Stratigraphic distribution of Globigerinoides ruber in the upper Middle Miocene to Early Pliocene of western Niger Delta, Nigeria

In well MJI-20, western Niger Delta of Nigeria, Globigerinoides ruber is found in samples ranging from Middle Miocene to lower Pliocene levels, and is the predominant taxon among the planktonic foraminifera recovered from upper middle Miocene onwards. Within the sampled interval, the species exhibits some morphological variations, especially in the shape and size of the apertures. Its occurrence can be divided into four zones.In the first zone, between 7,353 and 7,133 feet below seafloor, the species occurs frequently but not abundantly, with form variant c, succeeded by form d, in the latest part of Zone N. 13 (Globigerinoides ruber Zone). In the second zone (7,133 - 5,007 feet), specimens of Gds. ruber form d were recovered in only three samples and in very low numbers, representing the "pseudoextinction interval" of the species as described in the Caribbean, Atlantic, and Mediterranean in the lower part of Zone N.14. A transition between the second and third zone is characterised by the reappearance of Gds. ruber form c in the uppermost part of Zone N.14, at the top of a subzone characterized by abundant Globigerinoides obliquus obliquus. The third zone (4,787 - 4,504 feet) is the acme of Gds. ruber in this core, in association with Globigerinoides obliquus extremus and Globigerinoides ruber seigliei, suggesting Zones N.16-N.18. In this interval, forms c and b are both common. The fourth zone (4,476-4,034 feet), dominated by Gds. ruber form a, is above the last appearance of Gds. o. extremus and Gds. r. seigliei, and is interpreted as Pliocene (=N18-21), consistent with the occurrence of Pliocene palynomorphs in these samples.