Sedimentary Geology 166 (2004) 59 – 71
www.elsevier.com/locate/sedgeo
Evaporite mineralogy and geochemical evolution of the
Sambhar Salt Lake, Rajasthan, India
R. Sinha *, B.C. Raymahashay
Engineering Geosciences Group, Department of Civil Engineering, Indian Institute of Technology, Kanpur 208016, India
Received 1 April 2003; received in revised form 14 October 2003; accepted 28 November 2003
Abstract
The Sambhar Lake is the largest playa within the Thar desert of western India. A detailed mineralogical investigation was
carried out with bed rock and soil samples collected from the catchment area of the lake and with two deep cores obtained from
the lake bed. The clastic fraction of the lake sediment consists of quartz, alkali feldspar, mica, chlorite, amphibole and
weathering products such as kaolinite and goethite. The non-clastic evaporite fraction is dominated by calcite and halite. There
is a break in evaporite mineralogy at a depth of around 5 m. For example, gypsum is the major sulfate mineral below this depth
while in shallower horizons, its place is taken by an assemblage of thenardite, kieserite and polyhalite. Using the principle of
chemical divides, such variations in mineralogy have been explained in terms of a change in brine chemistry from K – Na – Ca –
Mg – SO4 – Cl to K – Na – CO3 – SO4 – Cl type. It is also suggested that at an earlier stage, the Sambhar Lake brine underwent
evaporation under the condition of Ca>alkalinity whereas in more recent times, the evaporite mineralogy has developed with
alkalinity>Ca. Dolomitisation of calcite and formation of Mg-clay helped Mg-removal. Presence of K-bearing evaporites in the
core sediments suggests that the evaporation of brine exceeded the halite saturation stage. 14C ages from one of the cores
indicate that the geochemical evolution of the lake spanned a period of more than 30 ka. This may have important paleoclimatic
implications.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Saline lakes; Thar desert; Lake sedimentology; Evaporites; Continental playas
1. Introduction
Geochemical evolution of salt lakes is governed
primarily by inflow composition, selective removal of
solutes and evaporative concentration. The formation
of evaporite minerals is dictated by the concept of
chemical divides (Hardie and Eugster, 1970; Eugster
and Hardie, 1978). A large number of publications are
* Corresponding author. Tel.: +91-512-257346; fax: +91-512250260.
E-mail address: [email protected] (R. Sinha).
0037-0738/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.sedgeo.2003.11.021
available on mineral – water equilibria and the controlling factors in the evolution of salt lakes (Eugster
and Smith, 1965; Hardie, 1968; Eugster and Jones,
1979; Spencer et al., 1985a,b; Wood and Sanford,
1990). A more recent review by Yan et al. (2002)
points out that there are significant deviations from
existing models perhaps influenced by local conditions such as lithological variations, weathering patterns, and inflow parameters. The stratigraphic
variation in the mineralogy of salt lake sediments is
also an indicator of past salinity conditions and
hydrological changes. Such variations have been
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R. Sinha, B.C. Raymahashay / Sedimentary Geology 166 (2004) 59–71
extensively used to deduce Quaternary paleoclimatic
fluctuations in many parts of the world (Wasson et al.,
1984; Last, 1990; Rosen, 2000; Schütt, 2000; Dean
and Schwalb, 2000; Last and Vance, 2002). We use
two deep cores from the Sambhar Salt Lake, located at
the eastern fringe of the Thar desert in western India
(Fig. 1), to infer the chemical evolution of the lake as
manifested in evaporite mineralogy.
The Sambhar Lake has attracted a lot of attention
over the past century and a number of hypotheses
have been proposed to explain its pronounced hypersalinity. Earlier workers have suggested a wind-borne
source of salt from the Rann of Kutch (Holland and
Christie, 1909), an inland sea (Tethys) during the
Tertiary (Godbole, 1952), and dissolution of halite
bed in the lake area (Bhattacharya et al., 1982). More
recent work, based on isotopic analysis of lake water
(Ramesh et al., 1993; Yadav, 1997), has clearly
refuted the marine origin and has suggested that the
lake brine is completely replenished by meteoric input
(precipitation) and surface runoff. Most of these
studies focused on the composition of lake brine and
local water bodies. It was felt that for a comprehensive
picture of the evolution of this lake, it is important to
document the nature of rock weathering in the catchment area together with a detailed data base of the
evaporite mineralogy. Our earlier work (Sinha and
Raymahashay, 2000) involved mineralogical studies
of shallow (f 1.5 m) auger hole samples from the
Sambhar Lake sediments and highlighted a cyclic
depositional sequence of clastics (quartz, illite, kaolinite) and evaporites (calcite, halite, thenardite). Maxima and minima of quartz/thenardite and thenardite/
calcite ratios determined from XRD patterns were
used as indicators of wet and dry phases. The presence
of a glycol expansive clay (smectite) in thenardite-rich
horizons suggested a mechanism of Mg-removal during evaporation. Gypsum was absent in all samples
and this was interpreted to indicate a low influx of Ca
into the lake during recent times.
