Rock control on evolution of Khorat Cuesta, Khorat UNESCO Geopark, Northeastern Thailand

2.1 General geology of the Khorat Plateau

The northeastern region of Thailand is characterized by elevated terrain ranging from 200 to 1,200 m a.s.l, forming what is known as the Khorat Plateau (Figure 1). This broad, shallow, basin-shaped region is slightly tilted to the southeast and is bounded by the Mekong River to the north and east, the Phetchabun Mountain Range to the west, and the Phanom Dang Raek Escarpment to the south [25]. The plateau spans nearly 200,000 km² across northeastern Thailand and parts of Laos and features a relatively flat surface at elevations of approximately 150–200 m a.s.l. Geologically, the Khorat Plateau is primarily composed of sedimentary rocks, predominantly clastic types, including mudstones, siltstones, sandstones, and conglomerates, as well as terrestrial rock salt formations. These sedimentary units were deposited during the Late Triassic to Cretaceous periods. Underlying these Mesozoic strata are Paleozoic formations, which are locally exposed, while Cenozoic semi-consolidated sediments and recent deposits overlie parts of the sequence [26,27,28,29,30,31]. The sedimentary succession of the Khorat Plateau has been subjected to at least five major regional tectonic events: Indosinian I, II, and III, as well as Himalayan Orogeny I (Paleogene) and Himalayan Orogeny II (Oligocene–Miocene) [32,33]. The regional uplift and folding of the Mesozoic sequences during the Himalayan Orogeny I resulted from the collision between the Indian-Australian and Eurasian tectonic plates. This tectonic episode is linked to the formation of the Phu Phan Range in the central Khorat Plateau, dated between 78 and 60 Ma [34,35]. The structural framework of the plateau is defined by low-angle antiforms and synforms with general NW–SE orientations, accompanied by conjugate parasitic folds trending NE–SW, indicating the principal tectonic stress directions during these deformation phases [36]. These structural features have played a critical role in shaping the modern landscape, including the formation of tablelands, mesas, and cuestas over millions of years. Subsequently, the Himalayan Orogeny II event caused additional uplift along the western margin of the plateau, leading to the development of steep escarpments and significant elevation contrasts that define the present-day topography. As a result of the plateau’s overall inclination to the east, nearly all drainage flows toward the Mekong River [25,37]. The Mesozoic sedimentary sequence of the Khorat Plateau is collectively referred to as the Khorat Group [38,39]. This group is predominantly characterized by reddish-colored clastic sedimentary rocks of terrestrial origin, including sandstones, siltstones, claystones, shales, and conglomerates, with a cumulative thickness exceeding 5,000 m [38,39]. Based on surface investigations, the Khorat Group is subdivided into nine formations arranged in an ascending stratigraphic order: Huai Hin Lat, Nam Phong, Phu Kradung, Phra Wihan, Sao Khua, Phu Phan, Khok Kruat, Maha Sarakham, and Phu Thok Formations [40,41]. In contrast, subsurface and seismic studies have delineated six principal formations: Upper Nam Phong, Phu Kradung, Phra Wihan, Sao Khua, Phu Phan, and Khok Kruat [32,42,43]. The sedimentological and paleontological characteristics of these formations strongly support interpretations of a variety of continental depositional environments [26,31,44,45,46,47,48,49,50,51,52,53,54].

For decades, the Mesozoic rocks in northeastern Thailand have been extensively studied across various disciplines, including lithology, petrology, stratigraphy, paleontology, geochemistry, and paleomagnetism [26,27,28,29,34,55,56]. These investigations have significantly contributed to understanding the geological history of the region, particularly the provenance of sediments, paleoenvironmental conditions, paleoclimate, and the tectonic evolution of the Khorat Plateau. Although several studies have examined the mechanical properties of these rocks, their focus has primarily been limited to engineering applications [57,58,59,60,61,62]. Yet, the geomorphological significance of rock properties on the Khorat Plateau remains largely unrecognized, making this study the first to explicitly investigate their control on landscape development.

2.2 Geology of the Khorat UNESCO geopark

The Khorat UNESCO Geopark is in Nakhon Ratchasima Province in northeastern Thailand. It is widely recognized for its striking geomorphological features and diverse paleontological resources [23,52]. Covering an area of approximately 3,167.38 km², the geopark is geographically situated between 14°40′48″N and 15°08′24″N, and 101°23′46″E and 102°23′53″E. The region is primarily underlain by the Mesozoic red beds of the Khorat Group (Figure 1), with locally occurring subcrops and outcrops of Cenozoic sediments [23,52]. Detailed accounts of the geology and paleontological significance of the Khorat Geopark can be provided elsewhere [23,52]. According to the recent geological mapping by the Department of Mineral Resources [63], as shown in Figure 3, the Phu Kradung, Phra Wihan, Sao Khua, Phu Phan, and Khok Kruat Formations are exposed within the geopark area, whereas the Maha Sarakham and Phu Thok Formations are not observed at the surface. The Mesozoic strata generally dip northeastward at angles of less than 10° (Figure 3). Recent fluvial sediments, particularly along the Lam Takong River and its tributaries, locally overlie the older bedrock units. The western boundary of the Khorat Geopark is defined by a prominent geomorphological feature known as the “Khorat Cuesta” [52,64,65]. This landscape consists of a pair of cuestas separated by a broad strike valley. The elevation of the valley floor is approximately 200 m a.s.l., while the cuesta ridges rise to elevations between 300 and 700 m a.s.l, forming small hills and mountains that dominate the local topography.

Figure 3

Geological map of the study area showing sampling localities and average bedding orientations.

