Geochemistry and provenance of Mesozoic sandstones in Khon Kaen Geopark: Implication for tectonics of the western Khorat Plateau of Thailand

Khon Kaen Geopark, representing an area of dinosaur fossil diversity, was selected for investigations to reveal the origin and tectonic setting of the Khorat Group. The area occupied by Mesozoic sedimentary rocks of four formal formations of the Khorat Group, namely the Phra Wihan Formation (PWF), Sao Khua Formation (SKF), Phu Phan Formation (PPF), and Khok Kruat Formation (KKF). A field investigation and macroscopic observations suggested that the immature sedimentary rocks of the study area are mainly clast-supported, pebbly sandstone and siltstone with few calcretes. The 50 rock samples that were selected for petrographical and geochemical investigations revealed that the sandstones of the PWF and PPF are quartz arenite and sublitharenite, with some subarkose, whereas those of the SKF are mainly subarkose and sublitharenite. In addition, the KKF dominantly presents sublitharenite with pebbles and calcretes. Mesozoic sandstones contain quartz, feldspars, various types of rock fragments, and accessory minerals (biotite, muscovite, zircon, and tourmaline), with siliceous, ferrous, and calcareous cement. Petrographic (Q–F–L) and geochemical (major and trace element) data suggested that the sources of sediments are mostly quartzose sedimentary rocks and some felsic-intermediate igneous rocks. Chondrite-normalized rare earth element patterns indicated that the origins of the studied sandstones are quartzose sedimentary rocks deposited in a passive continental margin or an upper continental crust. Geochemical traits of the sedimentary successions demonstrated that the provenance of the Khorat Basin prior to reworking by fluvial processes was situated in the passive continental margin or recycled orogen of the paleo-volcanic arc during the Mesozoic period.

Introduction fossil formations from the Khorat Group, comprising freshwater hypsodont sharks, actinopterygian fishes, turtles, crocodilians, and dinosaurs [16]. Through analysis of its vertebrate fossil content, the age of the SKF is dated to the early Cretaceous [17], whereas research on nonmarine bivalves suggests that this formation was deposited during the late Barremian [24].
The light grey sandstone and conglomerate of the PPF appear to be poorly sorted mediumto-coarse-grained quartz, green volcanic rocks, and chert, with expansive planar and trough cross-bedding. This formation was deposited in braided streams in a humid to semi-arid environment during the early Cretaceous [12,30]. Moreover, the reddish-brown stratum of the KKF (sandstones, siltstones, mudstones, and conglomerates, interbedded with gypsum lenses and calcrete nodules) was deposited in meandering rivers, in a semi-arid to an arid environment [30], during the early-middle Cretaceous (dated by freshwater bivalves [32]). The different lithologic and depositional aspects found in previous works [10,12] are of interest to the current work for identifying and classifying the petrographical and geochemical characteristics of the Mesozoic sandstones in detail.

Methodology
Initial field observations, samples, and measurements were taken in the Khon Kaen National Park (Fig 1) to determine the structure, orientation, etc. This area shows many geological structural features, including folds, faults, joints, and primary structures of sedimentary rocks. Fifty samples were collected from the early Cretaceous sedimentary rocks of the Khorat Group (PWF, SKF, PPF, and KKF) for study in petrographic and geochemical laboratories. Representative rocks were gathered to be studied in the petrographic lab for texture, mineral content, rock name identification (400-point counts, see S2 Table), and sedimentary characteristic classification. A petrographic examination was conducted on all 47 fresh rocks distributed among every rock unit in the studied areas. Moreover, at the Department of Geotechnology of Khon Kaen University, photomicrographs were taken using ZEN 3.4 (blue edition) core imaging software, using ZEISS (Carl Zeiss NTS Ltd., Oberkochen, Germany) imaging and Motic microscope BA310POL. Powder samples (200 μm) were made from finely chosen rock chips to detect chemical properties, individual rock names, and modification processes in the Department of Geological Sciences, Faculty of Science, Chiang Mai University.
For the geochemical study, 34 typical samples with the least amount of alteration, weathering, and replacement by secondary minerals (carefully evaluated under a polarizing microscope) were chosen for whole rock analysis using X-ray fluorescence (XRF76C) and inductively coupled plasma atomic emission spectroscopy (ICP95A). Analysis of the major oxides (SiO 2 , TiO 2 , Al 2 O 3 , Fe total , MnO, MgO, CaO, Na 2 O, K 2 O, and P 2 O 5 ) was performed using the Axios model at SGS (Thailand) Co., Ltd (lower reporting limit: 0.01%; Bangkok, Thailand), see S3 Table. For low-atomic-number elements, lithium metaborate fusion was employed, whereas, for higher-atomic-number elements, the pressed pellet technique was used. Fusion is a technique for analyzing key elements, which involves melting the sample with flux and casting it into a glass disc. In addition, trace elements (Rb, Sr, Zr, Y, Nb, Ni, Cr, V, Sc, Hf, Th, and Ta) and rare earth elements (REEs; La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb) were analyzed using the 5300DV ICP-OES and nexion300X ICP-MS of SGS-CSTC Standards Technical Services Co., Ltd. (Beijing, China), see S4 Table. Results

