A Shallow Water Case of Ordovician Marine Red Beds (South China): Evidence from Sedimentary Structures and Response to the Kwangsian Orogeny

Abstract

Ordovician marine red beds (OMRBs) are widely developed along the margins of Gondwana and represent distinctive limestone facies. These red beds are known for their diverse sedimentary structures and have been described by scholars as the “fashionable facies” in geological history. However, their characteristics and classification remain controversial. Multiple hypotheses about their origin have also hindered a clear understanding of these strata. Therefore, this study focuses on the Xiangxi area (South China) and presents a detailed analysis of the sedimentary structures of marine red beds, building on previous research on OMRBs in South China. Based on genetic features, we divide the most debated “nodule-like” and “cracked” structures—previously identified by earlier researchers—into ten subtypes. Three key genetic end-members are identified among these subtypes: breccia, patch, and argillaceous band. Detailed studies using microslab analysis, scanning electron microscopy, geochemistry, and paleontology were carried out on these three end-members. The results confirm that the Ordovician marine red beds were mainly deposited in a shallow marine environment, with the red coloration primarily derived from continental sources. As the sea level rose, the color of the red beds lightened, and the dominant sedimentary structures shifted from breccia end-members to argillaceous band end-members. Additionally, this study identified a vertically penetrating argillaceous band controlled by syndepositional compressive stress, which may be linked to NW-directed compression from the Kwangsian Orogeny. Evidence from tectonic styles, biofacies migration, and chronostratigraphy supports this hypothesis.

1. Introduction

Marine red beds (MRBs) are marine sedimentary formations with a distinctive reddish hue [1]. Their striking coloration has long attracted scientific interest. Research on MRBs started later than that on continental red beds and accelerated in the late 20th century, focusing on stratigraphy and paleontology [2]. MRBs differ from continental red beds in sedimentary environment, biota, and elemental composition. As a unique type of seafloor sediment [3], MRBs form a key component of the Earth’s “carbon–oxygen–iron cycles” [4,5,6,7,8]. They provide important clues for reconstructing paleo-oceans, paleo-climates, carbon cycling, and biological evolution [9,10].

Historically, MRB research has concentrated on the Cretaceous [11]. Since 2002, International Geoscience Programme (IGCP) projects have generated substantial findings. However, studies on MRBs from other geological periods are still limited. Phanerozoic MRBs have been identified from the Cambrian to the Paleogene [12]. Based on lithology, they can be classified as either limestone-dominated or clastic-/mudstone-dominated types [9]. Clastic-dominated MRBs are more common in the early Paleozoic—such as during the Cambrian [13,14] and Silurian [15,16]—with rare occurrences in the Devonian [17]. Limestone-dominated MRBs occur throughout the Phanerozoic [18,19,20,21,22,23,24,25,26,27]. These are not high-purity carbonate rocks but often contain elevated argillaceous material rich in high-valence iron, which contributes to their red color [28,29].

Ordovician marine red beds (OMRBs) are a distinctive type of limestone-dominated MRB [30]. They have been documented in China, Thailand, Morocco, India, and the Baltic region [28,31,32,33] (Figure 1a) and are geologically associated with the margins of Gondwana [34,35]. These OMRBs are characterized by high argillaceous and ferruginous content [27]. Nevertheless, comparative studies of OMRBs remain limited [25]. Significant variations are observed between different geological units in sedimentary characteristics, timing, and preservation form [25,26,28,33,36,37].

Among these characteristics, sedimentary structures are particularly notable. Compared to Cretaceous MRBs, OMRBs display a greater diversity of sedimentary structures. The proportions and morphologies of key components—such as breccias, argillaceous bands, and matrices—significantly influence the rock’s overall appearance, producing a variety of distinct patterns. Since the 1980s, researchers have used terms like “nodular”, “cracked”, “hoof-like”, “reticulate”, “augen-like”, and “worm-like” to describe the sedimentary structures of OMRBs in South China [47,48,49,50,51,52]. Among these, “nodular” and “cracked” are the most common. “Nodular” describes nodule-like structures within the limestone, while “cracked” refers to patterns resembling mud cracks or the veins on a turtle’s shell. However, these early names primarily described rock appearance rather than explaining origin. This focus on morphology has led to significant debate regarding how these structures formed [53,54,55,56]. Furthermore, the Ordovician period was a time of active tectonism around the Gondwanan margin [34,35,57,58], yet the potential influence of this tectonism on sedimentary structures is still poorly understood. Therefore, this study addresses two main objectives: (1) to classify these sedimentary structures based on their genetic origins, and (2) to interpret their formation by identifying key controlling factors. This study seeks to improve the precise description of OMRBs, aid in identifying their sedimentary environments, and provide more reliable comparative data for the widely distributed MRBs along the Gondwanan periphery.

2. Study Area

The OMRBs are widely distributed in South China. This study focuses on the Xiangxi area, located in the central Upper Yangtze Shelf (Figure 1b) [46,58]. During the Ordovician, this area occupied a low-latitude position south of the equator. Its paleogeography generally sloped from higher elevations in the northwest to lower ones in the southeast, with potential sediment sources from multiple directions, including the western and central Sichuan old land, central Guizhou old land, Cathaysian old land, and scattered underwater uplifts [59,60].

In the early Ordovician, the region was a stable carbonate platform. By the middle Ordovician, a suite of marine red beds had been deposited. In the Xiangxi area, these MRBs primarily occur in the Dawan and Kuniutan Formations (Figure 1c). The Dawan Formation consists of dark purple-red to brick-red, thick to very thick beds with high argillaceous content. It shows an abrupt color change from the underlying Honghuayuan Formation, which comprises grayish-white, thin to medium beds [29]. Fossils are rare in the Dawan Formation, with cephalopods and brachiopods being the main types. The Kuniutan Formation conformably overlies the Dawan Formation. It is characterized by purple-red, flesh-red, gray-green, and grayish-white, thick- to medium-bedded limestone with medium to high argillaceous content; the coloration becomes lighter upward. This formation is more fossiliferous than the Dawan Formation, containing cephalopods, brachiopods, trilobites, and conodonts. The Pagoda Formation overlies the Kuniutan Formation.

This study follows the lithostratigraphic scheme from the Dayong City regional geological survey report [61], which treats the Kuniutan and Pagoda Formations as being in direct contact (Figure 2). At the Sanbaidong section, the Dawan Formation is approximately 153 m thick, and the Kuniutan Formation is about 101 m thick. Based on conodont fossils, the red beds at the Sanbaidong section are from Oepikodus evae zone to Yangtzeplacognathus jianyeensis zone [27].

