Sedimentary Facies and Geochemical Signatures of the Khewra Sandstone: Reconstructing Cambrian Paleoclimates and Paleoweathering in the Salt Range, Pakistan

Abstract

Red sandstones of the Cambrian age are globally distributed and represent an important sedimentation phase during this critical time interval. Their sedimentology and geochemistry can provide key information about the sedimentation style, paleoclimatic conditions, and weathering trends during the Cambrian. In the Salt Range of Pakistan, the Khewra Sandstone constitutes the Lower Cambrian strata and consists of red–maroon sandstones with minor siltstone and shale in the basal part. Cross-bedding, graded bedding, ripple marks, parallel laminations, load casts, ball and pillows, desiccation cracks, and bioturbation are the common sedimentary features of the formation. The sandstones are fine to medium to coarse-grained with subangular to subrounded morphology and display an overall coarsening upward trend. Petrographic analysis indicates that the sandstones are sub-arkose and sub-lithic arenites, and dolomite and calcite are common cementing materials. X-ray Diffraction (XRD) analysis indicates that the main minerals in the formation are quartz, feldspars, kaolinite, illite, mica, hematite, dolomite, and calcite. Geochemical analysis indicates that SiO₂ is the major component at a range of 53.3 to 88% (averaging 70.4%), Al₂O₃ ranges from 3.1 to 19.2% (averaging 9.2%), CaO ranges from 0.4 to 25.3% (averaging 7.4%), K₂O ranges from 1.2 to 7.4% (averaging 4.8%), MgO ranges from 0.2 to 7.4% (averaging 3.5%), and Na₂O ranges from 0.1 to 0.9% (averaging 0.4%), respectively. The results of the combined proxies indicate that the sedimentation occurred in fluvial–deltaic settings under overall arid to semi-arid paleoclimatic conditions with poor to moderate chemical weathering. The Khewra Sandstone represents the red Cambrian sandstones on the NW Indian Plate margin of Gondwana and can be correlated with contemporaneous red sandstones in the USA, Europe, Africa, Iran, and Turkey (Türkiye).

1. Introduction

The Cambrian Period represents a pivotal time interval in the Earth’s geological history. Climate change, continental drift, the emergence of new life forms, and the catastrophic extinction of a significant percentage of species were the numerous events that occurred during this period [1]. The Earth’s surface temperature increased dramatically (from an average of 12 °C to around 22 °C), changing the global ice age that preceded this time. Polar or high-altitude glaciers hardly existed during the Cambrian [2], and the eustatic sea level was relatively high [3,4]. The continents were mostly low-lying deserts and alluvial plains, and during the Sauk transgression, the rising Cambrian sea encroached upon these regions, creating huge epicontinental seas [5,6,7].

Early Cambrian red sandstones, which are widely distributed across the globe (e.g., in India, Iran, Turkey (Türkiye), the Arabian Peninsula, Egypt, Poland, Sweden, the United States, and Canada), serve as valuable sedimentary records for reconstructing sedimentation styles, paleoclimatic conditions, and weathering patterns during this pivotal geological interval (e.g., [1,2,3]). As such, these have been the focus of numerous studies exploring Cambrian sea-level fluctuations [3,4,5], continental configurations, and biotic evolution [7,8], as well as sediment provenance, basin development, and chemical weathering trends [9,10,11,12,13,14,15]. These investigations generally indicate that red Cambrian sandstones were deposited in fluvio–deltaic settings under arid to semi-arid paleoclimatic conditions, characterized by low to moderate degrees of chemical weathering. Details on Cambrian correlation are provided in Chapter 7 (Regional and Global Correlation) to avoid repetition.

In Pakistan, the Salt Range is a crucial region for understanding the geological and paleoclimatic changes that occurred on the Indian Plate during the Late Neoproterozoic to Early Cambrian [8,9]. The first strata from the Indian Plate, identified as Cambrian, are found in the Salt Range [10]. The Khewra Sandstone is the lowermost unit of the Cambrian Jhelum Group and shows disconformable contact with the overlying Kussak Formation [9,10,11]. The formation consists of red–maroon sandstones with some shale in the lower part and is distributed throughout the Salt Range and the Khisor Range [12]. Despite being an essential reservoir rock in the area, the sedimentation style, paleoclimates, and weathering trend of the formation have not been focused on in detail. A greater understanding of the sedimentation style and weathering trend of the Khewra Sandstone may provide valuable information for regional and global reconstruction of the Cambrian paleoclimates and weathering trend. Hence, the present study focuses on the sedimentation style, the paleoclimate, and the weathering trend during the deposition of the Khewra Sandstone, in the Salt Range, Pakistan. This study uses outcrop data, petrography, bulk mineralogy (XRD), and bulk rock geochemistry data to contribute to our understanding of the sedimentation style, the paleoclimatic variation, and the weathering trend during the Cambrian.

2. Paleogeography and General Geology

During the Early Cambrian, Gondwana was the dominant landmass that formed following the breakup of the supercontinent Rodinia. It included present-day Africa, South America, Antarctica, Australia, Europe, India, and parts of Asia (Figure 1a) [13]. Gondwana rotated counter-clockwise, causing the eastern margin to drift northward [14,15]. By the end of the Cambrian, this continental mass had migrated southward again, likely reaching between 10° and 15° south latitude [6]. The Salt Range [16,17,18] and the northwestern margin of the Indian Plate [19,20] occupied a high-latitude paleogeographic position during the Cambrian, likely between 30° and 60° south of the equator [21,22,23,24]. At that time, the region lay along the northern passive margin of Gondwana and was influenced by arid to semi-arid climatic conditions, which favored the deposition of extensive evaporite and siliciclastic sequences [21,22,23,24,25]. The Khewra Sandstone was deposited unconformably over the salt and gypsum-rich evaporites of the underlying Precambrian Salt Range Formation [16,17], reflecting a transition from restricted evaporitic to more open marine and fluvial environments. The Indian Plate, as part of Gondwana, contributed to this paleogeographic setting [18,19,20,21,22].

Today, the Khewra Sandstone is part of the Himalayan foreland basin (Kohat–Potwar Plateau). The underlying Salt Range Formation plays a critical structural role as a detachment surface that accommodates thrusting associated with the Himalayan orogeny [10,17]. Some studies have noted an unconformity between the Khewra Sandstone and the Salt Range Formation, potentially marked by a magmatic intrusion known as the “Khewra Trap”. The “Khewra Trap” was named as “Khewrite” [27]. It has a purple–green color, is 6 m thick, and consists of highly decomposed radiating needles, probably composed of pyroxene [9]. However, the nature and significance of this intrusion remain insufficiently documented and warrant further clarification.

The Salt Range is prominently exposed along the east–west trending Salt Range Thrust (SRT), extending between the Jhelum River in the east and the Indus River in the west (Figure 1b,c) [28]. The Cambrian strata in this area, collectively referred to as the Jhelum Group, begin with the Khewra Sandstone, which is overlain sequentially by the Kussak, Jutana, and Baghanwala formations [8]. The pioneering study on the Khewra Sandstone was conducted by Wynne [29], who used the term “Purple sandstone series” for the formation. Noetling [30] used the term “Khewra Group” for this rock unit, and it was formally revised to the “Khewra Sandstone” by the Stratigraphic Committee of Pakistan in 1973 [31]. The Khewra Gorge near Khewra town in the eastern Salt Range is the type locality of the formation. Excellent exposures are found in the Nilawahan Gorge and Vasnal Village near Kallar Kahar in the central Salt Range. Khan Zaman Nala in the western Salt Range serves as the principal reference section of the formation [11].

3. Materials and Methods

3.1. Fieldwork

Stratigraphic sections of the Khewra Sandstone were studied at four locations, including the Khewra Gorge (KG), Nilawahan (NW), Vasnal (VS), and Khan Zaman Nala (KZ) in an east-to-west transect covering the eastern, central, and western Salt Range (Figure 1 and Figure 2). One hundred and seventy-seven (177) samples were collected, encompassing all facies variations (

Table 1. Geographic locations and sample details of the studied sections of the Khewra Sandstone.

