Magnetic Chronology of the Late Miocene Wulangou Red Clay in the Otindag Sand Field

Abstract

Asian environment is characterized by the uplift of the Tibetan Plateau, the formation of the Asian monsoon, and inland aridification during Cenozoic. However, there are significant controversies regarding the evolutionary patterns of the Asian monsoon and the historical records of inland aridification documented by different carriers in different regions. The Otindag Sand Field, one of the inland arid region in northwest China, is located at the eastern edge of the East Asian monsoon, which is highly sensitive to climate change. Owing to extensive Cenozoic aeolian deposits developed in the area, it serves as an ideal region for studying past climate and environmental changes. Previous research on the chronology of sandy land has primarily focused on the Quaternary, with limited attention paid to the stratigraphic age of Tertiary aeolian sediments, largely due to the absence of suitable geological markers. This study examined the basalt-lacustrine sand-aeolian red clay sedimentary sequence of the Wulangou section in the Otindag Sand Field. Paleomagnetism was used to determine the magnetostratigraphy of the aeolian red clay, and the 40Ar/39Ar isotope dating method was employed to obtain the absolute age of the basalt. Combined with stratigraphic correlations from the Baogeda Ula section, which is a standard stratigraphic section within the study area, and biostratigraphic ages of the Baodean Period, the ages of the lacustrine sand layers in the region were determined. This study established a complete chronological framework for the stratigraphic sedimentary sequence of the Wulangou section. Climate and environmental changes were analyzed based on the sedimentary characteristics, magnetic susceptibility, and grain size of the strata. The research results indicated that the red clay magnetic stratum captured the geomagnetic polarity sequence from C5n.2n to C4n.2n, spanning approximately 11.04–8.06 Ma. The ages of the three aeolian sand layers interbedded in the lower part of the red clay were 11.04–10.91 Ma, 10.51–10.44 Ma, and 10.37–10.23 Ma, respectively. Since the late Miocene, the climate of the Otindag Sand Field has been predominantly cold and dry, undergoing a fluctuation that transitions from cold and dry, then shifts to warm and wet, and ultimately reverts back to cold and dry. From 11.04 to 10.02 Ma, red clay and aeolian sand interbedding developed, characterized by low magnetic susceptibility, coarser particles, and a relatively cold and dry climate with strong aeolian sand activity. Between 10.02 and 8.91 Ma, the magnetic susceptibility reached a high value stage with a finer grain size and relatively warm and humid climate. The loess layer-paleosol in red clay were alternately developed. The presence of aeolian sand layers in the red clay indicated active aeolian sand processes in the region as early as 11.04 Ma. These climatic changes since the Late Miocene were closely tied to the East Asian monsoon, driven by the uplift of the Tibetan Plateau and global ice volume changes, reflecting the regional response to intensifying aridification in inland Asia under a cold and dry global climate.

Keywords: Otindag Sand Field, Magnetostratigraphy, 40Ar/39Ar isotope ages, Late Miocene, Red clay