TROPICAL GEOGRAPHY ›› 2019, Vol. 39 ›› Issue (2): 218-228.doi: 10.13284/j.cnki.rddl.003118

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Paleoenvironment of Eocene Red Beds in the Nanxiong Basin

Xie Zhenlong1a, Liu Xiuming1,2, Mao Xuegang1, Le Zhijun1a and Peng Chao1a   

  1. (1. a. State Key Laboratory Breeding Base of Humid Subtropical Mountain Ecology, School of Geographical Sciences, Fujian Normal University; b. Institute of Geography, Fujian Normal University, Fuzhou 350007, China; 2. Department of Environment and Geography, Macquarie University, Sydney NSW 2109, Australia)
  • Online:2019-03-05 Published:2019-03-05

Abstract:

Continental red beds are widely distributed in mainland China and have long been considered to be formed in inter-mountain river and lake environments, while the red strata are considered to be secondary oxidation formations in the late stage. While this is a kind of scientific explanation and logical reasoning, there is no further concrete argument. In this study, the Eocene red-bed paleogeocline section of the Nanxiong Basin was selected as the research subject, and the paleoenvironment was analyzed in combination with the characteristics of magnetic minerals and geochemistry. The upper and lower parts of the Gucheng section are red, and the middle part is cinerous. All colors and their depth change gradually with the sedimentary strata / layers, indicating that the color was formed simultaneously with the sedimentary strata / layers, and the red of the strata is the primary color. The saturation isothermal remnant magnetic (SIRM) and remnant coercivity (Bcr) results showed a strong positive correlation with redness (a*). The thermomagnetic curves demonstrated that the red layer samples were mainly hematite with incomplete antiferromagnetism. Diffuse reflectance spectroscopy (DRS) confirmed the existence of primary hematite and was the only magnetic mineral detectable. Hematite indicates that the sedimentary environment at that time should be oxidized at high temperature. In the middle of the section, there was a cinerous layer, which was previously interpreted as marine limestone by earlier researchers. However, instead of magnetic mineral signals, paramagnetic minerals were detected in the thermomagnetic measurements. DRS measurements after high temperature treatment also showed the presence of paramagnetic clay minerals. Geochemical element analysis showed that it was quite different from standard limestone, but similar to the typical loess in China. These data indicated that the limestone was not limestone. The upper and lower layers of the profile were red beds dominated by primary hematite, which represent the high temperature and strong oxidation environment on the surface. The formation is continuous and exhibits conformity. The “marine limestone” in the red beds of the two sections is not deemed to be limestone by chemical element analysis. Furthermore there was a high similarity between the cinerous layer and the red layer on both sides, upper crust and loess. The cinerous ash layer was more likely to have been formed by soil-forming processes under conditions of weathering and wetting. The ancient city section reflected the gradual change process of dry-heat oxidation-wetting redox and dry-thermal oxidation during the Eocene, which is of great significance for understanding the paleoenvironment of Nanxiong Basin.

Key words: red bed of Eocene Series, magnetic mineral, geochemical element, sedimentary environment, Gucheng section, Nanxiong Basin