Geochemical and Mineralogical Insights into Organic Matter Preservation in the Gondwana and Post-Gondwana Shale of the Lesser Himalayas, Nepal

Abstract

The depositional environments, weathering and provenance, organic matter enrichment, and preservation in the Gondwana and post-Gondwana units of the Lesser Himalayas, Nepal, are studied through geochemical and mineralogical analyses using petrography, X-ray diffraction, XRF, and ICP-MS. Mineralogical findings indicate that shales comprise 55% to 72% clay, 25% to 55% quartz, and less than 10% carbonate minerals, with a significant presence of illite, suggesting a transition from fluvial to shallow marine environments during post-Gondwana deposition. The thin sections of the post-Gondwana sandstone reveal an increase in quartz, feldspar, and plagioclase content, with rounded to sub-angular quartz grains indicating moderate transportation before lithification, resulting from the Indo-Asian collision. Geochemical data, including major, trace, and rare earth elements (REE), along with bivariate discrimination diagrams, reveal distinct environmental changes; Gondwana sediments exhibit oxic, arid conditions with continental provenance, while post-Gondwana deposits indicate humid environments favorable for organic matter enrichment, primarily sourced from felsic-intermediate igneous rocks. The TOC is less than 1 wt.% in the Gondwana and is 0.75 to 2 wt.% in the post-Gondwana shale, indicating better organic matter preservation. The existing geological structural data and the research findings highlight the pivotal role of Himalayan tectonism in enhancing the thermal maturity and hydrocarbon generation potential of organic-rich post-Gondwana shales, attributed to their substantial organic matter content.

1. Introduction

Geochemical analyses have been extensively used worldwide, reflecting detrital input, paleo-salinity, paleo-water depth, paleo-climatic conditions, and paleo-redox conditions in organic-rich shales [1,2,3,4]. These analyses have become crucial for understanding basin evolution, identifying source rocks, and determining paleo-deposition [5,6]. Depositional factors such as paleo-productivity, terrestrial input, paleo-salinity, and preservation conditions like redox conditions and paleo-water depth significantly affect the accumulation and preservation of organic matter in sedimentary deposits [7,8]. Similarly, the depositional environments modulate the distribution and concentration of elements in sedimentary deposits. Inorganic geochemical proxies, including specific element ratios and abundances, infer paleoenvironmental conditions and understand the processes influencing organic matter preservation and enrichment [9,10]. The major elemental oxides obtained from X-ray fluorescence (XRF) analysis of sedimentary rocks are essential for characterizing the provenance and discriminating the felsic, intermediate, and mafic parent rocks of the shales [11,12]. Additionally, the XRF reveals the degree of chemical weathering of source rocks [13,14] and documents the paleotectonic setting [15]. Similarly, the concentration of trace and rare-earth elements (REE) helps to determine the paleoenvironment and depositional conditions [16,17] in the sediments (clay-rich), which are valuable proxies to address their source characteristics and the tectonic setting of deposition [18]. The spatial and temporal distribution of sediments and their geochemical records can be used to restrict the complete exhumation history, foreland basin tectonics, and related foreland propagated thrust belts.

The Gondwana is a distinct stratigraphic framework primarily associated with the Gondwana supercontinent, which existed during the Late Paleozoic to Early Mesozoic era and consists of sedimentary rock layers with substantial lithological diversity, including sandstones, shales, clays, conglomerates, and coal seams. Gondwana and associated rocks, deposited in paralic facies and overlying Foreland Basin Sequences, have been identified within the Lesser Himalayan in different geographic locations in Nepal [19,20]. The Gondwana rocks are divided into three units based on deposition time: Lower Gondwana in the Late Carbonaceous–Permian, Upper Gondwana in the Jurassic–Lower Cretaceous, and post-Gondwana in the Paleocene–Middle Miocene [19]. The post-Gondwana is the foreland deposit, deposited during and after the India–Eurasian collision. The Gondwana exposures exist mainly within the basement of the Tansen syncloronium, lying unconformably over the Precambrian–Early Paleozoic Lakharpata Group [19]. The post-Gondwana Himalayan foreland basin is filled with hundreds to thousands of meter thicknesses of sediments that have preserved the records of Cenozoic Himalayan tectonics, such as exhumation and upliftment from the beginning to the termination stage of collision and post-collision [21]. Some of the past researchers have completed the stratigraphy, provenance analysis, and petrography of the Gondwana and a post-Gondwana in the Lesser Himalayas, Nepal [22,23,24,25]. Similarly, some past studies were centric on the impact of paleo-environment and climate [26], geochronology, detrital zircon U–Pb ages, Hf isotopic constraints, and trace element analysis [23,27,28]. In addition, some other literature focused on the structural and thermal evolution of the Gondwana sequence and the foreland basin in the Lesser Himalayas, Nepal [21,29,30,31,32]. However, the complete geochemical dataset still lacks the key features to comprehend the tectonic evolution of the Himalayas. Thus, the whole-rock geochemical analysis of the sandstone and shale sequence in a post-Gondwana can be used as a proxy to understand the provenance and tectonic evolution of the Himalayan orogeny [31,33].

Despite existing studies on the paleo-depositional environment and provenance analysis of Gondwana units in the Himalayan regions, there remains a significant gap in research concerning the enrichment and preservation of organic matter in the black shale of the Gondwana units, Lesser Himalaya, Nepal. This study addresses that gap by conducting a comprehensive geochemical and mineralogical characterization of shale and sandstone from various Gondwana units in the Tansen and Surkhet basins of the Lesser Himalayas. The research investigates explicitly the mineralogical and geochemical properties of these rock types to ascertain their provenance and paleo-weathering history. Utilizing advanced techniques such as petrography microscopy, X-ray Diffractometry (XRD), X-ray Fluorescence (XRF), and Inductively Coupled Plasma Mass Spectrometry (ICP-MS), this study seeks to enhance understanding of organic matter enrichment in these rock strata. The findings are expected to provide valuable insights into the region’s characteristics of hydrocarbon source rocks.

2. Geological Background

Tectonic and Stratigraphic Overview of Central Himalaya

The Nepal Himalayas occupy the central sector of the Himalayan arc and extend about 800 km which is divided into four tectonostratigraphic zones from south to north (Figure 1a): the Sub-Himalaya (SW), Lesser Himalaya (LH), Higher Himalaya (HH), and Tethys Himalaya (TH) [34]. The geological evolution is marked by the Gondwana sequence, deposited upon the Proterozoic Lesser Himalayan sequence during the Late Paleozoic to Mesozoic Era [35]. The Gondwana sequence in the Himalayas demonstrates discrete patches located in various geographical areas along the front of the Himalayan Thrust Fold Belt (HTFB). Several isolated Gondwana rock formations have been documented in the segments of western, central, and eastern Nepal, Sikkim/Darjeeling, Kameng, and the Siang region [19,20,36]. The Gondwana sequence is further divided into Lower and Upper Gondwana [19], characterized by initial glaciomarine deposits transitioning to meandering fluvial systems rich in coal, carbonaceous shale, and siltstone due to humid conditions [37]. Climate fluctuations led to changes in facies composition, with meandering river systems dominating, occasionally influenced by marine incursions.

The Tansen Basin exhibits distinct lithofacies, including conglomerate, coal, sandstone, carbonaceous shale, and gray shale-siltstone. The Tansen Group (Figure 1b), encompassing the Late Carboniferous to Early Miocene clastic rocks, is a reference for the Gondwana sequences in Nepal. Notable formations include the Sisne, Taltung, and Amile formations (Gondwana) and the Bhainskati and Dumri formations (post-Gondwana). The Sisne Formation comprises diamictite, shale, coal, and sandstone from glacio-marine to meandering deposits. It provides key insights into the paleoclimatic and depositional conditions of the region during the Late Carboniferous and Permian. In contrast, the Taltung Formation exhibits upward fining sequences indicative of fluvial activity, while volcanism is evidenced in the associated Aulis basalt [27,38], which points to volcanic activity contemporaneous with sediment deposition. The coal-rich Amile Formation originates from the Proterozoic and Cretaceous volcanic sources of Indian affinity [23]. During the Cretaceous to Palaeocene, the region experienced a humid and tropical climate that led to extensive chemical weathering of sandstone found within the Amile sediments. In contrast, the Bhainskati Formation consists of 100–150 m of organic-rich dark grey and black shales, representing the last marine sedimentary sequence in the southern Himalayas before and during the India-Asian collision [19]. The thick hematite- and kaolinite-rich oxisol at the uppermost part of this formation represents slight to gradual regression, indicating transitions to terrestrial conditions spanning approximately 15–20 million years [21,26]. Initially, the region experienced a transition from a marine to a brackish water environment during the Eocene, gradually shifting towards freshwater conditions at higher stratigraphic levels. This shift indicates a general marine regression, as evidenced by the presence of Nummulites and terrestrial mammal skeletal fragments in the sedimentary rocks, suggesting that the sea receded by the end of the Eocene [19]. The topmost unit, Dumri Formation, characterized by multistoried fluvial sandstones, laminated siltstone, and red-purple shale, was deposited after the Early Miocene unroofing of the Greater Himalayan sequence due to the MCT and the South Tibetan Detachment System (STDS) [31]. These post-Gondwana formations correlate with the Melpani, Swat, and Suntar formations in the Surkhet area (Figure 1c), indicating the regional geological consistency.

The post-Gondwana Bhainskati and Dumri Formations predominantly sourced from the erosion of crystalline rocks in the Himalayan orogenic belt during and after the collision of the Indian and Asian plates. These sediments are mainly clastic, comprising sandstone, mudstone, and siltstone, with varying clay mineral content, which reflects a dynamic sediment transport regime shaped by tectonic forces. The primary mode of sediment transportation was through fluvial systems, where river and stream processes were significantly influenced by tectonic deformation as the Himalayas uplifted. The Bhainskati Formation is characterized by evidence of shallow marine deposition, but the Dumri Formation features fluvial, floodplains, and possibly lacustrine environments, all resulting from active tectonic uplift leading to high-energy fluvial conditions that facilitated rapid sediment transport [19,21]. The clay minerals in the Bhainskati shale are primarily a product of diagenesis, occurring under relatively low temperatures and pressure. This mechanism leads to the transformation and alteration of initial sediments, including the breakdown of larger particles and the conversion of feldspar and volcanic minerals into clay minerals such as illite, smectite, and kaolinite [32,39]. Consequently, these diagenetic alterations significantly influence the mineralogical composition and character of the shale. Furthermore, the tectonic uplift of the Himalayas has resulted in increased pressures and temperatures within the sedimentary basins, which facilitate the recrystallization of clays and the cementation of sandstones, enhancing the overall complexity of the sedimentary rock assemblage in the Tansen Unit.

