Sedimentary Environment of Tertiary Recorded in the Yongdeng Section of Lanzhou Basin
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摘要: 对兰州盆地永登剖面第三系沉积进行了沉积学研究、并结合年代学研究结果分析了新生代以来沉积环境变化与气候变化.古新世末至早始新世沉积了一套扇三角洲相沉积(细柳沟组),为具交错层理的砖红色砂岩,顶界年龄约51 Ma.兰州地区在51 MaB.P.经历了一次大的环境变化,气候由相对温暖湿润转为半干旱半湿润,气候炎热,湖盆由淡水湖转为盐湖,蕴示着由副热带高压控制的干旱带北移,兰州地区进入受行星风系控制的干旱带.早始新世晚期至早渐新世早期(51～31.5 Ma B.P.),在近20 Ma时间内经历了微咸湖-盐湖阶段(野狐城组),砂泥岩沉积中含有大量石膏夹层.早渐新世,兰州地区向湿润方向转化,兰州盆地由咸水湖转化为淡水湖,可能反映了东南季风逐渐形成,中国大陆由早先行星风系控制转化为季风控制,前期东南方向干旱带消失,西北干旱带出现,兰州地区相对早先湿润.早渐新世中期至中中新世(31.5～15 Ma B.P.),兰州盆地在经历了三次大的沉积旋回后湖盆逐渐干枯,三个大旋回分别由河流相砂岩-微咸水滨湖相砂泥岩、泛滥平原相泥岩,河流相砂岩-淡水滨湖相砂泥岩-沼泽相、泛滥平原相泥岩,河流相砂岩-沼泽相、泛滥平原相泥岩组成.20 MaB.P盆地接受大量砂砾石沉积,反映出周缘山体快速抬升遭受剥蚀,可能蕴示着青藏高原抬升波及到兰州地区.Abstract: With the result of chronological research, sedimentology has been used to study the Tertiary sediments of Yongdeng Section of Lanzhou Basin and analyzed the changes of sedimentary environment and climate since Cenozoic.A set of fan delta deposits formed from the late Paleocene to early Eocene is red sandstone with crossstratification.The age of upper layers is about 51Ma.Great environmental changes took place in Lanzhou area at 51MaB.P.,which led to the climate transformation from warm and humid to semiarid and serihumid and climate was characterized by hot.Lake bed changed from freshw ater to salty, which implied the diversion toward north of arid zone controlled by semit ropical high-pressure belt.Lanzhou area became arid zone controlled by planetary wind system.The basin had a stage from slig ht saline-lake to salt-lake(Yiehusheng Formation)within the interval of about 20Ma(from late early Eocene to early early Oligocene(51 ～ 31.5Ma)), sand-mud stone contains plenty of interbeds of gypsum.The area was converting to moist climate in early Oligocene.Lanzhou Basin changed from saltwater lake to fresh lake,which probably indicates gradual formation of southeast monsoon.Mainland of China has been cont rolled by the monsoon instead of planetary wind system befo re.East-westward arid zone disappeared in early period and north-westward arid zone appeared.The area became moist comparing to before.Having endured three great cycles of sedimentation from the middle early Oligocene to middle Miocene (31.5 ～ 15Ma), the basinhad died gradually.The three cycles are respectively composed of sandstone of river facies —sand-mud stone of shore facies of light saline-lake, sandstone of river facies —sand-mud stone of shore facies of freshlake —mudstone of marsh facies and valley plain facies, sandstone of river facies—mudstone of marsh facies and valley plain facies.Plenty of the sediments of sand and gravel deposited in the basin at 20MaB.P., which reflects fast uplift of surrounding mountain mass and consequent degradation and implied that uplift of Qing hai-Tibean Plateau had influence on Lanzhou area.
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Key words:
- sedimentary environment /
- enviro nment of palaeoclimate /
- Lanzhou Basin /
- Tertiary
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