mesozoic–cenozoic history of the congo basin

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Journal of African Earth Sciences 43 (2005) 301–315

Mesozoic–Cenozoic history of the Congo Basin

Pierre Giresse *

Laboratoire d’Etudes des Geo-Environnements Marins, University of Perpignan, 66860 Perpignan, France

Received 1 September 2004; accepted 18 July 2005Available online 25 October 2005

Abstract

Geophysical surveys and drilling of deep wells have recently led to the recognition of underlying Precambrian basement, and to aninterpretation of the structural evolution of the Congo Basin. Deformation estimated as Late Cambrian to Early Ordovician correspondsto the late Pan-African event more accurately dated as end-early Cambrian in West Africa. Subsequently, Paleozoic deformation led towidespread erosion and the development of a marked regional unconformity. The 1000-m-thick mostly continental deposition during theCretaceous and Tertiary did not involve a noticeable subsidence process. There was no volcanism during this deposition, except at theEarly Cretaceous, with the advent of kimberlites that are distributed over the border of the Cuvette. As a consequence most of the dia-monds were transported northward or southward from upstream sources.

The Mesozoic sediments of the Congo Basin were formed in lacustrine or lagoonal basins close to the sea level as demonstrated bysome intercalations with marine fossils. In the eastern part of the basin, a limited marine connection during the Kimmeridgian was onlypossible with a gulf belonging to the young Indian Ocean. In southern Kasai, the same Kimmeridgian transgression is observed. In thenorthern part of the basin, a probable Cenomanian marine connection was suggested between the Tethys and the South Atlantic, and themarine deposition at Kipala suggests a connection with the Trans-Saharan corridor during the Late Cretaceous.

The geometry of the continental Mesozoic and Cenozoic deposits begins with beds overlying a widespread planation level unconfor-mity and/or the presence of gravel or conglomerate in the lower portion. The Sables Ocres Series and the Gres Polymorphes Series reston the planation levels of Late Cretaceous and mid-Tertiary ages respectively. Mechanical composition and morphoscopic charactersargue for a dominant eolian transport for the Gres Polymorphes and for a fluvial deposition for the Sables Ocres. The silicification pro-cess achieves its maximum extent during the Paleogene interval characterised by a micaceous mineral rich clay fraction in the Congobasin sediments. This process stopped at the beginning of the Neogene interval when the kaolinitic component increased significantly.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Congo; Sedimentary basin; Mesozoic; Cenozoic; Palaeogeography

1. Introduction

The Congo Basin is a broad downwarp centred on theCongo craton, extending into the two Congo Republics,the Central African Republic (CAR) and Angola (Fig. 1).For a long time, research into the Mesozoic and Cenozoicsediments of the Congo Basin was restricted to the study ofoutcrops in river valleys. Much of previous work donefocussed on the southern and eastern borders where fossi-

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liferous sequences are exposed. As a result of these studiesa series of lithostratigraphic units was recognised, enablingcorrelation with the less fossiliferous deposits of the north-ern and western Congo Basin. In addition to this, magneticand gravimetric surveys (Jones et al., 1960) as well as seis-mic data (Evrard, 1960) and two deep boreholes drilled atSamba and Dekese began to reveal hitherto unknown factsabout the whole Congo Basin and its contained sequences(Evrard, 1957; Cahen et al., 1959, 1960). Between 1973 and1981 renewed interest in the Congo Basin was stimulatedby 6000 km of magnetic profiles and drill-hole data pro-duced from the Mbandaka and Gilson localities. Theseindividual works remained largely unknown, their contents

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Fig. 1. General situation of the Congo Basin and the location of most of the geographical sites cited in the text.

302 P. Giresse / Journal of African Earth Sciences 43 (2005) 301–315

restricted until the Cuvette Centrale synthesis was pub-lished by Daly et al. (1991, 1992). Up to now this remainsthe most comprehensive source of information on theCongo Basin, its structure and its post-Paleozoic history.Some subsequent regional studies have added to our under-standing of the palaeogeographic and palaeoclimatologicevolution of the Congo Basin. Although these databaseson their own provide only discontinuous, partial recordsof the basin history, a more complete record can beobtained by comparing these with the records obtainedfrom numerous studies on the nearby Atlantic margin.

This paper is a summary of the known data and litera-ture pertinent to the Congo Basin. It sets out the subsur-face data and makes inferences regarding the probablebasin history, and its sedimentological and structural evo-lution, from the initial Gondwanide rifting and initialopening of the South and Central Atlantic through to thepresent. In general only more recent publications on theCongo Basin have been referred to in the text. The text fig-ures are all taken or adapted from other publications.Because of the difficulties caused by a lack of systematicand widely accepted lithostratigraphic and chronostrati-graphic names it was decided to summarise the stratigra-phy region by region.

2. Building a stratigraphy (Table 1)

The continental Mesozoic and Cenozoic depositsexposed in the Congolese Cuvette and its circumferenceare composed primarily of clayey sands or soft sandstonescharacterised by a nearly horizontal structure (Fig. 2).Through the first half of the 20th century, these slightlymonotonous formations were the subject of an abundantliterature sometimes encompassing conflicting viewpoints.The difficulties faced by the stratigrapher were numerous:

(1) At various sites, the runoff along slopes has reworkedthe superficial sediments and induced the depositionof a depositional blanket that buried the sides ofmany slopes as well as partly covering the early pla-nation surfaces located near the foot-slope. Thereworking was effective through various periods, upto the latest one, as attested by the presence of lithicartefacts in the proluvial sands. It was therefore pos-sible to confuse series of different ages, which werecharacterised by similar sedimentary facies.

(2) Correlations of well-developed erosion surfaces weretraced step by step, but these links became difficultto establish where the distance between each site

Fig. 2. (a) N–S section through the Tshikapa meridian from northern Kwango to Bunza Mount. 1. Basement, 2. Lualaba Series, 3. Kwango Series, 4. Grespolymorphes and Sables Ocres Series, 5. Pliocene–Pleistocene sands and gravels (from Cahen, 1954). (b) Planation levels and associated deposits of theKalahari System. S.n, Neogene surface; S.mt, Mid-Tertiary surface; S.fc, Late Cretaceous surface. 1, Sables Ocres Series; 2, Gres Polymorphes Series; 3,Mesozoic; 4, Precambrian (from De Ploey et al., 1968).

P. Giresse / Journal of African Earth Sciences 43 (2005) 301–315 303

was up to several tens of kilometres. For many years asilicified sandstone was used as a Paleogene referencebed throughout the Basin; observed variations of levelwere attributed to recurrent faulting. Then, it was sub-sequently established that silicification was regionallydeveloped under similar climatic conditions at variouslevels of the formation (Sekirsky, 1956).

