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Article

New Geochemical Insights into Pre-Khorat Paleoenvironments: A Case Study of Triassic–Jurassic Reddish Sedimentary Rocks in Thailand

by
Vimoltip Singtuen
1,*,
Burapha Phajuy
2 and
Punya Charusiri
3,4
1
Department of Geotechnology, Faculty of Technology, Khon Kaen University, Khon Kaen 40002, Thailand
2
Department of Geological Sciences, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
3
Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
4
Department of Mineral Resources, Ministry of Natural Resources and Environment, Bangkok 10400, Thailand
*
Author to whom correspondence should be addressed.
Geosciences 2025, 15(8), 324; https://doi.org/10.3390/geosciences15080324
Submission received: 28 July 2025 / Revised: 15 August 2025 / Accepted: 16 August 2025 / Published: 19 August 2025

Abstract

The Nam Phong Formation, a key unit of the pre-Khorat Group in the western Khorat Plateau, provides critical insights into the Mesozoic geological evolution of northeastern Thailand. This study presents the first integrated petrographic and geochemical investigation of the formation within Khon Kaen Geopark to reconstruct its Late Triassic–Early Jurassic depositional settings, provenance, and paleoclimate. A detailed stratigraphic section and five supplementary sites reveal litharenite and lithic wacke sandstones, interbedded with red paleosols and polymictic conglomerates. Sedimentary structures—such as trough and planar cross-bedding, erosional surfaces, and mature paleosols—indicate deposition in a high-energy braided fluvial system under semi-arid to subhumid conditions with episodic subaerial exposure. Petrographic analysis identifies abundant quartz, feldspar, and volcanic lithic fragments. Geochemical data and REE patterns, including diagnostic negative Ce anomalies, provide compelling evidence for provenance from active continental margins and oxidizing weathering conditions. These findings point to a tectonically active syn-rift basin influenced by climatic variability. Strikingly, the Nam Phong Formation exhibits paleoenvironmental and sedimentological features comparable to the modern Ebro Basin in northeastern Spain, highlighting the relevance of uniformitarian principles in interpreting ancient continental depositional systems.

1. Introduction

The Nam Phong Formation was originally classified as part of the Khorat Group [1,2,3]. However, subsequent investigations conducted by the Department of Mineral Resources have led to its re-evaluation, as it exhibits depositional environments markedly different from those of the overlying formations. This distinction has prompted its separation from the Khorat Group. Geographically, the Nam Phong Formation is predominantly distributed in the western part of northeastern Thailand, particularly in Khon Kaen, Chaiyaphum, Loei, and Nong Bua Lamphu provinces [4] (Figure 1a). It displays significant lateral variation in thickness, ranging from approximately 2500 m in the central basin to substantial thinning or complete absence along the basin margins [2]. Stratigraphically, the unit is subdivided into the Lower and Upper Nam Phong Formations, separated by a prominent unconformity [2]. Lithologically, the Nam Phong Formation consists primarily of reddish-brown micaceous sandstone, conglomerate, siltstone, and mudstone, with minor claystone. The Nam Phong Formation unconformably overlies the Rhaetian-aged Huai Hin Lat Formation, which reflects sedimentation in tectonically active basins during the Late Triassic [5]. Provenance studies indicate that sediments were derived from intermediate to felsic igneous rocks and quartzose sources, transported from uplifted continental arcs [6]. U–Pb zircon geochronology and Lu–Hf isotopic data reveal dominant age populations around 452 Ma and 290 Ma, linking them to early Paleozoic arc systems and Permian granitoids of the Indochina Terrane [7,8,9,10,11,12,13,14]. These signatures point to subduction-related magmatism associated with either the South China–Indochina collision or the Indosinian orogeny [7,8,9,10,11,12,13,14,15,16], highlighting the influence of arc-related volcanic activity and continental assembly on sedimentation across the region. The volcanic activity is interpreted to result from the convergence and collision of at least two continental terranes, where subduction-driven magmatism along active margins gave rise to extensive felsic to intermediate volcanism [7,8,9,10,11,12,13,14,15]. The Nam Phong Formation is interpreted to have been deposited initially in an alluvial fan setting, transitioning to a meandering fluvial system characterized by channel and floodplain deposits under a semi-arid paleoclimate [17,18].
The Nam Phong Formation has traditionally been assigned to the Late Triassic, primarily based on macrofossil and palynological evidence [3,21]. In contrast, the overlying Phu Kradung, Phra Wihan, and Sao Khua Formations have been placed within the Jurassic, and the Phu Phan and Khok Kruat Formations in the Early Cretaceous [2,22,23]. These age assignments are largely based on fossil assemblages comprising terrestrial vertebrates (including dinosaurs), freshwater bivalves, and palynomorphs—indicating non-marine depositional environments such as lakes, floodplains, and fluvial systems rather than marine settings [21,24]. The Nam Phong Formation, by comparison, exhibits even greater environmental complexity. Palynological data from the Lower Nam Phong Formation suggest a Late Triassic (Rhaetian) age, with assemblages dominated by gymnosperm pollen such as Corollina (syn. Classopollis) and Dicheiropollis, indicative of a warm, seasonally dry subtropical climate [21]. Recent discoveries of sauropod embryos and juvenile bones, interpreted from lacustrine or overbank deposits, suggest that deposition extended into the Early Jurassic [24]. Seismic studies further support the division of the Nam Phong Formation into two major units—a Lower and Upper Nam Phong—separated by a regional unconformity [22]. While the Lower Nam Phong has a relatively well-established Rhaetian age, the Upper Nam Phong remains poorly constrained. However, palynological evidence from the Phu Horm-1 well indicates it is no older than the Pliensbachian, and the absence of the Early Cretaceous marker Dicheiropollis etruscus supports a pre-Cretaceous, likely Jurassic, age [21]. The Phu Kradung Formation, which unconformably overlies the Nam Phong Formation, is interpreted as Middle to Upper Jurassic based on both macrofossil and palynological assemblages [2]. Its lower portions are characterized by alluvial deposits, transitioning upward into a high-energy fluvial system with meandering channels, which suggests deposition under a semi-arid paleoclimate [22,23]. The fossil content in these units—such as terrestrial vertebrates and freshwater mollusks—further affirms a continental setting dominated by rivers and ephemeral lakes rather than marine environments.
The Khon Kaen Geopark (KKGp), located along the western margin of the Khorat Plateau in northeastern Thailand, holds both scientific and geoheritage significance. It is currently under national evaluation for nomination as a UNESCO Global Geopark due to its outstanding geological diversity, paleontological heritage, and cultural importance. Geologically, the region is dominated by continental sedimentary sequences of the Khorat Group, which represent a major Mesozoic stratigraphic succession in Southeast Asia. These formations, in ascending order, include the Nam Phong, Phu Kradung, Phra Wihan, Sao Khua, Phu Phan, Khok Kruat, and Maha Sarakham formations [4,6,17,18], and are unconformably overlain by Quaternary alluvial deposits (Figure 1b). Of particular importance is the Nam Phong Formation, which forms the lowermost unit of the Khorat Group and represents a transitional sequence from pre-Khorat rocks. This formation comprises red sandstones and associated facies that record the initial stages of basin development, thus offering a valuable archive for understanding the paleoenvironmental and tectonic evolution of the region during the Late Triassic to Early Jurassic. Despite previous lithostratigraphic and paleontological studies in other areas, there remains a lack of detailed geochemical investigation to constrain the provenance, lithological classification, and tectonic setting of the Nam Phong Formation. In this study, we focus on the eastern sector of Khon Kaen Geopark, where the formation is well exposed as low-relief terrain and gently undulating plains. This area provides an ideal setting for analyzing sedimentary processes and reconstructing paleogeographic conditions across the Triassic–Jurassic transition. The primary objectives of this study are: (1) to determine the geochemical composition of representative sedimentary rocks in the Nam Phong Formation; (2) to evaluate their provenance, tectonic setting, and degree of chemical weathering using major, trace, and rare earth element proxies; and (3) to assess the implications of these geochemical signatures for the depositional environment, paleoclimate, and early evolution of the Khorat Basin.

