1 Introduction

Provenance studies of sandstones are crucial for understanding the source rock complex, chemical weathering, and tectonic setting. The detrital modes and geochemistry of terrigenous sediments are widely used to constrain the provenance of clastic rocks (Dickinson & Suczek, 1979; Ingersoll & Suczek, 1979; Dickinson, 1985; Roser & Korsch, 1986; Suttner & Dutta, 1986; Hayashi et al., 1997; Armstrong-Altrin et al., 2017, 2021; He et al., 2019; Noa Tang et al., 2020; Sayem et al., 2023). In addition, mineralogical and major element abundances of siliciclastic rocks were utilized to decipher the tectonic settings of the source terrain (Ingersoll & Suczek, 1979; Murphy, 2000; Roser & Korsch, 1986; Verma & Armstrong-Altrin, 2013). Furthermore, sediments originate from parent rocks through weathering processes, making them suitable candidates for provenance evaluation (Nesbitt & Young, 1982, 1984; Harnois, 1988; Wronkiewicz & Condie, 1987; Fedo et al., 1995; Cullers, 2000; Singh, 2010; Sayem et al., 2018).

The Pliocene Tipam Sandstone Formation is extensively exposed in the Chittagong Tripura Fold Belt (CTFB) of the Bengal Basin (Fig. 1c). Previous sedimentological studies from the CTFB have mainly focused on the underlying Miocene Surma Group sediments and the overlying Dupitila Formation (Alam et al., 2003; Dina et al., 2016; Rahman et al., 2014a, 2014b, 2020; Sayem & Rahman, 2012; Sayem et al., 2022; Yang et al., 2019, 2020; Yeasmin et al., 2024). In contrast, little is known about the Pliocene Tipam Sandstone Formation (Rahman et al., 2020). So far, no provenance studies have been conducted that integrate its petrography, geochemistry, and tectonic context. However, the origin of the CTFB sediments remains unclear. Numerous studies have indicated that the underlying Miocene Surma Group sediments were mainly derived from Himalayan Orogen (Najman et al., 2008; Rahman & Faulp, 2003; Rahman et al., 2014a, 2014b; Yang et al., 2019). In contrast to the Surma Group sediments, the younger zircon ages (< 200 ma) identified in the Tipam Sandstone Formation (Najman et al., 2012; Rahman et al., 2020) suggest a secondary detrital input from the Indo-Burma Ranges (IBR). Thus, the mixing of IBR and Burma magmatic inputs with Himalayan-originated detritus in the CTFB is still debated. The main purpose of this study is to investigate detailed petrography and major element geochemistry of the Pliocene Tipam Sandstone Formation from the Chittagong Tripura Fold Belt of the Bengal Basin. The major objectives are to infer the provenance, the intensity of chemical weathering, and tectonic environments of the Bengal Basin during the Pliocene.

Fig. 1
figure 1

a Location map of the Bengal Basin and its regional setting: b tectonic elements of the Bengal Basin and its surrounding areas (modified after Johnson & Alam, 1991; Sayem et al., 2022); c traditional stratigraphy of the CTFB [the red squares indicate the sampling structures; where SK, Sitakund Anticline; SP, Sitapahar; Anticline; BB, Bandarban Anticline; IN, Inani Anticline]

