Sedimentary Characteristics and Depositional Model of the Paleogene Dawenkou Formation in Eastern China: Insights from the Huanggang Depression

The Huanggang depression in eastern China is a significant Cenozoic salt-bearing basin that formed during the alternating dry and wet climate periods from the Eocene to the Oligocene. Despite the economic importance of the Huanggang depression, its saliferous model remains controversial. To address this issue, we conducted comprehensive analyses of the sedimentology and elemental geochemistry on the YZR1 borehole core, which hosts a relatively complete sedimentary record of the Huanggang depression, consisting of five lithofacies' assemblages. The combined lithofacies and geochemical ratios, including B/Ga, Sr/Ba, and V/(V + Ni), provide insights into the paleolake's evolution in the Huanggang depression. Our analyses indicated that the paleolake underwent a transition from a freshwater lake to a brackish water/saline lake, subsequently transforming into a salt lake, reverting back to a brackish water/saline lake, and ultimately returning to its original state as a freshwater lake. These changes are reflected in the sedimentary record and inform the six stages of evolution of the paleolake of the Dawenkou Formation. We propose two metallogenic models to explain the accumulation of the thick halite (LA1) and thin halite layers (LA2), respectively. LA1 is primarily dominated by halite deposition, forming in an extremely shallow water environment under arid climate conditions. In contrast, LA2 records the alternating deposition of halite, anhydrite, and mudstone, and formed in a shallow water environment under arid to semi-arid climatic conditions. LA1 has a much drier climate and higher salinity than LA2. Our results suggest that the salt-forming period in the Huanggang depression occurred from the late Eocene to the early Oligocene. The halite in the Huanggang depression formed in a shallow water environment, providing the basis for the halite deposition model of the depression. This study sheds light on the formation mechanism of halite in the Paleogene in eastern China.