Palynological and Paleoenvironmental Investigation of the Campanian-Lowermost Maastrichtian Asata/Nkporo Shale in the Anambra Basin, Southeastern Nigeria

Abstract

Ditch samples recovered from interval 2216 to 2716m of Nzam-1 well in the Anambra Basin, Nigeria were investigated for sedimentological, palynological age dating and paleoenvironment reconstruction. The stratigraphic successions in the well include intercalated gypsiferous shale, shale, sandy shale and shaly gypsum of varying gypsum content. Palynological zone of Milfordia spp acme zone was established. The zone is characterized by maximum development of Milfordia spp., Longapertites sp 3 and first uphole appearance of Buttinia andeevi. Other forms that mark the zone are regular occurrence of Cupanieidites reticularis, Syncolporites subtilis, Cingulisporites ornatus, Trichotomosulcites sp. 1, Periretisyncolpites sp., Auriculiidites sp., Tricolpites gigantoreticulatus, Foveotriletes margaritae, Cupanieidites reticularis, Auriculiidites sp. and Constructipollenites ineffectus. The top of the zone is marked by the final appearance of Trichotomosulcites sp. 1, Milfordia jardinei, Cupanieidites reticularis and relative increase in Longapertites marginatus, Monocolpites marginatus, stephanocolporate pollen. The interval is particularly marked by maximum development of Milfordia spp. depicting Campanian to Lowermost Maastrichtian age. The paleoenvironment of deposition is marginal marine in nature defined by higher percentage of peridinaceans such as Senegalinium spp. and Andalusiella spp. over Gonyaulacysta forms. The relative position of the sea level and climatic condition is manifested on the lithofacies, characterized by the intercalation of shale and gypsum representing different times of transgression and lowstand phases.