Organic geochemical characterization of Upper Cretaceous oxic oceanic sediments in Tibet, China: a preliminary study

Cretaceous strata, spanning the Berriasian to Maastrichtian, crop out widely in southern Tibet. The sedimentary sequence can be divided into the Gyabula, Chuangde and Zhongzuo formations. The Gyabula Formation, which is Berriasian-Coniacian in age, is composed mainly of black shales characterized by relatively high total organic carbon (TOC) levels and a positive organic carbon isotope excursion near the Cenomanian/Turonian boundary, which can be correlated to oceanic anoxic events (OAE2) or with the so-called Bonarelli Horizon in the western Tethys. The oceanic sediments of the Chuangde Formation are represented by red beds of Santonian-Campanian age, and are characterized by low TOC values, a negative organic carbon isotope excursion, and low nC17/nC27 and nC17/hopane ratios, which indicate that the primary production and burial of organic matter during this period was low. We suggest that the climate at the time was cool and the oceanic water oxygen-rich.