Page 152 - Sedimentology and Stratigraphy

Patterns in Fluvial Deposits 139

develops as a result of avulsion. Through time the of the detritus carried increase until the breach
channels occupy different radial positions and the reaches full size. Crevasse splay deposits are therefore
deposits form a fan-shaped body of sediment (see characterised by an initial upward coarsening of the
also alluvial fans, 9.5). sediment particle size. They are typically lenticular in
three dimensions. Channels within crevasse splays
may develop into new river channels and carry pro-
9.3 FLOODPLAIN DEPOSITION gressively more water until avulsion occurs.
The primary depositional structures commonly
The areas between and beyond the river channels are observed in floodplain sediments are:
as important as the channels themselves from the 1 very thin and thin beds normally graded from sand
point of view of sediment accumulation. When the to mud;
discharge exceeds the capacity of the channel, water 2 evidence of initial rapid flow (plane parallel lamina-
flows over the banks and out onto the floodplain tion) quickly waning and accompanied by rapid
where overbank or floodplain deposition occurs. deposition (climbing ripple lamination);
Most of the sediment carried out onto the floodplain 3 thin sheets of sediment, often only a few centi-
is suspended load that will be mainly clay- and silt- metres thick but extending for tens to hundreds of
sized debris but may include fine sand if the flow is metres;
rapid enough to carry sand in suspension. As water 4 erosion at the base of the overbank sheet sandstone
leaves the confines of the channel it spreads out and beds is normally localised to areas near the channel
loses velocity very quickly. The drop in velocity where the flow is most vigorous;
prompts the deposition of the sandy and silty sus- 5 evidence of soil formation (9.7).
pended load, leaving only clay in suspension (Hughes There is usually a general trend towards the deposi-
& Lewin 1982). The sand and silt is deposited as a tion of more overbank sediments further downstream
thin sheet over the floodplain, which may show cur- in a fluvial system. In the upper parts of the fluvial
rent ripple or horizontal lamination: rapid deposition depositional tract, the river valley is likely to be nar-
may result in the formation of climbing ripple cross- row, and as braided rivers laterally migrate from side
lamination (4.3.1). The remaining suspended load to side across the valley any floodplain deposits will be
will be deposited as the floodwaters dry out and soak reworked by channel erosion. Floodplain deposits
away after the flow has subsided. therefore sometimes have a lower chance of being
Sheets of sand and silt deposited during floods are preserved associated with braided river facies. In the
thickest near to the channel bank because coarser wider alluvial plain normally associated with the
suspended load is dumped quickly by the floodwaters lower parts of the fluvial depositional tract, meander-
as soon as they start flowing away from the channel. ing river deposits are commonly associated with a
Repeated deposition of sand close to the channel edge higher proportion of floodplain facies.
leads to the formation of a leve ´e, a bank of sediment at
the channel edge which is higher than the level of the
floodplain (Fig. 9.11). Through time the level of the 9.4 PATTERNS IN FLUVIAL DEPOSITS
bottom of the channel can become raised by sedimen-
tation in the channel and the level of water at bank- 9.4.1 Architecture of fluvial successions
full flow becomes higher than the floodplain level.
When the leve ´e breaks, water laden with sediment is The three-dimensional arrangement of channel and
carried out onto the floodplain to form a crevasse overbank deposits in a fluvial succession is commonly
splay (Fig. 9.11), a low cone of sediment formed by referred to as the architecture of the beds. The archi-
water flowing through the breach in the bank and out tecture is described in terms of the shape and size of
onto the floodplain (O’Brien & Wells 1986). The the sand or gravel beds deposited in channels and the
breach in the leve ´e does not occur instantaneously proportion of ‘in-channel’ deposits relative to the finer
but as a gradually deepening and widening conduit overbank facies. The thickness of the channel-fill
for water to pass out onto the floodplain. Initially only deposit is determined by the depth of the rivers and
a small amount of water and sediment will pass their width is governed by the processes of avulsion
through but the volume of water and the grain size and lateral migration of the channel. There is a