Lithofacies and paleo depositional environment of the rocks of Nkpuma-Akpatakpa, Izzi, Southeast Nigeria

Abstract

A detailed geological investigation of Nkpuma-akpatakpa, Izzi, Southeast Nigeria, done on a map scale of 1: 25,000, showed its lithofacies as brown shales, dark-grey shales, mudstones, siltstones and limestones. These lithofacies have been organized into two main lithostratigraphic units namely, brown shales (Unit A) and dark-grey shales (unit B). The brown colour of shales (unit A) suggests high content of ferric oxide due to intense weathering and low metamorphism caused by volcanism in the area. The dark colour of shale (unit B) suggests deposition in anoxic or reducing environment. Its dark-grey colour and the presence of bivalves suggest deposition in a low energy, marine environment. The abundance of illite which increases with depth indicates deeply buried shale not less than 3,500 m below surface. The presence of shelly limestone indicates shallow, warm, silt-free and clean marine environment. The intact shells of bivalves suggest deposition in a low energy protected shoreline where wave action is limited. Sedimentary structures like fissility and laminations, also suggest deposition in low energy marine setting. Pyroclastic rocks mapped in the area have been interpreted as volcanic tuffs and agglomerates interstratified with the shales.

Key words: Lithofacies, depositional environment, rocks, Nkpuma-akpatakpa, Izzi.