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

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 Reference Asmerom, Y.A., Jacobsen, S.B., Butterfield, N.J. and Knoll, A.H., 1991. Sr isotope variations in Late Proterozoic seawater: Implications for crustal evolution, Geochim. Cosmochim. Acta 55, 28832894, 1991. Alonso-Zarza, A. M., Sanchez-Moya, Y., Bustillo, M. A., Sopena, and Delgado, A., 2002. Silicification and dolomitization of anhydrite nodules in argillaceous terrestrial deposits: an example of meteoric-dominated diagenesis from the Triassic of central Spain. Sedimentology 49, 303-317. Barman, G., 1980. An analysis of Marwar Basin, western Rajasthan in the light of stromatolite, chronostratigraphy and utility. Geological Survey of India Miscellaneous Pulication 44, 292-297. Barman, G., 1987. Stratigraphic position of Marwar Supergroup in the light of stromatolites. Special Publication, Geological. Survey of India 11, p. 72-80. Berner, R. A., 1984. Sedimentary pyrite formation: An update. Geochim. Cosmochim. Acta 48, 605615. Bickle, M. J., Harris, N. B.W., Bunbury, J. M., Chapman, H. J., Fairchild, I. J., Ahmad, T., 2001. Controls on the 87Sr/86Sr ratios of carbonates in the Garhwal Himalaya, Headwaters of the Ganges. Jour. Geol. 109, 737–753. Bickle, M. J., Bunbury, J., Chapman, H. J., Harris, N. B. W., Fairchild, I., Ahmad, T., 2003. Fluxes of Sr into the headwater of the Ganges. Geochim. Cosmochim. Acta 67, 2567– 2584. Böttcher, M.E., Thamdrup, B., Vennemann, T., 2001. Oxygen and sulphur isotope fractionation during anaerobic bacterial disproportionation of elemental sulphur. Geochim. Cosmochim. Acta 65, 1601-1609. Brasier, M., McCarron, G., Tucker, R., Leather, J., Aleen, P. and Shields, G. 2000. New U-Pb zircon dates for the Neoproterozoic Ghubrah glaciation and for the top of the Huqf Supergroup, Oman. Geology, 28, 175-178. Burns, S. J., Haudenschild, U. and Matter, A. 1994. The strontium isotopic composition of carbonates from the late Preacambrian (c.560-540) Huqf Group of Oman. Chem. Geol. 111, 269-282. Canfield, D. E., 2001. Biogeochemistry of sulfur isotopes- Reviews in Mineralogy and Geochemistry 43, 607-636. Canfield, D.E., Raiswell, R., Westrich, J., Reaves, C., Berner, R.A., 1986. The use of chromium reduction in the analysis of inorganic sulphur in sediments and shales. Chem. Geol. 54, 149–155. Canfield, D. E . and Teske, A., 1996. Isotope fractionation by sulfate-reducing natural populations and the isotopic composition of sulfide in marine sediments. Nature, 382, 127-132. Canfield, D.E., Thamdrup, B., 1994. The production of 34S-depleted sulfide during bacterial disproportionation of elemental sulfur. Science 266, 1973-1975. 13 Claypool, G.E., Holser, W.T., Kaplan, I.R., Sakai, H., Zak, I., 1980. Age curves of sulfur and oxygen isotopes in marine sulfate and their mutual interpretation. Chem. Geol. 28, 199-206. Condon, D, Zhu, M, Bowring, S, Wang, W, Yang, A and Jin, Y., 2005. U-Pb Ages from the Neoproterozoic Doushantuo Formation, China. Science. 308, 95 – 98. Conway Morris, S., 1987. The search for the Precambrian-Cambrian boundary. American Scientist 75, 156-167. Conway Morris, S., 1993.. Ediacaran like fossils in Cambrian Burgess Shale-type faunas of North America. Palaeont. 