The arid region piedmonts: glacis and alluvial fans
375
transverse (transverse fibs) to the fan slope have been also described as products of sheet
flooding.
The incised channels dissect the fan apex and can be several metres deep. They
constitute paths for the transfer of sediments between the proximal and distal zones of
the fan, and consequently the feeding channels of the active depositional lobes. These
channels also contribute to fan sediment reworking by means of erosive work on their
incised channel beds. The resulting sedimentary facies are characteristic of confined flows
including fluvial sand and gravels mobilized during flooding events.
A distinctive sedimentation variety of alluvial fans are the so-called sieve lobes or sieve
deposits which were described by (Hooke, 1967) after laboratory studies with sand and
granule features resembling fans. Characteristic lobate deposits build up from the points
where the water flow loses its transport capacity due to its rapid infiltration into the
permeable fan substrate, commonly in the mid-fan intersection points. The coarsest
material is accumulated in the frontal sector of the depositional lobe, but upslope finer
sand-sized sediments prevail. Sieve deposits commonly develop during flooding events
and are mainly deposited in mid- and distal fan zones. They can also originate from
previously deposited debris flow lobes where overland flows sieve the finer material.
Commonly the result is a matrix-free, clast-supported fabric. In this last case, the origin of
the sieve lobes is secondary.
The three-dimensional analysis of alluvial fans provides evidence for the occurrence
of numerous and complex graded bedding features, which can be interpreted as climatic
or tectonic responses. Typically particle size decreases downslope with the coarser
material in the proximal areas and the finer material in the distal ones. The channelized
fluvial blocks and gravels, and debris flow deposits prevail in the proximal areas
(Figure 15.30c,d) (Harvey, 1997). According to Miall (1978) if the dominant sedimentary
facies are fluvial ones we have "Scott"- type sedimentary sequences. On the other hand,
the profusion of debris flow deposits distinguishes the so-called "Trollheim-type"
sequences (Figure 15.30a,b). In mid-fan areas sieve deposits can be developed, but sheetflood deposits commonly extend towards the distal parts of the fans. On the other hand,
strong changes in the distribution of sedimentary facies can be occur between untrenched
and proximally trenched fans; in the latter the more recent deposits are shifted towards the
distal areas of the fans (Figure 15.30e,f).
2.4.2. Reworking processes
As previously mentioned, alluvial fans mainly develop in response to high intensity
storm events, but the return period of these energetic events is important. In consequence,
the fan surface is exposed to a variety of reworking processes during the long periods
of sedimentary inactivity. During these periods the depositional surfaces created by the
primary sedimentary processes are largely modified. In addition, in active tectonic areas,
fan surfaces can be affected by ductile and brittle deformations, such as ground folding,
faulting (Figure 15.30f) and warping. These reworking processes are diverse and they
have been analysed by different authors (see Blair and McPherson, 1994b).
The most relevant reworking processes are the overland flows derived from ordinary
rainfall episodes that dominate the long periods of time between strong storm events.
375
transverse (transverse fibs) to the fan slope have been also described as products of sheet
flooding.
The incised channels dissect the fan apex and can be several metres deep. They
constitute paths for the transfer of sediments between the proximal and distal zones of
the fan, and consequently the feeding channels of the active depositional lobes. These
channels also contribute to fan sediment reworking by means of erosive work on their
incised channel beds. The resulting sedimentary facies are characteristic of confined flows
including fluvial sand and gravels mobilized during flooding events.
A distinctive sedimentation variety of alluvial fans are the so-called sieve lobes or sieve
deposits which were described by (Hooke, 1967) after laboratory studies with sand and
granule features resembling fans. Characteristic lobate deposits build up from the points
where the water flow loses its transport capacity due to its rapid infiltration into the
permeable fan substrate, commonly in the mid-fan intersection points. The coarsest
material is accumulated in the frontal sector of the depositional lobe, but upslope finer
sand-sized sediments prevail. Sieve deposits commonly develop during flooding events
and are mainly deposited in mid- and distal fan zones. They can also originate from
previously deposited debris flow lobes where overland flows sieve the finer material.
Commonly the result is a matrix-free, clast-supported fabric. In this last case, the origin of
the sieve lobes is secondary.
The three-dimensional analysis of alluvial fans provides evidence for the occurrence
of numerous and complex graded bedding features, which can be interpreted as climatic
or tectonic responses. Typically particle size decreases downslope with the coarser
material in the proximal areas and the finer material in the distal ones. The channelized
fluvial blocks and gravels, and debris flow deposits prevail in the proximal areas
(Figure 15.30c,d) (Harvey, 1997). According to Miall (1978) if the dominant sedimentary
facies are fluvial ones we have "Scott"- type sedimentary sequences. On the other hand,
the profusion of debris flow deposits distinguishes the so-called "Trollheim-type"
sequences (Figure 15.30a,b). In mid-fan areas sieve deposits can be developed, but sheetflood deposits commonly extend towards the distal parts of the fans. On the other hand,
strong changes in the distribution of sedimentary facies can be occur between untrenched
and proximally trenched fans; in the latter the more recent deposits are shifted towards the
distal areas of the fans (Figure 15.30e,f).
2.4.2. Reworking processes
As previously mentioned, alluvial fans mainly develop in response to high intensity
storm events, but the return period of these energetic events is important. In consequence,
the fan surface is exposed to a variety of reworking processes during the long periods
of sedimentary inactivity. During these periods the depositional surfaces created by the
primary sedimentary processes are largely modified. In addition, in active tectonic areas,
fan surfaces can be affected by ductile and brittle deformations, such as ground folding,
faulting (Figure 15.30f) and warping. These reworking processes are diverse and they
have been analysed by different authors (see Blair and McPherson, 1994b).
The most relevant reworking processes are the overland flows derived from ordinary
rainfall episodes that dominate the long periods of time between strong storm events.