We use three-dimensional seismic data to describe the architecture of shallow intrusive igneous sill and dike complexes intruded into the upper crust as well as the internal and external geometry of extrusive conical igneous mounds that were fed by the magma conduits. The conical mounds accreted on the seabed, directly above the tips of the basaltic dikes, between 54.9 and 54.6 Ma. In plan view the mounds are broadly circular, 1–1.7 km in diameter, and have a relief of 50–300 m. They occur above a complex network of linked cuspate-shaped sills and dikes intruded into Paleocene and Cretaceous sediments of the Faeroe-Shetland Basin, northeast Atlantic margin. The highly organized internal structure, consisting of bulbous layers arranged concentrically around a central axis, along with the clear vertical link to the underlying sills and dikes, indicates that they most likely represent pillowed basaltic lava and hyaloclastite mounds that were fed by the dikes and extruded onto the seafloor. Mounds of similar external geometry have been described from sidescan sonar images above active spreading centers in oceanic settings. However, such structures are rarely recognized in passive margin settings, and prior to this investigation our knowledge of their three-dimensional internal geometry has been largely speculative.

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