36, 593–635. Dasgupta, U., 1996. Marwar Supergroup evaporites, Rajasthan. In: Bhattacharya, A. (Ed.), Recent Advances in Vindhyan Geology. Gological Society of India 36, pp. 49-58. Dasgupta, U and Balaguda, S. S., 1994. An overview of geology and hydrocarbon occurrences in the western part of Bikaner-Nagaur Basin. Indian Journal. Petroleum Geology 3, 1-17. Dasgupta, S. P., Ramachandra, K. V., and Jairam, M. S., 1988. A framework analysis of Nagaur Ganganagar evaporite basin, Rajasthan. Indian Minerals 42, 57-64. DePaolo, D. J. and Ingram, B. L., 1985. High resolution stratigraphy with strontium isotopes. Science 227, 938-941 Derry, L. A., Brasier, M. D., Corfield, R. M., Rozanov, A.Yu., Zhuravlev, A.Yu., 1994. Sr and C isotopes in Lower Cambrian carbonates from the Siberian craton: A paleoenvironmental record during the ‘Cambrian explosion’. Earth and Planet. Sci. Lett. 128, 671-681 Dettmers, J., Brüchert, V., Habicht, K. S. and Kuever, J., 2001. Diversity of sulfur isotope fractionations by sulphate-reducing prokaryotes. Appl. Environ. Microbiol. 67, 888-894. Dey, R. C., 1991. Trans-Aravalli vindhyan evaporites under the semidesertic plains of western India: significance of depositional features. Jour. Geol. Soc. India, 37, 136-150. Glaessner, M.F., 1984. The Dawn of Animal Life: A Biohistorical Study. Cambridge Univ. Press, pp.244. Goldberg, T., Poulton, S. W. and Strauss, H., 2005. Sulphur and oxygen isotope signatures of late Neoproterozoic to early Cambrian sulphate, Yangtze Platform, China: Diagenetic constraints and seawater evolution. Precamb. Res., 137, 223-241. Goldhaber, M. B., 2003. Sulfur-rich sediments. In: Mackenzie, F. T., (Eds.) Treatise on Geochemistry. Elsevier Vol. 7, Pp 257-288. Gorin, G. E., Racz, L. G. and Walter, M. R., 1982. Late Precambrian-Cambrian sediments of Huqf Group, Sultanate of Oman. AAPG Bull. 66, 2628-2648. Grover, Swati,1997. Evaporite sequence along the borehole No.p/47, Lakhasar, Bikaner district, Rajasthan, India. Unpublished M.Sc Dissertation, University of Delhi, 46 p. 14 Habicht, K. S., Canfield D. E. and Rethmeier, J., 1998. Sulfur isotope fractionation during bacterial reduction and disproportionation of thiosulfate and sulfite. Geochim. Cosmochim. Acta, 62, 2585-2595. Harrison, A. G. and Thode, H.G., 1958. Mechanism of the bacterial reduction of sulphate from isotopic fractionation studies. Faraday Society Transaction 54, 84-92 Hoffman, P.F., Kaufman, A.J., Halverson, G.P., Schrag, D.P., 1998. A Neoproterozoic snowball Earth. Science 281, 1342-1346. Holser, W. T., 1977. Catastrophic chemical events in the history of the ocean. Nature 267, 403-408. Houghton, M.L., 1980. Geochemistry of the Proterozoic Hormuz Evaporites, Southern Iran. MSc thesis, University of Oregon. Wiley, New York, pp. 57–64. Hurtgen, M. T., Arthur, M. A., Suits, N. S. and Kaufman, A. J., 2002. The sulfur isotopic composition of Neoproterozoic seawater sulfate: implications for a snowball Earth? Earth Plant. Sci. Lett., 203, 413-429 Husseini, M. I. and Husseini, S. I. 1990. Origin of the Infracambrian salt basins of the Middle East, In Brookes, J. (Ed.), Classic Petroleum Provinces. Geological Society Special Publication, 50, pp. 279-292. Jacobsen, S. B., Kaufman, A. J., 1999. The Sr, C and O isotopic evolution of Neoproterozoic seawater. Chem. Geol. 161, 37–57. Kampschulte, A., Buhl, D. and Strauss, H., 1998. The sulfur and strontium isotopic compositions of Permian evaporites from the Zechstein basin, northern Germany. Geologisch Rundschau 87, 192– 199. Kampschulte, A., Bruckschen, P., Strauss, H., 2001. The sulphur isotopic composition of trace sulphates in Carboniferous brachiopods: implications for coeval seawater, correlation with other geochemical cycles and isotope stratigraphy. Chem. Geol. 175, 165– 189. Kampschulte, A. and Strauss, H., 2004. The sulphur isotopic evolution of Phanerozoic seawater based on the analysis of structurally