Deep-water channel and levee deposits are common depositional elements on modern and ancient continental slopes. Unlike their channel counterparts, the spatial and temporal evolution of levee stratigraphy is much less well understood, in part because of the typically more recessive nature of levee deposits in the ancient sedimentary record, and sparse, widely spaced core control or seismic images of insufficient resolution in the modern. Moreover, it is generally inferred that levee development, at least in part, precedes the main phase of channel filling, the reasons for which remain largely unknown. In the Isaac Formation of the Windermere Supergroup (Neoproterozoic) of east-central British Columbia, Canada, well-exposed levee deposits are divided vertically into packages, each consisting of a sand-rich lower part overlain sharply by a mud-rich upper part. The lower part (3–10 m thick) consists mostly of medium- to thick-bedded, upper medium- to coarse-grained, lower-division turbidites intercalated with thin-bedded, fine-grained, upper-division turbidites. Along depositional strike away from channel-fill margins, the thickness of lower-division turbidites exhibit a distinctive thickening and then thinning over a few hundreds of meters that results in a similar thickening and thinning of the entire lower part of a package. The upper part (3–16 m thick) consists mostly of thin-bedded, fine-grained, upper-division turbidites intercalated with uncommon medium- to thick-bedded, medium-grained, lower-division turbidites. Significantly, the thickness of very thin- and thin-bedded turbidites in the upper part generally decreases stratigraphically upward whereas the thickness of intercalated medium- and thick-bedded turbidites changes little. The lateral and vertical changes in these deposits suggest that channelized flows were initially coarse grained and moderately well-sorted, causing them to exhibit negligible density stratification, and therefore high flow efficiency. We interpret that the velocity maximum occurred above the height of the incipient channel margins, thereby allowing the lower, coarse-grained, dense part of flows to easily overspill and deposit thick-bedded, coarse-grained turbidites in the lower part of each package. The sharp contact with the upper part of each package marks the point when relief from channel floor to levee crest exceeded the height of the velocity maximum in average throughgoing turbidity currents. Above this height, density of the flow decreased abruptly and consisted of significantly finer-grained sediment that overspilled to form the upper, finer-grained part of each package. Later the makeup of the sediment supply changed to a more polydispersed grain-size distribution, which caused the throughgoing currents to be more density stratified. This enhanced near-bed stratification and concentration effects, which in addition to intense interfacial mixing, resulted in rapid kinetic energy loss, and promoted deposition in the channel.

中文翻译：

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河道化流密度结构对深海大堤沉积地层结构的影响

深水河道和大堤沉积是现代和古代大陆斜坡上常见的沉积要素。与河道对应物不同，堤坝地层学的空间和时间演化知之甚少，部分原因是在古代沉积记录中堤坝沉积物通常具有更隐性的性质，以及稀疏、宽间距的岩心控制或分辨率不足的地震图像在现代。此外，人们普遍推断，堤坝的发展，至少在一定程度上，先于河道充填的主要阶段，其原因在很大程度上仍是未知的。在加拿大不列颠哥伦比亚省中东部温德米尔超群（新元古代）的 Isaac 组中，暴露良好的堤坝沉积物被垂直分割成包裹，每个都由一个富含沙子的下部组成，上面有一个富含泥浆的上部。下部（3-10 m 厚）主要由中至厚层、上部中至粗粒、下层浊积岩夹有薄层、细粒、上层浊积岩组成。沿着远离通道填充边缘的沉积走向，下层浊积岩的厚度表现出明显的增厚，然后在数百米范围内变薄，导致整个包裹体下部的类似增厚和变薄。上部（3-16 m 厚）主要由薄层、细粒、上层浊积岩夹杂不常见的中至厚层、中粒、下层浊积岩组成。显著地，上部极薄层和薄层浊积岩的厚度一般随地层向上递减，而夹层中厚层浊积岩的厚度变化不大。这些沉积物的横向和垂直变化表明，渠道化的流动最初是粗粒和适度良好的分选，导致它们表现出可忽略不计的密度分层，因此流动效率高。我们解释说，速度最大值发生在初始通道边缘的高度以上，从而允许流动的较低、粗粒、密集部分容易溢出并在每个包的下部沉积厚层、粗粒浊积岩。与每个包装上部的急剧接触标志着从渠道底部到堤坝顶部的起伏超过平均通过浊度流中速度最大值的高度。在这个高度之上，流动的密度突然下降，并且由明显更细粒度的沉积物组成，这些沉积物溢出形成每个包裹的上部、更细粒度的部分。后来沉积物供应的构成变为更分散的粒度分布，这导致通过的水流更加密度分层。这增强了近床分层和浓缩效应，除了强烈的界面混合外，还导致快速的动能损失，并促进了通道中的沉积。流动的密度突然下降，并且由明显更细粒度的沉积物组成，这些沉积物溢出形成每个包裹的上部，更细粒度的部分。后来沉积物供应的构成变为更分散的粒度分布，这导致通过的水流更加密度分层。这增强了近床分层和浓缩效应，除了强烈的界面混合外，还导致快速的动能损失，并促进了通道中的沉积。流动的密度突然下降，并且由明显更细粒度的沉积物组成，这些沉积物溢出形成每个包裹的上部，更细粒度的部分。后来沉积物供应的构成变为更分散的粒度分布，这导致通过的水流更加密度分层。这增强了近床分层和浓缩效应，除了强烈的界面混合外，还导致快速的动能损失，并促进了通道中的沉积。