We have undertaken a detailed study of the geology and depositional environments of the Bandodani Formation in the Upper Cretaceous Izumi Group of Shimada-jima Island, north Tokushima, Shikoku, Japan. Shimada-jima Island is a well-known locality for the occurrence of the trace fossil
Archaeozostera, which had treated as the ancestor of the
Zosteracea seagrass. We have delineated the stratigraphic position of the
Archaeozostera-bearing interval in the Bandodani Formation distributed in Shimada-jima. Furthermore, we have constrained the depositional conditions suitable for the habitat of the
Archaeozostera-animals in submarine fan systems.
Facies analysis has revealed that the Bandodani Formation mainly comprises repeating channel-fill and inter-channel deposits, which were deposited in the frontal splay setting of a submarine fan system. Furthermore, a large-scale coarsening-upwards trend is observed throughout the formation and can be interpreted as the result of a prograding submarine fan system. In the studied section, the occurrence of
Archaeozostera is restricted to the mud-dominated lowermost interval. This implies that the
Archaeozostera-animals preferred relatively stable bottom conditions that were free from severe physical disturbances caused by abrupt and frequent aggradation and/or degradation of the seafloor associated with the sedimentation of the turbidites. Moreover, the ‘
Archaeozostera-bearing interval’ represents a useful marker horizon to identify the contemporaneous stratigraphic interval.
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