Sulfur And Strontium Isotopic Compositions Of Carbonate And Evaporite Rocks From The
Late Neoproterozoic-Early Cambrian Bilara Group (Nagaur-Ganganagar Basin, India):
Constraints On Intrabasinal Correlation And Global Sulfur Cycle
A. Mazumdar1* and H. Strauss2
1
2
National Institute of Oceanography, Dona Paula, Goa-403004, India
*Corresponding author
anmaz2001@yahoo.com
Geologisch-Paläontologisches Institut und Museum, Westfälische Wilhelms-Universität Münster,
Correnstrasse 24, D-48149 Münster, Germany
hstrauss@uni.muenster.de
ABSTRACT
Sulfur and strontium isotope ratios are presented for carbonate and evaporite rocks from the late
Neoproterozoic and early Cambrian Bilara and Hanseran Evaporite Groups, NW India. The sulfur
isotopic compositions of trace sulfate in carbonate rocks from the Bilara Group (27.2 to 42.0‰, avg:
33.8±3.1‰, n = 37) and for calcium sulfate from the Hanseran Evaporite Group (27.5 to 39.7‰,
32.4±3‰; n = 25) are in good agreement with previously determined sulfur isotope values from
evaporites and phosphorite deposits of terminal Neoproterozoic to early Cambrian age. Lithological
and geochemical results suggest the coeval nature of the Bilara Group and Hanseran Evaporite Group.
Fluctuations in the sulfur isotopic composition may at least partially be attributed to intrabasinal
bacterial sulfate reduction under sulfate limitation or diagenetic processes. These variations are
superimposed on the globally recognized sulfur isotopic enrichment that occurred between 600 and
500 Ma. Similarly, the strontium isotopic composition of Bilara carbonate rocks and Hanseran
evaporites are comparable to the contemporaneous global seawater
87
Sr/86Sr ratios, recording a
increase during post-Varangerian time. The rise in Sr isotopic ratio through the late Neoproterozoic
and early Cambrian and enrichment of sulfate in
34
S may
be attributed to high nutrient flux
associated with erosion and subsequent burial of sulfide through biogeochemical processes. This
process possibly dominated over the fluvial flux of 32S rich sulfate produced by pyrite weathering.
1
INTRODUCTION
Extensive fluctuations in the sulfur isotopic composition of marine sulfate have been recorded across
the time window straddling the terminal Neoproterozoic and early Cambrian (Strauss et al., 2001,
Schröder et al., 2004). Sulfate bearing rocks of this time are significantly enriched in the heavy sulfur
isotope (34S) compared to most of the geological record (e.g., Claypool et al., 1980, Walter et al.,
2000, Strauss, 1997, 2004), with examples from anhydrite deposits, phosphorites (Shields et al.,
1999, 2004) and limestone/dolomites (Goldberg et al., 2005). These perturbations have been
attributed to changes in the global marine sulfur cycle involving reduced (pyrite, organic bound
sulfur) and oxidized (sulfate) reservoirs. Enhanced burial of reduced sulfur, such as pyrite, would
shift the marine sulfate sulfur isotopic signal towards enriched values, whereas enhanced pyrite
weathering and a concomitant fluvial flux of dissolved sulfate would result in a decrease in δ34S
(Kampschulte and Strauss, 2004). The geochemical changes recorded across the terminal
Neoproterozoic and its transition into the early Cambrian have been linked to geological
(rearrangement of landmasses: e.g., McKerrow et al., 1992; Meert and Lieberman, 2004) and
biological (evolutionary: e.g., Glaessner, 1984; Conway Morris, 1987; 1993) changes.
Palaeogeographic reconstructions for this time by Gorin et al. (1982), McKerrow et al. (1992), and
Meert and Lieberman (2004) support an equatorial position of the northern part of Gondwanaland.
Several fault basins formed due to rifting/ wrenching in parts of Gondwanaland stretching from India
and Pakistan across the Arabian Shield to the central Iran where great volumes of carbonate
(dolomite/limestone) and evaporite deposited (Husseini and Husseini, 1990). Marine-evaporite and
carbonate deposits in northwestern India (Nagaur-Ganganagar basin), Pakistan (Salt Range), Oman
(South Oman salt basin), Iran (Kerman Basin) and Saudi Arabia formed in rift grabens that were in
close proximity to each other within a broad carbonate shelf along the western margin of east
Gondwanaland (Husseini and Husseini, 1990). Isotopic (stable and radiogenic) studies of chemical
sediments, i.e. carbonates and evaporites, provide a unique opportunity for interbasinal correlation.
Furthermore, these sediments have archived the compositional state of contemporary marine water
allowing the reconstruction of its temporal evolution. Carbonates (Bilara Group) and evaporites
(Hanseran Evaporite Group) of the Nagaur-Ganganagar basin (western India) constitute an important
link in a comprehensive correlation of sediments deposited along the northern margin of Gondwana
through terminal Neoproterozoic and early Cambrian time. Previous work on the sulfur isotopic
composition of sulfate rocks from the Ara Group, Oman Basin (Schröder et al., 2004), the Hormuz
2
Formation, Iran (Houghton, 1980) and the Hanseran Evaporite Group of Nagaur-Ganganagar basin
(Strauss et al., 2001) shows a large spread including highly enriched sulfur isotope values which can
be correlated with those recorded from other contemporary marine sulfates (Strauss et al., 2001;
Strauss, 2004; Schröder et al., 2004). However, owing to a lack of age control for the sampled
sections, to intrabasinal variations and to possible diagenetic factors, the global correlation of sulfur
isotopic perturbations, particularly in Precambrian successions, remains a challenging task.