(3) Throughout these generally terrestrial deposits, pale-ontological data are very scarce to the present day,and even in 1948, Polinard acknowledged that dis-tinction of Mesozoic and Cenozoic formations wasa difficult task and that estimation of their limitswas hazardous.

(4) The earliest authors emphasised various similarities ofthe Congo Basin deposits with the nearby series ofSouth Africa. The Karoo System of southern Africawas compared to the earlier Congo deposits, and theKalahari Series to the Cenozoic Congo deposits. Ifthe latter relation was rapidly proved to be useful,the first one was subject to successive re-appraisalinduced by the discovery of fresh-water and marinefauna in the Kisangani (Stanleyville) region.

It can be mentioned for the record that the Karoo Systemin southern Africa represents a thick sequence dated withthe help of a rich flora. It begins with the Late Carbonifer-ous–Permian fluvial–glacial deposits (Dwyka Formation)covered mainly by the Permian deposits of the Ecca Group,and then by Triassic deposits of the Beaufort Group. Theend-episode is represented by the Stormberg Group that isLate Triassic and Rhaetian in age (see the review by Catu-neanu et al., this volume). But to the north of southernAfrica, this Karoo System is reduced to limited preservationand the generally incomplete sequence is called the Lua-laba–Lubilash System. In the Congo Basin, the northern-most deposits of the presumed Karoo (Cornet, 1894) werelocated at ca. 4�N, in the Lualaba valley, where for many

years, it was believed that the Beaufort Group was to befound. The first mentioned data on stratigraphy and pale-ontology of these deposits attributed them to the upper partof the Karoo System and postulated them to be of Triassicor Rhaetian age (Leriche, 1925, 1938). This age, based onostracods and phyllopods fauna, was widely acknowledgedand identification of such beds extended as far as Brazza-ville where clayey beds cover the Precambrian basement(Nicolini and Roger, 1951). The Stanleyville beds, as theyare called, were known to be mainly of brackish-water affin-ity, deposited either in an inland sea or in extended lagoonsmore or less directly connected with the open ocean.

The discovery of a true marine fish fauna in the ‘‘limefine’’ beds on the eastern border of the Cuvette (de Saint-Seine, 1952, 1953, 1955; de Saint-Seine and Casier, 1962)gave a Kimmeridgian age or even a Purbeckian–Wealdianchronology for the upper layers, implying a significantlyyounger age for most of the Mesozoic deposits of theCongo basin. A late Jurassic age of these upper beds wasalso attested to by an ostracod fauna study (Grekoff,1960). As a consequence, a significant collection of publica-tions led to the suggestion of a new stratigraphic proposal(Cahen, 1954; Cahen and Lepersonne, 1954, 1978; Lep-ersonne, 1960) which in brief is as follows:

(1) The age of the lower part of the Lualaba Series isdefined as Upper Jurassic whereas the more grittyupper part (Loia Series) could not be accuratelydated, it may be still partly late Jurassic or EarlyCretaceous.

(2) In the type area the Lubilash Formation consistsmostly of Cenozoic sediments: the Lubilash cannottherefore be correlated with the Kwango which is ofUpper Cretaceous age.

(3) The Tertiary deposits previously assigned to beLubilash (Polinard, 1932; Lepersonne, 1945) wereassigned together to the Kalahari System (Cahen


et al., 1946). This system includes three series that aremainly exposed in the western Congo basin:

(1a) The Mesozoic lower formation named the KaminaSeries.

(2b) The overall Paleogene Series of ‘‘Gres Polymorphes’’is the middle unit. This name firstly proposed byCornet (1894) includes various silicified sandstones.These concretionary sandstones with chalcedonycement constitute the most characteristic facies, evenif some other sandstones are interlayered in olderformations.

(3c) The overall Neogene upper unit named the ‘‘SablesOcres’’ Series. These are of still undetermined age,older than the Lualaba Series but younger than theLukuga Series which is Late Carboniferous andPermian in age. Thus, so far, no Mesozoic series ofthe Congo Basin can be attributed to the UpperKaroo of South Africa.

3. Mesozoic deposits—present-day knowledge

Apart from the rather localised sites where useful refer-ences were provided by the discovery of a Kimmeridgian

Table 1Stratigraphic columns showing the major successive proposals defining the M

Central Katanga (Mortelmans, 1946)

Limons Series 1. Limons Ocres2. Irons crust

Miocene peneplanation

Gres polymorphes Series 1. Gres polymorphes2. Conglomerate (wind-worn pebb

Cretaceous peneplanation

Kamina Series Pink and red sands and clays

Kwango (Cahen and Lepersonne, 1954)

Kalahari System 1. Sables Ocres (120-m-thick)2. Gres polymorphes (80-m-thick)

Kwango Series 1. Nsele Group (100-m-thick)2. Inzia Group (105–200-m-thick)

Lualaba Series 1. Loia Group2. Stanleyville Group

Congo–Brazzaville (Cosson, 1955)

Sables Bateke Series (4–500-m-thick) Ba2

Ba1

Stanley–Pool Series (280-m-thick) SP3SP2SP1

Cuvette Centrale (Daly et al., 1992)

Kwango Formation TertiaryBokungu Formation Upper CretaceousLoia Formation Lower CretaceousStanleyville Formation Upper JurassicRegional hiatus

Haute Lueki Formation (Upper Karroo) Triassic–JurassicLate Palaeozoic deformation

Lukuga Formation (Lower Karroo) Permian–Upper Carboniferous

brackish-water or marine fauna (de Saint-Seine, 1952,1953, 1955; de Saint-Seine and Casier, 1962) and a LateJurassic ostracod fauna (Grekoff, 1960), the basis of thestratigraphy as presented by Cahen (1954), Cahen andLepersonne (1954), Lepersonne (1960) rests on two typesof classic observations, (1) the long distance over whichthese beds follow upon the planation level unconformityat the base of the series, (2) the presence of beds of gravelor conglomerate in the lower portion which coincides withthe appearance of a channelling surface.

The type region of the Lualaba Series is located in thenortheastern part of the Cuvette Congolaise. The nearlyhorizontal beds show frequent facies changes but theirpaleontological character is still incomplete. The preva-lent grey and green shades and argillaceous facies in thenorthern part are replaced by pink or reddish shadesand more gritty facies in the southern part. Generally,deposits thicken from the edge to the inner part of thebasin.