In the present work, we extend our data base to
much deeper levels using about 23 m deep cores
Fig. 1. Location of Sambhar Lake in Thar desert; BH1, BH2 and BH3 are sites of deep cores. Other sampling locations are indicated by
symbols.
R. Sinha, B.C. Raymahashay / Sedimentary Geology 166 (2004) 59–71
obtained from locations close the centre of the Sambhar Lake. These data enable us to draw inferences on
evaporative concentration of inflow waters including
major variations in brine chemistry over a longer time
scale.
2. Regional setting
The Sambhar Lake basin is bounded to the
northwest and west by the f 500 m high Aravalli
hills of Early and Middle Proterozoic age which also
divide the region into a subhumid eastern part and
semi-arid western part (Thar desert). The Sambhar
Lake falls in the rain shadow for south west monsoon with an annual rainfall of 100 – 500 mm. The
average annual temperature is 23 jC, with a maximum of 45 jC. The basement below the lake floor is
made up of Aravalli schists. The following physiographic information is available from Gopal and
Sharma (1994). The Sambhar is an elliptical and
shallow lake, with the maximum length of 22.5 km
running ENE –WSW. The width of the lake ranges
from 3.2 km to 11.2 km. The total catchment area of
the lake is 7560 km2, most of which lies to the north
and northeast. The lake occupies an area of about
225 km2 and the average depth of water is about 1 m
and the maximum depth is about 3 m. Two major
ephemeral streams, namely Mendha and Rupangarh,
feed the lake in addition to numerous rivulets and
surface runoff. The Mendha river, the largest feeder
stream, originates to the north east of the lake, then
flows southwest and enters the lake from the north
forming a small delta at the mouth. The river drains
an area of about 3600 km2, most of which is a
sandy, undulating plain, framed to the north, west
and east by residual Aravalli outcrops. River Rupangarh rises in the south near Ajmer city and flows NE
to enter the lake from the south draining a hilly area
of about 625 km2. In spite of a large catchment area,
the Sambhar Lake presently receives very little
runoff partly due to present climatic conditions and
partly due to human interference, e.g. small check
dams along the rivers and diversion of lake water to
reservoirs for salt production.
The surface of the Sambhar Lake presently undergoes complete desiccation every summer forming an
efflorescent crust. This crust essentially consists of
61
halite and calcite with minor amounts of dolomite,
carnallite, polyhalite and sylvite (Roy et al., 2001). It
dissolves when it comes in contact with fresh runoff
during the next rain and this process increases the
solute load of the lake brine. In terms of chemical
composition, the brine is known to be practically
devoid of Ca and Mg and high in Na (Bhattacharya et
al., 1982). The data supplied by Seshadri and Langalia (1961) as well as unpublished reports of the
Sambhar Salt Private Limited indicate that the feed
brine for salt crystallizers has density ranging from
5j to 25j Be which corresponds to specific gravity
1.036 – 1.208 g/cm3. NaCl, Na2SO4, Na2CO3 and
NaHCO3 are the main constituents amounting to
about 98% of the brine. Ramesh et al. (1993) report
that pH ranges from 8.46 to 8.93 and Cl from 115.22
to 143.21 g/l in the lake brine. The evolution of this
dominantly Na – Cl– SO4 – CO3 type of brine by evaporation of inflow waters along chemical divides has
been discussed later.
3. Methods
This work involved several field visits and sample
collection in and around the Sambhar Lake region
including the northern and southern catchment areas.
Rock and soil samples were collected from the
catchment areas for petrographic and mineralogical
investigations. A major activity was to organize deep
drilling to obtain samples from deeper horizons.
Continuous core samples were collected from three
locations in the Sambhar Lake bed (Fig. 1) using a
Calyx drilling machine and a double tube barrel.
