Sedimentology and relative sea level changes recorded in the Cretaceous Abeokuta Group, offshore Eastern Dahomey Basin, Nigeria: a core-based study

Abstract

The stratigraphy and depositional environment of Cretaceous to Paleocene sediments in the Nigerian sector of the Dahomey Basin remain contentious due to limited outcrops and boreholes. Previous knowledge of the lithofacies of older sediments in the onshore portion was largely inferred from geophysical exploration, while the sedimentology and depositional setting of the offshore section have not been documented in the literature. This study investigates the sedimentary facies and depositional environments of the sediments from the offshore Eastern Dahomey Basin, Nigeria, using core samples from well DN (depth: 2096–2150 m). Record of the vertical succession reveals lithologic characteristics such as channelized sandstone units, sand/heterolithic interbeds, varying sand/silt/mud heterolithic capturing alternations of bioturbated and non-bioturbated intervals, several Glossifungites demarcated discontinuity surfaces, and occasional fossiliferous sandstone. Many of the units/facies are recurrent, defined by sedimentary structures such as wavy, laser, lenticular, bidirectional beddings, cross bedding, consistent mud laminae and mud-rapes, coalified detritals, reactivation surfaces, and inclined heterolithic stratification. Six facies associations typical of ancient estuary-barrier mouth sediments were delineated, including fluvial channel, tidally influenced fluvial channel, intertidal flats, tidal sand bars, estuarine embayment, and shoreface. From the depth, 2150 to about 2114.5 m, the cores exhibit characteristics of a tide-dominated estuary bearing several transgressive surfaces of erosion; in contrast, 2114.5 to 2096 m corresponds to a regressive package of wave-dominated shoreface complex, characterized by initial predominance of suspension feeding structures at the lower shoreface and a generalized shallowing-upward succession. Trace fossil analysis reveals recurrent Skolithos and Cruziana assemblages, suggesting a brackish setting. The recurrence of Glossifungites ichnofacies helps identify surfaces of erosion/transgression and other stratigraphic surfaces which further strengthen an estuary setting for sediments. Abeokuta Group is a macrotidal estuary, and our findings suggest that Ise Formation represents a fluvial region, the Afowo Formation is a tide-dominated estuary, and Araromi Formation is the wave-dominated shoreface. This study provides new insights into the sedimentological evolution of the Abeokuta Group in the offshore Eastern Dahomey Basin.