Arenig trilobite associations from the Jiangnan Transitional Belt of Northern Hunan, China

Abstract

Stratigraphic ranges of 32 Ordovician trilobite taxa are recorded across the Zitai Formation (Arenig) and the Jiuxi Formation (Arenig-early Llanvirn) at four localities in northern Hunan, which represent outer shelf facies from the Jiangnan Transitional Belt region of the South China Plate. These are assigned to two benthic trilobite associations. The Nileid-Illaenid Association is characterized by the eponymous families as well as sometimes containing agnostids, Shumardia, Agerina and Ovalocephalus; it represents a persistent ecological composite of the cosmopolitan Ordovician Nileid and Illaenid associations. It is restricted to carbonate facies from the late early and early mid Ordovician of the Chinese regions of peri-Gondwana, having previously been recorded from Tarim. The Pseudopetigurus Association, characterized by the eponymous isocolid, also contains typically deep-water trilobite taxa such as raphiophorids and the mesopelagic cyclopygids Microparia ( Quadratapyge) and Pricyclopyge; despite occurring in clastic facies close to the lower limit of the photic zone (c. <200 m water depth), it is apparently restricted to South China. Faunal changes upsection at the two Zitai Formation localities are interpreted as reflecting the Arenig transgression, although a similar pattern is not displayed across the deeper water Jiuxi Formation.