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Sciences, Technologie & Développement ISSN 1029 - 2225

Ngon Ngon et al. Vol. 13, (2012), n°1, 20 – 35 20

Trace elements geochemistry of clay deposits of Missole II from the Douala

sub-basin in Cameroon (Central Africa) : a provenance study

NGON NGON G. F.*, BAYIGA E., NTAMAK-NIDA M. J., ETAME J., NOA TANG S. Department of Earth Sciences, Faculty of Science, University of Douala, P.O. Box 24157, Douala, Cameroon *E-mail : [email protected]

R. YONGUE-FOUATEU Laboratory of Applied Geology-Metallogeny, Department of Earth Sciences, Faculty of Science, University of Yaounde I, P.O. Box 812 Yaoundé, Cameroon

Abstract

Trace and rare earth element (REE) concentrations of the clay deposits of Missole II from the Paleocene-Eocene

N’Kapa Formation in the Douala sub-basin of Cameroon have been investigated to determine their provenance.

To carry out this study, X-ray diffraction and inductively coupled plasma mass spectrometry (ICP/MS) were

performed to determine respectively the mineralogical and chemical data of Missole II clayey materials. Clay

sediments are essentially made up by kaolinite, quartz, illite, goethite, anatase, minor amounts of K-feldspar and

occasionally hematite. The average value of Eu/Eu* (0.5), La/Sc (8.0), Th/Sc (0.99), La/Co (26.9), Th/Co (4.1),

and Cr/Th (8.3) ratios support essentially a felsic rocks source for these clay sediments. Total REE concentrations

of these clay sediments reflect the variations in their grain-size fractions. Chondrite-normalized REE patterns with

LREE enrichment, flat HREE, and negative Eu anomaly are attributed to felsic rocks source main characteristic of

Missole II clay sediments.

Key words: Clay deposits; Cameroon; Douala sub-basin; Missole II; Provenance; Trace elements.

Résumé

Les concentrations d’éléments trace et terres rares des dépôts d’argiles de Missole II de la Formation Paléocène-

Eocène de N’Kapa dans le sous-bassin de Douala au Cameroun ont été étudiées en vue de déterminer leur source

d’apport. Pour effectuer cette étude, la diffraction des rayons X et la spectrométrie par induction couple plasma-

mass (ICP/MS) ont été réalisées respectivement pour les analyses minéralogique et chimique des matériaux

argileux de Missole II. Les argiles sont essentiellement constituées de kaolinite, quartz, illite, goethite, anatase,

d’une petite quantité de feldspath potassique et occasionnellement de l’hématite. La valeur moyenne de ratios de

Eu/Eu* (0.5), La/Sc (8.0), Th/Sc (0.99), La/Co (26.9), Th/Co (4.1) et Cr/Th (8.3) soutient essentiellement une

source de roches felsiques. Les concentrations totales en REE de ces sédiments d’argiles reflètent les variations

de leurs fractions granulométriques. Les diagrammes de REE normalisés aux chondrites présentant un

enrichissement en LREE, de faible HREE et d’une anomalie négative en Eu sont attribués à une source de roches

felsiques, caractéristique principale des sédiments d’argiles de Missole II.

Mots clefs: Dépôt argileux, Cameroun, Sous bassin de Douala, Missole II, Source d’apport, Eléments en trace.

Vol. 13, n°1, 20 – 35



III. INTRODUCTION

In humid tropical region the trace elements

are particularly considered more resistant to

supergene phenomena notably to redistribution due

to alteration, weathering or the sediment

transportation and deposition processes (Mclennan

& Taylor, 1983; Condie et al., 1992; Singh &

Rajamani, 2001a, b). Due to their properties, trace

elements and notably REE have already been

extensively used as tracers of various geochemical

processes (Mclennan et al., 1990; Dupre et al.,

1996; Fralick, 1997; Laveuf et al., 2008; Khawar &

Noor, 2009). For their high field strength (ionic

charge/ionic radius) these elements are useful for

provenance analysis as they are insoluble and

usually immobile under surface conditions

(Bertolino et al., 2007). Because of their typical

behavior during mineral and geochemical

fractionation, weathering and recycling, they

preserve characteristics of the source rocks in the

sedimentary record (Taylor & McLennan, 1985;

Mclennan et al., 1993, 2003).

