Introduction

The Carboniferous period, a critical phase in Earth’s history, witnessed significant advancements in both terrestrial animal and plant life1. During this time, the Stegocephalia—a group of four-limbed vertebrates that includes true tetrapods—fully transitioned to a pentadactyl limb structure, having evolved from their lobe-finned fish ancestors in the preceding Devonian period2. Often referred to as the "Age of Amphibians," this period saw the diversification of early amphibians, particularly the temnospondyls, which emerged as dominant land vertebrates3. Additionally, the first amniotes appeared during the late Carboniferous4, leading to the evolution of two major lineages: synapsids, the ancestors of modern mammals, and sauropsids, which include today’s reptiles and birds3.

The elevated atmospheric oxygen levels during this period facilitated the remarkable evolutionary radiation of terrestrial arthropods, including arachnids like trigonotarbids and Pulmonoscorpius5, large myriapods such as Arthropleura6,7, and insects like the giant dragonfly-like Meganeura8. This period was also characterized by extensive forests and swamp ecosystems that covered vast portions of the land9. These dense plant communities, primarily composed of ferns, lycophytes, and other primitive vascular plants, played a pivotal role in the formation of the extensive coal beds that define the Carboniferous stratigraphy observed today10,11.

The Carboniferous rocks of Thailand date back to approximately 359 to 299 million years ago and are predominantly sedimentary in nature, comprising limestones, sandstones, and shales12,13. These formations, often fossil-rich, particularly with corals, brachiopods, and crinoids, suggest marine environments during the Carboniferous period12,13. The extensive outcrops of Carboniferous limestone can be found in west-central Thailand and along the western edge of the Khorat Plateau12,13,14. In northeastern Thailand, exposures occur across Loei (Wang Saphung) and Nong Bua Lamphu Provinces, extending over an area of approximately 20 to 40 km12,13 (Fig. 1a). The Carboniferous strata in this region lie conformably beneath Permian rocks and overlie Silurian-Devonian units, particularly in the western part of the northern region14,15.

Fig. 1
figure 1

Geologic maps related to Carboniferous studies. (a) Distribution of Carboniferous and Permian strata in Thailand, primarily derived from the 1:1 million scale geological map of Thailand (map modified from Ueno and Charoentitirat14). (b) Geologic map of Na Klang District, Nong Bua Lamphu Province, located along the western margin of the Khorat Plateau (geological data from DMR17). The maps were created using QGIS version 3.14.16 and Microsoft® PowerPoint for Mac version 16.90.2 (24,102,719).

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The Carboniferous rocks of northeastern Thailand are divided into two formations: the Wang Saphung Formation (equivalent to the subsurface Si That Formation) and the Nong Dok Bua Formation. The Wang Saphung Formation is further subdivided into two members, C1 and C2, which show an unconformable relationship with the underlying Silurian-Devonian rocks and a conformable transition into the overlying Permian strata. The Nong Dok Bua Member (C1) primarily consists of sandstone and shale, with occasional limestone lenses and conglomerates. Brachiopod fossils, dating from the Lower to Middle Carboniferous, are commonly found in this member12,13. Overlying the Nong Dok Bua Member is the Wang Saphung Member (C2), characterized by shale, siltstone, and intercalated limestone beds. This member contains plant fossils16, trilobites, and other taxa, further supporting its Carboniferous period12,13.

The Carboniferous rocks predominantly composed of calcareous shales, carbonates, and interbedded sandstones, also include coal, chert, and evaporite deposits, particularly in the Loei area14. The base of the formation rests on a mid-Carboniferous unconformity, with carbonates developing in the centre of the basin, while siliciclastic sediments dominate in peripheral areas. In addition, gypsum-anhydrite was deposited associated with carbonates in the Loei area during the Middle to Late Carboniferous18,19, suggesting deposition in a shallow-marine to tidal flat environment.

Notable Carboniferous fossil discoveries in Thailand include algae (e.g., Bersella), which are abundant in the middle Carboniferous of the Loei area18,20. Radiolarians have been extracted from cherts and siliceous shales, while foraminifera, particularly single-chambered varieties, are widespread in Carboniferous-Permian strata21,22. Corals are primarily represented by dendroid tubulata, solitary, fasciculate, and massive rugose forms, which were deposited during the Lower to Upper Carboniferous in the Loei region23. These findings underscore the importance of fossil assemblages in reconstructing the paleoenvironmental and tectonic history of the region.

