Late Precambrian and Phanerozoic Geology of Sierra Leone

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 Rokelidethermo-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 associated 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

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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.

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Formation which is composed of grey silty clays with interbedded orthoquartzite and subarkose beds. Allen (1968) interpreted these deposits as well-worked, possibly neritic zone sediments. The overlying Teye Formation is a thick, structurally complexsequence of purplish-brown to grey shales, interbedded with beds of greenish-grey sandstones 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 theMabole 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 equivalentTaia 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 onmetamorphosed 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 wouldalter 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- TheTabe Formation is overlain by theMakani ments of deposition might be possible.

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Allen(1969) considered that the Rokel River vertical axial planes.They die out N of the Rokel Group deposits represent the development and infil- River. A third phase of folding with concentric forms ling of ageosyncline, while Burke & Dewey(1973) and E-W upright axial planes is active in localized suggested that this basinwas thesite of continent E-W trending zones. collision tectonics. An intra-continental rift origin for Inthe northern part of thecountry, much of the the Rokel River Group basin is considered below. western margin of the Rokel River Group outcrop is an F1 thrust. Elsewhere, on the eastern margin and the southern part of the western margin, it is a heterolithic Structural geology and metamorphism unconformity, where bedding orientation varies from associated with the Rokelide Event horizontal to vertical dueto fault movements and Thestructure of theRokel River Group syn- associated folding. Indeed, the loci of F1 folds in the clinoriumhas been described by Allen(1967, 1969) Rokel River Group are probably located over fault- andMacFarlane et al. (1974).Bedding strikes are movement zones in the underlying Archaean granitic commonly SSE-NNW, with average dips of 20"W on basement. theeastern margin andup to 60"E on the western Metamorphic grades increase westwards from non- margin (Fig. 3). metamorphic lithologies in the E, to greenschist facies Allen recognized 3 phases of deformation. The first, in the W. Growth of sericite, actinolite,muscovite, associated with thrusting movements from the WSW, chloriteand biotite is typical in the volcanicsand producedbroken-bed type deformation, with thrusts pelites in the westernportion of theoutcrop, espe- parallelling bedding, and development of asymmetric cially in the N. An F1 schistosity is developed only in chevronfolds. These southward plunging folds are the W, and this is locally crenulated by Fz structures. close in style andhave SW-dipping axial planes. In There is no evidence for high grade metamorphism so siltstone-sandstone lithologies the folds are frequently typical of other Pan-African orogenic belts. disharmonic. An axial planar fracture cleavage is only The Rokelide Event in Sierra Leone developed the recognizable in theW, where deformation is most RokelRiver Group synclinorium,which plunges intense. The F1 folding is locally developed and is not northwards to become the southern extension of the penetrative in character, giving zones of intense defor- Falemidesorogenic belt in the westernSahara. In mationseparated by virtuallyundeformed rocks. In Sierra Leone, it reactivated Archaean brittle structures northern Sierra Leone, MacFarlaneet al. (1974) found in the basement rocks and produced fault-controlled that axial trends veered to the NNE and plunged to folding in the sedimentary strata of the cover. Part of the N. the western margin of the Group is a westerly dipping A second phase of folds, only recognized by Allen F1 thrust, similar in orientation to the Archaean thrust (1967), are gentle and concentric,with N-S trends and which forms most of the eastern margin of the Kasila

WEST < EAST INCREASINGDEFORMATION AND METAMORPHISM

Kasila Group

Tabe Frn.

GRANITIC BASEMENT

5km I FIG.3. Diagrammatic E-W cross-section through the Rokel River Group synclinorium. K, Kasewe Hills Formation; m, mylonite.

