In modern geologic usage the concept of basement includes six elements:
Spatial relation: Basement is in the lowest or basal rock unit occurring within a region.
Stratigraphic relation: Basement lies beneath an unconformity of regional extent. The formations above the unconformity are referred to as cover or mantle. The term cover is favored today because mantle is now applied also to the deeper structural layers of the earth.
Temporal relation: Basement is always older than its cover. The difference in age of the basement and cover may range from 10 million yr to over 2 billion yr.
Rock type: Characteristic of a basement complex is a hybrid assemblage of igneous and metamorphic rocks. The overlying cover is composed of sedimentary rocks, with or without thin interbedded pyroclastic or volcanic flow rocks.
Structure and texture: Basement rocks are usually highly deformed and dominantly crystalline, in contrast to cover rocks that are less deformed and preserve their clastic texture....
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Burwash, R.A. (1987). Basement . In: Structural Geology and Tectonics. Encyclopedia of Earth Science. Springer, Berlin, Heidelberg. https://doi.org/10.1007/3-540-31080-0_6
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