Introduction

The Sīma or Sema stones are intricately carved stone sculptures used as symbols or markers to delineate sacred spaces in Buddhist contexts. The previous research indicates that Sīmas were widely constructed in Northeast Thailand during the 12th–16th Buddhist centuries, a period coinciding with the flourishing of Dvaravati culture1,2. The 12th–16th Buddhist centuries correspond approximately to the 8th–12th centuries CE in the Gregorian calendar. This period is significant in Southeast Asian history for the spread and development of Theravāda Buddhism, particularly in regions like present-day Thailand, Laos, and Cambodia. It was marked by the establishment of religious monuments, increased production of Buddhist art and inscriptions, and the integration of Buddhist teachings into local cultures and governance. The tradition of erecting Sīma stones is believed to have developed from cultural and religious influences, potentially shaped by three key factors. First, megalithic traditions suggest that the practice may have originated from indigenous Southeast Asian beliefs that venerated ancestral spirits, as evidenced by megalithic practices3. Second, Buddhist rituals played a crucial role, as Sīma stones were used to demarcate the boundaries of sacred spaces or ceremonial areas, ensuring their sanctity2. Finally, sacred proxies refer to the symbolic function of Sīma stones as representations of sacred sites, similar to stupas, pagodas, or Buddha images, providing objects of veneration for devotees1.

Sīma stones were installed in various configurations, reflecting their symbolic and ritual purposes. The single placement of a stone was used to mark the location or boundary of a sacred space2,4. In grouped placement, multiple stones were positioned around mounds or sacred sites without a fixed directional orientation3. In contrast, directional placement involved the strategic arrangement of stones to mark cardinal or intercardinal directions, with configurations ranging from four to sixteen stones encircling religious structures such as stupas, reliquaries, or ordination halls (ubosots). These arrangements included single, paired, or tripled stones, emphasizing their ritual significance1.

The Sīma stone associated with the Dvaravati culture in Northeast Thailand were typically large stone blocks that exhibited variations in craftsmanship. Some were left as natural, unshaped stones or were only minimally worked2, while others were deliberately shaped into distinct forms. These included slab-type Sīmas, which featured a flattened, rectangular or slab-like appearance, and pillar-type Sīmas, which were carved into rectangular, hexagonal, or octagonal pillar forms3.

In addition to their structural diversity, Sīma stones were often adorned with intricate carvings that carried deep symbolic meanings. Some stones remained plain and undecorated, while others featured elaborate relief carvings. A common motif was the triangular relief, resembling a stupa, which symbolized the Buddha2. Another significant motif was the purna-ghata, or sacred water pot—a round vessel often embellished with floral designs and sometimes featuring spouts. This symbol, deeply rooted in ancient Indian beliefs, represented prosperity and auspiciousness3. Narrative scenes depicting Buddhist stories, including the life of the Buddha and key jataka tales, were also commonly carved onto these stones1. The Jataka Tales are a collection of moral stories from the Buddha’s past lives, teaching virtues like compassion, self-sacrifice, wisdom, and cooperation through animal and human characters. Additionally, many Sīmas bore representations of the Dharmachakra, or Dharma wheel, a central Buddhist symbol signifying the spread of Buddhism, which was often placed at the centre or along the edges of the stones2.

These artistic and functional characteristics highlight the religious and cultural significance of Sīma stones during the Dvaravati period. Serving both as sacred markers and as artistic expressions of Buddhist devotion, they played a crucial role in defining the spiritual landscape of early Thailand.

Globally, sacred Buddhist spaces are marked using various materials, yet the extensive use of stone for this purpose is uniquely concentrated in the Khorat Plateau region of Southeast Asia5. With the introduction of Buddhism in the 7th century, the plateau became a significant site for the erection of Sīma stones5, transforming parts of what is now Khon Kaen Geopark into prominent Buddhist sanctuaries and religious centres (Fig. 1). Although the tradition of Sīma stones has persisted into modern times, many have been relocated or repurposed. However, the property area preserves an exceptional collection of Sīma stones in their original Dvaravati-period context, providing a rare and invaluable testament to this once-flourishing cultural and religious practice.

