2.2 Fluvial Sediments
In cross-sections of a meandering system, the
interchannel floodplain deposits predominate and
make up the majority of the basin fill. Coal, carbonaceous shales and some freshwater limestones accumulate in poorly drained backswamps or lakes. Channel
sands are accompanied by levee and crevasse splay
sands. Aggradation of channel fills and floodplain deposits occurs either simultaneously {rapidly subsiding
flood basin), or the channels cut into pre-existing fluvial sediments.
Immediately after deposition and under the load ofyounger
sediments, these deposits are affected bydifferential compaction (cf. Sect. 13.2). This process leads to considerable deformation of fine-grained floodplain deposits and coal seams
near the channel fills, which themselves are little compacted.
Because compaction begins early in interchannel areas, peat
and subsequent coal seams may become thicker there than
near the channels, where they frequently split up into several
thin bands before pinching out (Fig. 2.19c).
The rate of vertical aggradation is a major factor controlling the total fluvial system. High rates are associated with abundant supplies of suspended load and
substantial subsidence. They tend to generate vertical
stacking of the channel deposits (point bars, etc., including relatively good preservation oflevee deposits,
see Figs. 2.8 and 2.19c). The contrary situation, however, leads to pronounced channel migration, lateral
stacking of channel deposits, and hence more reworking and removal ofthe levee and floodplain sediments.
The sedimentation rate of floodplain deposits in
terms of vertical upbuilding varies greatly. Some values mentioned in the literature are in the order of one
to several meters per thousand years (cf. Sect. 10.2).
Finally, we consider the evolution of river systems
from their upper reaches in mountain ranges to lowlands or to the sea (Fig. 2.20). Along the river courses,
fluvial sediments of some thickness accumulate and
are preserved only in subsiding regions, whereas in
between the river solely acts as transport system. The
gradient of the main channel of a fluvial system tends
to decrease systematically downstream. Thus, a complete idcalized system would include an alluvial fan
(steepest gradient) evolving to a braided stream, then
to a meandering and possibly anastomosing system
(lowest gradient; cf. Fig. 2.15). As part of this evolution, the bulk of the sediments becomes finer downstream where floodplain deposits predominate over
channel fiUs.
However, the rock types and climate in the hinterland also strongly control this evolution. The mineralogical composition of fluvial sands, for example, depends on both the nature ofthe source rocks and on the
climate and its change.
The mineralogie al composition of sands and sands tones and
thc degree of their "maturity" is frequently expressed in
quartz/feldspar/rock fragment percentages plotted in triangu49
lar diagrams (e.g. Pettijohn et al. 1987; Girty et al. 1988;
Bahlburg and Floyd 1999). If fluvial sandstones have not
been subjected to subsequent significant metamorphism,
such diagrams may be used as indicators ofthe paleoclimate
(e.g., Basu 1985; Suttner and Dutta 1986). Provided that the
petrographie composition of the source rocks was more or
less constant over a 10ng period of time, high feldspar and
rock fragment contents reflect an arid paleoclimate, while a
relative enrichment in chemically stable quartz indicates
warm and wet paleoenvironmental conditions. However,
high relief in thc source area diminishes the intluence of climate on the composition of sands, unless they have been exposed to extended weathering during alluvial storage
(Johnsson 1990). Other criteria, such as the varieties of'
quartz present, may serve to identifY different source areas.
Humid climates and abundant vegetation in the source
areas generally favor the formation of clay minerals and their
enrichment in fluvial deposits (cf. Chap. 9).
The influence ofthe tectonic and climatic history on an
entire fluvial system is demonstrated by the idealized
example in Fig. 2.20. It shows the large-scale lateral
and vertical facies changes in a subsiding montane basin and on a sediment-aggrading coastal plain. The
montane basin generally receives sediments from several small and medium local sources, indicated by
strongly differing transport directions and material of
possibly greatly varying composition. In contrast,
coastal plains are fed by a limited number of large
point sources, distributing their granulometricaUy and
compositionally mixed sediment load in a fan-like
fashion. This is accomplished by frequent changes in
river course (avulsion) over the plain with the aim to
evenly aggrade the depositional surface. The variation
in mean transport directions of such a radiating distribution system is much less than that of the montane
basin. Humid phases tend to provide more finegrained, quartz-rich material to the fluvial system (Fig.
2.20a, Stage 2) than do arid phases. Particularly in
combination with lowered relief and decreasing stream
gradient, such conditions favor mixed-load and predominantly suspended-load meandering and
anastomosing systems (Fig. 2.20b). High groundwater
levels may generate interchannel swamps and lakes.
This is only one of many other feasible tectonic,
climatic, and depositional scenarios realized in nature.
The above-mentioned general rules may be used to
identify the nature and facies associations of other fluvial systems differing from this example.
2.2.8 Reservoir Properties of Fluvial Sediments
Fluvial sediments frequently provide important reservoirs for groundwater and hydrocarbons (see, e.g.,
summary in Miall 1996). In this respect, the size and
geometry of sand and sandstone bodies (e.g. clastic
wedges, vaUey fiUs, ribbon and sheet sands) as weU as
their association with other sediment types (i.e. their
tectonic setting) are of major interest.
