Paleoenvironment, Paleoredox and Paleogeography of Mamu Shale Anambra Basin Southern Nigeria

Abstract

The Shales in Okada and Usen within Mamu Formation, Anambra Basin were evaluated for Paleogeography, Paleoenvironment and Paleoredox conditions by examining the foraminifera, polymorph assemblages and the trace elements concentration accordingly. the application of trace element composition ratios: V/Cr, V/Ni, U/Th, Ni/Co, V/(Ni+V)², V/(Sc)³ with the averages of 1x10^-3, 8x10^-4, 5.1x10^-1, 1.31, 9x10^-8 and 1x10^-5 respectively. These paleo–redox index for both Usen and Okada Shales indicates oxic condition in the ancient environment.  The foraminifera assemblages recovered are Ammobaculites sp., Ammobaculites irregulariformis sp., Ammobaculites fisheri crespin sp., Textularia sp. and Textularia earlandi sp. The forams recovered were of benthic arenaceous and calcareous foraminifera assemblages, typically of late Paleocene to early Eocene which coincides with marine incursion in the Anambra Basin. The presence of very few ammonites from the shale samples suggests some lithofacies control.  The palymorphs assemblages recovered are Triporites sp., Triorites sp., Hexacolpites sp., Araucariacites australis sp., Monoporites annulatus sp.,   Pseudoschizaea sp. and Arecipites sp. The presence of Arecipites in sedimentary rocks indicates the presence of palms in the local environment during the time the sediment was deposited. The recovered palymorphs indicates shales that were derived from terrestrial landscapes except Pseudoschizaea sp. that is found in wetlands and/or lacustrine environments. This suggested that the paleoecology at time the shale was deposited was predominantly land-based (terrestrial). The mixed terrestrial-marine environment and the oxic conditions would assist geologists to better assesses the potential for hydrocarbon accumulation and develop more effective exploration strategies.