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the flow. Deposition (i.e. the cessation of movement) is
brought about by friction with the underlying surface, re-
duction of gradient and by the reduction of the volume
of the flow partly by marginal deposition during trans-
port and the formation of debris-flow levees. The deposits
generally are massive and show matrix-supported clasts
(facies Gms of Rust, 1978, 1979, and Pierson, 1981), but
there is some interaction between the larger clasts, result-
ing in a clast fabric indicative of compression and internal
shearing (Wells and Harvey, 1987) (Figure 14.4(a)). This
may result in clast alignment parallel to the base of the
flow, but near-vertical clasts aligned across the flow at the
top and towards the front of a debris-flow lobe. The depo-
sitional topography is of the 'lobe and levee' type (Blair
and McPherson, 1998).
With higher water content and increased fluidity, pos-
sibly also smaller clast sizes, deformation is largely by
plastic flowage, which on deposition may produce a ten-
dency for clast alignment with the flow direction. Such
deposits would be common in fan-delta environments.
With even higher water content and increased fluidity, the
mixture behaves as a hyperconcentrated flow (Pierson and
Scott, 1985) near the threshold between transport by plas-
tic and turbulent flow. On deposition the water may drain
through the sediment, taking much of the sediment ma-
trix vertically down through the flow, leaving the clasts to
exhibit an open, matrix-poor, clast-supported, collapsed
fabric (Wells and Harvey, 1987).
tary structures (e.g. bedding, cross-bedding, interbedding
of gravels, cobbles and finer sediments, grading, reverse
grading, clast imbrecation; facies Gm, Gp, Gt of Miall,
1977, 1978). Sheetflood deposits may comprise thin
sheets of gravels and sands, often forming couplets (Blair
and McPherson, 1994a). The depositional topography on
alluvial fan surfaces will be much smoother than that on
debris-flow depositional surfaces It may resemble feature-
less sheets of imbricated gravels or may show an intricate
'bar and swale' topography (Figure 14.4(c)). One bar form
that has been described is the 'sieve bar' (Hooke, 1967;
Wasson, 1974), a lobate bar showing nose-to-tail and ver-
tical fining of the coarsest clasts, which sieve out the finer
sediments (Figure 14.4(d)).
On most fans, especially dry-region fans, whether
debris-flow or fluvially- dominant, sediment supply is
episodic, related to major storm and flood events (Beau-
mont and Oberlander, 1971; Whipple
et al
., 1998; Arzani,
2005; Mather and Hartley, 2005). On any one fan the
water-to-sediment ratio may vary during a storm (Wells
and Harvey, 1987) and may vary downfan (Pierson and
Scott, 1985), resulting in a complex of depositional facies,
but with the overall pattern, facies-dominance and the dif-
ferences between fans reflecting the gross catchment con-
trols over water and sediment production. As indicated
earlier, fans issuing from small steep catchments tend to-
wards debris-flow dominance, especially if the geology is
conducive to the production of fine sediments, and those
issuing from larger or less steep catchments tend to be
fluvially dominant (Harvey, 1984a, 1992a; Kostaschuk,
Macdonald and Putnam, 1986; Wells and Harvey, 1987).
In this way the style of sedimentation could be said to
be sediment-led. Ironically, the processes are event-based
and therefore could be said to be flood-led.
14.2.1.2
Fluvial processes
Beyond a threshold water content the flow becomes fully
turbulent, transporting coarse sediment by traction and
finer sediment by saltation and in suspension. This is
the realm of fluvial sediment transport, described in
many fluvial geomorphology and sedimentology texts
(e.g. Reinech and Singh, 1983; Schumm, 1977; Nilsen,
1982). Transport takes place either in relatively confined
channels (open-channel flow) either single- or multithread
channels, or as wide shallow unconfined sheetflow. Un-
der fluvial sediment transport, bedload transport capacity
depends on unit stream power (Richards, 1982), depen-
dent on flow depth and gradient. Deposition of bedload
clasts occurs as stream power falls below the transport
threshold, through inceased bed friction, reduced depth or
reduced gradient. The threshold depositional gradient is
much lower for tractional flows than for debris flows and
lower for channelised flows than for sheetflows.
On deposition the sediments exhibit much more organ-
isation than do debris -flow sediments (Figure 14.4(b)).
There is often a channelised or erosional base; then the
14.2.2
Post-depositional modification of
dry-region fan surfaces
In addition to purely depositional facies, arid area fans are
often characterised by other sedimentary features, related
to weathering and pedogenic processes, as well as by sur-
face modification through erosion (see Bull, 1991). Sur-
face modifications are time-dependent and have proved
invaluable in relative dating and correlating fan surfaces
(McFadden, Ritter and Wells, 1989; Bull, 1991).
Older fan surfaces in deserts are usually characterised
by a closely interlocked pavement of angular clasts (Fig-
ure 14.4(e)), below which is a fine silty soil that may fill
the spaces between the subsurface clasts or be entirely
clast-free. Such surfaces are described as desert pavement
