松辽盆地嫩江组白云岩结核的成因及其环境意义

GENESIS AND ENVIRONMENTAL SIGNIFICANCE OF THE DOLOMITE CONCRETIONS FROM THE NENJIANG FORMATION IN THE SONGLIAO BASIN,NORTHEAST ERN CHINA

关键词:  陆相盆地  白云岩结核  成因

Key words:  continental basin  dolomite concretion  genesis

摘要:

松辽白垩纪陆相盆地嫩江组泥质铁白云岩结核主要呈顺层状产于灰绿色粉砂质泥岩、页岩与介形虫层的互层层序中。其同位素组成δ¹³C(PDB)=1.105±2.4,δ¹⁸O(PDB)=-5.174±3.5,⁸⁷Sr/⁸⁶Sr=0.7078±0.0005。结核多不切割层理。结核内部具水平纹理。该类结核形成于湖平面快速波动过程中的浅水期,属沉积结核和早期成岩结核,椭圆状外形是碳酸盐同生胶结过程中始终受到上覆泥质沉积物持续压实作用的结果。白云岩为海水-淡水混合成因。

Abstract:

The argillaceous dolomite concretions occur mainly in the intercalations of the greyish green silty mudstones,shales and ostracod beds in the Nenjiang Formatin of the Cretaceous Songliao continental basin.Their isotopic compositions are:δ¹³C(PDB)=1.105±2.4, δ¹⁸O(PDB)=-5.174±3.5;⁸⁷Sr/⁸⁶Sr=0.7078±0.0005.Generally speaking,these dolomite concretions do not cut the adjacent beddings,and internally have horizontal lami-nations.It is inferred that the concretions were formed in the shallow-water stage during the rapid fluctuation of lake level,and are assigned to sedimentary and early diagenetic concretions.The authors interprete the dolomite as having a mixed sea water-fresh water origin,and the ellipsoidal shape of the concretions as the result of uninterrupted com-paction from the overlying muddy sediments during syngenetic cementation of the carbon-ate sediments.