Sedimentology And Paleoclimate Records Of Paleogene Strata In Tajik Basin

The history of Cenozoic inland aridification in Asia and its specific connection with the uplift of Tibet Plateau,the retreat of the Paratethys Sea,the global cooling of Cenozoic and other major geological and climatic events have been the focus of domestic and foreign paleoclimate research.Previous studies mainly focused on Loess Plateau and large-scale sedimentary basins and the northwest China,but lacked the Cenozoic geology-climate records in the arid areas outside China.The Tajik basin is located in the core of the arid region of central Asia,and a relatively complete early Cenozoic sedimentary sequence appears in the northeastern margin of the Tajik basin,which provides an excellent research carrier for the study of the environmental change history and the main controlling factors of the early Cenozoic in central Asia.Based on the previous chronology and sedimentology research of the working group,this paper focuses on the systematic Soil micromorphology,clay minerals and elemental geochemical analysis of the representative section of the Peshtova(PE)in the northeastern margin of the Tajik Basin to gain deep understanding of Early Cenozoic sedimentary environment evolution framework and paleoclimatic change history in the Tajik basin.This paper draws the following main conclusions and understanding:1)The results of soil micromorphology analysis showed that the grey-green lithology segment of the late Eocene strata in the Tajik basin contained a large number of microorganism paleontology fossils,which were shallow Marine facies deposits,and the common horizontal bedding of the tawn-brownish red lithology segment was fluvial facies deposits.The Oligocene strata are mainly unstratified,and the common traces of soil formation such as wormholes,iron-manganese cutan and clay coatings are aeolian deposits.The sedimentary structure of common horizontal bedding and fluvial action in Miocene strata is fluvial facies deposition.2)Clay minerals in early Cenozoic strata of Tajik basin include illite,chlorite,smectite,vermiculite,kaolinite,palygorskite,etc.The results of SEM analysis show that the late Eocene-Oligocene clay minerals are mainly clastic and no secondary clay minerals have been found.Secondary clay minerals with better crystallization degree are common in Miocene strata,which reflect the wetter climatic conditions of early Miocene.3)According to the elemental geochemical composition of fine-grained sedimentary rocks,the variation framework of the late Eocene to early Miocene chemical alteration index(CIA)of the Tajik basin and its source area was obtained.The results show that the CIA value of the late Eocene strata is between 62.5 and 67.1,with an average value of 64.9.The CIA values of the Oligocene strata ranged from 53.3 to 61.6,with an average of 56.9.The CIA values of the early Miocene strata ranged from 54.8 to 70.8 with an average value of 61.6.The variation trend of CIA and other chemical weathering indexes is generally consistent,which reflects that the chemical weathering intensity of the Tajik basin and its source area is higher in late Eocene,lower in Oligocene and higher in early Miocene.4)Combined with sedimentary facies,clay minerals and element geochemical indexes,we believe that the early-cenozoic climate of the Tajik basin has a changing trend of wet late Eocene,dry Oligocene and humid again since Miocene.