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Transcript of Sedimentation in the Kandi extensional basin (Benin and Niger): fluvial and marine deposits related...
Journal of African Earth Sciences 36 (2003) 185–206
www.elsevier.com/locate/jafrearsci
Sedimentation in the Kandi extensional basin (Benin and Niger):fluvial and marine deposits related to the Late Ordovician
deglaciation in West Africa
M. Konate a, M. Guiraud b,*, J. Lang b, M. Yahaya a
a D�eepartement de g�eeologie, Universit�ee de Niamey, BP 10 662 Niamey, Nigerb Biog�eeosciences-Dijon, UMR CNRS 5561, Centre des Sciences de la Terre, Universit�ee de Bourgogne, 6 Bd Gabriel, F21000 Dijon, France
Received 3 March 2002; accepted 8 April 2003
Abstract
The Lower Paleozoic detrital succession of the half-graben Kandi Basin in West Africa (Niger-Benin) is about 600 m thick and
rests unconformably on the Pan-African basement. Along the western edge of the basin, the base of the succession locally features
large glacial fault-bounded paleovalleys. These valleys are filled by the lowermost continental deposits of the W�eer�ee Formation
characterized by massive diamictites with dropstones, and coarse to conglomeratic sandstones associated with large-scale channel
structures and internal erosional truncations. The uppermost braided-river deposits of the W�eer�ee Formation deposited across the
entire basin are overlain by the Late Ordovician–Early Silurian storm and tidal sediments of the Kandi Formation, made up of
hummocky cross-stratified sandstones and siltstones. Computer-aided analysis of the populations of synsedimentary to synlithifi-
cation microfaults observed in the Late Ordovician to Early Silurian sediments shows evidence of extensional paleostress tensors
with a N90�E to N100�E horizontal r3 stress responsible for normal displacement along the Kandi Fault. The synsedimentary
normal activation of this major fault, inherited from the Precambrian, controls the spatial arrangement of the glacial, braided-
stream, storm to tidal, and offshore deposits as well as the deformation of the basin-fill into an asymmetric synsedimentary syncline
associated with progressive unconformities. The characterization of glacial features and Late Ordovician deposits from the bio-
stratigraphic distribution of traces of trilobites strongly supports the idea that the deposits of the Kandi Basin are contemporaneous
with the melting of the wide ice sheet which overlay the Afro-Arabian Shield during Late Ordovician times. The successive deposits
of the W�eer�ee and Kandi Formations reflect a gradual change from tillites, through glaciofluvial outwash conglomerates (Wa
Member), braided-stream sediments (Wb Member), and shoreface barrier sands (Ka Member), to offshore clays and sands (Kb
Member). They correspond to reworked, glaciofluvial to marine facies laid down by the Late Ordovician glacial retreat. The Kandi
Basin is therefore defined as a staging-post between the Late Ordovician––Early Silurian basins of the Sahara and those of South
Africa.
� 2003 Elsevier Ltd. All rights reserved.
Keywords: Fluvioglacial; Paleovalley; Marine deposits; Deglaciation; Late ordovician; Progressive unconformity; Half-graben; West Africa
1. Introduction
Late Ordovician to Early Silurian periglacial deposits
are widespread in West Africa (Fig. 1) and correlative
units of glacial origin also occur in central Arabia and
South Africa (Fig. 2). These deposits are represented by
the Late Ordovician upper tillites in the western Sahara
(Deynoux, 1980; Villeuneuve and Da Rocha Araujo,
1984; 1 and 2 of Fig. 2), central Sahara (Beuf et al., 1971;
* Corresponding author. Fax: +33-3-80-396783.
E-mail address: [email protected] (M. Guiraud).
0899-5362/03/$ - see front matter � 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0899-5362(03)00026-5
3 of Fig. 2), central Arabia (Vaslet, 1990; Abdulkaderet al., 1993; 4 of Fig. 2) and South Africa (Rust, 1981;
Fabre, 1988; 5 of Fig. 2). In the Taoudeni Basin, Late
Ordovician glacial deposits of the Tichitt Formation are
capped by Silurian shales and fluvial to marine sand-
stones of Devonian age (1 of Fig. 2). The basal glacial
unconformity locally features major paleovalleys which
are filled by thin diamictites described as tillites and
plugged by thick cross-bedded fluvioglacial sandstonesoverlain by marine shales. Paleovalleys of the same scale
characterize the glacial Tamadjert Formation exposed in
the Tassilis Basin (3 of Fig. 2) and occur within the co-
eval Sarah Formation along the border of the Arabian
Fig. 1. Structural map of West Africa.
