International Journal of Geosciences, 2011, 4, 640-647
doi:10.4236/ijg.2011.24065 Published Online November 2011 (http://www.SciRP.org/journal/ijg)
Mechanical Behavior of Pillow Lavas in Mako
Supergroup: Case of South Mako Hill
Déthié Sarr1, Meissa Fall1, Papa Malick Ngom2, Mapathé Ndiaye1
1Laboratoire de Mécanique et Modélisation-UFR Scienc es de l’Ingénieur, University of Thiès, Thiès, Senegal
2Département de Géologie, FST-UCAD, Université Cheikh Anta Diop of Dakar, Dakar, Senegal
Received July 9, 2011; revised August 24, 2011; accepted October 3, 2011
Abstract
This work shows the Kédougou-Kéniéba inlier (eastern Senegal) pillow lavas behavior from laboratory to
field. Some uniaxial tests are carried out on five types of specimens of pillow lavas. These types of speci-
mens are: microscopically healthy rock, fractured rock without filling, fractured rock filled with epidote,
chlorite and calcite and rocks with tension crack filled with quartz. The Young moduli and the uniaxial com-
pression strength are good for the healthy rock. The Young moduli fall slightly for facies with horizontal
cracks while uniaxial compression strength (Rc) varies slightly. For filled fractured specimens, Rc and Young
modulus (E) decrease remarkably. Decreases are most important for cracks filled with epidote, chlorite and
calcite than with quartz. That is due to the differences of rigidity between these materials. Also, the slope
stability of hillsides in this area depends on to these characteristics.
Keywords: Unconfined Compression Test—Uniaxial Compression Strength (UCT, Rc), JRC (Joint Rough-
ness Coefficient), Young Modulus (E), Roughness, Kédougou-Kéniéba Inlier, Lineaments, Dis-
continuities, Dihedral, Slope, Hillside
1. Introduction
The Kédougou-Kéniéba inlier has long been subject of
geological research and significant gold’s reserves are
found. Mains mining companies expand over and started
the gold extraction. It therefore becomes necessary to
know the rocks behaviors in this area. Basalts are mostly
cases the mineralized host rock formations. Knowledge
of the mechanical behavior of this rock became priority.
The pillow lavas are the basaltic facies more represented
in this area. It outcrops very well in this area with rela-
tively the same geological characteristic. So we choose
the Badian hill to conduct geomechanical studies on this
specimen. This hill is crossed by a very dense network of
fractures and tension cracks filled or not by secondary
crystallization. Some of these discontinuities may be
recognized in the field but for others, microscopic ob-
servations are necessary. In this paper, we will highlight
the impact of these fillings on the rocks behaviors. The
last paragraph of this paper consists to analysis of hill-
sides stabilities. For this, we are going to use the stereo-
graphic method.
2. Geological and Geographical Context
Proterozoic formations are located in the far south east of
Senegal (Figure 1(e)). This area is crossed by the Gam-
bia River and the Falémé (Senegal River flowing). It cli-
mate is soudano-sahelian. Birimian formations of eastern
Senegal are localized in this inlier (Figures 1(a) and (b))
which is a portion of the West African Craton (Figures
1(c) and (d)). This inlier consists of paleoproterozoic
formations surrounded by more recent formations (Upper
Proterozoic). Bounded on west by the Mauritanides chains,
in the east by the ”Plateau Mandingue” and on south by
the Medina Kouta basin, it subdivided in two Supergroups
[1]:
 The Mako supergroup located in the west part of the
inlier and dominated by volcanic formations. Many li-
tho- stratigraphic models are given by various authors
[2,5-11] to explain this volcanic domain.
 The Supergroup of Dialé-Daléma, which includes the
old series of Dialé and Daléma defined by Bassot [1,2].
It represents the eastern area of the inlier and is domi-
nated by sedimentary rock [1,2,12-14].