In the reference section of Bas–Lualaba (Table 1), theSeries begins with a basal conglomerate and sandstoneresting on an uneven topographic surface. Outcrop andwell data show that the overlying sequence comprises aseries of continental clastic rocks: calcareous, clayey or

esozoic–Cenozoic sedimentary evolution of the Cuvette Centrale

Middle Pliocene to Middle Miocene (30-m-thick)

Oligoceneles) Middle Cretaceous (20-m-thick)

Lower Cretaceous (80–90-m-thick)

NeogenePaleogene

UpperCretaceousLower Cretaceous or JurassicUpper Jurassic

Ochre loams NeogeneSoft sandstonesSilicified sandstones PaleogeneSoft sandstonesSoft sandstonesWhite compacted sandstones CretaceousStanleyville Group Upper Jurassic


bituminous sandstones, green clayey schists, red andmulticoloured marls and clays and bitumen-rich sand-stones. This sequence of 350 m in thickness constitutesthe Stanleyville Group cropping out as far as the northeast-ern and the northern limits of the Cuvette. The frequentlyarkosic deposits of the Loia Group are exposed in the val-leys of the Lomani and Tshuapa Rivers. Closer to thedepression of the Cuvette Centrale, the Lualaba Series con-sists mostly of the more gritty sediments of the Loia.

The Cuvette Centrale and its approaches are boundedby several Cretaceous units that represent a southernequivalent of the Continental Intercalaire or the NubianSandstones of North Africa. At the beginning of theCretaceous, the land surface experienced a slight upliftand subsequently suffered some deformation and plana-tion. The new deposits transgressed over all older ones.But if this Cretaceous surface occupied extensive surfacesthroughout the northern and the central part of the Cuv-ette, it is not, however, represented in the southern part.The sands of the Kwango Series are more argillaceous,coarser and better-sorted in the central and northernKwango than in the southwestern part and in the Kasai,which is assumed to have constituted the primary sourceof sediments. Kwango beds are nearly horizontal, but theirbasal surface shows a slight downwarp (1.55–1.70 m/km)to the western Kasai and the eastern Kwango. The upperlimit of the Series is a Late Cretaceous erosion surface that,in Kwango, gradually dips to the north (0.70 m/km)(Fig. 2a). This slight unconformity was later deformed(after the Kwango deposition).

The Kwango Series is mainly exposed in Kwango,Kasai, on the southern rim of the Cuvette Centrale, inthe Lukenie and Sankuru valleys and along the CongoRiver channel. Two sedimentary groups (geologic stages)were identified above a basal conglomerate with an arkosicmatrix that includes vertebrates remains of possible LateJurassic or Early Cretaceous age:

(1) The lower Inzia Group is a fine argillaceous sand,105–200-m-thick, sometimes fossil bearing. One bedcontains a fish fauna largely marine in origin and ofCenomanian–Turonian age (de Saint-Seine, 1953).This age was corroborated by later studies (Casier,1961).

(2) The upper Nsele Group, ca. 100-m-thick, appears asboth a coarser and a more varied deposit, but con-tains no fossils. It shows some textural evidence ofeolian accumulation.

The Kwango Series directly overlies the more or lesstruncated Lualaba Series but in the Kwango region, wherethe Lualaba Series is absent, there is no evidence of thisunconformity. In Kasai, the Kwango Series rests directlyeither above the Mesozoic, or above the Proterozoic base-ment. The same contacts are seen in the western part ofBas–Congo. A similar pattern is observed for the Carnotsandstones in Ubangui (CAR) where the first deposit is a

conglomerate with a clayey–sandy matrix whose brick-redcolour attests to a lateritic origin (Babet, 1935; Censier,1989). These ca. 300-m-thick kaolinitic sandstones restabove a weathered rock surface similar to old erosion sur-faces. In the southern Congo (Tshikapa and Lueboregions), the Kwango Series lies on a very uneven andwarped surface. Here, the basal conglomerate is discontin-uous and rather thin and overlies either the Lualaba Seriesor the basement where locally it can reach 10 m in thickness.

The Mesozoic sediments of the Congo Basin wereformed in lacustrine or lagoonal depositional environ-ments, close to sea level as indicated by some marine inter-calations. The presence (and often predominance) ofanalcime and carbonate (dolomite) in the clay fraction ofunweathered core samples indicates a very particular alka-line environment (Vanderstappen and Verbeek, 1964).

4. Cenozoic deposits—present-day knowledge

The Sables Ocres Series and the Gres PolymorphesSeries, often mentioned together under the name ‘‘Kala-hari’’ or ‘‘Systeme du Kalahari’’ occupy an important areaof southern Congo. Each of these two series rests on a pla-nation level (Fig. 2b). Toward the end of Cretaceous, a firstwarping episode led to the deepening of a NW–SE or N–Soriented basin before the deposition of Tertiary sedimentstook place. The regional dip of the Gres Polymorphes isN 1–2/1000, the slight unconformity between Cretaceousand Cenozoic formations is seen in the steeper dip of theCretaceous. A second truncation boundary indicates thetransition from the Gres Polymorphes Series to the SablesOcres Series. These two surfaces have been traced to thecoast, and from data obtained there and in the Western RiftValley (Cahen and Lepersonne, 1952) may be confirmed asrespectively of Late Cretaceous and mid-Tertiary ages. Alast surface is the end-Tertiary and its base level is that ofthe centre of the Congo Basin. These surfaces have under-gone a general uplift and subsequently suffered some defor-mation. Generally the downwarped portions of thesesurfaces are covered by sediments which are not representedin the uplifted portions. Each surface represents a level inthe landscape: the oldest and highest is separated from themiddle surface by an erosion scarp; the same relationprevails between the middle and lower surface (Fig. 2aand b).

Cahen and Lepersonne (1952) demonstrated that theKalahari Beds in the Kalahari region lie on the same ero-sion surface as do the Kalahari System in the Congo.The older erosion surface of the Congo Basin is traced stepby step through northeastern Angola in the Zambezi valleyabove the Katima Moliro rapids and into the NorthernKalahari area. It would appear that the Bateke (Congo–Brazzaville), Kwango, Lunda (Angola), Upper Zambeziand northern Kalahari are all basin-like morphological fea-ture distributed along a single, northeast-trending synclinalsystem. This presumed single basin stretches from parallelto 1�N to parallel to 20�S.


Within the main Congo Basin there are two main basin-like downwarped areas: the Kwango basin and the‘‘Cuvette Centrale’’.

In the Kwango basin, the deposition of sediments corre-sponding to the erosion of the uplifted areas has buried theLate Cretaceous surface. The mid-Tertiary surface is bur-ied beneath an accumulation of sands, which may haveblanketed the whole area but is now preserved only onthe depressed parts of the surface.

In the Cuvette Centrale Basin, end-Tertiary erosionremoved earlier surfaces and related deposits. In more ele-vated parts (Bas–Congo, Kasai and Katanga regions), thethin deposits accumulated here were also subject to erosion.