Fig. 1: Satellite imagery analysis, combined with Geographic Information System (GIS) techniques, was employed to identify and interpret archaeological sites within the Khon Kaen Geopark.
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A Digital Elevation Model (DEM) and 20-metre interval contour lines were analyzed using GIS software to enhance the understanding of topographic features and their relationship to site distribution and landscape context.

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Various types of archaeological evidence have been discovered through surface surveys in the agricultural areas of the ancient city of Non Nong Khu. These include Sīma stones made of sandstone, fragments of earthenware pottery, and other significant artifacts. The pottery fragments can be categorized based on their characteristics, including high-fired Khmer ceramics, high-fired Lan Xang ceramics, and Chinese porcelain. Additional finds include terracotta spindle whorls, grinding stone fragments, stone rollers, terracotta tobacco pipe fragments, sandstone sculptural fragments resembling votive plaques, grinding stones with wear marks, and slag remnants from iron smelting6.

Furthermore, an analysis of satellite imagery within the Khon Kaen Geopark area, utilizing Geographic Information System (GIS) techniques to identify archaeological sites, has revealed multiple significant locations. These include ancient settlements, iron smelting sites, salt production areas, cart tracks, and topographic features associated with past human activities. The latter comprises elevated landforms such as hills, mounds, ridges, knolls, and depressions, as well as distinctive terms in the local landscape lexicon—such as non (small hill), phon (isolated mound), kok (elevated ridge or rise), don (upland area or sandbar), kut (small water body or swamp), bao (hillock), rang (rocky area), pong (small pond), iet (cliff or rocky outcrop), and sok (slope or escarpment)—which often correspond to archaeological remains7, as illustrated in Fig. 1.

However, there has been no scientific study to explain the rock characteristics, rock names, composition, and geochemistry of Sīma stones to definitively identify their source. Such research would support archaeological data within the Khon Kaen Geopark, contributing to the understanding of the connection between natural resources, geology, and human culture from the past to the present. This would further enhance the potential of the area for future designation as a UNESCO Global Geopark.

The ancient site of Non Nong Ku or Ban Ruea is part of the Khon Kaen Geopark, located on the eastern outskirts of Phu Wiang Valley (Fig. 1). It exhibits geological characteristics of a plain, with an elevation of approximately 190 metres above sea level, and belongs to the Phu Kradung Formation within the Khorat Group8. The Tham Phra Kho Khat archaeological site lies in the southern area of Phu Wiang National Park, at the base of the Phu Wiang Mountain range, with a higher elevation of approximately 635 metres above sea level, classified under the Phra Wihan Formation of the Khorat Group (Fig. 2).

Fig. 2: Geological map of the Khon Kaen Geopark area, illustrating all study locations.
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The map highlights major lithostratigraphic units of Mesozoic sedimentary sequences of the Khorat Group. Geological base map modified from the Department of Mineral Resources8; coordinates and site locations were confirmed using handheld GPS during fieldwork.

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Additionally, the south-western part of the Phu Wiang Mountain, presumed to be a historical sandstone quarry or production site for Dvaravati Sīma stones6, is located within Phu Wiang National Park and is part of the Phra Wihan Formation in the Khorat Group as shown in Fig. 2.

Archaeological studies have not yet provided conclusive evidence as to whether the Sīma stones in the Non Nong Ku, Ban Ruea, and Tham Phra Kho Khat areas originate from the same source. Furthermore, it remains uncertain whether they were sourced from the Phu Wiang Mountain, which is believed to be a sacred site situated at a high elevation within the surrounding region. This study examines the lithology and geochemistry of Sīma stones from various locations, comparing them with rocks found in the areas where the Sīma stones were discovered. Additionally, lithological studies of nearby rock formations are conducted to determine the specific rock types.

Methods

The study commenced with the systematic compilation of data on Sīma stones within Khon Kaen Geopark, encompassing topographical characteristics, geological settings, and the historical development of both the geopark and its surrounding areas. This initial phase involved an analysis of lithology, geochemistry, and geological mapping, drawing upon prior research, established theoretical frameworks, and fundamental principles. Subsequent to the literature review, the research aimed to conduct petrographic and geochemical analyses to classify the lithological composition and determine the provenance of the stone materials used in the construction of Sīma stones, stone pillars, and other archaeological artifacts from Ban Ruea in the ancient city of Non Nong Khu. Additionally, the study examined Sīma stones at Wat Pa Don Ku, archaeological materials from Tham Phra Kho Khat, and rock samples from Sop Nam (confluence) of stream and Ban Kut Khaen near Ubon Ratana Dam (Fig. 2). A total of seven study sites were analyzed, with details on sample numbers, coordinates, site descriptions, and geological formation provided in Table 1.

