MID-CRETACEOUS GEOCHEMICAL ANOMALY AND THEIR RESPONSES TO SEA-LEVEL CHANGES IN TINGRI OF TIBET

A series of light dark gray marine clastic and carbonate rocks of middle Cretaceous (late Cenomanian to early Turonian) are well exposed in the Gongzha Section of Tingri in southern Tibet. The strata near the Cenomanian Turonian boundary can be subdivided into the Lengqingre and Gangbacunkou formations in ascending order, and the Cenomanian Turonian boundary located in the upper part of Lengqingre formation. The characteristics of microfauna and geochemistry from the strata show that tremendous changes took place in marine middle Cretaceous. In this paper, we mainly study the middle Cretaceous geochemical characteristics and their response to sea level changes in southern Tibet of China. The carbon, strontium and Uranium (U), Thorium (Th), Kalium (K) isotope composition of marine rocks are mainly controlled by global events that are related to the changes of sea level, such as relative amount of organic accumulation, changes in seafloor spreading rate and palaeoclimate variation etc. The δ 13 C curve of Tingri in middle Cretaceous is characterized by low high low cycle change, and the positive excursion of δ 13 C values happens near the Cenomanian Turonian boundary. Those characteristics coincide with sea level changes, accumulation of organic carbon and oceanic anoxic event during this period. The changes of U, Th and K abundance values near Cenomanian Turonian boundary in Tingri indicate that their compositions are higher during oceanic anoxic period because of increase of organic materials and clay minerals, which are the results from the changes of sea level. In southern Tibet, the 87 Sr/ 86 Sr ratio in middle Cretaceous is 0.7076. It is obvious that the value is rather low. We suspect that it should be related to the tectonic activity and big change of sea level in this time. In southern Tibet of China, middle Cretaceous is a period of short time expansion of the Tethyan ocean, which brings about sea level rises, accumulation of large amount of organic carbon and oceanic anoxic event. Some geochemical anomaly changes during this period, such as the positive excursion of δ 13 C values and U, Th and K abundance values, the relatively low 87 Sr/ 86 Sr ratio and so on, correspond to the sea level rises. There is no doubt that those geochemical anomaly changes are sensitive indicators of the sea level changes in middle Cretaceous.