In this paper we report the first set of data on sulfur and strontium isotopic compositions of carbonate
rocks from the Bilara Group and strontium isotope values for the correlative Hanseran Evaporite
Group of Nagauar-Ganganagar basin. The latter supplement previously determined sulfate sulfur
isotope data (Strauss et al., 2001). In general, the results facilitate intrabasinal and interbasinal
correlations. Moreover, an estimate of the broad depositional age is possible based on
chemostratigraphic grounds. In addition, possible causes for the marked perturbations in the sulfur
(and strontium) isotopic composition of seawater straddling the Precambrian-Cambrian time window
are discussed.
GEOLOGY
Stratigraphy
The Nagaur-Ganganagar Basin (Fig. 1A), western Rajasthan, India, is an elongated asymmetrical
sedimentary basin trending NNE-SSW and covering an area of over 100,000 km2. It is bounded by
the Aravalli Mountain Range in the east, the Delhi-Lahore subsurface ridge in the northeast and north
and the Devikot-Nachna subsurface high in the southwest. The Marwar Supergroup (Pareek, 1981)
constitutes the late Neoproterozoic-early Cambrian succession in this basin. It rests on Precambrian
gneisses, granites and rhyolites belonging to the Malani Group (Fig. 1B). Malani Group rocks reflect
polyphase igneous activity, which ranges in age from 780 to 680 Ma (Rathore et al., 1999), providing
a basal age limit for the Marwar Supergroup. The Marwar Supergroup consists of (i) Jodhpur Group,
(ii) Bilara Group and (iii) Nagaur Group in ascending order. The Jodhpur Group is a fluvio-marine
succession, comprised of cross-bedded, reddish sandstone with maroon shale. The beds dip gently 25o towards N and NW. Lenses of conglomerate derived from Malani Rhyolite locally underlie the
Jodhpur Group. The Bilara Group comprises dolomite and limestone with occasional clay beds and
conformably overlies the Jodhpur Group. The Bilara Group is overlain by a sequence of medium to
coarse grained, cross-bedded, reddish brown, sandstone belonging to the Nagaur Group (fluvial). The
Hanseran Evaporite Group (HEG, Fig. 1B) is thought to represent a coeval facies variant of the Bilara
3
Group (Dey, 1991) and is underlain and overlain by rocks from the Jodhpur and Nagaur groups,
respectively (Dasgupta et al., 1988; Kumar, 1999). The HEG has been encountered in subsurface
boreholes and is well developed in the central and northern parts of the Nagaur basin. A maximum of
seven evaporite cycles has been identified from the HEG (Dasgupta, 1996 and Kumar, 1999). Each
cycle is composed of dolomite, magnesite, anhydrite, halite, polyhalite and clay bands in ascending
order. Silvite has also been reported from some bore holes. The presence of organic-rich, finely
laminated shale layers and vuggy dolomite (emanating H2S smell when crushed) suggest intermittent
basinal anoxia possibly caused by density stratification during evaporation (Kirkland and Evans,
1981). Rocks of the Marwar Supergroup have also been reported from oil wells (Baghewala I and II)
on the western margin of the Nagaur-Ganganagar basin, with live oil shows in multiple zones
(Dasgupta and Balaguda, 1994). Based on geochemical characterization of the crude oil in the Bilara
and Jodhpur successions in Baghewala cores, Peters et al. (1995) correlated these oils with the late
Neoproterozoic-early Cambrian Huqf oil of southern Oman and Karampur-I oil of East Pakistan and
proposed an Infracambrian-Cambrian age for the Bilara carbonates.
Depositional Setup
The carbonate rocks of the Bilara Group are exposed in low-lying hills or quarries along the eastern
and southern borders of the Nagaur-Ganganagar basin. For the present study, samples were collected
near Bilara (TG), Dhanapa (DH, HD, HKSPH, K, ANS), Ransigaon (RAN) and Ghagrana (GAG)
villages (Figure 1A). Sedimentary structures observed suggest an arid-peritidal depositional milieu.
Horizontal tidal bundles along with current ripple laminations are commonly observed features (Fig.
2A). The horizontal lamination is characterized by alternating fine and coarse carbonate particles.
Ripple laminations often show truncations. These current structures typically suggest deposition in an
intertidal zone (Reinick and Singh, 1980). Microbial structures (Fig 2B) are ubiquitous throughout
the sections and often display a highly crinkled and contorted nature. The presence of mudcracks (Fig.
2C) suggests subaerial exposure in an arid environment. Well-preserved stromatolitic build-ups (Fig.
2D) have been observed at both Dhanapa and Bilara regions. Linked hemispheroidal and conoform
types have been recorded. The stromatolitic buildups constitute a reefal structure in Dhanapa region.
The stromatolites here are extensively chertified. Biohermal stromatolites viz., Collenia, Colloniella,
Cryptozoon and Irregularia have earlier been reported from the Bilara Group by Barman (1980;
1987).
