Trilobite biofacies and palaeogeographic development in the Arenig (Ordovician) of the Yangtze Block, China

Cluster analysis was conducted using Pearsonʹs coefficient based on the relative abundance data of 73 trilobite genera and subgenera in 72 collections, and fourteen Arenig (Floian–early Darriwilian) trilobite biofacies are recognized from the inner shelf to deep outer shelf of the Yangtze Block. The biofacies patterns and faunal dynamics across the eight successive time intervals suggest that the progressive changes in generic composition, diversity, and dominant components may have been affected by bathymetric gradients and lithofacies transition along the palaeoramp of the block, and by temporal constraints on the different trilobite clades. On the whole, the total faunal diversity reached its peak in the shallow outer shelf, from which it decreased both seawards and shorewards. The benthic forms generally exhibited increased endemicity shorewards, but atheloptic taxa were confined largely to the outer shelf, with the generic diversity increased seawards, while the mesopelagic trilobites were associated mainly with the deep-outer-shelf biofacies. The progressive alternations of trilobite biofacies spectra indicate that continual transgressions may have played an important role in a consecutive palaeogeographic development in the Yangtze Block, including mainly the drowning of the huge platform in the early Arenig (early Floian) (TI 2), the shoreward expansion of the deep outer shelf (TI 5) and the ongoing shoreward retreats of the inner shelf (TIs 6–7) in the Yangtze Sea, and the successive deepening of the southern Shaanxi sea especially in the late Arenig (Dapingian–early Darriwilian) (TIs 5–8) since its taking shape in TI 3.