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Article

Microbial Carbonates of Upper Triassic Doi Long Formation, Lampang Group: A Study of New Outcrop Localities in Northern Lampang, Central North Thailand

by
Kritsada Moonpa
1,*,
Mongkol Udchachon
2,
Jirattikarn Jainanta
1 and
Sathit Kanthata
1
1
Department of Geological Sciences, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
2
Centre of Excellence in Evolution of Life, Basin Studies and Applied Palaeontology, Palaeontological Research and Education Centre, Mahasarakham University, Maha Sarakham 44150, Thailand
*
Author to whom correspondence should be addressed.
Diversity 2025, 17(4), 299; https://doi.org/10.3390/d17040299
Submission received: 13 March 2025 / Revised: 16 April 2025 / Accepted: 17 April 2025 / Published: 21 April 2025
(This article belongs to the Section Microbial Diversity and Culture Collections)
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Abstract

:
The Doi Long Formation is the youngest record of Triassic deposits of the Lampang Group, Sukhothai Zone, central northern Thailand. The evolutionary history behind the presence of microbial limestone remains unknown, despite its high diversity and abundance in reef environments. This study documents new observations on the microbial carbonates of the Doi Long Formation in the northern Lampang Province. Eight new outcrop localities have been discovered and are analyzed in terms of sedimentological details. Microfacies description and fossil identification allow us to interpret the specific depositional environments of the bioconstructors of these mound-like build-ups. In the study localities, the Doi Long Formation consists of predominantly microbial carbonates, containing microproblematic organisms, microbialites (stromatolites), algae, and sponges. Five main microfacies are distinguished and several microproblematic organisms have been identified. The study localities yield microencrusters and microproblematica fossils, including Girvanella-like microstructures, Cladogirvanella, Garwoodia, Cayeuxia, Tubiphytes-like microorganisms, and shell fragments. Depositional environments corresponding to reef (mound-like microbial/microbialite build-ups) settings and lagoon environments have been proposed. Understanding the diversity within the Doi Long Formation is essential for clarifying the fossil assemblage and biological processes in reef ecosystems during the Late Triassic.

1. Introduction

The Triassic sequence in the Sukhothai Zone (or, in part, the Shan–Thai Terrane) extends from southern China to northwestern Lao, passing through north Thailand [1,2,3,4,5] (Figure 1a). The sequence has been studied comprehensively in southern China, northwestern Lao, and to a lesser extent in northern Thailand (Figure 1b). The wider area within the Sukhothai Zone in northern Thailand exposes marine intra-arc sediments of the Triassic Lampang Group and the Permian Ngao Group, comprising mainly sandstone, mudstone, and limestone [6,7,8]. The marine Triassic sedimentary rocks, known as the Lampang Group, consist primarily of carbonate rocks of the Pha Kan and Doi Long Formations (Figure 1c). There are numerous open issues regarding the timing of their deposition and the paleontological materials in which carbonate sediments were formed, as well as the paleoecology and environment (e.g., [5,9,10,11]). However, within the Sukhothai Zone, there is little documentation regarding the Upper Triassic rocks or fossils of the Doi Long Formation in the Lampang area. In addition, only a few studies have been reported [5,6,9,10]. Here we report the initial mapping, stratigraphy, and preliminary report on the microfossil assemblages of the Upper Triassic microbial carbonates from the Lampang area (Figure 2). This provides a valuable opportunity to enhance our understanding of carbonate sedimentation and integrate the data gathered over the past decade. Specifically, the study of microproblematic organisms provides new insights into our interpretation of the depositional environment. It offers a unique opportunity to further our understanding of carbonate sedimentation and combine the data accumulated over the last decade. This study presents the discovery of Upper Triassic microbial limestones in a newly examined section of the Doi Long Formation. Found in the Lampang region of central-northern Thailand (Figure 1a), these limestones exhibit generally good preservation. The paper further explores the microfacies characteristics of these limestones. This study provides the first comprehensive description and analysis of the microproblematic organisms and other notable fossils from the Doi Long Formation in this field. Their characteristics are examined in detail, particularly within the context of microfacies analysis.

2. Materials and Methods

2.1. Geological Background

The geotectonic framework of northern Thailand can be categorized into four zones, arranged from west to east: the Sibumasu Block, the Inthanon Zone, the Sukhothai Zone, and the Indochina Block [4,8,12,13,14,15] (Figure 1a). The Sukhothai Zone represents an island-arc system, consisting of Paleozoic high-grade metamorphic rocks as basement formations, Permian sedimentary rocks of the Ngao Group, Permo-Triassic volcanic rocks, and marine Triassic sedimentary rocks of the Lampang Group located in central north Thailand, which are unconformably overlain by a non-marine Mesozoic sedimentary sequence [1,3,6,13,16] (Figure 1b). The lithologies of the siliciclastic and volcanic rocks suggest a subduction-related tectonic environment, with the zone being regarded as a Permian-Triassic magmatic arc that developed along the western edge of the Indochina Block [15,17].
The Triassic strata of the Sukhothai Zone are conventionally referred to as the Lampang Group. This group is characterized by siliciclastic sedimentary rocks with carbonate rocks divided into five formations: the Phra That, Pha Kan, Hong Hoi, Doi Long, and Pha Daeng Formations, from bottom to top [6] (Figure 1c). Notably, the carbonate rocks of the Lampang Group, namely, the Pha Kan and Doi Long Formations, are prominently exposed in the central and northern parts of the Lampang Province (Figure 2). The studied limestones of the Doi Long Formation developed over several kilometers to the northeast and east of Lampang Province, central north Thailand (Figure 2). In terms of the geological maps of the study area, the formation is depicted on a 1:250,000 scale geological map of the Lampang region (see [7]). This study primarily focuses on the Upper Triassic limestone of the Doi Long Formation [18], consisting of fine-grained grey to light grey limestone. In the local area, peloids, algae, and stromatolites occur [5,10]. Additionally, some oncoids are present but are less prominent than in the Pha Kan Formation [6]. Yellowish-brown bands are commonly seen, with red limestone typically appearing at the top of the formation. From a paleontological point of view, the Doi Long Formation comprises bivalves (Trigonodus), serpulid worms, brachiopods, algae, ammonoids, corals, and crinoids [19]. Furthermore, the study of ammonoid fossils indicates a Late Triassic age for the formation [20]. Additionally, the foraminiferal assemblages are most likely attributable to the Carnian (early Late Triassic) period [10]. Consequently, this limestone offers a remarkable opportunity to gather new insights into the fossil assemblage within the Late Triassic period.

