2.2 Fluvial Sediments
43
Table 2.2. Main characteristics of selected braided river systems. (Some authors identify more types, but there
are no sharp boundaries between the different subenvironments). (After Miall1985)
Subenvironment, general
description
Proximal braided river
deposits (including lower
alluvial fans),
gravel-dominated
Braided river deposits (middle
reaches), rich in gravel
Mainly sandy
Distal braided river
deposits, sand-dominated
Silt and mud-dominated
Sandy river plains subject to flash
floods
Percentage
ofgravel
> 50
10-70
<10
Mainly sand
Silt and mud
Mainly sand
Prevailing lithofacies and
internal structure
Massive gravel
Planar and trough cross-bedded
gravel and sand, massive gravel
Trough and planar cross-bedded
sand
Trou~h and planar cross-bedded
sand
Massive and fine laminated silt,
mud and sand
Horizontal and low angle crossbedded sands
Minor Lithofacies a
Gp,Gt
Sp, St
Sr, Fl
Fm
Sh, Sr
Fl, Fm
Sh, Sr
SI, Gm
Fl,Fm
Sh, Sr
Gm,Fl
Fm
St
Sp
Sr
a For explanation of symbols, see Table 2.1
b Fining-upward channel fills cause distinct minor sedimentary cycles
downstream and lateraliy migrating sand and gravel bars,
planar and trough cross-bedding are the most important internal sedimentary structures.
Lateral migration and sudden abandonment of channels due to avulsion cause fining-upward channel fill
sequences 01 a few meters in thickness (Fig. 2.16c).
Such autocyclic sequences (cf. Sect. 7.1) are considered the most distinctive feature of this type ofbraided
river deposit.
In distal reaches ofbraided fluvial plains, runoff is
distributed by numerous shallow channels. The gradient and stream power of the system are lower than in
proximal areas, and sand is the dominant grain size
deposited.
Typical present-day examples of this situation are the distal
outwash plains of glaciated areas (sandar), but the same type
also occurs in non-glacial river systems.
The channel sands form large linguoid bars (Fig.
2.16d), sand waves, and smaller bed forms such as
dunes and ripples (cf. Fig. 2.9). In vertical sections,
planar cross-bedding prevails, whereas trough crossbedding is less common (Fig. 2.16e). Gravel may oc-·
cur in smallienses, and overbank fines are occasionally intercalated.
Fluvial cyclicity due to the filling and abandonment
of channels is much less pronounced in the distal regions than in the medium reaches of this depositional
system. In the most distal parts of outwash rivers, or in
regions which receive mainly fine-grained silts and
muds from their hinterlands, the river systems are characterized by low gradients and low-relief channels and
bars. The main sediment types are massive, laminated
and cross-Iaminated sandy silts.
Sand-dominated distal braidplain environments,
common in arid regions, are somewhat different from
the braided systems described so far. Here, runoff and
sediment accumulation are controlled by rare flash
floods over a broad river plain with very shallow channels. Flow may be either channelized or occur as sheet
floods. During waning flow, most ofthe sand is deposited essentially in sheets, displaying horizontal lamination resulting from upper flow regime conditions. Such
sands often grade vertically into finer and small-scale
trough cross-bedded layers (Fig. 2.16f).
Characteristic features of some subenvironments of
braided river systems are summarized in Table 2.2.
Experts distinguish more types of subenvironments
and provide detailed descriptions (e.g. Miall1996). An
example of a large-scale facies association is discussed
in Sect. 2.2.7 (cf. Fig. 2.19a and b).
43
Table 2.2. Main characteristics of selected braided river systems. (Some authors identify more types, but there
are no sharp boundaries between the different subenvironments). (After Miall1985)
Subenvironment, general
description
Proximal braided river
deposits (including lower
alluvial fans),
gravel-dominated
Braided river deposits (middle
reaches), rich in gravel
Mainly sandy
Distal braided river
deposits, sand-dominated
Silt and mud-dominated
Sandy river plains subject to flash
floods
Percentage
ofgravel
> 50
10-70
<10
Mainly sand
Silt and mud
Mainly sand
Prevailing lithofacies and
internal structure
Massive gravel
Planar and trough cross-bedded
gravel and sand, massive gravel
Trough and planar cross-bedded
sand
Trou~h and planar cross-bedded
sand
Massive and fine laminated silt,
mud and sand
Horizontal and low angle crossbedded sands
Minor Lithofacies a
Gp,Gt
Sp, St
Sr, Fl
Fm
Sh, Sr
Fl, Fm
Sh, Sr
SI, Gm
Fl,Fm
Sh, Sr
Gm,Fl
Fm
St
Sp
Sr
a For explanation of symbols, see Table 2.1
b Fining-upward channel fills cause distinct minor sedimentary cycles
downstream and lateraliy migrating sand and gravel bars,
planar and trough cross-bedding are the most important internal sedimentary structures.
Lateral migration and sudden abandonment of channels due to avulsion cause fining-upward channel fill
sequences 01 a few meters in thickness (Fig. 2.16c).
Such autocyclic sequences (cf. Sect. 7.1) are considered the most distinctive feature of this type ofbraided
river deposit.
In distal reaches ofbraided fluvial plains, runoff is
distributed by numerous shallow channels. The gradient and stream power of the system are lower than in
proximal areas, and sand is the dominant grain size
deposited.
Typical present-day examples of this situation are the distal
outwash plains of glaciated areas (sandar), but the same type
also occurs in non-glacial river systems.
The channel sands form large linguoid bars (Fig.
2.16d), sand waves, and smaller bed forms such as
dunes and ripples (cf. Fig. 2.9). In vertical sections,
planar cross-bedding prevails, whereas trough crossbedding is less common (Fig. 2.16e). Gravel may oc-·
cur in smallienses, and overbank fines are occasionally intercalated.
Fluvial cyclicity due to the filling and abandonment
of channels is much less pronounced in the distal regions than in the medium reaches of this depositional
system. In the most distal parts of outwash rivers, or in
regions which receive mainly fine-grained silts and
muds from their hinterlands, the river systems are characterized by low gradients and low-relief channels and
bars. The main sediment types are massive, laminated
and cross-Iaminated sandy silts.
Sand-dominated distal braidplain environments,
common in arid regions, are somewhat different from
the braided systems described so far. Here, runoff and
sediment accumulation are controlled by rare flash
floods over a broad river plain with very shallow channels. Flow may be either channelized or occur as sheet
floods. During waning flow, most ofthe sand is deposited essentially in sheets, displaying horizontal lamination resulting from upper flow regime conditions. Such
sands often grade vertically into finer and small-scale
trough cross-bedded layers (Fig. 2.16f).
Characteristic features of some subenvironments of
braided river systems are summarized in Table 2.2.
Experts distinguish more types of subenvironments
and provide detailed descriptions (e.g. Miall1996). An
example of a large-scale facies association is discussed
in Sect. 2.2.7 (cf. Fig. 2.19a and b).