Transformation of the middle–upper ordovician sedimentary environment of the north China plate under the caledonian tectonic background: a case study of the ordovician majiagou formation in the qishan area

This study documents an abrupt sedimentary and petrographic transition near the top of the Ordovician Majiagou Formation in the Qishan area, southwestern Ordos Basin, North China Plate. Detailed lithofacies analysis of two sections reveals a rapid shift from shallow-water platform mudstone and dolostone (with lamination and bioturbation) to deeper-water slope to base-of-slope deposits characterized by slump breccias and gravity-flow facies. Tuff/bentonite interlayers and widespread soft-sediment deformation indicate intensified syn-depositional disturbance during the late stage of Majiagou deposition. We interpret these observations as reflecting a major increase in accommodation and margin instability along the southern basin margin during late Ma VI time. However, because no new radiometric ages or geochemical fingerprints of arc-related volcanism are presented here, our tectonic discussion is framed as a sedimentological reinterpretation constrained by previously published age data rather than a direct chronological revision. Within this framework, the Qishan record suggests that tectonic influence related to Qinling Ocean subduction may have intensified by late Ma VI, potentially earlier than commonly inferred for the Pingliang stage. These results refine the depositional model for the Middle–Upper Ordovician transition and contribute to ongoing discussions of basin–orogen coupling along the southwestern margin of the Ordos Basin.

Introduction

The Cambrian–Ordovician transition, marked by the Huaiyuan movement, temporarily uplifted the North China Plate and exposed large areas as land (Cheng et al., 2012; Song et al., 2018; Tucker and Wright, 2009). An Early Ordovician transgression subsequently re-established marine sedimentation across the plate. During the Early to Middle Ordovician, the Ordos Basin, situated on the western margin of the North China Plate, was dominated by an epeiric carbonate platform (Guo et al., 2012; Guo et al., 2014; Hou et al., 2003; Wang et al., 2015), which evolved into a deep-marine basin during the Late Ordovician (Li and Li, 2008; Zhang et al., 2004; Zhang and Liao, 2006).

Tectonic evolution exerts a fundamental control on the formation, development, and environmental transformation of sedimentary basins. The distribution and evolution of sedimentary facies within basins are governed by basin tectonics, sediment sources, paleogeography, paleoenvironmental conditions, subsidence rates, sedimentation processes, and sea-level changes (Li H. et al., 2016; Li S. Z. et al., 2016; Ratanasthien, 1993; Lee et al., 2025). Together, these factors record the underlying crustal dynamics and paleoclimatic regimes (Song et al., 2018; Tu et al., 2016; Xiong et al., 2020; Xiong et al., 2021). Paleogeographic reconstructions therefore rely heavily on interpreting complex sedimentary records that may reflect tectonism, volcanic activity, seismic events, flood and gravity-flow deposits, oceanic anoxia and oxidation events, and biological proliferations or extinctions (Dong et al., 2010; Huang et al., 2011; Yang et al., 2019; Xie et al., 2024).

Despite extensive research, critical uncertainties remain concerning the precise onset and peak stages of the subduction of the Qinling Ocean. Constraining this timing is essential for understanding the geological evolution of the southern margin of the Ordos Basin. The Qishan area provides an excellent natural laboratory for investigating the Middle–Late Ordovician transition, owing to its clear record of rapid facies shifts and its proximity to the tectonically active Qinling Ocean domain.

Importantly, sedimentary facies shifts commonly represent an integrated response to multiple drivers rather than a unique proxy for tectonic timing. Studies of tectonically active basins have highlighted that depositional successions can be shaped by the interplay of syn-tectonic subsidence, basin geometry, sediment supply, sea-level fluctuations, and post-depositional modification, and robust tectonic interpretations often require integration with independent constraints such as provenance, structural data, and geochemical indicators (e.g., Lee et al., 2025; Xie et al., 2024). In this study, we therefore emphasize sedimentological evidence and discuss tectonic implications with appropriate caution and explicit limitations.

