藏北羌塘盆地上三叠统那底岗日组与下伏地层沉积间断的确立及意义

Identification of Sedimentary Gap between the Late Triassic Nadi Kangri Formation and Its Underlying Strata in the Qiangtang Basin,Northern Xizang (Tibet) and Its Geological Significance

藏北羌塘盆地上三叠统那底岗日组呈近东西带状展布,下部主要为凝灰岩、凝灰质砂岩夹流纹岩、玄武岩,上部主要为含砾砂岩、泥质粉砂岩和生物碎屑泥灰岩。笔者等野外调查中首次在下伏的上三叠统肖茶卡组顶部发现古风化壳,结合在那底岗日组底部识别出来的不同底界类型和底砾岩特征,证实那底岗日组与下伏地层之间存在沉积间断。早期磁性地层研究中也曾发现了该沉积间断的存在,间断时间被认为约2Ma。运用已有的生物地层资料确定肖茶卡组地层的沉积时间(晚三叠世诺利期—瑞替期),从而推测那底岗日组火山岩形成时代略晚于这个时间,很可能早于前人关于该套地层的时代认识。那底岗日组火山岩可能对研究东特提斯地区的晚三叠世生物绝灭、气候变化以及海退事件有重要意义。

The Late Triassic Nadi Kangri Formation, which delineates an E-W zoned array within northern Qiangtang basin northern Tibet, is mainly composed of lower volcaniclastic rocks with interbedded rhyolites, basalts and upper sedimentary sequences. A paleo-weathered crust is first identified at the top of the Late Triassic Xiaochaka Formation by our fieldwork in detail and petrochemical study. Combined with the different types of basal boundary and basal conglomerate observed in the bottom of the Nadi Kangri Formation, this paper confirms that a sedimentary gap was probably existed between the Nadi Kangri Formation and its underlying strata. The gap was actually observed by previous study of magnetic stratigraphy and was thought to have lasted about 2 Ma. The time span of the Xiaochaka Formation has also been constrained by related biostratum in present paper. It is inferred that the age of volcanism in the Nadi Kangri Formation is probably occurred between the Late Triassic Norian and Rhaetian, prior to the previous age-constraint of this Formation. It is proposed that the Nadi Kangri Formation volcanic rocks have potential significances in evaluating the Late Triassic biotic crisis, climate change, and regression event in the eastern Tethys.