3. Research Methods

To protect the core geoheritage of the red stone forest, we selected the Sanbaidong section, located approximately 4 km from the core area, which provides complete exposure of the red bed sequence. Fresh rock samples lacking secondary veins and fractures were collected along the section. Given the generally low dip angle of the strata (mostly less than 10°), the stratigraphic order correlates with altitude. Rocks were initially fractured with a sledgehammer. Fresh, vein-free samples were then extracted from the fragments using a fossil hammer. Samples were labeled with the pinyin abbreviations of their lithostratigraphic units: HHY, DW, GNT, and BT, from bottom to top.

Since “nodular” and “cracked” are the most common and controversial terms in previous studies, we first conducted a comprehensive review of literature on South China OMRBs since the 1980s, focusing on the description and genetic interpretation of these two structures. Second, using the Sanbaidong section in the Upper Yangtze region as a case study, we performed detailed identification and characterization of sedimentary structures from base to top, comparing them with previous descriptions. To understand the temporal evolution of these structures, we also analyzed strata below and above the red beds to identify changing patterns. There are varying definitions of lithostratigraphic units across regions (Dawan, Kuniutan, Pagoda, and Zitai Formations). This study focuses specifically on the Dawan and Kuniutan Formations of the Sanbaidong section.

Research methods included field investigation, microscopic observation, classification and statistics, scanning electron microscopy (SEM), and geochemical analysis. We collected 105 hand specimens and prepared 45 thin sections from various sedimentary structures. Microscopic observation used the Olympus BX41 microscope at Geological Geomatics Institute of Hebei. SEM analysis was conducted at Wuhan Sample Solution Analytical Technology Co., Ltd., using a JXA-Ihp200F electron probe microanalyzer. For silicate minerals, operating conditions were set at 15 kV accelerating voltage and 2 × 10⁻⁸ A beam current, with spot sizes of ~1 μm for normal elements and ~3 μm for volatile elements; analyzed mineral grains were ≥3 μm. A segmented counting time strategy was applied: for elements with concentrations < 1000 ppm, peak and background counting times were 30 s and 15 s, respectively; for other elements, 10 s and 5 s. Natural mineral standards calibrated most elements, with pure metal standards (Au, Ag, Co, V) used for some sulfide elements. All data were corrected using the ZAF procedure provided by JEOL.

Geochemical analysis focused on key elements such as Ca, Si, Al, Fe, the nutrient elements P and Cu, and the redox-sensitive elements Ce and Th. Major elements were analyzed following the national standard GB/T 14506.28-2010, using the fused pellet X-ray fluorescence spectrometry (XRF) method, with specific procedures detailed in [65]. Major-element analyses were performed with an Axios XRF spectrometer (precision ~0.2%). Trace elements were analyzed according to GB/T 14506.30-2010, using an iCAP-Qc inductively coupled plasma mass spectrometer, with a precision better than 98%, following the procedures outlined in [66]. Details of the geochemical analysis procedures are provided in Supplementary Materials Table S1. The results of geochemical analysis are provided in Supplementary Materials Table S2. The carbon and oxygen isotope results and corresponding paleosalinity calculations were based on [27].

Additionally, vertical argillaceous bands in the Sanbaidong section and surrounding outcrops were statistically analyzed, with 120 groups of data in Xiangxi area and 213 individual directions in the Anhui area (Supplementary Materials Table S3). These data were used to discuss the genesis of argillaceous bands.

4. Results

4.1. Petrological and Petrographical Characteristics of OMRBs

At the Sanbaidong section, the OMRBs display a diverse lithology. The sequence transitions upward from the gray bioclastic limestone of the Honghuayuan Formation to the marine red beds of the Dawan Formation, marking a distinct and abrupt lithological shift. The red beds at this section, as well as in other regional outcrops, are typically darkest at their base. Within the red bed interval, the Dawan and Kuniutan Formations exhibit specific variations: the overall color lightens from dark red upward, with occasional interbeds of grayish-white and grayish-green layers, and the argillaceous content varies. The upper part of the Kuniutan Formation is characterized by a lighter reddish color, predominantly flesh-red, which gradually fades into the grayish-white limestone of the overlying Pagoda Formation.

4.1.1. Mineral Composition

Non-Red Beds (Honghuayuan Formation)

The primary lithologies include gray micritic bioclastic limestone and grayish-white recrystallized sparite. Thin-section analysis reveals that the limestone comprises bioclasts, intraclasts, matrix, and terrigenous clastics. Bioclasts, predominantly spinoderms and bivalves, are randomly distributed and cemented by calcite with minor silica. Intraclasts are mostly rounded, sand-sized (0.2~1 mm), and scattered; they consist primarily of muddy microcrystalline limestone. The matrix is predominantly micritic, with subordinate sparite cement. Both components are calcite and exhibit a xenomorphic granular texture.

Red Beds (Dawan and Kuniutan Formations)

The red beds comprise brecciated bioclastic micritic limestone with sandy silt, argillaceous microcrystalline limestone, and argillaceous bands (Figure 3). Their color varies from purple-red and brown to flesh-red. The main components include brecciated clasts, clay minerals, iron oxides, calcite, terrigenous clastics, and bioclasts. The brecciated clasts are composed of calcite, bioclasts, and terrigenous clastics. Calcite occurs as xenomorphic microgranular crystals (0.01~0.05 mm) with a tight interlocking texture and is partially altered to limonite. Echinoderm and ostracod bioclasts are scattered within the rock and cemented by calcite. Terrigenous clastics are dominated by silt-sized quartz, with minor amounts of feldspar and mica; these grains are predominantly angular to subangular. The proportion of brecciated material can exceed 50% in the lower part of the section.

Under SEM (Figure 4), calcite is the dominant mineral, followed by quartz, with minor hematite, titanomagnetite, biotite, muscovite, chlorite, and rutile. Quartz grains are irregularly distributed and range from 10 to 80 μm in size. Hematite occurs as irregular aggregates or euhedral crystals, both typically less than 20 μm in size. Chlorite is present as fine veins, appearing gray-green; it is more abundant in the upper part of the Kuniutan Formation.

4.1.2. Elemental Composition

A total of 355 samples from the Sanbaidong section were analyzed for their elemental composition (Supplementary Material Table S1). The results indicate that CaO and SiO₂ are the main components of the red carbonate rocks, excluding loss on ignition (LOI). The CaO content ranges from 13.27% to 51.63% (avg. 36.62%), while SiO₂ ranges from 3.89% to 45.42% (avg. 20.05%). From the base to the top of the section, the evolutionary curves of CaO and SiO₂ exhibit a mirror-image relationship (Figure 5). The total iron (TFe) content ranges from 0.63% to 6.47%, with Fe³⁺ ratios varying between 9.11% and 92.06% (avg. 70.52%). The Al₂O₃ content ranges from 1.08% to 14.02% (avg. 6.29%), and Sr concentrations vary from 146 ppm to 445 ppm (avg. 275 ppm). The nutrient elements P and Cu are enriched in the lower part of the section and decrease upward (Figure 5). The redox-sensitive elements Ce and Th show complex, fluctuating trends in both the lower and upper parts (Figure 5).