2. Samples and Methods

Initial fieldwork, including stratigraphic logging and sample collection, was conducted in the eastern part of Khon Kaen Geopark, situated on the western flank of the Khorat Plateau (Figure 1). This region exposes a well-preserved and laterally continuous succession of sedimentary rocks belonging to the Nam Phong Formation. A total of ten rock samples were collected from five key localities for subsequent petrographic and geochemical analyses (Table 1).
Representative samples were prepared for petrographic study to determine textural properties, mineralogical composition, and sedimentary characteristics, including rock classification based on 400-point counting. Petrographic analyses were conducted at the Department of Geological Sciences, Chiang Mai University, and the Department of Geotechnology, Khon Kaen University. Thin sections were examined using a Motic BA310POL polarizing microscope, and photomicrographs were captured using ZEISS imaging equipment and ZEN 3.4 (blue edition) software (Carl Zeiss NTS Ltd., Oberkochen, Germany).
Seven representative sedimentary samples were selected for whole-rock geochemical analysis. These samples were derived from different lithologies—ranging from coarse-grained conglomerate and cross-bedded sandstone to fine-grained laminated sandstone—and were collected from distinct stratigraphic levels at each of the five study sites across the investigation area (Table 1). All samples exhibited minimal evidence of alteration, weathering, or secondary mineral replacement, as confirmed through petrographic examination, and were selected to capture the lithological diversity and spatial variability of the Nam Phong Formation in the study region. Although only seven samples were analyzed, they were rigorously selected based on prior lithological analysis to represent the key facies types and sedimentological variations observed in the formation. Moreover, the resulting geochemical data were compared with equivalent-age sandstones from other regions to contextualize provenance and tectonic setting.
Major, trace, and rare earth elements (REEs) were analyzed using a combination of X-ray fluorescence (XRF), inductively coupled plasma optical emission spectrometry (ICP-OES), and inductively coupled plasma mass spectrometry (ICP-MS). All analyses were performed at SGS Mineral Laboratory, People’s Republic of China. Major elements were determined using an Axios mAX XRF spectrometer (ANalytical’s, Malvern Panalytical B.V., Almelo, The Netherlands). Sample powders were finely ground using a ceramic grinding set and passed through a 200-mesh sieve. Pressed powder pellets were prepared for heavy elements, while lithium metaborate fusion was employed for light elements. The detection limit for major oxides was 0.01 wt%. Analytical accuracy was monitored using the GSR-3 certified reference material. Total iron was reported as Fe2O3, consistent with standard practice in sedimentary geochemistry.
Trace elements and REEs were measured using an Agilent 5800 ICP-OES system (Agilent Technologies Inc., Santa Clara, CA, USA) and a PerkinElmer NexION 300X ICP-MS system (PerkinElmer Inc., Waltham, MA, USA). Sample digestion was carried out using sodium peroxide flux. Certified reference materials GSD-2A and GSD-3A were used as calibration standards. All analyses were performed at SGS-CSTC Standards Technical Services Co., Ltd., Beijing, PR China. The typical analytical precision for most elements was better than ±5% relative standard deviation (RSD).

3. Results

3.1. Field Observation and Stratigraphy

The investigated section of the Nam Phong Formation at site NP1 (16°39′12.4″ N, 102°31′47.1″ E), located in Khon Kaen Province, northeastern Thailand, comprises a 16.5 m thick, continuous exposure dominated by medium-grained sandstone with prominent sedimentary structures (Figure 1a and Figure 2a–c). Situated in the central Khorat Plateau, this section represents a typical stratigraphic succession of the Nam Phong Formation.
Field observations indicate moderately dipping beds with well-preserved trough and planar cross-stratification, consistent with deposition under high-energy fluvial conditions. Stratigraphic logging (Figure 2d) defines three distinct intervals. The lower part of the section, from 0 to 8 m, consists of repeated successions of trough cross-bedded sandstone interbedded with thin reddish soil horizons composed of very fine to fine sand. The middle interval, from 8 to 12 m, is dominated by planar cross-bedded sandstone with improved sorting and fewer, thinner soil horizons. The upper interval, from 12 to 16.5 m, contains moderately to well-sorted, cross-bedded sandstone capped by fine sand horizons with red soil development.
Additional study sites provide a broader depositional and structural context. Site NP2 exhibits a fining-upward fluvial succession near the transition to the Phu Kradung Formation. Site NP3 features sandstone outcrops exhibiting multiple joint orientations, indicative of post-depositional tectonic deformation. Site NP4 presents an incomplete stratigraphic section but offers potential for lateral facies correlation. Site NP5 contains a basal conglomerate overlying volcanic basement, representing the initial phase of Nam Phong Formation deposition.

