关键词: 黑河中游, 水系演化, 重矿物组合, 早-中更新世, 祁连山北缘

Abstract:

The northern Qilian Mountains have been experiencing intense tectonic uplift and denudation since the late Cenozoic, which resulted in an exceptionally thick sediment building up in the Hexi Corridor. The mineral characteristics of these deposits preserved substantial information about the mountain uplift and drainage evolution. Heavy minerals are the stable components in the sedimentary cycle; its characteristics and assemblages are useful tools to acquire the sediment provenance information, by inference, drainage pattern evolution. With the purpose of deciphering the drainage evolution history in Jiudong Basin, we collected heavy mineral samples from two cores (DWJ and XKJD) and modern fluvial samples in Jiudong Basin. We find that: 1) The heavy mineral characteristics in the lower part of DWJ Core (140-69.7 m) are similar to those in the two tributaries, Bailang River and Maying River; 2) The heavy mineral characteristics in the upper part of DWJ Core (69.7-0 m) and the XKJD Core (65.6-0 m) are similar to those in the trunk Heihe River as well as two tributaries, Liyuan River and Shandan River. The concentration of heavy mineral grains in sand-sized terrigenous sediments may fluctuate considerably because of several factors, mainly the sedimentary processes and post-depositional dissolution. To eliminate the effects of these fators on heavy mineral assemblages as much as possible,we analyize the samples with same grain size (63-125 um), which means they represent similiar hydrodynamic condition. The presence of abundant less resistant minerals (e.g. amphiboles, epidotes) in cores indicates that chemical weathering is negligible and most unstable grains are preserved. Given the significant changes of both lithology and sedimentary facies around 70 m in DWJ Core, the change of heavy assemblages in cores could be attributed to the drainage reorganization of midstream of Heihe River. By comparing sedimentary features from DWJ Core with previous studies in the Hexi Corridor, we speculated that this drainage evolution and subsequent sedimentary environment change occurred between early and middle Pleistocene. This drainage reorganization of Heihe River in the Hexi Corridor happened during the Kunlun-Huanghe Movement, which allow us to attribute there organization to the tectonic movement surrounding this areas. As a whole, the paleodrainage evolution of the Jiudong Basin since the Pleistocene may be summarized as follows: during the Middle Pleistocene, Jiudong Basin were mainly sourced from Maying River and Bailang River, while the Heihe River flowed to the Minle Basin. At the end of the middle Pleistocene, the tectonic uplift of the Qilian Mountain caused a progressive reorganization of the drainage pattern of the Jiudong Basin. The provenance of the upper part of DWJ Core (69.7-0 m) and the XKJD Core (65.6-0 m) samples from trunk Heihe River indicates the northwestward shifting of Heihe River to its present-day position in the Jiudong Basin.

Key words: midstream of the Heihe River, drainage evolution, heavy mineral assemblages, early and middle Pleistocene, the northern Qilian Mountains