藏南贡扎剖面赛诺曼阶/土伦阶与三冬阶/坎潘阶界线附近地层的岩石磁学对比研究

Rock Magnetism of the Cenomanian/Turonian and Santonian/Campanian Boundary Strata in Gongzha, Southern Tibet

为了研究白垩纪中期大洋缺氧事件及其后古海洋环境的变化,对藏南贡扎剖面白垩纪赛诺曼阶/土伦阶和三冬阶/坎潘阶界线附近的浅海相沉积地层开展了详细的岩石磁学对比研究。结果显示,这两个时间段的沉积物中磁性矿物含量和粒度无显著区别,但赛诺曼阶/土伦阶地层中含有高矫顽力磁性矿物,如赤铁矿和针铁矿,而三冬阶/坎潘阶地层中则主要为低矫顽力磁性矿物,如磁铁矿。由于海平面位置在这两个时间段相近,海面变化对沉积物的磁学特征的影响很小。磁性矿物种类的变化可能主要是由于海洋沉积环境的变化所引起的。高矫顽力磁性矿物在赛诺曼阶/土伦阶的出现及其在三冬阶/坎潘阶的缺失,表明赛诺曼阶/土伦阶氧化程度可能比三冬阶/坎潘阶更高。这与深海沉积所记录的赛诺曼阶/土伦阶为缺氧以及三冬阶/坎潘阶为富氧的特征明显不同。这说明以江孜地区为代表的深海—半深海环境和以岗巴定日地区为代表的浅海环境对白垩纪中期气候变化有着不同的响应。深水和浅水环境的演化在这两个时间段的显著差异表明相应时期的大洋环流也可能比以往所认识的更复杂。

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.