Storm modification of shallow marine sand-bar deposits
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概要
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The lower part of the Tatsukushi Formation in the Miocene Misaki Group, southwestern Shikoku, represents storm-modified shallow marine sand-bar deposits. Within the lower part of the Tatsukushi Formation, two principal components are distinguished; interbedded sandstones and mudstones, and thick-bedded sandstones. Sandstone beds of interbedded sandstones and mudstones are mainly hummocky cross-stratified. These sandstones are interpreted as the deposits on the margin of sand bars, and represent intermittent as the sand supply on the mud floor chirefly during storms. Thick-bedded sandstones are mainly composed of thick, amalgamated hummocky cross-stratified sand-stones. These sandstones are interpreted as the deposits of main body of sand bars which were intensely affected by storms. The transition from interbedded sandstones and mud-stones to thick-bedded sandstones are either gradually or abrupt. The gradual transition implies outgrowth of a sand bar. The abrupt transition is marked by a remarkable erosion surface at the base of the thick-bedded sandstone unit, and is suggestive of deep erosion caused by intense outflow from river mouths. In the formation of the Tatsukushi sand bars, sand transport and deposition were basically controled by fluvial discharge from the river mouth. Storm waves reworked bar sands to mold hummocky cross-stratified sandstones. Consequently, records of fluvial-influenced sedimentation were mostly unpreserved in rock records.
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