Understanding the transportational and depositional setting of Panchet Formation, Purulia and Bankura districts of West Bengal, India---- Evidence from grain size analysis

Abstract

The rocks of Triassic Panchet Formation (TPF) of West Bengal, India, predominantly include sandstones of medium to fine grain-size, with subordinate shale and pebble horizons. Textural parameter like mean, standard deviation, skewness and kurtosis are calculated using standard methods to understand the transportation and the depositional environment of the sediments in a part of Panchet Formation. Granulometric analyses indicate the presence of fluvial environment interrupted with aeolian, beach and tidal environments prevailed during the time of deposition of the sediments in this part of Gondwanaland during the Triassic time.

Keywords

gondwanaland / panchet / textural parameters / environment / West Bengal

Introduction

Textural parameters of clastic sediments are often used to understand the history of sedimentation (Inman, 1952; Folk and Ward, 1957; Passega, 1957, 1964; Visher, 1969; Awasthi, 1970; Folk, 1974; Chakrabarti, 1977; Sengupta, 1977; Friedman, 1979; Pettijohn et al., 1987; Tucker, 1988; Buckley and Cranston, 1991). Although, some authors questioned the viability of application of granulometric methods (Schlee et al., 1964; Solohub and Kolvan, 1970), Martins (2003) showed the applicability of these methods for understanding of the depositional setup of sediments by analyzing a data set comprising thousands of samples from known depositional environments. These granulometric studies have been applied to decipher the environment of deposition of upper Gondwana sediments by Srivastava and Mankar (2009). From the sedimentological study of the Panchet Formation of the Dumdumi and Biharinath Hill area of Purulia and Bankura districts, West Bengal of eastern India (Fig. 1) earlier workers (Robinson, 1970; Ghosh et al., 1994; Bandyopadhyay et al., 2002) inferred fluvial channel and interchannel set-up. The tetrapod fauna of Panchet Formation is interpreted to be of fluviatile and lacustrine environment (Bandyopadhyay, 1999; Bandyopadhyay et al., 2002). The present study throws light on the temporal changes of granulometric parameters (mean grain size, sorting coefficient, skewness and kurtosis) of Panchet sediments. The results provide insight into the transportational and depositional pattern of the Panchet Formation as well as the validity of the earlier findings (Robinson, 1970; Ghosh et al., 1994; Bandyopadhyay et al., 2002).

Research area and geological backgroud

The Gondwana basins regarded as sub-basins of the Gondwana Master Basin (Acharyya, 1997) of India are situated along a linear belt. These basins are the NW–SE trending Son–Mahanadi basin, Pranhita–Godavari basin and the E-W trending Damodar Basin (Figs. 1(a),(b)). The Damodar basin, a part which is the area of the present study probably extends to the coal belts of Bangladesh in the east, below the Cenozoic cover of the Bengal Basin (Uddin, 1996). The Damodar Basin with its sediment pile overlies its basement the Chhotanagpur Granite Gneiss Complex (CGGC) (Mazumdar, 1988). The Dalma volcanic belt (1.48 Ga, Rb–Sr whole rock; Sengupta et al., 1994) on south of the Chhotanagpur Granite Gneiss, is suggested to tectonothermally approximately synchronously with these gneisses (Acharyya, 2000). The Precambrian CGGC forms the basement of the sedimentary rocks of Gondwana Supergroup (Dunn, 1929). The CGGC is dominantly granite gneissic with subordinate metabasic rocks of amphibolite to granulite grade and metasedimentary enclaves. The basin has faulted margins and both the Raniganj and Panchet rocks directly abut against the basement in the southern part. Evidences of syn- and post-sedimentation faulting are also numerous (Gee, 1932; Chakraborty and Ghosh, 2005). Exposures of Raniganj, Panchet and Supra Panchet Formations and scanty exposures of Barren Measures are present in the study area.

