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

Sills are important components of magmatic plumbing systems due to their role as storage features of magma. Previous studies have indirectly investigated sill propagation and architecture by using laboratory experiments, remote sensing, modelling and theory. These studies, however, often struggle to include the complexity of natural systems, which often includes strong interplay between host and intruder. To elevate the importance of host rock and magma interaction, we present the results from a study of combined UAV- and outcrop datasets from 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 shows 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, while 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. Supplementary material: The 3D model of the Mussentuchit Wash Sill (e.g. Figure 4) will be published on V3Geo.com (currently open-access database for 3D models) when the manuscript is published. Uninterpreted images of the sill will be published on figshare and are included as DR1.