Table 1 Details of the rock sample number, coordinates, study sites, and geological formation for lithological, petrographic, and geochemical analyses
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Additionally, the study incorporated topographic analysis through the use of a Digital Elevation Model (DEM) and 20-metre interval contour lines, analyzed within GIS and Google Earth Pro software. This approach facilitated the interpretation of archaeological site locations and landscape features within the Khon Kaen Geopark, enhancing the spatial understanding of their geological and cultural context.

This phase focused on the characterization of Sīma stones as well as the geochemical analysis of rock samples. Major oxides and trace elements were measured using a portable X-ray fluorescence (XRF) spectrometer, specifically the Olympus Vanta M Series analyzer. This device represents a significant advancement over its predecessor, the Olympus Delta, offering faster measurement speeds, shorter processing times, and improved detection limits—especially for low-energy elements such as Mg, Si, and Al. The M Series (VMR) features a highly sensitive, large-area silicon drift detector and a 50 kV X-ray tube with a Rh anode. This work used the Geochem Mode with integrated filter switching capabilities to optimize the detection of both light and heavy elements. Calibration was conducted using certified reference materials (CRMs) for silicate rocks, including USGS and Sigma-Aldrich standards, to ensure data reliability. Additionally, the intuitive software interface allows users to adjust calibrations as needed to suit different geological samples and matrices. The proprietary Axon Technology™ enables ultra-low-noise electronic performance, achieving higher X-ray counts per second and ensuring fast, accurate, and reproducible results. For geochemical analysis of archaeological artifacts, flat, clean, and unweathered surfaces were selected—free of algae or other surface contaminants. Ideally, the rock surface should be dry or have minimal moisture content, with fieldwork conducted on dry, sunny days to avoid the impact of weather conditions. As the artifacts could not be moved, in situ analysis was conducted using the portable XRF device. Each sample was analyzed at three different points for consistency, and the averaged results were used for interpretation. Data were processed using the Olympus Vanta software suite (v6.2), and outliers were removed based on internal quality control criteria (>95% confidence level).

Additionally, rock samples were collected from areas surrounding the archaeological sites for further petrographic analysis. A total of 11 thin sections, each 0.03 mm thick, were prepared from these samples. These served as representative specimens for petrographic examination using a Nikon polarized light microscope, integrated with Zeiss Light Microscopes and ZEN core free imaging software. This setup facilitated the classification of rock textures and lithologies. This multi-method approach enabled a comprehensive investigation of the geological and petrographic characteristics of the Sīma stones within the Khon Kaen Geopark, integrating established academic knowledge with original field data.

Results

Lithology

At the ancient site of Non Nong Khu, located in Ban Ruea Subdistrict, Phu Wiang District, a fragment of a Sīma stone or Buddhist boundary marker was discovered. The fragment measures 50 cm in width, stands 83 cm above the ground, and has a thickness of 27 cm. It is in a deteriorated condition and was found inverted on the surface, positioned along a laterite road adjacent to the northeastern section of the city moat (Fig. 3a). The sample exhibits a clastic texture, with a weathered brown exterior and a fresh maroon or reddish-purple interior. It is primarily composed of quartz grains, with sedimentary grain sizes ranging from very fine to moderately coarse sand. Additionally, minor gravel-sized grains constitute less than 5% of the total rock volume. The sample demonstrates moderate sorting, and no reaction was observed upon exposure to dilute hydrochloric acid.

Fig. 3: Physical characteristics of the studied samples.
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a Sīma stone (height: 83 cm), (b) stone pillar (height: 150 cm), and c archaeological artifacts from Ban Ruea in the ancient city of Non Nong Khu (geological hammer shown for scale; length: 38 cm). Archaeological materials from Tham Phra Kho Khat: d Sīma stone with a gilded Buddha image (height: 70 cm), and e sandstone cliff located near the Sīma stone with the gilded Buddha.

