In geology, the terms basement and crystalline basement are used to define the rocks below a sedimentary platform or cover, or more generally any rock below sedimentary rocks or sedimentary basins that are metamorphic or igneous in origin. In the same way the sediments and/or sedimentary rocks on top of the basement can be called a "cover" or "sedimentary cover".

[edit] Usage

On the European geological discussions the basement generally refers to rocks older than the Variscan orogeny.^{[citation needed]} On top of this older basement Permian evaporites and Mesozoic limestones were deposited. The evaporites form a weak zone on which the harder (stronger) limestone cover was able to move over the hard basement, making the distinction between basement and cover even more pronounced.^{[citation needed]}

In Andean geology the basement refers to the Proterozoic, Paleozoic and early Mesozoic (Triassic to Jurassic) rock units as the basement to the late Mesozoic and Cenozoic Andean sequences developed following the onset of subduction along the western margin of the South American Plate.^[1]

In a discussion of the Trans-Mexican Volcanic Belt of Mexico the basement include Proterozoic, Paleozoic and Mesozoic age rocks for the Oaxaquia, the Mixteco and the Guerrero terranes respectively.^[2]

The term basement is used mostly in disciplines of geology like basin geology, sedimentology and petroleum geology in which the (typically Precambrian) crystalline basement is not of interest as it rarely contains petroleum or natural gas.^[3]

[edit] See also

• Craton
• Platform
• Shield

[edit] References

• Parker, Sybil P. (Ed.). 1997. McGraw-Hill Dictionary of Geology and Mineralogy. New York: McGraw-Hill.
• Bates, Robert L. and Julia A. Jackson (Eds.) 1994. Dictionary of Geological Terms. American Geological Instutute. New york: Anchor Books, Doubleday Dell Publishing.
• Ben A. Van Der Pluijm (Editor), 1996, Basement and Basins of Eastern North America, Special Paper Geological Society of America ISBN 978-0813723082