A production mechanism and process of the dish- and lens-shaped shell concentrations observed in the lower part of Tatamigaura Sandstone Member of the Middle Miocene Tougane Formation distributed in the northeastern part of Hamada City, Shimane, Japan were examined from the ichnologic and paleontologic viewpoints. The dish- and lens-shaped shell concentrations treated herein are visible on the bedding plane as the circular patches having 7 cm to 30 cm in diameter. In cross section, on the other hand, they are observed as an isolated, concave-up crescent or lens, of which thickness ranges from 1 cm to 5 cm. These shell concentrations consist mainly of graded, disarticulated bivalves, most of which show convex-down orientation, with less number of gastropods.
In addition to these shell-concentrations, well-preserved specimens of pothole-like trace fossil
Piscichnus waitemata, which is interpreted to be the products of feeding behavior of rays, occur below the shell concentrations-bearing interval. The Tatamigaura
P. waitemata occurring in fine-grained sandstone can be seen an isolated structure filled with graded, fragmented barnacles derived from the overlying coquina bed and less number of shells of host sediment origin. Morphology, size, and internal fabric of lower part of the
Piscichnus are closely similar to those of the dish- and lens-shaped shell concentrations. These lines of evidence strongly suggest that the dish- and lens-shaped shell concentrations appear to have formed by biogenic currents by the feeding behavior of rays. Invisibility of the whole morphology of
P. waitemata associated with dish- or lens-shaped shell concentrations is due to (1) extremely low contrast in grain size between the fill and the host sediment and (2) selective obliteration of the morphologic record at the middle to upper parts of
P. waitemata in terms of overprinting by the younger one and/or subsequently occurred.
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