Bidirectional and unidirectional paleocurrents, inferred from orientation of shells in a Retceramus-concentrated bed from the middle Jurassic, Oshika Peninsula, Japan

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Mytilus-type and cylinder-type bioclasts in a shell bed of the Tsukinoura Formation, Oshika Group, were interpreted for paleocurrent analysis. The bioclasts, composed mainly of Retceramus cf. galoi, belemnites and plant fragments, are scattered on the shell bed; therefore they may sufficiently reflect the paleocurrent that formed the shell bed. Orientation of the beaks of R. cf. galoi and a solemarking in the shell bed suggest a current toward east-northeast. R. cf. galoi on the shell bed imbricate toward both northwest and southeast. In addition, the orientation of the longitudinal axis of belemnites, plant fragments and R. cf. galoi on the shell bed indicates waves having a southwest-northeast axis. The beaks of R. cf. galoi have the deviation of southeast direction. Thus the shell bed may have recorded both bidirectional and unidirectional paleocurrents. According to this interpretation, the shell bed would have been formed either by 1) a current toward east-northeast first, then waves having a southwestnortheast axis, or by 2) a combined-flow of a current toward east-northeast and waves having a southwest-northeast axis. If the southwest-northeast axis of the waves indicates the shoreline direction, the current toward east-northeast should have been parallel to the shoreline. This current may be a geostrofic current.
日本堆積学会の論文