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Group (Williams 1978), alongwhich mylonites are Africanthermo-tectonic event. It is generallyconsi- developed and retrogressive metamorphism occurred. dered(Reid & Tucker1972; Tucker & Reid1973) Fragments of materialfrom this Archaeanthrusting thatthe Saionia Scarp Group is of probablelate zoneare found within the basalbeds of theTabe Ordovician age. formation. The 2 parallel thrust zones die out south- The Saionia Scarp Group sedimentary sequence is wards in sympathy with the outcrop width and defor- 190 mthick and is intruded by several dolerite sills, up mationintensity in theRokel River Group. Near to 90 m thick, of probable Triassic age. The Grouphas Mano, no thrusts occur within the basal section of the been subdivided into the Moria and Waterfall Forma- Rokel River Group, and slightly further S, the basal tions(Reid & Tucker1972; Tucker & Reid1973). contact of the Group with the basement is clearly an The lower Moria Formation consists mainly of sand- unconformity. Innorthern Sierra Leone, the thrusts stones, with a basal conglomerate in places. The lower are widely spaced and enclose zones of basement with sandstones are feldspathic with well-developed heavy westerly increasing metamorphic grade (Allen 1969). mineral laminae occurring in truncated sets. The over- The coincidence of the thrusts with the outcrop of the lying sediments arewell-sorted quartz arenites exhibit- Group suggests a genetic link. This is borne out by the ing planar cross-bedding which in turn are overlain by deformation style, in which fault-controlled schuppen- redand purple laminated shales containing some like blocks of the underlying basement and their over- quartz silt. The sandstones of the Moria Formation are lying drape of thinsediment were activated by the considered to have been deposited under littoral and thruststo the W. The line of volcanicrocks of the shallow shelf conditions,while the overlyingshales Kasewe Hills Formation near the western margin also were perhaps deposited in a quieter water shelf envi- suggests thatlinear, rift-controlled volcanism took ronment (Reid & Tucker 1972; Tucker & Reid 1973). place, the structures dying out southwards. We prefer Reid & Tucker (1972) suggested that the basal beach a rift model(aulacogen) forthe origin of the syn- depositsmay have been deposited by thelate clinorium tothat put forward by Burke & Dewey Arenigiantransgression recorded over much of NW (1973),who suggested that it was formed by Africa. continent-continent collision tectonics. Neither do we The overlyingWaterfall Formation consists of regard the Group as having been formed as part of an 150 m of laminated argillites withlenticular coarse- island arc system, as implied by Gass (1977, fig. 3), for grained sandstone beds. The laminated mudstonesdis- which there is no evidence. play alternation of coarse and fine laminae which led During the deformation of the Rokel River Group Reid & Tucker (1972) to call them rhythmites. They rocks, the basement in the W and S of Sierra Leone containrandomly dispersed angular to sub-rounded was partially reworked by thrusting, and underwent a clasts up to 0.3 m in diameter, consisting of sedimen- thermaloverprint, giving K-Arages of c. 500 Ma tary, metasedimentary and granitic rocks. The lamina- (Hurley et al. 1971). Near to Port Loko, in NW Sierra tions of therhythmites are depressed and broken Leone,late-kinematic trondhjemites were intruded beneaththe clasts. Interbedded with therhythmites into the Kasila Group, and exhibit a Pan-African age, arecoarse-grained, sometimes graded, feldspathic determined by the Rb-Sr method on 1 sample. sandstones in lenticularbeds, exhibiting sole structures. TheWaterfall Formation hasbeen interpreted as having been deposited under glacio-marine conditions The Saionia Scarp Group (Reid & Tucker1972). The isolated clasts in the The Rokel River Group represents the W African rhythmitesareice-rafted dropstones, andthe FoldBelt in SierraLeone (Sougy 1962),while the feldspathicsandstones are considered to bechannel SaioniaScarp Grouprepresents the overlying ‘hori- turbidites probably formed through slumpingof coarse zontal sandstones’ (of authors). glacially-derived material (Tucker & Reid 1973). The Saionia Scarp Group covers an area of 50 km2 Quartz grainsremoved from the surface of speci- in northern Sierra Leone (Fig. l), and extends north- mens of Moriaand Waterfall Formation sediments wards intoGuinea, where laterally equivalentstrata and examined under the scanning electron microscope, are the ‘grts silicieux horizonteaux’ (Dixey 1925), now show surface textures which confirm interpretations of called the ‘grts blancs’ (Renaud & Delaire 1955). environments of deposition based on other sedimen- No fossils havebeen found in the SaioniaScarp tological criteria (Bull & Culver 1979). Quartz grains Group, but atTClimClC, a few km to the W, in Guinea, from the rhythmites show breakage blocks and con- the‘grts blancs’ are overlainconformably by black choidalfractures consistent with modification by a shales of the ‘sCrielitCe’ (DeChgtelat 1933), which mechanical process, possibly a glacial one. containSilurian graptolites (Sinclair 1928).The ThustheWaterfall Formation shows distinct SaioniaScarp Group rests unconformably onAr- lithological similarities with the Tibai Member of the chaeangranitic rocks and onRokel River Group Rokel River Group although tillite horizons have not conglomerates,the latter folded during the Pan- been definitely identified in the Saionia Scarp Group

