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phases of aggradation affecting both the wadi floor and
the fans can be identified (see Figure 14.18(c)), which
appear to be primarily responses to climatic changes. The
first aggradation phase (see above, Section 14.3.2.2; see
also Al Farraj and Harvey, 2000, 2004) appears to have
been caused by cold climates in the mountain source area,
though a tectonic influence cannot be entirely ruled out.
The second and third phases appear to be related to cli-
mates wetter than today. Under the arid climate of today,
there appears to be no excess sediment supply to the fans,
simply the transport of wadi sediments through the chan-
nel system. Because of the synchroneity of excess sedi-
ment supply to both the fans and the wadi, fan aggradation
coincided with rises in local base level and fan dissection
with wadi incision (Harvey, 2002b, 2003).
These three examples illustrate how alluvial fan dy-
namcs respond to interaction of the dynamic controls, but
the nature of the response depends on the precise timing of
sediment supply in relation to the timing of either tectonic
activity or base-level change.
specific 'slope gap' differentiating fluvial from alluvial
fan deposits is flawed.
A third myth, though not really a myth, relates to mis-
conceptions in sedimentary sequence models, particularly
in relation to coarsening-up sedimentary sequences. Many
descriptions of alluvial fan sedimentary sequences in the
rock record identify coarsening-up sequences as charac-
teristic. This may relate to the preservation potential of
proximal and distal environments As shown above (Sec-
tion 14.2.3; Figure 14.5(a)), coarsening-up may be charac-
teristic of distal fan environments on prograding fans, but
fining-up is more characteristic of proximal environments
on aggrading fans.
A fourth misconception is similar and relates to the
different emphasis on tectonic control in relation to the
rock record but climatic control of Quaternary alluvial
fan sequences. This may again be related to preservation
potential and to the different temporal and spatial scales
involved (see above, Section 14.4.1).
14.5.2
Significance to science
14.5.2.1
The distinctiveness of dry-region alluvial fans?
14.5
Discussion: significance
of dry-region alluvial fans
Alluvial fans occur in a variety of climatic regions, but
the conditions leading to the formation of alluvial fans are
particularly well developed in dryland mountain regions
(see Section 14.1.2). Furthermore, it is open to question
whether sediment transport and depositional processes
differ significantly between arid and humid regions. How-
ever, where there are important differences lie in the pro-
cesses of post-depositional modification of fan surfaces.
Interactions between fans and adjacent environments (e.g.
pluvial lakes, dunefields) often contain important palaeo-
climatic information (see Section 14.2.2). Soils developed
on dry-region fans not only allow correlation and relative
dating of fan surfaces but on induration the carbonate
horizons create indurated calcreted fan surfaces, modify-
ing infiltration rates and increasing erosional resistance
(see Section 14.2.2).
One important proviso is that the modern alluvial fans
in today's drylands are largely Pleistocene landforms, de-
veloped under climates that may have been different from
today's. While this may be an aid to palaeoclimate recon-
struction, caution is needed when linking modern process
observations to fan morphology.
14.5.1
Commonly held myths and
outdated concepts
Before discussing the significance of dry-region alluvial
fans, it would be appropriate to consider some of the
myths and misconceptions that persist in the literature.
The first relates to the common association of wet fans/dry
fans (Schumm, Mosley and Weaver, 1987) with wet cli-
mates/dry climates. Debris-flow or fluvial dominance is
primarily controlled by sediment supply and transport
mechanisms (see above Section 14.2.1). Arid and semi-
arid environments are more prone to flash floods than hu-
mid areas (Baker, 1977). Erosion in humid areas is more
likely to involve soils, providing the basis for debris-flow
activity. If anything, these two factors would be likely to
favour the development of fluvially (or at least sheetflood)
dominant fans in arid areas (Harvey, 1992a). Some corrob-
oration is afforded by the tendency for a climatic switch to
aridity to involve a reduction in debris-flow and other mass
movement processes and an increase in runoff-related pro-
cesses (Gerson, 1982; Gerson and Grossman, 1987).
A second 'myth' relates to the concept of the 'slope gap'
put forward by Blair and McPherson (1994b). As demon-
strated above (Section 14.3.1.3), Blair and McPherson
were correct in identifying different depositional slopes
14.5.2.2
Significance of alluvial fan research to
geomorphology and sedimentology
