PALAEOMAGNETISM OF NEOPROTEROZOIC FORMATIONS IN THE VOLTA BASIN

The Volta basin lies on the southern part of the West African craton, more precisely on the Leo (or Man) craton. The Dahomeyides chain is thrust onto its eastern fringe. The Volta basin is filled with Neoproterozoic to CambroOrdovician sediments. From bottom to top they are: the Boumbouaka Supergroup made of sandstone, microconglomerates, siltstones, shales and limestone lenses; the Pendjari or Oti Supergroup composed of a triad made of a diamictite, a cap carbonate and a silexitic complex that passes upwards to siltstones and shales; the Tamale Supergroup representing the molasse of the Dahomeyides chain. The three supergroups were sampled for palaeomagnetic study. The magnetic mineralogical study shows magnetite and hematite as carriers of magnetization. Mean palaeomagnetic directions are calculated on the high temperature components and yield a mean palaeopole, Plon=349.3°, Plat=44.1°, dp=26.6°, dm=33.7°, for formations with ages between 993±62 Ma and 660±9 Ma and a mean palaeopole, Plon=119.5°, Plat=71.2°, dp=19.8°, dm=38.1°, for sites dated between 635 Ma and 600 Ma which are, respectively, the ages of the marinoan glaciations in Volta basin and the Pan-African deformation, responsible of the Dahomeyides chain. The palaeolatitudes of the older formations about 44.9° S and that of the younger sites about 9.1° S show a migration of the West African craton from medium to low latitude during the Neoproterozoic, in conformity with the Snowball Earth hypothesis.


INTRODUCTION
The Volta basin is located in the southern part of the West African craton and stretches between longitudes 2° W and 3° E and latitudes 6 and 13° N (Fig. I).Its monoclinal western part grades eastwards to the folded external units of the Dahomeyides chain.The lithostratigraphy of the basin includes Neoproterozoic to Cambro-Ordivician sediments represented from bottom to top by the Boumbouaka, Oti and Tamale Supergroups which are separated by cartographic and/or erosional unconformities (Affaton, 1990).The base of the Middle Supergroup consists of a triad made of glaciogenic formations, cap carbonate and silexite that define a lithostratigraphic marker horizon in many basins on the West African craton.The glaciogenic formation in the Volta basin is considered the equivalent of that Jbeliat Group described in the Taoudeni basin situated in the north and attributed to the Marinoan glaciation (Deynoux et al., 2006).
In the Snowball Earth hypothesis (Kirschvink, 1992), the Sturtian and Marinoan glaciogenic formations are considered to be formed at low latitude (Hoffman et al., 1998).In order to constrain the palaeolatitude of formation of deposits in the Volta basin that include glacial sediments attributed to the Marinoan (Porter et al., 2004;Nedelec et al., 2007), seven sites were sampled for a palaeomagnetic study.Two sites, Boumbouaka 1 and 2 were located in the Boumbouaka Group of the Lower Supergroup.Four sites, Buipe, Koundjouare and Barkoissi 1 and 2 are attached to the triad of the Middle Supergroup.Buipe and Koundjouare belong to the cap carbonate of the glaciogenic formation and Barkoissi 1 and 2 to the horizon overlying the cap carbonate.The Kabalipe site is located in the Tamale Supergroup which constitutes the molasse of the Dahomeyides chain dated at Cambro-Ordovician.

Lithostratigraphy
The lithostratigraphy of the Volta basin (Fig. 2) consists of three supergroups separated by unconformities.The Boumbouaka lower Supergroup, about 1000 m thick, starts with the Dapaong Group which is made of sandstone, micro-conglomerates, siltstones and shale.It passes upwards to the Fosseaux-Lions Group composed of conglomerates, microconglomeratic sandstone, shale and siltstones containing limestone lenses.At the upper part, the Mount Bombouaka Group (or Yembouré) is made of feldspathic sandstone, ferruginous sandstone containing silty and /or conglomeratic intercalations, siltstones and more or less ferruginous shale.The Fosse-aux-Lions ferruginous Group is dated by the Rb-Sr method on fine argillaceous fractions at 993±62 Ma (Clauer, 1976).The Pendjari (or Oti or Afram) middle Supergroup, about 2500 to 4000 m thick, rests in cartographic and/or erosional unconformity pro-parte of glacial origin on the Boumbouaka Supergroup or directly on the Eburnean basement.It starts with the Sud-Bamboli Formation composed of glaciogenic sediments and cap carbonate and passes to the Barkoissi Formation made of clayey silexite containing intercalations of shale, siltstones and sometimes limestone.At the top, the Formation is made of siltstones and shales with intercalations of limestone lenses, sandstone or phospharenites and silexite bearing conglomerate.This Formation contains Chuaria Circularis, fossils of Vendian age (Amard and Affaton, 1984).It is dated by the Rb-Sr method on fine illite fraction at 660±9 Ma (Clauer, 1976).The Tamale upper Supergroup, about 500 m thick, rests in angular unconformity on the Pendjari Supergroup.It starts with the Yendi Group made up of conglomerates, shales and siltstones.At the upper part, the Kebia Group is composed of diamictites, polygenic conglomerate and sandstones passing upwards to micaceous sandstones, shales and siltstones then to finely bedded sandstones with intercalations of shales and massive feldspathic sandstones.This supergroup which represents the molasse filling the basin results from the active erosion of the adjacent Dahomeyides chain (Affaton, 1990).It is of Cambro-Ordovician age.

