GEOCHEMISTRY OF PAB SANDSTONE EXPOSED IN LAKI RANGE LOWER INDUS BASIN: PROVENANCE AND TECTONIC IMPLICATIONS

Geochemistry of the late Cretaceous Pab Sandstone deposited at the Laki Range, Southern Indus basin, was carried out in order to acquire a better knowledge of the source rock and paleoclimate. The unit in the studied region comprise predominantly sandstone, with some mudstone. The major element geochemistry of Pab Sandstone suggests that the samples were composed of abundant SiO 2 , Al 2 O 3 , CaO, and Fe 2 O 3 . Other major oxides comprise less than 2 wt. %. Major element discriminatory diagrams additionally imply that Pab sediments originated from a granitic source. The idea that the Pab sediments were generated by a granitic source is further supported by the rich concentration of trace elements including Zr, La Th, Ba, and Sr, in contrast to V, Ni, Cr, Co, and their ratios. It is conceivable that the Pab Sandstone was formed by sediments from granitic rocks, as indicated by the La/Sc and Th/Co model, as well as the TiO 2 and Zr plots. The measured Plagioclase Index of Alteration (PIA) values, which range from 54.35 to 84.89, and the Chemical Index of Alteration (CIA), which ranges from 53.81 to 79.60, indicate that the studied sediments experienced moderate to strong weathering under warm, humid climate conditions. Geochemical findings indicate that the Pab Sandstone sediments originated from the vast Indian Shield (Craton


INTRODUCTION
The term Pab Sandstone was first used by Vredenburg [1] which originated from the Pab Range.[2] designated the area of west of Wirahab Nai, at latitude 25°31'12" N and longitude 67°00'19" E, as the type section for the unit exhibiting a thickness of 490 meters.The Pab Sandstone in the Northern Laki Range is characterized by an exceptionally thick succession of sandstone, with some gravel beds and a relatively low volume of fine lithologies; such as silt and mud.The abundant cross bedding; both planar and trough, is reflected in this lithostratigraphic unit.Compared to planar cross bedding, trough cross bedding is more prevalent.It is overall red to reddish brown in color and medium to coarse grained in size, with moderate to high compaction.The sandstone and mudstone beds also contain other typical sedimentary structures like asymmetrical ripple marks and mud cracks.The lithologic trend in the gravel beds exhibit a discernible fining up at specific locations within the Pab Sandstone outcrops.The non-clastic sequences of the Fort Munro Formation lie beneath the unit, at few places in the study area whereas in most part of the study area lower contact of the formation is not exposed, while the Khadro Formation lies on top of it.Maastrichtian age is attributed to the Pab Sandstone strata based on fauna [1,3].The provenance analyses are typically performed to identify the source area of the sediment by utilizing different stratum properties.Since different types of source rocks produce different suits of detrital minerals, they can be used to reveal characteristics specific to the source rock from which they originated.The geochemical makeup of siliciclastic rocks is widely utilized to identify their composition, weathering history, and aid in the paleogeographic reconstruction of their original terrains.Additionally, information regarding provenance, tectonics, and paleoclimate can be obtained from their geochemical synthesis [4,5,6,7,8,9,10,11].
The various localities within the Sulaiman and Kirthar Fold-Thrust belts of the Middle and Lower Indus Basin represent the exposures of the late Cretaceous Pab sandstone.The Middle Indus Basin has been the focus of much research on this significant lithostratigraphic unit, primarily in relation to facies analysis, provenance, and reservoir characteristics [12,13,14,15,16].While the studies mentioned above provide a good overview of the deposition, sedimentary source, transportation, and diagenetic character of the Pab Sandstone, further discussion of the geochemistry including major and trace elements of the exposures of this rock unit in the Laki Range, Southern Indus Basin is needed.The current study aims to infer the provenance, tectonics, and paleoclimate utilizing geochemical analysis on a prominent stratigraphic section of the Pab Sandstone, namely the Burr Dhoro (BD) section in the Laki Range (Fig. 1).