Drilling mud or water was pumped down through an
outer tube and the core was captured in an inner
tube. Out of the three cores, the one from borehole
BH2 was the deepest (f 23 m) and therefore this
core was selected for detailed study. Limited work
was done with the cores from boreholes BH1 and
BH3 for comparison. The cores were split along the
axis and major stratigraphic units (Fig. 2) were
delineated on the basis of physical appearance and
broad lithological characteristics. Representative
samples were taken from different stratigraphic units
for mineralogical studies. Minerals were identified
by the X-ray diffraction (XRD) technique. Bulk
samples in powder form (air-dried or oven-dried at
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R. Sinha, B.C. Raymahashay / Sedimentary Geology 166 (2004) 59–71
Fig. 2. Detailed lithologs and major stratigraphic units of the cores from boreholes BH2 and BH3. Further subdivisions (a, b, c, etc.) of the units
are based on distinctive evaporite mineralogy (see Tables 2 and 3 for details).
35 jC) as well as smear slides prepared out of the
< 2 Am clay fraction were used. Heating to 550 jC
and glycolation were adopted, where appropriate, for
diagnostic tests for clay minerals. X-ray peak heights
were used as gross indicators of the relative proportions of minerals present. A total of 95 bulk samples
and 50 smear slides were studied during this work. A
total of 5 AMS 14C dates were obtained from the
organic fraction of sediment samples at the AMS
facility, Max Plank Institute of Biogeochemistry,
Jena.
4. Core lithostratigraphy and mineralogy
Borehole BH1 was very short, being located in the
peripheral region of the lake. It yielded only 0.6 m of
a blackish clay at the surface. This clay layer graded
to a fine sand layer followed by a weathered mica
schist layer. After a few intervening sandy layers, the
basement of mica schist was encountered at a depth
of 9.2 m.
Major evaporite minerals identified in the core
sediments from BH2 and BH3 are given in Table 1.
R. Sinha, B.C. Raymahashay / Sedimentary Geology 166 (2004) 59–71
Table 1
Major evaporite minerals in Sambhar Lake sediments as inferred
from X-ray diffraction patterns
Composition
Carbonates (Carb)
Calcite (Cc)
Aragonite (Ar)
Dolomite (Dol)
Nesquehonite (Nes)
Nahcolite (Nh)
Trona (Tr)
CaCO3
CaCO3
CaMg(CO3)2
MgCO33H2O
NaHCO3
NaHCO3Na2CO32H2O
Chlorides (Chlor)
Halite (Hal)
Carnallite (Cn)
Sylvite (Syl)
NaCl
KMgCl36H2O
KCl
Sulfates (Sulf)
Gypsum (Gyp)
Bassanite (Bs)
Thenardite (Th)
Mirabilite (Mb)
Kieserite (Ks)
Bloedite (Bd)
Glauberite (Gb)
Polyhalite (Pol)
CaSO42H2O
CaSO41/2H2O
Na2SO4
Na2 SO410H2O
MgSO4H2O
MgSO4Na2SO44H2O
CaSO4Na2SO4
K2Ca2Mg(SO4)42H2O
63
The clastic fraction of the sediments consists of
quartz, feldspar (mostly plagioclase and occasional
K-feldspar), mica, chlorite, occasional amphibole and
their weathering products such as kaolinite and goethite. Quartz is the most dominant mineral occurring
throughout the profile in variable amounts. Feldspar is
generally low in abundance and absent from many
samples. Mica and kaolinite are still lower in abundance except in certain horizons (e.g. 7.04 and 16.50
m in BH2). The non-clastic fraction of sediment is
dominated by calcite and halite which are present at
all depths albeit with variation in their relative
amounts. The other carbonate minerals present are
dolomite and rarely aragonite. In certain horizons, the
relative abundance of dolomite exceeds that of calcite.