The lithological characteristics of Nong Bua Lamphu Province are primarily sedimentary, ranging from Paleozoic to Mesozoic in age, with some occurrences of Permo-Triassic igneous and Silurian-Devonian metamorphic rocks17 (Fig. 1b). The Carboniferous rocks (C) are found in the Na Wang and Na Klang districts (this study) as well as the northern part of the Si Bun Rueang district. The Carboniferous Wang Saphung Formation in this province, which consists mainly of sandstone, shale, thin limestone beds, and conglomerate. These formations represent a shallow marine depositional environment, with fossils supporting this interpretation17.

Despite the presence of Carboniferous strata in Thailand, detailed studies—particularly those focused on the depositional environments and precise chronological frameworks—are limited. This research aims to address this gap by employing geochemical analysis to enhance the stratigraphic understanding of the Carboniferous rocks in Nong Bua Lamphu Province. Additionally, petrographic and palaeontological analyses will be conducted to identify index fossils and establish a detailed chronology of rock deposition. The results will provide critical data for reconstructing the tectonic evolution of Southeast Asia.

Methods

To enhance the present study, a comprehensive literature review was conducted, incorporating various materials, including maps and photographs, related to the Carboniferous rocks of Nong Bua Lamphu Province, located along the western margin of the Khorat Plateau. This research adheres to all relevant ethical standards and does not involve the use of non-human animals. The methodology employed for the analysis of these Carboniferous rocks comprised several key steps. Field investigations and borehole drilling were performed to obtain subsurface data, followed by detailed lithologic studies of the collected samples. Eight boreholes were drilled into limestone deposits using core drilling techniques, each reaching a depth of approximately 20 m. Borehole elevations and coordinates were precisely recorded using real-time kinematic (RTK) technology to ensure accuracy in spatial data collection. Additionally, surface sections at a nearby quarry were studied for geological data and clastic sampling. On-site lithological assessments were conducted using a hand lens for the preliminary identification and classification of rock units. From the borehole cores, 14 representative limestone samples were selected for detailed petrographic and paleontological analysis. Thin sections were prepared from rock samples for petrographic examination, which, typically measuring 30 μm (0.03 mm) in thickness, was conducted using a polarized light microscope to identify mineral compositions and fossil content. These samples were analyzed using ZEN core Imaging Software integrated with ZEISS microscopy systems, allowing for high-resolution examination. The petrographic and photomicrographic data yielded critical insights into the depositional environments and facilitated the correlation of stratigraphic sequences throughout the study area. Moreover, geochemical analyses were performed on fourteen carbonates and fourteen clastic rocks using an Olympus Vanta handheld XRF analyzer, which detected major and minor elements. The use of X-ray fluorescence (XRF) instead of carbonate staining for classifying dolomite offers a more precise, non-destructive method for elemental analysis. Unlike staining, which yields only qualitative data, XRF allows for quantitative assessment of elemental composition, enabling a more accurate and comprehensive characterization of carbonate deposits and associated mineral phases. The standard C series (VCR) model features a silicon drift detector, a 40 kV X-ray tube, and a rhodium (Rh) anode. In contrast, the M series (VMR) model is equipped with a large-area silicon drift detector and a 50 kV X-ray tube, also utilizing a Rh anode. This technique, capable of measuring elemental concentrations from ppm levels to nearly 100% by weight, was limited to elements with atomic numbers greater than twelve (12). To address the challenges posed by non-homogeneous textures in the studied rocks, each sample was analyzed three times, and the average results were calculated to ensure accuracy. The geochemical data were converted to oxide forms, aiding in rock classification, provenance determination, and the interpretation of depositional environments within the region. The results of these analyses, along with petrographic, paleontological, and geochemical data, were used to discuss and interpret the depositional environments of the Carboniferous rocks in the region. This integrated approach provided a robust framework for understanding the geological history and sedimentary processes that shaped the Carboniferous formations at Nong Bua Lamphu Province.

Results

Field observation and lithologic logs

The field study was carried out in two distinct areas: (1) eight boreholes drilled in the northern section of the alluvial plain, and (2) an examination of outcrops within a construction quarry located in the southern region (Fig. 2a). Borehole samples primarily comprised thick black limestone, characterized by abundant fossils and calcite veins, with sporadic thin siltstone interbeds observed in some cores. The limestone strata in the quarry are oriented towards the northwest, with bedding planes dipping at angles ranging from approximately 30 to 50 degrees to the north (Fig. 2b). This geological framework provides valuable insights into the structural and depositional history of the region.

