Study On The Strata And Environment Of The Late Pleistocene In The Central Western Sichuan Area

A preliminarily lithostratigraphic and magnetostratigraphic study of loess in the central western Sicuan Plateau shows that the loess deposition commenced at 200 kaBP, and began to spread out extensively during 120-130 ka B.P. The loess strata in this area can be divided into five units as followings: the postglacial paleosol S0, the last glacial loess L1, the last interglacial paleosol S1, the next to the last glacial loess L2 and the next to the last deglacial paleosol S2. Specifically, the thickness of the loess-paleosol couplets varies at different area.In this thesis, synthesized analyses of paleoenvironmental indicators of loess in the central western Sicuan Plateau are carried out. Magnetic susceptibility, which is regarded as a primary proxy of pedogenic intensity and/or dust accumulation rate, display a high coherency with the deep-sea oxygen isotope records. The carbonate content used as a proxy of the summer monsoon, keeps in line with the Indian Ocean monsoon as reflected by the median lithogenic grain size variations in RC27-61 core. While the median grain size, which is adopted as a proxy of the winter monsoon, shows a higher correlation to the Indian Ocean Monsoon than that to the GRIP records and the deep-sea oxygen isotope records.The paleoclimatic oscillations in this region can be reconstructed as the following stages:①during the time of 200-189.61kaBP, the summer monsoon was dominated;②during the period of 189.61-129.84kaBP, the winter monsoon was dominated with exceptional warmth and humidity during 140-155kaBP;③during the time of 129.84-73.91kaBP, the enhanced summer monsoon and declined winter monsoon displayed the characteristic of"three peaks with two troughs";④during the period of 73.91-58.96kaBP, the enhanced summer monsoon alternated with the enhanced winter monsoon;⑤during the time of 58.96-24.11kaBP, the enhanced winter monsoon coupled with enhanced summer monsoon at the first sub-stage, showing an extremely cold and humid environment; then the winter monsoon subdued and the summer monsoon began to dominant;⑥during the period of 24.11-12.05kaBP, the winter monsoon was dominated;⑦during the stage of 12.05-0kaBP, the winter monsoon prevailed at the beginning, and then followed by the altithermal period of the mid-Holocene. After that, the Neoglaciation began.During the late Pleistocene, a series of paleolake formed on the central western Sicuan Plateau. The thickness of the lacustrine deposits varies at different place, e.g., the thickness at Jiaocang reaches over 98 m, and only 7.6 m at Jiajun. Furthermore, the thickness of the Taiping lacustrine deposits and the Raoba lacustrine sediments are 44.8 m and 11.4 m respectively. The Moxi terrace formed during the Holocene. The period of 120kaBP-10kaBP is the reign of lacustrine formation on the central western Sicuan Plateau.There are 12 terraces of the Daduhe River, among which T1, T2 and T3 formed at 10～20kaBP, 30～70kaBP and 100～170kaBP respectively, and T4-T12 formed between the Early Pleistocene and mid-Pleistocene. There are 9 terraces of the Min River, among which T2 and T3 formed at 10kaBP and 20～30kaBP respectively.There are three neotectonic movements in the central western Sicuan Plateau during the late Pleistocene. The first one began from 200kaBP, ended at 120kaBP. The second one and the last one occurred during 30-50 ka BP and 10kaBP respectively.