Magnetostratigraphy Of Xifeng Loess-paleosol Sequences In Gansu Province

The Xifeng loess section in Gansu Province is one of the earliest studied loess-paleosol sequences in the Chinese Loess Plateau. Many studies in different aspects including paleoclimate and paleomagnetism have been done by the pioneering scholars. However, due to the low sampling resolution and the uncertainties in the stratigraphic divisions of the early research, high-resolution magnetostratigraphic studies of the whole section in this area which spans to the late Miocene are still lack. There are still some paleogeomagnetic abnormal results which are difficult to explain.In this thesis, we choose the full section of the Hujiayaoxian in Xifeng area, which spans the late Miocene to the Holocene, to perform the high-resolution magnetostratigraphic studies. The study shows that the Xifeng loess section not only has a good record of the geomagnetic polarity events, but also records two geomagnetic polarity anomalies, the results are as follows:The M/B boundary occurs at the top of soil S8; the upper and lower boundaries of the Jaramillo Normal Subchron occur at the lower part of soil S10 and the top of loess L13, respectively; the upper and lower boundaries of the Olduvai Normal Subchron occur at the lower part of loess L25 and at the transition zone between loess L26 and soil S26, respectively; the G/M boundary occurs at the lower part of loess L33.The events of Kaena and Mammoth, four subchrons of the Chron 3n, as well as 3An1 n, 3An2 n, 3Bn subchron are all well recorded in the Neogene red clay sequence and the oldest age of the section is 7.2 Ma.A normal polarity zone occurs in loess L9(N2-1 in this section). The author thinks that it is reasonable to explain this phenomenon with remagnetization, instead of corresponding it to Kamikatsura or Santa Rosa geomagnetic excursion.In addition, another geomagnetic polarity anomaly with a shorter duration(N2-2) below the lower boundary of N2-1 is estimated the age of 0.956 Ma by interpolation methods. This age is not consistent with the age of Santa Rosa. The author infers that the N2-2 is also the result of remagnetization. A normal polarity zone from soil S22 to loess L24 has been identified in this section. According to the average anisotropy of the samples, the author determines that the normal polarity zone is aqueous secondary loess-paleosol sequences. Assuming constant sedimentation rate between the lower Jaramillo and upper Olduvai polarity boundaries, the midpoint age of this normal polarity zone is estimated to be last about 1.56 ~ 1.52 Ma. The age is correlated with the excursion of Gilsa. The author infers that the normal polarity zone is the record of Gilsa for the aqueous secondary loess-paleosol sequences.The deposition rate increased rapidly during 3.2 ~ 2.58 Ma in the Xifeng Section, which indicates that the eolian deposits on the Chinese Loess Plateau indirectly recorded the great ice age in north hemisphere during the late Cenozoic.