186 M. Konate et al. / Journal of African Earth Sciences 36 (2003) 185–206
Shield in Saudi Arabia (4 of Fig. 2). Beuf et al. (1971)
and Vaslet (1990) suggest that these tillites may becontrolled by inherited faults in the underlying Pre-
cambrian basement. The paleovalleys are regarded as
the product of erosion and deposition by glaciofluvial
outwash rivers draining the north-eastern edge of the ice
sheet (Mc Clure, 1978; Vaslet, 1990). In South Africa,
Late Ordovician glaciogenic sediments have been de-
tected in the Cape Supergroup and exhibit glaciated
pavements which are associated with large scale foldsinterpreted as the result of subglacial deformation below
a large ice sheet (Rust, 1981).
The Late Ordovician–Early Silurian periglacial sedi-
ments are attributed to the major glaciation at the end of
Ordovician times reported for northern and southern
Africa, Arabia, Europe, North-East America, and South
America (see Eyles, 1993, for a review of the literature;
Crowell, 1999; Scotese et al., 1999). The consensus viewof the Early Paleozoic glaciation is one of successive
radial fluctuations of a wide ice sheet across the Afro-
Arabian Shield, bordered by extensive glaciofluvialoutwash fans (Deynoux, 1980; Vaslet, 1990). In North
Africa and Arabia (Fig. 2), the late Ordovician peri-
glacial sediments are overlain by the �Hot Shales� For-
mation recording the Early Silurian transgression that
followed the melting of the Hirnantian ice-cap (end of
Ordovician times; Luning et al., 2000). The Late Ordo-
vician glaciation was apparently a brief glacial episode
(Brenchley et al., 1994) estimated to have lasted some0.3 Ma (Sutcliffe et al., 2000).
Despite pioneering observations (Guiraud and et
Alidou, 1981; Alidou, 1987; Alidou et al., 1991), the
sedimentary succession and the tectono-stratigraphic
relationships characteristic of the Kandi Basin have not
yet been described in detail. Using mapping at 1/30 000
scale and a sedimentological analysis from the mea-
surement of key sections combined with a quantitativemicrofault analysis, this paper sets out the results of (1)
Fig. 2. (A) Correlations between the Lower Paleozoic Formations of basins of West Africa, Central Arabia, and South Africa (from Deynoux, 1980;
Beuf et al., 1971; Vaslet, 1990; Rust, 1981; Fabre, 1988). (B) Late Ordovician paleogeographic reconstruction of West Gondwana, indicating
approximate extent of grounded ice sheet (Beuf et al., 1971; Vaslet, 1990; Scotese et al., 1999; Sutcliffe et al., 2000). (C) Legend of Fig. 2A (Bassot
et al., 1963; Clauer, 1976).
M. Konate et al. / Journal of African Earth Sciences 36 (2003) 185–206 187
facies and geometric analysis of the basal fluviatile
periglacial deposits and overlying marine sediments of
Late Ordovician to Early Silurian age, and (2) investi-
gation of the relationships between the reworked fluvialto marine sedimentation and the deglaciation at the end
of Ordovician times.
2. Geological setting and stratigraphy of the basin-fill
The Lower Proterozoic shield of West Africa is
overlain by a 2000–3000 m thick Upper Proterozoic toPaleozoic sedimentary cover distorted by the Pan-Afri-
can and Hercynian fold belts (Fig. 1). The intracratonic
188 M. Konate et al. / Journal of African Earth Sciences 36 (2003) 185–206
Kandi Basin of N.E. Benin and S.W. Niger is some 160km long and 45–95 km wide (Figs. 1 and 3). It is
bounded to the west by the N20�E-oriented Kandi Fault
(Fig. 3), a major Pan-African crustal fault at the scale of
West Africa (Guiraud and et Alidou, 1981). The Lower
Paleozoic deposits are only about 600 m thick (Alidou,
1987; Konate et al., 1994; Konate, 1996) and are char-
Fig. 3. Geological map o
acterized by substantial lateral variations in facies andthickness. They are unconformably overlain in the
North-East and North, respectively, by Cretaceous and
�Continental Terminal� Cenozoic deposits (Fig. 3). The
Ordovician and Early Silurian detrital sedimentation
(Seilacher and Alidou, 1988; Konate, 1996; Fig. 4) is
subdivided into two distinct formations: the 500 m-thick
f the Kandi Basin.
Fig. 4. (A) Lithostratigraphic log of the Paleozoic sedimentary series of the Kandi Basin. (B) Legend of symbols used for the stratigraphic sections
(Figs. 4A, 7, 8 and 10).