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2 General geology

The geological evolution of the Bengal Basin (Fig. 1a) is intricately linked to the initial collision between the Indian and Eurasian plates, followed by subsequent interaction with the Burmese plate. Tectonically, the Bengal Basin is divided into three geotectonic regions, namely, the Indian Platform to the northwest; the Sylhet Trough and the Hatia Trough in the Central Deep Basin; and the Chittagong Tripura Fold Belt (also known as the Eastern Fold Belt, EFB) to the southeast (Alam et al., 2003) (Fig. 1b). The structural configuration of the Bengal Basin commenced with the rifting of the Gondwanaland during the Early Cretaceous period (Abdullah et al., 2022; Shamsuddin & Abdullah, 1997). The ongoing northward movement and eventual collision between the Indian and Eurasian plates led to the east–west oriented Himalayan Mountain ranges (Curray, 1991) during the Eocene. These orogenic processes were the primary contributors of sediments, leading to sedimentation in both the Indian Platform and the Central deep basinal part of the Bengal Basin (Abdullah et al., 2022; Alam et al., 2003; Najman et al., 2008, 2012). During the Oligocene–Miocene, the oblique collision between the Indian and the Burmese plates resulting the NNW-SSE trending Indo-Burman Ranges (Alam et al., 2003; Davies et al., 2003) to the east. In the Miocene, the Bengal Basin transformed into a remnant ocean basin due to the ongoing subduction of the Indian plate under the Burmese plate, and the CTFB became the major sediment depocenter (Alam et al., 2003). During the Mid-Pliocene, a fluvio-deltaic environment emerged as the ultimate marine regression, causing the sediment depocenter to relocate south of the basin (Alam et al., 2003; Davies et al., 2003; Abdullah et al., 2021a). By the Early-Middle Pleistocene, the persistent subduction of the Indian plate caused the compression of sediments resulting in their upliftment to form NNW-SSE trending anticlines within the CTFB (Abdullah et al., ).

Tectonically, the CTFB represents the western flank of the Indo-Burman Ranges and is separated from the eastern flank by the Kaladan Fault (Fig. 1b). The western boundary of the CTFB is marked by the Pleistocene Lalmai Hills (Abdullah et al., 2021b), while the Dauki Fault defines its northern limit. The CTFB in the Bengal Basin (Fig. 1b) contains an extremely thick (~ 3500 m) accumulation of Neogene clastic deposits (Alam et al., 2003; Davies et al., 2003), with sediments derived from the Indo-Burman Ranges and the eastern Himalayas (Reimann, 1993; Alam et al., 2003; Najman et al., 2008; Rahman et al., 2020; Yang et al., 2020. Because of the absence of a distinctive marker horizon and limited biostratigraphic zonation, the age correlation of these sediments remains unknown. The conventional stratigraphic scheme followed for the CTFB rock successions is illustrated in Fig. 1c and summarized in Table 1. The middle Miocene Bhuban Formation is the oldest rock unit in the CTFB followed by the Late Miocene Bokabil Formation (Reimann, 1993; Alam et al., 2003). The studied Tipam Group unconformably overlays the Bokabil Formation. The sedimentary process of this Group was associated with a marine regression (Davies et al., 2003; Gani & Alam, 2003). The Tipam group is divided into two formations, namely the Tipam Sandstone Formation and the Girujan Clay Formation. The Tipam Sandstone Formation is characterized by a yellowish grey to yellowish brown color, fine- to medium-grained, parallel bedded, large-scale trough, and planar cross-bedded sandstones (Fig. 2a–d). The mottled nature Girujan Clay Formation is poorly exposed in the CTFB. A braided fluvial environment has been interpreted for the Tipam Sandstone Formation (Alam et al., 2003; Davies et al., 2003; Gani & Alam, 2003). The Tipam Group is unconformably overlain by the Plio-Pleistocene Dupitila Sandstone Formation in the CTFB.

Table 1 Stratigraphic succession of the CTFB (modified after Johnson & Alam, 1991; Reimann, 1993; Alam et al., 2003; Davies et al., 2003; Gani & Alam, 2003)
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Fig. 2
figure 2

Outcrop photographs of the Tipam Sandstone Formation from Bandarban Anticline (a, d), Sitapahar Anticline (b) and Sitakund Anticline (c)

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3 Methodology

The sandstone samples were collected from the Tipam Sandstone Formation exposed in Bandarban anticline of the CTFB (Fig. 1b, c). The bulk samples were air-dried at room temperature prior to laboratory analysis. They were then treated with Canada Balsam to harden them, as most of the Tipam sandstones were moderately compacted. The rock samples were cut into smaller chips and polished with grinding powder of different sizes until they reached the desired thickness of 0.03 mm. Finally, the polished chips were mounted on glass slides using Araldite and covered with thin glass to avoid dust contamination. The modal composition of 20 samples was analyzed using a MEIJI ML 9000 polarizing microscope. At least 400 grains from each sample were examined using the point-counting method. The mineralogical compositions of the Tipam sandstones are listed in Table 2.