In this context, we examine the Ordovician Majiagou Formation exposed in the Qishan area. Our objectives are to: (1) document the sedimentological and petrographic characteristics of the abrupt transition from shallow-water carbonate platform deposits to deeper-water gravity-flow–dominated facies near the top of the formation; (2) evaluate depositional processes and plausible mechanisms responsible for this environmental shift, including both tectonic and non-tectonic drivers; and (3) discuss the implications of these sedimentary observations for basin–orogen coupling and the evolving tectonic influence along the southern margin of the Ordos Basin, within the constraints of existing chronological frameworks. The results provide a refined depositional model for the Middle–Upper Ordovician transition and offer a sedimentological perspective relevant to debates on the Qinling Ocean subduction history.

Geological setting

The study area is located on the southwestern margin of the Ordos Basin, which is bounded by the Yinshan Mountains to the north, the Luliang Mountains to the east, the Qinling Mountains to the south, and the Helan Mountains to the west (Table 1). The outcrops are situated north of Qishan town within the Weibei Uplift (Figure 1). In this region, dolomite reservoirs of the Ordovician Majiagou Formation constitute major targets for natural gas exploration (Chen et al., 2019; Huang et al., 2020a; 2020b; 2021).

Table 1

Table 1. Distribution of the Ordovician System in different regions of the Ordos Basin.

Figure 1

Figure 1. (a) Structural location map and (b) geological map of the study area.

The regional geological evolution was strongly influenced by Cambrian uplift and the subsequent Huaiyuan orogeny (Parnell, 2010; Weng et al., 2012), which caused uplift, exposure, and erosion along the southern margin of the basin and formed a prominent slope-break belt. These features support the interpretation of the southern basin margin as a carbonate platform rather than a homoclinal ramp (Dong et al., 2010; Zhao et al., 2007a; 2007b). A major transgression from the Qinling and Qilian oceans at the end of the Liangjiashan stage further shaped the southern depositional environment (Feng et al., 1998a; 1998b; Guo et al., 2012; 2014).

Ordovician strata in the study area comprise the Yeli, Liangjiashan, Majiagou, and Pingliang formations. The Lower Ordovician Yeli Formation unconformably overlies the Cambrian Gushan Formation to the west, but conformably overlies the Fengshan Formation in the eastern and southern parts of the basin. It is mainly composed of laminated dolomitic mudstone, wackestone, and packstone, representing the resumption of sedimentation after the Early Ordovician transgression. The overlying Liangjiashan Formation consists predominantly of medium–thick dolostone with interbedded chert bands.

The Middle Ordovician Majiagou Formation is commonly subdivided into six members and records three transgressive–regressive cycles (Li et al., 2012). Members Ma I–Ma III are dominated by thick to medium-thick sandstone, limestone, and dolomitic limestone containing abundant marine fossils. Members Ma IV and Ma V show increasing dolomite and argillaceous content from west to east, indicative of an open-platform environment with relatively minor sea-level fluctuations. South of Qishan, carbonate gravity-flow deposits and syndepositional deformation structures are well developed. Sedimentation of Ma VI reflects a significant influence of Caledonian tectonism, with marked thickness variations caused by erosion and differential subsidence.

The Late Ordovician Pingliang Formation exhibits distinct lithological differences across the basin. On the western margin, it is characterized by mud shale interbedded with carbonate gravity-flow deposits, whereas in the east it consists mainly of thin-bedded argillaceous limestone. Sedimentation during the Yeli–Liangjiashan interval records an evaporitic platform that transitions from intertidal–supratidal to more open-marine conditions southward. The Huaiyuan movement produced a regional hiatus between the Liangjiashan and Majiagou formations.

From the Middle Ordovician onward, widespread transgressions established extensive shallow-water carbonate platform sedimentation across the basin, gradually passing southwestward into slope deposits toward the Qinling trough. By the end of the deposition of the Majiagou Formation, open-platform conditions prevailed in the central basin, while marginal shoals, platform-margin slopes, and deep-water trough deposits developed toward the basin margins. During the generation of the Pingliang Formation, deposition continued along the southwestern margin, forming thick deep-water slope and trough successions locally exceeding 1,000 m.