S/No	Section	Location	Thickness (m)	Samples
				Total	Sandstone	Shale	Petrography	XRD	Geochemistry
1	Khewra Gorge (KG)	32°40′5.26″ N 73°0′13.68″ E	143	55	49	6	33	15	20
2	Nilawahan Gorge (NW)	32°38′13.19″ N 72°35′22.80″ E	78	53	48	5	26	9	16
3	Vasnal Village (VS)	32°42′54.13″ N 72°32′5.49″ E	22	19	17	2	12	4	8
4	Khan Zaman Nala (KZ)	32°32′6.88″ N 71°48′5.02″ E	67	50	42	8	29	12	16

).

Standard sedimentological procedures, e.g., [32], were followed to study the sedimentary structures and features, including observations of the lateral and vertical facies variations. Various types of cross-beds, parallel lamination, beds, deformation structures, ripple marks, erosional surfaces, and oxidized zones were identified in the field. Important features and structures were described and photographed, and lithological samples were collected from essential beds.

3.2. Petrography

Based on sedimentological variation, 100 samples were selected from four studied outcrops, and thin sections were prepared in the Department of Geology, Bacha Khan University, Charsadda, Pakistan. The grain size, shape, texture, and composition of each sample were studied using a Nikon ECLIPSE LV100ND microscope with a Nikon ECLIPSE 5MP HD camera (Nikon, Tokyo, Japan) in the sedimentology lab, National Centre of Excellence in Geology (NCEG), University of Peshawar, Pakistan. Point counting [32,33,34] was the primary focus of the petrographic studies, and 500 grains were counted for each sample (thin section). The quartz grains were petrographically subdivided into monocrystalline quartz (Q_m) and polycrystalline quartz (Q_p). Further, Q_m was subdivided into monocrystalline quartz with unit extinction (Q_mue) and monocrystalline quartz with undulose extinction (Q_muu). Likewise, polycrystalline quartz was subdivided into polycrystalline quartz with two to three quartz crystals (Q_pq(2–3)) and polycrystalline quartz with more than three quartz crystals (Q_pq>3). Chert was included in Q_p [35]. Feldspars (F) were categorized as alkali feldspars (K) and plagioclase feldspars (P). Lithic fragments (L) have the same meaning as rock fragments (R) [36] and include sedimentary lithics (L_s), metamorphic lithics (L_m), volcanic lithics (L_v), and metavolcanic lithics (L_vm). Plutonic lithic fragments were subdivided into constituent grains (visible under a microscope) following the Gazzi–Dickenson point counting method [35]. The total lithic fragments (L_t) are the sum of L_s, L_v, L_m, and L_vm [36]. The data obtained were plotted and interpreted following the practices of the existing literature (various QFL plots) [32,37,38,39,40,41,42].

3.3. X-Ray Diffraction (XRD) Analysis

Based on field observation and petrographic studies, 40 samples from four sections were selected for XRD analysis (Table 1). The XRD was applied to identify various mineral species and to determine the quantitative mineralogical composition of the Khewra Sandstone. The XRD analysis was conducted in the chemistry lab of the TAIYUAN University of Technology, China, using a HAOYUAN DX-2700B X-ray diffractometer (XRD) (Haoyuan Instrument Co., Ltd., Dandong, China), a Cu Kα radiation source (40 KV, 30 mA) at a scanning rate of 10°/min and 2θ ranging from 5° to 70°.

3.4. Geochemistry

Geochemical proxies are key tools in the paleoclimatic and paleoenvironmental interpretation of sedimentary rocks [26,43,44,45,46,47,48,49,50,51]. Therefore, geochemical analysis of the bulk sedimentary composition of 60 selected samples was conducted using X-ray fluorescence (XRF) spectroscopy at a commercial lab. Each sample was powdered, homogenized, and then analyzed for 90 to 120 s. The analysis included major and minor oxides and trace elements.

Various geochemical proxies were used to interpret paleoclimates and paleoenvironments of the Khewra Sandstone. These included the Chemical Index of Alteration (CIA) to estimate the intensity of chemical weathering and paleoclimate reconstruction [47], C-values to describe the paleoclimate [51], Al₂O₃ vs. CIA_molar and K₂O/Na₂O vs. CIA_molar plots to distinguish between the arid, subtropical, and tropical paleoclimate types [48], a Al₂O₃, CaO* + Na₂O, and K₂O ternary plot called the A-CN-K plot [48,52,53], and a binary plot of SiO₂ against the total Al₂O₃ + K₂O + Na₂O [37], as the quantity of Al, Si, K, and Na in sediments is strongly controlled by compositional variation [54,55]. Other proxies, such as Sr/Cu, Rb/Sr, and Sr/Ba, were also used to evaluate the paleoclimatic conditions during sedimentation [56,57]. Similarly, the V/Cr and Cu/Zn proxies were used to interpret the paleo-oxygenation level [58,59,60].

The combined analyses of field data, petrographic studies, XRD, and sediment geochemistry yielded reliable results, which helped us to interpret the depositional paleoenvironments and paleoclimate of the Khewra Sandstone.

4. Results

4.1. Outcrop Data and Lithofacies

Outcrop observations during the fieldwork (Figure 3a–k) identified five distinct lithofacies in the four studied sections of the Khewra Sandstone (

Table 2. A summary of the lithofacies and lithofacies associations identified within the Khewra Sandstone based on [61,62]. The table outlines key sedimentological characteristics, including the lithology, sedimentary structures, grain size, and interpreted depositional processes and environments for each facies and facies association.

Code	Lithofacies	Key Features	Vertical and Lateral Distribution	Lithofacies Association	Depositional Settings
CMFShL	Channel Margin and Floodplain Shale Lithofacies	Laminated, maroon and olive-green shale, low bioturbation	Lower parts, repeated in middle and upper parts of KG (3 cycles), KZ (4 cycles), NW (1 cycle); absent in VS	Channel margin, overbank	Fluvio–deltaic; channel margin and overbank areas
CMSL	Channel Margin Sandstone Lithofacies	Fine-grained sandstone, ripples, desiccation cracks, cross bedding, soft sedimentary deformation	Lower parts, some occurrences in middle and upper sections of KG, KZ, and NW	Channel margin, distributary channels	Fluvio–deltaic with low- to moderate-energy conditions; channel margin, distributary channels, delta front, mouth bars
DLDPL	Delta Lobe/Delta Plain Lithofacies	Interbedded fine sandstone and shale, cyclic variations in thickness	Present in all sections with interbedding cycles	Delta lobe, floodplain	Cyclic deposition indicating fluctuating flow energy; delta plain environment
CBSL	Channel Belt Sandstone Lithofacies	Medium to coarse-grained sandstone, tabular and tangential cross-beds, planar lamination, herringbone stratification	Middle parts, tabular cross-beds widespread in KG, NW, and KZ; herringbone cross-beds in KG and NW	Channel belts	High-energy depositional settings, such as fluvial and delta distributary channels and delta fronts
CBCL	Channel Belt Conglomerate Lithofacies	Clast-supported quartz conglomerate	Contact with the Kussak Formation, found at the top in KG and vs. only	High-energy river discharge, transition zone	High-energy fluvial settings marking transitions to deltaic conditions

). The characteristics of each lithofacies are discussed below:

4.1.1. Channel Margin and Floodplain Shale Lithofacies (CMFShL)

This lithofacies consists of olive green and maroon shale (Figure 3a). The maximum thickness of the shale is 75 cm in the KZ section, and the minimum observed thickness (4 cm) is in the KG section. The Channel Margin and Floodplain Shale Lithofacies (CMFShL) occurs in the lower part of the KG, NW, and KZ sections and is repeated three times in the KG outcrop, four times in the KZ outcrop, and is also observed in the uppermost part of the NW section, while the lithofacies was not observed in the vs. section. The shales are generally laminated and are overlain by channel margin sandstones.