Sporadic occurrences of Na-carbonates such as nahcolite, trona and Mg-carbonate such as Nesquehonite
have also been identified. Among sulfates, gypsum is
usually the dominant mineral associated with minor
and irregular occurrences of thenardite, glauberite,
bloedite, mirabilite, kieserite and polyhalite. In some
samples, we recorded major peaks of bassanite and it
is possible that this is an artifact of sample prepara-
Table 2
Lithostratigraphy, distinctive mineralogy and geochemistry of core BH2
Unit/depth
Lithology
Clastic minerals Evaporite minerals
3d (surface – 3.50)
Organic-rich mud
3c (3.50 – 5.0 m)
3b (5.02 – 5.50 m)
Mud
Silty sand
Q, Mx, F, K,
I, Chl
Q, F
Q, I, Chl
3a (5.50 – 6.50 m)
Mud
Q, K, I, Chl
Major
2c (6.50 – 10 m)
Mostly silty with
some silty sand in
upper layers
2b (10 – 13.20 m)
Alternate sand and
silt layers
2a (13.20 m – 15 m) Alternate sand and
silt layers
1b (15 – 16.70 m)
Mud with thin sand
layer
1a (16.70 m – base) Medium to coarse
sand with
carbonate concretions
Q, K, I, Chl,
Amph
Q, Mx F, K, I,
Chl
Q, K, I, F, Mx
Q, Mx, F, K, I,
Chl
Q, F, K, I, Chl
Minor minerals
Cc, Dol, Hal
Pol, Th, Ks, Gb,
Nes, Mb
Dol, Hal
Cc, Pol, Bd, Ar
Gyp/Bs, Hal, Cc, Dol, Th, Ks,
Pol
Gb, Nes
Gyp/Bs, Hal Cc, Dol, Pol, Th,
Mb, Anh
Cc, Gyp/Bs
Dol, Pol, Mb,
Bd, trona
Gyp/Bs, Cc,
Dol, Pol
Gyp/Bs, Cc,
Dol, Hal
Cc, Dol, Hal
Gyp/Bs, Pol, Cn, Syl
Cc, Hal
Dol
Relative dominance
Chlor>CarbHSulf
Chloride-rich
Chlor>CarbHSulf
Transitional
Sulf>Chlor>Carb
Sulfate-rich
Sulf>Chlor = Carb
Bd, Mb
Sulf>Carb = Chlor
Nes, Cn, Pol
Transitional
Carb>Chlor>Sulf
Carbonate-rich
Q, quartz; F, feldspar; Amph, amphibole; Chl, chlorite; Sm, smectite; K, kaolinite; I, illite; Mx, mixed layer clays (Chl – Sm). See Table 1 for
other abbreviations.
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R. Sinha, B.C. Raymahashay / Sedimentary Geology 166 (2004) 59–71
tion. In some other horizons, polyhalite occurs as the
main sulfate mineral (e.g. 5.5 and 11.5 m in BH2).
Halite is the main chloride mineral and is present at all
depths. Apart from halite, two other chloride minerals,
carnallite and sylvite, are present in many horizons.
The core from BH2 is divisible into three major
lithostratigraphic units and further subdivisions are
made on the basis of evaporite mineral assemblage
(Table 2). The lowermost Unit 1 (base to 15 m)
consists of medium to coarse sand layers with concretions of carbonates (mostly calcite) in the lower
part (Unit 1a) with distinct dolomite-rich horizons
and some associated halite and gypsum layers in the
upper part (Unit 1b). Sylvite occurs as the major
chloride mineral along with halite in uppermost
layers of Unit 1b (f 15.50 m). The detrital fraction
represented by quartz and feldspar remains high
throughout the unit.
The middle Unit 2 (15 – 6.5 m) consists of alternate
sandy and silty layers and can be divided into three
subunits on the basis of mineralogical characteristics.
The lower Unit 2a is gypsum-rich with thin dolomite
layers in the upper parts. The detrital component is
very high as reflected in high quartz content. The
middle Unit 2b has alternate gypsum-rich and carbonate-rich layers. Detrital quartz is generally low. The
upper Unit 2c is carbonate-rich (dominantly calcite).
The topmost organic-rich Unit 3 (surface to 6.5
m) consists of blackish clay and silty clay layers and
is quite heterogeneous. Characteristically, gypsum is
absent in most parts of this unit, the last occurrence
being at 5 m. Calcite, dolomite and halite dominate
this unit along with thenardite and kieserite. This
unit can be subdivided into several subunits (Fig. 2;
Table 2).
The chronostratigraphic data available for core
from BH2 are shown in Fig. 2. The age of the topmost
Unit 3 is very well constrained with 4 AMS 14C dates.
Using a gross linear extrapolation, the base of Unit 3
can be placed at f 12 ka. There is one 14C date of
20,800 F 220 BP available from a depth of 10.10 m
and this would place the base of Unit 2 at f 30 ka by
extrapolation. Unit 1 is completely devoid of any
organic material for 14C dating and therefore no age
estimates are available.
The borehole BH3 is located at the NW fringe of
the lake (Fig. 1). The clastic evaporite minerals
identified in the core are listed in Table 3. The
sedimentary stratigraphy above the basement schist
at f 13 m can be divided into three major units. The
bottommost Unit 1a is a sandy unit with thin (< 0.5
m) muddy layers followed by a thick sandy Unit 1b.
