洪泽凹陷赵集次凹阜宁组四段盐岩深水再沉积成因探讨

为了查明洪泽凹陷赵集次凹阜四段盐岩成因,利用录井、测井、古生物和地球化学资料,对苏北盆地洪泽凹陷赵集次凹古新统阜宁组四段(E₁f₄)的盐岩沉积特征进行了分析,发现阜四段含盐系地层主要发育石盐岩,硫酸盐岩次之,还有少量碳酸盐岩。石盐岩单层厚度大,纯度高,横向分布稳定。对比研究认为赵集次凹E₁f₄盐岩应为淮安凸起上白垩统浦口组(K₂p)盐岩经溶解搬运至洪泽凹陷赵集次凹深水湖盆中,使盐度过饱和后再沉积成因。赵集次凹E₁f₄比淮安凸起K₂p石盐岩矿层纯度高;盐岩洁净、盐间夹灰-深灰色泥岩,未见浅水蒸发构造;生物化石以陆相介形类为主,淡水滨浅湖的腹足类和轮藻化石稀少,在E₁f₄地层中存在仅见于K₂p地层中的Cicat ricosisporites pseudoanrifer化石以及Welwitschiapitesningensis、Cicatricosisporites pseudoaurifer等;两地盐岩的卤水类型均为硫酸盐-氯化物型,而赵集次凹E₁f₄盐岩重金属元素含量较低。赵集次凹阜四段存在两个主要成盐期:上盐亚段与下盐亚段,对比研究发现二者的盐岩类型、特征及成盐序列有较大差异,尝试根据盐类物质供给的成分和速率的不同建立了相应的成盐模式:下盐亚段为深水近源快速补给"石盐岩-无水芒硝岩-石盐岩再沉积"成盐模式;上盐亚段为深水近源缓慢补给"钙芒硝岩-石盐岩再沉积"成盐模式。

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

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.