Immobile elements like Al, Fe, Ti, Th, Sc,

Co, Cr, REEs, and particularly their ratios are useful

tracers of provenance as they are least affected by

processes such as weathering, transport and sorting

(Taylor & McLennan, 1985; Singh, 2009). Th/Sc,

La/Sc, La/Th, Th/Co ratios are especially sensitive

to the nature of source. They are useful to

distinguishing mafic and felsic sources. In fact, Sc

and Co as compatible elements are good tracers of

basic or less fractionated source component

particularly when compared with Th, which is

incompatible and thus enriched in felsic rocks

(Taylor & McLennan, 1985; McLennan et al.,

1990).

Clastic rocks may preserve detritus from

long-eroded source rocks and may provide the only

available clues to the composition and timing of

exposure of such source rocks (Armstrong-Altrin et

al., 2004). Among clastic sediments, clay-bearing

rocks have a much higher concentration of total

trace elements. It is for this reason that the trace

elements and notably the REE content of the clay-

rich sediments are used in order to establish the

sedimentary processes and to identify the

provenance. Based on this idea we have conducted

this study on clay deposits of the Missole II area.

Previous studies in this area were focused mainly on

stratigraphy (Njike Ngaha, 1984; Mooh, 2009;

Fowe, 2010) and on the prospection of the useful

clay deposits (Samba, 2010). Studies about the

geochemistry of clay-rich sediments in the Gulf of

Guinea basins are rare, particularly in the

Cameroonian basins.

This study presents the geochemical

signature of clay deposits of the Missole II area,

which aims at determining the sedimentary

processes in order to discern their provenance.

IV. GEOGRAPHIC AND

GEOLOGICAL SETTING

Missole II is located on the eastern part of

the Douala sub-basin (Cameroon, Central Africa)

between latitude 3°59’-3°54’ N and longitude 9°54’-

9°58’ E (Fig. 1). It is located within a humid

equatorial climatic zone. Annual rainfall ranges

between 3000 and 5000 mm, and the annual average

temperature is 26 °C (Olivry, 1986). The vegetation

is a dense rainforest transformed by the human

activities (Letouzey, 1985). The geomorphology of

the study area is a domain of the Cameroon coastal

plain; it has low altitudes (120-40m). This domain

shows hills with flat and sharp summits and is

deeply dissected by V and U shape valleys of

MBongo, Bongougou, Missolo and Bongo the main

rivers of the area. According to the geological map

of SNH/UD report (2005), the relative age of the

Missole II sediments is Paleocene-Eocene

corresponding to the N’Kapa Formation.



The study area is linked to the opening of

the South Atlantic Ocean. Several studies have been

done on the opening of the South Atlantic Ocean

(Fairhead & Okereke, 1987; Fairhead, 1988;

Guiraud & Maurin, 1991, 1992; Maurin & Guiraud,

1990, 1993; Pletsch et al., 2001). The theories

developed suggest that the West African margin

opened like a “zipper” from south to north and did

not reach Cameroon before the Barremian - Aptian

(Nguene et al., 1992; Meyers et al., 1996; Manga,

2008). By mid Aptian, the Cameroon margin

underwent major structural reconfiguration with the

onset of oceanic transform faulting and their margin

extensions resulting in a segmentation of the rift

structures of the margin (Benkhelil et al., 2002).

The Douala sub-basin represents the

northern part of the Cameroon’s Douala/Kribi-

Campo Basin, which is located in the Gulf of Guinea

between the Cameroon Volcanic Line and the Rio

Muni Basin (Equatorial Guinea). According to

Nguene et al. (1992), Benkhelil et al. (2002),

Lawrence et al. (2002) and SNH/UD (2005) report,

three major episodes of the geodynamic and

sedimentary evolution can be differentiated: (i) the

extensional rift phase in the Early Cretaceous; (ii)

the passage from rift to drift phase marked by the

accentuation of the transformed directions resulting

from a series of cross faults; (iii) the passive margin

wedge during the Late Cretaceous and Tertiary.