Fig. 2
figure 2

The topographic features and lithologic section of the studied area reveal key geological characteristics. (a) The satellite imagery displays the locations of boreholes, marked with red symbols, and the outcrop locations, marked with white symbols, providing a spatial overview of the study area. The satellite imagery was generated using Google Earth Pro (Map data ©2024 Google) version 7.3.6.9796 (64-bit) on a Mac OS X operating system (version 15.0.1) with an Apple M3 graphics driver (2.1 Metal—89.3). (b) A detailed lithologic section from the limestone quarry highlights the stratigraphy, showcasing the sequence of sedimentary layers. This section provides insights into the geological history and structure of the area, offering a clear depiction of the borehole and outcrop relationships within the limestone deposit.

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This geological section consists of thin to thick beds of sandstone and siltstone, overlain by thick to massive blackish-grey limestone, interbedded with black mudstone, calcareous shales, carbonaceous shales, and thin layers of dark grey sandstone, which contain fossilized coral remains. Representative clastic samples were collected for geochemical analysis under the OC code designation, with a total of 14 samples (seven samples of OC1 and seven samples of OC2). The rocks in this region are structurally affected by numerous strike-slip and reverse or thrust faults. Furthermore, certain areas exhibit signs of ancient sinkholes, likely formed due to the dissolution of limestone in shear zones.

A total of eight boreholes (designated as DH) were drilled into the limestone deposits within the study area using core drilling techniques, with each borehole reaching depths of approximately 20 m. The borehole cores from the study area can be correlated across three distinct sets along a southwest-to-northeast transect. These sets are as follows:

  • Set 1: DH8, DH6, and DH7—representing the northwestern portion of the study area.

  • Set 2: DH1, DH2, and DH3—forming the central portion of the transect.

  • Set 3: DH4 and DH5—situated in the southeastern zone near a quarry.

This correlation between the boreholes provides a comprehensive view of the subsurface stratigraphy, revealing lithologic continuity and variations across the study area. The aligned cores offer critical insights into the geological evolution, structural deformation, and depositional environments from southwest to northeast, facilitating a better understanding of the region’s subsurface conditions.

The limestone layer is estimated to be located five to ten meters beneath the surface, covered by two distinct soil formations: (1) inorganic clays with low to medium plasticity, including gravelly clays, sandy clays, and silty clays, and (2) inorganic clays with cohesive and compressible clay of high plasticity, containing a high proportion of minerals that make it greasy to the feel i.e. kaolinite (Fig. 3). The borehole cores revealed massive blackish-grey limestone rich in fossils, with boreholes DH2 and DH3 also showing interbedding with black shale and siltstone. Structural logs from the boreholes indicate the presence of numerous joints and faults, which align with the structural features observed at the mining outcrop.

Fig. 3
figure 3

Correlation of borehole cores across three distinct sets from southwest to northeast. Set 1 includes DH8, DH6, and DH7, located in the northwestern zone. Set 2 comprises DH1, DH2, and DH3, positioned within the central zone. Set 3 consists of DH4 and DH5, situated in the southeastern zone, adjacent to the quarry. The alignment of these boreholes highlights lithologic and structural variations across the study area, providing insights into subsurface geology.

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Petrography and paleontology

The petrographic analysis and classification of the rock samples in this study were conducted using the frameworks established by Dunham24 and Embry & Klovan25. The rock classification of all samples was identified as mudstone, wackestone, wackestone to packstone, floatstone, packstone, and packstone to grainstone (Table 1). Based on these carbonate categories, the depositional environment of samples in each borehole can be divided into three types: low-energy, high-energy, and low–high-energy environments as follows:

Table 1 The result of the petrographic analysis of fourteen samples from drill logs.
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Low-Energy Environment: Samples DH4-1 to DH4-4 and DH8-1 exhibit a matrix-supported texture, with grain content ranging from 5 to 40% (Fig. 4). This suggests that the depositional environment might be in the area where the energy of transportation was low for example within the lagoon or under wave-based area. However, some samples are composed of a diverse assemblage of transported grains and fossils, indicative of material carried into the lagoon. Fossil and grain types include crinoids, small foraminifera, gastropods, shell fragments, algae, brachiopod spicules, peloids, irregular meshwork (persistent laminae), and stylolites.

