Paleo-environment Characteristics Of The Lower Cambrian Longwangmiao Formation In Eastern Sichuan

Based on the study of field sedimentary characteristics and the observation of thin sections,combining with carbon and oxygen isotopes,X-ray diffraction as well as the major and trace elements test,the paleo-environment characteristics of the lower Cambrian Longwangmiao Formation in eastern Sichuan are researched.The results show that the lithology is mainly carbonate rocks,forming 3 sets of obvious sedimentary cycles of dolomite and limestone in vertical.The content of terrigenous materials is higher at the bottom to middle part of the formation,and lower at top.The?¹³C values of carbonate in the Longwangmiao Formation vary from-1.897‰to2.694‰with a mean value of 0.077‰and the?¹⁸O values vary between-10.771‰and-7.281‰,-8.457‰on average.Vertically,?¹³C values initially decreased until the middle part of the profile and then increased up-section.The carbon isotopic compositions possessed-negative values with frequent occurrences of positive or negative excursions at the lower to middle part of the formation and positive values with small fluctuations at the other parts,and it can be identified 6 times positive excursions and 6 times negative excursions.According to X-ray diffraction and the major and trace elements experimental results,CaO content is high and stable,in the range of 37.23%to 55.57%,with an average of 49.10%.MgO content is in the range of0.48%to 6.84%,with an average of 3.23%.Terrigenous detrital minerals content is relatively higher at the bottom,with a maximum at L43 sample,decreasing sharply later,and remains stable low-value.Through the reconstruction of paleo-ocean environment,Eastern Sichuan was marine environment during the lower Cambrian Longwangmiao stage.The salinity of seawater is high and distributional stability,with bottom-up gradually increased.The ratios of MgO/CaO and Sr/Cu indicates dry hot climate.Seawater temperature mainly distributed between 18?and 30?,suggesting a warm or hot subtropical climate.The ratios of Sr/Ba and Sr/Ca reflect that transgression-regression occurred 3 times during the Longwangmiao stage.Trace elements indicate the ancient REDOX conditions are mainly oxygen-lean and oxygen-poor,with oxygen-rich in stages.In the lower Longwangmiao Formation,terrigenous detrital minerals are more,and gradually decrease as time goes on.