Meaning of basement complex

base'ment com"plex

Pronunciation: [key]
— Geol. Geol.
  1. the undifferentiated assemblage of rock underlying the oldest stratified rocks in any region: usually crystalline, metamorphosed, and mostly, but not necessarily, Precambrian in age.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.