LATE CRETACEOUS OCEANIC OXIC EVENT IN TIBETAN HIMALAYAS

Late Santonian to early Campanian reddish colored mudstone intercalated with marlstone and limestone is exposed in Tibetan Himalayas. The latter strata, termed Chuangde Formation overlie the mid- Cretaceous "black shales". Detailed study of the red beds sequence demonstrates that it is characteristic by: 1) high Fe₂O₃ and low FeO; high content of iron trioxide is responsible for the red colour; 2) pattern of REEs show negative Ce anomaly; 3) the TOC is very low and ¦Â-carotene, which is sensitive to change in the oxidizing-reducing environment, is also relatively low; 4) ¦Ä¹³C values are lower than at any other correlative strata in the Tethys; ¦Ä¹³C is lowest at the base of the Chuangde Formation and increases stratigraphically upwards; 5) planktonic foraminifers are abundant in red limestones and marls, with highest frequency of biota occurrence in Tibetan Himalayas reached during late Santonian-early Campanian. All these evidences demonstrate that the red beds of the Chuangde Formation are especial sediments deposited in an oxidizing environment most probably as a result of high dissolved oxygen. The Late Cretaceous oceanic oxic event appear to signify complete reversal in ocean water chemistry and deep sea sediments diagenesis from the mid-Cretaceous, when organic rich black shales were deposited in dysoxic environment. Regional and global correlation of the upper Cretaceous red beds confirm that such deposits were widespread throughout the Neo-Tethys Ocean and do not only occur in deep ocean basins, below CCD, but extend up the continental slopes and rises. The "Ocean Oxic Event" can be result of various individual factors, or a result from a combination of factors. These include global eustatic sea level rise, decrease in supply of nutrients and organic carbon to the ocean reservoir, global cooling resulting in increased thermohaline circulation in the world oceans and major change in paleogeography, as a seaway allowing to circulate deep oceanic waters from the North Atlantic into southern ocean was established most probably in post Cenomanian time.