The lithostratigraphy of Douala sub-basin

consists of seven major Formations related to the

geodynamic and sedimentary evolution (Regnoult,

1986; Nguene et al., 1992, SNH/UD, 2005). The

syn-rift period represented by the Mundeck

Formation (Aptian-Cenomanian) is discordant to the

Precambrian basement and consists of continental

and fluvio-deltaic deposits (coarse sandstones,

conglomerates). The post-rift sequence includes: the

Logbadjeck Formation (Cenomanian-Campanian),

discordant to the Mundeck Formation and composed

Fig. 1 - Geological sketch map of Cameroonian coastal basins (SNH/UD, 2005)



of sand, sandstone, limestone, clay and

microconglomerates; the Logbaba Formation

(Maestrichien), mainly composed of sandstone, sand

and fossiliferous clay; the N'kapa Formation

(Paleocene-Eocene) is rich in marl, clay with lenses

of sand, fine to coarse crumbly sandstone; the

Souellaba Formation (Oligocene) lying

unconformably on N'kapa deposits and characterized

by marl deposits with intercalations of lenses and

channels of sand; the Matanda Formation

(Miocene), dominated by deltaic facies interbedding

with layers of volcanic deposits and unconformably

overlie all earlier deposits, and the Wouri Formation

(Plio-Pleistocene) which consists of coarse beds,

gravels and sand with a clayey matrix.

V. MATERIAL AND METHODS

Systematic sampling of sediments from

various geomorphic surfaces was done from two

road embankments along the Douala-Edéa road for a

distance of about 1.5 km and one well drilling on the

lower slope of the valley. A geological survey has

permitted the description of the four clayey profiles

in terms of texture, structure, distribution and color.

The color was obtained using a Munsell soil color

chart, and the terminology adopted for the

description was that of Miall (1996) and Postma

(1990).

Thirteen lithofacies were observed in the

three profiles of the road embankments and the

traditional well studied (Fig. 2, Table 1). The eight

clayey samples collected come from the most clayey

material layers (lithofacies F1, F2, F3) of the

profiles, with F3 the most laminated clayey layer.

Different analyses were performed on the

samples collected for mineralogical data at the

chemical laboratory of the University of Limoges

(France), and chemical data at the Geoscience

Laboratories (Geo Labs) of the Ontario Geological

Survey in Sudbury Ontario (Canada).

Mineralogical examinations were carried

out on bulk samples using a Brünker diffractometer

D8 ADVANCE with a copper source (λ = 1.5489 Å)

working under 40 kV and 40 mA. The exposure time

for qualitative analysis was 2 h. Samples were

pulverized with an agate mortar; the resulting

powder was picked up on a piece of tape before

being irradiated with CuKα radiation in the

diffractometer. The resulting diffraction spectra

were compared with a computerized data base of

common minerals, whose automatic mineral-

matching function was assisted by operator

identification of phases consistent with the known

compositions of the materials. Phase proportions

were estimated by the peak matching program

without calibration to synthetic mixtures of known

phase proportions. Semi quantitative analysis was

performed according to Charkravorty & Ghosh

(1991).

Trace and rare earth elements were

determined on bulk material by inductively coupled

plasma mass spectrometry (ICP/MS). Powders were

previously rusted then mixed with lithium

tetraborate before analyzing it with an ICP-MS

instrument, type Perkin - Elmer Elan 9000, for

lanthanide analyses. The IM-101 ICP-MS is a

lithogeochemical package that focuses on REE,

LILE, and HSFE in which the trace elements are

calibrated against solutions made up from single or

multi-elemental solution standards. The instrumental

precision of almost all lanthanides was above 5%

(2σ) for either all or 5 of the 6 compiled solutions

where the elements were above the limit of

quantification (LLoQ). Where the concentrations

approached the LLoQ (e.g., La and Pr in the trace-

element poor basalt standard BIR-1, or Eu in the

rhyolite standard RGM), the error increased is

between 5 and 8.5% (Burhnam & Schweyer, 2004).



