A review of continental sediments in Africa

Journal of African Earth Sciences, Vol. 10, No. l/2. pp. 1 - 25, 1990

0899 - 5362/90 $3.00 + 0.00 PergamonPress plc

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A Review of Continental Sediments in Africa ‘C. A. K~GBE and?p.F. BUROILIX ‘Rock View International, 10. rue Vandrezanne. Tour Onyx, 75644 Paris Cedex 13, France. 2134 part de Cassan 95290 L’Isle Adam, France. Abstract- The history of Continental Sediments in At&a dates back to the end of the Precambrian Era when the development of the Super Continent of Gondwana was completed by the. Pan African event Africa was at the centre of the Super Continent. The African chains were up-lifted and eroded during the late Precambrian and early Cambtian period. Subsidence and the fiiing-up of the intermountain basins commenced in different parts of the continent and sediientation was strongly influenced by climatic variations. Continental sediments are very extensive in Africa but knowledge of African sediients has been very general and often imprecise. This is due to the difficulties involved in the investigation of continental sediments that are usually poorly fossiliferous and highly oxidized. Their characteristic red colouration often obscures important aspects that could facilitate more detailed study. This article provides a general review of the major supergroups or groups of continental sediments in Africa It highlights important achievements of various national and international teams participating in the Geological society of Africa’s Working Group on African Continental Sediments (IGCP Projects 127 and 210). Their achievements include the recognition of several marine horizons within sequences that were previously considered as entirely continental in origin (e.g., Nubia Strata in NE Africa and Continental Terminal in coastal West Africa). More detailed stratigraphic subdivisions of continental beds have been facilitated by the presence of trace fossils, fossil wood, pokns and spores. The economic importance of African continental sediments are highlighted.

R&urn& L’histoire des sediments continentaux en Afrique peut itre commenc& h la Hindu P&ambrienquand la formation du super- continent gondwanien a Cd ache& par la crise panafricaine. L’Afrique se trouvait au milieu du super-continent. Les chaines panafricaines ont Cd 6rod6es a la fii du kecsmbrien et au debut du Cambrien. La subsidence et le remplissage des bassins intramontagneux debutent, a ces tpoques, en diverses regions du continent et la sedimentation a t5tCfortement influencee par les variations climatiques. Les sediments continentaux couvrent de vastes surfaces en Afrique, mais leurconnaissanceestrestee longtemps tres gCnQale et souvent imprecise. Ceci est dO aux difficult& rencontrees darts l’etude de ce type de terrains pauvres en fossiles et fortement ah&&. La rubkfaction caract&istique efface souvent des caractks qui auraient facilid des ttudes plus dbtaillkes. L’article prknte une revue gknerale des groupes principaux de sediments contiientaux en Afrique. ll met en evidence les resultats importants acquis par des kquipes nationales et intemationales qui ont participk au Groupe de travail de la SociCd g6ologique africaine sur les sediments continentaux d’Afrique (projets du PIGG 127 et 210). Leurs n%ultats compremtent l’identification de nombreux horizons marins consider& jusque 18comme entitement continentaux; par exemple. les G&s de Nubie en Afrique Nord-Grientale ou le Continental Terminal des zones &t&es d’Afrique Gccidentale. Les subdivisions stratigraphiques plus dkaillees dea series continentales ont Cte facilides par la prbence de traces fossiles et par la dCtermination de Vert&reS. de bois fossiles. de pollens et de spores. L’intQb kconomique des sediments continentaux est mis en evidence.

Nubia ‘Qpe Strata (Northeast Africa). . Rift Continental Sediments (East. West and Central Africa). The following major groups ofAfrican continental l Continental series associated with Atlasic orosediments are discussed. geny (NorthAfrica). l Karoo Super Group (South, Central and East - 7’b.eKazoosedfments range from lower Paleozoic A4-.4~...! to Jurassic. They are foundiin a number of inland Central, and basins, the southernmost and most imDortant of North Africa) which is located in South Africa. near the Cape l “Continental Terminal” &Vest. Central, and mountains and contains up to 8000 m of North Africa) sediments (Fig. 1). The stratigraphy of the Karoo l

INTRODUCTION

1

2

C. A. KOGBEami P. F. BUROW

Super Group is largely based on the flora and reptilian fauna. Four groups make up the Karoo Super Group. The oldest of the groups is the Dwyka, which is remarkable for the intercalation of very thick tillites. The Dwyka Group is about 800 m thick and it overlies Precambrian Basement, Proterozoic or Early Paleozoic sedimentary series, with angular unconformity. It is generally overlain by the Ecca Group consisting of 100-300 m of Permian sediments with occasional beds of coal, carbonaceous shales and feldspathic sandstones. The Beaufort Group overlies the Ecca beds. The lower portion of the group is Permian and the top is Triassic. The sediments are poor in plants but rich in reptilian fauna. The sequence of sandstones, mudrocks and volcanics that lie above the Beaufort Group is known as Stormberg Series. The Drakensberg Group is the topmost or youngest group of the Karoo Super Group. It is Jurassic in age and is made up of basalts, rhyolites and pillow lavas. The basal part of the group contains interbedded sediments in the Karoo basin. Karootype sediments outcrop extensively throughout southern. eastern and central Africa. The Beaufort Group has been recently intensively explored for uranium with encouraging results. Carbonaceous debris is invariably present in different parts of the Karoo. In Central Africa, Upper Precambrian continental sediments appear sometimes as part of folded chains, sometimes as tabular or undulating covers and often as a continuation of a neighouring chain. There are two chains: the Katangan and the West Congo. The Katangan is characterised by its E-W sweep. It crosses the Katanga and penetrates into the Zambian Copper belt. Its generally well-known stratigraphic series includes basal conglomerates, arkoses and feldspathic quartzites (identiiied only in Zambia). This is overlain by the Roan system, which ismade up of red chloritic dolomitic cemented sandstones, and the Mwashia system, which consists of black schists and feldspatic sandstones with local occurrence of boulder clays. Conglomerates, calcareous sandstones and reddish phyllades constitutes the upper portion of the Katangan chain. Metamorphism of the lower members of the Katangan may be fairly pronounced, but it decreases upwards in the Katangan and then disappears in the north. The We,st Congo chain extends over 1000 km between Gabon and Angola, but only the eastern portion is well known. The sediments vary from stromatolite limestone horizons alongside schists and quart&es. tillites of continental origin, to sandstone-schistose-limestone systems of over 5000 m in thickness. The Platfii Continental covers are widely developed in Central Africa, although often masked by

recent formations such as in the Congo basin. The oldest series of folded chains described above gradually change into these series of cover formations, diminishing in thickness. There are extremely few examples of Paleozoic covers in Central Africa. The Bilatian and Tassilis series have been distinguished as well as Karoo type series such as the Lukuga glacial series. Post-Karoo Jurassic to Quatemary continental deposits are well developed in the Kalahari basin, the Congo basin and the Chad basin. - Continental sediments in West Africa have been broadly subdivided by earlier workers into two broad groups: the ‘Continental Intercalaire” and the ‘Continental TmW” The Continental Intercalaire includes the continental formations overlying the marine Carboniferous beds and underlying the marine Cretaceous Formations of Cenomanian age. They consist of sandstones, mudstones. clays and basal conglomerates. The Continental Terminal includes Tertiary continental sediments overlying the marine Eocene beds and underlying the Quatemary deposits. These sediments are very poorly fossiliferous and their stratigraphy is not as detailed as that of the Karoo to the south or the Nubian-type strata to the northeast. Only lithostratigraphic subdivisions have so far been proposed, and they are strictly relevant to a local scale. In the West African coastal basins, the concept of the “Continental Terminal” has recently been revised [IGCP Projects 127 and 2 10). All Tertiary autochtonous weathered sediments of marine origin have been excluded from the Continental Terminal. The Continental Terminal should constitute a distinct lithologic facies with well-defined upper and lower boundaries that are marked by geodynamic events (Post-Eocene and Pre-Quaternary). The Muxzuk and Al Kufran basins of southern Libya represent two eastern intracratonic basins of North Africa. They are sedimentary basins of predominantly marine facies, with continental intercalations which are more frequent in Al Kufrah basin. The Post-Caledonian sequence in the Al Kufrah basins represents a sedimentary cycle of predominantly continental environment at the base, passing to open shallow marine during Silurian and Devonian, and locally to marine-lagoonal in the uppermost part. The Carboniferous sequence is mostly of continental environment with pronounced thickness. The presence of Upper Palaeozoic or Lower Mesozoic continental sediments in the most subsiding areas of the basin is only hypothetical. - Until very recently, the major part of the sedimentary sequence in Southern Egypt and Northern Sudan has been refered to as the ‘Nub&rt Sandstone,” consisting entirely of Continental