The Kwango region exhibits the most complete sectionof Kalahari sediments. The lower or Kamina Series isknown to fill old erosion channels. It is composed of sands,sandstones and gravels of late Cretaceous age. The 80–100-m-thick Serie des Gres polymorphes begins with abasal conglomerate that sometimes includes wind-wornpebbles (dreikanter). They are overlain by pale colouredsands and sandstones. In the middle part, silicification ofthe sandstones, limestones or breccias has occurred in someareas. These silicifications are scarcely associated, espe-cially in southern Congo, with fresh-water fauna of Cyprisand Physa and flora of Characea. On the basis of Ostracodfauna, the probable age of the Serie des Gres polymorphesis Eocene–Oligocene–Miocene (Grekoff, 1958). Ochre-coloured sands compose the 120-m-thick upper series.They consist of poorly argillaceous and azoic sands, with-out stratification that rests sometimes on a conglomeraticlevel with small-pebble quartz gravels, limonitic gravelsand iron-crust debris. The thickness of this conglomeraticlayer can be as much as several metres, but decreases tothe west, the north, and the east. Near the Katanga–Zim-babwe border, the sands are less than a few metres thick,and then disappear completely.

Lithological study of the middle and upper series (DePloey et al., 1968) provides additional information aboutthe nature of the successive sedimentary environments.

The Gres Polymorphes Series is stratified in thick layers,frequently finely bedded, and occasionally cross-bedded.The dominant colours are light grey, white, and yellow.The silicified deposits form layers with a very large scale len-ticular geometry. Some of these rocks result from the silicifi-cation of limestone and the calcareous cement of sandstones.Silica was supplied by diatomaceous debris. Diatoms areabundant in the limestone around some ponds (pans) ofthe modern Kalahari. By analogy, it is suggested that thesepans are amodern equivalent of the silicified limestone lensesof the Gres Polymorphes Series. Homogeneity of composi-tion, both vertically and horizontally, absence of coarse par-ticles, generally good sorting, high ratio of rounded grains/angular grains, all suggest an eolian transport. But the clayfraction characterised by the dominance of kaolinite andgoethite indicates a previous or simultaneous tropical weath-ering probably of a ferrallitic type.

By comparison, the Sables Ocres Series is composed offine sands without stratification and silicification. Thedominant colours are ochre or red–brown, fading to yel-lowish-ochre or light grey by discoloration near the sur-face. Equal portions of rounded grains and angularglittering grains, scarcity of chatter marks and a poorsorting coefficient exclude an eolian origin. It is concludedthat the mechanical composition and the shape of thegrains correspond to those of a typical fluvial deposit.The absence of stratification and the scarcity of the streamchannels results from the recurrent bioturbation of the soilfauna. Predominance of kaolinite and goethite, unequalrepartition of gibbsite and hematite indicate a hot andhumid palaeoclimate during the weathering process thataffected the kaolisols. These conclusions were then broadlycorroborated by the study of the eastern part of thePlateaux Bateke bordering the Cuvette with some indica-tion of an arid trend during the presumed Quaternaryupper deposits (Giresse and K�Vadec, 1971).

5. Deep framework of Congo Basin and tectonic evolution

Since 1957, geodetic and geophysics studies of successiveexpeditions of the Syndicat pour l�Etude Geologique etMiniere de la Cuvette Congolaise were carried out andbrought the first information related to the framework ofthe Congo Basin. The first magnetic survey (Jones et al.,1960) showed a regular pattern of the isodynamic linesand their prevailing E–W direction. The gravimetric survey(Jones et al., 1960) outlined the strongly negative andmonotonous character of the Bouguer anomalies through-out the Cuvette Centrale, only disturbed by less negativeanomalies to the north of Lodja and to south ofMbandaka. Seismic reflection and seismic refraction pro-files (Evrard, 1957, 1960) were complemented by two strati-graphic boreholes drilled in the two subsiding areas (Cahenet al., 1959, 1960). The surface topography was proved tobe more irregular than was previously suggested. In out-line, a prominent high axis in the central part of the basinindicates the presence of two smaller or sub-basins:

(1) A northern basin, parallel to the axis of the Lulongaand Busira rivers is dipping westward (from Ikela toSamba), and then, rises gently at the approach tothe Ubangui region. At Samba, the substratum recog-nised at 1170 m below the surface is composed of red-dish sediment, aged somewhere between Precambrianand Paleozoic.

(2) A southern basin, more prominent than the northernone, generally co-axial with the valley of the LukenieRiver between Kola and Dekese, hence risingtowards Lake Inongo. Reddish deposits similar tothose of the substratum of Samba were found at1680 m below the surface at Dekese, but older series,slightly or nonmetamorphosed, occur 3500–4000 mbelow the surface.


These various works took into account the progresslinked to the appraisal of the hydrocarbon potential ofthe Congo Basin (Nairn, 1978; Clifford, 1986). In the mean-time, two new exploration wells were drilled in 1981, at theMbandaka and Gilson localities (Daly et al., 1992). A sche-matic stratigraphical section across the Congo Basin indi-cates the upward attenuation of deformation. TheMesozoic beds (post-Karoo) are horizontal and indicate anearly constant (ca. 1 km) thickness (Clifford, 1986).

But the most comprehensive study of the tectonic evolu-tion of the Cuvette Centrale was derived from the lastpetroleum exploration (Daly et al., 1991, 1992). For thepurpose of this paper, we have summarised the portionsof the Precambrian and Paleozoic geological history thathave influenced the Mesozoic–Cenozoic deposition in someway. The Cuvette Centrale comprises a thick (up to 9 km)Early Cambrian to Recent sedimentary section. Six strati-graphic units (sequences) bounded by unconformities areseismically defined. Overlying the Proterozoic–Archaeanbasement, a carbonate-evaporite unit (sequence 1) wasreached in the Mbandaka well at a depth of 4884 m. Thisunit was locally deformed prior to or during the earlystages of deposition of Cambrian clastic sequence 2, thatprogrades into the basin from the west. Sequences 1 and2 were subjected to a major compressional deformation

Fig. 3. Sketch geological map of the Congo Basin and its regional setting showiGilson (G). The outline of the deep structures of the Cuvette is indicated: Kiriet al. (1992).

that led to emergence and to the development of a regionalunconformity. This deformation is estimated as Late Cam-brian to Early Ordovician by Daly et al. (1992), yet theyconsidered that it corresponds to the late Pan-Africanevent of Kennedy (1964), now well dated as the end of earlyCambrian in West Africa (Guiraud et al. (this issue)).Shales and arkosic sandstones of unit 3 are considered torange roughly from Late Ordovician to Devonian. TheCarboniferous–Permian glacial deposits form sequence 4.These deposits are presumed to be equivalent to the lowerpart of the Karoo system of South Africa (Dwyka/Ecca),called here the Lukuga Formation of the eastern CongoBasin. A major late Paleozoic deformation led to wide-spread erosion and the development of a marked regionalunconformity. This unconformity is overlain by sequence5 of regionally constant thickness (ca. 1 km). The age ofthis section ranges from Triassic to Recent and includesstrata of Upper Karoo age (Stormberg equivalent).