In the intra-cratonic Raniganj Basin, the basal Talchir Formation, consisting of glacial tillites, of Permian Damuda Group unconformably overlies the basement. Krishnan (1982) correlated the Talchir tillites with the Dwyka and Buckeye tillites of South Africa and Antarctica, respectively. The Talchir Formation is overlain conformably by sandstones of the Barakar and Ironstone Shale formations. The Talchir, Barakar and Ironstone shale formations in the Raniganj Basin are about 300, 650 and 350m thick (Krishnan, 1982). The youngest unit of the Damuda Group is the Raniganj Formation and it is overlain by the Lower Triassic Panchet Formation. Panchet Formation is overlain by Upper Triassic Supra-panchet Formation. The startigraphic column of Damodar Basin is presented in Table 1.

The Raniganj Basin is one of the few coal fields of Peninsular India where both the Lower Gondwana (Permian) and Upper Gondwana (Triassic–Lower Cretaceous) formations are present (Gee, 1932). The Permian rocks of the Raniganj Formation include an alternating sequence of sandstone, shale and coal. The white/gray sandstones are plagioclase rich and fine to medium grained. The shales are dark, rich in carbonaceous material.

The Panchet Formation is a coarsening upward sequence and has been divided into three parts (Robinson, 1970). The lower part of the formation is 0-100 m thick and comprises of micaceous, greenish or olivegreen colored, well-laminated siltstone, interbedded with yellow or buff colored sandstones beds (0.5-4 m thick). Framework grains comprise about 79.6% of the volume of the rock while matrix and cement make up about 5.3% and 14.9% volume of the rocks respectively. Common rock types of the Panchet Formation are arkosic arenites with rare occurrences of submature subarkose (Ghosh, 2008). Framework grains are subangular to subrounded, with occasional rounded grains. Matrix (~5%) consists of fine silt to clay sized terrigenous mica (mainly degraded biotite), quartz and feldspar mixed with finely disseminated iron oxide, chert and authigenic clay and iron-carbonate-silica cements (Ghosh, 2008). According to Robinson (1970) the lower part of Panchet Formation is dominated by fine-grained interchannel deposits.

The middle part of Panchet Formation has an average thickness of 200 m and contains red or chocolate colored laminated shaly siltstone (0.5-4.2 m thick) interbedded with buff colored sandstones (2.5-23 m thick). Buff or pale gray sandstones with thin red shale layers constitute the upper part having a thickness of 300 to 400 m. According to Robinson (1970) the middle part has both channel and interchannel facies.

The sediments in the upper part were deposited rapidly by large, high-energy river channel. The channel sand bodies in the upper part of the formation contain pebble beds with small quartz pebbles, feldspar chips and mud clasts. Pockets of bone fragments (lystrosaurus) are noted within this lithology. The tetrapod fauna of Panchet Formation is interpreted to be of fluviatile and lacustrine environment (Bandyopadhyay, 1999).

The present study area is within the Raniganj basin that forms the easternmost depository of the Damodar Basin (Figs. 1(b), (c)). The Raniganj Basin has a semi-elliptical, elongated shape, and covers an area of ca. 3000 km² in the interfluve of Damodar and Ajoy rivers. A gentle synclinal structure of the Panchet Formation has been reported by Gee (1932). The strata of the Panchet Formation have north and north-westward shallow inclination (~ 10°) in the study area which is on the southern limb of the syncline, south of the Damodar River). The overall paleocurrent trend is toward NW to NE (Veevers and Tewari, 1995).

Methodology and data collection

Sampling: Samples of Panchet Formation collected from Tentulrakha brook, Banspetali brook and high ground north-east of Dumdumi village were used for grain-size analysis. The sample locations are given in Fig. 1(c). To study variations of grain-size distribution over a long time period samples are collected from a section of about 150m which covers from the lower Panchet to middle part of middle Panchet formation. Thin sections are made from samples representing entire thickness of foresets of the cross strata as suggested by earlier workers (Otto, 1938; Purkait, 2006).

Grain size analysis: Sediments are divided into fine- and coarse-grained materials, with the lower boundary being located at the diameter of 0.063 mm. A polarizing microscope with micrometer ocular is used for measuring the long diameter of the framework grains and number frequency of grain size ranges are calculated for use in frequency curves and other statistical measurements.

