## 2 Materials and Methods  
Detailed sedimentological study was carried out on the Upper Cretaceous strata, especially the CORBs, in the Zhangguo section (Coordinates 28\({}^{\circ}\)58\({}^{\prime}\)52\({}^{\prime\prime}\) N; 89\({}^{\circ}\)48\({}^{\prime}\)13\({}^{\prime\prime}\) 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  
Figure 2: (**A**) Palaeogeographical location of the studied area (red dot) during the Late Campanian (\(\sim\)73.8 Ma). The base map is after Scotese (2021) [20]. (**B**) Satellite image of the studied area in the vicinity of Xigaze, southern Tibet. (**C**) Detailed geographical map showing location of the studied section (**D**) in Zhangguo, eastern Gyangze County (dash lines indicate the walking track). (**D**) Overview of the measured section covering the upper Gyabula, Chuangde, and lower Zongzhuo formations. at the Fossil Identification Center, China University of Geosciences (Beijing) (CUGB). While the foraminiferal preservation in the Zhangguo section exhibits varying degrees of recrystallization (Figures 3-10), 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 [PERSON] (1962) [21]. Sedimentological interpretations of microfacies were based on field observations and textural characteristics, as outlined by [PERSON] (1975) [22] and [PERSON] (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. Figure 3: Stratigraphic column and ranges of planktic foraminifer species of the Upper Cretaceous strata in the Zhangguo area. Note that four foraminiferal biozones including _Globotruncanita elevata_, _Contusotruncana plummerae_, _Radotruncana calcarata_, and _Globotruncanella havanensis_ zones were identified from the study area. Note: List of taxa cited with author attributions and year of publication see Appendix A.  
Figure 4: Planktic foraminiferal species observed in thin sections from the Zhangguo section of Rilang, Gyangze County, southern Tibet of China. (**1–3**) Heterobelix moremani (110); (**4**) Heterobelix navarroensis (115); (**5–14**) Heterobelix reussi (110); (**15–21**) Heterobelix carinata (**15–20**) 110; **21**. 115); (**22**) Laeviheterobelix pulchra (115); (**23**, **24**) Heterobelix globulosa (112); (**25**) Dorothia spheroidalis (115); (**26**) Dorothia? sp. (110); (**27**, **28**) Heterobelix cordites (**87**); (**29**) Heterobelix punctulata (**110**); (**30**, **31**) Heterobelix carinata (**110**); (**32–34**) Heterobelix plantata (**33**) 110; **34**. 103); (**35**) Heterobelix la bellosa (95); (**36**) Heterobelix cf. navarroensis (103); (**37**, **38**) Heterobelix globulosa (107); (**39**, **40**) Heterobelix moremani (107; 110). White scale bar = 50 \(\upmu\)m.  
Figure 5: Biserial planktonic foraminifera from the Zhangguo section of Rilang, Gyangze County, southern Tibet of China. (**1–3**) _Pseudotextularia nuttalli_ (**1**. 110; **2**, **3**.