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Marwar supergroup has been classified into three groups/ Formations; Lower Jodhpur group, middle Bilara group and upper Nagaur group. This group is sandwiched between Malani igneous suite and Bap boulder bed. Later, one more group/formation of rocks consisting of limestone, dolomite, gypsum, anhydride, halite etc has been described as Hanseran Formation stratigraphically above the Bilara Formation. However, closed resemblance of the lithological and C-isotope profile of Hanseran evaporites and Bilara carbonates allow intra-basinal correlation and suggest that they are coeval facies variants. In the present Mӧssbauer spectroscopic study of the sedimentary of samples of the Bilara formation of Marwar supergroup of the Bikaner-Nagaur basin to expose mineralogy and depositional environment from redox condition.

Mineralogical identification, investigation of the oxidation states and site population of the iron and identification of ironbearing minerals, and the semi-quantitative analysis of iron distribution.

The Bilara limestone formation, a shallow marine carbonate platform, records SSD features of widely varying geometry from three different stratigraphic levels through the succession. Separated by under formed strata, the deformed layers traceable on the outcrop scale over hundreds of metres are interpreted as products of seismic shacking. Fluidisation and liquefaction played a major role in their genesis, suggesting exposure to shock waves. SSD structures triggered by jerks are formed at various rheological states, viz., plastic, brittle and liquefied are recorded [4]. Two Neoproterozoic-Cambrian plays, namely, Jodhpur and overlying Bilara Group/Hanseran Evaporite Group (HEG) are envisaged in the study area of the basin. Presence of reservoir equivalent facies in fuviodeltaic sandstone of Jodhpur and fractured dolomites of Bilara/HEG group are established in this area. Continuity of the lower Halite cycles, top seal for the Jodhpur and lower Bilara reservoir were mapped across the area. However, upper halite cycles, interpreted as plausible seal for upper Bilara dolomite reservoirs, were found to be restricted in the northern part of the basin. Organic rich laminated dolomites and argillaceous litho-units within lower Bilara/HEG group spotted in the wells were correlated with the outcrops and presence of moderate quality of source rock is predicted in the unexplored part of basin [5]. Presence of Ediacaran discs in Jodhpur Sandstone and their absence in younger Bilara and Nagaur groups suggest the end of a typical taphonomic window mediated by microbial activity. Their global scarcity in younger assemblages indicates a more likely scenario of extinction or at least ecological restrictions [6]. Mössbauer spectroscopy is widely used for the study of geological samples including all types of sediments for mineralogical identification, investigation of the oxidation states and site population of the iron and identification of ironbearing minerals, and the semi-quantitative analysis of iron distribution in each mineral and lattice site [7]. It is well known that the oxidation state of iron metal in sediments is a measurement of the oxidation-reduction condition of sedimentation. It is the only technique that provides crucial information about the ferrous/ferric ratio in sediments. To get better insight into the application of 57Fe Mössbauer spectroscopy for geological samples [8].57Fe Mössbauer spectroscopy is particularly useful for the characterization of iron-bearing species because it probes the local environment of iron nuclei sensitivity. This method offers certain advantages over other conventional techniques such as chemical, optical, electron microscopic analysis, etc. Indeed, each technique has its own strength, but where Mössbauer spectroscopy can give results, it offers a quick, reliable and simple method. Being a non-destructive technique in the sense that the sample either in powder form or thin slice is not altered during the experiment also in a single run, one can get information about all the iron phases present in the sample by proper deconvolution of the Mössbauer spectrum. Mössbauer spectroscopy is also used widely to study organic-rich sediments (source rocks) from the different petroliferous basins. In fact, source rocks are tiny generators of oil/gas or both. Source rock characterization is one of the important aspects of the exploration of oil/gas. This distribution of minerals suggests that North Sea offshore sediments were deposited in a highly reducing environment. It is worthwhile to note that the offshore region is a major oil field off the North Sea. The detailed study of the chemical state of iron in subsurface sediments for four differ tiny of minerals including iron-bearing minerals, e.g. if the rate of sedimentation is fast it quickly cuts off sediment from the environment and this may favor the formation of minerals like pyrite which are diagenetically stable in reducing environment. Simultaneously organic matter also escapes oxidation and becomes more favorable for the generation of oil. But if the sedimentation rate is slow sediments will remain in contact with the atmosphere for more time. This may result in oxidation of organic matter making it unfavorable for the generation of oil and favors minerals like siderite, iron oxide etc. depending upon the degree of redox condition. This correlation can also be viewed that the physicochemical transformation of organic matter during the geological history of the sedimentary basin cannot be regarded as an isolated process. It is controlled by the same major factors that determine the variation of the composition of the inorganic solid phase of sediments; that is to say, biological activities in the early stage, and temperature and pressure afterward decide the evolution of both the organic matter and the inorganic solids [7-9]. 57Fe Mössbauer spectroscopy is particularly useful for the characterization of iron-bearing species because it probes the local environment of iron nuclei sensitivity. This method offers certain advantages over other conventional techniques such as chemical, optical, electron microscopic analysis, etc. Indeed, each technique has its own strength, but where Mössbauer spectroscopy can give results, it offers a quick, reliable and simple method.