Methods

To delineate the sedimentary environmental transition in the Qishan area, we combined detailed field observations, geological mapping, and petrographic analysis of thin sections. This methodological framework is particularly effective for recognizing sedimentary structures diagnostic of specific depositional processes—such as storm deposits, slump structures, and gravity-flow deposits—which are essential for reconstructing sedimentary environments and assessing tectonic influences.

Precise elevation measurements of lithofacies boundaries were obtained using standard geological survey tools, including tape measures and Jacob’s staffs, ensuring accurate stratigraphic correlation. Fieldwork involved systematic logging and photographing of stratigraphic sections, recording sedimentary structures, and collecting representative rock samples.

Sedimentary structures, including storm-generated beds, gravity-flow deposits, and slump features, were identified based on established criteria such as graded bedding, erosional bases, soft-sediment deformation, and characteristic layering patterns. Petrographic analysis focused on grain composition, matrix type, diagenetic features, and micro-sedimentary structures. The integration of these methods provided a robust dataset for reconstructing sedimentary environment changes and evaluating their implications for regional tectonic processes.

Results

Petrography characters of the eastern section

In the eastern section, the lower part of the Majiagou Formation is dominated by dolomitic mudstone and bedded dolostone. Bioturbation is common in the lower to middle intervals, whereas the upper part is characterized by laminated dolostone interbedded with monomict breccia and locally developed mud cracks. Regional stratigraphic correlation indicates that this succession corresponds to the Ma V member of the Majiagou Formation.

Based on lithology, sedimentary structures, and petrographic features, five representative lithofacies are identified.

Mottled dolostone

This lithofacies consists of dolomitic mudstone containing irregular dark mottles, typically ∼3 cm in diameter, which are locally interconnected (Figures 2a–c). Petrographic observations show that the mottles share the same mineral composition as the host matrix (Figure 2d), indicating pre-diagenetic bioturbation followed by pore infilling. Such features are characteristic of deposition under well-oxygenated, low-energy subtidal conditions.

Figure 2

Figure 2. Photographs of Mottled dolostone: (a,b) Mottled dolostone in field outcrop; (c) Polished hand specimen of Mottled dolostone; (d) Photomicrograph of mottled dolostone under the microscope.

Mottled and laminated dolostone

This facies combines bioturbation mottles with thin, parallel laminae (0.5–1 mm thick; Figure 3). The coexistence of lamination and biogenic disruption suggests fluctuating hydrodynamic conditions, consistent with deposition in a shallow subtidal to lower intertidal environment influenced by weak tidal currents.

Figure 3

Figure 3. Photographs of mottled and laminated dolostone: (a) Mottled and laminated dolostone in field outcrop; (b,c) Polished hand specimen of mottled and laminated dolostone; (d) Thin section of mottled and laminated dolostone under the microscope.

Microbial structure dolostone

Microbial structure dolostone is mainly developed within dolomitic mudstone and contains less than 10% allochems. Wavy to domal stromatolite-like lamination is common (Figure 4). Locally developed scour surfaces and intraformational debris indicate episodic storm disturbance superimposed on generally low-energy conditions. This facies reflects restricted shallow-water environments with periodic high-energy events.

Figure 4

Figure 4. Photographs of microbial structure dolostone: (a,b) Microbial structure dolostone in field outcrop; (c) Polished hand specimen of microbial structure dolostone; (d) Photomicrographs of microbial structure dolostone under the microscope.

Planar laminated dolostone

Planar laminated dolostone consists of parallel bright and dark laminae, each 0.5–1 mm thick (Figures 5a–f). The presence of mud cracks (Figure 5e) indicates intermittent subaerial exposure, suggesting deposition in an intertidal setting subject to episodic desiccation. These features record short-term environmental fluctuations within a generally restricted platform interior.

Figure 5

Figure 5. Photographs of planar laminated dolostone: (a,f) Planar laminated dolostone in field outcrop; (b,c) Polished hand specimen of planar laminated dolostone; (d,e) Photomicrograph of planar laminated dolostone under the microscope, and mud cracks in (e).

Monomict breccia

The monomict breccia is composed of well-sorted but poorly rounded dolostone clasts embedded in a fine dolomitic matrix (Figures 6a,b). Textural characteristics indicate episodic high-energy deposition, most plausibly related to storm-induced reworking or localized platform-margin instability. This facies provides evidence for intermittent energetic events within an otherwise low-energy carbonate system.