4
MINERALOGY AND PETROGRAPHY
Carbonate rocks of the Bilara Group range in composition from pure dolomite to pure calcite and
intermediate types with variable percentages of dolomite and calcite (Mazumdar and Bhattacharya,
2006, submitted). Dolomites are characterized by subhedral to euhedral crystals (Fig. 3A) with grain
size ranging form ~10-20 µm and display both planar and non-planar grain contacts. Compared to
dolomites, calcites are characterized coarse crystals (Fig. 3B). Anhydrite crystals although minor in
amount have been recorded in many carbonate samples. Anhydrite is present as crystal mush
(pseudomorphs after gypsum, Fig. 3C) or as isolated crystals within the carbonate matrix (Fig.3D and
E) which are often partially (peripheral alteration) or completely (phantom crystals) replaced by
carbonate. Needle shaped anhydrite crystals normally range in size from 60 to 100 µm (occasionally
500-600 µm) and show random orientation. In addition, they sometimes show dark inclusions of
organic matter. Silicified anhydrite has also been observed in the Bilara carbonates (Fig. 3F).
Anhydrite is replaced by silica as spherules which are in most cases are length slow chalcedony
(often riddled with inclusions of anhydrite). The chert spherules commonly show coalesced structure.
At least three silica types can be noted in figure 3E, outer quartzine, middle drusy quartz and inner
radiating mega quartz. Similar textures have been reported by Siedlecka (1972), Milliken (1979),
Alonso-Zarza et al.(2002). Owing to a strong matrix effect from calcite and dolomite, anhydrite could
not be quantified by x-ray diffractometry.
ANALYTICAL METHODS
Fresh pieces of carbonate rocks (30 to 40 gm) were powdered to 200 mesh size. Sample powders
were subjected to 24 Hr leaching in 0.5 M NaCl solution. This step dissolves the non-structural
sulfate (eg. anhydrite) owing to enhanced solubility in NaCl solution. However, it is difficult to
ascertain if minor anhydrite remains undissolved or as trapped grains within carbonate crystals. The
filtered residue was later dissolved in 6N HCl with constant stirring at room temperature (25°C). The
solution was filtered through a 0.45 micron cellulose nitrate membrane filter. The insoluble residue
amounted to 0.6 to 7% of the original sample and was mostly composed of quartz, clay (corrensite,
montmorillonite and illite), minor organic matter and pyrite. The filtrate was brought to boiling
followed by addition of BaCl2 solution. The suspension containing barium sulfate was boiled for half
an hour and then kept at around 90°C overnight for coarsening and purification of the BaSO4 crystals.
Low pH of the solution prevents precipitation of any carbonate phase. The BaSO4 precipitate was
filtered, dried and weighed.
5
The sulfur isotopic composition of pyrite from the insoluble residue was measured following wet
chemical extraction of sulfur. Chromium reducible sulfur (So +FeS2) was extracted with 1 M CrCl2
solution and 6N HCl in N2 atmosphere (Canfield et al., 1986). H2S produced by reduction of sulfide
was trapped as ZnS in zinc acetate solution (pH 11) and subsequently reprecipitated as Ag2S by
adding AgNO3 (Canfield et al. 1986). δ34S of pure BaSO4 and Ag2S precipitates were combusted with
V2O5 at 1150°C in an elemental analyzer. Measurements of the sulfur isotopic composition were
performed on a Finnigan MAT Delta plus IRMS with a continuous flow interface and an elemental
analyzer (EA or TC/EA) attached to it. All results are reported in standard delta notation (δ34S) as per
mil deviations from the VCDT (Vienna Canyon Diablo Troilite). Reproducibility of δ34S value is
better than ±0.3‰ based on repeated measurements of reference materials (NBS-127, IAEA S1, S2)
as well as internal lab standards. 37 samples were analyzed for their sulfate sulfur and 16 residues for
their sulfide sulfur isotopic composition.
The mineralogical (carbonate and clay) studies were carried out using Siemens D500 and PW3710 xray diffractometers. A semi-quantitative estimation of the relative proportions of dolomite and calcite
was made following Tennant and Berger (1957). Mn and Sr concentrations of the Bilara carbonate
samples were measured by ICP-AES (Seiko SPS 1500) at the University of Nagoya, Japan.
87
Sr/86Sr analyses presented in this study were conducted on micro-drilled samples from
petrographically selected areas of the apparently best preserved rock matrix. For Sr isotopic analyses
~2-4 mg of carbonate (6 samples) and sulfate powders (10 samples) were dissolved in 2 N ultra-pure
HCl (Kampschulte et al., 1998). Sr was extracted with HCl on a DOWEX AG-50W8 (200-400 mesh)
resin bed in 5ml columns. Sr was loaded with TaF5 on W filaments and analyzed on a VG sector 5430 in dynamic multicollector mode. Repeated analyses of the standard NBS 987 yielded 0.710295 ±
0.000030 (n = 12) for 87Sr/86Sr ratio.
RESULTS
Sulfate concentrations in the Bilara carbonates vary from 0.02 to 0.44% (avg: 0.08%±0.08) with 50%
samples falling within 0.018 to 0.05% (Table-1). The pyrite content could not be reliably quantified
by gravimetry owing to very low concentrations. Sulfate concentrations in the Bilara carbonates are
higher than those reported from normal Neoproterozoic carbonates (Hurtgens et al., 2002 and
Goldberg et al., 2005).
