2.2 Fluvial Sediments
In humid regions, alluvial fans have lower gradients
and are dominated by stream processes with marked
seasonal variations in runoff. The mid and lower fan
areas are vegetated and therefore less susceptible to
reworking. They are cut by a limited number of active,
narrow channels. The sedimentary processes in such
fans, particularly those of humid tropical regions, are
poorly known.
One of the largest present-day examples of this type is the
Kosi alluvial fan in India, draining the area around Mount
Everest (Wells and Dorr 1987). This fan has a length ofmore
than 150 km, but a very gentle slope (mean value 0.04°).
River flow varies greatly due to the monsoonal climate. The
channels are partly braided, and partly meandering and
anastomosed, but there are also many abandoned channels
forming oxbow lakes. The dominant grain size is sand and
finer-grained material; sheetfloods and mud flows are absent.
Minor primary sedimentary structures are frequently obscured and obliterated by vegetation and bioturbation. The
maximum thickness of these fan deposits is about 900 m.
The Kosi fan is thought to be analogous to the
depositional environment of ancient fluvial molasse accumulated in the foredeeps ofhigh mountain ranges.
In all alluvial fan deposits, current directions deduced
from sedimentary structures show radial flow patterns
from the fan head down fan. If measured in a limited
area, these directions display little variation (in comparison to meandering systems). Vertical sequence
profiles may be quite irregular without showing a particular trend, because they are controlled by several
factors. Flood events and debris flows are stochastic
processes with greatly varying recurrence intervals.
Nevertheless, they generate repeated successions
which are frequently a few meters thick.
Long-term trends in the evolution ofvertical alluvial
fan sections are mainly controlled by processes outside
of the depositional area (extrabasinal factors). Such
factors include the amplification or reactivation of relief in the hinterland, climatic changes affecting weathering conditions and erosion in the drainage area, or
the base level of fans entering a lake.
For example, coarsening-upward sequences (Fig.
2.l2a) may reflect fan growth during continuous faulting, i.e., uplift of the source area or subsidence of the
fan region. The fan then progrades toward the lowland.
Fining-upward sequences are generated if a short
phase of faulting is followed by retreat of the scarp
front and lowering of relief in the highlands (Fig.
2.12b). In paraglacial environments, the same phenomena can be generated in a different way. Periods of
glacial advance lead to coarsening-upward successions, whereas fining-upward sequences result from
glacial retreat.
Modem alluvial fans are associated with the source,
area containing the deposits of a confined stream, as
well as with talus or colluvium from the adjacent
slopes. In general, these facies types are not preserved
in ancient examples. Down-slope, alluvial fans grade
37
into other alluvial deposits, mostly those of a braided
river system flowing perpendicular or at high angle to
the outbuilding fans (cf. Fig. 2.7). The lower fan sediments, consisting predominantly of sand and mud, may
feed wind-blown sand transport systems or be associated with eolian sand (Fig. 2.22a). On abandoned fan
surfaces, vegetation starts to grow and soil forming
processes take place. Distal fans may reach lacustrine
environments, for example, playa lakes in arid regions
(Sect. 2.5).
Paleosols and the interfingering of fan deposits
with characteristic other facies types, such as playa
sediments or tillites and glaciolacustrine sediments,
perrnit the recognition of their paleoenvironment. Ancient fan deposits occasionally contain plant relics,
traces ofburrowing organisms, and other fossils which
may be useful in identification.
Fan Deltas
Coastal alluvial fans prograding into a lake or into the
sea form fan deltas (Figs. 2.11 band 2.13). As soon as
the streams, carrying a high bedload, reach the standing water body, they drop their coarse material at the
shore face and in pro delta foresets (cf. Sect. 2.5). The
intensity of reworking, sorting, and redeposition, as
weil as the transport of material along the shoreline,
depend on the hydrodynamic regime of the water-filled
basin. In the case of lakes and protected marine
embayments, fan pro gradation is little influenced by
these processes. Gravel and sand accumulate at the
mouths of streams until they become episodically unstable and move as subaqueous debris flows into
deeper water (cf. Sect. 5.4.1). There, they alternate
with muddy lake or marine deposits. On high-energy
coasts, some of the coarse material is transported from
the river mouth alongshore to adjacent beaches where
it forms distinctive beach gravel (cf. Sect. 3.1); some
sand and gravel is swept by storrns into deeper water.
Among a variety of phenomena, described e.g. by
Postma (1990), some processes commonly affecting
fan deltas should be briefly mentioned. Figure 2.13
depicts an alluvial fan or braided river entering an active tectonic graben occupied by a lake. The normal
facies association in such a basin fill is, from top to
bottom, a succession of topsets, foresets, and bottom
sets. Lateral switching of the river mouth, however,
complicates this simple facies pattern and leads to repeated displacements of the site where the coarsest
material is deposited. Thus, in conjunction with ongoing subsidence, several coarsening-upward sequences
may be generated on top of each other (Fig. 2.13).
Mass flow deposits, and silty and muddy turbidites
(Sects. 2.5.1 and 5.4.2) reflect the prograding delta
front in more distal regions. This model also applies to
proglaciallakes fed by melt-water streams (Sect. 2.1).
