CENOZOIC LACUSTRINE STROMATOLITES FROM THE SOUTHERN MARGIN OF THE JUNGGAR BASIN, NW CHINA AND ADJACENT AREAS: INDICATORS FOR PALAEOCLIMATIC AND TECTONIC EVOLUTION

Lacustrine stromatolites provide high-resolution and sensitive palaeoenvironmental and palaeoclimatic records of their respective drainage basins and beyond. A well-developed stromatolite bed was discovered within the clastic deposits of the Early Miocene Taxihe Formation in the southern margin of the Junggar Basin, north of the Tian Shan Mountains. Their sedimentological features indicate a near-shore stable environment of a shrinking lake. The study of the stable oxygen and carbon isotopes in this stromatolite bed revealed that δ¹⁸O V-PDB values range from -11.3‰ to -5.6‰ and those of δ¹³C V-PDB from -1.8‰ to 1.7‰. Lower values -10.2‰ and -8.2‰ respectively, were determined from nearby lacustrine limestones of Oligocene age. The co-variation and overall increasing trend of the δ¹⁸O and δ¹³C values indicate deposition in a closed basin during a period of aridification. This environmental change can be related to the major uplift of the Tian Shan Mountains, the southern border of the Junggar Basin, during the Early Miocene. Several δ¹⁸O and δ¹³C records of lacustrine stromatolites ranging in age from Oligocene to Middle Miocene were reported: from localities in the Northern Tibetan Plateau and western Qaidam Basin. The similarity of the individual stable isotope records in stromatolites from Northwest China, indicate an underlying regional cause. These stromatolites were deposited during the period of major uplift of the Tibetan Plateau and the Tian Shan Mountains. It is therefore concluded that the stromatolite records provide local snapshots of the effect of this major process on the atmospheric precipitation and environmental conditions in Northwest China.