The entire Unit 1 is carbonate-rich and the detrital
components such as quartz and feldspar are low. The
Table 3
Lithostratigraphy and distinctive mineralogy of core BH3
Unit/Depth
Lithology
Clastic minerals
Evaporite minerals
Major minerals
Minor minerals
Relative dominance
Chlor>CarbHSulf
Chlor = Carb>Sulf
ChlorHCarb>Sulf
Chlor>Carb>Sulf
Transitional
Carb>Chlor>Sulf
3d (surface – 0.5 m)
3c (0.5 – 1.8 m)
Fine sand
Organic-rich mud
Q, F, I, Chl
Q, F, I
Cc, Hal
Cc, Hal
3b (1.8 – 2.6 m)
3a (2.6 – 4.5 m)
2b (4.5 – 6.5 m)
Fine sand
Sandy silt
Alternate sand
and silt layers,
organic-rich in
upper parts
Mud
(organic-rich
at base)
Medium sand
Medium to
coarse sand with
carbonate
concretions
Q, I, Chl
Q
Q, I
Cc, Hal, Pol
Cc, Dol, Hal
Cc, Dol, Pol
Dol, Pol,
Dol, Bs, Bd, Cn,
Anh, Mb
Dol, Mb, Ks, Bs
Pol, Ks, Mb
Ks, Cn, Tn
Q, I
Cc, Dol, Hal
Pol
Q
Q
Dol, Cc, Hal
Dol, Cc, Hal
2a (6.5 – 8.9 m)
1b (8.9 – 11 m)
1a (11 m – base)
–
–
Carb>Chlor
Carb>Chlor
Q, quartz; Chl, chlorite; F, feldspar; K, kaolinite; I, illite; Mx, mixed layer. See Table 1 for other abbreviations.
Chloride-rich
Carbonate-rich
R. Sinha, B.C. Raymahashay / Sedimentary Geology 166 (2004) 59–71
middle Unit 2 starting at f 9 m is about 4.5 m thick
and consists of a lower Unit 2a made up of organicrich mud at the base followed by carbonate mud
with abundant halite and minor occurrences of polyhalite. The amount of quartz is moderate with minor
amounts of mica and no feldspar. The upper Unit 2b
consists of alternate sand – mud layers with an organic rich mud layer at the top. The sandy layers are
rich in sulfates, mainly polyhalite and thenardite with
minor occurrences of kieserite. The muddy layers are
65
rich in carbonates with calcite dominating at the base
and dolomite at the top of the subunit. Halite occurs
throughout the unit and carnallite appears in traces at
5.5 m. The topmost Unit 3 has alternate organic-rich
and sandy/muddy units which allow further subdivisions. The lower Unit 3a consists of organic-rich
mud with low detrital components such as quartz.
Halite is the main evaporite mineral along with
calcite and dolomite as major carbonates. Sulfates
are represented by traces of kieserite and mirabilite
Fig. 3. (a) Microphotograph showing weathered rim around alkali feldspar grain in a Quartzite sample from Mendha catchment (20 , XPL). (b)
Microphotograph showing sheared biotite flakes criss-crossed by iron oxide veins in a mica schist sample from Gudha hill, Rupangarh
catchment (5 , PPL).
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R. Sinha, B.C. Raymahashay / Sedimentary Geology 166 (2004) 59–71
and occasional but significant occurrence of polyhalite at f 3.50 m. Unit 3b is a sandy unit with very
high detrital component represented by quartz and
mica but no feldspar. This is also a halite-rich unit
with lesser proportions of calcite, dolomite, mirablite
and kieserite. Unit 3c is a muddy interval, organicrich at the base followed by carbonate mud. This is
essentially a chloride rich unit except at 0.52 m
where bloedite occurs in major amount and total
sulfates exceed that of chloride and carbonate. Other
minor occurrences are that of kieserite, mirabilite and
carnallite. Unit 3d is the surface aeolian cover of
sand with abundant quartz with feldspar and mica in
significant quantities. Evaporites are represented by
calcite and halite with minor amounts of polyhalite
and traces of dolomite.
5. Weathering in catchment area
Isotopic work in the Sambhar Lake area (Ramesh
et al., 1993; Yadav, 1997) has indicated that the lake
water is of meteoric origin and the concentration of
dissolved substances is influenced by the regional
weathering regimes. Therefore, a conscious effort
was made during this work to document the weathering of bed rocks in the catchment areas of the two
main feeder streams of the lake namely, Mendha and
Rupangarh (Fig. 1). Representative samples of fresh
rock, weathered rock and soil were collected for
identification of minerals.
The upper part of the Mendha catchment is dominated by residual hills comprised of massive jointed
quartzite which is frequently interbedded with feldspathic and micaceous layers. There are local crosscutting bands of amphibolite. Observation in hand
specimen, thin section and XRD studies showed that
the major minerals in fresh rocks in order to abundance are quartz, alkali feldspar, plagioclase, biotite,
muscovite and amphibole. The partly weathered rocks
show feldspar with weathered rims (Fig. 3a). A dark
colored rock exposed in a quarry near Govindi (Fig. 1)
shows weathering to kaolinite and a glycol expansive
smectite. The relevant weathering reactions have been
depicted in Fig. 4.
