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.

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: michel.guiraud@u-bourgogne.fr (M. Guiraud).

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

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).

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).

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-

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

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

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).

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

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).

Fig. 9. Stratigraphic section of the W�eer�ee Formation (W�eer�ee area, Fig. 3; see Fig. 4B for legend).

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).

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).

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).

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).

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).

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.

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).

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

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.

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).

Fig. 23. Paleostress field related to the N110�E extension of the Late Ordovician–Early Silurian.

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

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.

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).

Rawtheyan

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.

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.

References

Abdulkader, M.A., Makhlouf, I.M., Amireh, B.S., Khalil, B.P., 1993.

Upper Ordovician glacial deposits in southern Jordan. Episodes 16,

316–318.

Aigner, T., 1985. Storm depositional systems. In: Lectures Notes in

Earth Science, vol. 3. 317 p.

Alidou, S., 1987. Etude g�eeologique du bassin pal�eeo-m�eesozo€ııque de

Kandi Nord-Est du B�eenin (Afrique de l�Ouest), vol. 13. Publication

occasionnelle CIFEG, Paris. 119 p.

Alidou, S., Lang, J., Bonvalot, J., Roman, E., Seilacher, A., 1991.

Marine influences in the so-called continental sediments of the

Paleozoic-Mesozoic Kandi Basin (Northern Benin, West Africa).

J. Afr. Earth Sci. 12 (1/2), 55–65.

Bassot, J.P., Bonhomme, M., Roques, M., Vachette, M., 1963.

Mesures d�aages absolus sur les s�eeries pr�eecambriennes et

pal�eeozo€ııques du S�een�eegal oriental. Bull. Soc. G�eeol. Fr. 1, 401–405.

Beuf, S., Biju-Duval, B.-Y., Charpal, O., Rognon, P., Cariel, O.,

Bennacef, A., 1971. Les gr�ees du pal�eeozo€ııque inf�eerieur au Sahara.

S�eedimentation et discontinuit�ees. Evolution structurale d�un craton,

vol. 18. Publication Institut Franc�ais du P�eetrole, France, 464 p.

Brenchley, P.J., Marshall, J.D., Robertson, G.A.F., Long, D.G.F.,

Meidla, T., Hints, L., Anderson, T.F., 1994. Bathymetric and

isotopic evidence for a short-lived Ordovician glaciation in a

greenhouse period. Geology 22, 295–298.

Clauer, N., 1976. Chimie isotopique du strontium des milieux

s�eedimentaires: application �aa la g�eeochronologie de la couverture

du craton ouest-africain. Th�eese Doctorat d�Etat, Universit�ee Louis

Pasteur de Strasbourg, 256 p, unpublished.

Crowell, J.C., 1999. Pre-Mesozoic ice ages: their bearing on under-

standing the climate system. Geol. Soc. Am. Memoir. 192, 106 p.

Deynoux, M., 1980. Les formations glaciaires du pr�eecambrien terminal

et de la fin de l�Ordovicien en Afrique de l�Ouest. Deux exemples de

glaciation d�inlandsis sur une plate-forme stable. Trav. Lab. Sci.

Terre St.-J�eeroome, Marseille, 17, 554 p.

Ellis, P.G., Mc Clay, K.R., 1988. Listric extensional fault systems––

results of analogue model experiments. Basin Res. 1, 55–70.

Etchecopar, A., Mattauer, M., 1988. M�eethodes dynamiques d�analyse

des populations de faille. Bull. Soc. G�eeol. Fr. 8, 289–302.

Eyles, N., 1993. Earth�s glacial record and its tectonic setting. Earth

Sci. Rev. 35, 1–248.

Eyles, C.H., Eyles, N., Miall, A.D., 1985. Models of glaciomarine

sedimentation and their application to the interpretation of ancient

glacial sequences. Palaeogeogr. Palaeoclimatol. Paleoecol. 51, 15–94.

Fabre, J., 1988. Les s�eeries p�eel�eeozo€ııques d�Afrique: une approche. J.

