The deepest significantly thick, high density unit(s) within the geologic section of an area, which provide a major positive density contrast. The rocks above the major density contrast are usually younger sediments and/or volcanics, typically having densities ranging from approximately 1.9 g/cm³ to 2.6 g/cm³. Those below the major density contrast are usually older sedimentary, volcanic and/or crystalline rocks, typically having densities ranging from 2.6 g/cm³ to 3.0 g/cm³. High density basement may or may not be equivalent to crystalline and/or magnetic basement.