Geochemistry of the Permian-Triassic sequences of the Guryul Ravine section, Jammu and Kashmir, India: Implications for oceanic redox conditions

Abstract

Petrography and geochemistry including analysis of major, trace and rare earth elements have been carried out in the late Permian and early Triassic sediments of Guryul Ravine, Jammu and Kashmir, India to examine the palaeoenvironmental conditions at the Permo-Triassic boundary. A visible change in the lithostratigraphy from argillaceous∼carbonaceous mudstone in C Member (late Permian, Zewan Formation), to fine grained argillaceous siltstone with quartz in D Member-4 m below the Late Permian Event Horizon was observed. The XRD analysis divulges more terrigenous input below the PTB which is also reinforced by the dominance of quartz whereas is the dominant clay mineral is illite followed by chlorite. The K₂O+Na₂O vs SiO₂ plot indicates that the sediments at PTB were derived from andesite type of rocks (SiO₂ 52–63%) of intermediate composition. Major oxides SiO₂, CaO, Na₂O and MnO are most abundant in the D Member, whereas E Member is enriched in the Co, Ni, Cu, V and Zn indicating reducing conditions. Dominance of incompatible elements such as Ti, K, Rb, and Sr in finer shale fraction shows increased reworking of sediments. Moderate weathering is observed at PTB, whereas, below the LPEH, physical weathering is more. Y/HO ratio varies from 24–51 indicating that REEs are derived from shale source. The C_org:P is < 10:1 in the late Permian whereas it is > 10:1 in the early Triassic Period suggesting that the conditions transformed from oxidizing to reducing (maximum values noticed in sample no.5 (80:1)) indicating suboxic-anoxic conditions, which may be one of the causes of oceanic redox at PTB.