Widespread Thalassinoides facies from the upper Silurian of South China: A case study from the northwestern Hunan Province, middle to upper Yangtze Block

The upper Silurian of South China records the first, and perhaps the only occurrence of widespread (spanning over 1400 km), thick to very thick bedded Thalassinoides facies (TVTFs) after the Late Ordovician mass extinction. However, the palaeobiological, palaeoecological features and potential significance of these facies are yet to be demonstrated. A case study at northwestern Hunan Province, middle to upper Yangtze Block, reports the occurrence of TVTFs in a storm dominated open marine succession of the upper Silurian Xiaoxi Formation, which can be further assigned to lower offshore, upper offshore, offshore transition to distal lower shoreface, and proximal lower to middle shoreface settings. The regular changes of bioturbation intensity and maximum penetrating depth from various settings suggest that local energy levels, especially frequency of storms could have controlled the development of Thalassinoides, while their burrow sizes were generally confined by population densities. The Thalassinoides facies could have been constructed by unknown crustaceans, which generally conducted a deposit-feeding strategy. The return of widespread TVTFs in the tropical oceans of South China during the late Silurian could reflect a restored primary productivity, an elevated oxygen level, and a final recovery of benthic ecosystem to pre-extinction level after the Late Ordovician mass extinction. TVTFs could have remarkably shaped the late Silurian benthic environment in view of their high abundance, large burrow sizes, nature of deep tiered burrow systems and a deposit-feeding strategy.