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In-Depth Information
2008; Silva
et al
., 2008). A similar sequence, but operating
over longer timescales and over a larger spatial scale, has
been demonstrated for the Tertiary fans of the Ebro basin,
northern Spain (Hirst and Nichols, 1986; Nichols, 2005).
In the southeast Spain case, smaller fans developed around
the margins of the basins later in the Pleistocene, largely
in response to climatic change (Harvey, 1987, 1990).
Spatial variations in fan deposition partly reflect
proximal-to-distal variations in depositional processes
and sorting mechanisms. Debris-flow dominance in prox-
imal zones may give way downfan to sheetflood depo-
sition (Figure 14.5(b)) as debris-flow runout distances
(D'Agostino, Cesca and Marchi, 2010) may be limited
to the upper parts of a fan. Channel deposits may also
give way distally to sheetflood deposits as flow spreads
over the distal fan surfaces. Where this transition takes
place, often in midfan below the intersection point, sieve
deposits may be common (Hooke, 1967; Wasson, 1974).
In distal environments sand and silt sheets may interdig-
itate with thin gravel sheets. Within the sediments as a
whole there is normally a downfan decrease in clast size
and an increase in sorting (Bull, 1962a, 1963; Bluck, 1964,
1987; Lustig, 1975).
Channel switching and fanhead trenching will pro-
duce detailed variations in the distribution of sedimentary
units over the fan surface, resulting in a mosaic of fan
deposits and of depositional surfaces of different ages.
There is a contrast in spatial pattern between the random
pattern produced by channel switching near the apex of
an undissected aggrading fan and the more ordered age-
progression of deposits younging downfan and telescoped
(Bowman, 1978) into the fanhead trench of trenched pro-
grading fans (Figure 14.5(c)).
tion ranging from debris-flow to fluvial dominance,
dependent on the processes feeding sediment to
the fan.
(iii) Progradational fans are common, exhibiting proxi-
mal erosion within a fanhead trench and distal de-
position downfan from an intersection point. Such
fans may be mildly progradational, transferring sed-
iment from proximal to distal zones, or may be tele-
scopic (Bowman, 1978) and strongly progradational,
extending the fan boundaries distally.
(iv) Under conditions of excess flood power, fans may
undergo dissection. This may be focused on one of
three zones of the fan, proximally to form a fan-
head trench, in midfan, downfan from an intersection
point (see Section 14.3.2.2), or distally, in which case
it may reflect either erosion of the fan toe (Leeder
and Mack, 2001) or base-level change (see Section
14.3.2.3).
Given the topographic location and available accommoda-
tion space (Viseras
et al
., 2003), apart from the influence
of fan toe processes, the fan regime responds to water and
sediment supply conditions fed from the fan catchment.
Should these conditions change, then the fan regime will
respond, effectively by a change in plotting position on
Figure 14.7(b), resulting in either a change in the inten-
sity of processes or a change in the process regime of
the fan.
14.3
Factors controlling alluvial
fan dynamics
14.2.4
Alluvial fan morphology and style
Alluvial fan dynamics, the response of alluvial fan sys-
tems to external controls, depend on two sets of factors:
(a) those that are essentially passive over the timescale of
fan development and (b) those that are dynamic, chang-
ing within that timescale, and to which fan processes and
morphology respond. Passive factors include the fan set-
ting, dependent on the tectonic and geomorphic history
and influencing accommodation space and confinement
(see Section 14.1.1), catchment geology, affecting rock
resistance to erosion, and catchment size and relief, influ-
encing water and sediment supply. Of course, over long
timescales these factors can change through, for example,
tectonic uplift of the source area, progressive erosion, re-
ducing catchment relief or through river capture affecting
drainage area (Mather, 2000; Mather, Harvey and Stokes,
The morphological style of a fan reflects its recent history
and its current process regime, controlled by the critical
power relationships discussed earlier (Harvey, 2002b).
Four basic styles can be recognised (Figure 14.7(a)).
These are:
(i) A passive/inactive fan is a relict feature inherited
from a more active environment in the past. Under
present processes little or no sediment is yielded from
the feeder catchment and flood power is insufficient
to cause any erosion on the fan, floodwater merely
infiltrating into the fan sediments.