M. Konate et al. / Journal of African Earth Sciences 36 (2003) 185–206 189
conglomerates and continental sandstones of the W�eer�eeFormation at the base (Alidou, 1987; Konate, 1996)
overlain by some 80 m of sandstones and upper siltites
of the marine Kandi Formation.
The lithostratigraphic subdivision adopted here is the
same as that given by Alidou (1987), Seilacher and Alidou
(1988), and Alidou et al. (1991), supplemented by new bio-
stratigraphic and geological mapping data (Figs. 3 and 4).
Fig. 5. Arthrophycus simplex or Harlania-type galleries (linear), Late
Ordovician: Ka Member deposits, Tui sector (Fig. 3).
Fig. 6. Resting or temporary burrowing traces of Cruziana petraea-
type trilobites, Late Ordovician, Ka Member deposits, Kandi sector
(Fig. 3).
190 M. Konate et al. / Journal of African Earth Sciences 36 (2003) 185–206
2.1. The W�eer�ee Formation
The W�eer�ee Formation includes all of the sandstones
and conglomerates of the Kandi Basin (Figs. 3 and 4).
The poorly sorted detrital deposits of this Formation do
not provide any biostratigraphic material. These conti-
nental deposits have two distinct lithofacies:
• the lower section corresponding to the Wa Member(150 m thick) is composed of diamictite and breccia,
medium- to coarse-grained sandstones with quartzitic
boulders and faceted pebbles, and rare gneissic to
granitic pebbles, with tabular to trough cross-bedding
(Fig. 4). The Wa Member lies along the Kandi Fault
in the W�eer�ee-Goungoun sub-basin (Fig. 3).
• above, the Wb Member (about 350 m thick) consists
of granule-rich coarse sandstones with tabular totrough cross-bedding (Fig. 4) and crops out mainly
in the northern (Bangoun, Fig. 3) and south-eastern
(Gbessaka and L�eet�ee, Fig. 3) parts of the basin.
2.2. The Kandi Formation (Late Ordovician–Early Silu-
rian)
Two members are distinguished from ichnofossil as-sociations and sedimentary structures (Fig. 4):
• at the base, the Ka Member is some 45 m thick and
crops out in the vicinity of the western boundary
fault (Bodj�eekali: Fig. 3) and of the horsts on the
south-eastern boundary (Gbessaka, L�eet�ee, Segbana,
Fig. 3). The Ka Member sediments consist of me-
dium- to fine-grained sandstone deposits displayingsigmoidal bedding and herringbone cross-bedding.
In the region of Poria (Fig. 3), these deposits directly
overlie the kaolinized and ferruginized Pan-African
basement above an angular unconformity. In the
area of Tui and Poria, the lowermost deposits of
the Ka Member are formed by alternating beds of
fine-grained sandstones and siltstones with slightly
erosional furrows. Several traces of Cruziana petraeaand Arthrophycus (trilobites burrows) have been re-
ported in the deposits of the Ka Member (Alidou
et al., 1991; Seilacher and Alidou, 1988; Konate
et al., 1994; this study: Figs. 3, 5 and 6). These data
support a Late Ordovician age for the Ka Member
sediments (Seilacher and Alidou, 1988, Seilacher,
1994, Figs. 7 and 8).
• the overlying Kb Member (some 30 m thick) is welldeveloped in the basin center, and consists of siltstone
facies with hummocky cross-stratification. The bio-
stratigraphic distribution of traces of trilobites (Cru-
ziana acacencis) and worms (Arthrophycus or
Harlania, deep palmate form) in siltstone beds dates
these deposits from Early Silurian times (Seilacher
and Alidou, 1988; Alidou et al., 1991; Fig. 7).
2.3. The meso-cenozoic Formations
In the North of the basin, the Paleozoic Formations
are unconformably overlain by the fluviatile coarsesandstones and pebble conglomerates of the Lower
Cretaceous Send�ee Formation and by the post-Eocene
and pre-Quaternary sand-claystones of the �Continental
Terminal� (Alidou, 1987; Alidou et al., 1991).
3. Fluvial to marine depositional environments
Facies and depositional environments are character-
ized from observation of sedimentological sections
along the western edge of the basin and along an E–W
transect (Fig. 3). Continental lithofacies are described
using the nomenclature of Miall (1977, 1981, Table 1);details of the sedimentary structures and the content of
the lithofacies code are given in Fig. 4B and Table 1.