Table 2 Modal compositions (%) of the Tipam sandstones in the CTFB of the Bengal Basin
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On the other hand, 22 sandstone samples were chosen for geochemical analysis using X-ray fluorescence (XRF). The samples were crushed and powdered with a planetary ball mill (PM-200, Retsch, Germany) for 20 min and then treated with 30% HCl and 10% H2O2 to remove organic matter. The powdered samples were mixed with stearic acid in a 1:10 ratio and pulverized for at least two minutes. The mixture was then ladled into a 30 mm aluminum cap and sandwiched between two tungsten carbide pellets using a manual hydraulic press at 10–15 tons per square inch for 2 min. After slowly releasing the pressure, the pellet was prepared for X-ray analysis. The major elements were determined using an X-ray fluorescence (XRF) spectrometer, following the procedures of Goto and Tatsumi (1994, 1996), with a Rigaku ZSX Primus XRF machine equipped with a 4 kW Rh-anode X-ray tube at the Institute of Mining, Minerals and Metallurgy in Joypurhat, Bangladesh. A voltage of 40 kV and a current of 60 mA were employed to determine the heavy and light elements, respectively. Geological Survey of Japan (GSJ) Stream Sediments and USGS Rock Standards have been used to obtain the results for this study. Analytical uncertainty for major elements is about 2%. About 100 g of each sample was heated at 1000 °C to obtain the loss on ignition (LOI) before the treatment. Table 3 shows the percentage of the major elements for the Tipam sandstones.

Table 3 Geochemical compositions (%) of the Tipam sandstones from the CTFB of the Bengal Basin
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4 Results

4.1 Sandstone petrography

The Tipam sandstones are fine- to medium-grained, yellowish-grey to yellowish-brown in color (Fig. 2a–d). Quartz (Q) is the most dominant constituent (59.94%) in the Tipam sandstones and occurs as both monocrystalline and polycrystalline grains. Monocrystalline quartz (Qm) grains (Fig. 3a, b) make up ~ 56.92% of these sandstones. In contrast, polycrystalline quartz (Qp) covers 4.02% of the total lithic grains. Feldspar (F) covers about 8.93% of the total grains, whereas potassium feldspar (6.15%) dominates over the plagioclase feldspar (2.78%).

Fig. 3
figure 3

Photomicrographs showing the detrital composition of the Tipam sandstones: a monocrystalline quartz (Qm), polycrystalline quartz (Qp), and white mica (WM); b K-feldspar (K), biotite (B), mica-schist (Lm); c angurlar to subangular plagioclase feldspar (P), and white mica (WM); d sedimentary lithic grains (Ls), and clay cement (Ccl); e graphite-mica schist (Lm), and K-feldspar; f chert and white mica (WM)

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After quartz, micas (13.31%) are the dominant constituents in the Tipam sandstones, where white mica (WM, Fig. 3a, c, f) and biotite (B, Fig. 3b) occur in almost equal amounts with 6.68% and 6.63%, respectively. Among the lithic grains (L), sedimentary grains (Ls) dominate in the investigated sandstones with an average value of 7.42%. Ls grains (Fig. 3d) are present as shale, siltstone, and chert. Metamorphic rock fragments (Lm) comprise 2.51% of the detrital grains, and commonly occur as mica-schist (Fig. 3b), graphite-schist, quartz-mica schist, quartz-graphite-schist, quartz-graphite-mica schist (Fig. 3e), etc. However, volcanic lithic grains were not identifiable in the investigated sandstones. Chert (Fig. 3f) constitutes 3.64% of the total framework grains.

Chlorite grains (Ch) occur in minor amounts (0.61%). Detrital carbonate (C) is present only in a few samples (avg. 0.35%). The sandstones contain a small amount of matrix. Clays (Fig. 3d) are the dominant cement in the studied sandstones.

4.2 Major element geochemistry

The geochemical analysis reveals the highest concentrations of SiO2 with an average value of 74.99% (Table 3). Next to SiO2, Al2O3 is the dominant oxide in the Tipam sandstones, where the values range from 6.67 to 14.39% (avg. 11.24%). The concentration of Fe2O3 is relatively high (average 7.58%) and attains a wide range from 5.42 to 10.08%. The concentration of K2O (avg. 3.00%) dominates over the Na2O (0.70%). The average content of CaO and MgO represents 0.92% and 0.56%, respectively. The higher concentration of K2O and lower values of Na2O and CaO are congenial with the higher percentage of K-feldspar than plagioclase feldspar. TiO2 presents as a minor amount with an average value of 0.74%. The concentrations of MnO (avg. 0.11%) and P2O5 (avg. 0.08%) are very low in the Tipam sandstones.