D. SARR ET AL.
641
Figure 1. Kédougou-Kéniéba inlier in the African framework [2,3,4 modified] (a) Kédougou-Kéniéba inlier; (b) Eastern
Senegal; (c) West African Craton; (d) African Cratons; (e) Location of Kédougou-Kéniéba inlier.
Figure 2. Map of lineaments of the sector.
3. Mechanical Characterization
3.1. Field Data
3.1.1. Structural Data
By this map, we can remark that the whole area is cross-
ed by fracturations. In many cases, they appear like
schistosity of fracture but in little case we can have a
crenulated schistosity. It is the case in the rhyolitic tufs
showing a microfolding. Orientations of fractures are
NE-SW, N-S, NW-SE or approach to N-S and E-W axes.
Socking are also identify in the north east of Mako (in
the volcano-sedimentary rocks). Foldings are also pre-
sents under the form of straight folding and sometimes
inclined and knee folds. For igneous rocks, folds can be
remark after analyses of the schistosity of fractures or the
orientation of the axe of pillow lavas.
Measurements of mechanical parameters are carried
on the hill located on the interface of Badian-Bafoundou-
Maragoukoto and data are regrouping by discontinuities
affinities. So, these data make JRC, spacing and frequen-
cies of discontinuities.
3.1.2. JRC, Spacing, Frequencies and Opening of
Discontinuities
The JRC (Joint Roughness Coefficient) is estimated by
comparing the profile of the discontinuities with Barton
and Choubey charter [15-17]. It is given by the following
relation where JRCn is the exact value, (JRC0) the refer-
ence value, (Ln) the measured length and (L0) the refer-
ence length:
0
0,02
00
J
RC
n
nL
JRCJRC L





D. SARR ET AL.
642
The spacing between discontinuities is defined by
tracing a scaline and measuring the distance between
different discontinuities. From these data, we deduce the
spacing as the average distance between discontinuities.
Frequency (
f
) is defined by the following function
where X is the average distance:
1
f
X

Given the range of these JRC (Table 1), we can see
that we have smooth to slightly rough discontinuities.
This more or less smooth aspect of the discontinuities
confer them remarkable power of shear. So a small tan-
gential strength can caused displacement of joint. How-
ever for filling discontinuities, filled materials confer
them a greatest ability to resist to the strength. In fact, the
units located on both sides of cracks are welded like a
single entity. The filling materials are quartz, chlorite,
calcite or epidote.
In the field where discontinuities are great extension,
important sinuosities contribute to add the ability of the
rock to resist shearing strength caused by the imbrica-
tions of the bloc.
The Table 2 show that spacing of discontinuities are
very narrow to narrow for cracks, narrow for tension
cracks and moderate for fractures according to the ISRM
classification [18,19]. Opening s of the discontinuities are
variable. It varies from 7 to 15 µm for micro discontinue-
ties, 0.1 to 0.5 for cracks and 3 to 4 cm for tension cracks.
3.2. Experimental Data
3.2.1. Petrographic Data
Petrographic data on rocks and discontinuities are first
defined on the field and then by microscoscopic observa-
tion. Samples were recovered on the hill. The field ob-
servations show that the hill is cross by a large set of
tectonic structures. These pillow lavas show in micros-
copy, porphyric microlitic texture with microlite of pla-
gioclase and included pyroxene (Figures 3(a) and (b)).
The plagioclases show polysynthetic macle. These micro
discontinuities are either tension cracks filled or unfilled
fractures (Figure 4).
Table 1. Ranges JRC to discontinuities.
Type of Joint JRC values
Tension cracks 3 - 4
Unfilled fractures 2 - 5.2
Filled fractures 1 - 3
Cracks 0.5 - 1.5
Table 2. Ranges of variation of spacing and frequency.
Type of joint Spacing (m) Frequency
Tension cracks 0.08 - 0.11 9.1 - 12.5
Unfilled Fractures 0.11 - 0.30 3.3 - 9.1
Cracks 0.02 - 0.11 9.1 - 50
(a) (b)
Figure 3. Identification of minerals on the basalt (3.a healthy rock; 3.b fractured rock).

