Spectral Analysis of the Weathering of Yardang Buried Pterosaur Fossils——A Case Study of Yardang Near the No.2 Water Source of Dahaidao
ZHAO Wen-hua1, HAN Xiang-na1*, CHEN Cong1, WANG Xiao-lin2, 3, 4*
1. Institute for Cultural Heritage and History of Science & Technology, University of Science and Technology Beijing, Beijing 100083, China
2. Key Laboratory of Vertebrate Evolution and Human Origins, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing 100044, China
3. Center for Excellence in Life and Paleoenvironment, CAS, Beijing 100044, China
4. College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
Abstract The pterosaur fossils in the Hami area of Xinjiang are very important. The Hami Pterosaur Fauna is mainly buried in the Yardang of the Early Cretaceous lacustrine strata and enriched in tempestite. Once exposed up the ground, the fossils will undergo different degrees of natural weathering，especially when exposed to water or moisture. Due to the action of groundwater and intermittent runoff, serious salt efflorescence was found at the bottom of Yardang near the No.2 water source of Dahaidao, which caused the spalling of rock and the collapse of the upper Yardang. The weathering phenomenon of different layers from bottom to top are quite different, and the most intense weathering is found in the bottom rock. By sampling along Yardang stratigraphic section, the lithology and soluble salt types of each layer are determined by using scientific analysis technology, from a microscopic perspective to explain the mechanism of weathering of this Yardang. Polarized light microscopic observation and X-ray diffraction (XRD) results show that each stratum contains quartz, feldspar, calcite, and clay minerals. It is no difference in mineral composition, but there is a great difference in clay content. Accurate methods for identifying soluble salts types include ion chromatography (IC) to determine the content and types of soluble salt ions, Raman spectroscopy (Raman) and Fourier transform infrared spectroscopy (FTIR) to identify nitrate and sulfate, and the Scanning Electron Microscopy Energy Spectrometer (SEM-EDS) to distinguish chloride, nitrate, and sulfate. The results show that the soluble salts in the lowest stratum are mainly NaCl and Na2SO4, the middle stratum is NaCl and CaCl2, and the upper stratum is NaCl. The difference in salt type/content in rock layers is important for diverse weathering setting. Combined with the climate and geographical environment of this Yardang (extremely arid area, near the water source), this paper discusses the law of water and salt activities linked to the weathering setting, and illustrates the main weathering mechanism this Yardang. The weathering reasons of Yardang are that, the internal cause is the different lithology of each sedimentary layers, and the external cause is the comprehensive effect of water and salts (NaCl，Na2SO4，CaCl2). The strong water and salt activity caused the serious chemical weathering of Yardang with Hamipterus fossils. This weathering mechanism can also explain the strong weathering of Hamipterus fossils and their surrounding rocks after undergoing moisture erosion.
ZHAO Wen-hua,HAN Xiang-na*,CHEN Cong, et al. Spectral Analysis of the Weathering of Yardang Buried Pterosaur Fossils——A Case Study of Yardang Near the No.2 Water Source of Dahaidao[J]. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2022, 42(02): 561-567.
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