Bull. Soc. géol. Fr., 2008, t. 179, no 1, pp. 29-40
U-Pb single zircon grain dating of Present fluvial and Cenozoic aeolian
sediments from Gabon: consequences on sediment provenance, reworking, and
erosion processes on the equatorial West African margin
MICHEL SERANNE1, OLIVIER BRUGUIER1 and MATHIEU MOUSSAVOU2
Key-words. – West Africa margin, Continental margin, Denudation, Cenozoic, Sedimentary sources, Detrital sediment.
Abstract. – U-Pb ages obtained from detrital zircon from terrigenous sediments are used to determine the sources. Present fluvial sand-bars of the Ogooué river yield age spectra of detrital zircons in agreement with Archean and Early Proterozoic sources found in the drainage. The large proportion of Late Proterozoic zircons cannot be derived from primary
erosion of the watershed basement rocks, since there is no formation of that age in the area.
This later group of zircons is in good agreement with reworking of the aeolian Paleogene Batéké Sands, by regressive erosion in the upper reaches of the Ogooué river, as they contain a majority of Late Proterozoic age zircons.
The sources of Late Proterozoic zircons in the Batéké Sand are very distant, and transported and reworked – at least in
part – by aeolian processes. Our results, together with the widely distributed Paleogene sediments over continental Africa, suggests that Paleogene was a time of subdued erosion of the cratonic areas and extensive reworking, transport and
deposition within continental Africa. In contrast, our results from the Ogooué river indicate active present incision of
the cratonic area, erosion of the previous continental sediments, and export of the river bed-load to the continental margin. This temporal evolution of erosion-transport-deposition is correlated with the drastic climate change that occurred
during the Cenozoic, leading to a more efficient mechanical erosion, and it correlates with the increase of terrigenous
flux to the margin, observed during the Neogene.
Datations U-Pb de zircons détritiques d’alluvions actuelles et de sables éoliens cénozoïques
au Gabon : conséquences sur les sources sédimentaires, le remaniement et l’érosion sur la
marge équatoriale d’Afrique de l’ouest
Mots-clés. – Marge Afrique de l’Ouest, Marge continentale, Dénudation, Cénozoïque, Sources sédimentaires, Sédiments détritiques.
Résumé. – Les ages U-Pb obtenus sur zircons détritiques extraits de sédiments terrigènes sont utilisés pour déterminer
les sources. Les zircons issus des barres sableuses fluviatiles actuelles du fleuve Ogooué révèlent un spectre d’âge compatible avec des sources archéennes et paléoprotérozoïques, existant dans le bassin de drainage. La proportion élevée de
zircons néoprotérozoïques ne peut provenir de l’érosion primaire du substratum du bassin versant car il ne comprend
pas de formations de cet âge. Ce dernier groupe de zircons peut provenir du remaniement de la formation éolienne des
Sables Batékés, d’âge paléogène, affleurant à l’amont du bassin versant, car elle contient une majorité des zircons d’âge
néoprotérozoïque. La source des zircons néoprotérozoïques trouvés dans les Sables Batékés sont distantes ; ils ont été
remaniés et transportés par des processus éoliens. À la lumière de la grande extension des dépôts paléogènes sur le
continent africain, nos résultats suggèrent que le Paléogène était une période de très faible érosion du craton pendant laquelle dominaient remaniement et transport de sédiments sur de vastes parties de l’Afrique. Au contraire, nos résultats
sur les alluvions actuelles de l’Ogooué indiquent une incision très active du craton, une érosion des dépôts continentaux
antérieurs, et l’évacuation de la charge détritique des rivières vers la marge continentale. Ce changement de processus
d’érosion-transport-dépôt est corrélé au changement climatique majeur survenu pendant le Tertiaire, induisant une érosion mécanique plus efficace, et correspond à l’augmentation du flux terrigène observé sur la marge continentale, au
cours du Néogène.
INTRODUCTION
The occurrence of large terrigenous depocenters on continental passive margins give rise to the question of sediment
provenance. Erosion of elevated and active orogens can easily account for large sediment flux to continental margins,
as for example the erosion of the active Himalaya feeds the
Indian continental margins and the Indus and Bengal
deep-sea fans (e.g. [Clift et al., 2001]. On the equatorial
west African margin, the size of the Congo deepsea fan and
associated passive margin sedimentary sequences, is surprisingly large with respect to the tectonically stable hinter-
1. Géosciences Montpellier, cc.060, CNRS-Université Montpellier 2, 34095 Montpellier, France. [email protected]
2. Dépt de géologie, Université des sciences et techniques Masuku, B.P. 943, Franceville, Gabon.
Manuscrit déposé le 9 mars 2007; accepté après révision le 3 septembre 2007.