3.2. Petrography

Petrographic analysis of sandstone and conglomerate samples (NP1–NP5) shows different petrographic compositions (Figure 3), reflecting variations in provenance, depositional environments, tectonic setting, and diagenetic processes (Figure 4).
The sandstone samples (NP1–NP4) are predominantly composed of subangular to subrounded grains of quartz, plagioclase, feldspar, and lithic fragments, embedded within a fine- to medium-grained matrix (Figure 3a–g). These framework grains are accompanied by accessory minerals such as zircon, opaque minerals, biotite, muscovite, and chlorite. Textural features indicate moderate to poor sorting and grain-to-grain contacts that range from point to long contacts, with some concavo–convex contacts suggestive of pressure solution during compaction. Diagenetic features include calcite cement in some samples (e.g., NP1-2 and NP2-2), indicating secondary carbonate precipitation (Figure 3b,d).
The ternary classification diagram (Qt–F–Lt) following Pettijohn et al. [25] (Figure 4a) indicates that most samples plot within the subarkose and sublitharenite fields, with a few falling into the arkose and lithic arkose fields.
Provenance and tectonic settings were further evaluated using multiple discrimination diagrams. In the Qt–F–Lt diagram (Figure 4b) [26], the majority of samples cluster within the “recycled orogen” and “transitional continental” fields, implying derivation from recycled sedimentary rocks associated with uplifted orogenic belts. The Qm–F–Lt diagram (Figure 4c) [26] similarly places most samples in the “quartzose recycled” and “transitional recycled” fields.
Further resolution is provided by the Lm–Lv–Ls diagram (Figure 4d) [27], where samples are broadly distributed within the “mixed magmatic arc and subduction complex” and “suture belt” fields. Additionally, the Qp–Lv–(Ls+Lm) diagram (Figure 4e) [28] shows clustering within the “collision suture fold-thrust” and “arc orogen” fields.
Sample NP5, in contrast, is a volcanic clast from a polymictic conglomerate, showing a markedly different mineralogical composition, which is composed of euhedral to subhedral plagioclase, alkaline feldspars, and volcanic glass, with a small amount of quartz, hornblende, chlorite, and opaque minerals. These features point to an andesitic to latitic volcanic origin (Figure 3h). The presence of aligned plagioclase laths and interstitial chlorite may suggest subvolcanic texture and post-eruption alteration.

3.3. Geochemistry

3.3.1. Rock Name and Paleoclimate

Geochemical results are summarized in Table 2, with the complete dataset provided in the Supplementary Materials. Geochemical discrimination diagrams were employed to classify the sandstone samples and evaluate their compositional maturity, sedimentary recycling, and weathering intensity. These plots are widely used in sedimentary geochemistry to complement petrographic interpretations and provide quantitative constraints on provenance, mineralogical composition, and paleoclimatic conditions. Detrital geochemical discrimination diagrams (Figure 5) provide insight into the provenance, tectonic setting, weathering intensity, and paleoclimatic conditions of the Nam Phong Formation [25,29,30]. The classification diagrams, such as the Fe2O3 + K2O vs. SiO2/Al2O3 [29] and Na2O + K2O vs. SiO2/Al2O3 [30] plots, identify the sedimentary rocks as predominantly litharenites, greywacke, Fe-sandstone, and Fe-shale, reflecting moderate compositional and textural maturity derived from volcanic and recycled orogenic sources (Figure 5a,b). The SiO2 vs. (Al2O3 + K2O + Na2O) plot [31] suggests an evolutionary trend from semiarid to humid chemical weathering regimes (Figure 5c), indicative of multi-stage sediment recycling and variable paleoclimatic conditions.

3.3.2. Provenance and Environment of Deposition

A suite of geochemical discrimination diagrams was employed to assess the provenance, tectonic setting, weathering intensity, and sedimentary history of the Nam Phong Formation. The TiO2 vs. (Fe2O3 + MgO) diagram [31] indicates that most samples plot within the passive margin, active continental margin, and continental arc fields (Figure 6a), consistent with derivation from a tectonically active volcanic arc, likely associated with convergent margin processes. Similarly, the Log(K2O/Na2O) vs. SiO2 diagram [32] shows that all samples fall within the active continental margin field (Figure 6b), reinforcing an orogenic arc provenance linked to uplifted volcanic and plutonic source terrains.
The Al2O3/SiO2 vs. Basicity Index plot [33,34] further refines this interpretation (Equation (1)), with samples plotting in the evolved island arc and continental/dissected arc fields (Figure 6c), suggesting input from mature arcs that have undergone uplift, crustal reworking, and erosion.
Basicity Index = (FeO + MgO)/(SiO2 + K2O+Na2O)
Provenance discrimination using Discriminant Function 1 (Equation (2)) versus Function 2 (Equation (3)) [35] indicates that the analyzed samples predominantly cluster within the felsic igneous, quartzose sedimentary, and intermediate igneous provenance fields (Figure 6d). This distribution suggests derivation primarily from the upper continental crust, with potential contributions from recycled arc-derived sediments.
DF1 = −1.770TiO2 + 0.087Al2O3 + 0.170Fe2O3 − 1.5MgO + 0.616CaO + 0.500Na2O − 1.229K2O
DF2 = 0.445TiO2 + 0.07Al2O3 − 0.25Fe2O3 − 1.142MgO + 0.358CaO + 1.175Na2O + 1.428K2O
The La/Th versus Ce diagram [36], based on rare earth element (REE) ratios, further supports a quartzose sedimentary composition (Figure 7a), indicative of a mixed provenance involving reworked, compositionally mature sediments. Additionally, the La/Th versus Hf plot [37] reveals a trend toward felsic sources associated with passive continental margins and mixed felsic/basic origins (Figure 7b), along with a notable influence from andesitic arc components. Collectively, these geochemical signatures point to a multi-cycle sedimentary history influenced by both continental and arc-related sources. The Zr/Sc vs. Th/Sc plot [38] shows elevated Zr/Sc ratios across all samples, reflecting significant zircon enrichment and sediment recycling (Figure 7c).
The La–Th–Sc diagram [33] shows that most samples plot within the fields of granitic gneiss–metabasite mixtures and metabasite sources, as well as near the active margin boundary (Figure 8a). This suggests a composite source terrain comprising both felsic (granitic) and intermediate to mafic (metabasaltic) rocks. In the sediment-focused variation of the La–Th–Sc diagram [34], samples fall within fields representing clay, silt, sand, and gravel derived from mixed and metabasite sources (Figure 8b). This distribution supports a heterogeneous sediment supply from both felsic plutonic and amphibolitic/metabasaltic terrains. The Th–Sc–Zr/10 diagram [36] reinforces the tectonic interpretation, with samples plotting primarily in the continental island arc field (Figure 8c), with several trending toward the active continental margin. This pattern suggests deposition in a tectonically active arc setting influenced by continental arc systems. Finally, the La–Th–Sc diagram [39] confirms that the majority of samples are derived from an active continental margin setting (Figure 8d).