Petrography
The studied specimens included clastic rocks and non clastic rocks (Fig. 3).The clastic rocks of Boumbouaka 1 and 2 are made of fine grained 92 F. BOUDZOUMOU, D. VANDAMME, P. AFFATON AND J. GATTACCECA sandstones to siltites (ph.A).Quartz is the dominant mineral.Feldspars and micas are present in very small amount.The quartz and feldspar grains are sub-angular to sub-blunt.The cement is made of sparitic carbonate.It is abundant in some samples which can be classified as sandy carbonates.Grains of iron oxides are frequent.The carbonate rocks described are from the Buipe and Koundjouare display micritic to microsparitic limestone facies which have parallel or locally discordant laminae.These laminae are formed by an alternation of horizons with and without peloids (ph.B).The samples also show horizons (levels) containing needles and stars of epigenized gypsum and also of detrital quartz (ph.C).
Keystone-vugs or geodes, empty or infilled with microsparite or sparite, that is sometimes associated with quartz, are present in places (ph.B).Facies with algal mat structures appear in some samples (ph.D).These algal levels can alternate with peloids bearing levels.Lithoclasts and probably bioclasts bearing limestone facies and rare quartz and feldspar clastic grains are present.They are cemented by microsparite or sparite (ph.E and F).The lithoclasts can be micritic or recrystallized into sparite.Fibrous calcite generally, fills fissures observed in some samples.Styloliths are accompanied by iron oxide (ph.F).

Method
The samples were taken in the field using a portable petrol powered drill oriented with a solar or magnetic orienter and were cut in small cylindrical specimens (2.5 cm × 2.3 cm) then stored in a nonmagnetic chamber in which the measures of magnetization were taken.Samples were subjected to thermal demagnetization in order to isolate the Characteristic Remanent Magnetization (CRM).The magnetization of the silexite samples from the Barkoissi 1 and 2 sites was measured using a JR5 (Agico) continuous rotation magnetometer.That of the carbonates and sandstones from other sites was done with a SQUID (2G enterprises) cryogenic magnetometer.The directions of the Characteristic Remanent Magnetization (CRM) are determined by principal component analysis (Kirschvink, 1980) after selection of the linear segments on Zijderveld's (1967) orthogonal projection.The mean direction at each site was calculated using Fischer's (1953) statistics.The data were processed using Cogne's (2003) Paleomac 6.1 software.

PALAEOMAGNETISM OF NEOPROTEROZOIC FORMATIONS IN THE VOLTA BASIN
Three methods were used to determine the mineral carriers of magnetization.Two to three specimens from each site were subjected to thermomagnetic studies usiging an MFK1-MFA susceptibilimeter equipped with a CS3 furnace.The Lowrie's (1990) method was also used with application of fields 3T, 0.4T, and 0.12T respectively to three orthogonal axes of the sample.The acquisition of IRM by the samples was realized using an MMPMQ field pulse magnetizer.Finally, hysteresis

RESULTS
The samples carry a Natural Remanent Magnetization (NRM) with intensity between 0.4 and 8.2×10 -3 A/m.The mean at each site is shown in Table I.

Thermomagnetism
Nineteen samples, reduced to powder, were processed in argon and heated to temperatures of up to 670° C then cooled to room temperature.The results show a generally weak initial susceptibility signal situated between 0.3 and 14.1 -10 m 3 .The curves obtained are either reversible (Fig. 4a) or irreversible (Fig. 4b).The irreversible curves show an increase in susceptibility between 400° and 500°C during heating and about 580°C during cooling (Fig. 4b).Such an increase in susceptibility has been observed in the Gourma basin (Boudzoumou et al., submitted) and has no influence in the magnetization direction.Magnetite and hematite are indentified by the drop of the susceptibility respectively at 580°C (Fig. 4b) and at 650°C (Fig. 4a).
Lowrie's test.Lowrie's (1990) test is applied to 9 samples.Depending on the sample, the highest intensity is carried by the hard (Fig. 5a, b), the medium (Fig. 5c) and the soft fraction (Fig. 5d).The release temperatures for the hard fraction lie between 650°C and 670°C.Those for the medium fraction are, depending on the sample, situated at 575°C (Fig. 5 c, d) and at 650°C (Fig. 5 a, b).The soft fraction shows a release temperature of 575°C (Fig. 5a-d).These release temperatures lie between 575°C and 670°C and are characteristic of magnetite and hematite minerals respectively.

