J. geol. Soc. London, Vol. 136, 1979, pp. 605-618. Printed in Northern Ireland. Late Precambrian and Phanerozoic geology of Sierra Leone S. J. Culver & H. R. Williams SUMMARY:The post-Archaean geology of SierraLeone isreviewed and interpreted. Late Precambrian development of fault-controlled sedimentation of glacigenic, fluviatile, and marinedeposits with minor volcanics culminated in the Rokelide thermo-tectonic event at 550 Ma, resulting in a mildly metamorphosed NNW-trending synclinorium. Rokelide deposits areoverlain in northernSierra Leone by near-horizontal,shallow water sandstones and glacio-marinerhythmites of lateOrdovician age. Subsequent events were associated with sea-floor spreading activity, including development of coast-parallel dykes, a confluent cone- sheet, 2 alkaline complexes and kimberlites, all controlled by lineaments reactivated by the Mesozoic rifting of Africa from North America. Late Mesozoic downwarp of new continental margin produced’asedimentary basin into which marine, estuarine and fluviatile sediments were deposited. The geology of Sierra Leone has been the subject of is lacking but it was folded and slightly metamorph- major review papers by Dixey (1925)and Pollett osed by theRokelide Event (Allen 1968, 1969) (1951). Since then, much new work has been under- c. 500 Ma ago, the time of the Pan-African thermo- taken,and the geology of thecontinental shelf has tectonicevent (Kennedy 1964). The lithological and been the subject of several surveys. structural similarity of the Rokel River Groupwith the In this paper, previous work on the post-Archaean Falemian rocks of Senegaland Guinea-Bissau led (post 2500 Ma) geology of Sierra Leone is reviewed, Choubert & Faure-Muret (1971) to consider that it is but is interpreted in the light of recent studies by the of probable late Precambrian to early Cambrian age authors.This paper, together with oneon the Ar- andCulver et al. (1978)showed thatthe lowermost chaean geology of Sierra Leone (Williams 1978), has strata are of ‘Infracambrian’ age (c. 620 Ma). evolved from lecture material given to several genera- 6 formations were recognized by Allen (1968) in the tions of undergraduate students, to whom we owe a Rokel River Group (the Tabe, Makani, Taya, Mabole, debt for their continuing criticism. Taia and Kasewe Hills Formations) (Fig. 2). Geological events in Sierra Leone are dealt with in TheTabe Formation rests unconformably on Ar- order of decreasing age toassist in the building up of a chaean granitic basement and outcrops along the east- general geological history of thecountry. 2 major ern margin of the main Rokel River Group outcrop, eventscan be recognized: thedevelopment of the but is confined tothe southern end of thewestern Pan-AfricanRokelide synclinorium, andthe forma- margin. Outliers of theTabe Formation occur in tion, since early Mesozoic times, of a new continental northern Sierra Leone, both to the E and W of this margin with associated igneous and sedimentary pro- main outcrop (Fig. 1). The Tabe Formation has been cesses. subdivided into 3 members, theTibai, Taban and Dodo Members, the first 2 being glacigenic (Culver et al. 1978). TheTibai Member consists of 3 mainlithologies. The Rokel River Group and Poorly sorted conglomerates, originally interpreted as a marine trangressive deposit (Allen 1968), have been the Rokelide Event shown to be tillites (Culver et al. 1978). Closely as- sociated with the tillites are laminated siltstones and Stratigaphy and sedimentology fine sandstones with isolated granitic clasts which have of the Rokel River Group been interpreted aspossibly lacustrine rhythmites con- TheRokel River Group extends southward from tainingice-rafted dropstones (Vallance 1974;Culver Guineainto Sierra Leone where it occupies a belt et al. 1978).Interbedded with therhythmites are c. 30 km wide, trending SSE for some 225 km into lenticulargraded feldspathic sandstones which are southern Sierra Leone (Fig. 1). The group was defined considered to be channel turbidites. anddescribed by Dixey(1920, 1925) and was sub- TheTaban Member (formerly theTaban Forma- divided into 6 formations by Allen (1968). The lower- tion) is composed of feldspathic sandstones and was most formation of the Rokel River Group has been considered by Allen(1968) tobe post-Rokelide redefined (Culver et al. 1978)and has been further orogenic molasse. However, since the Taban Member subdivided into 3 members. exhibits greenschist metamorphism and folding consis- Fossil control of the age of the Rokel River Group tent with otherpre-orogenic Rokel River Group 0016-7649/79/0900-0605$02.00 @ 1979 The Geological Society Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/5/605/4885946/gsjgs.136.5.0605.pdf by guest on 26 September 2021 606 S. J. Culver & H. R. Williams 13O W 12” W COMPILATION FROM: D.O.S. GEOLOGICALMAP 1120 (19601, ALLEN (1969), BULLOM GRCUP (TERTIARY to RECENT) MACFARLANE et 01. (19741, VALLANCE (1974), BASIC L ALKALINE INTRUSIONS CULVER,S.J. and (TRIASSIC to JURASSIC) WILLIAMS,H.R. (unpub.1 SAIONIA SCARP GROUP (LATE ORDOVICIAN) LIBERIAN GPANITE- GREENSTONE TERRAIN i 110 W 120 W FIG. 1. Geological map of western Sierra Leone.Ba, Bagbe m m So, SongoComplex; Gb, Gbangbama;PL, Port hko;S, Sumbuya. The area of maximum coast-parallel dolerite dyke development is situated between the two dotted lines. