Spectral gamma-ray logs and palaeoclimate change? Permian – Triassic, Persian Gulf

Spectral gamma ray (SGR) logs are used as stratigraphic tools in correlation, sequence stratigraphy and most recently, in clastic successions as a proxy for changes in hinterland palaeoweathering. In this study we analyse the spectral gamma ray signal recorded in two boreholes that penetrated the carbonate and evaporate-dominated Permian–Triassic boundary (PTB) in the South Pars Gasfield (offshore Iran, Persian Gulf) in an attempt to analyse palaeoenvironmental changes from the upper Permian (Upper Dalan Formation) and lower Triassic (Lower Kangan Formation). The results are compared to lithological changes, total organic carbon (TOC) contents and published stable isotope (δ18O, δ13C) results. This work is the first to consider palaeoclimatic effects on SGR logs from a carbonate/evaporate succession. While Th/U ratios compare well to isotope data (and thus a change to less arid hinterland climates from the Late Permian to the Early Triassic), Th/K ratios do not, suggesting a control not related to hinterland weathering. Furthermore, elevated Th/U ratios in the Early Triassic could reflect a global drawdown in U, rather than a more humid episode in the sediment hinterlands, with coincident changes in TOC. Previous work that used spectral gamma ray data in siliciclastic successions as a palaeoclimate proxy may not apply in carbonate/evaporate sedimentary rocks.