2.2. Samples and Methodology

The newly mapped section along the Doi Huai Long and Doi Huai Nam Rin areas (more commonly known as the Doi Long in English language sources) consists mainly of a synform-oriented NNE–SSW (Figure 2) formation that follows the regional structural trend. The sequence consists of massive limestone units interbedded with thin-bedded limestone units, shale, and mudstone near the contacts with overlying shales and the lithic arenites and some limestones of the Hong Hoi Formation (Figure 3). Detailed lithostratigraphic columns of the studied sections are shown in Figure 3. The lithological details, fossil contents, and sedimentary structures are observed and investigated for distinct facies, mainly focusing on the more well-preserved localities. The carbonates of the Doi Long Formation crop out as steep cliffs, topographic highs, and karst topography with dense vegetation (Figure 4a). This limestone forms a moderately stratified structure with a total of several hundred-meter-thick, showing an observable stratigraphic relationship with the underlying or overlying unit in the field. In Section 1 and Section 2, the main components consist of fine-grained limestone calcareous algae and peloids with skeletal grains, such as bivalves, foraminifera, and gastropods. In addition, the limestone beds contain dark grey to scattered yellowish-brown-colored material (Figure 4b). Macrofossils are scarce but can be abundant locally in the middle to upper parts of the formation. However, the limestones are mostly thick-bedded to massive, with no bedding observed in Section 3 and Section 4 of the sequence (Figure 4c). Algal debris, stromatolites, and microbes are common to abundant in this limestone (Figure 4d–f). In Section 5 of the sequence, the limestone texture consists of algae, microbes, oncoids, and skeletal grains, with minor deposits of lime mudstone (Figure 4g,h). In all sections, the lower part of the sequence conformably overlays the shales and calcareous sandstones of the Hong Hoi Formation. However, the top of the Doi Long Formation is unconformably overlain by the limestone conglomerate of the lowermost part of the Pha Daeng Formation (Figure 3).
Samples from this limestone were utilized for micropaleontological analysis. According to our observations, the new micropaleontological content (chiefly comprising microproblematic organisms) and the stromatolite findings presented in the frame of this study are from localities in a wider area that is exposed in the Ban Tha Si and Ban Sob Chang areas of northern Lampang Province (Figure 2). Until now, all the reported biota of the studied carbonates have not been specifically documented. More than thirty samples were collected in the northeastern area of the Lampang Province (see the location in Figure 2). Slabbing samples and thin sections were obtained for petrographic work and processed with a microscope for fossil identification in the Department of Geological Sciences, Chiang Mai University, Chiang Mai, Thailand. In total, 40 thin sections from the rock samples (DL-01–08) were prepared for microfacies and microfossil examinations.

3. Results

3.1. Microfacies

The five main microfacies were established based on grain types, composition, and significant features [21,22,23,24]. Facies descriptions and their interpretation are summarized in Table 1; all microfacies are of shallow-water limestone types.

3.1.1. Bioclastic Wackestone (F1)

Bioclastc wackestone is mostly composed of bivalves, microproblematica such as Tubiphytes sp., and cyanobacteria (Figure 5a). Bivalve fragments range in size from 1 to 2 mm and represent 15–20% of the rock volume. Microproblematica and microencrusters (cyanobacteria) are large and angular, floating on the micritic matrix. Subordinate components include foraminifera, gastropods, aulotortid foraminifera, rare ostracods, and unidentified sparite-filled bioclasts. The matrix is of homogenous micrite.

3.1.2. Peloidal-Bioclastic Packstone (F2)

The primary components include peloids, aggregate grains, algal debris, and coated grains. Peloids and algal remains (ranging from 30% to a maximum of 40%) are subangular to well-rounded, poorly sorted, and randomly oriented (Figure 5b). Aggregate grains (sized 0.2–1.0 mm) represent 10–20% of the rock volume, consisting of peloids, oncoids, algal debris, and bioclast grains (Figure 5c). They are 0.2–1.0 mm in size. Subordinate components include microproblematica such as Tubiphytes sp., green algae (dasycladecean), bivalves, smaller foraminifera, and gastropods. The cement is of coarse sparry calcite, which is rarely micritic. Drusy calcite is observed as cement, sometimes displaying as thin, isopachous fibrous cement around particles. Major skeletal grains exhibit a thin to large micritic rim, especially echinoderm spines and fragments.

