Arid environments - Environmental Sedimentology - page 167

Table 5.4 Main soil orders associated with arid environments in order of abundance. (Adapted from Dregne 1976; Dunkerley & Brown
1997.)

Soil order Main characteristics Total land occupied

(106 km 2 )

Entisol Minimal horizon differentiation and modification of the bedrock 19.2

(sedimentary material)

Aridisol Dryness and /or salinity restricts plant growth throughout the year 16.6

Mollisol Thick, dark base-rich and organically rich topsoil or surface horizon (epipedon) 5.5

Alfisol Moderate base saturation, argillic horizon and seasonally available water for 3.1

plant growth

Vertisol Deep clay soils, characterized by cracking and shrink and swell characteristics 1.9

5.3.5.2 Desert pavements

also associated with evidence of vegetation such
as rhizoliths.

'Desert' or 'stone' pavements are typified by a
lag of stones, typically overlying a stone-free silt
(Fig. 5.17b). Where developed the stones act to
protect the surface from erosion, by breaking
up runoff flow paths. It is widely believed now
that the fines are largely aeolian in origin and
concentrate around the stone lag, passing into
the regolith, perhaps along desiccation cracks,
displacing the stones by pedogenic and aeolian
accretion (McFadden et al. 1984, 1987). The
stones are too large to follow and remain at the
surface. Some authors (e.g. McFadden et al. 1987)
have argued that the enrichment with clay and
carbonate restricts water penetration into the land
surface in the initial stages of surface develop-
ment, shedding more runoff and instigating gully
incision, which will eventually isolate the sur-
face, deactivating soil processes and facilitating
further pavement development. Any associated
coarse material at the surface can also affect the
surface hydrology. Lavee & Poesen (1991) found
through simulated experiments that in general
increased stone cover will enhance runoff gener-
ation. The response, however, is complex, as
the stone cover can also act as a mulch, trapping
moisture and encouraging plant growth. In such
situations vegetation may inhibit surface seal
development and encourage infiltration.

5.3.5 In situ landform modification

Once sediment has been deposited, depending on
its residence time, it can undergo some form of
in situ modification through pedogenic, biogenic
or hydrological processes. These processes vary
spatially in their intensity, creating a mosaic of
surface runoff sources and sinks during a rain-
storm. This is significant for sediment entrain-
ment, erosion and deposition as is illustrated
in section 5.4.3. The most significant in situ
modifications of the sediment surface are dis-
cussed in sections 5.3.5.1 to 4 below.

5.3.5.1 Arid region soils

The USA soil taxonomy classification system re-
cognizes five main soil orders (Table 5.4) for arid
regions. It is evident from this table that arid areas
are dominated by poor soils lacking in organic
matter and associated with limited plant growth.
Arid soils differ from soil developed in more
humid regions in a number of ways. Typically
accumulation is dominated by material from
external sources (e.g. aeolian dust), rather than
the in situ breakdown of parent material obser-
ved in more humid environments. In addition
desert soils are typified by long time-scales of
development. This facilitates the long-term build
up of minerals such as carbonate, or gypsum
in hyper-arid settings (Fig. 5.17a), developing
indurated 'petrocalcic' horizons (section 5.3.5.5).

5.3.5.3 Microphytic crusts

Although desert soils are typified by low
vascular plant cover, they can have abundant