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The stone pillar at Ban Ruea in the ancient city of Non Nong Khu, Phu Wiang District, measures 30 cm in width, 140 cm in height, and 20 cm in thickness. It is in a deteriorated condition, located in an agricultural area. The stone pillar is made of sandstone, exhibiting thin, planar and cross lamination (Fig. 3b). The rock sample has a weathered yellow-brown surface and a fresh grey surface, primarily composed of quartz. The sediment grains range from fine sand to medium-grained sand, with less than 2% of gravel present in the rock. The grains exhibit moderate sorting. The rock sample does not react with dilute hydrochloric acid. The archaeological artifacts from Ban Ruea in the ancient city of Non Nong Khu measure between 10 and 19 cm in length and approximately 4-6 cm in width, exhibiting similar characteristics (Fig. 3c). Most of these artifacts are made from sandstone, similar to the rock pillar.

The archaeological site of Tham Phra Kho Khat, located in Nong Phai Subdistrict, Chum Phae District, Khon Kaen Province, lies to the south of Phu Wiang National Park within the Phu Wiang Mountain area. Fragments of an unfinished Sīma stone were found at the site, measuring 25 cm in width, 70 cm in height from the ground, and 5 cm in thickness. The fragment is in a deteriorated condition, found on the surface of the ground in the Phu Wiang Mountain area (Fig. 3d). The rock sample exhibits a weathered brown surface and a fresh pale-grey surface, primarily composed of quartz. The sediment grains range from coarse sand to fine gravel and show moderate sorting, with less than 5% of gravel present in the rock. The rock sample does not react with dilute hydrochloric acid. In addition, the sandstone cave near the Sīma stone exhibits pale-grey, coarse-grained sandstone with small amounts of gravel and tiny iron-oxide veins. The sandstone at this site displays a significant amount of cross bedding in medium- to very thick beds (Fig. 3e), which is characteristic of the Phra Wihan Formation.

Petrographic data

The sandstone found in the areas where Sīma stone and stone pillar at Ban Rua were discovered exhibits a clast supported, with detrital grains ranging from 0.12 to 0.5 mm, corresponding to very fine to medium-coarse sand (Fig. 4a). The grains display sub-angular shapes with low sphericity and exhibit moderate to good sorting. The sandstone primarily comprises quartz and feldspars, with silica and iron acting as the cementing materials. The modal composition consists of 77–80% quartz, 14–18% feldspars, and 5–6% lithic fragments, classifying the rock as sublitharenite (Fig. 5) similar with sandstone from Sop Nam (confluence) and Ban Kud Kaen.

Fig. 4: Representative photomicrographs of sandstone samples.
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a Sandstone from the area of Sīma stone discovery at Ban Ruea in the ancient city of Non Nong Khu; b sandstone from the site of a discovered stone pillar at Ban Ruea; and c sandstone from Tham Phra Kho Khat, located at the summit of Phu Wiang Mountain. Left images: photomicrographs under plane-polarized light (PPL); right images: photomicrographs under cross-polarized light (XPL). Fld feldspars, Qtz quartz, Lf lithic fragments (rock fragments).

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Fig. 5
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Ternary diagram for sandstone classification based on relative proportions of quartz (Q), feldspars (Fs), and lithic fragments (Lt), as determined through petrographic analysis under thin section. Diagram modified from Pettijohn (1974)20.

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The sandstone from the Sop Nam (confluence) and Ban Kud Kaen also exhibits a clast-supported, with detrital grains ranging from 0.12 to 0.33 mm, corresponding to fine to medium-coarse sand (Fig. 4b). The grains are sub-angular to sub-rounded with low sphericity and display moderate sorting. Quartz and feldspars are the primary mineral components, with silica and iron acting as cementing materials. The modal composition consists of 55–65% quartz, 30–32% feldspars, and 3–15% lithic fragments, classifying the rock as litharenite (Fig. 5).

In contrast, the sandstone from Tham Phra Kho Khat exhibits a similar clast-supported, with detrital grain sizes ranging from 0.25 to 0.50 mm, spanning fine sand to very fine gravel (Fig. 4c). The grains are sub-angular to sub-rounded with low sphericity and display moderate sorting. This sandstone also comprises quartz and feldspars, with silica and iron as the primary cementing agents. The modal composition includes more than 90% quartz, with feldspars and lithic fragments constituting less than 10%, classifying the rock as sublitharenite to quartz arenite (Fig. 5).

