ABSTRACT: A detailed study on the distribution and morphology of Discoaster quinqueramus carried out within the Late Miocene NN11 zone of three basins (NigerDelta, Benin and Joint Development Zone of Nigeria/Sao Tome and Principe) in the Gulf of Guinea revealed the stratigraphic value of the taxon in terms of age and particulary the subzonal resolutions. This index calcareous nannofossil is widely distributed throughout the Niger Delta and the adjoining basins. The importance as a stratigraphic tool in regional and field-specific studies in the region, was found to be aided by the recognized size and shape variations, aswell as the distributional patterns throughout approximately a-three million year stratigraphic range. These three distinctive morphotypes - Discoaster bergenii, Discoaster berggrenii and Discoaster quinqueramus were clearly distinguished. They evolved as a medium-sized, 5-rayed, star-shaped form, with about a double distance relationship of the central area to the free ray length (Discoaster bergenii). They gradually developed into robust forms with a gradual reduction in the central area, having the free ray length approximately double of the central area (Discoaster berggrenii). Thiswas succeeded by formswith further reduction in the central area to the free ray lengthwhich sometimes tripled the central area (Discoaster quinqueramus) at the upper part of the zone. Knowledge of the size with shape variation, their quantitative relationship as well as the distribution of the morphotypes, have been used to determine their relative positions within the NN11 stratigraphic zone. These criteria enhanced the delineation of three stratigraphically important subzones thus â€“ Discoaster bergenii subzone, Discoaster berggrenii abundance subzone and Discoaster quinqueramus abundance subzone.
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