Overall, the OMRBs at the Sanbaidong section are characterized by “high Al, high Si, high Fe”. A sharp increase in silicon and aluminum components is observed at the base of the red bed sequence, correlating with its onset. In contrast, strontium content is very low at the base and increases upward, showing an inverse relationship with the red beds.

4.2. Subdivision of Sedimentary Structures

4.2.1. “Nodular” Structures

Previous descriptions of “nodular” structures are categorized into six subtypes in this study, each with distinct characteristics and genetic significance.

a.

Brecciated Structure (I-a)

Interpretation: Previous studies described “nodular” structures as consisting of “nodules” and matrix. Through field observations, this study confirms that the “nodules” are generally white and composed of bioclastic micritic limestone (Figure 3a), exhibiting a relatively uniform texture. The nodules are discrete, ranging in size from approximately 2 to 15 mm, although larger specimens exceeding 40 mm. Under the microscope, fossil boundaries within the nodules appear disrupted. The nodules and the surrounding matrix differ compositionally and exhibit sharp contacts. Pressure dissolution along some boundaries has led to recrystallization, forming growth rims. The matrix consists of micritic to microcrystalline limestone with relatively high clay and iron content. Microscopic examination reveals fractures of two types: primary and secondary. Primary fractures occur mainly within the breccia fragments and are filled with micritic calcite identical to the matrix, giving the fragments a fractured appearance. Secondary fractures are filled with sparry calcite. The breccia is predominantly matrix-supported, with a minor clast-supported portion; the breccia fragment content can exceed 50%.

This type of nodule displays a chaotic distribution, and the matrix shows no preferred orientation. Given their morphological resemblance to breccia fragments, this study classifies this subtype as a brecciated structure.

b.

Patchy Structure (I-b)

Interpretation: In previous studies, the white portions were described as “nodules” within a uniform matrix. These “nodules” range in size from approximately 3 to 40 mm. The matrix has characteristics similar to those of Type I-a, exhibiting high iron content and a distinctive bright red color. Thin-section analysis reveals that the boundaries between the “nodules” and the matrix are gradational, with no evidence of growth rims. Furthermore, fossil remains can be observed extending continuously from the nodules into the matrix (Figure 3b).

This type is markedly distinct from the brecciated structure (Type I-a), indicating a fundamentally different genetic origin. Therefore, it is inappropriate to group both under the broad term “nodular” structure. The key distinguishing feature of this type is the patchy, non-red appearance of the “nodules”. Consequently, this study classifies this category as a Patchy Structure.

c.

Brecciated–Patchy Structure (I-c)

Interpretation: This type represents a transitional form between the Brecciated Structure (Type I-a) and the Patchy Structure (Type I-b) (Figure 3c). Its defining characteristic is the relatively small size of the brecciated components, which range from approximately 0.5 to 3 mm. The patches can be classified into two morphological types: bedded and non-bedded.

The bedded patches exhibit features such as boudinage (sausage-like structures) or slight offsets along bedding planes. In contrast, the non-bedded patches display cross-bedding relationships or exhibit irregular morphologies in both plan and vertical views.

d.

Irregular Argillaceous Band Structure (I-d)

Interpretation: This type was previously described as consisting of grayish-white “nodules” within a red matrix, with the nodules exhibiting rounded and discontinuous morphologies at the macroscopic scale. However, both field and microscopic observations in this study reveal that its characteristic appearance is actually formed by the separation of micritic limestone by argillaceous bands (Figure 3d). These “nodules” are neither the product of transport under high-energy hydrodynamic conditions nor do they show gradational contacts with the matrix, indicating a genesis distinctly different from the types classified above. Numerous examples of this limestone type, controlled by argillaceous bands, are found in South China, such as the argillaceous banded limestone of the Late Devonian Wuzhishan Formation, which is widely interpreted as having formed in a slope environment.

Since the primary controlling factor of this subtype is the irregular development of argillaceous bands, this study terms it irregular argillaceous band structure.

e.

Argillaceous band–Patchy Structure (I-e)

Interpretation: Although the primary controlling factor of this type remains the argillaceous bands, it is also co-influenced by the presence of non-red patches. The development characteristics of the argillaceous bands are similar to those of Type I-d, predominantly appearing irregular or with a weak preferred orientation. The patches share identical characteristics with those in Type I-b, exhibiting irregular or banded shapes. The banded patches generally develop horizontally (Figure 3e), while the less common vertical patches exhibit cross-cutting relationships with bedding and display a certain degree of irregular dip.

f.

Mud shell Structure (I-f)

Interpretation: This type remains controlled by argillaceous bands but is typified by a distinct “mud shell” characteristic, where the bands enveloping the matrix form a thin, flaky layer approximately 0.1 mm thick, which is iron-rich and readily exfoliates (Figure 3f). Microscopic examination reveals that the calcite minerals within these “mud shell” structures exhibit a preferred orientation, indicative of stress compression—a feature absent in both Type I-a and Type I-b. Furthermore, the content of argillaceous bands in this type is exceptionally high, constituting over 80% of the rock volume by some estimates. Consequently, the enclosed matrix appears isolated, resembling “nodules” in form.

This study contends that this type differs significantly from the argillaceous banding described above, both macroscopically and microscopically. The observed mineral orientation likely indicates the influence of stress, justifying its classification as a distinct subtype termed mud shell structure.

4.2.2. “Cracked” Structures

“Cracked” structures are divided into four subtypes.

a.

Brain-Wrinkle Structure (II-a)

Interpretation: This type represents the primary subject of previous descriptions of “syneresis cracks”. Macroscopically, the rock exhibits a pattern resembling brain wrinkles or S-shaped waves. These ridges are typically 0.5 to 2 cm wide, are generally uniform, intersect at angles of approximately 120°, and have smooth junctions. The micritic limestone, enclosed by these brain-wrinkle patterns, forms irregular-shaped blocks measuring 5 to 15 cm in dimension, with most being 8 to 10 cm. Due to the compositional difference between the ridges and the micritic limestone blocks, the weathering surface of the rock appears uneven (Figure 3g).

Given that this sedimentary structure possesses a very distinct and uniform pattern of argillaceous ridges resembling brain wrinkles, this study designates it as a brain-wrinkle structure.

b.

