Depositional Environment and Paleoclimatic Significance of the Early Oligocene Yaxicuo Group in the Hoh Xil Basin

Key words: 
• depositional environment /
• climatic event /
• Yaxicuo Group /
• Early Oligocene /
• Hoh Xil /
• Tibet plateau

Abstract: The Hoh Xil basin, which is the largest Tertiary sedimentary basin in the hinterland of the Tibet plateau, is bounded on the north margin by the Kunlun Mountains and the South Kunlun suture zone, and on the south margin by the Tanggula Mountains and the Kangbataqing fault. The basin contains a Tertiary sediment pile of approximately 5.4 km thick of fluvial mudstone, sandstone, and conglomerate and 0.4 km thick of limestone, which was reported to have a closed relationship with the continental collision and early uplift of the Tibet plateau. From lower to upper, the Tertiary sediments include the Early Eocene to Early Oligocene Fenghuoshan Group, the Early Oligocene Yaxicuo Group, and the Early Miocene Wudaoliang Group. Here we clarify the facies analysis of the Yaxicuo Group, which was magnetostratigraphically dated as 32.0-30.0 Ma, and discuss its paleoclimatic significance. The Yaxicuo Group is 670.0 m thick and consists mainly of violet alternating sandstone and mudstone with intercalated gray layered and tubercular gypsolith. The standard section GG2 of the Yaxicuo Group is situated in the southern part of the Wudaoliang area of the Hoh Xil basin. The Yaxicuo Group consists mainly of lacustrine facies, but contains fluvial facies in the lower-middle part. The lacustrine facies includes shore, shallow lake, and salt lake subfacies. They are characterized by coarsening and thickening upward of sandstone and siltstone. The fluvial facies consists of point bars, natural levees, and flood plains and is characterized by sandstone and conglomerate that fine and thin upward. Paleocurrent directions were measured from sedimentary structures of, for examples, lamination, planar cross-bedding, parallel bedding, and climbing-ripple cross-lamination. They indicate that the depocenter of the Hoh Xil basin was migrated eastward and northward during the period of 32.0-30.0 Ma. The very thin stratified and nodulated types of gypsolith started to suddenly abundantly develop in the lower of the Yaxicuo Group at about 32.0 Ma ago. However, only a few layers of very thin stratified of gypsolith was formed in the lower part of the Fenghuoshan Group, whereas the other parts of sediments belong to fluvial and lacustrine environment with normal salinity. Therefore, the development of the gypsolith at least reflects the drying event in the earliest Oligocene, which was recorded in the northern Tibetan plateau. The paleoclimatic event could be correlated well to the global cooling and drying events of the earliest Oligocene.