The geological fieldwork has been conducted in Takream commune, Banan District, and Somlout commune, Somlout District in Battambang Province, the western part of the Tonle Sap sedimentary basin. The thickness of individual facies succession was measured from these two well-exposed Permian sedimentary outcrops, Takream and Somlout. Each stratigraphy interval’s sedimentary texture, structure, fossil content, and thickness have been recorded for facies analysis as evidence to interpret the sedimentary process and the details of the depositional environment.

There are 13 representative samples, five samples from Somlout and eight samples from Takream, calcareous shale (solid green triangle in the stratigraphy column), and limestone (solid red triangle in the stratigraphy column) (Figure 3, Figure 4), were collected during the construction stratigraphy column. These samples were prepared through laboratory works, petrography, X-ray fluorescence (XRF), and inductively coupled plasma mass spectrometry (ICP-MS).

The samples were cleaned, cut, and ground with diamond powder to get a thickness of 30 micrometers for petrography analysis to identify the minerals and fossil contents with Eclipse Ci-Pol. Moreover, samples were grounded to be powder and compacted in sample holders for XRF analysis with the model of Rigaku ZSX Primus III+. Thus, the oxide compound of these samples can be identified. The other parts of the samples were ground to 74 micrometers in a

ball mill of about 0.25 mg per sample and analyzed with ICP-MS at ALS analytical services in Vientiane, Laos. For this analysis, powder samples were digested with perchloric, nitric, hydrofluoric, and hydrochloric acids. The residue was topped up with dilute hydrochloric acid. The trace and major elements were plotted in indicating depositional conditions.

According to stratigraphy measurement, the stratigraphy column of Takream and Somlout outcrops consist of the same lithofacies of sediments, calcareous shale, and limestone, however, the results of petrography, XRF, and ICP -MS analysis showed that the properties of these facies are different in compositions (Table 1, Table 2). The details of lithofacies analysis and interpretation are described in the following sections.

According to the examined of petrographically analysis, Somlout shale consists mainly of clay minerals, less quartz, and fracture creaking of organic matter (black color), which indicates as the high temperature was applied after deposition (Figure 5(a), Figure 5(b)) [11] . This shale is dominated by clay −96.20%, and quartz −3.8%, and is classified as a clay-dominated lithotype [12] . Furthermore, limestone consists mainly of calcite grain, quartz, micrite, and fossil. They

are dominant grains, ranging from −46.79% −64.22% and micrite −35.78% −53.28%. Limestones are classified as wackestone and packstone [13] . The presence of substantial micrite in limestone is commonly interpreted to indicate the deposition under the low energy flow [14] . Therefore, fragmental fossils such as 2 mm of bivalve shells in S-L1 (Figure 5(c), Figure 5(d)), 1 - 2 mm bryozoan (Figure 5(e), Figure 5(f)), and 1 mm fusulinid foraminifera in S-L3 (Figure 5(g), Figure 5(h)) were found to be the depositional indicators.

Takream calcareous shale consists of carbonate mud, calcite minerals, and organic matter. It comprises carbonate mud −50.80% - 60.60%, quartz −30.20% to 43.00%, and clay minerals −6.20% to 9.20%. It is classified as silica-rich carbonate mudstone [12] . Under the microscope, sample T-S1 showed carbonate mud, calcite mineral, siliceous sponge spicules, and organic matter (Figure 6(a), Figure 6(b)). Limestone mainly comprises carbonate mud −22.60% to 96.80% and calcite grain −3.20% to 77.40% (Figure 6(c), Figure 6(d)). It is classified as packstone and lime-mudstone [13] .

Major oxide concentrations in calcareous shales and limestones of Somlout and Takream were obtained from XRF analysis (Table 1). Calcareous shale has a high concentration of SiO₂ ranging from 42.00% to 75.00% with a mean value of 62.68%, CaO ranges from 0.96% to 26.30% with a mean value of 12.01%, Al₂O₃ ranges from 0.97% to 10.05% with a mean value of 5.92%, and Fe₂O₃ ranges from 1.40% to 5.29% with a mean value of 3.56%. Meanwhile, limestone concentrations of SiO₂ range from 0.38% - 15.85% with the mean value of 9.97%, CaO ranges from 42.60% - 57.00% with the mean value of 47.99%, Al₂O₃ ranges from 0.04% - 2.49% with the mean value of 0.75%, and Fe₂O₃ ranges from 0.05% - 1.68% with the mean value of 0.56%. These represent the most abundant elements (Si, Ca, Al, and Fe), consistent with the abundant occurrence of carbonate minerals, quartz, and pyrite in calcareous shale and limestones of the Tonle Sap

sedimentary basin.

Trace element concentrations of shale and limestone of both locations in Battambang province were obtained from ICP-MS analysis and are presented in Table 2. The concentration of V, Cr, Ga, Rb, Nb, Ba, Th, Zr, Hf, Ce, La, Nd, W, and Y (mean value for shale 75.25, 65.00, 6.98, 18.00, 1.73, 65.20, 1.26, 40.50, 1.13, 10.00, 5.13, 5.55, 0.25, 9.25 and mean value for limestone 21.22, 31.11, 0.91, 1.23, 0.20, 41.64, 0.08, 3.78, 0.11, 9.27, 5.67, 5.02, 0.00, 8.50, respectively) are smaller than those of UC, except Sr which is higher than UC (1.52 for shale, and 3.29 for limestone).

