Sedimentary Environments Analysis And Susceptibility Measurement Of The Early Cretaceous Strata In The Longzhong And Liupanshan Basin

The magnetic susceptibility is a useful proxy in reconstructing paleoclimate, but its mechanism of variation is still unclear. Based on the analysis of the lithology, sedimentary environments and susceptibility value for the Early Cretaceous strata in the Longzhong basin and Liupanshan basin, northwest of the China, this paper provide a insight into the understanding to the facies-, tectonic-and climate-dependent variation of the magnetic susceptibility, and some conclusions as following:1. There are19lithologic sequences in the Linxia and Lanzhou subasin of the Longzhong basin, which constitutes the alluvial fan, braided river, meandering river, river delta, braided delta, shallow lake, shore lake and semi-deep lake sedimentary facies. Whereas, there are20lithologic sequences in the Liupanshan basin, which compose the alluvial fan, braided river, meandering river, shallow lake and semi-deep lake sedimentary facies.2. Magnetic susceptibility measurements indicates that magnetic susceptibility value is high in the Liupanshan basin, low frequency mass magnetic susceptibility ranges between0.28-82.6x10-8kg/m,the mean value is10×10-8kg/m,and the value in the margin of the basin (7.07×10-8kg/m3) is lower than those in the central basin (18.8×10-8kg/m3).However, the mean magnetic susceptibility value of the Hekou group in the Longzhong basin is lower than in the Liupanshan basin.In detail,the value in Linxia subbasin is significantly higher than in the Lanzhou subbasin. The low frequency mass magnetic susceptibility in the Linxia basin ranges from2.1to53.6x10-8kg/m3, the mean value is10.5×10-8kg/m3,whereas the low frequency mass magnetic susceptibility in Lanzhou basin ranges from1.5to23.9×10-8kg/m3,the mean value is7.9×10-8kg/m3In general, the variation trend in low and high frequency mass magnetic susceptibility are accordance in the two basins.3.Dependent on lithology and color, high magnetic susceptibility value of Hekou group in the Linxia subbasin corresponds to the oxidation silt and coarse gravely sandstone which is in the,oxidation and high energy water environment. However low value corresponds to deoxidation environment and fine siltstone except to mudstone. The high value in the Lanzhou basin corresponds to deoxidation environment and fine mudstone. For the Liupanshan group, the high mass magnetic susceptibility value in the northwest of the basin(section Huoshizai and Shikouzi) corresponds to red color and fine grained mudstone, the high value corresponds to high energy water and oxidation environment, such as the brown color and coarse particle sandstone in the central basin(the highest peak in Linpanshan). In the northeastern of the basin, the high value corresponds to still water and deoxidation environment, such as reduced color and fine particle marlstone.4.The analysis for magnetic susceptibility in various phase of the tectonic uplift in Longzhou basin, Higher magnetic susceptibility value indicates a steady tectonic uplift stage and hydrostatic environment, but the lower value indicates rapid and strong tectonic uplift stage and high energy water sedimentary environment. Meanwhile, high magnetic susceptibility value indicates hot and humid climatic status and high energy water sedimentary environment, whereas lower value indicates humid climate conditions and hydrostatic environment. However, these relationship is inconsistent to those in the Liupanshan basin. The cause for this contrary should be uncovered by the more study with the other proxies.