Page 153 - Sedimentology and Stratigraphy

140 Rivers and Alluvial Fans

tendency for nearly all rivers (meandering and and flows on the floodplain are faster and therefore
braided) to shift sideways through time by erosion of more likely to erode.
one bank and deposition on the opposite side. Lateral Rates of subsidence and the quantity of sediment
migration continues until avulsion of the river causes supplied to the floodplain also affect the architecture
the channel to be abandoned. If avulsion is frequent, of fluvial deposits (Fig. 9.17). With rapid subsidence
there is less time for lateral migration to occur and the and high sediment supply, aggradation on the flood-
architecture will be characterised by narrow channel plain will result in a high proportion of fine deposits. In
deposits (Fig. 9.17). Avulsion is frequent in rivers that regions of slow subsidence and reduced sediment sup-
are in regions of tectonic activity, where frequent ply to the overbank areas relatively more in-channel
faulting and related earthquakes affect the river deposits will be preserved (Bridge & Leeder 1979).
course, and in settings where overbank flooding is
frequent, resulting in weaker banks that make it
easier for the river to change course. 9.4.2 Palaeocurrents in fluvial systems
Lateral migration is slowed down if the river banks
are stable. Bank stability is governed by the nature of Palaeocurrent data are a very valuable aid to the
the floodplain: muddy floodplain deposits form stable reconstruction of the palaeogeography of fluvial
banks because clay is cohesive and is not easily deposits. It may be used to determine the location of
eroded. The type and abundance of vegetation are the source area from which the sediment was derived
also important because dense vegetation, particularly and it is possible to indicate the general position of the
grass with its fibrous roots, can very effectively bind mouth of the river and hence the shoreline. Sedimen-
the soils of a floodplain and stabilise the river banks. tary structures that can be used as flow indicators in
Vegetation also causes increased surface roughness, fluvial deposits include the orientation of channel
which slows overland flow. In arid or cold regions margins, cross-bedding in sandstone and clast imbri-
where vegetation is sparse, bank stability is decreased cation in conglomerate. An individual cross-bed is

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Fig. 9.17 The architecture of fluvial
deposits is determined by the rates of
subsidence and frequency of avulsion.