The formation of a non-stoichiometric mineral like
smectite is obviously complicated. It is likely that a
smectite with exchangeable Na can form by weathering of Na-rich silicates, e.g. albite under poor leaching
conditions where its three-layer structure is stable. On
the other hand, smectite derived as intermediate
weathering product can undergo further weathering
to form two-layer kaolinite where drainage conditions
are more extensive. Similarly, smectites and chlorite
with Fe, Mg in the structure can be weathering
products of mafic minerals like biotite and amphibole,
which are locally abundant in the catchment areas
around the Sambhar Lake.
Outcrops at the ridges near Palri and Gudha in the
Rupangarh catchment (Fig. 1) show a lithology similar to the Mendha catchment although the mafic
minerals (biotite, amphibole) are relatively more
Fig. 4. Schematic diagram showing inferred weathering reactions to explain the acquisition of major clastic minerals in lake sediments and
dissolved ions in water.
R. Sinha, B.C. Raymahashay / Sedimentary Geology 166 (2004) 59–71
abundant. For example, the crest of the Gudha ridge
shows a wide band of amphibolite cutting across
interbedded quartzite and quartz-biotite schist. In thin
section, sheared biotite flakes are criss-crossed by
numerous iron oxide veins (Fig. 3b). The soil profiles
at these locations and at the Korsina hill (Fig. 1) are
distinctively lateritic. The XRD analysis of these soils
show peaks of kaolinite and goethite suggesting their
derivation by weathering of mafic minerals.
Weathering of primary minerals like feldspar, biotite and amphibole to clay and iron oxyhydroxides
contributes K+, Na+,Ca2 +, Mg2 + along with bicarbonate and silica to river and ground water (Fig. 4).
The basic assumption is that CO2 saturated rainwater is the primary weathering agent. While major ions
and silica are released into river and/or ground water
67
by an exchange reaction with H+ ion derived from
dissolved CO2 (H2CO3, carbonic acid), Al is locked
up in the clay mineral and ferrous ion (Fe2 +) after
being oxidized to Fe3 + forms goethite. The co-existence of these primary and secondary minerals in the
catchment areas of Mendha and Rupangarh rivers
obviously points to a source of these dissolved constituents in the Sambhar Lake.
6. Discussion
During the progressive evaporation of water in a
saline lake, the sequence of minerals precipitated
follows the chemical divides proposed by Eugster
and Hardie (1978). During evaporation, saturation
Fig. 5. Inferred evaporation path for the Sambhar Lake brine; the left-hand branch is the possible evaporation path for the older brine (Sambhar I)
characterized by the dominance of gypsum; the right-hand side explains the development of present-day halite-rich brine (Sambhar II) (adapted
from Eugster and Hardie, 1978).
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R. Sinha, B.C. Raymahashay / Sedimentary Geology 166 (2004) 59–71
with respect to alkaline earth carbonates is reached
quickly. Therefore, calcite and high Mg-calcite precipitate during early stages of salinity increase. Subsequent precipitation of a mineral sequence of
sulfates, silicates and chlorides is controlled by the
relative concentration of Ca, Mg, HCO3, SO4 and Cl
(Fig. 5). Carbonate precipitation may also occur at
higher salinities (Schreiber, 1998) within the field of
gypsum or halite precipitation. This generally implies
external influences such as influx of bicarbonate and
Ca-rich brine through groundwater (Rouchy et al.,
2001). The evaporite mineralogy of sediment cores
from the Sambhar Lake supports this model in a broad
way and the following sections discuss the variations
in chemical pathways of the present and ancient brine.
Carbonates are the most widespread evaporites in
the sediments occurring at all depths but are particularly dominant in the lowermost units. Calcite is the
main carbonate mineral and some horizons have
dolomite in significant quantities. We interpret calcite
as an evaporite mineral since there are no major
carbonate rocks in the Aravalli source region (Heron,
1953; Gupta et al., 1982). Dolomite is also interpreted
to be authigenic, derived from a precursor calcite
under evaporitic conditions. The earlier work of
Yadav (1995) showed that in the present-day surface
and subsurface waters of the Sambhar Lake area, the
ratio mCa/mMg is less than K2c/Kd = 2:1 where Kc and
Kd are solubility products of calcite and dolomite,
respectively. This will favour dolomitisation of calcite
according to the reaction: 2CaCO 3 + Mg = CaMg
(CO3)2 + Ca. Further support of this idea is provided
by a strong correlation between dolomite content,
MgO/CaO ratio in insoluble fraction and d18O of
the carbonate fraction in the Sambhar Lake sediments
(Sinha and Smykatz-Kloss, 2003).