Figures 2 and 3 illustrate that the outer cuesta is formed by the Phra Wihan Formation, the intervening strike valley is developed within the Sao Khua Formation, and the inner cuesta is formed by the Phu Phan Formation. The outer cuesta is underlain by the Phu Kradung Formation, which lies outside the current boundary of the Khorat Geopark. To the east of the inner cuesta, the floodplain area is underlain by the Khok Kruat Formation. A brief stratigraphic overview of these formations, presented in ascending order, is provided below.

The Phu Kradung Formation [38] consists of siltstones, claystones, and micaceous sandstones characterized by Fe-oxide mottling, calcrete nodules, caliche, and nodular silcrete. Its thickness ranges from approximately 800 to 1,200 m. Based on palynomorphs, bivalves, and fossil vertebrates, this unit is dated to the Late Jurassic to Early Cretaceous [26,66,67].

The Phra Wihan Formation is composed almost entirely of yellowish-brown to white, fine- to coarse-grained, well-sorted sandstones of arkosic to orthoquartzitic composition, the Phra Wihan Formation includes only minor siltstones and mudstones. Its thickness varies from 100 to 250 m. The formation is dated to the Early Cretaceous based on fossil and palynomorph evidence [26] and more specifically to the Berriasian age based on sedimentological characteristics [68].

The Sao Khua Formation [38] comprises cyclic deposits of reddish claystones, siltstones, and fine- to medium-grained sandstones, along with conglomerates containing caliche, calcrete nodules, and nodular silcrete. These lithologies represent a low-energy fluvial environment characterized by meandering channels and floodplains, with thicknesses ranging from 200 to 700 m. The formation has yielded the richest and most diverse fossil vertebrate assemblages in Thailand, including hybodont sharks, actinopterygian fish, turtles, crocodilians, and both theropod and sauropod dinosaurs [69,70]. Its age is assigned to the Early Cretaceous (Valanginian to Barremian), based on palynological, invertebrate, dinosaur, and radiometric data [68,70].

The Phu Phan Formation [38] primarily consists of white to pale gray and pale yellowish-brown, medium- to coarse-grained sandstones and conglomeratic sandstones, with occasional siltstones and mudstones. Pebbly conglomerates with well-rounded quartz pebbles and mudstone intraclasts are also present [44]. The formation ranges from 80 to 140 m thick and is assigned an Early Cretaceous age (Hauterivian to Aptian), based on the ages of the bounding formations [26,68].

The Khok Kruat Formation is 400–700 m thick and composed predominantly of reddish-brown to maroon, fine- to medium-grained micaceous or arkosic sandstones. These are interbedded with calcareous conglomerates, siltstones, and mudstones [29]. This formation has yielded vertebrate fossils such as hybodont sharks, turtles, crocodilians, and various dinosaurs including theropods, ceratopsians, and ornithopods [71,72]. The unit is dated to the late Early Cretaceous (Aptian to Albian) based on its fossil content.

3.1 Geomorphological analysis

In this study, geomorphological analysis was conducted using geographic information system (GIS) and remote sensing techniques. Elevation data were derived from the Shuttle Radar Topography Mission (SRTM) Version 3 digital elevation model (DEM) at 30 m resolution, which was used to analyze slope, hillshade, and drainage patterns across the study area. Prior to analysis, the DEM was preprocessed in ArcMap 10.x using the Fill function to remove artificial sinks and raised cells, thereby ensuring hydrological consistency. The corrected DEM was then applied to generate surface coverage measurements, hillshade visualizations, topographic and geological maps, and drainage networks for geomorphological interpretation, which formed the basis of the subsequent discussion.

3.2 Field investigation

Field investigations were conducted in 2019, focusing on natural exposures in the western part of the Khorat Geopark. Eight sampling localities within the Phra Wihan, Sao Khua, and Phu Phan Formations were purposively selected for detailed observation. The selection emphasized sites with geotourism potential, providing additional information to support interpretation and storytelling in tourist field guides, as well as localities previously examined in earlier studies. The surveyed outcrops include both officially designated Khorat Geopark geosites [64] and naturally occurring exposures. Locations of the sampling localities are shown in Table 1 and Figure 3. However, access to some localities was constrained by dense vegetation in sanctuaries and protected areas.

At each locality, primary and secondary structures were examined, and rock types were identified in the field. Representative rock samples were then collected for laboratory analysis. In total, 32 rock samples were collected for this study. Samples were categorized based on their formation and labeled as 19PWxx, 19SKxx, and 19PPxx for the Phra Wihan, Sao Khua, and Phu Phan Formations, respectively. Sample set 19TYxx was also collected from a stratigraphic transition zone between the Sao Khua and Phu Phan Formations. Within this group, samples 19TY01–19TY06 and 19TY10 are assigned to the Sao Khua Formation, while 19TY07–19TY09 are attributed to the Phu Phan Formation. All samples were processed at the Geotechnology Laboratory, Suranaree University of Technology. Polished sections were prepared for petrographic analysis, during which physical characteristics, such as grain size, texture, types of gravel, and cementing materials, were documented.

3.3 Chemical analysis

Mineralogical compositions were determined from powdered samples using X-ray diffraction (XRD) analysis with a BRUKER D2 Phaser. XRD patterns were recorded over 2θ ranges of 5°–70° and 5°–60°, using a step size of 0.02°. The resulting data were analyzed using Bruker DiffracPlus EVA software to identify mineral phases, and relative abundances were estimated based on peak intensities. XRD analyses were conducted at the Synchrotron Light Research Institute, Nakhon Ratchasima.

