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Article

Mineralogy and Major Element Geochemistry of the Oligocene Barail Group Sandstones from the Sylhet Trough, Bengal Basin: Provenance and Tectonic Implications

by
Sunjuckta Mohinta
1,
Abu Sadat Md Sayem
1,
M. Julleh Jalalur Rahman
1,*,
Md Sha Alam
2 and
Rashed Abdullah
1
1
Department of Geological Sciences, Jahangirnagar University, Dhaka 1342, Bangladesh
2
Institute of Mining, Mineralogy and Metallurgy (IMMM), Bangladesh Council of Scientific and Industrial Research, Joypurhat 5900, Bangladesh
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(10), 985; https://doi.org/10.3390/min14100985
Submission received: 17 August 2024 / Revised: 25 September 2024 / Accepted: 26 September 2024 / Published: 29 September 2024
(This article belongs to the Section Mineral Geochemistry and Geochronology)
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Abstract

:
The origin of Oligocene sediments in the Bengal Basin and associated tectonic setting remain poorly understood. This study investigates the framework mineralogy and major element geochemistry of the Barail Group sandstones from the Sylhet Trough within the Bengal Basin to clarify the provenance and tectonic history of the Oligocene. Modal analysis (Q83F7L10) and geochemical data support a classification of sublitharenite to subarkose, some with Fe enrichment. The heavy mineral assemblage is dominated by opaque minerals, followed by ultrastable minerals with zircon > tourmaline > rutile. The sub-angular to sub-rounded sand grains with a compositionally moderate mature nature suggest that the sediments were deposited close to the source area. The mineralogical and geochemical provenance discrimination diagram suggests contributions from felsic igneous, sedimentary/metasedimentary, and low-grade metamorphic sources, with detritus derived from the Indian craton and proto-Himalaya region. Data suggest moderate to intense chemical weathering, indicative of low relief and a sub-humid to humid climate in the source area. The tectonic analyses indicate that the Bengal Basin transitioned from a predominantly passive margin to an active tectonic margin setting during the Oligocene.

1. Introduction

The Oligocene Barail Group sandstones in the Sylhet Trough of the Bengal Basin (Figure 1) play a pivotal role in deciphering the geological history of northeastern India and Bangladesh. These sandstones offer important insights into the provenance, paleoclimate conditions, and tectonic evolution during the Oligocene epoch. The Bengal Basin, characterized by its complex tectonic interactions between the Indian Plate, Eurasian Plate, and Burma Plate, experienced significant geological changes [1], making it an ideal setting for investigating sedimentary processes and tectonic dynamics.
The framework mineralogy, heavy mineral composition, and major element geochemistry are key tools for unraveling the provenance, tectonic history, and processes that have shaped these sandstones. By examining the mineral composition and geochemical signatures of sandstone deposits, various studies [2,3,4,5,6,7,8,9,10,11] inferred the nature of source rocks, the weathering history, and the paleoclimatic conditions prevailing during deposition. Additionally, these analyses shed light on the tectonic setting of the depositional basin [6,12,13,14,15,16,17], particularly concerning the uplift and erosion of the Himalayan orogen and its impact on sedimentation in the Bengal Basin.
In the Bengal Basin, the Tertiary sedimentary sequences are prominently exposed in the Sylhet Trough (Figure 1b). Previous research has primarily concentrated on the Miocene sequences, focusing on the basin-fill history and tectonostratigraphic reconstructions [1,18,19,20,21]. In contrast, the Oligocene Barail Group sediments in the Sylhet Trough remain less understood, with limited studies available [12,13,22]. Comprehensive analyses of provenance and tectonic context are still lacking. The origin of the Barail Group sediments is a subject of considerable debate. Uddin and Lundberg [12] proposed that these sediments were derived from cratonic sources rather than orogenic. Furthermore, the timing of Himalayan unroofing continues to be debated. While some studies suggest that significant Himalayan exhumation did not occur until the Miocene [12,13,22], others [23,24] propose that a strong collision and extensive Himalayan erosion took place during the Late Eocene. Therefore, the Oligocene sediments from Himalayan foreland basins, such as the Bengal Basin, are critical for addressing these unresolved issues.
This research aims to explore the mineralogical and geochemical properties of the Barail Group sandstones, with the objectives of identifying their source, examining the paleoclimatic conditions present during their formation, and evaluating the tectonic implications for the Bengal Basin. By conducting a thorough analysis, this study intends to contribute to a deeper understanding of provenance and tectonic history in one of the most geologically active areas of the Indian subcontinent.
Minerals 14 00985 g001
Figure 1. (a) Location map of Sylhet Trough in the Bengal Basin and surrounding regions, modified from [18]; (b) the geological map displaying the sample locations (blue steric symbols) and stratigraphic units exposed in the Sylhet Trough, northeastern Bengal Basin, Bangladesh, modified from [18].
Figure 1. (a) Location map of Sylhet Trough in the Bengal Basin and surrounding regions, modified from [18]; (b) the geological map displaying the sample locations (blue steric symbols) and stratigraphic units exposed in the Sylhet Trough, northeastern Bengal Basin, Bangladesh, modified from [18].
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2. General Geology

