Abstract
A magnetostratigraphic study has been carried out to constrain the age of the volcano-sedimentary Ngwa formation in the eastern part of the Dschang region. A stratigraphic section of about 80 meters thick corresponding to 26 sites has been sampled, and it is composed mainly of fine-grained sandstones, clays, lignite, volcanic sediment and tuffs. A magnetic study conducted on 56 samples shows one or two components of magnetization carried either by titanomagnetite, magnetite and Fe-sulphide. The section that was sampled shows one normal polarity and one reversed polarity. In the lower part of the section, a K-Ar radiometric dating was performed on the plagioclase minerals isolated from the tuffs level situated about 15 meters above the lignite seam, and gave an age of 20.1 ± 0.7 Ma. Constrained by this age, the observed polarity zones can be readily correlated with chrons C6An.1n-C6An.1r of the GPTS. This study suggests that the age of the lignite is comprised between 20.04 Ma and 20.21 Ma. The mean direction of the characteristic remanent magnetization documents a counterclockwise vertical axis rotation of about 8° with respect to the expected Lower Miocene direction derived from the Africa polar wander curve.
Highlights
Most studied and well known continental basins from Cameroon are Mesozoic intracontinental basins such as Mamfe [1,2], Mbere Djerem [3,4], Babouri Figuil [5], Mayo Oulo Lere [6,7,8], Hamakoussou [9,10] and Mayo Oulo-Léré [6].little attention has been paid to the continental Cenozoic sediments
To better constrain the magnetic mineralogy, we studied the acquisition of isothermal remanent magnetization (IRM), and the stepwise thermal demagnetization of three-axis differential IRM following the method of Lowrie [20]
We first analyzed the acquisition of IRM (Isothermal Remanent Magnetization) up to 800 mT and its subsequent thermal demagnetization
Summary
Most studied and well known continental basins from Cameroon are Mesozoic intracontinental basins such as Mamfe [1,2], Mbere Djerem [3,4], Babouri Figuil [5], Mayo Oulo Lere [6,7,8], Hamakoussou [9,10] and Mayo Oulo-Léré [6]. Some fragmentary sediment lithofacies were identified at Ngwa Village (eastern Dschang) as well as some enclaves of sedimentary rocks randomly distributed in a volcanic complex [11]. Very little is known about the features of these sedimentary rocks, their age or provenance. Capponi [12] suggested a Tertiary age for the Dschang lignite in comparison with lignite known in southern part of Nigeria. The lignite seams found in Dschang and Bamenda are commonly brownish to black in color and vary in thickness from a few centimeters to a maximum of 1.8 m. They are thinly laminated and contain fossils of leaves, fruits and wood fragments. This study is of great importance, since it is the first one carried out in a Neogene continental formation of Cameroon
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