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).

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.

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.

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.

Three lithofacies associations are recognized.

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).

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.

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).

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).

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).