3.4 Rock property testing

Selected rock samples were also subjected to mechanical testing. Dry density and water content were measured according to ASTM D6473–15 [73]. Rock strength was evaluated using the point load index test (ASTM D5731–16) [74], while durability was assessed using the slake durability index (SDI) test, extended to 10 cycles under room temperature conditions to evaluate long-term disintegration trends [58,75]. Mechanical testing was conducted at the Geomechanics Laboratory of Suranaree University of Technology.

4.1 Topographic and geomorphological analysis

The topographic and geomorphological analysis revealed the asymmetric topography characteristic of the Khorat Cuesta using GIS-based analysis. The elevation map highlights two prominent cuestas as outer and inner cuestas trending NW-SE, and cuesta ridges trending NE-SW along the western margin of the Khorat Plateau, with elevations ranging from approximately 200 to 1,200 m a.s.l. (Figure 4a), while Figure 4b exhibits the escarpment faces and dip slope using visual delineation. The intervening strike valley exhibits relatively lower elevations and a more subdued relief, reflecting preferential erosion of less resistant lithologies. The valley floor lies at 200–330 m a.s.l., while the cuesta ridges rise to 300–700 m a.s.l. These formed prominent hills and ridges that dominate the regional topography. The hillshade background enhanced the visualization of key geomorphic features, including rugged escarpment faces, ruiniform landforms, pedestal rocks, and mesas, within the inner cuesta.

Figure 4

Topographic and geomorphological characteristics of the Khorat Cuesta derived from SRTM DEM analysis. (a) Elevation map showing the outer and inner cuestas and intervening strike valley. (b) Hillshade visualization highlighting escarpment faces and dip slopes. (c) Slope map illustrating the steep escarpments (>25–35°) and gentle dip slopes (<10°). (d) Drainage network analysis showing consequent streams aligned with structural lineaments and dendritic to subdendritic patterns.

Slope analysis emphasized the asymmetry of the cuesta system. Escarpment slopes are consistently steep, with gradients exceeding 25–35°, whereas dip slopes present gentle slope inclinations with less than 10° and aligned with the regional northeast direction, as shown in Figure 4c and d. This sharp contrast highlights the role of lithological resistance and differential erosion in shaping the cuesta morphology.

Drainage analysis confirms the structural and lithological controls on fluvial incision. Drainages on the dip slopes of the outer cuesta are consequent streams, they are deeply incised and aligned with NE–SW-trending lineaments along the plateau’s western margin, whereas the strike valley exhibits dendritic to subdendritic drainage in weaker lithologies. In contrast, cuesta escarpments and dip slopes display subparallel drainage, indicative of bedding-controlled runoff and erosion.

Combined results of topographic and geomorphological analysis, including the elevation, slope, hillshade, and drainage analyses, provide robust geomorphological evidence that the Khorat Cuesta evolved through the combined influence of lithology, tectonic structures, and erosional processes.

4.2 Physical and mechanical results

4.2.1 Lithology

The analyzed rock samples comprise sandstones, conglomeratic sandstones, and siltstones, exhibiting a range of colors, textures, and cementing materials (Table 1; Figures 5–7). Samples from the Phra Wihan Formation (19PWxx) are predominantly fine- to medium-grained sandstones, generally noncemented, and primarily composed of quartz and feldspar, with minor opaque minerals (Figure 5). These samples exhibit compacted fabric textures surrounded by matrix material and can be easily scratched with a geological hammer, indicating relatively low hardness.

Figure 5

Polished slabs of the Phra Wihan samples. Scale bars = 1 cm. (a) 19PW01, (b) 19PW02, (c) 19PW03, (d) 19PW04, (e) 19PW05, (f) 19PW06, (g) 19PW08, (h) 19PW09, (i) 19PW10.

Figure 6

Polished slabs of the Sao Khua samples. Scale bars = 1 cm. (a) 19SK01, (b) 19SK02, (c) 19SK03, (d) 19SK04, (e) 19SK05, (f) 19SK06, (g) 19SK07, (h) 19TY01, (i) 19TY02, (j) 19TY03, (k) 19TY04, (l) 19TY05.

Figure 7

Polished slabs of the Phu Phan samples. Scale bars = 1 cm. (a) 19PP01, (b) 19PP02, (c) 19PP03, (d) 19PP04, (e) 19PP05, (f) 19PP06, (g) 19PP07, (h) 19PP08, (i) 19TY07, (j) 19TY08, (k) 19TY09, (l) 19TY10.

Samples from the Sao Khua Formation (19SKxx; 19TY01–05, 10) consist of fine-grained and conglomeratic sandstones that are cemented by calcareous material (Figure 6). Notably, sample 19TY04 is identified as calcrete, evidenced by visible cementation and a strong effervescence reaction when exposed to diluted hydrochloric acid (Figure 6k).

The Phu Phan Formation samples (19PPxx; 19TY07–09) include fine- to medium-grained sandstones and conglomeratic sandstones. Some samples – specifically 19PP02 and 19PP04 – contain significant calcite cement, as indicated by their physical appearance and mineralogical analysis (Figure 7b and d).

4.2.2 XRD analysis

Mineral compositions and their relative weight percentages were determined using XRD analysis. The results are summarized in Table 2 and illustrated in Figure 8. The primary mineral constituents identified across all samples include quartz, albite, muscovite, kaolinite, illite, chlorite, calcite, and dolomite. Quartz is the dominant mineral phase in all sandstone samples. Quartz content ranges from 66.93 to 89.12% in the Phra Wihan Formation, 57.86 to 93.59% in the Phu Phan Formation, and 42.82 to 67.69% in the Sao Khua Formation. In contrast, calcite content is notably higher in the Sao Khua Formation, ranging from 7.88 to 44.59%, whereas it is relatively low in both the Phra Wihan and Phu Phan samples.