A Reviewof ContinentalSedimentsin Africa non-marine deposits. Detailed study of the sequence by Klitzsch et al (1987) has clearly demonstrated that several marine horizons are intercalated within the Nubian Sandstone @enso Lato). The continental beds in northeastern Africa consist of massive white fluvial to deltaic sandstone interbedded with silty shale, siltstone and fine-grained sandstones of shallow marine origin. The beds date from Silurian to Upper Cretaceous and Tertiary. They contain the Nubian aquifer system which extends over an area of more than one million km2 from North Sudan and northeast Chad across southeastern Libya and the Western Desert of Egypt towards the Mediterranean. - In the West and Central Ajkican Rif System, a thick series of Lower Cretaceous Continental beds consisting of argillaceous sediments intercalated withcoarse detriticsediments (conglomerates.grits, coarse-to-fine sandstones) overlie the crystalline basement unconformably. These sediments can be correlated with the upper part of the Continental Intercalaire of the West African Inland basins. During the Middle to Upper Cretaceous. a series of transgressive phases resulted in the intercalation of Continental detritic facies intercalated within shallow marine deposits. The Mesozoic marine is unconformably overlain by the Continental Terminal. In East Africa, thick sedimentary covers were deposited in the Continental rift formed at the end of the Carboniferous. The basal part of the Karoo has been dated in Kenya as Upper Carboniferous to Permian and are said to be faulted against the basement with generally an upward-fining sequence. The basal beds are overlain by sandstones, and shales of Cretaceous to Tertiary age. - Contrary to generally held views, African continental sediments are not monotonous. They are often variable and can be excellent markers of different depositional environments. Fluviatile, near-shore, deltaic and lacustrine environments can often be distinguished from the characteristic sedimentary structures of the deposits. This has been well demonstrated in Nubia (Southern Egypt and Northern Sudan), in the Benue valley (West Africa), and in the Karoo-type locality in Southern Africa. Lithostratigraphic subdivision and correlation is often possible on a restricted or local scale. More precise stratigraphy has resulted from the results of recent intensive field investigations. It is important to stress that Continental sediments can be of great socioeconomic importance to the development of the continent as they provide abundant sources of industrial minerals (e.g., kaohnitic clays, glass sands, diatomites. ironstones, etc.) which constitute the basic raw material for the manufacture of many essential but imported products. The Continental sediments contain

3

acqtiers which provide or can provide large quantities of water, necessary for irrigation and drinking. They conta@,economic reserves of coal and uranium (South&lca, WestAfrica) andconstitute the source rocks for hydrocarbon produced from Cretaceous and younger sediments (Central Africa, West Africa). REGIONAL DESCIUPTION 80uTEERNAFRxc.A Most of Southern AiYica is underlain by a platform of ancient rocks, which is overlain by a series of Continental formations. The continental beds overlie the plafform discordantly and often constitute a very thick sequence, known collectively as the Karoo Super Group. The Karoo sediments are found in a number of inland basins, the most southern and most important of which adjoins the Cape mountains and contains about 8000 m of sediments. (Figs. 2). The stratigraphy of the Karoo Super Group is largely based on the flora and reptilian fauna. The Super Group has been subdivided into 5 groups (Dingle et al. 1983) (Fig. 3). l The Dwyka Group (oldest) l The Ecca Group l The Beaufort Group l The Stormberg Group l The Drakensberg Group (youngest) The Dwyka Group The Dwyka Group

is especially remarkable for the intercalation of very thick conglomerates or tillites. with striated pebbles. whose morainal origin is indisputable. In the North of the basin, the Dwyka sediments lie directly on old crystalline basements. Towards the South, they are separated from the basement by lower shales with ancestors of the Gl0ssoptert.sFlora. In Namibia, Dwyka sediments contain marine intercalations and are characterised by the genus Mesosaums, a reptile peculiar to the Gondwana Super Continent and adapted to aquatic life. The thickness of this basal group increases from North to South, reaching about 800 m. and the age is generally agreed to be Upper Carboniferous. The Ecca Group

TheEccaGroupconsistsof 100~300mof Permian sediments with occasional beds of coal, carbonaceous shales and feldspatic sandstones. They are character&d by a special Gondwanlan flora with Glossopterfs and Gangamopteris. They contain scarce reptilian remains. There is a degree of uncertainty as to whether the upper essentially non-carbonaceous rocks that locally overlie the Ecca Group in Namibia, Botswana. Northern Transvaal and Southern Zimbabwe are lateral

C. A. KOGBEand P. F. BUROILEI

CT : Continental Terminal Ci : Continental Intercalaire N : Nubian sandstones K : Karoo sediments

I

J

IJlJlJI II

Sedimentary Cover (Phanerozoic) with continental and marine beds (CT, Cf, N, K) Couverture .sedimentaire (Phantrozolque) avec series marines et continentales (CT,Ci, N. K) Continental carboniferous Carbonifire continental Crystalline basement and Palaeozoic (except Karoo and Continental Carboniferous) Socle crlstallin et Paltozorque (sauf Karoo et Carbonifere continental) Folded ranges (Hercynian and Atlasic) S&fes plissees (Hercynien et atlasique)

Fig. 1. African Continental and Coastal Basins Fig. 1 Bassins africains continentaux et cotiers

A Review of ContinentalSedimentsin Africa

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+

+

+

+

+

+

Fig. 2. Karoo Basin in Southern Africa (after Verniers& al.. 1989) Fig. 2. Bassins du Karoo en afrique australe (d’aprks Vernierset al., 1989)

v

RAKENSEERG

KAROO SUPER

2

GROUP

A I

6

CAPE SUPER

8 I 10-l Km

I

200

Km

3

HIATUS

Fig. 3. Relationship of the Karoo Super Group to the Cape Super Group Fig. 3. Relations entre les Supergroupes du Karoo et du Cap.

C. A. KOGBEand P. F. BUROLLET

facies equivalents of younger Ecca horizons, or whether they are local representatives of the Beaufort and Stormberg groups. On purely lithostratigraphic grounds, the latter interpretation is more acceptable. The sediments ofthe Ecca Group were deposited in predominantly lacustrine environment. They bear large coal seams in the northeast: in the South, the Green Ecca is formed by turbidites sedimented in an unstable trough. The Beaufort Group The lower portion of the Beaufort Group is Permian in age and the sediments are poor in plants but rich in reptilian faunas. They consist of grey. bluish-grey and greenish grey mudstones and siltstones with subordinate sandstones. Because of the complete succession and relative abundance of vertebrate fossils, the South African Beaufort sequence is frequently used as reference type for the Permo-Triassic in Africa. Mesozoic Strata (Stormberg Group and Drakensberg Group) The Mesozoic Strata within the Karoo Super Group lie towards the eastern side and only occupy about the third of the total outcrops. The succession here is relatively thick and easily differentiated into several well-defined formations which are taken as the type sequences. Further north, mesozoic strata are generally much thinner, with the exception of the uppermost units. The welldifferentiated sequence of the main Karoo basin cannot always be confidently recognized. It is apparent that this thinning is due to nondeposition or slow deposition rather than later erosion, During the Mesozoic, the modem morphology and outline of southern Africa evolved in a succession of important tectonic events. The Triassic to Jurassic period is characterised by intracontinental down-warping and the development of regional tensional forces resulting in rifting (Dingle et al. 19831. During the Cretaceous and Tertiary, southern Africa completed separation from its neighbouring parts of Gondwanaland (drifting). The present interior of the subcontinent was subjected to erosion in which the main direction of sediment transport was still centripetal. but a few rivers crossed the pericontinental rimcrest and drained the interior to the continental margins and adjacent ocean basins. The deposits constituted the upper part of the Karoo system as well as the Cretaceous and Tertiary systems in coastal basins in Zululand and Mozambique. Strata ofTriassic to early Jurassic age crop out in two main areas in southern Africa. The largest and best known is in Lesotho. in the Eastern Cape, Orange Free State,

South-Transvaal and East-Natal: whilst the other deposits occur as a series of outliers scattered in a broad arc to the north which stretches from Zululand and Swaziland in the southeast, through northern-Transvaal, Southern-Zimbabwe and Eastern Botswana, westwards into northwest Namibia (Verniers et al. 1989, Fig. 3). The area around Lesotho contains the youngest strata in the great syneclise that occupies the central southern part of South Africa. It contains rocks that range in age from Permian to Lowermost Jurassic and is known as the Karoo Basin. Whilst the present outlines of this area do not relate directly to the shape of the original depocentre. the term Karoo Basin is a useful and established geographical concept to describe the area which contains the classical Karoo lithostratigraphic sequences. In the basin, Lower Triassic strata rest conformably upon uppermost Permian rocks. They consist essentially of two lithostratigraphic units, which represent the upper division of the Beaufort Group: the rocks consist of a lower arenaceous unit with minor shales and mudstones and an upper argillaceous unit with minor sandstones and siltstones. These rocks overlie a series of grey. blue and green mudstones with minor sandstones of Upper Permian age. stormberg Group The sequence of sandstones, mudrocks and volcanics that lie above the Beaufort Group was known as Stormberg series. Recently Lock et al. (1974) placed the upper volcanic strata into the Drakensberg Group. so that the Stormberg Group now comprises predominantly sedimentary formations consisting of underlying glittering sandstones, grits and conglomerates with grey and black shale, mudstones and coals. The lower formation, named Molten0 Formation, is interpreted as a molasse deposited in the foredeep of the just-formed cape ranges with coal, vertebrates and Gondwanian-Euramerican Intermediate flora with Dtioidium. These are overlain by greyish-red and purple mudstones with minor yellow and grey sandstones. The top of the Stormberg Group consists of yellow and white massive and cross bedded feldspathic sandstones with minor greyish-green shales and silts. The Drakensberg Group The Drakensberg Group is Jurassic in age and consist of basalts. rhyolites and pillow lavas. The basal part of the group contains inter-bedded sediments in the Karoo basin (mostly eolianl. Basalts and dolerites resemble Western Africa volcanics. There were also Kimberlite extrusions, the

7

A Review of Continental Sediments inAfrica

deposits are erosional products orfgfnating from the peneplanationtithe Gondwana relief.They are unconformably deposited overthebasement within areas of low relief (local grabens. trough-valleys). WESTERN COASTAL BASINS OF usually poorly drained. which progressively deveCENTRAL AFRICA lop into swamps and lakes. From base to top, the South of the equator from Angola to Cameroon. sedimentary column is as follows: l coarse sediments followed by finer elements the coastal basins have some common features, although their detailed stratigraphy may vary. A (conglomerates, coarse sandstones, fine sandschematic stratigraphic column common to of all stones); l palustrine and lacustrlne deposits (clays and the basins is shown on Fig. 4. Gverlyingthe hrecambrianbasement are pre-salt fine grained sandstone with vegetable organic continental beds of Jurassic to Aptian age. These materialsl. These episodes may laterally comprise

products ofwhich include diamonds. Theywere reworked in ulterior continental cl@&ic deposits.

