黄骅坳陷歧口凹陷沙一下亚段湖相白云岩形成环境

Deposition Environment of the Lacustrine Dolomite in Sha-1 Lower Submember of the Qikou Sag, Huanghua Depression

歧口凹陷沙一下亚段白云岩发育在整体湖扩背景下,通过对研究区白云岩镜下鉴定、扫描电镜分析、地球化学分析测试,发现了海绿石、胶磷矿和颗石藻等海相指相性化石,Sr/Ba比值、V/Ni比值及Th/U比值均指示海相环境特征,87Sr/86Sr比值也表明湖盆受到过同期海水作用,证实了海侵作用是歧口凹陷沙一下亚段湖盆咸化的主要影响因素。海侵不仅为近海湖盆白云岩的形成提供了部分Mg2+,更重要的是改变了湖盆水体性质,促进了白云岩化作用。结合研究区古地貌的"洼——隆"相间特征以及碳氧同位素呈线性条带分布表现出的非常好的相关性,表明研究区沙一下亚段时期湖盆水体循环较差,封闭性很好,从而使得海侵导致的湖盆咸化环境得以保持,形成了白云岩有利的发育环境。

The dolomite of the Sha 1 lower submember in Qikou Sag was developed under the background of lake expansion. Through the microscopic identification, scanning electron microscopy analysis and geochemical analysis and testing on the dolomite samples in the study area, some fossils indicating marine facies were discovered such as glauconite, collophanite and coccolith. It was also found that the lake basin was once effected by the sea water during the same period through the analysis of Sr/Ba ratio, V/Ni ratio and 87 Sr/S6 Sr ratio, which demonstrates that the transgression activities not only provide part of Mg^2＋ for the formation of offshore lake basin dolomite, but more importantly, change the nature of the lake basin water so as to promote the dolomitization. The ancient landscape with alternating ＂depression＂ and ＂uplifting＂ in the study area, and good correlation of carbon and oxygen isotope by its linear distribution suggest the lake basin had a poor circulation and good closure during the Sha-1 lower sub member period, which led to sustained salinization environment of the basin caused by the transgression, ensuring a favorable environment for the development of the dolomite.