Microfossil Assemblages of the Subsurface Parts of the Enugu Formation: Proxies to Depositional Environment and Age of the Formation

Abstract

Two boreholes sunk into Enugu Formation within Enugu Metropolis were logged from the base to the top and samples collected were analyzed for palynomorphs and foraminifera in order to interpret the paleoenvironment and assign age to the formation. Four lithofacies identified include; dark fissile shale lithofacies, heterolithic lithofacies, sand laminated shale lithofacies and greyish shale lithofacies. Palynomorphs recovered consist of pteridophytic spores, angiosperm pollen, fungal spores and Botryoccocus braunii.  Echitriporites trianguliformis, Cythidites sps,. Retitricolporite sp., Retidiporite magdalenensis, Laevigatosporites sp., Zlivisporites blanensis among others were recorded. Five genera of arenaceous benthic foraminifera (Ammobaculites sps., Haplophragmoides sp.,, Trochamina sp., Ammotium sp. and Saccamina sp.) dominated by Ammobaculites sps. were also recovered. The coarsening upwards of the lithofacies succession from basal shale to sand laminated greyish shale lithofacies and the heterolithic lithofacies observed in borehole 1 is an attribute typical of a prograding sea. Evidences from the lithofacies and association, palynomorphs and foraminiferal assemblages showed sediment deposition in a marginal marine setting. Low abundance and diversity of the arenaceous foraminiferal species with the predominance of Ammobaculites sps. is characteristics seen in almost all the samples analyzed. These are suggestive of stressful environment due to low water salinity, rapid sedimentation with influx from terrestrial environment which resulted to stagnant and restricted low oxygen bottom water condition.

Keywords: Palynomorphs, Lithofacies, Foraminifera, Environment, Deposition, Shale