The linear correlation matrix of the major oxides against Al2O3 for the Tipam sandstones is displayed in Table 4, where almost all the elements show positive correlations except SiO2. The strong negative correlation of SiO2 (r = − 0.92) with Al2O3 suggests that hydrodynamic fractionation and sorting effects may control their distribution (Hossain, 2019). On the other hand, the moderate positive correlation between Al2O3 and Fe2O3 (r = 0.49) suggests an increase in aluminous clays or heavy minerals in the weathering products (Hossain et al., 2017). The strong positive correlations of Al2O3 with TiO2 (r = 0.69), and P2O5 (r = 0.50) suggest the presence of heavy minerals in the investigated sandstones (Sayem et al., 2023). The positive correlations of MgO (r = 0.55), CaO (r = 0.38), Na2O (r = 0.45), and K2O (r = 0.83) indicate that these oxides are associated with phyllosilicate minerals (Hossain, 2019). MnO exhibits a very poor relationship (r = 0.09) with Al2O3.

Table 4 Linear correlation matrix of the major oxides of the Tipam sandstones from the CTFB of the Bengal Basin
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The major element concentrations of the Tipam sandstones were normalized to the upper continental crust (UCC, Taylor & McLennan, 1985) values and displayed in Fig. 4. In general, most of the major oxides are depleted relative to the upper continental crust values. The SiO2 concentration is slightly higher than the UCC value, indicating higher concentrations of quartz-rich sediments. In contrast, Al2O3 is slightly depleted, probably with lesser amounts of clay components in the studied sandstones (Hossain, 2019; Sayem et al., 2023). Most of the sandstones are enriched in TiO2, Fe2O3, and MnO probably due to the presence of a higher proportion of aluminosilicate minerals and iron cement (Sayem et al., 2023). The strong depletion of MgO, CaO, and Na2O represents lesser concentrations of plagioclase feldspar in the investigated samples, which agreed well with the petrographic results (Table 2).

Fig. 4
figure 4

Major element concentrations of the Tipam sandstones normalized to Upper Continental Crust (UCC) values (after Taylor & McLennan, 1985). The red line indicates the average depletion/enrichment of the major oxides

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5 Discussions

5.1 Sediment classification and maturity

The sandstone classification proposed by McBride (1963) is widely accepted, where he classified sandstones using the QFL plot. The average detrital composition of the Tipam sandstones is Q77F11L12. According to this QFL diagram, the examined sandstones are primarily categorized as sublitharenite, with a few samples appearing in the lithic subarkose field (Fig. 5a). Additionally, the scatter plot of log(SiO2/Al2O3) versus log(Fe2O3/K2), commonly employed for sandstone classification (Herron, 1988), which indicate that the Tipam sandstones are predominantly litharenite. However, a few samples are scattered in the Fe-sand field (Fig. 5b). The petrographic and geochemical data are also used to determine sediment maturity. Long et al. (2012) introduced the ICV (index of compositional variability) to measure sediment maturity, where the ICV > 1.0 indicates immature sediments, and vice versa. The ICV value for the investigated sandstones ranges between 0.82 and 1.50, with an average of 1.27 (Table 3), indicating immature sandstone. The bivariate plot of ICV vs. CIA (Long et al., 2012) also reflects that the Pliocene Tipam sandstones are predominantly immature (Fig. 6a).

