塔里木盆地西南缘中生代沉积古环境恢复

Reconstruction of Mesozoic Sedimentary Paleoenvironment in the Southwestern Tarim Basin,Northwestern China

对塔里木盆地西南缘中生代沉积相和沉积环境的研究,不仅可以了解晚三叠世—晚白垩世塔里木板块南缘活动大陆边缘的历次碰撞事件,也为研究新生代中亚地区强烈的陆内变形活动提供了重要信息。本文利用野外地质调查结果和地球物理资料,在沉积相和地震相分析基础上,重建中生代沉积体系和沉积环境。结果表明:塔里木盆地西南缘中生界的原始分布受晚三叠世弧后前陆冲带冲断作用和造山后伸展作用形成的古地形控制。早侏罗世发育了一系列独立分割的小断陷,以巨厚的冲积扇、辫状河三角洲沉积为特征,其受控于NWW—SEE走向的正断层,而非NW—SE走向的古塔拉斯-费尔干纳右旋走滑断裂;早侏罗世晚期—中侏罗世广泛发育滨浅湖和河流-沼泽相沉积,可能存在一个连通了塔里木盆地南缘、吐拉以及柴达木盆地西缘的统一沉积坳陷;至晚侏罗世,冲积环境再次出现,发育的粗粒碎屑岩为早白垩世湖盆扩张前的产物;早白垩世,丰富的陆源碎屑在古昆仑山前形成多个相互叠置的冲积扇—扇三角洲—扇三角洲平原沉积体;晚白垩世,随着海水自西向东侵入,塔里木盆地西南缘陆相碎屑沉积向东逐渐退却,海相沉积占有优势。

The study of the Mesozoic sedimentary facies and environments of southwestern Tarim basin not only provides chance to understand the collision events in the south margin of Tarim block during Triassic to Late Jurassic, but information about continental deformation in mid-Asia during Cenozoic Era as well. Based on geological survey and geophysical data, Mesozoic sedimentary system and environment were reconstructed in combination with the analysis of sedimentary facies and earthquake phases. The results indicate that original distribution of the Mesozoic sediments in the southwestern margin of Tarim basin are controlled the paleo-topography resulting from pediment thrusting and post-orogenic extension during the Late Triassic. Several isolated fault-bounded sages developed during Early Jurassic are characterized by thick sediments of fluvial fans, braid-shaped river delta, and controlled by the NWW—SEE-trending normal faults but not by the Talas-Ferghana fault. Sedimentation of coastal lake and river-marsh facies dominated during the late Early Jurassic and Middle Jurassic. It was likely that there was unified sedimentary sag which connected the southern margin of Tarim basin, Tula and the western margin of Tarim basin. It was not until Late Jurassic that alluvial environment occurred again, forming coarse-grained clastic rock which was the product of lake-basin expansion in Early Cretaceous. In Early Cretaceous, abundant terrestrial debris formed a superposed deposit body of several alluvial fans-fan deltas-fan delta plain at the pediment. While at Late Cretaceous, with seawater transgression eastward from west, terrestrial clastic sediment of the southwestern margin of Tarim basin retreated eastward and gave the way to sea-faices deposit.