Deep-marine strata consisting of a sandy basal part overlain sharply by a muddier and mud-clast-rich upper part are increasingly being recognized in the deep-marine sedimentary record, and have been termed linked debrites, cogenetic debrite–turbidite beds, hybrid event beds, transitional-flow deposits, and bipartite beds. These composite strata are generally reported to develop downflow of clean (less muddy) sand, but the variable distance (hundreds of meters to tens of kilometers) over which this lithological change is reported to take place has resulted in a variety of physical depositional models. As such, the details about the origin and spatial evolution of these admixed sand–mud strata still remain poorly understood. Part of this uncertainty is that many earlier studies, with a few exceptions, have been based on macroscopic observations in drill core or discontinuous outcrops, and therefore interpreted bed continuity is based on similarity of textural or other sedimentological attributes. However, in slope to proximal basin-floor deposits of the Neoproterozoic Windermere Supergroup, and distal basin-floor deposits of the Ordovician Cloridorme Formation, exceptional outcrop exposure permitted individual two-part (bipartite) facies and their associated strata to be continuously traced and lithological changes to be documented along the strike of the outcrop.In both the Windermere and the Cloridorme, the bipartite facies consists of a lower sandy (25–60% matrix) part overlain sharply by a muddier (40–80% matrix), commonly mud-clast-rich upper part, and occurs downflow of muddy sandstone and upflow of sandy mudstone in a hundreds-of-meters long proximal to distal depositional continuum. These strata are then overlain everywhere by a thin- to very thin-bedded, traction-structured sandstone and/or silty mudstone cap. Notably, the interface separating the lower sand-rich part from the upper mud-rich part in the bipartite part of the depositional transect is always planar in the Windermere, but often alternates between planar and irregular in the Cloridorme. The continuum is interpreted to represent deposition downflow of an avulsion node that resulted in erosion of the local mud-rich seafloor and charged the flow with fine-grained sediment, resulting in increased effective fluid viscosity and significant changes in the characteristics, intensity, and transfer of fluid turbulence needed to maintain the particle suspension. Collapse of the suspension and development of overcapacity conditions resulted in rapid particle settling in the now rapidly depleting and negligibly sheared mud-enriched suspension that formed along the margins of the avulsion-wall jet in proximal-basin-floor and slope deposits (Windermere), or at the downflow terminus of the flow in more distal basin-floor deposits (Cloridorme). Additionally, when normalized to the total length of the continuum, the transition from muddy sandstone to bipartite facies and bipartite facies to sandy mudstone are equally proportioned in all examples, suggesting that, once formed, the depleting suspension evolved in a spatially systematic and dimensionally proportionate manner—a consistent depositional evolution difficult to reconcile with two mechanistically different parts to the flow. Moreover, differences in the morphology of the bipartite interface (i.e., planar vs. irregular) is not a primary depositional feature, but instead is interpreted to be a consequence of local postdepositional deformation of a previously continuous planar surface.

中文翻译：

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两部分（贫泥上覆富泥）及相关深水地层的地层特征和沉积成因：与可忽略剪切的富泥悬浮液中的颗粒沉降有关的横向沉积连续体中的成分

深海沉积记录中越来越多地识别出由砂质基底部分与较泥质和富含泥屑的上部急剧覆盖的深海地层，并被称为连接碎屑岩、共生碎屑岩-浊积岩层、混合事件床、过渡流沉积和二分床。据报道，这些复合地层通常会形成清洁（较少泥泞）沙子的下流，但据报道发生这种岩性变化的不同距离（数百米到数十公里）导致了各种物理沉积模型。因此，关于这些混合砂泥地层的起源和空间演化的细节仍然知之甚少。这种不确定性的一部分是许多早期的研究，除了少数例外，已经基于钻芯或不连续露头的宏观观察，因此解释的床连续性是基于纹理或其他沉积学属性的相似性。然而，在新元古代温德米尔超群的近端盆底沉积物斜坡和奥陶系克罗多姆组的盆底远端沉积物中，异常的露头允许对单个两部分（二部分）相及其相关地层进行连续追踪和岩性沿露头走向的变化有待记录。在温德米尔和克罗里多姆，二分相由较低的沙质（25-60% 基质）部分组成，上面覆盖着较泥质（40-80% 基质），通常是泥-富含碎屑的上部，近远部沉积连续体近百米长为泥质砂岩下流和砂质泥岩上流。然后，这些地层处处被薄层至极薄层、牵引结构砂岩和/或粉质泥岩盖层覆盖。值得注意的是，沉积样带二分部中下部富砂部与上部富泥部的分界面在温德米尔地区始终为平面，而在克罗里多姆地区则经常在平面和不规则之间交替出现。连续体被解释为代表撕裂节点的沉积向下流，导致局部富含泥浆的海底受到侵蚀，并使流中充满细粒沉积物，导致有效流体粘度增加，特征、强度、强度发生显着变化，以及维持颗粒悬浮所需的流体湍流转移。悬浮液的坍塌和产能过剩条件的发展导致颗粒迅速沉降在现在迅速消耗和可忽略不计的剪切富含泥浆的悬浮液中，该悬浮液沿近端盆底和斜坡沉积物（温德米尔）的撕裂壁射流边缘形成，或在更远端的盆底沉积物（Cloridorme）中的流下流终点。此外，当归一化到连续体的总长度时，从泥质砂岩到二分相和二分相到砂质泥岩的转变在所有示例中的比例相同，这表明，一旦形成，耗竭悬浮液以空间系统和尺寸比例的方式演化——一致的沉积演化难以与流动的两个机械不同部分协调一致。此外，二分界面形态的差异（即平面与不规则）不是主要的沉积特征，而是被解释为先前连续平面表面的局部沉积后变形的结果。