3.3.3. UCC Normalized Diagrams and REE Pattern

The geochemical composition of the Nam Phong Formation sandstones, as illustrated in the multi-element diagrams normalized to the upper continental crust (UCC) [40] and chondrite [41], is shown in Figure 9. The major oxide data (Figure 9a) reveal SiO2 contents comparable to those of the UCC, while Al2O3 and K2O concentrations are generally lower. MnO and Na2O contents are variable, with some values close to, lower than, or exceeding those of the UCC. Certain samples, particularly calcareous sandstones, exhibit elevated CaO. Most samples show lower Na2O and MgO relative to UCC values.
Trace element patterns (Figure 9b) show Th contents comparable to UCC, while Pb, Zr, and Hf are slightly lower. U contents display a distinct negative anomaly. Zr and Hf are elevated relative to some trace elements, while Sr, Ba, and Ni are notably lower than UCC. Cu and Zn values are equal to or exceed UCC levels.
Chondrite-normalized REE patterns (Figure 9c) display enrichment in light rare earth elements (LREE: La–Sm), a negative Ce anomaly, and a slightly negative Eu anomaly in some samples. These patterns resemble those of the upper continental crust [40], the Shijia Sandstone of China [42], and the Lomas Coloradas Formation in México [43].

4. Discussion

The sedimentological and geochemical characteristics of the Nam Phong Formation provide valuable constraints on the paleoenvironmental and paleoclimatic evolution of the Khorat Plateau during the Late Triassic to Early Jurassic [21]. Integration of field observations, petrographic analyses, and multi-proxy geochemical datasets from sites NP1–NP5 reveals a predominantly fluvial depositional system, punctuated by periodic subaerial exposure, episodic volcanic input, tectonic modification, and climatic variability.

4.1. Sediment Composition and Provenance

The Nam Phong sandstones are moderately to poorly sorted, composed of subangular to subrounded grains, and have a framework dominated by quartz, plagioclase, alkali feldspar, and volcanic lithic fragments. Accessory minerals—including zircon, chlorite, hornblende, and opaque phases—together with volcanic clasts at NP5, indicate a provenance from intermediate to felsic igneous rocks and recycled orogenic sources. The volcanic clast at NP5, consisting of euhedral plagioclase, volcanic glass, and minor hornblende, suggests an andesitic to latitic volcanic provenance, supporting deposition along a tectonically active continental margin, consistent with the geochemical evidence. Furthermore, the rounded to subrounded shapes of zircon grains point to significant sediment recycling from orogenic terrains, reinforcing the interpretation of mixed sources involving both uplifted continental crust and recycled sedimentary deposits.
Textural features reveal grain-to-grain contacts ranging from point to long, with concavo–convex boundaries suggestive of pressure solution during compaction [46]. Such relationships, along with the occurrence of matrix support in some samples, point to variable mechanical compaction and sediment reworking. Rounded to sub-rounded zircon grains observed in thin section indicate multi-cycle sediment recycling, consistent with derivation from uplifted orogenic terrains. Petrographic evidence indicates a moderate compositional maturity, with variability in feldspar and lithic content reflecting mixed sources from uplifted continental crust and recycled sedimentary deposits.
Framework grain compositions plot within litharenite to lithic wacke fields, consistent with rapid erosion and high-energy deposition. The Q–F–L diagram [25] places most samples in the “recycled orogen” field, indicating derivation from uplifted continental crustal sources subjected to erosion and recycling. Similarly, the Qm–F–Lt diagram [26] positions the majority of samples in the “quartzose recycled” and “transitional recycled” fields, reinforcing the interpretation of sediment reworking from older continental sources. This distribution suggests a composite provenance, potentially involving both continental arc-related and subduction zone-derived materials, further supporting a complex tectonic setting likely associated with continental collision and arc-related processes.
Geochemical discrimination diagrams further substantiate these petrographic interpretations. The use of ternary Th–La–Sc and Th–Sc–Zr/10 plots places the Nam Phong Formation predominantly within active continental margin and continental island arc fields. This approach, consistent with global applications of Th, La, and Zr concentrations to discriminate tectonic settings [47,48], has been successfully employed in the Changbaishan Tianchi volcanic field (NE China) to trace crustal evolution and magmatic inputs in arc-related regimes. Such cross-regional consistency reinforces the reliability of these discrimination systems for deciphering provenance and tectonic regimes in ancient sedimentary basins.
Chondrite-normalized REE patterns display pronounced LREE enrichment, flat to slightly fractionated HREE profiles, a negative Ce anomaly, and a slightly negative Eu anomaly in some samples. These patterns closely resemble those of diorite and monzodiorite from the Loei–Phetchabun Volcanic Belt (LPVB) [44,45], suggesting a direct genetic link between volcanic clasts within the Nam Phong sandstones and magmatic sources from the LPVB. This correspondence indicates that volcanic input into the Nam Phong Basin was at least partly derived from erosion of the Loei Volcanic Arc, reinforcing the tectonosedimentary connection between basin fill and adjacent volcanic arc/post-collision magmatism (Figure 10).
Comparative analysis reveals affinities with other red bed sequences—such as the Shijia Sandstone (Sichuan Basin, South China) and the Lomas Coloradas Formation (Cabullona Group, northern Mexico)—which, despite differing tectonic settings, share intermediate to felsic igneous provenance [42,43]. These formations, along with the El Antimonio Group in northwestern Mexico [49], record enrichment in LREEs, flat HREE patterns, and variable Eu anomalies, indicating mixed igneous sources and sediment recycling. The persistence of arc-derived geochemical signatures in both active and passive margin modern analogues, such as the La Pesca and Tesoro Altamira beach sands (northern Gulf of Mexico) [50], highlights the robustness of provenance signals across contrasting depositional settings.

