The biostratigraphic succession of acanthomorphic acritarchs of the Ediacaran Doushantuo Formation in the Yangtze Gorges area, South China and its biostratigraphic correlation with Australia

Abstract

Abundant large acanthomorphic acritarchs from the Ediacaran Doushantuo Formation in the Yangtze Gorges area, South China are becoming useful tools for biostratigraphic subdivision and correlation of the Ediacaran System despite limitations caused by facies control, taphonomic bias and taxonomic problems. Based on thin-section examination of chert samples collected from six sections of the Doushantuo Formation, a clear stratigraphic distribution of acritarchs has allowed two assemblages of acanthomorphic acritarchs (lower Tianzhushania spinosa assemblage and upper Tanarium anozos–Tanarium conoideum assemblage) to be established. These two assemblages, which are found in the lower (Member II) and the upper (Member III) Doushantuo Formation, respectively, are constrained stratigraphically to the first and the second positive δ13C excursions (EP1 and EP2), respectively, and are separated by a second negative δ13C excursion (EN2). The lower T. spinosa assemblage is dominated by the taxon T. spinosa. By comparison with the lower assemblage, the upper T. anozos–T. conoideum assemblage preserves more acanthomorphic acritarchs both in terms of number of individuals and in the diversity of the forms. Fifteen forms, Eotylotopalla delicata, Knollisphaeridium maximum (=Echinosphaeridium maximum), Meghystrichosphaeridium magnificum, Schizofusa zangwenlongii, Sinosphaera rupina, T. anozos, T. conoideum, Tanarium irregulare, Variomargosphaeridium litoschum, as well as the unnamed B, C, D, E, F and G forms, are the predominant species in upper T. anozos–T. conoideum assemblage, among them, the taxa T. anozos and T. conoideum occur in most great number and throughout the assemblage, whereas the species T. spinosa is presumed to have become extinct. The upper T. anozos–T. conoideum assemblage shares a number of species with the Ediacaran complex acritarch palynoflora (ECAP) in South and central Australia (including Ceratosphaeridium glaberosum, Gyalosphaeridium pulchrum, S. zangwenlongii, T. conoideum, T. irregulare and V. litoschum). In addition, the species T. spinosa is not present in the ECAP of Australia. Hence, the upper T. anozos–T. conoideum assemblage of the Doushantuo Formation can be correlated with the ECAP of South and central Australia, while the lower Tianzhushania assemblage appears to be missing in South and central Australia.