The sediments were classified according to their sand–silt–clay ratio as described by Shepard (1954). Grain size parameters (mean size, standard deviation, skewness and kurtosis) were computed using the graphical method (Folk and Ward, 1957). Graphic median value of Φ₅₀ denoting half of the particles by weight are coarser to it and half is fine. Graphic mean (Mz) is a measure of central tendency, which is calculated by the formula (Φ 16+ Φ 50+ Φ 84) / 3. The inclusive graphic standard deviation (σ_I) is the measure of sorting or uniformity of particles size distribution and it is calculated by the formula (Φ84 - Φ16) / 4+ (Φ95 - Φ5) / 6.6. The graphic skewness (Sk_i) measures the symmetry of the distribution or predominance of coarse or fine-sediments. It is calculated by the formula [(Φ 84+ Φ 16 - 2 Φ 50) / (Φ 84 - Φ 16) + (Φ 95+ Φ 5 - 2 Φ 50)/ (Φ95 - 2 Φ5)]. The negative value denotes coarse-skewed material, whereas, the positive value represents more material in the fine-tail i.e., fine-skewed.

The graphic kurtosis (K_G), is the peakedness of the distribution and measures the ratio between the sorting in the tails and central portion of the curve. If the tails are better sorted than the central portions, then it is termed as platykurtic, where as, leptokurtic, if the central portion is better sorted. If both are equally sorted then mesokurtic condition prevails. It is calculated by the formula (Φ 95 - Φ 5)/ 2.44 (Φ75 - Φ25).

Results

Frequency curves are drawn from number percent of grain sizes to obtain the grain size distribution of 20 samples. In general the sediments are coarse to fine-grained although a few samples have high percentage of fine-sand which can be attributed to fine-grained nature of the sample. An exceptionally different value of an individual sample (DGJ 1) is because of its gritty nature.

Textural parameters

For any given grain size distribution curve, a series of grain size parameters can be defined. The most frequently used parameters include mean grain size, sorting coefficient, skewness and kurtosis. In the present work, these four parameters are used to represent grain size characteristics of sediment (Table 2).

Graphic mean

The calculated values range from 1.46 to 4.259 Φ with an average value of 2.85 Φ. The average value denotes that the major class is of fine sand-size particles. A few samples (DGB-1c, DGB-1d, DGT-1a and DGJ1) having low values, 1.96 to 1.46 Φ, have comparatively more fractions of medium grain sediments (Table 2). The mean grain size decreases upward in Panchet Formation (Fig. 2(a)).

Inclusive graphic standard deviations

Four samples show well sorting, with σ₁ values ranging from 0.38 to 0.48 Φ, four samples are moderately well sorted with σ₁ values ranging from 0.53 to 0.56 Φ and three samples are moderately sorted, ranging from 0.74 to 0.92 Φ. In the very fine grained sediments poor sorting dominates showing σ₁ values between 1.00 to 1.84 Φ (Fig. 2(b)). Moderate sorting with an average value of 0.95 Φ is the general tendency of the Panchet sediments while sediments of the middle part of the Panchet Formation show poor sorting (1.001 to 1.836). In all the cases, verbal classification of sorting of Folk and Ward, 1957 has been used.

Graphic skewness

The graphic skewness (Sk_i) is a measure of symmetry of the grain-size distribution curve. The samples give skewness values ranging from - 0.42 to+ 0.59, i.e., from very-coarse skewed to very fine-skewed. Four samples fall in the near-symmetric category. Seven samples are dominantly fine-skewed and six samples are very fine-skewed. The coarse and very coarse-skewed categories are represented by two (sample nos. DGT 1a and DGB 1b) and one (sample no. DGP 2) samples respectively (Fig. 2(c)) i.e. positively skewed. Thus the sediments are mainly positively skewed.

Graphic kurtosis

The graphic kurtosis (K_G) or peakedness values range between 0.73 and 2.36. Six samples, dominantly leptokurtic (1.13 to 1.41), with better sorted central portion of the distribution (Fig. 2(d)). Four samples with large proportion of fine grains show very leptokurtic distribution (1.69 to 2.36). The platykurtic and mesokurtic distributions are shown by five samples each.

