Marine Cretaceous-Paleogene biofacies and ichnofacies in southern Tibet, China, and their sedimentary significance

Abstract

The study of biofacies is of great significance not only for biostratigraphy, but also for the interpretation of sedimentary environments and the reconstruction of paleogeography. The areas of Tüna, Gamba and Tingri of southern Tibet, China, were located in the north margin of the Indian Plate in the Cretaceous and Paleogene. The marine Cretaceous and Paleogene biofacies there can be divided into four groups using the following criteria: (a) the benthic biota and biogenic structures in the sediments; (b) the paleoecology of the main fossils; (c) the genesis of fossils. The four groups are euxinic biofacies, nektonic and planktic biofacies, benthic biofacies and redeposited biofacies, which include 24 main biofacies types. Like biofacies, ichnofacies are important indicators of paleoenvironments. The Cretaceous-Paleogene trace fossils in the Tüna and Gamba areas are attributable to Nereites, Zoophycos, Cruziana and Skolithos ichnofacies. Nereites ichnofacies indicates an open marine basin environment. Zoophycos and Cruziana ichnofacies indicate shelf margin and open sea shelf environments. The Skolithos ichnofacies reflects the environments of sand barrier deposited in the intertidal zone and the upper part of subtidal zone. The Zoophycos ichnofacies can further be subdivided into Thalassinoides-Planolites and Zoophycos-Chondrites subichnofacies which reflect aerobic and disaerobic conditions, respectively. The development of biofacies and ichnofacies in vertical profiles shows the sedimentary history from the Early Cretaceous to the Paleogene in Tüna, Gamba and Tingri areas. A regression occurred in Tuna and Gamba from the Neocomian to the Early Paleocene. The sedimentary environments went from euxinic basin and open marine basin, shelf margin, open sea margin, platform slope, platform margin, open marine platform to the sand barriers deposited in the intertidal zone or the upper part of subtidal zone. A transgression took place at the beginning of the middle Paleocene. The carbonate deposits formed in the open marine platform and open sea shelf environments overlay the sandstones of the Jidula Formation. The termination of marine deposits in these areas began in the Thanetian. The sedimentary history in the Tingri area differed from that in the Tüna and Gamba areas especially during middle Maastrichtian to Early Paleocene times. The sedimentary process in Tingri during this period took place in the lower part, the middle part of a fan and seaward prograding delta plains. The marine deposition in the Tingri area ended in Lutetian later than in the Tuna and Gamba areas.