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and the rhythmite laminationsin the Waterfall Forma- layersand lenses occur near the bases of individual tion are less laterally extensive than those in the Tibai zones,as do deformed lenses of leucogabbro;both Member.The Saionia ScarpGroup and the Rokel occuras xenoliths in olivinegabbro. Itseems likely RiverGroup are considered to be 2 temporally dis- thatinjection of a new magmapulse for each new tinct units which show lithological similarities due to zone tears away fragments of the sedimented floor and repetition of similardepositional environments with chilled roof zone of the existingmagma chamber. time. These fragments are then incorporated by settling, as beerbachite lenses and deformed leucogabbro pieces, Basaltic activity atthe initiation into the early formed basal cumulates developing from of Mesozoic rifting the newly cooling magma. Wells(1962) thought that the layering was either Early Mesozoic magmatism is basaltic and is directly dueto bottom crystallization, whichproceeded up- associated with the rifting apart of North America and wards, or to deposition from a laminar flow of cooling Africa. magma. He suggested that some of the layers on the Wside were too steep to havebeen deposited by The Freetown Complex sedimentation of cumulusgrains. However, some of the very steep dips may be due tomild regional crustal The Freetownlayered basic complex (Wells 1962) flexuringsince the intrusion of thebody, but this consists of a 6 km thick series of cumulate rocks of flexuringaffected the bodyas a whole, and is not gabbroiccomposition, containing layers of dunite, responsible for the gradual steepening of the dip sea- troctolite,olivine-gabbro, gabbro, leucogabbro and wards. anorthosite. Originally described as a batholith, it was Mineralogically, the Freetown Complex displays few shown by Baker & Bott (1961) to be sheet-like, and of the characters expected in large layered intrusions. because of its circular outline on the Freetown Penin- The minimalcryptic layering present is unrelated to sula,was assumed to be a funnel-shaped lopolithic height above the base of a zone or rhythm. Order of structure.However, a confluentcone-sheet structure cumulus crystallization is ilmeno-magnetite as immis- (Walker 1975) is more appropriate. The complex has cible droplets(Bowles 1978), olivine, pyroxene, been reliably dated as 193* Ma by Beckinsale et al. plagioclase; this gives rise to the following stratigraphy (1977). of rhythms and zones: Theoutcrop limits of the complexdefine an arc. Layeringdips radially inwards from this arc,and a Top:Anorthosite hypothetical 'centre' is deduced to lie 16 km WSW of Leucogabbro York. Layering and basal contacts are parallel, shallow Gabbro dipping (10-20") nearthe margins of theintrusion, Olivine gabbro becoming more steeply inclined as the 'centre' is ap- Troctolite proached (45-50'). Bottom: Ilmenomagnetite-rich troctolite. The layering may be divided into 4 major zones on the basis of large-scalevariations in mineralogyand Adcumulusgrowth to formheteradcumulates is topographicexpression. Wells (1962) suggested that most common in the anorthositic rocks found at the each zone is the result of a single magmatic event. The tops of zones. Elsewhere, adcumulus growth enlarges base of eachzone is olivine-rich,and thetop anor- cumulus grains, but traps liquid which reacts with and thositic. Within the zones are many rhythmic units on surroundsthe cumulusgrains, ultimately producing ahectometre scale, and withineach rhythm is a minuteamounts of quartzo-feldspathicmaterial. centimetre-to-metre-scalebanding. Both zones and Plagioclase compositions vary from An 59-64, while rhythmstend to bemore olivine-rich attheir bases variation in the composition of olivine is fromFo and plagioclase-rich at their tops, so in effect rhythms 58-71.Sulphides areubiquitous, but occur in small are small-scale duplications of zones. Rhythmic layer- amounts (Bowles 1978), either as crystallized droplets ing and banding is more developed in the basal por- of immiscibleliquid trapped asinclusions in olivine tions of zones and rhythms, while massive, lenticular andplagioclase, or aslate-stage hydrothermal veins anorthositic rocks, which are locally transgressive into and replacements. Copper, nickel, platinum and gold overlying rocks, characterize the tops of zones (Wells havebeen recorded in associationwith sulphides & Baker 1956). Current activity during cumulate for- (Stumpfl1966). mation caused the production of well-developed min- The well developed layering cannot be attributable erallamination, minor discordances, lensoid wispy to size-sorting by currents,because early formed layers,cross-stratification andrare mineral lineation grains atthe bases of zones are very fine. Hawkes within layers. Slumping of thelayered rocks is re- (1967) postulated that the layering was produced by corded, but as with cross-stratification, no consistent an easingof nucleation mechanism.The orderof crystal- directionsmay be deduced. Granular beerbachite lization is dependenton super-saturation, whilethe