Fig. 5
figure 5

Classification of the Tipam sandstones: a detrital classification after McBride (1963); b geochemical classification after Herron (1988)

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Fig. 6
figure 6

Sandstone maturity diagrams of a CIA (Chemical Index of Alteration) versus ICV (Index of Compositional Variability) (after Long et al., 2012); b Si2O/Al2O3 versus Qt (after Potter, 1978); c Si2O/Al2O3 versus (F + Lt) (after Potter, 1978); d (Na2O + K2O) versus (F + Lt) (after Pettijohn et al., 1972)

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Potter (1978) correlated the SiO2/A12O3 ratio with total quartz (Qt), and total feldspar plus total rock fragments (F + Lt) to examine the sediment maturity. SiO₂/Al₂O₃ is used to assess the relative abundance of silica (SiO₂) to alumina (Al₂O₃) in sandstones. The ratio is very high for quartz arenites and low for immature sandstones (Potter, 1998). Quartz is very resistant to weathering and often dominates in well-rounded and well-sorted mature sandstones. In contrast, feldspar (F) and lithic grains (Lt) are less resistant to weathering and are usually incorporated in immature sandstones. Thus, for mature sandstone, total quartz content (Qt) shows a positive correlation with SiO₂/Al₂O₃. On the other hand, a negative correlation exists between (F + Lt) and SiO₂/Al₂O₃. The results of this study show a moderate negative correlation (r = − 0.45) between SiO₂/Al₂O₃ and Qt (Fig. 6b), whereas a moderate positive correlation (r = 0.24) persisted between SiO₂/Al₂O₃ and F + Lt (Fig. 6c). These results indicate that the investigated sandstones are compositionally immature. In addition, the alkali content (Na2O + K2O) is very much applicable for the index of chemical maturity (Pettijohn, et al., 1972), where (Na2O + K2O) is positively correlated with (F + Lt) for mature sandstones. This relationship shows a negative correlation (r = − 0.17) for the investigated sandstones, which further supports the immature nature. Furthermore, angular to subangular grains (Fig. 3) and a relatively higher abundance of unstable feldspar and ductile grains indicate an immature nature. The immature nature of the investigated sandstones suggests that they were deposited close to their source region (Boggs, 2009).

5.2 Provenance

Prior to the collision, the Bengal Basin sourced its sediments from the Indian Craton in the Early Cenozoic (Uddin & Lundberg, 1998). The collision between the Indian and Eurasian plates resulted in the uplift of the Himalayas during the Oligocene–Miocene period (Critelli & Garzanti, 1994; Garzanti et al., 1987; Najman et al., 2012; Searle et al., 1999; Yang et al., 2019). Many of the Himalayan rivers, including the paleo-Brahmaputra, originated from the southern slopes of the Himalayas. These rivers transported detritus from the Higher and Lesser Himalayas, depositing it in the Sub-Himalayan Neogene foreland basins (Bracciali et al., 2015; Garzanti & Andò, 2007). During the Miocene, the paleo-Brahmaputra River mainly carried Himalayan and Trans-Himalayan sediments into the Bengal Basin (Bracciali et al., 2015; Garzanti & Andò, 2007; Najman et al., 2012; Rahman et al., 2020; Uddin & Lundberg, 1998; Yang et al., 2019). The river changed its course due to the uplift of the Shillong Plateau in the Pliocene–Pleistocene (Johnson & Alam, 1991), which led to the southward advance of the Eastern Himalaya's deformation front. Meanwhile, the deformation front of the Indo-Burman Ranges (IBR) progressively moved westward and began contributing IBR detritus to the Bengal Basin (Rahman et al., 2020; Uddin & Lundberg, 1998; Yang et al., 2019) during the Pliocene–Pleistocene period.

The modal composition of clastic rocks serves as a valuable clue in understanding their provenance (Dickinson & Suczek, 1979; Ingersoll & Suczek, 1979; Dickinson, 1985). The triangular diagrams of QFL and QmFLt are commonly used to distinguish recycled orogen, continental block, and magmatic arc provenances (Dickinson, 1985). According to the QFL diagram, the analyzed samples suggest a recycled orogen source (Fig. 7a). Meanwhile, the QmFLt diagram reveals a predominance of high monocrystalline quartz and feldspar, and a moderate level of lithic grains, characteristics of quartzose recycled provenance (Fig. 7b).

