Oligocene syndepositional lacustrine dolomite: A study from the southern Junggar Basin, NW China

Despite the continuously growing understanding of dolomite over the years, the dolomite problem remains unclear. Lacustrine dolomite found in the Anjihaihe Formation (E3a) on the margin of the southern Junggar Basin provides an opportunity to look into the dolomite formation process in a lacustrine environment. The Anjihaihe dolomite occurs as 10- to 20-cm-thick white-gray layers interbedded with lacustrine dark mudstone and silty mudstone. Based on scanning electron microscopy, X-ray diffraction, major and trace elements, and carbon and oxygen isotopes, as well as magnesium isotope analyses, the Anjihaihe dolomite is a syndepositional low-order dolomicrite formed under a reducing environment in a saline lake. Unlike the usual bacterial sulfate reduction and evaporation background of lacustrine dolomite, no bacteria or evaporite evidence is found in this study. Rather, increased Mg concentrations and carboxyl group catalysis may be the main factors that facilitate the dolomitization process. The results of a one-dimensional diffusion-advection-reaction numerical model of the Mg isotopes indicate that the dolomitization time, degree of dolomitization and diffusion in thin dolomite layers have little effect on the Mg isotopes in the Anjihaihe dolomicrite. Under the assumptions that seawater δ26Mg equals −0.82‰ and the fractionation factor is 1.75‰ at 25 °C, seawater as the Mg source for the Anjihaihe dolomicrite would be a plausible explanation and fits well with our other results. This explanation suggests that the contemporaneous adjacent proto-Paratethys might have provided the magnesium ions by episodic transgression and had a paleoenvironmental influence on the northern Tianshan area during the Oligocene.