4.2. Sedimentary Structures and Paleoenvironments

Field-based sedimentary logs from site NP1 document well-developed cross-bedding, both trough and planar types, which reflect strong unidirectional paleoflow typical of braided river systems [51], as shown in Figure 10. Interbedded red paleosol horizons and fining-upward successions record alternating phases of high-energy sedimentation and subaerial exposure, reflecting climatic shifts or episodic tectonic subsidence [52,53].
The vertical stacking of cross-bedded sandstone with paleosol layers is diagnostic of semi-arid to arid continental systems, where sediment supply is episodic and climate-controlled. Lateral facies variations between sites further illustrate this dynamic nature. At NP5, a basal polymictic conglomerate marks the onset of Nam Phong sedimentation, representing initial fluvial incision into older Permo–Triassic units. In contrast, NP2 features finer-grained units and reduced flow energy near the Phu Kradung Formation contact, suggesting a gradual transition from braided rivers to floodplain or lacustrine settings, likely driven by basin subsidence and regional climatic change [2,54].

4.3. Paleoclimate and Depositional Context in the Triassic–Jurassic

Geochemical proxies and REE patterns support a semi-arid to subhumid paleoclimate during Nam Phong deposition. Quartz dominance, moderate feldspar content, and depletion in mobile elements such as Na2O and MgO indicate chemical weathering under oxidizing, well-drained conditions [55]. Pronounced negative Ce anomalies in REE profiles further attest to oxidative weathering typical of terrestrial environments [56].
The reddish to purplish sandstone coloration, due to hematite coatings formed under alternating wet–dry cycles, suggests prolonged subaerial exposure and post-depositional stabilization [57]. During the Triassic–Jurassic transition, syn-rift tectonism formed half-graben structures in the Khorat Basin, accommodating thick fluvial successions. The alternation of sandstone and paleosol horizons, combined with mixed volcanic and continental crust provenance, reflects the interplay of tectonic subsidence, volcanic input, and climatic variability [53,58].

4.4. Implications for Basin Evolution

The Nam Phong Formation records the evolution of a tectonically active fluvial system influenced by intermittent volcanic input and climatic forcing. This unit represents a transitional stage in Khorat Basin development from older Paleozoic basement to Triassic–Jurassic continental sedimentation under semi-arid climatic regimes. Evidence for multi-cycle sediment recycling, indicated by heavy mineral concentration and compositional maturity trends, points to uplifted orogenic sources and prolonged subaerial exposure [59,60,61].
The tectonic evolution of the Nam Phong Formation parallels that of convergent-margin basins such as the Ebro Basin in northeastern Spain [62,63,64]. Both formed in plate collision settings—Nam Phong in a rift-to-post-rift environment associated with Indochina terrane accretion, and the Ebro Basin as a foreland basin during Iberian–Eurasian collision. In each case, volcaniclastic sediments from nearby arcs were delivered to subsiding basins; in the Khorat Basin from the Loei–Phetchabun Volcanic Arc, and in the Ebro Basin from the Pyrenean arc. This tectono-sedimentary analogy offers a valuable framework for interpreting the Nam Phong Formation as part of a syn-depositional arc-related system evolving under semi-arid paleoclimatic conditions.

5. Conclusions

New insights into the sedimentological, petrographic, and geochemical evolution of the Nam Phong Formation in northeastern Thailand highlight its deposition during the Late Triassic to Early Jurassic. The formation comprises litharenite to lithic wacke sandstones and polymictic conglomerates, interpreted to have formed within a high-energy braided fluvial system under semi-arid to arid conditions. Sedimentary features such as cross-bedding, ripple marks, and oxidized paleosol horizons reflect episodic subaerial exposure within an active continental margin setting. Petrographic classification reveals moderate compositional maturity, with most samples falling into subarkose and sublitharenite fields. Volcanic clasts with plagioclase, volcanic glass, and associated alteration textures further confirm direct volcanic input.
Geochemical signatures point to a mixed provenance dominated by felsic to intermediate igneous rocks, including arc-derived sources and recycled continental crust. Discrimination diagrams place most samples within “recycled orogen” and “transitional continental” fields, while tectonic setting diagrams suggest contributions from arc, subduction, and collisional zones. REE patterns—characterized by LREE enrichment, flat HREEs, and variable Ce–Eu anomalies—along with trace element ratios (e.g., Th/Sc, La/Sc, and low Cr–Ni) are consistent with derivation from the Loei–Phetchabun Volcanic Arc and resemble modern arc-derived beach sands from the northern Gulf of Mexico.
Regionally, the Nam Phong Formation shows strong lithological and compositional affinities with other Mesozoic red beds such as the Shijia Sandstone (South China), the Lomas Coloradas Formation, and the El Antimonio Group (Mexico), all of which are linked to volcanic arc and post-collisional sources. Additionally, the syn-rift basin architecture and sediment dispersal style closely resemble those of the Ebro Basin in northeastern Spain, reinforcing the relevance of modern analogues in interpreting Mesozoic continental basin development.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/geosciences15080324/s1. Table S1: Complete geochemical dataset of all analyzed samples.

