Discontinuities between picritic and crinanitic units in the Shiant Isles sill: evidence of multiple intrusion

Two internal discontinuities are described from the Tertiary, Shiant Isles sill (northwest Scotland). The lower discontinuity separates picrite from overlying picrodolerite and the recently discovered upper discontinuity separates picrite from underlying crinanite. The sill is thus divided into an inner, strongly differentiated, picrodolerite–crinanite unit and outer picritic units. The lower discontinuity is slightly transgressive with respect to the picrite. The discontinuities are interpreted as the contacts between an earlier-intruded picritic sill (about 30 m thick) and a slightly later-intruded, much thicker, crinanitic sill. The Shiant Isles main sill is therefore a multiple intrusion.