Lithofacies and environments of deposition within the Middle–Upper Cretaceous successions of Southeastern Nigeria

Abstract

This paper presents findings from detailed geologic mapping of the Cenomanian–Campanian sediment package of southern Benue Trough, outcropping in Afikpo area of Southeastern Nigeria. The aim was to integrate outcrop and sedimentological log data with lithofacies analyses to better understand prevalent depositional processes and environments of deposition, across the Amaseri–Ngodo outcrop sections. Ten lithofacies with their associated subordinate lithofacies were identified and grouped into five facies associations (FA 1–5) to aid depositional environment interpretation. FA 1 consists of silty-shale and shales of the mudstone (MS) lithofacies deposited within an oxygenated offshore/open shelf setting, probably below fair-weather and storm wave base. FA 2 comprises medium–coarse-grained sandstone (MCS), hummocky cross-bedded sandstone (sub-lithofacies 4 of CBS), bioturbated sandstone (BSs) facies, bioturbated siltstone (BSt) and planar parallel-laminated sandstone (PPLS) lithofacies, interpreted to be storm wave-dominated shelf deposits. FA 3 consists of tabular/planar and trough cross-bedded sandstones (sub-lithofacies 1 and 2 of CBS) and wave ripple-laminated sandstones (RLS) lithofacies interpreted as shorefaces deposits above fair-weather wave base. Herringbone cross-bedded sandstones (sub-lithofacies 3 of CBS) and bioclastic limestone (BcL) lithofacies constitute FA 4, interpreted as tidal inlet/lagoonal deposits. FA 5 is characterized by conglomeritic sandstone (CS) lithofacies, as well as CBS, PPLS, RLS lithofacies, deposited within fluvial channels. Furthermore, vertical stacking patterns reveal the occurrence of two facies successions: the fining-upwards/thinning-upwards (FU/TnU) succession (typical of deepening or shallow to proximal offshore marine environments), dominating the northern and central part of the study area; and coarsening-upwards/thickening-upwards (CU/TkU) succession (typical of shoaling or proximal offshore to shallow marine environment), dominating the southern part of the study area. Overall, these lithofacies represents deposits within the littoral (marginal marine) and sub-littoral (shallow marine) environments.