Distinguishing faults from flooding surfaces on spectral gamma-ray logs

Abstract

Elevated gamma-ray emission from discrete beds in sedimentary deposits may conventionally be interpreted as representing flooding surfaces or transgressive beds. Potassium (K), uranium (U), or (less commonly) thorium (Th) concentrations in outcrop or borehole successions cause such elevated gamma-ray emission and may be linked to the presence of specific mineral hosts. Furthermore, specific Th/K and Th/U ratios occur at correlated stratigraphic surfaces and form part of a pattern that reflects a sequence-stratigraphic hierarchy. Spectral gamma-ray logs from uncored boreholes or weathered cliffsides and acquired without full petrographic descriptions can intersect low-angle faults such as thrusts. Our study demonstrates that bedding-parallel faults can be mistaken for flooding surfaces. We document the spectral gamma-ray response through a range of visually obvious and cryptic faults that may serve as proxy examples for other areas. Finally, we derive a preliminary generic model for the origin of spectral gamma-ray variations in faulted sandstones, limestones, and metamorphic successions. This shows why fluid-rock interactions along bedding-parallel zones of deformation generate elevated K and U and depressed Th/K and Th/U. Our observations may aid subsurface studies of the complex stratigraphy below thrusts.