D. SARR ET AL.
643
Some of these fractures are filled with white crystalli-
zation recognized in microscopic analyses as calcitic and
epidotitic natures. Green crystallizations in the filonnets
of rock are chlorite and epidote. Calcite can be recog-
nized by it iridescent app earance. The surface of the prepa-
ration is dusty characteristic of the low metamorphism
grade. The calcite, epidote and chlorite reflect metamor-
phism in the green schist facies and an initial composi-
tion of basalts rich in calcium. Veins and veinules reflect
hydrothermalism.
3.2.2. D evices an d Experim en t a l Procedure
This device consists of a press type compression «Tinius
Olsen» with capacity varying between 12 kN to 600 kN
Depending on the material and experiences carried gauge
will be chosen. So we used the gauge 300 kN. In this
class, the ring of load includes major tick marks ranging
from 10 to 10 kN secondary and tertiary graduations are
respectively 0.5 and 0.25 kN. The first step of the ex-
perimentation is to carry out a series of tests so as to
choose the adapted loading speed to carry the test for a
time greater than or equal to 5 minutes. Second step con-
sists in applying gradually load to parallelepipedic sam-
ples. The specimen is placed between top and bottom
plates of the press in such a way to ensure that loading
speed is constant throughout the all test. The reading of
stress and corresponding displacement is made every 45
second.
3.2.3. Results and Discussions
Analysis of different experimental curves of these basalts
shows variations of Young’s moduli and compressive
strengths. The tests results are summarizing in Table 3.
Highest Young moduli are obtained with healthy ba-
salts (Figure 5(a)) without visible discontinuities. Thus,
the average modulus of the specimens is 13351 MPa.
After, we have successively the modulus of unfilled dis-
continuities (Figure 5(b)), tension cracks and microcraks
filled with quartz (Figure 5(c)), multidirectional cracks
(Figure 5(d)) filled with calcite and epidote (Figure
5(e)). The Young moduli (MPa) are respectively 12566,
10300, 9833 and 6647. The highest uniaxial compressive
strength are obtained for multidirectional cracks, fol-
lowed by the filled cracks and the healthy rock with rela-
tive same value of uniaxiale compressive strength (85
and 86.7 MPa) and the tensions cracks (75 MPa). The
weaker facies is corresponding to crack-basalts filled
with calcite and epidote (64.4 MPa).
These facts are due to of cracks present in the rock.
Indeed, microscopic analysis shows that our basalts are
micro-fractured. First, the mineralogy of these basalts
shows microliths of plagioclases and porphyric pyrox-
enes transformed partially to epidote and chlorite. These
micro-discontinuities affect mechanic parameters of the
rock.
For healthy basalts microstructures not affect compari ng
other specimens the Young moduli. The microdiscontinui-
ties are corresponding to macle, cleavage and veinlet pre-
sent in rock can affect the Young modulus. These micro-
discontinuities are also present to the other facies. But for
the uniaxial compressive strength a lot of quantities of
microfracturation limit the propagation of the fractura-
tions.
The second facies with upper modulus is the unfilled
cracks. For this facies, specimens are divided into two
parts separate by the discontinuity plan. So, for carrying
the test, we superpose the two parts. As these fracture
have a certain roughness that ensure good adhesion of
both parts and the load apply is normal, the fracturations
affect the Young modulus of material. Thus, the charac-
teristics involved in the variation of modulus are rela-
tively same as the first sample.
In third, we have the tension cracks filled with quartz.
Quartz is harder than o ther minerals o f th e rock . And th is
hardness of the quartz comparatively to plagioclase and
pyroxene caused deviation of charge during the load and
affect most pyroxene and plagioclase with micro discon-
tinuities like macle and cleavage. This is felt also if the
compressive strength is weak.
Table 3. Compression test results.