Fig. 4
figure 4

Photomicrographs of limestone under plane-polarized light from different boreholes: (a) DH1 at a depth of 12–13 m, (b) DH2 at a depth of 13–14 m, (c) DH3 at a depth of 11–12 m, (d) DH4 at a depth of 12–13 m, (e) DH5 at a depth of 6–7 m, (f) DH6 at a depth of 9–10 m, (g) DH6 at a depth of 14–15 m, and (h–i) DH7 at a depth of 13–14 m.

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High-Energy Environment: Samples DH2-1, DH5-1, DH5-2, DH6-1, DH6-2, and DH7-1 comprise 30% to 90% grain-supported textures, primarily cemented by calcite with some matrix content (Fig. 4). This composition indicates deposition in a high-energy setting, such as a winnowed area or platform margin. The grains in all samples are composed of small foraminifera, crinoids, calcispheres, peloids, bryozoan and algal fragments, ostracods, and both regular and irregular meshwork, alongside stylolites. Notably, quartz grains (0.1–0.2 mm) observed in sample DH5-2 indicate terrigenous input from a continental source. The presence of Carboniferous (Mississippian) Endothyra smaller foraminifera further supports the high-energy environment interpretation.

Transition from High to Low-Energy Environments: The change of energy from high to low energy through time was observed in the boreholes DH1 and DH3 (samples DH1-1&DH1-2 and DH3-1&DH3-2). Lower samples (DH1-2 and DH3-2) contain 5% grains (Tubiphytes fragments, peloids, bioclast fragments, and small foraminifera) in a matrix-supported texture, characteristic of a low-energy environment. In contrast, upper samples (DH1-1 and DH3-1) exhibit 30% to 40% grain-supported textures with predominant calcite cement, indicating deposition in a high-energy environment. Fossils found in the upper part of samples DH1-1 and DH 3-1 contain bryozoans, crinoids, brachiopods, gastropods and algae.

Geochemistry

The bulk chemical composition of the samples, including SiO₂, Al₂O₃, Fe₂O₃, MnO, MgO, CaO, Na₂O, K₂O, and P₂O₅, was determined using X-ray fluorescence (XRF). According to established classification criteria, carbonate rocks are typically characterized by containing more than 50% carbonate minerals. Most of the limestone samples analyzed were composed of over 50% calcite, with the exceptions of DH3-1 (46.9%) and DH3-2 (47.2%). The presence of CaO greater than 50% is indicative of calcite (CaCO₃). DH3-1 and DH3-2 also exhibited detectable levels of MgO, which suggests the presence of dolomite (MgCa(CO₃)₂) or magnesite (MgCO₃). Additionally, samples DH2-1, DH3-1, DH3-2, and DH5-2 contained SiO₂ and Al₂O₃, which are indicative of clay minerals or kaolinite (Al2Si2O5(OH)4) and some of quartz (SiO2). Based on the classification criteria outlined by Todd (1996), all samples were identified as pure limestone (composed solely of calcite). Specifically, samples DH3-1 and DH3-2 were classified as magnesian limestone due to their dolomite content.

Several geochemical discrimination diagrams are utilized to infer the depositional environment of limestones, each providing distinct insights based on elemental composition. Binary plots such as MgO versus FeO (Fig. 5a) and log MgO/ Al₂O₃ versus log K₂O/Al₂O₃ (Fig. 5b) are commonly used to examine key oxide ratios, indicating that samples DH3-1 and DH3-2 were deposited in a marine setting.

Fig. 5
figure 5

Chemical classification of Carboniferous limestone in the western Khorat Plateau. (a) Binary plot of MgO vs. FeO (diagram modified from Ratcliffe et al., 200726). (b) Binary plot of log MgO/Al₂O₃ vs. log K₂O /Al₂O₃ (diagram modified from El-Desoky et al. (2015)27). (c) Ternary plot of MgO-Fe₂O₃-SiO₂/Al₂O₃ (diagram modified from Ratcliffe et al. (2007)26). (d) Binary plot of Fe₂O₃ vs. Al₂O₃ (diagram modified from Zhang et al. (2017)28). (e) Binary plot of Al2O3/(Al₂O₃ + Fe₂O₃) vs. Fe₂O₃/ TiO₂ (diagram modified from He et al. (2019)29). (f) Binary plot of Fe + Ca vs. Ca (diagram modified from He et al. (2019)29). (g) Binary plot of Al₂O₃ vs. P₂O5 (diagram modified from Dhannoun and Al-Dlemi (2013)30). (h) Binary plot of CaO vs. MgO (diagram modified from Naseem et al. (2005)31). (i) Binary plot of Ca vs. Fe (diagram modified from Naseem et al. (2005)31).

