Geoscience Reference
In-Depth Information
Desert crusts and rock coatings
David J. Nash
8.1
Introduction
in the accumulation and preservation of thick sedimentary
sequences (see Nash and McLaren, 2007).
The majority of desert crusts are the product of spe-
cific arid zone hydrological or pedological processes. As
such, the occurrence of crusts as relict surficial deposits,
or in the geological record, may provide valuable palaeo-
climatic information. However, the processes and precise
environmental controls involved in crust formation are
not always well understood. Furthermore, there are sev-
eral structural forms of most of the main types of desert
crust. Some crusts closely resemble features that form un-
der markedly different environmental conditions - for ex-
ample, some silcretes may be the products of weathering
in the humid tropics while others form in predominantly
semi-arid regions. Hence, it is essential that the macro- and
micromorphological characteristics and chemical proper-
ties of desert crusts, plus the processes involved in their
formation, are fully appreciated before they are employed
as evidence in palaeoenvironmental reconstructions.
This chapter provides an overview of the main types of
desert duricrust, beginning with those that are found only
in hyper-arid regions (sodium nitrate deposits) through
to silcrete, which may form in wet-dry subtropical re-
gions (Figure 8.2). It also describes the major types of
rock coating, before considering the palaeoenvironmental
significance of duricrusts and rock coatings. Throughout
the chapter, the varieties of duricrust are treated separately.
Forms such as silcrete, calcrete and gypcrete, however, are
only the end members of a spectrum of duricrust types.
Hybrid or intergrade forms can occur when, for exam-
ple, diagenetic alteration of a duricrust results from water
percolation. A range of minerals may also be precipi-
tated within pores during the latest stages of cementation.
Thus it is possible to find not only silcrete and calcrete
Even in the driest regions of the world, the occurrence of
crusts at or near the land surface (termed duricrusts when
partially or fully indurated) testifies to the mobilisation
and precipitation of minerals in the presence of water. The
most widespread varieties of desert crust are calcrete (ce-
mented by calcium carbonate), silcrete (silica-cemented),
gypcrete (gypsum-cemented) and halite crust (or sal-
crete ). A number of other minerals form less widespread
hardened crusts, including ferricrete and dolocrete , and
a wide range of evaporite crusts composed of minerals
such as thenardite (Na 2 SO 4 ), mirabilite (Na 2 SO 4 ·
10H 2 O),
glauberite (Na 2 Ca(SO 4 ) 2 ) and epsomite (MgSO 4 ·
7H 2 O)
also exist. Even less common evaporites such as nitratite
(or nitratine, NaNO 3 ), natron (Na 2 CO 3 ·
10H 2 O) and trona
(NaH(CO 3 )
2H 2 O) may form crusts in desert basins in
volcanic regions such as the East African Rift Valley and
parts of the Atacama Desert of Chile.
Desert crusts have a variable geomorphological influ-
ence. Some are ephemeral features and have little long-
lasting impact upon the landscape. For example, halite
crusts are prone to rapid dissolution by water and may
only survive for a few months or years after formation.
Nevertheless, these and crusts such as gypcrete can have
a significant effect when they encase and immobilise sand
dunes (Watson, 1985a, 1985b; Chen, Bowler and Magee,
1991a, 1991b). Other crusts, such as calcrete and silcrete,
are persistent features in the landscape once indurated
(e.g. Pain and Ollier, 1995). Calcrete crusts tens of metres
thick are not uncommon and can mantle extensive areas
of desert terrain, protecting the underlying materials from
subaerial weathering and erosion (Figure 8.1). By reduc-
ing denudation rates, these crusts play an important role
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