Controlling the Iron Migration Mechanism for the Cretaceous Sediment Color Variations in Sichuan Basin, China

Abstract

Cretaceous continental sediments in Sichuan Basin, China, have different colors, and the reasons for their formation are not determined. Based on mineralogical and geochemical characteristics, red beds and nonred beds in the Upper and Lower Cretaceous sedimentary strata in the western Sichuan Basin are described and tested in this study. The test and analysis of the mineral composition, element content, and iron speciation of mudstone samples with gray-green, gray, and red colors in Cangxi, Bailong, and Guankou formations found that the change in hematite content directly causes the color difference of samples. For red mudstone, the average chemical index of alteration, chemical index of weathering, weather eluviation index (Ba), Ca/(Mg*Al), and Al 2 O 3 /SiO 2 index are 67.75, 79.94, 2.07, 0.26, and 0.26, respectively, indicating that chemical weathering is the most intense. The geochemical indexes corresponding to gray samples are 64.41, 74.91, 2.08, 0.19, and 0.24, respectively. Those corresponding to the gray-green samples are 62.30, 70.68, 2.17, 0.21, and 0.24, with the weakest chemical weathering. The ratio of Cu/Zn and the enrichment factor of V show that red and nonred bed samples are formed in weak oxidation and weak reduction environments, respectively. The red sample contains the highest content of hematite iron. The gray-green sample mainly represents paramagnetic ferrous in clay minerals. The geochemical contents of the gray sample’s three iron elements are slightly different, mainly trivalent iron. The change in iron speciation content in different color samples shows that the Fe element forming hematite in red bed samples may come from the weathering of source rock and clay minerals subjected to secondary weathering. At present, it is confirmed that different colors of samples are related to different weathering degrees of source rocks, which can be related to hot, dry/humid climates. It is necessary to distinguish the climate type in combination with other indicators.

Basic chemical compositions combination rules and quantitative criterion of red beds

“…For example, the content of Fe2O3 or hematite in the red beds is higher than that in the grey beds (Hu et al 2006). The chemical compositions of red beds (1536 groups data) with different geological ages and various lithologies such as conglomerate, sandy conglomerate, sandstone, siltstone, shale and mudstone were collected from the previous studies as shown in Supplementary Table 2 (e.g., (Gao et al 2017;Hong et al 2009;Jiang et al 2022;Kong et al 2018;Liu et al 2007;Liu et al 2020;Liu et al 2006;Uchida et al 2000;Wild et al 2017;Xue et al 2023;Yang et al 2016;Zhang et al 2008;Zhao et al 2005;Zhu et al 2003) May, 2023, using the following parameters: material = sedimentary and rock name = breccia. ), claystone (Supplementary Table 12 -3,790 groups data.…”