Figure 6

Figure 6. Photographs of monomict breccia: (a) Monomict breccia in field outcrop; (b) Polished hand specimen of monomict breccia; (c,d) Polymict breccia in field outcrop; (e) Polished hand specimen of polymict breccia; (f) Photomicrograph of polymict breccia under the microscope.

Collectively, the Ma V lithofacies assemblage records deposition in a restricted platform-interior setting characterized by low-energy conditions, intermittent tidal influence, episodic storm reworking, and short-lived subaerial exposure. The vertical stacking of these facies reflects gradual shallowing punctuated by high-energy events rather than a uniformly tranquil lagoon.

Petrography characters of the western section

In the western section, the Majiagou Formation is dominated by limestone rather than dolostone. The basal interval consists mainly of pale yellow to light orange clayey limestone with parallel bedding, indicating low-energy deposition with intermittent fine-grained input (Figure 7). The middle interval is characterized by skeletal limestone showing upward fining within individual beds (Figure 8b), whereas the uppermost part comprises thick polymict breccia marking the transition to the overlying Pingliang Formation, which begins with black shale.

Figure 7

Figure 7. Photographs of clayey limestone: (a,b) Clayey limestone in field outcrop; (c) Polished hand specimen of clayey limestone; (d) Photomicrograph of clayey limestone under the microscope.

Figure 8

Figure 8. Photographs of skeletal limestone: (a) Skeletal limestone in field outcrop; (b) Polished hand specimen of skeletal limestone; (c,d) Photomicrograph of skeletal limestone under the microscope.

Three lithofacies associations are recognized.

Clayey limestone

Clayey limestone includes mudstone, wackestone, and micrite containing 3%–15% grains (Figure 7). The fine-grained texture and parallel bedding indicate deposition under relatively calm hydrodynamic conditions, with periodic input of clay likely related to distal suspension fallout or low-energy currents.

Skeletal limestone

Skeletal limestone contains abundant bioclastic grains, predominantly shell fragments, with an upward decrease in grain size within individual beds (Figure 8). This graded bedding is characteristic of turbidity-current deposition, corresponding to the A division of the Bouma sequence. Such features indicate sediment transport downslope under waning flow conditions.

Polymict breccia

Polymict breccia consists of poorly sorted and poorly rounded limestone clasts (Figures 6c–f). The chaotic fabric and clast-supported texture indicate deposition by high-energy mass-transport processes, such as slumping or gravitational collapse along a slope. This facies marks the establishment of a slope to base-of-slope depositional environment at the top of the Majiagou Formation.

The western section records a transition from low-energy background sedimentation to gravity-flow–dominated processes, reflecting progressive slope development and increasing margin instability prior to deposition of the Pingliang Formation.

Sedimentary environment analysis

The lithofacies assemblages of the Ma V member in the Qishan area indicate deposition within a predominantly restricted platform-interior system under generally low-energy conditions. Bioturbation, microbial lamination, and tidal-flat indicators record a shallow-water environment evolving from subtidal to intertidal settings during a regressive phase. However, the presence of breccias, storm-related scour surfaces, and localized high-energy features suggests that this environment was intermittently disturbed by energetic events and may have been proximal to marginal shoal complexes. Accordingly, a simple lagoonal interpretation is likely an oversimplification of a more heterogeneous platform interior (Figure 9).

Figure 9

Figure 9. Compiled stratigraphic column of the eastern section. Facies indicate a predominantly restricted platform-interior setting with lagoon-like characteristics and intermittent tidal-flat exposure. Correlations are schematic and based on measured stratigraphic boundaries; thickness differences may reflect local accommodation and preservation.

In contrast, the upper Ma VI member is dominated by turbidites, slump breccias, and other gravity-flow deposits, indicating deposition in a slope to base-of-slope environment (Figure 10). The abrupt transition from restricted platform facies in Ma V to gravity-flow–dominated facies in Ma VI records a major reorganization of depositional conditions, characterized by rapid deepening, increased accommodation, and enhanced margin instability. While such changes are consistent with intensified tectonic influence, similar sedimentary responses may also arise from alternative mechanisms such as flexural subsidence or platform-margin collapse; these possibilities are explored further in the Discussion.

