Late precambrian glacial deposits from the rokelide fold belt, Sierra Leone

Abstract

Sedimentological details are given for the late Precambrian glacigenic rocks of Sierra Leone which occur at the base of the Rokel River Group. The Tibai Member is considered to be generally glacigenic, with tillites, rhythmites containing ice-rafted dropstones, and associated turbidite and possibly fluvio-glacial sandstones. The overlying Dodo Member is interpreted as a shallow-water, high-energy, perhaps partially intertidal deposits. The Taban Member, previously considered to be Rokelide (Pan-African) post-orogenic molasse, represents part of an outwash plain of fluvio-glacial sandstone sand gravels preserved in a down-faulted basin. The recording of these deposits in Sierra Leone extends the known range of the late Precambrian glaciation in West Africa. The Pan-African Rokelide orogenic belt in Sierra Leone is briefly compared with the higher-grade Pharusian and Dahomeyan mobile belts of West Africa. Late Precambrian depositional history in the Rokelide trough is similar to that recorded in the Taoudeni and Volta Basins and in the Pharusian and Dahomeyan fold belts but differes in detail due to variations in magnitude of the Pan-African orogeny from place to place. The Sierra Leonean glacial deposits are briefly related to late Precamrian polar wander paths.