Table 2

Mineral compositions of the studied rocks from XRD analysis (Qtz = Quartz, Abl = Albite, Mus = Muscovite, Kao = Kaolinite, Ill = Illite, Chr = Chlorite, Cal = Calcite, Dol = Dolomite)

Rock sample	Percentage of mineral content
	Initial								After 10 cycles of slake index test
	Qtz	Alb	Mus	Kao	Ill	Chr	Cal	Dol	Qtz	Alb	Mus	Kao	Ill	Chr	Cal	Dol
19PW01	85.12	1.48	3.82	6.28	0.98	2.23	0.00	0.09	86.04	0.93	1.4	6.58	1.87	3.01	0.05	0.13
19PW02	85.41	0.68	2.65	8.59	1.44	1.18	0.00	0.05	83.58	1.39	1.42	8.93	1.79	2.81	0.04	0.04
19PW03	89.12	1.27	3.01	3.89	1.85	0.86	0.00	0.00	86.91	1.02	0.54	4.59	2.43	4.31	0.05	0.15
19PW04	83.46	0.81	5.03	8.40	0.87	1.38	0.03	0.02	65.60	0.75	1.79	22.59	5.42	3.70	0.00	0.15
19PW05	81.79	0.61	1.64	9.74	5.05	1.14	0.03	0.00	74.35	0.43	2.89	12.97	7.31	1.99	0.06	0.01
19PW06	81.87	0.96	2.93	8.10	4.98	1.13	0.04	0.00	70.70	0.28	2.39	16.29	8.00	2.31	0.00	0.04
19PW08	82.44	1.33	3.72	10.04	1.14	1.21	0.06	0.05	66.11	0.62	1.24	23.84	6.00	2.05	0.05	0.09
19PW09	78.34	0.87	4.52	7.75	4.70	3.81	0.02	0.00	77.47	0.44	2.34	9.68	5.40	4.56	0.00	0.11
19PW10	66.93	7.37	5.55	4.50	9.42	5.91	0.17	0.16	68.82	3.69	2.20	6.61	15.41	3.11	0.08	0.09
19SK01	59.47	9.12	1.60	3.14	10.66	6.46	9.34	0.21	56.13	8.23	0.61	0.88	24.21	8.06	1.49	0.39
19SK02	49.28	1.25	4.29	3.04	9.50	1.31	30.57	0.76	51.02	2.17	0.63	1.14	14.47	6.37	23.24	0.97
19SK03	42.82	16.99	1.26	2.37	13.75	6.6	15.72	0.48	56.39	5.86	1.03	1.13	13.03	9.87	11.31	1.39
19SK04	54.58	4.51	3.96	2.88	3.17	2.86	26.87	1.17	61.37	6.82	0.39	1.01	4.64	7.62	17.68	0.47
19SK05	43.21	3.72	0.37	0.22	6.39	1.09	44.59	0.4	40.22	1.77	1.34	0.00	7.66	12.33	35.41	1.26
19SK06	58.84	3.98	0.90	0.81	6.94	2.2	25.55	0.77	63.52	4.28	0.32	0.29	18.09	7.24	1.10	5.15
19PP01	82.29	2.04	0.37	2.57	8.76	2.26	0.51	1.21	80.14	0.90	1.25	2.61	9.28	5.54	0.21	0.08
19PP02	57.86	8.47	2.59	4.81	7.00	2.06	16.75	0.46	62.40	4.17	4.86	2.13	10.96	5.02	10.45	0.01
19PP03	86.75	2.53	1.53	3.76	3.73	0.61	0.43	0.64	79.95	2.02	1.85	2.59	8.38	4.99	0.14	0.07
19PP04	82.48	1.18	0.40	11.48	0.75	0.63	2.76	0.31	74.93	0.81	2.30	3.29	10.54	7.43	0.30	0.40
19PP05	86.86	1.88	0.35	6.67	2.31	0.89	0.70	0.33	80.70	0.97	1.66	2.93	6.64	7.07	0.03	0.02
19PP06	79.27	2.99	0.36	8.82	6.47	1.47	0.52	0.11	61.67	3.62	3.42	5.41	14.58	11.00	0.07	0.25
19PP07	89.54	0.51	0.61	8.10	0.42	0.47	0.29	0.07	76.72	0.82	3.63	3.10	10.08	5.38	0.04	0.22
19PP08	93.59	0.28	0.52	4.25	0.17	0.48	0.54	0.17	75.74	1.32	1.50	3.74	8.49	8.44	0.17	0.6
19TY01	48.52	2.32	1.57	1.84	14.23	4.21	27.32	0.00	58.41	1.13	5.68	14.76	4.68	3.58	11.75	0.00
19TY02	67.69	2.75	3.46	12.19	2.76	3.28	7.88	0.00	38.94	1.34	5.48	1.86	4.00	0.56	47.82	0.00
19TY03	55.78	0.28	1.56	0.42	0.60	0.00	41.32	0.04	31.51	0.41	2.18	0.34	7.45	1.01	57.1	0.00
19TY04	49.39	7.28	3.26	2.24	15.59	4.77	17.48	0.00	25.79	0.32	4.82	0.17	14.95	2.41	51.53	0.00
19TY05	54.83	15.2	3.11	2.02	3.47	3.01	18.31	0.05	39.28	3.21	7.53	1.40	6.85	2.87	38.85	0.01
19TY07	72.59	5.47	2.16	15.9	3.84	0.00	0.04	0.00	57.29	6.36	3.81	5.81	14.18	11.56	0.84	0.15
19TY08	77.74	1.92	2.55	15.49	1.03	0.56	0.53	0.17	66.31	0.85	2.43	4.71	4.01	17.61	4.00	0.08
19TY09	90.49	1.98	0.83	5.40	0.58	0.56	0.09	0.08	73.07	1.02	1.54	4.53	2.59	17.13	0.01	0.12
19TY10	65.85	11.96	1.84	6.43	1.84	1.44	10.64	0.00	48.68	5.22	14.37	1.44	10.94	5.37	13.89	0.09

Figure 8

Percentage of mineral content of the rocks studied from XRD analysis.