Facies code Descriptions Colour

S0 Humiferous layer 5YR2/1

S1 Yellowish sandy-clay (2 - 6 m thickness) 5Y7/6

S2 Reddish sandstones and micro conglomerates bed 5R4/2

S3 Rusty and fine- to medium-grained sandstones (0.5 – 1 m thickness) 5R2/2

S4 Orangey yellow sandy-clay (1 – 2 m thickness) 5Y7/6

S5 Reddish micro conglomerates bed with gravels of quartz and ferruginous (1 – 2 m thickness)

5R2/2

S6 Yellowish grey medium to coarse sandstones 5Y8/1

S7 Fine yellowish grey sandstones 5Y7/6

S8 Yellow sandy-clay with gravels of quartz and some reddish fragments of ferruginous duricrust (2 – 3 m thickness)

5Y6/4

S9 Reddish ferruginous duricrust bed (1 m of thickness) 5Y2/2

F3 Purplish grey laminated clay with muscovite and some yellowish and reddish spots (2 – 4 m thickness)

5R4/2

F2 Grey silty-clay with yellowish, reddish and purplish spots 5Y6/1

F1 Mottled silty-clay with reddish, yellowish and greyish spots (3 - 4 m of thickness)

5Y6/1

Fig. 2 - Profiles of the road embankments and the valley. - (a) Profile of the lower slope (interfluves with altitude 60m); - (b) Profile of the upper slope (interfluves with altitude 60 m); - (c) Profile of the middle slope (interfluves with altitude 80 m); - (d) Profile of the valley. Samples were collected in the clayey material layers of F1 (M2A2b, M2P3b, M2P4a, M2P4b), F2 (M2A2a and M2P3a), and F3 (M2A3a and M2A3b) of the profiles.

Note that F1 and F2 appear at the base of the profiles of the road embankments in the interfluves with altitude 60 m, while F3 only appears in the profile of the valley with altitude 40 m.

Table 1. Facies descriptions of the Missole II representative profiles



VI. RESULTS AND DISCUSSION

4.1 Mineralogy

The bulk mineralogy of sediment samples

from the Missole II clay deposits has essentially

kaolinite > quartz > illite > goethite > anatase minor

amounts of K-feldspar and occasionally hematite

(Fig. 3). The clay mineralogy is similar in all

samples but differ in proportions. It is characterised

by poorly crystallised kaolinite (50 – 72%) and illite

(5 – 15%). This mineral suite reflects the

mineralogy of regional soils where kaolinite is the

major component (Segalen, 1995).

Nicolas (1957) and Roberts (1958) showed

that kaolinite as abundant mineral of these sediments

comes from slow decomposition of feldspars and

others rocks in sharp milieu during geological times.

Generally, kaolinite is found in nature within clayey

material in relation with iron hydroxides, quartz and

micas. The presence of illite and goethite in these

sediments characterises humid conditions.

Fig. 3 - XRD patterns of bulk clay samples from Missolle II area. A: Anatase; G: Goethite; He: Hematite; Il: Illite; K: Kaolinite; Q: Quartz; F: K-feldspar.



4.2 Trace elements geochemistry

Trace element concentrations of Missole II

clay sediments are reported in Table 2. In

comparison with average PAAS set (Post-Archaean

Australian average shale after Taylor & McLennan,

1985), the clay sediments are depleted in many

elements notably Cs, Rb, Sr, Cu, Co, Ba and Ni, and

enriched in Ta, Zr, Hf, Nb, Pb. In general, similar

values to PAAS are found for Cr, Ga, Th, U and V

(Fig. 4a).

