Paleoenvironmental Implication Indicated By Trace Elements Of The Ostracod Shell For Upper-middle Section Of Shangganchaigou Formation In The Western Qaidam Basin

Qaidam basin is the largest intermontane basin in the northern margin of QinghaiTibet plateau.It is an important Mesozoic-Cenozoic hydrocarbon-bearing basin in China.The thick sediments in the basin provide usable materials for studying the paleoenvironment and paleoclimatic evolution of the Cenozoic in the Qaidam Basin.The Ostracods in the sediments are good materials for paleoenvironment and paleoclimate,and trace elements and fossil itself have important significance in the study of paleoenvironment and paleoclimate.The study of the paleoclimate in the upper-middle sections of the Upper Ganchaigou Formation in the western of Qiadam Basin has great significance for the study of the Late Oligocene-Early Miocene inland aridification and the Qinghai-Tibet Plateau uplift.Through field geological investigation and drilling sampling,this paper collected 16 shell samples of Ostracod.The species of the shell fossils were identified and counted,and the shells Ca?V?Cr?Mn?Fe?Ni?Sr?Mo?Ba?Th ?U were tested.Through the variation characteristics of trace elements and ratios of the Ostracod shell with age,combined with the indication paleoenvironmental significance of the shell fossils,the variation characteristics of the trace elements in the upper-middle section of the Shangganchaigou Formation in the western Qaidam Basin and the paleoclimatic significance were discussed.The specific results are as follows:(1)The paleo-salinity index of the trace element ratio Sr/Ca,Ba/Ca and paleoredox index U/Ca,Mn/Ca,Th/U,V/(V+Ni)of Ostracod shell have obvious A(28.35~26.42 Ma)),B(26.42 ~ 23.08 Ma)and C(23.08 ~ 22.33 Ma)three stages of change characteristics,Stage B can be further divided into two stages with about 24 Ma.The overall values of Sr/Ca and Ba/Ca were lower in stage A,the mean values(0.002278 and 0.000539 respectively)were lower,higher in stage B,the mean values(0.003347 and 0.000796,respectively)were significantly higher,lower in the upper stage,and significantly higher in stage C,the mean values(0.003346 and 0.002059 respectively)were also higher,indicating that the paleo-salinity experienced an evolutionary process of lower,higher(lower in the upper stage),higher in the above three stages.The change characteristics of U/Ca are similar to those of Sr/Ca and Ba/Ca.Mn/Ca has the opposite trend.It has an upward trend in the A phase,with the highest average value(0.012866).In the B phase,the overall Mn/Ca is low.The average value(0.005153)is the lowest in the three stages,the upper part value is increased,the C stage Mn/Ca is significantly decreased,the average value(0.009809)is also lower,and the changes of Th/U,V/(V+Ni),etc.The trend is similar to Mn/Ca,indicating that the three-stage paleo-lake redox has undergone an evolutionary process of reducing the environment-oxidizing environment(upper-reductive-enhancement)-oxidation enhancement.Elements such as the shells Sr,Ba,U,Mn,and Th also indicate the above-mentioned paleoenvironmental evolution characteristics.(2)Through the quantitative calculation of the relative variation of the paleosalt and water level in the upper middle section of the Shangganchaigou Formation in the western of Qaidam Basin,the paleo-salt curve and the water level change curve have a good mirror image correspondence,and the paleo-salt evolution has obvious 28.35~26.42 Ma.The stage evolution characteristics of 26.42~24.3Ma,24.3~23.08 Ma,23.08~22.33 Ma,and the paleo-water level has obvious evolution characteristics of 28.35~27.3Ma,27.3~25Ma,25~23.08 Ma,23.08~22.33 Ma.(3)From the paleo-salt and paleo-reductive evolution characteristics of ancient lakes and the quantitative calculation of paleo-salt and Relative water level change,combined with the environmental indication significance of the shell fossils,the upper-middle section of the Shangganchaigou Formation in the western Qaidam Basin(28.35~22.33 Ma),the paleo-environment of ancient lakes has mainly experienced deep lakes(lower salinity and strong reduction)-shallow lakes(higher salinity and strong oxidizing)-semi-deep lakes(lower salinity and enhanced reductivity)-shallow lakes(increased salinity and enhanced oxidative property).the corresponding lake basin has undergone a relatively humid-drought-semi-wet-drought evolution.(4)The paleoclimatic evolution of the upper-middle section of the Shangganchaigou Formation in the western Qaidam Basin is mainly affected by the global climate evolution and the uplift of the Qinghai-Tibet Plateau,and has a certain relationship with the Deputy Tethys Sea.The climate evolution at this stage responded to the inland aridification of Asia,the early inland Miocene in the northwestern China,the Mi-1 glacial event,the late Oligocene warm period,and the early tectonic uplift of the Qinghai-Tibet Plateau.