西部黄土高原中新世风尘沉积剖面的磁性地层学和沉积学研究

Observation of the Miocene aeolian dust deposits in Qin'an area provide a long-timescale, almost continuous geological record for understanding significant environmental changes, such as the environmental evolution of Asian monsoons and the inland Asian aridification, and make possible new explorations of global and regional environmental evolutions and other relevant studies. However, the QA-I and QA-II sections that are previously investigated are near to each other, and there is a lack in understanding the spatial characteristics of changes in paleoenvironment. Therefore, new sections of Miocene aeolian dust deposits are needed to complete the present spatial framework. Meanwhile, it is necessary to carry out in-depth studies, which are already belated, on the depositional characteristics of these Miocene aeolian dust deposits.This thesis choose the Miziwan section and Shaogou section that are west to Liupan Mountain for magnetostratigraphic studies, results of which are compared with the QA-I section with respect to the lithology, magnetic susceptibility and paleomagnetic polarity stratigraphy. Quartz-fraction grain size of 300 samples from the QA-I section was analyzed. In addition, 30 out of these samples were determined in terms of the quartz morphological characteristics by using scanning electronic microscopy. On the basis of these data and the correlation with the Xifeng sequence of aeolian dust deposits, the following 3 conclusions are drawn as below:The upper and the lower boundary age of the Miziwan section are 11.6 Ma B.Rand 18.5 Ma B.R, respectively. The Shaogou section spans from 15.1 Ma B.P, to20.8 Ma B.R The lower boundary age of both sections is no older than that of theQA-I section. The Miziwan and the Shaogou sections can be well correlated withthe QA-I section in terms of the field lithological features, magnetic susceptibilityand magnetic polarity stratigraphy.Sedimentological evidence of the QA-I quartz grain size and quartz morphologicalcharacteristics further supports the previous conclusion that the QA-Iloess-paleosol sequence is of aeolian origin, and also verifies the accretionalfeatures in paleosols of this sequence in terms of the grain-size composition.Modifications of the grain-size composition of the original dusts due to later-stageweathering and pedogenesis are common characteristic of Miocene and Quaternary aeolian dust depositions. The above-mentioned processes further rework and refine the bulk grain size, which is more evident in paleosol layers. 3, Like the hipparion Red-Earth and the Quaternary aeolian deposits, the Miocene loess deposits are transported by near-ground winter monsoons. However, the average wind intensity evidenced by the quartz median grain size of Miocene loess, together with the maximum wind intensity by the quartz maximum grain size, is weaker than those of the Quaternary.