Biostratigraphy and Microfacies of Upper Cretaceous Oceanic Red Beds in the Northern Tethyan Himalaya: A Case Study from the Zhangguo Section, Gyangze, Southern Tibet, China

Abstract

The Cretaceous oceanic red beds (CORBs) and their implications for “oceanic oxic events” have been widely studied by geologists globally. In southern Tibet, CORBs are extensively distributed within the Upper Cretaceous strata of the northern Tethyan Himalaya (NTH). A well-exposed, CORB-bearing, mixed carbonate–shale sequence is found in the Zhangguo section of Rilang Township, Gyangze County. The Chuangde Formation in this section is characterized by well-preserved CORBs, which include reddish shale, limestone, marlstone, and interbedded siltstone. These CORBs are stratigraphically overlain by the Jiabula/Gyabula Formation (predominantly shale) and underlain by the Zongzhuo Formation (“mélange”). However, the precise age, depositional environments, and regional/global correlations of these CORBs, as well as their implications for synchronous versus diachronous oceanic oxic events, remain to be fully understood. In this study, a comprehensive analysis of foraminiferal biostratigraphy and microfacies is conducted for the CORB-bearing Chuangde Formation and the upper Jiabula (Gyabula) Formation in the Zhangguo section. Five planktic foraminiferal biozones including Dicarinella asymetrica, Globotruncanita elevata, Contusotruncana plummerae, Radotruncana calcarata, and Globotruncanella havanensis are identified through detailed biostratigraphic analysis, confirming a Campanian age for the Chuangde Formation and its CORBs. These findings are broadly correlated with typical Upper Cretaceous CORBs in pelagic–hemipelagic settings across the NTH in southern Tibet. Nine microfacies and four facies associations are identified within the Upper Cretaceous strata of Gyangze and adjacent areas through field and petrographic analyses. Notably, it is indicated that planktic foraminiferal packstone/grainstone CORBs were deposited in outer shelf to upper slope environments, while radiolarian chert CORBs are inferred to have formed in deep-water, basinal settings below the carbonate compensation depth (CCD).

1. Introduction

Red beds, characterized by reddish hues due to iron oxides, are globally widespread sedimentary sequences formed under oxidizing conditions (Turner, 1980) [1]. Marine red beds (MRBs), found in marine settings, are distinct from continental red beds (CRBs) (Fischer and Arthur, 1977; Basu, 1982) [2,3]. MRBs are significant in sedimentology, paleoceanography, paleontology, paleoecology, and paleoclimatology, offering insights into Earth’s redox history and climatic shifts (Fischer and Arthur, 1977; Wang et al., 2005; Li et al., 2021) [2,4,5].

First identified in the 19th century in the Mediterranean, Europe, and North Africa (Lyell, 1833) [6], MRBs were initially described lithologically. Mid-20th century advances linked their red coloration to hematite (Fe₂O₃) and oxidizing conditions. The late 20th century saw MRBs integrated into studies of oceanic anoxic events (OAEs), carbon cycling, and mass extinctions (Jenkyns, 1976; Arthur et al., 1987) [7,8].

Since the 21st century, high-resolution geochemical techniques and numerical modeling have advanced MRB research, revealing their roles in paleoenvironmental reconstruction, plate tectonics, and biological evolution (Li et al., 2011; Rong et al., 2019) [9,10]. MRBs are categorized into oceanic (basin), deeper water (slope), and shallow water (shelf) types based on depositional environments (Wang et al., 2009; Li et al., 2021) [5,11].

Cretaceous oceanic red beds (CORBs), first identified in the Carpathians and eastern Alps (Štúr, 1860; Gümbel, 1861) [12,13], are globally distributed in the Atlantic, Pacific, Indian, and Tethyan Oceans (Wagreich and Krenmayr, 2005; Wang et al., 2005) [4,14]. Their formation is linked to Late Cretaceous oceanic oxidation, iron cycle changes, global cooling, and sea-level fluctuations (Mansour and Wagreich, 2022) [15]. CORBs mark the transition from anoxic to oxic marine conditions (Wang et al., 2011) [16].

In the northern Tethyan Himalaya (NTH), CORBs are exposed as reddish limestones, marlstones, shales, siltstones, and radiolarian cherts, with key outcrops in Yamdrok Lake, Gyangze, Kangmar, Sakya, Gyirong, and Zanda (Figure 1). These deposits, dated from late Santonian to early Paleocene, exhibit a progressive east-to-west age trend (Wan and Ding, 2002; Wan et al., 2005; Li et al., 2011) [9,17,18].