The high in the central part of the basin corresponds totwo positive relief features: Kiri High in the NW and Lon-konia High in the SE (Fig. 3). This high follows a trendfrom WNW to ESE across the basin and is bounded by aset of faults linked to phases of deformation in the EarlyPaleozoic (Cambrian) to Late Paleozoic (Late Permian/Triassic). The two deformational events were mainly

ng the location of the boreholes at Dekese (D), Mbandaka (M), Samba (S),High (KH), Lonkonia High (LH), Inongo High (IH); adapted from Daly

Fig. 4. Geological sections across the Cuvette Centrale, based on seismic data. Post-Palaeozoic evolution postulated by Daly et al. (1992). Sequence 1.Proterozoic–Archaean; 2. Cambrian; 3. Ordovician–Devonian; 4. Permian; 5. Mesozoic–Cenozoic.


contractional and were linked to NE–SW shortening acrossthe Cuvette Centrale (Fig. 4). The Late Paleozoic deforma-tion has a similar geometry to that of the Early one, becauseit represents a compressional reactivation of many of theprevious structures. The Early Paleozoic episode is coinci-dent with the Pan-African event and as such appears tobe widespread in both Africa and South America (Kennedy,1964; Daly et al., 1992). Here the deformation appears to bea far-field (500 km) response to the collisional processesthat occurred along the western side of Africa during thelate stages of the East Congolian orogeny. The Pan-Africancrustal consolidation produced major basement lineamentsand faults whose trends formed precursor directions of themain structure for the future deformations. The Late Paleo-zoic contractional deformation was a previously unrecogn-ised event in Central Africa. This is compatible withintracontinental deformation resulting from distant colli-sional tectonics along the southern margin of Gondwana(roughly 2500 km away) and the generation of the Cape(South Africa) and Sierra Ventana (Argentina) Fold Belts.However, late Palaeozoic ‘‘Karoo’’ rift mechanisms seemunknown in the nearby Rift Basin in Niger, Chad and theCentral Africa Republic (Genik, 1992).

The continental deposition during the Cretaceous andTertiary did not involve a noticeable subsidence processbecause it was not linked to a failed rift and subsequentthermal relaxation. In this region, there was no known inci-dences that could be attributed to the breakup of Gondw-ana and the start of the separation between Africa andSouth America. We are thus very far from the 13,000 mof Early Cretaceous to Holocene continental and marineclastics recorded in the Niger and Chad basins or fromthe 7500 m of chiefly Early Cretaceous continental clasticsrecorded in the Central African Republic (Genik, 1993).

6. Main information brought by deep stratigraphic boreholes:

a summary

TheMesozoic–Cenozoic accumulation corresponds all inall to sequence 5 as recognised by Daly et al. (1992). This isrepresented by a banded-seismic package of regionally con-stant (ca. 1 km) thickness that progressively overlies anunconformity from east to west. The Haute Lueki Forma-tion ranges in age from Early Triassic to Middle Liassic,and passes upwards, with a Middle Jurassic hiatus, intothe structurally conformable Late Jurassic Stanleyville For-mation (Table 1).

The deposits classed as Paleozoic reach a thickness of320 m in the Samba well and 820 m in the Dekese wellrespectively. From Mesozoic to Cenozoic, the accumula-tion rates appear similar at both sites and exhibit a markedacceleration (ca. 800-m-thick deposits for 200 Ma) in sedi-ment accumulation rate that led to oversteppings of thelimits of the fault trough is observed at both sites.

The Samba corehole passed through ca. 1170 m ofsubhorizontal Mesozoic and Cenozoic deposits. The fre-quent occurrence of fossils (Ostracods, Phyllopods, pollen,Vertebrates) provides the basis for a well-established chro-nostratigraphic framework (Cahen et al., 1959; Vanderst-appen and Verbeek, 1964). The Dekese corehole cut715 m of Mesozoic and Cenozoic deposits that were corre-lated with the Samba section on the basis of lithologic pat-terns (fossil were relatively scarce) (Cahen et al., 1960).

Gritty and argillaceous deposits of the StanleyvilleGroup can be recognised in the Samba section, between1167 and 845 m below the surface, a total thickness of323 m. Conglomerates are frequently interbedded in thesandstones from the base at 1167 m up to 1080 m belowthe surface. From ca. 1080 to 1020 m, brown–red fine sand-


stones are the prevailing deposits and are associated withanalcime-rich calcareous or dolomitic deposits. Thissequence looks like the exposure of the type area. In thisborehole, the lower beds of the Group attain a thickness(150 m) that is generally greater than that found in the out-crops (10–50 m). This anomalous thickness could be linkedto a local deep part of the basin rather than to any generaltilting of the basin basement.

The top of the Stanleyville Group shows evidence oferosion prior to deposition of the Loya Group. The contactis more or less characterised by cut-and-fill features. TheLoya Group comprises frequently calcareous and finesandstone from 845 to 565 m below the surface. Its basalconglomerate includes abundant quartz, sandstone, andcarbonated rocks, with a pebble-size reaching up 2 cm. Inthe Dekese section, the Loya Group overlaps olderdeformed sediments of the Lukuga Group (Cahen et al.,1960).

Above 565 m below the surface, the Samba deposits areunfossiliferous and consist of a succession of soft sand-stones (up to 310 m), argillite and relatively consolidatedsandstones (up to 200 m), poorly consolidated argillitesand sands again (up to 90 m) and unconsolidated clayeysands. These deposits are stratigraphically undifferentiatedand comprise the upper part of the Cretaceous (BokunguFormation) and, predominantly, the Cenozoic deposits.

7. Marine incursions

It is somewhat surprising to find some evidence of mar-ine transgression in this moderately subsiding basin whosesurface level remained near sea level for a long period oftime.

The older marine deposits of the Basin are located in theeastern part, essentially in the Congo–Lualaba and Lomanivalleys, where argillaceous deposits of the StanleyvilleGroup are exposed. Most of these deposits include a faunathat is not especially marine. Fish remains indicate a lacus-trine or lagoonal setting (de Saint-Seine, 1955). In this basicand confined environment, analcime deposition wasinduced by weathering of albite-rich rocks (Vernet, 1961;Vanderstappen and Verbeek, 1964). Phyllopod faunas indi-cate mixed fresh-water and brackish conditions suggestinga swampy environment with probable seasonal desiccation(Defretin-Lefranc, 1967). Ostracods point to fresh-water orslightly brackish conditions, and no true-marine fauna wasidentified (all the species consistent with a terrestrial envi-ronment) (Grekoff, 1957).