QUATERNARY

OPEN OCEAN PARALLEL PARTING MARGIN STABILIZATION

-locally limestones and continental levels

-Erosion

OPEN OCEAN

-

carbonates EARLY TERTIARY?

DIVERGENT PARTING - dolomites - salt beds

LATE CRETACEOUS

---_----_ - sulfates DIVERGENT PARTING

- salt massive

LATE APTIAiJ

- basal sandstones

sandstones) -lateral deltaic, rive or slope detrital cones ___--_-_---CONTINENTAL DETRITAL UATERIAL FILLS IN THE RIFT VALLEYS

LANDSCAPE REJUVENATED BY ACTIVE FAULTS

-fine

to medium grain sandstones

LATE JURASSIC to APTIAN

--_--___-_ -coarse grained sandstones - conglomerates

Fig. 4. Synthesized stratigraphiccolumn of WesternCentral Africa Coastal basins (sUghtlymodifled from Schlumberger 1983) Fig. 4. Colonne stratigraphique synthetique des bassins &tiers ouest-africalns (kgtrement modifi& d’aprks Schlumberger 1983)

8

C. A.

bGBE

and P.

F. BUXOLLJZ

coarse deposits such as deltas or slope fans along the valleys edges. These deposits are those of the initial rifting phases. They are stratigraphically equivalent to a part of the Continental Intercalaire In Central Africa, Upper Precambrian continentof West. Central and North Africa but of difIerent al sediments appear sometimes as part of folded facies due to the restricted environments. The chains, sometimes as tabular or undulating covers grabens were very narrow in the southern part andoften asacontinuationofneighbouringchains. (Cuanza basin) and wider in the north, e.g., the There are two such chains: the Katangan and the West Congo (Tab. 11. Congo, Gabon, etc. This basal continental sequence is overlain by a sandy transgression level (Cuvo of the Cuanza The K&an@ chain Basin, N’ze. me Asso of Gabon), then by the thick The Katanga chain is characterizdby its cmved salt beds deposited in a Red Sea type environment and generally east-west sweep. It crosses the during the end of the lower Cretaceous (late Aptian- Katanga and penetrates into the Zambian copperAlbian). This second phase corresponds to a larger belt. Its stratigraphic series contains the following graben. a sort of proto-ocean, belonging to both the units which are generally well known: Brasilian and the African margins. It was, in fact, l basal conglomerates, arkoses and feldspathic a three-branched rift with a triple junction near the quartzites (identified only in Zambia): l the Roan System, which commences with red present location of the Niger delta. The southern branch evolution resulted in the South Atlantic chloritic dolomitic cemented sandstones followed Ocean. The western branch acted as a major shear by a series of metamorphic rocks with dolomites or zone since the lateAlbian; the northeasternbranch, magnesian limestones that portray varying degrees the Benue trough, was an active graben system of silicification; during Cretaceous, but then aborted during l the Mwashia system, which consists of black shists, felspathic sandstones with local occurPaleogene times. In the coastal basins the post-salt deposits are rence of boulder clay (glacial conglomerate); l the “large conglomerate,” which is a tillite or marine except in some internal fringe where they are continental or deltaic. The Neogene contains periglacial deposits; some continental sediments intercalated within l the Lower Kundelungu. which is essentially marine shales, with local carbonates. The strati- calcareous; graphic column is topped by quatemary forml the “small conglomerate,” which is also a tillite ations of continental sands or lagoonal sediments. (at least in the north); North of the equator, the basal continental l the Upper Kundelungu. which consists of redsequence continues to be present in all the coastal dish phyllades and sandstones, often feldspathic, basins northwest of Gabon but the salt beds are with cherts and some limestones. virtually absent in the Nigeria coastal basin and Metamorphism of the lower members of the westwards. The stratigraphic column is similarly Katangan may be fairly pronounced, but it decreacapped by Pliocene and recent continental sands ses upwards in the Katanga and then disappears or lagoonal sediments. During the Early in the north. The folding is very intense particularCretaceous. Angola, Congo and Gabon were loca- ly in the southern Katanga. The folds are overted at a higher latitude than they are presently, so turned. with overthrusting towards the north. the large graben had an arid climate. On the one hand, the shoulders were uplifted and the drain- The West Congo Chain The West Congo Chain extends over 100 km age was directed to the internal syneclises. prohibiting any important fresh water supply to the pre- between Gabon and Angola but only the eastern atlantic graben. On the other hand, Nigeria was part is known. The chain contains several stratistill in a wet tropical zone. graphic complexes subdivided into stages and In the northern Benue trough of Nigeria, a first horizons with various local names depending on phase of rifting created narrow continental gra- the country under consideration. The sediments bens during Barremian and Aptian ages. Then, in vary from stromatolite limestone horizons alongMiddle Albian. a larger collapse formed the wide side schists and quartzites, tillites of Continental basin with a median SW-NE trending hinge line. origin, sandstone-foliated limestone systems of Southeast of this line, sedimentation was fluvial in over 5000 m in thickness. These Precambrian the SE and deltaic in the northwest, with meander- sediments are often closely related and interwoven ing channels. The Cenomanian transgression is with metamorphosed folded beds with several the end of the continental sedimentation, and the igneous intrusions, granites, pegmatites, dolomiupper part of the Cretaceous series is palustrine tic lavas, etc. with coal measures.

A Reviewof Continental Sediments in Africa

Platform covers Precambrian continental platform covers are widely developed in Central Africa, although often masked by recent formations such as in the Congo basin. The oldest series offolded chains described above gradually change into these series of cover formations, diminishing in thickness. In the southeast of the Congo basin, the Katanga covers the molasse deposits of Bushimaye and the schistose-calcareous-sandstone series of Burundi which change in the east to the Bukoba Cranzanial series. To the west, the subsident series of the west Congo chain gradually change into a typical platform cover. This gives way in the northwest to the volcanic sedimentary deposits of the Francevillian Formation in Gabon, which yielded a radio-metric age of 1740 f 75 My (argillaceous fraction: 5 samples). Fossil vegetal traces also exist. Further north, formations of the same type but bearing a variety of local names outcrop. These are essentially schistose-sandstone series with carbonate intercalations and sometimes tillites. Slight local metamorphism (chlorite, sericite) and considerable doleritic occurrence are noticeable. West of the Kalahari basin, in Namibia, the Nama Group, unmetamorphosed Proterozoic, is a sedimentary cover. Some units, like the Schwarzkalk. are marine, but the lower and upper sandstone forrnations are continental or deltaic. lPalaeozoic covers There are extremelyfew examples of Palaeozoic covers in Central Africa. One of these is the ‘Bilatian”; the other is the Tassilis series in the north. The Bilatian was ascribed to the Palaeozoic in view of its pyrite concretions resembling Orthoceras. More recent geochronologic datings tend to favour an upper Precambrian age. The Tassilis series is of importance in the Borkou. Tibesti and Ennedi massifs attaining 2000 m. It consists essentially of sandstones ranging from the Ordovician to the Devonian. The top of the Devonian is lagoonal and the entire complex is distorted and even folded (Niger frontier). The folding was a distant and gentle effect of the Hercynian orogeny. lKaroo cycle The Karoo formations, which are entirely continental, attain their maximum development in Africa south of the equator. North of the equator, they extend within a minimum area. The northernmost outcrop of the Karoo has been described from the base of the Gabon basin. The following Karoo series are identifiable in Central Africa. lLukuga glacial aerier- This is equivalent to the Dwyka series. It is overlain by a subsident coal-

1cs10:1/Z-B

9

bearing Ecca series. a BeaUrortSeries of reduced thickness and a v@xnic-sedimentary Stonnberg series. particularlj%leveloped in Angola. An orogenie phase (often identified in Southern Africa) occurs between the Lower Karoo and the Stonnberg. lMeso-cenozoic

cover The Post-Karoo formations which extend from the Jurassic to the Quaternary can be divided into six great complexes corresponding to five major depositional basins. These are the Chad basin, the Congo basin, the Kalahari basin, the Rift valleys and the coastal basins which extend through the Benue valley. Gn the surface, the Chad basin is infilled with Continental Terminal deposits of Palaeoctne to upper Pliocene, in which flora has been found with Scophyte. The Continental Terminal deposits are underlain by Cretaceous sediments found in the subsurface or in outcroppings in the northwest (Tenere) corresponding to graben deposits. The main part of the Cretaceous is continental and lacustrine. but several Upper Cretaceous levels are marine. All these series are overlain by the Quatemary Chad Formation. Lake Chad contains 100 m of recent deposits. Further south in the Doba basin, very thick Mesozoic deposits have been identified. The Congo basin is infilled almost e~~~lusively with continental deposits ranging from the Jurassic to the Quatemary. These continental deposits are intercalated by three marine incursions of Upper Jurassic age to the northeast, of Albian age to the northwest and of CenomanianTuronian age to the south. The most complete Mesozoic sequence is seen to the north and is generally subdivided into the Kwango Series, the Lois Series and the Stanleyvile Series (Tab. 1). These series are separated by slight unconformities. The Kwango beds occur again in the Central African Republic where they are termed MaukaOuadda. The Cenozoic develops south of the basin as a sub-horizontal complex with scarcely any faulting (Me&mud 1982). The Kalahari Basin is infilled with Cenozoic and Quaternary deposits, “polymorph sandstones” with calcareous lenses. It is a huge fluvial lacustrine and eolian sedimentary complex due to the centripetal drainage net. It covers more than 380.000 km* in the Barotseland and high Zambesis Basin, 180.000 km* in Gvamboland and 210.000 km* in Kalahari proper. The Rift valleys are character&d by a series of lakes separating the Congo basin from East Africa. Their history dates back to tbe beginning of the Tertiary.The main tectonic phase dates from the end of the Pliocene. The sediments are of volcanc-lacustxlne origin.

