Study on Deep Water Resedimented Origin of Saline Sediments of E₁f₄ in Zhaoji Sag, Hongze Depression, Northern Jiangsu Basin

Key words: 
• Zhaoji sag /
• member 4 of Funing Formation /
• halite /
• thenardite /
• glauberite /
• salt-forming models

Abstract: The saline sediments' characteristics of member 4 of Funing Formation of Paleogene (E₁f₄) in Zhaoji sag of Hongze depression, North Jiangsu Basin were studied by means of mud logging, well logging, paleontological and geochemical data, in order to find out the origin of saline sediments of E₁f₄ in Zhaoji sag, Hongze depression. It's found that the E₁f₄ saline formation mainly develops halilyte, sulfate rock, and a small amount of carbonate rock, and saline mineral layers are relatively thick, pure and distribute steadily on the lateral direction. Through comparisons, it's proposed that much saline substance of Pukou Formation of Late Cretaceous (K₂p) in Huaian Uplift was dissolved and transported into the deep water of Zhaoji Sag, and then made the saline substance reach saturation, finally resedimented to form the saline formation of E₁f₄. Compared with the saline sediments of K₂p in Huaian Uplift, salt rocks of E₁f₄ in Zhaoji Sag are much purer and cleaner, and inter-salt rocks are mainly gray-dark or gray mudstone, and moreover, shallow water evaporation structures have not been discovered; ostracoda fossil is dominated among the aquatic fossil, but gastropod and charophyta fossils living in shallow water are poor, furthermore, the Cicat ricosisporites pseudoanrifer, Welwitschiapitesningensis and Cicatricosisporites pseudoaurifer fossils only existing in the K₂p Formation show in the E₁f Formation; the type of brine of the two places is sulphate-chloride type, but the content of heavy metal elements of E₁f₄ in Zhaoji sag is much less than that of K₂p in Huaian Uplift. The salt-forming period of E₁f₄ in Zhaoji sag can be divided into two stages: the Upper Salt Bed Sub-member and the Lower Salt Bed Sub-member. Through comparative study on the Upper Salt Bed Sub-member and the Lower Salt Bed Sub-member, it is found that there is a big difference on the types and characteristics of the saline sediment and sedimentary sequences, therefore, the salt-forming models could be tried to establish according to the differences in the rate and composition of salt supplies. The salt-forming model of the Upper Salt Bed Sub-member is "the resedimented model :Halite-Thenardite-Halite" in the saline substance supplied quickly, near source and deep water sedimentary environment; and the Lower Salt Bed Sub-member is "the resedimented model: Glauberite-Halite" in the saline substance supplied slowly, near source and deep water sedimentary environment.