Page 149 - Sedimentology and Stratigraphy

136 Rivers and Alluvial Fans

Meandering river

Scale Lithology MUD SAND GRAVEL Structures etc Notes

clay silt vf m vc gran pebb cobb boul
c
f
Overbank muds and
thin sands with soils
and roots

metres
Channel-fill
succession of
Fig. 9.12 The point bars on the inside bends of this mean- cross-bedded sands
and cross-laminated
dering river have been exposed during a period of low flow sands, fining-up.
in the channel. Lateral accretion
surfaces
perpendicular to
cross-beds.
are low angle, less than 158, and, because they repre-
sent the point-bar surface, are inclined from the river
bank towards the deepest part of the channel – i.e. Scoured base of
channel
perpendicular to the flow direction. The scale of the
cross-stratification will therefore be larger (as much
as the channel depth) than other cross-bedding, and it
will be perpendicular to any other palaeoflow indica- Fig. 9.13 A schematic graphic sedimentary log of mean-
tors, such as cross-bedding produced by dune migra- dering river deposits.
tion and ripple cross-lamination. The recognition of
lateral accretion surfaces (also know as epsilon
cross-stratification; Allen 1965) within the fining-
up succession of a channel-fill deposit is therefore a
reliable indication that the river channel was mean-
dering. The outer bend of a meander loop will be a
bank made up of floodplain deposits (9.3) that will be
mainly muddy sediment. Dried mud is very cohesive
(2.4.5) and pieces of the muddy bank material will
not easily disintegrate when they form clasts carried
by the river flow. These mud clasts will be deposited
along with sand in the deeper parts of the channel,
and will be preserved in the basal part of the channel-
fill succession (Fig. 9.13).
During periods of high-stage flow, water may take a
Fig. 9.14 A pale band across the inside of this meander
short-cut over the top of a point bar. This flow
bend marks the path of a chute channel that cuts across the
may become concentrated into a chute channel point bar.
(Fig. 9.14) that cuts across the top of the inner
bank of the meander. Chute channels may be semi-
permanent features of a point bar, but they are only The river flow may also take a short-cut between
active during high-stage flow. They may be recog- meander loops when the river floods: this may result
nised in the deposits of a meandering river as a in a new section of channel developing, and the
scour that cuts through lateral accretion surfaces. longer loop of the meander built becoming abandoned