- A

PAN AFRICAN

PRECAMBRIAN

PALEOZOIC

HERCYNIAN

CENOZOIC

Quaternary drifts of the Chad Basin

Littoral Sands

CAMEROON

Sandstones

Quaternary Sands, Top of the Chad Basin C.T.

CHAD Quaternary Sands of the Chad Basin

AFRICAN REPUBLIC

CENTRAL

Littoral Sands

GABON

sediments

Nine et Roan

Kundelungu

Katangan

Congolian

West

Basin

Nata

Coastal

Quaternary Sands of the

Littoral Sands

CONGO

Table 1. Central Africa - Continental sequences Tableau 1. Stquences continentales en afilque centrale

_

_.

Congo

System

West

off - shore

ZAIRE

West

Kalahari

Littoral

Congo

Sands

ANGOLA

_

A Reviewof ContinentalSedimentsin Africa We have discussed the coastal basins and the Benue trough which became active In the Lower Cretaceous. Marine transgression occurred during the Upper Aptian on the coast and during the Albian in the Benue valley. The oldest sediments in the coastal basins and the Benue valley are of continental origin (fluvio-lacustianl. At the end of the Cretaceous (Maastrichtianl. regression commenced and contlnental sedimentation prevailed from the Paleocene. This regressive phase was accompanied by the contemporaneous tectonic phase of the Tibestl and west Cameroon fracturing at the end of the Mesozoic. In southern Sudan, Cretaceous distension caused active subsidence of grabens infilled by lacustrine and fluvial sediments of Cretaceous and Lower Paleogene ages. This early rift system aborted during Eocene and is covered by recent alluvlum of Bahr-el-Ghazel. Bahr-el-Jebel and the White Nile. Rich oil reserves were discovered InBahr-el- Ghazel graben as In Chad Cretaceous troughs. WESTAFRICA Continental sediments in West Africa have been subdivlded by earlier workers into broad groups (Kilian 1931: Fur-on 1960; Greigert 1966. etc.). These consist of Continental Intercalaire (Permian to Lower Cenomanianl and Continental Terminal (Post-Eocene to Pre-Quatemary). These two ‘Super groups” of continental deposits outcrop extensivelyall overwest Africa, both within the inland basins (e.g.. Iullemmeden and Chad basins) and within the Coastal basins extending from Nigeria to Mauritania. The Continental Intercalaire includes the continental formations overlying the marine carboniferous beds and underlying the marine lower Cretaceous formations of Cenomanian age. The wide Carboniferous marine cover of the Western Sahara disappeared by regression, isolating the three main basins fI’induf, Taodeani and Regganl. The Continental Terminal Includes Tertiary continental sediments overlying the marine Eocene beds and underlying the Quatemaxy deposits. It was Kilian who, in 1931. first introduced the terminologies of ‘Continental Intercalaire” and ‘Continental Terminal” as defined above. The concept was to facilitate the lithostratigraphy of non-marine sediments that could not be easily differentiated due to the complete absence of fossils. The sediments are predominantly grits, sandstones, siltstones and clays. They are typified in the field by their characteristic redness due to the oxidation of the iron oxide in the cement of the sandstones. Since the introduction of the terms by Kilian. there has been an abuse of the usage, and any red

11

bed within the straigraphic horizons of the ContlnentalIntercalaireandcOntinentalTerminal have been assigned to either ofthe groups. Hecent workers have described marine horizons within the Continental Terminal and the Continental Intercalaire (Lappartient 1985: Lang et UL 1986: Kogbe 1972 and 1981). It has become obvious that the continental sediments within the coastal basins of West Africa are only partial@ equivalent to the series de9ned by Kilian within the Sahara Intracratonic basins. Continental Intemalaire (C.I.) In the Inland basins of West Africa (Iullemmeden and Chad basins), the Continental Intercalaire includes all the formations of continental origin overlying the marine carboniferous bed and overlain by the marine formations of Cenomanian to lower Turonian age. In the Ilullemmenden basin, they correspond to the ‘Gr& du Tegama” (cf. Chudeau 1909) and extend from the *Adrar des Ifores” as far as the Chad Republic. The sediments are very poorly fossiliferous and are not as precisely dated as the Nubia strata in the northeast or the Karoo strata in southern Africa. In 1952. H. Faure and F. Joulia proposed some subdivisions within the Continental Intercalaire in the west and the south of the Air massif. These are as follows (Tab. 2): I:The “Arkoses of Izegouandane” at the base, with a thickness varying from O-300 m; II: The ‘Gr& D’Agades” or the Agades sandstone group, with thickness varying from O-230 m: III: The ‘Irhazer clays,” with thickness varying from O-500 m and Assaouas Sandstones: IV: The Tegama Sandstone” at the top with thickness varying from 3004300 m. The sediments are very detrltic at the base, consisting of conglomerates and pebbly grits. They become progressively finer and more argillaceous upwards before becoming predominantly detrkic at the top. The subdivisions of the Continental Intercalaire is shown on Fig. 5. The age of the Continental Intercalaire is imprecise. The sediments are rich in fossil wood, and different authors have observed the close similarity between the facies and the European Wealdian. which Is known to be of Lower Cretaceous age. The lower limit is defined by the end of the Carbonlferous marine beds. The flora of the Continental Intercalaire consists of several species of Khxlmybn fArauccrrlawybd and Lepidudemkm (Boureau 1958; de Lapparent 1958; Kogbe and Lemoigne 1976) The earliest dicotyledons appeared in the Continental Intemalair-e.accompanied by podocarpaceans. especially in the Dibella Flora overlain by the Upper Cenomanian marine transgression. The wide

12

C.A. KOC~BE and P. F. BURO~Z

distribution of the Continental Intercalaireflora all over Africa implies the existence of a uniform climate throughout the continent during the transition from Jurassic to the Cretaceous. Vertebrate fqunas are very rich in Nfger (Taquet 1977). Near the Jurassic-Cretaceous limit in Irhazer. many prlnts have been observed (Theropods and Sauropods). In the Aptian site of Gadoufaoua, skeletons of Dinosaurs are described: Iguanodons with O~~anosuuncs nlgerkmsfs, Sauropods, Theropods and the Coelurosaurian Elaphosaums 4uidiensfs. They are associated with Crocodiles, Turtles, Fishes, Amphibians. etc.

Continental Tern&ml

The Continental Texminal outcrops extensively in West, North and Central Africa. KUan (1931) deflned the term to mean Sahara continental sediments of Eocene to Pliocene age. The terminology was employed in North. West and Central Afrka. It began with the Eocene regxssion and was formed by sandstones and sometimes lacustrine limestones. Recent investigations by the working Group on African continental sediments (IGCP project 127 and 2 10) revealed the extent to which the termino1ogyasoriginallydefinedbyKllianhasbeen abused.

Table 2. Stratigraphic subdivision of the “ContinentalIntercalaire”in Western AMca. (D = Discordance, R = Erosional Unconformity) Tableau 2. Subdivisions straugraphiques du Continentale intercalaire en afrlque de l’ouest (D = Discordance. R = Discordance de ravlnement)

Local Sequence (litho;~~~s~raphic

Age

z :' :

Early Cenomanian

z:

Farak

Maximum

Facies

thickness

Sandstone

(ml 200

Fluviatile

600

Fluviatile

$9

Albian

E-c

4. :

V

Tegama

Sandstone

Irhazer

D

clays Lacustrian

Wealdian Assaouas

Sandstone

(Abinky):

analcime Fluviatile

Jurassic Tchirezrine

Sandstone

Torrential Triassic Teloua

Sandstone

Clayey

Sandstone Lacustrian

Tammamait Permian

Sandstone R

G

z & :

H

Tejia

clayey

180

Sandstone Fluviatile

Izegouandane

Arkose

A Review of ContinentalSedimentsin Africa

.

LEGEND Tegama saidstone group Irhazer group Agades sandstone Izegouandane Arkose

>

C.I.

Tagora Series

Farazekat Sandstone I-3-eCambrian Basement Fig.5. Schematic structural section of the 1uIIemmeden bkWnwest of Air Mountains N-S section around the longitude 6O(after Grcigert 1966). Fig. 5 Coupe schtmatique du bassin des IuIIemmeden B I’ouest de I’AXr(d’aprts GrtIgert 1966)

DiEerent interpretations have been applied to the sediments by different authors. The difference is mainly due to the fact that altered Tertiary autochtonous marine sedlments have been confused with detritic sandy clays of post-Eocene age in the West African coastal basins: for instance, important

marine horizons have been described in areas formerly mapped as Continental Terminal in Nigeria. Benin and Togo (Kogbe 198 1: Lang et CA 19861. In the Senegalo-Maurltanian basin, the Continental Terminal consists of detrltaI sediments showing evidence of intense ferralitk altera-

14

C. A. KOGBE and P. F. BUROLLET

tion, with formation of ferruginous concretions and crustings. The sedimentary structures are generally obliterated by chemical alteration, particularly at the top. Discovery of fossiliferous layers confirm the marine origin of the Eocene and Miocene deposits hitherto described as continental (Lappartient 1985; Conrad and Lappartient 19671 in the West African coastal basin. The concept of the “Continental Terminal” has been revised to exclude all Tertiary autochtonous altered red beds of marine origin: Conrad et al provided a useful phases picture of Cenozoic tmm@es&e-regressive in the Senegal0 Mauritanian coastal basin from the reconstitution of fossil river beds on the basis of the chemically altered deposits. The Continental Terminal constitutes a distinct lithologic facies; the age of the Continental Terminal range from middle Eocene to Pliocene.

