Basement Rock

Basement rock is the crystalline igneous or metamorphic rock that underlies a sedimentary basin and marks the bottom of the layered sediment column. In the oceans the basement is the basaltic oceanic crust formed at a ridge; on continents it is granitic and metamorphic crust, often Precambrian. The sediment-basement interface is a strong seismic reflector that marine surveys map to measure sediment thickness and, with the magnetic-anomaly age of the basaltic basement, to date the seafloor. Ocean drilling that reaches basement recovers the pillow basalt at the top of the crustal section and dates the start of sedimentation on new ocean floor.