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

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.