Stratigraphic influence on emplacement and 3-dimensional structure of a large mafic sill in sedimentary strata

Sills are fundamental elements of volcanic plumbing systems emplaced, among other, in sedimentary basins. Even though sills are commonly considered simple, straight concordant igneous sheets, they are actually complex 3-dimensional objects. The detailed knowledge of the 3D structure of sills and their host rock is of primary relevance to better constraining the emplacement mechanisms and the impacts of sills on sedimentary basins. This study describes the results of 3-dimensional geological mapping of a large, well-exposed Early Cretaceous dolerite sill in Central Spitsbergen, Svalbard, Arctic Norway, using a combination of digital outcrop modelling and field mapping. The sill was emplaced within Upper Palaeozoic sedimentary formations of Svalbard. It is made of distinct segments emplaced at different stratigraphic levels of the host rock stratigraphy. The mapping shows a clear stratigraphic control on the intrusion morphology. The sill segments emplaced at the boundary between two formations, which marks a strong lithological boundary, are straight and very concordant. Conversely, the segments emplaced within a more homogeneous formation exhibit more complex, locally discordant shapes. The sill segments emplaced at distinct stratigraphic levels are connected by vertical steps, which formed through vertical faulting between the tips of the sill segments. The preferred NW-SE orientation of the steps and the thinning of the sill towards the SE suggests a propagation direction of the magma towards the SE. Our study shows how 3-dimensional knowledge of igneous intrusions is key for revealing their emplacement mechanisms.