Palynostratigraphy and Paleoenvironment of Afikpo Well_3, Afikpo Syncline, Nigeria

Abstract

The Afikpo Syncline constitutes the western segment of the southern part of the Benue Trough. The syncline orients in the northeast-southwest direction and unconformably overlies the pre-Santonian strata of the Benue Trough. This study investigates the sediments of Afikpo Well_3, drilled within the syncline, identifies its different palynozones, and interprets their paleoenvironments of deposition. Forty-two ditch-cutting samples recovered from the well at a 150 ft interval consist of mudstones, calcareous mudstones, carbonaceous shales, and calcareous shales. These samples were subjected to palynological analysis following the conventional acid maceration technique. A total of 637 palynomorph counts were recovered. These comprise 316 pollen grains, which make up 49.61%; 129 spores, which make up 20.25%; 9 fungi spores, which make up 1.41%; 17 microforaminifera wall linings, which make up 2.67%; and 166 dinoflagellate cysts, which make up 26.06%. Two palynozones were identified within the well. These include zone AF-01 (Monocolpites marginatus assemblage zone), which ranges from depths of 1500–1320 ft and is Campanian in age; and zone AF-2 (Dinogymnium sp. assemblage zone), which extends from depths of 50 ft–1320 ft and dates back to the Maastrichtian period. Based on the predominance of palmae pollen, together with the subordinate occurrence of dinoflagellate cysts, and the fair representations of spores (Cyathidite minor), the environment of deposition of the sediments ranges from a nearshore brackish water to a shallow marine environment. The shallow environment is further confirmed by the frequent occurrence of microforaminifera wall lining and the dominance of the peridinoid dinoflagellate cysts over the gonyaulacinoids.