摘要：
通过对兰州盆地凤凰山剖面沉积物（厚约2484 m）的常量元素（Si、Al、Ca、Fe、K、Mg、Na）和碳酸盐含量分析，发现元素含量变化特征可分为三类：（1）元素Al、Fe、K和Mg赋存于细颗粒矿物中，与粒度分选和风化强度关系密切；（2）元素Si主要赋存于石英中，能良好反映沉积相变化；（3）元素Ca、Na与化学沉积盐类的生成有关，元素比值（Na/Al和Ca/Al）能揭示沉积环境演变。元素比值变化表明兰州盆地第三纪古环境变化可划分为四个阶段：54—43 Ma为三角洲河流相沉积，气候较温暖湿润；43—33 Ma Na/Al比值较高，钠盐大量生成，进入盐湖发育阶段，气候开始变干；33—23 Ma Ca/Al比大幅度波动，地层中含有大量石膏层，盆地经历了半干旱—半湿润的气候环境波动；23—9 Ma盆地从咸水-半咸水湖转变为风尘主导的干旱环境。综合凤凰山剖面的元素、磁化率、色度和粒度结果，认为第三纪以来兰州盆地古环境经历了三次阶段性变干的过程（~43 Ma，~33 Ma和~23 Ma）；同全球气候和区域构造记录的对比表明，前两次变干可能同全球气候变冷关系密切，而早中新世的变干则可能是区域构造运动的结果。%Background, aim, and scope The Lanzhou Basin is rich in Tertiary mammalian faunas and experienced signiifcant sedimentary environmental evolution, making it an ideal site for paleontological, sedimentological, magnetostratigraphical and paleoclimatic studies. Located in the northeastern margin of the Tibetan Plateau and the western margin of the Loess Plateau, the thick Cenozoic sedimentary sequence in Lanzhou provides critical evidence for the tectonic history of the northeastern part of the Tibetan Plateau and contains a large number of paleoenvironment information. The Fenghuangshan section is situated at the south edge of the Lanzhou Basin with continuous and stable strata ranging from 9 Ma to 54 Ma. The Tertiary sedimentary systems of the basin are divided into three formations, i.e., the Xiliugou Formation, Yehucheng Formation and Xianshuihe Formation, in order of decreasing age. Previous work mostly concentrated in mammalian fossils, rock stratigraphic units and the sedimentary facies research by the use of indicators such as particle size, magnetic susceptibility, chroma in the Lanzhou Tertiary Basin. But the geochemical characteristics of Tertiary sediments in the Lanzhou Basin is hardly investigated. The variations of the elements content in the sediments can relfect the lfuctuations of dry and wet climate. To explore the element geochemical characteristics and infer the stage and signiifcance of the Lanzhou Basin Tertiary sediments, we analyse the major elements of the sediments in the Fenghuangshan section at 9 — 54 Ma, combined with the strata features and the changes of sedimentary facies.Materials and methods We picked out 235 samples in the Fenghuangshan section, which has a total thickness of~2484 m. The sample age was obtained by linear interpolation of the paleomagnetic ages. Major elements were measured by X-ray fluorescence spectrometry (XRF). The CM-700d spectral color measurement meter produced by Konica Minolta Company was used to test the color of sediments. The percentage content of carbonate was measured by gas method, using the formula: carbonate content = pneumatic × background value(measured value)/(temperature+273.15)/0.8309/sample quality to calculate the percentage content of carbonate samples.Results The results show that the major elements of sediments consist mainly Si, Al, Ca, Fe, K, Mg, Na in descending order, and the element content are weakly affected by carbonate content. According to the results of the correlation of each element content and Al, the seven elements were divided into three categories: (1) Fe (r2 = 0.9481), K (r2 = 0.7947), Mg (r2 = 0.8057) have a high positive correlation with Al. According to four element content curves, the results can be roughly divided into three parts with boundaries at 2012 m and 1438 m. (2) The relationship between Si and Al is more complex. There are little lfuctuations except that the elemental contents have large and frequent lfuctuation in the Yehucheng Formation and the lower part of the Xianhuihe Formation. (3) The correlation among Na (r2 = 0.1244), Ca (r2 = 0.1129) and Al are poor. The content of Ca was lower in the Xiliugou Formation, but increased signiifcantly from Yehucheng Formation (~2012 m) and was higher in the Xianshuihe Formation. The content of Na was quite low in the lower part of Xiliugou Formation and Yehucheng Formation, but generally high in the upper part of the Xiliugou Formation and the Xianshuihe Formation.Discussion The content of major element is closely related to sediment types and sedimentary facies, according to the results of the 7 major elements at the Fenghuangshan section. (1) Al, Fe, K, Mg that existed mainly in ifne-grain minerals relfect the effects of grain size sorting and/or weathering. In the period of 54 — 33 Ma, the contents of Al, Fe, K, Mg are low and stable, and associated with the coarse lfuvial facies sandstone, which has stronger transport power, related to warm and wet climate. The content of four elements have obvious periodic increase since 33 Ma, indicating the increasing of ifne particle input. Changes of element content suggested that the climate had a great dry-wet oscillation in 33 — 23 Ma. (2) Si enriched in quartz is well related to the changing sedimentary facies. In stage of 54 — 33 Ma, the high content of Si is related to coarse-grained fluvial sandstone. However, during 33 — 22 Ma, the content reduced and fluctuated frequently, which relfects the frequent interaction occurring of lfuvial-lacustrine facies (sandstone-mudstone). The Si content tends to be stable after 22 Ma, and the dust deposition dominated with the accidentally occurrence of lfuvial deposition. (3) Ca and Na may be associated with the formation of salts (i.e., salt and gypsum) during the chemical deposition, thus the Na/Al and Ca/Al ratios can be employed to infer the evolution of sedimentary environment. According to the element ratios and sedimentary facies research, the Tertiary paleoenvironmental evolution in the Lanzhou Basin can be divided into four stages: (1) lfuvial environment under a warm and humid climate at 54 — 43 Ma; (2) development of salt lake with high Na/Al ratios during 43 — 33 Ma; (3) lfuctuated salt lake from 33 Ma to 23 Ma with a signiifcant increase in Ca/Al ratio formed under semi-arid and semi-humid climate, and (4) shifts from salt-brackish lake to gradual aridiifcation environment lake after 23 Ma.Conclusions Combination of available proxy indicators including grain size, color, magnetic susceptibility and major elemental compositions suggests stepwise drying of the Lanzhou Basin occurred around 43 Ma, 33 Ma, and 23 Ma, respectively. Taking into account evidence of global cooling and regional tectonics, we tentatively attribute the ifrst two drying events to global cooling and the last drying in early Miocene to regional uplift of the northern Tibet Plateau.Recommendations and perspectives The results provide signiifcant stages of aridiifcation in the Lanzhou Basin. It is recommended that more work should be carried out to deepen the cognition of the Tertiary paleoenvironmental evolution in the Lanzhou Basin.