The study area (Sylhet Trough) is situated at the northeastern corner of the Bengal Basin (Figure 1a). From the Miocene and onwards, this area has been filled by orogenic sediments derived mostly from the eastern Himalayas and partly from the Indo–Burman Ranges [10,11,12,17,24,25,26,27,28]. However, the source of the Early Cenozoic sediments is least known. Tectonically, the area is very complex. It is separated from the Shillong Plateau by the Dauki Fault to the north and from the Chittagong Tripura Fold Belt (CTFB) of the Indo–Barman Range to the east by the Kaladan Fault (Figure 1a). The Eocene shelf-slope break is marked by its western limits. The southern limit of the trough is poorly defined.
The tectonic evolution of the Bengal Basin is related to the rifting (in Permo-Carboniferous time) and break-up of the Gondwanaland continent (in Late Cretaceous), followed by northward movement of the Indian plate that created sediment accommodation space in the stable shelf and deep basin [1,29,30]. Due to the initial collision, the Indian plate was subducted beneath the Lasha Terrain during the Paleocene–Eocene periods [1]. At this stage, the Jaintia Group (Table 1) deposited under a shallow marine environment [18,31]. The Late Oligocene–Miocene experienced a strong collision between the Indian and Eurasian plates, which generated a large influx of clastic detritus into the Bengal Basin [24]. During this tectonic event, the Oligocene Barail Group as well as the Miocene Surma Group clastic sediments were deposited in the Sylhet Trough [18]. The final and rapid phase of structural development took place during the Pliocene–Pleistocene, resulting in the suturing of the Burmese plate with the Indian plate [32]. At the same time, the Bengal Basin experienced a basin-wide marine regression, which initiated a fluvial environment in most of the basin [29]. During this event, the Pliocene Tipam and Pleistocene Dupitila Groups were deposited throughout the basin [31].
The Barail Group is one of the most widespread Cenozoic sedimentary sequences in the Sylhet Trough. Although Evans [33] divided the Barail Group into three lithostratigraphic units: (1) Renji Formation, (2) Jenam Shale Formation, and (3) Laisong Formation in the Assam Valley of India, the sequence is undifferentiated in the Sylhet Trough. Based on biostratigraphy, the Barail Group was assigned to the Oligocene Epoch [21].
In the Sylhet Trough, the Barail Group sedimentary sequence is undifferentiated into its formations. The sequence is entirely composed of siliciclastic rocks, and the basal part is unconformably overlying the Eocene Kapili Shale Formation (Table 1). Although there is a little lithologic variation between the lower and upper portions of the group, no distinct detrital variation is noticed between the sandstone layers. The lower part of the Barail Group consists of dark gray to bluish gray shale alternating with hard and compact, fine-grained pinkish sandstone (Figure 2a,b), while the upper sequence is characterized by fine-grained pinkish and yellowish-gray sandstone (Figure 2c,d) with occasional dark gray shale and silty shale. The Barail Group underlies unconformably (marked by the existence of a thick lateritic bed) under the Bhuban Formation (Figure 2e,f). The lateritic bed is characterized by the presence of ferruginous nodules, leached horizons, and characteristic reddish coloration typically associated with lateritic soils. The laterite bed formed during a period of subaerial exposure prior to the transgression [24]. The transgression subsequently halted pedogenesis by burying the laterite under transitional to marine deposits rather than forming concurrently with the soil. Facies and biostratigraphy analyses suggest a deltaic environment for the Barail Group sediments in the Sylhet region [21,31]. Alam [29] interpreted this sequence as tide-dominated deltaic and intertidal sub-environments. Moreover, Khanam et al. [18] postulated lateral facies changes in the Barail Group in its outcrops, indicating shallow marine to deltaic environments.

3. Materials and Methods

Fresh, unweathered sandstone samples were collected from the Barail Group (including the lower and upper sequence) exposures along the Tamabil–Jaintiapur road cut section in the Sylhet Trough (Figure 1b). The bulk samples were air-dried at room temperature before laboratory analysis. The moderately compacted samples were cooked with Canada balsam to make them harden. The samples were then cut into smaller chips and polished with grinding powders of varying grain sizes until they reached a thickness of 0.03 mm. The polished chips were mounted on glass slides using Araldite and covered with a thin glass layer to prevent dust contamination. Modal composition of 15 sandstone samples was analyzed using a standard petrographic microscope (MEIJI ML 9000), with 400–600 grains counted per thin section via point counting. Heavy mineral analysis was performed on 18 representative sandstone samples, focusing on grain sizes between 1 and 0.063 mm. For this analysis, 20 g of each sample underwent gravity separation using bromoform (CHBr3) with a density of 2.89 g/cm3 at 20 °C, following the procedure outlined by [34]. At least 300 grains were counted in each thin section. The petrographic compositions of the Barail sandstones are presented in Table 2.
Whole-rock geochemical analysis of 17 sandstone samples was conducted using X-ray fluorescence (XRF) at the IMMM, BCSIR in Joypurhat, Bangladesh. The samples were first crushed and pulverized using a planetary ball mill (PM-200, Retsch, Germany) for 20 min. They were then treated with 30% HCl and 10% H2O2 to remove any organic material. The resulting powdered samples were mixed with stearic acid at a 1:10 ratio and milled for an additional two minutes. The mixture was placed in a 30 mm aluminum cap and compressed between two tungsten carbide pellets using a manual hydraulic press at a pressure of 10–15 tons per square inch for 2 min. Once the pressure was slowly released, the pellet was ready for X-ray analysis. Major element concentrations were determined using an X-ray fluorescence spectrometer, following the procedures of [35,36]. The analysis was carried out with a Rigaku ZSX Primus XRF system equipped with a 4 kW Rh-anode X-ray tube. A voltage of 40 kV and a current of 60 mA were applied to detect heavy and light elements, respectively. Geological Survey of Japan (GSJ) stream sediments and USGS rock standards were used for calibration. The analytical uncertainty for major elements was approximately 2%. The concentrations of the major elements in the analyzed sandstones are presented in Table 3.