Author Contributions

Conceptualization, V.S. and B.P.; methodology, V.S., B.P. and P.C.; software, V.S.; validation, V.S., B.P. and P.C.; formal analysis, V.S., B.P. and P.C.; investigation, V.S., B.P. and P.C.; resources, V.S. and B.P.; data curation, V.S., B.P. and P.C.; writing—original draft preparation, V.S.; writing—review and editing, V.S., B.P. and P.C.; visualization, V.S., B.P. and P.C.; supervision, V.S., B.P. and P.C.; project administration, V.S.; funding acquisition, V.S. and B.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Science Research and Innovation Fund (NSRF) of Thailand, under the Fundamental Fund 2025, grant number 203190. The APC was funded by the Research Administration Division (RAD) of Khon Kaen University. The funders were not involved in the study design, data collection, analysis, decision to publish, or manuscript preparation.

Data Availability Statement

Data availability statements are available in the appropriate section.

Acknowledgments

The authors would like to express their sincere gratitude to the undergraduate students of the 46th cohort (G46) from the Department of Geotechnology, Khon Kaen University, for their valuable assistance during fieldwork and sampling. In particular, the contributions of Juthatip Khonman, Porawat Joosakoon, and Jakkrich Kulthong are gratefully acknowledged.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Regional geological map illustrating the distribution of the Nam Phong Formation (highlighted in purple) across the Khorat Plateau [4]. The study area is marked by a dashed box, with green circles indicating sample locations used for petrographic and geochemical analysis. (b) Geological map of Khon Kaen Geopark showing field data collection sites and geological cross-sections along lines A–A′ and B–B′, illustrating the folding structures within the area. Lithological data are adapted from DMR [4,6,19,20].
Figure 1. (a) Regional geological map illustrating the distribution of the Nam Phong Formation (highlighted in purple) across the Khorat Plateau [4]. The study area is marked by a dashed box, with green circles indicating sample locations used for petrographic and geochemical analysis. (b) Geological map of Khon Kaen Geopark showing field data collection sites and geological cross-sections along lines A–A′ and B–B′, illustrating the folding structures within the area. Lithological data are adapted from DMR [4,6,19,20].
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Figure 2. (a) Field photograph of the main stratigraphic section at Site NP1, displaying steeply dipping, cross-bedded sandstone units; a person (1.7 m tall) is included for scale. (b) Thin-bedded sandstone exposure at Site NP3. (c) Conglomeratic unit observed at Site NP5, with a 38 cm geological hammer for scale. (d) Stratigraphic column of the Nam Phong Formation at Site NP1, view facing south.
Figure 2. (a) Field photograph of the main stratigraphic section at Site NP1, displaying steeply dipping, cross-bedded sandstone units; a person (1.7 m tall) is included for scale. (b) Thin-bedded sandstone exposure at Site NP3. (c) Conglomeratic unit observed at Site NP5, with a 38 cm geological hammer for scale. (d) Stratigraphic column of the Nam Phong Formation at Site NP1, view facing south.
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Figure 3. Photomicrographs of sandstone and conglomerate samples under cross-polarized light (XPL). (a) NP1-1: Litharenite sandstone with quartz (Qz), k-feldspar (KFs), plagioclase (Pl), zircon (Zrn), lithic fragments (Lt), and opaque minerals (Opq). (b) NP1-2: Litharenite sandstone showing quartz, feldspar, calcite cement (Cc), muscovite (Ms), and opaque minerals. (c) NP2-1: Lithic wacke composed of abundant lithic fragments, quartz, feldspars, biotite (Bt), and matrix-supported grains. (d) NP2-2: Moderately sorted litharenite sandstone with quartz, feldspar, calcite cement, and opaques. (e) NP3-1: Lithic-rich wacke with lithic fragments, feldspar, and opaques in a fine matrix. (f) NP3-2: Fine-grained lithic wacke containing plagioclase, lithics, and opaques. (g) NP4: Poorly sorted lithic wacke with quartz, lithic fragments, and abundant opaque minerals. (h) NP5: Volcanic clast from conglomerate, showing euhedral plagioclase (Pl), feldspars, opaque minerals (Opq), and volcanic glass (Gs), indicative of felsic volcanic provenance.
Figure 3. Photomicrographs of sandstone and conglomerate samples under cross-polarized light (XPL). (a) NP1-1: Litharenite sandstone with quartz (Qz), k-feldspar (KFs), plagioclase (Pl), zircon (Zrn), lithic fragments (Lt), and opaque minerals (Opq). (b) NP1-2: Litharenite sandstone showing quartz, feldspar, calcite cement (Cc), muscovite (Ms), and opaque minerals. (c) NP2-1: Lithic wacke composed of abundant lithic fragments, quartz, feldspars, biotite (Bt), and matrix-supported grains. (d) NP2-2: Moderately sorted litharenite sandstone with quartz, feldspar, calcite cement, and opaques. (e) NP3-1: Lithic-rich wacke with lithic fragments, feldspar, and opaques in a fine matrix. (f) NP3-2: Fine-grained lithic wacke containing plagioclase, lithics, and opaques. (g) NP4: Poorly sorted lithic wacke with quartz, lithic fragments, and abundant opaque minerals. (h) NP5: Volcanic clast from conglomerate, showing euhedral plagioclase (Pl), feldspars, opaque minerals (Opq), and volcanic glass (Gs), indicative of felsic volcanic provenance.
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Figure 4. Petrographic diagrams: Diagrams of plotted petrographic data of sedimentary rocks in Khon Kaen Geopark. (a) Qt–F–Lt (rock name) classification diagram [25]; (b) Qt–F–Lt diagram with tectonic fields; (c) Qm–F–Lt diagram with tectonic fields [26]; (d) Lm–Lv–Ls diagrams with tectonic fields [27]; and (e) Qp–Lv–(Ls+Lm) diagrams with tectonic fields [28]. Qt: total quartz, Qm: monocrystalline quartz, Qp: polycrystalline quartz, F: feldspar (plagioclase and K-feldspar), L: lithic fragment, Lt: lithic fragment and polycrystalline quartz, Ls: sedimentary lithic grains, Lv: volcanic lithic grains, Lm: metamorphic lithic grains.
