PALEOCLIMATE VARIATIONS DURING LATE EOCENE TO EARLY OLIGOCENE IN WEIHE BASIN,CENTRAL CHINA,REVEALED BY CARBONATE MINERALS CONTENT
The Late Eocene-Early Oligocene climate temperature drop(ca.34 Ma,Eocene-Oligocene Transition,abbreviates EOT)is one of the significant Cenozoic events.To study the influence of EOT on climate change in East Asia,exploring the evolution of paleoclimate,and analysing the relationship between regional environmental change and the global climate transition events,are vital to understand the climate and environmental systems.The Weihe Basin(34°～35°40′N,107°～110°35′E),located in Shaanxi Province,Central China,covers an area of about 20,000 km2,with a watershed area of 130,000 km2.The Cenozoic deposits in the Weihe Basin are good archives for recording climatic and environmental changes.The Bailuyuan Formation sedimentary sequence(34° 16′03″N,109°12′26″E;635 m above sea level)in the Weihe Basin can be divided into three units from bottom to top:the lower part(413.95～346.75 m)is thick brown-red silty mudstone or siltstone with horizontal bedding,which is unconformity lying on the Honghe Formation.The middle part(346.75～233.5 m)is mainly composed of thick grey white sandstone interbedded with thin brown red siltstone or argillaceous sandstone.The upper part(233.5～0m)is interlayered with grayish-white sandstone and brownish-red mudstone,and is in unconformity contact with the overlying Lengshuigou Formation with low angles.Based on the fluvial-lacustrine sediments of Zhijiagou section in Weihe Basin during the period of 34.89～31.51 Ma,we obtained a high-resolution sequence of total carbonate and calcite contents(836 samples),analyzed by the method of Fourier transform infrared spectroscopy transferring(FTIR).In addition,we observed the sediments by thin rock slice and Scanning Electron Microscopy(SEM).Microscopic examination of thin sections of rock shows that variations of sediment grain size correspond well with the lithofacies.During 34.89～33.89 Ma,high content of total carbonate and calcite(with average of 16.10％and 11.93％,respectively)indicates that paleoclimate is relatively warm and arid,and the sedimentary facies indicate a transition from shallow lake to lake delta facies.During 33.89～33.09 Ma,low content of total carbonate and calcite(with mean of 4.56％and 2.75％,respectively)indicates a cold and relatively humid paleoclimate,and the sedimentary environment is a transition from lake delta to shore-lake and shallow-lake facies,the paleo-water level gradually rises.From 33.09 Ma to 31.51 Ma,variations of total carbonate and calcite contents(with average of 14.94％and 11.20％,respectively)are in good agreement with variations of lithology.During this time,sedimentary facies was shore-lake facies,and lake level may be higher than that of 34.89～33.89 Ma,but lower than 33.89～33.09 Ma.The sub-Tethys Sea may not change significantly during the study period(33.89～31.51 Ma),there was not an intensive uplift of the Tibetan Plateau synchronized with the wet-dry variations in the Weihe Basin,therefore the sea-land configuration changes and surface uplift could have less influence on the wet-dry changes in the Weihe Basin during the EOT.Combined with global temperature series,we speculate that the humidity variations in the Weihe Basin from the Late Eocene to Early Oligocene may be controlled by evaporation which was driven by global temperature variations,however the exact driving mechanism needs further study.
Weihe BasincarbonatecalciteEocene-Oligocene transitionpaleoclimate in a warm period
万方数据
维普
