甘肃张掖地区白垩系风成砂岩沉积序列:祁连山白垩纪隆升的沉积学响应

Cretaceous sedimentary succession of eolian sandstones in Zhangye Region of Gansu Province: Sedimentological response to the Cretaceous uplift of Qilian Mountains

白垩纪祁连山隆升过程及其沉积学响应的研究,对认识青藏高原的形成及其环境效应具有重要意义。受控于祁连山隆升过程的甘肃张掖地区下白垩统,包括赤金堡组、下沟组和中沟组,其基本特征是:(1)分布局限而且厚度上千米的赤金堡组,主要由旋回性发育的"冲积扇(或洪积扇)—辫状河—滨湖相风成沙丘"序列所组成,形成一种反映了"雨影效应"的粗碎屑岩序列;(2)下沟组总体为一套红层沉积,在盆地边缘发育更多的风成沙丘,可能代表了与祁连山更加快速隆升相对应的更加强烈的"雨影效应"时期的产物;(3)中沟组,从冲积体系演变为一个分布广泛的湖泊沉积体系,总体上属于潮湿气候背景下未受到"雨影效应"影响的产物,与祁连山均衡抬升存在成因联系;(4)研究区上白垩统总体缺失,可能与祁连山的卸载过程产生的山前地壳均衡反弹存在关联。因此,从赤金堡组到下沟组的风成砂岩序列、特殊的沉积相构成及其演化序列,不但成为窥视早白垩世祁连山隆升过程的物质记录,而且为早白垩世东亚大气环流格局的重建提供了另外一个重要的物质记录。

Studies on the uplift process of the Qilian Mountains and its sedimentological response are meaningful for the further understanding of the formation of the Tibetan Plateau and its impact of environ- ment in Asia. The Lower Cretaceous in the Zhangye Region of Gansu Province, which is genetically con- trolled by the uplift process of the Qilian Mountains, can be subdivided into the Chijinpu, Xiagou andZhonggou Formations. Their sedimentary features can be summarized as follows ： （ 1 ） a course clastic-rock succession can reflect the phenomenon of ＂rain shadow effect＂, i.e., the Chijinpu Formation with the re- stricted distribution in space and the thickness of more than 1000 meters, which is made up of the cyclic succession of sedimentary facies characterized by the ＂alluvial fan （or fluvial fan）-braided river-eolian sand dune of the lake shore facies＂ ; （2） a set of red beds that is composed of more eolian sand dunes of the lake shore facies in the margin areas of the basin, i.e., the Xiagou Formation, which probably repre- sents the product of a more obvious period of the ＂rain shadow effect＂ that is correspond to the rapid and strong uplift process of the Qilian Mountains ; （3） a succession from the alluvial system to the lake system with the more widespread spatial distribution, i.e., the Zhonggou Formation, which belongs to the product formed in moist climate setting without the rain shadow effect and is genetically related to the equilibrium uplift of the Qilian Mountains; （4） The generally absence of the Upper Cretaceous in the study area is probably genetic related to the unloading of Qilian Mountains that leads to the isostasy rebounding in front of mountains. Therefore, a particularly sedimentary succession with the development of eolian sandstones from the Chijinpu Formation to the Xiagou Formation in the study area, which is characteristic of a sedi- mentary succession of both a composition of special sedimentary facies and their evolutionary succession, not only becomes an important substance record for the further understanding of the Cretaceous uplift of the Qilian Mountains, but also provides another important record for the reconstruction of the atmospheric cir- cle pattern in the East Asia.