| The Quaternary climate of the Qaidam Basin is impacted and controlled by global changes, so its climate change record can be consistented with and compared with global change in the long-period and large-scale. Provenance, climate and the feature of palaeogeomorphology combined to effect the lacustrine sedimentation. When the provenance and palaeogeomorphology are pediocratic, the climate become the main factor.Cyclical changes in climate conditions could cause the changes of lake level,sediment supply and sedimentation types, then impact the lake basin deposit and the lake level fluctuation.With sedimentology and sequence stratigraphy theory, this paper focus on Changweitai section, combined with magnetic susceptibility,outcrop section and logging data, through field observation and laboratory micro-macro analysis, trying to resolve the lake level change style and its driving mechanism in the Shangganchaigou formation and Xiaganchaigou formation.then, provide reference and help for the study of the relationship between climate change and uplift of the Tibetan Plateau and global climate.First of all, based on field survey and thin section analysis, identify sedimentary structures and fossil lebensspur indicant,The sedimentary facies of Shangganchaigou formation and Xiaganchaigou formation is divided into alluvial fan and lacustrine facies,and then subdivided into 6 subfacies.Form old to new,vertical evolution characteristics of sedimentary environment indicate the lake surface increased and then decreased, meanwhile, reflecte the climate change form humid to arid.Secondly, taking magnetic susceptibility and gamma logging data as a proxy index of the paleoclimate and paleoenvironment, using the methods of linear regression and first differences to divide sedimentary cycles. Demonstrate different lithologies, different sandstone size and different mudstone colors on the effects of magnetic susceptibility:form mudstone to sandstone then to conglomerate, the peak of magnetic susceptibility was form high to low; the grain of sandstone change form coarse sandstone to sandstone , siltstone, the value of susceptibility alter form low to high;the susceptibility show opposite relationship with the grain size; when red or somethis oxidation colours appeared in mudstone, the peak of susceptibility always come to high value, intend the sedimentary temperature relatively higher, the climate towards aridity, and the level of water maybe lower in this situation; if the colour turns up to graygreen or balsam green, as these reducible colours, it represents a relatively warm and humid climate, meanwhile, the peak of susceptibility comes to lower value.Thirdly, using the cycle formation divided by magnetic susceptibility and gamma logging data, the lake level curve of Shangganchaigou formation and Xiaganchaigou formation drawed by Fisher diagram can visual infect high frequency lake level variations and long-period trend of lake level change.Both of the curves show the lake level changing from low to high, this result is similar with the outcome analysing by sedimentary environment.This good proof that the magnetic susceptibility and gamma logging data can be as a proxy of lake level change.At last, through filtration select a stratomere with steady deposition rate, according to power cspectral analysis of magnetic susceptibility and gamma logging data, find 4 frequency bands with different periods.The main frequency band is the first and the third frequency band.Though comparing various cycles and Milankovitch cycles, account the obliquity cycle(40.98ka) corresponding the cycle thickness range of 5-10m, the precession period corresponding the cycle thickness range of 2-4m.The climatic fluctuation effected by obliquity and precession of Milankovitch cycles is the main factor for controlling high frequency sedimentary cycle and lake level alternation of Shangganchaigou Formation and Xiaganchaigou Formation.
|
PDF Full Text Request