In this Group, only one bed, called ‘‘Lime fine’’, is anexception. It is only 43-cm-thick and consists of an alterna-tion of limestones and clayey laminae. The fish fauna indi-cates an obvious marine environment (de Saint-Seine andCasier, 1962). It is an impoverished fauna, rich in smallindividuals (some of them in a young state), developed ina shallow embayment which extended from East or North-east to West or Southwest for a few kilometres from Songa,before the calcareous bed changes to a sandstone. A Liassic

(early Jurassic) age was proposed for the fresh-water faunawhereas a Late Jurassic age was deduced from the interbed-ded marine fauna, suggesting a sort of archaism of thefresh-water fauna. In his more recent synthesis, Cahen(1983) suggested a Kimmeridgian age for most of the lowerbeds of this Group including the marine interval; the upperbeds are probably Purbeckian to Early Cretaceous.

In the fresh-water/brackish beds, the gritty deposits mayimply the influence of slight subsidence whereas the finerdeposits may be indicative of a higher stability. There is asort of rhythmic character to this flood-plain depositionthat points to some weak and probably distant tectonicactivity. These processes, the absence of marine Jurassicdeposits on the Atlantic coast of Africa and in the northernpart of the Congo Basin and the successive transgressionsand regressions recorded in eastern Africa (especially inAbyssinia and Sudan) suggest little possibility of an oce-anic connection. A short period of connection during theKimmeridgian was only possible through a gulf belongingto the Indian Ocean (Lepersonne, 1960) (Fig. 5a). This con-nection was probably related to the high sea level of theearliest Kimmeridgian and to the appearance of the Somaliand Mozambique rifts, which allowed separation of EastGondwana from Africa (Cecca et al., 1993). The impossi-bility of a connection with the Atlantic Ocean was also evi-denced by the deposition registered in the deep wells of theCuvette Centrale.

In the Brazzaville and Kinshasa subsoils (Nicolini andRoger, 1951; Egoroff and Lombard, 1962) a calcareous fos-sil-bearing argillite (0–20-m-thick) locally interbeddedbetween the Inkisi Sandstone (Precambrian or Paleozoic)and the Gres Polymorphes was intersected in several bore-holes. The Ostracod and Phyllopod faunas with Estheria,Estheriella and fish scales were initially considered asTriassic (Karoo). These faunas show similarities with theKimmeridgian fauna of the Stanleyville Group. But thissite is located at some 2500 km west of Mbandaka, so onthe basis of the above-mentioned features, these depositscan be attributed to small brackish and swampy hollowswithout connection to the Indian Ocean.

In southern Kasai, in the Tshikaya and Luebo regions,the Kwango Series was studied for its fossil-bearing depos-its. These deposits provide some Entomostracean and oneBivalve (Pteria sp) that indicate a marine influence, butSauropod remains are also reported from Late Jurassic toEarly Cretaceous beds (Cahen, 1954). Pteria would cer-tainly indicate a marine origin for these sediments, butthe Sauropod remains do not do so. This fauna providesonly a general overview of the stratigraphic position, ifits marine setting is verified, but in spite of the distancefrom the Lualaba valley, it should be related to the sameKimmeridgian transgression (Fig. 5b).

In Ubangui (CAR) and near Lame (Cameroon), fishremains with marine affinities were found in a calcareousbed and considered as Albian in age by Lepersonne(1960). In order to relate these deposits to the open ocean,we must consider the opening of the Trans-Saharan

Fig. 5. Location of successive marine (M) incursions in the Congo Basin, L: Lacustrine. The squares indicate the fossiliferous beds; the dots refer to welllocations, M: Mbandaka, D: Dekese, S: Samba, G: Gilson.


corridor, formed by intra-continental trench system andwhich allow the Cenomanian marine connection betweenTethys and the South Atlantic (Philip et al., 1993)(Fig. 5c). So this Cenomanian age seems more probablethan an Albian one especially since Albian deposits are ter-restrial in the Benue basin. A late Cenomanian age wasthen suggested by Schneider and Wolff (1992) and Ceccaet al. (1993).

This first connection would be strengthened later duringthe Kwango interval. In the northern Kwango, near Kipala,two beds belonging to the Inzia Group revealed a nearlycompletely marine fish fauna (from Cenomanian toTuronian) as well as Ostracods and Phyllopods that presentlittle stratigraphic significance. Using geographic logic, onecan suggest that these presumed marine depositions ofKipala were related to an Atlantic incursion. However, asall the Cretaceous formations of the Congolese Atlanticcoast indicate very shallow environments, it seems unlikelythat here there was a 300-km inland incursion of the shore-line. Several signs could, however, indicate a marine trans-gression coming from the north. The slight deformations atthe end of Jurassic times induced a small unconformitybetween the Loya Group and Stanleyville Group bedsand therefore probably closing of the Basin in the East

and opening in the West (Lepersonne, 1960). A connectionwith the Trans-Saharan corridor is suggested during theLate Cretaceous (Reyment and Dingle, 1987) (Fig. 5d).

From the Cenozoic, onward uplifts of the Cuvette bor-ders made any subsequent marine incursion impossible.

8. About drainage history into the Atlantic

The geomorphological development of Africa showsseveral continentwide denudation surfaces being formedsince the Late Cretaceous. On the basis of the chronologyof King (1963), these surfaces could be tentatively corre-lated with the regression surfaces of the shelf: each onsetof uplift initiated a period of renewed marine sedimenta-tion. After the Lutetian compression phase, a new north-ward displacement of the African plate since Miocenetime initiated a new distension phase (Le Pichon, 1968).The mid-Cenozoic deformation (northward and eastwardtiltings) induced a new drainage pattern that was nearlysimilar to the present one. The new relief initiated a newdrainage through the Albertine Rift Valley (Pickfordet al., 1993). However, local recurrent faulting may be dif-ferent from one area to the other. On the basis of the geo-morphological cyclical history introduced by Veatch


(1935), Cahen (1954) analysed the segments of drainagethat are mostly preserved in the southern and southwesternparts of the Congo Basin. The Late Pliocene–Early Pleisto-cene cycle was rejuvenated by the formation of the Stanley–Pool base level that initiated the present cycle. The suddenchange in direction of the Ubangui River after the conflu-ence with the Kemo River has suggested the hypothesisthat this river flowed previously towards the Chari andthen into Lake Chad. Then, it was captured by a small trib-utary of the Mpoko River across the Bangui–Zongo can-yon (Moguedet et al., 1990). As a consequence, thedrainage of the Congo Basin into the Atlantic began sev-eral times after the creation of the Stanley–Pool. So, theduration of the present cycle was probably less than 1 mil-lion years.