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rapid changes in mineral phases, producing the layer- represent basaltic material intruded into the Complex ing, is dueto cyclical under-coolingand delay in whilst it was still hot.The pegmatites represent hy- nucleation. As long as undercooling is small, precipi- drousresidual melts, trappedafter initial consolida- tated grains are large. When a critical stage of under- tion. Beckinsale et al. (1977) showed that the joint- cooling is reached, whereby abundant small grains are controlledleucocratic veins in thegabbro are hy- precipitated,as in the basalparts of zonesand drothermal in origin and need not represent residual rhythms, they are composed of relatively lower temp- granitic material produced by fractional crystallization. erature phases (more sodic and iron-rich). This typeof chemicalvariation seems more important than any Dolerite dykes and sills cryptic variation due to longer term fractional crystallization. In association with the nucleation of plagioc- Dolerite dykes and sills are found throughout Sierra lase in abundance at the tops of zones and rhythms, is Leone, (Fig. 1). Somehave been dated as Permo- the phenomenon of its partial transgressive nature into Triassic (Snelling 1966; White & Leo 1969), intruded therocks overlying it. Thisremobilization of anor- at much the same time as the Freetown Complex. thositic rocks is probably associated with the presence Dolerite dykes are most common close to the coast, of abundant water. The presenceof an increased water intruding Precambrian rocks and those of the Rokel vapourpressure at these stages is borneout by the River Group as a coast-parallel dyke swarm traceable development of replacement pegmatites, and coarsen- into Liberia (Fig. 4). Further inland, the dykes become ing of textures. thinner and occur with more varietyof trend, parallel- Associatedwith the Complex,but cutting it, are ling major NE, ESE faults. Visible intersections be- beerbachitedykes, pegmatites of pyroxeneand tweendykes arerare. Cataclasisparallel with dyke ilmeno-magnetite,and leucocratic veins. The dykes trends after intrusion (Mackenzie 1961), reinforces the

15O W l0OW

GUINEA

ATLANTICOCEAN

I5O W

FIG. 4. Sketch map showing the relationshipbetween magmatism and regional structure in SierraLeone and adjacent countries. B, Bagbe; C, Cape Mount; F, Freetown Complex; S, Songo; T, Tongo; Y, Yengema; GFZ, GuineaFracture Zone; SLFZ, SierraLeone Fracture Zone; solidcircles, kimberlite;dotted lines, hypothetical continental continuations of fracture zones; dashed line, major fault controlling kimberlite magmatism.