3. Samples and Methods

Extensive fieldwork was carried out in the section of Gondwana and post-Gondwana sequences in the Tansen unit (e.g., Butwal to Tansen section) and the Surkhet unit (Surkhet to Dailekh section) within the Lesser Himalaya, Nepal. Twenty-four samples (13 shales and 11 sandstone) from the Gondwana and the post-Gondwana units from the Tansen and Surkhet areas (Figure 2), and four samples from the Siwalik Group of the Tansen area (

Table 1. Global positioning system (GPS) coordinates and stratigraphy of Gondwana–Siwalik samples.

S. ID	Location		Rock Type	Age	Lithostratigraphic Division		Lithological Description
					Group	Formation
	Longitude (°E)	Latitude (°N)
F287	83.5447	27.7907	Shale	Late Carboniferous–Permian	Lower Gondwana	Sisne	Basal conglomerate, diamictite, fine-grained sandstone, and shale with few coal seams within shale beds, Bioturbated mudstone, and sandstone alternating band at the upper part
F300	83.4910	27.7922
F306	83.5440	27.7833
F297	83.5450	27.7910
F286	83.5462	27.7953	Shale	Jurassic– Early Cretaceous	Upper Gondwana	Taltung	Coarse to medium sandstone followed by fine sandstone–siltstone–mudstone– carbonaceous shale with volcanic tuff, overlain by conglomerate
F279	83.5437	27.7952
F316	83.5427	27.7963
F315	83.5414	27.7968	Sandstone	Early Cretaceous		Amile	Thick bedded ferruginous sandstone, a few coal seams are inter-band within quartzose sandstone, and a few shale layers are within sandstone.
F309	81.6045	28.6397
F282	81.5140	28.7347
F280	83.5164	27.7997	Shale	Middle Eocene	Post-Gondwana	Bhainskati	Black shale with few inter-bands of sandstone and coal seams was also observed. A few inter-layers of limestones overlain by the hematite band
F304	83.5327	27.7959
F298	83.5410	27.7965
F304	83.5327	27.7959
F301	81.5130	28.7376
F302	81.6152	28.6416
F303	81.5183	28.7387
F310	81.5149	28.7386	Sandstone				Alternating sandstone layer, gray to brown, fine to medium-grained sandstone and siltstone
F312	81.5195	28.7394
F314	83.5327	27.7959
F305	83.5927	27.7259
F311	81.5368	28.7385
F317	83.5156	27.8025	Sandstone	Early Miocene		Dumri	Purple to gray shale interbedded with gray sandstone, pebble conglomerates, and conglomeratic sandstones
F307	83.5291	27.7959
F281	83.4711	27.7219	Mudstone	Middle Miocene	Siwalik	Lower Siwalik	Mudstone and fine-grained sandstone inter-band.
F308	83.4711	27.7219	Sandstone
F285	83.4663	27.7432	Sandstone	Late Miocene–Pliocene		Middle Siwalik	Fine-coarse sandstone with inter-band of mudstone
F313	83.4715	27.7292

) were collected based on the formation, color, texture, and other morphological changes for subsequent mineralogical and geochemical analyses. Shales were dark gray to black, laminated, and sandstones were predominantly medium to coarse-grained, greyish-white, compact, and massive, with trough cross-stratified and planar cross-stratified bedding. The following methods were conducted in the Laboratory of Guizhou Coal Field Geological Bureau, Guizhou, China.

• Petrographic Analysis: Shale and sandstone samples were cut perpendicular to the bedding plane, and thin polished sections were prepared for the petrographic analysis. The polished thin sections were prepared according to the methods described by Littke et al. [41]. The analysis used reflected white light and fluorescence modes to observe mineral grains, organic matter, macerals, structures, pores, and fractures. Optical microscopy with polarized light was used to determine the mineralogical composition. Petrographic sections were prepared and observed under a polarized light microscope (ZEISS Axio Scope A1) in the Laboratory of China University of Mining and Technology, Xuzhou;
• X-Ray Diffraction (XRD): XRD was employed to assess the mineralogical characteristics of 13 shale and 13 sandstone samples. To prepare the samples for analysis, they were crushed and ground to a particle size of less than 200 mesh (74 µm). The mineralogical analysis utilized the X’ Pert Pro MPD DY3794 X-ray diffraction instrument (PANalytical, UK) to identify clay and non-clay minerals in sedimentary rocks, adhering to the SY/T 5163-2018 standard;
• Total Organic Carbon (TOC) Analysis: The TOC content of 11 representative shale samples was determined using an infrared sulfur analyzer, CS230 (LECO Corporation, Saint Joseph, MI, USA) following the methodologies outlined in the Chinese National Standard Method;
• X-Ray Fluorescence Spectrometry (XRF): XRF spectrometry was implemented to analyze oxides of major elements, including SiO₂, TiO₂, Al₂O₃, Fe₂O₃, FeO, MnO, MgO, CaO, Na₂O, K₂O, and P₂O₅ across nine shales and seven sandstone samples. Initially, the powdered samples were heated and fused into glass sheets using a mixture of anhydrous lithium tetraborate, lithium fluoride, and ammonium nitrate, which were subjected to heat in a muffle furnace to assess loss on ignition. Subsequently, the fused samples underwent heating to 800 °C for analysis with an ARL PERFORMX PerformX4200 XRF spectrometer instrument, (Thermo Fisher Technologies LTD, Waltham, MA, USA);
• Inductively Coupled Plasma Mass Spectrometry (ICP-MS): The analysis of trace elements, rare earth elements (REEs), and minor elements was performed using the NexION 2000 ICP-MS apparatus, (American PE companies, NY, USA) operating at 15–30 °C temperature. The process involved heating the powdered samples with hydrofluoric acid (HF) and nitric acid (HNO₃). After the initial evaporation to dryness, HNO₃ was reintroduced to the beaker and heated to 130 °C to dissolve the samples comprehensively. The resulting solution was then diluted with distilled water for trace element analysis.

4. Results

4.1. Petrography Analysis

The microscopic analysis of thin sections from Gondwana and post-Gondwana shale and sandstone reveals important sedimentary rocks’ characteristics. The petrological characteristics of the source rocks are vital in understanding the paleo-depositional environment, mineralogy, and their hydrocarbon potential. The rock types, mineral composition, and depositional environments shape the capacity of these formations to generate and store hydrocarbons, particularly in unconventional gas systems like shale gas. The organic petrographic analysis conducted in this study provides valuable insights into the diverse constituents of organic matter. The Gondwana shale (Figure 3a) of the Sisne Formation contains liptinite macerals, implying that the material originated from algal or bacterial ancestors in marine or lacustrine environments. The white areas in these images represent clay minerals, mainly illite. In Figure 3b, volcanic lithic wacke and volcanic rock fragments are observed in the shale sample of the Taltung Formation, along with kaolinite as a clay mineral. The predominance of vitrinite in the Eocene shale samples (Figure 3c,d) suggests a substantial contribution from terrestrial plants to organic matter deposition. The presence of pyrites in the Eocene shale implies preserving organic matter in periodically low oxygen concentrations in pore water and anoxic depositional conditions [42]. The fracture is observed clearly in the thin sections of Bhainskati shales (Figure 3c,d). Compared with sub-vertical fractures, the horizontal fractures exhibit an irregular surface in the Eocene shale (Figure 3c,d). Hydrocarbons can be preserved in bulk, organic, and inorganic kerogen [43]. The total organic content significantly affects shale rocks’ specific surface area, porosity, and permeability. The thin sections of sandstone (Figure 3e–h) from the post-Gondwana reveal that these primary intergranular pores are the predominant feature within the samples, indicating their importance in the overall porosity of the rock. The sandstone samples exhibit more quartz, feldspar, and plagioclase. The Amile Formation sandstone from the Surkhet area has large quartz grains and is compacted to each other (Figure 3d). The quartz grains are sub-angular to rounded, suggesting a moderate degree of transportation and abrasion before lithification. The Bhainskati sandstone samples comprise quartz as a major mineral, reflecting a critical record of the foreland basin dynamics resulting from the Indo-Asia continental collision and a few organic particles and clay minerals (Figure 3f–h). The heavy minerals are mainly magnetite and hematite in opaque and non-opaque minerals are zircon, tourmaline, and apatite (Figure 3).

4.2. Mineralogical Compositions and TOC Content

The major minerals identified by XRD analysis are quartz, clay, plagioclase, carbonates, and other minor minerals observed in shale and sandstone (

Table 2. Mineralogical composition, TOC content, and MIA of shale and sandstone samples.

S. ID	Group	CM	Clay Minerals				Q	Ca	Si	Pl	TOC	MIA
			K	C	It	I/S
	Shale
F287	Gondwana	43.29	12	18	68	2	36.56	0	0	20.15	0.45	64.47
F300		50.4	9	19	72	0	26.83	2.23	0.89	19.65	0.75	57.72
F306		47	8	26	66	0	29.26	9.98	0	13.76	-	68.01
F297		44.8	13	21	66	0	28.86	9.47		15.27	0.56	65.4
F286		72.61	57	42	1	0	27.39	0	0	0	0.65	100
F316		40.54	8	13	76	0	42.89	3.59	1.63	11.35	0.52	79.07
F298	Post Gondwana	70.15	10	16	74	0	29.85	0	0	0	-	100
F304		48.51	9	13	78	0	22.33	27.29	0	0	0.84	100
F280		59.45	11	17	58	14	40.55	0	0	0	1.56	100
F302		62.54	7	19	74	0	23.32	3.21	0	10.93	0.93	68.09
F303		69.53	9	13	72	6	30.47	0	0	0	0.63	100
F301		61.43	5	8	87	0	38.57	0	0	0	1.49	100
F281	LS	44.21	11	14	70	5	28.62	9.8	2.15	1.14	-	65.27
	Gondwana	Sandstone									KF
F282		1.39	28	71	1	0	71.16	0	27.45	0	0	100
F309		14.24	19	11	69	1	76.02	0	0.43	8.46	0.86	89.08
F315		19.38	8	13	79	0	69.52	0	0.78	10.32	0	87.07
F305	Post Gondwana	22.73	9	19	72	0	68.17	0	0.58	8.51	0	88.9
F314		21.49	13	33	0	54	77.9	0.61	0	0	0	100
F310		32.43	5	8	70	17	56.16	1.52	0	9.89	0	85.03
F311		28.96	5	10	66	19	58.06	0	1.27	11.71	0	83.22
F312		30.82	6	12	75	7	60.43	0	0.63	8.12	0	88.15
F307		18.38	9	14	77	0	70.01	0	0.65	10.96	0	86.46
F317		15.01	8	11	81	0	78.47	0	0.48	6.04	0	92.85
F308	Siwalik	13.72	14	9	76	1	48	18.77	1	8.7	3.75	79.4
F285		11.43	7	14	64	15	28.39	51.96	0.63	6.5	1.09	78.9
F313		14	10	13	67	10	32.9	41.08	0.72	7.63	3.66	74.45

CM, Clay Minerals; It, Illite; K, Kaolinite; C, chlorite; I/S, Illite/Smectite; KF, Potassium Feldspar; Si, Siderite, Ca, Carbonates; Pl, Plagioclase; Q, Quartz; MIA, Mineralogical Index of Alteration.