The petrographic classification based on the proportions of quartz, feldspars, and lithic fragments using the Q-Fs-Lt diagram (Fig. 5) indicated that the sandstone used for Sīma stone and the stone pillar at Ban Ruea was classified as sublitharenite. The sandstone from Sop Nam (confluence) and Ban Kud Kaen was classified as litharenite. However, the sandstone from the Tham Phra Kho Khat archaeological site was classified as quartz arenite and sublitharenite, which is consistent with the sandstones from the Phra Wihan Formation in the Phu Wiang Mountain9.

Geochemical characteristics

Numerous representative samples were analyzed for major oxides and trace elements (Table 2). The Sīma stones, archaeological artifacts, and nearby sandstones exhibited a wide range of silicon dioxide (SiO₂: 61.13–95.35 wt%) and aluminium oxide (Al₂O₃: 3.44–20.82 wt%) concentrations. Maroon sedimentary rocks from study sites 1, 2, 3, 5, 6, and 7 showed ferric oxide (Fe₂O₃) concentrations exceeding 1%, whereas the white sandstones—particularly those from Tham Phra Kho Khat (site 4)—had ferric oxide levels below 1%.

Table 2 Geochemical data of Sīma stones, archaeological artifacts, and nearby sandstones
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The geochemical classification was carried out using the chemical classification diagram for sandstone by Herron10, including the ratio of iron (III) oxide or ferric oxide (Fe2O3) to potassium oxide (K2O) and the ratio of silicon dioxide (SiO2) to aluminium oxide (Al2O3). The sandstones used to make Sīma stones, stone pillars, and the archaeological artifacts from Ban Ruea, as well as the rock samples from Ban Kud Kaen, were classified as Fe-sandstones, Fe-shale/siltstone, shale/siltstone, wacke, litharenite, and arkose (Fig. 6).

Fig. 6: Chemical classification of sandstone samples based on Herron’s diagram10, using log(SiO₂/Al₂O₃) versus log(Fe₂O₃/K₂O) ratios to distinguish between sandstone types10.
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The analysis includes newly collected sandstone samples from Ban Ruea, Wat Pa Don Ku, Tham Phra Kho Khat, and Ban Kud Kaen, along with comparative data from the Phra Wihan Formation reported by Singtuen et al.9.

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The sandstone used for the Sīma stones at Wat Pa Don Khu was classified as Fe-sandstone, litharenite, and wacke. The sandstones from Ban Ruea were classified as Fe-sandstone and litharenite, while the Sima at the Tham Phra Kho Khat were classified as Fe-sandstone, litharenite, and sublitharenite. The sandstone samples from Sop Nam (confluence) were classified as shale/silstone (Fig. 6), while the sandstone from the Phra Wihan Formation was classified as Fe-sandstone and Quartz arenite9.

The geochemical analysis results closely align with the petrographic observations. Well-sorted sandstone samples with a high quartz content exhibit elevated SiO₂ levels and low Al₂O₃ concentrations. In contrast, reddish or maroon sandstones display higher Fe₂O₃ content, attributable to the presence of ferric cement within the rock matrix. Additionally, sandstones containing abundant feldspars and lithic fragments show increased Al₂O₃ levels, as aluminium oxide is a principal component of feldspar minerals. These patterns are particularly evident in the sandstone samples and archaeological artifacts from Ban Ruea.

The mineral structure suggests that rubidium (Rb) can replace potassium (K+) ions in minerals like alkaline feldspars, biotite, muscovite, and others, while strontium (Sr) can substitute calcium (Ca2+) ions in minerals such as plagioclase, calcite, epidote, and related species11. Therefore, the Rb/Sr ratio is an important factor in identifying the composition of various rock types, including sedimentary grains and cementing materials, as well as assessing the degree of alteration. Similar Rb/Sr ratios in rocks may point to shared lithological properties and potentially indicate a comparable depositional age. These trace element analysis of rubidium (Rb) and strontium (Sr) ratios showed a significant correlation between these elements in sandstones from Sīma stones, stone pillars, and archaeological artifacts from Ban Ruea, as well as from Wat Pa Don Khu (Fig. 7).

