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

Abstract

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. Copyright © 2014 John Wiley & Sons, Ltd.

Reconstructing Eocene Eastern Indian Ocean Dynamics Using Ocean‐Drilling Stratigraphic Records

“…We thus conclude that Th/K could be used as a paleoclimate proxy in the Eastern Indian Ocean. We note, however, that some authors raised concerns about Th/K as a paleoclimate proxy in carbonate-rich marine successions (Ghasemi-Nejad et al, 2015;Li, Huang, et al, 2019). Yet, their conclusions are specific to the hinterland in the studied areas.…”

Multifractal characterization of the Coniacian–Santonian OAE3 in lacustrine and marine deposits based on spectral gamma ray logs

“…The Th/U and Th/K ratios in sediments fluctuate under a number of different controls such as clay mineral content, paleo-redox conditions and paleoclimate 53 , 76 .…”

“…The log gamma records the total natural gamma radiation while the spectral gamma ray log not only indicates the whole intensity of radioactivity, but also estimates quantitively the content of the main radioactive constituents (Th, U and K) in the formation. Because the concentration of natural radioactive elements is mainly a function of the depositional environment and diagenesis 102 , the spectral gamma ray log have been widely used for paleoenvironmental research [51][52][53] . The data used in this work are chiefly the spectral gamma ray logs data ( Fig.…”

“…Possible factors affecting the multifractal properties of Th-U and Th-K distributions in the OAE3 interval. The Th/U and Th/K ratios in sediments fluctuate under a number of different controls such as clay mineral content, paleo-redox conditions and paleoclimate 53,76 . Th/K ratio is for the most part a function of clay mineral content in shale formations 77 .…”

“…Third, the human factors have little effects on the measurement processes. Among the well log data, the spectral gamma ray has proved particularly useful for understanding paleoenvironmental changes 50 – 53 .…”