关键词: 白垩纪, 古近纪, 新特提斯, 重大地质事件, 微体古生物, 生物地层, 西藏南部

Abstract:

The Cretaceous-Paleogene is an important geological period, during which a series of major geological events occurred, including oceanic anoxic events (OAEs), Cretaceous oceanic red beds (CORBs), large igneous provinces (LIPs), Cretaceous/Paleogene biota mass extinction, Paleocene-Eocene thermal maximum (PETM) as well as the collision between Indian and Asian plates, and the closing of the Neo-Tethys seaway. The study of these major geological events is helpful for gaining a better understanding of paleoceanography, paleogeography, and the paleoenvironment. Tracking and capturing these major geological events and reconstructing paleogeography requires the establishment of an accurate chronostratigraphic framework, for which paleontological study is an important tool. A set of well-preserved marine sedimentary sequences of the Cretaceous-Paleogene was discovered in southern Tibet, which well recorded the evolutionary processes of the Neo-Tethys and the India-Asia collision. Additionally, the evolutionary process of organisms during the major changes of geological environment can also provide evidence for the co-evolutionary relationships among climate, environment and biology during extreme environmental changes. In the past ten years, we conducted a systematic paleobiostratigraphic study of the Cretaceous-Paleogene marine strata in south Tibet, on which this paper reviews the research progress.
Detailed biostratigraphic studies of two important global oceanic anoxic events, OAE 1d and OAE 2, were carried out in Gyangze, Gamba, and Zanda. Ninety-three species of 43 radiolarians were identified and they were divided into five radiolarian zones. The radiolarian fauna was significantly affected by OAE 1d at the Albian/Cenomanian boundary and OAE 2 at the Cenomanian/Turonian (C/T) boundary. Systematic studies of the Cretaceous foraminiferal biostratigraphy were carried out in Gamba and Zanda, from which 142 species from 65 foraminiferal genera were identified and they were divided into 19 planktonic foraminiferal zones. Biostratigraphic study of the foraminiferal showed that the Lengqingre Formation of the Qiangdong area in Gamba crosses the Cenomanian/Turonian boundary, marked by the first appearance of Helvetoglobotruncana praehelvetica. Global oceanic anoxic events, including a significant planktonic and benthic foraminiferal extinction event (OAE 2) occurred at the Cenomanian/Turonian boundary. When OAE 2 occurred, the dissolved oxygen content in the bottom waters significantly decreased, and the plankton-dwelling foraminifera Rotalipora in deeper waters gradually disappeared at the top of the R.cushmani Zone, while R.cushmani disappeared at the top of the R.cushmani Zone. After OAE 2, from the lower part of the H.helvetica Zone, fauna diversity and abundance began to increase. The foraminiferal extinctions found in southern Tibet closely resembled those found elsewhere in the world in time and scale, with plankton and many benthic species as the main victims.
Recent research progress in 5 study areas are summarized below. (ⅰ) Progress in the biostratigraphic study of the Cretaceous ocean oxygen-rich events. The oceanic red beds are widely distributed in the global oceans throughout the Phanerozoic, mainly after the oceanic anoxic events. They represent typical oxygen-rich sediments and play an important role in marine scientific research. Over the past years, we have been continuously studying the micropaleontology of the CORBs including mainly foraminifers, radiolarians, and ostracods. The results showed that the ages of the CORBs in southern Tibet are of obvious diachroneity, which are the late Santonian to Campanian in Gyangze, the Campanian in western Sagya, the Maastrichtian farther west in Saga and Gyirong, the early Paleogene in Zanda in westernmost Tibet, the late Santonian to early/late(?) Maastrichtian in southern Kangmar, and the middle Campanian further south in Yadong. During the oceanic oxygen-rich period, organisms showed characteristics of extreme prosperity.
(ⅱ) Progress in the K/Pg boundary biostratigraphic research. Based on the study of foraminifers from the Cretaceous-Paleogene marine strata in Xishan, Gamba, 112 species of 48 foraminiferal genera were identified, which included 65 species of 36 benthic foraminiferal genera and 47 species of 12 planktic foraminiferal genera, and 13 foraminifera zones in four sedimentary stages were recognized (TLK1-2, TP1-11). The results suggested: (1) An obvious foraminiferal extinction-recovery event occurred near the K/Pg boundary. The planktic foraminiferal Globotruncanca fauna and the larger benthic foraminiferal Lepidorbitoides, Omphalocyclus and Orbitoides, which once flourished in the Late Cretaceous, were almost extinct near the K/Pg boundary, and new foraminiferal fauna, represented by small benthic foraminifera such as Ranikothalia, Daviesina and Lockhartia, appeared in the early Paleogene. (2) The occurrence of larger benthic foraminifers, such as Alveolina, in East-Tethys was about 1 Ma later than in West-Tethys, supporting the hypothesis that benthic macroforaminifera migrated from west to east.
(ⅲ) Progress in biostratigraphic studies of the PETM event. The benthic foraminifera biostratigraphic studies were carried out in the Paleogene Zongpu section of Gamba. Thirty-four species from 28 benthic foraminiferal genera were identified and they were divided into three foraminiferal assemblages. The results showed that the Paleocene/Eocene(P/E) boundary should be placed between the Zongpu and Zhepure Formations, marked by the extinction of two genera, Miscellanea and Operculina, below the P/E boundary, and by the emergence and prosperity of two genera, Alveolina and Nummulites, above the boundary. The benthic foraminifera in this section exhibited the evolutionary characteristics of extinction and recovery near the P/E boundary. The analysis of the carbon and oxygen isotope values near the P/E boundary showed that δ ¹³C negative drift occurred at the P/E boundary, with a minimum peak of -8.5‰, and its change curve was consistent with the global carbon stable isotope events during the same period.
(ⅳ) Progress in the studies of the youngest marine deposits and the time of the disappearance of the Neo-Tethys. The final closure of the Neo-Tethys in Tibet meant that seawater was completely withdrawn from Tibet. Our study focused mainly on the biostratigraphic studies of the quantitative poly-phylum species in the youngest marine strata, which were commonly considered as possible locations of the youngest marine deposit in Tibet. Seventy-two species from 50 radiolarian genera were identified, and four radiolarian zones were recognized in the Sangdanlin and Beijia sections, Gyirong-Saga, representing the youngest marine deposit in Gyirong during the early Eocene. A total of 152 species of 35 planktic foraminiferal genera were identified and 11 zones were delineated. Categorization results also include: 42 species from 22 benthic foraminiferal genera and 3 assemblage zones; 57 species from 32 ostracod genera and 8 zones, 23 species from 14 calcareous nannofossil genera and 5 assemblage zones, 88 species from 63 dinoflagellate genera and 4 assemblage zones, 23 species from 18 palynological genera and 4 assemblages, 4 species from 4 charophytes genera, 29 species from 20 calcareous algae genera and 6 assemblages, and 11 species from 9 gastropod genera. These results indicated that the Neo-Tethys disappeared in the Yadong area after the late Eocene. Based on the above and previous research data, we speculate that the final closing of the Neo-Tethys in Tibet occurred after the late Eocene.

Key words: Cretaceous, Paleogene, Neo-Tethys, major geological events, micropaleontology, biostratigraphy, Southern Tibet