Inter-relationship of size parameters

The sediments are dominantly bimodal, with sand sized grains dominating. The sediments are moderately sorted due to presence of silt as a subordinate size class. Mean vs. standard deviation plots after Folk and Ward (1957) exhibits a smaller size range of the grains (Fig. 3(a)). The plots follow the trend of fluvial sands (Fig. 3(a)) of Folk and Ward (1957). The bimodality of present samples is confirmed by established sinusoidal curve of mean vs. skewness (Fig. 3(b)) of Folk and Ward (1957). Admixture of sand and silt sized grains lead to the sinusoidal nature. Positive or negative skewness are results of proportions of size classes in a grain mixture. Most of the samples are positively skewed with four samples showing negative skewness, within the mean-size range of 1.5 to 2.5 Φ (Fig. 3(b)). This is a clear indication of bimodality with sand dominance and presence of subordinate silt. According to the model plot of Folk and Ward (1957) for the relation of mean-size vs. kurtosis (Fig. 3(c)), mixing of two or more grain size-classes influence the index of kurtosis and an inverted 'V' trend is resulted due to scattering. Present values indicate that the mesokurtic (0.90 to 1.11) category is dominant, followed by leptokurtic (1.11 to 1.5) and platykurtic (0.90 to 0.67) respectively, in the size-class range of approximately 1.5-4.0 Φ, i.e., medium to very fine sand (Fig. 3(c)). Thus according to this plot the grain mixture is dominantly medium-sand and subordinate fine-sand and silt. As this character of the grain mixture makes the sorting poor, particularly in the tails, there are platykurtic to leptokurtic to very leptokurtic conditions also. The skewness vs. standard deviation plots (Fig. 3(d), after Folk and Ward, 1957) produce a scattered trend. Mixture of two modes in equal proportion in bimodal sediments or good sorting in unimodal sediments may be responsible for this type of plots. Prevalence of sand over subordinate silt mode is evident from the plot clustering in a single sector with deviation of skewness values in the negative sector due to presence of silt sized grains (Fig. 3(d)). The plot of mean vs. standard deviation is considered as an effective tool to differentiate between beach and river sands (Moiola and Weiser, 1968; Friedman, 1967). Friedman (1967) and Moiola and Weiser (1968) and later on Martins (2003) considered the bivariate plots between mean size, standard deviation and skewness are of great significance to differentiate between aeolian, beach and river environments. The trends of present values are clearly indicative of dominance of river sedimentation followed by dune and beach deposits (Figs. 3(a) and (d)).

Bimodal grain mixture controls the sorting vs. kurtosis plot of Folk and Ward (1957). Platykurtic and extensively poor sorting is due to equal proportion of two modes in a bimodal grain mixture. Subordinate proportion of grains finer than sand is evident from the scattered plots little outside of the pure sand region of the curve of Folk and Ward (1957). Dominance of medium-sized sand resulted in moderate sorting and prevalence of mesokurtic curves (Fig. 3(e)). Samples of silty sands are plotted near point A of the curve, while pure sands are plotted near point B and subequal mixtures of sand and silty particles are plotted near point C of the curve (Fig. 3(e)).

The plot of skewness vs. kurtosis follows a regular path of sinusoidal pattern as the mean-size changes and is dependent on two modes (Folk and Ward, 1957). Values for samples under study are mainly plotted in the shaded area which is represented by nearly pure sand and sand-silt mixture in the establish plot of Folk and Ward (1957) (Fig. 3(f)).

C-M plot of Passega and Mode of Transport

The relationships of ‘C’ or the coarser one percentile value in micron and ‘M’ which is median value in micron on log-probability scale are plotted to evaluate the hydrodynamic condition during sedimentation (Passega, 1957, 1964). The sediments of present study are mostly plotted in fields IV and V which are suspension sediments which may contain rolled grains smaller than 1 mm (Fig. 4(a)). All the sediment samples were collected from the cross-bedding foreset units representing bedload sediments. However, the fine-grained sediments may be attributed to the diagenetic effect for which a signature of suspension population is observed. These finer grains might have been transported for long distances in suspension before being rolled. Later on, Peiry (1988) has modified the Passega’s diagram and identified different facies of a fluvial regime (Fig. 4(b)). According to this plot sandstones of present study deposited mostly in the emerged bar facies followed by topset fill of a dead arm of a channel and submerged bank facies in a fluvial environment.

