Facies variebility and depositional processes of storm deposits

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Facies variability of storm deposits and its response to differentiation of strorm processes in upper shoreface to shelf environments were studied in the Miocene Muraolka Formation, Togane Formation and Misaki Group in southwestern Japan, and the preservation potential of storm and non-storm processes in a shallow marine environment was evaluated.The facies spectrum in the Muraoka storm deposits reveals that the storm beds were originated by waning combined-flows across the storm-wave base. Storm-induced currents deposited sand under significant oscillatory water-motion in a shallow depth. Offshore down slope, the oscillatory component of a flow rapidly diminished due to increasing water depth, whereas the unidirectional component was still intense enough to transport sand. The depositional current further transported and deposited finer sediments into deeper water below the storm-wave base.Within the succession of the Muraoka shelf deposits, thick sandstone beds tend to be covered by … More thick mudstones. Such couplets represent sedimentation controled by sand availability. Powerful currents generated by extraordinary storms carried considerable volumes of sand away from the coast and resulted in sand-starved condition for some long periods. During these periods, sand availability on the stormy shelf was extremely low, and storm events were seldom recorded in the shelf deposits.The shoreline deposits in hte Togane Formation provides an example of storm erosion and recovery stage deposition in an upper shoreface environment. Backwash action of storm waves or storm-enhanced rip currents awept 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 awells reworked much of the sediment previously transported offshore and returned them onshore in the form of migrating sand bars.Shallow marine bar deposits in the Misaki Group predominantly records storm processes. However, storms were seldom responsible for sand accumulation. Rather, sand transport and deposition were basically controled by fluvial discharge from the river mouth. Storm waves reworked were basically controled by fluvial discharge from the river mouth Storm waves reworked bar sands almost completely, and fluvial-influenced sedimentation records were mostly unpreserved. Less