Paleovalley systems: Insights from Quaternary analogs and experiments
Paleovalley systems: Insights from Quaternary analogs and experiments
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DOI：
10.1016/j.earscirev.2012.09.003
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发表时间：
2013-01-01
影响因子：
12.1
通讯作者：
Garvin, M.
中科院分区：
文献类型：
--
作者：
Blum, M.;Martin, J.;Garvin, M.
古代河谷系统长期以来被认为是地层记录中的重要特征，但随着第一代层序地层概念的出版而成为特别关注的焦点。本文从第四纪类比和实验研究的角度回顾了目前对古河谷系统的认识。古河谷系统可以包括不同的混合基岩冲积层、沿海平原和跨陆架段。混合基岩冲积段寿命较长，穿过年龄明显较老的基岩，并且具有整体退化的结构。相比之下，沿海平原和跨陆架段是对相对海平面变化的高频周期的非平衡响应：大多数沿海平原和跨陆架段形成为对相对海平面下降的几何响应，因为河流系统穿过沿海平原和内陆架斜层，并向盆地延伸以追踪海岸线。在切割和跨陆架延伸之后，横向河道迁移和同时发生的河道带沉积形成了山谷规模的特征。沿海平原和跨陆架古河谷的宽度是由这一时期沉积的河道带砂体的数量决定的。古河谷系统在源-汇沉积物路径中发挥着关键作用。早期的观点包括响应相对海平面下降的切口模型和完整的沉积物旁路模型。然而，这个模型经不起经验、理论或实验的检验。相反，在不断演变的山谷的切割、沉积和沉积物输出之间存在着复杂的动态：切割周期与沉积物输出最小值相对应，而横向迁移和河道带建设的周期则导致流向河口的流量增加。不断演变的山谷的沉积物输出以及跨陆架运输过程中排水系统的合并，在沉积物转移到陆架边缘以及与成因相关的斜坡到盆地底系统的过程中发挥着关键作用。根据河流和陆架的梯度以及高频海平面变化的幅度，河口和陆架边缘之间的连接可能发生在不同的时间段。晚第四纪的模拟和实验研究为古谷系统提供了另一种层序地层解释。在沿海平原古河谷中，底部河谷充填表面满足不整合面和经典定义的层序边界的标准：然而，该表面大部分与上覆河流沉积物具有相同的年龄，并且不对应于长期的切割和沉积物旁路。在跨陆架古河谷中，河流和三角洲或岸面沉积物之间的基底接触通常被解释为层序边界，但不是以切割和沉积物旁路为特征的不整合面。相反，该表面是分隔成因相关的河流地层和三角洲地层的相接触面：与沿海平原古河谷内的基底河谷充填面相关的表面，倾斜于跨陆架进积的三角洲和/或滨岸地层之下，这些地层由演化河谷本身内的沉积物供给，并且应该是下塌面。许多问题在未来值得关注。我们强调了解古谷系统形成和演化过程中的固有规模和物理过程。我们还建议同种异体强迫与自体动力学的相对作用，以及高频均衡调整的潜在意义应该成为未来讨论的主题。 (c) 2012 Elsevier B.V. 保留所有权利。
Ancient fluvial valley systems are long recognized as important features in the stratigraphic record, but emerged as a specific focus of attention with publication of first-generation sequence-stratigraphic concepts. This paper reviews current understanding of paleovalley systems from the perspective of Quaternary analogs and experimental studies.Paleovalley systems can include distinct mixed bedrock alluvial, coastal-plain, and cross-shelf segments. Mixed bedrock alluvial segments are long-lived, cut across bedrock of significantly older age, and have an overall degradational architecture. By contrast, coastal-plain and cross-shelf segments are non-equilibrium responses to high-frequency cycles of relative sea-level change: most coastal-plain and cross-shelf segments form as a geometric response to relative sea-level fall, as river systems cut through coastal-plain and inner shelf clinothems, and extend basinward to track the shoreline. After incision and cross-shelf extension, lateral channel migration and contemporaneous channel-belt deposition creates a valley-scale feature. Coastal-plain and cross-shelf paleovalley widths are set by the number of channel-belt sandbodies deposited during this time.Paleovalley systems play a key role in source-to-sink sediment routing. Early views included the model of incision and complete sediment bypass in response to relative sea-level fall. However, this model does not stand up to empirical, theoretical, or experimental scrutiny. Instead, there is a complex dynamic between incision, deposition, and sediment export from an evolving valley: periods of incision correspond with sediment export minima, whereas periods of lateral migration and channel-belt construction result in increased flux to the river mouth. Sediment export from evolving valleys, and merging of drainages during cross-shelf transit, play key roles in sediment transfer to the shelf-margin and genetically-linked slope to basin-floor systems. Connection between the river mouth and the shelf margin likely occurs for different periods of time depending on gradient of the river and shelf, as well as amplitude of high-frequency sea-level changes.Late Quaternary analogs and experimental studies provide an alternative sequence-stratigraphic interpretation for paleovalley systems. In coastal-plain paleovalleys, basal valley-fill surfaces meet criteria for an unconformity and a classically-defined sequence boundary: however, this surface is mostly everywhere of the same age as overlying fluvial deposits, and does not correspond to a long period of incision and sediment bypass. In cross-shelf paleovalleys, the basal contact between fluvial and deltaic or shoreface deposits is commonly interpreted as a sequence boundary, but is not an unconformity characterized by incision and sediment bypass. Instead, this surface is a facies contact that separates genetically-related fluvial and deltaic strata: the surface that correlates to the basal valley-fill surface within the coastal-plain paleovalley dips below cross-shelf prograding deltaic and/or shoreface strata, which are fed by deposition within the evolving valley itself, and should be the downlap surface.Many issues deserve attention in the future. We have stressed understanding the inherent scales and physical processes that operate during the formation and evolution of paleovalley systems. We also suggest the relative roles of allogenic forcing vs. autogenic dynamics, and the potential significance of high-frequency isostatic adjustments should be topics for future discussion. (c) 2012 Elsevier B.V. All rights reserved.