Sedimentary characteristics in the Coqen Basin, Xizang during the Middle Jurassic to the Early Cretaceous

The Coqen Basin in Xizang lies between the Bangong-Nujiang suture zone and Yarlung Zangbo suture zone, and once extended northwards and southwards, respectively centred by the Qiekan-Goicang-Asog rift zone as the center of deposition and subsidence during the Middle Jurassic to the Early Creatceous. (1) During the Middle-Late Jurassic, the rift zone was occupied by deep-water turbidites, radiolarian siliceous rocks, shallow-water carbonate rocks, clastic rocks and basic-ultrabasic rocks, while both sides of it by littoral and shallow-marine clastic rocks and carbonate rocks. The sedimentary sequences have the features of deepening- and shallowing-upward sequences. (2) During the early and middle stages of the Early Cretaceous, the rift zone was still composed of basic-ultrabasic rocks, deep-water flysch clastic rocks, radiolarian siliceous rocks, shallow-water carbonate rocks and clastic rocks. The shallow marine clastic rocks and limestone occur in the Risum-Ge'gyai-Tarico stratigraphic subprovince, while the littoral-delta clastic rocks and volcanic rocks appear in the Coqen-Xainza stratigraphic subprovince in the south and Muggar Kangri stratigraphic subprovince in the north. All these sedimentary sequences exhibit a deepening-upward sequence. (3) During the late Early Cretaceous, the Coqen Basin was mostly covered by platform carbonate rocks, with platform-margin reefal shoal deposits developed along the rift zone. The open platform limestones are arranged in the Risum-Ge'gyai-Tarico stratigraphic subprovince, while the restricted platform limestones and terrigenous progradational clastic rocks are observed in the Coqen-Xainza stratigraphic subprovince in the south and Muggar Kangri stratigraphic subprovince in the north.