4.1.2. Channel Margin Sandstone Lithofacies (CMSL)

The Channel Margin Sandstone Lithofacies (CMSL) comprises maroon colored, fine-grained, moderately well-sorted sandstones (Figure 3b). Thin-bedded, fine-grained sandstones are present in the lower portions of the KG, NW, and KZ sections. In contrast, sandstones in the vs. section occur in the upper part. The maximum thickness of these fine sandstones is ca. 85 cm in the KG section and ca. 80 cm in the KZ section. Ripple marks (mostly asymmetrical), including in-phase catenary ripples, occur in the KG section (Figure 3c). However, climbing ripples are present in the middle part of the KG outcrop. Desiccation cracks are present in the middle parts of the KG and KZ sections. The upper part of the CMSL displays load and cast structures (Figure 3d). Deformational sedimentary structures, including balls and pillows, slumps, and convolute beds, are seen in the lower parts of the KG and NW sections (Figure 3e).

4.1.3. Delta Lobe/Delta Plain Lithofacies (DLDPL)

The Delta Lobe/Delta Plain Lithofacies (DLDPL) contains the interbedding of fine-grained maroon sandstone and shale, which is present in the lower parts of all four studied sections (Figure 3f). The thickness of the lithofacies is generally 60–80 cm. The alternating sandstone beds that underlie the shale are thicker in the KZ section compared to other sections. The maximum thickness of alternating thicker sandstone layers with thinner shale layers from the KZ section is 13.5 m. The lithofacies is repeated four times in the KG, two times in the NW, once in the vs., and three times in the KZ section.

4.1.4. Channel Belt Sandstone Lithofacies (CBSL)

The Channel Belt Sandstone Lithofacies (CBSL) consists of medium to coarse and very coarse-grained sandstone with maroon and light grey colors. The sandstones display tabular cross-bedding, low-angle and tangential cross-bedding, and herringbone cross-stratification. Tabular cross-bedding occurs in all the studied sections. Low-angle and tangential cross-beds are restricted to the middle part of the KG section. The angle between these cross-beds is less than 20 degrees. Planar bedding in medium to coarse-grained sandstone is observed in the middle parts of all four studied sections and is repeated two times in the upper portion of the KG, NW, and KZ sections and three times in the vs. section. These vary in thickness from 60 cm (VS) to 8 m (KG). Planar cross-beds are present in the lower parts of the KG, NW, and KZ sections. Trough cross-beds are found only in the lower part of the KG section (Figure 3g), where the thickness of the trough cross-bedded strata is 8.54 m. Herringbone cross-stratifications occur in the middle parts of the KG, NW, and KZ sections (Figure 3h).

4.1.5. Channel Belt Conglomerate Lithofacies (CBCL)

The Channel Belt Conglomerate Lithofacies (CBCL) is a clast-supported conglomerate layer at the top of the Khewra Sandstone that marks contact with the overlying Kusak Formation (Figure 3i,j). This lithofacies is present in the KG and vs. sections and could not be observed in the NW and KZ sections. The clast size varies from coarse to very coarse pebbles (10 cm in diameter). The conglomerate mainly comprises quartz pebbles (Figure 3i) with some sedimentary, metamorphic, and volcanic rock fragments.

4.2. Petrography

Petrographic studies of the Khewra Sandstone indicate that quartz is the dominant component of sandstone, followed by feldspar and lithics (Figure 4 and Figure 5; Supplementary Material Table S1). Muscovite, biotite, rutile, zircon, and minor opaque minerals also occur (Figure 4a–f). Dolomite, iron oxide, and calcite are the dominant cementing materials (Figure 4b). Quartz averages 68.1% of the total framework mineralogical composition, ranging from 57.0 to 84.6%, and Q_m predominates, averaging 63.7%, while Q_p content averages 4.4%. The grains vary in size from fine to medium to coarse, and texturally they are angular, sub-angular to sub-rounded, while well-rounded and elongated quartz grains were also noticed. Generally, the grains are moderate to well-sorted (Figure 4d). The grain size in the NW and vs. sections is typically medium to coarse-grained, while in the KG and KZ sections, it starts from fine-grained sandstone or siltstone and coarsens in the upper parts. Point/tangential contacts are most common, while long-linear, concavo–convex, and suture contacts are also observed (Figure 4c). Feldspars are relatively fresh, and F in the framework grain averages 6%, ranging from 3.1 to 9.8%. K predominates, averaging 4.6%, while P averages 1.4% of the total framework composition (Figure 4c). The L contents range from 1.8 to 6.6% (averaging 3.7%) and include L_s = 2.0%, L_m = 0.6%, and L_v = 1.1% (Supplementary Material Table S1).

4.3. XRD Data

The XRD analysis of the Khewra Sandstone indicates that quartz is the dominant mineral (67.3%), followed by hematite (6.8%) and calcite (6.6%). Feldspar (both k-feldspar and plagioclase), illite, mica (mostly muscovite), kaolinite, and dolomite are the other common minerals (Figure 6). Quartz increases in the upper section of the KG, VS, and KZ sections while staying stable in the NW section. In contrast, dolomite decreases in the upper parts of the KG and KZ sections. Hematite is high in the shale samples from the middle part of the KG section. Feldspars are comparatively low in the NW section and relatively decreased in the upper part of the KZ and vs. sections. Illite, smectite, and chlorite are abundant in the associated shale samples. Kaolinite is also found in minor amounts throughout the formation.

4.4. Geochemistry Data

Geochemical analysis indicates that SiO₂ is the most significant component, ranging from 53.3 to 88.04% (averaging 70.40%), followed by Al₂O₃ ranging from 3.05 to 19.17% (averaging 9.27%), CaO ranging from 0.4 to 25.26% (averaging 7.38%), K₂O ranging from 1.23 to 7.42% (averaging 4.79%), MgO ranging from 0.2 to 7.42% (averaging 3.53%), and Na₂O ranging from 0.14 to 0.87% (averaging 0.45%), respectively (Supplementary Material Table S2). Comparatively, the average SiO₂ content is faintly higher in the NW section, averaging 72.91%. Meanwhile, Al₂O₃ shows the highest average in the KG section (10.20%).

4.4.1. Chemical Index of Alteration (CIA)

The average CIA value is 62.2, ranging from 50.7 to 71.8 in all four sections. The average CIA value for the KG section is 62.6, and it has a range of 50.8–71.7 (Supplementary Material Table S2). The average CIA values for the NW, VS, and KZ sections are 59.4 (ranging from 51.5 to 69.8), 58.8 (ranging from 50.7 to 65.1), and 62.2 (ranging from 56.1 to 71.5), respectively. The KG section shows a relative drop in the uppermost part, with a CIA value of (50.8). However, the CIA value remains generally low throughout the formation with minimal variation. Meanwhile, the KG section appears to have the highest CIA value in the shale (71.7). In general, the shales of the formation have continuously higher CIA values than their coarse-grained counterparts (siltstones and sandstones). In that order, the shale and fine-grained sandstone lithofacies such as the CMFShL, the CMSL, and the DLDPL show relatively high CIA values, and the CBSL of the middle and upper portions and CBCL show low CIA values, respectively.

4.4.2. The Al₂O₃ vs. CIA_molar and K₂O/Na₂O vs. CIA_molar Plots

The Al₂O₃ vs. CIA_molar and K₂O/Na₂O vs. CIA_molar plots (Figure 7a,b) show samples clustering in the range of (1–3) for CIA_molar and in the ranges of 3–19 and 1–39 for Al₂O₃ and K₂O/Na₂O, respectively. The Al₂O₃ values are comparatively high in the lower parts and slowly decrease in the upper parts of the formation. The highest values of Al₂O₃ and CIA_molar are plotted for the shales and siltstones. The lowest Al₂O₃ values are plotted for the coarser sandstone. The fine-grained sandstones/siltstones with desiccation cracks show Al₂O₃ in the 10%–13% range.

The K₂O/Na₂O ratio displays a significantly wide range (2.61–37.35) among all four sections. Its values decrease towards the upper part of the KG and KZ sections, while there are no major changes in the NW and vs. sections. The higher ratio (37.35) of K₂O/Na₂O occurs in the lower part of the formation, and the lowest value (2.61) relates to the upper part of the formation. Although the Na₂O content consistently remains below 0.9%, the larger fluctuations in the K₂O/Na₂O ratio are attributable to a sudden drop in Na₂O content.