In cross-sections of a meandering system, the
interchannel floodplain deposits predominate and
make up the majority of the basin fill. Coal, carbonaceous shales and some freshwater limestones accumulate in poorly drained backswamps or lakes. Channel
sands are accompanied by levee and crevasse splay
sands. Aggradation of channel fills and floodplain deposits occurs either simultaneously {rapidly subsiding
flood basin), or the channels cut into pre-existing fluvial sediments.
Immediately after deposition and under the load ofyounger
sediments, these deposits are affected bydifferential compaction (cf. Sect. 13.2). This process leads to considerable deformation of fine-grained floodplain deposits and coal seams
near the channel fills, which themselves are little compacted.
Because compaction begins early in interchannel areas, peat
and subsequent coal seams may become thicker there than
near the channels, where they frequently split up into several
thin bands before pinching out (Fig. 2.19c).
The rate of vertical aggradation is a major factor controlling the total fluvial system. High rates are associated with abundant supplies of suspended load and
substantial subsidence. They tend to generate vertical
stacking of the channel deposits (point bars, etc., including relatively good preservation oflevee deposits,
see Figs. 2.8 and 2.19c). The contrary situation, however, leads to pronounced channel migration, lateral
stacking of channel deposits, and hence more reworking and removal ofthe levee and floodplain sediments.
The sedimentation rate of floodplain deposits in
terms of vertical upbuilding varies greatly. Some values mentioned in the literature are in the order of one
to several meters per thousand years (cf. Sect. 10.2).
Finally, we consider the evolution of river systems
from their upper reaches in mountain ranges to lowlands or to the sea (Fig. 2.20). Along the river courses,
fluvial sediments of some thickness accumulate and
are preserved only in subsiding regions, whereas in
between the river solely acts as transport system. The
gradient of the main channel of a fluvial system tends
to decrease systematically downstream. Thus, a complete idcalized system would include an alluvial fan
(steepest gradient) evolving to a braided stream, then
to a meandering and possibly anastomosing system
(lowest gradient; cf. Fig. 2.15). As part of this evolution, the bulk of the sediments becomes finer downstream where floodplain deposits predominate over
channel fiUs.
However, the rock types and climate in the hinterland also strongly control this evolution. The mineralogical composition of fluvial sands, for example, depends on both the nature ofthe source rocks and on the
climate and its change.
The mineralogie al composition of sands and sands tones and
thc degree of their "maturity" is frequently expressed in
quartz/feldspar/rock fragment percentages plotted in triangu49
lar diagrams (e.g. Pettijohn et al. 1987; Girty et al. 1988;
Bahlburg and Floyd 1999). If fluvial sandstones have not
been subjected to subsequent significant metamorphism,
such diagrams may be used as indicators ofthe paleoclimate
(e.g., Basu 1985; Suttner and Dutta 1986). Provided that the
petrographie composition of the source rocks was more or
less constant over a 10ng period of time, high feldspar and
rock fragment contents reflect an arid paleoclimate, while a
relative enrichment in chemically stable quartz indicates
warm and wet paleoenvironmental conditions. However,
high relief in thc source area diminishes the intluence of climate on the composition of sands, unless they have been exposed to extended weathering during alluvial storage
(Johnsson 1990). Other criteria, such as the varieties of'
quartz present, may serve to identifY different source areas.
Humid climates and abundant vegetation in the source
areas generally favor the formation of clay minerals and their
enrichment in fluvial deposits (cf. Chap. 9).
The influence ofthe tectonic and climatic history on an
entire fluvial system is demonstrated by the idealized
example in Fig. 2.20. It shows the large-scale lateral
and vertical facies changes in a subsiding montane basin and on a sediment-aggrading coastal plain. The
montane basin generally receives sediments from several small and medium local sources, indicated by
strongly differing transport directions and material of
possibly greatly varying composition. In contrast,
coastal plains are fed by a limited number of large
point sources, distributing their granulometricaUy and
compositionally mixed sediment load in a fan-like
fashion. This is accomplished by frequent changes in
river course (avulsion) over the plain with the aim to
evenly aggrade the depositional surface. The variation
in mean transport directions of such a radiating distribution system is much less than that of the montane
basin. Humid phases tend to provide more finegrained, quartz-rich material to the fluvial system (Fig.
2.20a, Stage 2) than do arid phases. Particularly in
combination with lowered relief and decreasing stream
gradient, such conditions favor mixed-load and predominantly suspended-load meandering and
anastomosing systems (Fig. 2.20b). High groundwater
levels may generate interchannel swamps and lakes.
This is only one of many other feasible tectonic,
climatic, and depositional scenarios realized in nature.
The above-mentioned general rules may be used to
identify the nature and facies associations of other fluvial systems differing from this example.
2.2.8 Reservoir Properties of Fluvial Sediments
Fluvial sediments frequently provide important reservoirs for groundwater and hydrocarbons (see, e.g.,
summary in Miall 1996). In this respect, the size and
geometry of sand and sandstone bodies (e.g. clastic
wedges, vaUey fiUs, ribbon and sheet sands) as weU as
their association with other sediment types (i.e. their
tectonic setting) are of major interest.