Figure 10

Figure 10. Compiled stratigraphic column of the western section. Slump breccias and gravity-flow deposits indicate slope to base-of-slope processes and margin instability. Correlations are schematic; scale and thickness are based on field measurements, and uncertainty is mainly related to lateral facies variability and local erosion.

Discussion

Sedimentary response to basin-margin reorganization

The sedimentary record of the southwestern Ordos Basin reflects the combined influence of accommodation changes, depositional processes, and margin stability (Dong et al., 2010; Guo et al., 2014). In the Qishan area, the Ma V member is characterized by restricted platform-interior facies with intermittent storm reworking and episodic exposure, whereas the overlying Ma VI member records a pronounced shift to gravity-flow–dominated slope deposits (Figure 11). This sharp facies contrast indicates a fundamental reorganization of the depositional system at the basin margin rather than a gradual environmental adjustment (Shi et al., 2009; Wang et al., 2009).

Figure 11

Figure 11. Sedimentary Environment Change Diagram. During the ma V period, the depositional environment was a tranquil lagoon, where different sedimentary structures of dolomite were deposited in the subtidal, intertidal, and supratidal zones. In the ma VI period, the environment abruptly transitioned to a slope facies, characterized by the extensive development of slump breccia.

Importantly, the transition from platform to slope environments is accompanied by widespread soft-sediment deformation, slump breccias, and mass-transport deposits, suggesting rapid creation of accommodation and enhanced instability along the margin (Su et al., 2011; Xu et al., 2020). Such sedimentary features are commonly associated with externally forced perturbations affecting slope gradients and sediment redistribution, particularly in tectonically active basin margins (Yang et al., 2019; Lee et al., 2025).

However, sedimentary facies alone cannot uniquely constrain the underlying driving mechanism. Comparable platform-to-slope transitions have been documented in settings influenced by tectonic subsidence, flexural loading, or autocyclic collapse of oversteepened carbonate margins (Ratanasthien, 1993; Tucker and Wright, 2009). Therefore, the Qishan record is best interpreted as evidence for basin-margin reorganization involving increased accommodation and instability, while the specific tectonic processes responsible require evaluation within a broader regional and methodological context (Lee et al., 2025; Xie et al., 2024).

Regional comparison and basin-wide significance

Comparable facies transitions from shallow-water carbonate platforms to deep-water slope or basin deposits have been reported from multiple sections along the southern margin of the Ordos Basin near the Middle–Upper Ordovician boundary (Shi et al., 2009; Wang et al., 2009; Zhen et al., 2016). In many of these successions, breccias and gravity-flow deposits directly overlie shallow-marine carbonate platforms and are followed upward by fine-grained deep-water sediments. This recurring stratigraphic motif suggests that the environmental shift observed at Qishan was not localized but part of a basin-wide reorganization affecting the southern margin of the Ordos Basin.

The spatial coherence of these transitions implies large-scale controls on accommodation and depositional gradients along the basin margin. Although eustatic sea-level changes and climatic variability may have influenced background sedimentation, available records do not indicate first-order changes sufficient to explain the abruptness of the platform-to-slope transformation (Dai et al., 2020; Yang et al., 2015). Instead, the regional distribution of gravity-flow deposits suggests a common forcing mechanism operating along much of the southern basin margin during late Majiagou time (Dong et al., 2010; Guo et al., 2012).

Implications for tectonic influence and alternative mechanisms

The proximity of the southwestern Ordos Basin to the Qinling domain makes tectonic influence a plausible contributor to the observed sedimentary changes. Convergence between the Qinling Ocean and the North China Plate during the Ordovician has been widely recognized, with syn-depositional deformation, volcanism, and bentonite deposition documented in adjacent regions (Dai et al., 2019; Wang et al., 2015; Xu et al., 2020). Within this framework, enhanced tectonic loading or differential subsidence along the basin margin could reasonably account for the rapid deepening and slope instability recorded in the Ma VI member.