3.1.3. Cyanobacterial Packstone (F3)

Cyanobacterial packstone is composed of abundant cyanobacterial crusts and fragments (15–25%), along with peloids (20%) (Figure 5d). The most abundant organisms are clearly Tubiphytes sp., Tubiphytes-like organisms, Bacinella sp., and Microtubus sp., some of which retain their internal structures. Subordinate components include peloids, micritic particles, the debris of molluscan shells, rare gastropods, and a few benthic foraminifera such as Aulotortus sinuosus sp., Agathammina austroalpina sp., and Endotriadella wirzi sp. (see Section 3.2). The matrix consists of sparite, with some areas containing the remnants of micrite. Drusy mosaic cement is also present.

3.1.4. Microbial Packstone-Grainstone (F4)

Microbial packstone-grainstone is mainly composed of large fragments of cyanobacteria, encompassing microproblematica like the porostromate Garwoodia and the tubular forms of Cayeuxia, Cladogirvanella, and Microtubus. They range from 0.5 to 1.5 cm in size, forming millimeter- to centimeter-sized bushy masses, radial fans, or tangled masses, typically comprising small tubes with irregularly shaped bodies. The classification of microproblematic organisms is based on growth forms and filament patterns, with Cayeuxia being characterized by branching clusters of tubes (Figure 5e), while Garwoodia typically displays coarse, thin-walled, and radiating clusters. Cladogirvanella presents thin-walled tubed, tangled, aligned, or intermingled morphologies (Figure 5f). Subordinate components include peloids, algal fragments, aggregate grains, echinoderm fragments, and rare benthic foraminifera. Drusy sparite cement is abundant, along with large isopachous fibrous cement around the components.

3.1.5. Stromatolitic Algal Boundstone (F5)

Stromatolitic algal boundstone presents boundstone facies featuring algal-microbial crusts, microbial laminated stromatolites, or spongiostromata crusts (Figure 6a–c). The laminations consist of alternating layers of micrite and coarse sparry calcite cement (Figure 6b). The crusts range from a few millimeters to a few centimeters in thickness (Figure 6c). Light grey laminae are generally thicker than the darker layers, but some short, discrete dark laminae could occur in grey laminae (Figure 6d,e), thereby exhibiting greater variability of thickness. Subordinate components include debris of algae (dasycladacean green algae; see in Section 3.2), Tubiphytes, microproblematica such as Microtubus communis, fragments of bivalve shells, and smaller benthic foraminifera (Figure 6f). Skeletal debris, overall, is poorly sorted and set in a micritic or sparitic matrix. The benthic foraminifera assemblage includes Diplotremina sp., Endoteba sp., Endotriada spp., and Endoteba bithynica (see Section 3.2). Defined stromatolites with irregular top surfaces are overlain by unlaminated biomicrite. The top surface of the stromatolite is normally sharply defined but instead passes into non-laminated micrite without a break (Figure 6g). Blocky cement grew upon the crusts and sparry mosaic cement fills the residual voids.

3.2. Stromatolites, Microproblematic Organisms, and Other Significant Fossils

On the outcrop, the stromatolites show typical columnar to dome-shaped structures, contrasting with the surrounding rocks (Figure 4d,e and Figure 6). The micritic or bioclastic limestone beneath them may have acted as a stable substrate for the microbial communities responsible for forming the stromatolites. In the plan view, the stromatolites display subrounded to rounded shapes, featuring concentrically stacked, high-relief domes (Figure 7a). The domes typically range in diameter from 3 to 5 cm, with a few reaching up to 12 cm. Most of the domes are interconnected, with some encrusting one another. The height of the stromatolite columns is up to 20 cm, exhibiting a columnar or laminating macro-structure. These characteristic laminated patterns were observed both in the field and on the polished slabs (Figure 4d,e and Figure 7).
Microfacies observation reveals a diverse variety of skeletal microbial organisms and stromatolites, as well as non-skeletal microbialites that occur in the form of encrustations on other bioclasts, such as microbial lumps, micritic masses/patches, and peloids. Under a polarizing microscope, columnar stromatolites exhibit undulating laminations (Figure 7a,b). These consist of alternating dark and light grey micritic layers (Figure 7c). The dark layers are between 100 and 200 μm thick, with individual layers showing inconsistent thicknesses. Many of the laminae examined contain extensive fenestral fabrics (Figure 7d). The fenestrae are filled with calcite spar crystals, creating irregular mosaics, and are primarily found in thick, light grey micritic layers. They may be aligned vertically or parallel to the laminae, with no clear preference for orientation within the layers. Some microproblematic organisms occur preferentially in thick (of several micrometers) microbial encrustations on biogenic constituents like shells (Figure 7e,f).
Calcified cyanobacteria form bushy masses, radial fans, or tangled clusters ranging from millimeters to centimeters in size. These structures are often made up of tiny “tubes” (resulting from calcification within organic sheaths) or irregularly shaped bodies. Most of these cyanobacterial possess microfossils consisting of tubes organized in erect radial clusters of branched tubes (Figure 8). Cayeuxia is characterized by small nodules exhibiting a hemispherical outline, which are composed of outward radiating tubes that originate from a point located peripherally on the thallus. The tubes diverge upward at a narrow angle [25] (Figure 8a). Coarse, thin-walled, radiating clusters of tubes of Garwoodia can also be observed; these are associated with numerous microproblematica and calcareous sponges (Figure 8b,c). In addition, Cladogirvanella is composed of non-oriented intermingled tubes, characterized by thin-walled tangled, coiled, or aligned tubes (Figure 8d,e).
Significant foraminifera have been found in the Doi Long Formation in this study (Figure 9). They are dominated by involutinids (Aulotortus sp.), Endoteba sp., Endotriada spp., Endoteba bithynica, Duotaxis birmanica, occurring in association with Lenticulina sp., Nodosaria expolita, Nodosaria spp., and Diplotremina sp.