0,01

0,1

1

10

100

Ba Co Cr Cs Cu Ga Hf Nb Ni Pb Rb Sr Ta Th U V Zr Sc Y

Sam

ple

/PA

AS

M2A2a

M2A2b

M2A3a

M2A3b

M2P3a

M2P3b

M2P4a

M2P4b

Table 2. Trace element concentrations (in ppm) of the Missole II clay sediments

Profiles of the road embankments Profiles of the valley Profiles a b c c M2A2a M2A2b M2P3a M2P3b M2P4a M2P4b M2A3a M2A3b PAAS

Depth (m) 6.5 7.5 7 8 7.5 8 0.5 1

Ba 158.5 164.2 183.7 116.5 121.5 120.8 167.5 187.4 650

Co 3.17 3.02 5.18 4.77 6.73 5.94 2.57 2.54 23

Cr 108 141 153 123 183 154 110 89 110

Cs 0.376 0.389 0.584 0.552 0.57 0.548 0.36 0.417 15

Cu 8.6 9.2 6.9 13 15.1 15.9 4.5 4.5 50

Ga 23.61 24.4 28.64 23 30.09 27.64 22.15 20.71 20

Hf 12.41 12.97 16.44 16.83 14.69 13.86 12.31 13.36 5

Nb 31.204 31.281 50.464 44.653 42.37 41.171 29.325 28.517 19

Ni 12.2 11.7 20.5 19.2 27.2 24.8 7.2 7 55

Pb 33.4 34.7 144.6 46 27.5 24.3 29.4 26 20

Sc 14.9 15.6 14.7 18.5 20.4 21.1 13.5 13.3 16

Rb 11.88 12.22 11.83 9.69 10.75 10.82 11.03 11.8 160

Sr 31.2 30.6 110.1 36.9 31.7 27.5 18.8 17.1 200

Ta 1.9 1.925 3.067 2.809 2.64 9 2.556 1.881 1.834 0.026

Th 12.874 13.542 22.266 17.924 20.536 19.049 10.054 14.425 14.6

U 1.874 1.99 2.659 2.588 2.753 2.608 1.789 2.054 3.1

V 184 235.2 136.5 163.2 255.9 215.2 142.4 107.7 150

Y 8.62 8.68 17.28 19.47 10.21 8.94 39.92 20.65 27 Zr 482 498 642 637 561 534 469 503 210 For comparison average values are shown for sedimentary rocks of PAAS after Taylor and McLennan (1985).

Fig. 4 - (a) PAAS normalized traces patterns



The results of REE analysis are given in

Table 3 and are shown as chondrite-normalized

patterns and PAAS-normalized patterns in figures

4(b) and 4(c) respectively. ∑REE concentrations

vary widely in Missole II clay sediments (215.28-

1498.67 ppm). All analyzed samples have ∑REE

abundances higher than the average PAAS (185.3,

Taylor & McLennan, 1985). The LREE are enriched

relatively to PAAS, whereas the HREE are

impoverished. The abundances of REE

concentrations in clay sediments are partly due to

the presence of the REE-bearing minerals (as illite).

However, clay sediments of the profiles of the

interfluves are generally richer in REE than those of

the valley (respectively 259.46-1498.67 ppm against

215.28-299.52 ppm). In clay layers, REE content of

the profiles of the interfluves increase from the base

to the top while they increase inversely from the top

to the base in the profile of the valley (Table 3). This

observation shows the influence of the water

movement in clay sediments of the valley, which

transported trace elements towards the base of the

profile. Most clay sediments show similar chondrite-

normalized patterns with high LREE concentrations

and (La/Yb)N ratios ranging between 13.2 and 34.0.

The sample M2P3a exhibit a more distinct pattern

due to much higher enrichment in LREE and higher

fractionation between light and heavy REEs, with a

(La/Yb)N ratio of 114. Also, samples show negative

Eu anomalies (Fig. 4b) with (Eu/Eu*)N ratios

ranging between 0.58 and 0.65, and no Ce anomaly,

with the exception of one sample (M2A3a) with

slight negative Ce anomaly (0.81). Middleburg et al.