Fish-Scale Structure (II-b)

Interpretation: This type exhibits a macroscopic morphology resembling “fish scales.” The ridges are notably protruding on the weathering surface, are relatively narrow, measuring approximately 0.2~0.5 cm in width (Figure 3h). The enclosed matrix consists of bioclastic micritic limestone, forming shapes transitioning from hexagonal to elliptical. These units are relatively uniform in size, occur in clusters, and display a distinct preferred orientation.

The characteristics of the preferred orientation in this type differ from those observed in Type II-a. Therefore, this study designates it as a fish-scale structure.

c.

Grid Structure (II-c)

Interpretation: The grid structure is primarily defined by argillaceous bands with two dominant orientations: horizontal and vertical. These bands intersect at high angles (often ~90°), forming a grid-like to rhombic pattern. The rock layers are generally medium- to thin-bedded. The horizontal bands are aggregates of argillaceous material, approximately 0.4~1.0 cm thick, while the vertical bands are about 0.1~0.3 cm wide; overall, the horizontal bands are thicker (Figure 3i). The enclosed matrix blocks typically range from 4 to 8 cm in diameter. This structure is common in the Kuniutan and Pagoda Formations, where rock colors are relatively light, mostly light red or grayish-white. Critically, the vertical argillaceous bands cross-cut the horizontal bands, indicating a later formation phase.

Due to its distinctive grid-like morphology and the clear cross-cutting relationship of the bands, this study classifies this type as a grid structure.

d.

Lens-shaped Structure (II-d)

Interpretation: This type exhibits relatively homogeneous lithology with high overall argillaceous content, a distinctly red color, and a turbid matrix that obscures the argillaceous bands (Figure 3j). The matrix forms somewhat rounded bodies approximately 5 cm in diameter. Macroscopically, these aligned bodies show a preferred orientation, resembling arranged “lenses”—a characteristic that becomes more evident upon physical breakage. The rock is exceptionally hard. Microscopic observation reveals abundant ferruginous and argillaceous material, with few fossils present.

This structure is defined by its preferred orientation and high content of terrigenous material. Its appearance and genesis are distinct from those of the subtypes described above. Therefore, this study classifies it as a lens-shaped structure.

5. Discussion

5.1. Reassessment of Previous Sedimentary Structure Genesis

5.1.1. “Nodular” Structure Formation Assessment

Previous hypotheses (Eh) for the origin of the “nodular” limestone in the Ordovician marine red beds (OMRBs) of South China include: Eh1: Differential compaction and pressure solution. Eh2: Submarine sliding or slumping. Eh3: Periodic bottom-current effects. Eh4: Intermittent subaerial exposure and desiccation cracking. Eh5: Wave breaking, transport, and subsequent cementation (details in Supplementary Material Table S4).

Among these, Eh1 and Eh2 involve gravitational sliding and sediment flow of pelitic materials. Eh3 proposes that nodules and the matrix formed under different hydrodynamic conditions. The primary evidence for Eh4 (exposure origin) includes “V-shaped desiccation cracks” and iron oxide halos around breccias. Eh5 is characterized by the presence of well-sorted and rounded coarse gravels.

Differential compaction causes high-calcium sediments to deform plastically in a semi-consolidated state under vertical stress [50]. Ideally, in muddy limestone, calcium would dissolve first and migrate outward under pressure [55]. However, carbon isotope values in nodules are approximately 0.7‰ higher than in the matrix [54], which has been interpreted as evidence for a cold seabed current origin of the dissolved sediment. This hypothesis is challenged by the similar oxygen isotope values measured in both nodules and the matrix [67]. The subaerial exposure hypothesis (Eh4) is considered unreliable because the OMRBs on the Yangtze Block contain abundant marine fossils, and geochemical data consistently indicate a marine depositional environment (Figure 5). Although localized exposure surfaces may exist, leading to stratigraphic hiatuses [68,69], it is inappropriate to attribute the widespread and persistent “nodular” structures in the Yangtze area solely to exposure. The wave-effect hypothesis (Eh5) implies a relatively shallow-water environment, where the “nodules” could represent near-source limestone deposits. This is visible in some layers, such as at the base of the Sanbaidong section.

5.1.2. “Cracked” Structure Formation Assessment

Previous studies of “cracked” structures have primarily focused on those in the middle to upper parts of the OMRBs (i.e., within the Pagoda Formation). These structures typically consist of argillaceous seams and the enclosed matrix blocks, exhibiting variable seam widths but generally smooth intersections. Proposed hypotheses for their formation include: Eh1: Subaerial exposure and desiccation. Eh2: Formation as a deep-sea hardground. Eh3: Differential loading. Eh4: Biogenic origin. Eh5: Subaqueous shrinkage. Eh6: Diagenetic or tectonic origin (details in Supplementary Material Table S5).

Hypothesis Eh1 is considered unsuitable for explaining the “cracked” structures in South China. Similarly to the case for “nodular” structures, most of the Upper Yangtze area was characterized by a marine environment from the Middle Ordovician onward. Hypothesis Eh2 posits that the dense segments of the “cracked” structures formed under conditions of extremely slow sedimentation rates [70]. However, the Pagoda Formation lacks typical hardground features, such as extremely thin beds, hardground cavities, and abundant organic matter [56]. Hypothesis Eh3 primarily attributes the structures to the formation of load casts by sediments of differing densities [53]. Some researchers, however, argue that this phenomenon is rarely observable in the field [71]. Hypothesis Eh4 suggests that basin migration dynamics created the specific ecological niches required by Ordovician nautiloids, ultimately resulting in a distinctive facies [72]. Nevertheless, the traces of biological migration during the Great Ordovician Biodiversification Event are remarkably consistent. Furthermore, the widespread distribution of this sedimentary structure makes it difficult to reconcile with a simple spatiotemporal model based solely on biological activity. Hypothesis Eh5 proposes that the lithology and paleontological assemblages associated with the “cracked” structures indicate deposition in a quiet, low-energy submarine environment [73], during a transition from marine transgression to regression [47]. According to this view, lime-mud sediments below the water-sediment interface dehydrated and shrank to form the cracks [74]. The fact that the crack fillings are consistent with the rock’s own argillaceous components [51] supports an origin from subaqueous shrinkage during early diagenesis. In contrast, Hypothesis Eh6 emphasizes the role of pressure-solution effects on “cracked” structures during the middle to late stages of diagenesis [56,75]. Field observations from this study indicate that “cracked” structures are not strictly controlled by lithological position and exhibit diverse macroscopic morphologies. Previous studies have also considered the influence of paleotopography and microfacies on these structures [71]. The observed morphologies are associated with different depositional environments, including quiet water, intertidal, and near-subtidal zones. Although tectonic activity around Gondwana was active during the Ordovician [34,35,57], manifested in the Upper Yangtze area as paleo-uplifts and an increase in parallel unconformities [76], its direct impact on the formation of these specific sedimentary structures remains unclear.