For present study seven sedimentary samples were collected from Bilara formation of the Marwar supergroup of the Bikaner-Nagaur basin (BH-01 to BH-09 except BH03&06).In the present investigation, about 25gm of each sample (BH-01 to BH-09) were first ground to fine powder. Fractions of these powdered samples were used for Mössbauer study. The Mössbauer absorbers were prepared by using a fraction of these finely ground powder sediments in a sample holder (25 mm in diameter). The thickness of the absorbers was always kept constant. Mössbauer spectra were recorded at room temperature (300K) with a conventional constant acceleration spectrometer using a 10 mCi source 57Co in an Rh matrix. The isomer shift (IS) has been reported with respect to the centroid of a 25-µm- thick α-iron foil spectrum. Each spectrum was fitted using a computer program. This program assumes the spectrum to be the sum of Lorentzians. In most of the cases, width and intensity of the two halves of a quadrupole doublet or six halves in case of sextet were constrained to be equal. While in the case of sextet intensity of line 1 and 6, 2 and 5 and 3 and 4 were constrained to be equal. The relative intensities of various mineral components or sites were calculated by adding theareas of the two halves of the corresponding doublet or six halves of the corresponding sextet and are expressed as a fraction of the total area of resonant absorption. Solid lines in the spectra reported here represent computer-fitted curves and dots represent the experimental points. The χ2 values are generally obtained in this range. However, a little deviation in the X2 has been accepted on a few occasions when the iterations do not yield to further improvements in χ2. Such asituation arises when weak lines are present.

Mӧssbauer Spectra At Room Temperature



Figure-01: Mӧssbauer spectra of the samples(BH-01 to BH-09) of the Bilara Formation of Marwar supergroup of the Bikaner-Nagaur Basin
Table-01: Mӧssbauer parameters of the Bilara Formation:

Mössbauer spectra of samples BH-01 to BH-09 except BH-03&06 are displayed in Fig.1. The Mössbauer parameters are given in Table 1 for samples BH-01 to BHF-09 except Bh-03&06. It can be seen from Fig.1 and Table 1 that Mössbauer spectra of these samples (BH-01 to BH-09 except BH-03 & 06) exhibit presence of sextet and one sample also exhibit quadrupole doublet. Sextet has been shown by S1 and quadrupole doublet by D1.These sextets show anomalous (very low) magnetic hyperfine field (MHF). To the best of our knowledge the presence of such sextet with such low MHF value in sedimentary samples was not reported in literature, but magnetic hyperfine field range nearby goethite. These sextets may be attributed to very small (ultra-fine) oxide particles. Such ultrafine oxide particles are usually not expected to form in usual weathering processes. Quadrupole doublet Mössbauer parameters show presence of pyrite in all samples.

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