Page 146 - Sedimentology and Stratigraphy

River Forms 133

Fig. 9.5 Mid-channel gravel bars in a
braided river.

be an erosion surface representing the base of the
Braided river
channel and this will be overlain by a basal lag of
coarse clasts deposited on the channel floor. In a
Scale Lithology MUD SAND GRAVEL Structures etc Notes gravelly braided river the bar deposits will commonly
consist of cross-stratified granules, pebbles or rarely
clay silt vf m vc gran pebb cobb boul cobbles in a single set. A sandy bar composed of
c
f
stacked sets of subaqueous dune deposits will form a
succession of cross-bedded sands. As the flow is stron-
ger in the lower part of the channel the subaqueous
Overbank muds and dunes, and hence the cross-beds, tend to be larger at
thin sands with soils
and roots the bottom of the bar, decreasing in set size upwards.
Finer sands or silts on the top of a bar deposit repre-
sent the abandonment of the bar when it is no longer
actively moving. There is therefore an overall fining-
up of this channel-fill succession (Fig. 9.6). The thick-
metres ness may represent the depth of the original channel if

Channel-fill
succession of
cross-bedded sands,
decrease in
cross-bed set
thickness upwards,
fining-up
Scoured base of
channel

Fig. 9.6 A schematic graphic sedimentary log of braided
river deposits.

(Fig. 9.9). A characteristic sedimentary succession
(Fig. 9.6) formed by deposition in a braided river Fig. 9.7 Sandy dune bedforms on a mid-channel bar in a
environment can be described. At the base there will braided river.