5.2. Interpretation of Sedimentary Structure Genesis in This Study

Each sedimentary structure is theoretically underlain by a unique genetic mechanism. This study analyzes the principal factors controlling the ten sedimentary structures, focusing on color, breccias, and argillaceous bands (Figure 6). These factors are categorized into three genetic end-members: breccias, patches (non-red components), and argillaceous bands. Since color is a ubiquitous characteristic of all marine red beds, it is discussed first.

5.2.1. Genesis of Patches

Coloring Mechanism

The dominant color of the Ordovician marine red beds (OMRBs) is red, with intercalations of gray-green and gray-white layers. These color variations are primarily controlled by the types and concentrations of specific minerals [12,77]. Spectral analysis confirms that hematite is the primary chromophore responsible for the red coloration [78]. Goethite may also contribute to the color under non-dehydrated conditions [28]. In the gray-green veins found at the top of the red bed sequence, chlorite is identified as the main color-influencing mineral (Figure 4). In sedimentary environments, nanoscale to microcrystalline hematite and goethite are more effective at imparting color than their coarse-grained counterparts [9]. The hematite in the Sanbaidong section is predominantly microcrystalline, a characteristic also reported from the same stratigraphic horizon in the Tarim Basin at the northern margin of Gondwana [78]. Data from drill hole 1049C in the North Atlantic demonstrate that sediments develop a red color when the hematite content exceeds 1.5% [9,17], providing further evidence that hematite is the causative agent for the red hue.

At the elemental level, the coloring mechanism also predominantly involves iron [29]. The Fe³⁺/TFe ratio is key determinants of carbonate rock color. When the TFe is low, iron oxides cannot dominate over the white background of CaCO₃. The coloring threshold has been proposed, with TFe content around 1.5% and a Fe³⁺/TFe ratio of approximately 30% required for noticeable coloration [29] (Figure 5 and Figure 7). In the lower part of the Sanbaidong section, the TFe content frequently exceeds 5%, and the Fe³⁺/TFe ratio reaches 85%~90% in the middle to lower intervals.

Sources of Coloring Elements

In the Sanbaidong section, iron and aluminum contents show a strong positive correlation (R² = 0.953) (Figure 7). Marine carbonates typically contain little inherent aluminum, which is primarily derived from terrigenous sources [79,80]. Although the depositional environment of the OMRBs in South China is debated, being interpreted as either shallow-water [69,81] or deep-water [12,82], the lower part of the Dawan Formation in the Sanbaidong section exhibits high concentrations of TFe and terrigenous elements like aluminum (Figure 5). Grayish-green and grayish-white interbeds in the middle-upper parts of the sequence mostly occur in layers with low aluminum content. Nutrient elements such as P and Cu are typically low in normal surface seawater. In the lower red beds, however, low salinity and a limited biological presence [27] resulted in reduced consumption of these elements by biological processes. Consequently, their concentrations are high in the lower part of the section and decrease upward (Figure 5). This trend contrasts with the correlation observed for the scavenged elements. Paleosalinity shows a strong positive correlation with Sr and a negative correlation with Al (Figure 5). Therefore, elemental analysis suggests a shallow-water origin for the red carbonates in the Sanbaidong section, an interpretation that will be further explored below from the perspective of sedimentary structures.

Differences in Patches

As established, the red coloration is derived from trivalent iron. Accordingly, the non-red patches are characterized by the absence of trivalent iron. Two distinct types of non-red patches are observed in the Sanbaidong section: one is stratiform (Figure 8a), and the other occurs along argillaceous bands without a preferred orientation (Figure 8b).

The stratiform patches are intrinsically linked to the sedimentary sequence of the red beds and are governed by redox conditions during deposition. In contrast, patches associated with argillaceous bands predominantly coincide with secondary veins. Microscopic examination reveals that these veins are primarily composed of sparry calcite (Figure 8c). The formation of these patches is interpreted as follows: after the red beds formed in an oxidizing environment, reducing fluids migrated along micro-fractures, altering the local microenvironment around these pathways. The occurrence of non-red patches along veins and penetrating fossils is indicative of this later-stage reduction by fluid activity.

Therefore, two distinct genetic models are proposed for patch formation: one controlled by primary syndepositional oxidation conditions, and the other by secondary vein formation associated with post-depositional reducing fluids.

5.2.2. Genesis of Brecciated Structures

Brecciated structures occur exclusively at the base of the Sanbaidong section, where the lithology abruptly transitions from the underlying gray-white limestone to the red beds. This interval contains storm-derived deposits and shallow-water indicators such as algal layers, along with allochthonous breccias (Figure 9), suggesting deposition under high-energy hydrodynamic conditions. The breccia clasts differ compositionally from the surrounding matrix, exhibit growth rims, and show evidence of fracturing, collectively indicating a history of transport and rounding prior to final deposition.

Conodont fossils provide a basis for estimating the sedimentation rate of the marine red beds (MRBs) in the Xiangxi area. The MRBs at the Sanbaidong section have a total thickness of approximately 254 m and were deposited between 471 and 458 Ma [27,83]. Accounting for potential unconformities, the calculated sedimentation rate exceeds 19.4 m/Ma. Specifically, the Dawan Formation is 153 m thick, yielding a sedimentation rate of about 80.5 m/Ma, whereas the overlying Kuniutan Formation is 101 m thick, with a lower sedimentation rate of approximately 9.02 m/Ma. For comparison, the sedimentation rate of the Pagoda Formation in the Sichuan Basin is 4.68~7.67 m/Ma [84], and the overlying Wufeng Formation has a much slower rate of 0.27~0.69 m/Ma [67,85,86] (

Table 1. Sedimentary rates of different strata of the Ordovician.

Region	Stratigraphic Unit	Lithology	Estimated Age (Ma)	Sedimentation Rate (m/Ma)	References
Sichuan Basin, Well WX-2	Wufeng Formation	Siliceous mudstone	Lasting ~0.73	~0.27	[85]
Sichuan Basin, Qijiang	Wufeng Formation	Carbonaceous shale, marlite	445.2~440.8	~0.69	[86]
Hunan, Daming	Shizipu + Pagoda Formation	Bioclastic limestone	Lasting ~30	~2.66	[70]
Sichuan Basin, Qiaoting	Pagoda Formation	Micritic nodular/vein limestone	Lasting ~4.17 (astronomical cycles)	~7.67	[84]
Sichuan Basin, Liangcun	Pagoda Formation	Same as above	Lasting ~5.64 (astronomical cycles)	~6.37	[84]
Sichuan Basin, Sanquan	Pagoda Formation	Same as above	Lasting ~5.05 (astronomical cycles)	~4.68	[84]
Hunan, Sanbaidong	Kuniutan Formation	Nodular limestone, etc.	469.4~458.2 (conodont fossils)	~9.02	This study
Hunan, Sanbaidong	Dawan Formation	Nodular limestone, etc.	471.3~469.4 conodont fossils)	~80.5
Hunan, Yongshun	Nanjingguan-Fenxiang	Bioclastic limestone	Lasting ~7	~46.8	[70]

