Sedimentology and ichnology of the Xiaxishancun Formation of Qujing, Yunnan: Earliest Devonian marginal-marine environments and ecosystems in South China

The Silurian–Devonian transition experienced dramatic changes of climates, global carbon cycles and faunas, but detailed documentation of the environments and biotas during this transitional interval in the South China Block is needed. The upper Silurian–Lower Devonian sequence of Qujing, Yunnan, southwestern China, represents one of the rare stratigraphic records of this transition in the South China Block. In this study, the Xiaxishancun Formation of Qujing, a unit generally considered earliest Devonian in age, is studied in terms of sedimentology and ichnology. The results show that the Xiaxishancun Formation typically records the deposition of hyperpycnal flow systems, which have been rarely reported in the Paleozoic. Three facies associations, including mudstone-dominated association, heterolithic association, and sandstone-dominated association, represent different parts of the channels and lobes of hyperpycnal subaqueous deltas on the shelf. Nineteen ichnogenera and 32 ichnospecies were distinguished in the Xiaxishancun Formation, and grouped into two ichnoassemblages, the Lockeia-Ptychoplasma-Oravaichnium ichnoassemblage in hummocky/swaley cross-stratified sandstones, and the Cruziana-Diplichnites-Monomorphichnus ichnoassemblage in thin rippled sandstone beds. The hyperpycnal depositional processes of the Xiaxishancun Formation created nutrient-rich, oxygen-rich, siliciclastic environments that supported diverse benthic ecosystems. Our study provides new insights into the understanding of marginal-marine environments and biotas of South China, immediately following the Silurian–Devonian transition event.