Field observation and petrography
The sampling location comprised 20 sites distributed in the KKGp, excluding the protected area of the Phu Wiang National Park (Table 1). Fig 2 shows representative pictures of outcrops of road cuts, in situ rock, and geomorphological sites (e.g., rock pillar, mountain, and cliff) with planar bedding and cross-bedding with some calcretes. The main structure comprises multi-phases of syncline and high mountains surrounding a valley. Based on field observation, the studied area was divided into six areas (A, B, C, D, E, and F) for analyzing structural geology, as shown in Fig 1. The sedimentary stratum in area A presents the orientation E-W/10-17˚S, whereas the bedding of area B is approximately NW-SE and dips 40˚in the SW direction. In addition, areas C, D, and E present the same strike in NE-SW and dip values are 24-67˚NW. However, the strike of area F is NNW-SSE dipping 10-16˚in the NE direction. Thus, based on the data on the orientation of the geological structures, the KKGp may be affected by folding structures.
The PWF is made up of white-colored, thick-bedded, poorly to well-sorted, fine-to-coarse sandstone, pebbly sandstone, and siltstone. Moreover, the SKF comprises maroon, moderateto-well-sorted, very fine-to-very coarse sandstone interbedded with pebbly sandstone, and sandy siltstone. The uppermost part of the PWF is conformably overlaid by the SKF. In addition, the SKF can be divided into two parts: lower (medium-to-thick-bedded sandstone interbedded with siltstone) and upper (reddish siltstone, sandstone, limestone, conglomerate, calcrete layer, and fossils) parts.
There are nine excavation sites in the SKF of the KKGp, presenting with four associated sedimentary facies, as shown in Fig 3: 1) channel fill deposit, 2) crevasse splay deposit, 3) lake and flood plain deposit, and 4) over bank deposit [28]. The maroon pebbly sandstone of the KKF is gravel-rich with coarse-to-very coarse, moderate-to-well-sorted sand. Forty-seven lithological and petrographical studies conducted in 30 sites in the KKGp (Table 1) suggested that those rocks mainly comprise clast-supported, poor-to-well-sorted, subangular-to-angular shapes, and low-to-moderate sphericity clastic sedimentary rocks ( Fig  4). The cementation is composed of three types: siliceous, ferrous (Fe-rich), and calcareous. These clastic sedimentary rocks comprise quartz, feldspars, plagioclase, tourmaline, muscovite, zircon, and opaque minerals. In addition, these rocks comprise various rock fragment types: shale/argillite (very fine-grained with lamination), quartzose sedimentary rocks (siltstone/ sandstone), limestone (carbonate-rich), silicified rocks/quartzite (quartz-rich with sutured texture), and phyllite (very fine crystals with foliated texture), as well as felsic igneous rocks with some epidote and perthite.

Analysis no.
Phra Wihan Formation Sao Khua Formation Major Oxides (wt%) The sandstones in the KKGp were plotted in the diagrams of feldspars-lithic fragmentsquartz (Qt-F-Lt) and-monocrystalline quartz (Qm-F-Lt) with tectonic fields [35] mostly as recycled orogen with quartzose recycled, craton interior, and mixed sources (Fig 5B and 5C). In addition, the lithic fragments, metamorphic rocks (Lm)-volcanic rocks (Lv)-sedimentary rocks (Ls) diagram [36] shown in Fig 5D suggests that these rocks are related to rifted continental margin, mixed arc, and subduction complex. Moreover, these rocks were plotted in the diagrams of polycrystalline quartz (Qp) and lithic fragments [37] as collision suture fold-thrust ( Fig 5E).

