MARINE CRETACEOUS-PALEOGENE BIOFACIES AND ICHNOFACIES IN SOUTHERN TIBET, CHINA, AND THEIR SEDIMENTARY SIGNIFICANCE

The study of biofacies is of great significance not only for biostrati graphy, but also for the interpretation of sedimentary environments an d the reconstruction of paleogeography. The areas of Tuna, Gamba and T ingri of southern Tibet, China, were located in the north margin of th e Indian Plate in the Cretaceous and Paleogene. The marine Cretaceous and Paleogene biofacies there can be divided into four groups using th e following criteria: (a) the benthic biota and biogenic structures in the sediments; (b) the paleoecology of the main fossils; (c) the gene sis of fossils. The four groups are euxinic biofacies, nektonic and pl anktic biofacies, benthic biofacies and redeposited biofacies, which i nclude 24 main biofacies types. Like biofacies, ichnofacies are import ant indicators of paleoenvironments. The Cretaceous-Paleogene trace fo ssils in the Tuna and Gamba areas are attributable to Nereites, Zoophy cos, Cruziana and Skolithos ichnofacies. Nereites ichnofacies indicate s 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 ic hnofacies can further be subdivided into Thalassinoides-Planolites and Zoophycos-Chondrites subichnofacies which reflect aerobic and disaero bic conditions, respectively The development of biofacies and ichnofac ies in vertical profiles shows the sedimentary history from the Early Cretaceous to the Paleogene in Tuna, Gamba and Tingri areas. A regress ion occurred in Tuna and Gamba from the Neocomian to the Early Paleoce ne. The sedimentary environments went from euxinic basin and open mari ne basin, shelf margin, open sea margin, platform slope, platform marg in, open marine platform to the sand barriers deposited in the interti dal zone or the upper part of subtidal zone. A transgression took plac e at the beginning of the middle Paleocene. The carbonate deposits for med in the open marine platform and open sea shelf environments overla y the sandstones of the Jidula Formation. The termination of marine de posits in these areas began in the Thanetian. The sedimentary history in the Tingri area differed from that in the Tuna and Gamba areas espe cially during middle Maastrichtian to Early Paleocene times. The sedim entary process in Tingi during this period took place in the lower par t, the middle part of a fan and seaward prograding delta plains. The m arine deposition in the Tingri area ended in Lutetian later than in th e Tuna and Gamba areas.