东濮凹陷古近系含盐地层层序特征及成因分析

Sequence characteristics and genesis of the Eogene salt-bearing formation in Dongpu depression

东濮凹陷古近系含盐地层的层序划分除了依据常规的层序边界识别标志之外,还应该考虑盐韵律的发育特征,使其层序地层划分更为符合含盐地层的实际情况,据此将东濮凹陷古近系含盐地层划分为2个二级层序,10个三级层序,使得盐韵律的发育过程与层序的发育过程得到了统一。含盐地层的三级层序(低频层序)结构中表现出特有的规律:湖侵体系域和高水位体系域中的盐岩层横向展布面积大于低水位体系域中盐岩层的横向展布面积。在高频层序特别是五级层序对应的准层序地层单元中,盐岩层与砂岩层表现出同时异相的沉积关系,它们与下部的暗色泥岩层构成一个完整的准层序地层单元,盐岩的高频层序发育模式为准层序地层高位蒸发成盐。

A sequence demarcation that integrates the conventional markers of sequence boundaries and characteristics of salt rhythmite is more practical for the salt-bearing Eogene in Dongpu depression. According to this demarcation, the Eogene can be divided into 2 second-order sequences and 10 third-order sequences, ensuring the unity of salt rhythmite development and sequence growth. The salt-bearing third-order sequence （low-frequency） composition displays a peculiar regularity： the horizontal distribution area of salt-bearing formation in Transgressive Systems Tract （TST） and Highstand Systems Tract （HST） is larger than that in Lowstand Systems Tract （L ST）. Within the high-frequency sequences, especially the subsequence units corresponding with the fifth-order sequences, salt and sandstone layers are synchronous deposits of different faces, and they form a complete unit of subsequences together with the underlying dark mudstone layers. The developing mode of highfrequency salt-bearing sequence is believed to be formed by high-stand evaporation.