Microfacies Of Carbonate Rocks And Sedimentary Environment Of Cretaceous In Gamba-gyangze Area,Southern Tibet

The study area is located in the Gyangze-Gamba area of south Tibet,where a set of continuous Cretaceous marine sediments developed.Cretaceous carbonates are well developed and sedimentary types are various in Gyantze-Gamba area.Combined with the previous research data,in this thesis,the typical Cretaceous sections in Gyangze and Gamba areas were analyzed by field investigation and indoor thin section analysis.The research progress or understanding were obtained as flowing:Based on the study of carbonate rock of Cretaceous in the study area in southern Tibet,six kinds of microfacies in Gyangze were recognized: interclastic micritic limestone(JMF-1),argillaceous micritic limestone(JMF-2),bioclasts-intramolecular micritic limestone(JMF-3),bioclastic-bearing micritic limestone(JMF-4),bioclastic micrite(JMF-5),fine-grained feldspar graywacke(JMF-6);nine microfacies in Gamba: bioclastic limestone(GMF-1),bioclastic-lithic marl(GMF-2),micrite(GMF-3),fine bioclastic siltstone(GMF-4),planktonic foraminifera-and-ostracod-bearing packstone(GMF-5),plankton packstone(GMF-6),benthic foraminifera packstone(GMF-7),bioclastic packstone(GMF-8),bioclastic limestone(GMF-9)Based on the analysis of spatiotemporal variation of microfacies types in the study area,four facies were divided in Gyangze: platform margin slope facies(FZ-4),widesea shelf facies(FZ-2),deep-sea basin facies(FZ-1),and deep-water shelf facies(FZ-3).Five facies in Gamba area: restricted platform facies(FZ-8),platform front slope facies(FZ-4),open platform facies(FZ-7),wide-sea shelf facies(FZ-2),platform edge shoal facies(FZ-6).The analysis of sedimentary environment shows that during late Cretaceous,northern Tethys Himalayas was relatively deep compared with southern Tethys Himalayas,with a geographical pattern “deep in the north and shallow in the south”.The characteristics of the uppper Chuangde Formation in Gyangz indicates a sedimentary environment of deep-sea basin facies,indicating that sea level was at its highest level in history.The characteristics of the Zongshan Formation in Gamba indicates a sedimentary environment of the slope facies at the edge of the platform,with low sea-level and high oxygen content.At the same time,the collision between the Indian plate and the Eurasian plate may have occurred during this period.