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1.
A three‐dimensional numerical model of sediment transport, erosion and deposition within a network of channel belts and associated floodplain is described. Sediment and water supply are defined at the upstream entry point, and base level is defined at the downstream edge of the model. Sediment and water are transported through a network of channels according to the diffusion equation, and each channel has a channel belt with a width that increases in time. The network of channels evolves as a result of channel bifurcation and abandonment (avulsion). The timing and location of channel bifurcation is controlled stochastically as a function of the cross‐valley slope of the floodplain adjacent to the channel belt relative to the down‐valley slope, and of annual flood discharge. A bifurcation develops into an avulsion when the discharge of one of the distributaries falls below a threshold value. The floodplain aggradation rate decreases with distance from the nearest active channel belt. Channel‐belt degradation results in floodplain incision. Extrinsic (extrabasinal, allogenic) and intrinsic (intrabasinal, autogenic) controls on floodplain dynamics and alluvial architecture were modelled, and sequence stratigraphy models were assessed. Input parameters were chosen based on data from the Rhine–Meuse delta. To examine how the model responds to extrinsic controls, the model was run under conditions of changing base level and increasing sediment supply. Rises and falls in base level and increases in sediment supply occurred over 10 000 years. Rising base level caused a wave of aggradation to move up‐valley, until aggradation occurred over the entire valley. Frequency of bifurcations and avulsions increased with rate of base‐level rise and aggradation rate. Channel‐belt width varied with water discharge and the lifespan of the channel belt. Wide, connected channel belts (and high channel‐deposit proportion) occurred around the upstream inflow point because of their high discharge and longevity. Less connected, smaller channel belts occurred further down‐valley. Such alluvial behaviour and architecture is also found in the Rhine–Meuse delta. During base‐level fall, valley erosion occurred, and the incised valley contained a single wide channel belt. During subsequent base‐level rise, a wave of aggradation moved up‐valley, filling the incised valley. Bifurcation and avulsion sites progressively moved upstream. Relatively thin, narrow channel belts bordered and cut into the valley fill. These results differ substantially from existing sequence stratigraphy models. The increase in sediment supply from upstream resulted in an alluvial fan. Most bifurcations and avulsions occurred at the fan apex (nodal avulsion), and channel belts were the widest and the thickest here (giving high channel‐deposit proportion) due to their high discharge and longevity. The width and thickness of channel belts decreased down‐valley due to decreased discharge, longevity and aggradation rate. This behaviour occurs in modern alluvial fans. Intrinsic controls also affect floodplain dynamics and alluvial architecture. Variation of aggradation rate, bifurcation frequency and number of coexisting channel belts occurred over periods of 500 to 2000 years, compared with 10 000 years for extrinsic controls. This variation is partly related to local aggradation and degradation of channel belts around bifurcation points. Channel belts were preferentially clustered near floodplain margins, because of low floodplain aggradation rate and topography there. 相似文献
2.
Anatomy of an avulsion 总被引：12，自引：0，他引：12
NORMAN D. SMITH TIMOTHY A. CROSS† JOSEPH P. DUFFICY STEPHEN R. CLOUGH 《Sedimentology》1989,36(1):1-23
The Cumberland Marshes in east-central Saskatchewan, Canada, occupy over 5000 km2 and contain a variety of active and abandoned fluvial features, including straight to sinuous isolated channels, anastomosed channel systems, levees, and crevasse splays in addition to marshes, lakes and bogs. In 1873, an avulsion of the Saskatchewan River diverted most of its flow into a portion of the Cumberland Marshes (locally termed the breakout area), and altered the alluvial terrain as the invaded wetlands adjusted to the influx of sediment and water. These adjustments continue today, and so far over 500 km2 of wetlands have been affected by the avulsion. Avulsion-controlled modification of the wetlands involves the initiation and evolution of crevasse splays and splay complexes. Three intergradational forms are recognized, each associated with characteristic sand-body geometries. Stage I splays are small, lobate in plan, crossed by unstable distributary channels, and form wedge-shaped sheets which depositionally overlie fine-grained, organic-rich wetland sediments. Stage II splays and splay complexes evolve both spatially and temporally from Stage I splays. They are larger, contain dense networks of anastomosed channels, and form disconnected tabular sand bodies or continuous sand sheets, some of which incise underlying wetland sediments. Stage III splays develop from either Stage I or II splays and contain few but stable anastomosed channels that deposit isolated stringer sands encased in fine-grained floodplain sediments. Although sand bodies deposited by splays comprise important components of the evolving floodplain, various fine-grained facies occurring in levees, shallow lakes, abandoned splay channels, and interchannel floodplains dominate the avulsion deposits. The post-1873 record of deposition and terrain modification in the breakout area suggests four stages of floodplain evolution following avulsion. In the initial avulsion stage, new channels and splay complexes increase in numbers rapidly as diverted discharge of water and sediment overwhelm the adjacent floodbasin. The anastomosed stage is characterized by an approximate balance between rates of new channel and splay development, and abandonment of old ones. This stage continues for as long as new floodplain areas are invaded. The rate of new splay development eventually decreases as accessible floodplain becomes aggraded, forcing a higher rate of channel abandonment and concentration of remaining flow into fewer but larger channels (reversion stage). The result of reversion is eventual return to a single channel stage, completing the avulsive sequence and initiating a new alluvial ridge. For the Saskatchewan River, this final-stage single channel will likely produce a meander belt which occupies only a portion of the more extensive avulsion belt which preceded it. 相似文献
3.