Staudt and Schoonen (1995) reported structural sulfate (in carbonate rock)
concentrations ranging from 0.06 to 0.7% in evaporitic dolomite. High sulfate concentrations in
6
carbonates of Bilara Group could well be attributed to it’s arid and evaporitic depositional setup.
Presence of some residual anhydrite may also result in high sulfate concentrations as recorded for
some samples. However, mixture of two different sulfate sources viz., anhydrite and sulfate in
carbonate mineral lattice is not expected to cause any major difference is S isotopic composition.
This is corroborated by the fact that sample K-5 NaCl (NaCl filtrate: δ34S = 34.3‰) is similar to K-5
(NaCl leached residue: δ34S = 35‰; Table1). However, it is not expected in rocks with high pyrite
content owing to artifact sulfate production by pyrite weathering. In case of Bilara carbonate, pyrite
content is negligible.
The sulfur isotopic compositions for sulfate in the Bilara Group carbonates are given in Table-1 and
graphically represented in Fig 4. δ34SSO4 ranges from 27.2 to 42.0 ‰ (avg: 33.8±3.1 ‰, n = 37). The
sulfur isotopic composition of pyrite ranges from -2.5 to +21.6 ‰.
87
Sr/86Sr ratios for the carbonate rocks of Bilara Group (Table 2A) vary from 0.70817 to 0.70848
(0.70832 ± 0.000354, n=5) excluding one sample ANS-4 which shows a high value of 0.70915. It is
apparent from the data that the highest Sr isotope ratio corresponds to the lowest Sr concentration.
We have not considered Sr isotope ratio of ANS-4 in our interpretation owing to the possible
diagenetic alteration. Low Mn concentration, low Mn/Sr ratios (0.06 to 0.33) and oxygen isotope
ratios (-3.1 to 4.7‰) suggest minimum meteoric water alteration (Kaufman and Knoll, 1995) for the
Bilara samples measured for Sr isotope ratio. 87Sr/86Sr ratio for anhydrite from the HEG varies from
0.70812 to 0.708965 (average = 0.70856 ± 0.00026, n = 10) (Table-2B). In the absence of
geochemical tests for possible alteration of the Sr isotope ratio of anhydrites, we have tentatively
assumed that the Sr isotope ratios close to the well preserved Bilara carbonates as diagenetically least
altered. Petrographic studies of anhydrite and carbonates from cores P-47 and P-12 and P-4 of
Hanseran evaporites (Grover, 1997 and Mishra, 1998) show predominantly micritic anhydrite.
Anhydrites of P-47 and P-12 and P-4 were studied by Strauss et al. (2001) for sulfur isotopic
composition referred in the present paper. Sr isotopic compositions presented here are of the same
sample sets.
DISCUSSION
Intrabasinal Correlation
Owing to the lack of a proper basin evolution model and radiogenic dates, intrabasinal correlation of
the Bilara and Hanseran Evaporite Groups had so far been quite conjectural. However, newly
7
determined strontium and sulfur isotopic ratios from this study lend support to the contention that
rocks of the Bilara Group and the HEG are coeval facies variants. 87Sr/86Sr ratios (Table-2A and B)
and δ34SSO4 (Fig.4) values are close for the two Groups. Based on a correlation of carbon isotopic
studies, Mazumdar and Bhattacharya (2004) and Pandit et al. (2001) expressed similar views.
Strauss et al. (2001) recorded δ34SSO4 through a composite HEG succession which ranges from 29.7
‰ to 35.6‰ and a subsequent decline to 29.6‰. Such variation is also reflected in the carbonate
rocks of the Bilara Group with two maxima at around 31‰ and 36‰. Based on sedimentological and
geochemical data, it is possible to reconstruct the basinal evolution of Nagaur-Ganganagar basin
during terminal Neoproterozoic and early Cambrian times (Fig. 5). The Nagaur-Ganganagar basin
remained tectonically unstable as indicated by the presence of several steeply dipping faults (Kumar,
1999). Tectonic subsidence of the basin controlled the accumulation of a 652 m thick evaporite
succession. Based on borehole data, Kumar (1999) suggested an asymmetric depositional pattern for
the carbonate, sulfate and halite. Closed basinal conditions were established after the deposition of
the Jodhpur sandstone. The presence of several evaporite cycles (up to seven) in the HEG suggests
repeated episodes of marine water influx into a closed basin followed by evaporative concentration of
salts under arid conditions. It is apparent that the Bilara carbonates were deposited on the edges of the
basin whereas sulfate and halite were deposited in an asymmetric pattern in the central and northern
parts. By contrast to the HEG, no evaporite beds have been reported within the Bilara Group
carbonate succession. It may be concluded that the carbonate rocks of the Bilara Group essentially
represent marginal carbonate sediments of the Hanseran evaporite basin.
Intrabasinal sulfur isotopic variations
The observed sulfur isotopic composition measured in sulfate from carbonate rocks of the Bilara
Group and from evaporitic sulfate of the Hanseran Evaporite Group are in good agreement with
previously determined isotope values from evaporites and phosphorite deposits of late Ediacaranearly Cambrian age (Strauss., 1997; Strauss et al., 2001, Strauss, 2004, Shields et al., 1999, 2004).