This study aims to (1) analyze the well-preserved CORB sequence in the Zhangguo section (Gyangze, NTH), (2) establish stratigraphic correlations with global CORB deposits, and (3) evaluate the synchroneity and paleoceanographic implications of Upper Cretaceous CORB deposition.

2. Materials and Methods

Detailed sedimentological study was carried out on the Upper Cretaceous strata, especially the CORBs, in the Zhangguo section (Coordinates 28°58′52″ N; 89°48′13″ E) of Rilang (Suokim/Suojin) town, Gyangze County, southern Tibet of China (Figure 2). A total of 112 samples of limestone, marlstone, shale, and silty sandstone (No. RB1 to RB112) were collected from the upper Jiabula (Gyabula), Chuangde (CORBs), and lowest Zongzhuo formations for biostratigraphic and microfacies analyses.

Biostratigraphic analysis primarily relied on planktonic foraminifera, which were extracted from marlstones and limestones using the grinding method, which involved cutting rock samples into thin sections. A total of 34 thin sections, with particular emphasis on obtaining axial sections of foraminiferal tests, were subjected to detailed taxonomic examination. All specimens and corresponding rock samples are permanently housed at the Fossil Identification Center, China University of Geosciences (Beijing) (CUGB). While the foraminiferal preservation in the Zhangguo section exhibits varying degrees of recrystallization (Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Figure 8, Figure 9 and Figure 10), the majority of specimens retain sufficient morphological integrity to permit reliable identification at both genus and species levels.

Microfacies analysis was primarily conducted using thin sections. Carbonate rocks were classified according to their textures, following the classification scheme of Dunham (1962) [21]. Sedimentological interpretations of microfacies were based on field observations and textural characteristics, as outlined by Wilson (1975) [22] and Flügel (1972, 2010) [23,24]. The studied microfacies consist predominantly of limestones and cherts from the Zhangguo section in Gyangze (Figure 11). These were compared with microfacies from the Daba section in Kangmar (NTH) to assess variability in microfacies and changes in microfossils within CORBs.

3. Results

3.1. Planktic Foraminiferal Biostratigraphy

Abundant and well-preserved foraminiferal fossils were recovered from the Chuangde Formation (CORBs) in the Zhangguo section. Planktic foraminifera constitute the dominant elements of the analyzed assemblages, with the section’s biostratigraphic framework primarily established through detailed foraminiferal analysis (Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Figure 8, Figure 9 and Figure 10). Key diagnostic species were reliably determined through examination of fossil axial sections and rotary profiles.

The planktic foraminiferal assemblages are dominated by the following genera: Archaeoglobigerina, Bifarina, Contusotruncana, Dicarinella, Dorothia, Heterohelix, Globigerinelloides, Globotruncana, Globotruncanella, Globotruncanita, Laeviheterohelix, Muricohedbergella, Planoglobulina, Planoheterohelix, Pseudoguembelina, Pseudotextularia, Radotruncana, Rugoglobigerina, Rugotruncana, and Schackoina. Most genera are typical of Late Cretaceous planktic foraminiferal fauna that were widespread in the Neo-Tethyan ocean. The co-occurred benthic foraminiferal genera include Ammobaculites, Cibicides, Lenticulina, and Orbitolina.

These genera and species show temporal changes in the Chuangde Formation (CORBs) (Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Figure 8, Figure 9 and Figure 10). Important bio-events (Figure 3) include (1) the last occurrence (LO) (S42) of D. asymetrica, (2) the first occurrence (FO) (S54) of C. plmmerae, and (3) the first occurrence (FO) (S82) and (5) last occurrence (LO) (S113) of R. calcarata.

The well-preserved planktic foraminifera assemblages recovered from the Chuangde Formation (CORBs) in the Zhangguo area of Gyangze were systematically analyzed and classified into five distinct biozones based on the established planktic foraminiferal zonal scheme of Petrizzo et al. [25]. The biozonal framework, comprising the Dicariella asymetrica, Globotruncanita elevata, Contusotruncana plummerae, Radotruncana calcarata, and Globotruncanella havanensis zones, was successfully applied to categorize the foraminiferal species identified within the study section, as illustrated in Figure 3. This zonation not only provides a robust biostratigraphic framework but also enhances our understanding of the paleoecological and paleoenvironmental conditions prevalent during the deposition of the Chuangde Formation (CORBs).

