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 even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet. This means that the sill does not cut across preexisting rocks, in contrast to dikes which do cut across older rocks.
Sills are always parallel to beds (layers) of the surrounding country rock. Usually they are in a horizontal orientation, although tectonic processes can cause rotation of sills into near vertical orientations. They 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 the "upper" and "lower" 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 of vesicles (bubbles) where gases escaped into the atmosphere. Because sills generally form at depth (up to many kilometers), the pressure of overlying rock prevents this from happening much, if at all. Lava flows will also typically show evidence of weathering on their upper surface, whereas sills, if still covered by country rock, typically do not.
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[edit] Associated ore deposits
Certain layered intrusions are a variety of sill that often contain important ore deposits. Precambrian examples include the Bushveld, Insizwa and the Great Dyke complexes of southern Africa, the Duluth intrusive complex of the Superior District, and the Stillwater igneous complex of the United States. Phanerozoic examples are usually smaller and include the Rùm peridotite complex of Scotland and the Skaergaard igneous complex of east Greenland. These intrusions often contain concentrations of gold, platinum, chromium and other rare elements.
[edit] 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, examples include the Whin Sill and sills within the Karoo basin.[1][2] The geometry of large sill complexes in sedimentary basins has become clearer with the availability of 3D seismic reflection data.[3] Such data has shown that many sills have an overall saucer shape and that many others are at least in part transgressive.[4]
[edit] Other meanings
"Sill" may also refer to the rise in depth near the mouth of a fiord caused by the terminal moraine of the previous glacier.
[edit] See also
[edit] References
- ^ Holmes, A. 1978. Pinciples of Physical Geology, Van Nostrand Reinhold (UK)Co.Ltd., 720pp.
- ^ Polteau, S., Mazzini, A., Galland, O., Planke, S. & Malthe-Sørenssen, A. 2008. Saucer-shaped intrusions: Occurrences, emplacement and implications, Earth and Planetary Science Letters 266, 195–204.
- ^ Thomson, K. & Hutton, D. 2004. Geometry and growth of sill complexes: insights using 3D seismic from the North Rockall Trough, Bulletin of Volcanology, 66, 364-375.
- ^ Planke, S., Rasmussen, T., Rey, S.S. & Myklebust, R. 2005. Seismic characteristics and distribution of volcanic intrusions and hydrothermal vent complexes in the Vøring and Møre basins, In: Doré, A. G. & Vining, B. A. (eds) Petroleum Geology: North-West Europe and Global Perspectives — Proceedings of the 6th Petroleum Geology Conference, 833–844. Petroleum Geology Conferences Ltd. Published by the Geological Society, London.
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