羌塘盆地东部晚三叠世—早中侏罗世沉积环境转变研究——来自地质浅钻岩芯的证据

Late Triassic to Early—Middle Jurassic depositional environment transformation process study in the eastern Qiangtang Basin:evidence from the record by the core samples

位于羌塘盆地东部的羌资-16井首次在羌塘盆地钻遇了完整的上三叠统—中下侏罗统地层,清晰的记录了盆地东部晚三叠世—早中侏罗世时期的沉积环境转变。本文在对羌资-16井岩芯的野外详细观察和室内分析的基础上,结合前期在该区实施的羌资-7井和羌资-8井资料,综合分析了羌塘盆地东部晚三叠世—早中侏罗世过渡时期的沉积环境变化特征。上三叠统地层主要包括波里拉组、巴贡组和鄂尔陇巴组,分别形成于局限台地、三角洲和火山喷发环境,标志着羌塘盆地前陆盆地演化结束和新一轮裂谷盆地的开启。中—下侏罗统雀莫错组底部紫红色底砾岩沉积代表了裂谷盆地开启后的初始沉积,形成于河流沉积环境,而后随着南部海水侵入,逐渐过渡为陆缘近海湖沉积环境。总体上,晚三叠世—早中侏罗世时期,羌塘盆地东部表现为前陆盆地海相沉积的消亡→裂谷盆地的开启→裂谷盆地早期充填的演化过程。晚三叠世诺利期开始(~220 Ma),羌塘盆地晚三叠世海相沉积结束,逐渐向早中侏罗世陆相沉积转变,开始了羌塘侏罗纪盆地的沉积演化历史。

Objectives: The drilling Well QZ-16,located in the eastern part of the Qiangtang Basin,first recovered a complete Upper Triassic to Lower Jurassic strata and clearly recorded the sedimentary environment transformation process in the eastern part of Qiangtang Basin during the Late Triassic to Early—Middle Jurassic Epoch. On the basis of comprehensive analysis of the core samples of the Well QZ-16 along with the data of the Well QZ-7 and the Well QZ-8 in this area,the sedimentary environment transformation characteristics of the Late Triassic to Early—Middle Jurassic transition period in the eastern part of the Qiangtang Basin are comprehensively analyzed.Methods: In this paper,field observation and indoor thin-section analysis of the core samples of the Well QZ-16 along with the Well QZ-7 and the Well QZ-8 were carried out. The trace elements of the fine clastic rock samples from the Quemo Co Formation of the Well QZ-16 in the Quemo Lake(Quemo Co) area were measured. And the zircon U-Pb age of tuff layer of the E’erlongba Formation was determined by LA-ICP-MS.Results: The Upper Triassic strata mainly consist of the Bolila Formation,Bagong Formation and E’erlongba Formation,which were deposited in restricted platform,delta and volcanic eruption environment,respectively,representing the end of the foreland basin revolution and the onset of the new Qiangtang Mesozoic Basin. The purplish-red conglomerate deposits of the Lower—Middle Jurassic Quemo Co Formation represent the initial deposition after the opening of the rift basin,which formed in fluvial environment,and then gradually transferred to the offshore lacustrine sedimentary environment with the seawater transgression from south. Overall,this period was characterized by the disappearance of the foreland basin→ opening of the rift basin→ early filling process of the rift basin. The Late Triassic marine sediments in the Qiangtang Basin gradually transformed to continental sediments during the Norian period( ~ 220 Ma),which also represented the beginning of the Qiangtang Jurassic Basin evolution history.