GEOLOGICAL SETTING
The study area is part of the Laki Range in Southern Indus Basin, Sindh, Pakistan.The Pab Sandstone stratigraphic section exposed in the northern extremities of the Laki Anticline of the Laki Range (Survey of Pakistan Topographic sheet number 35 N/16 that is part of Hunting Survey Corporation geologic map number 11 Bela 35 J-N) is selected for present study.This anticline is part of the Laki Range which in turn is fold and thrust belt, and is oriented in a North -South direction at regional scale.Moreover, this anticline has steep eastern flank and moderately dipping western flank, thus it is asymmetrical in nature.A north-south thrust fault system intersects the steep eastern slope, with potential displacement of up to 1000 meters [17].The oldest rocks in the base are exposed when this anticline reaches its maximum crest height in the northern extremes of the Laki Range.These rocks are subsequently being worn by ephemeral rivers.
The elevation of the Manchhar Formation (Miocene-Pliocene) and recent sediments show that the deformation of this anticline persisted well into the Pleistocene, even after the Miocene [17].The oldest unit in the region is the Fort Munro Formation, while the youngest unit is the Dada Conglomerate.The region is made up of exposed Cretaceous to Recent sedimentation records.Other than that, the Southern Indus Basin does not have any exposed Cretaceous to Recent age sedimentary strata.The occurrence of many lithologic characteristics in the area suggests that the entire sedimentation stack indicate events of sea level variations.In this location, basaltic flows in Paleocene age strata are also observed [18,19].The research area is bordered on the west by the Kirthar Range and Axial Belt, which includes Bela Ophiolites and on the east by the Thar desert and the Nagar Parker igneous complex (Indian shield).

SAMPLING AND ANALYTICAL METHODS
During field work, the rock samples were taken from the Burr Dhoro (BD) section, with a focus on the Pab strata.From the targeted region, a total of sixteen (16) samples were collected.Particular attention was given to recording every difference in lithology during the sampling process.Out of all the collected samples, nine (09) representative samples were chosen for the major and trace element geochemistry.Prior to the number these samples were labeled with the prefix BD designating the name of the section sampled.The description of the selected samples is given in Table 1.The S4 Pioneer X-Ray Fluorescence Spectrometer of the Bruker AXS, Germany, was utilized for the geochemical analyses comprising the major and trace elements for the chosen samples.The XRF apparatus was operated using "Spectra Plus," a common software program that measures the various elements included in the rock samples at various detection limits.Geochemical elemental analyses was performed using the Geo Quant, which is utilized for marketed earth materials.For the rock samples under study, the XRF machine's working parameters were 7 mA of current and 27 kV of voltage.Using the Geomajor application of XRF, the major element oxides (SiO2, Al2O3, TiO2, Fe2O3, MnO, CaO, MgO, Na2O, K2O, P2O5) were determined.For major elements, analytical precision is typically less than 2% relative.The USGS rock standards were used for the calibration.Major element data was reported using normalized percentages.[20] as well as [21] have evaluated the XRF analytical method in extensive detail.Using pressed powder pellets, the aforementioned X-ray fluorescence spectrometer has also been used to determine trace element concentrations.The best fraction of the chosen samples was meticulously chosen and thoroughly crushed.Additionally, extremely finely powdered samples have been homogenized using the conning and quartering procedure to circumvent the impact of particle size.

Major Oxides
The whole rock geochemistry reported in present study is based on the investigations of nine (09) samples.The Table 2 presents the results of the major element geochemical investigation of the Pab Sandstone samples exposed at the BD section.Table 2 displays the ratios of the major oxides of BDS samples.The Pab Sandstone in studied section has high concentrations of silica, iron, and aluminum (66.43%, 6.58%, and 14.62%, on average) respectively, whereas all other oxides are found in lesser proportions.The SiO2 concentration of the Pab Sandstone at BD section is the dominantly prevalent major oxide in all the studied samples; ranging from 58.96% to 69.95% with an average of 66.43%.The Si/Al ratio in the Pab Sandstone sediments varies considerably.Since the SiO2/Al2O3 ratio rises with increasing quartz concentration at the expense of less resistant minerals during weathering, transportation, and recycling, it is employed as a maturity indicator for sandstones [22,23].Furthermore, low ratios of these variables refer to clay-rich shales and are utilized to determine mineralogical maturity; greater SiO2/Al2O3 ratios points to quartz-rich sandstone [24].ratio was utilized by [23] to distinguish between the sandstone of Active Margin and Passive Continental Margin.According to [23] sandstones in active-margin settings have K2O/Na2O ratios smaller than 1, whereas those in passive-margin settings have ratios larger than 1.The average K2O/Na2O ratios at BD section are 2.86%, which indicate that the sediments under study belong to the passive margin.
Phosphorous oxide (P2O5), shows the presence of apatite, is present in the analyzed sediments at BD section and varies from 0.19 to 0.65%, with an average value of 0.41% in the studied samples.