Bull. Soc. géol. Fr., 2008, no 1
30
SÉRANNE M. et al.
land (fig. 1) [Leturmy et al., 2003]. Discussions on the mass
balance bear on 1) long-term geodynamics of continental
margins, allowing renewed uplift of the hinterland [Lucazeau et al., 2003], 2) tracing the respective contribution of
major, versus numerous small rivers [Bentahila et al.,
2006], and 3) on the temporal evolution of sediment flux to
the margin, due to the interaction of climate and tectonics
[Lavier et al., 2001; Séranne and Anka, 2005].
U-Pb ages obtained from detrital zircon from sediments
yield results that can be interpreted in term of signature of
sedimentary sources (e.g. [Bruguier, 1996; Bruguier et al.,
1997; Avigad et al., 2003]). Analysis of zircons from clastic
formations of different ages and different depositional environment, within a river drainage allows : a) to trace the sedimentary sources, b) to document temporal evolution of
the sources, and c) to infer changes in erosion-transport-deposition processes within the margin hinterland.
The Ogooué river basin (fig. 1), adjacent to the giant
Congo river, is the third African fresh-water hydrological
source to the Atlantic [Mahé et al., 1990], and a significant
contributor to the sedimentation of the equatorial west African margin [Mougamba, 1999]. Sandy alluvium of the lower reaches of the Ogooué river gives a representative
sample of the different lithologies found within the drainage, modulated by varying erosion processes. Such fluvial
sediment provides information on the erosion-transport processes that presently feed the equatorial West African margin. Similarly, analyses of the older continental sandstones
formation found in the drainage basin, yield information on
the sources during ancient (Cenozoic) times. The aim of
this contribution is to investigate consequences of the temporal variations of sedimentary sources and the evolution of
the erosional-depositional cycle during the Cenozoic, on the
continental margin hinterland.
GEOLOGICAL AND MORPHOLOGICAL SETTING
The structural framework of equatorial west Africa consists
of an Archean basement (the Congo craton) surrounded by
Early and Late Proterozoic belts, and unconformably overlain by a Phanerozoic sedimentary cover. The latter can be
split into a pre-Cretaceous sequence (with very scarce exposures), and a younger sequence linked to the rifting of
Gondwana. The Cretaceous to Present coastal basin records
the Atlantic rifting and continental margin development
[Reyre, 1984; Teisserenc and Villemin, 1989; Séranne et
al., 1992], while the internal Congo basin (« Cuvette Centrale ») records the long-term evolution of the intra-cratonic
zone [Giresse, 1982; Giresse, 2005].
The morphology of the onshore part of the Gabon continental margin is dominated by a lowland coastal plain occupied by the wide Ogooué delta, downstream of Lambaréné
(fig 2). The Congo craton and Proterozoic orogenic belts
present NW-trending ridges up to 1000 m altitude (Mont de
Cristal and Chaillu). Most of the Ogooué river drainage is
set over this basement. To the East, the edge of the Batéké
plateau makes the water divide between the Ogooué and the
Congo watersheds, and the western boundary of the “Cuvette Centrale”. This relief is gently sloping east, toward
the Congo river, whereas it is highly dissected to the west
by active headward erosion of numerous tributaries of the
Ogooué river. Ogooué drainage basin is about 215,000 km2
and includes the Ivindo and the Ngounié rivers as main
FIG. 1. – Geological map of equatorial West Africa and river drainage that feeds detrital sediments to the continental passive margin. The depocenters along
the margin as well as the Congo and Ogooué deep-sea fans are indicated by an isochron map compiled from seismic data [Emery et al., 1975 ; Anka, 2004].
FIG. 1. – Carte géologique de l’Afrique de l’Ouest équatoriale et des réseaux de drainage alimentant la marge continentale passive. Les dépôts-centres de
la marge ainsi que les éventails sous-marins du Congo et de l’Ogooué sont indiqués par la carte isochrones compilant les données de sismique réflexion
[Emery et al., 1975 ; Anka, 2004].
Bull. Soc. géol. Fr., 2008, no 1
U-PB SINGLE ZIRCON GRAIN DATING OF FLUVIAL AND AEOLIAN SEDIMENTS FROM GABON
tributaries. At the city of Lambaréné, where the river enters
the coastal plain for its final track, it has an average yearly
discharge of 4700 m3/s (although highly variable throughout the year) and an average suspended sediment load of
19.7 106 t/y [Syvitski et al., 2005]. In spite of an average
discharge one order of magnitude smaller than the Congo,
the Ogooué has a comparable sedimentary flux (22.7 106 t/y
for the Congo). These values suggests that the Ogooué watershed is a zone of active erosion and thus a significant present-day source of terrigenous sedimentation on the
equatorial west African continental margin. This might not
have been true throughout geological time (e.g. [Séranne
and Anka, 2005].
