Storm sedimentation and its preservation potential in shallow marine sand-bar deposits

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Storm flow dynamics and the mechanism of storm flow generation were studied in the Miocene Muraoka Formation, To^^^gane Formation and Misaki Group in southwestern Japan, and the preservation potential of storm processes in ancient, shallow marine sand-bar deposits were examined.The storm deposits in the Muraoka Formation reveal clear facies spectrum from hummocky cross-stratified sandstones to base-cut-out turbidites. This facies spectrum reflects the change in storm flow characteristics across the storm-wave base and is suggestive of the generation of turbidity currents as a storm-induced depositional current. A turbidity current interpretation is supported by off-shore directed paleocurrents shown both by sole marks of hummocky cross stratified sandstones and by ripple cross laminations in turbidites. Thick, storm sandstone beds tend to be covered by thick mudstones. Such couplets represent storm sedimentation controlled by sand availability. Powerful currents generated by extraordin … More ary storms carried considerable volumes of sand away from the coast and resulted in sand-starved condition in coastal zone for some long periods. Lack in coastal sand prevented the generation of powerful turbidity currents even in major storm events. Consequently sand availability on the stormy shelf was extremely low, and storm events were seldom recorded in the shelf deposits.The shoreline deposits in the To^^^gane Formation preserve the sedimentation records of a storm-fairweather cycle in an upper shoreface environments. Backwash action of storm waves or storm-enhanced rip currents swept sediments into the shoreface and emplaced coarse materials as a lag on the erosion surface produced in the peak stage of a storm. During the declining and following stage of storms, long-period swells reworked much of the sediment previously transported offshore and returned them onshore in the form of migrating longshore bars, recovering the shore profile. Longshore bars did not weld to the beachface, and barred system produced in the recovery stage was maintained during fairweather periods.The Misaki Group provides a good example of storm-reworked, shallow marine sand-bars.The Misaki sand-bar deposits are dominantly hummocky cross stratified. However, storms were seldom responsible for sand accumulation. Rather, sand transport and deposition were basically controlled by fluvial discharge from the river mouth. Storm waves reworked bar sands almost completely, and fluvial-influenced sedimentation records were mostly unpreserved. Less