西藏波密地区晚古生代地层层序及盆地演化分析

The Late Palaeozoic stratigraphic sequences and basin evolution in the Bomi region, Xizang

关键词:  界面  火山活动  层序  红色磨拉石  波密  西藏

Key words:  boundary  volcanic activity  sepuence  red molasse  Bomi  Xizang

摘要:

西藏波密及邻区松宗、然乌一带,下石炭统诺错组与中上泥盆统松宗组之间的层序不整合界线是藏东南地区冈瓦纳北缘晚古生代盆地性质转变的重要界面。界线之下的松宗组为稳定的碳酸盐岩台地沉积;界线之上,以大规模的火山活动、盆地裂解为标志,伴随着沉积盆地的持续沉陷和相对海平面的上升,沉积了以石炭系诺错组和来姑组为代表的向上变深序列,相对海平面在晚石炭世达到了顶点,以来姑组上部的含铁质板岩和大套的浊积岩为标志。其后,以洛巴堆组为代表的晚石炭世末期-二叠纪的沉积记录,则代表了一个缓慢的向上变浅的沉积层序。在这个向上变深再变浅的沉积盆地演化过程中,火山活动呈现了明显的由强转弱的变化轨迹。中侏罗统马里组陆相红色磨拉石不整合堆积于下伏地层之上,表明本区在晚三叠世-早侏罗世经历了一次规模宏大的褶皱造山事件。

Abstract:

The unconformity between the Lower Carboniferous Norco Formation and Middle-Upper Devonian Sumzom Formation in Bomi and its adjacent areas such as the Sumzom and Rawu zones is believed to be an important boundary that represents the conversion of the nature of the Late Palaeozoic basins on northern Gondwana in southeastern Xizang. The Sumzom Formation beneath the unconformity is interpreted as stable carbonate platform deposits, while large-scale volcanism and basin disintegration appear above the unconformity where the deepening-upward depositional sequences represented by the Carboniferous Norco and Laigu Formations rested down following the steadily subsidence of the sedimentary basins and relative rising of sea level. The maximum sea level rise during the Late Carboniferous was manifested by the occurrence of the ferruginous slates and turbidites in the upper part of the Laigu Formation. The successive latest Late Carboniferous to Permian deposits represented by the Luobadui Formation recorded a gradually shallowing-upward depositional sequence. The deepening- and shallowing-upward depositional cycle in the basin has disclosed the evolutionary path from a strone one to a weak one of volcanic activity. The angular unconformable contact of the red continental molasse deposits of the Middle Jurassic Mali Formation with the underlying strata shows that a major orogenic event once took place in the study area during the Late Triassic to the Early Jurassic.