Peat is most compressible of all natural soils. Compaction of peat layers potentially leads to substantial amounts of land subsidence. Peat is common in many distal parts of Holocene deltas, which are often densely populated. It is known that land subsidence due to peat compaction may have serious societal implications in such areas, as it may cause damage to construction works and lead to land inundation. Effects of peat compaction on the natural evolution of deltas are however poorly understood, whereas this might be an important control on delta evolution at both local and regional scales.The main objective of this paper is to review current knowledge concerning the peat compaction process and its effect on delta evolution in Holocene settings, and to identify gaps in this knowledge. An overview is given regarding: 1) the compaction process, 2) presumed and potential effects of peat compaction on delta evolution, 3) field methods to quantify peat compaction and 4) numerical models to calculate the amount and rate of peat compaction.Peat compaction and formation influence channel belt elevation, channel belt geometry and channel belt configuration. Last-mentioned aspect mostly concerns the influence of peat compaction on avulsion, which is one of the most important processes controlling delta evolution. Interactions between peat compaction, peat formation and avulsion have seldom been studied and remain unclear, partly because factors such as peat type, organic matter content, sediment sequence composition and groundwater table fluctuation are so far not taken into account. Peat compaction and formation potentially influence avulsion as 1) a decrease in accommodation space created by peat compaction underneath a channel causes superelevation and/or an increase in lateral migration, 2) the high cohesiveness of peat banks inhibits lateral migration, which increases bed aggradation, decreases sediment transport capacity and hence increases crevassing frequencies, which possibly evolve into an avulsion, although the low regional gradient in peatlands will hinder this, and 3) peat compaction and oxidation in flood basins following groundwater table lowering leads to relief amplification of channel belts. At delta scale, variations in compaction rates might stimulate the occurrence of nodal avulsions.To quantify effects of peat compaction on delta evolution, and to determine the relative importance of different factors involved, field research should be combined with numerical models describing peat compaction and formation. The model should be validated and calibrated with field data. 相似文献
4.
河流辫-曲转换特点与废弃河道模式 总被引：4，自引：2，他引：2
通过总结河流变迁规律,明确了辫-曲转换特征,建立了辫-曲过渡型河流的沉积序列;通过分析弯曲度与废弃河道沉积特征的相互关系,提出不同类型河流中存在分叉分流、串沟取直及颈项截直三类废弃河道,并提出三类废弃河道的分类依据,进而建立了相应的沉积模式。通过解剖北京西山军庄二叠系河流相野外露头,分析了辫状河、以曲流河及其过渡型河流的沉积特征与演变特点,识别出该露头中发育的废弃河道,进而明确该露头中产生辫-曲转换的地质因素是不同气候条件下,由于沉积供源的差异导致河道类型的变化。 相似文献
5.
西江和北江三角洲区的水沙特点及河道演变特征 总被引：11，自引：0，他引：11
通过对珠江三角洲区有关水文泥沙资料的分析, 初步揭示了西江和北江在流速与流量、水位、含沙量、断面宽深比之关系方面的差别。结合河道平面及断面形态和河道冲淤特性, 探讨了其河型特点及成因。认为研究区西江为典型的网状河流, 而北江仍处于网状河流的发育初期。该区网状河流多河道的形成遵循冲裂机制, 人工堤围加速了河道沉积、抑制了河漫滩地的淤积, 从而使河道及河漫滩地的加积表现为不均一性。 相似文献
6.
7.