(1988) suggested that significant REE fractionation

occurs during the advanced stages of weathering. In

fact, high fractionation observed in Missole II clay

sediments is due to the weathering materials. Also,

Braun et al. (1993) showed that LREE enrichment

in clay sediments should be resulted from weathered

materials. This LREE enrichment and HREE

depletion in Missole II clay sediments characterize

some weathered materials of South Cameroon

basement (Bayiga et al., 2011).

4.3 Provenance

REE, Th, and Sc are most useful for

inferring crustal compositions, because their

distribution is not significantly affected by

diagenesis and metamorphism and is less affected by

heavy-mineral fractionation than that for elements

such as Zr, Hf, and Sn (Cullers et al., 1979; Bhatia

& Crook, 1986; Wronkiewicz & Condie, 1987; Cox

et al., 1995; Mclennan, 2001; Armstrong-Altrin et

al., 2004). REE and Th abundances are higher in

felsic than in mafic igneous source rocks and in their

weathered products, whereas Co, Sc, and Cr are

more concentrated in mafic than in felsic igneous

rocks and in their weathered products (Armstrong-

Altrin et al., 2004). Furthermore, ratios such as

Eu/Eu*, La/Sc, Th/Sc, La/Co, Th/Co, and Cr/Th are

significantly different in mafic and felsic rocks

source and can therefore provide information about

the provenance of sedimentary rocks (Cullers et al.,

1988; Wronkiewicz & Condie, 1989; Condie &

Wronkiewicz, 1990; Cullers, 1994). In this study,

those ratios of the Missole II clay sediments are

similar to the values of sediments derived essentially

from felsic rocks source (Table 4), suggesting that

these clay sediments probably were derived from

felsic rocks source.

However, for fractionated crust, Th/Sc,

Th/Co, La/Sc ratios are high and for mafic rocks

they are low. Typically, for post Archaean UCC the

ratio of Th/Sc is ~ 1, for granitic rocks it is higher

and for Archaean and mafic it is less than 1 (Singh,

2009).



Table 3. Rare earth element concentrations (in ppm) of the Missole II clay sediments

Profiles of the road embankments Profile of the valley

Profile a b c d

M2A2a M2A2b M2P3a M2P3b M2P4a M2P4b M2A3a M2A3b PAAS

Depth (m) 6.5 7.5 7 8 7.5 8 0.5 1

La 80.94 79.27 354.29 105.27 79.93 64.43 60.99 68.15 38

Ce 159.79 155.29 808.66 228.99 163.5 130.57 95.54 137.81 80

Pr 14.658 14.004 73.868 22.186 15.54 12.549 8.373 14.159 8.9

Nd 40.23 37.11 219.56 71.17 47.89 39.26 27.29 49.78 34

Sm 3.806 3.693 20.967 8.286 4.733 4.181 4.587 9.523 5.6

Eu 0.725 0.708 3.31 1.535 0.792 0.708 0.909 1.851 1.1

Gd 2.026 1.968 7.858 4.542 2.202 2.038 3.983 6.29 4.7

Tb 0.314 0.308 0.817 0.625 0.329 0.292 0.642 0.884 0.8

Dy 1.992 1.946 3.946 3.801 2.066 1.856 4.51 4.933 4.7

Ho 0.39 0.388 0.679 0.778 0.437 0.377 1.063 0.866 1

Er 1.226 1.251 1.977 2.378 1.384 1.228 3.303 2.387 2.9

Tm 0.212 0.216 0.3 0.364 0.225 0.2 0.475 0.341 0.4

Yb 1.609 1.664 2.096 2.538 1.695 1.521 3.124 2.218 2.8

Lu 0.256 0.269 0.341 0.401 0.275 0.246 0.489 0.325 0.43

∑REE 308.17 298.08 1498.67 452.86 320.99 259.46 215.28 299.52 185.33

LREE 299.42 289.37 1477.35 435.90 311.59 250.93 196.78 279.42 172.3

HREE 8.75 16.73 21.31 16.96 9.40 8.46 18.49 20.09 13.03

LREE/HREE 34.22 17.30 69.31 25.69 33.13 29.64 10.64 13.91 13.22

(Ce/Ce*) N 1.00 1.01 1.13 1.07 1.02 1.01 0.81 0.98 1.02

(Eu/Eu*) N 0.65 0.65 0.58 0.64 0.59 0.59 0.62 0.65 0.66

(La/Yb)N 33.99 32.19 114.22 28.03 31.87 28.62 13.19 20.76 9.2

Abbreviation normalization of REE to chondrites after Taylor and McLennan (1985)



