Shallow‐marine Cretaceous oceanic red beds from the southern Tethyan Himalaya, Tibet, western China: Biostratigraphy, microfacies analysis, and global correlations

Abstract

The “Cretaceous oceanic red beds (CORBs)” associated with “oceanic oxic events” have become a hot topic for geologists from around the world. In southern Tibet, the CORBs are widely distributed in the Upper Cretaceous of the northern Tethys Himalayas, but they have never been reported from the southern Tethyan Himalaya (Tibet, China). A mixed Upper Cretaceous carbonate‐shale succession is well exposed in the Gyabukeqing section of Guru town, Yadong County, southern Tibet, which tectonically belongs to the southern Tethyan Himalaya. These sedimentary strata constitute the transition from the Gangbacunkou Formation shale towards the Zongshan Formation limestone through the Jiubao Formation alternations of marl and shale. The Gangbacunkou Formation is characterized by grey‐greenish shales occasionally interspersed with laminated marls, followed by the Jiubao Formation composed of marls with interbeds of shales, which is overlain by the limestones of the Zongshan Formation. A 5‐m‐thick purplish marl bed was found from the lower Jiubao Formation in the Gyabukeqing section, and these reddish marls are assignable to shallow marine red beds. Detailed foraminifer biostratigraphy established five planktic foraminifer zones: Dicarinella covcavata, D. asymetrica, Globotruncanita elevata, G. ventricosa, and Radotruncana calcarata throughout the entire Upper Cretaceous succession in the Gyabukeqing section. The reddish marl beds are constrained as middle Campanian in age as they are calibrated to the middle part of the foraminifer Radotruncana calcarata Zone. As a comparison, the typical CORBs of pelagic–hemipelagic settings are also documented from the Upper Cretaceous of the northern Tethys Himalayas in southern Tibet. A total of 10 microfacies were recognized from the entire sequence of the Upper Cretaceous outer shelf‐slope settings in the Yadong area, southern Tibet. The Yadong red marl beds represent the sedimentation of the foraminiferal biomicrite/foraminiferal wackestone and mudstone/mudrock microfacies, pointing to a shallow marine continental shelf margin setting. Global biostratigraphic correlations indicate that the Yadong red marl beds are coeval with the typical CORBs of the Gyangze and Kangmar documented from the northern Tethys Himalayas in this study or reported previously from the same region as well as elsewhere in the world. The Yadong red beds also are comparable in lithology and biostratigraphy with the shallow marine red beds documented from Europe, and they all may represent the contemporaneous deposition of the CORBs in shallow shelf. These shallow marine CORBs share similar depositional mechanisms: undergoing a phase of iron and manganese enrichment in seawater due to injection of deep sea anoxic water to shallow shelf, followed by oxidization in shallow settings or being oxidized during diagenetic process or a response to the oxygenation of coeval CORBs in deep seas.