4.4.3. The C-Values

The C-value (C = {(∑Fe + Mn + Cr + Ni + V + Co) + (∑Ca + Mg + Sr + Ba + K + Na)} (Figure 7c) plot is used to interpret the paleoclimate. The average C-value for the present sandstones is 0.19, ranging between 0.05 and 0.65, and the average for the shales is 0.67, ranging from 0.32 to 1.2. The highest C-value is seen in the shales from the middle portion of the KG section.

4.4.4. Chemical Maturity Plot (SiO₂ vs. Al₂O₃ + K₂O + Na₂O)

The chemical maturity plot (SiO₂ vs. Al₂O₃ + K₂O + Na₂O) displays samples clustering between 50 and 90 on the y-axis and 5 and 30 on the x-axis (Figure 7d). The SiO₂ content ranges from 53.3 to 85.5 throughout the formation, where in the KG it ranges between 53.3 and 85.5, in the NW ranges from 66.1 to 78.0, in the vs. range from 61.1 to 74.3, and in the KZ range from 58.2 to 84.3. The Al₂O₃ + K₂O + Na₂O ranges from 4.7 to 24.9 in the studied sections, where in the KG, it ranges between 4.7 and 24.9, in the NW ranges from 9.9 to 21.4, in the vs. range from 11.9 to 23.04, and in KZ ranges from 8.5 to 23.8. The SiO₂ (quartz) increases upward in all sections in contrast to feldspars; hence, the highest SiO₂ concentrations are associated with lower Al₂O₃ + K₂O + Na₂O values. The shale from all four sections contains < 63% SiO₂ with more than 22% Al₂O₃ + K₂O + Na₂O.

4.4.5. A-CN-K (Al₂O₃-CaO*+ Na₂O-K₂O) and A-N-K (Al₂O₃-Na₂O-K₂O) Plots

In both the A-CN-K and A-N-K plots, the samples cluster near the A-vertex along the A-K side in the muscovite position (>90%) (Figure 8a,b). Three shale samples from the KG section and one sample each from the NW, VS, and KZ sections cluster near the illite position (<10%). Fine-grained sandstone clusters near muscovite, while medium- and coarse-grained sandstones cluster close to muscovite and shift slightly toward the K-feldspar position, respectively. The Al₂O₃ and K₂O display a wide range for the KG, VS, and KZ sections, while in the NW section, these display uniformity in concentration. Na₂O indicates stable levels within the vs. section and low to moderate variability in the KZ, NW, and KZ sections, respectively. The CIA values for the samples are plotted to the left side of the A-CN-K plot to reflect the chemical weathering intensity corresponding to the composition evolution of the samples.

4.4.6. Al₂O₃/Na₂O vs. CIA and Al₂O₃/K₂O vs. CIA Plots

The Al₂O₃/Na₂O vs. CIA and Al₂O₃/K₂O vs. CIA plots were used to evaluate the paleoclimatic conditions and weathering trend during the deposition of the Khewra Sandstone (Figure 9a,b). The Al₂O₃/Na₂O ratio averages 27.4 and varies between 6 and 77.4. The highest Al₂O₃/Na₂O value is marked in the CMSL of the lower part of the KG section. The average Al₂O₃/K₂O ratio in all four sections is 2.1, ranging from 0.8 to 2.8, with slightly higher values corresponding to shale. However, the lowest Al₂O₃/K₂O ratios occur in the DLDPFL.

4.4.7. Weathering Index of Parker (WIP) vs. CIA Plot

The WIP values are plotted on a logarithmic scale along the Y-axis. The WIP vs. CIA plot shows samples clustering between 10 and 100 on the Y-axis (Figure 10). All The highest WIP values are obtained from the shale samples (CMFShL) of the lower part of the KZ section. In contrast, the lowest WIP values are obtained from the coarse-grained sandstone (CBSL) of the uppermost part of the KG section. In the KG, VS, and KZ sections, the WIP values increase upwards except in the shale samples, while they stay more or less the same in the NW section.

4.4.8. K/Na, Sr/Ba, Rb/Sr, V/Cr, and Cu/Zn Ratios

The K/Na, Sr/Ba and Rb/Sr ratios are used for paleoclimate and paleo-weathering trends, and the V/Cr and Cu/Zn ratios are used to evaluate paleo-oxygenation levels. Presently, data for the KG section (type locality) is included (Figure 11). The K/Na ratio is relatively high (>10) in the lower part of the KG section, particularly in the CMFShL and CMSL lithofacies, reaching higher values (>30) in KG-13 (41.79) and KG-16 (38.04). The middle and upper parts display low values (<10), with the lowest value of 2.92 in KG-51. The Sr/Ba ratio yielded low values (<0.3) throughout the formation, with a peak value (0.31) in the shale samples (e.g., KG-34) of the middle part. The Rb/Sr ratio generally indicates low values (<1) in the formation, and the lowest value of 0.27 corresponds to KG-51 in the upper part. The lower part of the formation yielded comparatively higher values (Figure 11, Supplementary Material Table S2). The V/Cr ratio of the formation is always below 2. The lowest ratio of 0.1 corresponds to KG-16. The Cu/Zn ratios for the formation always remain below 3, and the lowest value of 0.01 occurs in KG-34. The highest value of 2.92 occurs in KG-54 (the uppermost part).

5. Discussion and Interpretation

5.1. Depositional Lithofacies and Lithofacies Associations

5.1.1. Channel Margin and Floodplain Lithofacies Associations (CMFLAs)

The presence of the CMFShL that features laminated shale with an olive-green and maroon color is indicative of a fluvio–deltaic setting in low-energy, channel margin, or overbank-floodplain areas [70,71]. This reflects subaerial episodic sedimentation in channel margin and the overbank-floodplain regions of fluvio–deltaic settings during flood/high water levels [72,73,74,75], a characteristic of shallow deltaic or marginal marine settings [76]. The laminations indicate low bioturbation and undisturbed sedimentation in protected channel margins or deltaic overbank settings [77,78]. Such laminations also reflect water column stability, where minimal mixing occurs, supporting a shallow marine or marginal marine interpretation [78,79]. The vertical up-section repetition of the CMFShL in the studied sections suggests cyclic depositional events, likely driven by the combined interplay of flood pulses, channel migration, and sea-level fluctuations typical of fluvio–deltaic settings [80]. The absence of the CMFShL in the vs. section may suggest relatively higher energy, possibly a more proximal setting, where fine sediments could not settle [81,82,83].

Fine sandstones of the CMSL with thin beddings suggest deposition in low-energy environments such as abandoned distributary channels or delta fronts [84,85]. Ripple marks generally represent deposition in shallow, subaerial fluvio–deltaic environments characterized by fluvial and tidal processes [86]. Asymmetrical ripples indicate a unidirectional transport (generally a fluvial process), while in-phase catenary ripples suggest regular tidal action [87,88]. Thus, the CMSL supports sedimentation in the delta front, mouth bars, or channel margin/distributary channels, where fluvial currents dominated but were modulated by tidal influences [89]. The presence of load and cast structures suggests rapid sedimentation events, potentially linked to flooding or high-energy conditions (on channel margins), causing the deformation of water-saturated sediments [90]. The desiccation cracks in the KG and KZ sections imply a fluvio–deltaic setting with alternating wet and dry periods [91,92,93]. Deformational sedimentary structures reflect soft-sediment deformation due to rapid sedimentation and liquefaction in deltaic, fluvial, or shallow marine environments [94]. Ball and pillow structures, slumps, and convolute beds indicate instability from gravitational forces or seismic activity [95]. Deformation occurs when rapidly deposited, water-saturated sediments are loaded or destabilized, leading to slumping or internal sediment deformation before lithification [96]. Thus, the closely associated depositional features allow the CMFShL and CMSL of the Khewra Sandstone to be grouped into the CMFLA.