4.2.4 SDI test

For each sample, ten rock lumps weighing approximately 50 g each (total weight of 500 g) were subjected to slake durability testing. The standard two-cycle wet-dry procedure was performed to determine the second-cycle SDI (I _d₂). Additional tests were extended from the third to the tenth cycle to observe long-term durability behavior. The results are plotted as a function of the number of cycles in Figure 9.

Figure 9

Slake durability index for 10 cycles of the studied rock samples. (a) Phra Wihan Formation. (b) Sao Khua Formation. (c) Phu Phan Formation. (d) Rock samples from Khao Tum Yae locality.

Samples from the Phra Wihan Formation display a linear degradation trend, with 19PW05, 19PW08, and 19PW10 deteriorating at notably faster rates than the others (Figures 9a and 10a–i). The I _d2 values of all samples exceed 85.00%, which corresponds to medium-high to high durability according to the Slake Durability Classification of Gamble [76]. At the tenth cycle, all samples exceed 60.00%. Samples from the Sao Khua Formation exhibit more consistent and durable behavior across all ten cycles (Figures 9b and 10j–o). Similarly, the calcrete sample 19TY04 shows comparably high durability (Figure 11a–c).

Figure 10

Selected samples of Phra Wihan and Sao Khua Formations before, after 2 cycles, and after 10 cycles of Slake Durability Index Test. 19PWxx for Phra Wihan Formation. 19SKxx, 19TY01-19TY03 for Sao Khua Formation. (a) 19PW05, (b) 19PW05-2 cycles, (c) 19PW05-2 cycles, (d) 19PW08, (e) 19PPW08-2 cycles, (f) 19PPW08-10 cycles, (g) 19PW10, (h) 19PPW10-2 cycles, (i) 19PPW10-10 cycles, (j) 19SK01, (k) 19SK01-2 cycles, (l) 19SK01-10 cycles, (m) 19SK03, (n) 19SK03-2 cycles, (o) 19SK03-10 cycles.

Figure 11

Selected samples of Sao Khua and Phu Phan Formations before, after 2 cycles, and after 10 cycles of Slake Durability Index Test. 19TY04-60, 19TY10 for Sao Khua Formation, 19PPxx for Phu Phan Formation. (a) 19TY04, (b) 19TY04-2 cycles, (c) 19TY04-2 cycles, (d) 19PP01, (e) 19PP01-2 cycles, (f) 19PP01-10 cycles, (g) 19PP04, (h) 19PP04-2 cycles, (i) 19PP04-10 cycles, (j) 19PP08, (k) 19PP08-2 cycles, (l) 19PP08-10 cycles, (m) 19TY07, (n) 19TY07-2 cycles, (o) 19TY07-10 cycles.

Samples from the Phu Phan Formation display a linear degradation trend (Figure 9c and d). The I _d₂ values of all samples, except for three, exceed 85.00%, which corresponds to medium-high to high durability according to Gamble [76]. Exceptions include samples 19PP01 (79.93%) and 19TY07 (84.33%), both classified as medium durability, and sample 19PP04 (34.79%), which falls under the low durability category. Samples 19PP01 (Figure 11d–f), 19PP04 (Figure 11g–i), and 19TY07 (Figure 11m–o) exhibited significantly faster degradation compared to other samples. The relationship between estimated uniaxial compressive strength and slake durability indices (after 2 and 10 cycles) is presented in Figure 12.

Figure 12

Estimated uniaxial compressive strength as a function of slake durability index for 2nd and 10th cycles.

5.1 Rocks of the Phra Wihan, Sao Khua, and Phu Phan Formations

The samples from the Phra Wihan Formation examined in this study were collected from three localities along the outer Khorat Cuesta (Figure 3) and are classified as fine- to medium-grained sandstones. Mineralogically, they are composed primarily of quartz and albite, with accessory kaolinite, muscovite, illite, chlorite, and some opaque minerals. Despite forming part of the high-relief escarpment, these rocks are generally noncemented and exhibit compacted fabrics with a soft matrix, easily scratched with a geological hammer. Previous studies have investigated the lithology and provenance of the Phra Wihan Formation sandstones [55,56,77,78]. Sandstones from Chaiyaphum and Khon Kaen Provinces are similarly composed of quartz and feldspar, along with albite, kaolinite, mica, and opaque minerals, closely resembling the samples from the Khorat Geopark. Field exposures often display various types of cross-bedding, interpreted as products of high-energy, shallow braided channels with subordinate meandering systems [44]. These sandstones contribute to the formation of rugged, erosion-resistant landforms, including runiform landforms and pedestal rocks at notable geosites. Sandstones at the Hin Chang Si geosite can be classified into silica-cemented and noncemented types, which directly influence their degrees of weathering [78].