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Additionally, ternary plots like the MgO-Fe₂O₃-SiO2/Al₂O₃ diagram (Fig. 5c) suggest a marine depositional environment for DH3-1 and DH3-2, while samples DH2-1 and DH5-2 were associated with nonmarine and deltaic settings. Further refinement of depositional interpretations is achieved through binary plots such as Fe2O3 versus Al₂O₃ (Fig. 5d) and Fe + Ca versus Ca (Fig. 5e), which confirm that samples DH2-1, DH3-1, DH3-2, and DH5-2 were deposited in environments related to marine settings.

The Al₂O₃/(Al₂O₃ + Fe₂O₃) ratio plotted against Fe₂O₃/TiO₂ (Fig. 5f) offers a robust framework for distinguishing between marine and continental settings, integrating trace element data. This analysis classified samples DH2-1, DH3-1, DH3-2, and DH5-2 as having been deposited in a continental margin setting, with DH3-1 and DH3-2 also indicating pelagic marine deposition.

Water depth within depositional environments can be inferred through binary plots that examine oxide ratios. The Al₂O₃ versus P₂O5 plot (Fig. 5g) correlates aluminium and phosphorus concentrations to water depth, revealing that none of the samples were associated with shallow water depositional environments. The CaO versus MgO (Fig. 5h) and Ca versus Fe (Fig. 5i) plots further explore the geochemical balance of calcium, magnesium, and iron, providing evidence of deep marine conditions during limestone deposition. These geochemical diagrams are valuable tools for reconstructing paleoenvironments, offering nuanced insights into the depositional processes, water depth variations, and environmental conditions that shaped limestone formation.

Additionally, the southernmost part of the mining section encompasses very fine- to coarse-grained, clastic sedimentary rocks. Those samples were analyzed geochemistry together with interbedded sandstone in DH2 and DH3 boreholes. Based on the ratios of Log(SiO2/Al2O3) and Log(Fe2O3/K2O), the rocks are identified as shale, Fe-shale, wacke (OC2-1, OC2-3, OC2-4, and OC2-6), and litharenite (DH2Y) as shown in Fig. 6a. This classification aligns with the grain size analysis, indicating a provenance predominantly from mafic rocks, with minor contributions from felsic and intermediate rocks (Fig. 6b). Tectonic setting analysis of the sedimentary rocks, using Fe2O3 + MgO vs. Al2O3/SiO2 (Fig. 6c) and Fe2O3 + MgO vs. TiO2 (Fig. 6d), shows that the distribution of the analyzed samples relates to volcanic arc setting despite of DH2X was fall in passive margin environments.

Fig. 6
figure 6

Chemical classification of Carboniferous sedimentary rocks in the western Khorat Plateau. (a) log(Fe2O3/K2O) and log(SiO2/Al2O3) fields (diagram modified from Herron32) and (b) SiO2 versus Al2O3/TiO2 (diagram modified from Le Bas et al.33). Discrimination diagrams were modified from Bhatia34 for sedimentary rock samples (c) Fe2O3 + MgO versus Al2O3/SiO2 and (d) Fe2O3 + MgO versus TiO2.