The Vanadium (V) and Chromium (Cr) are essential indicators for redox conditions to estimate the degree of oxygen dissolved in water during the sediment deposited [15] . Cr is usually incorporated within the detrital clastic fraction of s sediment where it may substitute for Al within clays, be adsorbed, or occur as

Table 2. Trace elements concentrations (ppm) of Somlout and Takream samples.

chromite. Vanadium in contrast may be bound to organic matter by the incorporation into porphyrins and is concentrated in sediments deposited under reducing conditions. The ratio V/Cr has been suggested as an index of palaeo-oxygenation, oxic to anoxic bottom waters environment [15] [16] . It has been proposed that the value of V/Cr ratio is less than 2 suggests the oxic condition (the oxygen concentration range from 2 - 860 ml/L), 2 to 4.25 indicates the dysoxic condition (the oxygen concentration range from 2 - 0.2 ml/L), and more than 4.25 suggests suboxic to anoxic condition [13] [15] [17] . Somlout samples have a V/Cr value ranging from 0.93 to 3.87, shale sample values are 3.30 and 3.87 are suggested as dysoxic conditions and other samples are suggested as oxic conditions (Figure 7(a)). Takream samples have a V/Cr value ranging from 0.28 to 1.30, which is suggested as the oxic condition.

The Uauthigenic value less than 5 ppm, from 5 ppm to 12 ppm, and higher than 12 ppm, indicated oxic, dysoxic, and anoxic conditions, respectively [18] . The average of Uauthigenic of Somlout and Takream samples is equal to 0.42 ppm and 2.28 ppm, respectively, which suggests to oxic condition environment (Figure 7(b)). Moreover, the oxic condition is generally defined as a reducing environment [15] [19] .

The chemical precipitate is resulted from the interaction between sediments and calcium. Calcium is a major chemical constituent of seawater [20] . The Ca/(Fe + Ca) ratios of argillaceous sediments can provide a direct estimate of paleosalinity throughout the fresh water to marine water [16] [20] . The values for modern sea samples are more than 0.8, whereas the values for modern lake samples are less than 0.6 [16] . In addition, [12] [16] suggests that a high Sr/Ba ratio value of more than 0.8 indicates high salinity, while a low Sr/Ba value of less than 0.5 indicates low salinity water or freshwater environment. Shale samples from Somlout have the value of Ca/(Fe + Ca) (Figure 7(c)) and Sr/Ba (Figure 7(d)) range from 0.15 to 0.18 and 0.24 to 0.32, averaging 0.17 and 0.28, respectively, indicating a freshwater lake environment [16] . In limestone samples in this formation range from 0.96 to 0.98 and 0.95 to 110, averaging 0.97 and 49.11, respectively, which suggests high salinity seawater environment. This condition indicated that the lithologies (shale and limestone) in Somlout samples were deposited in two different conditions involving sea-level fluctuation. The Ca/(Fe + Ca) and Sr/Ba value of both calcareous shale and limestone of the Takream sample range from 0.91 to 0.97 and 11.8 to 1450, averaging 0.97 and 387.14, respectively, which indicate a high salinity seawater environment.

The Al₂O₃/(Al₂O₃ + Fe₂O₃) ratio can be used as an index to determine the sedimentary environment, with a value ranging from 0.4 to 0.7 in the oceanic basin, indicating a deep-water, and from 0.5 to 0.9 in the continental margin [16]

(Figure 7(e)). The concentration of Al₂O₃ is affiliated with aluminosilicate phases; it is an excellent indicator for terrigenous input, and Fe₂O₃ and TiO₂ are rich in the metalliferous mid-oceanic ridge [22] . Thus, Fe₂O₃/TiO₂ ratio in the mid-oceanic ridge is higher than the continental margin. Moreover, the cross-plot value of Fe₂O₃/TiO₂ with Al₂O₃/(Al₂O₃ + Fe₂O₃) can be used to distinguish the continental margin from the mid-oceanic ridge [16] [22] . The Al₂O₃/(Al₂O₃ + Fe₂O₃) ratio of Somlout samples ranges from 0.50 to 0.66, all falling into the overlapping area of the oceanic basin and continental margin. The Al₂O₃/(Al₂O₃ + Fe₂O₃) ratio of Takream samples ranges from 0.36 to 0.63, mostly falling into or near the overlapping area of the oceanic basin and continental margin.

Based on paleontological aspects, the presence of fusulinid and bivalve shelve was commonly suggested in a shallow water environment [23] [24] , while bryozoan mainly indicated a marine environment [25] , because these types of micro-fossil mostly live in these conditions. The recognized fossils of bryozoans, fusulinids, and small foraminifers, are skeletal or fragmental debris, indicating that the depositional environment must be a barrier [26] . Somlout depositional condition might be a significant regression depending on the lithostratigraphy of limestone at the bottom, followed by shale at the top [26] .

Takream fine-grained shale and limestone are suggested to form in a quiet marine water setting, a lagoon [27] .