Energy Dispersive X-Ray Spectroscopy of the Sohnari Member of Laki Formation from Southern Indus Basin of Pakistan

This study determines the geochemical and depositional environment analysis of the sediments of the Sohnari Member of the Laki Formation, Northern Kirthar Fold Belt of Pakistan. The Energy-Dispersive-X-Ray Spectroscopy (EDS) technique is used for the detection of major elements and the effects of shifting depositional climatic conditions of six representative samples which were acquired from the Sohnari Member of the Laki Formation at Lakhra area, Sindh, Pakistan. The sedimentological studies clarify that the sediments the Sonahri Member are relatively immature and most migrated in clastic mode. The availability of Silica shows that the Member was formed due to biochemical precipitation and detrital mode and was deposited at a fast rate of sediment deposition under the fluvio-deltaic depositional system. This is also deduced that the rapid rate of sediment deposition might be created a reducing atmosphere and allowing for the mineralization of sulphur.


Introduction
Sohnari member is the basal part of the Laki Formation which overlies the Ra-DOI: 10.4236/ojg.2021. 116011 184 Open Journal of Geology nikot group and the Lakhra Formation of the Lakhra area [1] and [2]. Its basal contact is with Ranikot Group and got is unconformable obvious and its upper contact is with chat member of Laki Formation [3] [4] [5] [6]. Our study is focused on a geochemical investigation of the Sohnari member of the Laki Formation to interpret its origin and climate ( Figure 1). SM of the Laki Formation is also thought to be a distinct origin from the Lakhra Formation and the overlying Chat member. Due to different characters the Sonhari member ( Figure 3). It is worth including the position from the Laki formation SM is a section of clastic sedimentary rock transported beginning the east also placed among the Chat member and the Lakhra Formation. The SM is known as basal Laki laterite and represents the very slight regression possibly a reduction in the rate of the sinking of the continental shelf in the Indian Shield of Indian Plate and the peat is accumulated very close to the sea level.

Stratigraphy of the Area
The general stratigraphy of the study area is very much interesting in the research. The oldest stratigraphic unit is Bara Formation (Middle Paleocene age), [9]. The second oldest unit is Lakhra Formation (Late Paleocene). And the third one is the Laki Formation of the Eocene age. The fourth youngest stratigraphic unit is the Siwalik group i.e. Manchar Formation of (Mio-Pliocene age). General stratigraphy of the studied area is presented in (Table 1). The second oldest stratigraphic unit of this area is the Lakhra Formation of (late Paleocene) age [2], Lakhra Formation was also proposed as a name for the "Upper Ranikot (limestone)" and "Upper Ranikot" of later workers, after Lakhra, Laki range [10]. The Formation is predominantly grey limestone with yellowish staining and weathering brown and buff, with some orange-brown and pinkish-brown patches. The lower section is made up of sandstone., with sand interbeds. In the upper section, there are sandstone and shale interbeds. The Bara Formation is uniformly overlain by the Formation. It is uncomfortably overlain by the Laki formation in the Kirthar province [2].

Sohnari Member
The thickness of the Sonhari member is about 25 feet [11]. SM is very well developed in the meeting Jhimpir area of Thatta district. The SM sometimes there are thin beds of sandy yellow limestone with assilina granulose. The thickness of the SM is varying from place to place (10 to 25 feet). SM is well developed in onger, mettign and Jampir areas. It is Lanticular, thickness varies from 1 to 20 meters. In the Lakhra area, the thickness ranges from 0 to 8 meters. The SM is missing in drill no L-25 and L-27 of the Lakhra area. Lenticular beds of limonite, ochre white clay (fire clay) and lignite coal seams of varying thickness or characteristic unit for the member and have also the commercial importance and The Ranikot and the Laki have interacted. There is a layer of ferogeneous claystone and siltstone that ranges from 17 to 19 feet thick and is known as laterite. It's a concrete color with sporadic white spots, and it's moderately strong ( Figure 3). These lateritic beds are on the contact of the Formation are present in the Lakhra area [12]. The surface exposure of the Sohnari member consists of the highly ferruginous claystone and the sandstone of the variegated colors varies from yellowish-brown, rusty brown, dark yellowish, brown, white, cheery red, and The SM is the basal part of the Laki Formation which overlies the Ranikot group and the Lakhra Formation of the Lakhra area [14]. It has an uncomfortably marked basal contact with the Ranikot group and an upper contact with the Chat member of the Laki Formation. The Sohnari member of the Laki Formation is also thought to be distinct from the underlying Lakhra Formation and the overlying Chat member ( Figure 3).