1

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Sam

ple

/Ch

ond

rite

M2A2a

M2A2b

M2A3a

M2A3b

M2P3a

M2P3b

M2P4a

M2P4b

Fig. 4 - (b) Chondrite normalized REE patterns

Fig. 4 - (c) PAAS normalized REE patterns of the Missole II clay deposits (after Taylor and McLennan, 1985).

0,1

1

10

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Sam

ple

/PA

AS

M2A2a M2A2b M2A3a M2A3b

M2P3a M2P3b M2P4a M2P4b



Th/Sc values for the studied samples vary

between 0.74 and 1.08, with the exception of one

sample (M2P3a) with Th/Sc=1.51 that suggest

influence of a granitic source. All the other samples

have Th/Sc values similar to that of the UCC (0.75,

Taylor & Mclennan, 1985) and Paas (0.91).

Incompatible trace element abundance of shale

reflect that of the average upper-continental crust,

but lower in absolute abundances due to the

presence of sediments with lower Th, REE and

other trace element abundances such as sandstones,

carbonates, and evaporates. Therefore, the

variability found in Missole II clays may be related

with granulometry and the abundance of quartz that

act as a dilute of some trace element abundances.

However, Th/Sc vs Zr/Sc can be used to observe

igneous differentiation but also to see sediment

recycling. Fig. 5 showed that Missole II clayey

materials are plotted to the UCC source and also

shows sediment recycling (McLennan et al., 2003).

Moreover, the relative REE patterns,

(La/Yb)N and the size of the Eu anomaly also have

been used to infer sources of sedimentary rocks

(Taylor & McLennan, 1985; Wronkiewicz &

Condie, 1987). They are useful to differentiate the

mafic and felsic source. Felsic rocks generally show

fractionated chondrite normalized REE patterns

with higher (La/Yb)N ratios and prominent Eu

anomalies, in contrast the mafic rocks have less

fractionated chondrite normalized REE pattern with

low (La/Yb)N ratios and little or no Eu anomalies

(Taylor & Mclennan, 1985). Clay sediments in this

study show strongly fractionated chondrite

normalized REE patterns with higher (La/Yb)N

ratios with averages 16.78 and 44.82 (Table 3) for

clay sediments of the profile in the valley and the

profiles of the interfluves respectively, and

prominent Eu anomalies (0.62 in average), suggest

felsic rock source as those from Tertiary and

Precambrian crystalline environment of the South

Cameroon coastal plain basement;

Profiles of the road embankments Profile of the valley

Profile a b c d

M2A2a M2A2b M2P3a M2P3b M2P4a M2P4b M2A3a M2A3b PAAS Depth (m) 6.5 7.5 7 8 7.5 8 0.5 1

Th/U 6.87 6.81 8.37 6.93 7.46 7.30 5.62 3.56 4.70

Th/Co 4.06 4.48 4.30 3.76 3.05 3.21 3.91 5.68 0.63

Cr/Th 8.39 10.41 6.71 6.86 8.91 8.08 10.95 6.17 7.53

Zr/Sc 32.35 31.92 43.67 34.43 27.5 25.31 34.74 37.82 13.13

Th/Sc 0.86 0.87 1.51 0.97 1.01 0.90 0.74 1.08 0.91

La/Co 25.54 26.25 68.40 22.07 11.88 10.85 23.73 26.83 1.65

La/Sc 9.39 9.13 20.50 5.41 7.83 7.21 1.53 3.30 2.40

La/Th 6.29 5.85 15.91 5.87 3.89 3.38 6.07 4.72 2.60

Table 4. Element ratios



Table 3. Rare earth element concentrations (in ppm) of the Missole II clay deposits