Geochemistry
Thirty-four representative samples were analyzed for major oxides, trace elements, and REEs (  [38] in Fig 6A, the PWF was classified as quartz arenite and subarkose, with a small amount of arkose, whereas rock samples from the SKF were identified as arkose and subarkose. The sandstones of the PPF were  [34], the PPF sandstone was plotted in the quartz arenite, lithic arenite, subarkose, and sublitharenite fields (Fig 6B). In addition, the SKF sandstone was classified as subarkose and sublitharenite, the PPF as quartz arenite and sublitharenite, and the KKF as sublitharenite. According to the isotopic systems of zircon-specific and whole-rock geochemistry of granitoids in Sukhothai Terrane and the western part of Indochina [39,40], the geochemical characteristics are relatively consistent with the lithic arenite and sublitharenite in KKGp. However, SiO 2 has been increased, while Al 2 O 3 has been lost during feldspars and other inconsistent minerals weathering and alteration. The ratios of Th/U in PWF, SKF, PPF, and KKF are 2.579-4.525, 3.293-5.769, and 3.168-3.882, respectively that lower than Th:U of comparison granitoid (8.0-11.8) indicating that both Pb and Th have been lost from the zircon during subsequent tectonic events and sedimentary process.

Provenance
The relationships of major oxides (TiO 2 , Fe 2 O 3 , MgO, K 2 O, Na 2 O, SiO 2 , and Al 2 O 3 ) can be used to classify the tectonic settings of sedimentary rocks [41][42][43][44][45]. The sandstones of the KKGp were mostly plotted in the passive margin field, whereas some fell in the active continental arc (Fig 7A-7C). The basicity index can be calculated by Eq 1.
Ternary trace elements classification diagrams were used to classify provenance and tectonic settings. The La-Th-Sc discrimination diagrams [46] in Fig 8A and 8B suggest that the studied sandstones in the KKGp were from mixed sources (granitic gneiss metabasite, clay, silt, sand, and gravel). In addition, based on the Th-Sc-(Zr/10) discrimination diagram [41] and La-Th-Sc discrimination diagram [41] in Fig 8C and 8D, these sources were related to the continental arc setting. Thus, the PWF, SKF, PPF, and KKF of the Khorat Group may be formed from sediments of mixed sources (felsic-intermediate rocks) of the continental arc.  [43], (C) basicity diagram [44,45], and (D) discriminant function analysis using major oxides [42]. https://doi.org/10.1371/journal.pone.0284974.g007 Based on the discriminant function analyses using major oxides [42], the sandstones were mostly classified as quartzose sedimentary (Fig 7D). Furthermore, some very fine-grained sandstones of SKF, siltstones of PWF, and calcareous sandstones of PWF were plotted in the intermediate igneous provenance field because these rocks have high CaO (wt%) from soluble cement, which affects the CaO value in the discriminant functions. However, similarities appeared when comparing REEs and immobile trace elements in the La/Yb versus Ce diagram [48], suggesting that the sandstones of the KKGp were formed from quartzose sedimentary provenances (Fig 9A). In addition, the La/Th versus Hf classification diagram [49] suggests that the sandstones of the KKGp were formed from the intermediate arc and felsic passive margin sources (Fig 9B). Furthermore, the Th/Sc versus Zr/Sc diagram [50] indicates that the sandstones are related to the upper crust and sediment recycling (Fig 9C).  [45], (B) La-Th-Sc discrimination diagram [44], (C) Th-Sc-(Zr/10) discrimination diagram [46], and (D) La-Th-Sc discrimination diagram [47]. https://doi.org/10.1371/journal.pone.0284974.g008

