Transition from a hydrologically partitioned to an integrated lake in the Cretaceous Junggar Basin, Central Asia

Abstract

We report sedimentary microfacies analysis of Lower Cretaceous algal-laminated limestones interbedded with siliciclastic sediments in the Junggar Basin. We use these data to fill a knowledge gap in the Jurassic–Late Cretaceous paleoenvironmental and paleoclimate reconstructions of Central Asia. To decipher paleoenvironments and genetic mechanisms, we applied optical petrography, cathodoluminescence and fluorescence imaging, Raman spectroscopy, stable isotopic, and scanning electron microscopic analysis. Some oncoids exhibit alternating dark organic carbon-rich micrites and light sparry calcite laminae with unequal thicknesses and binding structures of microbiota. Formed by both algal binding and inorganic calcite precipitation, in-situ mixing happened during the formation of oncoidal, ooidal, and ostracodal grainstones in an agitated lakeshore and thromobolites in a relatively low-energy environment. Sparry carbonate cortices peeled off from the oncoids were transported to the shallow lake and densely packed with micrites and mud. Based on the degree of covariance between δ18O and δ13C and the wide range of δ18O, the paleo-lake was inferred to be a semi-open, shallow, and hydrologically partitioned lake in the Early Cretaceous, but evolved into a closed but well-mixed lake in the Late Cretaceous. We infer that this change is caused by the aridification in the Late Cretaceous. The Early Cretaceous lacustrine carbonates in the Junggar Basin show overall more positive δ13C than the counterparts in the Upper Cretaceous, which is attributed to the 12CO2 absorption by microbiota. Sulfate reduction is revealed by negative sulfur isotopes of concomitant thin-layered gypsums but it exerts no measurable impact on the δ13C of the carbonates.