Gravity stratification in the Freetown Basic Igneous Layered Complex,Sierra Leone,West Africa

An area of 78 sq. km in the vicinity of Freetown is mapped for the first time. By systematic mapping and study of about 1,500 thin sections, nine macro layered units have been recognized in this lowest exposed part of the intrusion. Each unit grades upward from magnetite or olivine-rich layers, through pyroxene-rich to plagioclase-rich rock. Field data of fine-scale layering, ‘false-bedded’ layers and slumping are given. These and other sedimentary structures and features indicate that the Freetown magma was convective. It is suggested that rhythmic layering in the Freetown basic intrusion originated from slow accumulation of density-sorted crystals which precipitated simultaneously from the magna-pulses.