Parameters E (MPa) Rc (MPa)
Healthy basalts 13,351 85
Unfilled fractures 12,566 86.7
Tension cracks w ith quartz 10,300 75
Fractures filled with calcite and
epidote 6647 64.4
Multiple fractures 9833 93.11
Figure 4. Microscopic appearance of discontinuities.

D. SARR ET AL.
644
(a) (b)
(c) (d)
(d)
Figure 5. Stress-strain relations of Proterozoic pillow lavas.
Specimen with multiple cracks and variables fillings
shows low Young modulus but greater than for the facies
with epidote and calcite. This is the result of the trend
balance between the quartz filling and epidote and calcite
filling. In fact, th e high quantity of discontinuities reduces
remarkably the rigidity of the material. But the bifurca-
tions caused by the micro discontinuities reduce the uni-
axial compressive strength.
The specimen with a filling of calcite and epidote shows
the lowest moduli of rigidity du e to the differential rigid-
ity between those minerals and the rock matrix composi-
tion. But, unlike the prev ious filling, in this case the ma-
terials filled the discontinuities are softer than the rock
minerals. This will facilitate the reduction of the rigidity

D. SARR ET AL.
645
of the samples but also its very low compressive strengths.
The pillow lavas of Mako show the uniaxial compres-
sive strength of healthy rock (Rc) and unfilled crack rock
(JCS) are substantially identical and the average is more
or less most important for cracks unfilled rock than
healthy rock. This is due to the presence of microfrac-
tures (photo 2) with most often opening of 15 µm and the
compression tests are realized on unaltered joints.
4. Slope Stability of Hillsides of Collin
4.1. Analysis of Dips and Pole’s Densities of
Fractures
An overall analysis shows that discontinuities are of vari-
ables dip (Figure 6) and diverse orientations given by den -
sity poles (Figure 7). The hillside Wassadou-Badian the
density map shows an grouping of poles in focus area NE,
SE, SW and NW with a higher concentration in NW and
SW the dip direction is also variable but with most con-
centration from south to north when we go from Wassa-
dou to Badian. In wake of Dalakoy (after Wassadou) we
have two sides; the one is running between Wassadou
and Dalakoy and the second is along the extension of
Dalakoy from the hill. In the first hillside poles are con-
centered on NE, SE, NNE and ENE with low dips for
discontinuities to NW, SW and SE. Concentrations of
poles are oriented NNE-SSW and NWSE and orienta-
tions of discontinuities NE-SW and NNW-SSE and NE-
SW. In the second hillside vergences are NNE, S, E, SE
and SW. the hillside parallel to Badian-Bafoundou d irec-
tion, poles are concentered on NW, NE, N and S area
with low dip directions excepted the NW area with high
dips. The left hillside of shows poles densities grouped
on a great circle N-S with little concentrations on SE
with low dips excepted dips on SE. These dips are on all
directions.
4.2. Stability of the Hillsides
This section studies the possibility of sliding of rocks by
stereographic method. For this, all fracturations crossed
are represented in a stereogram. So, for this analysis, we
must have the orientation of the hillside and of the bed-
rock. On the case of this area, the bedrock is favorable to
the sliding for all hillsides.
For the stability analysis of hillsides representing in
these it is necessary to cons ider “pierced” of plans forming
dihedral angle, the “meet” of the intersection between those
plans and the bedrock and the “release” of one of plans
constitute dihedral. While the bedrock is in all our cases
favorable, the “meet” is always verify.
The hillsides are representing by green great circle and
blue is the direction of sliding. Others colors represent the
discontinuities. So, in our stereographic representations,
Figure 8 (a) shows possibility of sliding along lines of
intersection of plans formed dihedral angles to the NNE
and NNW directions but Figure 8 (b) represents possi-
bilities of sliding along the intersection associate to a
possibility of sliding along one of the two plans consist-
ing the dihedral to the NNW. In Figure 8(c), we have
also possibilities of sliding along the line of intersection
to NW and SW and sliding along one of the two plans to
WSW. Figures 8(d) and (e) show only sliding along line
to NW and in Figure 8(f) we have the same behavior but
to NW and SSW. In Figure 8(j) that sliding has done to
NW and WSW. Figure 8(h) shows sliding along lines to
NNW, ENE and ESE, and sliding along one plan to NE
and E. Figure 8(i) shows sliding along the lines of inter-
section and one of planes formed dihedral to all the area
located from E to SSW. For hillside of Figure 8(i), we
have some slide to E and S along lines and to SE along
one of the planes. The Figure 8(k) shows possibilities of
sliding to S SSE and SSW.