4. Results

4.1. Framework Mineral Composition

The Barail sandstones were predominantly pink in color, fine-grained, hard, and compact, with grains that ranged from angular to sub-angular and sub-rounded shapes (Figure 3). The framework mineralogy primarily consists of quartz, feldspar, rock fragments, and mica. Quartz is the most abundant component in the Barail sandstones (Table 2), with monocrystalline quartz (Qm) being more prevalent than polycrystalline quartz (Qp) (Figure 3). Qm grains account for 59.07% of the total framework grains, while Qp is present in smaller quantities (avg. 3.60%). Qm grains frequently exhibit undulose extinction (Figure 3b). Qp grains typically comprise 2–3 crystals (Figure 3a). The quartz grains are generally fresh and free of inclusions, with grain boundaries being mostly sutured, though slightly curved and straight boundaries are also observed.
Feldspars were found in most sandstones, comprising an average of 5.19%. The feldspar grains were fresh, angular to sub-angular, with less abundant mica inclusions (Figure 3b,c). The most common types identified were orthoclase, microcline, and albite, which frequently exhibit twinning. K-feldspar (avg. 3.92%) was more abundant than plagioclase (avg. 1.28%) (Table 2). Twinned orthoclase (Figure 3b) and microcline were the predominant K-feldspars observed in these sandstones.
Rock fragments were the second most abundant detrital particles in the total framework grains, comprising 7.72%, following quartz. The sandstones primarily contained sedimentary lithic grains (Ls) and metamorphic lithic grains (Lm) (Figure 3b,d). Metamorphic lithic grains (5.34%), including various aggregates such as mica–schist, graphite–schist, quartz–mica–schist, quartz–graphite schist, and quartz–graphite–mica schist, etc., were more prevalent than sedimentary lithic grains (2.38%). Volcanic lithic grains were not identified. Micas (Figure 3c,d) were found in trace amounts in the Barail sandstones, with muscovite (0.93%) being more abundant than biotite (0.61%) (Table 2). Chert (0.36%) and chlorite (0.33%) were present in minor quantities. The sandstones also contained a significant amount of matrix (1.7%) and cement (1.54%). Quartz overgrowth (Figure 3a,b), iron (Figure 3b), and clays (Figure 3c,d) were the predominant cementing materials in the analyzed sandstones.

4.2. Heavy Mineral Composition

In the Barail Group sandstones, the heavy mineral assemblage exhibits low diversity (Table 2, Figure 4), predominantly consisting of ultrastable and opaque minerals. Opaque minerals account for 28.87% of the total heavy minerals. Zircon is the most abundant transparent mineral, comprising 21.37% of the total heavy mineral content. Zircons are typically colorless, exhibiting sub-angular to sub-rounded and prismatic forms (Figure 4a,d). Tourmaline is the second most abundant transparent mineral, making up 13.09% of the heavy fraction, generally occurring in prismatic form with pale brown to pale yellow coloration (Figure 4a–c). Rutile (Figure 4a,b), characterized by deep red, reddish-brown, or yellowish-brown hues, averages 10.80%. The sandstones show a moderate ZTR index, ranging from 53.76% to 69.80%, with an average of 63.67% (Table 2).
The garnet concentration ranges from 0.00% to 15.21%, with an average of 6.49%. These garnets are typically colorless and exhibit anhedral forms. Pyroxene is present in colorless, irregular shapes (Figure 4d) and constitutes 5.98% of the total heavy minerals. Hornblende, with an average concentration of 3.79%, displays bluish-green to brownish-green hues and is characterized by prismatic, elongated, and columnar morphologies (Figure 4b,c). Epidote is present at a concentration of 2.68%, characterized by pale green or pale brown coloration and irregular morphology (Figure 4b,c). The average concentrations of apatite and chloritoids are 1.75% and 1.48%, respectively (Table 2). Kyanite (0.33%), sillimanite (0.72%), and staurolite (0.99%) are found in very minor quantities within the sandstones. These findings highlight the dominance of an ultrastable heavy mineral assemblage, where zircon > tourmaline > rutile, followed by a significant presence of opaque minerals. In contrast, the meta-stable heavy mineral assemblage is composed of garnet > pyroxene > hornblende > epidote. High-grade metamorphic minerals such as kyanite, sillimanite, and staurolite are rare in the Barail sandstones.

4.3. Major Element Geochemistry

Geochemical analysis reveals a significant concentration of SiO2, followed by Al2O3. SiO2 concentrations range from 62.32% to 83.96% with an average value of 74.21% (Table 3). Al2O3 content varies between 7.93% and 20.63%, averaging 13.12%. These high SiO2 and Al2O3 levels indicate substantial quartz and aluminosilicate content, respectively. The mean Fe2O3 concentration is 6.77%, suggesting the presence of Fe-rich minerals in the sandstones. K2O averages 2.36%. The lower concentrations of CaO (0.09%) and Na2O (0.19%) are consistent with the reduced abundance of plagioclase feldspar (Table 2). TiO2 has an average concentration of 1.10%, indicating the presence of titanium-rich heavy minerals such as rutile and ilmenite. Mineralogical results also show a relatively high rutile content (10.80%) in the sandstones. Other oxides, including MgO (avg. 0.68%), MnO (0.04%), P2O5 (0.13%), and ZrO (0.07%), are present in minor quantities.
The Barail Group samples were normalized against upper continental crust (UCC) values [37]. Overall, the major elements’ concentrations were lower than UCC values (Figure 5). The average SiO2 concentration in the sandstones closely matched the UCC value, while the average Al2O3 concentration was slightly depleted. Most of the sandstones exhibited enrichment in TiO2 and Fe2O3. The significant depletion of CaO and Na2O suggests a lower abundance of plagioclase feldspar, consistent with petrographic observations (Table 2).

4.4. Sandstone Classification and Maturity

Mineralogical and geochemical analyses were conducted to classify the Barail Group sandstones (Figure 6). The sandstones exhibited a diverse composition based on their major framework and elemental constituents. The average modal composition, Q83F7L10 (Table 2), predominantly classifies these sandstones as sublitharenite (Figure 6a), with only a few samples showing characteristics of sub-arkose. Utilizing the geochemical classification method proposed by [38], which employs the log(SiO2/Al2O3) vs. log(Fe2O3/K2O) diagram, the Barail sandstones were primarily identified as Fe sand and litharenite to sublitharenite types (Figure 6b). A small subset of samples exhibited shale and wacke characteristics.
The sub-angular to sub-rounded quartz and ultrastable heavy mineral grains (Figure 3 and Figure 4), along with a moderate ZTR index (63.67%), indicate that the sandstones are mineralogically sub-mature. According to [40], the SiO2/Al2O3 ratio is correlated with total quartz (Qt) and the ZTR index to assess sediment maturity. A strong positive correlation between SiO2/Al2O3, Q, and the ZTR index suggests high compositional maturity, and vice versa. In this study, a moderate positive correlation is observed between SiO2/Al2O3 and Q (r = 0.52) and the ZTR index (r = 0.27) (Figure 7a,b), indicating that the samples are compositionally sub-mature. Additionally, alkali content (Na2O + K2O) is an important indicator of chemical maturity [41], where (Na2O + K2O) is negatively correlated with SiO2/Al2O3 and positively correlated with (F+Lt) for mature and well-rounded sands (Figure 7c). The Barail Group sandstones exhibit a strong negative correlation (r = −0.91) between (Na2O + K2O) and SiO2/Al2O3 (Figure 7d) and a weak positive correlation (r = 0.29) between (Na2O + K2O) and (F+Lt), further supporting the classification of these sandstones as sub-mature.

