Late Cenozoic Evolution Of Sedimentary Environments Of Lop Nor And Salt Resources Effects

Lop Nur,generally known as the ‘sea of death' and ‘Asian arid polar',which is a fault basin located in the easternmost of Tarim Basin of China.Since the late Cenozoic,the basin experienced multi-phase sedimentary environmental evolution under the superimposed influence of tectonics,climate and provenance,leading to the formation of the Quaternary super-large brine potash deposit.By far,previous studies mainly focused on potash formation,and enormous progress in potash theory model and exploration has been achieved.However,the sediments in this region are dominated by evaporites,which greatly limited the application of traditional chronological methods,and hindered the establishment of stratigraphic chronologic frame as well as the further investigation of the relationship between phased environmental changes and formation of salt or potash.In this contribution,the first scientific drill for deep potash exploration in Lop Nur,LDK01,is investigated in combination with field geological surveys,which is aimed to build the chronologic frame and to reconstruct the phased evolution processes of the sedimentary environments in Lop Nur since late Cenozoic,and to investigate their responses to the regional climatic and tectonic events and relationship between the formation of salt and potash.The following conclusions are achieved:1.For first time,the chronological frame in the salt lake region of Lop Nur since early Pleistocene is established,by using U-series unequilibrium dating method of salt minerals including gypsum,glauberite and halite crystals.On basis of correlation with regional stratigraphic and this study,the boundaries among Early and Middle Pleistocene(0.78 Ma),Middle and Late Pleistocene(0.13 Ma),Pleistocene and Holocene of borehole(0.0115 Ma)LDK01 are set at stratigraphic depth of 263.80 m,43.10 m and 2.70 m,respectively.2.The phased evolution process of sedimentary environments of Lop Nur is revealed based on the chronological frame and related paleoenvironmental proxies.Since early Pleistocene ~0.61 Ma,the basin was in the stage of fault depression,which is characterized by fluvial-alluvial environments and dominated by semi-humid to semi-arid climate with dramatic oscillation.0.9 Ma~0.61 Ma,resulted from weakening of surrounding tectonics and shift of the basin into continuous depression from earlier fault depression,the sedimentary systems changed from unstable alluvial-delta to lacustrine or saline lake environment,which is characterized by decreasing of coarse sediments and increasing of fine sediments and wide occurrence of gypsum-bearing mud under an arid and semi-arid climate condition.0.61 Ma~0.25 Ma,it is the stage of disintegration of big lake of Lop Nur and the formation of Luobei Sag characterized by the glauberite giant deposited due to strong evaporation and concentration with arid increased.0.25 Ma~0.15 Ma,it was in the stage of climate transmitting dominated by general humid climate as characterized by dilution of lake with clastics increased,salts decreased and sediments shift to gray green mud-bearing gypsum or gypsum-bearing muds.0.15 Ma~0 Ma,it was in stage of shrinking to desiccation of salt lake characterized by extreme arid climate although interrupt with short-term humid.With increasing evaporation and concentration,potassium mineral such as sylvite and carnallite formed at the top of salt-bearing series.3.For the first time we discover and ascertain the Pliocene sediments in Dapingtai area in eastern Lop Nur is in aeolian origin,provding direct evidences for aridification of Tarim Basin.Based on the studies of lithology,detrital zircon U-Pb age data,REE and correlation with the modern Taklimakan Desert,we suggest the desert sediments is part of ancient Taklimakan Desert,desertification in the basin has developed a certain scale and landscape of basin should be characterized by co-occurrence of salt lake and desert.4.Phased evolution process of sedimentary environments in Lop Nor during the late Cenozoic is strongly controlled by multi-factors of regional tectonic and climate events.In early period,under oscillation of tectonics and climates condition,potassium was preliminarily accumulated and source beds formed in the basin.In the late stage,potash accumulated in a very short interval with a stable tectonic and extreme arid climate.Therefore,we suggest the formation of potash deposits in Lop Nor has characteristics of ‘multi-phase accumulation in early period and short-term explosive formation of potash in late period' and it's a metallogenic response to coupled tectonics,climate and provenances.