5.1.2. Delta Plain Lithofacies Association (DPLA)

The interbedded sandstone and shale of the DLDPL indicate cyclic changes in flow energy and sediment supply, which may suggest deposition in channel margin and delta plain settings [61]. Similar deposition can also occur in distributary channels and inter-distributary areas with periods of alternate calm (shale) and high-energy events (sandstone) [97,98]. The alternation signifies episodic sedimentation, with fluctuations between energetic flow (coarser sandstone deposition) and quiescent periods (fine-grained shale deposition), typical of deltaic environments where sediment supply varies with fluvial processes and water level changes [99,100]. The presence of alternate thicker sandstone beds with thinner shale beds suggests progradation and aggradational conditions in delta lobe systems of the fluvio–deltaic settings [101,102,103,104]. Therefore, the DLDPL represents the DPLA within the Khewra Sandstone.

5.1.3. Channel Belt Lithofacies Associations (CBLAs)

The presence of tabular cross-beds in the CBSL highlights an active fluvial system where medium to coarse-grained, moderately to well-sorted sandstones were deposited in channel belt/bar setting [55,105,106]. Planar bedding reflects low-energy water flow and steady sedimentation typical of fluvio–deltaic environments [107]. Low-angle and tangential foreset cross-beds imply deposition in lower-energy areas, such as river mouths or the delta front, where sediment accumulates as the water flow slows [108]. The planar bedding in the medium to coarse-grained sandstones indicates relatively high energy conditions and upper flow regimes in channel belt depositional settings [109].

The variation from light grey to maroon colors suggests periods of oxidation and reduction, with maroon suggesting more oxidizing conditions [110]. Thickness variations (60 cm to 8 m) reflect sedimentation rates and accommodation space changes, with thicker beds indicating periods of higher sediment influx and/or more significant accommodation space [111]. Planar cross-beds represent deposition by unidirectional currents in fluvial, deltaic, or shallow marine settings [84,112]. Trough cross-beds indicate deposition in moderate-energy environments with unidirectional currents, typical of fluvial or shallow marine settings, particularly within channels [113,114]. Herringbone cross-stratification forms by alternating cross-beds inclined in opposing directions, indicating fluctuating flow directions influenced by tidal and variable fluvial forces [115].

The occurrence of pebbly beds of the CBCL in the upper part of the formation suggests a high-energy fluvial environment, likely channel belt deposits [86]. The moderate sorting indicates periods of fluctuating energy conditions during high-energy events (e.g., floods), where larger pebbles were mobilized, while lower-energy conditions allowed the deposition of finer sediments in the pore spaces, resulting in a bimodal sedimentary fabric [116,117]. Clast-supported conglomerates suggest dynamic sediment transport conditions typical of high-energy environments [118]. Very coarse pebbles (10 cm in diameter) imply energetic sediment transport during periods of increased fluvial discharge [119,120,121]. The uppermost conglomerate bed of the formation may indicate localized deposition, likely reflecting a high-energy flood event [112,122]. Therefore, the CBCL indicates a transition zone influenced by high-energy flow conditions typical of fluvial channels or delta fronts [123,124]. The channel belt features of the CBSL and CBCL allow these to be placed in the CBLAs.

5.1.4. Vertical Facies Architecture

Shifts in the depositional environment can be interpreted by the vertical arrangement of the lithology and grain size variations [125,126,127,128]. As per “Walter’s law of facies association”, the coarsening upward succession observed in the formation signifies regression and progradation of the delta and/or sea-level fall [129,130]. Therefore, the presence of shale in the lower part of the Khewra Sandstone indicates a sea-level high stand (transgression) and deposition in relatively deeper water [131,132,133,134]. The overlying medium to coarse-grained sandstones of the CMSL and the CBSL support deposition during progressive regression and sea-level low-stand [135]. Therefore, the overlying coarser-grained delta front and delta plain sandstones indicate delta progradation [136,137]. Similarly, the apparent upward thickening of the sandstone beds of the CBSL in all four sections and the occurrence of CBCL in the uppermost parts of the formation also suggest an increase in sediment supply and sea-level fall, thereby supporting the delta progradation throughout the deposition of the Khewra Sandstone [138,139].

5.2. Petrography

The framework grain composition, size, sorting, roundness, and matrix are the petrographic features of sandstone that reveal several key aspects of the depositional environment, paleoclimate, and weathering trend during sedimentation [140,141,142]. The present petrographic results suggest a mixed depositional environment system dominated by fluvio–deltaic and deltaic shallow-marine settings. The high amount of quartz with a sub-angular to sub-rounded texture and moderate to well-sorted grains signifies fluvio–deltaic sedimentation [143,144,145,146,147,148,149,150,151].

Subangular to subrounded coarse-grained sandstones are characteristic of fluvio–deltaic systems, possibly distributary channels, where higher energy promotes the transportation and deposition of coarse material [149,150,151,152,153,154,155]. In contrast, a matrix of fine-grained clays indicates deposition in quiet-water, much-protected deltaic environments such as back swamps, tidal flats, or lagoons [156,157,158]. The upward coarsening sequence points to deltaic progradation [159]. The present sandstones are classified as sub-arkose and sub-litharenite (Figure 5a), suggesting a transitional depositional environment, typically between continental and marine settings. These sandstones are commonly associated with fluvial, deltaic, and shallow-marine settings where sediments are derived from uplifted areas and transported toward a basin [160,161,162].

The composition of the framework grains, especially the amount of quartz and the common occurrence of feldspar and lithics, aligns with arid conditions [163,164]. The relatively low alteration of feldspars and the presence of fresh feldspars suggest an arid to semi-arid paleoclimate during deposition [165,166]. Quartz is highly resistant to both chemical dissolution and mechanical disintegration, and its dominance, along with the occurrence of fresh, unaltered feldspars, suggests that mechanical weathering rather than chemical weathering played a more significant role in sediment production [167,168]. In tropical or humid climates, feldspars tend to weather rapidly into clay minerals such as kaolinite due to chemical weathering [169,170]. However, in the present case, the fresh and unaltered feldspars suggest limited chemical weathering, resulting in the prevalence of an arid climate lacking moisture and hydrolysis [171,172,173,174].

The paleoclimate, paleo-weathering, and their control on sandstone mineralogy can be obtained by QFL, QFR, binary log/log plots of Q_P/(F + R) vs. Q_t/(F + R), and ln(Q/F) vs. ln(Q/R) plots [33,38,39]. Presently, the samples are clustered in the sub-arkose arenite field along the QF side of the plot, suggesting a semi-humid climate during deposition (Figure 5a,b). The binary log/log diagram (Figure 5c) is a sensitive plot for the indication of paleoclimatic conditions [40]. This plot indicates that the Khewra Sandstone was deposited in semi-arid to semi-humid climatic conditions. The ln(Q/F) vs. ln(Q/R) plot (Figure 5d), which is generally used to represent the Cumulative Chemical Weathering Index (CCWI), suggests that the weathering intensity in the region was low to moderate, as determined by the equation Iw = C × R, following Weltje’s methodology [40].

Overall, the framework composition plots (Figure 5a–d) generally do not support the presence of a hot and humid paleoclimate during the deposition of the Khewra Sandstone. Instead, these indicate semi-arid to semi-humid paleoclimatic conditions. However, since the QFL plots consider only three parameters (quartz, feldspars, and lithic fragments) and are limited to sand-sized grains, their effectiveness in distinguishing between semi-arid and semi-humid climates is constrained [40]. This limitation in paleoclimatic interpretation based solely on framework mineralogy was addressed in the present study by incorporating comprehensive bulk sediment geochemistry. This approach not only includes fine-grained sediments but also accounts for a broader range of geochemical variables simultaneously [40,48,52]. The significance of bulk sediment geochemistry is further elaborated in Section 5.4.

5.3. XDR Data

The common occurrence of feldspars in the present samples suggests mechanical denudation [167,175,176,177]. This supports the deposition of fine-grained sandstone/siltstone and shales, with higher feldspar content, under low chemical weathering conditions [162,178,179]. The relative drop of feldspars in the upper parts of the KZ and vs. sections suggests increased weathering, possibly linked to climatic shifts from arid toward more humid, warm conditions [180].