In humid regions, alluvial fans have lower gradients
and are dominated by stream processes with marked
seasonal variations in runoff. The mid and lower fan
areas are vegetated and therefore less susceptible to
reworking. They are cut by a limited number of active,
narrow channels. The sedimentary processes in such
fans, particularly those of humid tropical regions, are
poorly known.
One of the largest present-day examples of this type is the
Kosi alluvial fan in India, draining the area around Mount
Everest (Wells and Dorr 1987). This fan has a length ofmore
than 150 km, but a very gentle slope (mean value 0.04°).
River flow varies greatly due to the monsoonal climate. The
channels are partly braided, and partly meandering and
anastomosed, but there are also many abandoned channels
forming oxbow lakes. The dominant grain size is sand and
finer-grained material; sheetfloods and mud flows are absent.
Minor primary sedimentary structures are frequently obscured and obliterated by vegetation and bioturbation. The
maximum thickness of these fan deposits is about 900 m.
The Kosi fan is thought to be analogous to the
depositional environment of ancient fluvial molasse accumulated in the foredeeps ofhigh mountain ranges.
In all alluvial fan deposits, current directions deduced
from sedimentary structures show radial flow patterns
from the fan head down fan. If measured in a limited
area, these directions display little variation (in comparison to meandering systems). Vertical sequence
profiles may be quite irregular without showing a particular trend, because they are controlled by several
factors. Flood events and debris flows are stochastic
processes with greatly varying recurrence intervals.
Nevertheless, they generate repeated successions
which are frequently a few meters thick.
Long-term trends in the evolution ofvertical alluvial
fan sections are mainly controlled by processes outside
of the depositional area (extrabasinal factors). Such
factors include the amplification or reactivation of relief in the hinterland, climatic changes affecting weathering conditions and erosion in the drainage area, or
the base level of fans entering a lake.
For example, coarsening-upward sequences (Fig.
2.l2a) may reflect fan growth during continuous faulting, i.e., uplift of the source area or subsidence of the
fan region. The fan then progrades toward the lowland.
Fining-upward sequences are generated if a short
phase of faulting is followed by retreat of the scarp
front and lowering of relief in the highlands (Fig.
2.12b). In paraglacial environments, the same phenomena can be generated in a different way. Periods of
glacial advance lead to coarsening-upward successions, whereas fining-upward sequences result from
glacial retreat.
Modem alluvial fans are associated with the source,
area containing the deposits of a confined stream, as
well as with talus or colluvium from the adjacent
slopes. In general, these facies types are not preserved
in ancient examples. Down-slope, alluvial fans grade
37
into other alluvial deposits, mostly those of a braided
river system flowing perpendicular or at high angle to
the outbuilding fans (cf. Fig. 2.7). The lower fan sediments, consisting predominantly of sand and mud, may
feed wind-blown sand transport systems or be associated with eolian sand (Fig. 2.22a). On abandoned fan
surfaces, vegetation starts to grow and soil forming
processes take place. Distal fans may reach lacustrine
environments, for example, playa lakes in arid regions
(Sect. 2.5).
Paleosols and the interfingering of fan deposits
with characteristic other facies types, such as playa
sediments or tillites and glaciolacustrine sediments,
perrnit the recognition of their paleoenvironment. Ancient fan deposits occasionally contain plant relics,
traces ofburrowing organisms, and other fossils which
may be useful in identification.
Fan Deltas
Coastal alluvial fans prograding into a lake or into the
sea form fan deltas (Figs. 2.11 band 2.13). As soon as
the streams, carrying a high bedload, reach the standing water body, they drop their coarse material at the
shore face and in pro delta foresets (cf. Sect. 2.5). The
intensity of reworking, sorting, and redeposition, as
weil as the transport of material along the shoreline,
depend on the hydrodynamic regime of the water-filled
basin. In the case of lakes and protected marine
embayments, fan pro gradation is little influenced by
these processes. Gravel and sand accumulate at the
mouths of streams until they become episodically unstable and move as subaqueous debris flows into
deeper water (cf. Sect. 5.4.1). There, they alternate
with muddy lake or marine deposits. On high-energy
coasts, some of the coarse material is transported from
the river mouth alongshore to adjacent beaches where
it forms distinctive beach gravel (cf. Sect. 3.1); some
sand and gravel is swept by storrns into deeper water.
Among a variety of phenomena, described e.g. by
Postma (1990), some processes commonly affecting
fan deltas should be briefly mentioned. Figure 2.13
depicts an alluvial fan or braided river entering an active tectonic graben occupied by a lake. The normal
facies association in such a basin fill is, from top to
bottom, a succession of topsets, foresets, and bottom
sets. Lateral switching of the river mouth, however,
complicates this simple facies pattern and leads to repeated displacements of the site where the coarsest
material is deposited. Thus, in conjunction with ongoing subsidence, several coarsening-upward sequences
may be generated on top of each other (Fig. 2.13).
Mass flow deposits, and silty and muddy turbidites
(Sects. 2.5.1 and 5.4.2) reflect the prograding delta
front in more distal regions. This model also applies to
proglaciallakes fed by melt-water streams (Sect. 2.1).