Figure 6. Dip directions map of discontinuities.
Figure 7. Densities diagram map of discontinuities.

D. SARR ET AL.
646
Figure 8. Stereographic plots of discontinuities cross on different hillsides.
5. Conclusions
Previous studies allow us to conclude that the area of
Mako is characterized by discontinuities and microdiscon-
tinuities oriented N-S, E-W, NE, NNE, NW and NNW. I t
is brittle discontinuities (fractures) and ductile disconti-
nuities (folds). The hill of Badian shows like others for-
mations of the domains th e same discontinu ities. A finest
study of this hill show fractured and microfractured and
microfractured unfilled or filled with calcite, epido te and
quartz. The spacing of the discontinuities are moderate
that is causing block rarely metric. The mechanical be-
havior of this rock is also influenced by the presence of
imperfection. Thus, the application of normal charge on
a discontinuity makes decrease the Young modulus but
the compressive uniaxial strength varies only slightly. If
the discontinuity of th e ro ck is filled , the ba salts beh av ior
will depend to the nature of filling. Quartz significantly
decreases the Young’s modulus and uniaxial compres-
sive strength. But the reduction of the characteristic of
the rock is more important when the material is filled by
epidote and chlorite. But if the specimen contains 3 or 4
direction of discontinuities the Young’s modulus de-
creases while the compressive uniaxial strength increase.
This hill shows a complex instability with plane slid-
ing associate to a sliding along the line of intersection
between two planes that formed dihedral angles.
6. Acknowledgements
Authors would like to Acknow ledge Mr. Amsata Thiam,
technical Director of SENBUS industries sa for his great
contribution in this work, allowing as to us the supplies
of his manufactory for the samplings. Acknowledge also
to Dr. Edmond Dioh, Head of the Department of Geology
of the IFAN-Cheikh Anta Diop (Dakar), most of the mi-

D. SARR ET AL.
647
croscopic analysis had been done in his laboratory. The
authors would like to acknowledge Mouhamadou Lamin e
Lo (Assistant Professor—EPT/Thiès) for his guidance
and valuable input in this research project.
7. References
[1] J. P. Bassot, “Etude Géologique du Sénégal Oriental et de
ses Confins Guinéo-Maliens, ” Thèse Doctorat és Science
de la terre, Université de Clermont Ferrand, Clermont
115 pages.
[2] J. P. Bassot, “Etude géologique du Sénégal oriental et ses
et ses confins Guinée-Maliens,” Mémoire BRGM, 1966,
40 pages.
[3] G. Rocci, “Essai d’Interprétation des Measures Géochro-
nologique,” La Structure de l’ouest Africain. Science de
la Terre, Vol. 10, No. 3-4, 1965, p. 461.
[4] N. Clauer, R. Caby, D. Jeanette and R. Trompette, “Geo-
chronological of Sedimentary and Metasedimentary pre-
cambrian Rock of the West African Craton,” Precam-
brian Research, Vol. 18, No. 1-2, 1982, pp. 53-71.
[5] J. P. Bassot, “le Complexe Volcano-Plutonique Calco-
Alcalin de la Rivière Daléma (Est Sénégal): Discution de
sa Signification Géodynamique dans le cadre de l’Oro-
genèse Éburnéenne (Pr otérozoique inférieur), 1987.