More detailed lithostratigraphic subdivisions are only possible on a restricted scale within each sedimentary basin. lUeeoz&c and Tertiary Continental formations intercalated within marine strata- During the Mesozoic and Lower Tertiary periods, there were a series of transgressive-regressive episodes in West Africa resulting from fluctuative levels of the sea floor. The sedimentary sequence is characterised by lateral facies. The continental sediments consists of inter-fingering continental and marine deposits of deltaic plain deposits, fluviatile deposits and channel-fill sediments, and can attain several hundreds of metres in places (NigerDeltaBenue Valley. EASTERN AFRICA

Mozambique-Tanzania There are two sedimentary basins in Mozambique which were developed from Albian-Aptian to Miocene above a downwarped surface of Karoo rocks. The larger southern basin extends from South Africa to a line located about 160 km north of the Zambezi river. It reaches a maximum width of about 440 km in an east-west direction. The smaller, northern basin lies astride of the northern border of Mozambique. and extends intoTanzania. It has a maximum width of 120 km from west to east along the lower course of the Rovuma river. Both basins have their major axis trending roughly parallel to the present day coast. They are represented by a belt of Lower to Middle Cretaceous continental to transitional sediments running parallel to the Karoo and basement outcrops in the west, grading eastwards and upwards into Upper Cretaceous, Eocene and Oligocene marine beds. Three of the units into which the Karoo was classically subdivided by South African geologists

are represented in Mozambique. These are the Stormberg series (volcanics and sandstones), the Beaufort series (sandstones and shales) and the Ecca series (shales, siltstones. sandstones and coals) (Fig. 6). The Dwyka series, tillites and shales, is not known in Mmbique. A characteristic of the Albian-Aptian deposits resting directly above the Karoo and post-Karoo is a deposition of a considerable thickness of dark shales with rare sandy intercalations. This formation is known as the Domo shales and constitute a continuous subsurface feature extending for an average width of about 50 km inland from the present day coast. The Domo shales grade westward into coeval and younger continental deposits. The entire sedimentary section of post Karoo age, as well as part of the Karoo section, was affected by rifting during the break up of this part of Africa. The sedimentation is partly controlled by the development of the East African Rift tectonics, with little or no development of compressional folds but only of mild broad arching. In the coastal region of Tanzania, predominantly continental Karoo sediments more than 3000 m thick were deposited, mainly in down faulted zones which continued to develop into Jurassic times. The subsequent sedimentary history was that of a normal shelf environment with several transgress&e-regressive phases, during the course of which 4000 m of marine Mesozoic and up to 4500 m of Tertiary sediments built out the present continental shelf and slope. Karoo rocks crop out in the north of Malawi mainly between 10’ and 11” south of the equator.

The beds are preserved in small N-S trending grabens and elongated down-faulted troughs on the unconformably underlying basement complex gneisses. The Karoo succession in northern Malawi consists of the Dwyka. Ecca and (lower) Beaufort Groups, and has a total thickness of about 900- 1350 m. Kenya-Somalia The Lamu basin of coastal Kenya contains up to 10,000 m of sediments ranging in age from Carboniferous-Permian (Karoo)to Quaternary.The earliest marine beds are middle Jurassic in age, and most of the subsequent Mesozoic and Tertiary stages are represented in the overlaying sedimentary succession. The succession within the Larnu basin is not known in great detail, since very few deep wells have been drilled and surface outcrops are virtually columns of the known strata of the basin from Jurassic to Quaternary. Evaporites, especially halite, were encountered in boreholes and they extent largely eastwards off-shore.

A Review of Continental Sediments in Africa

15

lark grey shales with thin sand stonesbands. Ammonites msent. Limestone is detrital, brown and slightly silty Dark grey or brown shales, silty to sandywith Ammonitcs md thin lenticular interbeds of grey limestone.

Boulder bed at base contains boulders of limestone. Grey-brown sandy micaceous shale with sandstone bands. Dark grey oolitic limestone with shale intercalations. Coarse grained cross-bedded sandstone with siltstone intercalations and fossil wood (Dadoxylon).

lnterbedded very fine-grained sandstone and grey to purple shale.

Sandstones

Cross-bedded falsebedded sandstone fine to medium gained slightly micaceous - Blue-grey.

Chumvi

Khaki shales and siltstones with thin layers of sandstones.

Chumvi

Beds

Beds

Blue micaceous shales with fish remains at the base. ~juned sandstones with Grey laminated siltstones and finedmg. Interbedded grey, frequent ripple marks and cross silty carbonated shales.

Hard, grey feldspathic sandstone.

Lower Taru Grit

PRE - CAWBRIAN

F/

Fig. 6. Paleozoic-Mesozoic

tlBasementr~

Blacksandy shale with bedsof feldspathic sandstone: arkosic grit and conglomerate. Also rare pink rubbly limestone.

Hornblende grains and biotite schist with pegmatitic lenses.

succession in Coastal Kenya (West of Mombasa).

16

C.

A. KOGBEand P. F. BUROILET

In Somalia, although continental pre-Jurassic deposits do not outcrop, a substantial thickness of Jurassic rocks rests above the metamorphic basement in the central western portion of the Manders-Lugh basin. The region began its sedimentary history probably between the late Paleozoic and early Mesozoic, with major rifts or large grabens that broke up the GondwanaIand continent and were at the origin of the Indian Ocean. Northeast Africa

Formerly, the major part of the sedimentary sequence in southern Egypt and northern Sudan has been refered to by the general term ‘Nubian sandstone,” and consists entirely of continental, non-marine deposits. After 1958. oil companies explored southeastern Libya, and their geologists identified several Paleozoic marine horizons on both sides of Tibesti and around Kufra-basins (Burollet 1963: El Hasherni 1978; Jacque 1962; Vittimberga and Cardello 1963 etc.). Detailed study of the sequences by Klitzsch et aL 1987 has clearly demonstrated that several marine horizons can be recognised intercalated within the Nubian sandstones. These marine horizons have been differentiated and mapped. They are shown on Fig. 7. Early Palaeozoic strata extend from the Ennedi mountains in northern Chad into neighbouring areas of Sudan and the southern edge of the Kufra Basin. Early Ordovician interbedded fluviatile sandstones and conglomerates have been identified just above the basement around 2 l”58’N and 25O08’Ein northern Sudan, where they are intercalated with highly bioturbated marine sandstone. The formation is unconformably overlainby marine and fluvial sandstones of Silurian age. In some areas of southern Egypt and northern Sudan, sandstones of Silurian age rest directly on folded basement of Precambrian age. According to Klitzsch (op. cit.), the best exposures of these sediments can be seen at the Umm Ras Passage near the Egyptian-Libyan border. where they consists of about 400 m of massive white fluvial to deltaic sandstones interbedded with silty shales, siltstones and fine-grained sandstones of shallow marine origin. The Sflurian strata are overlain in large parts of the Kufra Basin, in Libya and Chad by a massive sandstone formation which in Libya is called Tadrart Sandstone (Klitzsch 1970). Very similar beds overlie the Sflurian Umm Has Formation in southern Egypt and northern Sudan, where they rest with a slight erosional disconformity on the older strata. In the main part of Libya Tadrart Sandstone is a fluvial sediment grading northward to deltaic or coastal facies.

50-100 m of shallow marine to coastal marine strata with intercalations of fluvio-delta& sandstones of Lower Carboniferous overlie the Devonian Tadrart sandstone. These beds constitute the Wadi MaIik Formation. Glacial tilhtes are extensively represented in southern Egypt and northern Sudan. These gIacial sediments rest on strata of early Carbotierous age and may be late Carboniferous in age. They may be equivalent to the Dwyka Formation of the Karoo sequence in central and Southern Africa. During the Permo-Juriassic to Lower Jurassic times, tectonic movement resulted in regional uplifting along the Sudanese-Egyptian border. These movements were probably initiated in Middle or Late Carboniferous times and ended during the Jurassic. Paleozoic and older strata were eroded from the uplift and the eroded material was transported southward by a system of braided rivers, which deposited the material into a shallow continental basin. In most of northwestern Sudan, these Permo-Triassic to LowerJurassic beds constitutethe LakiaArbainFormation.The LakiaArbain Formationconsists of severalfiningupwards cycles of relatively immature sandstones, partly trough cross-bedded. partly tabular. The whole sequence is very immature and the age is not very precise. However, the upper and lower boundaries are well dated and correspond with the late Carboniferous (to possibly Early Permian) and late Jurassic respectively. The time span is therefore Permian to middle Jurassic. Upper Jurassic to Lower Cretaceous strata were deposited in southern Egypt and northern Sudan after the structural movements of the late Carboniferous to early Jurassic time were reversed, and a normal northward drainage resulted in the deposition of mainly fluvial sediments along both sides of the Sudanese-Egyptian border as well as further north. This fluviatile deposition was briefly interupted by a shallow marine transgression in Aptian times. In areas where no clear differentiation is possible, the Upper Jurassic to Lower Cretaceous Strata are called GiIf Kabir Formation and Selima Formation (Fig. 7). In areas where the upper marine strata can be separated, they are mapped and described as the Six Hills Formation or basal elastics. The marine Cretaceous strata constitute the middle and upper portion of the sedimentary sequence in this part of Africa. The beds have been subdivided into several formations of marine and non-marine origin (Klitzsch 1987). The upper cycle is made up of Nubian-type strata consisting mainly of non-marine continental strata with intercalations of Aptial, Cenomanian and Senonian marine sediments deposited during the upper Cretaceous transgressions that have been recorded elsewhere