Afr. Earth Sci. 7 (1), 1–40.

Ghienne, J.F., Deynoux, M., 1998. Large-scale channel fill structures

in Late Ordovician glacial deposits in Mauritania, western Sahara.

Sediment. Geol. 119, 141–159.

Guillocheau, F., 1991. Stratigraphie s�eequentielle des bassins de plate-

forme: l�exemple D�eevonien armoricain, Th�eese Doctorat d�Etat,

Universit�ee Louis-Pasteur, Strasbourg, 257 p.

Guiraud, M., 1991a. M�eecanisme de formation du bassin cr�eetac�ee sur

d�eecrochements multiples de la Haute-B�eenou�ee (Nig�eeria). Bull.

Centres Rech. Explor. Prod. Elf-Aquitaine 15 (1), 11–67.

Guiraud, M., 1991b. Les d�eecrochements s�eenestres N60�E du bassin de

la Haute-B�eenou�ee (NW Nig�eeria): un exemple de r�eegulation et de

r�eeorientation de la contrainte horizontale r1 sur d�eecrochements.

C.R. Acad. Sci. Paris 313, 929–936.

Guiraud, R., et Alidou, A., 1981. La faille de Kandi (B�eenin), t�eemoin du

rejeu fini-cr�eetac�ee d�un accident majeur �aa l��eechelle de la plaque

africaine. C.R. Acad. Sci. Paris 293, 779–782.

Guiraud, M., S�eeguret, M., 1987. Soft-sediment microfaulting related

to compaction within the fluvio-deltaic infill of the Soria strike-

slip basin (northern Spain). In: Jones, M.E., Preston, R.M.F.

(Eds.), Deformation of Sediments and Sedimentary Rocks (Spe-

cial Publication), 29. Geological Society of London, pp. 123–

Guiraud, M., Laborde, O., Philip, H., 1989. Characterization of

various types of deformation and their corresponding deviatoric

stress tensors using microfault automatic analysis. Tectonophysics

170, 289–316.

Huguen, C., Guiraud, M., Benkhelil, J., Mascle, J., 2001. Synlithifi-

cation deformation processes of the Cretaceous sediments of the

Ivory Coast-Ghana transform margin: a way to detail the margin

history. Tectonics 20 (6), 959–982.

Konate, M., Guiraud, M., Alidou, S., Clermonte, J., Drouet, J.J.,

Lang, J., 1994. Stucturation et dynamique s�eedimentaire du bassin

pal�eeozo€ııque en demi-graben de Kandi (B�eenin, Niger). C.R. Acad.

Sci. Paris 318, 535–542.

Konate, M., 1996. Evolution tectono-s�eedimentaire du bassin

pal�eeozo€ııque de Kandi (Nord B�eenin, Sud Niger). Th�eese Doctorat

Universit�ee de Nancy I, 302 p, unpublished.

Luning, S., Craig, J., Loydell, D.K., Storch, P.B., Fitches, B., 2000.

Lower Silurian �Hot shales� in North Africa and Arabia: regional

distribution and depositional model. Earth Sci. Rev. 49, 121–

Mc Clay, K.R., 1996. Recent advances in analogue modelling: uses

in section interpretation and validation. In: Buchanan, P.G.,

Nieuwland, D.A. (Eds.), Modern Developments in Structural

Interpretation (Special Publication), 99. Geological Society, pp.

201–225.

Mc Clure, H.A., 1978. Early Paleozoic glaciation in Arabia. Palaeo-

geogr. Palaeoclimatol. Palaeoecol. 25, 315–326.

Miall, A.D., 1977. A review of the braided-river depositional

environment. Earth Sci. Rev. 13, 1–62.

Miall, I.C., 1981. Early Paleozoic Pakhuis tillite, South Africa. In:

Hambrey, M.J., Harland, W.B. (Eds.), Earth�s Pre-Pleistocene

Glacial Record. Cambridge University Press, Cambridge, pp. 113–

Petit, J.P., Proust, F., Taponnier, P., 1983. Crit�eeres de sens de

mouvement sur les miroirs de failles en roches non calcaires. Bull.