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/5/605/4885946/gsjgs.136.5.0605.pdf by guest on 26 September 2021 Late Precambrian and Phanerozoic geology of Sierra Leone 607 Formation which is composed of grey silty clays with interbedded orthoquartzite and subarkose beds. Allen (1968) interpreted these deposits as well-worked, pos- sibly neritic zone sediments. The overlying Teye Formation is a thick, structur- ally complexsequence of purplish-brown to grey shales, interbedded with beds of greenish-grey sand- stones from 5 mm to 0.5 m thick. These deposits were interpretedas turbidites by Allen(1968). Grading, sole structures, cross-lamination and mud clasts are all present in sandstonesintercalated with shaly layers. However,some sequences consist almost entirely of siltstone andshale and may be distal turbidites,but they can as readily be interpreted as quiet water outer shelf sediments. The intermittent exposure and com- plexstructure preclude any attempts at elucidating palaeocurrent directions. Overlying the Teye Formation are the lateral equiv- alents, the Mabole and Taia Formations; interbedded with them is the Kasewe Hills Formation. The Mabole TEYE Frn. Formation is composeddominantly of shaleswith interbedded siltstones, orthoquartzites, subarkoses and arkoses.Ripple-marks, cross-bedding, detrital specu- larhaematite and well-rounded orthoquartzites led Allen(1968) to suggest thatthe Mabole Formation MAKANI Fm. sediments were deposited in a shallow water, probably deltaic environment. In northern Sierra Leone, Allen (1968)observed sedimentary clasts in conglomerates at the base of the Mabole Formation; these suggest that a period of non-deposition, perhaps caused by the early stages of the Rokelide tectonism, followed Teye FIG.2. Stratigraphic succession of the Rokel River Formation sedimentation. Group. K. H. Fm., Kasewe Hills Formation. Tibai, To the W of the Mabole Formation is the laterally Dodo and Taban Members comprise the Tabe equivalent Taia Formation. It consists of greymud- Formation. stonesand shales with a few interbedded silty and sandy layers. Allen(1968, 1969) interpreted these strata, we consider that thesedeposits are pre- depositsaspro-delta sediments situated off the orogenic.They rest conformably on metamorphosed Mabole Formation deltas. TibaiMember tillite and havebeen interpreted as The Kasewe Hills Formation, which occurs in lenses braided stream deposits which formed a fluvio-glacial up to 2 km thick within the Mabole and Taia Forma- outwash plain (Culver et al. 1978). tions, is composed of volcanic tuffs and lavas of vary- Thusthe basalbeds of theRokel River Group ing composition(Allen 1968).Andesites are domin- recordthe presence of anextensive ice-sheet over ant, but spilites, basalts and tuffs of dacitic composi- SierraLeone. Several ice advancesand retreatsare tion also occur. Allen (1968, 1969) considered that the probably recorded by the several tillite horizons and lavasand tuffs wereerupted fromseveral volcanic there is some evidence, in the form of glacially trans- centreswithin or in closeproximity tothe Mabole ported pebbles, that the ice-sheet moved over Sierra Formation delta complex. Pillowed spilites show that Leone from the E. someeruptions were sub-aqueous, while air-borne Closelyassociated with theglacigenic deposits are pyroclastics are alsopresent, both interbedded with feldspathicsandstones of theDodo Member, which lavas, and further from the volcanic centres, interbed- overlie the Tibai Member but may be laterally equi- ded with Taia Formation pro-delta deposits. valent in places.These cross-bedded and ripple- Most mapping of the Rokel River Group has been markedsandstones have been interpreted (Allen of areconnaissance nature. It is likely thatdetailed 1968; Vallance 1974; Culver et al. 1978) as shallow mapping, across strike, of river and stream exposures marine, possibly intertidal deposits, and they probably would alter considerably the present outcrop config- represent a generally post-glacial rise in sea-level and uration of the various formations of the Rokel River subsequent marine transgression. Group. Similarly, modified interpretations of environ-