). Clay minerals accounted for 43%–72% of the samples, dominated by chlorite and illite with rare kaolinite and mixed-layer illite/smectite, followed by quartz (23%–42%), carbonates (3%–27%), and plagioclase (10%–20%) in shale. The Gondwana shale exhibits clay minerals <50%, whereas the post-Gondwana shale consists of (60%–70%) of clay minerals (Table 2). The Gondwana shale exhibits plagioclase; however, the post-Gondwana shales are free of plagioclase. Small quantities (<2%) of feldspar, pyrite, and siderite were observed in a few samples. Illite constitutes 58%–87% of clay minerals, averaging 73%. The others are chlorite at 8%–26% (average 15.4%) and kaolinite at 5%–13% (average 9%), from the highest to the lowest. However, the Upper Gondwana, Taltung Formation sample exhibits kaolinite with the highest concentration at 57%, chlorite at 42%, and illite at only 1% (Table 2). The TOC content in 11 shale samples ranges from 0.5% to 1.56% (Table 2). The Gondwana shale exhibited a TOC of ≤0.75%, while the post-Gondwana shale had a higher value (up to 1.56%). Similarly, the sandstone reveals that quartz accounted (56%–78%), clay minerals for (14%–32%), followed by plagioclase (6%–12%). The representative Lower Siwalik sandstone sample exhibits higher quartz content than the Middle Siwalik and the clay mineral ranges between 11.5 and 14% (Table 2). The Siwalik sandstones exhibit many carbonates, 18.77% in the Lower Siwalik and (41 to 51.9) wt.% in the Middle Siwalik (Table 2).

Furthermore, the Mineralogical Index of Alteration (MIA) measures weathering intensity, reflecting the relative concentration of stable quartz at the expense of unstable feldspars. It can quantify the degree of weathering in the source region and is not affected by sorting or abrasion [13,44]. The MIA is calculated as in Equation (1).

$$\mathrm{M}\mathrm{I}\mathrm{A}=(\mathrm{Q}\mathrm{u}\mathrm{a}\mathrm{r}\mathrm{t}\mathrm{z}/(\mathrm{Q}\mathrm{u}\mathrm{a}\mathrm{r}\mathrm{t}\mathrm{z}+\mathrm{P}\mathrm{l}\mathrm{a}\mathrm{g}\mathrm{i}\mathrm{o}\mathrm{c}\mathrm{l}\mathrm{a}\mathrm{s}\mathrm{e}+\mathrm{K}-\mathrm{f}\mathrm{e}\mathrm{l}\mathrm{d}\mathrm{s}\mathrm{p}\mathrm{a}\mathrm{r}\left)\right)\times 100$$

(1)

It is depicted graphically on a Q–Pl–KF triangular plot, with quartz (Q), plagioclase (Pl), and K-feldspar (K-f) at the apices (Figure 3a).

4.3. Major Elements

The distribution of the major elements in the representative shale and sandstone samples from the Gondwana and post-Gondwana (

Table 3. Correlation coefficient matrix for shales of Gondwana and post-Gondwana.

	SiO2	Al2O3	Fe2O3	CaO	MgO	SO3	TiO2	K2O	Na2O	MnO2	P2O5	CIA
SiO2	1.000
Al2O3	−0.550	1.000
Fe2O3	−0.827	0.839	1.000
CaO	−0.202	−0.695	−0.290	1.000
MgO	0.512	−0.759	−0.721	0.365	1.000
SO3	−0.683	−0.125	0.194	0.816	−0.085	1.000
TiO2	−0.306	0.741	0.575	−0.576	−0.811	−0.288	1.000
K2O	−0.240	−0.612	−0.232	0.865	0.566	0.596	−0.616	1.000
Na2O	0.508	−0.714	−0.591	0.310	0.780	−0.327	−0.553	0.564	1.000
MnO2	−0.081	0.083	0.259	0.082	−0.470	0.174	0.060	−0.282	−0.408	1.000
P2O5	0.138	0.285	−0.040	−0.295	−0.458	0.042	0.257	−0.604	−0.680	0.645	1.000
CIA	−0.436	0.920	0.700	−0.644	−0.826	−0.016	0.711	−0.746	−0.908	0.282	0.595	1.000

) from the Tansen and Surkhet basins exhibit variations in the concentration of major oxides. Notably, SiO₂ enrichment relative to Al₂O₃, K₂O, and CaO is observed in shale, with the Lower Gondwana averaging 64.36 wt.%, Upper Gondwana 53.54 wt.%, and the post-Gondwana 56 wt.%. Al₂O₃ varies between 12.47–20.85 wt.%, with the highest levels in Gondwana. Fe₂O₃ content ranges between 5.085–11.44 wt.% in the Lower Gondwana, 6.66–9.49 wt.% in post-Gondwana, and peaks in the Upper Gondwana at 9.54–13.34 wt.%. MnO contents in shales range from 0.1–0.22 wt.% and are stable proxies during thermal diagenesis. The reciprocal values of different major oxides in shale from the Lower Gondwana, Upper Gondwana, and the post-Gondwana are, on average, given as SiO₂/Al₂O₃ 5.2, 2.6, and 3.2, respectively. Similarly, K₂O/Na₂O from 3.76–4.51, 10.78–12.29, and 15.78–19.50, respectively, K₂O/Al₂O₃ from 0.27,0.15, and 0.20; Al₂O₃/TiO₂ average 17.32, 7.39 and 13.58 respectively. Furthermore, major oxide concentrations in sandstones in the post-Gondwana exhibit SiO₂ content ranging from 79.63–88.99 wt.%, Al₂O₃ varying from 0.89% to 5.88%. The post-Gondwana sandstone exhibits 7.49–8.88 wt.% Al₂O₃ in Bhainskati Formation, while Dumri Formation has 4.72% Al₂O₃, and Na₂O is 0.09–0.19 wt.% (Figure 4b). The correlation coefficient matrix for shales and sandstone of Gondwana and post-Gondwana are presented in Table 3 and

Table 4. Correlation coefficient matrix for sandstone of Upper and post-Gondwana.

	SiO2	Al2O3	Fe2O3	CaO	MgO	SO3	TiO2	K2O	Na2O	MnO2	P2O5	CIA
SiO2	1.000
Al2O3	−0.598	1.000
Fe2O3	−0.116	−0.162	1.000
CaO	−0.727	0.033	−0.192	1.000
MgO	−0.975	0.677	−0.047	0.724	1.000
SO3	0.285	−0.442	0.474	−0.276	−0.391	1.000
TiO2	−0.151	0.698	−0.119	−0.243	0.234	−0.499	1.000
K2O	−0.734	0.969	−0.119	0.189	0.785	−0.351	0.527	1.000
Na2O	−0.946	0.445	−0.094	0.887	0.941	−0.372	0.076	0.582	1.000
MnO2	−0.068	−0.543	0.765	0.208	−0.062	0.327	−0.313	−0.494	0.063	1.000
P2O5	−0.149	−0.338	0.925	0.003	−0.035	0.644	−0.427	−0.217	−0.011	0.774	1.000
CIA	0.403	0.171	−0.458	−0.360	−0.265	−0.533	0.694	−0.057	−0.302	−0.326	−0.723	1.000

, respectively. The SiO₂ in shale is positively correlated with MgO, Na₂O, and P₂O₅ and negatively correlated with other major elements (Table 3). In contrast, SiO₂ of sandstone is negatively correlated with all the other major oxides (Table 4). SiO₂ demonstrates a negative correlation with Al₂O₃ in shale and sandstone (Table 3 and Table 4). The SiO₂ content is likely associated with quartz, clay minerals, and plagioclase, whereas Al₂O₃ is predominantly linked to the preferential inclusion of aluminous clays in shale. The weak correlation between SiO₂ and Al₂O₃ suggests that SiO₂ primarily exists as detrital silicates and is not significantly influenced by clay minerals [45]. The K₂O/Al₂O₃ ratio serves as an indicator of the original composition of ancient sediments, with distinct ranges for clay minerals (0.0 to 0.3) and feldspars (0.3 to 0.9) [7]. The average K₂O/Al₂O₃ ratios in the examined shale and sandstone samples (

Table 5. Major oxide concentrations (wt.%), their ratios, and some calculated indices of shales and sandstones from the Tansen and Surkhet area, Nepal.

