A hyperpycnal flow forms when a relatively dense land-derived gravity flow enters into a marine or lacustrine water reservoir. As a consequence of its excess of density, the incoming flow plunges in coastal areas, generating a highly dynamic and often long-lived dense underflow. Depending on the characteristics of the parent flow (flow duration and flow rheology) and basin salinity, the resulting deposits (hyperpycnites) can be very variable. According to flow duration, land-derived gravity flows can be classified into short-lived or long-lived flows. Short-lived gravity flows last for minutes or hours, and are mostly related to small mountainous river discharges, alluvial fans, collapse of natural dams, landslides, volcanic eruptions, jökulhlaups, etc. Long-lived gravity flows last for days, weeks or even months, and are mostly associated with medium- to large-size river discharges. Concerning the rheology of the incoming flow, hyperpycnal flows can be initiated by non-Newtonian (cohesive debris flows), Newtonian supercritical (lahars, hyperconcentrated flows, and concentrated flows) or Newtonian subcritical flows (pebbly, sandy or muddy sediment-laden turbulent flows). Once plunged, non-Newtonian and Newtonian supercritical flows require steep slopes to accelerate, allow the incorporation of ambient water and develop flow transformations in order to evolve into a turbidity current and travel further basinward. Their resulting deposits are difficult to differentiate from those related to intrabasinal turbidites. On the contrary, long-lived Newtonian subcritical flows are capable of transferring huge volumes of sediment, freshwater and organic matter far from the coast even along gentle or flat slopes. In marine settings, the buoyant effect of interstitial freshwater in pebbly and sandy hyperpycnal flows can result in lofting due to flow density reversal. Since the excess of density in muddy hyperpycnal flows is provided by silt-clay sediments in turbulent suspension, lofting is not possible even in marine/saline basins. Muddy hyperpycnal flows can also erode the basin bottom during their travel basinward, allowing the incorporation and transfer of intrabasinal sediments and organic matter. Long-lived hyperpycnal flow deposits exhibit typical characteristics that allow a clear differentiation respect to those related to intrabasinal turbidites. Main features include (1) composite beds with gradual and recurrent changes in sediment grain-size and sedimentary structures, (2) mixture of extrabasinal and intrabasinal components, (3) internal and discontinuous erosional surfaces, and (4) lofting rhythmites in marine/saline basins.

中文翻译：

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高密度（超密度）流和沉积物

当相对密集的陆源重力流进入海洋或湖泊水库时，就会形成超重流。由于密度过高，流入的水流在沿海地区骤降，产生高度动态且通常持续时间长的密集底流。根据母流的特征（流动持续时间和流动流变学）和盆地盐度，由此产生的沉积物（超重晶石）可能变化很大。根据流动持续时间，陆源重力流可分为短寿命流和长寿命流。短寿命重力流持续数分钟或数小时，多与山地河流小流量、冲积扇、天然水坝坍塌、滑坡、火山爆发、冰河湖等有关。 长寿命重力流持续数天、数周甚至数天几个月，并且主要与中到大型河流排放有关。关于流入流的流变学，超重流可以由非牛顿流（粘性泥石流）、牛顿超临界流（火山泥流、超浓缩流和浓缩流）或牛顿亚临界流（卵石、砂质或泥质沉积物湍流）引发）。一旦下降，非牛顿和牛顿超临界流需要陡坡来加速，允许环境水并入并发展流动转变，以演变成浊流并进一步向盆地移动。它们产生的沉积物很难与与盆地内浊积岩有关的沉积物区分开来。相反，长寿命的牛顿亚临界流能够输送大量的沉积物，远离海岸的淡水和有机物质，即使沿着平缓或平坦的斜坡。在海洋环境中，间质淡水在卵石和沙质超重流中的浮力效应可能会由于流动密度反转而导致放样。由于泥质超重流中的过量密度是由湍流悬浮中的粉砂粘土沉积物提供的，因此即使在海洋/咸水盆地中也不可能进行放样。泥质超重流在向盆地移动过程中也会侵蚀盆地底部，从而使盆地内沉积物和有机质混合和转移。长寿命的超重流沉积物表现出典型特征，可以明确区分与盆地内浊积岩相关的那些沉积物。