西藏尼玛北部新生代盆地沉积记录及控盆机理

通过野外地质调查和地层对比,将尼玛北部盆地新生代陆相地层定为牛堡组。根据岩石组合和沉积特征分析,尼玛北部盆地牛堡组可划分为扇三角洲相、湖泊相和冲积扇相。扇三角洲相可进一步划分为扇三角洲前缘和前扇三角洲2种亚相;湖泊相可划分为半深湖—深湖和滨湖—浅湖2种亚相。盆地的演化特征可分为盆地形成初始期、盆地扩张期和盆地萎缩期,3期的演化可分别对应牛堡组的一段、二段和三段。尼玛北盆地发育的各个阶段都跟古气候变化和构造活动有很大的联系,两者共同影响着盆地发育的各个阶段。结合前人的研究资料,认为尼玛盆地的发育时代为早白垩世末期—晚白垩世初。根据盆地边缘相与半深湖—深湖沉积相伴生、牛堡组底部发现火山岩夹层等沉积特征,可以推断尼玛盆地是一个具有走滑拉张性质的盆地。

Cenozoic basin sedimentary record and basin-controlling mechanism of northern Nyima, Tibet

On the basis of field investigation and stratigraphic correlation, the Cenozoic strata of the northern Nyima basin are classified as Niubao Formation. According to an analysis of rock assemblage and sedimentary features, fan delta facies, lacustrine facies and alluvial fan facies are identified in Niubao Formation in northern Nyima basin. The fan delta facies can be subdivided into fan delta front subfacies and pre-fan delta subfacies, whereas the lacustrine facies can be subdivided into semi-deep lake subfacies, shore-shallow lake subfacies and shore lake subfacies. It is believed that the sedimentary setting went through three evolution stages, i.e., initial period of basin formation, expansion period and shrinking period of the basin, The evolution stages of basin correspond to various sections of Niubao Formation. Stages of development of the basin are related to paleoclimate and tectonics in northern Nyima. It was not a single factor that controlled the basin development but a combined factor that affected all stages. Based on previous researches, the authors hold that the formation of basin began in the period from the end of Early Cretaceous to earliest Late Cretaceous. Based on the sedimentary characteristics that the margin facies is accompanied by semi-deep lake subfacies in the basin, and volcanic rock occurs on the bottom of Niubao Formation, it is concluded that Nyima basin has the characteristics of strike-slip fault systems.