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hypothesisthat the dyke trends are fault-controlled. The Songo Ijolite Coast-paralleldykes with a high NE dip, showing TheSongo Ijolite (Baker et al. 1956) is apoorly bifurcation and en echelon structures, were described exposed body 17 km E of the Freetown Complex (Fig. by Rollinson (1974) and may indicate a slight trans- 1). Generallycovered by Bullom Groupsands, it is current shear component operative during their intru- intrusive intothe Archaean Kasila Group,but no sion. Crustal dilation due to dyke intrusion appears to contacts are visible. Pavement exposures over an area be generally below 5%, though in the area W of Bo of 0.3 km2 consist of ijolite and derived bauxitic ma- (Allen 1967), extension is in the order 20%, with of terial.Boreholes indicate that the ijolite forms an dyke widths of up to 200 m quite common. irregularridge oriented WNW-ESE. Layering as The dykes show a variation in mineralogy, but there defined by pyroxene elongation is inclined to the N at is a consensus of opinion that they are consanguinous. 70-90". Arcuate and convex to the S, the trend of the Some contain olivine and orthopyroxene, while others steep layering possibly indicates that the exposed ijo- show only relicts of these minerals. All dykes consist lite is part of aring complex. A 5 mdyke of fine- mainly of a zoned labradoritic plagioclase and brow- grained phosphatite cuts the ijolite for 500 m, with an nish synneusis clots of augite occurring as subophitic ESE trend. A K-Ar radiometric age of 150 10 Ma intergrowths.Porphyritic dykes, containing abundant f hasbeen determined for pyroxene from the ijolite 3 cm laths of plagioclase with marginalsubparallel (Andrews-Jones1968). alignment of crystals, also occur. Magnetite, ilmenite, The ijolite is massive, medium grained, dark grey, and graphic quartz-feldspar intergrowth are common with elongated black pyroxenes. The cm-scale banding accessoryphases. Uralitic hornblende,biotite and is due to variations in the ratio of nepheline to diop- leucoxene are found as autometamorphic minerals in sidic pyroxene. Accessory minerals include magnetite, the large, coarse-grained bodies. The mineralogy and ilmenite, apatite and zeolites. Biotite and aegirine are chemistry indicate a tholeiitic magma composition. present as secondary minerals. Dolerite sills form resistant caps of 2 major moun- The dyke cutting the ijolite is ultrabasic, composed tains in Sierra Leone, Bintumani (1948 m) and Saionia of rockbridgeite,aluminous strengite, goethite, war- Scarp (1000 m). Sills occur at progressivelylower dite,and cacoxenite, making thename 'phophatite' levels when traced westwards, forming similar tabular most appropriate (Vorma 1961). structures up to 120 mthick in the Rokel River Group at a height of 150 m (MacFarlaneet al. 1974; Vallance 1974). The progressive decrease in sill elevation with The Bagbe Alkaline Complex respect to sea level towards the coast may be evidence TheBagbe Alkaline Complex (Wilson 1965)con- for a post-Triassic continental margin subsidence as- sists of nepheline syenites, perthosites, alkaline granite sociated with the opening of the Atlantic Ocean and and fenitized Archaean granitic country rocks (Fig. 1). the formation of new continental shelf (Bott 1973). The earliest eventassociated with the Complex is an The small sills on Bintumani are banded on a metre- alkali metasomatism along a fracture zone, which ulti- scale,suggesting cumulate formation (MacFarlane et mately desilicated the Archaean granitic host material. al. 1974). The 3 separate sills on Saionia Scarp make up atotal intrusion thickness of 200m, and these Dykes and minor intrusions of nepheline syenite were intruded within the fenitizedzone, and a late-stage indicatea large scale mineralogical variation due to crystal settling (Strasser-King, pers. comm.). hydrousalteration produced sodalite, cancrinite and The sills have mineralogical compositions similar to carbonate. that of the dykes, and are presumed to be structurally Andrews-Jones(1968) suggested that the Bagbe and temporally related. and Songo Complexes are coeval. However, Hurley et (1971) determined a single Rb-Sr age of 2100 Ma Withinthe kimberlite breccias of Koidu,basaltic al. xenoliths have been found. Their grain size is much from the Bagbe Complex, but this may reflect a relict smaller than that found in dykes and sills of the area, Archaean age in the fenitized granitic rocks. as they probably represent fragments of an extrusive basaltic layer present during Cretaceous times that has Kimberlites since been removed by erosion (Hubbard 1967). Considerable research into the kimberlites of Sierra Alkaline and kimberlitic activity Leone followed the realisation that alluvial diamond deposits d.erived from them were nearing exhaustion After the basalticmagmatism associated with the early (Williams 19 Williams1975). The relativelyrecent stages of rifting, there followeda phase of alkaline discovery of kimberlite may be attributed to the in- plutonism. Magmatism terminated with kimberlite ac- tense weathering, such that only very rarely may kim- tivity duringthe Cretaceous period, when rotation berlitebe determined at the surface. However, with poles changed position as south America rifted from expertise,kimberlite may bedetected in freshly Africa (Williams & Williams 1977). trenched lateritic profiles.

