Highly Variable Precambrian Fluvial Style Recorded In the Nelson Head Formation of Brock Inlier (Northwest Territories, Canada)
Highly Variable Precambrian Fluvial Style Recorded In the Nelson Head Formation of Brock Inlier (Northwest Territories, Canada)
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布洛克因利尔 (Brock Inlier) 尼尔森角地层 (加拿大西北地区) 记录的高度可变的前寒武纪河流样式
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发表时间：
2016
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影响因子：
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通讯作者：
R. Rainbird
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作者：
A. Ielpi;R. Rainbird
摘要沿着西北地区现代布罗克河的∼1 Ga Nelson Head地层壮观的峡谷暴露，为评估编织到弯曲河道的河流系统的植被前沉积提供了难得的机会。我们分析了这一地点16条叠置河道带的沉积学、构造学和沉积演化，显示出比典型的前寒武纪河流解释的沉积风格和地貌动力学更大的变异性。基于沉积结构和古水流综合分析的河流平面形态推断揭示了复杂的河道-平原演化模式，可识别出四个沉积阶段：(1)洪泛盆地-展布进积，并伴有边界风成改造；(2)以深分枝和突出的侧向增积为特征的游荡型河道带沉积；(3)向局限于冲积河谷内、以下游增积为主的浅水辫状河道带沉积；(4)受海洋影响的辫状河道带沉积，其下游、侧向和上游混合增积。总体而言，所研究的河道带具有显著的地貌动力学复杂性和地貌变异性，挑战了所有前志留纪河流都具有较差的沟道化和低弯曲度的假设。观测到的河体也有宽度：厚度比，与植被上升后的河流有很强的重叠。在游荡型和辫状型河道带，复杂的河道运移、沙坝淤积和解剖是河道内横向速度带发育、沉积地形引起的局部古水流扰动和撕裂等共同作用的结果。与区域范围和古水流以及相关地层单位向西北方向越来越远的特征相一致，所观察到的干流河道的规模与能够横穿劳伦斯克拉通数千公里的成熟水系相一致。
Abstract Spectacular canyon exposures of the ∼ 1 Ga Nelson Head Formation along the modern Brock River, Northwest Territories, provide a rare opportunity to assess the deposits of pre-vegetation, braided to sinuous-channelized fluvial systems. We analyze the sedimentology, architecture, and depositional evolution of 16 stacked fluvial-channel belts at this site, demonstrating greater variability in sedimentary style and morphodynamics than is typically interpreted for Precambrian rivers. Inferences on fluvial planview based on integrated analysis of depositional architecture and paleoflow reveal complex patterns of channel-planform evolution, and four depositional stages are recognized: (1) floodbasin-splay progradation with bounding episodes of eolian reworking; (2) deposition in wandering-channel belts characterized by deep anabranches and prominent lateral accretion; (3) progressive shift towards shallow, braided-channel belts confined within alluvial valleys and dominated by downstream accretion; and (4) deposition by marine-influenced braided-channel belts with mixed downstream, lateral, and upstream accretion. Overall, the studied fluvial-channel belts point to significant morphodynamic complexity and geomorphic variability, challenging the assumption that all pre-Silurian rivers shared poor channelization and low sinuosity. Observed channel bodies also have width:thickness ratios strongly overlapping with those of rivers postdating the rise of vegetation. In both wandering- and braided-channel belts, complex patterns of channel migration, bar accretion, and dissection were the result of co-acting processes such as development of transverse-velocity zones within the channels, local paleoflow disturbance induced by depositional topography, and avulsion. In concert with the regional extent and paleoflow, and increasingly distal character of correlative stratigraphic units towards the northwest, the magnitude of the observed trunk-fluvial channels is consistent with a mature drainage capable of transecting the Laurentian craton over thousands of kilometers.