3.1.1. Dicariella asymetrica Zone

The Dicariella asymetrica Zone is a total range zone, defined by Postuma (1971) [26] as the stratigraphic interval of total range of the Dicarinella asymetrica. In Zhangguo, the top of this zone is defined by the LO of Dicariella asymetrica at S42, and the base is undetermined. Key planktic species include Archaeoglobigerina blowi, Bifarina bohemica, Contusotruncana fornicata, Dicariella asymetrica, D. concavata, Globigerinelloides ultramicra, Globotruncanita elevata, Globotruncanita stuartiformis, Muricohedbergella flandrini, Heterohelix moremani, and H. reussi. This zone is dated as Santonian in age.

3.1.2. Globotruncanita elevata Zone

The Globotruncanita elevata Zone is a partial range zone, defined by Postuma (1971) [26] as the stratigraphic interval with Globotruncanita elevata, from the last occurrence (LO) of the Dicarinella asymetrica to the first occurrence (FO) of Globotruncana ventricosa, which is later replaced by Contusotruncana plummerae according to Petrizzo et al. (2011) [25]. In Zhangguo, the base of this zone is determined by the LO of Dicarinella asymetrica at S43, and the top is defined by the FO of Contusotruncana plummerae at S54. Key planktic species include Archaeoglobigerina blowi, Bifarina bohemica, Contusotruncana fornicata, Globigerinelloides prairiehillensis, G. ultramicra, Globotruncana arca, G. bulloides, G. linneiana, Globotruncanita elevata, Globotruncanita stuartiformis, Muricohedbergella flandrini, Heterohelix moremani, H. reussi, and Radotruncana subspinosa. This zone is dated as early Campanian in age.

3.1.3. Contusotruncana plummerae Zone

The Contusotruncana plummerae Zone is an interval zone defined by Petrizzo et al. (2011) [25] as the stratigraphic interval from the first occurrence (FO) of the nominal taxon and the FO of R. calcarata. In Zhangguo, the base of this zone is delineated by the FO of C. plummerae at S54, and the top is determined by the FO of R. calcarata at S82. Key planktic species include: Archaeoglobigerina blowi, Bifarina bohemica, Contusotruncana fornicata, C. plummerae, Globigerinelloides prairiehillensis, G. ultramicra, Globotruncana arca, G. bulloides, G. linneiana, G. neotricarinata, Globotruncanita elevata, Globotruncanita stuartiformis, Muricohedbergella flandrini, Heterohelix moremani, H. reussi, Radotruncana subspinosa, Rugoglobigerina rugosa, Schakoina multispinata, etc. This zone is dated as middle Campanian in age.

3.1.4. Radotruncana calcarata Zone

Radotruncana calcarata (Cushman, 1927) [27], a globotruncanid planktonic foraminifer, is taxonomically debated, potentially assigned to Radotruncana (El-Naggar, 1971) [28], Globotruncanita (Reiss, 1957 [29]; emended by Robaszynski et al., 1984) [30], or Globotruncana. It is distinguished by its chamber spines (peripheral extensions of chambers along sutures by tapering tubolospines) (Longoria and VonFeldt, 1991; Wagreich, 2012) [31,32].

The Radotruncana calcarata Zone (TRZ, Herm, 1962) [33] is a globally recognized Late Campanian biozone, defined by the total range of R. calcarata. It spans low- to mid-latitude successions and is constrained to a brief interval (~806 kyr, 74.3/73.9–75.1/74.7 Ma; Longoria and VonFeldt, 1991) [31]. Initially misattributed to the Maastrichtian base in Tethyan sections (Caron, 1985) [34], it was later corrected to the Late Campanian (Schönfeld and Burnett, 1991) [35].

