Peridinioid Dinoflagellate Cysts from the Oligocene of the Niger Delta, Nigeria

Abstract

An abundant and diverse assemblage of Oligocene peridinioid dinoflagellate cysts has been recovered from 15 boreholes in the Niger Delta. Seven new species belonging to Lejeunecysta, two new species of Phelodinium and one new species of Selenopemphix are proposed. Four new species, tentatively assigned to Lejeunecysta, are described and left in open nomenclature, due to uncertainty as to generic rank. Lejeunecysta fallax is emended. INTRODUCTION In the course of oil exploration, palynological analyses were carried out on fifteen boreholes of subsurface Tertiary sediments in the Niger Delta (text-fig. 1). The Oligocene samples studied yielded rich dinoflagellate cyst assemblages. Specimens of Lejeunecysta (Gerlach) Artzner and Dbrh6fer, 1978, Phelodinium Stover and Evitt, 1978, and Selenopemphix Benedek, 1972 emend. Bujak, 1980 are particularly abundant. The present paper formally describes seven new species of Lejeunecysta, two new species of Phelodinium and one new species of Selenopemphix. Lejeunecysta fallax (Morgenroth) Artzner and D6rh6fer, 1978 is emended, and four new species, tentatively assigned to Lejeunecysta, are described and left in open nomenclature, owing to the uncertainty at generic rank. Many problems confront one when trying to describe and speciate an abundant and morphologically intergrading group such as the Oligocene peridinialean dinoflagellate cysts. In fact, the species of Lejeunecysta (as Lejeunia Gerlach, 1961) have hitherto been for the most part poorly defined. We here make an attempt to define certain morphological criteria for speciating the Lejeunecysta cysts, in order to identify potentially useful stratigraphical markers. Harland (personal communication, 1981) believes that the archeopyle shape and position, a continuous or discontinuous paracingulum, compression (as it reflects the natural three-dimensional shape of the cyst), and overall size are important criteria for speciating these cysts. Additionally, we believe that the ornamentation features of the autophragm, the shape and length of the antapical horns, the occurrence of an apical horn, its shape and length, and the length/breadth cyst ratio have also to be considered as criteria for speciation. These criteria are, above all, particularly important for the laevigate cysts, which up to now have been generally placed in either Lejeunecysta hyalina (Gerlach) Artzner and D6rh6fer, 1978 or Lejeunecysta fallax. The type material of this study is stored at AGIP S.p.A., SGEL Service (Geological Studies and Laboratories), Palynology Unit, S. Donato Milanese, Milan, Italy. The specimen coordinates are from the Leitz Orthoplan microscope, and all microphotographs were made with an Orthomat camera. GEOLOGIC SETTING AND STRATIGRAPHY The Eocene-Pliocene sediments of the Niger Basin are essentially deltaic sands and shales, which reflect the different environments of demicropaleontology, vol. 29, no. 2, pp. 126-145, pls. 1-7, 1983 126 This content downloaded from 157.55.39.195 on Thu, 14 Apr 2016 06:27:24 UTC All use subject to http://about.jstor.org/terms Micropaleontology, vol. 29, no. 2, 1983 TEXT-FIGURE 1 Location map showing Niger Delta region and studied areas. OML 60 and 61 are the Nigerian AGIP Oil Company permits in which studied wells are situated. position. In the Niger Basin three main lithostratigraphic units are known: 1) Benin Formation: sands with a few shale intercalations: sometimes lignite occurs in thin streaks. The sands are considered to represent deposits of a continental upper delta plain, while the shales may be interpreted as backswamp deposits. 2) Agbada Formation: alternate sandstones and sand bodies, typical of a lower tidal coastal plain-littoral environment, with marine shale layers. 3) Akata Formation: prodelta-open marine shales. The age of these formations ranges from Eocene to Recent, owing to progradation of the Niger Delta (Short and Stauble, 1967; AGIP-NAOC, internal reports). In this paper, we consider only the Oligocene sedi-