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Kimberlite occurs mainly as dykes, but 2 pipes and a1977). The W Africankimberlites of Mesozoicage dyke-enlargement ring complex structure are also re- form an ENE-WSW trending zone from Sierra Leone corded (Hall 1968; King 1972). 2 main areas contain to northern Ghana, along the lines of 2 fracture zones kimberlite,the Yengema and Tongo Leases, 50 km (Fig. 4). Kimberlite fields occur where these continen- apart (Figs. 1, 4). Kimberlite is foundelsewhere tal continuations are transected by reactivated regional but only as isolated dykes. The dyke zones are bor- Archaeanstructures. The combination causes deep dered to the E and W by 2 N-S trending regional fault crustal faults to coincide with deep mantle movement zones, and withinthese faults the dykes generally zones,allowing the rise of magmasfrom very great trend WSW-ENE, cutting other Archaean faults and depths. The concidence of kimberlite intrusions with Permo-Triassic dolerites. The kimberlites are dated as sea-floor spreading structures suggests that the timing Cretaceous (Bardet & Vachette 1966), and the pres- of kimberlite magmatism is coincident with change in ence of feldspathicsandstones, shales and basaltic platemovement directions (Williams & Williams rocks in thekimberlite breccias indicates erosion of 1977; Sclater et al. 1977). pre-Cretaceousrocks not represented at the present erosion level. Estimates of this erosion are upwards of theorder of 300 m,based on thequantities of The Bullom Group and continental diamondsreleased into the drainage system (0. F. King, pers. comm.). shelf sedimentation In the Yengema Lease, the dykes occur as a series Bullom Group of arrays of averagetrend 070°, between2 major The faults. Individual dykes trend in an en echelon manner Bullom Groupsediments occupy the low-lying from 050 to OSO", significantly different from the main coastal plain of Sierra Leone (Fig. 1). These deposits swarmdirection. Locally, trends are affected when extend up to 50 km inland and are found at heights of dykes intersect with faults. Thedykes occur as vertical, up to 40 m above present sea level. Outcrops are rare curvilineararrays, with individualdykes between and generally poor with the exception of 25 m high 50mm and 1m wide and up to 300m long. The sea-cliffs at Bullom, N of Freetown. dykes,some dilational and others replacive,contain The Bullom Group consists of a laterally variable abundant xenoliths of the granitic host rock (Williams sequence of poorlyconsolidated, near horizontal, & Williams 1975). often iron-stained gravels, sands and clays with occa- Progressiveenlargement of dykes atintersections sionalintraformational laterites and lenticular seams with faults ultimately leads to pipe formation. 2 small of lignite. pipes, less than75 m in diameter,and dyke-a The clays are generally kaolinitic, red, purple and enlargementcomplex, are found in the Yengema white in colour and in the Bullom cliffs contain plant Lease surrounded by 3 m aureoles of fenite and cal- remains.Beetle elytra (Dixey 1921), gastropods and cite, talc and analcite mineralization. Breccias within lamellibranchs have also been found in shallow hori- the pipescontain avariety of mantleand crustal zons (Junner 1929). In a borehole drilled to a depth of xenoliths (Hubbard 1967; Grantham & Allen 1960). c. 120 m (100 m below present sea level), E of the In the Tongo area, only dykes are found, which are Freetown Peninsula, a sparse fish and mollusc fauna of similarcomposition, structural setting and age as obtained from borehole sludges indicated an age not those in the Yengema Lease. older than Eocene and possibly as young as Miocene The kimberlite rock is usually a porphyritic to fine- fornear basalsediments (White & Gorodiski, in grainedgrey togreen material composed predomin- Mackenzie 1961). antly of serpentine, phlogopite, olivine, pyroxene and Thesands, sometimes graded, butrarely cross- carbonate.Pyrope and picro-ilmenite megacrysts are bedded, are generally poorly sorted, with a clay mat- commonand are usedas indicator minerals during rix; partially disintegratedfeldspars occur. Quartz prospecting. The rock is cut by numerousveins of grains are very angular and under the scanning elec- calcite and serpentine, the former often fenitizing the tron microscope show no evidence of marine or pro- adjacent wall-rocks. The magma shows signs of chil- longed fluvial activity. Interbedded with the sands are ling against theupper crustalxenoliths, which are occasional grit beds, stringers of rounded quartz peb- frequently very well rounded. The penetration of the bles, and horizons of kaolin clay clasts. Intraforma- magmainto the granitichost rocks on anintimate tional laterites occur within the sands and often form scalesuggests that it was of a verylow viscosity puddingstone horizons. Rare,thinly bedded calcareous (Grantham & Allen 1960). clays and grits havealso been recorded (Mackenzie The regionalstructural control of kimberlitemag- 1961). matism in Sierra Leone and West Africa as a whole is Lenticular lignite bedsup to 2m thick (Pollett based onthe association of possiblemajor ocean 1950) have been recorded in shallow boreholes E of fracture zone continuations in continents with funda- the Freetown Peninsula and also at Sherbro in south- mentalArchaean lineaments (Williams & Williams ern Sierra Leone where they have given radiocarbon

