Abstract:

 A marine Cretaceous organicrich sequence deposited extensively in Southern Tibet, and the black shale therein correlated with CenomanianTuronian Anoxic Events has been receiving extensive interest. However, the TOC content is much lower than the black shale in West Tethys. Therefore, understanding the dominated mechanism of the organicrich sediments is important to the problem. We selected 4 cross sections in  Southern Tibet,which have been studied more intensively by the predecessors, namely, the Gamba ZongshanⅡ, the Tingri Gongzha, the Gyangze Jiabula and the Chuangde, to calculate the sedimentation rate, accumulation rate and paleoproductivity by using related data. The result showed that the highest sedimentation rate occurred at CenomanianTuronian, and the Gamba, Tingri and Gyangze areas had different sedimentation rate owing to their different deposited environments. Moreover, when the sedimentation rate <4 cm/ka, the content of TOC will increase with the increasing sedimentation rate; but there is no obvious correlation at the higher sedimentation rate. The primary paleoproductivity in Gyangze is lower, and the formation of the black shale is not the result of high paleoproductivity; it may be related to anoxia.  Although calculated paleoproductivity in Gamba and Tingri is also lower, the excursion of carbon isotope and speculated coastal upwelling indicates that the Cretaceous organicrich sediments had high primary paleoproductivity. The condition of paleooceanography e.g. high paleoproductivity and anoxia, which is excellent for the deposition of organicrich sediments, existed in Gamba and Tingri area. The interplay of these two factors together with the sedimentary rate controlled the deposition of the organicrich sediments. The cause of low TOC content and  thus of low calculated paleoproductivity was the intensive weathering at the late stage of diagenesis.

Key words: Key words: sedimentation rate; accumulation rate; paleoproductivity; TOC; Southern Tibet