Stratigraphically limited intervals from the Lower Eocene Willwood Formation contain laterally extensive carbonaceous shales and ribbon sandstone networks associated with channel avulsion. We present data from one such interval that documents the avulsion sequence. Vertical sections measured along the outcrop of this interval are similar and comprise a basal carbonaceous shale overlain by fine-grained deposits on which weakly developed, hydromorphic paleosols formed. The paleosols enclose and are locally incised by ribbon sandstones, some of which cut down to and partly through the carbonaceous shale. The ribbons have width/thickness ratios between 3 and 13. Some ribbons cluster at a particular stratigraphic level, which, together with paleocurrent trends, suggests that they formed channel networks. Sections are capped by yellow-brown paleosols showing moderate pedogenic development. We suggest that the carbonaceous shales developed in low-lying topogeneous swamps in distal portions of the floodplain far from the trunk channel. Such a location set limits on the sediment that they received. The mudrocks with weakly developed paleosols and associated ribbon sandstones are interpreted as crevasse-splay complexes resulting from avulsion of the trunk river. The ribbon sandstones represent ancient feeder channels of the avulsion complex. The rapid influx of avulsion deposits appears to have been crucial to preserving the organic material, and this study reveals an important and as yet uncharacterized link between trunk channel processes and the accumulation of organic-rich deposits in distal alluvial swamps. Similar deposits are found in other stratigraphic units in the Rocky Mountain region, and the development of these and other organic-rich deposits should be reassessed in terms of channel avulsion. 相似文献
8.
Alessandro Ielpi Martin R. Gibling Arden R. Bashforth Corinne Lally Michael C. Rygel Saif Al‐Silwadi 《Sedimentology》2014,61(6):1659-1700
Vegetation is a major driver of fluvial dynamics in modern rivers, but few facies models incorporate its influence. This article partially fills that gap by documenting the stratigraphy, architecture and palaeobotany of the Lower Pennsylvanian Boss Point Formation of Atlantic Canada, which contains some of the Earth's earliest accumulations of large woody debris. Braided‐fluvial systems occupied channel belts of varied scale within valleys several tens of metres deep and more than 12 km wide, and their deposits predominantly consist of sandy and gravelly bedforms with subordinate accretionary macroforms, high flow‐strength sand sheets and rippled abandonment facies. Discrete accumulations of clastic detritus and woody debris are up to 6 m thick and constitute at least 18% of the in‐channel deposits; they represent lags at the base of large and small channels, fills of minor channels and sandy macroforms that developed in central positions in the upper parts of channel fills. Sandstones with roots and other remnants of in situ vegetation demonstrate that vegetated islands were present, and the abundance of discrete channel fills suggests that the formation represents an anabranching, island‐braided sandbed river, the earliest example documented to date. Although some sphenopsid and lycopsid remains are present, most woody fragments are derived from cordaitalean trees, and the evolution of this group late in the Mississippian is inferred to have exerted a significant influence on fluvial morphodynamic patterns. The formation records a landscape in which active channel belts alternated with well‐drained floodplains colonized by dense, mature forests and local patches of pioneering, disturbance‐tolerant vegetation. Lakes and poorly drained floodplains dominated by carbonate and organic deposition, respectively, were also present. A large supply of woody debris triggered channel blockage and avulsion, and active channel margins and islands within the channel belts were initially colonized by pioneer vegetation and subsequently stabilized by large trees. A similar alternation of stable and unstable conditions is observed in modern braided rivers actively influenced by vegetation. 相似文献
9.
This paper examines the spatial distribution of the functional systems in the different belts of settlements around Budapest. The surburban belts have an important role in the capital's labour market, places of employment and recreation with 45 settlements. The paper reviews the structure of the Budapest agglomeration from the core to the three inner, middle and outer belts comprising the suburbs, the semi-urban settlements, the suburban villages, the satellite towns, and the satellite villages. Then the system of the functional urban centres are reviewed, The author establishes the concept of the main, the division, the district and the settlement group centres in the Budapest agglomeration. The different levels of attached functions, such as international, national, regional and local are also described with each centre type. The areal distribution of the centre system of Budapest is shown in maps too. 相似文献
10.
Lawson M. Smith 《Engineering Geology》1996,45(1-4):139-165
The Lower Mississippi Valley (LMV) has been one of the most intensively studied alluvial valleys in the world in terms of it's geological and geomorphic framework and history. A brief outline of the history of the major geological and geomorphological investigations of the LMV is provided. The results of these investigations are discussed in terms of the fluvial geomorphic framework of the valley and the apparent significant changes in the regime of the Mississippi River during the Late Wisconsinan and Holocene stages.