Still, it is important to assess whether the observed isotope signatures were overprinted by
fluctuations in the basinal sulfur geochemistry. This is also pertinent when considering the significant
spread in the sulfur isotopic composition of seawater sulfate over this time window (Fig.6). The
presence of foetid dolomite and dark organic rich carbonate as well as shale layers in both, the Bilara
Group and the Hanseran Evaporite Group, suggest intermittent anoxia in the environment due to
salinity stratification in a closed basin following fresh influx of seawater. High algal biomass and
8
sulfate should promote extensive bacterial sulfate reduction (BSR). Bacterially mediated
dissimilatory sulphate reduction involves reduction of dissolved SO4 to H2S via a series of complex
enzymatically catalyzed biochemical reactions coupled with organic matter oxidation (Goldhaber,
2003; Megonigal et al. 2003 and references therein). Thereby, hydrogen sulfide is enriched in
relative to
34
32
S
S (Harrison and Thode, 1958; Detmers et al., 2001). H2S reacts with easily reducible
ferric compounds (e.g., FeOOH) forming pyrite (Berner 1984). However, under iron limited
conditions, H2S may either be lost from the system or trapped by organic molecules as organo-sulfur
compounds. Very low pyrite contents in rocks of the Bilara and Hanseran Evaporite groups suggest
iron limited conditions. In addition, under sulfate limited conditions, bacterial sulfate reduction (BSR)
would lead to enrichment in 34S in the residual sulfate following Rayleigh fractionation. Schröder et
al. (2004) reported a mean value of 39.4‰ (32 to 46‰) for δ34SSO4 from the Ara Group (Oman)
evaporites and attributed it to BSR under sulfate limited conditions. Sediments of both, the Bilara
Group and the Hanseran Evaporite Group show δ34SSO4 values of up to 42‰, suggesting a similar
situation.
Keeping in view the shallow marine conditions that prevailed in the Nagaur-Ganganagar basin during
the development of the evaporite sedimentary package, oxidation of H2S at the oxic-anoxic interface
cannot be ruled out. Subsequently, this may also lead to disproportionation of elemental sulphur (S0)
or sulfur-bearing intermediates such as thiosulfate (S2O32-) and sulfite (SO32-). It is well documented
that disproportionation is associated with a large sulphur isotope fractionation between residual
sulphate and sulphide by depleting the sulphide in
34
S through repeated steps of sulphide oxidation
and disproportionation (Canfield and Thamdrup, 1994, Habicht et al., 1998; Böttcher et al., 2001).
Absence of highly
34
S depleted pyrite in Bilara carbonates is possibly due to overprinting by late
diagenetic pyritisation in the sediment from 34S enriched residual fluids during burial.
Late Neoproterozoic sulfur isotopic enrichment
As a consequence of a large residence time for sulfate (20×106 years) compared to the ocean mixing
time (~1500 years), contemporary marine sulfate precipitates (mainly calcium and barium sulfate)
record the same isotopic composition. Hence, the sulfur isotopic composition of marine sulfate is
considered an important geochemical proxy for evaluating the global marine geochemical evolution
and a further option for interbasinal correlation. Despite possible basinal effects, the heavy δ34S
values recorded for sulfate sulfur from the Bilara Group and the Hanseran Evaporite Group are
9
consistent with the general trend of the global sulfate sulfur isotope curve through the terminal
Neoproterozoic and Cambrian time interval (Fig.6). The rise in the sulfur isotope ratio towards the
late Neoproterozoic (avg: 32‰, Strauss, 2004) relative to the early Neoproterozoic (avg: 18 ‰)
reflects a major change in sulfur isotope mass balance in the contemporary global ocean. The sulfur
isotopic composition of seawater sulfate is a function of pyrite deposition and preservation in marine
sediments and pyrite weathering and subsequent fluvial transport as dissolved sulfate into the ocean.
The former process is a result of bacterial sulfate reduction favoring the incorporation of the light 32S
isotope into the pyrite and, hence, leads to an increase in the
34
S of dissolved sulfate and
contemporary sulfate mineral precipitates. On the other hand pyrite weathering is expected to
contribute
32
S enriched sulfate to the dissolved sulfate pool.
Alternatively, a rise in
34
S of oceanic sulfate has been proposed in connection with phases of
enhanced continental weathering (e.g., Kampschulte et al., 2001; Kampschulte and Strauss, 2004).
The latter process delivers substantial amounts of nutrients to the ocean, triggering an increase in
primary productivity. Subsequent delivery of organic matter to the sediment in turn results in
increasing rates of bacterial sulfate reduction, extracting a higher proportion of the lighter isotope 32S
and leaving behind a
34
S enriched residual global sulfate pool (e.g., Strauss, 2004). The sulfur
isotopic composition of the contemporary dissolved sulfate would depend on the relative significance
fluvial input of sulfate from terrestrial pyrite weathering and marine sulfide burial. On the other hand,
Shields et al. (1999) invoked the hypotheses of increase in isotopic discrimination between sulfide
and sulfate by bacterially mediated disproportionation reaction (Canfield and Teske, 1996)
Two contrasting hypotheses have been proposed to explain the rise in the sulfate sulfur isotopic
composition (by pyrite burial and sulfate limitation) through the Ediacaran to early Cambrian: a
salinity driven oceanic stratification with deepwater anoxia (Holser, 1977) or the development of an
ice-covered global ocean in connection with the Snowball Earth hypothesis (Hoffman et al., 1998),
both followed by subsequent upwelling of water masses enriched in 34SSO4 onto the shelf regions.