	F287	F300	F286	F279	F280	F304	F301	F303	F281	F282	F315	F310	F312	F314	F317	F285	PAAS	UCC
	Shale									Sandstone
SiO2	63.83	64.88	51.66	55.42	64.88	46.98	59.13	53.30	53.07	79.63	87.04	83.44	83.51	88.99	87.72	55.31	62.80	66.00
Al2O3	12.73	12.20	20.61	19.56	16.38	15.16	18.10	19.75	17.90	0.89	5.88	7.49	8.88	3.82	4.72	5.77	18.90	15.20
Fe2O3	5.33	4.84	13.34	9.54	6.60	9.49	7.38	10.30	7.68	14.23	2.58	4.48	2.96	6.50	1.98	2.79	6.50	5.00
MnO2	0.12	0.08	0.21	0.10	0.27	0.18	0.05	0.10	0.08	0.32	0.06	0.11	0.04	0.24	0.03	0.19	0.10	0.08
MgO	3.01	4.01	0.86	1.00	1.63	1.90	2.03	2.67	3.02	0.61	0.69	1.16	0.94	0.42	0.38	2.91	2.20	2.20
CaO	4.96	4.52	0.33	0.60	1.29	9.60	0.47	0.57	2.98	0.97	0.41	0.47	0.39	0.38	0.32	16.48	1.30	4.20
Na2O	0.92	0.74	0.27	0.24	0.14	0.23	0.26	0.37	0.19	0.09	0.10	0.09	0.09	0.09	0.09	0.23	1.20	3.90
K2O	3.46	3.34	2.91	2.95	2.73	3.63	2.93	3.20	5.06	0.33	1.47	1.66	2.10	0.17	1.15	1.95	3.70	3.40
TiO2	0.74	0.70	2.35	3.25	1.40	0.98	1.64	1.12	0.85	0.44	0.59	0.59	0.74	0.80	0.53	0.52	1.00	0.50
P2O5	0.18	0.16	0.19	0.35	0.54	0.28	-	-	0.15	0.39	0.10	0.09	0.08	0.11	0.07	0.13	0.20	0.20
CIA	70.60	71.68	85.66	85.08	84.48	78.75	83.99	83.37	76.69	63.57	77.88	80.28	79.57	91.61	78.02	70.54	75.60	57.58
ICV	1.46	1.49	0.98	0.90	0.86	1.72	0.82	0.93	1.11	18.73	0.99	1.13	0.81	2.19	0.94	4.31	0.85	1.27
CIW	87.37	89.18	97.45	97.60	98.32	97.06	97.21	96.39	97.92	83.18	96.71	97.65	98.01	95.50	96.33	92.62	88.73	66.09
WIP	37.60	36.46	26.16	26.43	24.65	32.79	27.55	31.21	45.19	3.96	13.24	15.21	18.49	2.47	10.27	20.48	40.62	57.50
PIA	42.00	42.60	73.38	71.13	66.46	40.29	69.72	69.28	49.14	24.56	56.11	60.04	59.16	81.84	56.85	15.64	60.56	0.00
SiO2/Al2O3	5.01	5.32	2.51	2.83	3.96	3.10	3.27	2.70	2.96	89.47	14.80	11.14	9.40	23.30	18.58	9.59	3.32
Al2O3TiO2	17.20	17.43	8.77	6.02	11.70	15.47	11.04	17.63	21.06	2.02	9.97	12.69	12.00	4.78	8.91	11.10	18.90	3.62
K2O/Na2O	3.76	4.51	10.78	12.29	19.50	15.78	11.27	8.65	26.63	3.67	14.70	18.44	23.33	1.89	12.78	8.48	3.08	1.15
K2O/Al2O3	0.27	0.27	0.14	0.15	0.17	0.24	0.16	0.16	0.28	0.37	0.25	0.22	0.24	0.04	0.24	0.34	0.20
MgO/Al2O3	0.24	0.33	0.04	0.05	0.10	0.13	0.11	0.14	0.17	0.69	0.12	0.15	0.11	0.11	0.08	0.50	0.12
Fe2O3/K2O	1.54	1.45	4.58	3.23	2.42	2.61	2.52	3.22	1.52	43.12	1.76	2.70	1.41	38.24	1.72	1.43	1.76
MnO2/TiO2	0.16	0.12	0.09	0.03	0.19	0.18	0.03	0.08	0.09	0.10	0.73	0.19	0.05	0.30	0.06	0.36	0.77	0.12
Log (Fe2O3/K2O)	0.19	0.16	0.66	0.51	0.38	0.42	0.40	0.51	0.18	1.63	0.24	0.43	0.15	1.58	0.24	0.16	0.24
Log (SiO2/Al2O3)	0.70	0.73	0.40	0.45	0.60	0.49	0.51	0.43	0.47	1.95	1.17	1.05	0.97	1.37	1.27	0.98	0.52
Na2O/K20	0.27	0.22	0.09	0.08	0.05	0.06	0.09	0.12	0.04	0.27	0.07	0.05	0.04	0.53	0.08	0.12	0.32
Log (Na2O/K2O)	−0.58	−0.65	−1.03	−1.09	−1.29	−1.20	−1.05	−0.94	−1.43	−0.56	−1.17	−1.27	−1.37	−0.28	−1.11	−0.93	−0.49

CIA—Chemical index of alteration; ICV—Index compositional variation; CIW—Chemical index of weathering; PIA—Plagioclase Index of Alteration; WIP—weathering index of Plagioclase; PAAS—Post-Archean Australian shale; UCC—Upper Continental Crust.

) are <0.3, suggesting that illite is the predominant clay mineral in these samples. The Siwalik Group sandstone exhibits the lowest SiO2, with 55.31 wt.%, and the highest CaO of 16.48 wt.% (Table 5).

The Chemical Index of Alteration (CIA) is a geochemical measure of weathering intensity that assesses the conversion of feldspars to clay minerals and serves as a proxy for reconstructing paleoclimatic conditions, considering other factors [6,13,14,46]. The CIA is expressed as Equation (2).

$$\mathrm{C}\mathrm{I}\mathrm{A}=\mathrm{M}\mathrm{o}\mathrm{l}\mathrm{a}\mathrm{r}(\mathrm{A}\mathrm{l}₂\mathrm{O}₃/(\mathrm{A}\mathrm{l}₂\mathrm{O}₃+\mathrm{C}\mathrm{a}\mathrm{O}\mathrm{*}+\mathrm{N}\mathrm{a}₂\mathrm{O}+\mathrm{K}₂\mathrm{O}\left)\right)\times 100$$

(2)

CaO* is only the CaO in silicate minerals [46]. If CaO ≤ Na₂O, then CaO* equals CaO; if CaO > Na₂O, Na₂O is used as CaO*. The CIA in shale is positively correlated with Al₂O₃ and Fe₂O₃ and negatively with others (Table 3), but in sandstone, it is positively correlated only with SiO₂ and Al₂O₃ (Table 4).

CIA and weathering paths can also be evaluated using plots of A–CN–K (Al₂O₃–CaO* + Na₂O–K₂O) [46]. The average CIA value of the shale of the Lower Gondwana is 71, Upper Gondwana is 85, and the post-Gondwana ranges from 82, which is higher than the PAAS (75.6). In contrast, the CIA values for Upper Gondwana sandstone range from 63 to 77, whereas the post-Gondwana sandstone exhibits CIA values ranging from 77.85 to 91.61.

The Index of Compositional Variability is given in Equation (3).

$$\mathrm{I}\mathrm{C}\mathrm{V}=(\mathrm{F}\mathrm{e}₂\mathrm{O}₃+\mathrm{K}₂\mathrm{O}+\mathrm{N}\mathrm{a}₂\mathrm{O}+\mathrm{C}\mathrm{a}\mathrm{O}+\mathrm{M}\mathrm{g}\mathrm{O}+\mathrm{M}\mathrm{n}\mathrm{O}+\mathrm{T}\mathrm{i}\mathrm{O}₂)/\mathrm{A}\mathrm{l}₂\mathrm{O}₃)$$

(3)

For these analyzed samples from different units of Gondwana, an ICV value between 0.5 and 1.6 was indicated (Table 5).

4.4. Trace and REE Elements

Trace and rare earth element (REE) data for 16 samples are detailed in

Table 6. The concentration of rare earth elements (ppm) in shale and sandstones compares with PAAS.

	F287	F306	F286	F279	F280	F301	F303	F281	F315	F282	F310	F312	F314	F317	F285	PAAS
	Shale									Sandstone
La	62	62.3	104	200.1	96.9	72.6	54	88.6	43.7	29.9	40.7	54.2	37.6	43.6	44.9	38.2
Ce	140.5	138	214	346.6	202.7	172	106.6	174.9	100	77.3	92.1	123	97.7	99.7	100	79.6
Pr	13.4	13.5	22	34	20.3	14.4	10.7	17.3	9.87	8.76	9.61	12.4	9.75	9.78	9.81	8.83
Nd	51.6	51.7	92	127.6	82.8	61.9	45	68	38.7	38.5	38.3	48.5	41	38.2	37.6	33.9
Sm	9.69	9.37	19.1	20	16.3	11.8	8.44	13.1	7.37	8.49	7.35	9.35	8.16	7.35	7.01	5.55
Eu	2.01	1.75	4.92	5.25	3.96	2.43	1.73	2.73	1.37	2.19	1.52	2	1.54	1.35	1.35	1.08
Gd	10	9.65	19.6	18.9	17.3	11.4	7.72	13.8	7.68	9.52	7.34	9.49	7.17	7.6	7.25	9.66
Tb	1.34	1.29	2.67	2.24	2.33	1.42	0.95	1.9	1.05	1.36	0.99	1.37	0.95	1.04	0.97	0.77
Dy	7.88	7.55	15.1	11.7	13.2	8.69	5.69	11	6.15	8.26	5.65	8.28	5.5	6.03	5.71	4.68
Ho	1.5	1.46	2.71	2.07	2.47	1.5	0.98	2.1	1.18	1.57	1.06	1.62	1.04	1.15	1.13	0.99
Er	4.52	4.42	7.53	5.86	6.94	4.93	3.33	6.16	3.48	4.48	3.07	4.81	3.14	3.47	3.38	2.85
Tm	0.6	0.6	1	0.8	1	0.6	0.4	0.9	0.5	0.6	0.4	0.7	0.5	0.5	0.5	0.41
Yb	4.5	4.4	6.8	5	6.5	4.44	3.1	6	3.5	4.3	3	4.8	3.2	3.5	3.4	2.82
Lu	0.63	0.63	0.93	0.69	0.9	0.58	0.4	0.84	0.5	0.59	0.43	0.69	0.44	0.49	0.48	0.43

and

Table 7. Trace elemental concentration in shale and sandstone.