Macroscopic sedimentological observations (lithology,
Table 1
Lithofacies characterizing the continental deposits of the W�eer�ee Formation in the W�eer�ee and Goungoun sections (Figs. 9 and 10)
Lithofacie
types
Lithological description Sedimentary structures Interpretation
Gmg Boulders (maximum size 70 cm) and non jointed
pebbles, with sandy–silty to silty matrix
Dropstones, clusters bedforms as thick as 20–50
cm, non-erosive basal contact
Glacial tillites with debris
flow reworking
Gmm Structureless and matrix-supported breccia,
maximum size of blocks lower than 30 cm
Coarsening upward or fining upward Angular grain flows
Gt Polymictic diamictites, with immature sandy–
silty matrix, matrix-supported conglomerates
Trough cross-bedding, erosional basal contact
overlain by pebbles and granules, metre-thick
beds
Channels deposits with
pebbles
Gp Polymictic diamictites, with scarce brecciated
elements (W�eer�ee area), sandy–silty matrix with
granules
Oblique planar cross-bedding, non-erosional
basal contact
Linguo€ııd conglomeratic bar
deposits
St Very coarse-grained sandstones, with granules
and few rounded pebbles, sandy–silty matrix,
occurrence of lenticular-shaped silty layers
20–30� dip trough cross-bedding, erosional basal
contact
Channel deposits: coarse-
grained sandstones with
granules and pebbles
Sp Very coarse-grained sandstones, with granules
and rounded pebbles, sandy–silty to silty matrix
20–40� dipping trough cross-bedding, fining-up-
ward succession
Transverse sand bar deposits
Sh Fine to coarse-grained sandstones, with rare
granules, sandy–silty matrix
Planar bedding Vertically aggraded sand
deposits
Sm Medium to very coarse-grained sandstones, with
rare granules and pebbles
Slight erosional basal contact Channel infilling (coarse
sands)
Fig. 7. Cruziana ichnospecies, distinguished by scratch patterns and behavioral features, used as index fossils in otherwise non-fossiliferous Paleozoic
sandstones (Seilacher, 1994).
M. Konate et al. / Journal of African Earth Sciences 36 (2003) 185–206 191
sedimentary structures, trace fossils) characterize eight
lithofacies associated with specific marine depositional
environments (Table 2). Paleocurrent directions were
determined from sedimentary structures such as trough
cross-bedding and planar or sigmoidal bedding (Figs. 17
and 18). The basin-fill succession is interpreted from
changes in depositional environments which reveal
variations in relative sea level. The W�eer�ee (Fig. 9) andGoungoun (Fig. 11) sections on the western edge of the
basin exhibit details of the continental deposits of the
W�eer�ee Formation; the Tui (Fig. 12) and Poria (Fig. 13)
sections illustrate the succession of marine sediment
lithofacies of the Kandi Formation.
3.1. The continental deposits of the W�eer�ee Formation
Geological mapping combined with sedimentologicalanalysis show that the conglomeratic sandstones and
breccias of the Wa Member are spatially restricted to the
western, faulted edge of the basin and crop out over a
Table 2
Lithofacies characterizing the marine deposits of the Kandi Formation in the Tui and Gbass�ee sections (Figs. 12 and 13)
Lithofacies Lithological description Sedimentary structures Interpretation
Shb Fine to medium-grained sandstones Herringbone cross-bedding, bundle structures,
irregular basal contact, reactivation surfaces
draped with thick clay
Tidal bars (foreshore)
Sl Fine-grained sandstones to siltstones with worm
traces, occurrence of mud balls
Low-angle cross-bedding (<10�) Backshore and foreshore
deposits
Sh Fine-grained sandstones to siltstones with worm
traces, occurrence of mud balls
Planar bedding Channel deposits
(foreshore)
Sps Whitish graded coarse-grained sandstones with
pebble conglomerates and clayey beds, Harlania
(Arthrophycus)
Sigmoidal bedding, erosional basal contact Foreshore sand bar
Shcs Dark siltstones with Harlania (Arthrophycus) Vertically accreted hummocky, cross stratifica-
tion, current ripple marks or wave ripple marks
Storm deposits (lower
shoreface to upper
offshore)
Swb Micaceous sandstones with mud balls Swaley cross-stratification erosional hummocky
cross stratification, wave interference ripple
marks
Storm deposits (upper
shoreface)
Sm Poorly graded coarse-grained sandstones and
pebble conglomerates with coarse sandy matrix
Massive structure, non-erosional basal contact Storm deposits
Fhcs Ferrugineous micaceous siltstones with trace
fossils (Cruziana et Harlania)
Vertically accreted hummocky cross stratification,
lens-shaped erosional furrows, current ripple
marks or oscillation ripple marks
Storm deposits (upper
offshore)
Fl Fine-grained sandstones to siltstones with Scolite Alternatively coarse-grained and fine-grained
siltstone laminations
Upper offshore
Fig. 8. Fossil traces in the siltstones–sandstones of the Kandi Formation (modified from Seilacher and Alidou, 1988). Cruziana sp.: Probable
trilobite burrow with fine longitudinal striation (possibly made by expostites).
