Abstract
Lithofacies, bounding surfaces and sedimentary architectural elements exposed in two ridges at Uturu being quarried for construction sands were analysed to determine the paleoenvironment of deposition and the factors that control the deposition of sand units. Mainly outcropped is Ajali Formation overlying locally exposed Mamu Formation in Western Afikpo basin. Lithofacies identified include: Trough cross bedded medium- to coarse-grained (St), Planar cross bedded fine- to coarse-grained sandstone (Sp), Small scale planar cross bedded sandstone (SSp), Lenticular mudstone (Fm), Heterolithic sandstone/mudstone (Fsm), Horizontal stratified sandstone (Sh), Cross ripple laminated sandstone (Sr), Reddish muddy sand (Fl), Siltstone (SSm) and Shale (Fsh). The associations of lithofacies and bounding surfaces gave four fluvial and one marine architectural element. The fluvial elements which mainly characterized the Ajali Formation include: Channel-fill (CH), Macroforms Accretion (MA). Flood-Plain Fines (FF) and Channel Abandonment Fines (CAF). Offshore-shoreface fines (OSF) element defined marine Mamu Formation. The profiles of the ridges show dominance of MA followed by CH while FF is limited in occurrence and in some zones pinch-out to lenticular inter-bar mudstone. The MA is characterized by planar cross beddings, reactivation surfaces, internal grading, steep dipping ferruginized accretion surfaces and abrupt flat top which indicate mid-channel bars deposition in typical sandy braided fluvial depositional system. Generally, there is vertical aggradation/ amalgamation of channel deposits and dominance of sheet alluvial architecture. Low rate of channel avulsion, moderate rate of lateral migration and aggradation, variable discharge rate and high rate of sediment supply and subsidence were considered as factors that controlled the deposition and preservations of sand units. This study provided an understanding of mesoscopic heterogeneities and compartmentalization style inherent in hydrocarbon bearing sandy braided reservoirs which can be used as analog model for its development.
Highlights
The aim of this paper is to document the lithofacies types, bounding surfaces and architectural elements that unequivocally defined the paleoenvironments of deposition and factors that control the deposition and preservations of sand units in the outcropped Formations by the reason of their exposure at the Uturu sands quarry sites
The discontinuity of some of the sandstone ridges suggests sandy braided fluvial successions and their evolvement in locations where depressions as a result of differential tectonic subsidence occurs along NW- SE trending fluvial channels that cut into the pre-existing marine deposit called Mamu Formation
The analysis of Lithofacies and architectural elements exposed in two ridges at Uturu being quarried for constructions sands revealed the followings: 1. The Ajali sandstone and Mamu Formation comprised of fluvial and Marine sedimentary architectures respectively
Summary
The terrain of Uturu in Nigeria is characterized by thickly vegetated ridges and depressions, with elevations above mean sea level that ranged from 127 to 320m. The Ajali Formation whose facies and internal architecture are well exposed at Uturu sands quarrying sites has been studied by many authors at different locations. It was named Ajali sandstone and described as false bedded sandstone by (Reyment, 1965; Simpson, 1954). Some researchers through petrographic analysis of samples classified it as quartz arenite and sub-feldspathic arenite respectively (Hogue and Ezepue, 1977; Uzoegbu and Ikwuagwu, 2016).
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