5. Discussion

5.1. Bulk Chemical Compositon

In Figure 8, linear correlations between major oxides and Al2O3 are presented. SiO2 exhibits a strong negative correlation with Al2O3 (r = −0.97) (Figure 8a), implying that their distribution may be influenced by hydrodynamic fractionation and sorting processes [42]. The strong positive correlations observed between Al2O3 and TiO2 (r = 0.93) and P2O5 (r = 0.93) (Figure 8h,i) typically indicate a close association between aluminosilicate minerals, such as clay minerals, and titanium-bearing heavy minerals like rutile and ilmenite within the sediments or weathering products [11,17]. This suggests that the distribution of these minerals is likely governed by similar geological processes, including weathering, sedimentation, or diagenesis, which affect both aluminum- and titanium-rich minerals similarly. The moderate positive correlation between Al2O3 and Fe2O3 (r = 0.74) (Figure 8b) suggests an association between aluminum-rich and iron-rich minerals, such as diagenetic clay minerals, micas, and iron or clay cement, particularly in the Barail sandstones (Figure 3c,d). The strong positive correlation between Al2O3 and MgO (r = 0.89) (Figure 8d) may indicate the presence of clay minerals like chlorite or illite, which contain both aluminum and magnesium. Moderate to strong positive correlations of Al2O3 with CaO (r = 0.61), Na2O (r = 0.82), and K2O (r = 0.96) (Figure 8e–g) suggest these oxides are associated with phyllosilicates or micaceous and clay minerals [11,42]. MnO, however, shows a very weak correlation with Al2O3 (Figure 8c).

5.2. Provenance

Before the collision between the Indian and Eurasian Plates, sediment deposition in the Bengal Basin was likely derived from the Indian craton [12]. Several studies suggest that the initial collision between India and Eurasia began in the Early Eocene [23,24]. Due to this collision, the paleo-Brahmaputra originated on the southern slopes of the Himalayas, which subsequently transported eroded detritus, and deposited it in the foreland basins, including Bengal Basin [8,43]. However, other research [1,12,13] indicates that significant Himalayan thrusting and erosion occurred during the Early Miocene.
The detrital modes and geochemical compositions of sandstones serve as reliable indicators for interpreting provenance [2,4,6,8,10,11,17,26,44]. The QmFLt triangular plot [4] is commonly employed to distinguish between major provenances, such as continental blocks, magmatic arcs, and recycled orogens. In this plot (Figure 9a), the prevalence of high monocrystalline quartz, moderate to high total lithic grains, and low feldspar content suggests quartzose recycled and craton interior provenances for the Barail Group sediments. Roser and Korsch [6] identified four provenance fields: mafic igneous, intermediate igneous, felsic igneous, and quartzose recycled. In the discriminant function diagram, most of the samples are concentrated within the quartzose recycled field, with two samples falling within the mafic igneous provenance and three within the intermediate igneous provenance (Figure 9b).
The Al2O3/TiO2 ratio was plotted against SiO2 to characterize the composition and nature of the source rocks for the sandstones [2]. In this diagram (Figure 9c), most samples align with the felsic igneous field, with only a few suggesting an intermediate igneous origin. The TiO2/ZrO ratio is also a useful indicator for determining sedimentary rock provenance, where values less than 55 indicate felsic igneous origins, values between 55 and 199 suggest intermediate igneous origins, and values greater than 200 point to mafic igneous rocks [44]. For the Barail Group sandstones, the TiO2/ZrO ratios range from 5.91 to 39.67 (avg. 17.46; Table 3), indicating a felsic igneous provenance. The predominance of ultrastable heavy mineral suites (zircon, tourmaline, rutile), opaque minerals, and garnet supports low- to medium-grade metamorphic, felsic igneous, and metasedimentary sources [12]. The ternary plot of &–A–POS [8] for the heavy mineral suites further indicates low-grade metamorphic sources for the investigated sandstones (Figure 9d).
The observations above indicate that the primary provenance for the Oligocene Barail sandstones was the Indian craton, aligning with the findings of [12]. They [12] also proposed that the opaque minerals likely originated from the flood basalts of the Indian peninsula. The moderate textural maturity and sub-angular to sub-rounded detrital grains suggest little transport distance and minimal reworking. The presence of recycled sources points to the reworking of detrital grains, likely derived from the initially uplifted Himalayan regions.

5.3. Weathering and Paleoclimate

Various weathering indices (CIA, CIW, PIA) were used to evaluate the intensity of chemical weathering (Table 3). The CIA (chemical index of alteration) indicates the progressive alteration of plagioclase and potassium feldspar into clay minerals [46]. CIA values for the Barail Group sandstones range from 74.91 to 86.41, with an average of 82.09, suggesting that the sandstones were likely derived from a moderately to strongly weathered zone. Similarly, the CIW (chemical index of weathering) measures the degree of alteration of feldspar into clay minerals [47]. The CIW values range from 95.37 to 100, with an average of 97.95, indicating strong chemical weathering. The PIA (plagioclase index of alteration), proposed by [7] as an alternative to CIW for clarifying plagioclase weathering, also shows high values for the samples, ranging from 94.05 to 100, with an average of 97.46, reflecting strong chemical weathering in the source region.
The A–CN–K [Al2O3–(CaO* + Na2O)–K2O] triangular diagram is commonly employed to assess weathering intensity in the source area [10,11,17,20,42]. Based on this diagram [3], the sandstones under study indicate a moderate level of chemical weathering (Figure 10a). Furthermore, the degree of chemical weathering can also be evaluated using the A–CNK–FM [Al2O3–(CaO* + Na2O + K2O)–(Fe2O3 + MgO)] triangular plot [46]. In this diagram (Figure 10b), most samples are dispersed just above and along the feldspar–smectite join line, trending towards illite and muscovite, which also suggests a moderate to strong intensity of chemical weathering. Furthermore, the abundance of ultrastable heavy minerals (zircon, rutile, and tourmaline) and low diversity of heavy mineral content suggest intense chemical weathering.
The degree of chemical weathering is closely linked to the relief and climate of the source area. The higher chemical weathering intensity suggests reduced tectonic activity and low relief, often occurring in warmer, more humid conditions [48]. The aforementioned weathering indicators show that the sandstones experienced moderate to strong chemical weathering, indicating that sub-humid to humid climate conditions existed in a low-relief source area during the Oligocene. The relationship between Qt/(F+Lt) and Qp/(F+Lt) illustrates the climate conditions present in the source area [5]. According to this diagram (Figure 11a), the source region experienced sub-humid to humid climate conditions. Additionally, the ln(Q/L) vs. ln(Q/F) model [48] indicates that the Barail Group sediments were derived from a low-relief area with moderate weathering and a humid climate (Figure 11b).