Trace Elements
The trace element results of studied samples from the BD section are given in Table 4, whereas the Table 5     Sandstone sediments as reported by [11].
The existence of potassium feldspar, muscovite, biotite, and illite is suggested by the increased K2O relative to Na2O values, according to [46].Major oxides including SiO2, CaO, MgO, Na2O, and K2O were measured and plotted on a ternary diagram proposed by [31] in order to determine the provenance of the Pab Sandstone.Most of the samples in this diagram are dispersed near granitic sources, but some are plotted in between basaltic and granitic fields; this could be because of the minor contributions from the mafic sources.In the southeast of the research area, the Indian shield rocks contain minor intrusions in the form of diorite and dolerite dikes of various compositions and reported by many authors (e.g., [33,34,35,36,37,38]).

Geochemical parameters including TiO2
and Zr were used by [28] to distinguish acidic,

Provenance and Tectonics
To determine the provenance type, the major oxide parameters such as SiO2, CaO, MgO, Na2O, and K2O, as well as their      According to [42] weathering results in the decrease of alkali and alkaline earth elements.
To evaluate the paleoweathering trend, the Chemical Index of Alteration (CIA) and Plagioclase Index of Alteration (PIA) have been used for last few decades.The chemical weathering impact is minimal when the CIA value is less than 50, and it is highly significant when the CIA value is greater than 75.
According to

CONCLUSIONS
Based on geochemical investigations of Late Cretaceous Pab Sandstone exposed in the Laki Range, the most common oxide found is silica, which is followed by aluminum oxide, calcium oxide, iron oxide respectively; all other oxides are present in lesser amount. In

Fig. 1 .
Fig. 1.The geological map showing the studied section along with lithologies, in the inset the index map of Pakistan marking the study area are found in the investigated sediments as a result of the general abundance of quartz, though all the silica is not always the part of crystal structure of quartz but some of the silica is also accommodated within the clays along with certain other silicate minerals.Al2O3, which ranges from 12.19 weight percent to 20.68 weight percent, is the second most common oxide in the studied sandstone and can be related to the occurrence of certain clay minerals.[15, 11] reported low amount of feldspar in Pab Sandstone and suggested the potential alteration of feldspar into certain clay minerals.The overall abundance of Al2O3 in studied sediments may possibly be related with the alteration of feldspar.Fe2O3 and CaO range from 2.13 to 13.64 weight percent and 2.19 to 11.12 weight percent respectively in the sediments of Pab Sandstone; greater Fe2O3 is associated with iron oxide cement, while CaO is representative of calcite cement.As per [45] there exists a correlation between the K2O content and the orthoclase feldspar and mica content, whereas the plagioclase feldspar and Na2O content are linked.The investigated sediments exhibit deficiencies in K2O and Na2O, which aligns with the petrographic observations of Pab intermediate and mafic source; the studied sediments were plotted in this model and depict strong affinity with the acidic rocks; though few samples were plotted close to intermediate fields suggesting slight input from basic sources.The trace elements like La, Sc, Th, Co, and the ratios of La/Sc and Th/Co, were used to identify the source, whether it was felsic or basic.Employing the La/Sc and Th/Co ratios from the [40] scheme, it is evident that the BD section samples (Fig. 4) fall into the acid rocks field.Based on the La/Sc and Th/Co model suggested by [40] it is possible that granitic rocks contributed sediments to the Pab Sandstone.According to [39] felsic igneous rocks, including granite, typically have elevated levels of Sr and Ba.The studied sediment have higher Sr and Ba values suggesting granitic source for the Pab Sandstone sediments at BD section.Based on the current whole rock geochemical data along with some previous studies (e.g., [12, 47, 16, 11]), it is established that Pab Sandstone sediments were extracted from the granitic rocks of the Indian shield which are exposed in the southeast of the research area in Pakistan (Nagar Parkar Igneous Complex) as well as in India (Malani Igneous Suite).On the extreme weathering conditions in the source region are concluded for studied sediments of the Pab Sandstone.
relationships are crucial.Consequently, they are employed in the current study to examine the provenance of Pab sediments; and is accomplished by applying the Si/10-(Ca+Mg) -(Na+K) ternary plot-based categorization approach that was suggested by [31].The major oxide values when plotted on ternary model the majority of the samples are dispersed close to granitic field; this type of plotting may be explained by the relatively high proportion of SiO2 found in the sediments under study (Fig. 2).Consequently, confirming the granitic provenance of Pab sediments as opposed to mafic, ultramafic, or metamorphic sources.The source of the Pab sediments is thus thought to be the granitic rocks of Indian Shield, as indicated by the geochemical data.