The Batéké Sands unconformably overly the Archean
craton and its Late Proterozoic cover, as well as the Late
Proterozoic Panafrican metasediments. River incision in the
Cuvette Centrale shows that they are also unconformable
over Late Cretaceous terrigenous sediments, and are covered by Quaternary fluvial-lacustrine sediments. They belong to the Kalahari system [De Ploey et al., 1968] which
extends across Africa south of the Equator, and they are
correlated with the Cenozoic formations “Continental Terminal” of western Africa [Lang et al., 1990]. Two major sedimentary sequences have been distinguished within the
continental Batéké formation : the Grès Polymorphes and
the Sables Ocres [Cahen, 1954; Le Marechal, 1966]. More
recent unconformable formations are described in the Cuvette Centrale [Giresse and K’vadec, 1971] and in the coastal area of Gabon [Peyrot, 1998] and Congo. These younger
31
unconsolidated sand deposits characterise fluvio-lacustrine
environments more or less reworked by aeolian processes;
they are not present in the studied area. Dating such continental formations is difficult due to the scarcity of preserved fossils and pollen. However there is an agreement on a
post-Late Cretaceous and Pre-Miocene age (i.e. Paleogene)
for the Grès Polymorphes and a Neogene age for the Sables
Ocres [Cahen, 1954; Le Marechal, 1966; De Ploey et al.,
1968].
SAMPLING
Ogooué sands were sampled at Lambaréné (fig. 2), on the
surface of a sand bar in the active bed of the river. It
consists of medium to coarse, well sorted sand, that is classically transported as bed load and accumulated as sand
bars during the two yearly high water periods (May – June
and November – December). Such bars are formed, migrate
and disappear frequently, which indicate that the sediments
sampled are typical of the current sediment bed-load of the
river. Lambaréné being located downstream of all the major
tributaries of the Ogooué, this sample is thought to represent a reasonably good mix of all the different sources of
the drainage area.
Batéké sands were taken from the Lékoni canyon, located
close to the Ogooué eastern water divide (fig. 2). The site is
a tourist attraction due to the circa 150 m-incision by regressive erosion of the steep edge of the plateaux Batéké
(650 m altitude), displaying the reddish-purple to ocre sandstone beds. The unconsolidated sands are being actively
eroded and slope processes feed the Leconi river (tributary
of the Ogooué river) with bed-load material.
The sample comes from the top of the Paleogène Grès
Polymorphes formation (fig. 3), a white to pink, medium,
well sorted and well rounded sand formation that presents
several metres high foresets of aeolian sand dunes. Above
the sample, the yellow to light brown massive sand formation is attributed to the Neogene Sables Ocres, which is not
complete as it is eroded.
ANALYTICAL METHODOLOGY
FIG. 2. – Geological map of the Ogooué drainage basin (thick ticked line)
and surrounding area. Stars indicate the two sampling localities. Small figures are radiometric ages obtained from basement rocks and compiled by
[Caen-Vachette et al., 1988 ; Ledru et al., 1989].
FIG. 2. – Carte géologique du bassin versant de l’Ogooué (en pointillés
gras) et des zones environnantes. Les étoiles indiquent les deux localités
d’échantillonnage. Les âges radiométriques obtenus sur le substratum sont
indiqués [Caen-Vachette et al., 1988 ; Ledru et al., 1989].
Zircon grains from the sand samples were concentrated by
conventional heavy liquids techniques and were subsequently processed by magnetic separation using a Frantz isodynamic separator. Flawless zircons (free of visible inclusions
and fractures) from the least magnetic separates were
hand-picked under alcohol and then embedded in epoxy resin with chips of a matrix-matching standard material
(zircon standard G91500) [Wiedenbeck et al., 1995]. The
mount was then slightly polished to expose the internal part
of the grains, which in the case of the Ogooué sand were
observed by scanning electron microscope (SEM) using the
back-scattered electron (BSE) mode. After repolishing to
remove the carbon coating, the mount was then cleaned
with soap, ultra-pure MQ water and dried with alcohol before its introduction in the ablation cell. U-(Th-)Pb analyses
were performed by laser ablation ICPMS at Geosciences
Montpellier, University of Montpellier 2, following the analytical procedure outlined in Bruguier et al. [2001] and given in earlier reports [Neves et al., 2006]. The spot size of
the laser beam was 51 µm and the laser was operated using
Bull. Soc. géol. Fr., 2008, no 1
32
SÉRANNE M. et al.
an energy density of 15J/cm2 at a frequency of 4 Hz, which
resulted in a c. 40-50 000 cps on the 206Pb isotope (i.e.
3000 cps/ppm of Pb). Pb/Pb and U/Pb ratios of unknowns
were calibrated against the G91500 zircon crystal which
was repeatedly measured and used to correct for interelement fractionation and mass bias. Errors measured on the
standards and on each unknown were added in quadrature to
produce the results quoted in table I. The later reports only
the analyses for which 204Pb was not detected (i.e. no common Pb correction).
