The arid region piedmonts: glacis and alluvial fans
377
calcium carbonate cementation of fan surfaces. In rich-carbonate semi-arid zones these
processes can give rise to the occurrence of petrocalcic soils.
Wind action is an important reworking process in arid regions. Windflow produces the
pick-up and winnowing of fine particles from the fan surface that can eventually give
place to the development of desert pavements. The exposed gravels undergo aeolian
abrasion resulting in the generation of ventifacts sometimes coated with desert varnish.
The sand-sized particles transported by windflows can also be deposited on to the fan
surface giving place to continuous sand-sheets or generate dune- type accumulations
around obstacles, such as plants. These aeolian deposits can attain an important thickness
in favourable "leeside-slopes" of the fans giving place even to relevant disturbances on
fan channels.
The weathering of fan surfaces includes the action of mechanical, chemical and
biological processes. Mechanical weathering, such as salt crystal growth, mineral
hydration-dissecation cycles and gelifraction, disintegrate and fragment the clasts and
they are conditioned by the abundance in water and their salt content. Biological
weathering, or bioturbation, is generated by the burrowing and material removal produced
by the activity of plants and fauna. Calcium carbonate precipitation near the fan surface
produces the development of petrocalcic soils (Gile and Hawley, 1966; Machette, 1985;
Harvey, 1987a). The cementation of fan sediments generates resistant calcrete crusts,
which prevent the erosion and reduce the infiltration of water on fan surfaces (Harvey,
1990, 1997).
2.5. Dynamics of alluvial-fan development
The alluvial fans are the landforms of faster development rates within arid areas
(Mabbutt, 1977). Average accumulation rates in alluvial fans of the Death Valley (USA)
are of 1 m every 1000 years (Hooke, 1972). Alluvial fans are dynamic geomorphic
systems showing progressive morphologic changes during their development (Harvey,
1997). This author proposed a model for fan evolution in which the relationships between
the fan- surface and fan- channel profiles control the balance of aggradation an incision
in mid and distal fan areas (Harvey, 1987a, 1997) (Figure 15.31). Types A and B are
characterized by the occurrence of fanhead entrenchment and distal aggradation from
the mid-fan intersection point. In Type C, scour rather than deposition takes place at
the intersection point, and in Types D and E this scour is sufficiently effective to create a
series of headcuts, which may shift to distal areas as a consequence of a progressive
downfan migration of successive intersection points. These successive headcuts can
eventually link together as a mid-distal fan trench, dissecting the fan throughout its length
(Type F). Fan dissection can also occur by base level-induced headward erosion from
the fan-toe (Type G). This can be considered a key model for the understanding of fan
aggradation and dissection dynamics, in which tectonic disturbances, climatic changes,
human activity, and extreme storm events play a relevant role in the modification of the
dominant erosive or depositional processes.
There are other models of fan development described in the classic literature, such as
that proposed by Davis (1905) based on their cyclical conception of landscape evolution.
This author, proposed a juvenile fan stage in which the higher relief continuously provides
377
calcium carbonate cementation of fan surfaces. In rich-carbonate semi-arid zones these
processes can give rise to the occurrence of petrocalcic soils.
Wind action is an important reworking process in arid regions. Windflow produces the
pick-up and winnowing of fine particles from the fan surface that can eventually give
place to the development of desert pavements. The exposed gravels undergo aeolian
abrasion resulting in the generation of ventifacts sometimes coated with desert varnish.
The sand-sized particles transported by windflows can also be deposited on to the fan
surface giving place to continuous sand-sheets or generate dune- type accumulations
around obstacles, such as plants. These aeolian deposits can attain an important thickness
in favourable "leeside-slopes" of the fans giving place even to relevant disturbances on
fan channels.
The weathering of fan surfaces includes the action of mechanical, chemical and
biological processes. Mechanical weathering, such as salt crystal growth, mineral
hydration-dissecation cycles and gelifraction, disintegrate and fragment the clasts and
they are conditioned by the abundance in water and their salt content. Biological
weathering, or bioturbation, is generated by the burrowing and material removal produced
by the activity of plants and fauna. Calcium carbonate precipitation near the fan surface
produces the development of petrocalcic soils (Gile and Hawley, 1966; Machette, 1985;
Harvey, 1987a). The cementation of fan sediments generates resistant calcrete crusts,
which prevent the erosion and reduce the infiltration of water on fan surfaces (Harvey,
1990, 1997).
2.5. Dynamics of alluvial-fan development
The alluvial fans are the landforms of faster development rates within arid areas
(Mabbutt, 1977). Average accumulation rates in alluvial fans of the Death Valley (USA)
are of 1 m every 1000 years (Hooke, 1972). Alluvial fans are dynamic geomorphic
systems showing progressive morphologic changes during their development (Harvey,
1997). This author proposed a model for fan evolution in which the relationships between
the fan- surface and fan- channel profiles control the balance of aggradation an incision
in mid and distal fan areas (Harvey, 1987a, 1997) (Figure 15.31). Types A and B are
characterized by the occurrence of fanhead entrenchment and distal aggradation from
the mid-fan intersection point. In Type C, scour rather than deposition takes place at
the intersection point, and in Types D and E this scour is sufficiently effective to create a
series of headcuts, which may shift to distal areas as a consequence of a progressive
downfan migration of successive intersection points. These successive headcuts can
eventually link together as a mid-distal fan trench, dissecting the fan throughout its length
(Type F). Fan dissection can also occur by base level-induced headward erosion from
the fan-toe (Type G). This can be considered a key model for the understanding of fan
aggradation and dissection dynamics, in which tectonic disturbances, climatic changes,
human activity, and extreme storm events play a relevant role in the modification of the
dominant erosive or depositional processes.
There are other models of fan development described in the classic literature, such as
that proposed by Davis (1905) based on their cyclical conception of landscape evolution.
This author, proposed a juvenile fan stage in which the higher relief continuously provides