Comparison with previous studies
The major and minor oxides normalized analyses were compared with the results of the upper continental crust (UCC) from [51], as shown in Fig 10A. The graph shows that SiO 2 content is close to that of the (UCC; TiO 2 , Al 2 O 3 , Fe 2 O 3 , MgO, CaO, Na 2 O, K 2 O, and P 2 O 5 content tended to be lower than that in the UCC, but in some samples, MnO and CaO values were higher than that in the UCC. The study of the lithology and petrography showed that the samples had a calcareous cement binder, resulting in high CaO and MnO content of the samples, consistent with the results of the chemical analysis. Trace elements and REEs in the rock samples were compared with those in the UCC from [52], and the result is presented in Fig 10B. The graph shows that the amount of Sc, V, Cr, Cu, Ni, and Zn tended to be lower than that in the UCC, indicating that the rock sample contained very few mafic minerals, which are found in small quantities when the rock is high in SiO 2 , excluding some calcrete samples (KK3-1) presenting high Cu (wt%). In addition, the lower Sr content than that in the UCC was owing to the high CaO content of the rock samples. Moreover, some sandy mudrock (KK7-2) presented high Ba content, suggesting that the rock sample contained a very large amount of potassium-bearing minerals (alkaline feldspars, mica, and clays). From the graph in Fig 10, it can be seen that only rock samples from the SKF (KK3 and KK6-8) have high Zr, Hf, and Y content, suggesting that these rocks may have a high zircon proportion in rock fragments, which is consistent with the petrographic classification of the rocks as lithic wacke, lithic arenite, and feldspathic lithic arenite. Furthermore, the provenance discrimination diagram from the major and minor oxide data (TiO 2 -Al 2 O 3 -Fe 2 O 3 -MgO-CaO-Na 2 O-K 2 O) [42] suggested that the PWF, SKF, PPF, and KKF originated from quartzose sedimentary rocks (Fig 7D).
Numerous studies have shown that the chemical composition of clastic rocks is partly influenced and controlled by tectonic settings, with different tectonic landscapes exhibiting different geochemical characteristics [41][42][43][44][45]. There are three main types of tectonic margins: passive continental margin, active continental margin, and oceanic island arc; these were classified using major and minor oxides in Fig 7A-7C. These tectonic classifications [41][42][43][44][45] revealed that most are deposited in the passive continental margin or intracratonic basin. Furthermore, REE tectonic classification diagrams [41] demonstrate the samples were deposited in either a passive continental margin or intracratonic basin. Rare Earth Elements (REEs) are  [48], and (C) Th/Sc versus Zr/Sc diagram [49].
https://doi.org/10.1371/journal.pone.0284974.g009 not mobile during weathering so are more reliable than major elemental analysis. Therefore, it was concluded that most rock samples are associated with passive continental margins or intracratonic basins (Fig 8). When comparing REEs in chondrite from [52,53] concluded that the sandstones in the study area originated from quartzose sedimentary rocks deposited in the passive continental margin. REE patterns from Shijia sandstone [53] and REE patterns (Khorat Group) from this research were found to be similar (Fig 11) and their patterns also according to felsic igneous rocks from Qinling Orogenic Belt [54]. Therefore, it was concluded that the studied rock samples had similar origins and tectonic landscapes to those in the previously mentioned research. Although the rock samples from the two studies were collected at very different ages, the REEs within the rocks were not lost. Thus, the use of REEs is quite accurate and it can be concluded that studies of provenances and tectonic settings using rare earth data have some degree of reliability.
Previous studies [10,58,59] have suggested that the late Triassic Huai Hin Lat and Nam Phong Formations originate from intermediate to felsic igneous rocks and quartzose sedimentary rocks and accumulate in the active continental margin to the passive continental margin or intracratonic basin (Fig 12). Based on lithology, petrography, and geochemistry, it has been suggested that the Jurassic to Cretaceous Phu Kradueng Formation, PWF, SKF, PPF, and KKF [60] originate from quartzose sedimentary rocks and accumulate in the passive continental margin or intracratonic basin (Fig 12).
The Huai Hin Lat deposition in the sedimentary basin was created by the late Paleozoic collision of the microcontinents (Fig 13A) at a higher latitude than the present location or near Southeast Tibet [62]. Therefore, intermediate and felsic igneous rocks, as well as quartzose sedimentary rocks, were transported from the western part and accumulated in the basin [63]. Meanwhile, U-Pb geochronology and Lu-Hf isotopic signatures of the Huai Hin Lat formation (HHF) suggest that the western margin of the Khorat Plateau may have been part of an early Paleozoic arc system [64][65][66][67][68][69][70], which is consistent with the prominence of early Paleozoic zircons, or approximately 452 Ma [71,72], and similar to the South China Terrane [73][74][75][76]. In addition, the 290 Ma zircon isotopic signature in the HHF is consistent with crystallization ages from granitoids within the Indochina Terrane that span from 310 to 203 Ma [54,[77][78][79][80][81][82][83][84] and is more evolved than the Khao Khwang Fold-Thrust Belt [85,86]. Therefore, these ages may be associated with Late Carboniferous to Middle Permian subduction-related volcanic arcs as a result of the early stages of the South China collision with Vietnam and Indochina [73,76], or the similarly timed Indosinian orogeny caused by the collision of the Sibumasu and Indochina Terranes [58,83].
Over time, these two microcontinents became more stable, and the Nam Phong Formation occurred ( Fig 13B) from sediments of felsic igneous rocks and quartzose sedimentary rocks [59], bounded below and above by major unconformities, deposited in the NW-SE trending basin in similarity to south Vietnam and NE Cambodia [84]. The ostracod assemblage and microfacies in the lower Jurassic Nam Phong Formation, above the oldest known dinosaur footprints site in Thailand, indicate deposition in a shallow and low-energy fluvial-lacustrine regime [85]. During this period, a contractional fold belt in the basin developed during the Early and Middle Jurassic affected by the Jurassic tectonic of the southeastern Indochina Terrane [84].
Many researchers suggest that the Khorat Group was essentially deposited in a layer-cake fashion, accompanied by local faulting [58,88], whereas Racey described the Khorat Group as being deposited in a craton interior setting or intracontinental basin [12]. In addition, stratigraphic investigation described that the Khorat Group in the Lom Kao (the western part) and Phu Phan Mountain (the eastern part) as being generally oriented in a westerly direction from  [86,87]. The location of the Khorat Basin and transportation (black arrow) of sediments are modified from [32,70]. the South China Terrane [63]. Considering the orientations of the cross-bedding data in the field, the paleocurrent pattern in the PWF and PPF was in a westerly direction [32].
Collisional orogens are one of the factors that affect the sedimentary rift basin of the tectonic passive continental margins [39,89]. This event may apply to the Khorat Basin, which was developed as a series of major half-grabens in the extensional event during the Triassic (Fig 13C). In addition, the Indosinian orogeny during the early Cretaceous may have caused the collapse of the over-thickened crust, and multi-sequence continental sediments were deposited during the Jurassic and Cretaceous [90]. Usually, the intracontinental basin is flanked by mountains on both sides. The drainage transports sediments to the basin on each side until it accumulates as a stratum. Moreover, dinosaur footprints (small theropods Carmelopodus isp.) were discovered in the PWF [16,91,92], supporting the premise that dinosaurs lived in this terrane during the Jurassic-Cretaceous in a braided stream environment [30].
Based on the age constraints of detrital zircons in the Khorat Plateau (PWF-PPF), the sedimentary rocks were from a major early Cretaceous drainage disruption across Southeast Tibet [60]. According to Jurassic-early Cretaceous zircons (U-Pb dating and U-Pb-Hf compositions) from the Khorat Plateau [60], the sedimentary rocks of the PWF, SKF, PPF, and KKF were largely recycled from multiple source terranes, with some possibly transported from the Sibumasu terrane or terrane in the western flank (Fig 13A), which is consistent with our petrographic and chemical results. According to the cross-section illustrated in Fig 1, the KKGp shows the fold structures (Fig 13B) that may be affected by the extensive Palaeogene deformation and neotectonics (Miocene-Pleistocene) during the Himalayan orogeny event [99]. The fold and strike-slip fault structures along the NW-SE direction in the Khorat Plateau represent the dominant tectonic deformation fabric evolved by compression [100]. The regional structures and tectonic evolution were recorded by fold structures in the KKGp, supporting that tectonically this region is located on the western margin of the composite Indochina Terrane and deposited in diverse tectonic settings.

