Cretaceous black shale and dissolved oxygen content——A case study in southern Tibet

Abstract: Black shales have special lithological, paleontological and geochemical features, which are mainly represented by changes in faunal assemblage and abundance, an increase of organic carbon content and positive excursion of carbon stable isotopes. High surface water productivity is of paramount importance in organic matter deposition. Black shales were worldwide during the Mid Cretaceous and well marked the Cenomanian-Turonian transition. In southern Tibet, the extinction rates for planktonic foraminiferal species are 34%, and up to 50% for the benthos at the C/T boundary. A short positive δ13C excursion that shows a maximum value of 2.74‰ occurs at the top of the Cenomanian. The bioturbation almost disappeared; microburrows are pyritized; foraminiferal chambers are filled with pyrite microspherical clusters. Other geochemical indicators also show different degrees of anomalies. These features suggest that different degrees of oxygen-poor to oxygen-deficient states occurred in water layers at different depths of the ocean. The shales deposited simultaneously are the result of preservation of abundant organic carbon in the anoxic environment. The causes for the occurrence of this phenomenon are the influences of the sea-level rise and climatic change in the Mid Cretaceous. The changes of the conditions of the Tibetan Tethys Sea were markedly affected by the global and regional sea-level rises and the Mid Cretaceous black shale formed thus have the features consistent with those of the global shales.