Dolomite and calcite indicate carbonate cementation, and their presence, particularly the high dolomite content in the lower part of the KG section, suggests conditions conducive to carbonate precipitation linked to phreatic zone diagenesis, rich in saline/marine groundwater [181,182]. The decrease in dolomite in the upper parts of the KG and KZ sections could reflect a shift towards more clastic-dominated environments. This is possibly tied to changing sea-level or reduced carbonate productivity and the presence of fresh groundwater in the pore spaces of the sandstones during the diagenesis [183]. This, together with the occurrence of iron oxide (hematite), is typically associated with oxidative conditions, potentially tied to subaerial exposure and vadose zone diagenesis or oxygenated shallow marine conditions during sedimentation [184,185].

Clay minerals (kaolinite, illite, smectite, and chlorite) generally occur in low quantities, implying hydraulic sorting and differential removal of the fine sediments during deposition [186,187]. Kaolinite typically forms under warm, humid conditions with intense chemical weathering [188,189]. Its low average throughout the formation suggests semiarid to subtropical conditions [190]. In contrast, illite, smectite, and chlorite are usually associated with lower-temperature diagenetic processes and show low chemical weathering during their deposition [191,192]. Their presence may indicate colder and arid climatic conditions or limited chemical weathering, reflecting climatic variability during the deposition of the Khewra Sandstone [193,194,195,196].

5.4. Bulk Geochemistry

This study employs various geochemical proxies derived from major oxides and trace elements to interpret the intensity of chemical weathering and paleoclimatic conditions during the deposition of the Khewra Sandstone (Figure 7, Figure 8, Figure 9, Figure 10 and Figure 11). To maintain clarity and conciseness, a synthesized interpretation and discussion of these proxies is presented below.

5.4.1. Chemical Weathering Intensity

The CIA is a reliable proxy for interpreting the intensity of chemical weathering, with higher values reflecting progressively intense chemical weathering [64,197], and is used for paleoclimatic reconstructions [42]. The low average CIA value (61.2) for the Khewra Sandstone suggests poor chemical weathering and arid climatic conditions (Figure 8, Figure 9, Figure 10 and Figure 11). Even the high CIA values (up to 71.8) in fine-grained shale samples are still considered moderate, implying semi-arid conditions [198,199,200,201]. The minimal variation in the CIA throughout the studied sections suggests an overall regional arid to semi-arid paleoclimate characterized by poor to intermediate chemical weathering [202,203,204].

As poor chemical weathering is generally linked to arid and semi-arid climatic conditions, the mola CIA plots (Al₂O₃ vs. CIA_molar and K₂O/Na₂O vs. CIA_molar plots) [50] can confirm this interpretation. Presently, these plots indicate an arid climate for the formation (Figure 7a,b). The high amount of Al₂O₃ in the lower parts and its relative decrease upward indicate a decrease in clay/shale content within the formation in the vertical direction [205,206,207]. The fluctuation in the K₂O/Na₂O ratio depends on feldspar alteration. Na and K are mobile elements found mostly in plagioclase and k-felspar, respectively. Thus, the sudden drop in Na₂O in the KG-13 and KG-16 samples of the CMSL lithofacies in the lower part of the KG section may be related to plagioclase alteration. Moreover, the low K₂O/Na₂O ratio suggests poor chemical weathering, potentially tied to an arid climate [208,209,210].

The C-values [50] are used for paleoclimatic interpretation and were used here to validate the molar CIA results. Higher C-values (>0.8) indicate humid conditions, values of 0.4–0.8 indicate semi-humid conditions, while lower values (<0.4) suggest semi-arid to arid paleoclimatic conditions [211]. Presently, the average C-value for the Khewra Sandstone is 0.25 (Figure 7c), indicating deposition under arid paleoclimatic conditions. The majority of the samples (63%) with C-values less than 0.2 indicate deposition under an arid climate. Of the remaining, 23% have C-values of 0.2–0.4, indicating semi-arid conditions. The remaining samples (dominantly shales in the lower part) have values ranging from 0.4 to 0.8, indicating semi-arid to semi-humid paleoclimate, and may be related to increased weathering in wetter climates [212]. The higher Fe₂O₃ in the shales may be an alternative reason for slightly higher C-values in the shale, rather than intense weathering in a humid paleoclimate [211].

The chemical maturity plot (SiO₂ vs. Al₂O₃ + K₂O + Na₂O) is a relationship between silica (and thus quartz), alumina (Al₂O₃), and alkali elements (K₂O + Na₂O) (and thus feldspars) that provides insight into paleo-weathering and paleoclimatic conditions [65]. The clustering of samples in the semi-arid to semi-humid fields (Figure 7d) indicates that the Khewra Sandstone was deposited under poor to moderate weathering and, hence, an arid to semi-arid paleoclimate [26,213]. The high SiO₂ content of the sandstones forces these samples to be plotted close to the humid field, higher on the y-axis (50–90), in all the sections, and may yield a misleading interpretation for a humid paleoclimate. Therefore, shales/clays, the weathering products of coarse-grained rocks, are the more reliable indicators of weathering conditions [49,64]. Presently, shale samples from all four reflect semi-arid to arid paleoclimatic conditions (Figure 7d).

The A-CN-K and A-N-K ternary plots indicate paleoclimatic conditions and paleo-weathering trends and include reference minerals and standards for correlation [213]. The cluster of samples near the illite–muscovite position along the A-K side indicates low to moderate chemical weathering that is likely in arid to semi-arid settings (Figure 8a,b). The low CIA values (plotted to the left side of the triangles) support this interpretation [63,214].

The debate that Na₂O, K₂O, and Al₂O₃ may be related to plagioclase and k-feldspar’s alteration allows the use of Al₂O₃/Na₂O vs. CIA and Al₂O₃/K₂O vs. CIA plots to interpret paleoclimate and paleo-weathering linked to such alterations [215]. Generally, the low ratios of Al₂O₃/Na₂O and Al₂O₃/K₂O show less alteration of plagioclases and k-feldspar, respectively, while their high ratios suggest their intense alteration [216]. Presently, both plots show low to moderate chemical weathering under an arid to semi-arid paleoclimate (Figure 9a,b) [50]. The slightly high Al₂O₃/K₂O in the shale samples from the lower and middle parts of all four sections may indicate k-feldspar alteration [217]. In addition, the Al₂O₃/K₂O ratio in shales can also be influenced by their compositional characteristics, as the clay minerals (illite, kaolinite, and chlorite) are typically associated with shales [218,219]. However, the highest ratio of Al₂O₃/Na₂O in KG-13 (77.39) and KG-16 (69.40) in the CMSL’s lower part of the KG section suggests plagioclase alteration in moderate chemical weathering [220,221,222].

The WIP vs. CIA plot can also validate the weathering conditions indicated by the CIA [66]. The cluster of samples on the WIP vs. CIA plot indicates a weathering trend between low and moderate, likely under arid to semi-arid climatic conditions (Figure 10). The CMFShL and fine-grained CMSL lithofacies, which exhibit high WIP values, indicate relatively moderate weathering, while samples from the DLDPL lithofacies show comparatively low WIP values, suggesting mild or weak chemical weathering. The sample’s population on the plot unanimously negates strong weathering and rules out hot and humid paleoclimatic conditions [220,221].

5.4.2. Paleoclimate, Depositional Setting, and Paleo-Oxygenation Level

The K/Na, Sr/Ba, Rb/Sr, V/Cr, and Cu/Zn ratios help interpret the paleoclimate, the weathering intensity, and the paleo-oxygenation levels of the depositional environment [60,223,224]. Higher K/Na ratios indicate the chemical weathering trend of plagioclase and k-feldspar (discussed in the previous section). Under arid to semi-arid climatic conditions, the plagioclase content (and hence Na content) is not differentially removed due to poor chemical weathering in these settings, resulting in low K/Na ratios [26,169]. Presently, the K/Na ratio shows low values (<20), thereby indicating poor chemical weathering and deposition under arid to semi-arid paleoclimate (Figure 11). The increase in the K/Na ratios in KG-13 (41.79) and KG-16 (38.04) shows an increased alteration of plagioclase and possibly a climate shift [225]. An alternative mechanism for high K/Na ratios in these samples may be the lack of plagioclase in the parent rocks. However, sediment source switching for such a short period is highly unlikely to have occurred. In general, the high K/Na ratios in the shale can be attributed to the presence of illite-rich clay minerals, as revealed in the XRD (Figure 6).