The LMV occupies the broad deep synclinal trough of the Mississippi Embayment which extends from Cairo, Illinois, to the Gulf of Mexico in a slightly sinuous north-south trend. The embayment is filled with a north to south thickening wedge of non-marine and marine sediment ranging in age from Jurassic to Holocene. The major landscapes of the LMV may be considered in four regions: (1) a narrow active meander belt in a broad valley of Late Pleistocene valley train in the northern third; (2) a wide mosaic of interwoven Holocene meander belts in the middle third; (3) a relatively narrow valley of the Atchafalaya Basin bounded on each side by narrow meander belts in the upper part of the lower third; and (4) the broad distributary wedge of the deltaic plain in the southernmost region of the valley. The valley trains vary in age and landform with the oldest occurring as slightly dissected low ridges and the youngest as broad flats separated by shallow interwoven former braided channels. Meander belts formed throughout the Holocene are comprised of low natural levee ridges flanking abandoned courses and bordered by crescent-shaped oxbow lakes and ridge and swale topography. In the middle third of the valley, meander belts are separated by expansive backswamps of very little relief. The deltaic plain is also exceptionally flat, interrupted by the low natural levee ridges of the abandoned deltaic distributaries.
The floodplain of the LMV is a complex mosaic of fluvial features and landscapes within the four landscape regions. Included in this mosaic are abandoned channels and courses, lateral accretion topography of ridges and swales, natural levees, crevasses and crevasse channels, distributary channels, backswamps and rimswamps, alluvial fans and aprons, valley trains (braided stream terraces), lakes and lacustrine deltas, terraces, and the alluvial valley bluff.
Changes in the hydraulic regime of the Lower Mississippi River (LMR) since the Late Pleistocene have played a major role in the development of the landscape of the valley. The most important regime change was the diminishment of the influence of Wisconsinan glaciation in the upper Midwest and the resultant evolution of the Mississippi River from a broad braided outwash channel to a more narrow but sinuous meandering channel at the end of the Pleistocene. During the Holocene, the Mississippi River undoubtedly responded to major climatic changes, rising sea level, tributary stream influence, and possibly tectonism, diapirism, and subsidence through the growth and evolution, and abandonment of it's meander belts and deltas. 相似文献
11.
Distributary channel systems are an important component of deltaic systems, but details of their branching pattern, stream‐order, internal variability and relation with adjacent levée, bay and bayhead delta are rather poorly documented in ancient examples. Photomosaic and measured sections collected along a gooseneck‐shaped canyon in southern Utah allow direct mapping of the branching pattern of an ancient distributary system. The main channel belt is ca 250 m wide and narrows to ca 200 m downstream of the branching point. A subordinate channel belt, ca 80 m wide, branches off of the main channel, forming a distinctly asymmetrical branching pattern. Water discharge in the main channel is estimated to be 85 to 170 m3 sec?1. Comparison with palaeodischarge estimates of trunk rivers mapped in previous studies suggests that the branching documented in this study probably is a fourth‐order split. The distributary channels are characterized by a U‐shaped geometry filled with medium‐grained, cross‐bedded sandstone, and are dominated by lateral accretion, suggesting limited lateral migration and moderate sinuosity. Tidally influenced facies and limited trace fossils indicate direct marine influence. The distributary channels erode into adjacent levée and underlying heterolithic bay‐fill deposits, and the marine influence suggests that they were deposited on a lower delta plain, rather than on a non‐marine floodplain. The subordinate channel fed a bayhead delta, suggesting that it was formed by a partial avulsion, rather than bifurcation around a mouth bar, as is more characteristic of terminal distributary channels. Channel‐floor drapes, bar‐accretion drapes and abandoned channel fills within the sandstone channel belts represent the most important heterogeneity from the perspective of reservoir characterization. 相似文献
12.
