Rust, B. R., 1972. Structure and process in a braided river. Sedimentology, 18:221-245.The Donjek is a proglacial braided river with a coarse heterogeneous bedload ranging from -7 to + 8 (o. The three reaches studied comprise, in downstream order, a zig-zag reach formed by the interaction of the river and tributary fans, a straight reach, and a meandering reach, all with internal braids. The straight to meandering junction is unusual in that slope and discharge both increase.The dominant bed-forms are longitudinal gravel bars, which migrate only during flood. Their internal structure is poorly defined horizontal bedding, which suggests that gravel deposition takes place on the upper bar surfaces, rather than on foreset slopes at the downstream margins. At lower stage sand accumulates in wedge-shaped units lateral to the bars, with internal high-angle cross-stratification and ripple cross-lamination; other sedimentary structures are rare. A simple facies model can be constructed on the basis of two trends: a proximal-distal trend, and an active-stable trend. Both result in the increased abundance of fine-grained sediment.Random directional measurements give good estimates of the river trend from small-scale structures (mainly ripples) and from channels, the latter giving the closest approximation. The variability of ripple orientation always exceeds that of channels. Both increase in the meandering reach, although the divergence of the vector mean from the river trend is not significantly greater than in the straight reach.
Abundant sand‐sized mud aggregates in the Cooper and Diamantina Rivers, Lake Eyre Basin, Australia are attributed to bedload transport of aggregates formed in deeply‐cracked floodplain soils. The conditions required for formation of pedogenic mud aggregates are: (i) abundant clay containing at least minor swelling clay, and (ii) a climate with at least seasonally hot dry periods. The worldwide distribution of these soils (Vertisols) suggests that a significant amount of mud is transported as pedogenic aggregates by modern rivers.
Ancient analogues in which mud aggregates and Vertisol profiles have been recognized are the Jurassic East Berlin Formation (Connecticut, USA) and the Carboniferous Maringouin Formation (New Brunswick and Nova Scotia, Canada). The dominant red mudstones of these formations are interpreted as mainly bedload sediments deposited by sheet floods in semi‐arid palaeoclimates. The Triassic Hawkesbury Sandstone (NSW, Australia) also contains sand‐sized mudstone aggregates, thought to be pedogenic, but its paleosol and other facies point to formation in a wetter palaeoclimate. The indications are that bedload transport of mud as pedogenic aggregates was as significant a process in ancient rivers as it is at present.
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