substituted sulphate in carbonates. Chem. Geol. 204, 255-286. Kaufman, A. J., Jacobsen, S. B. and Knoll, A.H. 1993. The Vendian record of Sr and C isotopic variations in seawater: Implications for tectonics and paleoclimate. Earth Plant. Sci. Lett. 120, 409-430. Kaufman, A. J. and Knoll, A. H., 1995. Neoproterozoic variations in the C-isotopic composition of seawater: stratigraphy and biogeochemical implications. Precambrian Res. 73, 27–49 Kirkland, D. W. and Evans, R., 1981. Source-Rock potential of evaporitic environment. AAPG Bull. 65, 181-190. Kumar, V., 1999. Eocambrian sedimentation in Nagaur-Ganganagar evaporite basin, Rajasthan. Journal of Indian Association of Sedimentologists 18, 201-210. 15 Mazumdar, A and Bhattacharya, S.K., 2004. Stable isotopic study from late Neoproterozoic-early Cambrian (?) sediments from Nagaur-Ganganagar basin, western India: possible signatures of global and regional C-isotopic events. Geochemical Journal, 38, 163-175 McKerrow,W. S., Scolese,C. R. and Brasier,M. D., 1992. Early Cambrian continental reconstruction. Jour. Geol. Soc. London 149, 599-606. Meert, J. G., and Lieberman, B. S., 2004. A palaeomagnetic and palaeobiogeographic perspective on latest Neoproterozoic and early Cambrian tectonic events. Jour. Geol. Soc. London 16, 1-11. Megonigal, J. P., Hines, M. E. and Visscher, P. T., 2003. Anaerobic Metabolism: Linkages to Trace Gases and Aerobic Processes. In: Holland, H. D. and Turekian, K. K. (Eds.), Treatise on Geochemistry, Vol. 8, Biogeochemistry. Elsevier Pergamon. pp. 317-424. Melezhik, V.A., Gorokhov, I.M., Fallick, A.E. and Gjelle, S., 2001.Strontium and carbon isotope geochemistry applied to dating of carbonate sedimentation: an example from high-grade rocks of the Norwegian Caledonides. Precambrian Res.108, 267–292. Milliken, K. L., 1979. The silicified evaporite syndrome – two aspects of silicification history of former evaporite nodules from southern Kentucky and northern Tennessee. Jour. Sed. Petrol. 49, 245-256. Mishra, A,1998. Mineralogy and petrography of the Hanseran Evaporite Group of Rocks, Bore Hole no. P-4, Near Lakhasar, Bikaner District, Rajasthan, Unpublished M.Phil Thesis, University of Delhi, 44 p. Misi, A and Veizer, J., 1998. Neoproterozoic carbonate sequences of the Una Group, Ireci Basin, Brazil: chemostratigraphy, age and correlation. Precambrian Res. 89, 87-100. Montanez, I., Osleger, D. A., Baneer, J. L., Mack, L .E. and Musgrove, M., 2000. Evolution of the Sr and C Isotope Composition of Cambrian Oceans. GSA Today 10, 1-7. Nicholas, C. J., 1996. The Sr isotopic evolution of the oceans during the ‘Cambrian Explosion’. Jour. Geol. Soc. London 153, 243-254. Palmer, M. and Edmond, J. M., 1989. The strontium isotope budget of the modern ocean. Earth Plant. Sci. Lett. 92, 11–26. Pandit, M. K., Sial, A. N., Jamrani, S. S. and Ferreira, V. P., 2001. Carbon isotopic profile across the Bilara group rocks of Trans-Aravalli Marwar supergroup in western India: implications for Neoproterozoic-Cambrian transition. Gondwana Research 4, 387-394. Pareek, H. S., 1981. Basin configuration and sedimentary stratigraphy of western Rajasthan. Jour. of Geol. Soc. India 22, 517-527. Peryt, T. M., Halas, S., Kovalevych, V. M., Petrychenko, O. Y. and Dzhiordidze, N. M., 2005. The sulphur and oxygen isotopic composition of Lower Cambrian anhydrites in East Siberia. Geological Quarterly, 2005, 49, 235-242. 