Identification of an enhanced input of dissolved material into the ocean, resulting from continental
weathering, has been assessed through the
87
Sr/86Sr ratio of marine chemical sediments (limestone,
dolomite, barium sulfate and calcium sulfate). Continental weathering represents one of two principal
sources which govern the isotopic ratio of strontium in the ocean (Bickle et al., 2001, 2003).
Temporal variations in
87
Sr/86Sr ratio record changes in the balance between Sr fluxes to the ocean
from hydrothermal fluid-rock interaction at mid-ocean ridges and from continental weathering
10
(Palmer and Edmond, 1989). The 87Sr/86Sr composition of the seafloor hydrothermal flux is buffered
at low values of ~0.703-0.705. On the other hand continental weathering contributes Sr with a high
87
Sr/86Sr value (0.709-0.730) via riverine flux to the ocean. Unaltered marine carbonates and
evaporites faithfully record ambient seawater
87
Sr/86Sr, given the negligible fractionation of Sr
isotopes and the homogenous distribution of Sr isotopes in seawater (DePaolo and Ingram, 1985;
Veizer et al., 1999).
Compilation of Sr isotopic composition (Fig. 6) for late Neoproterozoic and Cambrian (Asmerom et
al.,1991, Brasier et al.,2000, Derry et al., 1994, Jacobsen and Kaufman, 1999, Kaufaman et al., 1993,
Melezhik et al. 2001, Misi and Veizer, 1998, Montanez et al., 2000, Nicholas, 1996) is characterized
by a rapid rise in
87
Sr/86Sr ratios from a low of ~0.7065 to 0.7070 at around 610 Ma to a high of
0.7085 at 580 Ma and form a plateau up to the base of Nemakit-Daldynian. Jacobsen and Kaufman
(1999) reported a further rise to 0.07087 in Nemakit-Daldynian and subsequent rapid fall to a low of
0.7080 in Tommotian followed by a steady rise to 0.7095 up to late Cambrian (Derry et al., 1994,
Nicholas, 1996). The rise in 87Sr/86Sr through the terminal Neoproterozoic and early Cambrian has
been linked to orogenic activities related to the break-up of Rodinia and subsequent assembly of
Gondwana (Pan-African-Brasiliano orogeny: Asmerom et al.,1991, Burns et al., 1994; Derry et al.,
1994; Montanez, 2000) which resulted in a major phase of continental weathering and riverine flux of
highly radiogenic Sr. 87Sr/86Sr ratios of possibly least altered carbonates of Bilara Group (0.70817 to
0.708418) and evaporites of HEG (0.70812 to 0.708442) are consistent with a terminal Neproterozoic
to early Cambrian age.
Looking at the sulfate sulfur isotopic composition, δ34SSO4 shows a significant rise following the level
of the Varangerian glaciation and remains high until the late Cambrian (e.g., Shields et al., 1999;
2004; Strauss, 2004). This increase in δ34S observed during this study would be consistent with a
scenario in which post-glacial upwelling of
precipitation of chemical sediments
34
S-enriched deep-water onto the shelf and subsequent
accompanied by the incorporation of this distinct isotopic
signature. It would also be consistent with a scenario in which the enhanced delivery of nutrients
from continental weathering resulted in increasing primary production and greater subsequent
remineralization of sedimentary organic matter, causing sulfate limitation and a Rayleigh-type
isotopic fractionation. The biogeochemical process responsible for burial of lighter sulfur was
apparently more significant than its contribution via pyrite weather and fluvial flux in to ocean.
Hence the rises in Sr and S isotopic compositions through late Neoproterzoic-Early Cambrian and
11
possible erosional and nutrient flux into contemporary oceans are apparently linked processes as
observed in Phanerozoic with δ34S and 87Sr/86Sr correlation (Veizer et al., 1999).
CONCLUSIONS
Carbonate and evaporite rocks of the Bilara Group and the Hanseran Evaporite Group, Marwar
Supergroup, are coeval facies variants. Sedimentological studies suggest an arid carbonate tidal flat
type depositional environment for the Bilara Group rocks. These Bilara carbonates form the marginal
carbonate facies of the Nagaur-Ganganagar basin. Simultaneously, thick deposits of anhydrite and
halite formed in the central and northern part of the basin. Sulfur and strontium isotope ratios for
sedimentary rocks from both groups are quite similar and justify the interpretation that sediments
from both groups were deposited within the same age brackets.
The sulfur isotopic composition for samples from both groups is in good agreement with the globally
observed trend of
34
S-enriched marine sulfate sulfur during Ediacaran and much of Cambrian time.
Intrabasinal variations in the sulfur isotopic composition of dissolved sulfate via bacterial sulfate
reduction and/or via disproportionation of sulfur intermediates might have caused additional
fluctuations in the sulfur isotopic composition of dissolved sulfate and subsequent chemical
precipitates. Such an effect would be superimposed on the global sulfur isotopic signature, making
global correlations more challenging. Apparent rise in Sr isotopic ratio through the late
Neoproterozoic and early Cambrian and enrichment of sulfate in 34S may tentatively be attributed to
high nutrient flux associated with erosion and subsequent burial of sulfide through biogeochemical
processes. This process presumably dominated over the fluvial flux of
32
S rich sulfate produced by
pyrite weathering.
Acknowledgement
Funding for this research by the Deutsche Forschungsgemeinschaft is gratefully acknowledged. The
Alexander-von-Humboldt Foundation supported the stay of Aninda Mazumdar.
