Implications of simple flexure theory for the formation of saucer-shaped sills

Saucer-shaped dolerite sills have been identified in several sedimentary basins. They are characteristically sub-circular in plan, with an inner sill connected to a steeply inclined arcuate sheet extending upwards from the edge of the inner sill, and in some cases have raggedly developed flat outer rims. By applying elastic plate theory to the overburden, we obtain an expression for the maximum radius of a circular sill in terms of emplacement depth, excess magma pressure, elastic constants of the overburden and the tensile strain at failure. This expression predicts that the radius of the flat base of a saucer-shaped sill increases with emplacement depth, as observed in the western Karoo Basin, South Africa. Some saucer-shaped sills display an elliptical morphology. From consideration of the flexural strains around the periphery of a horizontal elliptical sill, we infer that saucer-shaped elliptical sills have been fed from below by feeder dykes oriented along the major axis of the ellipse. Flat outer rims would have developed at the final stage of each intrusive episode, when the excess magma pressure was decreasing towards equilibrium with the overburden load. Consequently, flexural strains would also have decreased, so continued intrusion might preferentially occur along a bedding plane.