1. The igneous and metamorphic rocks that exist below the oldest sedimentary cover. In some areas such as shields the basement rocks may be exposed at the surface. 2. A complex, usually of igneous and metamorphic rocks, that is overlain unconformably by sedimentary strata. Also known as basement rock. 3. Acrustal layer beneath a sedimentary one and above the MohoroviSc discontinuity. 4. The ancient continental igneous rock base that lies beneath Precambrian rocks. Also known as basal complex; basement complex.
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