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Discussion

The chemical classification of Carboniferous limestone in the western Khorat Plateau is assessed through various geochemical plots, including binary plots of MgO vs. FeO26, log MgO/Al₂O₃ vs. log K₂O/Al₂O₃27, Fe₂O₃ vs. Al₂O₃28, Al₂O₃/(Al₂O₃ + Fe₂O₃) vs. Fe₂O₃/TiO₂31, Fe + Ca vs. Ca29, Al₂O₃ vs. P₂O₅30, CaO vs. MgO31, and Ca vs. Fe31, as well as the ternary plot of MgO-Fe₂O₃-SiO₂/Al₂O₃26. These classifications provide insights into the limestone deposits, suggesting their formation in a continental margin setting or a shallow-marine environment near a tidal flat to a subtidal zone. Several geochemical proxies are fundamental to reconstructing the depositional environments of limestones, including the ratios of Al₂O₃/(Al₂O₃ + Fe₂O₃)29, Ca/(Ca + Fe)35,36, and Ca/Mg and Mg/Ca37. The Al₂O₃/(Al₂O₃ + Fe₂O₃) ratio serves as a key indicator for differentiating depositional settings, with values between 0.5 and 0.9 characteristic of continental margins, and lower values between 0.4 and 0.7 indicative of oceanic basin environments29. Similarly, the Ca/(Ca + Fe) ratio is used to infer the salinity of the depositional environment, with freshwater environments typically exhibiting ratios below 0.4, brackish environments showing values from 0.4 to 0.8, and saline environments having values exceeding 0.836. The Ca/Mg ratio is particularly significant in understanding the chemical conditions influencing limestone deposition. It provides insights into the degree of evaporation and salinity in the depositional environment. In highly evaporative conditions, the Mg/Ca ratio increases as seawater becomes progressively enriched in magnesium37. Magnesium, unlike calcium, is susceptible to redistribution during diagenesis, particularly under the influence of meteoric water, while calcium remains relatively stable38,39. These geochemical indicators provide a robust framework for reconstructing the paleoenvironmental conditions that influenced limestone deposition. The geochemical data from this study suggest that the limestones were deposited in a transitional environment, likely between a non-marine or deltaic setting and a marine environment near an active continental margin. Evidence supports both shallow and deeper water deposition (Fig. 5). Furthermore, the limestones are associated with the deposition of clastic sediments derived from multiple sources, consistent with a volcanic arc setting (Fig. 6).

Stratigraphic and paleontological investigations of brachiopods, trilobites, conodonts, microvertebrates, and gastropods, conducted by researchers from Australia, England, Japan, and the United States, have revealed that Thailand’s fossil record exhibits strong correlations with those from Burma, China, Australia, and South America40,41,42,43,44,45,46,47. These findings suggest that the Indochina terrane was part of the Gondwana supercontinent during the early Paleozoic, remaining in close proximity to Australia throughout the late early Paleozoic40. It subsequently drifted away from Gondwana during the Devonian period40. Since its likely early to mid-Paleozoic separation from Gondwana, the Indochina terrane has remained within the paleo-equatorial region50,51. In northeastern Thailand, particularly in the Khorat Plateau, Upper Paleozoic shallow-marine carbonate rocks are extensively distributed along the terrane’s margin50. The initial phase of thrusting and folding recorded in Carboniferous rocks was driven primarily by the closure of Paleotethys. Additionally, the Indosinian Orogeny had a substantial impact on the topography and stratigraphy of western Indochina, with regional-scale thrusting and folding further modified by Cenozoic strike-slip deformation50,51. This tectonic activity displaced originally adjacent facies, as evidenced by fossil assemblages, age data, and fault indicators documented in this study.

Our study integrates lithological, petrographic, geochemical, paleontological, and structural analyses to reconstruct the Carboniferous depositional environment. A block model illustrating the proposed setting, distinct from previous models, would enhance understanding of spatial relationships, facies distribution, and depositional processes (Fig. 7a). Evidence indicates that the Carboniferous strata were deposited in a forearc basin setting related to subduction processes along the Nan Suture, which contributed to the formation of the C1 unit during the Mississippian, and the Nakhon Thai Block, which formed the C2 unit during the Pennsylvanian (Fig. 7b). Volcanic arcs, active in the Loei and Laos regions from the Middle Devonian through the Carboniferous (Fig. 7a), were a significant factor, leading to the deposition of tuffaceous sandstones and volcaniclastics48,49,50. The basin evolution from the Middle Devonian through the early Carboniferous in the Loei Fold Belt, part of the Indochina terrane, supports the hypothesis of a back-arc basin setting, contrasting with the previously proposed oceanic and seamount models21,22, like Doi Chiang Dao Limestone50. The closure of the basin during the Mississippian was driven by compressional tectonics of the Nakhon Thai Block (Fig. 7b), leading to uplift and the widespread deposition of syn-orogenic coarse siliciclastics52.