5.4. Tectonic Implications

The tectonic setting of a depositional basin can frequently be identified by analyzing the detrital composition of clastic deposits [10,11,12,13,17,27,49]. The triangular plot of Qp–Lvm–Lsm [48] is a widely recognized tool for distinguishing sand derived from various tectonic settings, such as collision orogens and rifted continental margins. This diagram indicates that the Barail sandstones exhibit mixed signals, pointing to origins from both a rifted continental margin and collision orogen (Figure 12a).
The major element compositions (%) of clastic rocks offer important insights into the tectonic history of the depositional basin [6,14,15,16]. The discriminant diagram for DF1 (Arc–Rift–Col)m1 vs. DF2 (Arc–Rift–Col)m1 [16] suggests that the Barail sandstones primarily reflect a rifted continental margin with some influence from a collision orogen (Figure 12b). The Al2O3/SiO2 vs. (Fe2O3 + MgO) bivariate diagram [14] (Figure 12c) points to a tectonic setting transitioning from a passive to an active margin. Additionally, the SiO2 vs. K2O/Na2O tectonic discriminant plot [15] shows a predominantly passive margin setting, with some samples scattered within the active margin and continental rift settings (Figure 12d).
These findings reveal the tectonic history of the Himalayan Orogen during the Oligocene. During this period, tectonic environments transitioned from a dominant passive margin to an active margin setting. Provenance studies indicate that the Oligocene Barail sediments were sourced from craton-derived and recycled materials (Figure 9a). This suggests that the Barail sediments mainly originated from the Indian craton and were deposited in a passive margin environment prior to the collision between the Indian and Eurasian Plates. The provenance analysis also suggests cratonic sources for the Oligocene sediments. Furthermore, the occurrence of recycled materials suggests the erosion and uplift of older sediments due to the initial collision between the Indian and Eurasian Plates. The moderate to strong chemical weathering indicates low relief in the source area [48]. Although previous studies [1,21,23,24] suggested a strong collision during the Oligocene period, the limited occurrence of high-grade metamorphic heavy minerals, such as kyanite and sillimanite observed in this study (Table 2), suggests that the major exhumation of the Himalayas might not have occurred until the Oligocene. Instead, an initial soft collision, resulting in proto-Himalaya uplift, is more likely to have taken place.

6. Conclusions

  • The Oligocene Barail sandstones mainly comprise monocrystalline quartz with low feldspar and lithic fragments, classifying them as sublitharenite to subarkose types.
  • The results suggest that the Barail Group sediments originated from felsic igneous, low-grade metamorphic, and recycled sources like the Indian craton and proto-Himalayan regions.
  • Mineralogical and geochemical shreds of evidence show moderate to strong chemical weathering, implying a sub-humid to humid climate and low relief in the source area during the Oligocene.
  • The tectonic discrimination diagrams and detrital compositions suggest that during the Oligocene, the Bengal Basin setting transitioned from a passive to an active margin setting, pointing to the proto-Himalayan uplift due to the initial collision between the Indian and the Eurasian plates.

Author Contributions

Conceptualization: A.S.M.S.; methodology: S.M. and A.S.M.S.; software: A.S.M.S. and S.M.; validation: S.M. and M.S.A.; formal analysis: S.M. and M.S.A.; investigation: S.M., A.S.M.S. and R.A.; data curation: S.M. and A.S.M.S.; writing—original draft preparation: S.M.; writing—review and editing: A.S.M.S., M.J.J.R. and R.A.; funding acquisition: A.S.M.S. and M.J.J.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially partly supported by the Faculty of Mathematical and Physical Sciences (Fiscal Year: 2022–2023), Jahangirnagar University, Dhaka, Bangladesh.

Data Availability Statement

The data that support the findings of this study are available within the article.

