Abstract:

Resolving sedimentary processes and constraining the ages of the Hualongdong site in Dongzhi County, Anhui Province, are crucial for researching ancient human evolution, as well as its living environment and adaptive survival behaviors. This paper comprehensively analyzes the geological, geomorphological, and sedimentary characteristics of the deposits at the Hualongdong site and its surroundings. In addition, we also examine the ²³⁰Th-²³⁴U-²³⁸U dating results of the speleothem debris and animal fossils collected during the excavation. It is confirmed that the deposits at the Hualongdong site formed from a mixture of cave deposits and surrounding rock fragments during the collapse of an ancient karst cave. The transition from the ancient karst cave to the accumulation at the Hualongdong site occurred in three main stages: the cave formation and development period, a relatively stable deposition period, and a rapid collapse and accumulation period. The collapse of the ancient karst cave and the formation of mixed deposits at the site occurred later than approximately 216 thousand years before present (ka BP), and ancient cave deposits containing fossils are the main components of the mixed accumulation at the site. Along with the dating results attained previously, the ²³⁰Th-²³⁴U-²³⁸U dating results of the recently discovered flowstone within the deposits indicate that the cave underwent at least two accumulation periods during its stable sedimentation phase, with early deposition occurring prior to 600 ka BP and later deposition spanning from around 331 to 275 ka BP. The mixing of deposition across at least two depositing episodes adds complexity to the chronological study of the site. By integrating the ²³⁰Th-²³⁴U-²³⁸U dating results of all available speleothem, fossil teeth and bones, this study speculates that the burial age of the most complete human skull fossil discovered at the Hualongdong site can be bracketed between 275 and 331 ka BP, placing it in the late Middle Pleistocene. We suggested that Uranium-series dating of representative fossil specimens can offer vital insights for the chronological study of the site and is strongly recommended for understanding the evolution of the fauna and ancient humans at this site.

Key words: Hualongdong site, deposition process, speleothem, mammalian fossils, U-series dating