Fig. 7
figure 7

Provenance discriminating diagrams for the Tipam sandstones: a, b Q–F–L and Qm–F–Lt triangular plots (after Dickinson, 1985); c major element discrimination function F1 versus F2 (after Roser & Korsch, 1988); d bivariate diagram of SiO2 versus Al2O3/TiO2 (after Le Bas et al., 1986)

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Apart from modal composition, the provenance of clastic rocks also mirrored their geochemical composition (Armstrong-Altrin et al., 2021; Hayashi et al., 1997; Li et al., 2020; Nesbitt & Young, 1982, 1984; Rahman et al., 2020; Roser & Korsch, 1986, 1988; Sayem et al., 2023; Suttner & Dutta, 1986; Yeasmin et al., 2024). Based on major element composition, Roser and Korsch (1988) proposed a provenance discriminant function diagram which is divided into four distinct provenance types, such as felsic igneous, intermediate igneous, mafic igneous, and quartzose recycled. Within this diagram, the majority of the studied samples are dispersed prominently within the quartzose recycled field (Fig. 7c), with two samples plotted within the mafic igneous source, and two in the intermediate igneous. Although only one sample scatter at the felsic field most of the samples show affinity along the boundary between the quartzose sedimentary and felsic igneous provenances.

The Al2O3/TiO2 is graphed against SiO2 (Le Bas et al., 1986) to delineate the source rock composition. This diagram (Fig. 7d) displays that felsic igneous is the dominant source for the Tipam Sandstone Formation. In addition, the TiO2/Zr ratio (Hayashi et al., 1997) serves as another valuable proxy in determining the provenance of clastic rocks, with values below 55 indicative felsic igneous, between 55 and199 suggesting intermediate igneous, and values exceeding 200 pointing to mafic igneous sources. For the Tipam sandstones, the TiO2/Zr ratio ranges between 5.91 and 39.67 (Table 3), indicating a felsic igneous provenance. Conversely, the metamorphic lithic grains (Lm) in the studied sandstones indicate the presence of meta-sedimentary and metamorphic source rocks.

Based on the above discussions, it is evident that the Tipam sandstones were derived predominantly from recycled sedimentary/meta-sedimentary, felsic igneous, and metamorphic sources. The abundance of K-feldspar (Table 2) in the Pliocene Tipam sandstones, compared to Miocene Surma Group sandstones (Dina et al., 2016; Sayem & Rahman, 2012), suggests a granitic source, likely derived from deep crustal Higher Himalayan Crystalline Sequence (HHCS, Uddin & Lundberg, 1998). The absence of granitic terrain in the IBR devoids the possibility of it being a source rock for the Pliocene sediments in the Bengal Basin. Additionally, the absence of volcanic lithic fragments (Lv) excludes the IBR as a potential source. In contrast, angular to subangular shape, and fresh feldspars (Fig. 3b, c) suggest nearby sources, indicating that the Sub-Himalayan sedimentary and the IBR may be potential sources of the Tipam sandstones. Rahman et al. (2020) identified younger zircon ages (< 200 ma) in the Tipam sandstones from the CTFB, suggesting IBR as a possible source. The rich of monocrystalline quartz, sedimentary and metamorphic lithic fragments (Table 2), points to the Lesser Himalayan Sequence (LHS) as a potential source, which is composed of sedimentary and low-grade metamorphic rocks (Singh, 2010; Uddin & Lundberg, 1998). Geochemical relationships among various major oxides can also reveal the dominant source area (Singh, 2010). The scatter plot of Na2O vs. K2O (Fig. 8a) highlights LHS as a dominant source. The CaO versus TiO2 plot also indicates a significant contribution of sediments from the LHS (Fig. 8b), with only two samples representing HHCS. Therefore, the mineralogical and geochemical findings of this study indicate that the CTFB primarily sourced its sediments from the LHS, with smaller contributions from the HHCS, Sub-Himalayan sources, and the IBR during the Pliocene.

Fig. 8
figure 8

Source area discrimination diagrams (a, b) for the Tipam Sandstone Formation (fields are after Singh, 2010), where LHS, Lesser Himalayan Sequence; HHCS, Higher Himalayan Crystalline Sequence

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5.3 Paleoweathering

Different chemical indices were employed to assess the degree of chemical weathering (Table 3). Chemical Index of Alteration (CIA) reflects the progressive transformation of feldspar into clay minerals (Nesbitt & Young, 1982). CIA values for the sandstones of the Tipam Formation range from 58.09 to 77.42, averaging 66.56, suggesting that these sandstones likely originated from a weak to moderately weathered region. Likewise, the Chemical Index of Weathering (CIW) (Harnois, 1988) assesses the extent of feldspar alteration into clay minerals. CIW values span from 70.83 to 96.51, with an average value of 82.03, indicating moderate chemical weathering. On the other hand, Fedo et al. (1995) suggested the Plagioclase Index of Alteration (PIA) as an alternative to CIW, which aims to specifically clarify plagioclase weathering. PIA values for the studied sandstones ranging between 62.66 and 95.37 (average 76.52) indicate weak to moderate weathering.

