新疆博格达山初始隆升时间的地层学标定

Stratigraphic constraints on the initial uplift age of Bogda Shan,Xinjiang,north-west China.

博格达山位于东天山北缘,是准噶尔盆地与柴窝堡凹陷的分界岭。它的初始隆升时间是一个意义重大而又长期争论的基础地质问题。地表地质调查和人工地震勘探结果证实,石炭纪以来最早的区域性角度不整合发育于白垩系与前白垩纪地层之间。博格达地区斜层理、叠瓦状砾石迎水面等古流向示向构造研究结果证实,博南、博北沉积物扩散方向在早二叠世均指向南、侏罗纪时指向博格达主脊、而白垩纪及其后则背离博格达主脊。博格达山南、北砾石成分统计结果显示,侏罗系具双物源特征,而白垩纪物源区则为博格达山。据此推论,博格达山初始隆升发生在侏罗纪末-白垩纪初。基于博格达山初始隆升发生在侏罗纪、白垩纪转换之际,有理由相信博格达山地区在海西期未卷入天山造山带,在早中生代期间柴窝铺凹陷与准噶尔盆地同属一个盆地。

The Bogda Shan, located on the northern margin of East Tianshan, forms the watershed between the Junggar basin and the Chaiwopu depression. Its initial uplift time has been debated for a long time, and is of important geotectonic significance.Field surveys and seismic exploration show that the lowest regional angular unconformity older than the Carboniferous developed between the Cretaceous and the pre-Cretaceous strata. Several measurements have been made of cross-bedding and stoss surfaces of imbricate pebbles. Statistical analysis of the data indicates that a major change in sediment dispersal pattern occurred at the end of the Jurassic or the beginning of the Cretaceous. In the Jurassic, the sediment dispersal pattern is convergent, and the Bogda area functioned as a water catchment. In the Cretaceous, the sediment dispersal pattern is divergent, and the Bogda area functioned as a watershed. A statistical study of the composition of clasts in the Jurassic and Cretaceous conglomerates suggests that these Cretaceous conglomerates are different from the Jurassic conglomerates, and are composed mainly of volcanic gravels derived from nearby areas. On the basis of these studies we conclude that the initial uplift of Bogda Shan took place between the latest Jurassic and the earliest Cretaceous.Therefore we suggest that 1the Bogda Shan is not an integral part of the Tianshan orogenic belt; and 2 the Chaiwopu depression is part of the early Mesozoic Junggar Basin.