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 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