In the core from BH2, the sediments below a depth
of 5.5 m (Units 2a through 3a) have an evaporite
assemblage of gypsum, calcite, dolomite, halite with
minor thenardite. This is in contrast with the sediments
in the upper 5.5 m (Units 3b through 3d) where gypsum is conspicuously absent and the sulfate mineral assemblage is represented by thenardite (Na2SO4)
with or without kieserite (MgSO4H2O). This suggests
a change in the brine evaporation chemistry corresponding to this depth. It is possible that during an
early stage of lake evolution when deeper sediments
were deposited, Ca concentration in the brine was
higher than alkalinity on an equivalent basis (2mCa>allkalinity). The brine therefore evolved along the left
branch of the chemical divide shown in Fig. 5 depositing calcite and gypsum. Quantitative removal of Ca
led to mSO4>mCa. As gypsum continued to precipitate,
the principle of binary salt formation (Drever, 1997)
would dictate that SO4 (the species present in larger
concentration) progressively increased while Ca (present in lower concentration) decreased to a low value. At
this stage, the evaporation path looped back to carbonate precipitation and followed the right branch of the
chemical divide in Fig. 5. Further evaporation under the
condition of alkalinity greater than Ca and Mg led to the
precipitation of Mg-clay, e.g. chlorite –smectite mixed
layer together with thenardite in the younger (shallower) sediments. Along with transport from catchment
area, precipitation in evaporative, alkaline basin is
another mode of origin of Mg-smectites listed by
Chamley (1989). It is likely that Mg was also removed
by dolomitisation of early formed calcite and precipitation of kieserite (present only in top 5 m of the
sequence).
An additional point, compared with the Eugster
and Hardie (1978) model, which has emerged from
this work is the occurrence of K-bearing minerals
such as polyhalite. Polyhalite, a highly soluble K – Mg
sulfate, occurs in many horizons throughout the vertical profile, with or without gypsum. Its occurrence is
quite typical of low MgSO4 deposits in continental
playas and is an indication that the brine progressed
beyond halite saturation (Ingebritsen and Sanford,
1998). Minor occurrences of glauberite, bloedite and
mirabilite can be attributed to different reasons. Glauberite is an authigenic mineral and it generally forms
at the expense of gypsum deposited earlier by reaction
with Na-rich brines (Hardie, 1968). Mirabilite is
generally not expected in closed basin evaporite
deposits as it would also re-dissolve to form glauberite
and halite. Its occurrence in the Sambhar Lake, albeit
in low quantities, may be attributed to subsurface
leakage of brine to achieve a steady state salinity to
precipitate mirabilite (as in West Texas and New
Mexico lakes, Ingebritsen and Sanford, 1998).
Further evidence of the importance of subsurface
inflow comes from the nature of occurrence of evaporites. Although the Sambhar Lake has more than 15
m of chemical sediments, we rarely find beds of pure
evaporites except a few very thin (f 1 cm) layers of
R. Sinha, B.C. Raymahashay / Sedimentary Geology 166 (2004) 59–71
gypsum. On the other hand, all the evaporite minerals
are intimately intermixed with the detrital fraction.
Therefore, they appear to be subaqueous interstitial
deposits. It is likely that the interstitial water had a
higher salinity than the overlying lake water because
of sediment – water interactions as observed for marine sediments by Siever et al. (1965). At the Sambhar
Lake, the evaporite-rich horizons possibly developed
under conditions when evaporation was high and
groundwater inflow was sufficient as suggested by
Rosen (1991, 1994).
Halite is the most important chloride mineral and it
dominates the surface horizons (Units 3c and 3d). This
is a reflection of the present-day brine of K – Na – CO3 –
SO4 – Cl type from which halite is the final evaporation
product. Further, the occurrence of two K-bearing
chloride minerals, carnallite (KMgCl36H2O) and sylvite (KCl) once again indicates that the evaporating
brine exceeded halite saturation at these stages. The
occurrence of these two minerals in a particular horizon
is usually mutually exclusive and this is a reflection of
post-halite brine composition. The relative abundance
of Mg will cause precipitation of carnallite and a brine
poor in Mg will result in sylvite (Ingebritsen and
Sanford, 1998).
