SEDIMENTARY CHARACTERISTICS OF THE UPPER TRIASSIC LANGJIEXUE GROUP IN SOUTHERN QIONGJIE, TIBET

The Langjiexue Group, herein refers to the lightly metamorphosed Upper Triassic terrigenous rocks exposing to the east of the northsouth directed and active YangbajingQiongdui Fault, and being constrained by faults both southern and northern sides. It is mainly composed of slate (phyllite) and lightly metamorphic clastic rock, in which the sandstone is of lithoclastic and / or feldsparthic graywack. The clastic grains in sandstone are of worse sorting and roundness, and both the component and texture maturities are relatively low, somewhat due to diagenesis. There are many sedimentary structures such as flute, load cast, parallel crossbedding, ripple, etc. remained in the strata, and varieties of the Bouma sequence and depositional cycles are readily recognizable in the field. According to a postulated normal stratigraphic sequence of the Songre Formation, Jiangxiong Formation to Jiedexiu Formation in the Langjiexue Group, it is proposed that the lithofacies evolution has a tendency of outermiddle fan sandstoneslate package to middleinner fan gravel sandstoneslate, which could be separated by sandstone and conglomerate series at the middle Lower Member of the Jiedexiu Formation. By combination of herein lithofacies, and southward paleocurrent and recycled orogenic belt tectonic background of sources cited in an unpublished report, it is supposed that there could be a bigger lobular turbiditic fan progressed southward in the study area and its vicinity.