Terrestrial sediments are generally rich in Ba and depleted in Sr, while marine sediments are usually the other way around [226,227], thereby making the Sr/Ba ratio a good proxy for identifying a shift from terrestrial to marine environments [228,229,230]. Generally, freshwaters (and thus fluvial and terrestrial settings) have low Sr/Ba ratios (<1.0), brackish waters (e.g., delta front settings) have Sr/Ba ratios of 1.0–3.0 that increase to 3.0–8.0 in saltwater (e.g., prodelta), and normal marine settings have Sr/Ba ratios of > 8.0 [225]. In the present case, the Sr/Ba ratio is always <0.5 (Figure 11), strongly supporting deposition in fluvial/terrestrial settings. Sr and Ba behave differently in response to chemical weathering and leaching. Sr is more easily dissolved and carried away by water than Ba. However, in arid to semi-arid climatic conditions where there is less rainfall, and surface water is limited and Sr can still be leached (removed) by any available water, while Ba tends to stay in place because it does not dissolve as easily [231]. This means sandstones retain more Ba and lose more Sr over time, leading to a lower Sr/Ba ratio in the rock (Figure 11).

Rubidium (Rb) is easily adsorbed during weathering processes by illitic clays, while Sr is leached in the process [232]. This makes the Rb/Sr ratio a good indicator of the intensity of chemical weathering in paleoclimatic interpretations [128]. In the present case, the Rb/Sr ratios have a low range of 0.27 to 1.17, indicating poor chemical weathering and, thus, arid to semiarid paleoclimatic conditions during the deposition of the Khewra Sandstone (Figure 11).

The interpretation of the weathering trend in the absence of paleo-oxygenation data can be misleading. The V/Cr ratio is used to interpret paleo-oxygenation levels of the depositional environments [233]. A V/Cr ratio of >4–5 is generally considered high and indicates reducing, often anoxic conditions, typically in marine or organic-rich depositional environments. A V/Cr ratio of <2 is generally considered low and indicates oxidizing conditions, common in continental or terrestrial depositional settings [233,234]. In the present scenario, the V/Cr ratios are <2, suggesting oxic conditions in continental/terrestrial environments (Figure 11) [235,236].

The V/Cr ratios as paleo-oxygenation level proxies can be verified by the Cu/Zu ratio. The Cu/Zn ratio in sandstones depends on the depositional conditions. In oxic environments, Zn is often more mobile compared to Cu, and in reducing conditions, Cu tends to be more stable and enriched compared to Zn [237,238]. Therefore, a lower Cu/Zn ratio suggests more oxygen-rich conditions, and a higher Cu/Zn ratio might indicate reducing conditions [239]. In the present case, the Cu/Zn ratios are generally <2 (Figure 11), indicating deposition of the Khewra Sandstone under oxic conditions.

6. Paleoclimate and Sedimentation

6.1. Paleoclimate and Paleo-Weathering Trend

The lithofacies and lithofacies association (Table 2), petrographic analysis (Figure 5), XRD analysis (Figure 6), and geochemical proxies (Figure 7, Figure 8, Figure 9, Figure 10 and Figure 11) used in this study provide coherent evidence for an arid to semi-arid paleoclimate with low to moderate chemical weathering during the deposition of the Khewra Sandstone. The vertical grain-size increase observed in the formation indicates the gradual onset of fluvial conditions and an associated sea-level fall.

The angular to subrounded, poor to moderate sorting of the quartz grains and the fresh to weakly altered nature of the minerals like feldspar and lithics (Figure 4) suggest more mechanical weathering than chemical disintegration and align with arid conditions [206,240,241]. The QFL and the Q_P/(F + RF) vs. Q_t/(F + RF) plots (Figure 5a–c) suggest semi-arid to semi-humid climatic conditions. The ln(Q/F) vs. ln(Q/R) plot (Figure 5d) shows that the weathering intensity in the region was low to moderate.

The common occurrence of feldspars and clay minerals like illite, smectite, and chlorite, as observed in the XRD analysis (Figure 6), implies low chemical weathering under arid to semi-arid conditions. The low average of kaolinite negates intense leaching and dissolution, negating tropical conditions [242,243]. Illite, smectite, and chlorite are generally the products of mechanical/physical weathering under colder and arid climatic conditions [244,245] and thus reflect deposition of the Khewra Sandstone in arid to semi-arid conditions.

The geochemical proxies, including the low to moderate CIA value (<72) and the positions of samples in the arid fields of Al₂O₃ vs. CIA and K₂O/Na₂O vs. CIA plots, indicate arid conditions of deposition (Figure 7a,b). Similarly, the C-values and the chemical maturity plot (SiO₂ vs. Al₂O₃ + Na₂O + K₂O) further support arid to semi-arid deposition conditions for the Khewra Sandstone (Figure 7c,d). The A-CN-K and A-N-K ternary plots confirm the low-to-moderate chemical weathering intensity. The bivariate plots of CIA vs. Al₂O₃/K₂O and CIA vs. Al₂O₃/Na₂O (Figure 9a,b) and the WIP vs. CIA plot (Figure 10) provide further supporting evidence for low to moderate weathering conditions and, hence, arid to semi-arid conditions during the Khewra Sandstone’s deposition. The elemental ratios (K/Na, Sr/Ba, and Rb/Sr) also support weathering under arid to semi-arid climatic conditions (Figure 11).

6.2. Depositional Environments and Sedimentation Style

The sedimentary structures and features observed in the outcrop, lithofacies, and lithofacies associations indicate deposition of the Khewra Sandstone in fluvio–deltaic and deltaic to shallow marine sub-environments. The CMFShL indicates a fluvio–deltaic environment with episodic sedimentation in channel margin and overbank/floodplain areas. Similarly, the occurrence of the CMSL suggests a fluvio–deltaic setting with low-to-moderate energy conditions in the channel margin to delta-top areas. The presence of DLDPL points to a cyclic deposition, indicating fluctuating flow energy in a delta-plane environment. The prominent sandstones of the CBSL suggest high-energy depositional settings, such as channels fill, point bars, or deltaic plains, and the presence of the CBCL indicates high-energy fluvial settings marking transitions to deltaic conditions. The coarsening upward sequence points to delta progradation, where the depositional environment shifts from a deeper shallow marine to fluvial settings over time. The Khewra Sandstone’s thickness variation across the eastern, central, and western Salt Range may highlight local tectonic influences or differences in subsidence rates influenced by the accommodation space available for sedimentation. The elemental proxies, including Sr/Ba, Rb/Sr, V/Cr, Ni/Co, and Cu/Zn (Figure 11), indicate deposition in well-oxygenated conditions. Therefore, considering all the available evidence, the depositional system of the Khewra Sandstone can be interpreted as a fluvio–deltaic to deltaic shallow marine environment, where sedimentation occurred under well-oxygenated floodplains, distributary channels, mouth bars, the delta front, and the delta plain (Figure 12).

7. Regional and Global Correlation

Cambrian red sandstones have a global scale distribution (Figure 13) and have remained the subject of several studies in various localities, including the Lalun Formation in the Shirgesht area of Central Iran, the Zabuk Formation in south-eastern Turkey (Türkiye), the Mt. Simon Sandstone in western Ohio, Midcontinent, and the Hickory Sandstone in Texas, USA, the Sandstone succession in the Podlasie region of East Poland, the Araba Formation in the east Sinai of Egypt, the Hardeberga Formation in Scania of southern Sweden, and sandstone from the Anti-Atlas region (Morocco), exhibit similarities with the Khewra Sandstone in the lithological, depositional environment, and paleoclimate characteristics.