Exposures of the Sao Khua Formation are limited in the study area, as the valley between the outer and inner cuestas is predominantly covered by soil and fluvial deposits. The samples analyzed were collected from southeastern localities (Figure 3), including sites that had been previously examined [79]. These rocks are classified as fine-grained and conglomeratic sandstones, primarily composed of quartz, albite, kaolinite, illite, muscovite, and chlorite, with significant calcite content. Notably, sample 19TY04 is identified as calcrete based on its dense texture and vigorous reaction with diluted HCl. The previous study [44] reported that the Sao Khua Formation is mainly composed of fluvial sandstones and floodplain mudstones, with occasional sand-matrix-supported caliche conglomerates. However, lithological variations across the formation have been documented in different locations [78]. Results of Daorerk [79] who investigated 14 sections of the Sao Khua Formation on the Khorat Plateau were interpreted as lacustrine and fluvial depositional environments. They found the formation to consist mainly of calcareous sandstones, siltstones, claystones, and calcrete horizons, with minor conglomerates and freshwater limestones. The samples in this study correspond well to the conglomerate–sandstone–calcrete horizon lithofacies described by Daorerk [79].

The Phu Phan Formation samples were collected from the inner cuesta localities within the Khorat Geopark (Figure 3) and are classified as fine- to medium-grained sandstones and conglomeratic sandstones. They are primarily composed of quartz, albite, kaolinite, and muscovite. Samples 19PP02, 19PP04, and 19TY10 contain a notably high percentage of calcite. According to the previous study [44], the Phu Phan Formation consists mainly of quartz arenites with grain sizes ranging from fine to coarse sand, deposited in high-energy braided river channels. It also includes floodplain/lacustrine siltstones and mudstones and locally developed pebbly conglomerates. The Phu Phan sandstones in this study can be differentiated from those of the Phra Wihan Formation by their higher angularity and a greater abundance of pebble-sized clasts.

5.2 Relationships between rock strength and physical properties

The dry density of the studied rock samples is positively correlated with their point load strength indices, as illustrated in Figure 13a. The estimated UCS, derived from I _s(50) values, allows the classification of the rocks according to hardness grades (Figure 13b) following the International Society for Rock Mechanics [80]. Samples with densities below 2.5 g/cm³ fall within the range of weak rocks (R2) to strong rocks (R4), whereas samples with higher densities range from medium-strong rocks (R3) to very strong rocks (R5). All R5-classified rocks are restricted to the Sao Khua Formation (samples 19SK01–05 and 19TY10) and are characterized by very fine-grained sediments with few or no pebble inclusions. In contrast, R2-classified rocks include samples 19PW05, 19PW08, 19PW10, 19PP01, 19PP03, 19PP04, 19PP07, 19PP08, and 19TY07. These samples exhibit coarser textures and abundant translucent quartz grains (Figures 5 and 7). Overall, the very fine-grained samples tend to show higher density and greater strength, reflecting the influence of grain size on compaction and cementation. A bivariate analysis of petrophysical properties reveals a clear linear relationship between porosity and bulk density.

Figure 13

Point load strength index (a) and estimated uniaxial compressive strength (b) as a function of bulk density.

As shown in Figure 14, samples with higher porosity correspond to lower bulk densities. The correlation coefficient (R ² = 0.7386) indicates a strong negative relationship, which is expected for siliciclastic rocks with similar matrix densities. These findings align with previous studies on the mechanical behavior of sandstones: the lower porosity and increased grain packing enhance UCS [81]; UCS and Young’s modulus in sandstone decrease with increasing porosity, water content, and mean grain size [82]. It is suggested by Pappalardo et al. [83] that sandstone strength is influenced by compactness and lithic fragment content, while porosity and pseudo-matrix components contribute to mechanical weakening. Recent advances using micro-computed tomographic imaging further confirmed that pore structure and the texture coefficient are strongly correlated with both Young’s modulus and compressive strength [84].

Figure 14

Point load strength index and estimated uniaxial compressive strength as a function of bulk density.

5.3 Relationships between rock strength and chemical composition

The relationship between rock strength and mineral composition – specifically the contents of quartz, calcite, mica, and clay minerals – is illustrated in Figure 15. Although all studied rock samples contain more than 40% quartz, several Phra Wihan and Phu Phan samples with high quartz content exhibit relatively low strength (Figure 15a), suggesting that quartz alone is not a sufficient predictor of mechanical performance. In contrast, calcite content appears to have a more direct influence on rock strength (Figure 15b). All R5-classified rocks contain more than 10% calcite, whereas R2-classified rocks contain less than 3%. The Sao Khua Formation samples, which are classified predominantly as R4 and R5, are characterized by a higher proportion of calcite, reinforcing the role of calcareous cement in enhancing rock strength. No clear trends were observed between rock strength and the content of mica or clay minerals (kaolinite, illite, chlorite), as shown in Figure 15c. Previous studies have highlighted the influence of chemical composition on the mechanical behavior of sandstones. Melo et al. [85] observed that the dissolution of kaolinite cement in medium- to coarse-grained sandstones was a key driver of weathering processes, leading to the development of surface erosional features such as furrows, ponds, and depressions. Lin et al. [86] found that chemical corrosion significantly altered sandstone properties by increasing total porosity and the proportion of macropores, thereby weakening the rock. Their experiments revealed that corrosion was most severe in diluted H₂SO₄, followed by NaOH and distilled water. Similarly, Chen et al. [87] demonstrated that variations in solution pH affected sandstone strength, peak strain, and elastic modulus – first increasing, then decreasing – when exposed to Na₂SO₄ solutions. In the context of Thailand, Phakdimek et al. [90] showed that the compressive and tensile strengths of sandstones from the Phra Wihan, Sao Khua, and Phu Phan Formations deteriorated after prolonged chemical weathering. After 300 wetting cycles using acidic water (pH = 5), both rock density and strength declined, with Sao Khua sandstones showing the highest sensitivity – losing 30–50% of their original mechanical performance. The findings of this study support those results, indicating that the initial high strength of the Sao Khua samples is likely attributable to increased calcite content, which enhances cohesion but is also vulnerable to chemical dissolution.