In Zhangguo, the base and top of this zone are marked by the FO of R. calcarata at S82 and the LO of R. calcarata in S113, respectively. Important planktic foraminiferal species include Archaeoglobigerina blowi, Bifarina bohemica, Contusotruncana fornicata, C. patelliformis, C. plicata, C. plummerae, Globigerinelloides prairiehillensis, G. ultramicra, Globotruncana arca, G. bulloides, Globotruncana falsostuarti, Globotruncana hilli, G. lapparenti, G. linneiana, Globotruncana lunaris, G. mariei, G. neotricarinata, G. ventricosa, Globotruncanita atlantica, G. elevata, G. pettersi, G. stuarti, G. stuartiformis, Muricohedbergella flandrini, Heterohelix carinata, H. cordites, H. globulosa, H. labellosa, H. moremani, H. navarroensis, H. planata, H. punctulata, H. reussi, H. striata, Laeviheterohelix dentata, Laeviheterohelix pulchra, Pseudoguemblina costulata, Pseudotextularia elegans, P. nuttalli, Pseudotextularia punctulata, Radotruncana calcarata, R. subspinosa, Rugoglobigerina rugosa, Schakoina multispinata, etc. The age of this zone is inferred as late Campanian.

3.1.5. Globotruncanella havaensis Zone

The Globotruncanella havaensis Zone is a partial range zone (PRZ) described by Caron (1978) [36], which spans the interval with Globotruncanella havaensis from the LO of R. calcarata to the first occurrence of Globotruncana aegyptiaca. In Zhangguo, the base of this zone is marked by the LO of R. calcarata at S113, and the top is undetermined. Important elements (planktic species) include Contusotruncana fornicata, C. patelliformis, Dorothia spheroidalis, Globigerinelloides prairiehillensis, Globotruncana arca, G. bulloides, G. forsostuarti, G. lapparenti, G. linneiana, G. ventricosa, Globotruncanita stuarti, G. stuartiformis, Globotruncanella havaensis, Heterohelix carinata, H. globulosa, H. navarroensis, H. reussi, H. striata, Laeviheterohelix dentata, L. pulchra, and others. The age of this zone is inferred as late Campanian.

Based on the foraminiferal biostratigraphy, the age of the CORBs of the Chuangde Formation in the study area should be within the Campanian.

3.2. Microfacies Analysis

In this study, detailed microfacies analyses were performed on the Campanian strata, with a particular focus on the CORBs, exposed at the Zhangguo section in Gyangze and the Daba section in Kangmar. These sections are representative of the typical CORBs within the northern Tethyan Himalayan zone. Through comprehensive petrographic examination and paleontological analysis of thin sections, eight distinct microfacies (MF) were distinguished from the study sections (Figure 11 and Figure 12).

3.2.1. MF1: Peloidal Marlstone

This microfacies is characterized by the presence of peloids, which are predominantly small (<0.2 mm), sub-rounded to sub-angular microcrystalline grains. Genetically, these peloids are interpreted as fragmented pieces of carbonate mud, essentially small intraclasts. However, due to their diminutive size (<200 μm), they are classified within the peloid category (Figure 11A; S90). The poor sorting and the variability in grain morphology (ranging from sub-rounded to angular) suggest that these grains likely originated from the reworking of weakly lithified carbonate mud. The matrix, composed of lime mud and clay, indicates deposition in a low-energy environment. The peloids, although derived from higher-energy settings, were transported and deposited as suspension particles. Such depositional conditions are consistent with slope–basin environments situated in front of the shelf margin.

3.2.2. MF2: Laminated Packed Wackestone

This microfacies is characterized by red laminated limestone, with laminae thickness ranging from 0.3 to 0.5 mm (Figure 11M,N). The red layers are rich in iron, primarily composed of hematite (Fe₂O₃), while the gray layers consist mainly of planktonic foraminiferal bioclasts and micritic calcite. This microfacies is predominantly observed in the lower (e.g., Figure 11M, sample S53) or upper (Figure 11N, sample S91) intervals of the Chuangde Formation in the Zhangguo section. The modern red iron-bearing carbonate mud in the Atlantis II Deep of the Red Sea is formed by the mixed deposition of hydrothermal iron and pelagic calcareous planktonic remains, with its depositional environment characterized by hydrothermal activity and oxidizing conditions. Based on lithofacies characteristics and modern sedimentary analogs, it is inferred that the formation environment and mechanism of the red laminated limestone in this microfacies are similar, likely representing an oxidizing deep-sea basin environment. The iron source may be related to submarine hydrothermal activity or terrigenous input, and the periodic alternation of laminae reflects cyclical changes in redox conditions or sedimentation rates.

