Facies, provenance and paleoclimate interpretation using spectral gamma logs: Application to the Lower Cretaceous of the Scotian Basin

Abstract

Spectral gamma logging has been useful in some settings for correlation of wells, for identification of sediment facies, and for interpretation of hinterland paleoclimate. We have taken advantage of the hundreds of geochemical analyses of sandstones and shales from conventional core that are available from the Scotian Basin to test the relationship of the elements K, U and Th measured by spectral gamma to sedimentary facies. These elements are also known to be useful in characterising provenance of sediment to different parts of the basin. The geochemical and spectral gamma data both show that there are subtle differences in K, U and Th in different lithofacies, such as higher Th/U in low sedimentation rate facies, but they are not sufficiently distinctive to identify the lithofacies of any unknown interval. The main control on abundance of these elements is the effect of grain size, that is the proportion of sand to mud in sediment. Volcanic sources have a high K/Th ratio, and early Albian sediments with relatively high volcanic component have unusually high K/Th. In distal shale-prone successions, the grain size effect is sufficiently small that climate controlled variations in Th/K ratio are recognised in the Early Cretaceous, with the ratio being higher at times of humid weathering on land. The variations in Th/K ratio through time are broadly similar to those known from Western European sections, notably with topmost Hauterivian and topmost Aptian humid events. Overall, this study provides an assessment of the use and pitfalls of spectral gamma logs in a deltaic succession.