The Bithordites ichnofabric in the Pleistocene ocean current-generated sand ridge complex

Abstract

The ichnofabric characterised by Bichordites monastiriensis, a fossil burrow of the echinocardiid spatangoid echinoid, is first reported from the large-scale subaqueous dune (sand wave) deposits of the ocean current-generated shelf sand ridge complex of the middle Pleistocene Ichijiku Formation of the Boso penisula, Japan. It consists of dominant B. monastiriensis and associated Alcyonidiopsis, large Macaronichnus, Piscichnus, Scolicia, Skolithos and unnamed escape structures. Similar Bichordites-dominant ichnofabrics have been reported from Neogene to Quaternary sand wave deposits formed by various agents (such as tidal currents, storm-induced currents, or fair-weather wave-generated currents) whose depositional environments range from the deep sea floor to the shallow shelf. Although sand wave systems are commonly characterised by rapid sedimentation and highly shifting substrates, preventing colonisation by most benthic animals, rapid burrowers such as the echinocardiids were likely able to inhabit these regions regardless of the dominant depositional agents or the water depths of the habitats. The fossil record of Bichordites dates back to the Oligocene, although the trace fossil became particularly common in the Miocene and the known tracemakers (i.e., the echinocardiids and the maretiids) appeared in the Eocene. Moreover, these tracemakers now exhibit global distributions; therefore the ichnofabric described here may be used as an indicator of highly shifting substrates, such as those found in sand wave systems, in the Cenozoic.