17

A Review of ContinentalSedimentsin Africa

P

I

R

.--

ECAMBRI

A

N(

I

Fig. 7. Correlation chart of strata in Northern Sudan and bordering areas in Southern Egypt (Klitzsch et al 1987) Fig. 7. Corr&lationsstratigmphiques entre le nord Soudan et les r&gionsvoisines du sud de 1’Egypte (d’aprb Klitzsch et al. 1987) in northern and western Africa. TheAswan Formation constitutes one of the most important upper Cretaceous continental strata in southern Egypt. It rests on the Precambrian basement and consists of fluviatile deposits (interrupted by paleosols) which represent the beginning of sedimentation in middle Cretaceous times just before the Cenomanian transgression. The Aswan Formation corresponds to the Lower Nubia Group and is well exposed in the west bank of the Nile at Aswan. where it is 20-30 m thick. The Aswan Formation is conformably overlain by well-dated marine beds of Cenomanian age and is therefore Albian or early Cenomanian in age. The recent sub-division of the Nubian Strata (sense lato) further illustrates the fact that with detailed study, which can only be attained with significant financial as well as logistic resources, most of the large African continental sequences can be found to contain important intercalations of marine strata which are often obscured by weathering. NORTH AFRICA CONTINENTAI, SERIES In North Africa there are two different structural domains: the Saharan platform and syneclises. and the Atlasic zones. Pabozoic After the IS 10:1/2-c

Pan-African

orogemc

crisis,

Late

Precambrian molasse infilled intramountain depressions as the ‘Strie pourpx-ee de l’Ahnet” northwest of the Hoggar. or the Mhrizidie Formation northwest of the Tibesti. Then the whole Sahara was covered by a Cambrian sheet of coarse-grained sandstone, fluvial with braided streams in the south and the center, grading to transitional or shallow marine facies to the north like in the Jeffara deep boreholes. After the Ordovician transgressions gentle uplifting of the shields was the site of Late Ordovician large ice fields. The Silurian transgression invaded the syneclises: however on the emerged shields appeared the first continental floras (Hoffmeister 1959; Massa and Jaeger 1971). Sandstones prograded northward and infilled the basins. The Lower Devonian Tadrart Sandstone (Ijerane Series. in Algeria) was a wide fluvial sheet unconformably overlying various terms of the Silurian series (Burollet and Manderscheid 1967: Legrand 1967). F&h flora was described. especially with Lycophyts (Massa and Nicol-Lejal 197 1, Massa and Moreau-Benoit 1976). At the end of the Visean. after deposition of the Mrar Formation, gentle mouvements are recorded around Hoggar and ‘Dbesti. In Morocco and in the Ougarta ranges of Western Algeria there was a real Hercynian erogenic crisis; in South Tunisia and in Mpolitania. the Saharan platform was uplifted, partially eroded, and faulted in the northern part,

C. A. KOGBE and P. F. BUROU~ET

forming steps which correspond to the southern limit of a Palaeo-Tethys invaded by transgressive I-ate Carboniferous and Permian seas. The Upper Permian series is formed by continental red sandstones and clays. In Atlas ranges of Algeria and Tunisia, Palaeozoic series are not known except in the massifs of Kabylia, wandering terranes accreted to the African margin during the Miocene Atlasic orogeny. The Kufra basin was less subsident than the Murzuk or Ghadames areas. In Kufra. a part of the Palaeozoic is continental as the upper Devonian and the carboniferous series of Jebel Avenat. Mesozoic The Continental Intercalaire, with thick deltaic sequences, covered the syneclises of the Sahara and the south of the Atlasic unstable shelf. It is described in detail in another chapter of this journal. In Central Tunisia, the deltaic progradations finish with the 600-800 m thick fluvial coarsegrained Boudinar Sandstone (Hauterivianl.

Cenozoic During the Pakeocene. large surfaces of the Sahara were still covered by shallow seas, which had reached their maximum extension with the Maastrichtian transgression (Fig. 8). In Eocene, there was a progressive regression in Sahara as several islands emerged in North Africa, where tectonic movements caused a widespread unconformity in Middle Eocene series and supplied large quantities of sand during Oligocene and Miocene times. Morocco - The end of Eocene shows continental beds, i.e., the Hadida Formation of Quarzazate and of the southern High Atlas. Oligo-Miocene sandstones and conglomerates are described along the plain of Haouz near Marrakech. They are folded and dip steeply near the range, and are cut by Villafranchian conglomerates. South of Tafilalt lacustrine, sediments finishing by a limestone horizon form the minor hammada of Boudenib (north) and of Boulaouaiche (south); the Oligocene age is reported from Clavator and Umfcoloria fauna (Lavocat 1954: Dresch et al. 1952; Conrad 1969). Near the High Atlas, Gorler et al. (1987) described the detritic lacustrine AIt Kandoula Formation (Early Miocene to Pleistocene): stromatolithes, oncolithes. Characeae. Molluscans. Ostracods. Fishes and Mammals: large olistolithes of Jurassic limestone are interbedded. The Miocene begins oftenwith reddish continental deposits, including conglomerates. Marine transgressions invaded the basins and the intermountain lows during Middle or Late Miocene (Choubert et al. 1962; Charriere 1984. etc.). Continental

levels are known in the upper part of Messinian, like in the post-nappe troughs ofthe Rif(Wemli in Steininger et al. 1985). In the Bharb and the Rif, the marine Pliocene series grade upward to coarse continental elastics capped by Villafranchian red beds. Westem.A@er&resembles Morocco. Inthe marine Miocene CheliffBasin, conglomerate fans are known along the borders. Upper Miocene evaporites are associated with lacustrine levels including diatomites. At the end of Pliocene there are fluvial sands and conglomerates. North of the Grand Erg Occidental and south of the Saharan Atlas, after the Miocene orogenies, widely spread Piedmont sediment are capped by lacustrine limestones or by massive calcretes. They are the Hammadas, the oldest and largest being Pontian or Pliocene: the second one, which is patchy, is reported asvillafranchian (Conrad 1969: Callot 1987) (Fig. 91. Eastern Algeria. Tunisia and Western Libya- The Algers-Ghardafa meridian is a major palaeogeographic limit. On the eastern side, Eocene seas covered large surfaces in Melrhir and Grand Erg Oriental areas. Upper Palaeogene and Neogene continental series are very thick in the Melrhir Basin and in the Gafsa-Tozeur area of Tunisia. In Atlasic Eastern Algeria, continental facies are known in Upper Eocene with evaporites and varicolored shales, during Upper Oligocene (Boghari Series.) in some parts of Miocene, especially Messinian, and at the upper part of Pliocene. The Constantine intramountainous basin is infilled by coarse elastics of Middle and Late Miocene age 300- 100 m thick, overlain by trachytes and andesites dated 10.9 ? 0.5 M.a. and 9.3 + 0.5 M.a. (Bellon 1976). In Central and Southern Tunisia during Eocene times, emerged areas are characterized by calcretes and red beds with Bulfmae. Palaeocyclotusand Characeae includingRaskyeZlapecIcf(Abdeljaoued et aL 1987): Bou Loufa Formation. In the southern Gulfof Gabes, boreholes have found Eocene lacustrine shale called Tanit Formation. With the Oligocene times began a large supply of sandy material, marine at first, thencoarsening up to Aquitanian gravel-sand. This is the For-tuna Formation, capped in some places by the red beds of Messiouta (Burollet 1956). Middle Miocene is generally marine, followed by a thick body of iluvial sandstone, i.e., the Beglia Formation, with a richvertebrate fauna. In eastern and northern Tunisia, thick molasses (Saouaf p.) are shallow marine, brackish, lagoonish or fluvial, especially in the upper part (continental Segui Formation). Lacustrine limestone is associated with some evaporites in the Messinian Oued Be1 Khedim Formation. The Upper Pliocene is

A Review of Continental Sediments in Africa

19

/

\

x k Y

. v----ml

\

I

I

’

t +

_

Fig. 9. Mio-Pliocene Conttnental sedtments of North Afr-tca. 1. Continental Aquitanian and late Oligocene formations: 2. lakes, marshes, and playas of upper Miocene and Pliocene basins : 3. Hammarks; 4. Post-erogenic molasses in Atlas lakes and marshes ln Fezzan : 5. Lakes and playas of Western and Central Sahara ; 6. Pan&c transitional f&es of Tunisia ; 7. Limits of Miocene Seas : 8 Main deltas. Names of places : Algeria and Momcco : H.D : Draa Hamma H.G Cutr Hammada. BA : Benf : Abbes; Libya and Egypt : 2 : Zella , B : Brak, M : M~Izu~, Gi : Gtalo, Bh : Baharyh. H Hurgada. Fig. 8.8ediments continentaux mio-pb&nes en aftique du nord. : 2. LacS, marah et pfayas du miocene superieur et du pliocene : 1. Formations conttnentafes ac~uitank~c~ et O&$XBkrIeS SUpCkURS 3. Hammadas: 4. Molasses post orogkriques de l’atlas : lacs et mamls du Fezzan : 6. lacs et playas du Sahara occidental et central ; 6. Fades paraliques de Tunisie ; 7. Limites des mers miocenes : 8. Principaux deltas.