One of the most spectacular consequence of this lastdrainage would be the capture by a coastal steam of thewaters of Stanley–Pool initiating the drainage of the Cuv-ette Centrale (Giresse, 1982). However, for Jansen (1984),the maximum age of the Congo deep sea fan would havebeen determined by the initial opening of the South Atlan-tic. The greater part or 70% of the sediments in the Congodeep sea fan was already supplied before the Tertiaryboundary, 66 million years ago. Comparison of formationthicknesses with those in nearby profiles from outside thereach of the fan (Uchupi and Emery, 1974; Emery et al.,1975) leads to the conclusion that during both the Creta-ceous and Cenozoic sedimentation in the Congo fan wasrapid. Today, the recognition of the true Congo fan seemsstill a set of problems and is a confidential topic of researchfor private companies involved in hydrocarbon surveys. Itseems that the minimum age of the Congo deep sea fanwould be Miocene (personal communication by Elf CongoCompany).

9. Kimberlites and diamond-bearing gravels

In the Congo Basin, no volcanic rocks are associatedwith the Mesozoic–Cenozoic deposition. In South Africa,the last volcanic occurrences are the basaltic dischargesthat during the Liassic overlapped the Stormberg Series.The absence of volcanic rocks in post-Karoo Jurassicdeposits has consequently been interpreted as evidencefor a quiet tectonic setting. One exception is suggested bythe kimberlitic intrusions during the Cretaceous.

In Katanga, the kimberlitic intrusions of the BushimaieRiver (Bakwanga) cut the Loya Series. The diamond fieldsof Kasai (Congo) and Lunda (North-Angola) owe theirorigin to the kimberlitic pipes, one of which was locatedin Lunda. Taking into account the presence of diamondsin the basal conglomerate of the Kwango Series, a pre-Kwango intrusion has been suggested (Cahen, 1951;Polinard, 1951). The occurrence of kimberlite older thanthe Kwango Series emphasises the significance of theunconformity between the two series. Thus, the ages ofthese pipes is certainly pre-Late Cretaceous and is probablyEarly Cretaceous. The intrusion of kimberlites and their

subsequent erosion would have taken place between theWealdian and the Albian (Lepersonne, 1960).

While in Bakwanga, the diamond deposits were formeddirectly from the weathering and the erosion of the kimber-lites, the Kasai–Lunda ores are found in Cretaceous grittyformations (Kwango or Lunda Series) that act as second-ary deposits and aid the concentration of diamonds (inBardet, 1974). So far, it is believed that most of the dia-monds of the Kwango region, especially those from theTshikapa River deposits, were transported northward fromAngolan provenances. This assumption is supported by thenorthward decrease in the size of the diamonds and thegeneral slope of the platform from SE to NW which indi-cates the flow direction. However, sources nearer Tshikapaare possibly buried beneath the Kalahari blanket. Down-stream and approximately 450 km to the W and NW ofTshikapa, small diamonds were found in the KwangoRiver. In addition to this, small diamonds found in CongoRiver terraces, near Brazzaville, possibly represent by-products of the Kwango River (Nicolini, 1952). But inthe same area, small diamonds were recovered in a 6-m-thick coarse layer that was found at ca. 30 m below the sur-face by drilling (Giresse, 1990). This layer corresponds tothe local equivalent of the Kwango basal conglomerate.This discovery was strengthened by the occurrence of dia-mond traces in the drainage area of the nearby PlateauxBateke. Because no Cretaceous kimberlites are known toexist in this northern zone of the Congo Basin, it suggestedan upstream source originating from the immediatelyneighbouring Proterozoic ranges. The existence of met-akimberlites is recognised at two distinct sites 500 kmapart: at Mitzic (Gabon) and at Komono (Congo). Butthe overlying Niari tillite and Inkisi large deltaic body arenot diamond bearing. A primary kimberlitic intrusion tak-ing place immediately before Early Cretaceous is postu-lated. Far to the northwest, some 800 km from theKwango field, field diamonds were discovered near Kelle(Lebango region) in the dissected remnants of basal Creta-ceous conglomerate above the basement. These ores pro-vide a link to the deposits of the CAR. In the westernUbangui, the secondary diamond deposit appears to behosted in the arkosic grits and conglomerates of the LowerKwango beds. The current bedding and general northerlydip of the sediments indicate that the sediments were trans-ported from the south (Delaune and Delany, 1958). But asin the Brazzaville area, the primary diamond deposit isunknown. The horst uplift between the Chad Basin andCongo Basin induced inversion of relief. This new surfacedip probably induced a significant erosion of the primary(or secondary) formation. Here too, the absence of dia-monds in the early Lukuga eliminates the possibility of aCretaceous primary origin (Bardet, 1974).

Finally, it seems that the Early Cretaceous kimberliticintrusions were most probably a general feature of centralAfrica and more widely of the Africa–Brazil parts ofGondwana. The intrusions have been interpreted as beinga result of deep shock of the earth�s crust down to the outer


mantle, basement source of kimberlite. It appears that thediamondiferous pipes in central Africa are distributed overthe border of the Cuvette du Congo. This observationwould be the key to the interpretation of the main oresof central Africa: more of the pipes are located in the junc-tion or contact area of two ‘‘mega-ring structures’’ bothsurrounding a large subsiding basin, interpreted as a truesyncline (Bardet, 1973).

10. Paleoclimates and paleoenvironments

Because pollen grains are poorly preserved in the coarseand more or less oxidised deposits of the Mesozoic–Ceno-zoic Congo Basin, few palynologic studies have been suc-cessful (Maheshwari et al., 1977). As a result of this,most of the knowledge about the paleoclimatic evolutionof this region is based on geochemical evidence and com-parison of diverse sedimentary environments with the mar-ine sediments of the adjacent Atlantic Basin (Giresse,1982). Effectively, the mineralogical evolution of the twoadjacent basins indicates a relative parallelism of thegenetic processes.

The Atlantic deposits are relatively rich in feldsparsup to the middle Albian, in which they are especially wellpreserved in a calcareous matrix, with their abundancedecreasing markedly and tending to disappear towardsthe Tertiary deposits. On the margins of the Cuvette, feld-spars are abundant in the Jurassic Lualaba Series (Cahen,1954), and in the Loya and Kwango Series exposed in theeast and the south of the Basin (Cahen, 1954), but areextremely rare in the Paleogene Gres Polymorphe Serieswhich has an essentially siliceous composition. In the Car-not sandstones (in the Central African Republic), the lowerseries (presumed Early Cretaceous) include arkosic gritsand fine feldspathic sandstones whereas the upper series(presumed Late Cretaceous) consists of purely quartz-sands; however, kaolinite is commonly distributed throughthe whole series.

