砂岩碎屑组份变化对库车坳陷和南天山盆山演化的指示

DETRITAL MODES OF SANDSTONES AND THEIR IMPLICATIONS FOR BASIN-MOUNTAIN EVOLUTION BETWEEN THE KUQA DEPRESSION AND SOUTH TIANSHAN MOUNTAINS

为研究库车坳陷中-新生代物源变化与南天山构造演化的关系,对该坳陷中-新生界砂岩进行了系统取样,应用Dickinson砂岩分析方法详细研究了其沉积背景和物源变化。库车坳陷中-新生代沉积物源始终为再造山旋回带,自老到新砂岩中不稳定岩屑组份的含量有规律地变化:从三叠系到侏罗系,岩屑组份中沉积岩岩屑含量最高;白垩系中变质岩岩屑增多;新生界特别是新近系,碳酸盐岩岩屑所占的比例很大。岩屑组份的这种变化反映了天山地区隆升和剥蚀的一个历史过程。随着山脉隆升幅度和剥蚀强度的加大,古生界变质岩和碳酸盐岩逐步接受剥蚀,并在白垩纪以后成为盆地沉积物的主要物源。根据风化指数半定量计算的结果,结合前人古气候的研究成果,早-中三叠世该区沉积物风化程度低,推测此时地形高差较大;在晚三叠世至晚侏罗世该区风化程度较高,地形起伏较低;白垩纪是一个转折期,沉积物风化程度由强转变为弱,地形高差在该期也有所增大;古近纪延续了这种趋势。新近纪以后地形高差显著加大,沉积物风化程度明显降低,说明此时南天山的隆升速率和库车坳陷的沉积速率都很快,源区与沉积区的距离很近。

In order to understand the relationships between Meso-Cenozoic provenance changes in the Kuqa Depression and tectonic evolution of the South Tianshan Mountains, the sandstones were sampled systematically, the detrital modes of which and their implications for sedimentary setting and provenance were studied. The results indicated that the tectonic setting of provenance was, mainly, a recycled orogen during Meso-Cenozoic, and the contents of unstable lithic fragments changed regularly with the time. From Triassic to Jurassic, the unstable lithic fragments were mainly sedimentary ones. The metamorphic fragments increased in Cretaceous. The limeclasts accounted for a large proportion in Cenozoic, especially in Neogene. The change trends of lithic fragments indicated that the South Tianshan Mountains underwent a gradual erosion and unroofing process. With enhance of uplift and erosion, the Paleozoic metamorphic rocks and limestones of the South Tianshan were eroded gradually, which became primary provenance to the Kuqa Depression from Cretaceous. According to the results of paleoclimate study and estimate of semi-quantitative weathering index, the weathering extent of sediments was low and the topographic relief between the Kuqa Depression and South Tianshan was obviously in EarlyMiddle Triassic ; from Late Triassic to Late Jurassic, the weathering extent of sediments became high and the topographic relief became gentle. The Cretaceous might be a key transitional period, when the weathering extent of sediments was low and the topographic relief high affain, and this situation kept on to Paleogene. Nevertheless, the weathering extent of sediments became low and the topographic relief became high remarkably in Neogene, since the uplift rate of the South Tianshan and sedimentary rate of the Kuqa Depression being stronger, and the distance between the both becoming closer.