Permo-Carboniferous Clastic Sediments Of The Yongzhu Area, Xainza, Tibet And Their Possible Sedimentary Environments

From late Carboniferous to early Permian, glacierization affected the most areas of Gondwana. The related deposits formed by glaciation are widely distributed in Africa, Australia, Antarctica, India, and Pakistan. The Lhasa Block is located between the Bangong-Nujiang suture and Yarlung Zangbo suture, where the deposits were affected by the cold climate in Gondwana, forming pebbly mudstone and pebbly sandy shale. The record of glacial-marine sedimentation is also found in Tibet and Yunnan.Apart from the glacial-marine sedimentation and the cold water fauna formed by the affection of cold climate in Gondwana, normal littoral and shallow marine clastic rocks and carbonate rocks are also found. The lithology of Nibajiayo section in Yongzhu area, Xainza in Tibet is mainly siliciclastic, including conglomerate, sandstone, pebbly sandstone, pebbly mudstone (mixed conglomerate), siltstone, and shale, of which the components of gravel are complicated, mainly including the debris such as quartzite, sandstone, mudstone, basalt, and metamorphic rock. Several gravels have good psephicity.In the lower to middle part of the section, brachiopod fossil Neospirifer sp. with thick shell can be seen. In the middle of the section, small cross beddings develop in the fine sandy formation, and parallel beddings in the varved mudstone and siltstone. In the upper part of the section, we can see the sediments of shallow marine bioclastic limestones, containing the brachiopods Aulosteges ingense Hosking,Punctocyrtella nagmargensis (Bion) with thick shell in the limestone formation. On the basis of the field observations and the statistic analysis of thin sections, the author identified the sedimentary environment of clastic rocks from the Nibajiayo section.The clastic rocks of Nibajiayo section are mainly littoral and shallow marine sedimentation, of which sandstone and siltstone with good grain size and sorting, relatively bad psephicity, and relatively high maturity. In local formations, there are gravels, whose diameter larger than 5 centimeters with good psephicity, sparsely distributed in muddy sandstones. We speculate that they are the characteristics of debris flow deposits. Due to the affection of cold climate, the formations of conglomeratic sandstone and conglomeratic muddy sand show the characteristics of littoral and marine facies deposits. We can see gravels and debris distributed in the sandy mudstone and muddy sand formations. The colors of these rocks are mostly gray, glaucous, or mignonette, which have some relationship of the affection by the cold climate, indicating that the depositional environment was weak reduction environment at that time.