Lab ID	F287	F306	F286	F279	F280	F301	F303	F315	F282	F310	F312	F314	F317	F281	F285
	Shale							Sandstone
Be	2.26	2.09	4.45	4.18	3.31	3.05	3.37	1.13	0.55	1.4	1.56	0.47	1.17	4.57	1.53
V	98.5	87.6	577	232	171	-	-	46.8	151	72.3	95.5	77.8	46.6	161	53.5
Cr	136.8	116.7	360.8	628.9	217.9	-	-	176.4	232	175.5	205.3	246.5	205.7	159	74.9
Co	17.1	15.4	40.4	40.3	49.1	32.5	34.1	27.1	31.7	16.7	12.2	31.8	27.1	23.8	7.28
Ni	42.4	35.5	87.4	183.5	95	132.5	109.8	48.3	84.2	60.2	41.2	53	49.1	59.4	24.1
Cu	26.97	27.33	38.93	72.48	52.32	-	-	75.12	11.46	39.12	25.56	32.13	75.43	47.57	18.53
Li	32.9	36.0	47.6	23.3	37.7	72.5	66.7	17.8	8.0	21.3	23.9	19.5	17.4	40.4	17.8
Ga	24.3	20.8	40.5	31.5	30.6	29.15	30.77	11	3.47	13	15.3	9.01	11.1	34.5	10.5
As	11.2	11.6	31	9.69	104	-	-	19.2	36.6	16.3	12.5	16.7	19	30.3	7.65
Rb	179.1	183.6	144.3	128	176	166	203	77.7	5.75	81.9	112.1	5.35	77.7	290	90.9
Sr	136	146	122	329	127	106	102	31.6	35.1	37.7	43	52.6	31.3	83.2	134
Y	31.1	29.8	45	40.4	50	42.0	26.9	23.8	29.7	19.9	32.9	19.2	23.6	44.4	24.8
Zr	366	417	733	748	586	250	183	484	140	330	663	507	484	398	189
Nb	16.9	16.3	45.5	166.3	31.7	34.1	19.8	11.3	6.78	10.9	15.9	16.3	11.2	24.4	11.1
Mo	0.46	0.5	1.88	1.77	1.63	-	-	4.22	3.57	2.09	3.35	3.7	4.32	2.29	0.72
Cd	0.39	0.4	0.54	0.6	0.7	0.24	0.21	0.63	0.3	0.6	0.72	0.53	0.33	0.46	0.36
Ge	2.96	3.36	4.01	3.07	3.82	2.80	2.83	2.40	1.86	2.69	2.86	2.55	2.42	4.03	2.24
In	0.08	0.06	0.16	0.12	0.11	0.09	0.10	0.04	0.03	0.04	0.05	0.03	0.03	0.12	0.04
Cs	11.3	9.68	13.4	10.3	11.8	13.1	11.9	3.25	0.44	4.24	5.5	0.38	3.24	17.3	4.69
Ba	849	593	965	1581	678	423	303	487	40.8	798	1441	66.1	487	1302	371
Sc	17.8	13.8	34.3	26.4	21.4	27.4	28.0	6.74	8.03	8.47	9.83	6.02	6.79	20.6	6.93
Te	0.02	0.02	0.13	0.04	0.11	0.55	0.54	<0.01	0.17	0.02	0.02	0.07	0.01	0.05	0.02
Mn	960	677	1667	670	2222	-	-	443	2470	822	216	1834	437	687.1	1688
Th	24.3	23.7	24.2	31.1	27.5	-	-	16.5	8.11	14.4	19.2	14.6	16	32.3	14

, respectively, showing total REE (∑REE) values ranging from 310.17 to 780.81 μg/g in shale and 195.82 to 281.61 μg/g in sandstone. The concentration of light rare earth elements (LREEs) exceeds that of heavy rare earth elements (HREEs), consistent with typical REE distributions in shales [47]. The LREE/HREE ratios range from 12.94 to 15.75 in shale and 8.25 to 13.74 in sandstone, higher than the PAAS and UCC (La/Yb)_N values, which reflect LREE/HREE ratios, range from 3.17 to 4.98, indicating LREE enrichment (

Table 8. Ratios of trace element concentration of shales and sandstones from different units.

Lab ID	F287	F306	F286	F279	F280	F301	F303	F281	F315	F282	F310	F312	F314	F317	F285	PAAS	UCC
∑REE	310.170	306.820	512.160	780.810	473.600	368.209	249.015	407.330	225.250	195.820	211.520	281.610	217.690	223.760	223.490	184.860	146.370
Eu/Eu*	0.624	0.562	0.777	0.825	0.721	0.641	0.655	0.620	0.556	0.744	0.632	0.649	0.615	0.552	0.579	0.643	0.650
Ce/Ce*	1.142	1.117	1.052	0.985	1.071	1.241	1.041	1.047	1.130	1.119	1.091	1.115	1.196	1.131	1.116	1.016	1.027
∑LREE	289.200	286.470	475.420	752.450	440.260	346.039	234.152	378.430	208.890	174.660	196.920	259.340	202.920	207.580	207.920	171.910	136.280
∑HREE	20.970	20.350	36.740	28.360	33.340	22.170	14.863	28.900	16.360	21.160	14.600	22.270	14.770	16.180	15.570	12.950	10.090
LREE/HREE	13.791	14.077	12.940	26.532	13.205	15.609	15.754	13.094	12.768	8.254	13.488	11.645	13.739	12.829	13.354	13.275	13.506
La/Sc	3.476	4.529	3.015	7.583	4.525	2.647	1.930	4.291	6.487	3.722	4.803	5.516	6.245	6.424	6.476	2.388	2.206
Th/Sc	1.362	1.723	0.705	1.179	1.284	0.000	0.000	1.564	2.449	1.009	1.699	1.954	2.425	2.357	2.000	1.000
La/Co	3.626	4.045	2.562	4.965	1.974	2.234	1.582	3.723	1.613	0.943	2.437	4.443	1.182	1.609	6.168	1.661	1.765
Th/Co	1.421	1.539	0.599	0.772	0.560	0.000	0.000	1.357	0.609	0.256	0.862	1.574	0.459	0.590	1.923	0.635	0.629
Cr/Th	5.630	4.924	14.909	20.222	7.924	-	-	4.923	10.691	28.607	12.188	10.693	16.884	12.856	5.350	7.534	7.757
Ba/Sr	6.243	4.062	7.910	4.805	5.339	3.977	2.989	15.649	15.411	1.162	21.167	33.512	1.257	15.559	2.769	3.250
Sr/Ba	0.160	0.246	0.126	0.208	0.187	0.251	0.335	0.064	0.065	0.860	0.047	0.030	0.796	0.064	0.361	0.308
Sc/Cr	0.130	0.118	0.095	0.042	0.098	-	-	0.130	0.038	0.035	0.048	0.048	0.024	0.033	0.093	0.145	0.164
Cr/V	1.389	1.332	0.625	2.711	1.274	1.806	1.541	0.988	3.769	1.536	2.427	2.150	3.168	4.414	1.400	0.733	1.383
Y/Ni	0.733	0.839	0.515	0.220	0.526	0.317	0.245	0.747	0.493	0.353	0.331	0.799	0.362	0.481	1.029	0.491	1.100
V/Cr	0.720	0.751	1.599	0.369	0.785	0.554	0.649	1.013	0.265	0.651	0.412	0.465	0.316	0.227	0.714	1.364	0.723
V/Ni	2.323	2.468	6.602	1.264	1.800	1.359	1.684	2.710	0.969	1.793	1.201	2.318	1.468	0.949	2.220	2.727	3.000
V/(V+Ni)	0.699	0.712	0.868	0.558	0.643	0.576	0.627	0.730	0.492	0.642	0.546	0.699	0.595	0.487	0.689	0.732	0.750
Ni/Co	2.480	2.305	2.163	4.553	1.935	4.079	3.221	2.496	1.782	2.656	3.605	3.377	1.667	1.812	3.310	2.391	1.176
La/Yb	13.778	14.159	15.221	40.020	14.908	16.347	17.378	14.767	12.486	6.953	13.567	11.292	11.750	12.457	13.206	13.546	13.636
La/Th	2.551	2.629	4.277	6.434	3.524	-	-	2.743	2.648	3.687	2.826	2.823	2.575	2.725	3.207	2.616	2.804
Zr/10	36.600	41.700	73.300	74.800	58.600	25.017	18.313	39.800	48.400	14.000	33.000	66.300	50.700	48.400	18.900	21.000	19.000
Th/Sc	1.362	1.723	0.705	1.179	1.284	0.000	0.000	1.564	2.449	1.009	1.699	1.954	2.425	2.357	2.019	0.913	0.787
Zr/Sc	20.519	30.313	21.350	28.346	27.363	9.127	6.549	19.275	71.848	17.425	38.939	67.470	84.211	71.307	27.259	13.125	13.971
Sr/Cu	5.043	5.342	3.134	4.539	2.427	-	-	1.749	0.421	3.063	0.964	1.682	1.637	0.415	7.232	-
Ga/Rb	0.136	0.113	0.281	0.246	0.174	0.175	0.152	0.119	0.142	0.603	0.159	0.136	1.684	0.143	0.116	0.125
Rb/Sr	1.317	1.258	1.183	0.389	1.386	1.560	1.996	3.486	2.459	0.164	2.172	2.607	0.102	2.482	0.678	0.800

). The V/Cr ratios for the different units of Gondwana sediments show a range of 0.37–0.78 except for a sample from the Upper Gondwana shale exhibits, i.e., 1.6. The Ni/Co ratios in the investigated Gondwana shale range from 2.06 to 4.55, and in sandstone between 1.78 and 3.38. The Ba/Sr ratios in shale from the Gondwana range from 4.06 to 7.9, post-Gondwana from 2.99 to 15.65, and sandstones up to 33.51 (Table 8).

Regarding chondrites, REE patterns with Eu and Ce anomalies are commonly used to infer sediment source regions [48]. The shale samples exhibit negative Eu anomaly, indicating a reducing depositional environment [49,50]. The REE patterns also provide insight into sedimentary provenance [48], with a high LREE/HREE ratio suggesting a felsic source and vice versa for a mafic source [12].

Europium anomaly (Eu/Eu*) is calculated in Equation (4) after McLennan [51] and Cerium anomaly (Ce/Ce*) in Equation (5) after Worrall and Pearson [52].

$$Eu/{(Eu}^{*})={Eu}_{CN}/(\sqrt{({Sm}_{CN}\ast {Gd}_{CN}}\left)\right)$$

(4)

after McLennan [51]

$$Ce/{Ce}^{*}=\left({Ce)}_N/\sqrt{({La}_N\ast {Pr}_N}\right)$$

(5)

after Worrall and Pearson [52]

Similarly, the normalization of rare earth elements (REEs) utilizes the post-Archean Average Shale (PAAS) as the Equation (6).

$$\left(\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}\right)\mathrm{N}=\left(element\right)sample/\left(element\right)PAAS$$

(6)

after Kala, Turlapati [53].

5. Discussion

5.1. Paleo Depositional Environment

High-resolution elemental analyses have revealed paleo-productivity, paleo-salinity, paleo-water depth, paleo-redox conditions, and detrital influx within shales [3]. The dominance of quartz and significant clay minerals in sandstone indicate deposition in a fluvial or shallow marine environment with considerable reworking. Sedimentation in the Gondwana basin was initiated during the Early Permian over the Archean crystalline basement.