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dates of 30000-34000 years(Raufuss, in Hawkes Pleistocene, Bullom Group deposits will certainly ex- 1970). tendwestwards ontothe continental shelf. No Thepoor palaeontological control limits detailed boreholedata are available,but several offshore understanding of the history of deposition of the Bul- geophysical surveys have been undertaken. lom Group.However, a reasonable picture canbe obtained by comparingthe Group with the well- The continental shelf documented Cenozoic deposits in the Senegal coastal basin tothe N. Thisbasin extends around 500 km Sheridan et al. (1969) recorded the possible presence inland and contains, beneath Tertiary deposits similar of 2-3 km of Cambrian to Devonian strata overlain by to the Bullom Group rocks, Jurassic and Cretaceous more than 5 km of possible Cretaceous to Quaternary marine shales, sandstones and limestones. sedimentsalong a seismic refraction profile running Marinedeposition continued in the SenegalBasin SW from Freetown (Fig. 5). This is in agreement with until the Upper Eocene, when a regression, possibly Jones & Mgbatogu (1977), who considered Cretaceous caused by epeirogenic uplift (Bond 1978), heralded a strata on the outer shelf off Sierra Leone to be over period of intensive erosion under sub-arid conditions. 2 km thick. Sheridan et al. (1969) suggested that the Non-marine detrital sediments (‘continental terminal’) Palaeozoic strata, resting on crystalline basement, are dominated by argillaceousmottled sandstones, of the seaward extension of the Palaeozoic sedimentary white to pink or red colour, were deposited on marine rocks of the Guinea Basin. They terminate seawards at Eocene strata. These deposits are considered to have the old line of rupturebetween Africa and the been deposited by repeated surface wash of extensive Americas (Fig. 5), and off Guinea and Guinea-Bissau glacis (Michel 1973). Amarine transgression in the form the WSW-ENE trending Guinea Arch (Sheridan Miocenecaused small embayments to form in the et al. 1969) which separatesthe Sierra Leone Basin Senegal coastline, but major post-Miocene epeirogenic from the SenegalBasin. The possibleCretaceous to uplift of Africa(Dixey 1956; Bond 1978) broughta Quaternary sediments recorded in the profile are the return to the ‘continental terminal’style of sedimenta- seaward extension of the Bullom Group. The history tion. of marine transgressions and regressions recorded in TheEocene marine deposits recorded in the the strata of the Senegal Basin is probably recorded Senegal Basin are possibly represented in the Sierra also in these shelf deposits. Leone Bullom Group Basinby the fossiliferous and Bathymetric, magnetic, seismic reflection and sedi- calcareousstrata recorded at the bottom of deep ment studies have been undertaken on the continental boreholes. The overlyingwhite, pink andred sands shelf and upper slope off Guinea, Guinea-Bissau and represent non-marine detrital sediments eroded from SierraLeone, by McMasterand co-workers. Several extensive quartz-rich laterite cover (Mackenzie, pers. published papers were summarized by McMaster et al. comm.) andtransported seawards bywater courses (1971). having a wadi regime. Definite Miocene marine strata The late Tertiary and probably theearly Pleistocene have not yet been recognized, andmay not be present were marked by continued upbuilding and outbuilding onland in Sierra Leone. of the continental shelf over the Guinea Arch to the N Wadi-typetransportation and alluvial-fandeposi- of Sierra Leone. During the middle of the late Pleis- tion in SierraLeone probably continued into the tocene,major deltaic construction occurred in this Quaternary.During sub-arid glacial maxima area to form the now submarine Bissagos, Orango and (Sarnthein 1978), infrequentbut heavyrains caused Nunezdeltas (Fig. 6). Off SierraLeone, St. Ann’s extensivesheet erosion and rivers incisedtheir barrier spit (Fig. 6) extended offshore towards the NW courses.During humid inter-glacial and interstadial by longshore drift and caused inland westward-flowing periods(Sarnthein 1978), puddingstonelaterites drainage to be diverted also to the NW. Large delta formed by surfaceconcentration of ironoxides complexes are absent off Sierra Leone, as most sedi- (Michel 1973). Rivermouths in SierraLeone were ment was by-passedthrough well developed floodedduring interglacial periods. Estuarine clays valleys/submarinecanyons (Fig. 6) tothe deep-sea penetrated in boreholesaround the Rokel estuary floor (McMaster, De Boer & Ashraf 1970). The late recordthis ria formation,while interbedded lignites Pleistocene and Holocene marine transgression caused indicate periods of emergence when rate of sedimenta- reworking of continental shelf sediments.Several tionexceeded rate of sealevel rise and mangroves majortransgression stillstands occurredat -90 m, becameestablished. Thelate Pleistocene and -80m, -55 m, -45 m, -35 m,and -25 m levels. Holocene sea level rise has flooded the lower courses Theseare in reasonableagreement with thosepro- of numerous rivers andstreams and subsequent es- posed for the Texas Gulf Shelf (McMaster, Lachance tuarine sedimentation is widespread today (e.g. Tucker & Ashraf 1970). At the present day many active rivers 1973). reachthe coast in SierraLeone but little modern AS sealevels lower than that of thepresent day deposition takes place on the middle and outer shelf, haveoccurred several times during the Tertiary and since thesediment load is trapped in estuariesand

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CONTINENTAL RISE CONTINENTAL SLOPE CONTINENTAL SHELFSIERRA LEONE

TERTIARY 8.