Material & Methods
The detailed geological fieldwork was carried out and rock samples were col-

Sampling
These

Result
Six rock samples from the Sohnari member were collected, prepared, and analyzed by the Advance Research Laboratory of the Centre for Pure and Applied Geology, University of Sindh, Jamshoro. Energy-dispersive x-ray spectroscopy (EDS) is a technique for analyzing elements and determining the chemical properties of a sample form of XRF [15] and [16]. It is a form of spectroscopy that involves analyzing a sample by interactions between electromagnetic radiation and matter, such as detecting X-rays imitated by matter in response to being struck with charged particles [17]. Its classification abilities are due in large part to the general premise that each element has a unique atomic structure, which enables X-rays that are products of an element's atomic structure to be differentiated from one another is centered into the sample being analyzed to induce the release of distinctive X-rays ( Figure 4). When an atom in a sample is at rest, it includes ground state (or unexcited) electrons in distinct energy ranges or electron shells, which are expelled from the shell, leaving an electron-hole in its place. An electron released from an exterior, higher-energy shell, and a lower-energy shell in the shape of a specimen can be quantified with an energy dispersive spectrometer.
The elemental composition of the specimen can be measured since x-ray radiation is a result of the temperature differential between the two shells as well as the atomic structure of the elements from which it was emitted.
Kaolinite and illite hold high concentrations [20]. Kaolinite, chlorite, illite, and Montmorillonite Clay minerals are present in Sohnari members of the Laki Formation. The maturity index of the analyses rock samples was determined using the Al 2 O 3 /Na 2 O 3 ratio, as suggested by [21] and [22]. The Al 2 O 3 /Na 2 O ratios (6.62 to 20.80) mean that the sediments analyzed are younger and are shipped as detrital particles. The Al 2 O 3 /SiO 2 ratio of the analyzed rock samples, suggests that sediments of Sohnari member of the Laki Formation do not contain a substantial amount of minerals found in clay (Table S2).

Iron (Fe 2 O 3 ):
The iron concentration in Sohnari member samples studied (N = 6) ranges from 2.75% to 37.54%, with an average of 2.75% to 16.17%). To determine the relationship between iron and clay minerals, the Fe 2 O 3 /Al 2 O 3 ratio is used [23] and [24]. The Fe 2 O 3 /Al 2 O 3 ratios of the samples analyzed show a poor association with clay minerals (Table S2). The Mn/Fe ratio has been used to classify depositional conditions in sedimentary rocks [25] and [26]. The observed Mn/Fe ratios indicate that the Sohnari member of the Laki Formation was deposited nearshore to offshore. The Low Mn/Fe ratio is a consequence of this leaching process. Strong chemical weathering can cause a high concentration of Fe 2 O 3 to develop [27].

Magnesium (MgO):
The mg/Al 2 O 3 ratio measured from the MgO distribution in the analyzed (N = 6) samples ranges from 0.92% to 1.65%, (Table S1), with mean values of (1.05%). ratio calculated according to [24] for the presence of Montmorillonite and Clay minerals, and the mg/Al 2 O 3 (Table S2). The occurrence of MgO in montmorillonite lattices is suggested by its weak positive correlation with iron.
Sodium (Na 2 O): The concentration of (Na 2 O) in the Sohnari member of the Laki Formation ranges from 1.79% to 7.85%, with a mean of 3.61% (n = 51) samples (Table S1).
In contrast to other cations including Ca 2+ and Mg 2+ , the montmorillonite (smectite) produces fewer Na+ in the inter-layered position [20] and [25] used the Na/K ratio to calculate the sediment deposition rate. These Na/K ratios suggest a medium to high rate of sediment deposition in the samples analyzed.
The Sohnari member of the Laki Formation (K 2 O) concentration ranges from 0.51% to 0.80%, with a mean value of 0.51% and 0.21%. As compared to other cations (calcium and magnesium), the lattices of montmorillonite often contain small amounts of potassium and sodium oxides [20], and [28] used the Na/K ratio to calculate the rate of sediment deposition and the crystallinity of Illite. The presence of depleted illite and a low to high rate of sediment deposition was demonstrated by the Na/K ratios of the examined samples. The K 2 O/Al 2 O 3 ratio is used to determine the amount of sand in a sediment sample [24], the density of sand in sediments, and the fast rate of sediment deposition was suggested by the K 2 O/Al 2 O 3 ratio of the samples analyzed.

Sulphur (S):
The sulphur concentrations range from 0.72% to 0.52%, with an average value of 0.52% and 0.37% (Table S1). Sulfur content of shales is on average below 1%.
(0.25%), the average concentration of European Paleozoic shales is 0.32% [29] and [30]. The Sohnari member of the Laki Formation Sulphur in the samples indicates the presence of metabolizable organic matter, sulfate-reducing bacteria, and dissolved sulphide in the depositional conditions. [31] knows that the presence of sulphides in some Pacific Ocean trenches is due to a heavy content of organic matter and a high rate of sediment deposition. The Sohnari member of the Laki Formation sediments' rapid deposition rate may have resulted in reducing conditions during deposition, which may explain the presence of sulphur in the studied samples.

Conclusions
The Sohnari Laki Formation member is mostly quartz (low), with biogenically precipitated or detrital silica as the source of the silica.