The Palaeogene Desert Deposits In The Jianchuan Basin, Eastern Margin Of Tibetan Plateau (SW China), And Its Significance

The eastern margin of Tibetan Plateau had been located in the arid band which was dominated by the planetary circulation until the modern East Asia Monsoon was established. Abundant evaporites that are closely related to the desert events were developed in this area. However, the exact records of the desert existence and the direct evidences of the atmospheric circulation haven’t been reported up until now. Thick red sandstones with large-scale high angle cross-beddings had been widely distributed in the Paleogene basins such as the Jianchuan Basin, Gongjue Basin and Yanyuan Basin, which were widespread in the eastern margin of Tibetan Plateau (SW China), but it is still controversial about their exact origin. To some extent, this restricts revising the Paleogene paieogeographic pattern in the eastern margin of Tibetan Plateau and reconstructing the evolutionary process of the Cenozoic arid belt and palaeoclimate in China.For this reason, this paper took the Eocene Baoxiangsi Formation in the Jianchuan Basin (western Yunnan Province) as study object and carried out systematic sedimentology studies, including field geological investigation, thin section authentication, grain size analysis, electron microscope scanning and palaeocurrent direction reconstruction, etc. Furthermore, the paleogeographic significances and palaeoclimatic implications from the evolutionary sequence and palaeocurrent changing regularity of the Baoxiangsi Formation in the Jianchuan Basin were also discussed. The following major conclusions can be drawn:(1)The important sedimentary characteristics of these thick red sandstones from the Eocene Baoxiangsi Formation in the Jianchuan Basin include:very high textural maturity and mineral maturity, the distribution characteristics of grain sizes coinciding with typical aeolian sands, and generally developed standard aeolian grain surface features and large-scale cross-beddings dipping at high angles. All these sedimentary characteristics are clearly different from aqueous deposits, but perfectly consistent with typical aeolian sandstones, indicating that these red sandstones should be attributed to aeolian origin.(2)In the middle Eocene epoch, the Jianchuan Basin had the major controlling factors led to forming desert deposits and developed the essential matched facies of the basin-range desert. Combined with the sedimentary characteristics of the Baoxiangsi Formation, it is demonstrated that the lower-middle part of the Baoxiangsi Formation should be basin-range desert sedimentary system. From the basin margin to the basin center, the sedimentary environments are, in order, pluvial fans, ephemeral rivers, aeolian dunes, deflational inter-dunes and sabkha, while the upper part of the Baoxiangsi Formation should be fan delta sedimentary system. This environmental drastic change of the Baoxiangsi Formation probably occurred in the late middle Eocene epoch.(3)By measuring the wind directional indicator of the Eocene desert in the Jianchuan Basin, the palaeoprevailing wind directions and their changing regularity were revealed. In the early stage, the dominant prevailing winds were westerlies, indicating that the study area was indeed controlled by the planetary wind system then; while in the late stage, the dominant prevailing winds transformed into northwesterlies with weak precipitation, implying the monsoonal circulations might have initiated. This transformation of the palaeoprevailing wind directions in the Jianchuan Basin could record the early transitional process from the planetary circulations to the monsoonal circulations.(4)On the basis of the discovered Paleogene aeolian deposits at present, it is confirmed that there were an aeolian dune enrichment zone in the eastern of Tibetan Plateau during the early-mid Paleogene, and we can’t excluded the possibility that it was an enrichment zone of Paleogene deserts, which consisted of series of small basin-range desert systems. Its existence was not only the direct evidence for the fact that the low-mid latitudes of China had been controlled by the subtropical high-pressure belt then, but also reliable sedimentary records for the fact that the southern boundary of the arid belt hadn’t drift northwards; while its disappearance was the sedimentary response to the northward migration of the southern boundary of the arid belt.(5)The Paleogene palaeoclimate evolutionary sequence in the study area was as follows:arid (hot)-semiarid (hot)-arid (hot)-subhumid (slightly hot)-humid (warm)-moist (warm). Its turning point should be the termination of the Eocene desert aeolian environments and the start of the following fluviolacustrine aqueous environments in the Jianchuan Basin. Furthermore, it is also confirmed that there were series of arid-humid transition events occurred successively from south to north in the eastern of Tibetan Plateau during the mid-late Eocene epoch. It is inferred that this could be related with the unremitting and irreversible evolutionary process of the East Asia Monsoon since the late middle Eocene.