Acknowledgments

The first author expresses his gratitude to Jahangirnagar University for financial support (2022–2023 fiscal year) in this project. The authors extend their appreciation to the IMMM, BCSIR for granting access to their XRF laboratory for the successful completion of the experimental works.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 2. Outcrop photographs of the Barail sandstones: (a) trough cross-bedded pinkish sandstone; (b) planar cross-bedded pinkish sandstone; (c) thick-bedded yellowish-brown sandstone; (d) trough cross-bedded yellowish sandstone; (e,f) lateritic bed marks the unconformity between the Oligocene Barail Group and Miocene Surma Group deposits.
Figure 2. Outcrop photographs of the Barail sandstones: (a) trough cross-bedded pinkish sandstone; (b) planar cross-bedded pinkish sandstone; (c) thick-bedded yellowish-brown sandstone; (d) trough cross-bedded yellowish sandstone; (e,f) lateritic bed marks the unconformity between the Oligocene Barail Group and Miocene Surma Group deposits.
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Figure 3. Photomicrographs of framework minerals of the Barail sandstones: (a) dominance of angular to sub-angular, and sub-rounded monocrystalline quartz (Qm), polycrystalline quartz (Qp), and silica cement (Qc); (b) undulatory monocrystalline quartz (Qmu), K-feldspar (K), sedimentary lithic grain (Ls), silica cement (Qc), and iron cement (Ic); (c) plagioclase feldspar (P), K-feldspar (K), biotite (B), quartz–mica–schist (Q-M-S), chert, and clay cement (Clc); (d) muscovite (M), mica-schist (M-S), graphite–schits (G-S), and clay cement (Clc).
Figure 3. Photomicrographs of framework minerals of the Barail sandstones: (a) dominance of angular to sub-angular, and sub-rounded monocrystalline quartz (Qm), polycrystalline quartz (Qp), and silica cement (Qc); (b) undulatory monocrystalline quartz (Qmu), K-feldspar (K), sedimentary lithic grain (Ls), silica cement (Qc), and iron cement (Ic); (c) plagioclase feldspar (P), K-feldspar (K), biotite (B), quartz–mica–schist (Q-M-S), chert, and clay cement (Clc); (d) muscovite (M), mica-schist (M-S), graphite–schits (G-S), and clay cement (Clc).
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Figure 4. Photomicrographs of characteristic heavy minerals (a–d) of the Barail Group sandstones; here the notation used is zircon (Z), rutile (R), tourmaline (T), kyanite (Ky), staurolite (St), hornblende (H), sillimanite (S), epidote (Ep), garnet (G), chloritoid (Ch), pyroxene (Py).
Figure 4. Photomicrographs of characteristic heavy minerals (a–d) of the Barail Group sandstones; here the notation used is zircon (Z), rutile (R), tourmaline (T), kyanite (Ky), staurolite (St), hornblende (H), sillimanite (S), epidote (Ep), garnet (G), chloritoid (Ch), pyroxene (Py).
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Figure 5. The major element percentages of the Barail Group sandstones were normalized and plotted against upper continental crust (UCC) values in reference to [37]. The red line indicates the average value of the major oxides.
Figure 5. The major element percentages of the Barail Group sandstones were normalized and plotted against upper continental crust (UCC) values in reference to [37]. The red line indicates the average value of the major oxides.
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Figure 6. Classification of the Barail Group sandstones from the Sylhet Trough: (a) Q (quartz)–F (feldspar)–L (lithic fragments) triangular classification after [39]; (b) bivariate log(SiO2/Al2O3) vs. log(Fe2O3/K2O) after [38].
Figure 6. Classification of the Barail Group sandstones from the Sylhet Trough: (a) Q (quartz)–F (feldspar)–L (lithic fragments) triangular classification after [39]; (b) bivariate log(SiO2/Al2O3) vs. log(Fe2O3/K2O) after [38].
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Figure 7. Scatter plots of sediment maturity diagrams: (a,b) Si2O/Al2O3 with Q (total quartz), and ZTR index, (c,d) (Na2O + K2O) with Si2O/Al2O3, and (F+Lt).
Figure 7. Scatter plots of sediment maturity diagrams: (a,b) Si2O/Al2O3 with Q (total quartz), and ZTR index, (c,d) (Na2O + K2O) with Si2O/Al2O3, and (F+Lt).
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Figure 8. The scatter plots display the distribution of major oxides (%) against Al2O3 (a–i) for the Barail Group sandstones.
Figure 8. The scatter plots display the distribution of major oxides (%) against Al2O3 (a–i) for the Barail Group sandstones.
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Figure 9. Provenance discriminating diagrams for the Barail Group sandstones: (a) Qm (monocrystalline quartz)–F (feldspar)–Lt (total lithic fragments) triangular plots after [4]; (b) geochemical discrimination function F1 vs. F2 [45], the blue steric sign indicates the position of upper continental crust (UCC) value, and AN—andesite, BA—basalt, DA—dacite, RH—rhyolite, RD—rhyodacite; (c) SiO2 vs. Al2O3/TiO2 bivariate plot after [2]; (d) ternary diagram of &–A–POS after [8], where POS—pyroxene + olivine + spinel, A—total amphiboles, &—all transparent heavy minerals are not included in the other two poles.
Figure 9. Provenance discriminating diagrams for the Barail Group sandstones: (a) Qm (monocrystalline quartz)–F (feldspar)–Lt (total lithic fragments) triangular plots after [4]; (b) geochemical discrimination function F1 vs. F2 [45], the blue steric sign indicates the position of upper continental crust (UCC) value, and AN—andesite, BA—basalt, DA—dacite, RH—rhyolite, RD—rhyodacite; (c) SiO2 vs. Al2O3/TiO2 bivariate plot after [2]; (d) ternary diagram of &–A–POS after [8], where POS—pyroxene + olivine + spinel, A—total amphiboles, &—all transparent heavy minerals are not included in the other two poles.