Besides the weathering indices, the A–CN–K and (A–K)–C–N diagrams (Fig. 9a, b), also reflect weak to moderate chemical weathering in the source area for the Tipam sandstones. Similarly, the ICV versus CIA diagram (Fig. 6a) also reveals weak to moderate chemical weathering. The weathering intensity of the investigated sandstone is further assessed by the A–CNK–FM triangular plot (Fig. 9c; Nesbitt & Young, 1982), where the samples fall above the feldspar-chlorite but below feldspar-smectite join line. These results further suggest a weak to moderate weathering profile in the source area. The MFW (mafic–felsic–weathering) triangular plot is used to assess the degree of chemical weathering (Ohta & Arai, 2007), where the “M” and “F” and “W” apexes indicate mafic, felsic, weathering parent igneous rocks, respectively. In this plot (Fig. 9d), the Tipam sandstone samples are scattered at a considerable distance from the granite-basalt join line and show moderate to strong weathering trends toward the proximity of the weathering apex (W). Although most of the weathering indices and discriminant diagrams show weak to moderate chemical weathering for the investigated samples only a few samples suggest strong weathering as reflected in the PIA values as well as in Fig. 9d. These variations in weathering intensity across different samples might be linked with the complex interplay of weathering processes and sediment recycling.

Fig. 9
figure 9

Triangular plots of: a A(Al2O3) − CN(CaO* + Na2O) − K(K2O) (Nesbitt & Young, 1984); b (A–K)(Al2O3 − K2O) − C(CaO*) − N(Na2O) (Fedo et al., 1995); c A(Al2O3) − CNK(CaO* + Na2O + K2O) − FM(Fe2O3 + MgO) (Ohta & Arai, 2007); where, CaO*, CaO in silicate phase; IWT, ideal weathering trend; UCC, upper continental crust composition; Ka, kaolinite; Gb, gibbsite; Chl, chlorite; Sm, smectite; Mu, muscovite; Pl, plagioclase feldspar; Ksp, K-feldspar; An, anorthite; By, bytownite; La, labradorite; Ad, andesine; Og, oligoclase; Ab, albite; Bio, biotite; Hbl, hornblende; Cpx, clinopyroxene; M, mafic igneous rock; F, felsic igneous rock; W, weathering of parent rocks

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The relative abundances of quartz, feldspar, and lithic grains in sandstones have frequently been used to demonstrate the paleoclimate conditions of the provenance (Basu, 1985; Suttner & Dutta, 1986; Yeasmin et al., 2024). One of the simplest methods for interpreting source-area climate is to use a bivariant plot of Qp(F + Lt) versus Qt(F + Lt) (Suttner & Dutta, 1986). This diagram primarily suggests sub-humid climate conditions in the source areas of the Tipam sandstones (Fig. 10a). On the other hand, the ln(Q/L) ln(Q/F) plot by Weltje et al. (1998) indicates that the sediments of the Tipam Sandstone Formation were eroded from slightly weathered source rocks in areas with moderate relief and sub-humid climate condition (Fig. 10b).

Fig. 10
figure 10

The scatter plots displaying the paleoclimate conditions: a Qt/(F + Lt) versus Qp/(F + Lt) after Suttner and Dutta (1986); b ln (Q/L) versus ln (Q/F) after Weltje et al. (1998), where, 0, arid climate, high relief, unweathered; 1, sub-humid climate, moderate relief, slightly weathered; 2, humid climate, low relief, moderately weathered; 4, intensely weathered

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5.4 Tectonic setting

The tectonic environment of a depositional basin is widely inferred by analyzing the modal composition of clastic rocks (Dickinson, 1985; Dickinson & Suczek, 1979; Ingersoll & Suczek, 1979; Yeasmin et al., 2024). QpLvmLsm and LmLvLs triangular plots (Ingersoll & Suczek, 1979) are commonly used to differentiate different kinds of tectonic settings. According to the QpLvmLsm plot, the Tipam Formation sandstones are indicative of collision orogen (Fig. 11a). A suture belt setting is suggested by comparatively higher Ls, low Lm, and absence of Lv lithic grains (Fig. 11b; Table 2). The collision orogen and suture belt settings is indicative of active continent margin (Dickinson, 1985; Dickinson & Suczek, 1979; Ingersoll & Suczek, 1979).

