Depositional Models for Braided Alluvium

Abstract

Abstract Braided alluvial deposits form in rivers and on alluvial fans and plains, and comprise three lithotypes: gravel-, sand- and silt-dominant. The gravel lithotype is characterised by framework-supported gravel, which in proximal deposits is mainly horizontally bedded and imbricate, occurring together with matrix-supported (debris flow) gravel in alluvial fan deposits. Horizontally bedded gravel accumulates on longitudinal bars, which are thought to originate as primary bedforms of high stage flow. Distal gravel deposits are characterised by cycles in which trough cross-bedded framework-supported gravel fines upwards to sand (commonly trough cross-stratified) and massive to laminated mud. The coarser components of the cycle are interpreted as deposits of an active braided tract; grain size decreases as the tract aggrades and becomes inactive. Proximal sandy braided deposits typically lack mud in primary layers, but it is abundant as intraclasts on erosion surfaces. Horizontally stratified, low-angle (<10°) stratified, and trough cross-stratified sand are also abundant, in vertically and laterally variable successions. Distal sandy braided deposits commonly have fining-upward cycles, lateral continuity, and a significant primary mud content. They are transitional to deposits of meandering systems. Silty braided deposits are typically traction- rather than suspension-laid, and lack association with coarser channel sediments. They are rare today, because most fine-grained alluvium accumulates on floodplains of meandering systems, for which vegetation is an important stabilising component. Terrestrial vegetation was essentially absent before Late Paleozoic times, when braided alluvium, including the silt-dominant type was widespread, particularly on alluvial plains of humid regions.