). This marked difference indicates a relatively fast sedimentation rate for the MRBs, with the Dawan Formation exhibiting one of the highest rates recorded for the Ordovician period. This high rate is consistent with the global sea-level rise during the Ordovician. Regarding the water depth during the deposition of the upper red beds, studies indicate that the Pagoda Formation limestone lacks high-energy sedimentary structures such as cross-bedding, hummocky cross-stratification (HCS), swaley cross-stratification (SCS), tempestites, and scour structures. Its lithology is predominantly micritic limestone and bioclastic wackestone, lacking grain-supported fabrics or sparite cements. These characteristics point to a low-energy depositional environment indicative of relatively deep and quiet water conditions. The fossil assemblage, dominated by deep-water-type trilobites (e.g., the Cyclopyge fauna), small-shelled Follomena brachiopods, small thin-shelled ostracods, and well-preserved large orthoconic nautiloids (Sinoceras), further supports a low-energy, oxygen-deficient, and relatively deep-water setting. This assemblage corresponds to Benthic Assemblage (BA) 4–5, typically representing water depths between 80 and 120 m [67]. Consequently, the water depth during the deposition of the Pagoda Formation likely did not exceed 120 m. Integrating the evidence from sedimentation rates and paleobathymetry, the water depth for the OMRBs is estimated to have ranged from 10 to 120 m. In geological terms, shallow marine environments—representing the natural extension of continental shelves—are generally defined as depths down to approximately 130~200 m. Therefore, the OMRBs in the Xiangxi area are interpreted to have been deposited in a shallow-water environment.

From a paleogeographic perspective, structural subsidence in the late Early Ordovician was variable [87]. Compression from the southeast induced uplift and possible emergence of the Wuling Mountains [59,75], triggering a sea-level regression recorded in the Honghuayuan Formation and the development of a paleoweathering crust (5~30 cm thick) in the Jishou-Fenghuang area [75]. This tectonic activity increased sediment supply to the Dawan and Kuniutan Formations in the Guzhang-Zhangjiajie area, potentially altering sediment provenance [60]. The presence of storm deposits and algal limestones further supports this environmental shift. Moreover, lower δ¹⁸O values in the red carbonates and an increase in terrigenous material within the Dawan Formation suggest enhanced continental and atmospheric influence during early diagenesis [27]. This evidence aligns with the transition from a low-energy shallow marine environment during Honghuayuan Formation deposition to a tidal flat setting during Dawan Formation deposition [61].

Fossil abundance in the Sanbaidong section is low in the lower and middle parts of the Dawan Formation, as indicated by low conodont yields and rare nautiloid fossils. The first nautiloid fossil appears near bed DW-72 (Figure 5). In contrast, fossil abundance increases in the Kuniutan and Pagoda Formations. Given that nautiloids are mobile predators with a narrow salinity tolerance [72], the environmental conditions during the early red bed deposition were likely unfavorable for them.

Therefore, the brecciated structures in this section formed in a shallow-water environment characterized by strong terrigenous input. This serves as a different case in South China, correlating with facies changes from siltstone and shale.

5.2.3. Genesis of Argillaceous Bands

Syndepositional Argillaceous Bands

Argillaceous bands are generally associated with deeper-water and slope environments and form during sedimentation. Under certain pressure conditions, water-rich sediments dehydrate and contract around nuclei, forming irregular polygons with obtuse angles [52,88]. At the Sanbaidong section, these bands exhibit density variations along strike and mostly do not cross-cut bedding planes.

Studies have shown that the δ¹⁸O values of argillaceous bands in the Ordovician marine red beds are similar to those of the surrounding matrix [67] (Figure 10). Since δ¹⁸O is a sensitive indicator of diagenetic fluid influence and given that meteoric or syndepositional fluids can have δ¹⁸O values as low as –30‰, any interaction with such fluids can readily alter the original isotopic signature [89,90,91,92]. In situ analysis reveals that the δ¹⁸O values of both the bands and the matrix are stable at approximately −6‰, indicating a lack of significant post-depositional alteration by external fluids. The significant difference in δ¹³C between the argillaceous bands and the matrix may reflect biological activity, such as the influence of algae or other microorganisms on carbon cycling. Biological processes preferentially incorporate light carbon (¹²C), leading to its enrichment in clay-rich bands [93]. The consistent δ¹⁸O values between components, which align with bulk isotope data from the section [27], further support a syndepositional origin for the argillaceous bands. This conclusion is consistent with findings from the Huangnitang section in Zhejiang Province, where a similar isotopic trend was observed between nodules and the matrix, despite differences in absolute values [54]. In summary, multiple lines of evidence confirm that most argillaceous bands are primarily of syndepositional (primary) origin.

The brain-wrinkle structures may represent a distinctive case. Field observations show that their argillaceous bands branch at angles of approximately 120°, with no detectable density differences between branches (Figure 3g). The preservation of this geometric regularity during dehydration strongly indicates formation in a horizontally stable depositional setting. The sedimentary beds hosting these structures contain minimal terrigenous clastic material, consistent with deposition under highstand sea-level conditions. Furthermore, the absence of hurricane-induced disruptions in the paleoequatorial zone, as noted by [67], would have allowed the undisturbed accretion of CaCO₃ meganodules. These characteristics collectively identify the brain-wrinkle-like argillaceous bands as indicators of a low-energy, non-sloping paleoenvironment.

Modified Argillaceous Bands

The vertical argillaceous bands in the OMRBs of the Xiangxi area may have a complex, polygenetic origin. Given their distinct characteristics compared to bands formed purely by syndepositional processes, they are interpreted here as having a significant post-depositional component. At the Sanbaidong section, these vertical bands display the following key features: (1) They can vertically cross-cut bedding planes from top to bottom or vice versa; (2) Their mineralogical composition is identical to that of the horizontal bands; (3) They clearly truncate pre-existing horizontal bands; (4) They exhibit a consistent, oriented climbing direction; (5) These oriented bands are developed only in the middle-upper part of the section; (6) They do not disrupt or cut through fossils.

Several genetic hypotheses could account for these observations: (1) syndepositional gravitational differentiation, (2) post-depositional differential compaction, (3) modification by later tectonic stress, and (4) syndepositional compressional stress.