16 Peters, K. E., Clark, M. E. Das Gupta, U., McCaffrey, and Lee, C.Y. 1995. Recognition of an Infracambrian source rock based on biomarkers in the Baghewala-1 oil, India: AA PG Bulletin 79, 1481-1494. Rathore, S. S., Venkateshan, T. R. and Srivastava, R. K., 1999. Rb-Sr isotopic dating of Neoproterozoic (Malani Group) magmatism from south west Rajasthan, India: Evidence of a younger Pan-African thermal event by Ar-Ar studies. Gondwana Research 2, 246-271. Reinick, H, E and Singh, I.B. (1980) Depositional Sedimentary Environments. Springer-Verlag, Berlin. Schröder, S., Scheiber, B. C., Amthor, J. E. and Matter, A., 2004. Stratigraphy and environmental conditions of the terminal Neoproterozoic-Cambrian period in Oman: evidence from sulphur isotopes. Jour. Geol. Soc. London 161, 489-499. Shen, Y., Schidlowski, M., Chu, X., 2000. Biogeochemical approach to understanding phosphogenic events of the terminal Proterozoic to Cambrian. Palaeogeography, Palaeoclimatology. Palaeoecology 158, 99–108. Shields, G., Kimura, H., Yang, J., Gammon, P., 2004. Sulphur isotopic evolution of NeoproterozoicCambrian seawater: new francolite-bound sulphate δ34S. Chem. Geol. 204, 163–182. Shields,G. A., Strauss, H., Howe, S. S. and Siegmund, H., 1999. Sulphur isotope composition of sedimentary phosphorites from the basal Cambrian of China: implications for NeoproterozoicCambrian biogeochemical cycling. Jour. Geol. Soc. London 156, 943-955. Siedlecka, A., 1972. Length-slow chacydony and relicts of sulphates- evidence of evaporitic environments in the Upper Carboniferous and Permian red beds of Bear Island, Svalbard. Jour. Sed. Petrol. 42, 812-816. Staudt, W. and Scoonen, M. A. A., Sulfate incorporation into sedimentary carbonates. In: Vairavamurthy, M. A. and Schoonen, M. A. A. (Eds.), Geochemical transformation of sedimentary sulfur. American Chemical Society pp 332-345. Strauss, H., 1997. The isotopic composition of sedimentary Palaeogeography, Palaeoclimatology, Palaeoecology 132, 7-118. sulfur through time. Strauss, H., 2004. 4Ga of seawater evolution: evidence from the sulphur isotopic composition of sulphate. In: Amend, J.P., Edwards, K.J., Lyons, T.W. (Eds.), Sulphur Biogeochemistry—Past and Present. GSA, Special Paper 379, Boulder, pp. 195–205. Strauss, H., and Banerjee, D. M. and Kumar, V., 2001. The sulfur isotopic composition of Neoproterozoic to early Cambrian seawater-evidence from the cyclic Hanseran evaporites, NW India. Chem. Geol. 175, 17-28. Tennant, C. B. and Berger, R. W., 1957. X-Ray determination of dolomite-calcite ratio of carbonate rock. American Mineralogist 42, 23-29 17 Valladares, M. I., Ugidos, J. M., Barba, P., Fallick, A. E. and Ellam, R. M., 2006. Oxygen, carbon and strontium isotope records of Ediacaran carbonates in Central Iberia (Spain) Precambrian Research Veizer, J., Ala, D., Azmy, K., Bruckschen, P., Buhl, D., Bruhn, F., Carden, G.A.F., Diener, A., Ebneth, S., Godderis, Y., Jasper, T., Korte, C., Pawallek, F., Podlaha, O.G., Strauss, H., 1999. 87Sr/86Sr, δ13C and δ18O evolution of Phanerozoic seawater. Chem. Geol. 161, 59–88. Vinogradov, V. I., Pokrovskiy, B. G., Pustylnikov, A. M., Muravyev, V. I., Shatsiy, G. V., Buyakayte, M. I and Lukanin, A.O., 1994. Isotopno-geochimicheskie osobennosti I vozrast verkhenodokembriyskikh otlozheniy zapada Sibirskoy platoformy. Litol. Polezn. Iskop., 4, 49-76. Walter, M. R., Veevers, J. J., Calver, C. R., Gorjan, P., Hill, A.C., 2000. Dating the 840–544 Ma Neoproterozoic interval by isotopes of strontium, carbon, and sulfur in seawater, and some interpretative models. Precambrian Research 100, 371–433. 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. 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V V V V V V V V V 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