3.2.3. MF3: Bioclastic Wackestone

This microfacies is characterized by a matrix-supported texture, with bioclasts comprising 10–30% of the rock volume, including planktonic foraminifers (Figure 11C,D). The abundance and composition of clasts exhibit significant variability at the centimeter scale. The heterogeneous sizes and shapes of bioclasts, combined with the matrix-supported fabric, indicate deposition in low-energy environments, likely below the fair-weather wave base or storm wave base. Within the study area, the Cretaceous oceanic red beds (CORBs) of the Chuangde Formation, which are characterized by interbedded mudstone and limestone (e.g., samples S95 and S115), are representative of this microfacies.

3.2.4. MF4: Loosely Packed Wackestone

This microfacies shares similarities with MF3 but is distinguished by a higher proportion of bioclasts (30–50%) (Figure 11E; sample S85) and is typically expressed as thicker beds in outcrop. The texture is matrix-supported, with the interstitial space between grains often being smaller than the size of individual clasts. These characteristics suggest deposition in low-energy environments, likely with a significant supply of bioclasts derived from the upper water column.

3.2.5. MF5: Planktonic Foraminiferal Packstone

This microfacies is characterized by a grain-supported texture, predominantly composed of identifiable planktonic foraminifers (Figure 11G,J). The interstitial matrix consists of muddy material and microcrystalline cements, which account for approximately 25–50% of the rock volume (e.g., samples S110, S88). Based on its textural and compositional features, this microfacies is interpreted to have been deposited in either protected environments behind the shelf-margin–shoal complex or in upper slope settings adjacent to the shoal complex.

3.2.6. MF6: Planktic Foraminiferal Grainstone

This microfacies is characterized by a grain-supported texture with microcrystalline calcite cement. The clasts are predominantly densely packed planktic foraminifers, accounting for >75% of the rock volume (Figure 11B,I). Notably, Figure 11I displays distinctive “pane fabric” features. This microfacies is predominantly observed in the lower (e.g., sample S45; Figure 11I) and upper (e.g., sample S112; Figure 11B) intervals of the Chuangde Formation in the Zhangguo section, exhibiting a distinctive red coloration. Based on its textural and compositional attributes, this microfacies is interpreted to have been deposited in high-energy environments, likely within a shelf margin shoal complex or as grain flow deposits in slope settings.

3.2.7. MF7: Radiolaria-Bearing Chert

This microfacies is characterized by a matrix of red cryptocrystalline quartz, with clasts predominantly composed of radiolarian tests (Figure 11P). The abundance of radiolarian clasts exhibits significant variability, ranging from approximately 1% to 30%. This microfacies is identified in the middle and upper Chuangde Formation of the Daba section in Kangmar, as well as in the upper part of the Chuangde Formation in Gyangze (Figure 12). The presence of this microfacies is indicative of a deep-water basin depositional environment, consistent with the pelagic conditions associated with the accumulation of radiolaria-bearing chert.

3.2.8. MF8: Radiolarian Chert

The matrix consists of red cryptocrystalline quartz, densely populated with radiolarian tests. This radiolarian chert is predominantly composed of planktonic radiolarians with siliceous skeletons, which are characteristic of deep-water environments located below the carbonate compensation depth (CCD) in the ocean. The radiolarian clasts are highly abundant, typically exceeding 30% and in some cases reaching up to 75%. This microfacies is identified in both the lower and upper Chuangde Formation of the Daba section in Kangmar (Figure 11Q), as well as in the upper part of the Chuangde Formation in the Zhangguo and Chuangde sections in Gyangze (Figure 12). These occurrences strongly corroborate the interpretation of a deep-water pelagic depositional environment for this microfacies.

3.2.9. MF9: Lithoclastic Wackestone/Floatstone

The matrix is predominantly composed of micrite, within which densely packed lithoclasts (floats) are embedded. Lithoclasts (floats) constitute over 60% of the rock volume, as evidenced by sample S104. Pressure solution stylolites, marked by arrows, are concentrated around the lithoclasts (floats) and are embedded within the micritic matrix. The fabric exhibits sutured laminated to sutured packed textures, indicative of significant compaction and diagenetic processes. This microfacies is observed in the upper part of the Chuangde Formation in the Zhangguo section in Gyangze.

The presence of lithoclastic wackestone/floatstone, characterized by high lithoclast content and pressure solution features, suggests a depositional environment influenced by gravitational transport and reworking of carbonate material. This is consistent with a deep-water slope setting, where sediment gravity flows, such as debris flows or turbidity currents, are common. The sutured fabric further supports a depositional environment subjected to significant overburden pressure, typical of slope or basinal settings. These features collectively reinforce the interpretation of a deep-water slope depositional environment for this microfacies.

