Palynofacies analysis of the Permian-Triassic transition in the Amb section (Salt Range, Pakistan): implications for the anoxia on the South Tethyan Margin

Abstract

The uppermost Chhidru Formation and the lower part of the Mianwali Formation were sampled in the Amb Valley, Salt Range, Pakistan for the study of the particulate organic matter (POM) content in order to evaluate the depositional environment during the Permian–Triassic transition. The POM content was assigned to four distinct palynofacies (palynofacies A–D). Palynofacies A recovered from siltstone within the white sandstone unit of the Upper Permian Chhidru Formation indicates a shallow marine oxic shelf setting. Recorded from the siltstone intercalations in the Kathwai Member of the basal Mianwali Formation, the Griesbachian palynofacies B is characterised by abundant acritarchs indicating a transgressive event. Palynofacies C recovered from the siltstone of the lowest Ceratite Marls (middle Dienerian) is dominated by terrestrial organic particles and indicates shallowing of the depositional environment, whereas 40 cm above, palynofacies D represents transgressive oxygen depleted conditions. The comparison with sections from the Australian Tethyan margin shows that oxygen depleted conditions occurred during the Griesbachian in the Perth Basin, while in the Bonaparte Basin oxygenated conditions prevailed. Hence, oxygen depleted facies do not correspond to a single, synchronous Permian–Triassic oceanic anoxic event but depend on local geography and bathymetry.