Alkaline sill intrusions in sedimentary basins: emplacement of the Mussentuchit Wash Sill in San Rafael Swell, Utah

Abstract

Sills are important components of magmatic plumbing systems because of their role in magma storage. Previous studies have indirectly investigated sill propagation and architecture by using laboratory experiments, remote sensing, modelling and theory. However, these studies often struggle to include the complexity of natural systems, which commonly involves strong interplay between hosts and intrusions. To highlight the importance of host rock and magma interaction, we present the results from a study of combined unmanned aerial vehicle and outcrop datasets from a world-class 1.3 km long, 30 m high 3D exposure of a 12 m thick alkaline trachybasalt sill in Mussentuchit Wash, San Rafael Swell, Utah. The sill intruded into Jurassic, dominantly sandy, sedimentary rocks. Results of this study show that the propagation of the Mussentuchit Wash Sill features both fracture-driven and complex non-brittle fluid interaction emplacement, which are strongly influenced by local sedimentology and presence of porewater. Segregated melt emplaced progressively within the sill during emplacement is used to document the evolution of sill inflation. The fracture-driven propagation is initiated along sedimentary discontinuities through hydrofracturing, whereas the non-brittle fluid interaction is caused by the presence of local porewater within the sedimentary host rocks. This suggests that local lithology may exert strong control on the architecture and morphology of sills in sedimentary basins.