新疆天山地区早更新世沉积及其下限

EARLY PLEISTOCENE DEPOSITS AND ITS LOWER BOUNDARY(Q/N) IN TIANSHAN MT.,XINJIANG REGION

经过对新疆天山地区第四纪地层的广泛、综合研究，提出该区早更新世由两套沉积组成：下部西域砾岩层，为随青藏高原隆升而发生的山前磨拉石堆积，后受盐水沟运动影响，岩层褶皱，组成山前成排的背斜构造，并受剥蚀。上部为五梁司砾石层，按岩性分为两层：下层为山麓冰川前缘冰水堆积；上层为经长途搬运的间冰期冰水沉积物。五梁司层与西域岩层呈不整合接触，并构成天山山前最高台地顶部。根据对安集海和独山子两剖面的地层岩性、古地磁、孢粉、重矿物等的综合对比研究，确定新疆第四系下限应依西域砾岩层底界年龄而定。本文指出西域砾岩层底界特征，并根据古地磁测试确定其底界位于古地磁柱高斯正极性时凯恩纳负极性亚时的顶面，年龄为２．９２ＭａＢ．Ｐ。本文还分析了新疆早更新世的构造背景和沉积环境。

Based on the comprehensive study on the Quaternary strata in the Tianshan Mt.in Xinjiang region, this paper proposes that the Early Pleistocene of that area consists of two sedimentary sequences:(1) The lower part──Xiyu Conglomerate Bed consists of greyish black, thickbeded conglomerate interbedded with sandstone and argillaceous sandstone, which is the molasse sediments in piedmont formed by the uplift of the Qinghai-Tibet Plateau,and later then folded by the Yanshuigou Movement and distributed as the anticlinal zone along the northern piedmont of Tianshan Mt. The Xiyu Conglomerate Bed is often in comformable contact with the underlying Grey-Brown Bed of Neogene Period.So, it is difficult to fix and date the lower boundary of the Quaternary (Q/N).(2) The upper part──Wuliangsi Conglomerate Bed, consists of two subbeds:the lower one consists of grey-yellowish sandstone, mudstone and breccia beds, which are the glacio-fluvial deposits in glacier-front in piedmont. The upper one is greybrown-reddish sandstone and well-rounded conglomerate beds which are the glaciofluvial deposits after a long distant transporting in interglacial stage. There exists an unconformity between the Wuliangsi Bed and the underlying Xiyu Conglomerate Bed. The Wuliangsi Bed forms the top deposit of the highest terrace in the Tianshan Mt. piedmont (Fig. 1).The key to solve the question of lower boundary of Quaternary in Xinjiang region is to find the boundary plane between the continuous sections of the Xiyu and the Grey-Brown Beds and to date its age. In area of Usn-Manas on the northern side of Tianshan Mt., the above-mentioned two Beds are continuously accumulated and well exposed. The sections at An jibal and Dushanzi here are selected for discussion based on the following considerations: 1) A continuous sequence involving the two units; 2) Completely exposed; 3) The Anjihai section bears well-dating fossil,Equns sanmenensis; 4) Rocks have not been affected by tectogenesis and metamorphism; 5) Exposures are accessible.In the field we confined our measurement to the lithologically transitional segment between the Xiyu Conglomerate and Grey Brown Beds because their exposures stretch for a too long distance. Respectively in the Anjihai and Dushanzi sections we made petrographic analysis, palaeomagnetic dating, sporopollen analysis, and heavy mineral analysis. The multiple studies and analyses conducted recently have produced a clearer understanding of the lower limit of the Quaternary in Xinjiang.The lithology and the paleomagnetic polarity columns of the sections at Anjihai and Dushanzi are shown in Fig. 2. The test and analysis reveal that both the polarity columns are predominatedly occupied by normal polarity interrupted by two reversed events. Taking into account the stratigraphical correlation over the whole Xinjiang, we are sure that our measurement in the sections grasped the lithologically transitional segment from the Pliocene sequence to the Xiyu Conglomerate, which is about 2.5 Ma B. P. It follows that the dominant normal polarity corresponds to the Gauss, and the two reversed events respectively to the Kaena and Mammoth. Giving a glance at the polarity and lithological columns, we can find the following facts:1. The Matusyama Gauss boundary goes upwards beyond the coverage of the both sections, which suggests an age older than 2.48 Ma B.P.2. The upper limit of the Kaena coincides with the evident lithological change line above which coarse-grained sediments previal. In Dushanzi section, the upper limit is represented by the boundary between Members 4 and 3, the former consists chiefly of conglomerate with sandy mudstone, upwards passing into Member 5 of entirely sandy conglomerate; while the beds underlying Member 3 predominatedly comprises sandy mudstone occasionally with sandy conglomerate, downwards gradating to Member 1 of mudstone. In Anjihai section, the limit lies within unit 13 in. top part of Member 4, about sin below the lithological change line that separates the uniform sandy conglomerate (Member 5) from the underlying f