RESULTS
Ogooué sand
The results of laser ablation U-Pb analyses of 38 detrital
zircons (44 spot analyses) from the Ogooué sand are shown
in figure 4. The zircon population is constituted by various
morphologies, including rounded, sometimes pitted grains,
and euhedral to sub-euhedral zircons which suggests a varied provenance. The zircon grains analysed have ages ranging from 580±8 Ma (1σ) to 3062±9 Ma (1σ). The age
spectrum is dominated by Archean (2.8-3.1 Ga), Paleoproterozoic (1.95-2.10 Ga) and Neoproterozoic (580-800 Ma)
age peaks and also includes one concordant grain at
2455±10 Ma (1σ). Although the above age groups are well
constrained by concordant analyses, many zircon grains are
discordant, which reflects postcrystallisation disturbances
(see fig. 4). This is consistent with the Scanning Electron
Microscope (SEM) imaging which reveals that some grains
yield complex internal structures such as recrystallized domains or metamorphic overgrowths (fig. 5). Since the spot
size was generally larger than the observed zircon domains,
it is likely that the discordant analyse reflect straddling by
the laser beam of various domains rather than Pb loss from
the crystal lattice. The occurrence of low-U zircons with
high discordant degree (see analyses 1 and 2) is consistent
with this view and pleads against Pb loss enhanced by radiation damage to the zircon lattice [Silver and Deustch,
1963]. These analyses have not been used in the discussion
since discordance makes it difficult to determine the age of
the source rock from which the zircons derived. However
they point to a metamorphic overprint of the source area,
possibly, but not exclusively during the Pan-African
(550-700 Ma) event. In the cumulative probability diagram
of figure 6, where the heavy plain line represents age grouping defined by analyses which are less than ±5% discordant, it can be seen that the main detrital input derived from
Archean rocks with a sharp age peak at c. 2.95 Ga. This
component constitutes 46% of the concordant zircon grains
analysed. The Paleoproterozoic age peak (19%) has a maximum at around 2.0 Ga, whereas Neoproterozoic zircons
(31%) fall in three age groupings at 570-600 Ma,
650-670 Ma and c. 800 Ma.
Batéké sand
A total of 44 analyses of zircons from the Batéké sand has
been performed. Dated grains range in age from Archean to
Neoproterozoic (fig. 7). Three analyses, concordant at
2620±9 Ma, 2669±3 and 2877±8 (1σ), document the occurrence of Archean rocks in the source area. However, in contrast to the present-day Ogooué sand, the age spectrum is
Bull. Soc. géol. Fr., 2008, no 1
FIG. 3. – Synthetic stratigraphic column of the Batéké Sands from the Lékoni locality. The « Grès Polymorphes » Formation consists of fluvial
sandstones and aeolian sand characterised by cross stratifications several
metres high. The top of the sequences are altered, which gives the distinctive multicolored aspect to the formation.
FIG. 3. – Colonne stratigraphique synthétique des Sables Batékés dans la localité de Lékoni. La formation des « Grès Polymorphes » consiste en des
grès fluviatiles et éoliens peu consolidés, caractérisés par des stratifications internes de plusieurs mètres de haut. Le sommet des séquences, altéré
donne la coloration multicolore spécifique de la formation.
dominated by Neoproterozoic zircons, which represent ca.
60% of the analysed grains whereas the proportion of
Archean grains is only 7%. The Neoproterozoic component
yields major age peaks at c. 700-750 Ma and 800-820 Ma,
and a subordinate peak at c. 620 Ma (fig. 8). Other age
peaks include Early Neoproterozoic to Late Mesoproterozoic grains (c. 980 Ma and c. 1100 Ma) and Paleoproterozoic grains at c. 1.8 Ga, c. 2.0 Ga and a tight grouping at c.
2.1 Ga. The youngest concordant grain analysed was dated
at 576±3 Ma (1σ).
TESTING POSSIBLE SOURCES FOR THE
OGOOUÉ SANDS
The Ogooué watershed extends over the Congo Craton formed by Archean and Paleoproterozoic gneisses (fig. 2). It is
therefore expected that erosion of the Congo Craton will
provide a large proportion of Archean zircons. The Archean
gneisses of the Chaillu (southern part of the drainage) the
Mont de Cristal (in the north) of the regional basement,
which have yielded a variety of ages from 2650 to 3090 Ma
[Caen-Vachette et al., 1988; Ledru et al., 1989] account for
the oldest zircon grains found in the Ogooué sands. Several
somewhat younger orogens and magmatic events are found
in the present Ogooué watershed. A regional West Central
African belt, extends from Cameroon to Congo [Ledru et
al., 1989; Feybesse et al., 1998]. The initial stages of activation of the mobile belt between Archean cratons is marked by emplacement of plutonic rocks between 2515 and
U-PB SINGLE ZIRCON GRAIN DATING OF FLUVIAL AND AEOLIAN SEDIMENTS FROM GABON
33
TABLE 1. – U-Th-Pb laser ablation ICP-MS results for zircons from the Ogooué and Batéké sands
All grains were selected from non magnetic separates at full magnetic field in Frantz magnetic separator.