Based on the microfacies characteristics (e.g., planktic foraminiferal packstone and grainstone) and microfossil assemblages (e.g., calcimicrobes, abundant planktic foraminiferal assemblages, silicimicrobes, and monotonous to paucispecific radiolarian assemblages), the planktic foraminiferal packstone and grainstone are interpreted to have been deposited in outer shelf to upper slope environments, proximal to the shelf margin shoal complex. In contrast, the radiolarian cherts are inferred to have accumulated in deep-water, oceanic basin environments situated below the carbonate compensation depth (CCD). This distinction highlights the contrasting depositional settings associated with these microfacies, reflecting the transition from shallower shelf-slope systems to deeper pelagic basins.

4. Discussion

The Chuangde Formation (CORBs) is a widely distributed stratigraphic unit in the northern Tethyan Himalaya that contains abundant planktic foraminiferal and/or radiolarian fossils. Biostratigraphically, the Chuangde Formation in southern Tibet has been previously assigned to a range from upper Santonian to Campanian/Maastrichtian, but the age of its topmost part is still in dispute (e.g., Wan et al., 2005; Li et al., 2011) [9,18].

4.1. Facies Associations and Depositional Environments

Based on the integration of field observations and detailed microfacies analyses, the lithofacies of the upper Jiabula (Gyabula) Formation and the Chuangde Formation can be systematically classified into four distinct facies associations, (1) lower slope–basin, (2) lower slope, (3) upper slope, and (4) upper slope–shelf margin, as illustrated in Figure 12 and Figure 13.

The lower slope–basin facies association consists mainly of shale and marlstone (MF1), with occasional siltstone or radiolarian chert (MF7–8) interbeds (Figure 12). Within the upper Jiabula Formation (S1 to S25), this facies association is typified by black shales interbedded with cherts, which we interpret as indicative of deposition in basinal environments (Kneller and Branney, 1995; Stow and Johansson, 2000) [37,38]. This facies association is also identified in the upper (S94 to S97 and S106 to S109) Chuangde Formation (Figure 12 and Figure 13A–C), where it is manifested by laterally extensive, parallel-laminated red to cherry red shales, peloidal marlstone (MF1), and radiolarian cherts (MF7–8). The presence of radiolarian cherts (MF7 and MF8) in the upper Chuangde Formation may indicate basinal environments below the carbonate compensation depth (CCD).

The lower slope facies association is predominantly characterized by bioclastic wackestone with variable clast content (MF3–4), interbedded with shale layers, slump blocks, and breccias composed of foraminiferal grainstone (MF6). These facies exhibit a distinctive red to cherry red coloration and are primarily observed in the lower Chuangde Formation (Figure 12 and Figure 13C). In contrast to the basin facies, this association is marked by a higher proportion of carbonate components and, more significantly, the presence of slump blocks and breccias composed of grainstone. The foraminiferal grainstones are interpreted as having originated from the shelf margin and subsequently transported into slope environments via submarine slides or sheet flow deposits (e.g., Mullins and Cook, 1986; Tucker and Wright, 1990) [39,40,41]. Notably, the slump blocks and breccias are relatively thin and infrequent, leading us to infer that they were deposited in distal or lower slope environments.

The upper slope facies association is predominantly characterized by bioclastic wackestone (MF3 and MF4) and slump blocks exhibiting brecciation (MF6), which are prominently observed within the middle-upper Chuangde Formation (Figure 12). The slump deposits display partial brecciation at their basal and frontal regions, while internally, they reveal syn-depositional folding structures (Figure 13F). Despite the presence of localized deformations, the slump units exhibit significant lateral continuity, which enables them to be consistently traced across the outcrop scale. These features are interpreted as indicative of submarine slide events originating from the shelf margin, occurring within proximal or upper slope environments (e.g., Cook and Mullins, 1983; Mullins and Cook, 1986) [39,40,42].