Figure 4. Petrographic diagrams: Diagrams of plotted petrographic data of sedimentary rocks in Khon Kaen Geopark. (a) Qt–F–Lt (rock name) classification diagram [25]; (b) Qt–F–Lt diagram with tectonic fields; (c) Qm–F–Lt diagram with tectonic fields [26]; (d) Lm–Lv–Ls diagrams with tectonic fields [27]; and (e) Qp–Lv–(Ls+Lm) diagrams with tectonic fields [28]. Qt: total quartz, Qm: monocrystalline quartz, Qp: polycrystalline quartz, F: feldspar (plagioclase and K-feldspar), L: lithic fragment, Lt: lithic fragment and polycrystalline quartz, Ls: sedimentary lithic grains, Lv: volcanic lithic grains, Lm: metamorphic lithic grains.
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Figure 5. Geochemical diagrams for sedimentary rock samples used to infer rock name, weathering intensity, and paleoclimate. (a,b) Log(SiO2/Al2O3) vs. Log(K2O/Na2O) [29] and Log(SiO2/Al2O3) vs. Log(SiO2/K2O) [25] plots showing rock name classification. (c) SiO2 vs. (Al2O3 + K2O + Na2O) diagram [30] indicating degrees of chemical weathering. Symbols (#) indicate sampling sites within the Nam Phong Formation (e.g., #1 = NP1, #2 = NP2). Samples from site NP1 are highlighted in the pink field, whereas samples from site NP2 are shown in the sky-blue field. Filled black circles represent individual samples analyzed.
Figure 5. Geochemical diagrams for sedimentary rock samples used to infer rock name, weathering intensity, and paleoclimate. (a,b) Log(SiO2/Al2O3) vs. Log(K2O/Na2O) [29] and Log(SiO2/Al2O3) vs. Log(SiO2/K2O) [25] plots showing rock name classification. (c) SiO2 vs. (Al2O3 + K2O + Na2O) diagram [30] indicating degrees of chemical weathering. Symbols (#) indicate sampling sites within the Nam Phong Formation (e.g., #1 = NP1, #2 = NP2). Samples from site NP1 are highlighted in the pink field, whereas samples from site NP2 are shown in the sky-blue field. Filled black circles represent individual samples analyzed.
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Figure 6. Major oxides discrimination diagrams for sedimentary rock samples used to infer provenance, tectonic setting, and source composition. (a) Discrimination diagram of (Fe2O3 + MgO) vs. TiO2 [31] used to identify tectonic setting. (b) K2O/Na2O vs. SiO2 [32], a tectonic setting discrimination. (c) Basicity Index vs. Al2O3/SiO2 plot [33,34] used to differentiate between evolved and immature island arcs and continental settings. (d) Discriminant Function Analysis plot [35]. Filled black circles represent individual samples analyzed. Fields and trends are based on published classification schemes for sedimentary rocks and tectonic settings.
Figure 6. Major oxides discrimination diagrams for sedimentary rock samples used to infer provenance, tectonic setting, and source composition. (a) Discrimination diagram of (Fe2O3 + MgO) vs. TiO2 [31] used to identify tectonic setting. (b) K2O/Na2O vs. SiO2 [32], a tectonic setting discrimination. (c) Basicity Index vs. Al2O3/SiO2 plot [33,34] used to differentiate between evolved and immature island arcs and continental settings. (d) Discriminant Function Analysis plot [35]. Filled black circles represent individual samples analyzed. Fields and trends are based on published classification schemes for sedimentary rocks and tectonic settings.
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Figure 7. Trace elements discrimination diagrams for sedimentary rock samples used to infer provenance, tectonic setting, and source composition. (a) La/Yb vs. Ce diagram [36] for sedimentary provenance. (b) La/Th vs. Hf ratios [37] to constrain source composition and arc-type sediment input. (c) Th/Sc vs. Zr/Sc plot [38] indicating sediment recycling and zircon concentration trends. Filled black circles represent individual samples analyzed.
Figure 7. Trace elements discrimination diagrams for sedimentary rock samples used to infer provenance, tectonic setting, and source composition. (a) La/Yb vs. Ce diagram [36] for sedimentary provenance. (b) La/Th vs. Hf ratios [37] to constrain source composition and arc-type sediment input. (c) Th/Sc vs. Zr/Sc plot [38] indicating sediment recycling and zircon concentration trends. Filled black circles represent individual samples analyzed.
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Figure 8. Ternary discrimination diagrams of sedimentary rock samples used to infer provenance, tectonic setting, and source composition. (a,b) La–Th–Sc diagrams [33,34] for provenance and tectonic discrimination. (c) Th–Sc–Zr/10 diagram [36] for tectonic setting and source characterization. (d) La–Th–Sc diagram [39] emphasizing tectonic environment and sediment source types. Filled black circles represent individual analyzed samples.
Figure 8. Ternary discrimination diagrams of sedimentary rock samples used to infer provenance, tectonic setting, and source composition. (a,b) La–Th–Sc diagrams [33,34] for provenance and tectonic discrimination. (c) Th–Sc–Zr/10 diagram [36] for tectonic setting and source characterization. (d) La–Th–Sc diagram [39] emphasizing tectonic environment and sediment source types. Filled black circles represent individual analyzed samples.
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Figure 9. Geochemical normalized distribution patterns of the sandstone samples (NP1–1 to NP4). (a) Major element concentrations normalized to upper continental crust (UCC) values [40], showing enrichment and depletion trends, notably elevated CaO in the calcareous sandstone sample (indicated by a black star). (b) Trace element concentrations normalized to UCC values [40], indicating variable depletion/enrichment patterns, with significant U depletion in some samples. (c) REE patterns normalized to chondrite values [41]. The REE distribution of Shijia Sandstone [42], sandstones of the Khorat Group from Khon Kaen Geopark [6] (orange field), and sandstone of the Lomas Coloradas Formation, Cabullona Group, México (pale reed field) [43] is compared to UCC, Loei Volcanic Belt [44,45], suggesting similarities in source characteristics and sedimentary processes.
