A revised stratigraphy of the Bida Basin, Nigeria

Abstract

The Bida Basin is a NW-SE trending inland basin, stretching from Shegwa (NW) to Dekina (SE). It is approximately 350 km long and varies in width from 75 to 150 km. It is roughly elliptical in ground plan and runs perpendicularly to the western margin of the NE-SW trending Benue Trough Complex. Unlike other sedimentary basins in Nigeria, it is characterized by the absence of volcanics, carbonates and rocks of Tertiary age. Dedicated fieldwork carried out over the past four years revealed that the basin is underlain by mainly continental sediments with minor occurrence of marginal/shallow marine to freshwater flood-plain deposits. The initial basin fill is characterized largely by several cycles of simple, but amalgamated and/or stacked fanglomerates (alluvial fan) systems developed from several single stream breaches onto the proximal setting of the basin. These fanglomerates directly overly the Basement Complex and display a fining upward sequence. This unit is generally accepted as the Lokoja/Bida Sandstone. Our studies revealed that the Lokoja/Bida Sandstone is overlain directly by the oolitic/pisolitic ironstone of the Agbaja/Sakpe Ironstone which is in turn succeeded by a sequence of rocks generally referred to as the Patti/Enagi Formation, which is comprised of predominantly fine to medium-grained, grey and white sandstones, grey clays, carbonaceous silts and shales, and ubiquitous concretionary ironstone bands. This observation is opposed to the widely accepted view of previous workers that considered the Agbaja Ironstone as the youngest formation in the basin. This study further revealed that the sequence of rocks used to define the Patti Formation does not exist at the location of Mount Patti in Lokoja, rather, it is found in road cut sections, quarries and boreholes around the Ahoko Village. Following the rules for stratigraphic nomenclature, it is hereby proposed that the name ‘Patti Formation’ should be changed to ‘Ahoko Formation.’ A new geological map was made for the basin which clearly demonstrates the relationship of the formations.