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Figure 10. Paleoweathering discriminant diagrams for the Barail Group sandstones: (a) A–CN–K diagram after [3]; (b) A–CNK–FM ternary diagram after [46], where A—Al2O3, CN—CaO* + Na2O, K—K2O, CNK—CaO* + Na2O + K2O, FM—Fe2O3 + MgO, UCC—upper continental crust, Pl—plagioclase feldspar, Ksp—K-feldspar, B—biotite, Mu—muscovite, Sm—smectite, I—illite, Ka—kaolinite, Gb—gibbsite, chl—chlorite, Cpx—clinopyroxene, Hbl—hornblende.
Figure 10. Paleoweathering discriminant diagrams for the Barail Group sandstones: (a) A–CN–K diagram after [3]; (b) A–CNK–FM ternary diagram after [46], where A—Al2O3, CN—CaO* + Na2O, K—K2O, CNK—CaO* + Na2O + K2O, FM—Fe2O3 + MgO, UCC—upper continental crust, Pl—plagioclase feldspar, Ksp—K-feldspar, B—biotite, Mu—muscovite, Sm—smectite, I—illite, Ka—kaolinite, Gb—gibbsite, chl—chlorite, Cpx—clinopyroxene, Hbl—hornblende.
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Figure 11. Paleoclimate discriminant diagram: (a) Qt/(F+Lt) vs. Qp/(F+Lt) after [5]; and (b) ln(Q/L) vs. ln(Q/F) after [48], where, 0—unweather, arid climate, high relief; 1—slightly weathered, sub-humid, moderate relief; 2—moderately weathered, humid climate, low relief; 4—intensely weathered.
Figure 11. Paleoclimate discriminant diagram: (a) Qt/(F+Lt) vs. Qp/(F+Lt) after [5]; and (b) ln(Q/L) vs. ln(Q/F) after [48], where, 0—unweather, arid climate, high relief; 1—slightly weathered, sub-humid, moderate relief; 2—moderately weathered, humid climate, low relief; 4—intensely weathered.
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Figure 12. Tectonic setting discriminant diagram for the Barail Group sandstones: (a) Qp–Lvm–Lsm triangular after [49], where Qp—Polycrystalline quartz, Lvm—lithic meta-volcanic fragments, Lsm—lithic meta-sedimentary fragments; (b) multi-major elements discriminant diagram of DF1 (Arc–Rift–Col)m1 vs. DF2 (Arc–Rift–Col)m1 after [16]; (c) Al2O3/SiO2 vs. (Fe2O3 + MgO) after [14], where PM—passive margin, ACM—active continental margin, CIA—continental island arc, OIC—oceanic island arc; (d) SiO2 vs. K2O/Na2O after [15].
Figure 12. Tectonic setting discriminant diagram for the Barail Group sandstones: (a) Qp–Lvm–Lsm triangular after [49], where Qp—Polycrystalline quartz, Lvm—lithic meta-volcanic fragments, Lsm—lithic meta-sedimentary fragments; (b) multi-major elements discriminant diagram of DF1 (Arc–Rift–Col)m1 vs. DF2 (Arc–Rift–Col)m1 after [16]; (c) Al2O3/SiO2 vs. (Fe2O3 + MgO) after [14], where PM—passive margin, ACM—active continental margin, CIA—continental island arc, OIC—oceanic island arc; (d) SiO2 vs. K2O/Na2O after [15].
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Table 1. Traditional stratigraphy of the Sylhet Trough, Bengal Basin (the wave line indicates unconformable boundary) (modified after [18,31]).
Table 1. Traditional stratigraphy of the Sylhet Trough, Bengal Basin (the wave line indicates unconformable boundary) (modified after [18,31]).
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Table 2. Results of the petrographic composition (%) of Barail Group sandstones from Sylhet Trough, Bengal Basin.
Table 2. Results of the petrographic composition (%) of Barail Group sandstones from Sylhet Trough, Bengal Basin.
Sample NoBS03BS04BS06BS07BS10BS11BS12BS13BS14BS15BS19BS20BS22BS23BS24BS28BS29BS30Average
Framework minerals (%)
QuartzQm54.3463.9949.0163.1361.1358.4367.0657.7367.1066.3656.0356.3055.9959.0950.33---59.07
Qp2.681.503.271.715.093.014.175.302.682.522.843.036.513.745.90---3.60
FeldsparK5.956.024.913.196.085.103.054.031.790.343.384.535.031.154.21---3.92
P2.501.172.302.421.481.070.480.321.430.421.221.551.480.141.20---1.28
MicaM1.191.843.891.240.490.561.120.000.810.170.801.000.380.430.00---0.93
B1.500.832.600.410.490.440.000.000.310.000.440.090.770.500.72---0.61
Chert0.000.250.590.010.690.000.280.250.041.190.340.330.041.160.22---0.36
Chlorite1.450.001.900.001.140.000.000.210.000.080.000.090.000.000.00---0.33
Lithic GrainLs2.340.501.002.001.002.211.162.212.872.003.153.643.103.295.29---2.38
Lm8.407.681.795.673.934.593.584.272.515.325.398.115.697.495.68---5.34
Lv0.000.000.000.000.000.000.000.000.000.000.000.000.000.000.00---0.00
Matrix1.501.501.273.281.361.342.210.571.351.071.412.202.941.841.70---1.70
Cement0.531.190.321.072.811.181.580.591.971.071.691.341.882.453.47---1.54
Porosity17.6313.5327.1515.8714.3222.0815.3024.5117.1519.4623.3217.7816.2018.7221.27 18.95
Q Recalculated to 100%74.8281.0083.9583.0184.1482.5889.5985.3489.0389.5181.7776.8980.3483.8977.44---82.89
F11.098.8811.577.179.608.284.445.894.120.986.387.888.361.737.45---6.92
L 14.0810.124.479.826.269.145.968.786.869.5111.8515.2311.3014.3915.12---10.19
Lt13.419.686.069.3810.029.818.9111.788.069.8411.3814.7815.3014.5116.87---11.32
Qm/Qp20.2942.5914.9736.9012.0119.4016.0710.8925.0426.3019.7018.588.6015.828.53---16.42
F+Lt21.8616.8613.2714.9817.5715.9712.4516.1311.2810.6015.9720.8721.8015.8022.28---16.51
Heavy Minerals (%)
Zircon 24.3430.6921.0320.3344.8131.4411.9526.6418.5022.9313.1019.3719.8517.7817.9213.0212.8118.2521.37
Tourmaline 10.323.0013.2112.0910.086.9119.4713.2712.007.3217.5011.1612.966.4413.9619.5528.0818.2513.09
Rutile 8.429.3210.559.418.7211.4810.4614.0716.005.857.5016.0510.9912.789.9510.0010.8412.0610.80