LAYER 1 t CRETACEOUS? LAYER 2

OCEANIC CRUST LAYER 3 I MANTLE

~~~ ~ FIG. 5. Interpreted structural section of the continental margin off Sierra Leone, based on seismic refraction data. Vertical exaggeration X 20 (after Sheridan et al. 1969).

along the inner shelf (McMaster & Lachance 1969). dykescould be simple extension features developed Themodern coastline of SierraLeone exhibits a during the pulling apart of Gondwanaland.Alterna- widevariety of depositionalerosional environments. tively, May(1971) proposed that they are part of a Sandy beaches, lagoons, estuaries, mangrove swamps, muchlarger dyke system, radial in form,involving sand spits and cliffs are all represented.The wide most of the continental masses bordering theN Atlan- variety of sediments produced by these environments tic. He suggested that. when the 3 continents of the suggests thatthe Tertiary and Quaternary Bullom Americas and Africa are reassembled in their pre-drift Groupdeposits on thecontinental shelf, formed by position, the dykes are all convergent on an area now migration of depositional environments westwards and represented by the Blake Plateau of the Bahamas and eastwards with regressionsand transgressions, will the western Senegal Basin. exhibit a rapid and complex lateral and vertical varia- Alkali magmatism, developed 30 Ma after basaltic tion in character. This is certainly the case with the activity ceased, resulted in the formation of the Bagbe Bullom Group deposits onshore. and Songo Complexes. Structurally, the Songo Com- plex is located onthe continental extension of the Guinea Fracture Zone, just inland from the Freetown Structural setting of Mesozoic Complex. The Bagbe Complex is close to the Moro- to Recent events GbewaFault system (Wilson 1965) adjacent to the Sierra Leone Fracture Zone (McMaster et al. 1975). Events within this periodhave been dominated by Kimberlitic magmatism in Sierra Leone is probably sea-floor spreading following rifting of Gondwanaland related to the loci of the continental continuation of in the early Jurassic (Jones & Mgbatogu 1977). The transform fault fracture zones, with individual kimber- development of transform fracture zones in new ocean lite fields located between pairs of N-S trending faults. crust, parallelling ancient weaknesses in the adjacent Isolatedkimberlite dykes occur elsewhere in Sierra continentalcrust, has determined the loci of both Leone, but their structural setting is not understood. igneous activity (Williams & Williams 1977),and Boththe alkaline and kimberlitic intrusives indicate sedimentation (McMaster et al. 1975). thegradual waning of geo-isothermsfollowing the TheFreetown Complex is aconfluent cone-sheet sea-floor spreading episode initiated by crustal separa- developed at the intersection of the Guinea Fracture tion. zoneand the Atlantic protorift. The coast-parallel Jones & Mgbatogu(1977) gave evidence for an

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;iul

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extensive rift off the coast of Sierra Leone, floored by zones. This tectonicactivity and thegeneral subsi- oceanic crust by Oxfordian time (157 ma) and caused dence of the continental shelf after rifting is a direct by drift of N America from Africa. The thick deposits consequence of the Mesozoic break-up of Gondwana- of probable Cretaceous age (2 km) indicate considera- land. ble downwarp of the newly formed continentalshelf at McMaster et by an early stage. al. (1975) mapped ACKNOWLEDGMENTS.We wish to acknowledge aliterature geophysical means thethickness of sediment on the search by Martin Thorp, cartographic assistance from Larry continentalshelf close to the coastline, and found a Isham andDon Dean, use of thelibrary facilities at the closecorrelation between sediment thickness and Geological Survey of Sierra Leone,and financial support faultingassociated with the transform fault fracture from the Diamond Corporation of West Africa.

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Received 3 August 1978; revised typescript received 21 May 1979. STEPHENJOHN CULVER, Department of Paleobiology,National Museum of Natural History, Smithsonian Institution, Washington, D.C., U.S.A. HOWARDRAYMOND WILLIAMS,Department of Geology, University of Leicester, Leicester LE1 7RH.

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