Facies variebility and depositional processes of storm deposits

研究了日本西南部中新统村老卡组、东根组和三崎群风暴沉积的相变异性及其对上岸-陆架环境层流过程分异的响应，评价了浅海环境中风暴和非风暴过程的保存潜力。风暴引发的洋流使沙子沉积在浅水深处的显著振荡水流作用下。在近海下坡，水流的振荡分量随着水深的增加而迅速减弱，而单向分量仍然很强，足以输送沙子。在村冈陆架沉积序列中，厚厚的砂岩层往往被…覆盖更多厚厚的泥岩。这种对联代表了受沙可获得性控制的沉积作用。异常风暴产生的强大洋流将大量沙子带离海岸，导致一段很长时间内沙子匮乏。在此期间，风暴陆架上的沙可利用性极低，风暴事件很少记录在陆架沉积中。Togane组的海岸线沉积提供了上岸环境中风暴侵蚀和恢复阶段沉积的一个例子。风暴波或风暴增强的激流的反冲作用将沉积物冲进岸面，并在风暴高峰期产生的侵蚀面上作为滞后堆积粗质物质。在风暴减弱和随后的阶段，长周期的水井重新加工了以前运往近海的大部分沉积物，并以迁移沙坝的形式将它们带回岸上。三崎群的浅海砂坝沉积主要记录了风暴过程。然而，风暴很少导致沙粒堆积。相反，泥沙的输送和沉积基本上是由河口的河流排放控制的。风暴波改造基本上是由河口河道流量控制的，风暴波几乎完全改造了沙坝，受河流影响的沉积记录大多没有保存下来。较少

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