Hydrological differentiation, source composition, and chemical weathering influence the chemical composition of sedimentary suites. As sediments mature, quartz replaces feldspar and lithic fragments, leading to increased SiO₂ and decreased Fe₂O₃, Al₂O₃, K₂O, MgO, and CaO. These sediments are chemically classified (Figure 4a) as lithic arenite due to the dominance of silica in the matrix and cement fractions of the sandstones, based on the log (SiO₂/Al₂O₃) versus log (Fe₂O₃/K₂O) diagram [54]. Few sandstones fall into the Fe sand category due to their ferruginous nature (Figure 5a). The samples plotted on a K₂O versus Na₂O diagram as described by Crook [55] fall within the quartz abundance area, indicating quartz-rich shale and sandstones (Figure 5b). Among SiO₂, CaO, and Al₂O_3, sandstones and shale samples have a variable enrichment of SiO₂, as shown by the ternary plot of these major oxides (Figure 6a). The variation in the concentrations of minerals in various units of Gondwana is due to the different depositional environments. The dominance of illite followed by quartz in shale and sandstone indicates a source area rich in weathered feldspar and a significant input from continental crust materials. The highest concentration of illite (>50%) as clay minerals and 14%–20% of plagioclase in the shale refers to a shallow oxic deposition environment during the Gondwana deposition (Figure 5b). The depositional conditions suggest a range of redox environments, influenced by both glacial meltwater and fluvial systems, typically characterized by fluctuating redox states due to variations in sedimentation rates and organic matter input during glacial periods.

Figure 5. (a) Chemical classification of the Gondwana and post-Gondwana shale and sandstone using major oxide data (log (Fe₂O₃/K2O) vs. log (SiO₂/Al₂O₃)) after Herron [54] and (b) quartz richness analysis Na₂O–K₂O diagram after Crook [55].

Figure 5. (a) Chemical classification of the Gondwana and post-Gondwana shale and sandstone using major oxide data (log (Fe₂O₃/K2O) vs. log (SiO₂/Al₂O₃)) after Herron [54] and (b) quartz richness analysis Na₂O–K₂O diagram after Crook [55].

The Upper Gondwana shale indicates a robust reducing depositional environment in anoxic settings with restricted water circulation, while the post-Gondwana showed an aquatic environment deposited at shallow sea level [26]. Alumina is a good measure of the detrital flux, and the positive correlation of TiO₂ with Al₂O₃ (r² = 0.741, Table 3) suggests a relatively constant detrital source. The Mn/TiO₂ ratio is a reliable and widely accepted approach to evaluating depositional conditions [56]. The samples analyzed possess MnO/TiO₂ ratios less than 0.5, suggesting a shallow marine or fluvial continental sedimentary environment. Sc/Cr ratio slope analysis [57] of the analyzed shale and sandstones in the Lower Gondwana samples show mixed influences. The coal seams and associated organic-rich sediments in the Amile Formation suggest deposition in a swampy, deltaic, or low-energy floodplain environment. These settings were influenced by alternating episodes of fluvial sedimentation and organic matter accumulation under humid conditions. The predominance of kaolinite reflects the intense chemical weathering of feldspar-rich source rocks in a tropical to subtropical climate. This result is identical to the past study that highlighted the humid paleoclimate and prolonged weathering in the source regions during the Late Cretaceous and Paleocene [58].

Figure 6. (a) The Ternary diagram showing the major oxide concentration of shale and sandstones (SiO₂-Al₂O₃-CaO) [59] and (b) Ternary diagram of the major clay mineral groups (Illite–Kaolinite–Chlorite) [60] of the different units of Gondwana and Siwalik.

Figure 6. (a) The Ternary diagram showing the major oxide concentration of shale and sandstones (SiO₂-Al₂O₃-CaO) [59] and (b) Ternary diagram of the major clay mineral groups (Illite–Kaolinite–Chlorite) [60] of the different units of Gondwana and Siwalik.

In contrast, the Upper and the post-Gondwana samples reflect hydrothermal deposition. The Bhainskati Formation marks a transition to marine sedimentation typical of shallow marine environments. This formation exhibits characteristics aligned with more stable deposition conditions, where redox states may favor anoxic or suboxic conditions, especially in deeper marine settings. The environment shifts from the terrestrial, glacially influenced habitats of the Gondwana to a marine environment dominated by finer sediments, suggesting a shift to less turbulent hydrodynamic conditions [19]. This hydrothermal was evidenced by a linear correlation between the V/Cr and V/Sc ratios in shale samples from the Tansen area, reflecting changes in the redox environment. The V/Cr ratios range from 0.37–1.6, i.e., below 2 indicates oxic deposition conditions [61], suggesting that these sediments were deposited in an environment with well-oxygenated bottom waters, typical of shallow marine or continental settings. The Ga/Rb ratio is <0.28 for all shale samples, and only two sandstone samples exceed this value, suggesting a warm and arid climate [62] during deposition (Figure 7). The Sr/Cu ratio is also sensitive to climatic changes, and the Sr/Cu ratio in the Lower Gondwana samples is greater than 5 with a value of 5.04–5.34 (Table 2), indicating a warm–humid to dry and arid climate [63] during the deposition and indicates conditions with limited organic matter preservation, further supporting an oxic depositional environment. Furthermore, the post-Gondwana and the Siwalik shale samples are within 1.3 and 5.0, suggesting a warm-humid depositional environment. According to Sakai [19], the Bhainskati Formation was primarily deposited in a marine setting, transitioning into brackish and eventually freshwater environments as a result of regional regression. The sedimentary facies include organic-rich dark grey to black shales, minor sandstones, and siltstones. These deposits suggest transportation by suspension and turbidity currents in the deeper marine settings, with occasional contributions from fluvial and deltaic processes in shallower environments [19] that imply changes in sedimentation due to marine transgressions and regressions associated with tectonic activity and global sea-level changes. The petrographic characteristics of quartz grains suggest derivation from a predominantly granitic source with some contribution from metamorphic rocks. The presence of micas in sandstone signifies the weathering and erosional origin of pre-existing rocks, which were subsequently transported and deposited. The existence of authigenic mica reveals that they were formed in place during the diagenesis process. The distinct change in the depositional environment from marine to continental and the shift in sediment provenance from cratonic India to the Himalayan orogenic belt highlight the effects of orogenic activity and adjustments within the overall basin dynamics [21,31].

Some of the studies by DeCelles (2001, 2004) highlighted the role of the Himalayan tectonic uplift during the India–Asia collision in influencing sediment supply. The post-Gondwana Bhainskati Formation received sediments primarily from the unroofing of Proterozoic basement rocks and the Greater Himalayan Sequence. Fluvial systems likely delivered clastic material to a shallow marine shelf, where finer-grained sediments were deposited in relatively low-energy settings. The presence of organic-rich shales points to anoxic conditions during sediment deposition, further emphasizing the dominance of suspension transport mechanisms [21,29]. Clay minerals in the Bhainskati Formation provide valuable insights into the weathering processes and sediment provenance. Kaolinite in the upper portions of the Bhainskati Formation indicates intense chemical weathering under warm and humid conditions. Hematite and kaolinite-rich oxisols at the topmost layers suggest prolonged exposure to subaerial weathering during a phase of marine regression [19]. DeCelles (2004) elaborates on the clay mineral assemblages, noting contributions from weathered feldspathic and volcanic source rocks. The presence of illite and chlorite in the lower sections of the formation reflects moderate physical weathering of primary minerals, likely derived from the crystalline rocks of the rising Himalayas. As tectonic activity intensified and weathering conditions evolved, kaolinite enrichment in the upper stratigraphic levels marked a shift towards stronger chemical weathering processes [21]. The V/Cr ratios in the analyzed samples range from 0.37 to 1.60, indicating oxidizing conditions, as values below 2 suggest a more oxidizing environment [61].

The cerium anomaly (δCe) values range from 1.01 to 1.2 (Table 5), implying a slight positive anomaly, suggesting oxic conditions since values above 1 indicate aerated environments [63]. Additionally, the Rb/Sr ratio is higher than the PAAS, further supporting the notion of a warm and humid climate during the deposition of Gondwana shale alongside increased weathering intensity. Freshwater deposits are confirmed by the Sr/Ba ratios, which are consistently below 1 in all tested samples. The Sr/Ba ratios of the analyzed samples exhibit values less than 0.6 (Table 8), consistently noted in freshwater sediments, whereas marine sediments generally exceed values of 1.0 [64]. The high Sr/Ba ratio is suggested to reflect changes in water salinity (Sr/Ba < 0.6: freshwater; 0.6 < Sr/Ba < 1.0: transition; Sr/Ba > 1: saline water) [65]. Our results indicate that the analyzed samples are of a fresh to transitional state of salinity. The gas shale quality increases during periods of enhanced salinity in the bottom waters, leading to anoxic conditions. This indicates that the low TOC content in the analyzed samples is due to fresh water and shallow marine depositional environments during the deposition of the post-Gondwana Bhainskati Formation.

Figure 7. A plot of Ga/Rb versus Sr/Cu ratios showing paleoclimatic conditions after Ding et al. [65]. LG, Lower Gondwana; PG, post-Gondwana; UG, Upper Gondwana.

Figure 7. A plot of Ga/Rb versus Sr/Cu ratios showing paleoclimatic conditions after Ding et al. [65]. LG, Lower Gondwana; PG, post-Gondwana; UG, Upper Gondwana.

The paleo-depositional environment of the Gondwana and post-Gondwana units within the Lesser Himalayas, Nepal, reflects a dynamic interplay of geological processes spanning millions of years. During the Permian period, glacial deposits formed in emergent basins at high southern latitudes, signaling a period of global climatic change. Over time, these glacial environments gave way to meandering fluvial systems characterized by the deposition of coal, shale, siltstone, and carbonaceous shale [37]. The Lower Gondwana unit was deposited within a continental rift valley (Figure 8a), and the Upper Gondwana reflected deposition in a passive continental margin setting. Aulis basalt with plagioclase porphyritic basalt repeatedly shows the eruption of volcanoes and erosion and sedimentation as fluvial beds (Figure 8b) during the late Jurassic to early Cretaceous [27,38]. Gondwana sediments are sourced from the Greater Himalayas and the Tethys Himalayas, or both of these units share the same source rocks, as there is a possibility of Indian affinity detritus in the Gondwana sequence [23]. At the same time, the post-Gondwana reflects a primary Himalayan provenance resulting from the collision of the Indian plate with the Eurasian plate (Figure 8e), further elucidating the tectonic influences on sediment deposition throughout the Gondwana units [28,66]. The post-Gondwana sediments represent the final marine sedimentary sequence in the southern Himalayas, suggesting a transition to terrestrial conditions driven by the Indo-Asian plate collision, leading to seawater’s westward retreat (Figure 8c). Eocene Foreland deposit has been interpreted as an indication of an aquatic environment allowing productive shallow sea level conditions [26]. The Eocene organic-rich shales and fossiliferous limestone, accumulated in a shallow-marine back-bulge depozone between a migrating fore bulge and the Indian craton [19,31], is probably deposited in extreme floodplains under tropical climatic conditions. Plant fossils in the post-Gondwana Bhainskati Formation point to a tropical rainforest climate during the Early and Middle Eocene, suitable for accumulating coaly source rocks.