Figure 15

Estimated uniaxial compressive strength as a function of chemical composition: (a) compared with silica content; (b) compared with calcite content; and (c) compared with mica and clay minerals content.

5.4 Relationships between rock durability and physiochemical properties

The durability of the studied rock samples is closely associated with their physical and chemical characteristics, like their strength behavior. Results from the SDI test indicate that samples from the Sao Khua Formation – very fine-grained to fine-grained sandstones with calcite cement, including the calcrete sample 19TY04 – degraded slowly over multiple test cycles. In contrast, samples from the Phra Wihan and Phu Phan Formations, which have coarser textures, higher quartz contents, and little to no calcite cement, exhibited significantly greater weight loss during the durability tests. These findings suggest that grain size, bulk density, and the presence of calcite cement are key factors influencing the long-term durability of the tested sandstones. Fine-grained and calcite-cemented rocks tend to resist slaking and mechanical breakdown more effectively than coarser, poorly cemented equivalents. In this study, we applied the extended 10-cycle SDI test following the methodology presented by Ward and Bunnag [38] to simulate prolonged mechanical weathering. Among the samples, 19PP01 and 19PP04 exhibited a distinct nonuniform degradation pattern, where an initially weak outer layer rapidly disintegrated, followed by a slower rate of weight loss. This pattern is reflected in their concave SDI curves, indicating layered structural weakening. Several previous studies have explored the durability of Khorat Group sandstones in relation to their physiochemical properties. Torsangtham et al. [60] conducted 100-cycle slake durability tests under dry, wet, and acidic conditions on samples from the Phra Wihan, Sao Khua, and Phu Phan Formations. Their results demonstrated that all sandstones were sensitive to water, especially to acidic environments, showing rapid degradation under such conditions. They emphasized the role of cementing materials, particularly kaolinite, as a key factor affecting long-term durability. Patanapongsonti et al. [62] simulated erosional processes using 80-cycle SDI testing and found that Phu Phan sandstones were highly water sensitive, exhibiting increased porosity and reduced density under both wet and dry cycles. Conversely, Phra Wihan sandstones showed less water sensitivity but degraded more quickly under dry conditions, indicating a contrasting weathering mechanism. Overall, the results of this study confirm that Khorat Group sandstones exhibit significant variations in texture and composition and are susceptible to both mechanical and chemical weathering. The Phra Wihan and Phu Phan Formations are more influenced by mechanical disintegration, while the Sao Khua and parts of the Phu Phan Formations are more vulnerable to chemical weathering, especially in the presence of acidic moisture.

5.5 Discussion on rock control in the Khorat Cuesta evolution

The Khorat Cuesta represents a recent geomorphic expression of Mesozoic strata within the Khorat Plateau, shaped over an extended period by both tectonic and surface processes. The formation of the Khorat Plateau is primarily attributed to tectonic movements associated with the Himalayan orogeny [11,34,35]. Charusiri et al. [88] outlined four tectonic phases: (1) initial deformation of sedimentary rocks within the Khorat Basin (∼70 ± 12 Ma), forming the primary cuesta landscape; (2) major uplift and erosion (∼50 ± 5 Ma), elevating the plateau and enhancing cuesta development; and (3)–(4) subsequent strike-slip and vertical-slip faulting events (∼34 ± 3 Ma and ∼21 ± 2 Ma). Uplift and erosion during the Tertiary progressed at estimated rates between 0.12 and 0.004 mm/year. The presence of wind and water gaps along the western and southern margins of the Khorat Plateau provides evidence of a superimposed paleo-drainage system sourced from highlands to the west. Surface processes, including fluvial erosion and mass wasting, have further shaped the region through the Cenozoic [37].

Our study demonstrates that the physical, chemical, and mechanical properties of sandstones significantly influence their strength and durability, and thereby their geomorphic role in cuesta evolution. The rock samples from the Phra Wihan, Sao Khua, and Phu Phan Formations show natural variability in texture, grain size, cementation, and mineral composition. Field observations reveal that Phra Wihan sandstones are typically massive, compacted, and noncemented, except for iron oxide-stained zones. The Sao Khua Formation, generally medium to thick-bedded, comprises conglomerates, sandstones, siltstones, and mudstones with calcareous cement and localized calcrete horizons. Mudstones are exposed locally with average thicknesses less than 5 m [79, this study]. Phu Phan Formation rocks are typically coarse grained and range from medium- to very thick-bedded sandstone and conglomeratic sandstone.

Contrary to common assumptions, quartz-rich sandstones are not always strong or durable. Phra Wihan and Phu Phan samples range in strength from R2 to R4 (Figure 13b), and several degrade rapidly during slaking (Figures 9–11). In contrast, the Sao Khua samples, although finer-grained, exhibit both higher strength and durability, which we attribute to their higher density and calcite content. These results highlight the significance of grain size and cement composition, particularly calcite, in influencing mechanical behavior.

Cenozoic tectonic activities initiated the formation of the cuesta landscape and were later shaped by weathering and erosion.

