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In-Depth Information
crusts. However, there is evidence that crystal habits dif-
fer depending on the depositional environment. Surface
efflorescences of halite can be made up of fibrous crys-
tals, while cubic or trigonal pyramids crystallise in porous
media (Eswaran, Stoops and Abtahi, 1980).
The mineralogical and chemical characteristics of halite
crusts are closely related to the environment of deposi-
tion (see Chapter 15). In lacustrine settings, the initial
chemical composition of the evaporating brine controls
the sequence of minerals that precipitate from it by a pro-
cess termed fractional crystallisation (Hardie and Eugster,
1970; Eugster and Hardie, 1978). Evaporite minerals as-
sociated with primary halite and gypsum deposits such as
celestite (SrSO
4
) (Evans and Shearman, 1964; Magee,
1991) and polyhalite (K
2
Ca
2
Mg(SO
4
)
halite crust, which redissolves upon flooding and repre-
cipitates by evaporation after flood-borne clastic depo-
sition. By this mechanism the halite crust is preserved
as the uppermost horizon, with clastic sediment progres-
sively accumulating 'beneath' the halite layer (Chivas,
2007). The saline mudflat areas surrounding perennial
saline lakes and playas are associated with the devel-
opment of halite efflorescences and intrasediment (dis-
placive) evaporite minerals. In some systems, these may
be zoned, with the most soluble minerals towards the low-
est portion of the mudflat (e.g. Saline Valley, California;
see Hardie, 1968). Dry mudflats, where the groundwater
table is deeper, are associated with minor efflorescences
and intrasediment salts related to fluctuating water tables
(Smoot and Lowenstein, 1991).
The formation of some terrestrial halite crusts is a pe-
dogenic process. In the Namib (Watson, 1983b) and At-
acama Deserts (Ericksen, 1981), halite crusts are illuvial
accretions formed by similar processes to pedogenic cal-
cretes and gypcretes. Salt deposited at the surface as dust
or in fog moisture in the Namibian and Chilean Deserts
is leached into the solus where it recrystallises when the
soil moisture evaporates. Provided the soil's moisture stor-
age capacity is not exceeded, the salts accumulate as an
illuvial horizon. Amit and Yaalon (1996) describe cu-
bic halite accumulations within mature reg soils in the
Negev Desert, formed by crystallisation from supersatu-
rated soil solutions at the depth of maximum water pen-
etration. Highly soluble salts are less likely to persist be-
cause even infrequent saturation of the solus will cause
flushing. The different solubilities, and therefore vertical
mobilities (Yaalon, 1964), of the common crust-forming
minerals can result in the formation of two-tiered crusts
if rainfall is sufficient to mobilise less soluble salts but
not to flush the more soluble. The less soluble salt will
accumulate in an illuvial horizon above the more soluble
one. Calcretes above gypsum crusts are found in North
Africa (e.g. Horta, 1980) and gypsum crusts above halite
crusts in the central Namib Desert.
2H
2
O) (Holster,
1966) are formed by diagenesis. Pedogenic halite crusts
may contain significant quantities of accessory minerals,
particularly sulfates and, less commonly, nitrates. For ex-
ample, XRD and SEM analyses of saline soils from the
Las Vegas Basin, USA, identified the presence of halite,
bloedite, eugsterite, gypsum, hexahydrite, mirabilite, se-
piolite thenardite, vivianite and possibly kainite (Buck
et al.
, 2006).
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8.3.3
Mode of formation
The requirements for the development of a halite crust in
a lacustrine setting are a source of solutes, their transport
to an accumulation basin and their evaporation. Hardie,
Smoot and Eugster (1978) and Smoot and Lowenstein
(1991) identify a number of subenvironments where this
combination of factors occurs. In perennial saline lakes
(e.g. Lake Chad, the Dead Sea and the Caspian Sea),
evaporite minerals including halite accumulate as cumu-
lus crystals that initially precipitate at the water surface
and sink to the lake floor. Evaporite crusts may also be pre-
cipitated directly on shallow lake beds. Logan (1987), for
example, describes upward-pointing chevron crystals of
halite forming units up to 5 m thick in Lake McLeod,
Western Australia. Halite deposits may exhibit ripple
or cross-bedded structures if subaqueously reworked. In
playa environments (Figure 8.4(a) and (b)), salts accu-
mulate during successive cycles of flooding and evapo-
ration (Lowenstein and Hardie, 1985). Subaqueous salt
hoppers and rafts, together with inwashed clastic sedi-
ments, may bury and preserve earlier layered salts. Dis-
placive and diagenetic salts, including gypsum and halite,
may also precipitate within pore spaces. Ultimately, a
polygonal surface crust may form, with halite efflores-
cences emerging from the pressure ridges between poly-
8.4
Gypsum crusts
8.4.1
General characteristics
Gypsum crusts (often referred to as gypcretes) (Figure 8.5)
have been defined as 'accumulations at or within 10 m
of the land surface from 0.10 to 5.0 m thick containing
more than 15 % by weight gypsum
and at least 5.0 %
by weight more gypsum than the underlying bedrock'
(Watson, 1985a, p. 855). Gypcretes have received less at-
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