准噶尔盆地南缘二叠纪—三叠纪构造-沉积环境与原型盆地演化*

准噶尔盆地南缘(简称“准南”)的构造-沉积演化历史以及原型盆地性质一直以来备受争议。依据沉积环境分析、地层对比以及沉积演化研究,结合火成岩年代学、大地构造学等研究成果,探讨了该区二叠纪—三叠纪多期次的伸展—挤压环境转换及沉积盆地性质转变。晚石炭世,准南西段处于北天山洋壳向伊犁地体俯冲的末期,沉积环境以滨浅海为主,为残留洋盆地;准南东段以半深海相碳酸盐沉积为主,发育典型的双峰式火成岩,显示为陆内的伸展环境。早二叠世,准南以滨浅海相细粒碎屑岩沉积为主,发育同沉积断裂和伸展垮塌变形构造,表现为陆内裂陷盆地的特征。中二叠世,准南仍以滨浅海相为主,但其沉积速率明显加快,沉积厚度变大,整体上表现为以热力沉降为主的坳陷盆地。晚二叠世,北天山和博格达地区普遍发育冲积扇或扇三角洲,上二叠统泉子街组和中二叠统红雁池组之间呈明显的角度不整合接触,沉积环境发生突变,均显示北天山快速冲断隆升,表明该时期准南为陆内压陷盆地。早三叠世,准南快速冲断结束,该区进入相对稳定的发展阶段,以发育滨浅湖相细粒沉积物为主,表现为弱挤压的陆内压陷盆地的特征。中晚三叠世,由于持续湖侵,沉积盆地范围进一步扩大,北天山被削高补低,准南乃至整个准噶尔盆地进入统一的内陆湖泊演化阶段,整体上以滨浅湖相—半深湖相沉积为主,表现出陆内坳陷盆地的特征。综合上述原型盆地性质和沉积环境分析,可将准南二叠纪—三叠纪构造-沉积演化划分为4个阶段:晚石炭世—中二叠世为后碰撞伸展阶段,晚二叠世为北天山挤压冲断阶段,早三叠世为弱挤压压陷和削高补低阶段,中晚三叠世为稳定拗陷和准平原化阶段。

Tectonic-depositional environment and prototype basin evolution of the Permian-Triassic in southern Junggar Basin

The tectonic-depositional environment and prototype basin evolution of the Permian-Triassic in the southern Junggar Basin have long been debated. Detailed sedimentary environment,stratigraphic correlation and sedimentary evolution analyses,combined with collection and sorting out of relevant data, have been carried out on the Permian-Triassic strata. Hereby,we discussed a multicycle of compressional and extensional transformation in the Permian-Triassic,and proposed the prototype of the sedimentary basin. In the Late Carboniferous,the west segment of the southern Junggar Basin situated at the end of the North Tianshan Ocean crust subducted southward below the Yili Block,and dominated by littoral and neritic facies,showing the characteristic of a remnant ocean basin. However,the east segment of the southern Junggar Basin is mainly composed of bathyal carbonate deposits and a set of bimodal volcanic rocks,implying that it formed under an extensional environment. In the Early Permian,they both primarily contain fine grained clastic rocks of the littoral-shallow facies. Moreover,syn-depositional faults and collapse deformations can be observed,indicating an intracontinental rift basin. Later on,the deposition rates is faster and the thickness is thicker. Thus,the prototype basin has been transformed into the depression basin in the Middle Permian. Until the Late Permian,the depositional environments were characterized by the alluvial fan or fan delta deposits in the North Tianshan and Bogda area. Also,the Upper Permian Quanzijie Formation is in unconformity relation with the Middle Permian Hongyanchi Formation. Hence,it implied the rapid uplift of the North Tianshan, formed an intracontinental compressional basin. In the Early Triassic,because of the end of the uplift,the basin entered into a relatively stable stage,which formed a weak intracontinental compressional basin dominated by a series of fine grained sediments of coastal shallow-lake facies. In the Middle-Late Triassic,with the occurrence of continuously transgress of the basin,the North Tianshan have been eroded,which was consistent with the expansion of the lacustrine basin and the widespread fine-grained shallow-deep lacustrine sediments. Taken together,we suggested that the basin of the Late Carboniferous to Middle Permian belongs to post-collision extensional depression stage. Nevertheless,a short thrust stage happened in the Late Permian. In the Triassic,the plate squeezing action weakened and Junggar Basin gradually entered a depression-subsidence stage from the intense compressive stage. The Early Triassic is a weak compression and denudation stage,and the Middle-Late Triassic is a stable depression and peneplanation stage.