A newly discovered earliest Triassic chert at Gaimao section, Guizhou, southwestern China

Abstract

The Permian–Triassic (PTB) boundary section at Gaimao (Huaxi District, Guiyang, Guizhou Province) records a unique lithological transition between the Permian and Triassic in South China. Thus, bioclastic limestones of the Changxing Formation are overlain by radiolarian-bearing siliceous rocks of the Dalong Formation and ultimately limestone/mudstone alternations of the Shabaowan Formation. Conodont sampling of the section revealed well-preserved elements that enabled a conodont biostratigraphy to be established. The occurrences of Hindeodus julfensis and Hindeodus typicalis in Changxing Formation bioclastic limestone indicate a Late Permian age. Four conodont zones have been established in Dalong and Shabaowan Formations. In ascending order those are the Isarcicella isarcica Zone, Neogondolella (or Clarkina) planata Zone, Neogondolella krystyni Zone and Neospathodus dieneri Zone. The presence of typical Griesbachian conodonts I. isarcica and Isarcicella staeschei in upper Dalong Formation indicates that this formation's radiolarian-bearing siliceous mudrocks are of earliest Triassic age and not restricted to the Permian as previously thought. Thin section analysis reveals that the upper Dalong Formation contains spheroidal radiolarians but not a diverse assemblage of Permian holdover taxa. The new data proves that, following the end-Permian mass extinction of radiolarian, the abundance of these siliceous planktons was maintained at least locally enabling chert deposition to persist. Thus, the Gaimao section provides an exception to the ‘Early Triassic Chert Gap’, the first of its kind in Palaeo-Tethys, and indicates that the end of chert deposition does not always mark the end-Permian mass extinction.