It is clear from the above discussion that a pronounced variation in the evaporite mineral assemblage
in the stratigraphic column represented by the core
from BH2 points to sharp temporal changes in the
chemical environment of the lake. Such chemical
transformations with time have been frequently observed in saline lakes around the world, for example,
Searles Lake (Eugster and Smith, 1965), Dead Sea
(Neev and Emery, 1967), Ceylone Lake (Last, 1989,
1990) and North Ingebrigt Lake. Several reasons have
been cited for such transformations including change
in brine chemistry related to source area and/or
climate, chemical stratification of lakes, and recycling
of salts by fractional dissolution of efflorescent crusts.
In Sambhar Lake, located at the margin of the Thar
desert, such changes could be climatically driven. As
discussed earlier, the total time span involved in the
formation of these deposits is more than 30 ka, and
therefore, Quaternary climatic fluctuations may be the
prime factor in pronounced salinity variations in the
Sambhar Lake.
We also note a marked spatial variation in evaporite mineral assemblage between the cores from bore-
69
holes BH2 and BH3. Unlike the core from BH2,
gypsum is absent throughout the core from BH3 and
the major sulfate mineral present is polyhalite. Our
observation is strikingly different from the situation at
the Bristol Dry lake where the concentration of
gypsum in the peripheral region in contrast to halite
in basin center was interpreted to be of ‘displacive’ in
origin. At the Sambhar Lake, the absence of gypsum
in the peripheral region is interpreted to be a manifestation of the lake basin configuration. The core
from BH2 located close to the center of the lake
perhaps represents the complete sedimentation history
of the lake. It is likely that the most complete
evaporation sequence did not occur at the location
of borehole BH3 producing only the carbonate – chloride assemblage similar to the upper part of the core
from BH2. This suggestion, however, needs to be
verified by more rigorous analysis.
7. Conclusions
Among the saline lakes in the Thar desert of
western India, the Sambhar Lake is the largest. From
a comprehensive study of the lake and its evaporite
mineralogy, we draw the following conclusions:
1. From the geomorphic and hydrologic setting and
the fact that the lake undergoes complete desiccation in summer months forming an efflorescent
crust, the Sambhar is classified as a ‘playa’.
2. Bedrocks in the catchment areas of two feeder
streams of the lake show evidence of kaolinisation
of feldspar and alteration of mafic minerals to
goethite. These tropical weathering processes can
supply the solutes to the lake through surface runoff.
3. A break in evaporite mineralogy is observed at a
depth around 5 m in the core from borehole BH2.
For example, while gypsum is the main sulfate
below this depth, it is absent in shallower horizons
where thenardite, kieserite and polyhalite represent
the sulfate component. Calcite and halite occur at
all depths. This indicates a sharp change in brine
chemistry.
4. Applying the mineralogical data to the Eugster
and Hardie (1978) evaporation model, it is
inferred that during the early stages of the
evolution of the Sambhar Lake, Ca concentration
70
R. Sinha, B.C. Raymahashay / Sedimentary Geology 166 (2004) 59–71
in the brine was higher than alkalinity which
favoured precipitation of gypsum. During a more
recent evaporation path, alkalinity greater than Ca
concentration gave an assemblage of carbonates
and chlorides associated with thenardite and Mgclays. The overall brine chemistry therefore
changed from K – Na – Ca –Mg – SO4 –Cl to K –
Na – CO3 – SO4 – Cl type (Fig. 5).
5. Our mineralogical data suggest a major difference
from the Eugster and Hardie (1978) model in terms
of formation of K-bearing evaporite minerals such
as polyhalite, carnallite and sylvite. The occurrence
of these minerals indicates that the brine exceeded
halite saturation several times during the evolution
of the lake.
6. The complete absence of gypsum in the core
sediments from the peripheral borehole BH3
compared to the central borehole BH2 has been
interpreted in terms of basin configuration. In other
words, the evaporation sequence was more complete at BH2 in comparison with BH3.
7. Based on chronological data from the core from
BH2, our study records changes in the chemical
environment of the Sambhar Lake for over 30 ka.
If coupled with other proxies, this may have
important implications for paleoclimatic fluctuations in the Thar desert.
Acknowledgements
The authors are grateful to Department of Science
and Technology, Government of India for financial
assistance through a sponsored research project on
the Sambhar Lake. The authorities of the Sambhar
Salt Private Limited provided the much-needed
logistic support during the entire tenure of the
project. We are also thankful to our students, S.K.
Panda, Sudip Ganguly, Asim Chatterjee, P.D. Roy
and M. Vasantha who have contributed during the
last few years in many ways. A part of the analysis
was carried out at the Institute of Mineralogy and
Geochemistry, Karlsruhe University, Germany where
one of the authors (RS) was supported by the
Alexander von Humboldt Foundation. We are
indebted to Dr. S.P. Harrison at the Max Plank
Institute of Biogeochemistry, Jena for providing us
the 14C AMS dates.
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