Basement (geology)
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".
On the European and North American continents the basement contains generally every rock older than the Variscan orogeny. On top of this Variscan 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.
Some geologists object to the use of the term basement because they see it as too general an expression for all metamorphic and igneous formations. It is therefore used mostly in disciplines of geology like basin-geology, sedimentology and exploration geology (for exploration of hydrocarbons the basement is not interesting: it rarely contains oil or gas).
Sources
- 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.