Similarly, past studies reveal that the northern Indian shield provided the sole source of the Upper Gondwana, whereas the post-Gondwana reflects the Himalayan provenance (Figure 8d) [31]. A regional unconformity occurred during the Eocene-Oligocene transition, with the fluvial deposits overlapping the forebulge unconformity in the Early Miocene [31]. Significant erosion and quasi-plaining at the top of the Eocene Formation resulted from low-angle non-integration with the overlying Miocene Formation, characterized by meandering river deposits (Figure 8d).

Figure 8. Schematic deposition and tectonic sedimentary evolution model of different Gondwana units of the Nepal Himalayas. Depositional and sediment provenance model diagram (a–d) and (e) Tectonic model diagram showing the Gondwana and post-Gondwana units within the Lesser Himalaya and major thrusts, namely, Ranimatta Thrust RT, Main Central thrust MCT, Main Boundary Thrust MBT, and Main Frontal Thrust MFT in the Nepal Himalaya (modified from [67]).

Figure 8. Schematic deposition and tectonic sedimentary evolution model of different Gondwana units of the Nepal Himalayas. Depositional and sediment provenance model diagram (a–d) and (e) Tectonic model diagram showing the Gondwana and post-Gondwana units within the Lesser Himalaya and major thrusts, namely, Ranimatta Thrust RT, Main Central thrust MCT, Main Boundary Thrust MBT, and Main Frontal Thrust MFT in the Nepal Himalaya (modified from [67]).

5.2. Weathering and Its Impact on Sediment Composition

Chemical weathering significantly influences clay mineral formation, with montmorillonite transforming into illite as thermal maturity increases, eventually becoming a late diagenetic product [57]. The weathering history of the source area can be quantified through the CIA [46], PIA [14], and CIW [12,14,46]. Most shale and sandstone samples show illite dominance and supremacy of illite in the post-Gondwana sediments, reflecting the impact of Himalayan tectonic events (MCT/MBT) on nearby source rocks. Transport distance increases illite content while decreasing kaolinite along with the climate alternations. Shales enriched in trace metals and iron sulfides suggest sedimentary and terrigenous inputs [1], with high Fe₂O₃ content in the Upper and post-Gondwana units due to oxidative weathering. The Lower Gondwana shales have higher SiO₂ than PAAS but lower than UCC, while Upper and post-Gondwana shales have lower SiO₂ (Table 5) [48]. Al₂O₃, TiO₂, and K₂O align closely with PAAS and UCC. The K₂O/Na₂O ratio ranges from 3.76 to 19.70, higher than PAAS and UCC, suggesting Na substitution for K during later diagenesis and weathering [14] and migration during weathering. The CIA values of the Upper Gondwana (63–77) indicate moderate to intense weathering conditions [68] of the source rocks, while the post-Gondwana (70–90) experience higher weathering intensity under warm, humid conditions. The chemical alteration results with low CIA values in the Lower Gondwana samples may reflect cool and arid conditions during deposition (Figure 9a). This wide variation may be due to frequent climatic oscillations between warm-humid and semi-humid-semiarid conditions. Sandstones show higher SiO₂ than shale and CIA values (77–91), indicating intensification weathering in source regions (Figure 4b). The ideal weathering trends (IWT) run parallel to the A–CN edge from pristine sources (CIA ~50). Data often diverge from IWT, trending towards illite–muscovite due to post-depositional K-metasomatism [14]. The CIA and CIW are interpreted similarly, with a value of 50 for fresh upper continental crust and about 100 for highly weathered materials with complete removal of alkali and alkaline-earth elements [6]. In our studies, the CIW values are higher than that of CIA and consistently increase for the younger deposits.

The index of chemical variability (ICV) measures chemical weathering and identifies the compositional maturity of sediments based on alumina abundance relative to other oxides except silica [69]. Sediments with ICV values >1 are considered compositionally immature and originate from first-cycle processes in tectonically active environments. In contrast, sediments with ICV values <1 are classified as compositionally mature, indicating active recycling and significant weathering in tectonically stable or cratonic settings [69]. The Gondwana shale samples exhibit ICV values over 1, while the post-Gondwana shale samples range from 0.82 to 0.93, and most sandstone samples also show values above 1 (Table 5). These results suggest that the sediments are linked to alteration/weathering products like illite, chlorite, and kaolinite, deriving from tectonically active sources after active recycling for post-Gondwana sediments and mixed sources of Gondwana rocks. The dominance of quartz grains, along with a low presence of feldspars and rock fragments in the analyzed sandstones, indicates a prolonged period of intensive chemical weathering in a warm, humid climate. The combined ICV, CIA, and WIP values indicate moderate weathering at deposition for most samples (Figure 9). The ranges of CIA values for shale samples indicate early to vital chemical weathering stages, with sediments primarily losing Ca- and Na-bearing silicate minerals due to plagioclase weathering, highlighting that sediments are derived from parent rocks with compositions similar to UCC. The Plagioclase Index of Alteration (PIA) monitors the weathering of the plagioclase [14] in silicate rocks, as the mineral has a higher abundance in the clastic rocks derived from bare igneous rocks, and it also possesses a high dissolution nature. The PIA values are less than 50 in the Lower Gondwana and more significant in the Upper and the post-Gondwana, suggesting progressive weathering and alteration of feldspars, with more advanced weathering in the later periods. Sediments derived from intensely weathered sources fall near the Q apex with high MIA values (80–100), whereas incipiently weathered sediments plot at lower MIA (50–70), indicating variable weathering levels related to climatic influence. Gondwana shale samples exhibit lower MIA, indicating minimal weathering and cold and arid conditions [44], and the post-Gondwana shale has higher MIA (Table 2), probably due to intense chemical weathering in warm and humid conditions (Figure 3a).

Distribution patterns of major oxides normalized to PAAS values (Figure 10a) and normalized to UCC (Figure 10b) show different characteristics. Most oxides (Figure 9) have mean normalized values of less than 1, indicating extensive weathering and erosion. Comparing the proportions of the major elements with values obtained from PAAS shows that the studied shale samples are enriched in Fe₂O₃, MnO₂, and TiO₂ and highly deficient in Al₂O₃ and Na₂O, more than the proportion of PAAS (Figure 10a). About 30% of Al₂O₃ and CaO and 80% of metallic oxides like Fe₂O₃, MnO₂, and TiO₂ in the studied shale samples are enriched when normalized to PAAS (Figure 10a). These findings suggest that the sedimentation during the post-Gondwana period potentially occurred in a dynamic environment characterized by variations in salinity and redox states, which influenced manganese and titanium distributions. Regarding titanium, its association with the clay fraction is well-established, often serving as an indicator of sediment provenance and injection intensity, especially sedimentation in the Himalayan regions. The high concentrations of titanium observed in our shales suggest significant contributions from terrigenous sediments, reflecting active weathering and erosion processes accompanying the tectonic movements in this region. Diagenetic processes cause the enrichment of Mn, Ti, and Ni in manganese deposits [70]. Meanwhile, the sandstone samples normalized with UCC exhibit different characteristics, i.e., enriched with Fe₂O₃, MnO₂, and CaO is indicative of varied depositional environments [70]. Similarly, about 30% of the studied sandstone is enriched for Fe, Mn, and Ti oxides when normalized with UCC. In contrast, Al₂O₃, MgO, CaO, Na₂O, K₂O, and P₂O₅ in the study are deficient after normalization (Figure 10b).

5.3. Provenance and Tectonic Setting

The high percentage of quartz and the significant presence of plagioclase in the Gondwana and the post-Gondwana sandstone suggest a continental source, likely from weathered igneous rocks, such as granites or granodiorites, which further indicates a source area rich in weathered feldspar. The ternary plot of these oxides (Figure 6) shows a relationship between different units of Gondwana sediment deposition and diagenesis, particularly in the enrichment of SiO₂ relative to Al₂O₃ and CaO, which implies the source variations in Gondwana and post-Gondwana deposits. REE is valuable for provenance studies and paleoenvironmental reconstructions [16,17]. The PAAS normalized REE distribution for the sediments shows nearly flat REE curves (Figure 11), whereas the chondrite normalization shows downward-sloping curves towards the La-Lu element (Figure 10b). The samples demonstrate negative europium anomalies concerning the Gondwana units. Eu depletion of comparable scale characterizes nearly all post-Archean sedimentary strata, suggesting the hallmark of an earlier event in a more reducing igneous environment than in the upper crust. Most post-Gondwana sandstone and shale samples show positive cerium anomalies, emphasizing an oxic depositional environment. The Gondwana (mainly Upper Gondwana) sandstone is sourced from an Indian basement craton, and the detritus in the post-Gondwana Formation was sourced from the Tethys Himalayan sediments, Asian Plate [23,32]. A recent study concluded that the source for Gondwana and post-Gondwana sequences must be the Greater Himalaya (or Tethys Himalaya) or the same source that provided sediments for the Greater Himalaya and the Tethys Himalaya [28].

The post-Gondwana sandstone samples display uniform patterns characterized by fractionated light rare earth elements (LREEs) with a La_N/Sm_N ratio ranging from 0.488 to 0.733 and heavy rare earth elements (HREEs) with a Gd_N/Yb_N ratio between 0.101 and 0.317. These samples show a negative europium (Eu) anomaly, with Eu/Eu* values ranging from 0.56 to 0.74 (average 0.6), indicating their derivation from a source area dominated by felsic rocks and minimal contributions from highly fractionated sources [6]. Moreover, the sample exhibits LREEs (6.80–20.81) and HREEs (8.367–21.14) with positive Eu anomaly (Figure 11a) normalized with PAAS and negative Eu anomaly normalized with chondrite (Figure 11b). The total rare earth elements (∑REE) exceeding 300 ppm further suggest that these shales primarily derive from felsic source rocks.