7.1. Correlation with Iran and Turkey (Türkiye)

The Lalun Formation in the Shirgesht area, Central Iran, consists of felsic-rich, quartzose sandstones that suggest a mixed shallow-marine, fluvio–deltaic, and tide-dominated coastal depositional system. The texturally and compositionally immature strata of the formation indicate poor chemical weathering and thus arid to semi-arid conditions [246]. The formation represents Cambrian siliciclastics within a peri-Gondwana epicontinental basin situated along the northeast margin of the Arabian Shield [247,248]. Thus, both the Khewra Sandstone and the Lalun Formation were deposited on similar Gondwanan margins and represent fluvio–deltaic to shallow marine deposition under arid to semi-arid climates, with sediment influx from craton sources [247].

The Zabuk Formation of Turkey (Türkiye) is a thick succession of Early Cambrian, mixed, fluvial–shallow marine siliciclastic that was deposited on the northern margin of Gondwana. The detrital assemblage of the formation implies poor chemical weathering, suggesting an arid to semi-arid paleoclimate [249]. Thus, the formation represents an Early Cambrian fluvial–shallow marine mixed siliciclastic system on the northwestern margin of Gondwana (the northern margin of the Arabian Plate) similar to the Khewra Sandstone.

7.2. Correlation with the USA

The Cambrian Mt. Simon Sandstone in western Ohio represents siliciclastic sedimentation under tidally influenced transgressive barrier–fluvial systems with tidal channels and mixed flats. The formation is the product of broad continental interior deposition with poor chemical weathering under semi-arid to perihumid settings [250,251]. Similarly, the Hickory Sandstone is a siliciclastic Cambrian succession in Texas, USA, and consists of medium to coarse-grained sandstones, conglomerates, and siltstones. Cross-beds, graded beds, and ripple marks are the common features in the formation, and the formation was deposited in fluvial to deltaic shallow marine settings [252,253]. These formations indicate an Early Cambrian continental to marginal marine depositional system with coastal depositional energy and arid to semi-arid paleoclimatic conditions like those interpreted for the Khewra Sandstone in the present work.

7.3. Correlation with Egypt and Morocco

In eastern Sinai, Egypt, the Early Cambrian Araba Formation consists of sandstones, siltstone, and shale. Cross-bedding, ripple marks, and graded bedding are common sedimentary features. The textural and compositional immaturity of the formation indicates its deposition in a continental-shallow marine setting characterized by poor chemical weathering under an arid paleoclimate [254,255,256]. The formation presents lithological similarities with the Khewra Sandstone and was deposited on the Gondwanan margin with sediments sourced from the Arabian–Nubian Shield.

Similarly, the Cambrian sandstones from the Anti-Atlas region, Morocco, consist of poorly sorted quartz and unaltered feldspars. The formation represents fluvial–deltaic and sheet sands deposition under an arid to semi-arid Gondwanan setting [257]. The formation shares a similar provenance to the Khewra Sandstone, linked to Pan-African orogeny, and was deposited under a similar climatic regime.

7.4. Correlation with Sweden, Poland, and Norway

The Cambrian Hardeberga Formation in Scania, southern Sweden, displays sedimentary structures such as hummocky cross-stratification, ripples, and tabular cross-beddings. It is interpreted to have been deposited as a fluvio–deltaic shallow marine depositional unit under an arid to semi-arid paleoclimate on the Baltica continental margin [258]. Similarly, the Cambrian succession in the Podlasie region, East Poland, consists of conglomerate, sandstone, heterolith, and mudstone. Sedimentary features include massive, parallel lamination, cross-lamination, ripple cross-lamination, flaser and wavy lamination, and bioturbations, and indicate a fluvio–deltaic shallow marine environment. The presence of angular quartz and fresh feldspars indicates poor chemical weathering and sedimentation under an arid to semi-arid paleoclimate at the Baltic’s western rifted margin [259,260]. Thus, the Hardeberga Formation and Podlasie successions share striking similarities in depositional settings, weathering trends, and paleoclimatic conditions with the Khewra Sandstone. This allows correlation between the Early Cambrian sedimentation on the Baltica continental margin and the northwestern Indian Plate margin in a Gondwanan setting.

The Breidvika Formation from Northern Norway is dated as Lower Cambrian and comprises mudstone, siltstone, and sandstone. It includes sedimentary structures such as ripples, cross-bedding, and hummocky cross-stratification. The formation is suggested to have been deposited in a deltaic environment, showing cyclic coarsening upward sequences under well-oxygenated conditions [261]. Thus, the formation represents a deltaic-shallow marine Baltica equivalent of the Khewra Sandstone.

The Nagaur Sandstone from Rajasthan, India, is dated to the Lower Cambrian period [9]. It primarily consists of silty shale and sandstone. The formation is interpreted to have been deposited in a deltaic environment, and the region likely experienced a warm, arid to semi-arid climate during sedimentation. Similarly, the Amin Formation from North Oman spans from the Lower to Middle Cambrian and comprises sandstone, siltstone, shale, and conglomerate. The depositional environment for this formation is illustrated as an alluvial fan and aeolian, and the deposition occurred under arid to semi-arid climatic conditions [262,263]. Likewise, in Jordan, the Salib Formation from the Lower Cambrian strata includes sandstone, shale, and conglomerate. It was deposited as a fining upward sequence in braided stream–alluvial depositional settings under arid to semi-arid conditions [264]. These formations represent stratigraphic equivalents of the Khewra Sandstone in the nearby Gondwanan terrains and indicate sedimentary provenance associated with the Pan-African orogeny.

The lower Cambrian Cerros San Francisco Formation in Uruguay consists mainly of sandstones and siltstones. The depositional setting for this formation is described as a shallow marine environment with a fining upward sequence [265]. Also, the Billy Creek Formation, from the Lower to Middle Cambrian strata in South Australia, comprises claystone, siltstone, and salt pseudomorphs. Deposited in a fluvio–deltaic environment, this formation displays a coarsening upward trend [188,212]. Furthermore, Namibia’s Lower Cambrian Stockdale Formation consists of shale, sandstone, and calcareous nodules. The formation shows a fluvio–deltaic deposit with a coarsening upward sequence [266,267]. All these stratigraphic units reflect textural angularity and compositional immaturity that is attributed to poor chemical weathering under arid to semi-arid paleoclimatic conditions, similar to those that prevailed during the deposition of the Khewra Sandstone.

Figure 13. Correlation of Khewra Sandstone with regional and global red sandstones [9,187,246,249,250,251,254,257,259,261,263,264,265,266].

Figure 13. Correlation of Khewra Sandstone with regional and global red sandstones [9,187,246,249,250,251,254,257,259,261,263,264,265,266].

8. Conclusions

The Lower Cambrian Khewra Sandstone (Salt Range, Pakistan) primarily consists of maroon–red sandstone with minor shale and siltstone in the lower sections.

• The formation exhibits a coarsening upward succession with moderate to well-sorted, angular to sub-rounded, fine to coarse-grained, cross-bedded sandstones.
• Lithofacies analysis and lithofacies associations support deposition environments that include fluvio–deltaic to shallow marine settings, encompassing distributary channels, channel margins, floodplains, delta plains, and mouth bars.
• Framework mineralogy classifies the sandstones as sub-arkoses and sub-litharenites, indicating limited chemical weathering and deposition in arid to semi-arid paleoclimates.
• XRD analysis advocates for sedimentation under arid to semi-arid climatic conditions.
• Geochemical proxies support deposition in oxic, arid to semi-arid environments with poor to moderate chemical weathering intensity.
• Lithological and environmental attributes, along with paleoclimate and weathering trends, align the Khewra Sandstone with globally recognized Cambrian red sandstone successions from the USA, Europe, Africa, Iran, Turkey, Arabia, and India.

These findings enhance the understanding of Early Cambrian sedimentary systems along the Gondwanan margin. Further research, including single-grain provenance studies, isotopic analysis, and geochronology, is recommended to clarify the formation’s relation to the Pan-African orogeny and the broader Gondwana Assembly.