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morphological sequences and morphometry (Harvey,
1978, 1984a, 1984b, 1987, 1988, 1992a, 1992b, 2002a;
Somosa et al. , 1989; Silva et al ., 1992), and more recently
with fan evolution and dynamics in relation to tectonic,
climatic and base-level controls (Harvey, 1990, 1996,
2002b; Calvache, Viseras and Fernandez, 1997; Harvey
et al ., 1999, 2003; Viseras et al ., 2003; Silva et al .,
2008). In addition there have been important studies of
calcretes on fan surfaces, a characteristic of Pleistocene
fan surfaces in this region (e.g. Alonso-Zarza et al ., 1998;
Stokes, Nash and Harvey, 2007).
flood power and sediment supply by changing the style or
rate of sedimentation or by switching between erosional
and depositional regimes, they may preserve in their sedi-
ments and morphology a sensitive record of environmen-
tal change in their source areas, particularly as affecting
sediment supply. Indeed, this may be a much more sen-
sitive and complete record than that preserved in fluvial
sediments further downstream, which tend to respond pri-
marily to variations in flood power (Macklin and Lewin,
2003) and would be more affected by changes in coupling
relationships.
There are, however, two problems. One is dating. In
contrast with fan deposits in humid areas, which may
preserve organic materials suitable for radiocarbon dating,
such material is rare in dry-region fan sediments. On the
other hand, advances in luminescence and cosmogenic
nuclide dating have potential (see Section 14.5.2.3).
The other problem relates to scale and the preserva-
tion potential of fan sediments. For modern or Quater-
nary fans there is little problem, but for fan sediments
preserved in the rock record, larger bodies of sediment,
especially those at basin margins, are more likely to be
preserved than those within valley systems. This may lead
to a mismatch between interpretations or models based on
the rock record and those based on modern fans. One of
the challenges facing alluvial fan research has been to re-
late observations on modern or Quaternary fans to those
made on ancient fan sediments. One approach that shows
some potential has been to apply the sequence stratigra-
phy concept, normally applied to marine sediments in the
rock record, to the Quaternary alluvial fan sediments of
the central valley of California (Weissmann, Mount and
Fogg, 2002; Weissmann, Bennett and Lansdale, 2005).
Instead of sea-level change controlling sequence bound-
aries, interstratified palaeosol horizons are seen as the key
and Quaternary climates the primary control of the se-
quence. Glacials caused fan aggradation and interglacials
caused fan trenching and soil formation on the abandoned
fan surfaces. In this way, from a three-dimensional inter-
pretation of the changing fan morphology and sediment
sequence a meaningful environmental reconstruction has
been achieved.
14.1.3 The role of alluvial fans within dryland
fluvial systems
With their location between sediment-source areas and ei-
ther enclosed sedimentary basins or arterial river systems,
alluvial fans have an important role in either coupling
or buffering dry-region fluvial systems. In the long term
alluvial fans act as major sediment stores, trapping the
coarse fraction of the incoming sediment. Locally, de-
position will occur if the sediment supply rate is greater
than the transporting capacity, in other words, if the actual
stream power through the fan is less than that needed to
carry the sediment through the system, a threshold defined
by Bull (1979, 1991) as the threshold of critical stream
power. Spatial variations in stream competence will cause
variations in deposition on the fan in relation to stream
power behaviour (Figure 14.3(a)). Temporal variations
will cause variations in sedimentation rate or style or even
switches between net aggradation on, or dissection of, the
fan surface (Figure 14.3(b)).
Fans undergoing aggradation, whether untrenched and
proximally aggrading or fanhead trenched and distally
aggrading (telescopic or prograding fans; see later in Sec-
tion 14.2.4), will act as buffers within the fluvial system,
breaking the continuity of sediment movement between
the sediment source area and either the sedimentary basin
or the main channel system downstream (Harvey, 2002c,
2010). The implications are important not only in the con-
text of sediment movement through the system but also in
the system's response to environmental change. Alluvial
fans may absorb such changes by varying their rates or
patterns of sedimentation (Harvey, 1987) and inhibit the
downstream transmission of the effects of such changes.
On the other hand, where fans become dissected through-
out their length there may be continuity of coarse sediment
movement from the source area to arterial drainage. For
these reasons the coupling/buffering role of alluvial fans
is fundamental to the dynamics of sediment movement
through the fluvial system.
14.2 Process and form on dryland
alluvial fans
14.2.1 Sediment supply, transport and
depositional processes
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