All analyses have 204Pb below detection limits and the quoted ratios are only corrected for Pb/Pb mass bias and U/Pb inter-element fractionation (uncorrected for common Pb). * stands for radiogenic
Disc. (%) is percentage discordance assuming recent lead losses.
TABL. I. – Résultats des analyses U-Th-Pb par ICP-MS ablation laser pour les zircons de l’Ogooué et des sables Batéké. Tous les grains ont été sélectionnés parmi la fraction non magnétique traitée par un séparateur magnétique Frantz. Toutes les analyses ont un 204Pb inférieur au seuil de détection et les
rapports ne sont corrigés que de la discrimination de masse pour le Pb/Pb et du fractionnement du rapport inter élément U/Pb. (*) indique les isotopes radiogéniques. Disc (%) indique les pourcentages de discordance résultant des pertes en plomb récentes.
Bull. Soc. géol. Fr., 2008, no 1
34
SÉRANNE M. et al.
FIG. 4. – Concordia diagram of the laser ablation U-Pb analyses of the detrital zircons from the present alluvium from the Ogooué (38 grains). Boxes 1, 2,
3, focus on the Archean, Paleoproterozoic and Pan African groups.
FIG. 4. – Diagramme Concordia des analyses U-Pb par ablation laser des zircons détritiques des alluvions actuelles de l’Ogooué (38 grains). Les inserts
1, 2, 3 focalisent sur les groupes archéen, paléoprotérozoïque et panafricain.
2435 Ma [Feybesse et al., 1998], and presently cropping out
east of Lambaréné. Erosion of these formations may have
provided the zircon grains dated at 2455±10 Ma. According
to Feybesse et al. [1998], a second stage of plutonic rocks
generation occurred as late-orogenic granites emplacement
between 2040 and 1920 Ma ago. Erosion of these late stage
Eburnean rocks such as the Fougamou and Lecoué granites
(Guerrot and Mayaga-Mikolo in Bouchot and Feybesse
[1996]) is the most probable source of the 1.95 – 2.10 Ga
detrital zircons identified in the Ogooué sands.
From that period onward, the area remained remarkably
devoid of younger plutonic events, but recorded deposition
of the Middle Proteozoic (Francevillian) sediments dated
around 1750 Ma [Weber, 1968]. The later may have reworked older zircons from the neighbouring Congo craton and
mobile zones; but they may also include sediments derived
from further away and therefore, of unknown age.
The Late Proterozoic Pan-African orogeny and its multiple branches straddling the African continent is characterized in the studied area by reactivation of older structures:
the West-Congolian belt reworked older Eburnean (2 Ga)
basement [Maurin et al., 1991] and pre-Pan-African rift-related magmatism (1 Ga) [Tack et al., 2001]. However, the
later plutonic activity is located south of the study area, outside the Ogooué drainage. The sedimentary basin located in
Bull. Soc. géol. Fr., 2008, no 1
the foredeep of the Pan-African belt, exposed within the
Ogooué drainage, may have collected detrital zircons eroded from the thrust nappes of Paleoproterozoic terrains
[Caen-Vachette et al., 1988], but the lack of Pan-African
magmatic activity in the West-Congolian belt, prevented
crystallisation of zircons of Late Proterozoic age.
The source of younger age populations found in the
Ogooué sand (570-800 Ma) is therefore questionable, since
all magmatic and high-grade metasedimentary rock cropping out in the watershed, are significantly older. One possible source for detrital zircons of Pan African age could be
the reworking of younger terrigenous sedimentary formations such as the Cenozoic Batéké Sand, whose 550-750 m
altitude plateau makes the easternmost part of the Ogooué
watershed.
TESTING POSSIBLE SOURCES FOR THE BATÉKÉ
SANDS
Deposition of the Cenozoic Batéké sands partly results from
aeolian transport; therefore, the sources are not restricted to
a well defined watershed. Analyses of the different detrital
zircon age groups suggest possible sources for the sediments.
U-PB SINGLE ZIRCON GRAIN DATING OF FLUVIAL AND AEOLIAN SEDIMENTS FROM GABON
Similarly to the source for Ogooué sands, the Archean
(2.9 and 2.6 Ga) and the Paleoproterozoic (2.1 Ga) age
groups may be derived from the Congo craton that is underlying the Batéké sand formation, and from the Eburnean intrusions in the surrounding mobile belts [Caen-Vachette et
al., 1988; Ledru et al., 1989; Penaye et al., 2004; Lerouge
et al., 2006].