Fig. 11
figure 11

Tectonic setting discrimination diagram for the Tipam sandstones: a, b Qp–Lvm–Lsm Qm–Lv–Ls triangular plots after Ingersoll and Suczek (1979); c SiO2/Al2O3 versus K2O/ Na2O bivariate plot after Roser and Korsch (1986), where, A1, continental island arc, A2, oceanic island arc; d SiO2 versus (K2O/Na2O) bivariate plot after Murphy (2000)

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The geochemical concentrations are also important for tectonic setting delineation (Bhatia, 1983; Bhatia & Crook, 1986; Murphy, 2000; Roser & Korsch, 1986; Verma & Armstrong-Altrin, 2013). The tectonic discriminate diagram of Roser and Korsch (1986) favors an active margin tectonic setting for Tipam sandstones (Fig. 11c). The active margin setting is also confirmed by SiO2/Al2O3 versus log (K2O/Na2O) diagram (Murphy, 2000) (Fig. 11d). Despite a few samples being spread out in a passive margin environment in both diagrams, this aligns with the tectonic past of the Himalayan Orogen. Prior to the collision of the Indian and Eurasian Plates, the northern part of the Indian Plate exhibited a passive margin environment, where Paleozoic to Early Cenozoic sedimentary rocks were laid down along the Tethyan Himalaya (Garzanti et al., 1987; Najman et al., 2012; Uddin & Lundberg, 1998). These rocks began to rise with the uplift of the Himalayas. The earlier-mentioned provenance analysis suggests that the Tipam Sandstone Formation has recycled sources. The reworked sediments from older passive margin deposits might have been mixed into the younger active margin sediments, resulting in mixed signals in the geochemical data. Furthermore, Verma and Armstrong-Altrin (2013) proposed a new model of the tectonic setting for the depositional basins by using the discriminant function of DF1 (Arc–Rift–Col)m1 versus DF2 (Arc–Rift–Col)m1, where the arc and collision fields represent the active margin, while the rift field indicates the passive margin setting. In this plot (Fig. 12), the studied samples are entirely plotted in the collision field, which implies an active margin tectonic setting for the Tipam sandstones.

Fig. 12
figure 12

Tectonic discriminant diagram of DF1 (Arc–Rift–Col)m1 versus DF2 (Arc–Rift–Col)m1 after Verma and Armstrong-Altrin (2013)

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6 Conclusions

The Tipam sandstones from the CTFB of the Bengal Basin display an average modal composition of Q77F11L12, which classifies them as sublitharenite to lithic subarkose. The mineralogical behavior of the studied sandstones is consistent with the geochemical composition, characterized by higher concentrations of SiO2, Al2O3, and Fe2O3, and lower values of TiO2, MnO, CaO, MgO, Na2O, and K2O values. The abundance of feldspar and ductile grains, the negative correlation between Si2O/Al2O3 and total quartz (Qt), and the higher ICV (> 1.0) values indicate that the Tipam sandstones are texturally and compositionally immature. The provenance discriminant diagrams suggest a recycled sedimentary provenance with dominance from felsic igneous sources. The weathering indices of CIA, PIA, and CIW, along with the triangular plots of A–CN–K, (A–K)–C–N, A–CNK–FM, and MFW, reflect a weak to moderate degree of chemical weathering under sub-humid climate conditions. The petrographic and geochemical results suggest that the Tipam sandstones were deposited in an active margin tectonic setting, with the majority of sediments originating from the Lesser Himalayan Sequence and a lesser portion from the Higher Himalayan Crystalline Sequence, Sub-Himalaya, and Indo-Burma Ranges during Pliocene period.