Abstract
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 Cambro- Ordovician sediments. From bottom to top they are: the Boumbouaka Supergroup made of sandstone, micro-conglomerates, 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. KEYWORDS: West African craton, Volta basin, Virtual Geomagnetic Pole, Palaeolatitude, Snowball Earth
Highlights
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)
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
They define Virtual Geomagnetic Poles (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
Summary
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). 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. 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. Buipe, Koundjouare and Barkoissi 1 and 2 are attached to the triad of the Middle Supergroup. The Kabalipe site is located in the Tamale Supergroup which constitutes the molasse of the Dahomeyides chain dated at Cambro-Ordovician
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