Conclusion
The geological record of the KKGp involves Mesozoic sandstone of the Khorat Group, including the PWF, SKF, PPF, and KKF, representing the western part of the Khorat Plateau. The SKF is the main stratum for fossil exploration, especially dinosaur footprints and dinosaur bones. According to field observation, lithology, petrography, and geochemistry, the conclusions and interpretations regarding the sedimentary rocks of the KKGp are as follows: • Field investigation and lithological data suggest that Mesozoic sandstones of the Khorat Group have several sequences of maroon and white clastic sedimentary successions, which consist mainly of sandstone, pebbly sandstone, and siltstone interbedded with calcrete, conglomerate, and some evaporite layers.
• Using Qm, Qp, F, Lv, Lm, and Ls, petrographic data classified the provenance from diverse tectonics, including recycled orogen with quartzose recycled, craton interior, mixed sources, subduction complex, and collision suture fold-thrust. • The sandstones in the KKGp were predominantly sourced from the exposed proximal rocks in the terrane, from the northwestern recycled orogen, based on their maturity, grain characteristics, and previous U-Pb Lu-Hf isotopic signatures.
• This publicity could increase the international scientific value of KKGp allowing it to achieve the status of a UNESCO Global Geopark and facilitate development of sustainable geotourism. However, the results of this study based on the relatively small dataset and geochemistry remain inconclusive. Thus, additional electron probe microanalyses (EPMA) for heavy metal minerals and isotopes are required for additional resolution.
Supporting information S1