[ 28 ]
distinguished the main igneous rock types, such as the mafic, intermediate, and felsic, using the concentrations of TiO2 and Zr.The Pab Sandstone sediments from the BD section primarily plot in the field of acidic rocks according to this TiO2 against Zr model, with only a small number of samples falling into the category of intermediate rocks (Fig. 3).In the southeast of the research area acidic igneous rocks like granites, microgranites, aplites and rhyolites are exposed which may have contributed sediments for the Pab Sandstone.However, the mixing of many sources may be the reason for the scattering of a few samples around intermediate fields.A small number of mafic dikes are also seen in the Indian shield rocks, which may have provided some sediments for the Pab Sandstone together with the granitoids.

5. 2 Paleoweathering
Prior to mobility or transportation, the weathering conditions at the source region should be examined as the main oxides concentration might be impacted by diagenetic processes [41, 48, 49].The tectonics that exposes the source rocks and provides information on the relief, along with the composition of the source rock and climatic conditions are the key factors governing the weathering conditions of the source region [42].Alkali and alkaline earth elements can be used to assess the weathering history of sediments [43].
addition to major oxides trace elements such as Th, Zr, Ba, Sr and La are observed in abundance in contrast to Sc, Co, V, Cr and Ni.The major oxide and trace element data along with different discrimination diagrams suggest the felsic/granitic source dominantly in comparison to mafic and ultramafic rocks.The results of major oxide analyses are consistent with the trace element geochemistry findings.The CIA and PIA values indicate that the examined sediments experienced moderate to intense weathering conditions.The vast landmass of the Indian Shield (Craton) is the source of the Pab Sandstone sediments.DECLARATIONS Conflicts of interest Authors have none to declare

Table 1 .
Description of the studied samples from BD section.
Mudstone; thick to very thick bedded, reddish in appearance loose to moderately compacted.05 BD-06 Sandstone; thick to very thick bedded massive, light red to dark red in color medium to coarse grained, compacted.06 BD-07 Sandstone; thick bedded, massive, dark brown to reddish colored, fine to medium grained and compacted.07 BD-08 Sandstone; thick to very thick bedded, light brown colored, medium to coarse grained, highly compacted and cross-bedded.08 BD-09 Sandstone; thick bedded, light brown reddish colored, very fine to medium grained, highly compacted and cross-bedded.09 BD-10 Sandstone; thick bedded, massive, dark brown in color, medium to coarse grained, highly compacted.

Table 2 .
Major element concentrations (wt.%) of selected samples of Pab Sandstone from BD sectio

Table 3 .
Ratios of major elements of selected samples of Pab Sandstone from BD section.

Table 4 .
Trace element concentrations (in ppm) of studied samples from BD section.

Table 5 .
Ratios of trace elements of studied samples from BD section.

Table 2
, the CIA values of the studied sediments range from 53.81 to 79.6,