Abstract

This paper describes some of the results from a 3D seismic-based analysis of the mechanics of igneous sill emplacement in sedimentary basins. Detailed 3D interpretation of igneous intrusions flanking the Corona Ridge in the Faroe-Shetland Basin has led to the discovery of a sill (the Corona Sill) with a previously unrecognized morphology. Two potential feeder sources have been interpreted for the broadly rectangular intrusion, of which the surveyed portion measures approximately 15 by 4 km. The Corona Sill has a linear NW margin and two lobate protuberances along the SE side. Arcuate ridges that radiate from a central point at the intersection of the lobes cover the imaged surface of the sill. The ridges have wavelengths of 220-350m and amplitudes in the range 25-50m.

The ridged morphology on the surface of the Corona Sill has not previously been described from any seismic or outcrop-based study of igneous sills. The ridges are interpreted to have formed as a direct result of the propagation mechanism, and are thought to have been influenced by the viscosity of the magma, host sediment and the depth of intrusion, which is likely to have been within 400 m of the sediment-water interface. It is suggested that during very shallow intrusion of viscous magma into soft, waterlogged sediments, magma is able to spread, creating a geometry similar to that expected for a high viscosity lava flow.

Ridges were formed by compression of a more rigid outer layer of magma in the sill, retarded by the solidifying sill front. Forward movement of the surface layer is likely to be caused by viscous drag from within the sill body. The ridges on the sill top surface are a kinematic indicator for the flow direction of the magma, enabling identification of the feeder zone, which displays a clear link to an underlying sill. It is demonstrated that 3D seismic data has significant untapped potential for the study of magma transport and intrusive processes in the upper crust