LOWER TRIASSIC ICHNOFOSSIL ASSEMBLAGES FROM SOUTH CHINA: AN EXAMPLE OF A DEEP MARINE REFUGIUM

In northern Guizhou Province, South China, the Lower Triassic (Induan) Daye Formation preserves a diverse and robust ichnofossil assemblage that includes several of the archetypal elements of the Nereites ichnofacies. The host sediments are interpreted here as distal calciturbidites comprising interbedded, planar laminated micrite and argillaceous micrite partings. Micrite beds (2-10cm thick) are characterized by predominantly pascichnial ethologies, the most common elements of which are Taphrehelminthopsis, Glockeria, and Oldhamia; additional constituents include Chondrites, Cochlichnus, Gordia, Mammelichnus, Neonereites, and Phycodes. Argillaceous micrite partings are characterized by a diminutive, less diverse assemblage comprising Helminthoida, Phycosiphon, Cosmoraphe, and rare Planolites; this assemblage is regarded as an ambient pre-turbidite fauna. Based on ichnological and lithological data the depositional environment is interpreted as outer bathyal to upper abyssal, representing the deepest water trace fossil assemblage known from the Early Triassic.

The importance of documenting this fauna is twofold. First, trace fossils provide an in situ record of benthic biota, which, due to the predominance of soft body fauna in deep-water environments, is rarely preserved. Second, the current ichnological dataset for the Early Triassic is weighted toward shallow marine ichnofacies, namely Skolithos and Cruziana. This implies that shallow marine environments acted as ecological refugia for post-extinction survivor communities. This interpretation is corroborated by abundant evidence for anoxic, dysoxic, or euxinic oceanic conditions during this time. Without a record of deep-water ichnofossils, it might be argued that these ethologies went extinct at the Permian-Triassic boundary and were subsequently reinvented. Instead, we conclude that the ichnofossil assemblages of the Daye Formation are representative of a deep-marine ecological refugium, from which deep-water benthos radiated with the return of more suitable ocean conditions later in the Triassic.