A Reviewof Continental Sediments in Africa continental, ending with red beds and a massive calcrete reported as Late Villafranchian. Rich Vertebrate fauna was described in Upper Pliocene and Earliest Pleistocene, e.g., near the Ichkeul Lake. Northeast of the Sahamn Africa: Eustem Libya., Egypt East of the Misurata Meridian, Saharan low lands are in contact with the Mediterranean. Palaeogeneand Neogeneseas invaded largelySyrte. Cyrenaica and the Western Desert of Egypt. They received detritic supply from large rivers coming from the southern continent that had wide deltas. Similarly supplied were the Qasr es Segha or Fayum areas in Egypt or the Priabonian delta of Gebel Coquin [Dar et TaIha) in Libya with a very rich fauna ofvertebrates (Arambourg and Magnier 1961). A later delta was located near Zella (Libya) in Oligocene times and it gave Palaeomastodonts. Phyomta wintoni Megdohydraxpalueotheri.oides.

Crocodiles, etc. (Arambourg and Magnier 1961). The third delta family is Early Miocene as in Jebel Zeltene or Marada of Libya, Moghra or the lower Nile valley in Egypt (Fig. 9). Apart from the ccast. the Pliocene series are continental as near Augila (‘Vertebrate remains rich Sahabi Formation), near Helwan or Gar el Muluk around Cairo, or along the Gulf of Suez and the Red Sea. During the Coenozoic. the climates were generally warm and arid; erogenic and epirogenic movements caused erosions and detritic supply, which may be related also to more humid phases. One may underline the importance of large meridian limits, especially the Algeer-Ghardia-Gao line corresponding to the Mzab arch and separating centripetal hydrographic organisation on the western side from a divergent pattern of the lowlands on the eastern side. CORRELATIONS Knowledge ofAfrican Continental Sediments has attained a stage where broad regional correlations can be attempted. Figure 10 is an example of correlations of diverse levels of the Karoo with the Continental Intercalaire and the Nubian Sandstones. It is also possible to correlate the ContinentalTerminal with the upper portion of the Nubian Sandstone (Fig. 10). Continental sequences of Cretaceous-Tertiary age are intercalated within marine deposits in western. northern and northeastern Africa. These beds can be correlated in detail in diiferent parts of Africa due to the precise datation of their upper and lower boundaries. Precambriancontinental sequencesaremuch more difficult to correlate as they are poor in fossils and are often altered.

21

ECONOMIC AND PALRORNVIRONMRNTAL

IMFORTANCR

Detailed stratigraphic, structural and sedimentologic study of the Nubian Strata has provided important data of hydrogeologic signiikance on the Nubian aquifer system of the eastern Sahara. In this area, a basement high divides the Sudanese part of the Misaha Trough (Klitzsch et al.. 1987) and the Selima platform in the south from the Dakhla Basin in the north. In previous hydrogeologicalinvestigations,the sediment thickness above the basement high between Gebal Uweinat and Aswan was estimated to be in the order of a hundred metres. Thus, the amount of the ground water influx across this basement high seemed to be negligible. Recent investigations by Klitzsch et al has shown, however, that the ground water influx was greatly underestimated (Thorweihe 1982). The fresh water reserves of the eastern Sahara are mainly contained in the vast Nubian Aquifer System which extends over an area ofmore than 1 million km2 from north Sudan and northeast Chad. across southeast Libya and the Western Desert of Egypt, towards the Mediterranean. Huge sandstone deposits of varying thickness and thus varying transmissivity form the predominant aquifer type (Thonveihe 1982; Ezzat 1974) except in the north where calcareous deposits are significant. In Algerian and Tunisian Sahara, the Continental Intercalaire is a prolific aquifer, exploited by many wells and feeding the main oases: southwest of the Hoggar. in the Tanezrouft. it gives water to the new settlements of Bordj el Moktar near the Malian border. Wycisk (1984) has demonstrated the importance of continental sediments as environmental indicators. He subdivided the Permo-Triassic to Cretaceous sediments of northwestern Nubia on both sides of the Egyptian/Sudanese border into three main units of distinct environments of deposition: l fluvial deposits of Permo-Triassic to Early Jurassic time; l fluvial and near shore deposits of Jurassic to Middle Cretaceous time; l braided river deposits reflecting a fluctuating fluvial depositional environment with interfingered thick paleosols and thin shallow marine deposits. Although proven commercially exploitable mineral resources of the Karoo sequence are relatively modest in comparison with the overall mineral wealth of Southern Africa, they are in no way insignificant. They very probably hold greater potential than has been generally conceded. The Beaufort Group has been recently intensively

C.

22

A. KOGBEand P. F. BUROUET

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Fig. 10. Attempt to correlate some major Continental sequences in Africa. Fig. 10. Essai de correlation des grandes sequences continentales en afrique.

A Review of Continental Sediments in Africa

explored for uranium with very encouraging results. Continued exploration for uranium has led to recent discoveries of mineralization at unspecified localities within the Stormberg Group and Drakensberg Group. Although uranium mineralization has been recorded from most ofthe facies of Beaufort Group. significant occurrences are mainly in rocks of the high sinuosity facies where the sandstone to mudrock ratios are about 1:4. These rocks display numerous permeability barriers. The mineralized zones are usually in the thickest parts where there are numerous interfingerswith mudrocks, discontinuous silt and mudstone beds. Mineralizedzones are often separated by wide barren zones, and range from small lens a few metres long to 7 m. Mineralized sand are dark grey to black. although the rock is sometimes bleached white or yellowbrown. Carbonaceous debris is invariably present, and the host sandstones are arkosic with carbonaceous cement, sulphides and volcanic debris. Prolific coal mines are exploited in the Karoo basin. High geothermal gradient and numerous dolerite dykes have destroyed hydrocarbons, the remains of which are found as cristalline ozokerite or ‘pseudo-coal.” The Continental Terminal is of great economic significance because of its hydrological importance. In Nigeria, the Gwandu Formation is the most comprehensively studied Formation in the Sokoto Basin. The aquifers of the Formation cover an area of 22.000 km2. From the eastern contact with the Kalambaina Formation, and its southern unconformable contacts with the older Cretaceous Formations, it thickens northwestwards. The Gwandu Formation aquifers are characterized by remarkable lateral and vertical variation in constitution and degree of induration. Four main aquiferous zones, separated from each other by various thicknesses of clays, are known. They are designated Upper Zone I, Upper Zone II. Middle Zone and Lower Zone (Ogilbee and Anderson, 1965; Otezie, 1976). The recharge areas for all the aquifers is in the eastern side of the outcrop areas, but Upper Zone I is exposed to recharge over almost all its surface area. The Gwandu aquifers are the most important source of ground water in the Sokoto state of Nigeria. This is evidenced by the high ratio of successful boreholes drilled recently into the Formation to combat the Sahelian drought. In addition, the Formation is important because of the abundant ferruginous deposits that constitute its capping. Although a detailed study of the concentration of the different types of ferruginous deposits has yet to be carried out, there is no doubt that the amount of iron in the deposits is quite high. and can be economically exploited. In the

23

past, the ironstone constitutes the raw material for home-based smelters and small scale iron industries. The iron slags at Kalgo, near Bimin-Kebbi, confirm this observation. In the neighbouring Niger Republic, the existence of three important levels of ferruginous deposits within the Continental Terminal were highlighted by Dubois (1979). Although these levels may not be recognized on a regional scale, similar interestingzones exist where the Continental Terminal outcrops. These fen-uginous beds will require further detailed study in each area. The Gwandu Formation (CT) is famous for its clay, which has a very high kaolin content. It exists in very large amounts and could be investigatedfor possible utilization as raw material for ceramic industries. The sands and gravels are also useful industrial materials, and are already extensively exploited in the construction industry. Field evidences support the view that the Continental Terminal may contain some radioactive minerals such as uranium. In the NigerRepublic, economic reserves of uranium has been exploited for over 10 years. The economic importance of African Continental Sediments extends into the field of petroleum exploration,where excellentlacustrine source rocks of continental origin are known to be directly related to the new discoveries of oil and gas in Chad and other Central African basins. In Algerian and Tunisian Sahara Triassic Sandstones are reservoirs of several oil and gas fields as Hassi Rmel. El Box-ma. Makherouga. . etc.; the hydrocarbons migrated there from Silurian source beds. REFERENCES

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sedimentology.ExcursionGuidebook, Ext. A3, 89127, Tunis. Arambourg. C. and Magnier. Ph. 1961. Gisements de vertebr&s dans le basssin tertiaire de Syrte (Libye). CR. Acad Set. Paris. t. 252. 1181-l 183.

Bellon.H. 1976. stries magmatiquesn&ogtneset quatemairesdu pourtour de la M&liter-ran&e occidentale. comparkes dans leur cadre gkochronomttrique: Implications geodynamiques. These Dr. Park&may. . Boureau. E. 1958. Paleobotanique Aiiicaine. Evolution des flores disparues de I’Afrique nordtquatorlale. &IL SC. Corn. et Ifist. et SC. 64pp. Burollet. P.F. 1956. Contribution 8 l’etude stratIgraphIque de la Tunisie centrale. Ann. Mines GM.. Tunis, 18, 350 pp. Burollet, P.F. and Manderscheid. E. 1967. Le Devonlen en LIbye et en Tunisie. Intern. Symp. on the DevonIan System, Oswald Ed., Calgary, Vol. 1. 285-302. Busson, G. 1967. Le Mesozoique SaharIen. l&e part.: Lkxtreme-Sud TunIsten. C.N.R.S.. Paris, 194 pp.