[6] A. Dia, “Caractères et Significations des Complexes Mag-
matiques et Métamorphiques du Secteur de Sandikounda
—Laminia (Nord de la Boutonnière de Kédougou—Ké-
niéba, Est du Sénégal),” Un Modèle Géodynamique du
Birimien de l’Afrique de l’Ouest, Thèse d’Etat, Univer-
sité de Dakar, 350 pages.
[7] P. M. Ngom, “Contribution à l’étude de la Série Biri-
miennes de Mako Dans le Secteur Aurifère de Sabodala
(Sénégal Oriental),” Thèse de Doctorat du 3e Cycle, Uni-
versité de Nancy I., 1985, 134 pages.
[8] D. P. Diallo, “Caractérisation d’une portion de croûte
d’Âge Protérozoïque Inférieur du Craton Ouest Africain:
Cas de l’Encaissant des Granitoïdes Dans le Supergroupe
de Mako (Boutonnière de Kédougou),” Implications Géo-
dynamiques. Thèse d’Etat, Université Cheikh Anta Diop
de Dakar, 1994, 466 pages.
[9] P. M. Ngom, “Caractérisation de la Croûte Birimienne
dans les Parties Centrale et Méridionale du Supergroupe
de Mako,” Implications Géochimiques et Pétrogénétiques,
Thèse d’Etat Université Cheikh Anta Diop de Daka, 1995,
240 pages.
[10] E. Dioh, “Caractérisation, Signification et Origine des
Formations Birimiennes Encaissantes du Granite de
Diombalou (Partie Septentrionale de la Boutonnière de
Kédougou—Sénégal oriental),” Thèse d’Etat, Université
Cheikh Anta, Diop de Dakar, Sénégal, 1995, 447 pages.
[11] S. Cissokho, “Etude Géologique du Secteur de Mako
(Partie Méridionale du Supergroupe de Mako, Bouton-
nière de Kédougou—Kéniéba, Sénégal Oriental): Impli-
cation sur la Diversité Magmatique,” Thèse de 3e cycle,
Université Cheikh Anta Diop de Dakar, 2010, 214 pages.
[12] Bassot J. P. et Caen Vachette, “Données Géochronolo-
giques, Géochimiques Nouvelles sur les Granitoides de
l’Est du Sénégal: Implication dans l’Histoire Géologique
du Birrimien de Cette Region,” African Geology Edition,
1984.
[13] P. M. Ndiaye, “Etude Géologique et Métallogénique de la
Partie Septentrionale du Granite de Saraya: Secteurs de
Missi ra, Wassangara, Frandi. Sénégal Oriental,” Thèse
Doctorat du 3ème Cycle Université Cheikh Anta Diop
Dakar, 1986, 109 pages.
[14] P. M. Nidaye, “Evolution au Protérozoïque Inférieur de la
Région Est Saraya, Supergroupe de Dialé—Daléma Sé-
négal Oriental: Tourmalinisation, Altérations Hydrother-
males et Minéralisations Associées,” Thèse d’Etat Uni-
versité Cheikh Anta Diop de Dakar, 1994, 372 pages.
[15] N. Barton and V. Choubey, “The Shear Strength of Rock
Joints in Theory and Practice,” Rock Mechanics and Rock
Engineering, Vol. 10, No. 2, 1977, pp. 1-54.
[16] M. J. A. Leal-Gomes, “Some New Essential Questions
about Scale Effects on the Mechanics of Rock Mass
Joints,” South African Institute of Mining and Metallurgy,
International Society for Rock Mechanics 10th Congress
Technology Roadmap for Rock Mechanics, Vol. 2, 2008.
[17] Jaboyedoff, “Caractérisations Géométriques Simples des
Discontinuités dans un Massif Rocheux,” Quanterra, In-
ternational Independent Center of Climate Change Impact
on Natural Risk Analysis in Mountains Area, 13 pages.
[18] Zhao, “Propriétés des discontinuités EPFL-ENAC- LMR,”
2008, 94 pages.
[19] ISRM, “Technol ogy Road map for R ock Mechani cs,” So uth
African Institute of Mining and Metallurgy, Johannesburg,
2003.