Sponges are one of the major faunistic components of the Burgess Shale-type fossil Lagerstätte of Chengjiang (Cambrian Stage 3, South China). Although pyritization is often invoked as a key process linked to the early diagenetic preservation in the Chengjiang Fauna, the unweathered specimens analyzed lack evidence of pyrite. Energy dispersive X-ray (EDX) analyses of these specimens show a robust and continuous film of organic carbon. Most of the Chengjiang sponges underwent extensive weathering and diagenetic alteration. The biogenic silica transformed in opal-CT (crystalline type) was dissolved leaving a cavity, successively filled by different minerals, or possibly underwent dissolution under acidic water conditions combined with oxygenation. This resulted in clay and iron oxide replication (framboids or micro-sized crystals) of the sponge spicules. Organic material of the original cellular layer is preserved as depleted organic carbon film.
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Forchielli, A., Steiner, M., Hu, S., & Keupp, H. (2012). Taphpnomy of Cambrian (stage 3/4) sponges from Yunnan (South China). Bulletin of Geosciences, 87(1), 133–142. https://doi.org/10.3140/bull.geosci.1225