Figure 9. Geochemical normalized distribution patterns of the sandstone samples (NP1–1 to NP4). (a) Major element concentrations normalized to upper continental crust (UCC) values [40], showing enrichment and depletion trends, notably elevated CaO in the calcareous sandstone sample (indicated by a black star). (b) Trace element concentrations normalized to UCC values [40], indicating variable depletion/enrichment patterns, with significant U depletion in some samples. (c) REE patterns normalized to chondrite values [41]. The REE distribution of Shijia Sandstone [42], sandstones of the Khorat Group from Khon Kaen Geopark [6] (orange field), and sandstone of the Lomas Coloradas Formation, Cabullona Group, México (pale reed field) [43] is compared to UCC, Loei Volcanic Belt [44,45], suggesting similarities in source characteristics and sedimentary processes.
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Figure 10. Tectonic and sedimentological model illustrating the depositional environment of the Nam Phong Formation during the Late Triassic to Early Jurassic period within a convergent margin setting. The Nam Phong Formation was deposited in the Khorat Basin, located between the Indochina blocks, and is characterized by fluvial sediments derived from braided stream systems. Volcaniclastic sediments were transported from the Loei–Phetchabun Volcanic Arc and deposited into the basin. This tectonosedimentary setting reflects syn-depositional activity associated with arc volcanism and crustal convergence during the early Mesozoic era.
Figure 10. Tectonic and sedimentological model illustrating the depositional environment of the Nam Phong Formation during the Late Triassic to Early Jurassic period within a convergent margin setting. The Nam Phong Formation was deposited in the Khorat Basin, located between the Indochina blocks, and is characterized by fluvial sediments derived from braided stream systems. Volcaniclastic sediments were transported from the Loei–Phetchabun Volcanic Arc and deposited into the basin. This tectonosedimentary setting reflects syn-depositional activity associated with arc volcanism and crustal convergence during the early Mesozoic era.
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Table 1. Sample codes, corresponding locations, and geographic coordinates of the studied sites in the Nam Phong Formation.
Table 1. Sample codes, corresponding locations, and geographic coordinates of the studied sites in the Nam Phong Formation.
SiteLocationSample Code
NP116°39′12.4″ N 102°31′47.1″ ENP1-1, NP1-2, NP1-3, NP1-4
NP216°42′34.1″ N 102°25′58.9″ ENP2-1, NP2-2, NP2-3
NP316°37′14.5″ N 102°23′53.8″ ENP3
NP416°35′27.5″ N 102°23′55.9″ ENP4
NP516°45′59.2″ N 102°30′45.0″ ENP5 (volcanic clast)
Table 2. Major oxides (wt%), trace elements (ppm), and rare earth elements (REEs, ppm) of sandstone samples of the Nam Phong Formation from the Khon Kaen Geopark.
Table 2. Major oxides (wt%), trace elements (ppm), and rare earth elements (REEs, ppm) of sandstone samples of the Nam Phong Formation from the Khon Kaen Geopark.
SampleNP1-1NP1-2NP2-1NP2-2NP2-3NP3NP4
Major oxides (wt%)MgO0.480.380.81.750.650.520.5
Al2O38.336.5311.1412.1310.677.9617.2
SiO280.0672.6577.4259.9878.1383.8765.38
Na2O2.982.663.993.284.172.837.91
K2O0.580.370.411.040.390.450.16
CaO0.237.790.38.120.210.231.19
TiO20.470.350.450.550.470.280.83
MnO0.040.080.270.120.30.080.18
Fe2O3t2.541.992.773.923.031.913.66
P2O50.040.040.040.080.01<0.010.41
Trace elements (ppm)Ba14669.834079.8232232409
Cr44354951412683
Cs1.30.91.12.510.90.6
Cu61472725532651
Hf3335323
Li0.0020.0010.0020.0020.001<0.0010.001
Nb3234318
Ni33522931501444
Rb20.715.816.436.51517.57.2
Pb155<5<5151117
Sc6<57107<519
Sr84.510389.314395.460.6527
Th42.85.55.43.23.45.3
U0.580.090.530.40.640.911.72
V69505776704754
Y7.27.414.718.615.831.639.5
Zr11183.811615912964.8117
Rare Earth Elements (ppm)La8.9728.816.920.55853.2
Ce21.21537.830.338.842.976
Pr2.861.986.74.435.4412.0512.05
Nd10.87.527.918.422.246.250.3
Sm2.21.65.33.64.18.59.2
Eu0.60.461.310.941.262.22.27
Gd1.761.573.843.313.536.997.82
Tb0.370.250.70.620.591.121.34
Dy1.851.623.283.322.966.117.38
Ho0.360.340.550.650.591.11.39
Er1.020.911.541.871.552.893.79
Tm0.190.150.240.320.250.420.56
Yb10.91.41.71.42.22.9
Lu0.20.170.220.30.250.330.45
Fe2O3ᵗ = total iron expressed as Fe2O3.
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Singtuen, V.; Phajuy, B.; Charusiri, P. New Geochemical Insights into Pre-Khorat Paleoenvironments: A Case Study of Triassic–Jurassic Reddish Sedimentary Rocks in Thailand. Geosciences 2025, 15, 324. https://doi.org/10.3390/geosciences15080324

AMA Style

Singtuen V, Phajuy B, Charusiri P. New Geochemical Insights into Pre-Khorat Paleoenvironments: A Case Study of Triassic–Jurassic Reddish Sedimentary Rocks in Thailand. Geosciences. 2025; 15(8):324. https://doi.org/10.3390/geosciences15080324

Chicago/Turabian Style

Singtuen, Vimoltip, Burapha Phajuy, and Punya Charusiri. 2025. "New Geochemical Insights into Pre-Khorat Paleoenvironments: A Case Study of Triassic–Jurassic Reddish Sedimentary Rocks in Thailand" Geosciences 15, no. 8: 324. https://doi.org/10.3390/geosciences15080324

APA Style

Singtuen, V., Phajuy, B., & Charusiri, P. (2025). New Geochemical Insights into Pre-Khorat Paleoenvironments: A Case Study of Triassic–Jurassic Reddish Sedimentary Rocks in Thailand. Geosciences, 15(8), 324. https://doi.org/10.3390/geosciences15080324

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