4. Discussion

4.1. Microfacies Interpretation

4.1.1. Back-Reef/Lagoon Environment

F1 and F2 represent the only thin-bedded deposits in the sequence, which are characterized by dark wackestone to packstone (Figure 5a–d). This facies, which is nearly laminated, occurs at the base and top of the formation and predominantly comprises a micrite matrix with minor amounts of bioclast grains, such as Tubiphytes sp., calcified cyanobacteria, microbialites, foraminifera, ostracods, and shell fragments. F3 is characterized by well-sorted peloidal packstone to grainstone, which is associated with dasycladacean green algae (Figure 9z–bb), and microencrusters (Figure 5d). Occurrences of green algae debris and cyanobacteria confirm their deposition within the upper photic zone [26,27] (Figure 9z–bb). This association is typical of open-lagoon environments within the upper photic zone, which were characterized by regular changes in water energy, above the fair-weather wave base (FWWB) [21,24,27].

4.1.2. Reef Setting

F4 microfacies are found as packstone to grainstone, suggesting a depositional setting characterized by very variable water energy (Figure 5e,f). Additionally, the presence of peloids, coated grains, and green algal debris confirms their deposition within the upper photic zone, indicating a relatively shallow subtidal area in a stable environment [24,27,28,29]. Microbial features (cyanoids, stromatactis structures, and peloids) associated with cyanobacteria in F5 indicate a moderate energy environment with water circulation in the normal marine setting, where regular changes in water energy led to the absence of mud and a lack of micritization [23,24,30,31,32]. F5 is characterized by stromatolitic algal boundstone, also including some laminated micro- and mesostructures and various micritic masses/patches associated with microproblematic organisms (Figure 5). According to the components and field observations, F5 is interpreted as microbial build-up or mound-like deposits, possibly a small-scale reef in a lagoonal setting or occurring on a bank margin [23,24,33,34,35,36,37,38]. In addition, the fenestral features record the common lenses of macroscopic skeletal organisms such as shell fragments, suggesting a subtidal interior platform setting (Figure 7d) [31].
The occurrence of microbial laminations suggests that the environment was generally low in energy, unlike the high-energy conditions typical of reef fronts or rims, where corals, sponges, and algal frame-builders flourish [23,24,25,37,39,40]. Our samples indicate that stromatolites and the preserved microbial structures formed an organic framework in a shallow subtidal setting [10,41,42] (Figure 7). The grain-supported fabric showed various microbial crusts and microproblematica, and the reefal cavities of the F5 facies can be interpreted as being diagnostic of reef environments [24]. The interstitial spaces within microbial encrusters are sometimes filled with isopachous fibrous or drusy calcite cements, pointing to active water circulation and submarine cementation [24,43]. Based on these observations, we can suggest that the external part of the platform was not dominated by frame builders (no strong and continuous reef barrier) but rather comprised small, alternating microbial build-ups/small bioconstructions and tidal channels.

4.2. Microbial Communities and Fossils

The Doi Long Formation discussed in this study belonged to the Carnian age and is characterized by a variety of microbial fabrics [5,9,10]. The dome-shaped stromatolites and irregular microbial grainstone with wrinkle structures represent the dominant components in this limestone unit (Figure 6 and Figure 7). This co-existed with numerous microproblematica, echinoderms, calcareous algae, and sponges, which might be formed in a euphotic zone with shallow water, normal salinity, and relatively high-energy conditions [44,45,46,47]. Various microproblematic organisms (Girvanella-like microstructures, Cladogrivanella cipitensis, Cayeuxia, and Tubiphytes-like organisms) and calcified cyanobacteria could be recognized in the limestones. Microbialites and microproblematic organisms constituted a large volume of the microbial build-up, generating various microbial products and microstructures such as stromatolites, oncoids, micritic masses, and aggregates (Figure 6 and Figure 7). Small micritic intraclasts can be derived from the mechanical destruction of upward-growing stromatolites and the nodular and laminar growth forms of calcified cyanobacteria [23,24]. Microproblematica and microbial carbonates played a vital role in their construction and evolution [24,25,37,48,49,50]. This suggests that calcifying cyanobacteria and microproblematic organisms served as the main bioconstructors of these mound-like build-ups during the Late Triassic in the Lampang area of northern Thailand. They differ significantly from those in other regions, where different reef types and encrusting organisms were more prominent during the Carnian period (e.g., [4,28,42,46]). These findings likely suggest that encrusting microproblematica and algae were flourishing and served as the main mound builders during the Late Triassic period in the Lampang area of northern Thailand. In addition, numerous age-significant foraminifera have been found in the Doi Long Formation, and their assemblage indicates a dominant Carnian age. The following species have been identified: Aulotortus sinuosus, Endoteba bithynica, Duotaxis birmanica, Nodosaria spp., Nodosaria expolita, Diplotremina astrofimbriata, and Endoteba sp. (Figure 9). This assemblage is of well-known Triassic species, which suggests a Carnian age [5,9,10,51,52,53,54,55]. Additionally, Aulotortus sinuosus is a widely recognized Triassic involutinid species that first appeared during the Ladinian period and became increasingly prevalent in the Late Triassic [4,5,9,10,51]. Dasyclad algae, interpreted as lagoonal deposits, are the most common organisms in the Doi Long Formation, which suggests the Middle and Late Triassic shallow-marine carbonates as their setting [5,24,56].

