THE DEPOSITIONAL SEQUENCE AND PALAEOCLIMATIC AND PALEOENVIRONMENTAL VARATIONS AT LUSHI BASIN,CENTRAL CHINA DURING THE MIDDLE EOCENE
Eocene is one of the most important warm periods in Cenozoic era.Reconstruction of paleoclimate and paleoenvironment variations can help us to understand more about the Eocene world.Here we report a comprehensive analysis of age control,sedimentary sequence,and environmental indication of Eocene deposits in Lushi Basin,East Qinling Mountains,Central China.Lushi Basin(33°57′～34° 10′N,110°50′～111°14′E)is a rifting basin with an area of approximately 300 km2.Over 1000 m thick fluvial-lacustrine sediments of the Eocene continuously deposited in the basin,making it suitable for reconstructing Eocene climate and environment.(1)The youngest age peaks of detrital apatite fission tracks(AFT)and zircon U-Pb dating analyses can constraint the oldest depositional age of sampling sequence.Our youngest AFT age peak(63±12 Ma)and youngest zircon(ca.60 Ma)show that the depositional sequence in Lushi Basin is younger than ca.60 Ma.This result supports the previous biostratigraphy studies suggested that fossil assemblages of mammals in Zhangjiacun Formation to Lushi Formation can be compared to the Middle Eocene.(2)The lowest strata,Zhangjiacun Formation(73.3 m thick)shows transition of sedimentary facies from alluvial fan to river,and then to flooded plain or shallow lake,recorded the process of basin extension and early lake formation.Lushi Formation(265.4 m thick)overlying on Zhangjiacun Formation,mainly consists of carbonates and mudstones,recorded sedimentary faces of shallow lake→deep lake or semi deep lake→shallow lake or flood plain→ shallow lake or semi deep lake from the bottom to top,indicating the expansion,shallowing and recovery of Lushi paleolake.The uppermost strata,Dayu Formation(652.3 m thick)shows sedimentary facies transition from river to flood plain,and finally to alluvial fan,indicate that alluvial fans and rivers continued to be active after the vanish of lakes.(3)Paleoclimate and paleoenvironment variations revealed by sedimentary facies,fossils and types of chemical deposits in Lushi Basin shows a semi-humid to semi-arid climate during the Middle Eocene.At that time,there were active surface runoff,lack of eolian activities,forest,and lakes from freshwater to brackish water,with large diversity of animals lived around.Sediments in Lushi Basin provide important evidence for climatic and environmental variations of Central China during the Middle Eocene.
万方数据
维普
