Interpretation of gamma-ray logs: The distribution of uranium in carbonate platform

High gamma-radioactivity in carbonates is usually ascribed to uranium of detrital minerals and organic matter, and to thorium and potassium of clays. The present Study based on Urgonian marls and marly limestones (France) shows that some of the most radioactive values correspond instead to some 'pure' limestones. These peaks are generally associated with a sequence boundary or a maximum flooding surface. Low-level gamma-spectrometry and ICP-MS analyses show that although high radioactivities are mostly associated with uranium, there is no obvious correlation between uranium enrichment and lithology. Also, correlation between high radioactivity and argillaceous beds might not be systematic.