Garnet 7.305.865.523.6710.382.0010.493.563.675.002.508.371.990.009.9410.5210.8715.216.49
Kyanite 0.500.001.290.000.000.000.000.000.000.980.001.900.000.000.000.800.490.000.33
Epidote 2.332.501.890.005.572.612.381.012.505.373.502.373.963.008.911.101.321.132.86
Staurolite 1.340.000.520.000.000.000.004.671.001.462.502.791.490.000.500.500.990.000.99
Hornblende 2.242.502.732.643.304.453.902.273.509.761.003.122.366.111.881.074.938.763.70
Sillimanite 0.790.600.000.000.000.961.002.400.000.003.001.801.480.000.500.000.000.520.72
Pyroxene 7.428.397.589.337.729.235.404.772.504.393.003.025.623.229.703.755.626.915.98
Apatite 0.003.501.551.330.940.482.993.274.001.465.501.861.430.560.000.000.002.581.75
Chloritoid 1.890.660.720.670.000.000.503.741.110.981.500.933.456.672.970.720.130.001.48
Opaque 30.2632.9832.6738.677.0828.6129.9917.9031.8032.8537.5025.4931.9741.6722.3838.9822.4616.3428.87
Others 2.860.000.761.861.421.831.482.423.421.661.901.772.461.781.400.001.480.001.58
ZTR Index61.7764.1766.5268.2168.4569.8059.8165.7668.1853.7660.9662.5164.3763.4353.8969.7566.7158.0463.67
POS7.428.397.589.337.729.235.404.772.504.393.003.025.623.229.703.755.626.915.98
A2.242.502.732.643.304.453.902.273.509.761.003.122.366.111.881.074.938.763.70
G+HgM9.936.467.323.6710.382.9611.4910.634.677.448.0014.864.960.0010.9311.8212.3515.738.53
&50.1649.6749.7045.6971.5354.7549.2264.4257.5345.5650.5053.5155.1049.0055.1144.3854.6552.2752.93
Qm—monocrystalline quartz, Qp—Poly crystalline quartz, K—K-feldspar, P—plagioclase, M—muscovite, B—biotite, Ls—sedimentary lithic grains, Lm—metamorphic lithic grain, Lv—volcanic lithic grain, Lt—total lithic grains + Qp, Lvm—lithic meta-volcanic, Lsm—lithic meta-sedimentary, Q, F, L—values were recalculated to 100%, POS—pyxroxen + olivine + spinel, A—total amphiboles, &—all transparent heavy minerals not included in the other two poles.
Table 3. Major element concentrations (%) of the Barail Group sandstones from the Sylhet Trough of the Bengal Basin.
Table 3. Major element concentrations (%) of the Barail Group sandstones from the Sylhet Trough of the Bengal Basin.
Sample No.BS03BS0421BS06BS07BS10BS11BS12BS13BS14BS15BS19BS20BS22BS23BS24BS28BS29Average
SiO264.8978.0258.0580.8983.9758.9181.1283.7580.880.7953.3380.870.3380.4862.3379.7283.3574.21
TiO21.541.191.920.670.791.610.770.661.090.731.460.710.921.11.680.970.811.10
Al2O319.7312.2723.928.937.9620.647.947.139.0111.5320.5210.2413.9110.0720.7710.038.3613.12
Fe2O37.85.218.776.783.929.136.615.975.794.2412.534.6310.745.38.684.894.176.77
MnO0.050.040.040.030.030.040.060.040.130.020.050.0400.030.040.040.030.04
MgO1.170.281.570.190.681.420.530.150.40.261.660.380.320.281.230.680.350.68
CaO0.090.050.130.060.030.190.050.060.090.040.190.160.130.050.070.050.090.09
Na2O0.440.150.470.080.240.220.080.050.110.090.320.0600.130.420.30.130.19
P2O50.150.070.120.050.050.090.130.030.080.050.10.060.10.90.180.040.030.13
K2O3.591.724.191.191.744.181.750.991.531.574.161.712.431.563.922.151.752.36
ZrO0.080.090.090.060.060.050.060.050.180.040.040.040.080.050.070.070.060.07
CIA81.6985.8981.9786.4174.9180.4978.5785.783.3386.4979.9783.0284.1383.8781.8178.0779.2282.09
PIA96.7298.5596.3798.4894.7697.1399.5798.1298.0898.9996.210099.0297.1397.7794.0595.6197.44
CIW97.3598.7797.0498.795.9597.7510098.498.4399.1497.0197.71100.0197.698.2295.3796.5797.88
SiO2/Al2O33.296.362.439.0610.552.8510.2211.758.977.012.607.895.067.993.007.959.976.88
Al2O3/TiO212.8110.3112.4613.3310.0812.8210.3110.808.2715.7914.0514.4215.129.1512.3610.3410.3211.93
TiO2/Zr18.2512.9121.2211.5712.8229.4912.8112.115.9116.9439.6719.7811.7520.2623.1114.1614.0617.46
K2O/Al2O30.180.140.180.130.220.200.220.140.170.140.200.170.170.150.190.210.210.18
K2O/Na2O8.1611.478.9114.887.2519.0021.8819.8013.9117.4413.0028.50-12.009.337.1713.4614.13
Na2O + K2O4.031.874.661.271.984.401.831.041.641.664.481.772.431.694.342.451.882.55
Log(SiO2/Al2O3)0.520.800.390.961.020.461.011.070.950.850.410.900.700.900.480.901.000.78
Log(Fe2O3/K2O)0.340.480.320.760.350.340.580.780.580.430.480.430.650.530.350.360.380.48
Log(Na2O/K2O)−0.91−1.06−0.95−1.17−0.86−1.28−1.34−1.30−1.14−1.24−1.11−1.45-−1.08−0.97−0.86−1.13−1.12
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Mohinta, S.; Sayem, A.S.M.; Rahman, M.J.J.; Alam, M.S.; Abdullah, R. Mineralogy and Major Element Geochemistry of the Oligocene Barail Group Sandstones from the Sylhet Trough, Bengal Basin: Provenance and Tectonic Implications. Minerals 2024, 14, 985. https://doi.org/10.3390/min14100985

AMA Style

Mohinta S, Sayem ASM, Rahman MJJ, Alam MS, Abdullah R. Mineralogy and Major Element Geochemistry of the Oligocene Barail Group Sandstones from the Sylhet Trough, Bengal Basin: Provenance and Tectonic Implications. Minerals. 2024; 14(10):985. https://doi.org/10.3390/min14100985

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Mohinta, Sunjuckta, Abu Sadat Md Sayem, M. Julleh Jalalur Rahman, Md Sha Alam, and Rashed Abdullah. 2024. "Mineralogy and Major Element Geochemistry of the Oligocene Barail Group Sandstones from the Sylhet Trough, Bengal Basin: Provenance and Tectonic Implications" Minerals 14, no. 10: 985. https://doi.org/10.3390/min14100985

APA Style

Mohinta, S., Sayem, A. S. M., Rahman, M. J. J., Alam, M. S., & Abdullah, R. (2024). Mineralogy and Major Element Geochemistry of the Oligocene Barail Group Sandstones from the Sylhet Trough, Bengal Basin: Provenance and Tectonic Implications. Minerals, 14(10), 985. https://doi.org/10.3390/min14100985

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