Little is known about the evolution and gradual abandonment of the fluvial channel and associated sedimentary architecture preserved in the ancient sedimentary rock record. This research aims to analyze the changes and evolution of a gradually abandoned fluvial valley and associated within-channel deposits and architecture from the rock record. This is achieved through detailed photogrammetry, outcrop sedimentology and quantification of palaeochannel parameters for the Oligocene palaeovalley fill of the north Western Desert basin of Egypt. The three-dimensional vertical Oligocene outcrops and trenches of the study area provide the opportunity to analyze sedimentary facies, ichnology, architectural elements and evolution of the gradually abandoned fluvial channel belt. This paper presents a detailed model for the gradual abandonment and evolution of the fluvial palaeovalley through the stepwise transition from braided to meandering channel systems. This includes the transition from a bedload-dominated, low sinuosity, braided channel that prominently features downstream-accreted, mid-channel and compound bars, to a small, mixed and suspended load-dominated intermediate sinuosity (underfit) channel that is characterized by heterolithic-dominated point bars, floodplain and mud-plugs via a large, sand-dominated, intermediate sinuosity channel with laterally accreted bars of transitional stage. The gradual abandonment of the fluvial palaeovalley is also associated with a successive decrease of the channel width and depth from >800 to 166 m and from 12 to 5 m, respectively. The successive abandonment is also associated with a gradual increase in the diversity and abundance of Scoyenia–Termitichnus ichnofacies groups, and vice versa in the Skolithos ichnofacies group. The successive decrease in channel width with gradual abandonment of the Oligocene palaeovalley is probably attributed to autogenic gradual avulsion, rather than allogenic. Comparison between the Oligocene palaeovalley fill and the modern analogues indicate the similarities in channel pattern changes and channel dimension through filling stages and differ from most modern counterparts because it is aggradational. This research enhances the understanding of the sedimentological characteristics and stratigraphic architecture of the gradually abandoned fluvial channels in the rock record and introduces insights for prediction of within-channel facies and architectural heterogeneities of the fluvial channel reservoirs.

中文翻译：

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逐渐废弃的渐新世河流古河谷填充物的演化：阿布-罗阿什地区，西北沙漠，埃及

人们对古代沉积岩记录中保存的河流河道和相关沉积结构的演化和逐渐废弃知之甚少。本研究旨在从岩石记录中分析逐渐废弃的河流河谷以及相关的河道内沉积物和建筑的变化和演化。这是通过详细的摄影测量、露头沉积学和对埃及西北部沙漠盆地渐新世古河谷填充物的古河道参数进行量化来实现的。研究区的三维垂直渐新世露头和海沟为分析逐渐废弃的河流河道带的沉积相、地层学、建筑元素和演化提供了机会。本文通过从辫状河道系统到蜿蜒河道系统的逐步过渡，提出了河流古河谷逐渐废弃和演化的详细模型。这包括从以下游积聚、中间通道和复合杆为突出特征的以床载为主的低曲率编织通道过渡到以异石为特征的小型、混合和悬浮载荷为主的中间曲率（欠拟合）通道- 主要的点坝、洪泛区和泥塞，通过一个大型的、以砂为主的、中间曲折的河道和过渡阶段的横向增生坝。河流古河谷的逐渐废弃还与河道宽度和深度分别从 >800 到 166 m 和从 12 到 5 m 的连续减小有关。Scoyenia–Termitichnus ichnofacies 群，在Skolithos 中反之亦然异相组。随着渐新世古河谷的逐渐废弃，河道宽度的连续减少可能归因于自体逐渐撕脱，而不是同种异体。渐新世古河谷充填与现代类似物之间的比较表明，在充填阶段的通道模式变化和通道尺寸方面有相似之处，并且与大多数现代对应物不同，因为它是累积的。这项研究增强了对岩石记录中逐渐废弃的河道沉积学特征和地层结构的理解，并为预测河道储层的河道内相和结构非均质性提供了见解。