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[Objective] Marine red beds are distributed across various stages in geological history, are often interpreted as products of special environments, and have attracted widespread attention. The Sowa Formation in the Jurassic Qiangtang Basin is a set of sedimentary rocks composed of clastic and carbonate mixed sediments. In the Sowa Formation exposed in Bilocuo, Shuanghu county, Tibet, the red beds are mainly composed of carbonate sediments. [Methods] This study explored the causes of th red color and significance in the deposition and diagenesis of the red beds based on sedimentological, mineralogical, and geochemical methods. [Results] Field sedimentology and downhole thin section identification of these red beds revealed that the rocks are composed of grain-supported limestone, with abundant debris from cephalopods, echinoderms, bivalves, bryozoans, gastropods, and other benthic organisms. Biogenic debris and terrestrial quartz debris are the main components of the ooids, which display characteristics of a high-energy beach environment. Spectroscopic studies and scanning electron microscopy indicate that the main cause of the red color is submicron- to micron-sized pseudo-hexagonal and tetragonal hematite crystals. The coloration minerals are concentrated in the ooid cortex structure, and the cementing material is virtually devoid of coloration minerals. This suggests that during early diagenesis, particularly during the formation of ooids and peloids, iron-loving microorganisms played an important role in capturing iron oxides. Furthermore, the geochemical analysis of carbonate minerals shows that the redness is positively correlated with the flux of terrestrial debris input. Additionally, it exhibits a positive Ce/Ce* anomaly (1.1-1.3) and a pattern of enriched-middle rare earth elements "bulge", indicating that the iron oxides underwent reductive dissolution.[Conclusions] Based on these findings, the study reconstructed the red coloration process of these red beds. The input of terrestrial iron-bearing minerals provided the material basis for the red coloration, and then, during early diagenesis, reductive fluids dissolved the iron-bearing minerals, providing free iron ions for iron-loving microorganisms, allowing them to recrystallize and ultimately form hydroxyl iron and cause red coloration through chemical or biological processes. [ABSTRACT FROM AUTHOR] |
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