The clay fractions of the Atlantic Basin are relativelyrich in micaceous minerals (montmorillonite and illite),but show an increasingly kaolinitic composition up to theTuronian and Senonian times. This kaolinitic compo-nent increases significantly from the Miocene (Giresse,1982). Analogously, in the Congo Basin, this boundarybetween the smectite-dominant assemblage and the kaolin-itic- dominant assemblage is observed during the Creta-ceous deposition of the Brazzaville undersoil (Giresse,1990). Similarly, in southern Congo, the characteristic clayminerals of the Cretaceous period (montmorillonite, illite-mica, feldspar) are very different from the kaolinitic Ceno-zoic assemblage (De Ploey et al., 1968). If one attributes adetrital origin to the minerals one has to consider the differ-ence as being due to a general change in paleoclimatic con-ditions at the end of the Cretaceous. However, the presence(and often the predominance) of analcime and carbonate(dolomite) in the clay fraction of unweathered core samplesindicates a particular, but local alkaline environment.

When they are exposed, these same deposits have lost theiranalcime, possibly as a result of a tropical surface weather-ing leading to montmorillonite neoformation (Vanderstap-pen and Verbeek, 1964). This may, however, be due to aclimatic trend, and the erosional surfaces of the LateCretaceous, Late Paleogene or Late Cenozoic show evi-dence of humid conditions (setting of iron-crusts). Eachred accumulation is characterised by ubiquity of kaoliniteand goethite and unequal repartition of gibbsite and hema-tite. The clay mineral association kaolinite–iron oxyhy-droxides, characteristic for each series, indicates that theerosion affected kaolisols of a ferrallitic type, which wereformed under hot and humid paleoclimates. Palygorskitewas present in noticeable amounts in the Paleogene andLate Cretaceous formations and corresponds to chemicalsedimentation in perimarine basins in a hot climatewith sharp contrasts in humidity (Chamley et al., 1984).Palygorskite disappeared after mid-Eocene times (Robert,1982). The Early Senonian paleomargin, located in Gabonand Cabinda, provides the earlier evidence for marinedeposit silicification. These processes reached a maximumextent during the Paleogene when phosphates acted asthe host formation (Giresse, 1980; Giresse et al., 1984; Gir-esse and Baloka, 1995). In the Cuvette region, this silicifica-tion corresponds to the late Cretaceous and showswidespread development during the Kwango Series andthe Gres Polymorphes Series. These more or less continu-ous processes stopped in the two basins at the beginningof Neogene interval. These climate-driven processes arecharacterised by a shift to more arid conditions attestedby eolian mobilisation and great eolian contrasts (Giresse,1990). These series were mainly eolian formations depos-ited under an arid or desertic climate, and marked seasonaldryings are also suggested by the study of phyllopod fauna(Defretin-Lefranc, 1967). During the Paleogene, rainfallwas high enough to produce ponds or swamps with abun-dant fresh-water ostracods, gastropods and characea.Some of these rocks result unquestionably from the silicifi-cation of limestones and the calcareous cements of sand-stones. It is suggested that the sedimentary conditionsmust have been similar to those of certain comparativelyhumid parts of small lakes and marshes, characterised bymore or less sandy limestones, with fresh-water faunaand flora. The Sables Ocres Series, according to De Ploeyet al. (1968), correspond to those of an alluvial deposit,and are not eolian.

Climatic successions are punctuated by distinctiveevents that have been recognised by both field and miner-alogical characteristics:

(1) Around the Paleozoic–Mesozoic boundary, climaticconditions extended uniformly over large surfacesof the Congo Basin and evidenced a gradual trendfrom glacial conditions toward semi-arid to tropicalconditions through Late Permian and Triassic times.From Jurassic times, a recognisable tropical signatureis observed in the deposits (Lepersonne, 1960).


(2) A hot and slightly arid climate is evidenced during theEarly Cretaceous enhancing the development ofextended lacustrine or lagoonal surfaces.

(3) In Albian and Cenomanian deposits, the feldspathiccomponent decreases (Q/F = 2–4) and the red levelsincrease and reveal a general pattern of wide-spread humid conditions. Change toward morehumid conditions, higher water tables characterisedTuronian–Senonian times and resulted in iron exportand development of dissolution cavities in quartzgrains; this last process provides a great number ofsilt-size particles to the marine deposition.

(4) At the Late Cretaceous–Paleogene boundary, amarked drying trend led to a partial desiccation ofthe Cuvette and to an eolian mobilisation. Recurrentsilicifications are slightly synchronous in the marineand the continental basins. Q/F values rise up to 6–8, and then to 8–12.

(5) During the Neogene, red deposits re-appeared withslightly localised iron-crust. Q/F ratio is alwayshigher than 10. The iron supply to the Atlanticreached its height and controlled significant glauco-nite formation processes on its margin (Giresse,1982).

11. Conclusions

For a long time, the study of the Mesozoic and Cenozoicdeposits of the Congo Basin was restricted to outcropsalong valleys and much of the previous works in the basinfocussed on the very scarce fossiliferous sequences alongthe southern and eastern borders. Relatively recent geo-physics data and drilling of deep wells have improved ourunderstanding of the Basin but there is still a frequent dis-continuity of our knowledge. Previous studies were inter-rupted some 40 or 50 years ago and some relativelyrecent studies have no other regional equivalent or arepartly confidential. So, a complete and synthetic palaeoen-vironmental reconstruction is still within the future.

Nevertheless, some major phases of basin developmentare recognised: (1) after the Late Paleozoic deformation,the continental Mesozoic and Cenozoic deposits are char-acterised by a nearly horizontal structure that did notinvolve a noticeable subsidence process; (2) during theMesozoic, some high sea levels are attested to by short peri-ods of marine incursion in lacustrine or lagoonal basinsnear to sea level: from the Indian Ocean (Kimmeridgian),from a marine connection between the Tethys and theSouth Atlantic (Cenomanian) or from the ‘‘Trans-Saharancorridor’’ (Late Cretaceous); (3) from the Cenozoic, themoderate uplift of the Cuvette borders made remote anypossibility of a marine incursion; (4) kimberlite intrusionsseem to be distributed over the southern, the westernand, possibly, the northern border of the Cuvette duCongo; (5) as paleontological evidence is very scarce, cor-relation of Mesozoic and Cenozoic deposits was based on

well-developed erosion surfaces and on some paleoenviron-mental characteristics; (6) the sedimentary paleoenviron-ments of the Gres Polymorphes Series (�Paleogene)indicate a prevalent eolian deposition, an active silicifica-tion process, and a moderate hydrolysis of the soils hori-zons; (7) the Sables Ocres Series (�Neogene) correspondsto the onset of a fluvial deposit that closely parallels a dra-matic change to wetter conditions recorded by prevalentferrallitic pedogenesis and a general increase in kaolinitethrough time.

Acknowledgements

This paper benefited from critical reviews by Rene Gui-raud (University of Montpellier), Russel Shone (Universityof Port-Elizabeth and Ian Haddon (Council for Geosci-ence, South Africa) on an earlier version and the guest edi-tor Pat Eriksson (University of Pretoria). I especially thankthem for suggesting many improvements to the originalEnglish form.

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mesozoic–cenozoic history of the congo basin

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