The upper slope–shelf margin facies association is found in the near-top Chuangde Formation (S113 to S115; Figure 13A) and the basal Zongzhuo Formation (Figure 12 and Figure 13C). In the upper Chuangde Formation, it primarily consists of foraminiferal wackestone (MF4) and packstone (MF5), with occasional internally deformed slump blocks (MF6). Alongside abundant planktonic foraminifera, a considerable number of benthic foraminifera (e.g., Eouvigerina aculeata, Exsculptina cf. jacobi, Lenticulina sp., Miliammina sp., Neoflabellina sp., and Orbitolina sp.) are present in both wackestones and packstones. The in situ packstone beds with benthic foraminifera suggest deposition near the shelf margin, close to the fair-weather or storm wave base, while the slump blocks indicate upper slope environments where semi-consolidated sediments slid, fragmented, and moved basinward (Cook and Mullins, 1983) [42]. In the basal Zongzhuo Formation, this facies association is characterized by megabreccias (Figure 13C). The clasts in these megabreccias are composed of foraminiferal grainstones (MF6), but the megabreccias are matrix-supported, poorly sorted, and disorganized, often with erosional contacts with underlying shale or chert (e.g., Figure 13C). These megabreccias likely represent debris flow deposits in shelf margin–upper slope environments, formed by episodic shelf margin collapses (e.g., Mullins and Cook, 1986) [39,40].

4.2. Distribution and Age of the CORBs

The Late Cretaceous oceanic red beds (CORBs) are prominently developed across the North Tethys Himalaya (NTH) in southern Tibet, China, with discontinuous outcrops spanning from east to west in the regions of Yamdrok, Gyangze, Kangmar, Angren, Gyirong, Zhongba, Zhada, and Zanskar (Figure 14). Regional stratigraphic correlations, based on lithologic associations and fossil assemblages (primarily planktic foraminifera and radiolaria), indicate that the ages of CORBs in this region exhibit significant spatial variability. Specifically, the CORBs in the Gyangze region are dated to the late Santonian to Campanian (Wan et al., 2005) [43], while those in the Kangmar region span from the late Santonian to Maastrichtian (Li et al., 2011) [9], whereas in Gyirong, they extend from the Campanian to the Paleocene (Wan and Ding, 2002; Wan et al., 2005; Wang et al., 2017) [17,18,43,44], and the Zhada region records CORBs of early Paleocene age (Wan et al., 2005) [18].

A notable trend emerges in the age distribution of CORBs across the NTH, with a general tendency for younger ages towards the east. For instance, in Zanskar, Kangmar, and Gyangze, the CORBs are primarily Campanian, while in Gyirong and Zhada, they range from Maastrichtian to early Paleogene (Figure 14).

The depositional environments of the CORBs were traditionally interpreted as being confined to continental slope and deep basin settings, with their absence in shelf or shelf margin environments being widely accepted (e.g., Wan et al., 2005; Wang et al., 2005; Li et al., 2011) [4,9,18,43]. However, our study in the Gyangze region and adjacent areas reveals that CORBs are not limited to deep marine settings but instead occur across a broad spectrum of depositional environments, ranging from deep basins to slopes and even shelf settings (e.g., Figure 12). This finding aligns with the recent identification of shallow-marine, purple limestone beds within the upper Campanian in the Guru area of Yadong (Li et al., 2021) [5], which is tectonically situated within the Southern Tethys Himalaya (STH) (Figure 14).

Paleogeographic reconstructions further demonstrate that Campanian-aged CORBs were geographically extensive, spanning from shelf to basin environments (Figure 15). However, we emphasize that the thickness and temporal duration of the CORBs in different paleogeographic settings likely exhibit significant variability. This variability is primarily controlled by the extent to which Fe(II)-rich (or ferruginous) seawater infiltrated oxic depositional environments, highlighting the complex interplay between geochemical conditions and depositional processes during the Late Cretaceous.

5. Conclusions

This study presents a comprehensive analysis of foraminiferal biostratigraphy and microfacies from the Cretaceous oceanic red beds (CORBs) exposed in the Zhangguo section of Gyangze, southern Tibet, China. Five planktic foraminiferal biozones including Dicarinella asymetrica, Globotruncanita elevata, Contusotruncana plummerae, Radotruncana calcarata, and Globotruncanella havanensis were identified within the Chuangde Formation, confirming a Campanian age for these deposits (CORBs). Additionally, nine distinct microfacies were recognized, providing further insights into the depositional environments and processes. Integrating detailed field observations with microfacies analyses propose that the CORBs in the Gyangze region were deposited across a spectrum of environments, ranging from deep basin to slope and extending to shelf margin settings. Comparative analysis of the Gyangze CORBs with those from other sections in the Tethys Himalaya and globally suggests that many Campanian CORBs, particularly those in deep-sea environments, may have been deposited synchronously.