摘要： 兰州盆地是青藏高原东北缘系列盆地中最早接受沉积的地区之一,也是探讨高原隆升与古气候演化的理想场所。通过大量野外露头观察,综合岩石学特征、古生物化石及沉积地球化学等多种指标,对兰州盆地始新世—中中新世沉积演化进行了全面研究。结果表明,兰州盆地始新统西柳沟组桔红色、砖红色砂岩应属于风成沙漠沉积,而非前人认为的扇三角洲或河湖相沉积。西柳沟组沉积之后,兰州盆地进入河流—三角洲—湖泊演化阶段: 始新世—渐新世早期(野狐城组沉积期)气候总体干热,形成以滨浅湖(含干盐湖)为主、间或出现小规模三角洲的沉积格局; 早渐新世晚期(咸水河组沉积早期)开始发育三角洲沉积,之后水体范围逐渐扩大,至晚渐新世浅湖沉积发育,该时期古环境由半干旱半湿润的开阔林地—灌丛演变为干旱气候背景下的灌丛—草原。 早中新世初期(咸水河组沉积中期)发育三角洲平原沉积,之后快速过渡为三角洲前缘—浅湖沉积,代表了湖盆在新生代最后一次扩张,古气候以半干旱为主; 中中新世(咸水河组沉积晚期),湖盆逐渐萎缩消亡,兰州盆地整体演变为河流相沉积,仅在局部残存小型洼地,古环境仍为温带半干旱的灌丛—草原。

关键词: 青藏高原, 兰州盆地, 新生代, 沉积相, 古环境

Abstract: As one of the earliest Cenozoic sedimentary basins located on the northeastern margin of the Tibetan Plateau, the Lanzhou Basin is an ideal area to study the plateau uplift and paleoclimatic evolution. Based on the outcrop observations, lithological characteristics, paleontological fossils and sedimentary geochemistry, comprehensive studies were carried out on the sedimentary evolution of the Lanzhou Basin during the Eocene-Middle Miocene. The results show that the orange or brownish red sandstone of the Eocene Xiliugou Formation represents the eolian desert deposits, rather than the fan delta or the fluvio-lacustrine facies. After the deposition of the Xiliugou Formation, fluvial-delta-lacustrine facies occurred in the basin. The Yehucheng sedimentary period was characterized by a hot dry climate, forming inshore shallow lake deposits, together with small-scale delta. From the late Early Oligocene (i.e., the early stage of the Xianshuihe Period), delta deposition developed, and then the water body gradually expanded. Until the Late Oligocene, the inshore shallow lake deposits developed, and the paleoenvironment changed from the semihumid-semiarid open woodlands and shrub to arid shrub and grassland. At the beginning of the Early Miocene (i.e., the middle stage of the Xianshuihe Period), the delta plain deposits developed, and then rapidly evolved into the delta front and shallow lacustrine environments, representing the final expansion of the Cenozoic lake in the Lanzhou Basin dominated by the semi-dry climate. During the Middle Miocene (i.e., the late stage of the Xianshuihe Period), the lake basin began to shrink and gradually disappeared, which was replaced by the fluvial sedimentary system, and the small-scale bottomlands were only locally distributed. The paleoenvironment was predominated by semiarid shrub and grasslands.

Key words: Tibetan plateau, Lanzhou Basin, Cenozoic, sedimentary facies, palaeoenvironment

中图分类号: 

• P531