4.3. Importance of the Microbial Doi Long Limestone and Its Implications

The occurrence of this microbial limestone in association with microproblematic organisms, stromatolites, and other significant fossils of the Doi Long Formation indicates a shallow marine carbonate setting. The Upper Triassic microbial deposition reported in this study is quite similar to other eastern and western Tethys realms and Pacific/Panthalassa domains. For example, microbial-dominated carbonates have been widely reported in western Tethys areas, such as Upper Triassic limestone from northern Calabria, Italy [50], the Upper Triassic dolomite carbonate platform in the eastern southern Alps [57], Upper Triassic microbialitic facies in dolomitized carbonate platforms in southern Italy [58], and Upper Triassic platform margins and microbial bioconstructions [59]. Microfacies 4 and 5 are microbial-dominated facies, which are typical of a shallow, open marine environment. These microbial characteristics have been reported worldwide, including Lime Peak, Canada, the southern Alps, northwest and south Tethys, China, Indochina, and southwest Japan [5,60,61,62,63,64,65,66,67]. In addition, the microbial characteristics in this study are very similar to other microbial assemblages, such as stromatolites from southwest China and the limestones of Lesno Brdo, central Slovenia, and central Oman [42,68,69]. In addition, the Upper Triassic Doi Long limestones present the characteristics of typical shallow-water microbial carbonate deposition in the Sukhothai Zone in northern Thailand [5,10]. Microproblematica and other encrusted organisms such as corals, sponges, and foraminifera are very common in the samples. The occurrence of these organisms clearly indicates the reef settings (mound-like microbial/microbialite build-ups) and lagoon environments that are interpreted through the distinction of microfacies [5,23,24].
The most prominent features of the Doi Long limestone are lenses and masses of microbial grainstone and stromatolitic algal boundstone. As mentioned, some grainstones and boundstones appear to have grown in the form of columnar shapes, while in others, microbialites exhibit irregular layers of sediment. Cyanobacteria (Garwoodia, Cayeuxia, Cladogirvanella, and Microtubus) with microproblematic organisms (Tubiphytes) were observed in this study (Figure 8). These observations correspond with the high-growing microbial communities commonly reported in other Carnian platforms [4,5,63,65,70]. Therefore, the Doi Long Formation records the growth of a microbial reef ecosystem during the Late Triassic, which was distributed into the northwestern Lao PDR and southern China [4].
After the end-Permian extinction, global distribution and the presence of organic build-ups gradually increased [63,71,72,73]. By the end of the Middle Triassic period, extensive build-ups could be found in many global areas, including around the Western Tethys and Neo-Tethys, the Panthalassa oceans, and the Pacific regions [73,74]. However, due to the lack of reports regarding the microbial limestone in mainland Southeast Asia, microproblematica and other significant fossils in the Doi Long Formation are defined as the main community, which provides pivotal information regarding reef evolution in the volcanic arc domain of the Sukhothai Zone, Southeast Asia. However, the Upper Triassic bioconstructors of the mound-like build-ups of the Doi Long Formation represent small-scale microbial build-ups and lagoon environments that were not necessarily part of a continuous patch reef, as observed in contemporaneous carbonate platforms [50,63,67]. Based on these field observations, the occurrence of microfacies F4 and F5, and facies distribution, we suggest that the microbial limestone of the Doi Long Formation developed on a shallow marine platform acting as a “small-scale reef” during the Carnian period (Late Triassic), which corresponds with previous studies [4,5,10].
The Doi Long Formation is, therefore, essential to characterize and conceptualize how mound-like microbial/microbialite build-ups developed at that time. The presented data is a new and significant step to improve our knowledge of Upper Triassic environments in the volcanic arc domain, and, more generally, our understanding of small-scale microbial build-ups and limestone formation.

5. Conclusions

This study presents the first comprehensive integration of microfacies and microproblematica associations within the limestone of the Doi Long Formation, Sukhothai Zone, northern Thailand. Eight new outcrop localities have been discovered and analyzed in the northern part of the Lampang area. Five facies, corresponding to characteristic depositional environments, are described and interpreted herein. The fauna predominantly consists of calcified cyanobacteria, microproblematic organisms, microbialites, sponges, and gastropods. The compositional and textural features of the main microfacies types suggest a speculative depositional profile mainly characteristic of lagoon and reef (mound-like microbial/microbialite build-ups) settings. Microbial limestone sediments composed of microbial boundstone and sparse-allochem packstone to grainstone, consisting mainly of microproblematic organisms (Girvanella-like microstructures, Cladogrivanella cipitensis, Cayeuxia, Bacinella sp., Microtubus sp., and Tubiphytes-like organisms), stromatolites, calcified cyanobacteria, foraminifera, and shell fragments, which are common in this limestone. These suggest the shallow marine sedimentation of the bioconstructors of the mound-like build-ups. This study provides new data that can be incorporated into paleogeographic and paleoenvironment reconstructions of this limestone to better understand marine life evolution during the Late Triassic period.

Author Contributions

Conceptualization, K.M.; methodology, K.M. and M.U.; formal analysis, K.M. and J.J.; fieldwork, K.M., S.K. and J.J.; data curation, K.M. and M.U.; writing—original draft preparation, K.M., M.U. and S.K.; writing—review and editing, K.M. and M.U.; project administration, K.M.; funding acquisition, K.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Science CMU Research Fund from the Faculty of Science, Chiang Mai University, grant number R000032728.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Data can be made available upon request by reaching out to the corresponding author.

