Magnetostratigraphy of deep drilling core 15YZK01 in the northwestern Qaidam Basin (NE Tibetan Plateau): Tectonic movement, salt deposits and their link to Quaternary glaciation

摘要

The thick Cenozoic sediments in the Qaidam Basin contain considerable information about the uplift of Tibetan Plateau, climate change and salt deposits. In this paper, a palaeomagnetic study was performed on the 1400-m-long drilling core 15YZK01 from the Yiliping playa in the northwestern Qaidam Basin to obtain a timescale documenting this information. The polarity sequence consisted of 17 pairs of normal and reversed zones and can be correlated well with subchrons C2Ar-C1n of the geomagnetic polarity timescale GPTS 2012 (from ca. 4.1 to 0.002 Ma). Three periods of high sedimentation rates occurred at ca. 4.1-3.207 Ma, 2.581-1.945 Ma, and 0.781-0.002 Ma. These 3 periods correspond to 4 episodes of tectonic activity on the Tibetan Plateau: the Qingzang Movement (3.6 Ma), the B phase of the Qingzang Movement (2.6 Ma), the Kunlun-Huanghe Movement (1.1-0.7 Ma) and the Gonghe Movement (0.15 Ma). The dips of the formations in the drilling core indicate that the Yiliping area was uplifted at ca. 3.236 Ma. After this uplift, the sedimentation rate sharply decreased from 39.1 cm/ ka to 19.2 cm/ka. In the Pliocene, the Yiliping area mainly featured a freshwater environment, and the earliest gypsum sediment formed at ca. 3.530 Ma after the Qingzang Movement. After the B phase of the Qingzang Movement, the Yiliping area alternated between freshwater and semi-saline lake environments, and 20 layers of gypsum-bearing strata began to appear since 2.103 Ma. After the Kunlun-Huanghe Movement, the Yiliping area alternated between semi-saline and saline lake environments, and salt deposits began to form at 0.733 Ma or 0.746 Ma in the middle Pleistocene. Four salt-forming periods in the Yiliping area can be correlated with Quaternary glaciations on the Tibetan Plateau: the S1 deposit corresponds to the Wangkun Glaciation; the S3 and S4 deposits and the mirabilite-bearing stratum between them correspond to the Penultimate Glaciation; and S5 began to form in the Last Glaciation. The calculated formation age of S2 has to be considered with caution, and it is very close to the timing of the Wangkun Glaciation. A reduction in the water supply of the salt lakes during the Quaternary glaciations was an important driving factor in the formation of salt deposits in the Qaidam Basin, and the Quaternary glaciations were closely related to the tectonic activity of the Tibetan Plateau. (C) 2017 Elsevier Ltd and INQUA. All rights reserved.