Fig. 7: Bivariate plot of rubidium (Rb) and strontium (Sr) concentrations in sandstone samples from archaeological sites within Khon Kaen Geopark.
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Group I and II reflect compositional trends of Phu Kradung rocks—Rb-depleted/Sr-enriched and Rb-enriched/Sr-depleted, respectively—while Group III represents the geochemical signature of Phra Wihan rocks.

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These findings suggest that the sandstones used in the archaeological artifacts and Sīma stones at Ban Ruea likely originated from a common local source within the vicinity of the site. Based on trace element composition, they can be classified into two distinct groups: Group I, characterized by rubidium depletion, and Group II, enriched in rubidium. Similarly, the sandstones from Tham Phra Kho Khat and those from the Phra Wihan Formation (Group III) exhibit comparable trace element signatures, indicative of a shared provenance. Both are marked by concurrent rubidium and strontium depletion, consistent with the geochemical profile of quartz-rich sandstones.

Discussion

The Khon Kaen Geopark, located on the Khorat Plateau, exemplifies the intricate interplay between human history, culture, and the natural environment. The region’s distinctive geological composition—particularly its sedimentary rock formations such as sandstone—has played a central role in shaping local cultural practices, notably the crafting and use of Sīma stones for designating sacred Buddhist spaces, possibly during the Dvaravati period5. The area’s geography, characterized by fertile river basins and mountainous terrain, has historically influenced settlement patterns, trade routes, and cultural exchange. The sustainable use of natural resources, including the exploitation of local stone for religious and agricultural structures, illustrates a deep-rooted relationship between humans and their environment. As such, Khon Kaen Geopark serves as a living record of the enduring impact of geology and geography on cultural evolution.

This study presents a comprehensive lithological and geochemical characterization of sandstone samples associated with archaeological sites across the geopark. Lithological analysis indicates that the Sīma stones and stone pillars at Ban Ruea, along with those at Wat Pa Don Ku, Sop Nam (confluence), and Ban Kud Kaen, are composed of maroon-coloured, ferruginous sandstones. These units exhibit medium to thick bedding and medium- to fine-grained textures, with planar and cross-laminated sedimentary structures—features consistent with the Phu Kradung Formation. In contrast, the sandstone at Tham Phra Kho Khat is a coarse-grained, quartz-rich unit with pronounced cross bedding, indicative of the Phra Wihan Formation.

Geochemical data, particularly the major oxide ratios (Fe₂O₃/K₂O and SiO₂/Al₂O₃), support the classification of most sandstone samples—especially those from Ban Ruea, Ban Kud Kaen, and the Phra Wihan Formation—as iron-rich sandstones (Fe-sandstones). In contrast, the Sīma stones at Wat Pa Don Khu are primarily classified as litharenite and wacke. Most sandstone samples from Sop Nam (confluence) are categorized as siltstone, while the sample from the Phra Wihan Formation is identified as quartz arenite. Some samples, including those from Ban Ruea and Tham Phra Kho Khat, display geochemical signatures characteristic of both Fe-sandstone and litharenite, suggesting compositional heterogeneity and the potential mixing of materials from multiple sources.

Trace element analysis, especially Rb/Sr ratios, reveals strong correlations among sandstones from Ban Ruea and Wat Pa Don Ku, as well as between Tham Phra Kho Khat and the Phra Wihan Formation, suggesting a shared geological provenance. Sandstones from Ban Ruea, Ban Kud Kaen, and associated archaeological materials can be divided into two geochemical groups: Group I, defined by rubidium depletion, and Group II, enriched in rubidium. The enrichment and depletion of rubidium are linked to the presence of specific mineral phases such as alkaline feldspars (KAlSi₃O₈), biotite (K(Mg,Fe)₃AlSi₃O₁₀(F,OH)₂), and muscovite (KAl₃Si₃O₁₀(F,OH)₂), supporting their correlation with the lithological and petrographic profiles of the maroon Phu Kradung sandstone. Similarly, samples from Tham Phra Kho Khat and the Phra Wihan Formation exhibit co-depletion of Rb and Sr—a geochemical signature consistent with quartz-rich sandstones deficient in feldspars and mica—further supporting the inference of a shared geological origin.