192 M. Konate et al. / Journal of African Earth Sciences 36 (2003) 185–206
Fig. 9. Stratigraphic section of the W�eer�ee Formation (W�eer�ee area, Fig. 3; see Fig. 4B for legend).
M. Konate et al. / Journal of African Earth Sciences 36 (2003) 185–206 193
strip some 1–4 km wide and some 140 km long. The WaMember is therefore a basin-margin facies that changes
vertically (W�eer�ee-Goungoun sector) and grades laterally
into the Wb Member toward the basin axis. The sedi-
ments of the Wb Member are granule-rich coarse
sandstone deposits that fine eastward (Fig. 3).
3.1.1. The glacial diamictites and sandstones of the Wa
Member
In the south, the W�eer�ee section (Fig. 9) is characterized
at its base by dark brown deposits of the Wa Member,
consisting of poorly sorted polymictic conglomerateswith erratic boulders scattered in a sandy-silt matrix.
Cobbles and pebbles are generally subrounded to an-
gular. Elsewhere, meter-sized boulders and cobbles are
frequent within the fine-grained massive sediment
(lithofacies Gmg). Meter-sized isolated blocks in non-
jointed pebbles with a sandy–silty to silty matrix (Fig.
15) are interpreted as dropstones formed by melt-out of
debris carried in icebergs as defined by Visser (1983) andEyles et al. (1985). These observations suggest that the
Gmg facies is a glacial deposit reworked by debris flow
Fig. 10. Geological map of W�eer�ee area: a fault-bounded paleovalley filled by the large-scale conglomeratic to coarse-grained sandstones of the Wa
Member (see Fig. 4B for legend).
194 M. Konate et al. / Journal of African Earth Sciences 36 (2003) 185–206
processes. Preserved glacial structures include massive
diamictites with dropstones, large-scale breakage rocks,
scarce polished surfaces described as ‘‘roche mou-
tonn�eee’’ features and faceted clasts that are typically
bullet-shaped but without any striation. About 80% of
the clast types correspond to nearby basement rocks
(quartzites, granites, gneiss) whereas 10% of the clasttypes are related to exotic basement rocks (amphibo-
lites). ‘‘Far-traveled’’ extrabasinal clasts are strong evi-
dence of glacial action. Toward the top of the section,
the Wa Member gives way to conglomerates and the
very coarse trough and planar cross-bedded sandstones
(Gt, Gp, St, Sp) of the Wb Member braided-stream
deposits (Figs. 9 and 16).
In the W�eer�ee area, the conglomerate to sandstone
bodies of the Wa Member follow a N10�E striking
feature that is prominent on aerial photographs. De-tailed geological mapping at the scale of 1/30 000 reveals
that these deposits form a single, straight ribbon 1–2 km
wide, about 35 km long, and 50–70 m thick (Fig. 10).
Fig. 11. Stratigraphic section of the W�eer�ee Formation (Goungoun area, Fig. 3; see Fig. 4B for legend).
M. Konate et al. / Journal of African Earth Sciences 36 (2003) 185–206 195
This ribbon displays large-scale channel structures
marked by several internal erosional truncations, mas-
sive diamictites with dropstones preserved in places, andcoarse to conglomeratic sandstones. It is fault-bounded,
aligned parallel to the regional tectonic structures (Pre-
cambrian foliation, fault traces) and its southern ter-
mination has a characteristic U-shaped floor. Similar
large-scale channel-fill structures of Late Ordovician age
have been reported in the central Sahara by Beuf et al.
(1971), in Mauritania by Deynoux (1980) and by Ghi-
enne and Deynoux (1998), and in central Arabia byVaslet (1990). At regional scale, the channel-fill struc-
tures are parallel to paleo-ice-flows as indicated by
striated pavements and glaciotectonic structures. Ghi-
enne and Deynoux (1998) claim that interaction betweenstructural trends and channel-fill orientation can be ex-
plained by ice flowing along a tectonically constrained
topography. These observations enable us to interpret
the N10�E conglomerate to sandstone ribbon in the
W�eer�ee area as a large-scale glacial fault-bounded pa-
leovalley incised by the Late Ordovician ice sheet and
filled by glacial outwash deposits. Similar features in-
terpreted as glacial paleovalleys have also been detectedin the Thya, Goungoun, and Gu�een�ee areas (Fig. 3).
Fig. 12. Tui stratigraphic section (Fig. 3; see Fig. 4B for legend).
