Occurrence of some campanian-maastrichtian organic-walled microfossils from Enugu Shale, Anambra Basin, Southeastern nigeria: Implications for age and paleoenvironments

Okechukwu Nicodemus Ikegwuonu,Kingsley Chukwuebuka Chiadikobi,Osita Igwebuike Chiaghanam,Nnamdi Enock Nwakoby,Emmanuel Ude Aniwetalu

doi:10.4314/gjgs.v19i1.11

Abstract

Detailed geological field mapping and sampling of the Enugu Formation in the Anambra Basin has been carried out in order to re-examine the age of sediments and reconstruct their paleoenvironments of deposition. A total of ten (10) outcrop samples of shale were subjected to palynological laboratory examination, using conventional method of acid demineralization and maceration techniques for recovering acid-insoluble organic-walled microfossils from sediments. Two main lithological units were distinguished: - carbonaceous fissile shale and siltstone. A late Campanian - Earliest Maastrichtian age was assigned based on index palynomorphs marker taxa Longapertites marginatus (overwhelming abundance), Monocolpites marginatus, Zlivisporis blanensis, and Echitriporites trianguliformis. The age designation was strengthened by the occurrence of a well-known stratigraphic age-diagnostic organic-walled microplankton Coronifera tubulosa, Senegalinium spp. and Andalusiella polymorpha. Palynomorphs of environmental value include Cyathidites minor, (a tree fern of wet, forested, tropical to temperate regions, usually most developed in mountainous / highland terrains under moist and equable climate); Spinizonocolpites baculatus/echinatus, Longapertites marginatus, Mauritidites crassibaculatus and Moncolpites marginatus, which are palm pollen that inhabit similar brackish water as the mangrove. A non-marine to marginal marine depositional setting has therefore been proposed for the Enugu Formation.

Highlights

The Anambra Basin of southeastern Nigeria is a funnelshaped, concave south structure which was filled with sediments of Campanian-Maastrichtian age (Murat, 1975; Umeji, 2000) (Fig. 1)
MATERIALS AND METHODS The materials for this study were obtained by sampling the shale intervals of the Enugu Formation
The carbonaceous fissile shale samples generally yielded moderately rich palynomorph assemblage. Terrigenous species such as fern spores were the most abundant in almost all the examined samples, followed by pollen species of mangrove affinity. Marine species such as dinoflagellate cysts recorded very few presence.The peridiniacean species with a proximate cyst affinity predominates over the gonyaulacaceans

Summary

Introduction

The Anambra Basin of southeastern Nigeria is a funnelshaped, concave south structure which was filled with sediments of Campanian-Maastrichtian age (Murat, 1975; Umeji, 2000) (Fig. 1). The Nkporo Group (Nwajide, 2006) forms the oldest unit in Anambra Basin, to which the Enugu Formation belongs. Simpson (1954, p.34) described the soft grey-blue shale in the channels of Asata and the Ogbete Rivers, and noted that very thin lenses of ‘‘vitrinite’’ may occur in dark shales, an unusual 2.5cm band of coal crops out near the top of the formation. Ladipo et al (1992) reports that the facies of the Nkporo Group are inferred to be pro-delta to delta front environments. They noted that the shaly aspects of the group, with their mixed arenaceous and planktonic

Methods

Results

Conclusion