Acknowledgments

This study was supported by grants from the Science CMU Research Fund, Faculty of Science, Chiang Mai University. We are grateful to Steve Kershaw (Brunel University London) for suggesting the relevant microbial carbonates and Clive Burrett and Mahasarakham University for this cooperation and providing the background of the microbial fossils in the northern Thailand. We would like to thank Yannawat Ngernlumyong for data collection and the field trip. We also thank assistant editor and four anonymous reviewers for their useful comments and suggestions. Finally, we acknowledge the Department of Geological Sciences, Chiang Mai University, for the petrographic laboratory.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a). Tectonic subdivisions of northern Thailand, showing the main Indochina continental blocks, the Sibumasu Block, Sukhothai Zone, and bounding Paleo-Tethys and back-arc basin suture zones (after [3,8]). Thin dotted line: national boundary, black arrow: location of terrane. (b). The location of the study area is indicated in the Sukhothai Zone (black rectangle) (after [6,8]). (c). General stratigraphy of the Lampang Group and Song Groups and the Sukhothai Zone, which consists of the Doi Long Formation (blue-shaded area with a limestone symbol).
Figure 1. (a). Tectonic subdivisions of northern Thailand, showing the main Indochina continental blocks, the Sibumasu Block, Sukhothai Zone, and bounding Paleo-Tethys and back-arc basin suture zones (after [3,8]). Thin dotted line: national boundary, black arrow: location of terrane. (b). The location of the study area is indicated in the Sukhothai Zone (black rectangle) (after [6,8]). (c). General stratigraphy of the Lampang Group and Song Groups and the Sukhothai Zone, which consists of the Doi Long Formation (blue-shaded area with a limestone symbol).
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Figure 2. Geological map of the study area, representing the position of the studied localities and the distribution of the marine Triassic Doi Long Formation (Dl) in the Lampang Province, central northern Thailand (after [7]).
Figure 2. Geological map of the study area, representing the position of the studied localities and the distribution of the marine Triassic Doi Long Formation (Dl) in the Lampang Province, central northern Thailand (after [7]).
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Figure 3. Detailed stratigraphic columns of the studied sections of the Doi Long Formation.
Figure 3. Detailed stratigraphic columns of the studied sections of the Doi Long Formation.
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Figure 4. Field photographs of the investigated Triassic limestone section in the Lampang area, central northern Thailand. (a) General view of the limestone of the Doi Long Formation (from the road to Ban Sob Chang). (b) Massive and bedded limestones, exposed in the sample site of DL-3. (c) Limestone unit in the DL-5 location, showing a steep cliff. (d) Stromatolitic limestone at the bottom of the limestone cliff shown in (c), showing the laminae; the camera lens cover is 4.5 cm in diameter. (e) Vertical section showing upright, branching stromatolite columns with well-defined laminae; the pen is 0.8 cm in diameter. (f) Close-up view of branched columns of stromatolite; the pen is 0.8 cm in diameter. (g) Photograph showing the macroscopic features of the laminated texture of stromatolite, from locality DL-6. (h) Close-up of the boxed area in (g), showing the growth layer of the stromatolite (white arrow).
Figure 4. Field photographs of the investigated Triassic limestone section in the Lampang area, central northern Thailand. (a) General view of the limestone of the Doi Long Formation (from the road to Ban Sob Chang). (b) Massive and bedded limestones, exposed in the sample site of DL-3. (c) Limestone unit in the DL-5 location, showing a steep cliff. (d) Stromatolitic limestone at the bottom of the limestone cliff shown in (c), showing the laminae; the camera lens cover is 4.5 cm in diameter. (e) Vertical section showing upright, branching stromatolite columns with well-defined laminae; the pen is 0.8 cm in diameter. (f) Close-up view of branched columns of stromatolite; the pen is 0.8 cm in diameter. (g) Photograph showing the macroscopic features of the laminated texture of stromatolite, from locality DL-6. (h) Close-up of the boxed area in (g), showing the growth layer of the stromatolite (white arrow).
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Figure 5. Photomicrographs showing typical microfacies of the studied samples collected in the Lampang area, central northern Thailand. (a) Bioclastic wackestone, characterized by a micritic matrix with rare shell fragments (yellow arrows). Sample from DL-1 locality. (b) Bioclastic wackestone is composed of large microproblematica (Tubiphytes sp.) (yellow arrow), encrusted with a thin microbial crust (white arrow), all set in a micritic matrix. Sample from DL-2 locality. (c) Fine-grained peloidal packstone is largely composed of sorted peloids, together with coated grains and microproblematica, all cemented by microsparite. Sample from DL-3 locality. (d) Microproblematica, including Cayeuxia and Girvanella-like veins or tubes (yellow arrows) and aggregate grains in algal peloidal packstone. The rest of the material consists of peloids (red arrows) and isopachous fibrous calcite cement around the components (white arrows). Sample from DL-4 locality. (e) Cayeuxia, showing the branching and fan-like longitudinal section (red arrow). Sample from DL-5 locality. (f) Photomicrograph showing the tangled and linear filaments of Girvanella-like micritic veins. Sample from DL-4 locality. All scale bars are equal to 0.5 mm.
