| Popis: |
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. |