The next age group (1850-1780 Ma), not present in the
Ogooué sands, does not correspond to a major episode of
continental crust accretion in Africa. However, the Adamawa-Yadé domains of Cameroon have yielded numerous Paleoproterozoic ages [Toteu et al., 2004]. The migmatite
rocks presently making the comparatively small basement
exposure of the Lambaréné horst yielded an age of 1840 Ma
[Caen-Vachette et al., 1988]. Further away, in southeastern
Africa, the basement rocks of the Bangweulu block (N.
Zambia, Tanzania, and Zaire) consists of rocks ranging
from 2000 to 1800 [Rainaud et al., 2005].
Similarly to the Ogooué sands, the Batéké sands are
characterised by a marked lack of ages between 1750 and
1100 Ma, in spite of the large occurrence of granitoids of
this age range in Cameroon [Toteu et al., 2004; Tchakounté
et al., 2007] and south-central Africa [Johnson et al., 2005].
Neoproterozoic zircons are by far the most frequently
found in the Batéké sands. Two small subgroups : the 1100
and 970 Ma age peaks, may correspond to the onset of
35
Rodinia break-up, which generated extension-related magmatism in the western edge of the Congo craton [Tack et al.,
2001]. These groups also correlate with magmatic events
documented in Cameroon [Toteu et al., 2006]. The largest
and youngest group, centred on 750 Ma corresponds to the
Neoproterozoic Panafrican orogeny, that led to the assembly of Pangea. Such a widespread orogeny involved thrusting, folding and metamorphisms of marginal sedimentary
sequences and reworked older crustal blocks, but did not
consistently involve syn- to late-kinematic intrusion of granitoids. In particular, the closest Panafrican outcrops (West
Congolian belt and the southern Central African belt, fig. 9)
seem devoid of any granitoid of that age. On the opposite,
the widespread occurrence of syn- and post-kinematic intrusions in the Central African belts of Cameroon yielded Panafrican ages [Toteu et al., 2001]. Other granitoids of
known Panafrican ages are found at a greater distance in the
Damara belt, in south-central Africa [Johnson et al., 2005],
along the Red Sea basement outcrops [UNESCO, 1986] and
in the Hoggar [Caby, 2003]. The actual source of Panafrican
zircons found in the Batéké sands is difficult to identify for
several reasons :
• Due to the fluvial and aeolian sedimentary environment of the Batéké sands, they can be transported either by
the river (and the source can be sought within the Cenozoic
river drainage basin), and/or they can be transported by the
FIG. 5. – Scanning electron microscope (SEM) images of detrital zircons grains from the Ogooué Sands revealing complex internal structures such as recrystallized domains or metamorphic overgrowths. Such succesive zircon growths are probably responsible for discordant ages observed on the Concordia
curve figure 4 (see text for discussion).
FIG. 5. – Images au microscope électronique à balayage (MEB) de zircons détritiques des alluvions de l’Ogooué révélant des structures internes complexes, telles que des recristallisations ou des auréoles de croissance. Ces auréoles sont probablement à l’origine des âges discordants observés sur les
courbes concordias de la figure 4 (voir texte pour discussion).
Bull. Soc. géol. Fr., 2008, no 1
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SÉRANNE M. et al.
wind (and possibly come from outside the river watershed).
The Batéké sands belong to the Cenozoic sandstones covering most of the Congo cuvette; since there are no mapped
Panafrican granitoids within the Congo watershed
[UNESCO, 1986], a long-distance aeolian transport must be
invoked to account for the Neoproterozoic zircons.
• Alternatively, the Batéké sands rework earlier,
post-Panafrican, extensive continental clastic formations,
derived from the erosion of Neoproterozoic terranes. As a
result, zircons reworked from the post-Panafrican sedimentary sequences are mixed with zircons delivered by Paleogene erosion. This especially applies to the extensive
Cretaceous terrigenous formations that are underlying the
Cenozoic sandstones of the Congo Cuvette, and has already
been documented by [De Ploey et al., 1968]. The problem
of the origin of zircons becomes more complex if they
are derived from reworked sedimentary formations of
pre-Atlantic rifting, as South American plate sources should
then be considered. In any case, the Batéké sands are dominantly derived from erosion of several, distant sources, with
a minimum of 500 km (for Cameroon source zone).
IMPLICATION FOR EVOLUTION OF EROSION
PROCESSES DURING CENOZOIC
The contrasted detrital zircon composition of the two studied samples clearly shows a change in the sources for the
host sediment. We interpret this change as characterizing a
change of erosion and transport processes that led to the deposition of the Batéké Sands during the Paleogene and of
the Ogooué Sands in the present time.
