The arid region piedmonts: glacis and alluvial fans
369
geological periods. Neotectonics constitutes the main control in fan location and
development in these areas. Many of the old sedimentary sequences of alluvial fans are
commonly interpreted from a tectonic point of view (Harvey, 1997). On the other hand, the
tectonic activity is not limited to the main range-front fault (Figure 15.24), but ground
deformations and faulting may also affect the entire fan system. These recent tectonic
ground dislocations may affect the feeder channels of the fans giving rise to relevant
channel offsets and displacement with the consequent shifting on the locus of the active
sedimentation (Figure 15.25).
Tectonic activity can also affect fan development by means of direct modification of
the fan slope. When the uplift rate of the mountainous front is greater than the incision
rate of the fan channel (Figure 15.26), fan slope increases and sedimentation occurs in the
proximal zone. On the other hand, if the channel incision rate exceeds the uplift rate the
active deposition shifts to the mid- or distal zones of the fan. Figure 15.26 illustrates how
during limited aggradation conditions the development of fanhead trench in the proximal
zone transfers the active deposition to distal locations (Bull, 1964a, 1977). This process
gives rise to segmented fan surfaces constituted by fan segments of different ages located
at different topographic positions. These different fan segments can display a different
degree of incision, as well as soil development (calcretes, desert pavements, and varnish,
etc.). These segmented fans (Figure 15.27) still have a very controversial origin, and
different hypothesis, based on recent extreme events, and complex responses to climatic
and tectonic changes, have been proposed (see Cooke et al., 1993).
Finally, Blair and McPherson (1994b) consider a set of extrinsic factors related to the
effect of neighbouring environments on fans. In this way, fans can be affected by aeolian
sedimentation, because windblown deposits can modify or even obstruct the fan channels.
The development of sandsheets or dunes in distal fan locations may occasionally promote
channel ponding. The fluvial channels developed in downfan locations can capture
the main fan channel, triggering an abrupt fall in the fan base-level and its subsequent
dissection. If the alluvial fans are bounded by either lacustrine or marine environments,
the subaerial fan flows may transform into fandeltas. In these cases, sea- or lake- level
changes directly affect the fan base-level influencing its erosional or sedimentary
behaviour. Finally, volcanic activity can generate lava flows that can interfere and modify
the internal drainage of fans. The occurrence of ashfall within the drainage basins favours
the generation of debris flows from the steep slopes during storm events.
2.4. Sedimentary processes
The sedimentary processes involved in fan development are of two types, primary and
secondary (Blair and McPherson, 1994a,b). The primary processes, or sediment supply
processes, are those responsible for the transportation of sediment from the drainage basin
to the mountain front locations where alluvial fans develop. The dominant transport
mechanism will be strongly conditioned by the water to sediment ratio in the fan flows, as
well as by the availability of fine, mud-sized materials within the drainage basin (Wells
and Harvey, 1987; Harvey, 1997). A low concentration of fines and high water/sediment
ratios favour the occurrence of fluvial transport of coarse clasts by traction. When the
content of fines increases the clasts can be matrix-supported and mobilized in a debris flow
369
geological periods. Neotectonics constitutes the main control in fan location and
development in these areas. Many of the old sedimentary sequences of alluvial fans are
commonly interpreted from a tectonic point of view (Harvey, 1997). On the other hand, the
tectonic activity is not limited to the main range-front fault (Figure 15.24), but ground
deformations and faulting may also affect the entire fan system. These recent tectonic
ground dislocations may affect the feeder channels of the fans giving rise to relevant
channel offsets and displacement with the consequent shifting on the locus of the active
sedimentation (Figure 15.25).
Tectonic activity can also affect fan development by means of direct modification of
the fan slope. When the uplift rate of the mountainous front is greater than the incision
rate of the fan channel (Figure 15.26), fan slope increases and sedimentation occurs in the
proximal zone. On the other hand, if the channel incision rate exceeds the uplift rate the
active deposition shifts to the mid- or distal zones of the fan. Figure 15.26 illustrates how
during limited aggradation conditions the development of fanhead trench in the proximal
zone transfers the active deposition to distal locations (Bull, 1964a, 1977). This process
gives rise to segmented fan surfaces constituted by fan segments of different ages located
at different topographic positions. These different fan segments can display a different
degree of incision, as well as soil development (calcretes, desert pavements, and varnish,
etc.). These segmented fans (Figure 15.27) still have a very controversial origin, and
different hypothesis, based on recent extreme events, and complex responses to climatic
and tectonic changes, have been proposed (see Cooke et al., 1993).
Finally, Blair and McPherson (1994b) consider a set of extrinsic factors related to the
effect of neighbouring environments on fans. In this way, fans can be affected by aeolian
sedimentation, because windblown deposits can modify or even obstruct the fan channels.
The development of sandsheets or dunes in distal fan locations may occasionally promote
channel ponding. The fluvial channels developed in downfan locations can capture
the main fan channel, triggering an abrupt fall in the fan base-level and its subsequent
dissection. If the alluvial fans are bounded by either lacustrine or marine environments,
the subaerial fan flows may transform into fandeltas. In these cases, sea- or lake- level
changes directly affect the fan base-level influencing its erosional or sedimentary
behaviour. Finally, volcanic activity can generate lava flows that can interfere and modify
the internal drainage of fans. The occurrence of ashfall within the drainage basins favours
the generation of debris flows from the steep slopes during storm events.
2.4. Sedimentary processes
The sedimentary processes involved in fan development are of two types, primary and
secondary (Blair and McPherson, 1994a,b). The primary processes, or sediment supply
processes, are those responsible for the transportation of sediment from the drainage basin
to the mountain front locations where alluvial fans develop. The dominant transport
mechanism will be strongly conditioned by the water to sediment ratio in the fan flows, as
well as by the availability of fine, mud-sized materials within the drainage basin (Wells
and Harvey, 1987; Harvey, 1997). A low concentration of fines and high water/sediment
ratios favour the occurrence of fluvial transport of coarse clasts by traction. When the
content of fines increases the clasts can be matrix-supported and mobilized in a debris flow