Geochemical characteristics and their paleoenvironmental significance of eyelid-eyeball limestones:a case study of the first Member of Maokou Formation in central Sichuan Basin
Based on core observation and rock thin section identification,core samples of eyelid-eyeball limestones in the first Member of Maokou Formation are selected for major elements,trace elements,carbon and oxygen isotopes and rare earth element(REE)tests,and the geochemical characteristics and their paleoenvironmental significance of two lithofacies,eyelid and eyeball limestones in central Sichuan Basin are discussed.It is found that:(1)The mud mass fraction of eyelid limestone ranges from 10％to 45％,and that of the eyeball limestone is less than 10％.(2)The average values of δ13CPDB of eyelid and eyeball limestones are 4.38‰ and 3.84‰ respectively,the average values of δ18OPDB are-5.39‰ and-6.00‰ respectively,and the average of total w(REE)are 10.54 and 3.43 μg/g respectively;Ce negative anomaly and Eu positive anomaly are found in both eyelid and eyeball limestones,and the anomaly degree of eyeball limestone is higher.(3)The mass fractions of different major oxygenated compounds are significantly correlated with each other.The correlation coefficient of eyelid limestone ranges from 0.52 to 0.95(mean value is 0.69)and that of eyeball limestone ranges from 0.91 to 0.99(mean value is 0.96).(4)The mass fraction of trace elements,w(REE)and terrigenous elements of eyelid limestone are significantly higher than those of eyeball limestone,reflecting differences of original sedimentary materials between the two lithofacies and the higher input of terrigenous material in eyelid limestone.(5)Both eyelid limestone and eyeball limestone were deposited in an anoxic environment of high salinity seawater under dry and hot climate,with similar average paleowater depth,and average paleowater temperatures are 19.9 and 22.5 ℃ respectively.The climate was hotter during the eyeball limestone sedimentary period,and the periodic change of paleoclimate was the main factor of the difference in original sedimentary material.These research results are of great significance to the understanding of the understanding of eyelid-eyeball limestone genesis in the first Member of Maokou Formation in Sichuan Basin.
eyelid-eyeball limestonegeochemistrypaleoenvironmentthe first Member of Maokou Formationcentral Sichuan BasinSichuan Basin
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