Archaeological evidence from previous studies—particularly sandstone carvings from Non Nong Khu—highlights two prominent historical cultural phases: the Dvaravati–Lopburi period (14th–18th Buddhist centuries) and the Lan Xang period (22nd–24th Buddhist centuries)6. The earlier phase is characterized by artifacts including earthenware, Khmer ceramics, grinding stones, stone rollers, and elaborately carved sandstone Sīma stones. The later phase introduces high-fired Lan Xang ceramics, Chinese porcelain, and terracotta tobacco pipes6, suggesting shifts in cultural influences and trade networks over time. Fragments of earthenware, grinding stones, stone rollers (i.e. archaeological artifacts from Ban Ruea in Fig. 3c), and various ceramic materials are commonly found throughout the Khon Kaen Geopark area, although they are often incomplete or fragmentary in form.

At Tham Phra Kho Khat, a rock shelter features a seated sandstone Buddha oriented eastward, measuring 23 cm across the chest, 43 cm in base width, and 64 cm in height. The sculpture’s head, reconstructed in cement and coated with gold paint, hints at its restoration or reuse. A previously present sandstone marker, possibly a Sīma stone, was removed, but three fragments were later recovered—one bearing an engraving of a central stupa axis. These attributes suggest the site dates back to the Dvaravati period, approximately between the 12th and 16th centuries CE12.

Sīma stones found across northeastern Thailand, especially in the Dvaravati cultural context, typically consist of large sandstone blocks—either natural or roughly hewn into pillars or slabs—and often feature ornamental carvings. These stones, central to Dvaravati Buddhist ritual architecture, are frequently sourced from local geological formations12. Notably, Phu Phra Bat Historical Park, a UNESCO World Heritage Site in neighbouring Udon Thani Province, represents the most prominent expression of this tradition5,13. While materials used for sacred boundary markers vary by region4, the widespread use of sandstone on the Khorat Plateau is distinctive in Southeast Asia and likely reflects regional adaptations of Buddhist religious practices5.

Following the spread of Buddhism to the region during the 7th century CE, the erection of Sīma stones became a widespread practice across the Khorat Plateau5. In other areas of Thailand, these boundary markers were constructed from regionally available materials. For example, in Nakhon Sawan, central Thailand, Dvaravati-period Sīma stones were made of phyllite and volcanic rocks14, whereas those in Sukhothai were primarily slate and phyllite11. The Khmer temples of the Angkor complex also employed sandstone from the Khorat Group in the construction of Sīma stones, stupas, and sculptures during the Dvaravati era15,16,17,18. The same lithological group was used in the construction of megalithic stone jars in the Lao People’s Democratic Republic19. Although many Sīma stones have since been relocated or repurposed, the preservation of in situ examples within the Khon Kaen Geopark provides invaluable insights into the historical landscape and the socio-religious fabric of the region.

Ultimately, the Khon Kaen Geopark offers valuable insights into the relationship between cultural heritage and the geological landscape. The use of indigenous sandstone for crafting religious artifacts and Sīma stones in antiquity suggests a sophisticated understanding of local natural resources and their cultural applications. Through integrated lithological and geochemical analyses, this study provides important perspectives on historical patterns of resource utilization and cultural expression in the region.

However, there are several limitations to this study that should be acknowledged. First, the geochemical and lithological data presented here are based on a limited number of sandstone samples, which may not fully capture the regional variability of geological formations. While these samples provide insights into the characteristics of sandstone used in specific archaeological sites, they do not necessarily represent the full range of materials that may have been utilized across the entire Khon Kaen Geopark during the period in question. Second, the archaeological context of the Sīma stones, while informative, remains incomplete, as not all sites are well-preserved, and many artifacts are fragmentary. This limits the ability to draw definitive conclusions about the extent and nature of cultural practices across the region. Finally, while this study suggests connections between the local geological formations and the crafting of Sīma stones, it does not provide direct evidence of the socio-religious functions or the precise historical timeline of their use, leaving room for further investigation.

The presence of in situ Sīma stones within their original archaeological contexts offers evidence of the region’s long-standing cultural practices and the historical significance of these materials. These findings highlight the complex interplay between humans and the environment over time, reflecting historical interactions that have shaped the cultural landscape of the Khon Kaen Geopark.