosting the Skolithos Ichnofacies, and indicates that the boundary between reflects transgressive ravinement. The position of the paleoshoreline and its migration are delineated by the relationship between the Psilonichnus Ichnofacies and the overlying Skolithos Ichnofacies.