The sedimentology and context of Late Ordovician glacial marine sediments from Sierra Leone, West Africa

Abstract

Glacial marine deposits (the Waterfall Formation of the Saionia Scarp Group) from northern Sierra Leone, West Africa, correlate with sequences in Guinea and Senegal and indicate a Late Ordovician age. The sediments (150 m in thickness) are laminated mudstones (rhythmites) of alternating fine laminae and lenticular, graded, coarse laminae a few millimetres thick. Dispersed clasts of various rock types occur within the rhythmites, which also locally contain slump structures. Interbedded with the rhythmites and dropstones are turbidites, with sole structures, amalgamated and graded beds. Transport directions are from the north. Deposition from low-density melt-water suspension currents derived from a shelf glacier is suggested for the rhythmites, with the clasts emplaced by iceberg rafting. Underlying the glacial sediments are subarkoses with heavy-mineral laminae and cross-bedded, super-mature quartz arenites attributed to a shallow-water tidal origin. The occurrence of glacial marine sediments in Sierra Leone provides a southwestern boundary to the ice-sheet which is well-documented as covering much of the Sahara and Northwest Africa during Late Ordovician time, and further emphasizes the great area affected by this glaciation.