Topographic analysis (Figure 16) shows that drainages on the dip slopes of the outer cuesta are deeply incised and aligned with NE–SW–trending lineaments along the plateau’s western margin [36]. However, the escarpment itself remains steep and relatively intact, with limited headward erosion. Our results suggest that mechanical weathering processes, such as hydraulic action and abrasion, are the dominant factors in the Phra Wihan sandstones. Geomorphic features at Wat Khao Chan Ngam, a renowned tourist geosite (location A in Figure 16), including sandstone caves, jointed blocks, and mushroom rocks (Figure 17), reflect the influence of water-driven sculpting. At Wat Khao Saad (location B in Figure 16), sandstone gulls, buttresses, and lateral potholes along the contact between the Phra Wihan and Sao Khua Formations suggest ancient strike-slip stream action, which is illustrated in Figure 18a and b.

Figure 16

Topographic map of the study area with localities of geomorphic features.

Figure 17

Geomorphic features at Wad Khao Chan Ngam locality. (a) Jointed sandstone block; (b) sandstone cave; and (c) mushroom rock.

Figure 18

Geomorphic features of the Phra Wihan Formation. (a) and (b) sandstone walls with lateral potholes at Wad Khao Saad locality and (c) rockfall along Highway No.2 (October 2020).

In contrast, landslides or mass wasting are notably absent in the Khorat Cuesta, despite its prominent steep escarpments and dip slopes. While minor rockfalls have been observed along Highway No. 2 (locality C in Figures 16 and 18c), mainly at the lithologic contact between the Phra Wihan and Phu Kradung Formations. No large-scale slope failures or landslide scars were detected during the wet seasons between 2017 and 2024 (Figure 19). This finding is supported by landslide detection models employing the differential bare soil index (dBSI) in combination with slope filtering [89,90]. The model defines slope failure zones as areas with gradients exceeding 10 degrees and deposition zones as areas with slopes below 10 degrees [90]. The dBSI is calculated by subtracting pre-event bare soil index (BSI) values by post-event values, capturing the spectral response associated with changes in bare soil cover. Areas affected by landslide scars or surface exposure are characterized by negative dBSI values, typically visualized in red polygons (Figure 19). Based on these results, no landslide activity was detected on the escarpment or adjacent steep slopes. However, in the event of prolonged heavy rainfall 3–4 days and exceeding 400 mm, landslides may be potentially initiated in strongly weathered sandstone. Such failures are often associated with well-developed joints, dense cracking, fractures, and separation planes that intersect slope faces or mountain flanks [91,92]. The minor rockfalls observed are likely attributable to contrasts in hydraulic conductivity between the two formations, particularly at location C in Figure 16, and as illustrated in Figure 18c.

Figure 19

Spatial and temporal distribution of landslide scars on Khorat cuesta from 2017 to 2024.

The strike valley, between 250–330 m a.s.l., lies within the Sao Khua Formation, which is typically regarded as a weak and easily erodible unit. However, our data show that Sao Khua sandstones have higher compressive strength and slake durability than those of the Phra Wihan and Phu Phan Formations. Despite this, chemical weathering – especially dissolution of calcite – plays a significant role in weakening the formation over time, facilitating sediment transport by fluvial processes. Streams within the Sao Khua Formation drain northeast and southeast, eventually merging with the Lam Takong and Lam Phra Pleang Rivers (Figure 16).

The inner cuesta, formed from the Phu Phan Formation, is characterized by dissected hills and mesa-like landforms (Figure 16), generally lower and narrower than the outer cuesta, with less-developed dip streams. The higher rate of weathering and erosion in this unit is attributed to the physical heterogeneity of the conglomeratic sandstones and pebbly conglomerates, which were deposited in high-energy fluvial environments. Poorly sorted gravels may be dislodged, acting as abrasive agents that intensify mechanical breakdown. Several geosites within the Khorat UNESCO Geopark preserve potholes within the Phu Phan Formation, including the Si Khio Stone Quarry – a Khmer-era site at location D (Figure 16), as illustrated in Figure 20a. These 2–3-meter-wide potholes are intersected by NE–SW joints aligned with Cenozoic regional lineaments [36], indicating that they likely formed in paleochannels prior to the plateau’s deformation. Moreover, hilltop potholes recently discovered at ∼350 m a.s.l. at Pha Sam Sip Sang (location E in Figure 16; Figure 20b) are interpreted as indicators of ongoing mechanical weathering, contributing to the progressive degradation of the Phu Phan Formation in exposed areas [93].

Figure 20

Potholes in the Phu Phan formation. (a) Fluvial pothole of the ancient stream at Si Khui stone quarry. (b) Hilltop potholes on the Pha Sam Sip Sang hill.

The evolution of the Khorat Cuesta appears to differ markedly from comparable landforms elsewhere. For instance, Ward [7] attributed the development of strike valleys in the Coal Cliff Cuesta, Utah, USA, to incision rates and tectonic uplift – a model unsuitable for northeastern Thailand, where tectonic activity has long been dormant. Similarly, the framework proposed by Abe et al. [94], which emphasizes landslides and translational failures, is not applicable, as such processes are absent in the Khorat Cuesta. Instead, our findings support the view that cuesta development is primarily governed by different weathering processes acting on sedimentary strata with contrasting properties. In the Khorat Cuesta, mechanical weathering dominates the Phra Wihan and Phu Phan Formations, while chemical weathering – particularly acid dissolution of calcite – affects the Sao Khua Formation. This might be influenced by Cenozoic volcanic activity in nearby regions, which may have led to acidic atmospheric conditions. Volcanic events of regional significance, including 20–11 Ma eruptions in Lam Narai–Chaibadan (central Thailand); 11.03–8.82 Ma events in Wichian Buri; 3.28–0.92 Ma events along the southern Khorat Plateau margin are recorded [95]. These events may have generated acidic rainfall, accelerated chemical weathering processes, and enhanced the development of the parallel valley system associated with the Sao Khua Formation.