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reverse grading, weak imbrication and clasts
may be matrix-supported (Costa 1988).
Research has shown that fluvially dominated
fans may be restricted to certain lithologies such
as those associated with high-grade metamorphics
(see for example the work by Harvey 1984, 1987),
which source few clays and are dominated by
coarser material. The topographic constraints are
typically larger, lower relief catchments, which
can produce higher water to sediment ratios
from the slopes. These types of drainage basin
become increasingly abundant with the progres-
sive erosional development of the catchment
(i.e. as weathered slope material is progressively
exhausted and larger, more open, lower relief
catchments develop). The spatial extent of fluvial
alluvial fans is typically greater than for those
deposited by other processes as the potential
transportation distance for the fluvial flows is
greater, but the gradient needed to maintain
transport is less. Thus fluvial-dominated fans
tend to be larger and possess a gentler apex-to-
toe gradient than those dominated by other flow
processes (Harvey 1987, 1990).
Steel (1984), Postma (1986) and Costa (1988)
for a discussion of mass flow processes). Debris
flows are associated with catchments that gener-
ate sufficient fine material to lend shear strength
to the flow and are supplied predominantly by
slope material, i.e. small, steep catchments (Wells
& Harvey 1987). These conditions are typically
met when the catchment is developed on sedi-
mentary and low-grade metamorphic geologies
(Harvey 1990), and before the drainage network
has had time to reduce relief in the fan catch-
ment. Debris flows require a steeper gradient to
maintain transport and tend to have a smaller
run out distance than comparable fluvial flows,
so that debris-flow-dominated fans are typically
smaller and steeper than corresponding fluvial-
dominated fans.
5.3.3.3 Aeolian processes
Aeolian processes are not commonly associated
with alluvial fans in the literature, yet observa-
tions of fans in arid areas contradict this. It is
not uncommon to find impeded dunes (section
5.3.4.2) developed against fan apexes, or devel-
oped in topographic lows at the fan toe. The
aeolian material may subsequently be reworked
by debris and streamflow processes on the fan
surface or buried by debris flows. Thus in some
areas aeolian processes can be significant in
sourcing external fine material, and locally
affecting flow processes on the fan.
5.3.3.2 Debris-flow processes
The transport and deposition of sediment by
debris-flow processes (Fig. 5.11) may be (i)
confined within channels, or (ii) as an uncon-
fined lobe. Debris flows behave as a plastic
(Costa 1988) and may be channelized with
levees in the upper parts of the fans, and more
unconfined and lobate in the more distal parts
of fans. The types of flow will be determined by
the water to sediment ratio and the type of fine
sediment available. Essentially the flows can be
(i) cohesive or (ii) non-cohesive, depending on
the shear strength of the flow resulting from the
sediment concentration (typically 70 -90% by
volume, Costa 1988) and nature of fines. The
resulting deposits of debris flows typically lack
sorting, may possess inverse grading (towards
the base) and normal grading (towards the
top), are matrix-supported and lack much in the
way of internal structure, but will vary in their
internal organization depending on water con-
tent and cohesivity of the flow (see Nemec &
5.3.4 Aeolian bedforms
Aeolian sand deposits in arid environments
cover approximately 5% of the Earth's land sur-
face and aeolian silt (loess) covers a further 10%.
Dunes can vary in size from
200 m
in height and can represent the deposition of
one windstorm event or, for larger systems, the
deposition of sediment over millennia. Dependent
on source area, dunes are typically composed of
quartz sand, but can include any material capable
of being wind blown (e.g. gypsum, volcanic ash,
shell fragments). Where dunes are composed of
carbonate or gypsum, cementation of the dunes
may occur early in their development.
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