Soc. G�eeol. Fr. 25 (7), 589–608.

Rust, I.C., 1981. Early Paleozoic Pakhuis Tillite, South Africa.

In: Hambrey, M.J., Harland, W.B. (Eds.), Earth�s Pre-Pleisto-

cene Glacial Record. Cambridge University Press, Cambridge,

pp. 113–117.

Scotese, C.R., Boucot, A.J., Mc Kerrow, W.S., 1999. Gondwanan

palaeogeography and palaeoclimatology. J. Afr. Earth Sci. 28,

99–114.

Seilacher, A., 1994. How valid is Cruziana stratigraphy. Geol.

Rundsch. 83, 752–758.

Seilacher, A., Alidou, S., 1988. Ordovician and Silurian trace fossils

from Northern Benin (W-Africa). N. Jb. Geol. Pal€aaont. Mh.,

Stuttgart 7, 431–439.

Seranne, M., Benedicto, A., Labaume, P., Truffert, C., Pascal, G.,

1995. Structural style and evolution of the Gulf of Lion Oligo-

Miocene rifting: role of the Pyrenean orogeny. Marine Petroleum

Geol. 12, 809–820.

Sutcliffe, O.E., Dowdeswell, J.A., Whittington, R.J, Theron, J.N.,

Craig, J., 2000. Calibrating the Late Ordovician glaciation and

mass extinction by the eccentricity of Earth�s orbit. Geology 28,

967–970.

Vaslet, D., 1990. Upper Ordovician glacial deposits in Saudi Arabia.

Episodes 13, 147–161.

Villeuneuve, M., Da Rocha Araujo, P.R., 1984. La stratigraphie du

bassin pal�eeozoique de Guin�eee (Afrique de l�Ouest). Bull. Soc. G�eeol.

Fr. 26, 1033–1039.

Visser, J.N.J., 1983. Glacial-marine sedimentation in the Late Paleozoic

Karoo Basin, southern Africa. In: Molnia, B.F. (Ed.), Glacial-

Marine Sedimentation. Plenum, Mt. Kisco, NY, pp. 667–701.

Sedimentation in the Kandi extensional basin (Benin and Niger): fluvial and marine deposits related...

Documents

Transcript of Sedimentation in the Kandi extensional basin (Benin and Niger): fluvial and marine deposits related...

The first Hirnantian (uppermost Ordovician) odontopleurid trilobite from Western Gondwana (Argentina)

Enrolment in a Middle Ordovician agnostoid trilobite

Style and sequence of extensional structures in tessera terrain, Venus

Deglaciation explains bat extinction in the caribbean

Ordovician conodonts from the Thong Pha Phum area, western Thailand

Early Ordovician (Floian) ostracods from the Cordillera Oriental, Northwest Argentina

The role of extensional structures on experimental calderas and resurgence

Animated models of extensional basins and passive margins

First Ordovician Foraminifera from South America: A Darriwilian (Middle Ordovician) fauna from the San Juan Formation, Argentina

The link between volcanism and deglaciation in Iceland

The hexactinellid sponge Cyathophycus from the Upper Ordovician of the Argentine Precordillera

Cirque glacier activity in arctic Norway during the last deglaciation

The Montecristo monzogranite (Northern Tyrrhenian Sea, Italy): a collisional pluton in an extensional setting

The oldest Ordovician foraminifers (Oepikodus evae conodont Zone, Floian) from South America

Continental Extensional Tectonics - Geological Society ...

Mesozoic extensional tectonics in the southeast Iberian Chain

A NEW, PHYLOGENETICALLY SIGNIFICANT EARLY ORDOVICIAN ASTEROID (ECHINODERMATA)

CONTROLS ON DOLOMITIZATION IN EXTENSIONAL BASINS

A new aglaspidid arthropod from the Lower Ordovician of Wales

Review of Ordovician Pelecypods