Figure 5. Photomicrographs showing typical microfacies of the studied samples collected in the Lampang area, central northern Thailand. (a) Bioclastic wackestone, characterized by a micritic matrix with rare shell fragments (yellow arrows). Sample from DL-1 locality. (b) Bioclastic wackestone is composed of large microproblematica (Tubiphytes sp.) (yellow arrow), encrusted with a thin microbial crust (white arrow), all set in a micritic matrix. Sample from DL-2 locality. (c) Fine-grained peloidal packstone is largely composed of sorted peloids, together with coated grains and microproblematica, all cemented by microsparite. Sample from DL-3 locality. (d) Microproblematica, including Cayeuxia and Girvanella-like veins or tubes (yellow arrows) and aggregate grains in algal peloidal packstone. The rest of the material consists of peloids (red arrows) and isopachous fibrous calcite cement around the components (white arrows). Sample from DL-4 locality. (e) Cayeuxia, showing the branching and fan-like longitudinal section (red arrow). Sample from DL-5 locality. (f) Photomicrograph showing the tangled and linear filaments of Girvanella-like micritic veins. Sample from DL-4 locality. All scale bars are equal to 0.5 mm.
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Figure 6. Photomicrographs showing typical stromatolitic algal boundstone microfacies of the Doi Long Formation from the DL-6 locality. (a) Polished slab showing small irregular columns, undulating laminations showing alternating dark and light grey laminae. (b) Thin section showing a stromatolite on the transverse section, separated by a dark matter network and iron-rich material. (c) Zoomed-in view of stromatolite, showing interbedded accumulation of microbial unit and micritic unit. (d) Photo showing the macroscopic features of plain view of stromatolites. (e) Close-up view of the patchy biomicrite of parts of stromatolite column in transverse section with Tubiphytes sp. (red arrows). (f) Defined stromatolite irregular top surface, overlain by unlaminated micrite with smaller foraminifera (white arrow). (g) The detail of the top surface of stromatolite showing is not sharp but instead passes into non-laminated micrite without a break. Scale bar is 500 μm, except for (a,d), which is 1 cm.
Figure 6. Photomicrographs showing typical stromatolitic algal boundstone microfacies of the Doi Long Formation from the DL-6 locality. (a) Polished slab showing small irregular columns, undulating laminations showing alternating dark and light grey laminae. (b) Thin section showing a stromatolite on the transverse section, separated by a dark matter network and iron-rich material. (c) Zoomed-in view of stromatolite, showing interbedded accumulation of microbial unit and micritic unit. (d) Photo showing the macroscopic features of plain view of stromatolites. (e) Close-up view of the patchy biomicrite of parts of stromatolite column in transverse section with Tubiphytes sp. (red arrows). (f) Defined stromatolite irregular top surface, overlain by unlaminated micrite with smaller foraminifera (white arrow). (g) The detail of the top surface of stromatolite showing is not sharp but instead passes into non-laminated micrite without a break. Scale bar is 500 μm, except for (a,d), which is 1 cm.
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Figure 7. Macro- and microscopic views of the stromatolite: (a) Polished slab showing columnar stromatolites with slightly to strongly upward-convex structures from bottom to top. Sample from DL-6 locality. (b) Small irregular columns in vertical profile. (c) Close-up view of the laminations, showing alternating dark and light grey micritic laminae. (d) Fenestral vug-type fabric within the microfacies in F5, showing calcite spars filled in between the grain components. Sample from DL-7 locality. (e) Photomicrographs of encrusting organisms on bivalve shell fragments, surrounded by clotted micritic aggregates in the white dotted line. (f) Zoom view of bivalve shell fragment (white dotted line area), surrounded by darker micrite of microbial origin. Interstitial spaces are filled with peloidal sediments and/or marine cement. All microphotographs are under plane-polarized light. Scale bars: (b,e) 500 μm; (f,c) 250 μm.
Figure 7. Macro- and microscopic views of the stromatolite: (a) Polished slab showing columnar stromatolites with slightly to strongly upward-convex structures from bottom to top. Sample from DL-6 locality. (b) Small irregular columns in vertical profile. (c) Close-up view of the laminations, showing alternating dark and light grey micritic laminae. (d) Fenestral vug-type fabric within the microfacies in F5, showing calcite spars filled in between the grain components. Sample from DL-7 locality. (e) Photomicrographs of encrusting organisms on bivalve shell fragments, surrounded by clotted micritic aggregates in the white dotted line. (f) Zoom view of bivalve shell fragment (white dotted line area), surrounded by darker micrite of microbial origin. Interstitial spaces are filled with peloidal sediments and/or marine cement. All microphotographs are under plane-polarized light. Scale bars: (b,e) 500 μm; (f,c) 250 μm.
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Figure 8. Photomicrographs of the microproblematic organisms of this study. (a) Small nodules exhibiting a hemispherical outline are composed of outward radiating tubes of Cayeuxia (yellow arrow) associated with microencrusters (white arrow). Sample from DL-6 locality. (b) The microencruster Garwoodia. Sample from DL-7 locality. (c). Microbial lamination encrusting the sponge (Uvanella; S) and numerous microproblematica (Baccinella; blue arrows, Microtubus; yellow arrow). Interframework pores are filled with drusy cement (white arrow). Sample from DL-7 locality. (d). Close-up view of Cladogrivanella cipitensis Ott, showing tangled and linear filaments (red arrow). Sample from DL-5 locality. (e). Girvanella-like microstructures showing thin-walled tubed, tangled, aligned, or intermingled morphology. Sample from DL-8 locality. All scale bars are 500 μm.
