The Lacustrine Carbonates In The Cenozoic Hoh Xil Basin And Paleoenvironment Change

The lacustrine carbonates in the Cenozoic Hoh Xil Basin of northern Tibet are studied in this research. The article is trying to clarify the evolution of the lake systems' environment during Eocene-Oligocene and Miocene and its major contibutor, with studies in detail of the petrologic characteristics, X-ray diffraction, SEM research, the carbon and oxygen isotopes composition and the trace elements content of the lacustrine carbonates. The results of petrologic studies show that the lake systems trended to been closed-basins from the open lakes during late Eogene and early Neogene, with a increasing salinity and a decreasing biology variety. Bothδ¹³C andδ¹⁸O values show a stable positive trend while the samples get younger in the stratigraphies, with a sharp positive shift at the line of Oligocene Yaxicuo Group/Miocene Wudaoliang Group. These characteristics and the positive covariance betweenδ¹³C andδ¹⁸O of Miocene Wudaoliang Group lacustrine carbonates further suggest of the change of the lacustrine depositional setting; Major/trace-element contents are considered to be potential indicators of the different carbonate depositional environments. The allogenic (aeolian) elements(Ti, K, Na), which have high positive covariance with Al element, show sharp decrease at the line of Oligocene/Miocene whereas the authigenic (lacustrine) elements show the opponent. The Na: Ti and Na: K ratios, which are considered to be indicators to the degree of aridity of paleoclimate condition, show both similar trends that probably reflect more dry and hotter climatic conditions in Miocene. By the contrast studies to the uplifting of the plateau and Global-changing of climates during late Eogene and the earliest Neogene, we suggest that the prominent change in sedimentary pattern of the lake systems in Hoh Xil Basin mainly reflects the influence of the palaeoclimatic evolution of the internal plateau, which is mostly due to the uplift. Thus, we suggest that the palaeoaltitude of the Tibetan Plateau have been so high that played a major role in the climate systems of the internal plateau during this period.