Abstract
A total of sixty-eight (68) core samples retrieved from eleven wells penetrating the Mamu Formation in the Anambra Basin, Southeastern Nigeria were studied for their palynomorph content in order to determine the age and paleoenvironment of deposition of the Mamu Formation. The wells are characterized by intercalation of laminated fine grained sand and light-dark grey fissile shale with interbeds of coal seams. The lower part with dark grey shale is characterized by the maximum development of Longapertites marginatus Acme Zone, dated Middle Maastrichtian. The upper part defined by intercalation of sand and shale with coal seam layers interbedding belong to Spinizonocolpites baculatus Assemblage Zone. This zone is characterized by the occurrence of Spinizonocolpites baculatus, S.echinatus, Gemmazonocolpites sp, Constructipollenites ineffectus, Syncolporites marginatus, Periretisyncolpites gigantus, Zlivisporites blanensis, Retidiporites magdalenensis, Distaverrusporites simplex, Foveotriletes margaritae, L.vaneendenburgi, Proxapertites cursus, Proteacidites sp, Buttina andreevi, Periretisyncolpites sp and Monocolpites marginatus. All these forms characterize Upper Maastrichtian age on the basis of the recognized diagnostic forms in the wells. The palynomorph abundance and diversity peaks and fossil assemblage of Mamu Formation show similarity with South American forms characterized by colder and warmer climate. These are directly related to eustatic change in sea level resulting to transgression and regression phases. The marine incursion resulted in Mid-Maastrichtian leading to the deposition of basal shale sequence of Mamu Formation while the coal seams were deposited during the regressive phases in the Late Maastrichtian.
Highlights
The Mamu Formation occurs extensively in the Anambra Basin in southeastern Nigeria (Fig. 1)
The Maastrichtian Coal Measures in the Anambra Basin were investigated for their miospore content
Lithostratigraphic description carried out on the core samples revealed that the wells are characterized by intercalations of white to light grey, fine grained laminated sandstone and light to dark grey, fissile shale with interbeded coal seams
Summary
The Mamu Formation occurs extensively in the Anambra Basin in southeastern Nigeria (Fig. 1). The middle part of the stratigraphic section is characterized by the assemblages of Gemmamonocolpites sp, Foveotriletes margaritae, Proxapertites cursus, Monocolpites marginatus, Buttinia andreevii, Periretisyncolpites giganteus, Periretisyncolpites sp, and relatively high frequency of Monosulcites sp The assemblages of these forms are similar to those described for Late Maastrichtian sediments by Lawal and Moullade (1986). Other forms present at the (166-163 m) are Proteacidites sp, smooth trilete spore, Zlivisporites blanensis and relative occurrence of Monosulcites (Fig. 14a) This assemblage of pollen and spores is similar to those reported by Van Hoeken-Klinkenberg, (1964, 1966) for the Upper Cretaceous pollen recovered in the Mamu Coal Measures. The presence of Maastrichtian markers such as Cingulatisporites ornatus, Buttinia andreevi, Retidiporites magdalenensis, Monocolpites marginatus, Longapertites vaneendenburgi, Proxapertites cursus, Auriculiidites sp and dinoflagellates cysts such as Batiacasphaera sp, Senegalinium sp and Andalusiella polymorpha are indicative of brackish to shallow marine environments. Few forms such as monocolpate pollen, trilete spores, fungal spore, and algae present could suggest a lacustrine depositional setting for the sediments
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