Profiles of the road embankments Profile of the valley Profile a b c d

M2A2a M2A2b M2P3a M2P3b M2P4a M2P4b M2A3a M2A3b PAAS

Depth (m) 6.5 7.5 7 8 7.5 8 0.5 1

La 80.94 79.27 354.29 105.27 79.93 64.43 60.99 68.15 38

Ce 159.79 155.29 808.66 228.99 163.5 130.57 95.54 137.81 80

Pr 14.658 14.004 73.868 22.186 15.54 12.549 8.373 14.159 8.9

Nd 40.23 37.11 219.56 71.17 47.89 39.26 27.29 49.78 34

Sm 3.806 3.693 20.967 8.286 4.733 4.181 4.587 9.523 5.6

Eu 0.725 0.708 3.31 1.535 0.792 0.708 0.909 1.851 1.1

Gd 2.026 1.968 7.858 4.542 2.202 2.038 3.983 6.29 4.7

Tb 0.314 0.308 0.817 0.625 0.329 0.292 0.642 0.884 0.8

Dy 1.992 1.946 3.946 3.801 2.066 1.856 4.51 4.933 4.7

Ho 0.39 0.388 0.679 0.778 0.437 0.377 1.063 0.866 1

Er 1.226 1.251 1.977 2.378 1.384 1.228 3.303 2.387 2.9

Tm 0.212 0.216 0.3 0.364 0.225 0.2 0.475 0.341 0.4

Yb 1.609 1.664 2.096 2.538 1.695 1.521 3.124 2.218 2.8

Lu 0.256 0.269 0.341 0.401 0.275 0.246 0.489 0.325 0.43

∑REE 308.17 298.08 1498.67 452.86 320.99 259.46 215.28 299.52 185.33

LREE 299.42 289.37 1477.35 435.90 311.59 250.93 196.78 279.42 172.3

HREE 8.75 16.73 21.31 16.96 9.40 8.46 18.49 20.09 13.03

LREE/HREE 34.22 17.30 69.31 25.69 33.13 29.64 10.64 13.91 13.22

(Ce/Ce*) N 1.00 1.01 1.13 1.07 1.02 1.01 0.81 0.98 1.02

(Eu/Eu*) N 0.65 0.65 0.58 0.64 0.59 0.59 0.62 0.65 0.66

(La/Yb)N 33.99 32.19 114.22 28.03 31.87 28.62 13.19 20.76 9.2

Abbreviation normalization of REE to chondrites after Taylor and McLennan (1985)

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

0 10 20 30 40 50

Th

/Sc

Zr/Sc

upper continental crust sediment recycling

mantleClay sediment

Fig. 5 - Th/Sc vs Zr/Sc diagram to reveal the main source composition (after McLennan et al., 1990).



V. CONCLUSION

The mineralogy of clay deposits of Missole

II from lower slope in valley and interfluves are

made up by kaolinite, quartz, illite, goethite, anatase,

minor amounts of K-feldspar and occasionally

hematite. Many trace elements, such as Cs, Rb, Sr,

Ba, Co, Cu and Ni are depleted in the clay sediments

relatively to PAAS, whereas others, mainly Ta, Hf,

Zr and Nb, are enriched. REE present similar trends

with high LREE and low HREE, and are

systematically enriched in clay deposits. Chondrite-

normalized REE patterns show negative Eu

anomalies and high fractionation between LREE and

HREE. The LREE are enriched in relation to PAAS,

but the HREE present lower concentrations. Th/Sc,

La/Sc, La/Th, Th/Co and Cr/Th ratios showed clay

sediments essentially derived from felsic rocks

source when fractionated chondrite normalized REE

patterns also indicate felsic rocks source.

ACKNOWLEDGEMENTS

The authors are grateful to the staff of the chemical

laboratory of the University of Limoges, France and

chemical data at the Geoscience Laboratories (Geo

Labs) of the Ontario Geological Survey in Sudbury

Ontario (Canada).

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