Figure 8. Photomicrographs of the microproblematic organisms of this study. (a) Small nodules exhibiting a hemispherical outline are composed of outward radiating tubes of Cayeuxia (yellow arrow) associated with microencrusters (white arrow). Sample from DL-6 locality. (b) The microencruster Garwoodia. Sample from DL-7 locality. (c). Microbial lamination encrusting the sponge (Uvanella; S) and numerous microproblematica (Baccinella; blue arrows, Microtubus; yellow arrow). Interframework pores are filled with drusy cement (white arrow). Sample from DL-7 locality. (d). Close-up view of Cladogrivanella cipitensis Ott, showing tangled and linear filaments (red arrow). Sample from DL-5 locality. (e). Girvanella-like microstructures showing thin-walled tubed, tangled, aligned, or intermingled morphology. Sample from DL-8 locality. All scale bars are 500 μm.
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Figure 9. Photomicrographs of the Triassic foraminifera of the Doi Long Formation, northern Lampang. (a) Diplotremina sp. (from DL-4). (b,c) Endoteba sp. (from DL-4). (d–f) Endotriada spp. (d,e) from DL-4; (f) from DL-3). (g–i) Endoteba bithynica (from DL-4). (j–l) Duotaxis birmanica (j,k) from DL-3; (l) from DL-2). (m) Lenticulina sp. (from Dl-2). (n–s) Aulotortus sinuosus ((n,q,s) from DL-3; (o,p) from DL-2; (r) from DL-4). (t,u). Nodosaria expolita (from DL-4). (v,w). Nodosaria spp. ((v) from DL-4; (w) from DL-3). (x,y). Diplotremina astrofimbriata ((x) from DL-3; (y) from DL-4). (z–bb). Dasycladacean green algae debris in the transverse section ((z,aa) from DL-4; (bb) from DL-8). Scale bar is 250 μm, except for (t–w) at 100 μm.
Figure 9. Photomicrographs of the Triassic foraminifera of the Doi Long Formation, northern Lampang. (a) Diplotremina sp. (from DL-4). (b,c) Endoteba sp. (from DL-4). (d–f) Endotriada spp. (d,e) from DL-4; (f) from DL-3). (g–i) Endoteba bithynica (from DL-4). (j–l) Duotaxis birmanica (j,k) from DL-3; (l) from DL-2). (m) Lenticulina sp. (from Dl-2). (n–s) Aulotortus sinuosus ((n,q,s) from DL-3; (o,p) from DL-2; (r) from DL-4). (t,u). Nodosaria expolita (from DL-4). (v,w). Nodosaria spp. ((v) from DL-4; (w) from DL-3). (x,y). Diplotremina astrofimbriata ((x) from DL-3; (y) from DL-4). (z–bb). Dasycladacean green algae debris in the transverse section ((z,aa) from DL-4; (bb) from DL-8). Scale bar is 250 μm, except for (t–w) at 100 μm.
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Table 1. Summary of microfacies types characterizing the Doi Long Formation, north Thailand.
Table 1. Summary of microfacies types characterizing the Doi Long Formation, north Thailand.
Sample LocalitiesMicrofaciesNon-Skeletal ComponentsSkeletal ComponentsInterpretation
DL-1Bioclastic wackestone (F1)Major: inhomogeneous micrite with minor quartzRare skeletal grains (shell fragments and foraminifera)Lagoon
DL-2Bioclastic wackestone (F1)Major: peloids (microbial, reworked mud grains, micritized grains) Major: bivalves, microproblematica such Tubiphytes sp.
Subordinate: foraminifera and rare ostracods		Lagoon
DL-3Peloidal bioclastic packstone (F2)Major: micritized grainsMajor: microproblematica such as Tubiphytes sp. and Microtubus sp., and calcareous algae
Subordinate: debris of echinoderm, foraminifera, bivalves, and gastropods		Lagoon
Cyanobacteria packestone (F3)Major: peloids (microbial, reworked mud grains, micritized grains)Major: Microproblematica such Tubiphytes sp., Bacinella sp., and Microtubus sp., debris of algae, bivalves, and green algae (dasycladecean)
Subordinate: foraminifera and ostracods 		Back-reef, Lagoon
DL-4Cyanibacteria packestone (F3)Major: peloids (microbial, reworked mud grains, micritized grains)Major: microproblematica and fragments of green algaeBack-reef, Lagoon
DL-5 to DL-6Microbial packstone- grainstone (F4)Very rareMajor: microproblematica such as Tubiphytes sp., Cladogirvanella sp., Bacinella sp., and Microtubus sp., foraminifera, green algae (dasycladecean), and echinoderm,
Subordinate: bivalve and ostracods		Reef
Stromatolitic algall boundstone (F5)NoneMajor: algal-microbial crusts and microbial laminated stromatolites, and microproblematic organisms (Cayeuxia sp., Garwoodia sp., and Tubiphytes sp.)
Subordinate: smaller foraminifera		microbial build-up or mound-like
DL-7 to DL-8Cyanobacteria packestone (F3)Mainly peloid grains, large isopaquous and blocky cementsMajor: fragments of microproblematica
Subordinate: bivalve shells and foraminifera		Back-reef, Lagoon
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Moonpa, K.; Udchachon, M.; Jainanta, J.; Kanthata, S. Microbial Carbonates of Upper Triassic Doi Long Formation, Lampang Group: A Study of New Outcrop Localities in Northern Lampang, Central North Thailand. Diversity 2025, 17, 299. https://doi.org/10.3390/d17040299

AMA Style

Moonpa K, Udchachon M, Jainanta J, Kanthata S. Microbial Carbonates of Upper Triassic Doi Long Formation, Lampang Group: A Study of New Outcrop Localities in Northern Lampang, Central North Thailand. Diversity. 2025; 17(4):299. https://doi.org/10.3390/d17040299

Chicago/Turabian Style

Moonpa, Kritsada, Mongkol Udchachon, Jirattikarn Jainanta, and Sathit Kanthata. 2025. "Microbial Carbonates of Upper Triassic Doi Long Formation, Lampang Group: A Study of New Outcrop Localities in Northern Lampang, Central North Thailand" Diversity 17, no. 4: 299. https://doi.org/10.3390/d17040299

APA Style

Moonpa, K., Udchachon, M., Jainanta, J., & Kanthata, S. (2025). Microbial Carbonates of Upper Triassic Doi Long Formation, Lampang Group: A Study of New Outcrop Localities in Northern Lampang, Central North Thailand. Diversity, 17(4), 299. https://doi.org/10.3390/d17040299

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