Sill (geology)
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or along the direction of foliation in metamorphic rock. A sill is a concordant intrusive sheet, meaning that it does not cut across preexisting rock beds. Stacking of sills builds a sill complex[1] and a large magma chamber at high magma flux.[2] In contrast, a dike is a discordant intrusive sheet, which does cut across older rocks.
Formation
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 to foliation 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 surrounding country 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 (
Associated ore deposits
Certain
Transgressive sills
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 as transgressive. The geometry of large sill complexes in sedimentary basins has become clearer with the availability of 3D seismic 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 the Whin Sill and sills within the Karoo basin.[8][9]
See also
References
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- ISSN 0022-3530.
- S2CID 128669544.
- ISSN 0026-461X.
- S2CID 128713713.
- ISBN 9781862391642.
- ISBN 9783642124969.
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