Early Cenozoic saucer-shaped sills of the Faroe Islands: an example of intrusive styles in basaltic lava piles

The Early Cenozoic igneous activity of the North Atlantic Igneous Province that generated widespread sill complexes in sedimentary basins at the NW European margins also generated various intrusive systems in the contemporaneous basaltic lava pile of the Faroe Islands. The Faroe Island Basalt Group comprises seven formations with a total thickness of about 6.5 km, of which four major formations are built up of tholeiitic lava flows, each being several hundred metres thick, and three thinner formations are mostly built up of volcaniclastic lithologies, each being a few metres to a few tens of metres thick. The largest sills are exposed as partly saucer-shaped bodies in the three uppermost formations, where inner gently dipping basal sill sections gradually give way to more steeply inclined discordant outer rims that commonly cut several hundred metres into overlying lava flows. Numerous subvertical and moderately inclined dykes ranging in thickness from c. 0.5 to c. 4.0 m intersect the areas affected by sill intrusion, but only inclined dykes or sheets have been positively identified as sill feeders. Locally controlled rotations of least principal stress axes 3 during initial sill intrusion or propagation may have been an important contributing factor in determining the overall geometry of the investigated intrusions.