陇中盆地古近纪—新近纪地层学与古气候学研究进展

Review of the stratigraphy and paleoclimatology study of the Paleogene — Neogene Longzhong Basin

位于我国西北部的陇中盆地古近纪—新近纪(第三纪)地层出露较好,并且产出大量哺乳动物化石和植物化石,这为西北地区地层对比和古气候研究提供了良好材料。近几十年,众多学者在陇中盆地的各次级盆地(如西宁盆地、兰州盆地、临夏盆地等)开展了大量古生物学、生物地层学、磁性地层学和古气候学研究,为探讨陇中盆地的构造演化过程和气候演变过程提供了重要参考。本文对陇中盆地第三纪地层年代学和古气候研究已取得的进展进行了初步总结归纳,并提出了需要进一步研究的问题。

Background aim and scope The well-developed long continuous Paleogene — Neogene fluvio-lacustrine strata（the Tertiary red beds） are exposed in the Longzhong Basin, Northwestern China. The abundant mammalian faunas and phytolite in the red beds provide valuable material for the stratigraphic correlation and paleoenvironmental reconstruction. In recent decades, paleontological, sedimentological, magnetostratigraphical and palynological studies have been conducted in the Longzhong Basin（e.g., Xining Basin, Lanzhou Basin and Linxia Basin）. The environmental evolution in the Longzhong Basin since Cenzoic is likely to be associated with the land-sea redistributions the uplift of the Tibetan Plateau and/or the global cooling. Materials and methods In this study, we review the achievements mentioned above based on the stratigraphic and paleoclimatic research published in recent decades and bring out questions to be solved in future studies. Results The Tertiary beds in the Xining Basin unconformably overlie the Cretaceous Minhe Group, and the initiation of Tertiary beds began at least 55 — 52 Ma ago. Between 52—17 Ma, the continuous deposits in this basin implied a relatively stable sedimentary environment. The Tertiary beds of the Lanzhou Basin unconformably overlie the Cretaceous Hekou Group with an basal age of over 58 Ma. Given the formation of the basin systems in and around the Tibetan Plateau at that time, we speculate that the initiation of Paleogene deposits in these two basins at 58 — 52 Ma may be linked to the Indian-Eurasian continental collision at 65 — 55 Ma. The low sedimentary rate before late Paleogene（32 — 29 Ma） implied a relatively weak tectonic activity in the northeast margin of the Tibetan Plateau after the collision of Indian and Eurasian continent. However, the sedimentary rate increased from 32 — 29 Ma in both basins and a hiatus was observed in the Lanzhou Basin at the boundary of Yehucheng Formation and Xianshuihe Formation（~31 Ma） implying an active tectonic movement in the Longzhong Basin. The increased subsidence by 29 Ma at the latest in the Linxia Basin also provides evidence for the tectonic movements in the Longzhong Basin at that time. Furthermore, the sedimentary response of the Qaidam Basin to the uplift and denudation of the source area was observed around 30 Ma. The evidences support a widespread tectonic activity in the northeastern margin of the Tibetan Plateau between 32 — 29 Ma. Later, more tectonic movements were observed in the Longzhong Basin in the Late Oligocene（25 — 20 Ma） and Late Miocene（8 — 6 Ma）. The Lithology and cyclostratigraphic analysis, pollen records, n-alkane biomarker proxy, and environmental magnetism indicate the cooling and aridification trend in the Xining Basin from 52 Ma to 25 Ma. Of particular importance is the permanent disappearance of gypsum layers after 34 Ma, which was attributed to the aridification resulting from the global cooling at the Eocene-Oligocene Climatic Transition（EOT）. The gypsum layers in the Xining Basin and Lanzhou Basin suggest a playa lake environment in the western Longzhong Basin under hot and dry background in the Eocene. The phytolith evidences in the Late Oligocene（~31 Ma） suggest that the climate in the Lanzhou Basin is warmer and more moist than that of today. The red clay has been deposited on the eastern Longzhong Basin since at least 25 — 22 Ma, which may implies the existence of an arid area in the central Asia and the formation of the East Asian winter monsoon. The magnetic susceptibility in the red clay recorded the influence of the Middle Miocene Climate Optimum（17.5 — 13.5 Ma） on the eastern Longzhong Basin. The sedimentary records and mammalian fossils in the Linxia Basin show that further aridifi cation initiated at 8 — 6 Ma, which may be associated with the uplift of the Tibetan Plateau. Discussion The evolution of the Longzhong Basin is linked to two stages of the Indian-Eurasian continental collision at ~60 Ma and ~30 Ma. The environmental evolution of the Longzhong Basin is consistent with global climate before 25 Ma, especially during the EOT. This suggests that the Longzhong Basin was dominated by the Westerlies, which came from the North Atlantic and brought climate signals from the Ocean. After 25 Ma, however, the atmospheric circulation has been changed signifi cantly, leading to the formation of the East Asian winter monsoon and the aridifi cation in central Asia. The Tibetan Plateau may have reached a certain altitude which can influence the regional environment significantly. ConclusionsThe evolution history of the sub-basins（Xining basin, Lanzhou basin, Linxia Basin and LongxiJingning Basin） of Longzhong Basin have much similarities with each other, which may be affected by the northeast outgrowth of the Tibetan Plateau. The paleoclimate of the Longzhong Basin is related to both the global cooling and the uplift of the Tibetan Plateau. Recommendations and perspectivesThe explanation of coexist of aeolian deposits（in the Longxi-jingning Basin） and fl uvio-lacustrine sediments（in the Xining and Lanzhou Basin） in the late Oligocene and early Miocene in the Longzhong Basin remained controversial. The paleoclimatic evolution in the Longzhong Basin during the key climate transition periods（e.g. Eocene-Oligocene Climate Transition and Middle Miocene Climate optimum） needs further study. The Longzhong Basin should be treated as a whole when we try to deal with above questions.