Fig. 7
figure 7

Models of the Carboniferous depositional environment in the western part of Khorat Plateau, northeast Thailand. (a) Schematic tectonic model and evolution of sedimentary deposits. (b) Tectonostratigraphic column for Carboniferous rocks. The diagrammatic reconstructions are based on and incorporate modifications from previous studies on the tectonic evolution of Indochina19,48,49,50,51,52,53,54,55,56,57,58,59.

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The volcanic arcs associated with subduction persisted from the Late Carboniferous through the early to middle Permian, introducing pyroclastic materials into the clastic sedimentary sequences53. Furthermore, the development of a shallow carbonate platform occurred in this region during the overall regressive conditions of the Middle-Upper Pennsylvanian epoch or Moscovian–Kasimovian ages18 (Fig. 7a). The evolving basin environment impacted Pennsylvanian carbonate deposits and the Permian Saraburi formations14,54, which include carbonate platforms, deep-sea deposits, and turbidite sequences. This tectonic framework is consistent across the southwestern margin of the Indochina terrane, particularly in the Khao Khwang Fold-Thrust Belt, where carbonates and arc-related clastics were deposited from the Late Carboniferous to the Permian50,51,52,53. Additionally, the gypsum-anhydrite deposits associated with carbonates in the Loei area indicate deposition in a shallow-marine to tidal flat environment during the Middle to Late Carboniferous18. The sequences analyzed in this study reflect fluctuations in eustatic sea level and regional tectonic activity, possibly comparable to Lower–Middle Eocene deposits in the central Nile Valley and west-central Sinai, Egypt55. Each sequence comprises facies associations indicative of lowstand, transgressive, and highstand system tracts. The thick, fossil-rich black limestone interbedded with black shale, siltstone, and occasional evaporites—resembling carbonate sequences in the western Ordos Basin, China56—supports a detailed paleoenvironmental reconstruction. Evidence points to warm paleotemperatures, suboxic to anoxic conditions, and elevated paleosalinity56. Additionally, similarities with black shales in the Middle Yangtze region, China57, suggest a transition from relatively deep to shallow marine facies, accompanied by shifts from oxygen-rich to anoxic, and even euxinic, conditions before oxygen levels increased. A biological bloom, characterized by increased biodiversity, significantly elevated total organic carbon (consistent with hydrocarbon sources from five petroleum wells58) in the carbonates, leading to greater fossil abundance. Additionally, early meteoric water infiltration facilitated the dissolution of gypsum, resulting in its discontinuous distribution as gypsum-anhydrite (Fig. 7a). The tropical shallow-marine system persisted from the Upper Pennsylvanian to the Permian, coinciding with the expansion of Paleotethys and a probable reduction in salinity, which diminished evaporite deposition during the Carboniferous-Permian transition. Elevated oxygen levels further promoted the formation of limestone deposits (Saraburi Group) characterized by lower carbon content and increased biodiversity.

In conclusion, this study offers significant new geological insights into the Carboniferous sedimentary rocks of the Khorat Plateau within the Indochina terrane, utilizing a comprehensive multidisciplinary approach. By integrating lithological, petrographic, geochemical, stratigraphic, and paleontological data, it fills existing gaps in the understanding of the depositional environments and chronological framework of these Carboniferous rocks. Carbonate textures, classified using the Dunham and Embry & Klovan systems, range from mudstone to grainstone, reflecting deposition in a shallow marine setting during the Carboniferous (Mississippian) period, with energy levels varying from low to high. The clastic rocks, including shale, Fe-shale, wacke, and litharenite, suggest formation in a volcanic arc-related environment. The findings, supported by key paleontological evidence, including the presence of Endothyra foraminifera, provide critical insights into subduction events during the early Carboniferous and tectonic extension from the late Carboniferous to Permian58. These insights further clarify Late Carboniferous sediment deposition, though estimating the original thickness remains challenging due to deposition within a horst and graben system. This interpretation is supported by the orientation of faulting, which is consistent with previous studies60. Correlating stratigraphic sequences from multiple boreholes will enhance the understanding of sedimentary deposition patterns, facies distributions, and the influence of tectonic processes on sedimentation. This approach is critical for future research, as it will facilitate the development of more refined paleoenvironmental models and enhance our understanding of the tectonic forces shaping sedimentary basins, particularly at the boundary between Carboniferous and Permian rocks in the surrounding region.