196 M. Konate et al. / Journal of African Earth Sciences 36 (2003) 185–206
3.1.2. The braided stream deposits of the Wb Member
In the area between Goungoun and Gu�een�ee, the Wb
Member consists of matrix-supported and poorly
sorted breccia (lithofacies Gmm, Table 1 and Fig. 14).
These deposits are nearly structureless (rare planar or
trough cross-bedding, Fig. 11). The sequence consistsof alternating coarse-grained and fine-grained depo-
sits. These planar and rarely trough cross-bedded strata
form layers up to several meters thick and are inter-
preted as the product of traction in a braided-stream
system, marked by an irregular flow regime (Konate,
1996). Along the basin axis, the very coarse sandstones
of the Wb Member are characterized by trough
and planar cross-bedding (lithofacies Sp, Fig. 17)related to braided-stream-type sedimentation over-
flowing the glacial paleovalley (Konate et al., 1994;
Fig. 13. Poria stratigraphic section (Fig. 3; see Fig. 4B for legend).
M. Konate et al. / Journal of African Earth Sciences 36 (2003) 185–206 197
Konate, 1996). A few features in the deposits of the
Wa and Wb Members, including rare dropstones and
till-type facies, point to a proglacial influence (Konate,
1996).
Fig. 14. Structureless and matrix-supported breccia from the W�eer�eeFormation (Wb Member), tilted 50–70� eastward: lithofacies Gmm,
Gu�een�ee sector (Fig. 3).
Fig. 17. Medium- to coarse-grained sandstones with planar cross
bedding (lithofacies Sp, fluviatile deposits of the W�eer�ee Formation,
Thya sector, Fig. 3).
Fig. 16. Boulder scour in structureless matrix-supported breccia from
the W�eer�ee Formation (lithofacies Gmm, Wb Member, W�eer�ee sector,
Fig. 3).
Fig. 15. A dropped meter-sized quartzite boulder in non-jointed
pebbles, with sandy–silty to silty matrix (subglacial tillite, lithofacies
Gmg, Wa Member, W�eer�ee sector, Fig. 3).
Fig. 18. Fine sandstones with sigmoidal bedding (lithofacies types Sps)
and erosional hummocky cross stratification (lithofacies Swb); tidal
deposits of the Kandi Formation, Ka Member, Bodj�eekali sector
(Fig. 3).
198 M. Konate et al. / Journal of African Earth Sciences 36 (2003) 185–206
3.2. The marine deposits of the Kandi Formation
The Kandi Formation, which is some 75 m thick,
crops out extensively in the Kandi Basin (Fig. 3). These
marine deposits, composed of fine sandstones and
siltstones, overlie the continental deposits of the W�eer�eeFormation above a major unconformity correspond-
ing to a ravinement surface emphasized by granule-rich sandstones with rounded material. Two members
are identified from lithofacies associations (Figs. 3
and 4).
3.2.1. Storm and tidal deposits of the Ka Member
The deposits of the Ka Member crop out close to theboundary zones in the East and West of the basin (Fig.
3). Analysis of the Tui section (Fig. 12) shows that the
Ka Member is formed at the base by coarse-grained and
Fig. 19. Directions of palaeocurrents associated with Ka Member marine deposits of the Kandi Formation.
M. Konate et al. / Journal of African Earth Sciences 36 (2003) 185–206 199
fine-grained siltstone laminations, showing load or ball-
and-pillow structures, with frequent bioturbation fea-
tures (lithofacies Fl). These rhythmic layers related tolaminites are associated with an inner offshore envi-
ronment (Fig. 12). These strata grade upward into
white, fine- to medium-grained sandstones, with amal-
gamated swaley cross-stratification and erosional hum-
mocky cross-stratification (HCS), characteristic of a
shoreface environment (lithofacies Shc and Swb) that
passes into foreshore deposits (lithofacies Sl). In theBodj�eekali and Gbass�ee sectors, the upper levels of the Ka
Member consist of well-sorted, fine- to medium-grained
sandstone bars several decimeters to a meter thick, with
sigmoidal bedding (lithofacies Sps, Fig. 18). There are
Fig. 20. Fine sandstones and siltstones with hummocky cross strati-
fication showing vertical accretion (lithofacies Shcs-Fhcs, upper off-
shore deposits of the Kandi Formation, Kb Member, Tui sector,
Fig. 3).
200 M. Konate et al. / Journal of African Earth Sciences 36 (2003) 185–206
many reactivation surfaces, often draped with thick clay.