Log-probability curves and depositional environment

A classic work of Visher (1969) showed depositional environment of a sand deposit can be deciphered from the grain size distribution. Visher (1969) plotted the cumulative percentage of sand on a probability scale as a function of phi ø, where ø = - log 2 (grain diameter in mm). The advantage of a probability scale is that normally distributed data fall on a straight line. Visher (1969) showed that log-probability plots of grain size distributions fit well to three connected straight lines which can be used to differentiate between traction, saltation and suspension lodes in sediments.

Since the publication of Visher’s (1969) paper, further studies have applied his methodology to determine the depositional environments of unconsolidated sands and sandstones (e.g., Glaister and Nelson, 1974). Middleton (1976) and Sagoe and Visher (1977) have investigated the theoretical basis for the transition grain sizes.

The lognormal subpopulations in bedload and suspended load are identifiable (cf. Visher, 1969). The sediment transport was mainly carried out by saltation (Fig. 5). The suspension was sparse, as can be observed in Fig. 5. Although cumulative curves comparing the samples of the flood and the low water phases are similar, the segment belonging to the saltational transport was better defined during low water periods.

Suspension and saltations domains of size-populations are shown by the present samples (Fig. 5). The traction between saltation and suspension is normally near 2Φ value. The rolling load commonly represents minor (≤5%) quantity of poorly-sorted sediments while the suspension load may vary from about 5 to 25%. The saltation load is comparatively better sorted than the suspension population and is occasionally divided under influence internal forces responsible for rolling and sliding, into two sub-populations, truncating around 1.5Φ value (Visher, 1969). In the present study saltation population is dominant. Sediments of Panchet formation indicate fluvial, natural levee, tidal inlet, wave zone, and beach and dune environments of deposition (Table 3).

Discussion

Inter-relationship of size parameters suggest that the sediments are moderately sorted (Fig. 3(a)) and bimodal in character, with dominating sand sized grains and subordinate silt-size grains which is established by their plots on sinusoidal curve in mean vs. skewness bivariate diagram (Fig. 3(b)) of Folk and Ward (1957). Bivariate plots like mean vs. standard deviation (Fig. 3(a)) and skewness vs. standard deviation (Fig. 3(d)) suggest that most of the samples having character of river deposits with subordinate dune and beach deposits.

C-M plots (Passega, 1957, 1964; Peiry, 1988) of present samples suggest that sediments were deposited mostly in the emerged bar facies followed by topset fill of a dead arm of a channel and submerged bank facies in a fluvial environment (Fig. 4).

Log-probability Curves proposed by Visher (1969) show that the sediments were carried out mainly by by bedload (saltation and rolling) while the suspension load was meager (Fig. 5). Sediments of Panchet formation indicate fluvial, natural levee, tidal inlet, wave zone, and beach and dune environments of deposition (Table 3). Bandyopadhyay (1999) and Bandyopadhyay et al. (2002 and references therein) described terrestrial tetrapod fauna from this site. Based upon this information it is interpreted that the beach and dune deposits of Panchet Formation were formed on lake shore, where some aeolian activity might have been active.

Earlier workers, such as Robinson (1970), Ghosh et al. (1994), Bandyopadhyay (1999) voiced about the fluvialtile character of the Panchet sediments. Grain-size analyses of present work reveal for the first time the existence of beach and inland dunes in Panchet Formation those developed around inland lakes during early Triassic Period.

Conclusions

From grain size and petrographic analyses of Gondwana sandstones of Dumdumi and Biharinath areas of Damodar Basin it can be concluded that the sediments are dominantly medium sand sized with subordinate fine and very fine sand, moderately well sorted to poorly sorted, very fine to very coarse skewed and dominantly leptokurtic. Sand dominated bimodal nature of the sediment with subordinate silt is understood from different bivariate plots between mean, skewness, standard deviation and kurtosis. From various standard plots it is understood that the sediments are mainly fluvial with signature of lake-shore and aeolian environment. Fluvial regime of sedimentation with subordinate beach environment is also revealed from then log-probability plots. Sediments were in suspension and saltation before being deposited.

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