藏南珠穆朗玛峰地区三叠系层序地层及沉积演化——从陆表海盆地到裂谷盆地

Triassic Sequence Stratigraphy and Sedimentary Evolution in the Qomolongma Area,Southern Xizang (Tibet) ——from Epicontinental Sea to Basin

珠峰地区的三叠系形成于大陆拉伸背景。自下而上可划分为12个三级沉积层序和5个层序组,分属于2个中层序。发生在Induan阶初期(约250Ma),Anisian阶早期(约239Ma),Carnian阶初期(约231Ma)和Norian早期(约223Ma)的4个海侵事件最为重要,可作为跨越板块的地层对比标志。藏南地区在三叠纪经历了从泛大陆到大陆裂谷的构造演化,早—中三叠世以陆表海盆地浅水环境为主,晚三叠世以深水断陷盆地为特征。晚三叠世晚期,与长期全球海平面下降相伴随,过量的陆源碎屑输入造成该地区由深水盆地转化为河流作用明显的三角洲平原环境。

The Triassic in the Qomolongma area, southern Xizang(Tibet), was deposited under an extensional tectonic setting from the Pangea supercontinent to continental rift. From the Induan to Rhaetian, 12 depositional sequences (3rd-order) have been recognized, which can be grouped into 5 sequence sets and belong to 2 mesose-quence (2nd-order). Four marine transgressions, respectively at the beginning of the Triassic (ca. 250 Ma), Early Anisian (ca. 239 Ma), Early Carnian (ca. 231 Ma) and Early Norian (ca. 223 Ma), are particularly of significance and can be used as markers for stratigraphic correlation widely across continents. The study shows that during the Triassic the Qomolangma area experienced a sedimentary evolution from epicontinental sea to rift basin with the turning point at the Late Carnian (ca. 228 Ma). During the Early and Middle Triassic, the area was under epeiric sea, with carbonate ramp to mixed shoal environments predominant. In the Late Carnian , strong extension initiated listric faulting and rapid basement subsidence, thus resulting in the onset of rift basin. From the late Norian to Rhaetian, it manifested as a rapid basin filling process in the area. Coupled with the long -term global sea -level fall in the latest Triassic, the excessive terriginous influx led to the shifts of environment from deep -water prodelta to shore and finally to fluvial plain.