All of these observations together with the occurrence of
tidal bundle structures, herringbone cross-bedding, andopposing (NE and SW) paleocurrent directions (Fig. 19)
are indicative of the tidal origin of the sand deposits of
the Ka Member in the northern and south-eastern sec-
tors of the basin.
3.2.2. Storm deposits of the Kb Member
These deposits are made up of siltstone strata inter-
bedded with micaceous fine sandstones in undulating
beds several decimeters to a meter thick, with lens-
shaped erosional troughs (lithofacies Fhcs, Figs. 12 and
13). In many outcrops, the beds display sedimentary
structures (HCS, erosive furrows) characteristic of theemplacement and reworking of sediments under storm
action (e.g. Aigner, 1985; Guillocheau, 1991). Analysis
of the geometry of the erosional furrows at the base of
the beds, of the vertically accreted HCS (Fig. 20) and, at
the top, of the beds of wave interference ripple marks
(Guillocheau, 1991) relates sedimentation to an upper
offshore environment.
4. Synsedimentary normal activation of the Kandi Faultand basin architecture
Geological mapping of the western edge of the basin
(Tui sector, Fig. 21A) shows that the whole of the de-
trital infilling is deformed by an asymmetrical synclinal
structure, the axis of which is parallel to the major
N20�E faults. This structure is associated with several
intraformational angular unconformities (e.g. D1 andD2, Fig. 21B), and a divergent pattern of Paleozoic
deposits revealed by the eastward variation of dip values
that change over distances of less than 500 m from 70�
to 10�. The synsedimentary character of this structure isshown by the eastward thickening of the Paleozoic series
and by the arrangement of the overlying deposits of the
Kandi Formation (Member Kb) in a progressive un-
conformity fossilizing the underlying tilted sedimentary
beds.
The microtectonic analysis is based on 51 stations of
microfault populations distributed across the entire
Precambrian basement, the Paleozoic deposits, and theCretaceous sediments (Konate, 1996). About 2200
brittle to synlithification microfaults were considered
with an average of 25 microfaults per station. Close to
the Kandi fault zone, the Precambrian basement ex-
hibits Late Pan-African subvertical mylonitic foliations
(Fig. 10). The population of the brittle microfaults
which displace the foliation planes have been analyzed
by using the computer-aided Etchecopar method(Etchecopar and Mattauer, 1988). The calculated pa-
leostress tensors are of the compressional strike-slip type
as defined by Guiraud et al. (1989); they are character-
ized by a N100�E horizontal maximum compressive
stress, r1, responsible for the reverse displacement of the
Kandi Fault during the late Pan-African phase (Fig.
22A). The sandy and silty Paleozoic deposits are fre-
quently cross-cut by several normal microfaults (Fig.22B) of characteristic synsynsedimentary to synlithifi-
cation types (Guiraud and S�eeguret, 1987; Huguen et al.,
2001). These microfaults are associated with significant
changes in bed thickness and they can clearly be differ-
entiated from usual brittle microfaults (Petit et al.,
1983). The analysis of the populations of synsedimen-
tary to synlithification microfaults observed in the Late
Ordovician to Early Silurian sediments shows evidenceof extensional paleostress tensors with a N90�E to
N100�E horizontal stress, r3 (Fig. 22C). The regional
stress field related to the Late Ordovician–Early Silurian
extension is obtained by interpolation of the extensional
stress tensors (Fig. 23). It is characterized by uniform
trajectories of horizontal stress, r3, which are oriented
N110�E and imply normal displacement along the
Kandi fault and the secondary faults bordering the L�eet�eeand Gb�eess�ee horsts. The Paleozoic deposits, the Pre-
cambrian basement, and the Early Cretaceous sediments
are also disrupted by later brittle microfaults with syn-
kinematic quartz recrystallization. This brittle defor-
mation is associated with the Late Cretaceous strike-slip
to strike-slip compressional episodes (Fig. 22C) related,
respectively, to N–S to N140�E horizontal shortening in
the neighboring Benue Basin (Nigeria, Guiraud,1991a,b).
The progressive syntectonic tilting of the continental
and marine Paleozoic strata is therefore related to the
normal motion of the Kandi Fault (Fig. 21A). The half-
graben structure and the syncline-shaped geometry of
the Kandi Basin acquired during the Paleozoic exten-
sional phase (Konate, 1996) are governed by the acti-
Fig. 21. (A) Geological map of Tui area showing the Kandi boundary fault and the geometry of the synsedimentary syncline deforming the syn-
tectonic Paleozoic deposits (location in Fig. 3). (B) E–W section showing the progressive unconformities and the general divergent configuration of
the syntectonic Paleozoic deposits.
