In geology, asill is a tabularsheet intrusion that hasintruded between olderlayers ofsedimentary rock, beds of volcaniclava ortuff, or along the direction offoliation inmetamorphic rock. A sill is aconcordant intrusive sheet, meaning that it does not cut across preexisting rock beds. Stacking of sills builds a sill complex[1] and a largemagma chamber at highmagma flux.[2] In contrast, adike is a discordant intrusive sheet, which does cut across older rocks.
Sills are fed by dikes,[3] except in unusual locations where they form in nearly vertical beds attached directly to a magma source. The rocks must be brittle and fracture to create the planes along which the magma intrudes the parent rock bodies, whether this occurs along preexisting planes between sedimentary or volcanic beds or weakened planes related tofoliation in metamorphic rock. These planes or weakened areas allow the intrusion of a thin sheet-like body of magma paralleling the existing bedding planes, concordant fracture zone, or foliations. Sills run parallel to beds (layers) and foliations in the surroundingcountry rock. They can be originally emplaced in a horizontal orientation, although tectonic processes may cause subsequent rotation of horizontal sills up to near vertical orientations.
Sills can be confused with solidified lava flows; however, there are several differences between them. Intruded sills will show partial melting and incorporation of the surrounding country rock. On both contact surfaces of the country rock into which the sill has intruded, evidence of heating will be observed (contact metamorphism). Lava flows will show this evidence only on the lower side of the flow. In addition, lava flows will typically show evidence ofvesicles (bubbles) where gases escaped into the atmosphere. Because sills form below the surface, even though generally at shallow depths (up to a few kilometers),[4] the pressure of overlying rock means few if any vesicles can form in a sill. Lava flows will also typically show evidence ofweathering on their upper surface, whereas sills, if still covered by country rock, typically do not.
Certainlayered intrusions are a variety of sill that often contain importantore deposits.Precambrian examples include theBushveld,Insizwa and theGreat Dyke complexes of southern Africa; and theDuluth intrusive complex alongLake Superior, and theStillwater igneous complex of the United States.Phanerozoic examples are usually smaller and include theRùmperidotite complex[5] ofScotland and theSkaergaard igneous complex of eastGreenland. These intrusions often contain concentrations ofgold,platinum,chromium and other rare elements.
Despite their concordant nature, many large sills change stratigraphic level within the intruded sequence, with each concordant part of the intrusion linked by relatively short dike-like segments. Such sills are known astransgressive. The geometry of large sill complexes in sedimentary basins has become clearer with the availability of 3Dseismic reflection data.[6] Such data has shown that many sills have an overall saucer shape and that many others are at least in part transgressive.[7]
Examples include theWhin Sill and sills within theKaroo basin.[8][9]