The high SiO₂ content in post-Gondwana shale and sandstone and significant Fe₂O₃ and MnO concentrations indicate derivation from the continental crust and possible tectonic uplift and erosion. Na₂O concentrations above 1% are likely due to plagioclase breakdown in the source region. K₂O/Na₂O ratios are lower (<5) in Gondwana and maximum (>10) in the post-Gondwana. The high K₂O/Na₂O ratios (3.76 to 19.70) suggest derivation from felsic rocks, likely granitic or gneissic, as these ratios are characteristic of source areas rich in K-feldspar and mica. The Al₂O₃/(Al₂O₃ + Fe₂O₃) ratio of the shale samples ranges from 0.64 to 0.95, with an average of 0.88, indicating sedimentation in a continental margin. The Al₂O₃/TiO₂ ratio serves as a provenance index, with values spanning from 8 to 21 wt.%. The range suggests a predominant felsic to intermediate source (Figure 12a), consistent with sediments from a continental arc or active continental margin [72]. The tectonic environment of sedimentary rocks significantly impacts their clastic components. Clastic rocks contain various geochemical components that may have come from different tectonic settings [62]. Plotting TiO₂ and Zr ratios of the sandstone and shale in the Lower Gondwana confirms the felsic source. In contrast, the Upper to post-Gondwana shale supports felsic to intermediate igneous rock as the source of clastic input (Figure 12b).

The interplay between tectonism and depositional systems significantly influences the geochemical compositions of sediments [6]. Active tectonism with steep slopes enables rapid sediment discharge, while tectonic quiescence with gentler slopes allows longer residence times and more weathering during transport. The REE abundances in source rocks and the weathering conditions in source areas are the major factors controlling the REE in the accumulating sediments. Hence, REE in clastic sedimentary rocks is widely used as the primary indicator to identify the provenance [18]. The La-Th-Sc discriminant diagram distinguishes between continental and oceanic island arcs, while the Th-Sc-Zr/10 diagram differentiates active from passive continental margins [18]. Plotting all shale and sandstone samples from the Gondwana and the post-Gondwana units on a La-Th-Sc (Figure 13a) discrimination ternary plot reveals that the sandstone source areas predominantly derive from Oceanic Island Arc (OIA) and Th-Sc-Zr/10 (Figure 13b) indicate mixed source sandstones mainly from Oceanic Island Arc and shale from Continental Island Arc.

Like the Th/Sc ratios, the La/Sc ratios (Table 8) mainly lie within the felsic rock range (2.5–16.3) [12], indicating mixed sources with a dominant felsic contribution. Specifically, the La/Sc ratios range from 0.33 to 11.51, averaging 4.20 (Table 8), suggesting contributions from felsic sources. The tectonic setting (Figure 14a) can be revealed using a log K₂O/Na₂O vs. SiO₂ (a) discriminant diagram [15] and discriminant function diagram (Figure 14b) [73], which indicates that the analyzed samples are from the active continental margins (Figure 14).

5.4. Organic Matter Enrichment and Preservation

Organic matter preservation and enrichment in shale are primarily influenced by redox conditions, paleoclimate, depositional environments, and tectonic settings. Submarine hydrothermal activity, warm and humid climates, and suboxic to anoxic conditions contribute significantly to organic richness. For instance, the Gondwanan basins transition from glaciomarine deposits in high-latitude areas to Permian fluvial systems consisting of coal, shale, siltstone, and carbonaceous shale [37], with climate fluctuations affecting facies composition [74]. Trace elements like Ni and Cu, often associated with OM, indicate high primary productivity and OM flux under reducing conditions [4]. The presence of these elements in the post-Gondwana shale is higher than in the Gondwana shale (Table 7), especially alongside high barium concentrations, highlighting significant OM deposition in marine settings due to nutrient influx from hydrothermal activity [63]. V/(V + Ni) ratios in the analyzed samples range from 0.49 to 0.87, indicating dysoxic to anoxic conditions [17] during the deposition (Figure 15), except for one sample from the Gondwana unit (Table 8). The post-Gondwana Eocene shale is enriched with vanadium or nickel, suggesting algal-derived organic matter encountered anaerobic conditions early in its settling, enhancing the preservation and enrichment of organic matter during this depositional environment. The low V/Cr ratios (<2) and Sr/Cu values (>5) indicate an oxic depositional climate less favorable for the preservation of organic matter due to oxidative degradation. The result of the analyzed samples (Table 8) suggests that the paleo depositional environment fluctuates regularly from shallow marine to fluvial during the deposition of post-Gondwana deposits.

The post-Gondwana, Bhainskati Formation experienced rapid and frequent transgressions and regressions, leading to sedimentary changes and multi-layer thin coal seams [19]. The organic matter primarily derives from marine organisms like sponges and diatoms, leading to the initial formation of organic-rich shale. The northward shift of the Indian plates enhances hydrothermal fluid upwelling, further contributing to black shale formation during the post-Gondwana deposit. Although long-term transgressions reduced productivity, their initial impact fostered conditions that favored OM preservation. Moderate Ba/Sr ratios indicate fluctuating productivity and preservation conditions, particularly in marine-influenced settings. Previous analyses in the same area reveal a TOC content ranging from 0.40% to 2.80% across various shale samples in the post-Gondwana [68]. However, in the present study, the TOC of 11 samples ranging from 0.56 to 1.56% indicates low to moderate organic carbon content (Table 2), amplifying predominantly continental sources of organic matter. The better preservation of OM in post-Gondwana shallow marine shales above 1% of TOC, enriched with clay minerals, contrasts with the lower preservation rates in the Gondwana fluvial shales. These higher clays protect the organic matter from decay and enhance preservation [1]. The depositional environment was primarily oxic with moderate preservation of organic matter, consistent with terrestrial and shallow marine settings during post-Gondwana deposition.

The tectonic activity associated with the Himalayan orogeny has played a crucial role in influencing the maturity of organic matter within both Gondwana and post-Gondwana shales. Due to significant burial depths and intense tectonic stress during this period, the organic matter has undergone considerable thermal alteration, reaching an over-mature stage suitable for shale gas generation. Previous studies on vitrinite reflectance (Ro%) of the analyzed samples from this area indicate a range of 1%–3%, suggesting that the samples are in the mature to overmature stage. Additionally, the pyrolysis results reveal a type III kerogen, which is conducive to dry gas generation [75]. As the Himalayas were exhumed, numerous local structural features emerged along the orogenic belt, substantially affecting the post-Gondwana shales. The development of various tectonic structures following the deposition of post-Gondwana shales, notably the Main Central Thrust (MCT), Main Boundary Thrust (MBT), Ranimatta Thrust (RT), along with local faults and folds, had a profound impact on geological conditions (Figure 8e). The overturning associated with the MBT and RT resulted in the creation of submerged tectonic traps, significantly increasing the source rocks’ burial depth. This alteration provided optimal conditions for the thermal maturation of hydrocarbons in areas like Tansen and Surkhet. Moreover, the activation of the MBT and the occurrence of local structural features have thrust the Gondwana and post-Gondwana strata within the Lesser Himalayan zone. This thrusting has transformed primary oil and gas reservoirs into secondary reservoirs, enhancing their potential for resource extraction [67]. The geological and tectonic activity, coupled with the favorable mineralogical composition of the post-Gondwana shale, is a prospective target for unconventional gas exploration in the Lesser Himalayan region.

6. Conclusions

The geochemical and mineralogical characteristics of shales and sandstones from Gondwana and post-Gondwana units in the Tansen and Surkhet basins exhibit distinct variations in mineral content and depositional environment.

• The petrographic analysis of the Gondwana shale contains liptinite macerals, suggesting lacustrine environments, whereas vitrinite in the Eocene suggests a substantial contribution from terrestrial plants to organic matter deposition in marine environments. The Upper and post-Gondwana sandstone samples exhibit more quartz over feldspar and plagioclase. The quartz grains are sub-angular to rounded, suggesting a moderate degree of transportation and abrasion before lithification;
• In the Gondwana unit, quartz and plagioclase are prominent, especially within the sandstones, which indicates a significant influx of clastic material. In contrast, the post-Gondwana unit predominates illite as the primary clay mineral, reflecting intense weathering at the source. SiO₂ is the most abundant major element, followed by Al₂O₃ across the Gondwana and the post-Gondwana units. The CIA values of the analyzed samples exceed those of the PAAS, indicating moderate to extreme weathering of the source area;
• The geochemical analysis demonstrated through ratios and plots, such as La-Th-Sc, suggests that these sediments were predominantly derived from weathered rocks, with a significant contribution from granitic sources. The pronounced enrichment of LREE relative to HREE further supports a felsic provenance. The sedimentary records reveal variation in the paleo-depositional environment over geological time and the movement of the Indian plate towards the Eurasian plate;
• The elemental ratios analyzed suggest an oxic, dry, and cold climate during the deposition of the Lower Gondwana sediments, transitioning to a warmer environment during the deposition of the Upper and post-Gondwana units. This climatic shift is evidenced in preserving organic matter, with Gondwana shale displaying low TOC, attributed to oxic conditions and dilution by terrestrial inputs in contrast to the post-Gondwana shales. This result concludes that shales were primarily deposited in shallow marine environments and predominantly continental sources of organic matter. These findings suggest the shales are poorly enriched and have poor preservation of organic matter in the Gondwana shales compared to the post-Gondwana counterparts, aligning with deposition in an oxic environment;
• The paleo-depositional environment of the Gondwana and post-Gondwana units in the Lesser Himalayas of Nepal reveals a complex interplay of geological processes and climatic shifts. The Lower Gondwana was deposited in a continental rift valley, while the Upper Gondwana represents a passive continental margin. Gondwana sediments are predominantly sourced from the Greater and the Tethys Himalayas, with evidence of Indian-affinity detritus. In contrast, the post-Gondwana sediments primarily reflect the Himalayan provenance arising from the continental–continental collision;
• The tectonic evolution of the Himalayas has significantly influenced the sediment source and depositional processes in the post-Gondwana deposits. The organic matter in these sediments is highly matured and prone to gas generation, attributed to the Himalayan tectonism. The shifts in tectonic activity and related erosion and weathering patterns have profoundly impacted the sediment composition and organic matter preservation observed in these sequences.