Conceptual framework for assessing the response of delta channel networks to Holocene sea level rise
Douglas J. Jerolmack 《Quaternary Science Reviews》2009,28(17-18):1786-1800
Recent research has identified two fundamental unit processes that build delta distributary channels. The first is mouth-bar deposition at the shoreline and subsequent channel bifurcation, which is driven by progradation of the shoreline; the second is avulsion to a new channel, a result of aggradation of the delta topset. The former creates relatively small, branching networks such as Wax Lake Delta; the latter generates relatively few, long distributaries such as the Mississippi and Atchafalaya channels on the Mississippi Delta. The relative rate of progradation to aggradation, and hence the creation of accommodation space, emerges as a controlling parameter on channel network form. Field and experimental research has identified sea level as the dominant control on Holocene delta growth worldwide, and has empirically linked channel network changes to changes in the rate of sea level rise. Here I outline a simple modeling framework for distributary network evolution, and use this to explore large-scale changes in Holocene channel pattern that have been observed in deltas such as the Rhine-Meuse and Mississippi. Rapid early- to mid-Holocene sea level rise forced many deltas into an aggradational mode, where I hypothesize that avulsion and the generation of large-scale branches should dominate. Slowing of sea level rise in the last ～6000 yr allowed partitioning of sediment into progradation, facilitating the growth of smaller-scale distributary trees at the shorelines of some deltas, and a reduction in the number of large-scale branches. Significant antecedent topography modulates delta response; the filling of large incised valleys, for example, caused many deltas to bypass the aggradational phase. Human effects on deltas can be cast in terms of geologic controls affecting accommodation: constriction of channels forces rapid local progradation and mouth-bar bifurcation, while accelerated sea level rise increases aggradation and induces more frequent channel avulsion. 相似文献
13.
14.
基于过程的分流平原高弯河道砂体储层内部建筑结构分析--以大庆油田萨北地区为例 总被引：4，自引：1，他引：3
河道砂体内部结构已成为影响剩余油分布的决定因素。本研究旨在弄清三角洲平原高弯曲河道砂体内部结构,指导开发调整。解剖分为复合河道、单一河道、基本结构要素及要素内部结构等四个层次。基于三角洲平原上高弯河流的形成过程,依据河泛沉积分布及河道沉积体内部沉积特征识别单一河道。依据砂体厚度分布及砂体测井响应所体现的砂体所处的河道位置识别最终废弃河道,依据测井响应特征识别中间废弃河道,通过中间废弃河道及最终废弃河道位置关系,分析河道摆动特征,追踪河道迁移演化。按照河道演化过程识别点坝及其平面展布。在点坝内部通过岩芯、对子井分析确定侧积体的垂向厚度、倾向、倾角,进而获取侧积体平面发育频率,建立点坝内部的结构模型。研究基于河流演化过程,将河流过程与沉积结果统一起来,从过程角度解释了砂体形成,使解剖结果更合理更准确,可很好地指导剩余油预测分析。 相似文献
15.
茂兰喀斯特自然保护区撂荒地时空演变、机制及其植被恢复 总被引：5，自引：3，他引：2
本研究以高分辨率遥感影像和实地调查为基本数据源,运用Arcmap和Fragstats软件,研究了茂兰自然保护区弃耕撂荒地的空间分布与动态演变。结果表明:整个保护区的弃耕地主要集中在缓冲区和实验区;从2005年到2015年,弃耕地的空间分布范围明显减少,斑块面积逐渐增加;2005年和2010年,撂荒地数量与距聚落距离呈正相关,2015年,撂荒地主要分布在距聚落<200 m、600~800 m和>1 000 m的缓冲范围;弃耕地植被恢复型演替主要分布在核心区和缓冲区。研究结果为全面反映保护区的人地关系演变和生态恢复情况提供了参考。 相似文献
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17.
The General Characteristics and Distribution of the Glaucophane-Schist Belts of China 总被引：1，自引：0，他引：1
Dong Shenbao 《《地质学报》英文版》1990,64(1):101-114
On the basis of the temperature variation, the glaucophane-schist facies Gan be classified into two faciesgroups, the glaucophane-lawsonite facies group and the glaucophane-greenschist facies group, and forms vari-ous facies series with other metamorphic facies. Glaucophane-schist belts of China are of different types withregard to their average T-P gradients and tectonic environments. They are: (1) Proterozoic intracratonicglaucophane-schist belts, (2) Caledonian intracratonic glaucophane-schist belts, (3) Palaeozoic pericratonicglaucophane-schist belts along the northern margin of the Chinese Platform, and (4) Meso-Cenozoicglaucophane-schist belts related to subduction. The initiation and evolution of glaucophane-schist metamorphism are related to their tectonic environ-ments. Most glaucophane-schist belts of China were formed in sialic environments. First the crust began tobreak up and eventually oceanic crust was formed. It seems that a non-uniformitarian mechanism may be in-volved as this evolution is related to the change of the configuration of the heat flow within the crust. 相似文献
18.
