Abstract:

：In order to study the paleoceangraphic conditions following the mid-Cretaceous oceanic anoxic events （OAEs），we have carried out a rock magnetic study of the Cretaceous shallow marine strata at the Cenomanian/Turonian and Santonian/Campanian boundaries at Gongzha，Southern Tibet, China. The results show that mineral magnetic properties at the two intervals are largely similar including mineral content and particle size, except that the Cenomanian/Turonian boundary statra contain high coercivity magnetic minerals such as hematite and goethite; whereas. Santonian/Campanian boundary strata are dominated by low coercivity minearls such as magnetite. Becaue sea levels at these two intervals are similar， the effects of sea level change on the magnetic characteristics are minimal. The difference in the types of magnetic minerals is probably mainly due to changes in the marine environment. The presence of high coercivity magnetic minerals in the Cenomanian/Turonian boundary strata, but absence in the Santonian/Campanian boundary strata suggest that the shallow marine condition at Gongzha during the Cenomanian/Turonian was more oxic than that during the Santonian/Campanian interval. This observation is different from what is seen in deep ocean where anoxic conditions characterized the Cenomania/Turonian interval and oxygen-rich conditions occurred during the Santonian/Campanian interval. It can be concluded that the deep-bathyal environment represented by Gyantse area and the shallow marine environment represented by Gamba-Tingri area responded differently to climate change in the mid-Cretacrous. And the evolution of the shallow water and deep water were probably different during these time periods and the ocean circulation in these periods may be more complicated than previously thought.

Key words: Tibet, Cretaceous, Cenomanian/Turonian, Santonian/Campanian, Rock magnetism