Detailed orientation measurements of vertical argillaceous bands around the Sanbaidong section yielded 120 data points, indicating a dominant orientation between 38° and 57° (Supplementary Materials Table S3). A similar directional trend has been documented in Guizhou Province [94] and Anhui Province (Supplementary Materials Table S3). While the first two hypotheses (gravitational differentiation and differential compaction) can explain the local preferred orientation of the bands, they cannot adequately account for their consistent orientation across such a broad regional scale.

Regarding the hypothesis of modification by later tectonic movements, some scholars have attributed similar features to the Late Yanshanian tectonic movement (J₃–K₁) [94]. However, carbonate rocks are highly prone to pressure dissolution and recrystallization. Crucially, neither recrystallization textures within these argillaceous bands (observed in thin section and SEM; Figure 3 and Figure 4) nor pressure-solution effects on fossils within the host rock have been identified. These observations make it difficult to reconcile the features with multiple episodes of orogeny and compressional uplift. Therefore, the characteristics are more consistent with an influence of syndepositional (penecontemporaneous) stress. This stress would have promoted the directional propagation and deformation of argillaceous bands or micro-fractures within the semi-consolidated lime mud, leading to a consistent expulsion direction for the water-rich argillaceous material (Figure 11). Although faults and joints related to the Yanshanian orogeny in this region share a similar orientation, the composition, mineralogy, and isotopic signatures of the argillaceous bands suggest that they are not products of later tectonic modification.

The syndepositional compressional stress is attributed to the Kwangsian Orogeny, an intraplate orogenic event that progressed during the Ordovician in the study area (Figure 12). First defined by Ding Wenjiang based on an angular unconformity between the Devonian Lianhuashan Formation and the underlying early Paleozoic “Longshan Series” in Xing’an, Guangxi, this orogeny is regarded as South China’s response to the Caledonian tectonic cycle [95]. It involved the interaction and eventual amalgamation of the Yangtze and Cathaysia blocks, establishing a fundamental tectonic framework for the region [96]. During the formation of the oriented argillaceous bands, the orogeny was in its incipient stages, not yet causing large-scale tilting or igneous activity. Nonetheless, non-destructive compressional stress transmitted from the margin of the South China Block could have influenced the stable Yangtze region, leaving a record within the carbonate rocks.

Intraplate orogeny is widely recognized as a far-field response to tectonic activity at plate margins [97,98,99]. For ancient orogenic events, however, the original plate boundaries are often obscured by subsequent tectonic reorganization, making it challenging to pinpoint the specific dynamic sources [100]. This explains the absence of direct evidence for the origin of the syndepositional compressional stress. Despite this limitation, a syndepositional compressional stress origin is strongly supported by three lines of evidence from this study: (1) compatibility with the regional tectonic style, (2) consistency with biozone migration patterns, and (3) alignment within chronological error margins.

The current focus and primary challenge in studying the Kwangsian Orogeny concern its dynamic mechanism and tectonic model [96,101]. Proposed driving forces include subduction of the ancient Japanese arc to the east [102], convergence with potential blocks to the southeast [103], interactions with the South China Sea and Australian blocks to the south [104], collision with the North Vietnam block to the southwest [105], or combined effects from these sources [101]. Petrographic evidence indicates that the Kwangsian Orogeny was directional and episodic, generally involving multi-stage tectonic events that propagated from the southeast to the northwest [106,107], characteristic of an intraplate orogenic process [101,108]. Consequently, the tectonic force likely originated from the southeast, potentially as a multi-source driver with southeastern dominance [101,109]. This SE-directed compressional stress couples effectively with the NE-trending paleogeographic underwater highs in the region. Such stress would generate NE-trending folds and tensile joints, providing migration pathways for primary argillaceous material within unconsolidated, plastic strata and ultimately leading to the formation of vertical argillaceous bands overlying horizontal ones.

Figure 12. Explanatory model of the Kwangsian Orogeny (modified after [106,108]). (a), The influence range of the Kwangsian Orogeny. The force directions on the South China Block may come from the directions indicated by the black arrows. (b), The migration direction of the biofacies is coupled with the southeastern potential block. Blue lines represent biofacies, brown lines represent Silurian coastlines, at this time the depositional center had migrated far to the northwest. The gray circle is the climbing direction of vertical argillaceus bands of sites in Sanbaidong section and Anhui Province (details in Supplementary Materials Table S3).

Figure 12. Explanatory model of the Kwangsian Orogeny (modified after [106,108]). (a), The influence range of the Kwangsian Orogeny. The force directions on the South China Block may come from the directions indicated by the black arrows. (b), The migration direction of the biofacies is coupled with the southeastern potential block. Blue lines represent biofacies, brown lines represent Silurian coastlines, at this time the depositional center had migrated far to the northwest. The gray circle is the climbing direction of vertical argillaceus bands of sites in Sanbaidong section and Anhui Province (details in Supplementary Materials Table S3).

Biostratigraphic studies [106,109,110,111] reveal a diachronous uplift across South China from the Late Ordovician to Early Silurian. The uplift initiated in the southeastern coastal areas (e.g., Taishan, Guangdong) during the Nemagraptus gracilis Zone (early Sandbian) and progressed northwestward, reaching the eastern Xuefeng Mountain area (e.g., Xinhua, Hunan) by the Cystograptus vesiculosus to Coronograptus cyphus zones (late Rhuddanian). This progression resulted in an angular unconformity between Devonian and underlying strata in the Pearl River Basin, and a paraconformity on the Yangtze Platform [101,106]. The migration pattern of biozones from the Cathaysia to the Yangtze region aligns with a northwestward weakening of the tectonic stress.

Chronologically, the Kwangsian Orogeny commenced around the Sandbian Age (~456 Ma). Associated magmatic activity spans from ~460 to 380 Ma, peaking at ~435 Ma [112,113,114], while metamorphic ages range from 450 to 420 Ma [101,115,116]. This timeline indicates that the onset of the Kwangsian Orogeny was nearly synchronous with the development of the vertical argillaceous bands near the top of the red beds, which are constrained by the conodont Yangtzeplacognathus jianyeensis zone (~458 Ma) (Figure 13).

Therefore, based on the consistent tectonic style, biozone migration pattern, and chronological alignment, we propose that the early-stage compressional stress of the Kwangsian Orogeny was a critical factor influencing the formation of the oriented argillaceous bands. This finding suggests a unique tectono-sedimentary coupling mechanism recorded within the red beds.

5.2.4. Causes and Statistics of Each Subtype

In summary, the key genetic elements for each subtype are consolidated in

Table 2. Key genetic elements of different subtypes in this study.

