Stratigraphic Distribution of Globigerinoides ruber in the Upper Middle Miocene to Early Pliocene of Western Niger Delta, Nigeria

Abstract

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 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. INTRODUCTION The species Globigerinoides ruber (d'Orbigny) is generally described as being characteristized by 3 chambers in the last whorl with a high arched primary aperture positioned symmetrically above the sutures between the penultimate and antipenultimate chambers on the umbilical side, and two or more secondary or substituted apertures on the spiral side. Cordey (1967) differentiated Gds. ruber from the commonly suggested synonym Globigerinoides subquadratus Br6nnimann, on the grounds that the former has 31/2 chambers in the penultimate whorl and the latter has 4 chambers. Questions arise, however, in distinguishing between a half and a whole chamber. Separating the two taxa, according to Cordey (1967), is only possible when the tests are dissected and the ontogenetic development is taken into account. Globigerinoides ruber is quite cosmopolitan, with a welldocumented occurrence in the tropical and sub-tropical regions such as the Caribbean, Mediterranean, Atlantic, Indian and Pacific oceans, ranging from the earliest Miocene to the present. Its first appearance coincides with that of the Globigerinoides trilobus complex in the Early Miocene (Bolli and Saunders, 1985) in the upper part of Blow's Zone N.5 (Blow, 1969). There are two schools of thought on the morphology and stratigraphic distribution of the species. One school considers that the specimens from the Early and Middle Miocene (below the pseudoextinction zone) should be included in Gds. subquadratus, and would give the name Gds. ruber only to younger specimens (cf. laccarino 1985). The second school treats all of these specimens as one species, with various explanations of the interrupted occurrence in the upper Middle Miocene (cf. Bolli and Saunders 1985; Martinotti 1990). The reappearance of the species is, however, recorded at different levels. Some authors put this in the upper part of the upper Middle Miocene Zone N. 14 Globorotalia mayeri Zone (Liska 1985), some in the Upper Miocene (Bolli 1966a; Bermudez and Bolli 1969) and some in the lower Pliocene (Bolli 1966b; Thunell and Belyea 1982). This species is also known to include well recognized variants. Liska (1985, p. 377) figured some specimens of Gds. ruber from the Middle and Late Miocene of Trinidad and Jamaica that show variations in the shape, size and position of the primary aperture. Those pictured with a side view show a more or less reduced spire. Some of the author's figures are also comparable with those figured by Bismuth et al. (1994, p. 520) from the Pliocene of Tunisia. Boltovskoy (1968) and Adegoke et al. (1971) distinguished variations in this taxon in Recent forms from the Gulf of Guinea based on the heights of the spire, and some authors also used the color of the tests to identify forma, especially in Recent specimens. Comparable morphological variation is also observed in the Miocene to Pliocene specimens obtained from well MJI-20 well in the Niger delta. Having studied the various forms in some detail, I conclude that all of the specimens in our samples, from middle Miocene to the top of the core, should be considered as one species, Globigerinoides ruber, because the differences, while consistently appearing in populations of a given age interval, are rather subtle. Therefore, what is recorded as Globigerinoides subquadratus in the literature will be ascribed here to Gds. ruber. The various morphological forms that have been recognized in the MJI-20 well samples are therefore described for reference purposes, without ascribing the subtle morphologic variations to a formal taxonomic level. Interestingly, some of the forms reach abundance acmes at different intervals, where they are also associated with micropaleontology, vol. 44, no. 2, pp. 173-186, text-figures 1-4, plates 1-3, 1998 173 This content downloaded from 157.55.39.58 on Wed, 16 Nov 2016 04:24:09 UTC All use subject to http://about.jstor.org/terms Victor B. Adeniran: Stratigraphic distribution of Globigerinoides ruber; Middle Miocene to Early Pliocene, western Niger Delta, Nigeria TEXT-FIGURE 1 Location map of OKW 73, MJI and BRM 1 wells in the western part of the Niger Delta. Inset is the map of Africa and Nigeria, and the study area along the west coast of Nigeria. some more diagnostic species, such as Globigerinoides obliquus obliquus Bolli (upper Zone N. 14), Globigerinoides obliquus extremus Bolli and Saunders (Zones N.16-21) and Globigerinoides ruber seigliei Bermuidez and Bolli (Zones N. 16-18). The latter species is particularly helpful in dating the occurrence of Gds. ruber in the well, as well as the correlations made with similar occurrences in other regions. This work is an aspect of a broader project that attempts to produce biozones that could be correlated across depo-belts in the Neogene of the Niger Delta, emphasizing the possible role of the variation in Gds. ruber because of its abundance in these sections. The stratigraphic distribution of the species is compared with other occurrences in tropical and sub-tropical regions, for possible correlations. This could be helpful in determining absolute ages within its interval of occurrence in the well, especially because other index species are rare. Chronostratigraphic ages in this study are in reference to the zonations of Blow (1969, 1979) and Bolli et al. (1985, p. 6). REVIEW OF THE GEOLOGY The Niger Delta is a typical regressive clastic sequence, subdivided into three lithostratigraphic formations (text-figure 4) based on their depositional characteristics (Short and Stauble 1967). The oldest unit, the Akata Formation, consists of marine black shales interbedded with occasional sands and silts considered to be turbidites and continental-slope channel fills. This is overlain by the Agbada Formation, which is characterized by alternating sand and shale sequences believed to have been deposited under paralic conditions. The youngest is the Benin Formation, predominantly made up of coarse sands that have been described as continental in origin, although in the eastern part of the delta, the formation contains some deep-water clay fills that have been described as submarine canyon deposits (Burke 1972). The delta has been variously described as balanced (Galloway 1975) or wave dominated (Weber 1987). The modern delta be-