12
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18
Figure Captions
Figure-1. (A) Map showing the location of the Nagaur-Ganganagar Basin. Sampling locations are
marked with arrows. (B) Generalized lithologs of the Marwar Supergroup showing Bilara and
Hanseran Evaporite Groups. The Hanseran Evaporite Group is extensively developed in the central
and northern parts of the basin and has been mapped through borehole studies (Kumar et al., 1999).
Figure-2. Sedimentary structures in carbonate rocks of Bilara Group (A) Tidal laminites and
truncated ripple structure (arrow). (B) Crenulated microbial structures (C) Mudcracks in dolomicrite.
(D) Domal stromatolites. Diameter of coin = 26 mm, Scale length = 15 cm.
Figure-3. (A) Fine grained interlocking euhedral to subhedral dolomite crystals. (B) Coarse calcite
crystals in limestone, (C) Anhydrite mush in sparry dolomite. Anhydrite crystals show partially
corroded grain boundaries and replacement by dolomite. (D) Needle shaped isolated anhydrite
crystals within micritic dolomite. Dark colour of the crystals is possibly due to organic enrichment.
Anhydrite crystals show peripheral alteration or complete replacement by carbonate. (E) Anhedral,
isolated anhydrite crystals within sparry dolomite. (F) Anhydrite nodules replaced by silica. Note the
structural zoning within the nodules.
Figure-4.A and B Histograms showing sulfur isotopic composition of sulfate in Bilara and Hanseran
Evaporite Groups. Average compositions are marked with arrows.
Figure-5. A schematic basin evolution model for the Marwar Supergroup depicting temporal and
spatial relationship between the Bilara and Hanseran Evaporite Groups.
Figure-6. Temporal variations of sulfur and strontium isotopic compositions of sea waters through
late Proterozoic and Cambrian. Sulfur isotopic compositions recorded for terminal Proterozoic and
Cambrian from different basins are plotted for comparison. 1= Doushantuo Fm. (China, Shen et al.,
2000), 2. Bilara and Hanseran Evaporite Group (this study and Strauss et al., 2001), 3. Danilovo
(Siberia, Vinogradov et al.,1994 cf: Peryt et al.,2005), 4. Hormuz Formation (Iran, Houghton, 1980),
5. Ara Group (Oman, SchrÖder, 2004), 6. Zhongyicun (China, Shields et al., 1999), 7 = Amadeus
Basin, Australia (Claypool et al., 1980; Walter et al., 2000), 8. Yuhucun Fm. (China, Shields et al.,
1999, Shen et al., 2000), 9. Angarskya (Siberia, Claypool et al., 1980), 10. Motskya (Claypool et al.,
1980), 11. Doushantuo Formation (Shields et al., 2004), 12A to 12D. (Doushantuo, Dengying,
19
Nemakit-Daldynian and Tommotian of Yangtze Platform, China, Goldberg et al., 2006). Upper age
limit for Doushantuo Formation from Condon et al. 2005.
The range for strontium isotopic ratios for the presumably least altered carbonates and evaporites of
Bilara Group and Hanseran Evaporite Groups (HEG) are plotted as double sided solid arrows. Sr
isotopic ratios of HEG anhydrites showing possible diagenetic alteration is represented by double
sided dotted arrow. VG = Varanger glaciation, N-D = Nemakit-Daldynian, Tom = Tommotian, Atd =
Atdbannian, Bot = Botomian.
Table Captions
Table-1. Mineralogy and sulphur isotopic compositions of sulphate and chromium reducible sulphide
(CRS) in Bilara carbonates. CDT = Canyon Diablo Troilite
Table-2. Strontium isotopic composition of carbonates and anhydrites of Bilara and Hanseran
evaporite Groups respectively.
20
72 o
73
o
74
o
N
GANGANAGAR
30 o
INDIA
SURATGARH
ta
n
29 o
DI
A
Pa
kis
PUGAL
IN
DULMERA
LAKHASAR
28 o
BIKANER
NACHNA
NAGAUR
PHALODI
Ghagrana
DHANAPA
Ransigaon
POKARAN
GOTAN
27 o
JODHPUR
BILARA
88km
0
A
*
26 o
NagaurCenozoic
Bilara
Jodhpur Malani Basement
Baghewala II
Middle Eocene
Lower Eocene
Lower Jurassic
Lower Paleocene
Permo-Carboniferous
XX X X X
Nagaur Gp..
75-500 m
450 m
Reddish-brown/buff and grey Sst
with puple to greenish shale with
or without chert bands
Unconformity
V
B
Evaporite cycle with
dolomite, anhydrite
halite and potash salts and Clay
seams
Bilara Gp.
XX X X X X
X XX X
XX X X
V
V
V V V
V V V
Unconformity
Red Gritty and pebbly Sst., Siltstone
and clay
Pondlo Fm
Cherty dolomite and limestone
with some interbeds of siltstone
and claystone
Gotan Fm
Interbedded sequence of dolomite
and limestone
Dhnapa Fm
Dolomite and dolomitic
limestone with chert lenses
Girbhakar Fm
Gritty and pebbly Sst with few
interbeds of sandy shale
Sonia Fm.
