Facies Associations and Successions in Amaseri Sandstones, Southern Benue Trough, Nigeria: Implications for Interpretation of Depositional Environments and Palaeogeography

Abstract

The purpose of this study is to identify and describe sandstone facies and facies associations of Amaseri Sandstone and its succession as a means of assessing the depositional environments. The data set used consists of logged outcrop sections describing critical lithofacies parameters such as composition, lithology, paleoflow, photo montages of some bedding planes, textures, fossils and sedimentary structures including biogenic structures studied on outcropping vertical sections of the Amaseri Sandstones in the study area.  The lithofacies interpreted from the Amaseri Sandstone consist of seven main types; some of which represent Bouma sequence ranging from Ta to Td. The interpreted lithofacies are: cryptobioturbated, conglomeritic, siliceous sandstone (Ta); bioturbated, shelly, conglomeritic, calcareous sandstones (Ta); bioturbated, conglomeritic, arenaceous sandstone (Ta); bioturbated, parallel laminated, calcareous sandstone (Tb); ripple/wavy laminated sandstone (Tc); cryptobioturbated mudstone (Td); and cross bedded, arenaceous sandstones. These lithofacies were integrated with physical sedimentary structures, biogenic structures, body fossils content and nature of bedding contacts to interpret facies associations. The facies associations interpreted are: proximal fan facies associations deposited by debris flows and represented by the conglomeritic sandstones (Ta); outer fan facies associations deposited by turbidity currents and represented by bioturbated, parallel, calcareous sandstone (Tb) and ripple/wavy laminated sandstone (Tc); mid-fan facies association represented by crptobioturbated mudstone (Td); and offshore to shoreface facies association represented by cross bedded arenaceous sandstone.The palaeoflows interpreted from the rose diagram constructed from the azimuths of the trough cross beds indicate two directional modes, the north-west and the south-west flows. The petrographic studies show that the grain shapes are dominantly sub-angular to sub-rounded, moderately sorted and texturally and compositionally immature indicating that the sediments were not long transported. The combined palaeoflow and petrographic results show that the Amaseri Sandstones were sourced from the nearby Calabar Flank and Oban Massif as indicated by the northwestery palaeoflow and from the Basement Complex in areas around Ogoja as indicated by the southwestery palaeoflow mode. The limestone cobble/boulders forming the rip-up clast in the conglomeritic sandstone may be sourced from the Mfamosing Limestone located within the Calabar Flank.

Keywords: Amaseri Sandstone, Facies Association, Palaeoenviroments, Palaeogeography and Benue Trough