Different Developing Mechanism Of Fine-grained Sediments In Cenozoic Saline Lakes In The Qaidam Basin And Its Geological Implications

Fine-grained sediments refer to clay-silt sediments with a particle size of less than 62 μm.They are mainly composed of silt,clay minerals,carbonate,organic matter,and carbonaceous compounds.Fine-grained sedimentary sediments are usually considered as potential source rocks,and are a good indicator of the paleoclimate and paleoenvironment.A better understanding of the origin of fine-grained sediment deposition can improve oil and gas exploration,help reconstruct the paleoenvironment,and understand the Earth’s evolution.However,the origin of fine-grained sediment deposition is still controversial,and the related research is considered as one of the most difficult challenges in sedimentology.The Qaidam Basin is the largest Cenozoic saline lacustrine basin in the Tibetan Plateau.The great thickness of fine-grained sediments is deposited in the Paleogene western Qaidam depression and Neogene Yiliping Depression respectively.These two sets of fine-grained sediments are also deposited in the depocenter of the saline lacustrine basin at that time.On the one hand,as a witness of the evolution of paleoclimatic and paleogeographic patterns in the Tibetan Plateau and even Central Asia,the study of these Neogene lacustrine fine-grained sediments is of significance.On the other hand,in aspect of the hydrocarbon exploration in the Qaidam Basin,comparing to the Paleogene lacustrine fine-grained sediments which is the main source rocks and hydrocarbon-bearing reservoirs,there is no significant hydrocarbon discovery in the Neogene lacustrine fine-grained sediments.The vastly different exploration results suggest different causes.At present,there is no systematic research on the specific differences between them and the root causes of their differences.The Qaidam Basin has experienced significant climatic-tectonic-paleogeographic evolution and transformation in the Cenozoic,including the retreat of the Paratethys Sea,the uplift of the western ranges(Pamir-Tianshan-Altai Mountains),the uplift of the Tibetan Plateau,and the global temperature changes.However,there is a lack of in-depth research on the link between the climate-tectonic system of Central Asia and the Qaidam Basin,which may fundamentally hinder the understanding of the difference of Cenozoic fine-grained sediments in the Qaidam Basin.In view of the above problems,based on the comparative study of the Paleogene and Neogene fine-grained sediments in the saline lacustrine basin,this paper clarifies the developing differences of the fine-grained sediments deposition in the Qaidam Basin and its primary factors.The research results mainly include the following aspects:(1)The fine-grained sediments in the Paleo/Neogene in the Qaidam Basin are dominated by fine-grained mixed sediments,and the relative contents of siliciclastic,carbonate and clay components are all less than 50%.The siliciclastic,carbonate,and clay components in the Neogene fine-grained rocks are mixed sedimentary in a ratio of nearly 1:1:1,while the content of siliciclastic components in the Paleogene fine-grained sediments is lower,and the relative content of carbonate is higher.(2)The Neogene fine-grained sediments are deposited in the weak oxidizing-oxidizing conditions with an average sedimentation rate of nearly 300 m/Ma,while the Paleogene fine-grained sediments are deposited in the dysoxic-anoxic and more saline environments,with an average sedimentation rate of about 200m/Ma.The Neogene paleowater depth fluctuates in the range of about 15-35 m in a shallow lake environment,whereas the Paleogene fine-grained sediments are deposited in a semi-deep lake environment.The Neogene fine-grained sediments are deposited in semi-arid and arid cold climate conditions,while the Paleogene fine-grained sediments are deposited in warmer/hotter climate conditions.(3)The uplift of the Tibetan Plateau and the retreat of the Paratethys Sea in the early Cenozoic are the main controlling factors of the depocenter of the Paleogene saline lacustrine basin in the Qaidam Basin,and the fine-grained sediments are cause of deep-water in fluvial-lacustrine source-to-sink system controlled by regional faults.The changes in mountain topography of Central Asia and global temperature are important factors affecting the evolution of the basin since the Neogene,and the fine-grained sediments are cause of shallow-water storms in a wind and fluvial-lacustrine source-to-sink system.(4)Although tectonic uplift in Central Asia such as the NE Tibetan Plateau uplift and western ranges uplift,nearly penetrates the Neogene,at least in ～16-9.5 Ma,global temperature change is the primary factor of regional climate,and the NE Tibetan Plateau uplift is the secondary factor.From ～9.5 to 4.3 Ma,although the global cooling continues to affect the Central Asia,the influence of the interaction between the mid-latitude westerlies and western ranges on the hydroclimatic pattern is manifested in this period,together with the NE Tibetan Plateau uplift,controlling the regional climate.Since ～4.3 Ma,the intensified aridity in Asian interior may be the result of combined effects of global cooling and the western ranges uplift.(5)The saline lacustrine basin in the Paleogene depocenter has good primary productivity,due to warm climate and westerly water vapor.The wind and fluvial-lacustrine source-to-sink is not conducive to increasing the organic matter abundance in the Neogene.