Hysteresis cycles
The hysteresis cycles were realized on 9 samples subjected to a 1000 mT field.The results reveal saturated and non-saturated magnetic minerals.The curves obtained, corrected for slope, are variable in shape (Fig. 6): (I) A normal shape (Fig. 6a); (2) A wasp shaped size (Fig. 6b) attributed to a bimodal distribution of the hard and soft coercive fractions that may be due to a difference in grain size of the same mineral or to different magnetic minerals (Raposo et al., 2003(Raposo et al., , 2006;;Piper and Darabi, 2005) like magnetite and hematite; (3) The specimens show a largely open shape (Fig. 6c) that characterizes the presence of strong coercivity ferromagnetic grains such as hematite.The thermal demagnetization curves indicate one to three principal components, carriers of magnetization.In the samples with one component, the latter is destroyed at temperatures, either of 575°C or of 670°C characteristic of magnetite and hematite respectively (Fig. 7a).For the samples with two components (Fig. 7b), the first component is destroyed at 470°C.It is a carrier of secondary magnetization.The second component is destroyed at 575°C or 670°C.In the samples with three components, the first two components are destroyed at 300 and 470°C respectively.The last component is destroyed at 575°C or 670°C.

DISCUSSION
The data provided by magnetic mineralogy show that magnetization is carried by magnetite and hematite.One, two or three components were identified in the samples.The mean directions of the sites are located mainly in the NW and NE quadrants.Only the Koundjouare site occurs in the SW quadrant.The oldest formations of Boumbouaka 1 and 2 consist of fine to very fine sandstone, feldspathic with carbonate cement.They define VGP, Plon=342.6°,Plat=47.9°,dp=21°, dm=25.5°and Plon=355.2°,Plat=39.9°,dp=9.8°,dm=10.9°respectively, yielding high palaeolatitude values of 49.6°S and 59.6°S.These high palaeolatitudes are those that are attributed to the West African craton in models of the reconstruction of the supercontinent of Rodinia before 750 Ma.Such high palaeolatitudes found at Boumbouaka 1 and 2 well confirm the age between 993±65 and 660±9 Ma attributed to these two virtual geomagnetic poles.Nevertheless, the two VGP are close to those of the Adma Diorite, Plon=344°, Plat=34°, dp=16°, dm=17°, described in the Adrar of Iforas, dated between 620 and 590 Ma (Morel, 1981).

CONCLUSION
The palaeomagnetic study of the Volta basin has improved the data base of the Neoproterozoic Formations of the West African craton.The magnetic mineralogy consists of magnetite and hematite, carriers of magnetization.The mean direction obtained from the high temperature components allow the calculation of the VGP's at the different sites.The Boumbouaka sites 1 and 2 are made of fine to very fine grained sandstones and define a medium palaeopole of Plon=349.3°,Plat=44.1°,dp=15.4°,dm=18.2°dated between 993±65 Ma and 660±9 Ma.The associated palaeolatitude is 44.9°S which places the West African craton in a medium palaeolatitude as in the reconstruction models of the Rodinia supercontinent before its fragmentation (Meert and Torsvik, 2003).The Marinoan cap carbonate of Buipe and Koundjouare and the Barkoissi 2 silexites that overlie them give a medium palaeopole of Plon=119.5°,Plat=71.2°,dp=9.7°,dm=18.6°dated between 635 Ma and 600 Ma.This palaeopole is associated with a palaeolatitude of 9.1°S which now places the West African craton at a low palaeolatitude in conformity with the Snowball Earth hypothesis which stipulates that all continental masses were located at low latitudes during the Marinoan epoch (Kirchvink, 1992;Hoffman et al., 1998;Hoffman and Schrag, 2002).The two palaeolatitudes of the two medium palaeopoles show a migration of the West African craton from medium to low latitude toward the end of the Neoproterozoic.

Fig. 7 .
Fig.7.Thermal demagnetization of characteristic samples from Barkoissi 1 and Boumbouaka 1 sites.a) Intensity curves as a function of temperature; b) [Zijderveld's diagrams (The open/ full circles correspond to projections in the lower / upper hemisphere): 1b -one component; 2b-two components; 3b-three components]; c) Stereographic projections of demagnetization data.

Table 1 :
Mean directions of Volta basin sites.