Previous Studies’ Descriptions	Subtypes in This Study	Key Genetic Elements
“Nodular”	Brecciated structure (I-a)	Strong hydrodynamics, shallow water environment
	Patchy structure (I-b)	1. Short-term reducing environment; 2. Reducing fluid
	Brecciated–patchy structure (I-c)	Transitional type between I-a and I-b
	Irregular argillaceous bands (I-d)	Sloping environment during deposition, Compaction and pressure dissolution
	Argillaceous band–patchy structure (I-e)	Transitional type between I-b and I-d
	Mud shell structure (I-f)	High argillaceous content, Compaction and pressure dissolution, compressive stress
“Cracked”	Brain-wrinkle structure (II-a)	Structural quiescence, quiet water, no slope
	Fish-scale structure (II-b)	Similarly to II-a but with regional compressive stress
	Grid structure (II-c)	Alternating lime-mud deposition, modified by regional compressive stress
	Lens-shaped structure (II-d)	Turbid, rich in terrigenous material

. The cause of subtype I-a is attributed to high-energy hydrodynamic conditions in a shallow-water setting, analogous to typical sedimentary breccias, with clasts likely derived from adjacent bioclastic limestones. In contrast, subtype I-b originates from two distinct mechanisms: short-term reducing conditions in primary settings, corresponding to stratiform patches; and the activity of reducing fluids along epigenetic veins, accounting for patches associated with veins or certain argillaceous bands. Subtype I-c represents a transitional form between I-a and I-b, while I-d formed syndepositionally on a slope. Similarly, I-e is transitional between I-b and I-d. Subtype I-f developed syndepositionally in argillaceous-rich settings, where subsequent compaction and pressure dissolution under stress produced a mud-shell fabric. Moving to type II structures, II-a formed during tectonic quiescence in quiet water below wave base, devoid of hurricane disturbance. II-b constitutes a special case of II-a wherein regional stress governed the dewatering direction and argillaceous band orientation. The formation of II-c involved two stages: initial development of carbonate-mud interbeds creating a “calcite-dominant and mud-dominant” heterogeneity, followed by diagenetic regional stress that generated vertical bands with a unified northeast climb direction, plausibly linked to northwest-directed compression and potentially triggered by the Kwangsian Orogeny. Finally, II-d formed in shallow, turbid, terrigenous-rich environments.

Based on their primary timing of formation within the sedimentary process (temporal attribute), the genesis of the ten sedimentary structure subtypes can be categorized into three sequential stages: primary deposition, diagenesis, and secondary alteration. Subtypes such as the brecciated, irregular argillaceous banding, and brain-wrinkle structures were predominantly formed during primary deposition. Argillaceous bands with a preferred orientation developed during the diagenetic stage, mainly in early diagenesis. In contrast, patches associated with later reducing fluids formed during the secondary alteration stage (Figure 14).

These sedimentary structures display a range of distinctive key characteristics that require careful attention during field identification. Accordingly, this study compiles and summarizes their field identification criteria, microscopic compositional features (as observed in the Sanbaidong section), and representative schematic diagrams (Figure 14). Although diagnostic criteria are provided to differentiate these subtypes, a comprehensive, multi-faceted evaluation remains essential in practical field investigations to accurately determine the nature of the sedimentary structures.

5.3. Linking to Sedimentary Environments

At the Sanbaidong section, the ten sedimentary structures display a distinct vertical distribution pattern (Figure 13). Breccia end-member structures are confined to the middle-lower part of the red bed sequence. In contrast, argillaceous band end-member structures occur in the middle-upper part and show increasing variety upward. So-called “cracked” structures are actually restricted to the Kuniutan and Pagoda Formations in the upper half of the succession, which is overlain by the black graptolite-rich shales of the Wufeng and Longmaxi Formations. Overall, a clear transition in the dominant controlling end-members is observed from the base to the top of the red bed interval.

These sedimentary structures also correlate with specific depositional environments. Type I-a structures formed nearest to terrigenous sources, influenced by storm processes. Structures related to argillaceous bands developed in relatively deeper-water settings, with subtypes like I-f, II-a, and II-b forming within this broader context. Directional features, such as those in grid-like and fish-scale structures, are associated with the inflection points of submarine folds, corresponding to paleotopographic highs or platforms. In local depressions, higher argillaceous content and pressure promoted the formation of mud-rich band-shell structures. Within the vast epicontinental seas fringing Gondwana, these sedimentary structures formed complex assemblages, exhibiting a far greater diversity than those found in Cretaceous oceanic red beds. Together, they define a distinctive sedimentary signature characteristic of shelf red beds (Figure 15).

Around the periphery of the Gondwana continent, shallow water environments also served as the primary setting for the development of OMRBs. In Australia, OMRBs include red dolomites, evaporites, siltstones, and limestones exhibiting mottling structures, all indicative of shallow-water deposition [39,40]. The Lachlan Orogen in New South Wales contains horizontally bedded and cross-bedded red siltstones and red siliceous shales [117]. In Normandy, France, red mudstones and conglomerates overlie Early Ordovician Cerisy-la-Salle sandstones (which contain Caradocian fossils), suggesting a continental or shallow-marine origin [42]. In North America, hematite-rich sandstones within the retroarc sequences of New York State are genetically linked to rapidly uplifted highlands [41,44]. Similarly, red limestones in southern Sweden contain shallow-water fossil assemblages comprising gastropods, echinoderms, and ostracods. Widespread unconformities spanning the Devonian-Ordovician boundary across many regions point to active tectonic phases along the Gondwanan margin [34,35,42]. Collectively, these examples demonstrate that the Ordovician marine red beds around Gondwana exhibit not only a vast geographical distribution but also a remarkable diversity in lithology and tectonic setting.

It is important to note that sedimentary structures in OMRB sections across South China are not uniform due to facies variations. Changes in microfacies can lead to significant macroscopic differences in sedimentary structures. From the Upper to the Lower Yangtze regions, key controlling factors include variations in water depth, the presence of submarine highs, redox conditions, and the distribution of biotic zones. The exceptionally high complexity of the OMRB sequences in South China underscores the substantial potential for future comparative studies.

6. Conclusions

This study systematically reviews and reinterprets the sedimentary structures within the Ordovician marine red beds (OMRBs) of South China, based on detailed analysis of the Sanbaidong section in the Xiangxi area. The principal conclusions are as follows:

(1)

A new genetic classification scheme is proposed, subdividing the sedimentary structures into ten distinct subtypes. These subtypes are categorized into three key genetic end-members: the breccia, patch, and argillaceous band end-members.

(2)

The genesis of these structures is elucidated through the three end-members. The breccia end-member indicates deposition in a shallow-water, high-energy environment. The patch end-member reflects control by syndepositional redox conditions and the later influence of reducing fluids. The argillaceous bands have a dual origin, forming either under primary depositional conditions or from syndepositional compressive stress.

(3)

The development of vertically penetrating argillaceous bands was controlled by syndepositional compressive stress, likely related to NW-directed compression during the early stages of the Kwangsian Orogeny.