2.2 Fluvial Sediments
41
MUD/SILT
FINE SAND
COARSE SAND
GRAVEL
SUSPENDED·LOAD
MIXED·LOAD
BED·LOAD
ALLUVIAL FANS -
Trollheim
SG
1
!ncrease
.------= BRAIDPLAINS=---In stream
power and
sediment
load
- - - RIVERS - - - -- -- - -
Platte
Brahmaputra
SB
. ," .
. "'.
OF
~FM ~ ' ====
_ • • • • : "
• : ' , _ . : - I
~
Lower Mississippi
~LA ' · ~s:G
. " , • • • 10 • • • • • _
ARCHITECTUAAL ELEMENTS
GB gravsl bar/bedforms
SB sandy bedforms
le
Increasing sinuosity
-:'Z
FM foreset macroforms
LA lateral accretion
;i"J%S7 Gravel/sand bars
Fig. 2.15. Channel types of various fluvial systems
(overview) in relation to the gradients and sediment
Glaeiofluvial outwash rivers also belong to this low-sinuosity, multiple-ehannel eategory. Both the valley gradient and
the stream power are relatively high, the latter often intermittently so.
From all these characteristics, it can be inferred that
braided systems are dominated by their bedload, i.e.,
they carry and deposit mainly gravel and sands. Their
sediments therefore consist predominantly of channel
and channel-flank deposits, while silty and muddy
floodplain facies are subordinate.
An exeeption to this mle has been deseribed from an Eoeene
fluvial system in the South Pyrenean foreland basin
(Bentham et al. 1993). Here, the ehannel belt was laterally
eonfined and rapid sediment aggradation led to a high pereentage of fine-grained overbank deposits.
In eommon types ofmultiple-ehannel bedload rivers, longitudinal gravel bars are eharaeteristie ehannel bed forms in
proximal reaehes (Fig. 2.16a). They develop as a result of
intermittent elast aeeretion under flow passing obliquely over
the bar. The bars therefore migrate not only downstream parallel to the eurrent, but also laterally. The resulting planar
sheets of gravel show indistinet, emde horizontal stratifiealion. The interstiees in the gravel are usually later filled with
sand during low water periods, but in this system sandy beds
are relatively rare in the proximal zone, unless sand is the
dominant grain size delivered by the souree area.
SG sediment gravity flows
OF overbank fines
IC isolated channels
loads of the rivers. (Modified after Orton and Reading 1993). See text for further explanation
Sandy beds develop best at somewhat higher topographie
elevations within the braided system next to the aetive ehannel system. Here, minor channels aetive during floods may
drop their bedload during waning flow.
A typical vertical sequence of a proximal braided system is shown in Figure 2.16b. The dominant sediments
are multistory gravel units, originating from channel
bars. Many bars are somewhat graded. Interbedded
with the gravels are thin lenses of sand representing
deposition in abandoned channels or sand wedges at
the edge of bars. In places, one can observe repeated
successions offining-upward gravel-sand sequences, 1
to 2 m thick, but in general it is difficult to identify the
bottoms of former channels and the geometry of their
fills. The reason is that the underlying and neighboring
sediments also consist largely of gravel.
Downstream, the predominantly gravelly beds grade
into beds consisting partly of smaller pebbles and sand.
In addition, the bed forms and internal sedimentary
structures progressively decrease in size.
In the lower, more active channels, bar gravels dominate,
whereas sands and pebbly sands are common at higher topographie elevations. In rarely flooded areas, some silt and mud
may be deposited and preserved. In the total assemblage, the
gravel content varies between 10% and 70%. As a result of
41
MUD/SILT
FINE SAND
COARSE SAND
GRAVEL
SUSPENDED·LOAD
MIXED·LOAD
BED·LOAD
ALLUVIAL FANS -
Trollheim
SG
1
!ncrease
.------= BRAIDPLAINS=---In stream
power and
sediment
load
- - - RIVERS - - - -- -- - -
Platte
Brahmaputra
SB
. ," .
. "'.
OF
~FM ~ ' ====
_ • • • • : "
• : ' , _ . : - I
~
Lower Mississippi
~LA ' · ~s:G
. " , • • • 10 • • • • • _
ARCHITECTUAAL ELEMENTS
GB gravsl bar/bedforms
SB sandy bedforms
le
Increasing sinuosity
-:'Z
FM foreset macroforms
LA lateral accretion
;i"J%S7 Gravel/sand bars
Fig. 2.15. Channel types of various fluvial systems
(overview) in relation to the gradients and sediment
Glaeiofluvial outwash rivers also belong to this low-sinuosity, multiple-ehannel eategory. Both the valley gradient and
the stream power are relatively high, the latter often intermittently so.
From all these characteristics, it can be inferred that
braided systems are dominated by their bedload, i.e.,
they carry and deposit mainly gravel and sands. Their
sediments therefore consist predominantly of channel
and channel-flank deposits, while silty and muddy
floodplain facies are subordinate.
An exeeption to this mle has been deseribed from an Eoeene
fluvial system in the South Pyrenean foreland basin
(Bentham et al. 1993). Here, the ehannel belt was laterally
eonfined and rapid sediment aggradation led to a high pereentage of fine-grained overbank deposits.
In eommon types ofmultiple-ehannel bedload rivers, longitudinal gravel bars are eharaeteristie ehannel bed forms in
proximal reaehes (Fig. 2.16a). They develop as a result of
intermittent elast aeeretion under flow passing obliquely over
the bar. The bars therefore migrate not only downstream parallel to the eurrent, but also laterally. The resulting planar
sheets of gravel show indistinet, emde horizontal stratifiealion. The interstiees in the gravel are usually later filled with
sand during low water periods, but in this system sandy beds
are relatively rare in the proximal zone, unless sand is the
dominant grain size delivered by the souree area.
SG sediment gravity flows
OF overbank fines
IC isolated channels
loads of the rivers. (Modified after Orton and Reading 1993). See text for further explanation
Sandy beds develop best at somewhat higher topographie
elevations within the braided system next to the aetive ehannel system. Here, minor channels aetive during floods may
drop their bedload during waning flow.
A typical vertical sequence of a proximal braided system is shown in Figure 2.16b. The dominant sediments
are multistory gravel units, originating from channel
bars. Many bars are somewhat graded. Interbedded
with the gravels are thin lenses of sand representing
deposition in abandoned channels or sand wedges at
the edge of bars. In places, one can observe repeated
successions offining-upward gravel-sand sequences, 1
to 2 m thick, but in general it is difficult to identify the
bottoms of former channels and the geometry of their
fills. The reason is that the underlying and neighboring
sediments also consist largely of gravel.
Downstream, the predominantly gravelly beds grade
into beds consisting partly of smaller pebbles and sand.
In addition, the bed forms and internal sedimentary
structures progressively decrease in size.
In the lower, more active channels, bar gravels dominate,
whereas sands and pebbly sands are common at higher topographie elevations. In rarely flooded areas, some silt and mud
may be deposited and preserved. In the total assemblage, the
gravel content varies between 10% and 70%. As a result of