Stratigraphic Influence on Emplacement and 3-Dimensional Structure of a Large Mafic Sill in Sedimentary Strata

Abstract

Sills are fundamental elements of volcanic plumbing systems emplaced in sedimentary basins. Even though sills are commonly considered simple, planar concordant igneous sheets, they are actually complex 3-dimensional objects. Detailed knowledge of the 3D structure of sills and their host rocks is of primary relevance to better constrain the emplacement mechanisms and the impacts of sills on sedimentary basins. This study describes the results of 3-dimensional geological mapping of a large (~14 × 9 km), 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. Sill segments emplaced at the boundary between two formations, which mark a strong lithological and rheological boundary, are straight and concordant. Conversely, segments emplaced within a more homogeneous formation exhibit more irregular, locally discordant shapes. The sill segments emplaced at distinct stratigraphic levels are connected by steeply dipping steps, which formed through dilatant shearing 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.