24

C. A. K~GBEamdP. F. BUROILFI

Busson, G. 1970. Essai de synthpse des dorm&s des sondages aIg&m-tuntstens. Centre Rech. Zones Ax-ides, CNRS, S. gtologie, 11, Paris, 811 pp. Callot. Y. 1987. Gtomorphologie et pal&environments de I’Atlas Saharien au Grand Erg occidental: dynamique tolienne et paltolacs holoctncs. Thtse Dr. SC. Nat. Univ. P. et M. Curie, Paris, No. 87-21, 474 pp. Charritre. A 1984. Evolution ntogene de bassins continentaux et marIns dans le Moyen-Atlas central (Maroc). Bull. Sot. C&oI. I+ance (7). tXXVI- 6, 11271136. Choubert, G. and Faure-Murct. A. 1956. Lexique stratigraphique du Maroc. Notes M&m Serv. g&oI. Mcuoc. 134, 165 pp. Choubert, G.. Faure-Muret. A., Hamel. C.H., Hottinger. L., Lorenchet de Montjamont. M. and Suter. G. 1962. Introduction stratigraphique in Chevalier, J.P. Notes et M&n Seru. g&l. Mamc, 173.7-15. Chudeau. R 1909 Sahara Sudanese 9/p. Cleverly, RW. and Bristow, J.W. 1979. Revised volcanic stratigraphy of the Lebombo monocline. Trans. Geol. Sot. S. AfrL 82. 227-230. Conrad, G. 1969. L.‘PuoIut&n continentale post-hercynienne du Sahara ak$rlen (Saoura, Erg-ChechTanezmujl, Ahnet-Moydti. Centre Rech. Zones arides. CNRS 527 pp. Conrad, G. and Lappartient, J.R. 1987. Le ‘Continental terminal”, sa place dans l’evolution gtodynamique du bassin s&negalo-mauritanien durant le Cenozolque. Jour. of Ajiican Earth ScL 6.1, 45-60. Dingle, R., Siesser. W.G. and Newton, A.R. 1983. Mesozoic and Tertimy Geology of Southern Africa. A. A. Balkema publishers. 375 pp. Dresch J.. Gigout. M.. JoIy. F.. Le Coz. J. and RaynaI. R. 1952. Aspects de Ia G&omorphoIogte du Manx. XIX Congr. Giol. Intern. AIger. Monographies regionales. s.3 Maroc. n.3. 182 pp. Dubois, D. 1979. Etude geologique de formations oolitiques ferrugineuses du bassin des IuIIemmeden. These 3eme cycle Universites de Niamey et d’orlitans 123 pp. El Hashemi. M.M. 1978. SCdimentologie et paltogeographie des series dttritiques de I’Egypte occidentale, du Cambrien au Ctnomanien. These Dr. SC. Nat., Strasbourg, 148 pp., 16 pp. Faure and Joulia, 1952. Le Cambrien h L’Est de Zinder (Niger). Arch. Div. Fr. des Mines et Geol. Dakar. Furon, R. 1960. Gedogie de I’Afrlque. Payot ed. 2” ed.. 400 p. Gorier. K.. Helmdach, F.F. and Zucht. M. 1987. Conti nentaI Neogene south of the Central High Atlas Mountains. In: CurrentResearchLnAfrlcanEarthSclences (edited by Mathels and Schandelmeier). Balkema. Rotterdam, 133- 136. Greigert. J. 1966. Description des formations cretacees et tertiaires du bassin des IuIIemmeden (Aliique Occidentale). PubI. Dti. Mines et de la Ckdaqie, R&publique du Niger. 2.234 pp. (ed B.RG.M., Paris). Hoffmeister, W.S. 1959. Lower Silurian plants from Libya-MlcTopaIeonm@y I%.3. 331-334. Jacque. M. 1962. Reconnaissance geologique du Fezzan Oriental, Notes et M&m. Camp. Francabe &s R%roles, Paris, 8. 44 pp.

Jaeger, J.J. and. Martin. J. 197 1. Decouverte au Maroc des premiers micro mammi&es du Pontien d’AMque C.R. Acad. Sci. Parts. 2Q2.2 155-2 158. Johnson. M.RM. Botha, B.J.V.. Hugo, P.J.. Keyser, A.W.. Turner. B.R. and de la Winter, R 1976. Btnaty report on Stratigraphic nomen.cIoture tn the Kamo Sequence. Unpub. report to South African Committee for stratigraphy by the Karoo Working Group. Keyser. R.W. and Smith, R.M.H. 1979. Vertebrate biozonation of the Beaufort Group with special reference to the Western Karoo Basin. Ann. Ged. Sum S. Afr... 12/ for (1977-78). 1-35. Kihan. C.M. 193 1. Des principaux complexes continentaux du Sahara. C.R. Sac. G&l. F?., V, 109. Kitching, J.W. 1977. A short review of the Beaufort Zoning in South Africa. In: 2nd Gondwana Sym. Rut. and Papers P. C-S. lR., 309-312. Klitzsch, E. 1970. Die Strukturgeschichte der ZentraIsahara. neue Erkenntnisse zum Bau und zur Palaeogeographie lines Tafellandes. GeoI. Rundschau 69. 2, 459-527. Kktzsch. E. et al. 1987. Research in Egypt and Sudan. Special Research Project Arid Areas S.F.B. - 69. 19841987. Berlinergeowiss. Abh &A)75.2.628 pp. Kogbe. C.A. 1972. Petrology of Maastrichtian and PostPaleocene Formations of the Nigerian Flank of the Iuliemmeden basin. Geolcx~ische Rundschau 62/l, 197-201. Kogbe. C.A. and Lemoigne, Y. 1976. Fossil wood from the Gundumi and 1110Formations (Continental Intercalaire) of northwestern Nigeria. Proc. 13th AMcan Micropal Coll. he-Ife. Nigeria. Kogbe. C.A. 1981. Cretaceous and Tertiary of the Iullemmeden Basin in Nigeria (West Africa). Cretaceous Research 2, 129- 186. Lang. T.. Kogbe, C.A., Ahdou, S.. AIzouma, K., Dubois. D., Houessou, A and Mchet. J. 1986. Le Siderolithique du Tertiaire Ouest-africain et le concept de Continental Terminal. Bull. Sot. G&I. fiance. 8, II, 4. 605622. de Lapparent. AF. 1947. Interpretation stratigraphique des series continentales entre Ohanet et Bourara (Sahara Central). C.R. Acad. Set. Paris. 227,1106- 1107. de Iapparent, A.F. 1953. Repartition des gisements de Vertebres et d’invertebres actuellement connus dans le ‘Continental Intercalaire” du Sahara. Bull. Sot. G&oI. Fr..6, III. 451-456. de Lapparent, AF. 1958. Sur les Dinosauriens du ‘Continental Intercaiaire” du Sahara Central. C.R. Acad. Sci. Parts, 246. 1237- 1240. de Lapparent, A.F. and Mongin. D. 1958. NouvelIes ricoltes de Lamelltbranches hmniques dans le C&ace inferieur du Niger (Airique). Bull. Sot. G&Z. J+. 7, 148153. Lappartient. J.R. 1985. Le “Continental Terminal” et le Pleistocene ancien du Bassln Sentgalo-Mauxitanien. These de Do&rat es Sciences Naturelles, Univ. AixMarseille, 290 pp. Lavocat. R. 1954. Reconnaissance geologique dans les hamadas des conflns algero-marocains du Sud. Notes et M&m. Serv. g&I. Maroc 116, 148~.

A Review of ContinentalSediients in Africa Legrand Ph. 1967. Le D&onien du Sahara alg&ien, Intern. Symp. on the Devonian System, Oswald Edit. Calgary, 1,245-284. Lock B.E., Paverd A.L. and Broderick, T.J. 1974. Stratigraphy of the Karoo volcanic rocks of the Barkly East district. Trans. GeoL Sot. ofS.Afrlca77.117-129. Massa. D. and Jaeger, H. 197 1. Dorm&es stratigraphiques sur le Silurien de l’ouest de la Libye. in Colloque Ordovicien-Silurien. Brest, M&n. BRGM73, pp. 313321, Parts. Massa, D. and Moreau-Benoit. A. 1976. Essai de synthese stratigraphique et palynologique du systeme devonien en Libye occidentale. Rev. Inst. I+. P&role, 31. 2. 287-333. Massa. D. and Nicol-Lejal. A. 197 1. Le Devonien P Lycophytes de la Libye Sud-occidentale. Consequences paleophytogeographiques. C.R. Acad. Scf. Paris, 273, 1182- 1185.. Mestraud. J.L 1982. G&ologie et ressources minimlesde la Republique Centrafricaine. M&n. BRGM, 60.185 p. Ogilbee and Anderson 1965. Hydrogeology of the Chad Basin, Nigeria. Unpublished Report. United states Geological Survey (USAID-Project).

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Otezie. G.E. 1976. The hydrogeology of the northwestern Nigeria Basin. In: Geology ofNigeria (edited by Kogbe. CA.), Elizabethan Piibl. Co. Lagos, 373-390. Said, R 1971. Explanatory notes to accompany the geological map of Egypt. GeoZSurv.ofl$ypt ,88.1.123. Schlumberger Ed. 1983. Well Evaluation Conference. Aiiique l’ouest 1983.7 chapters. Taquet. Ph. 1977. Les decouvertes r&entes de Dinosaures du Jurassique et du Cretace en Ah-ique, au Proche et Moyen-Orient et enInde. M&n h Ser. Sot. 01. France, 8.325-330. Thorweihe. U. 1982. Hydrogeologie des Dal&la Beckens (Agypten) Berliner geowiss AM. (A). S&53 p. Berlin. Verniers, J., Jourdan, P.P.. Paulis. RV.. Frasca-Spada. L. and De Bock F.R. 1989. The Karroo Graben of Metangula. Northern Mozambique. J. AfrL Earth ScL 9. 137-158. Vittimberga. P. and Cardello, R 1963. Sidimentologieet petrographic du Pal&ozoique du bassin de Kufra. Reu. Inst. F?. P&role, 16.11. 1546- 1558. Technip. Paris. Wycisk, P. 1984. Depositional environments of the Mesozoic Strata from northwestern Sudan. Berliner geowiss. Abl. (AI , 50.8 l-97. Berlin.