西藏荣玛地区上三叠统日干配错组沉积环境及其构造意义

Sedimentary Environment and Tectonic Significance of the Upper Triassic Rigain Pünco Formation in the Rongma Region,Tibet,China

西藏荣玛地区上三叠统日干配错组发育厚度较大的碳酸盐岩夹少量碎屑岩沉积,在野外剖面实测的基础上,对这套碳酸盐岩进行了岩相与微相分析。通过对碳酸盐岩样品显微薄片观察分析,共识别出13个微相。根据岩石特征及其组合类型划分出6个沉积相:陆源碎屑滨岸相、局限台地相、开阔台地相、台地边缘浅滩相、台地边缘礁相和斜坡相。分析结果表明,晚三叠世荣玛地区主要为陆源碎屑滨岸-浅海碳酸盐台地环境,夹有较深水的斜坡相沉积,共发生了4次海侵-海退旋回,并间隔有若干次小规模的海平面升降变化。区域上,班公湖—怒江洋北缘晚三叠世的沉积环境大体一致,且广泛发育晚二叠世—晚三叠世之间的区域不整合,故认为班公湖—怒江洋的开启时间为晚三叠世之前,且东西段呈准同时开启模式打开。

The Upper Triassic Rigain Pünco Formation is composed of thick carbonate sequences with some clastic interbeds in Rongma(Nima,Tibet).A total of 13 carbonate microfacies were identified through thin-section microscopic analysis.According to the petrological characteristics and assemblage types,six sedimentary facies were recognized,i.e.,detrital littoral,restricted-platform,open-platform,platform-edge bank,platform-edge reef,and slope facies.The results show that the Late Triassic Rongma was dominated by shallow-marine carbonate platform environment with local bathyal slope facies.During this period,four major transgression-regressive cycles may have occurred,together with several smaller sea-level fluctuations at intervals.Regionally,the sedimentary environment in the northern margin of the Bangong-Nujiang Ocean was similar to that in the Late Triassic,and the regional unconformity between the Upper Permian and Upper Triassic sequences was widely developed.Therefore,we considered that the Bangong-Nujiang Ocean was opened in an east-west quasi-simultaneous mode before the Late Triassic.