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

Haoyuan Jiang; Yanqing Xia; Jiyong Li; Shanpin Liu; Mingzhen Zhang; Yongchao Wang

doi:10.1021/acsomega.1c04893

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

ACS Omega. 2021 Dec 20;7(1):480-495. doi: 10.1021/acsomega.1c04893. eCollection 2022 Jan 11.

Authors

Haoyuan Jiang^{1

2

3}, Yanqing Xia^{1

3}, Jiyong Li^{1

2}, Shanpin Liu^{1

3

4}, Mingzhen Zhang^{1

3}, Yongchao Wang^{1

2}

Affiliations

¹ Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China.
² University of Chinese Academy of Sciences, Beijing 100049, China.
³ Key Laboratory of Petroleum Resources, Gansu Province, Lanzhou 730000, China.
⁴ Lanzhou University, Lanzhou 730000, 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 Al₂O₃/SiO₂ 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.