The sources of present-day Ogooué sediments are all
within the river watershed. Our results give evidence for active mechanical denudation of the craton, while headward
erosion of the poorly lithified Batéké Sands provides a
FIG. 6. – Cumulative probability diagram of Ogooué Sands analyses. The heavy plain line represents
age grouping defined by analyses which are less
than ± 5% discordant. This defines uneven groups:
almost half of the zircons are Archean in age, one
fifth are Paleoproterozoic and nearly one third are
Neoproterozoic (Panafrican).
FIG. 6. – Histogramme des âges obtenus sur les
zircons des sables de l’Ogooué. La ligne continue
représente les âges définis par les analyses qui présentent moins de 5 % de discordance. Trois groupes
sont ainsi définis : quasiment la moitié des zircons
sont archéens, un cinquième sont paléoprotérozoïques et presque un tiers sont néoprotérozoïques
(Panafricain).
Bull. Soc. géol. Fr., 2008, no 1
secondary source for alluvium. The sources of the Cenozoic
Batéké Sands gives evidence for predominant erosion of the
Panafrican belts and secondary input from Archean craton.
These contrasted sedimentary sources reflect a different distribution of exposed Archean, Proterozoic and Panafrican
terrains. The low proportion of Archean input in the Batéké
Sands may be due to more extensive sedimentary cover of
the cratons during the Paleogene than presently.
Indeed, headward erosion of the Batéké Sand hills to
the east, and of the Atlantic rift margin sediments to the
west [Anka, 2004] suggests that at least the margins of the
Achean cratons were covered until the Neogene. Unpublished data and ongoing studies on apatite fission track analyses [see Anka, 2004] as well as regional studies of the
uplift-erosion of the onshore Atlantic margin [Séranne and
Anka, 2005] document erosional denudation of the craton,
now appearing as a window between the Congo cuvette and
the coastal Mesozoic-Cenozoic basins. In addition, extensive weathering mantles that developed over long-term periods, during the Cretaceous and Paleogene [Tardy and
Roquin, 1998; Gunnell, 2003] may have protected the basement from mechanical erosion.
One drastic difference in the Paleogene vs Present erosion-transport-deposition systems comes from the obvious
observation of extensive Paleogene sediment distribution
over wide areas of continental Africa [Lepersonne, 1961;
Guiraud et al., 2005; Haddon and McCarthy, 2005] which
suggests that depositional processes were dominant over
erosion, and that extensive areas were subjected to deposition or reworking of terrigenous sediments. In contrast,
Quaternary continental deposition is restricted to the lower
part of the Cuvette Centrale and the lowermost Ogooué delta, while incision and erosion processes dominate morphogenesis of most of the region.
U-PB SINGLE ZIRCON GRAIN DATING OF FLUVIAL AND AEOLIAN SEDIMENTS FROM GABON
FIG. 7. – Concordia diagram of the laser ablation U-Pb analyses of the detrital zircons from the Paleogene Batéké Sands (44 grains).
FIG. 7. – Diagramme concordia des analyses U-Pb par ablation laser des
zircons détritiques de la formation paléogène des Sables Batéké
(44 grains).
Alternatively – or possibly in conjunction with – different forcings on the mechanical erosion and weathering processes active during Batéké Sand deposition can account for
the observed change. The increased amplitude of high-frequency climate changes driven by glacial – interglacial successions that occurred during the Neogene, and that
drastically increased in Plio-Pleistocene times, may have favoured mechanical erosion [Séranne, 1999; Molnar, 2004].
Geomorphology of equatorial west Africa clearly indicates
Neogene river incision, and dismanteling of possible old alterite cover [Peyrot, 1998; Tardy and Roquin, 1998].
37
Geochemical analyses of the Congo river at Brazzaville
indicate that present mechanical erosion rates (8 t/km2/y)
outweighs chemical erosion rates (5 t/km2/y) in the upstream watershed [Gaillardet et al., 1995]. Similar climate in
the adjacent Ogooué basin would suggest a similar proportion of mechanical and chemical erosion. However, the relatively large suspended load (19.7 t/y for an area of 0.14
106 km2) presently carried by the river compared with that of
the Congo 22.8 t/y for an area of 3.7 106 km2) [data from
Harrison, 2000; http://www.wsag.unh.edu/index.html] clearly indicate a denudation rate one order of magnitude higher
in the Ogooué than in the Congo watersheds (0.06 mm/y and
0.002 mm/y, respectively). The different proportions of lithologies exposed in the two watershed considered (dominantly
sandy in the Congo, and dominantly cratonic in the Ogooué),
and / or the higher mean slope gradient in the Ogooué basin,
may account for such a discrepancy. The amount and distribution of erosion in equatorial west Africa during the Tertiary has been analysed by extrapolation of geological relict
surfaces [Leturmy et al., 2003]. It shows that the basement
in the Ogooué watershed has suffered denudation ranging
from several hundreds of metres up to 1 km. Such values of
denudation are consistent with the high values gradient for
the coastal rivers (such as the Ogooué) compared with the
low gradient characterizing the Congo tributaries [Leturmy
et al., 2003]. High strontium isotopic ratios from Pleistocene sediments, cored off the Ogooué mouth [Bentahila et
al., 2006], are in agreement with a sedimentary source mostly derived from the Archaean craton underlying the Ogooué drainage, and in strong contrast with the much lower
values characterizing the strontium isotopic ratio of the rivers draining the Batéké sands. Accordingly, low radiogenic
values from the Batéké sands indicate that they have a much
younger source, or includes a significant proportion of a
younger component. These independant sets of data support
FIG. 8. – Cumulative probability diagram of Batéké
Sands analyses. In the Paleogene Batéké Sands,
60% of the analysed zircons are Neoproterozoic
(Panafrican) in age, while the remaining older
grains are spanning the Paleoproterozoic and
Archean.
