PROVENANCE AND PALEOENVIRONMENTS OF THE SHALES OF PATTI AND ENAGI FORMATIONS IN BIDA BASIN, NORTH CENTRAL NIGERIA

EVIDENCE OF MARINE COVERAGE IN THE CAMPANIAN – MAASTRICHTIAN

Abstract

The lateral equivalent Campanian- Maastrichtian shales of Patti and Enagi formations in the Bida Basin were investigated for provenance and paleoenvironments through inorganic geochemistry and foraminiferal paleoecological studies. The shales exposed at Ahoko, Patti Formation and at subsurface in a borehole at Gulu, Enagi Formation are grey to black and massive with about 10m and 28m thicknesses, respectively. Gulu shales consist mainly of benthonic foraminifera assemblages with representative species of Ammotium sp, Ammobaculites sp, Haplophragmoides sp, Trochammina sp, Miliamina sp, Pseudobilivina sp., Spiroplectammina sp., and Textularia sp. while the shale from Ahoko included similar benthic foraminifera and macro fossils like mulluscs. Their paleoecology suggests a slightly reduced saline marine environment ranging from brackish to the continental shelf. Vanadium to nickel ratio ranging from 0.6 to 3.3 in Ahoko and 0.2 to 52.6 in Gulu suggests mixed marine and terrigenous (fluvial) sources under dysoxic to oxic conditions with more marine influence in the shales of Gulu. Paleo-oxygenation indicated by V/Cr ratio of 0.1 to 4.3 in Ahoko and 0.6 to 6.3 in Gulu which are below 4.5 shows that paleo-conditions were more oxic with very sparse period of anoxic conditions for those above 4.5. SiO₂ and Al₂O₃ are the most abundant oxides of the shales with average values of 57.6% and 26.9% in shales from Ahoko, and 51.3% and 27.5% in Gulu shale respectively suggesting more influx of terrigenous and argillaceous sediments of the fluvial settings in the south. Average Al₂O₃/TiO₂ of 14.3 in Ahoko, and 23.4 in Gulu indicates that their sediments were sourced from intermediate igneous and felsic rocks respectively. Their degree of weathering estimated from the chemical index of alteration (CIA) ranges from 94 - 107, thus indicating a high degree of chemical weathering in hot and humid tropical climates. K₂O/Na₂O against SiO₂ and other indices suggest an active continental margin with a mixture of intermediate, mafic, and felsic igneous rock for the source of the sediments for the shales of Patti and Enagi formations. The paleo-conditions of the studied shales are similar across the basin and this suggests a wide coverage of sedimentation of marine shale across the basin during the Campanian to Maastrichtian which could be a potential petroleum source or sealing rocks in the Bida Basin.