The life cycle of crevasse splays as a key mechanism in the aggradation of alluvial ridges and river avulsion

Abstract

Accommodation space in the unconfined distal part of low‐gradient fluvial fans facilitates abundant floodplain deposition. Here, the development of crevasse splays plays a key role in the aggradation of alluvial ridges and subsequent river avulsion. This study presents an analysis of different stages in the evolution of crevasse splays based on observations made in the modern‐day Río Colorado dryland fluvial fan fringing the endorheic Altiplano Basin in Bolivia. A generic life cycle is proposed in which crevasse‐splay channels adjust towards a graded equilibrium profile with their lower‐lying distal termini acting as a local base level. Initial development is dominantly controlled by the outflow of floodwater, promoting erosion near the crevasse apex and deposition towards the splay fringes. When proximal incision advances to below the maximum level of floodplain inundation, return flow occurs during the waning stage of flooding. This floodwater reflux leads to a temporary repositioning of the local base level to the deeper trunk‐channel thalweg at the apex of the crevasse‐splay channels. The resultant decrease in the floodplainward gradient of these channels ultimately leads to backfilling and abandonment of the crevasse splay, leaving a subtle local elevation of the floodplain. Consecutive splays form an alluvial ridge through lateral amalgamation and subsequent vertical stacking, which is mirrored by the aggradation of their parent channel floor. As this alluvial ridge becomes increasingly perched above the surrounding floodplain, splay equilibration may cause incision of the levee crevasse down to or below its trunk channel thalweg, leading to an avulsion. The mechanisms proposed in this study are relevant to fluvial settings promoting progradational avulsions. The relatively rapid accumulation rate and high preservation potential of crevasse splays in this setting makes them an important constituent of the resultant fluvial stratigraphy, amongst which are hydrocarbon‐bearing successions. Copyright © 2018 John Wiley & Sons, Ltd.

Anatomy and facies distribution of terminal lobes in ephemeral fluvial successions: Jurassic Tordillo Formation, Neuquén Basin, Argentina

“…The present succession does not record deposits of channels with a high degree of incision, nor of lateral or frontal accretion bar deposits, as have been described for other terminal systems (Donselaar et al ., ; Owen et al ., ; Van Toorenenburg et al ., ). Regarding the architecture of the middle to distal sector of the system, it is the first time that fluvial terminal‐lobe deposits have been described in terms of their complex internal architecture, which displays evidence for compensational stacking and a thickening‐upward internal arrangement.…”

“…This characteristic has not been described as being so dominant in other terminal fluvial systems, such as the Ebro, the Rio Colorado and the Morrison Formation (Donselaar et al ., ; Weissmann et al ., ; Li & Bristow, ; Owen et al ., ; Van Toorenenburg et al ., , ), but has been generally recognized for systems with significant variations in discharge regimes (Fielding, ; Fielding et al ., , , ; Plink‐Björklund, ; Colombera & Mountney, ).…”

“…The composite silty‐sandstone bodies are interpreted to represent the accumulation in the off‐axis zone of terminal lobe deposits (Sáez et al ., ; Fisher et al ., ; Weissmann et al ., ). These elements may include crevasse splay deposits, considering that some studies on modern systems have shown that, towards the terminal sector of active and abandoned rivers, crevasse splays form amalgamated thin‐bedded sand sheets which merge with the terminal splay sand of the river (Donselaar et al ., ; Burns et al ., , ; Van Toorenenburg et al ., ). The cross‐sectional transition from the sandy axis sector to the silty off‐axis sector of the lobe is not clear‐cut (Fig.…”

“…Compensation suggests avulsion (through the ‘incisional avulsion’ mechanism, sensu Mohrig et al ., ) of the small‐scale distributive channels feeding the lobes, with flows forced towards topographic depressions (Buehler et al ., ; Weissmann et al ., ). Part of these deposits could also represent crevasse splays, although a distinction between terminal and crevasse splays is rendered difficult by the fact that, distally, crevasse splays form amalgamated deposits which merge with the terminal lobe deposits (Donselaar et al ., ; Li & Bristow, ; Van Toorenenburg, ; Van Toorenenburg et al ., ).…”

“…Recently, authors have described deposits of modern terminal splays, characterizing their morphology, their stages of development, their constituent sedimentary facies and the styles of fluvial termination (Lang et al ., ; Tooth, ; Billi, ; Fisher et al ., ; Donselaar et al ., ; Van Toorenenburg et al ., ). However, relatively little is known of the mechanisms by which these terminal deposits accumulate and become preserved in the sedimentary record, especially within terminal lobes.…”