FIG. 8. – Histogramme des âges obtenus par analyse des zircons des sables Batéké. Dans cette formation paléogène, 60 % des zircons analysés sont
d’âge protérozoïque (Panafricains), alors que les
grains plus vieux sont répartis dans le Paléoprotérozoïque et l’Archéen.
Bull. Soc. géol. Fr., 2008, no 1
38
SÉRANNE M. et al.
present evolution of equatorial west Africa, provide age
spectra that are used to identify the sedimentary sources. In
addition, the sources of the sediments can be used to infer
varying modes of erosion and transport. Results of the study
support the views that erosion and transport processes active in equatorial west Africa have drastically changed during the Cenozoic. In spite of a limited database, which can
be increased and associated with other sedimentary source
tracers, the results are consistent and lead to the following
reconstruction.
During a long period spanning most of the Atlantic
post-rift period, through to the Paleogene, the area was subjected to little denudation, which rarely affected the
Archean basement, and the cratonic areas were the locus of
extensive reworking and deposition of terrigenous sedimentary sequences. Fluvial and aeolian reworking of distant and
varied sources account for the detrital zircon age spectra
(dominated by Neoproterozoic-Panafrican ages) found in
the Batéké sand. The extensive outcrops of continental Cretaceous and Tertiary sequences suggest a rather flat paleogeography with subdued relief (although there is no
indication of elevation).
FIG. 9. – Sketch of the basement map of Africa showing Archean cratons
surrounded by Paleoproterozoic and Panafrican belts. Note that Panafrican
granitoids are not evenly distributed in the Panafrican belts : the possible
sources of zircons of that age are therefore restricted. In addition, presence
of a Phanerozoic cover (stippled areas) further reduces the number of potential sources of detrital zircon in Cenozoic terrigenous sediments.
FIG. 9. – Schéma cartographique du substratum d’Afrique montrant les
cratons archéens entourés des ceintures paléoprotérozoïque et panafricaine. Remarquez que les granitoïdes panafricains sont distribués de manière inhomogène dans les chaînes panafricaines : les sources potentielles
de zircons d’âges correspondants sont restreintes. De plus, la présence
d’une couverture phanérozoïque (zones pointillées) réduit d’autant plus le
nombre de sources possibles de zircons détritiques dans les sédiments terrigènes cénozoïques.
the views that recent (Neogene?) mechanical erosion of
equatorial Africa has become more efficient, has incised
deeper and older basement previously protected, and that
the rate has increased with time [Séranne and Anka, 2005].
CONCLUSION
The U-Pb ages of detrital single zircon grains, from two distinct sedimentary environments related to the Cenozoic to
During the Neogene, the previous Cretaceous to Paleogene sedimentary cover is incised by rivers, and the cratonic
basement is exposed to erosion, whose rate seems to increase steadily until Present. The poor chronostratigraphy
does not allow precise dating of this change, and a higher
resolution sampling would be needed to bracket the change.
Such change occurs in conjunction with the Tertiary climate
change which improved the rate of erosion [Séranne, 1999]
and with renewed uplift of the onshore Atlantic margin and
of the southern part of the African plate [Lavier et al.,
2001]. This evolution of the continental area of equatorial
Africa fits with the sedimentary record of the continental
passive margin off Gabon - Congo - Angola, which displays
an increase in terrigenous sedimentation during Neogene,
and the growth of the Congo deep-sea-fan [Anka and Séranne, 2004; Séranne and Anka, 2005].
Acknowledgements. – Field work was supported by Total. We are gratefull
to Masuku University for the technical and logistical support provided, and
to Bachir Ledounga for his precious help during field work. Reviews by
Pierre Giresse and Sadrack Toteu have helped clarify several points of the
paper.
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