A Simple Analysis of Influential Factors for Cretaceous Marine Organic-rich Sediments in Southern Tibet

Cretaceous marine organic-rich sediments are extensively found in southern Tibet, and they consist of black shales formed by the Cenomanian-Turonian Anoxic events that have received considerable interests. However, the total organic carbon (TOC) content in this area is much lower than that found in the black shales of western Tethys. Therefore, understanding the control mechanisms of the organic-rich sediments is important. In this study, four representative sections in southern Tibet, Zongshan section in Gamba area, Gongzha section in Tingri area, and Gyabula and Chuangde sections in Gyangze area, were selected to quantitatively calculate the sedimentation rate, accumulation rate, and paleoproductivity using related published data. The result showed that the highest sedimentation rate occurred at the Cenomanian-Turonian transition, and the Gamba, Tingri, and Gyangze areas showed different sedimentation rate because of their different sedimentary environment. Furthermore, when the sedimentation rate is < 4 cm/ka, the content of TOC increased with the increasing of the sedimentation rate. However, with the higher sedimentation rate, no obvious correlation exists. The primary paleoproductivity in the Gyangze area is low, and formation of the black shales is not the result of high paleoproductivity. It may be related to the preserved anoxia. Although the calculated paleoproductivity in Gamba and Tingri areas is 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 are excellent for the deposition of organic-rich sediments, existed in the Gamba and Tingri areas. The interplay of these two factors together with the sedimentary rate controls the deposition of the organic-rich sediments. The low TOC content and low values of calculated paleoproductivity were the result of intensive weathering at the later stage of diagenesis. The low TOC content during the Cenomanian-Turonian in Tingri area is also partly related to its grain size.