M. Konate et al. / Journal of African Earth Sciences 36 (2003) 185–206 201
Fig. 22. (A) Plotting of the calculated paleostress tensor and associated reactivation of the Kandi fault. (B) Synsedimentary normal microfaults
deforming the conglomerates and sandstones of the Wa Member, W�eer�ee area. (C) Calculation of the extensional paleostress tensor related to the
population of synsedimentary normal microfaults measured in the deposits of the Wb Member (Bangoun area). 1. Location of the poles of the fault
planes in a Mohr diagram. 2. Histogram of angular differences between calculated and measured striations. 3. Stereographic projection of the fault
planes and the principal stress axes (measured and calculated striations are indicated respectively by arrows and by circles).
202 M. Konate et al. / Journal of African Earth Sciences 36 (2003) 185–206
Fig. 23. Paleostress field related to the N110�E extension of the Late Ordovician–Early Silurian.
M. Konate et al. / Journal of African Earth Sciences 36 (2003) 185–206 203
vation of the N20�E Kandi Fault, which was a late Pan-
African overthrust reactivated as a normal fault. Rela-
tionships between ramp syncline extensional basin andthe associated synsedimentary regional syncline are de-
scribed in the abundant literature (e.g., Ellis and Mc
Clay, 1988; Seranne et al., 1995; Mc Clay, 1996). The
synsedimentary normal activation of the Kandi Faultcontrols the spatial arrangement of the glacial (Wa),
SERIES STAGES BIOZONE Sea Level
RISELlanoverian
Rhuddanian acuminatus
persculptus
Member Kb
Member Ka(Early Silurian)
Late Ordovician
Deposits ofthe Kandi
basin
Attribution tocorrelative
global eventsEarly Siluriantransgression
Fig. 24. Control exerted by the synsedimentary extensional deformation on the distribution of the Paleozoic facies in the Kandi Basin.
204 M. Konate et al. / Journal of African Earth Sciences 36 (2003) 185–206
braided-stream (Wb), storm to tidal (Ka), and offshore
deposits (Fig. 24A and B). Motion of the Kandi normal
Fault also controlled paleorelief features (Kandi and
Poria Horsts).
Ashgillian HirnantianRawtheyan
extraodinarius
pacificus
(Late Ordovician)Member Wa
deglaciationMember Wb
Fig. 25. Stratigraphic subdivision and infened eustatic curve charac-
terizing the Late Ordovician (Hirnantian) glaciation-deglaciation and
the global scale Early Silurian transgression (modified from Brenchley
et al., 1994). The stratigraphic succession of the Kandi Basin is
attributed to these correlative global events.
5. Discussion: The syntectonic W�eer�ee and Ka deposition––a reworked fluvial to marine facies laid down by the Late
Ordovician glacial retreat
The sedimentary series of the Kandi Basin are made
up of reworked fluvial to marine deposits and record the
Late Ordovician glacial retreat and the early Silurian
marine transgression (Fig. 25). Our evidence for this
consists of: (1) analogy with stratigraphic patterns in
West Africa, Central Arabia, and South Africa, in-
cluding the emplacement of Early Silurian transgressive
marine clays and Late Ordovician periglacial deposits(Figs. 2 and 4); (2) the Late Ordovician ages yielded by
trace fossil assemblages and attributed to the Ka
Member deposits of the Kandi Basin (Seilacher and
Alidou, 1988; Alidou et al., 1991; Figs. 7 and 8); and (3)
the identification of a single pre-Silurian glaciation in
the Paleozoic (Scotese et al., 1999).
The spatial arrangement of the glacial (Wa), braided
stream (Wb), storm to tidal (Ka), and offshore (Kb)
deposits was controlled by synsedimentary normal ac-
tivation of the Kandi Fault. During Late Ordovician
times, terrestrial glaciogenic deposits, including diamic-
tites and fluvial outwash, were prevalent in the southern
Hoggar and Hodh areas, while glacially related sandy
marine deposits were dominant in the northern Hoggarand in the Adrar of Mauritania (Ghienne and Deynoux,
1998). These observations characterize the Kandi Basin
as a staging-post between the Late Ordovician–Early
Silurian basins of the Sahara and those of South Africa.
M. Konate et al. / Journal of African Earth Sciences 36 (2003) 185–206 205
Acknowledgements
This work was part of theme 3 �Factors determining
the stratigraphic signal� of UMR CNRS 5561
Biog�eeosciences. It was supported by Burgundy-Cotonou
and Burgundy-Niamey inter-university agreements. The
authors are very grateful to M. Deynoux and P. Turner
for their helpful remarks on the original manuscript. We
are also grateful for the useful reviews made by L. B.Aspler, Th. Kreuser and P. Eriksson.
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