Sst, Sh, minor chert, cherty
dolomite, limestone
Unconformity
V
Malani Igneous suite/
Delhi Supergroup
Unconformity
Tunkalian Fm
X
100-300m
50-640 m
Nagaur Gp..
Hanseran
Evaporite Gp.
100-652 m
XX X
X XX X X
?
Late Neoproterozoic
?
Red, grey or maroon Sst., Siltstone
and claystone
Jodhpur Gp.
Early Cambrian
Unconformity
Unconformity
V
V
V V V Malani Igneous suite/
V V V
Delhi Supergroup
Figure1
Figure -2
Figure-3
6
Hanseran Evaporite Group
Number of Analyses
5
4
Avg.
3
2
1
0
26
28
30
32
34
36
38
40
42
44
Number of Analyses
12
Bilara Group
10
Avg.
8
6
4
2
0
26
28
30
32
34
36
38
40
234
δ SSO4 (‰CDT)
42
44
46
46
Nagaur Gp.
Unconformity
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V
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B
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V
V
V
V V
Figure 5
V
V
V
V
VV
V
V
V
V
V
V V
V
V
A
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
Basement
complex
V
V
V
aaaa
aaaa
aaaa
Metamorphics Sandstone Limestone/
Dolomite
V
CaSO4
V
V
V
V
V
V
V
V
V
V
Halite/
Potash Salts
Water
level
Cambrian
Late Neoproterozoic
5
0.7100
54.3 ‰
1
0.7095
0.7090
2
42
11
(635Ma)
0.7085
3
0.7080
10
12D
(635Ma)
BILARA
87
38
8
12A
6
34
9
7
HEG
4
12B
86
Sr/ Sr
87
Sr/86Sr
46
12C
d34S
30 (‰)
VG
0.7065
620
Bot
22
Atd
0.7070
Tom
26
N-D
0.7075
600
18
580
560
Age uncertainty
540
520
500
Age
34
87
Sr/86Sr
d S
Mean
Fig.6
Diagenetic
Least Altered
87
Sr/86Sr range for Bilara Group and HEG
Table-1
Sample number
TG/6/3/5
Tg/7/3/7
TG 8-7
TG 8-10
TG 9
TG 10
TG 11
TG 15
TG 16
TG 17
TG 22(2)
TG 29
TG 32
TG 36
TG37
TG 39
TG 43
K1
K2
K5
K8
K 10
K 11
K 13
K 14
DH/11/3/1
DH /11/3/25
Gag-8
GAG10
GAG 15
ANS 1
Ran 7
Ran-1
HKSPH 3
HKSPH-4
HD/9/3
HD 9/3/4
K5-NaCl
Mineralogy
60%calcite
Calcite
Calcite
Calcite
60% Calcite
Calcite
Dolomite
Dolomite
Dolomite
Dolomite
Dolomite
Calcite
Calcite
Calcite
Calcite
Calcite
Calcite
nm
Calcite
Calcite
Calcite
Calcite
Dolomite
Dolomite
Dolomite
Dolomite
Dolomite
Calcite
Dolomite
Dolomite
Calcite
Dolomite
nm
nm
nm
nm
nm
NaCl filtrate
Strctural SO4% δ34S-sulfate ‰ (CDT)
0.07
30.6
0.44
31.8
0.06
31.3
0.04
31.1
0.02
30.0
0.04
34.2
0.04
31.7
0.04
33.5
0.03
27.3
0.04
31.0
0.02
28.7
0.03
31.4
0.03
31.5
0.10
31.0
0.16
31.6
0.06
32.9
0.04
31.1
0.08
36.4
0.26
36.3
0.07
35.0
0.08
36.1
0.20
35.7
nm
35.7
0.07
34.4
0.09
35.9
0.10
36.5
0.11
36.3
0.03
34.5
0.10
33.4
0.04
34.4
0.06
36.2
0.03
41.8
0.03
42.0
0.04
36.0
0.04
35.7
0.05
33.0
0.09
34.4
34.3
δ34S-CRS ‰ (CDT)
nd
21.65
nd
19.89
nd
3.21
nd
nd
nd
-2.75
nd
nd
-2.54
nd
12.41
11.3
nd
12.14
nd
nd
nd
17.49
nd
10.56
nd
8.34
14.2
nd
nd
8.26
nd
10.99
nd
nd
7.45
nd
11.2
Table-2A
Sample
ANS-1
K8
K9
K10
ANS-4
DH11/3/17
Mineralogy
cal
cal
dol+cal
cal
dol+cal
dol+cal
delta18O
-1.97
-3.1
3.75
1.65
1.5
4.66
Mn/Sr
0.11
0.22
0.25
0.06
0.33
nm
Mn(ppm)
16.7
36.4
37.8
21.7
38.8
nm
Sr(ppm) Sr87/Sr86
147
0.70835
164.9
0.70825
151
0.70837
320.3
0.70817
116
0.70915
nm
0.70848
nm = not measured
Table-2B
Sample
550.1
558
562.8
573.35
590
599.5
613.75
641
273.8
704
Mineralogy
Anhydrite
Anhydrite
Anhydrite
Anhydrite
Anhydrite
Anhydrite
Anhydrite
Anhydrite
Anhydrite
Anhydrite
Sr87/Sr86
0.708706
0.708592
0.708553
0.708442
0.708778
0.70816
0.708965
0.708722
0.70859
0.708122
IR%
2.10
0.80
1.83
3.70
0.80
nm