Abstract:

Kunbei area of Qaidam Basin is in front of Kunlun Mountains in the southern part of Western Qaidam Basin,Qinghai Province,China.It is adjacent to the Qinghai-Tibet Plateau.Therefore,the tectonic activities are unusually frequent.Furthermore,very thick sedimentary strata have remained from the Cenozoic Era.Therefore,it is an ideal area for studying the paleoenvironment and paleoclimatic evolution.Fresh drilling core samples were collected,and trace elements were detected in the lower Cenozoic mudstone samples of Well Qian 26 in Kunbei area.The changes of trace element contents and their correlations were studied,and the ratio of characteristic elements was further calculated.The results showed that the average contents of Cu,U,Cr,and Sr in the Lulehe Formation were 18.34×10^-6,3.24×10^-6,141.68×10^-6,and 107.10×10^-6,respectively,near the stratum boundary,while those in the Lower Ganchaigou Formation were 28.17×10^-6,3.35×10^-6,192.07×10^-6,and 217.05×10^-6,respectively.Compared to the Lulehe formation,the content of trace elements in the lower part of the Xiagangchaigou Formation increased obviously,indicating that the sedimentary environment has changed.The correlation between U and Th,the indicator elements of terrigenous clastic rocks,was poor (R² = 0.174 5).This indicates that U was mainly autogenic,so the increase in U content may indicate an increase in the degree of reduction of the original sedimentary body.The weak correlation between Cr and U R²= 0.420 9) indicates that the change of water environment had some influence on the enrichment of Cr.The correlation between Cu and U was poor (R² = 0.123 1),indicating that the original sedimentary environment had limited influence on the enrichment of Cu,and the increase of its content may be more controlled by the deposition of organic matter and terrigenous debris sources.The correlation between Sr and U was poor (R² = 0.192 9),indicating that water environment was not the main factor controlling Sr enrichment.The values of U/Th and V/Cr were 0.19-0.35 and 0.10-0.13,respectively,indicating that the paleoenvironment of the study area was oxidized water depositional environment.The enhancement of U/Th and V/Cr indices in the lower part of Lower Ganchaigou Formation indicates that the water body deepened and the reducibility increased.Based on the Sr/Cu ratio and the evolution of fossil species,it is inferred that the paleoclimate near the stratum boundary has undergone drought-humidity-drought cycles.It can be seen that the evolution of dry and wet paleoclimate led to changes in water depth,which in turn led to changes in the redox conditions of the water environment.This is also the reason for the differential enrichment of trace elements.

Key words: Qaidam Basin Kunbei area, Lulehe Formation-Xiaganchaigou Formation, geochemistry, oxidation-redution environment, Paleoclimate