为揭示黄河口清水沟河道长时段的冲淤演变规律并建立其冲淤计算方法,分析了清水沟1976—2015年的时空冲淤演变过程,采用河床演变的滞后响应模型,考虑河口来水来沙及河道延伸与蚀退的影响,建立了清水沟累计冲淤量的计算方法。结果表明:1976—1980年改道初期清水沟改道点上游先冲后淤,改道点下游淤滩塑槽,淤积量随着下游河道展宽而增加,1980年后改道点上、下游河道冲淤过程趋于一致;受水沙条件等因素影响,1980—1986年清水沟主槽冲刷展宽,之后主槽淤积萎缩;1996年清八改汊和2002年小浪底水库"调水调沙"原型试验以来,河道转淤为冲,2002年后河道冲刷速率随时间指数衰减;河床演变的滞后响应模型可计算清水沟长时段的冲淤过程,该方法可为预测未来清水沟冲淤演变趋势提供科学参考。 相似文献
19.
Tjalling de Haas Wilco van den Berg Lisanne Braat Maarten G. Kleinhans 《Sedimentology》2016,63(6):1596-1619
Alluvial fans develop their semi‐conical shape by quasi‐cyclic avulsions of their geomorphologically active sector from a fixed fan apex. On debris‐flow fans, these quasi‐cyclic avulsions are poorly understood, partly because physical scale experiments on the formation of fans have been limited largely to turbidite and fluvial fans and deltas. In this study, debris‐flow fans were experimentally created under constant extrinsic forcing, and autogenic sequences of backfilling, avulsion and channelization were observed. Backfilling, avulsion and channelization were gradual processes that required multiple successive debris‐flow events. Debris flows avulsed along preferential flow paths given by the balance between steepest descent and flow inertia. In the channelization phase, debris flows became progressively longer and narrower because momentum increasingly focused on the flow front as flow narrowed, resulting in longer run‐out and deeper channels. Backfilling commenced when debris flows reached their maximum possible length and channel depth, as defined by channel slope and debris‐flow volume and composition, after which they progressively shortened and widened until the entire channel was filled and avulsion was initiated. The terminus of deposition moved upstream because the frontal lobe deposits of previous debris flows created a low‐gradient zone forcing deposition. Consequently, the next debris flow was shorter which led to more in‐channel sedimentation, causing more overbank flow in the next debris flow and resulting in reduced momentum to the flow front and shorter runout. This topographic feedback is similar to the interaction between flow and mouth bars forcing backfilling and transitions from channelized to sheet flow in turbidite and fluvial fans and deltas. Debris‐flow avulsion cycles are governed by the same large‐scale topographic compensation that drives avulsion cycles on fluvial and turbidite fans, although the detailed processes are unique to debris‐flow fans. This novel result provides a basis for modelling of debris‐flow fans with applications in hazards and stratigraphy. 相似文献
20.
Little is known about controls on river avulsion at geological time scales longer than 104 years, primarily because it is difficult to link observed changes in alluvial architecture to well‐defined allogenic mechanisms and to disentangle allogenic from autogenic processes. Recognition of Milankovitch‐sale orbital forcing in alluvial stratigraphy would provide unprecedented age control in terrestrial deposits, and also exploit models of allogenic forcing enabling more rigorous testing of allocyclic and autocyclic controls. The Willwood Formation of the Bighorn Basin is a lower Eocene fluvial unit distinctive for its thick sequence of laterally extensive lithological cycles on a scale of 4 to 10 m. Intervals of red palaeosols that formed on overbank mudstones are related to periods of relative channel stability when gradients between channel belts and floodplains were low. The intervening drab, heterolithic intervals with weak palaeosol development are attributed to episodes of channel avulsion that occurred when channels became super‐elevated above the floodplain. In the Deer Creek Amphitheater section in the McCullough Peaks area, these overbank and avulsion deposits alternate with a dominant cycle thickness of ca 7·1 m. Using integrated stratigraphic age constraints, this cyclicity has an estimated period of ca 21·6 kyr, which is in the range of the period of precession climate cycles in the early Eocene. Previous analyses of three older and younger sections in the Bighorn Basin showed a similar 7 to 8 m spacing of red palaeosol clusters with an estimated duration close to the precession period. Intervals of floodplain stability alternating with episodes of large‐scale reorganization of the fluvial system could be entirely autogenic; however, the remarkable regularity and the match in time scales documented here indicate that these alternations were probably paced by allogenic, astronomically forced climate change. 相似文献