General remarks to application of gamma-ray spectrometry in sedimentary archives of foreland basins

The spectral gamma-ray data (GRS) are frequently used as lithofacies proxies. However, GRS character of siliciclastics is primarily driven by their mineral and chemical composition, which is related to provenance and effect of... [ view full abstract ]

The spectral gamma-ray data (GRS) are frequently used as lithofacies proxies. However, GRS character of siliciclastics is primarily driven by their mineral and chemical composition, which is related to provenance and effect of chemical weathering (chemical maturity) of siliciclastics. To provide a better insight into these relationships, we conducted a comprehensive study of GRS in outcrop areas of turbidite systems in three foreland basins, the Nízký Jeseník Culm Basin (Variscan orogeny; Lower Carboniferous), the Silesian Unit of the Western Carpathians fold-thrust belt (Carpathians orogeny; Upper Cretaceous) and the Grès d´Annot Formation in southeastern France (Alpine orogeny; Oligocene). The main aim of this work was exploration of sensitivity of GRS data to facies changes and variations in detrital composition of sediments.
Detailed lithofacies logging and nearly 2500 points measured by RS-230 portable spectrometer were carried out on 68 outcrops. Each measurement was performed with full contact with the rock. Samples of characteristic facies were measured using RT-50 laboratory gamma-ray spectrometer. GRS data were confronted with results of complex petrographical and geochemical study of sediments. Thin-sections of sandstones were studied using optical and cathodoluminescence microscopy and compositional groups of Gazzi-Dickinson method were identified. Heavy minerals were identified through a binocular microscope and counted semi-quantitatively. Chemical composition of detrital grains was analysed by electron microprobe and composition of mudstones were studied using X-ray powder diffractor.
Generally low mineral and chemical maturity of arenites is reflected in low compositional contrast between sand grain size, sandstone matrix and mudstones. Th concentrations are the most sensitive to facies changes, due to link of Th to accessory heavy minerals in sandstones and to clay minerals in mudstones. Contrariwise, K concentrations are usually poor indicators of lithofacies due to the presence of chemically and mechanically unstable K-bearing components in all grain sizes. Consequently, good sensitivity of GRS to grain size suggests high chemical maturity (“dilution effect” of non-radioactive quartz grains) whereas the low contrast between GRS values in sandstones and mudstones indicates low chemical maturity and prevalence of mechanical weathering over chemical one. Important changes in concentrations of radioactive elements correlate with important changes of composition of the main detrital components of sandstones and they can obscure facies changes. However, GRS data can be good proxies of compositional changes in sediments which can reflect provenance changes in source area adjacent to foreland basins or proximal-to-distal changes within large depositional systems.
The work was supported by the project GA14-18183S.