Microfacies Analysis, Geochemistry and Depositional Environment of Turonian Nkalagu Limestone, Southeastern Nigeria

Opatola, A. O., Oluwajana, O. A., and Agbim, C.
Keywords: Microfacies, Nkalagu Formation, Wackestone, Bioclast, Depositional Setting
Tropical Journal of Science and Technology 2020 1(1), 1-15. Published: June 29, 2020


Abstract

The integration of petrographic, microfacies and geochemical data were used to interpret the depositional settings of the Turonian Nkalagu Limestone within Lower Benue. Three different microfacies were delineated within the limestone beds, these are: bioclastic mudstone-wackestone facies, bioclastic wackestone facies and Siliceous lime mud facies. The Bioclastic mudstone-wackestone and the Bioclastic wackestone facies are generally matrix-supported, fossil-rich with whole fossils of brachiopods, gastropods and disaggregated fossils such as shell fragments. They possess abundant carbonate of about 61.1%, the lithoclasts are generally angular to sub-angular showing poor-sorting. They are composed of very fine-grained aggregates of micritic matrix/carbonate mud with dispersed and random orientation of bioclasts which suggests deposition in warm, shallow sub-tidal environment. The Siliceous Lime Mud Facies are generally low in fossil content with presence of pellets dispersed in lime mud and lithoclasts of quartz and feldspar which indicates varying periods of siliciclastic influx. Dispersion of pellets in lime mud hints deposition in protected shallow-marine platforms in low to medium energy conditions. The pattern of diffraction for limestone sample shows decrease in calcite concentration from Bioclastic wackestone, Bioclastic mudstone-wackestone and Siliceous lime mud respectively. Calcite constitutes about 61.1%, with 12% quartz and 19% albite in Bioclastic wackestone. Muscovite occurs in tiny amounts as well as kaolinite (2.0%) and gypsum (1.9%). The Siliceous lime mud is silica rich with 22.6% quartz, 13.9% albite and 7.8% k-feldspar. Calcite only occurs in 52.7% with illite and pyrite of 2.0% and 1.0% respectively. The trace elements show higher concentrations in the Siliceous lime mud faces than bioclastic wackestone and bioclastic-mudstone wackestone facies. The trace element thus shows diagnostic pattern of distribution that differentiate deepening water depth of the depositonal setting. The integration of microfacies, X-ray diffraction and trace element concentration therefore pointed towards a shallow marine depositional setting.