Depositional framework and stratigraphy of the Konshisha area, southern Benue trough

Abstract

The study focuses on the stratigraphic succession and depositional environment in areas around Konshisha, southern Benue Trough. The area is underlain by the Ezeaku Formation and Konshisha River Group. Integrated outcrop, textural and petrographic analysis aided inference of the depositional framework of sediments in the area. Three lithofacies were identified – micaceous sandstone (Facies A), limestone (Facies B), and grey shale (Facies C). Textural analysis indicated that the sandstones are fine grained with mean size ranging from 2.2 to 2.8 phi, very poorly to poorly sorted and deposited in a fluvially-influenced shallow marine setting. Petrographic analysis of the sandstone showed that the sediments have undergone compaction as a result of tectonic stress, shown by the elongate shape of some quartz grains, dominant line contact, and the tilted nature of the beds. Sandstones are both compositionally and texturally sub-mature, implying a proximal source. Results from the petrographic analysis of the limestones reveal micrite as the dominant matrix, with skeletal fragments, pelloids, recrystallized bivalve shells and some mineral grains (quartz and muscovite) present. The depositional environment is probably lagoonal, evidenced by the fine lamination of the limestone beds and low diversity. This study suggests an initial transgressive phase in the Turonian, with the deposition of the grey shale and limestone facies in a shelf – lagoonal setting, and a subsequent regressive phase that led to the deposition of micaceous, ripple-laminated sandstones in a fluvially-influenced shallow marine environment.Keywords: Stratigraphy, petrography, facies, depositional environment, Turonian