CARBONATE RETICULATED RIDGE STRUCTURES FROM THE LOWER MIDDLE TRIASSIC OF THE LUOPING AREA, YUNNAN, SOUTHWESTERN CHINA: GEOBIOLOGIC FEATURES AND IMPLICATIONS FOR EXCEPTIONAL PRESERVATION OF THE LUOPING BIOTA

Abstract

Exceptionally preserved carbonate reticulated ridge structures are documented from the lower Middle Triassic Guanling Formation of the Luoping area, eastern Yunnan Province, southwestern China. Macrostructures suggest these reticulated ridge structures are morphologically similar to the growth-related structures in ancient and modern microbial mats. Other such features as minute load structures and pyrite crystals beneath the reticulated layers on bedding surfaces are overlain by a black carbonaceous shale layer. Clay minerals in the reticulated layers are oriented parallel to bedding at the top, and they usually form wavy laminae. These characteristics indicate that these carbonate reticulated ridge structures share a similar microstructure to their siliciclastic analogs, and are also comparable with that of wrinkle structures. SEM imaging shows that many tiny objects—coccoid spheres, rod-like structures and filamentous fibrils—are distinct within those ultra-thin, wavy laminae, suggesting similar accretion and growth models to modern microbial mats. Accordingly, the Luoping reticulated ridge structures are of biogenic origin. These microbial mats are associated with the early Middle Triassic Luoping Biota and they are thought to have played a crucial role in the preservation of the fossils through sealing and microbial coats. An SEM-EDS analysis on fossil fragments and their surrounding matrix reveals that microbial mats also played an important role in the phosphatization of Luoping fossils.