Nevertheless, alternative mechanisms must also be considered. Flexural subsidence related to distal orogenic loading, reactivation of pre-existing intraplate faults, and autocyclic collapse of carbonate platform margins are all capable of generating gravity-flow deposits and slope geometries similar to those observed at Qishan (Tucker and Wright, 2009; Yang et al., 2019). Variations in carbonate productivity and sediment supply may further modulate the sedimentary response (Li H. et al., 2016). Given the absence of new structural data, direct geochronological constraints, or geochemical fingerprints of arc-related magmatism within the studied sections, it is not possible to conclusively discriminate among these mechanisms using sedimentary evidence alone (Lee et al., 2025; Xie et al., 2024).

Accordingly, tectonic influence is interpreted here as a viable, but not exclusive, explanation for the observed basin-margin reorganization.

Implications for the timing of Qinling Ocean subduction

Previous studies based on zircon U–Pb ages from bentonites and volcanic rocks suggest that subduction along the southern margin of the North China Plate initiated during the Middle Ordovician and intensified toward the Middle–Late Ordovician boundary (Wang et al., 2015; Xu et al., 2020). A commonly cited interpretation places the strongest tectonic expression during deposition of the Pingliang Formation, as indicated by widespread gravity-flow deposits and volcanic activity (Xu et al., 2020).

The new sedimentological observations from Qishan demonstrate that gravity-flow deposits, slump breccias, and tuff/bentonite interlayers were already well developed by late Ma VI time. These features indicate that significant margin instability and enhanced accommodation were established prior to widespread Pingliang deposition (Shi et al., 2009; Zhen et al., 2016). Within the limits of the available data, this observation suggests that tectonic influence along the southwestern margin of the Ordos Basin may have intensified earlier than traditionally emphasized.

Crucially, this inference is based on sedimentological evidence integrated with previously published chronological constraints and does not constitute a direct revision of the tectonic timescale. Rather, it represents a sedimentological reinterpretation of basin-margin response to regional convergence during the late stages of Majiagou deposition, consistent with recent methodological perspectives on interpreting sedimentary records in tectonically active basins (Lee et al., 2025; Xie et al., 2024).

Conclusion

1. 1. Lithofacies associations and sedimentary structures in the Ma V member of the Ordovician Majiagou Formation at Qishan indicate deposition within a predominantly restricted platform-interior system under generally low-energy conditions. The facies succession records a regressive evolution from subtidal to intertidal environments, punctuated by episodic high-energy reworking and intermittent exposure.

2. 2. The upper Ma VI member is dominated by gravity-flow–related facies, including turbidites and slump breccias, locally associated with tuff or bentonite interlayers. This assemblage documents a rapid transition from restricted platform deposition to slope to base-of-slope environments, reflecting a major reorganization of depositional conditions along the southwestern margin of the Ordos Basin.

3. 3. The sedimentological evidence indicates that the abrupt platform-to-slope transformation observed at Qishan cannot be readily explained by gradual environmental change alone. Instead, it reflects a rapid increase in accommodation and margin instability affecting the basin margin during late Majiagou time.

4. 4. Integrating the new sedimentological observations with previously published chronological constraints suggests that tectonic influence related to Qinling Ocean convergence may have intensified by late Ma VI time, prior to widespread deposition of the Pingliang Formation. Because this study does not present new geochronological or structural constraints, this interpretation is framed as a sedimentological reinterpretation within existing frameworks rather than a direct revision of tectonic timing.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.

Author contributions

DZ: Writing – original draft. XW: Writing – review and editing, Resources. RL: Data curation, Conceptualization, Writing – review and editing, Formal Analysis. BZ: Funding acquisition, Investigation, Methodology, Writing – review and editing. XW: Visualization, Writing – review and editing, Validation, Software, Supervision. AX: Investigation, Writing – review and editing, Software. KF: Writing – review and editing, Methodology, Project administration.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This research was supported by the National Key Research and Development Program of China (Grant No. 2025ZD1009805-02) and the International Geoscience Programme (IGCP-652). The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.

Acknowledgements

The authors would like to thank Daniel Lehrmann, Jack Koellmann, Lei Qiang, and Xue Chunling for their assistance during fieldwork.

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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The author(s) declared that generative AI was not used in the creation of this manuscript.

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